ADDENDA . . . 231
INDEX . . . 233
{1}
PAPER-MAKING.
INTRODUCTORY NOTE.
The raw materials of the paper-maker are primarily the vegetable
fibrous substances; in addition to these there are various articles
which are employed as auxiliaries, either in the preparatory or
finishing processes to which these fibrous materials, or the web of
paper are subjected. The latter class are of subsidiary importance,
more especially from our present point of view.
In insisting upon the recognition of first principles, we cannot
overrate the importance of a thorough grasp of the constitution of the
plant fibres, as the necessary foundation for the intelligent conduct
of paper-making, and to this subject we will at once proceed.
Careful study of a mature plant will show that it is made up of
structural elements of two kinds, viz. fibres and cells, which, to use
a rough parallel, we may liken in function to the bricks and mortar
of a house. It is the former which admit of the many extended uses,
with which we are familiar, in the arts of spinning and weaving, and
which constitute the fabrics which are the most indispensable to our
civilised life. For the most part, as we know, fibres and cells are
aggregated together into compound tissues, and a process of separation
is therefore a necessary preliminary to the utilisation of the former.
The cotton fibre is the only important exception to this general
condition of distribution. Here we have the seed envelope or perisperm,
converted into a mass {2} of fibres, and these by a spontaneous
process accompanying the ripening, so isolated as to be immediately
available. Next in order in point of simplicity of isolation, are those
fibrous masses, or tissues, which, although components of complex
structures, exhibit a greater cohesion of their constituent fibres than
adhesion to the contiguous cellular tissues with which they go to build
up the plant. Into such a tissue the “bast,” or inner bark layer of
shrubs and trees, more especially those of tropical and sub-tropical
regions, frequently develops, and it is, in fact, this bast tissue,
graduating in respect of cohesion of its constituent fibres, from a
close network such as we have spoken of, to a collection of individual
fibres or fibre-bundles disposed in parallel series, which supplies
the greater part of the more valuable of the textile and paper-making
fibres; we may instance flax, hemp, and jute, each of which is the
basis of an enormous industry. According to the degree of adhesion of
the bast to the contiguous tissues, or, in another aspect, according to
its lesser aggregate development, so is the difficulty of isolation and
the necessity of using processes auxiliary to the mechanical separation
of the tissue.
It is worthy of note here that the Japanese paper with which we
are in these times so familiar, is prepared by the most primitive
means from the bast of a mulberry (_Broussonetia papyrifera_); the
isolated tissue, consisting of a close network of fibres, is simply
cut and hammered to produce a surface of the requisite evenness, and
the production of a web of paper is complete. In isolating the bast
fibres employed in the textile industries, a preliminary partial
disintegration of the plant stem is brought about by the process of
steeping or retting, by which the separation of fibre from flesh or
cellular tissue is much facilitated.
Last in order of simplicity of distribution, we have the fibres
known to the botanist as the fibro-vascular bundles of leaves and
monocotyledonous stems, these bundles being irregularly distributed
through the main cellular mass, and consequently, by reason of adhesion
thereto, much more {3} difficult of isolation. For this and other
reasons, more or less in correlation with natural function, we shall
find this class of raw material lowest in value to the paper-maker.
It is necessary at this stage to point out that the work of the
paper-maker and that of the textile manufacturer are complementary one
to the other, and the supply of fibrous raw material is correspondingly
divided: it may be said, indeed, that the paper industry subsists
upon the rejecta of the textile manufactures. The working up of
discontinuous fibre elements into thread, which is the purpose of the
complicated operations of the spinner, is conditioned by the length
and strength of these ultimate fibres. Paper-making, on the other
hand, requires that the raw material shall be previously reduced to
the condition of minute subdivision of the constituent fibres, and
therefore can avail itself of fibrous raw material altogether valueless
to the spinner, and of textile materials which from any cause have
become of no value as such. To the raw materials of the paper-maker,
which we have briefly outlined above, we must therefore add, as a
supplementary class, textiles of all kinds, such as rags, rope, and
thread.
Having thus acquired a general idea of the sources of our raw
materials, we must study more closely the substances themselves, and
first of all we must investigate them as we should any other chemical
substance, i.e. we must get to understand the nature and properties
of the matter of which the vegetable fibres are composed. While these
exhibit certain variations, which are considerable, the substances
present a sufficient chemical uniformity to warrant their being
designated under a class name: this name is cellulose. The prototype of
the celluloses is the cotton fibre.
{4}
CHAPTER I.
CELLULOSE: THE CHEMICAL PROPERTIES OF TYPICAL MEMBERS OF THE CELLULOSE
GROUP, WITH REFERENCE TO THEIR NATURAL HISTORY.
Plants are so far built up of cellulose that it may be called the
material basis of the vegetable world. Plant tissues, however, seldom,
if ever, consist of pure cellulose, but contain besides, other products
of growth either chemically combined with the cellulose or mechanically
bound up with the tissue, which are, according to the nature of their
union, removable either by means of fundamental chemical resolution
or by the application of simple solvents. A general method for the
isolation of cellulose consists in exposing the moist tissue to
the action of chlorine gas or of bromine water in the cold, and
subsequently boiling in dilute ammonia; repeating this treatment until
the alkaline solution no longer dissolves anything from the tissue or
fibre. The cellulose is then washed with water, alcohol, and ether,
and dried. Obtained in this way, or in the form of bleached cotton, or
of Swedish filter paper, it is a white substance, more or less opaque,
retaining the microscopic features of the tissue or fibre from which it
has been isolated. Its sp. gr. is 1·25–1·45. Its elementary composition
is expressed by the percentage numbers (Schulze)
C 44·0 44·2
H 6·3 6·4
O 49·7 49·4
or by the corresponding empirical formula, viz. C_{6}H_{10}O_{5}.
These numbers represent the composition of the ash-free cellulose.
Nearly all celluloses contain a certain proportion, {5} however small,
of mineral constituents, and the union of these with the organic
portion of the fibre or tissue is of such a nature that the ash left on
ignition preserves the form of the original. It is only in the growing
point of certain young shoots that the cellulose tissue is free from
mineral constituents. (Hofmeister.)
As already indicated, cellulose is insoluble in all simple solvents;
it is, however, dissolved by certain reagents, but only by virtue of a
preceding chemical modification. An exception to this is to be found,
perhaps, in the ammoniacal solution of cupric oxide (Schweitzer’s
reagent), in which it dissolves without essential modification, being
recovered by precipitation, in a form which is chemically identical
with the original, though differing, of course, in being structureless,
or amorphous. This reagent may be employed in a variety of forms, but
the following method of using it is to be recommended as the most
certain in its results. The substance to be operated upon is intimately
mixed with copper turnings in a tube which is narrowed below and
provided with a stopcock. Strong ammonia is poured upon the contents of
the tube and, after being allowed to stand for some minutes, is drawn
off and returned to the tube; the operation is several times repeated
until the solution of the substance is effected. In order to facilitate
the oxidation of the copper by the atmospheric oxygen, a current of
air may be aspirated through the apparatus. The solution of the oxide
prepared in this way is more effective in its action on cellulose
than that obtained by dissolving the precipitated hydrate in ammonia.
Cellulosic tissues in contact with this reagent are seen to undergo
a disaggregation of their fibres, which swell up, become gelatinous,
and disappear in solution. On adding an acid to the viscous solution,
a precipitate of the amorphous cellulose is obtained in the form of a
jelly resembling hydrated alumina; after washing and drying, it forms
a brownish, brittle, horny mass. The cellulose is also precipitated
upon simply diluting the viscous solution with water and allowing it
to stand {6} 8–10 days in a closed vessel. From this observation it
was inferred by Erdmann that the cellulose could not be considered
as dissolved in the strict sense of the word, but the experiments
of Cramer upon the osmotic properties of the solution proved this
inference to be unfounded, and that cellulose is actually dissolved by
the ammoniacal solution of copper oxide.
On treating the ammonio-cupric solution of cellulose with metallic
zinc, this metal precipitates the copper, replacing it in the solution,
and producing the corresponding ammonio-zincic solution of cellulose,
which is colourless. Some of these solutions are lævo-gyrate.
Cellulose, in those forms to which the application of the term has
been hitherto restricted, is a comparatively inert substance, and its
reactions are consequently few. One of these is available for the
identification of cellulose, and is chiefly used in the microscopical
examinations of tissues: this is its reaction with iodine. Cellulose
is not coloured blue by a solution of iodine excepting under the
simultaneous influence of hydriodic acid, potassium iodide, sulphuric
acid, phosphoric acid, or zinc iodide or chloride. The solution is
prepared in the following way: zinc is dissolved to saturation in
hydrochloric acid, and the solution is evaporated to sp. gr. 2·0; to
90 parts of this solution are added 6 parts potassium iodide dissolved
in 10 parts of water; and in this solution iodine is dissolved to
saturation. By this solution cellulose is coloured instantly a
deep-blue or violet. For the identification of cellulose in the gross,
mere inspection is usually sufficient; confirmatory evidence is
afforded by an observation of the action of the ammonio-copper reagent,
and of the absence of reaction with chlorine water. (See p. 18.)
Cellulose in its earlier stages of elaboration has no action upon
light; but with age it acquires the property of double refraction, not,
as has been shown by experiment, by virtue of its state of aggregation,
but of its molecular constitution (Sachs). {7}
_Animal Cellulose._—The mantles of many of the mollusca, e.g. the
_Pyrosomidæ_, _Salpidæ_, and _Phallusia mammillaris_, contain a
resistant substance which, after isolation by chemical treatment,
has been found to be identical both in composition and properties
with vegetable cellulose. Cellulose has also been stated to occur in
degenerated human spleen and in certain parts of the brain.
_Compounds of Cellulose._—The chemical inertness of cellulose is a
matter of everyday experience in the laboratory, where it fulfils
the important function of a filtering medium in the greater number
of separations of solids from liquids. Its combinations with acids
and with basic oxides are, as might be expected, few and of little
stability. It has been shown by Mills that cellulose (cotton) in
common with certain other organic fibrous substances, when immersed in
dilute solutions of the acids or basic oxides, condenses these bodies
within itself at the expense of the surrounding solution, which is
proportionately weakened. This effect of concentration is sufficiently
uniform and constant to lead us to assign it to a chemical cause, and
the view is strengthened by a consideration of the relative effects
upon the various acids and bases which have been investigated, and
brought to the following numerical expression:—Weight of cotton 3
grm. (with 6·9 per cent. H_{2}O and 0·05 per cent. ash)—i.e. 2·893
anhydrous fibre. Strength of solution about 0·5 grm. of the reagent in
250 cc.
Temp. Time. Weight absorbed.
H_{2}SO_{4} 4° C. (39° F.) 3 min 0·00495
HCl „ „ 0·00733
NaOH „ „ 0·02020
The molecular ratio of the absorption, in the two latter, is
3 HCl : 10 NaOH, and it is noteworthy that the same ratio was observed
for silk.
Cellulose removes barium hydrate from its solution in wafer to form
with it an insoluble compound. On adding lead acetate to the solution
of cellulose in the ammonio-copper reagent, so prepared as to contain
no carbonate, a {8} precipitate is obtained consisting of a compound of
cellulose with lead oxide, but in variable proportions. The compound
C_{6}H_{10}O_{5}PbO is formed by the action of finely-divided lead
oxide upon the above solution. Quite recently it has been shown
(O’Shea, _Chem. News_, May 28th, 1886) that when dilute solutions of
lead are passed through ordinary filter paper, a certain amount is
retained which cannot be removed by washing.
Cellulose does not combine with metallic salts, a fact which has been
established incidentally to researches upon the mode of action of
mordants.
The combinations of cellulose with acid radicles (ethereal salts) are
both definite and stable.
_Triacetyl Cellulose_ [C_{6}H_{7} (C_{2}H_{3}O)_{3} O_{5}] is formed
by treating cellulose with six times its weight of acetic anhydride at
180° C. (356° F.). The product of the reaction is a syrupy solution
from which the compound in question separates on dilution with water as
a white flocculent precipitate.
Triacetyl cellulose is insoluble in alcohol and in ether, but soluble
in glacial acetic acid. It is easily saponified by boiling with a
solution of potassium hydrate, the cellulose being regenerated. No
derivative containing more than three acetyl groups has been obtained;
but a mixture of the mono-and di-acetyl cellulose is formed in treating
cellulose with only twice its weight of acetic anhydride, the formation
of these bodies being unattended by their solution.
Whenever cellulose, in any form, is brought into contact with strong
nitric acid at a low temperature, a nitro product, or a nitrate, is
formed. The extent of the nitration depends upon the concentration of
the acid, on the time of contact of the cellulose with it, and on the
state of the physical division of the cellulose itself.
Knop, and also Kamarsch, and Heeren, found that a mixture of sulphuric
acid and nitric acid also formed nitrates of cellulose; and still later
(1847), Millon and Gaudin employed a mixture of sulphuric acid and
nitrates of soda or potash, which they found to have the same effect.
{9}
Several well characterised nitrates have been formed, but it is a very
difficult matter to prepare any one in a state of purity, and without
admixture of a higher or lower nitrated body.
The following are known:—
Hexa-nitrate, C_{12}H_{14}O_{4}(NO_{3})_{6},[1] gun cotton. In the
formation of this body, nitric acid of sp. gr. 1·5, and sulphuric acid
of sp. gr. 1·84 are mixed, in varying proportions, about 3 of nitric to
1 of sulphuric (sometimes this proportion is reversed), and cotton is
immersed in this at a temperature not exceeding 10° C. (50° F.) for 24
hours: 100 parts of cellulose yield about 175 of cellulose nitrate. The
hexa-nitrate so prepared is insoluble in alcohol, ether, or mixtures of
both, in glacial acetic acid or in methyl alcohol. Acetone dissolves
it very slowly. This is the most explosive gun-cotton. It ignites at
160°–170° C. (320°–338° F.). According to Eder the mixtures of nitre
and sulphuric acid do not give this nitrate. Ordinary gun cotton may
contain as much as 12 per cent. of nitrates soluble in ether-alcohol.
The hexa-nitrate seems to be the only one quite insoluble in
ether-alcohol.
