THE DYEING AND COLOURING OF PAPER PULP
Nearly all papers, even those commonly regarded as white, are dyed
with some proportion of colouring matter. With the ordinary writing
and printing papers the process is usually confined to the addition of
small quantities of pigments or soluble colours sufficient to _tone_
the pulp and correct the yellow tint which the raw material possesses
even after bleaching. In the case of cover papers, tissues, and similar
coloured papers, the process is one of dyeing as it is generally
understood.
The colouring matters which have been employed by the paper-maker are--
PIGMENTS.
(A) Added to the pulp in the form of mineral in a finely divided state.
_Yellow._--This colour is obtained by the use of _ochres_, which are
natural earth colours of varying shades, from bright yellow to
brown.
_Red._--Ordinary red lead.
Various oxides of iron, such as Indian red, Venetian red, red
ochre, rouge.
_Blue._--_Smalts_--An expensive pigment prepared by grinding cobalt
glass.
_Ultramarine_--A substance of complex composition prepared by
heating a mixture of china clay, carbonate of soda, sulphate
of soda, sulphur, charcoal, and sometimes quartz, rosin and
infusorial earth.
_Prussian Blue_--A compound prepared by adding potassium
ferrocyanide to a solution of ferrous sulphate.
_Brown._--Natural earth colours, such as sienna, umber, Vandyke brown.
_Black._--Lamp-black, bone-black, Frankfort black.
(B) Produced by the reaction of soluble salts upon one another when
added to the pulp in the beating engine.
_Yellow._--_Chrome Yellow_--The paper pulp is first impregnated with
acetate of lead, and potassium or sodium bichromate added. This
precipitates the chromate of lead as a yellow pigment.
_Chrome Orange_--The addition of caustic alkali to the bichromate
solution converts the chrome yellow into an orange.
_Blue.--Prussian Blue_--The paper pulp impregnated with iron salts
is treated with potassium ferrocyanide. The blue colour is at once
obtained.
_Brown.--Iron Buff_--A light yellow-brown colour due to the
precipitation of ferrous sulphate by means of an alkali.
_Bronze._--Manganese chloride followed by caustic soda.
SOLUBLE COLOURS.
(A) Natural Dyes. These colouring matters are now seldom used.
_Yellow and Brown._--The vegetable extracts, such as fustic,
quercitron, cutch, turmeric, have practically all been replaced by
aniline colours.
_Red._--Madder (Turkey red), Brazilwood, cochineal (a dye obtained
from dried cochineal insects). Safflower.
_Black_.--Logwood, used in conjunction with an iron salt. Cutch, used
with an iron salt.
(B) Coal Tar Dyes. The dyeing and colouring of paper pulp by means of
the artificial organic substances has become a matter of daily routine,
the expensive natural dyes and the ordinary pigments having been almost
completely superseded. The numerous colouring matters available may be
classified either by reference to their chemical constitution or simply
on general lines, having regard to certain broad distinctions.
If the latter classification is taken, then the dyes familiar to the
paper-maker may be divided into--
(a) Acid dyes, so called because the full effect of the colouring
matter is best obtained in a bath showing an acid reaction.
(b) Basic dyes, so called because the colour is best developed in an
alkaline solution, without any excess of mordant.
(c) Substantive dyes, which do not require the use of a mordant, as
the colour is fixed by the fibre without such reagents.
Some of the most frequently used colouring matters are shown in the
accompanying table on page 202.
The distinction between _acid_ and _basic_ dye-stuffs is largely due
to certain characteristics possessed by many of them. Thus magenta,
which is the salt of the base known as Rosaniline, belonging to the
basic colouring matters, a group of dyes which do not possess the
fastness of colour peculiar to acid dyes, has a limited application.
But by treatment with sulphuric acid magenta is converted into an
acid magenta, and this dye has wider application than the basic salt.
Similarly the basic dye called aniline blue is insoluble in water,
and therefore has only a limited use, but by treatment with sulphuric
acid it is converted into alkali blue, soluble blue and so on, which
dissolve readily in water and are good fast colours. The acid dyes
generally have a weaker colouring power than the basic dyes, but they
produce very even shades.
-------+-------------------+-----------------+-----------------
Colour.| Acid. | Basic. | Substantive.
-------+-------------------+-----------------+-----------------
Yellow | Metanil yellow. | Auramine. | Cotton yellow.
and | Paper yellow. | Chrysoidine. | Chrysophenine.
