/CH(OH).CH(OH)
CO >CH2
\CH(OH).CH(OH) ,
the main linking of such units in the complex taking place as between
their respective CO and CH2 groups in the alternative enolic form
CH-C(OH). This view gives expression to the genetic relations of the
celluloses to the ligno-celluloses, to the tendency to carbon
condensation as in the formation of coals, and pseudo-carbons, to the
relative resistance of cellulose to hydrolysis, and its other points of
differentiation from starch, and more particularly to the ketonic
character of its carbonyl (CO) groups, which is also more in harmony
with the experimental facts established by Fenton as to the production
of methyl furfural.
The probability, however, is that no simple molecular formula adequately
represents the constitution of cellulose as it actually exists or indeed
reacts. On the other hand, it has been suggested that cellulose is to be
regarded as representing a condition of matter analogous to that of a
saline electrolyte in solution, i.e. as a complex of molecular
aggregates, and of residues (of monose groups) having distinct and
opposite polarities; such a complex is essentially labile and its
configuration will change progressively under reaction. The exposition
of this view is the subject of a publication by Cross and Bevan
(_Researches on Cellulose_, ii. 1906). The main purpose is to give full
effect to the colloidal characteristics of cellulose and its
derivatives, with reference to the modern theory of the colloidal state
as involving a particular internal equilibrium of amphoteric
electrolytes.
The typical cellulose is a white fibrous substance familiar to us in the
various forms of bleached cotton. Other fibrous celluloses are equally
characteristic as to form and appearance, e.g. bleached flax, hemp,
ramie. It is hygroscopic, absorbing 6 to 7% its weight of moisture from
the air. When dry, it is an electrical insulator, and has a specific
inductive capacity of about 7: when wetted it is a conductor, and
manifests electrolytic phenomena.[1] It is insoluble in water and in the
ordinary solvents; it dissolves, however, in a 40-50% solution of zinc
chloride, and in ammoniacal solutions of copper oxide (3% CuO, 15% NH3):
from these solutions it is obtained as a highly hydrated, gelatinous
precipitate, from the former by dilution or addition of alcohol, from
the latter by acidification; these solutions have important industrial
application. Projected or drawn into a precipitating solution they may
be solidified continuously to threads of various, but controlled
dimensions: the regenerated cellulose, now amorphous, in its finer
dimensions is known as artificial silk or lustra-cellulose. These forms
of cellulose retain the general characters of the original fibrous and
"natural" celluloses. In composition they differ somewhat by combination
with water (of hydration), which they retain in the air-dry condition.
They also further combine with an increased proportion of atmospheric
moisture, viz. up to 10-11% of their weight.
_Derivatives._--Important derivatives are the esters or ethereal salts
of both inorganic and organic acids, cellulose behaving as an alcohol,
the highest esters indicating that it reacts as a trihydric alcohol of
the formula n[C6H7O2(OH)3]. The nitrates result by the action of
concentrated nitric acid, either alone or in the presence of sulphuric
acid: the normal dinitrate represents a definite stage in the series of
nitrates, and the ester at this point manifests the important property
of solubility in various alcoholic solvents, notably ether-alcohol. Such
nitrates are the basis of collodion, of artificial silk by the processes
of Chardonnet and Lehner, and of celluloid or xylonite. Higher nitrates
are also obtainable up to the limit of the trinitrate, which is
insoluble in ether or alcohol, but is soluble in nitroglycerin,
nitrobenzene and other solvents. These higher nitrates are the basis of
the most important modern explosives.
Cellulose reacts directly with acetic anhydride to form low esters; in
the presence of sulphuric acid the reaction proceeds to higher limits;
the triacetate is soluble in chloroform. The acid sulphuric ester,
C6H8O3(SO4H)2, is obtained by the action of sulphuric acid, but its
relation to the original cellulose is doubtful. The monobenzoate and
dibenzoate are formed by benzoyl chloride reacting on alkali-cellulose
(see below). Cellulose xanthates are obtained from carbon bisulphide and
alkali-cellulose; these are water soluble derivatives and the basis of
"viscose," and of important industries. Mixed esters---aceto-sulphate,
aceto-benzoate, nitrobenzoyl nitrates, aceto-nitro-sulphates--have also
been investigated.
Cellulose (cotton), when treated with a 15-20% caustic soda solution,
gives the compound C6H10O5.H2O.2NaOH, alkali-cellulose, the original
riband-like form with reticulated walls of the cellulose being
transformed into a smooth-walled cylinder. The structural changes in the
ultimate fibre determine very considerable changes in the dimensions of
fabrics so treated. The reactions and structural changes were
investigated by J. Mercer, and are known generally as "mercerization."
In recent years a very large industry in "mercerized" fabrics (cotton)
has resulted from the observation that if the shrinkages of the yarns
and fabrics be antagonized by mechanical means, a very high lustre is
developed.