[Footnote 1: To represent the series of cellulose nitrates so as
to avoid fractional proportions, the ordinary empirical formula
is doubled and the nomenclature has reference to this double
molecule.]
Penta-nitrate, C_{12}H_{15}O_{5}(NO_{3})_{5}. This composition
has been very commonly ascribed to gun-cotton. It is difficult,
if not impossible, to prepare it in a state of purity by the
direct action of the acid on cellulose. The best method is the one
devised by Eder, making use of the property discovered by de Vrij,
that gun-cotton (hexa-nitrate) dissolves in nitric acid at about
80°–90° C. (176°–194° F.) and is precipitated, as the pentanitrate,
by concentrated sulphuric acid after cooling to 0° C. (32° F.); after
mixing with a larger volume of water, and washing the precipitate with
water and then with alcohol, it is dissolved in ether-alcohol, and
again precipitated with water, when it is obtained pure.
This nitrate is insoluble in alcohol, but dissolves readily
{10} in ether-alcohol, and slightly in acetic acid. Strong
potash solution converts this nitrate into the di-nitrate,
C_{12}H_{18}O_{8} (NO_{3})_{2}.
The tetra- and tri-nitrates (collodion pyroxyline) are generally
formed together when cellulose is treated with a more dilute nitric
acid, and at a higher temperature, and for a much shorter time (13
to 20 minutes), than in the formation of the hexa-nitrate. It is not
possible to separate them, as they are soluble to the same extent in
ether-alcohol, acetic ether, acetic acid or wood spirit.
On treatment with concentrated nitric and sulphuric acids, both
the tri-and tetra-nitrates are converted into penta-nitrate and
hexa-nitrate. Potash and ammonia convert them into di-nitrate.
Cellulose di-nitrate, C_{18}H_{13}O_{8} (NO_{3})_{2} always results as
the final product of the action of alkalis on the other nitrates, and
also from the action of hot, somewhat dilute, nitric acid on cellulose.
The di-nitrate is very soluble in ether-alcohol, acetic ether, and in
absolute alcohol. Further action of alkalis on the di-nitrate results
in a complete decomposition of the molecule, some organic acids and
tarry matters being formed. The reactions and resolution products of
this body have, however, been but slightly studied, and apparently not
at all with the view to elucidate anything respecting the constitution
of cellulose itself.
_Cellulose and Chlorine._—Dry chlorine gas has no action upon
cellulose, but the presence of water determines an indirect oxidising
action. This oxidising action results from the decomposition of water
by chlorine with formation of hydrochloric acid and liberation of
oxygen. The reaction, however, can only take place in the presence of
light.
_Cellulose and Oxygen._—When cotton is exposed for some time to the
action of a solution of bleaching powder, in the cold, and with access
of air, e.g. when a piece of white calico is moistened with the
solution, squeezed out and exposed to the air, it is found to undergo
gradual disintegration accompanied by a change in composition, and an
elementary {11} analysis of the product shows it to contain less carbon
and more oxygen than the original cellulose. The following percentage
numbers indicate the composition of these oxidised derivatives of
cellulose, and the progress of the oxidation:—
12 hours’ exposure. 24 hours’ exposure.
C 43·78 43·47 43·00 42·90
H 5·85 6·13 6·28 6·18
O 50·37 50·40 50·72 50·92
Other oxidising agents produce similar results; even by exposure to
air and light, cellulose is slowly converted into these oxidised
derivatives.[2] From their mode of formation, they have been termed
oxycelluloses, and to distinguish them from a series of more
highly oxidised derivatives, produced by the action of nitric acid
upon cellulose, which they nevertheless resemble in many of their
characteristics, the prefix α is employed. The following are the
distinguishing features of the α oxycelluloses as represented by the
more extreme of the above mentioned products. It reduces Fehling’s
solution at the boiling temperature, and the cuprous oxide is deposited
upon the fibre in a state of intimate union, producing the effect of
an orange dye. It attracts the basic colouring matters from their
solutions and is dyed to a full shade, the depth of colour being
proportionate to the amount of oxidation to which the cellulose has
been subjected. See also p. 43. Treated with a warm solution of
phenylhydrazine salts in water, it is coloured a bright lemon-yellow.
Its most remarkable property is its attraction for the vanadium
compounds, which is so powerful that combination may be proved to
take place when this element in the form of chloride is presented to
the oxycellulose in an aqueous solution containing not more than 1 in
1,000,000,000,000 parts.
[Footnote 2: Witz. Bull. Soc. Ind. Rouen, _X._ 416, and _XI._ 189.]
The β oxycellulose resulting, as already indicated, from the
action of dilute nitric acid upon cellulose, will be subsequently
considered, under the head of the decompositions of cellulose, to
which the reaction which we have been considering may be regarded as
transitional. {12}
_Decomposition of Cellulose._—_Acids._—Of the decomposition of
cellulose by artificial means, i.e. by the action of reagents, the
most important are those which disclose the relationship which it
bears to the group of so-called carbo-hydrates. Cellulose is, indeed,
a highly _elaborated_ carbo-hydrate, differing from dextrin and
starch, not so much in its essential constitution as in the molecular
arrangement which has been impressed upon it in and through the life of
the plant. There are numerous observations in biological science which
demonstrate the close connection, and in many cases the physiological
equivalence of the members of this natural group, which includes the
sugars, dextrin, starches, and cellulose. The following facts show the
constitutional or chemical ties which underlie their interdependence.
_Amyloid._—When cotton is treated with 30 times its weight of sulphuric
acid somewhat diluted (1 part H_{2}O, 4 parts H_{2}SO_{4}) it quickly
dissolves, the solution being at first gelatinous, becoming afterwards
syrupy. On pouring this solution into water, a white flocculent
precipitate is formed, consisting of a substance closely resembling the
cellulose from which it is derived, differing, indeed, only in being
amorphous, and in being coloured blue by iodine. This substance, from
its resemblance to starch, has been called _amyloid_. When sufficiently
dehydrated it constitutes a swollen starchy mass, which dries to a
brittle transparent solid. This transformation appears to consist in a
simple hydration of the cellulose molecule, thus:—
Cellulose. Amyloid.
2 (C_{6}H_{10}O_{5}) + H_{2}O = C_{12}H_{22}O_{11}.
The modification of cellulose, which occurs on the conversion of
unsized paper into the so-called parchment paper, by exposure for a
short time to the action of strong sulphuric acid, and subsequent
washing and drying, consists doubtless in a superficial conversion of
the cellulose into _amyloid_, or a body closely resembling it.
_Hydracellulose._—Closely related in composition to the above {13}
described amyloid, is the substance called hydracellulose, into which
cellulose is converted by exposure for some time to the action of
moderately strong sulphuric, or hydrochloric acids (sp. gr. 1·35).
This substance is also formed by the action of very weak acids, if the
exposure of the cellulose be sufficiently prolonged. Its formation is
accompanied by the complete disintegration of the cellulose. If the
action of strong sulphuric acid be prolonged, a further modification
ensues, dextrin or sugar being formed. These products have not been
sufficiently studied to have established their exact identity with
any of the substances known under these group-names. The dextrin has
a close resemblance to the dextrin obtained from starch; the sugar
is susceptible of fermentation; but beyond this nothing specific is
known of these bodies. Their general properties, however, and mode of
formation, sufficiently establish the fact that cellulose itself is
constituted of similar molecules.
The action of zinc chloride solution upon cellulose is similar to that
of sulphuric acid.
Cellulose in contact with iron, and in presence of air and moisture, is
converted into a sugar and a gummy substance, which latter is converted
into a sugar on boiling with dilute acids. Oxidised by potassium
permanganate or bichromate in presence of acetic acid, it is converted
into glucose, dextrin, and formic acid.
_Cellulose and nitric acid: β Oxycellulose._[3]—On digesting cellulose
with nitric acid (sp. gr. 1·3) at 100° C. (212° F.), a considerable
quantity of oxalic acid is formed; but after prolonged digestion, a
residue remains, which yields but very slowly to the action of the
acid. This substance, which is white and flocculent, when thrown upon
a filter and washed with water, combines with the latter to form a
gelatinous hydrate. It dissolves in dilute alkalis, but without forming
a stable compound with the base. It usually amounts to about 30 per
cent. of the cellulose acted upon. Its elementary composition is
expressed by the formula {14} C_{18}H_{26}O_{16} (C = 43·4, H = 5·3
per cent.). Treated with a mixture of concentrated sulphuric and nitric
acids, it dissolves, and, on pouring the solution into water, the
compound C_{18}H_{23}O_{13}(NO_{3})_{3} separates as a white flocculent
precipitate. In its essential properties therefore β oxycellulose
exhibits a close resemblance to cellulose itself. This oxycellulose is
distinguished from the members of the cellulose group by dissolving
in concentrated sulphuric acid with development of a pink colour. It
appears to stand in very close relationship to the group of pectic
substances, which are oxidised derivatives of cellulose.
[Footnote 3: Cross and Bevan, Chem. Soc. Journ., xliii. p. 23.]
_Alkalis._—Concentrated solutions of the caustic alkalis in the cold
attack and disintegrate cellulose but slowly. The action is in many
cases merely superficial. Dilute solutions of the alkalis are without
action upon cellulose even at the boiling temperature, unless the
condition of oxidation is super-added; in this case, acid bodies are
found. The extreme product of the oxidation of cellulose in presence of
the caustic alkalis is oxalic acid; this acid has indeed been prepared
on the commercial scale by the “fusion” of cellulosic substances,
with mixture of sodium and potassium hydrates. Malic acid has been
obtained as an intermediate product of this decomposition. By potassium
permanganate in presence of potassium hydrate, both in solution,
finely divided cellulose is oxidised to a syrupy mixture of acids
resembling in composition and properties Frémy’s meta-pectic acid. A
similar decomposition is effected by concentrated solutions of the
hypochlorites, employed cold.
_Water at High Temperature._—Cellulose itself is only slightly affected
by exposure to a temperature of 200° C. (392° F.) but in contact with
water at this temperature, in sealed glass[4] most part into dark
brown insoluble products, a portion, however, passing into solution in
the water, imparting to it a deep yellow colour. A large quantity of
carbonic acid is {15} formed, and the solution contains in addition to
formic acid an appreciable quantity of pyrocatechol. The formation of
this aromatic substance from cellulose is, perhaps, the most noticeable
feature of the decomposition.
[Footnote 4: The action of alkaline substances dissolved from the
glass must be taken into account in this decomposition.]
_Decomposition by means of ferments, &c._—The soluble ferment of the
foxglove has been found to convert cellulose into glucose and dextrin.
The fluid from the vermiform appendix of the rabbit has also been found
to digest cellulose with liberation of marsh gas and formation of a
soluble compound which reduces cupric oxide in alkaline solution.
_Humus._—In the natural decay of cellulosic structures a large quantity
of marsh gas is formed, and solid products are formed, which are
usually known as humic or ulmic substances. These bodies are amorphous,
are largely dissolved by alkaline solutions, and are attacked by
chlorine gas in presence of moisture, yielding yellow coloured
chlorinated derivatives whose composition has a general resemblance to
the chlorinated compounds obtained from the compound celluloses, to be
hereafter described.
The most important part which ulmic substances play in the economy of
nature, is in the composition of soils. It is doubtful whether they
act directly as fertilisers, but by their action upon the mineral
constituents of soils they contribute to the supply of these necessary
elements of growth to the plant.
_Synthesis of Cellulose._—A transformation of certain soluble
carbo-hydrates into cellulose, which may be regarded as a proximate
synthesis of cellulose, has recently been brought to light. This is
set up spontaneously in beet-juice, and results in the formation of
a hard white substance, insoluble in boiling dilute potash, soluble
in the ammonio-copper reagent, converted by sulphuric acid into
dextrin and sugar, and by nitric acid into oxalic acid. On adding
some of these cellulose lumps to a solution of pure cane-sugar, a
further transformation of the saccharose into the same substance is
brought about. At the same time there is simultaneously produced a
gummy substance, which is precipitated by {16} alcohol as a white
amorphous caoutchouc-like mass, of the same composition as cellulose,
but swelling up with water, and otherwise differing in its physical
properties from cellulose. This latter substance is also formed by the
action of diastaste upon a solution of saccharose.
The process of cellulosic fermentation may be represented by the
equation:
n. (C_{12}H_{22}O_{11}) = n. (C_{6}H_{10}O_{5}) + n. (C_{6}H_{12}O_{6})
Saccharose. Cellulose. Glucose.
A similar transformation takes place under the influence of certain
fatty seeds, e. g. those of rape and colza; and it is probable that
the formation of cellulose in living plants may take place at the
expense of saccharose and under the influence of ferments. In support
of this it has been established that in the sugar-cane, the formation
of wood—i.e. cellulose—is accompanied _pari passu_ by a decrease
in saccharose. More recently, A. Brown (_Chem. Soc. Journ._, 432,
1886) has investigated the formation of cellulose by the “vinegar
plant” growing in solutions of the carbo-hydrates, e. g. dextrose in
yeast-water. The cells elaborate an extra-cellular fibrin, which acts
as a “cell-collecting medium,” and they possess therefore a two-sided
activity, i.e. the property above mentioned, in addition to their
strictly fermentative activity. The cellulose film in question was
found to contain 50 to 60 per cent. of pure cellulose. It is noteworthy
that in a solution of levulose the growth of the “plant” is unattended
by fermentative action, 33 per cent. of the substance being, on the
other hand, transformed into cellulose.
_Modifications of Cellulose in the Plant. Compound Celluloses._—It has
been already stated that cellulose occurs in nature for the most part
in a form different from that in which we have hitherto considered it,
viz. either combined or mixed with other substances which are of such
a nature that they may be removed by a chemical process, more or less
simple. One of the most characteristic of these modified celluloses
constitutes the jute fibre, the isolated bast of an Indian plant,
{17} which has an enormous industrial application for the coarser
textile purposes. The chemistry of jute throws light on the course of
lignification, the process by which cellulose is modified into wood,
and in studying it in preference to wood we have the advantage of
studying a simple tissue.
Like cellulose, jute dissolves in cuprammonia, and is similarly acted
upon by the concentrated acids. By nitric acids it is converted into
nitric ethers, which are yellow coloured, but in other respects closely
resemble the pyroxylins. They are entirely soluble in acetone.