Orange.| Orange II. | |
| Naphthol yellow S.| |
| Quinoline yellow. | |
| | |
Red. | Fast red A. | Rhodamine. | Congo red.
| Cotton scarlet. | Paper scarlet. | Benzopurpurin.
| Erythrine. | Safranine. | Oxamine red.
| Ponceau. | Magenta. |
| | |
Blue | Water blue 1 N. | Methylene blue. | Azo blue.
and | Fast blue. | Victoria blue. |
Violet.| Acid violet. | New blue. |
| | Indoine blue. |
| | Methyl violet. |
| | Crystal violet. |
| | |
Brown | Naphthylamine | Bismarck brown. |
| brown. | Vesuvine. |
| | |
Black | Nigrosine. | Coal Black B. |
| Brilliant black B.| |
| | |
Green | | Diamond green. |
| | Malachite green.|
-------+-------------------+-----------------+-----------------
The difference in the composition of the basic and acid dyes is
taken advantage of in the dyeing of paper pulp to secure a complete
distribution of the colouring matter upon the pulp, with the result
that the intensity of colour is increased, its fastness strengthened,
and the process of dyeing generally rendered more economical. This is
effected by the judicious addition of a suitable acid dye to the pulp
already coloured with the basic dye.
The direct colouring matters have but a very limited application for
paper dyeing owing to their sensitiveness to acids and alkalies.
In the colouring of paper pulp, attention is given to many important
details, such as:--
_Fading of Colour._--Some loss of colour almost invariably occurs even
with dyes generally looked upon as fast to light. The shade or tint of
the paper is affected not only by exposure to light, but by contact of
the coloured paper with common boards on which it is often pasted. The
alkalinity of straw boards, for example, is frequently one source of
serious alteration of colour, and the acidity of badly made pastes and
adhesives another.
In all such cases, the dyes must be carefully selected in order to
obtain a coloured paper which will show a minimum alteration in tint
by exposure to light or by contact with chemical substances. This is
particularly necessary in coloured wrapping paper used for soap, tea,
cotton yarn, and similar goods.
_Unevenness of Colour._--The different affinity of the various
paper-making fibres for dyes is apt to produce an uneven colour in the
finished paper. This is very noticeable in mixtures of chemical wood
pulp or cellulose and mechanical wood pulp. The ligno-cellulose of
the latter has a great affinity for basic dyes, and if the required
amount of dye is added to a beater containing the mixed pulps in
an insufficiently diluted form, the mechanical wood pulp becomes
more deeply coloured than the cellulose. If the former is a finely
ground pulp, the effect is not very noticeable, but if it is coarse,
containing a large number of coarse fibres, then the paper appears
mottled. The defect is still further aggravated when the paper is
calendered, especially if calendered in a damp condition. In that case
the strongly coloured fibres of mechanical wood are very prominent.
When dyes have been carelessly dissolved and added to the beating
engine without being properly strained, unevenness of colour may often
be traced to the presence of undissolved particles of dye.
_Irregular Colour of the two Sides._--Many papers exhibit a marked
difference in the colour of the two sides. When heavy pigments are
employed as the colouring medium, the under side of the sheet, that is,
the side of the paper in contact with the machine wire, is often darker
than the top side. The suction of the vacuum boxes is the main cause
of this defect, though the amount of water flowing on to the wire, the
"shake" of the wire, and the extent to which the paper is sized are all
contributory causes. By careful regulation of these varying conditions
the trouble is considerably minimised.
The under surface of the paper is not invariably darker than the top
surface. With pigments of less specific gravity the reverse is found to
be the case. This is probably to be explained by the fact that some of
the colouring matter from the under side is drawn away from the paper
by the suction boxes, and the pigment on the top side is not drawn away
to any serious extent, because the layer of pulp below it acts as a
filter and promotes a retention of colour on the top side.
It is interesting to notice that this irregularity sometimes occurs
with soluble dyes, as for example in the case of auramine. The
decomposition of this dye when heated to the temperature of boiling
water is well known, and the contact of a damp sheet of paper coloured
by auramine with the surfaces of steam-heated cylinders at a high
temperature brings about a partial decomposition of the dye on one side
of the paper. Generally speaking, acid dyes are more sensitive to heat
than basic dyes.
The presence of china clay in a coloured paper is also an explanation
of this irregular appearance of the two sides. China clay readily forms
an insoluble lake with basic dyes, and when the suction boxes on the
machine are worked with a high vacuum the paper is apt to be more
deeply coloured one side than another.
