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T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEJUSTRY
Vol. 14, No. 10
Application of the Direct Dyes in Coloring Paper’ By Walter C. Holmes TECHNICAL LABORATORY, DYESTURR? DEPARTMENT, E. I. DU PONT DE NEMOURS & CO., WILXINCTON, DELAWARE
With paper materials in general, the best results in dyeing direct dyes in sized papers are obtained when provision is made to insure maximum retention in the form of dye and the precipitation of residual color in the form of some late other than the aluminium lake. The color should be dyed with salt, the amount of alum used in sizing reduced to a minimum, and the addition of the alum preceded by that of calcium chloride or other suitable metallic salt, which will precipitate a late of comparafioely desirable characteristics. With ground wood furnishes, on the other hand, the best results are obtained if the absorption of the dye is preoented entirely by preceding the addition of the color by that of the metallic salt which i s most suitable. In the dyeing of mixed furnishes containing ground wood, the same principles of color application should be followed in conjunction with the customary prooisions for insuring uniformity of coloring. The methods outlined will increase the brightness of the dyeings without incurring the disadoantages attendant upon the employment of basic dyes in conjunction with direct dyes. By means of the employment of copper sulfate in the case of numerous indioidual dyes and dye combinations, a considerable range of good shades of great fastness may be obtained.
HE DIRECT, or substantive, dyes are soluble sodium
T
salts of various color acids, characterized by an affinity for cellulose which is sufficiently strong to render practical their application in coloring paper materials without the intervention of precipitating or mordanting agencies. Their employment, accordingly, is peculiarly advantageous in coloring unsized, or lightly sized, papers and SUCK products as “mottling fiber.” The same property, on the other hand, restricts their utility in certain respects, and necessitates suitable precaution in their general application. They are further differentiated from dyes of other classes by the relative insolubility of the lake compounds which they form in beater operation and the pronounced affinity of the latter for cellulose. As a class, they are characterized by comparatively excellent fastness to light and by tinctorial strength and brightness of shade which are only moderate. A minor disadvantage attendant upon their employment is a tendency to mottled effects. The solubility, the completeness of color retention, with the accompanying advantage of freedom from color in back-waters, and the relatively slight tendency to result in two-sided effects in the sheet, are excellent. In addition to characteristics common to the class, individual dyes possess, in numerous instances, distinctive qualities which make them of special value for paper application. The unique and complete stability of Pontamine Yellow SX and Pontamine Yellow CH to the action of hypochlorites; and the extreme resistance of the latter to alkali, whereby it is exceptionally qualified for employment in coloring papers in which soaps are to be wrapped, may be cited by way of example.2 . 1
Presented before the Division of Dye Chemistry at the 63rd Meeting
of the American Chemical Society, Birmingham, Ala., April 3 to 7, 1922. Such individual properties of the direct dyes, as well as those of colors of other classes, as are of essential interest to the paper manufacturer have been compiled in a chart t o be included in a chapter on “The Coloring of Paper” in a vocational-educational textbook which will receive eatly publication by the American Pulp and Paper Association.
The direct dyes find only limited application in the production of very light shades in consequence of the difficulty encountered in effecting uniformity of coloring. Their employment in the minor varieties of color application on paper is also somewhat limited, although Pontamine Black EX is utilize4 extensively in dipping tissues, and various direct dyes, of which the yellows are the more important, are used in coating and coloring on the calender as well as in dipping. In the coloring of blotting papers and of other unsized, or lightly sized, products and that of “mottling fiber” intended for use in “granite” or mottled papers, they are used exclusively except for a minor employment of basic colors. The principal consumption of the direct colors, however, is in the beater coloring of sized papers in all except the lightest shades. I n this extensive and important field, their employment has become general in American practice, and may be expected to increase further a t the expense of other classes of colors as their merits receive more complete recognition, as improvements in dyeing practice are effected, and as the demand for color permanence in paper becomes more insistent, more particularly in view of the progress which is being made in the development of direct dyes of superior qualities. The usual practices which obtain in the beater coloring of sized papers with the direct colors are not such as will develop their maximum tinctorial power or brilliance or, in numerous instances, their maximum fastness to light. It does not appear that the fundamental importance of the role which metallic lake compounds of the dyes play in the mechanism of the color retention has been recognized. It is upon the regulation of such lake formation that effective coloring depends. With all metallic elements except the alkalies the direct dyes form lakes which are insoluble even at beater dilutions and which manifest a remarkable affinity for cellulose. I n the beater dyeing of sized papers, the formation of such lakes is unavoidable since it is impossible to effect complete absorption of the dye by the stock. Both dye and lake contribute to the total coloring effect obtained. The essential mechanism of the dyeing is that of a limited absorption of the dye itself (the soluble sodium salt of the color acid), followed by the precipitation upon the stock of such portions of the dye as remain in the liquor, in the form of the aluminium lake (the insoluble aluminium salt of the color acid), upon the addition of the alum required in the sizing. The total percentage retention of the color is very high, except when very heavy shades are run, but that factor alone is not decisive in evidencing the efficiency of the practice. The true effectiveness of the dyeing is dependent upon the degree in which the color is retained in the more advantageous form, which is, invariably, that of the dye. When dyed in the form of dye the color is superior in both tinctorial strength and in brightness of shade, in degrees which are variable with different dyes but which are usually of a material order. With certain exceptions, into which other considerations enter, which will be noted subsequently, it is essential in obtaining optimum results, therefore, that the retention of the color should be in the form of dye, so far as may be possible, with a minimum degree of lake formation; and the effectiveness of any dyeing practice must be rated on the basis of the degree in which that condition is obtained. I n ordinary trade practice, wherein no provision is made to