Jute differs from cellulose in the following respects: its percentage
composition (excluding ash) is
C 47·0 48·0 per cent.
H 5·9 5·7 „
O 47·1 46·3 „
It is harsher to the touch, and its colour varies from grey to brown;
it combines directly with the greater number of the organic colouring
matters, removing them from solution, i.e., becoming dyed with them;
it is coloured deep yellow by immersion in a solution of aniline
sulphate; moistened with a solution of phloroglucol and afterwards
with hydrochloric acid, it gives a deep red coloration; with pyrrol
also in presence of hydrochloric acid it gives a deep carmine colour;
it is attacked and partially converted into soluble products by a
number of reagents which have no action, under similar conditions, upon
cellulose. Certain of these we must consider more in detail.
_Iodine._—Jute absorbs iodine from its solution in potassium iodide.
This action is characteristic of all lignified tissues, the quantity of
iodine taken up being proportional to the amount of lignification. If
the physical conditions remain the same, the amounts of iodine absorbed
will be found to be constant for each particular fibre. This constancy
points to the formation of a definite chemical compound, probably
resembling the blue compound of starch and iodine. This property may
be made available for the estimation of the {18} amount of mechanical
wood-pulp in papers; it will be more fully considered elsewhere.
_Bromine._—Jute, exposed to the action of bromine water in the cold,
combines with the halogen, being oxidised at the same time, and the
products may be dissolved by boiling the modified fibre in weak
alkaline solutions. On repeating this treatment four or five times, a
residue of cellulose is ultimately obtained, amounting to 72 or 73 per
cent. of the original fibre: the cellulose is obtained in the form of
the disintegrated fibre elements. On allowing the halogen to act in
presence of an alkali, _i.e._, as hypobromite, not only is its action
intensified and accelerated, but its _modus operandi_ more clearly
revealed. The chief products are in this case, carbonic acid, bromoform
CHBr_{3} (which may be isolated by distillation), and cellulose. At
the same time the yield of cellulose is considerably less than by the
former method, the cellulose itself being attacked by the hypobromite.
_Chlorine._—Dry chlorine gas has no action upon jute even when the
temperature is raised to 140° C. (284° F.), but the presence of
moisture determines rapid combination, at ordinary temperature,
accompanied by evolution of heat. The colour of the jute changes to
a bright yellow, which, on treatment of the fibre with a solution
of sodium sulphite, gives place to a pure magenta. This reaction is
characteristic of all lignified tissues, and is a valuable aid in the
micro-chemical examination of plant structures. For this purpose, the
reagent may be employed in the form of chlorine water; the sections or
small fragments to be examined are allowed to remain in this solution
a short time, and then mounted for observation in a dilute solution of
sodium sulphite. By combination with chlorine the fibre substance or
_ligno-cellulose_ is partially resolved. Alcohol dissolves away from
the chlorinated fibre, a bright yellow substance, which is precipitated
in yellow flocks on pouring the solution into water. This chlorinated
derivative has the composition C_{19}H{18}Cl_{4}O_{9}. By this formula,
as well as by its colour reactions, and by its yielding protocatechuic
acid on fusion with potash, it is {19} shown to be related to the
trihydric phenols, and to the tannins which are derivatives of these.
After dissolving away this compound by exhaustive treatment with
alcohol, the fibre still gives a brilliant reaction with sodium
sulphite, showing that a portion is still held back in combination
with the cellulose, or cellulose residue. On treating the residual
fibre with boiling nitric acid, a considerable quantity of chloropicrin
CCl_{3}NO_{2} is formed, and it is probable that the union of the
molecule C_{19}H_{18}Cl_{4}O_{9} with the cellulose may be effected
by the aldehyde[5] molecule CCl_{3}COH; each group being contained
in the original lignocellulose, the action of the chlorine tending
to disturb the atomic equilibrium and to rearrange the atoms into
groups, which in their modified form have less mutual coherence. If the
chlorinated fibre be directly boiled with the sodium sulphite solution,
its resolution into cellulose and soluble non-cellulose derivatives
is complete: and this treatment constitutes the most simple and rapid
method of estimating the cellulose in lignified tissues. It is only
necessary finally to wash the cellulose with hot water containing a
little acetic acid (when placed in a funnel it acts as its own filter),
and then with, alcohol, when it may be dried and weighed. In order to
ensure the resolution of the fibre by a single chlorination (in the
case of jute, and the like), it must, previously to exposure to the
gas, be boiled in a dilute (1 per cent.) solution of potassic hydrate.
In the case of wood and other more resistant structures, it will be
necessary to repeat the chlorination.
[Footnote 5: Furfural may also be isolated from the chlorinated
jute; the survival of this aldehyde is noteworthy.]
The percentage of cellulose yielded by this method is, in the case
of jute, usually 2–3 per cent. higher than by the bromine method.
Moreover, if the temperature be maintained at 0° C. (32° F.), by
placing the fibre, which is to be exposed to the action of the gas,
in contact with pounded ice, the percentage may be still further
increased, amounting {20} in some cases to 80–82. Corresponding to
this increased yield, the cellulose is obtained in long filaments.
It will be seen, therefore, that the cellulose isolated by chemical
treatment from a _compound cellulose_ is affected both in character
and quantity by the process employed, and it is affected in a much
greater degree than the cellulose itself exposed to the same treatment,
_after isolation_. The composition of the cellulose obtained in this
way differs from that of celluloses, such as cotton, which exist in
the plant in an isolated and more fully formed condition; it contains
43 per cent. C and 6 per cent. H, corresponding to the formula n
[3 C_{6}H_{10}O_{5}H_{2}O.] The composition of this cellulose will
be seen to be identical with that of certain of the oxycelluloses
previously described; and its properties are, moreover, those of an
oxycellulose. These facts go to show that the jute fibre substance,
and the substances allied to it, are compounds of cellulose with
other molecules, _i.e._ they are _compound celluloses_. They may be
conveniently grouped under the term _ligno-cellulose_.
_Dilute Mineral Acids._—Under the action of dilute sulphuric or
hydrochloric acid (5–10 per cent.) at 60°–80° C. (140°–176° F.), a
considerable portion of the fibre substance is dissolved. The action
of either reagent is of course purely hydrolytic; and the hydrolysis
is of the simplest character, since the substance dissolved as well as
the residual fibre are found to have the same percentage composition.
Moreover, the residual fibre, though having undergone a profound
structural modification, and having been converted into a disintegrated
mass of brittle fragments, still preserves the chemical characteristics
of the original, unaltered. Thus it is converted by the action of
chlorine into a similar complex of derivatives, yielding on resolution
by sodium sulphite solution 75 per cent. of cellulose; also when acted
upon by alkaline hypobromite it yields bromoform. Under the action of
these hydrolytic reagents, therefore, the fibre shows itself to be
essentially a chemical whole. At the boiling temperature, these acids
resolve as well as dissolve the fibre substance, and the volatile
products are acetic acid and the peculiar aldehyde {21} _furfural_
C_{4}H_{3}O.COH. The yield of this aldehyde may be raised with suitable
precautions to 15–20 per cent. of the raw fibre. The action of dilute
nitric acid upon the fibre differs from that of the above-mentioned
acids, in effecting its resolution into soluble derivatives and
insoluble cellulose; the yield of the latter when the reaction is
conducted at 60° C. (140° F.), with 5 parts HNO_{3} is 60 per cent.
only, showing that in the process of isolation some of the cellulose
itself is attacked and converted into soluble products.
Schulze’s method of isolating cellulose from wood, and from the
ligno-celluloses generally, consisted in macerating them for 12 to
14 days with 8/10 their weight of potassium chlorate dissolved in
12 parts by weight of nitric acid (sp. gr. 1·10), and completing
the resolution by afterwards boiling with dilute ammonia. By the
action of a more concentrated acid, 50 per cent. HNO_{3}, at 80° C.
(176° F.), a soluble derivative is obtained which has the composition
(C_{25}H_{40}NO_{25}). It is a syrupy, highly acid body, dyeing animal
fibres a deep yellow shade, and giving with the earthy bases salts
of the formula (C_{25}H_{32}NO_{25}M_{4}), which are precipitated by
alcohol from their aqueous solution in the form of bright yellow flocks
that dry to a yellow powder.
_Dilute Alkalis._—The action of boiling dilute solutions of the caustic
alkalis upon the ligno-celluloses is similar to that of the dilute
acids, viz., purely hydrolytic, causing no essential change in the
composition of the fibre substance.
_Water at High Temperatures._—The ligno-celluloses undergo a profound
modification when heated with water at elevated temperatures. They
are to a great extent converted into soluble products, of which
a large proportion are the volatile bodies, acetic and formic
acids and furfural. The residual fibre is found to be more or less
disintegrated, and of a dark-brown colour, but preserving many of
the chemical features of the original. A similar disintegration
is caused by exposing the ligno-celluloses in a dry state to an
atmosphere of superheated steam, the effect being much increased if
the ligno-cellulose be previously bleached by any of the ordinary {22}
processes, i.e., under oxidising conditions. In these decompositions
oxidation, as well as hydration, plays an important part. It is found
that the presence of small quantities of reducing agents, e.g.,
sulphites, exerts a remarkable effect in retarding and often entirely
preventing these changes. It is probable that they act in obviating an
initial change, which, with a cumulative reaction on the fibre, effects
the profound resolutions above described. (See p. 71.)
_Concentrated Hydrochloric Acid._—When exposed to the concentrated
aqueous acid at ordinary temperatures, ligno-cellulose undergoes a
profound structural modification, and is largely converted into soluble
bodies. The disintegrated residue reacts with chlorine similarly to the
original jute, but the quantity of cellulose isolated by subsequent
treatment with alkaline solutions is only 50 per cent. of the original.
Since jute cellulose, after isolation, exhibits under similar treatment
a much smaller conversion into soluble products, 10 per cent. as
compared with 33, we have here further evidence that the cellulose is
present in the fibre in a condition chemically different from that of
the isolated cellulose.
_Sulphuric Acid._—On triturating the fibre with the concentrated acid,
it dissolves to a brownish-purple solution; on pouring this solution
into water, a substance is precipitated in dark-brown flocks; when
dried it has the following composition:
C 64·4
H 4·4
O 31·2
The solution yields on distillation furfural and acetic acid.
_Nitric and Sulphuric Acids._—(1 : 2 by vol.)—By the action of these
acids ligno-cellulose is converted into yellow-coloured explosive
compounds, preserving all the external features of the original fibre,
and showing a close resemblance to the pyroxylins.
The increase in weight resulting from the fixation of the {23} nitric
acid residue is approximately equal to that of cotton “nitrated” under
the same conditions.
They are freely soluble in acetone. On examination they prove to be
homogeneous. If the fibre be warmed with the mixture after the first
reaction is completed, it dissolves. The solution is found to contain
oxalic, succinic, and suberic acids, but no aromatic nitro-derivatives,
whereas if ligno-cellulose contain a benzene nucleus, as has been
supposed, such derivatives could not fail to be formed under these
conditions.
_Animal Digestion of Ligno-cellulose._—It has long been known
that the urine of herbivorous animals contains hippuric acid, or
benzoyl glycocine, as a normal constituent, and it has been shown
that the benzoyl radicle necessary to form this body by reacting
with the glycocine of the liver, is a product of the digestion
of ligno-cellulose. On the other hand, it has been shown that
ligno-cellulose contains no benzene compounds as such. We have
therefore, in this fact additional evidence that a portion of the
complex ligno-cellulose molecule is in what we may term a potentially
aromatic condition.
_Distribution of Ligno-celluloses._—As already indicated, the
distribution of the ligno-celluloses is extremely wide; they are met
with in every variety of vegetable tissue, and their presence is
readily ascertained by the reaction of chlorination already described.
Of these ligno-celluloses most closely allied to jute, we may mention
the stony concretions of pears (glycodrupose) and the main tissue of
fir wood (glycolignose).
_Decay of Ligno-cellulose._—When a mass of ligno-cellulose (jute)
is allowed to lie for a long period in a damp state, it undergoes
resolution accompanied by structural disintegration. The soluble
products of this change are acid bodies of the pectic class, and
astringent substances closely resembling the tannins. In this
decomposition also we have evidence of the plastic condition of the
ligno-cellulose molecule, of the variety of its modes of resolution,
according to the various directions in which it is attacked.
_Decomposition by Heat._—The celluloses burn in the air with {24} a
quiet luminous flame. When heated out of contact with the air, they
are completely resolved into gaseous and volatile products on the
one hand, and a residual black mass, which, although containing a
high percentage of carbon, and approximating in its properties to the
element itself, nevertheless contains hydrogen and oxygen in essential
chemical union with the carbon, in consequence of which it is expedient
to apply the term pseudo-carbon to this as to all similar substances.
The volatile products of the destructive distillation of pure cellulose
have not been studied, but those of certain of the ligno-celluloses
(woods) have been exhaustively investigated, and have moreover, a very
considerable commercial importance. They divide themselves into two
groups (_a_) soluble in water, and containing, besides water, acetic
acid, methyl alcohol, methylic acetate and acetone; (_b_) insoluble in
water, or wood tar, containing tolyene, xylene, cymene, naphthalene,
chrysene, anthracene, retene, pittacal, cedriret solid paraffins, &c.
The quantity of the pseudo-carbon (charcoal) obtained is, in the case
of the hard woods, about 25 per cent. of their weight. The liquid
portion of the products amount to some 50 per cent., and contains
about one-fifth of the carbon of the original wood; the remainder of
the carbon is eliminated in the gaseous form (CO, CO_{2}, &c.). The
proportion of acetic acid is 3–4 per cent., the tar amounting to 7–8
per cent.
The coals themselves may be regarded as pseudo-carbon derivatives of
celluloses, formed by a process of molecular condensation, the true
nature of which remains a matter of speculation. In this view, the
whole of our vast series of aromatic or benzene compounds, derived as
they are from the products of the destructive distillation of coals,
may be traced back to a cellulose origin.