_The Machine Backwater._--Economy in the use of dyes to avoid a loss
of the colouring matter in the "backwater," or waste water from the
paper machine, is only obtained by careful attention to details of
manufacture on the one hand and by a knowledge of the chemistry of
dyeing on the other. The loss is partly avoided by regulating the
amount of water used on the machine, so that very little actually goes
to waste, and further reduced by ensuring as complete a precipitation
of the soluble dye as possible.
The _acid_ dyes generally do not give a colourless backwater, and all
pulps require to be heavily sized when acid dyes are used.
The _basic_ dyes are more readily precipitated than the acid dyes,
particularly if a suitable mordant is used, even with heavily coloured
papers. The addition of an acid dye to pulp first coloured with a basic
dye is frequently resorted to as a means of more complete precipitation.
_Dyeing to Sample._--The matching of colours has been greatly
simplified through the publication of pattern books by the firms who
manufacture dyes, in which books full details as to the composition
of the paper, the proportion of colour and the conditions for maximum
effects are fully set out. The precise results obtained by treating
paper pulp with definite proportions of a certain dye, or a mixture
of several dyes, is determined by experimental trials. A definite
quantity of moist partially beaten and sized pulp, containing a known
weight of air-dry fibre, is mixed with a suitable volume of water at a
temperature of 80° to 90° F. and the dye-stuff added from a burette in
the form of a 1 per cent. solution. If preferred a measured volume of
a 1 per cent. solution of the dye can be placed in a mortar, and the
moist pulp, previously squeezed out by hand, added gradually and well
triturated with the pestle.
The dyed mixture is then suitably diluted with water, made up into
small sheets of paper on a hand mould or a siphon mould, and dried.
The effect of small additions of colour to the contents of a beating
engine is frequently examined in a rough and ready way by the
beaterman, who pours a small quantity of the diluted pulp on the edge
of the machine wire while the machine is running. This gives a little
rough sheet of paper very quickly.
The comparison of the colour of a beaterfull of pulp with the sample
paper which it is desired to match is also effected by reducing a
portion of the paper to the condition of pulp, so that a handful of the
latter can be compared with a quantity of pulp from the engine. This is
not always a reliable process, especially with papers coloured by dyes
which are sensitive to the heat of the paper machine drying cylinders.
_Detection of Colours in Papers._--The examination of coloured papers
for the purpose of determining what dyes have been employed is a
difficult task. With white papers which have been merely toned the
proportion of dye is exceedingly small, and a large bulk of paper has
to be treated with suitable solvents in order to obtain an extract
containing sufficient dye for investigation.
With coloured papers dyed by means of pigments, the colour of the
ash left on ignition is some guide to the substance used, a red ash
indicating iron oxide, a yellow ash chromate of lead, and so on.
With papers dyed by means of coal tar colours the nature of the
colouring matter may be determined by the methods of analysis employed
for the examination of textile fibres.
The following hints given by Kollmann will be found useful:--
Tear up small about 100 grammes of paper, and boil it in alcohol, in
a flask or a reflux condenser. This must be done before the stripping
with water, so as to extract the size which would otherwise protect
the dye from the water. Of course the alcohol treatment is omitted
with unsized paper. The paper is now boiled with from three to five
lots of water, taking each time only just enough to cover the paper.
This is done in the same flask after pouring off any alcohol that
may have been used, and also with the reflux condenser. The watery
extract is mixed with the alcohol extract (if any). Three cases may
occur:--(1) The dye is entirely stripped, or very nearly so. (2) The
dye is partly stripped, what remains on the fibres showing the same
colour as at first or not. (3) The dye is not stripped. To make sure
of this the solution is filtered, as the presence in it of minute
fragments of fibre deceive the eye as to the stripping action. In the
first two cases the mixed solutions are evaporated down to one half on
the water bath, filtered, evaporated further, and then precipitated by
saturating it with common salt. The dye is thrown out at once, or after
a time. It may precipitate slowly without any salt. The precipitated
dye is filtered off and dried. To see whether it is a single dye or a
mixture, make a not too dark solution of a little of it in water, and
hang up a strip of filter paper so that it is partly immersed in the
solution. If the latter contains more than one dye they will usually
be absorbed to different heights, so that the strip will show bands of
different colours crossing it. If it is found that there is only one
dye, dissolve some of it in as little water as possible, and mix it
with "tannin-reagent," which is made by dissolving equal weights of
tannin and sodium acetate in ten times the weight of either of water.