Similar, but less definite compounds, are formed with the oxides of
lead, manganese, barium, iron, aluminium and chromium. These
derivatives, which also find industrial applications in the dyeing and
printing of fabrics, differ but little in appearance from the original
cellulose, and are without influence on its essential characteristics.
_Decompositions._--Hydrolysis:--By solution in sulphuric acid followed
by dilution and boiling the diluted solution cellulose hydrolyses to
fermentable sugars; this reaction is utilized industrially in the
manufacture of glucose from rags. Hydrochloric acid produces a friable
mass of "hydrocellulose," probably C12H22O11, insoluble in water, but
readily attacked by alkalis, with the production of soluble derivatives;
some dextrose is formed in the original reaction. Hydrobromic acid in
ethereal solution gives furfurane derivatives. Cold dilute acids have no
perceptible action on cellulose. The actions of such acids are an
important auxiliary to bleaching, dyeing and printing processes, but
they require careful limitation in respect of concentration and
temperature. Cellulose is extremely resistant to the action of dilute
alkalis: a 1-2% solution of sodium hydrate having little action at
temperatures up to 150 deg. hence the use of caustic soda, soda ash and
sodium silicate in bleaching processes, i.e. for the elimination of the
non-cellulose components of the raw fibres. Oxidation in acid solutions
gives compounds classed as "oxycelluloses," insoluble in water, but more
or less soluble in alkalis; continued oxidation gives formic, acetic and
carbonic acids. Oxidation in alkaline solution is more easily controlled
and limited; solutions of bleaching powder, or more generally of
alkaline hydrochlorites, receive industrial application in oxidizing the
coloured impurities of the fibre, or residues left after more or less
severe alkali treatments, leaving the cellulose practically unaffected.
This, however, is obviously a question of conditions: this group of
oxidants also oxidize to oxycellulose, and under more severe conditions
to acid products, e.g. oxalic and carbonic acids. Certain bacteria also
induce decompositions which are resolutions into ultimate products of
the lowest molecular dimensions, as hydrogen, carbon dioxide, methane,
acetic acid and butyric acid (Omeliansky) (_Handb. Techn. Mykologie_ [F.
Lafar] pp. 245-268), but generally the cellulose complex is extremely
resistant to the organic ferments. Cellulose burns with a luminous flame
to carbon dioxide and water; dry distillation gives a complicated
mixture of gaseous and liquid products and a residue of charcoal or
pseudo-carbon. Chromic acid in sulphuric acid solutions effects a
complete oxidation, i.e. combustion to water and carbonic acid.
_Ligno-celluloses._--These compounds have many of the characteristics of
the cellulose esters; they are in effect ethereal compounds of cellulose
and the quinonoid lignone complex, and the combination resists
hydrolysis by weak alkalis or acids. The cellulose varies in amount from
80 to 50%, and the lignone varies inversely as the degree of
lignification, that is, from the lignified bast fibre of annuals, of
which jute is a type, to the dense tissues of the perennial
dicotyledonous woods, typified by the beech. The empirical formula of
the lignone complex varies from C19H22O9 (jute) to C26H30O10 (pine
wood). In certain reactions the non-cellulose or lignone constituents
are selectively converted into soluble derivatives, and may be separated
as such from the cellulose which is left; for example, chlorination
gives products soluble in sodium sulphite solution, by the combination
of unsaturated groups of the lignone with the halogen, while digestion
with bisulphite solutions at elevated temperatures (140 deg.-160 deg.)
gives soluble sulphonated derivatives. This last reaction is employed
industrially in the preparation of cellulose for paper-making from
coniferous woods. These reactions are "quantitative" since they depend
upon well-defined constitutional features of the lignone complex, and
the resolution of the ligno-cellulose takes place with no further change
in the lignone than the synthetical combination with the substituting
groups. The constituent groups of the lignone specifically reacting are
of benzenoid type of the probable form
HC
//\
HC // \CO
| |
H2C \ /CO
\ /
CO ,
deduced from the similarity of the chlorinated derivatives to
mairogallol, the product of the action of chlorine on pyrogallol in
acetic acid solution (A. Hantzsch, _Ber._ 20, p. 2033). The complex
contains methoxy (OCH3) groups. There is also present a residue which is
readily broken down by oxidizing agents, and indeed by simple
hydrolysis, to acetic acid. Another important group of actual
constituents are pentosanes--partially isolated as "wood gum" by
solution in alkalis--and furfural derivatives (hydroxy furfurals)
derived from these. The actual constitutional relationships of these
main groups, as well as the localization of the methoxy groups, are
still problematical.