Pseudo-carbons are obtained as products of the action of various
reagents upon the celluloses, and other of the so-called
carbo-hydrates. These reagents, such as sulphuric acid, act in virtue
of their dehydrating power; and the recognition of this fact, together
with the supposed “carbonaceous” {25} character of the product,
led to the erroneous conclusion that the carbohydrates are in such
decompositions simply resolved into carbon and water; a conclusion
which seems to derive additional warrant from the peculiar numerical
relationship which exists between the C, H, O atoms of all the members
of the group. Their relation is expressed in the general formula C_{n}
H_{2(n−m)} O_{(n−m)}, and in the somewhat misleading term carbohydrate,
which is applied to the whole group. We now know that the removal of
water from these bodies by the action of dehydrating agents—including
heat—follows the ascertained laws of chemical dehydration, involving
molecular condensations and rearrangement, and that the pseudo-carbons
are the extreme terms of a series of such condensations or cumulative
resolutions. The matter, however, is not as yet sufficiently
investigated to enable us to state with any preciseness the mechanism
of these changes. Still this general statement will enable us to avoid
many of the erroneous views which have existed on the subject, and in a
measure to anticipate the results of future investigation.[6]
[Footnote 6: It is worthy of mention that by the action of
chlorine in presence of water, and by the action of concentrated
nitric acid upon the cannel coals, substitution derivatives are
formed resembling those obtained by the action of these reagents
respectively upon the ligno-celluloses.]
_General conclusions._—The aldehydic character of the group of
ligno-celluloses is typical of the constituents of plant tissues
generally, and is, there can be no doubt, extended to the celluloses.
Indeed the evidence of biological observation goes to show that the
ligno-celluloses are formed, by a process of chemical modification,
from some more purely cellulosic tissue, which preceded it, and
with this modification there is, nevertheless, a retention of the
general features of the original cellulose. In the domain of animal
chemistry, recent investigation has also shown that the development
of proteid tissues finds its chemical expression in their aldehydic
characteristics, or as it has been put, the immediate physical cause
of life is to be found in the _chemical tension_ of aldehydes. The
meaning of this expression will be readily grasped on {26} reference
to the properties of the aldehydes as a group; their power of combining
with oxygen; of uniting by their own molecules, to polymerise; of
uniting by way of dehydration; of combining with the aromatic alcohols;
a consensus of properties which shows them to possess an activity
surpassing that of any other group of compounds. The evidence of
biology, added to our knowledge of the properties of these substances,
would lead us therefore to define cellulose as a condensed aldehyde
allied to the sugars, and likewise containing alcoholic OH groups; and
ligno-cellulose a compound cellulose, containing a cellulose nucleus or
residue so combined with certain groups which may be included in the
term _non-cellulose_, that it yields, under the action of reagents,
products belonging either to the aromatic, the furfural, or the fatty
group.
_Adipo-cellulose._—Cork or Cuticular Tissue.—From the mode of formation
of cork and cuticular tissues, it has been concluded that they are also
modified celluloses. The aggregate ultimate composition of cork is
represented by the following percentage numbers:
C 65·7
H 8·3
O 24·5
N 1·5
Cork, however, unlike the jute fibre, is by no means chemically simple,
but can be resolved by the action of mere solvents into a number of
proximate constituents, such as acids, a variety of fatty bodies,
nitrogeneous bodies, &c.
Cuticular tissues, such as constitute the covering of fruits, are more
simple in composition; the cuticle of the apple after purification, has
the following percentage composition:
C 73·66
H 11·37
O 14·97
The cuticular substance of cotton, straw, esparto, &c., are doubtless
similar bodies. These numbers point to a {27} remarkable similarity
in composition to the fats, and, indeed, from the results of his
study of this tissue, Frémy concluded that it was a fat in everything
but its physical properties, of which we may mention insolubility in
alcohol and in ether, and infusibility. This tissue, however, as also
cork, contains a cellulose residue, which may be isolated by any of
the methods of treatment given for ligno-cellulose. Under the action
of boiling nitric acid these tissues are resolved into cellulose
on the one hand, and a series of fatty acids or products of their
decomposition, suberic and adipic acids, &c., on the other; the latter
amounting in the case of cork to 40 per cent. of the weight of the
substance treated. The cellulose obtained by this treatment is but 2–3
per cent.; this number, however, represents only the amount which has
survived a treatment which we know to be destructive to cellulose. If,
on the other hand, cork be resolved by treatment with sodium sulphite
solution at 100lb. pressure, or 166° C. (331° F.), a soft mass is
obtained, preserving the structural features of the original cork,
until subjected to slight pressure, when it falls to a cellular mass.
From this, cellulose is isolated by any of the less drastic processes
above described, and is found to amount to 9–10% of the original cork.
As in ligno-cellulose, we have evidence of a transition from cellulose
to the tannins, so in cork and cuticular tissue we have evidence of
the metamorphosis of cellulose into fats, a fact indicated in the term
_adipo-cellulose_, which we have applied to the compound celluloses
constituting these protective plant tissues. This metamorphosis is
doubtless a very complex process, and would appear to involve the
formation of tannins also, at least as a subsidiary result. Still,
the essential feature of the change is the production of the peculiar
fat-like substances which have been described; and with due regard
to the limitation pointed out, the views here advanced represent the
results of the investigations of the subject as far as they have
proceeded.
It may be observed that the general characteristics of the {28}
adipo-celluloses as of the ligno-celluloses are those of a complex
aldehyde.
_Pecto-Cellulose._—The compound cellulose described under the names
adipo-cellulose and ligno-cellulose have this feature in common, that
they are richer in carbon than cellulose. On the other hand there is
a large number of cellulosic tissues which contain in the aggregate
less carbon, i.e., in which the cellulose is combined or mixed with
substances of lower carbon percentage than itself. Such tissues, or
rather the substances which compose them, are resolved by simple
treatment with dilute alkaline solutions into insoluble cellulose,
and soluble bodies of the pectic class. Thus the purified bast of the
Russian flax, i.e., the raw flax, is found to contain—
43·7 per cent. C.
5·9 „ H.
On boiling with an alkali it loses in weight about 20 per cent.,
the substance dissolved being identical in properties with Frémy’s
pectic acid, a substance containing 42 per cent. C and 4.8 per cent.
H = C_{16}H_{22}O_{15}. Many other bast fibres, as well as cellulose
tissues, exhibit similar properties, and although this branch of the
subject has been but little investigated, there is sufficient evidence
for constituting a special class of compound celluloses under this
term, pecto-cellulose, having properties indicated in the type selected
above.
In addition to pectic acid there exists in, or can readily be obtained
from certain plants, a series of bodies of a similar nature, such
as pectose, pectin, parapectic acid and metapectic acid. The most
important of these are pectic acid and metapectic acid.
Pectic acid, though it rarely exists ready-formed in the plant, can
readily be obtained from it by the action of weak alkalis. It is best
procured by boiling the pulp of turnips with a 10 per cent. solution
of sodium carbonate for about half an hour. If an acid be added to
the filtered liquid, pectic acid is precipitated as a transparent
colourless jelly, which {29} dries up to a transparent horny mass. By
the prolonged action of alkalis or acids, pectic acid is converted into
metapectic acid (C_{8}H_{14}O_{9}), which may be obtained as a syrupy,
strongly acid liquid.
From what we know of the properties of the members of the pectic group,
and therefore of the pecto-celluloses, it will be readily seen that
the latter are very liable under the action of alkalis to undergo
conversion into soluble derivatives.
Conversely, a study of the action of alkalis upon plant substances,
enables us to determine to what extent the latter partake of the nature
of the pecto-cellulose, the importance of which to the practical paper
maker we need hardly point out. For a further development of this idea
(see p. 43).
{30}
CHAPTER II.
PHYSICAL STRUCTURE OF FIBRES.
We now have to treat of the fibrous raw materials from the point of
view of form or structure, which is, of course, a very important
factor in determining the quality of the paper or other fabric into
which they are manufactured. It is sufficiently evident that the
strength of paper is primarily due to the cohesion of its constituent
fibres. When paper is torn, the edges present a fibrous appearance,
and observation teaches us that, other things being equal, the greater
the manifestation of fibrous structure the stronger will be the paper.
If a paper be thoroughly wetted, its tensile strength is reduced to a
minimum, and if subjected to a slight strain we get, not a tearing, but
a pulling asunder of the fibres. If this be performed under a lens, the
structure of the paper is more clearly seen, and it will be appreciated
to what extent the qualities of a paper are the aggregate of the
qualities of its constituent fibres. A more careful dissection of the
paper shows that these fibres, which are the _ultimate fibres_ of the
plant, as distinguished from the bundles of these, or _filaments_,
which the spinner employs, are interlaced in all directions. To produce
this effect of interweaving, and to insure that uniformity which is an
essential feature of good paper, we have among others the following
contributory causes: (1) the deposition of the fibres from suspension
in water; (2) the composition of the pulp with regard to the reduction
of the fibrous bundles, and the isolation of the individual fibres; and
(3) the _structure_ of these ultimate fibres. It is with the last that
we are chiefly concerned at present.
To convey a general notion of the influence of the structure {31}
of fibres upon fabrics, we shall with advantage travel outside
our immediate province to consider briefly the woollen and silk
manufactures in relation to this point. Wool is, as we know, a
discontinuous fibre, and its structure is that represented in Fig.
1, the most conspicuous feature being its broken surface, consisting
apparently of imbricated scales.
[Illustration: FIG. 1.]
The silk fibre, on the other hand, is a dual cylinder, spun by the
worm in a continuous length, and with a perfectly smooth surface.
Now, it would not be to our purpose to point out that in starting
from a discontinuous simple fibre, to produce a continuous, therefore
necessarily compound one, a very different treatment or process of
spinning is required from that which the opposite condition renders
practicable. We will rather consider the influence of structure upon
materials manufactured from these fibres. It is obvious that the wool
fibres, brought into contact with one another, tend to interlock;
whereas silk fibres if rubbed or pressed together, simply slide
over one another; the result in the fabric is by multiplication of
the effect, a shrinking or contracting in length and breadth. This
interaction of the fibres, and the phenomena to which it gives rise,
is known as the _felting_ of wool and woollen goods; this tendency,
for the contrary reason, is not seen in silk fabrics. The production
of paper from a disintegrated fibrous mass or pulp introduces
similar considerations. That paper will be the stronger in which the
constituent fibres are the better felted, and the degree in which
felting takes place will depend to a great extent upon the form or
microscopic peculiarities of the fibres. This is only one of the
more obvious inferences to be drawn from the structure of fibres to
the qualities of the papers which they compose. Other {32} equally
important practical bearings will be seen to attach to the microscopic
study of our fibrous raw materials, and to the consideration of this
branch of the subject, we now ask the careful attention of the student.
MICROSCOPICAL EXAMINATION.—Under the head of “Microscopic Features”
we must include everything which has to do with the structure of the
vegetable fibres, as well as their organisation and distribution
in the plant. In the analysis of “organised” structures we employ
the two methods. (1) of _dissection_; (2) _examination_ by means
of the microscope; in other words, we first isolate the part under
investigation by a mechanical process and then proceed to the optical
resolution or analysis of the part. Having by analysis acquired a
knowledge of the _parts_, we study their mutual relations in the
structure they compose—we integrate our knowledge, so to speak—by means
of sections of the structure, cut so as to preserve the cohesion of
the parts in section, and yet in so fine a film as to appear under
the microscope to be virtually a plane surface. These points are
illustrated in the drawings given.
It is impossible for us to deal specially with the subject of the
microscope and its manipulation. The microscope, as a revealer of
natural wonders is one thing; as an instrument of scientific discovery,
quite another. For the latter, the student must train himself by
systematic work, and should especially concentrate his attention upon
some _one_ branch of natural history, however restricted.
We shall assume, in our treatment of the subject, a knowledge of
the microscope as an instrument of research, such as can be easily
acquired in a few weeks of work under the guidance of a teacher or
of one of the excellent manuals which now abound. We also assume a
certain acquaintance with the elements of vegetable physiology, which
it will be seen is necessary for a full grasp of the subject. Such an
acquaintance, also, may be easily acquired, under direction, in a few
weeks of work.
[Illustration: FIG. 2.]
[Illustration: FIG. 3.]
We have before alluded to the differences presented by {33} mono and
dicotyledonous stems in regard to the distribution of their fibrous
constituents. In illustration of this, we may cite Figs. 2, 3, which
represent, (2) a section of the aloe, (3) a section of the jute plant.
The available fibres are in (2) the fibro-vascular bundles (_f_),
which are irregularly distributed throughout the main mass of cellular
tissue, and {34} in (3) the bast fibres (_f_), which constitute a
definite and separate tissue. We have already alluded to the practical
consequences of this typical difference of distribution, in regard to
processes of separating these fibres on the large scale.
This process we have explained is necessarily simpler in the case of a
fibrous _tissue_, definitely localised; and this may be demonstrated
by a superficial examination of a young branch of an exogen. As we
know, the bark tissues are easily stripped from the underlying wood.
If now we work up the former in a mortar, with a little water, we soon
perceive the separation of the compound tissue into cellular matter
on the one hand and _fibres_, the latter being more or less long
and silky, according to the plant from which isolated. They vary in
length from one millimetre to several centimetres, and are aggregated
together in the plant in such a way as to constitute bundles, often
of very considerable length; the general arrangement being comparable
with that of the tiles in the roof of a house. It is important to
distinguish the fibre-bundles from the elementary or normal _fibres_,
and to this end they are designated by the term _filament_. Bast
_fibres_ are flexible and fusiform, terminating gradually in a point
at either end, as represented in Fig. 4; bast _filaments_, built up of
these fibres, containing often as many as twelve in the bundle, are
usually cylindrical, but exhibit the widest differences in regard to
the aggregation, in degree as well as number of their constituents. It
is obvious that while the spinner has to do with these _filaments_, the
paper maker works up the ultimate fibre constituents or _fibres_. It is
also an obvious corollary from this distinction that a fibrous material
which from “weakness” is unavailable for textile application, may yet
be perfectly “strong” from the paper maker’s point of view; in other
words, the individual fibres may be strong, but have little cohesion
in the filaments. As we proceed, the student will see more and more
the practical bearings of this branch of the study, and will perceive
the inferences to be drawn from the investigation of {35} minute
relationships to manufacturing processes and their products.
[Illustration: FIG. 4.]