If there is a precipitate there is a basic dye, if not, an acid dye.
In the former case mix the strong solution of the dye with concentrated
hydrochloric acid and zinc dust, and boil till the colour is destroyed.
Then neutralise exactly with caustic soda, filter, and put a drop of
the filtrate on to white filter paper. If the original colour soon
reappears on drying, we draw the following conclusions:--
(_a_) The colour is red; the dye is an oxazine, thiazine, azine, or
acridine dye, _e.g._, safranine. (_b_) It is orange or yellow; the dye
is as in (_a_), _e.g._, phosphine. (_c_) It is green; the dye is as in
(_a_), _e.g._, azine green. (_d_) It is blue; the dye is as in (_a_),
_e.g._, Nile blue, new blue, fast blue, or methylene blue. (_e_) It
is violet; the dye is as in (_a_), _e.g._, mauveine. If the original
colour does not reappear on drying, but does so if padded with a 1 per
cent. solution of chromic acid, we draw the following conclusions:--
(_a_) The colour is red; the dye is rhodamine or fuchsine, or one of
their allies. (_b_) It is green; the dye is malachite green, brilliant
green, or one of their allies. (_c_) It is blue; the dye is night blue,
Victoria blue, or one of their allies. (_d_) It is violet; the dye is
methyl violet, crystal violet, or one of their allies.
If the original colour does not reappear even with chromic acid, it was
in most cases a yellow or a brown, referable to auramine, chrysoidine,
Bismarck brown, thioflavine, or one of their allies.
If the tannin reagent produces no precipitate, reduce with hydrochloric
acid and zinc, or ammonia and zinc, and neutralise and filter as in the
case of a basic dye. The solution when dropped on to white filter paper
may be bleached (_a_), may have become a brownish red (_b_), may have
been imperfectly and slowly bleached (_c_), or may have undergone no
change (_d_).
(_a_) If the colour quickly returns the dye is azurine,
indigo-carmine, nigrosine, or one of their allies. If it returns only
on padding with a 1 per cent. solution of chromic acid, warming, and
holding over ammonia, some of the dye is dissolved in water mixed
with concentrated hydrochloric acid, and shaken up with ether. If the
ether takes up the dye, we have aurine, eosine, erythrine, phloxine,
erythrosine, or one of their allies. If it does not, we have acid
fuchsine, acid green, fast green, water blue, patent blue, or one
of their allies. If the colour never returns, heat some of the dye
on platinum foil. If it deflagrates with coloured fumes, the dye is
aurantia, naphthol yellow S., brilliant yellow, or one of their allies.
If it does not deflagrate, or very slightly, dissolve a little of the
dye in one hundred times its weight of water, and dye a cotton skein
in it at the boil for about fifteen minutes. Then rinse and soap the
skein vigorously. If the dyeing is fast with this treatment we have a
substantive cotton yellow or thiazine red; if it is not, we have an
ordinary azo dye. (_b_) The dye is an oxyketone, such as alizarine.
(_c_) The dye is thiazol yellow, or one of its allies. (_d_) The dye is
thioflavine S., quinoline yellow, or one of their allies.
If the dye is not stripped by alcohol and water, it is either inorganic
or an adjective dye, such as logwood black, cutch, fustic, etc.; and we
proceed according to the colour as follows:--
If it is red or brown, the dyed fibre is dried and divided into two
parts. One is boiled with bleaching powder. If it is bleached entirely
or to a large extent, the dye is cutch. If the bleach has no action,
incinerate some of the dyed fibre in an iron crucible and heat the
ash on charcoal before the blowpipe. If a globule of lead is formed,
we have saturn red. The second portion is boiled with concentrated
hydrochloric acid. If there is no action, we have Cologne umber; if
there is partial action, we have real umber; if the dye dissolves
completely to a yellow solution, we have an ochre; if the solution is
colourless instead of yellow, and chlorine is evolved during solution,
we have manganese brown.
If the colour is yellow or orange, boil with concentrated hydrochloric
acid. If we get a green solution and a white residue, we infer chrome
yellow or orange. If we get a yellow solution, we boil it with a drop
or two of nitric acid and then add some ammonium sulphocyanide. A red
colour shows an ochre or Sienna earth.