Certain colour reactions are characteristic, though they are in some
cases reactions of certain constituents invariably present in the
natural forms of the ligno-cellulose; which may be removed without
affecting the essential character of the lignone complex. Aniline salts
generally give a yellow coloration, dimethyl-para-phenylenediamine gives
a deep red coloration, phloroglucin in hydrochloric acid gives a crimson
coloration. Reactions more definitely characteristic of the lignone
are:--ferric ferrocyanide, which is taken up and transformed into
Prussian blue throughout the fibre, without affecting its structure,
although there may be as much as a 50% gain in weight; iodine in
potassium iodide solution gives a deep brown colour due to absorption of
the halogen, a reaction which admits of quantitative application, i.e.
as a measure of the proportion of ligno-cellulose in a fibrous mixture;
nitric acid gives a deep orange yellow coloration; digested with the
dilute acid (5-10% HNO3) at 50 deg. the ligno-celluloses are entirely
resolved, the lignone complex being attacked and dissolved in the form
of nitroso-ketonic acids, which, on continued heating, are finally
resolved to oxalic, acetic, formic and carbonic acids.
_Derivatives of Ligno-cellulose._--By reaction with chlorine jute yields
the derivative C19H18Cl4O9, soluble in alcohol, and in acetic acid; this
derivative has the reactions of a quinone chloride. By reaction with
sodium sulphite it is converted into a hydroquinone sulphonate of deep
purple colour. The reaction of the ligno-celluloses (pine wood) with the
bisulphites yields the soluble derivatives of the general formula
C26H29O9.SO3H (containing two O.CH3 groups). Jute reacts with nitric
acid in presence of sulphuric acid to form nitrates; and with acetic
anhydride to form low acetates. It reacts with alkaline hydrates with
structural changes similar to those obtained with cotton; and by the
further action of benzoyl chloride and of carbon bisulphide upon the
resulting compounds there result the corresponding benzoates and
xanthates respectively. But these synthetical derivatives are mixtures
of cellulose and lignone derivatives, and so far of merely theoretical
interest.
_Decompositions of Ligno-cellulose._--In addition to the specific
resolutions above described which depend upon the distinctive chemical
characters of the cellulose and lignone respectively, the following may
be noted: to simple hydrolytic agents the two groups are equally
resistant, therefore by boiling with dilute acids or alkalis the groups
are attacked _pari passu_. Weak oxidants may also be used as bleaching
agents to remove coloured by-products without seriously attacking the
ligno-cellulose, which is obtained in its bleached form. Nitric acid of
all strengths effects complete resolution. Chromic acid in dilute
solutions combines with the lignone complex, but in presence of
hydrolysing acids total oxidation of the lignone is determined. The
principal products are oxalic, carbonic, formic and acetic acids. This
reaction is an index of constitution. Generally, the lignone is attacked
under many conditions and by many reagents which are without action upon
cellulose, by virtue of its unsaturated constitution, and its acid and
aldehydic residues.
_Cuto-cellulose._--A typical cuto-cellulose is the cuticle (peel) of the
apple which, when purified by repeated hydrolytic treatment and finally
by alcohol and ether, gives a product of the composition C = 75.66%, H =
11.37%, O = 14.97%. Hydrolysis by strong alkalis gives stearo-cutic
acid, C28H48O4, and oleo-cutic acid, C14H20O4 (Fremy). Cork is a complex
mixture containing various compound celluloses: extraction with alcohol
removes certain fatty alcohols and acids, and aromatic derivatives
related to tannic acid; the residue is probably a mixture of cellulose,
ligno-cellulose, cerin, C20H32O and suberin; the latter yields stearic
acid, C18H36O2 and the acid C22H42O3. The cuto-celluloses have been only
superficially investigated, and, with the exception of cork, are of but
little direct industrial importance.
_Industrial Uses of Cellulose_.--The applications of cellulose to the
necessities of human life, infinitely varied in kind as they are
colossal in magnitude, depend upon two groups of qualities or
properties, (1) structural, (2) chemical. The manufactures of vegetable
textiles and of paper are based upon the fibrous forms of the naturally
occurring celluloses, together with such structural qualities as are
expressed in the terms strength, elasticity, specific gravity. As
regards chemical properties, those which come into play are chiefly the
negative quality of resistance to chemical change; this is obviously a
primary factor of value in enabling fabrics to withstand wear and tear,
contact with atmospheric oxygen and water, and such chemical treatments
as laundrying; positive chemical properties are brought into play in the
auxiliary processes of dyeing, printing, and the treatment and
preparation in connexion with these. Staple textiles of this group are
cotton, flax, hemp and jute; other fibres are used in rope-making and
brush-making industries. These subjects are treated in special articles
under their own headings and in the article FIBRES. The course of
industrial development in the 19th century has been one of enormous
expansion in use and considerable refinement in methods of preparation
and manufacture. Efforts to introduce new forms of cellulose have had
little result. Rhea or ramie has been a favourite subject of
investigation; the industry has been introduced into England, and
doubtless its development is only a question of time, as on the
continent of Europe the production of rhea yarns is well established,
though it is still only a relatively small trade--probably two or three
tons a day total production. The paper trade has required to seek new
sources of cellulose, in consequence of the enormous expansion of the
uses of paper. Important phases of development were: (1) in the period
of 1860 to 1870, the introduction of esparto, which has risen to a
consumption of 250,000 tons a year in the United Kingdom, at which
figure it remains fairly steady; (2) the decade 1870 to 1880, which saw
the development of the manufacture of cellulose from coniferous woods,
and this industry now furnishes a staple of world-wide consumption,
though the industry is necessarily localized in countries where the
coniferous woods are available in large quantities. As a development of
the paper industry we must mention the manufacture of paper textiles,
based upon the production of pulp yarns. Paper pulps are worked into
flat strips, which are then rolled into cylindrical form, and by a final
twisting process a yarn is produced sufficiently strong to be employed
in weaving.