We shall say but little as to the necessary equipment. (1) A dissecting
microscope, for dissecting under a lens, magnifying the object to 40
or 50 diameters. (2) An ordinary student’s microscope with lenses for
magnifying to 100 and 300 diameters. This is adequate to the work,
though of course, it may be an advantage in certain cases, to be
provided with higher powers. (3) A glass slide, carrying an engraved
scale of centimetres and millimetres for measuring the lengths of
objects, and a micrometer, divided into 1/10 mm. for measuring
diameters. It is also important to be able to determine the degree of
enlargement under any particular combination of lenses, and for this
purpose to possess a micrometer eye-piece, with a millimetre scale
divided into hundredths. (4) An effective microtome and the usual
mounting accessories.
A very important feature in the diagnosis of fibres, more especially
in regard to the composition of the fibre substances, is the effect
produced by treatment with various reagents. Certain of these reactions
we have already {36} indicated. We shall now give the details of
composition of the several solutions which will be required.
_Neutral Mounting Solutions._—It is advantageous in mounting
transparent objects to employ a solution of the same refractive power
as the substance itself. For this purpose, pure glycerin or a syrupy
solution of calcium chloride may be employed; it is expedient to mix
with either reagent a small proportion of acetic acid. The designation
“neutral” has reference to the fact that these reagents are without
sensible action on the fibres.
_Iodine Solution._—We have previously described the preparation of
a solution, giving the characteristic blue reaction with cellulose
directly. It is, however, often preferable to bring about this
colouration in another way, and the following are the solutions
employed:—1 gramme of potassium iodide is dissolved in 100 cc. water,
and the solution is saturated with iodine; it is preserved in stoppered
bottles, containing a few fragments of the element, so as to keep up
the saturation of the solution.
The accessory solution, dilute sulphuric acid, which is employed
to determine the reaction between the cellulose and the iodine, is
prepared as follows:—2 volumes of concentrated glycerin are mixed with
1 volume water, and to the mixture an equal volume of oil of vitriol
(1·78 sp. gr.), is slowly added, so as to prevent as far as possible a
rise of temperature. The effect of the glycerin is very remarkable in
preventing the distortion of the objects under the action of the acid,
which in other respects remains uninfluenced.
By way of verification of this iodine test, which is somewhat
capricious, it is advisable to test the reagents with a standard
substance. The best for the purpose is a linen yarn which has been
_partially_ bleached. Under the action of the reagents the fibres
composing this yarn, which must, of course, be suitably “teazed out”
for mounting, are coloured a light blue, the centre, however, showing
a yellow line, {37} marking the distribution in the interior canal of
a non-cellulose fibre constituent. Should these effects not appear,
it may be concluded that the acid requires to be strengthened. On the
other hand, too great a concentration is equally to be avoided; it is
evidenced by causing a _distortion_ of the fibre, easily recognised by
comparison with the fibre mounted in a neutral medium.
_Chlorine Water._—One of the most characteristic reactions of lignose,
or lignified cellulose, is that of combining with chlorine. The
reaction of the chlorinated derivatives with sodium sulphite solution,
is an important feature in the microscopic diagnosis of lignified
fibres and cells. The reagent is prepared by dissolving chlorine to
saturation in water. The sodium sulphite solution is prepared by
dissolving the crystallised salt in 20 parts distilled water.
_Aniline Sulphate Solution._—With this reagent lignose gives a
characteristic deep yellow colouration. A convenient strength is a 2
per cent. solution of the salt. The colour is more quickly developed
if the reagent is acidified; a few drops of sulphuric acid should
therefore be added.
_Solutions of the Aniline Colours._—Some of these are of importance
in enabling the microscopist to differentiate plant tissues. The
“affinities” of the fibre substances for these are very various in kind
and intensity. The phenomena of staining cannot be adequately treated
in our histological scheme, which is necessarily very restricted. We,
therefore, merely mention the more important colours which are used in
staining, viz., magenta, methylene blue, eosine, diphenylamine blue. A
convenient strength is a solution of 1 in 2000. (See also p. 43.)
The employment of the aniline sulphate solution, as well as of the
solutions of the aniline colours, presents no difficulties, and
therefore needs no detailed description. The former strikes a more or
less deep yellow with lignose; the aniline colours stain or dye the
tissue or fibre more or less deeply, according to its composition, and,
as it is a reciprocal action, {38} according also to the composition
of the colouring matter. In following up this subject, the student will
require to consult works on vegetable histology.
_Preparation of the objects._—The necessary preliminary to the
examination of the fibres themselves is their isolation. This is
accomplished either by means of the dissecting microscope, or more
roughly, according to circumstances. Having obtained the filaments,
they are boiled in a 10 per cent. solution of sodium carbonate until
sufficiently softened to yield easily to the “teazing” needles. In
certain cases the boiling must be supplemented by trituration in a
mortar; this, or some similar operation, is especially necessary when
the fibres are embedded in a mass of cellular tissue (parenchyma) e. g.
in the fibro-vascular bundles of monocotyledons.
Sections of the filaments are prepared by cutting in a microtome, the
filaments being previously agglutinated into a stiff bundle by means of
any of the usual stiffening solutions, and after drying, embedded in
wax in the usual way. Sections of fresh stems and tissues are cut with
a “section” razor.
Having prepared the objects, their examination under the microscope
necessarily divides itself into:—(1) the determination of external
features; (2) the diagnosis of chemical composition. The fibres
themselves will be individually considered in regard to microscopic
features.
There is one aspect of these structural features, however, which admits
of more general treatment, and in respect to this we may anticipate
with advantage, viz., the dimensions or simple elements of form. The
importance of the determination of the length and diameter of both
filaments and fibres will be readily appreciated by an inspection of
the following table, in which the numbers are given for several of the
more important.
A careful study of this table in relation to the application of these
several fibres, will show that the correlation of the latter with these
ultimate dimensions is close and essential. {39}
TABLE OF LENGTHS OF RAW FIBRES (FILAMENTS) AND DIMENSIONS OF
CONSTITUENT CELLS AND FIBRES.
---------------------------------------------+-------------+-----------+---------------------
| | | Diameter of Cells.
| Length of | Length of +-----------+---------
| Filament. | Fibres. | Extreme. | Normal.
---------------------------------------------+-------------+-----------+-----------+---------
| cm. | cm. | 1/100mm. | 1/100mm.
A. _Seed hairs._ Filaments composed | | | |
of individual cells. | | | |
| | | |
_Cottons._ | | | |
_Gossypium barbadense_ (Sea Island) | 4·05 | 4·05 | 1·92–2·79 | 2·52
_Gossypium acuminatum_ | 2·84 | 2·84 | 2·01–2·99 | 2·94
_Gossypium arboreum_ | 2·50 | 2·50 | 2·00–3·78 | 2·99
| | | |
_Bombax heptaphyllum_ | 2–3 | 2–3 | 1·9–2·9 | ..
| | | |
B. _Bast fibres._ Filaments or fibre | | | |
bundles, made up of individual | | | |
fibre-cells aggregated together. | | | |
| | | |
_Flax._ Linum usitatissimum | 20·140 | 2·0–4·0 | 1·2–2·5 | 1·6
| | | |
_Hemp._ Cannabis sativa | 100–300 | .. | 1·5–2·8 | 1·8
| | | |
_China Grass._ Boehmeria nivea | .. | 22·0 | 4·0–8·0 | 5·0
| | | |
_Ramie._ Boehmeria tenacissima | .. | 8·0 | 1·6 | ..
| | | |
_Jute._ | | | |
Corchorus capsularis | 150–300 | ·8 | 1·0–2·0 | 1·6
Corchorus olitorius | 150–300 | ·8 | 1·6–3·2 | 2·0
| | | |
_Paper mulberry._ Broussonetia | | | |
papyrifera | .. | ·7–2·1 | .. | 3·6
| | | |
_Linden bast._ Tilia grandifolia | .. | 1·1–2·6 | .. | 1·5
| | | |
C. Fibro-vascular bundles | | | |
| | | |
_New Zealand Flax._ Phormium tenax | 80–110 | 2·5–5·6 | ·8–1·9 | 1·3
| | | |
_Aloe._ Aloe perfoliata | 40–50 | 1·3–3·7 | 1·5–2·4 | ..
| | | |
_Esparto._ Stipa tenacissima | 10–40 | 0·5–1·9 | ·9–1·5 | ..
---------------------------------------------+-------------+-----------+-----------+---------
A very important point in the diagnosis of a raw material, and next in
order of treatment, is the degree of purity of the substance, in so
far as this is related to structure. The fibres may be associated with
cellular tissue, or with cellular debris, if they have undergone the
retting process or other treatment for separation; or with “encrusting
and intercellular substances” in various proportions. In the latter
case the association with the fibres is usually much more intimate;
they are in fact essential constituents of the fibre _bundles_ {40}
(bast-fibres, fibro-vascular bundles), whereas the former we may
regard as “foreign matter.” We may, however, distinguish between the
normal incrustation of the fibre-cells, and such an incrustation of
the filaments as would be described as a loose adhesion of non-fibrous
matter. The latter is seen in such tissues as the bast of the
adansonia, and the fibro-vascular bundles of the aloes. These are
points with which observation alone can familiarise the student; as
experience grows he will find it increasingly easy to follow _general_
distinctions, and in proportion as he uses his own faculties, so he
will be able to generalise for himself. He will find this equally
true of the second section of the microscopic examination, i.e., the
micro-chemical diagnosis of fibres. Under this head is included the
observation of the behaviour of fibres towards the various reagents
above described. In addition to their microscopical employment it is
useful to note their effect on fibres in the gross, both in their
natural state and after treatment with bleaching agents.
In applying the iodine reaction, attention must be paid to the
following details of manipulation. Place the object (dry) upon the
glass slide, moisten with a few drops of the iodine solution, cover
with a glass slip and examine under the microscope. Note the effects,
which are those of the iodine alone. Then remove the iodine solution
by means of blotting paper, and introduce the sulphuric acid by the
method of “irrigation.” The colouration of the cellulose (blue-violet)
is immediate; it has the effect moreover, of bringing out more clearly
a number of the structural details of the fibre.
We have already treated of the resolution of the raw fibres into
cellulose and non-cellulose constituents by processes which convert
the latter into soluble derivatives. The student will derive much
instruction from following up the attendant structural disintegration
with the aid of the microscope. The chemical dissection of lignified
fibres by the alternate action of bromine water and alkalis, should
be studied by mounting specimens of the fibre at all stages and {41}
carefully noting all the changes which occur. The more drastic action
of chlorine should also be studied by mounting the chlorinated fibre
(in water) and then irrigating with the alkaline solvent (caustic
alkali or sulphite) and noting the stages in the completion of the
disintegration.
{42}
CHAPTER III.
SCHEME FOR THE DIAGNOSIS AND CHEMICAL ANALYSIS OF PLANT SUBSTANCES.
It will be convenient at this point to present to the reader a general
scheme for the chemical analysis of fibrous raw material.
We have already discussed (p. 4) the methods for the determination of
cellulose in fibrous substances. This is to the paper-maker the most
important constituent, and is that upon which his calculations of the
value of a raw material are based. Not only, however, is it of the
greatest importance for him to be able accurately to determine the
_amount_ of cellulose, but it is necessary to be able to form some
idea of the _nature_ of the cellulose and also the ease with which it
can be obtained from the fibre. This latter point can only be properly
established by direct trial of the fibre under the conditions existing
in the factory. It should also be borne in mind that these conditions
influence to a considerable degree the amount of cellulose, actually
obtained in practice, and that therefore the percentage shown by a
careful determination in the laboratory should only be taken as an
indication of the amount likely to be realized. The student will be
able, from what has been said before of the properties of the different
celluloses, to apply his knowledge to a determination of any particular
specimen. In order to supplement this knowledge, it may be well at this
particular point to cite the chief results of an investigation bearing
upon this point by Webster (Chem. Soc. Journ., 1883, p. 23).
His method of investigation consisted in (1) estimating {43} the
cellulose; (2) determining the loss sustained by the fibre when boiled
for 5 minutes, and also for 60 minutes, with an alkaline solution
(caustic soda 1 per cent. Na_{2}O), the former being taken to represent
the purely solvent action, the difference between the two amounts being
an indication of the _degrading_ action of the soda; (3) determining
the ultimate composition (C H and O) of the fibre substance. It should
be observed that the specimens were carefully averaged and purified by
boiling in glacial acetic acid and in alcohol. On carefully considering
the following table it will be seen that the chief points brought
out by Webster’s investigation are (1) the association of a low
carbon percentage with pectous properties and considerable degrading
action on long boiling in alkali. The bearing of this will be better
understood in connection with a discussion of the properties of pectic
acid (p. 28) and the oxycelluloses (p. 13). (2.) The influence of the
composition of the original fibre on the isolated cellulose.
The solutions of aniline sulphate and of the aniline colours give
valuable aid in a diagnosis of plant substances. Webster observed, as
was to be expected, that the yellow colour produced could be taken as
an indication of the amount of lignification, those fibres which were
of a pectous nature showing little or no colour. On the other hand,
when the fibres were treated with a neutral solution of the so-called
alkali-blue it was found that the converse took place, highly pectous
fibres being dyed much more deeply than those which were lignified.
(Compare the action of aniline colours on oxycellulose, p. 11.)
If the student require more precise knowledge of the nature of
a fibrous material than that afforded by a determination of the
cellulose, the following method of analysis will prove of service.
{44}
FIBRO-VASCULAR BUNDLES OF MONOCOTYLEDONOUS PLANTS.