If the colour is green, boil with caustic soda lye. If the fibre turns
brown, we have chrome green. If no change takes place, boil with
concentrated hydrochloric acid. A yellow solution shows green earth; a
red colour logwood plus fustic.
If the colour is blue or violet, boil with caustic soda lye. If the
fibre turns brown, we have Prussian blue. If no change takes place,
boil with concentrated hydrochloric acid. A yellow solution shows
smalts. If the colour is destroyed, and the smell of rotten eggs is
developed, we have ultramarine.
If the colour is black, warm with concentrated hydrochloric acid
containing a little tin salt. If the black is unchanged, we have a
black pigment. If we get a pink to deep red solution we have logwood
black.
By means of the tests above detailed at length the group to which the
dye belongs is discovered, and often the actual dye itself. Once the
group is known it is generally easy, by means of the special reactions
given in many books, _e.g._, in Schultz and Julius's "Tabellarische
Übersicht," to identify the particular dye.
When one has to deal with a single dye and simply desires to
determine its group, the following table, due to J. Herzfeld, will
suffice. Originally intended for textiles, it will serve, with some
modifications here made in it, for the rapid testing of paper.
1.--RED AND REDDISH BROWN DYES.
Boil the paper with a mixture of alcohol and sulphate of alumina. If
no dye is extracted or a fluorescent solution is formed, we have an
inorganic pigment, or eosine, phloxine, rhodamine, safranine, or one
of their allies. Add bleaching powder solution, and heat. If the paper
is bleached, add concentrated hydrochloric acid. A violet colour shows
safranine or an analogue. If there is no colour, but the fluorescence
disappears, we have eosine, phloxine, rhodamine, or one of their
allies. If the paper is not bleached test for inorganic colouring
matters. Cutch brown is partly but not entirely bleached.
If the alumina solution gives a red or yellow solution without
fluorescence, add to it concentrated sodium bisulphite. If bleaching
takes place, heat a piece of the paper with dilute spirit. A red
extract shows sandal wood, fuchsine, etc. If there is little or no
extract, we have acid fuchsine or one of its allies. If the bisulphite
causes no bleaching, boil a piece of the paper with very dilute
hydrochloric acid. If the colour is unchanged, heat another piece of
the paper with dilute acetate of lead. If no change takes place, we
have an azo dye. If the colour turns to a dark brownish red, we have
cochineal or the like. If the boiling with very dilute hydrochloric
acid darkens the colour we have a substantive cotton dye.
2.--YELLOW AND ORANGE DYES.
Heat some of the paper with a not too dilute solution of tin salt in
hydrochloric acid. If the colour is unchanged, with a colourless or
yellow solution, boil some more paper with milk of lime. A change to
reddish or brown shows turmeric or a congener. Absence of change shows
phosphine, quinoline yellow, or a natural dye-stuff. If the acid tin
solution turns the paper red, and then quickly bleaches it to a pale
yellow, we have fast yellow, orange IV., metanil yellow, brilliant
yellow, or the like. If the tin turns the paper greyish, heat another
portion with ammonium sulphide. A blackening shows a lead or iron
yellow. If there is no change, we have naphthol yellow, auramine,
azoflavine, orange II., chrysoidine, or one of their allies.
3.--GREEN DYES.
Heat a sample of the paper in dilute spirit. If the spirit acquires
no colour, warm for a short time with dilute sulphuric acid. If both
paper and solution become brownish red, we have logwood plus fustic. If
this fails, boil with concentrated hydrochloric acid. A yellow solution
shows green earth. If this fails, boil with concentrated caustic soda.
Browning shows chrome green. If the spirit becomes blue, it is a case
of paper which has been topped with blue on a yellow, brown, or green
ground. The solution and the insoluble part are separately tested. The
case is probably one of an aniline blue dyed over a mineral pigment. If
the spirit becomes green, heat with dilute hydrochloric acid. If the
fibre is completely or nearly bleached, and the acid turns yellow, the
dye is brilliant green, malachite green, or one of their allies.
4.--BLUE AND VIOLET DYES.
Heat some of the paper with dilute spirit. If the alcohol remains
colourless, we have Prussian blue or ultramarine. If it becomes blue
or violet, shake some of the paper with concentrated sulphuric acid.
A dirty olive green shows methylene blue, and a brownish colour shows
spirit blue, water blue, Victoria blue, methyl violet, etc. If the
spirit turns yellow, and the colour of the paper changes, we have wood
blue or wood violet.