What we may call the special cellulose industries depend upon specific
chemical properties of cellulose, partly intrinsic, partly belonging to
the derivatives such as the esters. Thus the cellulose nitrates are the
bases of our modern high explosives, as well as those now used for
military purposes. Their use has been steadily developed and perfected
since the middle of the 19th century. The industries in celluloid,
xylonite, &c., also depend upon the nitric esters of cellulose, and the
plastic state which they assume when treated with solvent liquids, such
as alcohol, amyl acetate, camphor and other auxiliaries, in which state
they can be readily moulded and fashioned at will. They have taken an
important place as structural materials both in useful and artistic
applications. The acetates of cellulose have recently been perfected,
and are used in coating fine wires for electrical purposes, especially
in instrument-making; this use depends upon their electrical properties
of high insulation and low inductive capacity. Hydrated forms of
cellulose, which result from treatment with various reagents, are the
bases of the following industries: vegetable parchment results from the
action of sulphuric acid upon cellulose (cotton) in the form of paper,
followed by that of water, which precipitates the partially
colloidalized cellulose. This industry is carried out on "continuous"
machinery, the cellulose, in the form of paper, being treated in rolls.
Vulcanized fibre is produced by similar processes, as for instance by
treating paper with zinc chloride solvents and cementing together a
number of sheets when in the colloidal hydrated state; the goods are
exhaustively washed to remove, last traces of soluble electrolytes; this
is necessary, as the product is used for electrical insulation. The
solvent action of cupro-ammonium is used in treating cellulose goods,
cotton and paper, the action being allowed to proceed sufficiently to
attack the constituent fibres and convert them into colloidal
cupro-ammonium compounds, which are then dried, producing a
characteristic green-coloured finish of colloidal cellulose and
rendering the goods impervious to water. The important industry of
mercerization has been mentioned above; this is carried out on both
yarns and cloth of cotton goods chiefly composed of Egyptian cottons. A
high lustrous finish is produced, giving the goods very much the
appearance of silk.
Of special importance are the more recent developments in the production
of artificial fibres of all dimensions, by spinning or drawing the
solutions of cellulose or derivatives. Three such processes are in
course of evolution, (1) The first is based on the nitrates of cellulose
which are dissolved in ether-alcohol, and spun through fine glass jets
into air or water, the unit threads being afterwards twisted together to
constitute the thread used for weaving (process of Chardonnet and
Lehner). These processes were developed in the period 1883 to 1897, at
which later date they had assumed serious industrial proportions. (2)
The cupro-ammonium solution of cellulose is similarly employed, the
solution being spun or drawn into a strong acid bath which instantly
regenerates cellulose hydrate in continuous length. (3) Still more
recently the "viscose" solution of cellulose, i.e. of the cellulose
xanthogenic acid, has been perfected for the production of artificial
silk or lustra-cellulose; the alkaline solution of the cellulose
derivative being drawn either into concentrated ammonium salt solutions
or into acid baths. This product, known as artificial silk, prepared by
the three competing processes, was in 1908 an established textile with a
total production in Europe of about 5000 tons a year, a quantity which
bids fair to be very largely increased by the advent of the viscose
process, which will effect a very considerable lowering in the cost of
production. The viscose solution of cellulose is also used for a number
of industrial effects in connexion with paper-sizing, paper-coating,
textile finishes, and the production of book cloth and leather cloth,
and, solidified in solid masses, is used in preparing structural solids
which can be moulded, turned and fashioned.
For the special literature of cellulose treated from the general point
of view of this article, the reader may consult the following works by
C.F. Cross and E.J. Bevan: _Cellulose_ (1895, 2nd ed. 1903),
_Researches on Cellulose_, i. (1901), _Researches on Cellulose_, ii.
(1906). (C. F. C.)