----------------------+------------+------------+--------------+--------------+--------------+
| | | Ananassa | Musa | Phormium |
| Agave | Yucca | sativa | paradisiaca | tenax |
| Americana. | gloriosa. |(Pine Apple.) | (Manilla). | (New Zealand |
| | | | | Flax). |
----------------------+------------+------------+--------------+--------------+--------------+
1. Yield of cellulose | 81·2 | 80·8 | 76·3 | 62·8 | 86·3 |
by chlorine method | | | | | |
| | | | | |
2. Character of |Fibres free.|Fibres free.| Fibres |Fibres | Fibres |
isolated cellulose | | | agglutinated.|agglutinated. | partially |
| | | | | agglutinated.|
| | | | | |
3. Loss in boiling 1 | 9·9 | 14·5 | 9·6 | 18·6 | 5·8 |
per cent. caustic | | | | | |
soda (5 minutes) | | | | | |
| | | | | |
4. Ditto (60 minutes) | 14·6 | 16·5 | 19·0 | 31·7 | 9·9 |
| | | | | |
5. Ash | 1·0 | 1·3 | 1·0 | 1·6 | 0·9 |
| | | | | |
6. Elementary C. | 45·9 | .. | 42·5 | 42·6 | 44·4 |
composition H. | 6·1 | .. | 5·6 | 5·6 | 5·9 |
| | | | | |
7. Lignified or |Lignified. |Lignified. |Lignified. |Lignified. |Lignified. |
otherwise. Aniline | | | | | |
sulphate reaction | | | | | |
----------------------+------------+------------+--------------+--------------+--------------+
BAST FIBRES OF DICOTYLEDONOUS PLANTS.
----------------------+---------------+------------+------------+--------------+-------------+------------
| | Urtica | Crotolaria | | Linum | Corchorus
| Boehmeria |heterophylla| juncea | Hibiscus |usitatissimum| capsularis
| Puya. | (Nilgherry | (Sunn). | strictus. | (Flax). | (Jute).
| | Nettle). | | | |
----------------------+---------------+------------+------------+--------------+-------------+------------
1. Yield of cellulose | 83·8 | 95·0 | 76·9 | 63·0 | 82·0 | 75·0
by chlorine method | | | | | |
| | | | | |
2. Character of |Fibres |Fibres free.|Fibres free.| Fibres |Fibres free. |Fibres free.
isolated cellulose |partially | | | partially | |
|agglutinated. | | | agglutinated.| |
| | | | | |
3. Loss in boiling 1 | 15·6 | 2·6 | 5·3 | 13·7 | 7·8 | 17·2
per cent. caustic | | | | | |
soda (5 minutes) | | | | | |
| | | | | |
4. Ditto (60 minutes) | 24·1 | 7·3 | 10·7 | 25·5 | 16·6 | 18·6
| | | | | |
5. Ash | 3·7 | 1·0 | 0·9 | 2·0 | 1·7 | 1·1
| | | | | |
6. Elementary C. | 41·8 | 42·9 | .. | .. | 43·7 | 47·1
composition H. | 6·0 | 5·9 | .. | .. | 5·9 | 5·9
| | | | | |
7. Lignified or | Not | Not | Lignified. | Lignified. | Not |Lignified.
otherwise. Aniline | lignified. | lignified. | | | lignified. |
sulphate reaction | | | | | |
----------------------+---------------+------------+------------+--------------+-------------+------------
About two grms. are taken and dried at 110–115° C. until the weight
remains constant: the loss represents water. The fibre is then treated
with a hot mixture of benzene and alcohol, dried as before and weighed:
the loss is fat, wax, {45} &c. The residue from this treatment is then
repeatedly exhausted with boiling water and again dried at 110–115° C.
and weighed: the loss is put down as aqueous extract. The exhausted
fibre is then treated for cellulose by either of the methods given on
p. 19. The ash is determined in a separate portion.
{46}
CHAPTER IV.
AN ACCOUNT OF THE CHEMICAL AND PHYSICAL CHARACTERISTICS OF THE
PRINCIPAL RAW MATERIALS.
This chapter contains the results obtained from an investigation of the
different plant fibres by the methods indicated in the two previous
chapters.
In the following table the fibres are classified according to the
reactions which they give with iodine solution.
CLASSIFICATION OF PLANT FIBRES ACCORDING TO THEIR COLOUR
REACTION WITH IODINE SOLUTION (VÉTILLART).
A. B. C.
Seed Hairs. Dicotyledonous Monocotyledonous
Bast Fibres. Fibres corresponding
to Bast Fibres.
Blue reaction
Cotton. Linen. Straw.
Hemp. Esparto.
China grass. Pine apple.
Paper mulberry
Sunn.
Yellow reaction
Jute. New Zealand flax.
Yucca.
Hibiscus. Aloe.
Manilla hemp.
Class A. COTTON.—Genus, _Gossypium_. Order, _Malvaceæ_. Seed hairs,
consisting of ultimate fibres. Length, 2·0–4·0 c.m., diameter,
0·012–0·037 mm. Mean ratio, 1250. Illustrated in Fig. 5: _a_, sections;
_b_, longitudinal views; _c_, ends; mag. 300.
Microscopic features.—Fibres simple, opaque, flattened, always more or
less twisted upon itself, side wall membranous, showing striæ. {47}
Sections.—Simple, oval, irregular, central cavity often containing
granules.
Micro-chemical reaction.—Blue with iodine solution.
[Illustration: FIG. 5.]
General chemical characteristic.—Pure cellulose basis with slight
admixture of colouring matters, &c.
COMPOSITION OF RAW FIBRE.
Church. Müller.
Cellulose 91·15 91·35
Fat 0·51 0·40
Aqueous extract (containing
nitrogeneous substances) 0·67 0·50
Water 7·56 7·00
Ash 0·11 0·12
Cuticular substance (by difference) .. 0·63
E. Schunck in his investigation of cotton bleaching (Chem. News, 17,
p. 11) finds that in boiling with sodium carbonate solution the fibre
loses 5 per cent. in weight. Some portion of the dissolved substances
is precipitated on acidifying and is found to contain a wax, brown
resinous and colouring matters, and pectic acid. {48}
Forms in which employed.—Fibrous refuse from the decortication of the
seeds; spinning waste, threads, rags, new and old.
Class B. FLAX.—Genus, _Linum_. Order, _Linaceæ_, Bast fibres and
filaments. Length of fibres, 25–30 mm.; diameter, 0·02 mm., ratio,
1200. Illustrated in Fig. 6: _a_, sections of the fibres, isolated
and in groups; _b_, the fibres viewed longitudinally, one showing the
creases produced by repeated bending; _c_, ends; _a′_, sections of
fibres situated near the butt of the plant; mag. 300.
[Illustration: FIG. 6.]
Microscopic features.—Fibres transparent, regular, with tapered ends,
side walls thick, consequently central canal small, smooth externally,
sometimes slightly striated. Filaments easily split up into fibres.
Sections.—Polygonal, regular, angles more or less acute, lumen
represented by point; slight indications of concentric arrangement of
fibre substance.
Micro-chemical reaction.—Blue with iodine solution.
General chemical characteristic.—Pecto-cellulose.
Composition of raw fibre.—This varies with the different species, and
is doubtless also influenced by variations in the {49} processes of
retting. The following are the analyses of two samples of Belgian flax
(heckled):—
Müller.
Cellulose 81·99 70·55
Fat and wax 2·37 2·34
Aqueous extract 3·62 5·94
Pectous substances 2·72 9·29
Water 8·60 10·56
Ash ·70 1·32
The chemistry of the flax fibre has been investigated by Kolb.
Forms in which employed.—Scutching refuse, spinning waste, threads,
rags, new and old.
HEMP.—_Cannabis sativa._ Order, _Cannabinacæ_. Bast fibres: length,
22 mm.; diameter, 0·022 mm.; ratio, 1000. Illustrated in Fig. 7:
_a_, _a′_, sections of groups of fibres of the first and second zone
respectively; _b_, fibres seen longitudinally; _c_, ends; mag. 300.
[Illustration: FIG. 7.]
Microscopic features.—Compact bundles. Fibres show striæ and fissures,
and often fibrillæ, detached or adherent. The central canal almost
obliterated, ends of fibres large and flattened. The bundles show fine
transverse markings. {50}
Section.—Well marked concentric zones of fibre substance, irregular in
outline.
Micro-chemical reaction.—Blue and yellow reaction with iodine solution,
the joint result showing green colouration.
General chemical characteristic.—Pecto-cellulose.
COMPOSITION OF RAW FIBRE (ITALIAN HEMP).
Müller.
Cellulose 77·13
Fat and wax ·55
Aqueous extract 3·45
Pectous substances 9·25
Water 8·80
Ash 0·82
Forms in which employed.—Scutching refuse, spinning waste, threads,
cuttings and rope ends.
SUNDA or SUNN HEMP.—Genus, _Crotalaria_. Order, _Papilionaceæ_. Bast
filaments: length, 7–8 mm.; diameter, 0·03; ratio, 200 : 1.
Microscopic features.—Generally similar to those of hemp.
Micro-chemical reactions.—With iodine various, from blue to yellow.
With aniline sulphate, slight yellow.
General chemical characteristics.—Pecto-cellulose (with some
ligno-cellulose).
The following are the results of analysis of the raw fibre (H. Müller):—
Cellulose 80·01
Fat and wax ·55
Aqueous extract 2·82
Pectous substances 6·41
Water 9·60
Ash 0·61
This fibre, exported from India and the Sunda Islands, has found
employment in Spain and Portugal, but up to the present has not been
adopted to any extent in other parts of Europe.
CHINA GRASS, RHEA, RAMIE.—Genus, _Bōhmeria_. Order, _Urticaceæ_.
Bast filaments: length, 120 mm.; diameter, 0·05; ratio, 2400 : 1.
Illustrated in Fig. 8: _a_, section of a {51} bundle of fibres; _b_, a
fibre seen longitudinally; _c_, ends; mag. 300.
[Illustration: FIG. 8.]
Microscopic characteristics.—Irregular in form and length; often
conspicuous in respect of latter (see Table, p. 39); fibre sometimes
cylindrical, either smooth or striated, sometimes flattened; central
canal well developed, often containing granules; extremities of fibres
rounded, spatulated or lance-shaped. Sections marked by numerous
concentric layers, often showing radiating striæ.
Micro-chemical reaction.—Blue to violet, with iodine solution.
General chemical characteristics.—Pecto-cellulose.
Composition of raw fibre (H. Müller):—
Cellulose 75·83 62·50
Fat and wax ·20 0·56
Aqueous extract 6·29 9·76
Pectous substances 6·07 12·00
Water 8·74 9·55
Ash 2·87 5·63
Forms in which employed.—Up to the present the price has been
prohibitive. Has been the subject of successful {52} experiments;
imparts great tensile strength to paper. The portions available will be
as in flax.
COMMON NETTLE.—_Urtica dioica._ Order, _Urticaceæ_. Bast filaments:
length, 27 mm.; diameter, 0·05; ratio, 550. Microscopic characteristics
similar to the above.
The fibres of this plant were, in the olden times, separated in the
same way as flax, and worked up into cloth. At present it has no
practical interest to the paper-maker, but deserves attention at the
hand of the student, as the most easily accessible for the purpose of
study. It is, moreover, possible that, under cultivation, it may yet
become a source of raw material for paper-making.
JUTE.—_Corchorus._ Order, _Tiliaceæ_. Bast: length, 2 mm.; diameter,
0·022; ratio, 90. Illustrated in Fig. 9: _a_, section of bundles of
fibres; _b_, fibres seen longitudinally; _c_, ends; mag. 300.
[Illustration: FIG. 9.]
Microscopic features.—Compact bundles; fibres smooth.
Micro-chemical reactions.—Yellow brown, with iodine; yellow, with
aniline sulphate; bright yellow, with chlorine water; changed to
carmine by treatment with sodium sulphite solution. {53}
General chemical characteristic.—Ligno-cellulose. The chemistry of jute
as the type of ligno-cellulose has been treated on p. 17.
Composition of raw fibre (Müller):—
Cuttings or Butts.
1st Quality. (Root ends).
Cellulose 63·76 60·89
Fat and wax 0·38 ·44
Aqueous extract 1·00 3·89
Non-cellulose, or lignin 24·32 20·98
Water 9·86 12·40
Ash ·68 1·40
Forms in which employed.—Threads, butts, bagging.
BAST TISSUES.—We have alluded, in our opening remarks, to the broad
division of the bast of exogen or dicotyledonous plants into coherent
and non-coherent tissues. We have now considered the more important
of the latter, and we find that they are obtained from the stems of
annuals. We find also that while they constitute a tissue, in the sense
of being definitely localised, the constituent fibres are disposed in
parallel series of independent bundles or filaments, which are isolated
by a mechanical operation after the whole stem has undergone the
preparatory retting process. In exogens of longer and larger growth,
the bast, as might be expected, becomes a coherent compound tissue,
which is, in many cases, easily detached from the underlying wood. We
shall mention three of these as receiving very important applications;
though only one has been applied to any extent by the papermakers of
this country. This, which we shall consider first, is the bast of
the _Adansonia digitata_, or baobab, a tree which flourishes in the
tropical regions of the West Coast of Africa. It is exported chiefly
from Loanga, in the form of fibrous lumps of a brown colour. These
are seen to be composed of a network of bast bundles, which are but
slightly coloured, intersected by medullary rays of a dark brown
colour. The microscopic features of the fibre are those which are
generally characteristic of the bast of exogens; we do not know that we
could cite any whereby it could be identified with certainty. {54}
The fibres give a yellow colouration with aniline sulphate, but they
contain only a small proportion of lignose. The following are the
results of analyses of this bast:—
Cellulose 49·35 58·82[7]
Fat and wax 0·94 0·41
Aqueous extract 13·57 7·08
Pectous substances 19·05 15·19
Water 10·90 13·18
Ash 6·19 4·72
[Footnote 7: Made up of 13·75 cellulose from medullary tissue,
45·07 cellulose of fibres.]
_Linden Bast_ (_Tilia Europæa_) is the raw material employed in the
manufacture in Russia of the mats so largely used in this country for
wrapping furniture and heavy goods, and also by gardeners for a variety
of purposes. One of the features of this bast is the strong cohesion
of the fibres in the bundles or filaments. They are resolved, but with
difficulty, on long boiling, with a solution of carbonate of soda, the
soluble products being of a mucilaginous nature. This bast has not been
applied to any extent by the paper-makers of this country.
_Bast of the Paper Mulberry_ (_Broussonetia papyrifera_).—This
product deserves mention, not from its importance to the European
papermaker, but because of its application to the manufacture of the
peculiar papers of the Chinese and Japanese. The special features
of this, and the other basts which are similarly employed in these
countries—_Edgworthia papyrifera_, _Broussonetia kaempferia_, are (1),
the ease with which the coherent fibrous tissue is separated from
the parenchymatous tissue which accompanies it; (2), its comparative
freedom from medullary rays; (3), the great length and fineness of the
fibres. These properties conduce to its ready conversion into a well
felted paper, of great tensile strength and remarkable softness.
