CHAPTER X
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_Yellow and Brown._--The vegetable extracts, such as fustic, quercitron, cutch, turmeric, have practically all been replaced by aniline colours.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.