Class C.—These are whole stems of monocotyledonous annuals. The
isolation of fibres for paper-making from these plants depends upon a
chemical process of resolution; the {55} pulp obtained is, therefore,
a complex of the various orders of cell-fibres contained in the plant.
While the pulp consists for the most part of the vessels of the
fibro-vascular bundles, it contains in addition the serrated cuticular
cells which are so characteristic of this group. They therefore present
a general similarity in microscopic features; there are, however,
certain individual characteristics, such as the form and dimensions of
particular cells, which serve for the identification of the various
pulps. Where such occur they will be indicated.
[Illustration: FIG. 10.]
ESPARTO.—_Stipa tenacissima_ and _Lygeum Spartum_.
Order, _Gramineæ_. Bast fibres of fibro-vascular bundles. Length,
1·5 mm.; diameter, 0·012 mm.; ratio, 125. Illustrated in Figs. 10,
11. In Fig. 10, the fibro-vascular {56} bundles _f_ are seen spread
throughout the interior of the leaf, but the intervals, instead of
being occupied by parenchyma, with large cells and thin walls, are
filled with a compact mass of fine solid fibres _f′_; _e_, external
epidermis; _e′_, internal epidermis; mag. 100. In Fig. 11, _a_ is a
section of a group of fibres; _b_, fibres seen longitudinally; _c_,
ends; mag. 300.
[Illustration: FIG. 11.]
Microscopic features.—Short, smooth, cylindrical, uniform in diameter,
central canal very small, extremities rounded, truncated and bifurcated.
Section.—Minute, generally oval, sometimes polygonal; central cavity
represented by a point.
Micro-chemical reaction.—Both blue and yellow with iodine solution.
In examining a paper containing esparto under the microscope, the
pulp will be found to contain, in addition to the fibres of the
fibro-vascular bundles, a certain number of the cuticular cells (see
Fig. 12), together with some of the fine hairs which are seen in the
section (Fig. 10). These are very characteristic.
General chemical characteristics.—Pecto-cellulose mixed with some
ligno-cellulose. {57}
Composition (Hugo Müller):—
Spanish. African.
Cellulose 48·25 45·80
Fat and wax 2·07 2·62
Aqueous extract 10·19 9·81
Pectous substances, &c. 26·39 29·30
Water 9·38 8·80
Ash 3·72 3·67
STRAW.—Order, _Gramineæ_.
Microscopic features.—Generally similar to those of esparto. There are,
however, differences of shape and dimensions of the serrated cuticular
cells, which differentiate the various kinds of straw from each other
and from esparto.
[Illustration: FIG. 12.]
In Fig. 12 are shown these cells, from maize-straw (_a_ and _b_); from
rye-straw (_c_) and from esparto (_d_). The {58} following table gives
the dimensions of the cells from different kinds of straw:—
Length. Breadth.
Barley 0·103–0·224 mm. 0·012–0·014 mm.
Rye 0·086–0·345 „ 0·012–0·016 „
Wheat 0·152–0·449 „ 0·018–0·024 „
Oats 0·186–0·448 „ 0·012–0·017 „
Another distinctive feature of straw-fibre is the presence in it of a
number of oval cells, derived from the pithy matter attached to the
inside of the stem. These are clearly shown at _b_ (Fig. 13), which
represents the general appearance of straw pulp.
[Illustration: FIG. 13.]
General chemical characteristics.—Both ligno-cellulose and
pecto-cellulose.
The following are the results of analyses of straws (Hugo Müller):—
{59}
Winter Wheat. Winter Rye.
Cellulose 46·60 47·69
Fat and wax 1·49 1·93
Aqueous extract 8·07 0·05
Non-cellulose or lignin 28·49 26·75
Water 9·85 11·38
Ash 5·50 3·20
BAMBOO AND SUGAR CANE.—Order, _Gramineæ_.
From the close botanical relationship of these products to the stems
of the gramineæ of our own climate, their microscopic features are, as
might be expected, similar to those of straw. The similarity is further
shown by the chemical composition (Hugo Müller):—
Air-dried
Cellulose 50·13
Fat and wax ·78
Aqueous extract 10·56
Lignin and pectous substances 24·84
Water 8·56
Ash 5·13
NEW ZEALAND FLAX.—_Phormium tenax._ Order, _Lineaceæ_. Fibro-vascular
bundles of the leaves.
Microscopic characteristics.—Length of fibres, 9 mm., diameter 0·016
mm., ratio, 560. Fibres are fine, regular and smooth; the walls are
uniform, central canal small, extremities vesicular. The fibres have
little cohesion in the bundle. Sections round or polygonal. Illustrated
in Fig. 4, p. 35.
Micro-chemical reaction.—Yellow, with iodine solution. Characteristic
deep red colouration with concentrated nitric acid (Müller).
General chemical characteristic.—Ligno-cellulose. It contains 86·3 per
cent. of cellulose.
MANILLA HEMP.—_Musa textilis_, Abacá. Order, _Musaceæ_. Fibro-vascular
bundles of leaves.
Microscopic characteristics.—Length of fibres, 6 mm.; diameter, ·024
mm.; ratio, 250. Fibres, white lustrous; the walls are uniform; central
cavity large and very apparent. Fibres easily detached. {60}
Sections round or polygonal. Illustrated in Fig. 14: _a_, section of
bundle of fibres; _b_, fibres seen longitudinally; _c_, ends; mag. 300.
[Illustration: FIG. 14.]
Micro-chemical reaction.—Yellow with iodine solution.
Composition (Hugo Müller):—
Cellulose 64·07
Fat and wax ·62
Aqueous extract ·96
Lignin and pectous substances 21·60
Water 11·73
Ash 1·02
WOOD.—Nearly, if not the whole of the chemical wood pulp used in this
country is obtained from trees belonging to the order _Coniferæ_
(Gymnospermæ), more particularly from the genera _Abies_ and _Pinus_.
In America, however, poplar and other woods are largely employed. The
coniferæ yield a larger proportion of pulp than most other woods, the
individual fibres, moreover, are longer, and for these reasons it is
generally preferred. On the other hand, however, poplar is more readily
acted upon by reagents. Fig. 15 gives the microscopic appearance of
{61} the fibre of the common white fir. It is characterised by the
presence of numerous pitted vessels (Fig. _a_).
[Illustration: FIG. 15.]
Pine wood consists essentially of a compound cellulose, resembling in
most of its properties the jute fibre (see p. 16). With iodine solution
it gives a deep yellow colour. The chemical composition of some of the
more important woods will be seen from the following analyses. (Müller.)
----------------+------+------+-----+-----+-------
|Birch.|Beech.|Lime.|Pine.|Poplar.
+------+------+-----+-----+-------
Cellulose[8] | 55·52| 45·47|53·09|56·99| 62·77
Resin | 1·14| 0·41| 3·93| 0·97| 1·37
Aqueous extract | 2·65| 2·41| 3·56| 1·26| 2·88
Water | 12·48| 12·57|10·10|13·87| 12·10
Lignin | 28·21| 39·14|29·32|26·91| 20·88
----------------+------+------+-----+-----+-------
[Footnote 8: For the amounts of cellulose actually obtained in
practice see p. 77.]
The above results are calculated on the ash-free wood. The ash varies
from about 0·3 to 0·7 per cent.
{62}
CHAPTER V.
PROCESSES FOR ISOLATING CELLULOSE FROM PLANT SUBSTANCES.
We are now in a position to discuss generally the various methods
by which the paper-maker obtains cellulose from the different raw
materials. The special treatments necessary for each of the more
important of these will be given more fully subsequently. These
methods, though they vary considerably with the different materials
to be treated, may be roughly divided into two groups—the alkaline
and the acid processes. The former, which we will call _a_, comprises
all processes in which caustic soda, carbonate of soda and lime, or
mixtures of these are employed; the latter, _b_, the various processes
which have been introduced of late years, involving the use of
sulphurous acid, either alone or in combination with a base, such as
lime or magnesia.
The processes in group _a_ may be applied to the treatment of every
known fibrous vegetable material; those in group _b_ are at the present
time applied exclusively to the preparation of pulp from wood.
Group _a_ may be conveniently subdivided according to the nature of the
materials operated upon.
It is obvious that those materials, such as bleached cotton and linen
threads and rags, which have already undergone treatment by the textile
manufacturer, and are therefore more or less already in the state of
pure cellulose, require but little chemical treatment at the hands of
the paper-maker, whose attention is therefore chiefly directed to the
removal of such adventitious matters as size, grease, &c. This can
generally be effected by the employment of weak solutions of caustic
soda, or even of lime at a low pressure. {63}
We now have to consider the treatment of those materials which consist
of the compound celluloses.
All methods for the isolation of cellulose from the compound celluloses
depend upon a hydrolytic resolution of their constituents, i.e. a
splitting up by combination with water into cellulose on the one hand,
and a series of soluble derivatives on the other. This, as will be
shown, may to a certain extent be accomplished by the action of water
itself at a high temperature. As, however, the products of such an
action are acid bodies, which, if allowed to remain in contact with the
cellulose, would injuriously affect it, and would induce the production
of complicated bodies, the removal of which from the cellulose would
become increasingly difficult, it is necessary to have present a body
such as caustic soda, which by combining with these acid bodies,
removes them as such from the sphere of action.
In certain raw materials, such as straw and esparto, we have, in
addition to the compound celluloses which form, so to speak, the
groundwork, a certain proportion of fatty and resinous bodies, whose
removal is brought about by the action of the caustic soda converting
them into soaps.
The compound celluloses may be divided, as we have seen, into three
classes—pecto-celluloses, ligno-celluloses, and adipo-celluloses. The
last-named being present in only a few paper-making materials, and then
only in very small proportion, is without any practical interest. We
are concerned therefore with the two former only.
(1) _Pecto-celluloses._—From what we have seen of the nature of the
pecto-celluloses, it follows that for their resolution a tolerably
simple treatment with soda will be sufficient.
The chief members of this group in which the paper-maker is interested,
are esparto and straw. The former being but very slightly lignified,
its resolution can be effected at a very low pressure; in fact it is
even possible by using tolerably strong solutions to do it by boiling
in open vessels.
Straw, on the other hand, though it possesses certain features in
common with esparto, is more lignified, and in {64} consequence,
necessitates a much more energetic action to complete its resolution.
It is therefore usual to boil it at somewhat high pressures (40–70
lb.), and with larger quantities of soda than are demanded in the case
of esparto. It is, moreover, essential, so to regulate the treatment,
that even the most resistent portions of the straw, such, for example,
as the knots, shall be completely resolved, otherwise unbleached
portions are liable to find their way into the finished paper. By
increasing the quantity of soda, it is possible to boil under reduced
pressure, and _vice versâ_; this may be taken as a general principle
applicable in all other cases.
We now come to the second class of celluloses, that is to say, the
ligno-celluloses. Of these, perhaps the most important are jute,
manilla, and wood.
For their _complete_ resolution, it is imperative to employ very strong
solutions at very high temperatures and pressures; thus in the case
of wood it is necessary to boil at temperatures corresponding to a
pressure of 100–120 lb. per square inch.
The use of strong caustic soda solutions at high temperatures is
attended with very serious objections, apart from the question of
cost. Among these may be mentioned the destructive action of the soda
upon the cellulose itself, involving a considerable loss of pulp;
the de-hydration, condensation, and oxidation of the non-cellulose
portion of the fibre (which, as we have seen (p. 25) is of the nature
of an aldehyde, and therefore peculiarly susceptible of such changes),
and the formation of dark coloured products which remain intimately
combined with the residual cellulose; the inconvenience of working at
very high temperatures and pressures.
With a view to removing the difficulties attending the use of soda,
various processes have from time to time been proposed, in which the
hydrolytic agent is a solution of sulphurous acid in water, either
alone or in combination with a base. These processes are comprised
under group B. {65} Several of them have lately acquired great
commercial and industrial importance, and a clear understanding of them
is therefore essential to the student of paper-making.
Though applicable to lignified fibres generally, they have only been
applied up to the present time to wood, and it is in this connection,
therefore, that we propose to consider them.
In order to bring out more clearly the fundamental differences
underlying the two classes of processes, it will be well to consider,
somewhat in detail, these processes, which, although they have now
a merely historical interest, will assist the student in thoroughly
grasping the rationale of the modern and improved methods.
Water Process.—Wood is to a certain extent resolved by treatment with
water at a high temperature, the degree of disintegration being,
within certain limits, proportional to the temperature at which it is
digested. The volatile products of the resolution are chiefly acids
of the acetic series, furfurol and terpenes. The non-volatile soluble
products are for the most part acids. This process was studied by
Fry, who, in 1866, made it the subject of a patent. He found that
under the most favourable conditions of treatment, the yield of brown
pulp amounted to 70 per cent. of the wood employed. The objections
to the process were sufficiently obvious: the pulp produced was of
low quality, and the soluble products acted powerfully on the iron
digesters employed. The chemical causes of its failure lie in the
prominence of the conditions which determine re-combinations of
the products of resolution with one another, viz. oxidation, high
temperature, and the presence of acids. It is worthy of note that in
one experiment the amount of acids produced from 100 parts wood were
equivalent to 3·4 parts of soda (Na_{2}O). This was by far the most
serious defect in the process, and it eventually caused its abandonment
in favour of the older method of treatment with caustic alkali.[9]
[Footnote 9: See Journ. Soc. of Arts., February 1883, p. 223.]
Alkali Processes.—The first to develop the alkali process {66} were
Watt and Burgess, in America (1853–1857), and Houghton in this country.
To follow up the several modifications introduced is unnecessary; they
differ from one another in the economical balance which they strike
between temperature and quantity of chemicals: thus Watt and Burgess
employed a lye of 12° Baumé, and digested at 60 lb. pressure (equal
to 152° C.), whereas Houghton used a lye of 4° B., at 100–180 lb. One
great defect of these processes was the gradual neutralisation of the
alkali, necessitating as a compensating condition the employment of a
very high temperature towards the end of the operation. To overcome
this objection, Ungerer devised a process which aimed at securing a
properly graduated action of the lye. This process, being based on
an eminently rational view of the action of the alkali, is worthy of
a more extended notice, especially as the principle on which it is
founded is capable of wide application. A system of nine vertical
digesters, connected together in series by means of pipes and cocks, so
that the liquid contents can be transferred from one to the other in
succession, is the main feature of the method employed. In connection
with this system there is a double service of pipes, by which the
hot soda lye at 90 lb. pressure, and cold water, may be respectively
introduced. The operation is so conducted that in five of the digesters
wood is being treated at a high temperature, in three the pulp is being
washed, while the remaining one is being charged with fresh wood.
The whole round of operations may be described thus, numbering the
digesters according to their position in the series:—Nos. 1, 2, and 3,
Washing the finished pulp; 4, Final treatment with lye at the extreme
pressure of 90 lb.; 5, Digesting at lower pressure with lye previously
employed in 4; 6, Treatment at still lower pressure with lye which has
already done duty in 4 and 5; and so on to 8, where the liquor, having
passed through the series, is employed at 15–20 lb. pressure on a fresh
charge of wood. The washings are conducted in such a way that the water
which is introduced into 1 displaces the solution in each digester
without {67} the last being allowed to escape. The introduction of the
fresh liquor at 4 is similarly accomplished, 1, 2, and 3 being cut off
by closing the cocks which control the ingress and egress of liquid.
The pulp produced by this method is of a sufficiently pale colour to
be capable of application for many purposes without bleaching. It is
converted into a perfectly white pulp by treatment in the ordinary way
with from 3 to 4 per cent. of bleaching powder, calculated in the dry
pulp.
As far as it goes the method is practically perfect, and it appears
to have given sound results, at least as regards the main product.
The principal disadvantages were the complications attending such an
elaborate system of working which would doubtless make it costly.
We have next to consider a class of processes which are based primarily
upon the action of an alkali, but which introduce the simultaneous
action of a soluble sulphide, generally that of sodium. As far back as
1855 Jullion patented in this country a process of this nature, which,
however, does not appear to have been applied to wood. The results
obtained with such materials as straw and esparto appear to have been
very unsatisfactory, and to have led to the early abandonment of the
method.
Recently the increased attention which has been given to wood as a
raw material has led to a revival of these sulphide processes for its
treatment. We believe that at least two processes of this class are
being worked on the continent, viz. those of Dahl and Blitz.
That of the former consists in heating wood, at pressures varying
from 60–120 lb. per square inch, with a solution of the following
composition:—
Sodium sulphate 37
Sodium carbonate 8
Sodium hydrate 24
Sodium sulphide 28
Various compounds 3
───
100
{68}
This solution is obtained only after the process has been some time at
work, and it then remains tolerably constant.
The wood, to commence with, is boiled with a solution containing a
large quantity of sodium sulphate, with a small quantity of sodium
hydrate, obtained by boiling sodium sulphate with lime. The liquor from
this operation is then evaporated and calcined, the result being a
fused mass which, when dissolved in water, yields a liquor containing
the above-mentioned ingredients. The loss of chemicals is made up by
the addition of fresh quantities of sodium sulphate.
From experiments which the authors have made with this process, it
appears, that while it is perfectly true that pulp can be obtained,
it is necessary to employ either very strong solution or very high
pressures. This is equivalent to saying that either the hydrolytic
action is weak, or the tendencies to reversal of the hydrolysis are
strong, and indeed both are to be predicted from the nature of the
solution. One great objection to the process is that during the boiling
a quantity of very foully-smelling compounds are produced.
The process of Blitz consists in heating wood to from 50–60 lb.
pressure, with a solution of sodium sulphide, containing an
infinitesimal proportion of vanadate of ammonia.
It is difficult to see what influence the presence of the costly
vanadate of ammonia can have on the result. Actual experiment on this
point has failed to convince us of its efficacy.
Although in these sulphide processes a strongly reducing agent is
present, its effect in preventing oxidation is more than outweighed by
the weakening of the hydrolytic action of the soda, both directly by
its combination with sulphur, and indirectly by the complication which
it induces by combination with the soluble bye-products. Further, as
regards the final product, they offer no advantage over the caustic
alkali processes. There is, however, a considerable advantage in point
of economy, which should lead to their adoption in districts where the
production of malodorous gases is permitted. {69}
Acid Processes.—We now come to consider the processes which are
based upon the hydrolytic action of acids as a primary condition.
We have seen that in the water process this condition obtains as a
secondary result, and that while it aids in resolving the wood, it also
determines the limit of the action, in consequence of its promoting
secondary combination amongst the soluble products. The use of powerful
oxidising acids, such as nitric and nitro-hydrochloric, induces an
action which is sufficiently drastic to cause a complete resolution
of the non-cellulose constituents of the wood with the formation of
soluble products. The use of these acids has been made the subject of
numerous patents.
The process of Barre and Blondel consisted in digesting the wood in
the cold for twenty-four hours in 50 per cent. nitric acid, by which
it was converted into a soft fibrous mass. This was boiled for some
hours in water, afterwards in a solution of sodium carbonate, and
finally bleached in the ordinary way. Many other patents have been
taken out in this direction, but none have met with practical success,
owing doubtless, to the high cost of nitric acid, and the difficulty
of recovery. A process has lately been patented by Young and Pettigrew
(No. 14,988, 1884) in which they claim the use of either nitric or
nitrous acids; the acid fumes which are evolved being condensed and
reconverted into nitric acid. The process has not yet, however, been
tried on a practical scale.
The resolution of wood by the hydrolytic action of non-oxidising acids,
is for reasons already stated, very imperfect. An important feature,
however, of this action is the production of glucose as a bye-product,
and a process having the double object of producing this, together
with a brown pulp, was patented by Bachet and Machard in 1864, and
worked for some time in Switzerland. The process consisted in boiling
the wood for twelve hours in dilute hydrochloric acid; the solution,
which contained glucose to the amount of 20 per cent. of the weight
of the wood, was drawn off, neutralised with chalk, and fermented by
the addition of {70} yeast. Alcohol was separated from the fermented
liquor in the usual way. The residual pulp, after being ground with
stones, was made into coarse packing paper. The process failed
commercially.
The next processes in the order of the development of our subject,
but not chronologically, are those involving the use of reducing
acids, i.e. those which combine with their hydrolytic function a
de-oxidising or anti-oxidising action. Of these the only one which
has been practically investigated is sulphurous acid. A full account
of the earlier discoveries in regard to the use of this reagent, for
the special purpose under consideration, will be found in the _Papier
Zeitung_, 1884, No. 51, p. 1927; this should be consulted by those who
are interested in the history of a subject which has now grown to be of
the first importance.
The first patented process claiming the use of sulphurous acid, in the
form of its aqueous solution, for the purpose of disintegrating wood,
was that of Tilghmann (1866). So far as is known, however, the results
obtained by the inventor with the acid alone were such as to lead him
to abandon it for solutions containing a certain proportion of base.
The reasons, doubtless, were the greater facility of preparation and
simplicity of application of such solutions.
More recently Professor Pictet, of Geneva, on the basis of the
experience acquired by him in the preparation and employment of
sulphurous acid in his patent freezing processes, has revived and
perfected a process which in other hands had proved impracticable.
The method and apparatus employed will be found fully described in the
author’s specification. The process consists in digesting the wood in
closed vessels with an aqueous solution of the acid (10–12 per cent.
SO_{2}) at 176°–194° F. (80°–90° C.), and under the corresponding
pressure (60–75 lb. per square inch). A complete disintegration of
pine wood is effected in about twenty-four hours’ treatment. The
yield of pulp is, we are informed, from 40–50 per cent. of the wood
(_Pinus sylvestris_); it is also stated to be readily bleached {71}
with a moderate amount of bleaching powder, and to yield a pulp of
the highest quality. Further, the recovery of the sulphurous acid, for
which special apparatus is provided, is practically complete (95–98 per
cent.). In view of the probable utilisation of the bye-products, it is
interesting to observe that they are obtained in a form in which they
can very readily be treated.
The process being as yet undeveloped on a large scale, we cannot do
more than recommend it to the consideration of paper-makers. At the
same time we would draw attention to the features of interest which
it possesses in regard to the general aspect of our subject. The
resolution which it effects is obviously one of simple hydrolysis, and
it affords an indirect proof of the chemical complications which attend
all other processes for disintegrating wood which employ temperature
much above the boiling-point of water. The soluble bye-products are
obtained in the simplest form, and may be isolated by mere evaporation.
As regards the chemical features of the pulp, they will presumably be
different from those of any other, whether on the side of advantage or
not it remains to decide. Provided the process fulfils the condition
of economical working, we have no hesitation in predicting for it a
considerable development.
The difficulties attending the employment of a gaseous acid have
doubtless operated in deterring many, in addition to the originator,
from a complete investigation of its action, and have led them to adopt
in preference one of its several compounds with bases, the employment
of which defines a group of processes to which we now desire to call
attention. There is a variety of processes agreeing in the essential
feature of employing a bisulphite as the disintegrating reagent, and
differing from one another only in the less essential details of method.
The general principle of these processes will be readily understood
from what has gone before. The chief agency is the hydrolytic action
of sulphurous acid, aided by the conditions of high temperature and
pressure: and the subsidiary {72} agencies are—(1) The prevention of
oxidation; (2) The removal from the sphere of action of the soluble
products of resolution in combination with the sulphite as a true
double compound. This mode of union is well known to be peculiarly
characteristic of all aldehydes, to which class we have shown that the
non-cellulose constituents of wood belong. The combination is expressed
by the general equation—
M.CHO + SO_{3}HNa = M.CH(OH/SO_{3})Na.
Aldehyde. Sodium Double compound.
bi-sulphite.
(3) The removal of a portion of the constituents in combination with
the base, i.e. with expulsion of sulphurous acid.
The verification of these views has been afforded in the course of an
investigation of the bye-products from the Ekman process, undertaken by
the authors for Messrs. Thomson, Bonar & Co., the proprietors of the
patent.
As the various bisulphite processes differ chiefly in their working
details, the essential feature of the employment of a solution of
sulphurous acid, combined with a certain amount of base, being common
to them all, it will not be necessary to do more than examine one or
two typical cases. We shall select those of Ekman and Partington, the
latter being based upon Francke’s patent, with certain modifications
and additions.
(1) The Chemicals employed.—In both cases sulphur is used as the source
of the sulphurous acid. The Ekman process starts from magnesite, a
native carbonate of magnesia. This is converted by calcination into
magnesia, and in this state is introduced into lead-lined towers,
through which is passed sulphurous acid gas, obtained by burning
sulphur in suitable furnaces, while a stream of water trickles down
from the top. The supply of the gas being properly adjusted, a
continuous production of a solution of magnesium bisulphite of uniform
strength is obtained. To secure this uniformity, constant attention
is necessary, and further, the loss of sulphurous acid by oxidation
to sulphuric acid, and by diffusion, is, in consequence of its
distribution over so large a surface, {73} and with free exposure to
the air, necessarily very large. It is unnecessary, however, to point
out that the preparation of the magnesium bisulphite is capable of
very considerable modification; in fact, we understand that numerous
improvements in this direction have already been introduced by the
proprietors of the process.
Partington’s method, in addition to the advantage of starting
from lime, the cheapest of all basic substances, is free from the
disadvantages referred to as attending the use of a tower. It consists
in passing the sulphurous acid gas through a series of three vessels
connected together with pipes. Into these vessels a charge of milk of
lime is introduced, and the two first being closed air-tight, the gas
is passed in. The third remains open to the air, and from it escapes
a continuous stream of nitrogen, resulting from the removal of the
oxygen by the burning sulphur, from the stream of air which is blown
through the sulphur retort at a pressure of 5 lb. to the square inch.
The expenditure for chemicals is under 1_l._ per ton, whereas, by the
Ekman process, it has been variously estimated at from 2_l._ to 3_l._
10_s._ per ton. Provided therefore, that as good pulp can be obtained
by Partington as by Ekman, the former offers considerable advantages
both from point of view of simplicity and economy. Doubtless the cost
of chemicals could be somewhat reduced in both systems by the adoption
of a cheaper form of sulphur, such as pyrites.
Preparation of the Wood.—Although the acid sulphite processes can,
by suitable modifications, be made available for the preparation of
cellulose from almost all kinds of wood, yet it is necessary, in
order to insure uniformity of result, not only to take care that the
wood is carefully selected, but that it is freed from bark and roots,
which, if allowed to remain, would be imperfectly pulped, and would
be exceedingly difficult to bleach. The most suitable kind of wood is
_Pinus sylvestris_.
After the bark has been carefully removed the logs are sawn into boards
of ordinary sizes. This treatment {74} exposes the knots, which are
then removed either by boring or cutting out with an axe. The pieces
are then passed through a machine, which cuts them or rather breaks
them into fragments about an inch long. In order that the acid sulphite
may as quickly as possible enter into the wood, these fragments must be
crushed and opened out by passing through heavy rollers: moreover, the
wood is cut _across_ the grain, by which means it is more or less split
up into laminæ. A machine for this purpose has been patented by Fry,
Ransome, and Wilkie (No. 11,389, 1884).
The Boiling Process.—Ekman uses a lead-lined, jacketed cylindrical
boiler, suspended on trunnions, so that it can be turned upside down
for discharging. The pressure in the outer jacket is 70 lb. per square
inch, while inside the boiler it is 90 lb., the difference being due
to the tension of the sulphurous acid gas. The duration of boiling
is from ten to twelve hours. Partington, on the other hand, boils
with live steam, and therefore is enabled to use a boiler of simpler
construction. His boilers are spherical, lead-lined and rotary. The
pressure employed does not exceed 60 lb. per square inch, the time of
boiling being about sixteen hours.
Various other sulphite processes have been from time to time devised,
which differ but little either in principle or in working detail from
those described, and therefore a brief mention of them will suffice.
Mitscherlich employs a bisulphite of lime in somewhat weaker solution
than that employed by Francke or Partington, and in order to produce
the same result he is obliged to extend the time of boiling to from 40
to 60 hours.
Graham, after treating the wood with a mono-sulphite, injects into
the boiler either a solution of a bisulphite or gaseous or liquid
sulphurous acid.
Flodgvist uses a solution prepared by dissolving bones in sulphurous
acid solution.