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T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
increase the absorption of the dye, less than one-third of the color present in dyeing 6-lb. shades is generally absorbed as such and escapes precipitation in lake form. Quantitative data on the absorption of two representative direct dyes in 6-lb. shades with bleached sulfite and ground wood furnishes are given in Table I. The stock received a beating corresponding in duration to average beater practice before the introduction of the color. TABLE I-PERCENTAGE COLORRETENTION I N 6-LE. DYEINGS TIME Salt BLEACHED GROUND Min. Temperature Lbs. SULFITE WOOD PONTAMINE BLACKE X C O N C . 30.4 25.5 15 Room 0 29.6 24.2 30 Room 0 ' 60 Room 0 31.5 27.0 44.6 50.5 15 140' F. 0 58.8 61.3 15 Room 40 65.7 80 67.3 15 Room 61.5 40 56.5 30 Room 67.2 68.3 30 Room SO I5 140° F. 40 64.2 68.6 P O N T A M ISKY N ~ BLUE 5 B X 40 63.5 50.8 15 Room 38.1 15 140' F. 40 50.8 0 25.4 28 6 30 Room 19.0 30 140' F. 0 .... 40 68.3 60.3 30 Room 60.8 30 Room 80 70.0
The majority of dye concerns advocate the practice of dyeing their direct dyes with 40 lbs. of salt and at a temperature of 140" F.; a recommendation which is usually ignored by the paper trade unless very heavy shades are to be run. The general specification of the elevated temperature is ill advised, both by reason of the deleterious effect which its employment has upon the sizing of the paper and from consideration of coloring alone. With such dyes as are improved in retention thereby, the gain is not materially more considerable than that which may be effected with the employment of salt alone, whereas the absorption of other dyes is inhibited by heat in a decided degree. The employment of salt, on the other hand, is not attended by unfavorable effects in general and is decidedly effective in promoting color absorption. The advantage to be derived thereby varies with individual dyes in proportion to the degree of superiority of the dye over its lake in respect to brightness and strength, With such colors as Pontamine Sky Blue 5BX, the lake of which is inferior to the dye in only a moderate degree, a 6-lb. dyeing made with 40 lbs. of salt is equivalent in strength to a 7-lb. dyeing without salt, and is decidedly brighter. With Pontamine Fast Pink BL, vi-hich is typical of dyes which are more seriously affected by conversion into their aluminium lakes, a 6-lb. dyeing with 40 lbs. of salt is fully equivalent in strength to a 9-lb. dyeing in general practice, and is brighter to an extreme degree. The improvement in strength and brilliance cited, together with an attendant minor advantage of a more complete retention of color in heavy shades, is entirely consequent upon the limitation of lake formation which the employment of salt effects. A further improvement, which is material in degree with the majority of direct colors, may be effected by modification of the character of lake formation. The aluminium lakes of the dyes have particularly unfavorable characteristics. With the great majority of colors the calcium, barium or magnesium lakes are decidedly brighter and more attractive in shade, while even the iron lakes are often appreciably superior to those formed with alum. It is due to these facts that the presence of mineral impurities in the beater, such as may occur in the water or find introduction in the stock, is beneficial rather than otherwise in its influence on the shades obtained in dyeing sized papers with direct dyes in the usual practice. It should be noted, incidentally, that the preliminary addition of alum to the beater in order to effect the removal of such mineral impurities is, therefore, decidedly injudicious in so far as iegards its influence on the shades obtainable with the direct colors.
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The best results in dyeing the direct dyes in sized papers, therefore, are obtained only when provision is made which insures a maximum color retention in the form of dye and a precipitation of residual color in the form of such metallic lakes as are most suitable in the case of the individual dyes. As a general practice, it may be recommended that the calcium lakes be utilized, and that precipitation be effected by means of the addition of suitable amounts of calcium chloride. A subsequent addition of alum is essential in obtaining the best results in respect to sizing, but the amount of alum employed, which will be less than is normally necessitated, should be restricted so far as is possible. By compliance with the principles of color application outlined above, results are obtained which are, in general, greatly superior to those obtained in usual practice. A limited number of direct dyes which find application on paper exhibit a decided susceptibility to such conditions of acidity as obtain in the paper beater in usual practice; with such it is customary to advocate the employment of limited amounts of soda ash. An excess of soda ash over the amount required to neutralize the acidity of the alum is attended by serious interference with the satisfactory sizing of the paper, and by color losses consequent upon the instability of the color lake to alkalies, but the use of restricted quantities has no more serious consequences than a possible increase in frothing. With colors of this type the improvement effected by the employment of salt and calcium chloride is of about the same order as that obtained by a resort to the use of soda ash. It is probably impossible to employ such dyes in the beater coloring of sized papers in such manner as to develop their maximum excellence of shade without entailing some measure of disadvantage in certain other respects. It is fortunate that their employment is becoming less necessary by reason of the increasing availability of dyes of similar shade and superior qualities.
GROUND WOODFURNISHES The general practice advocated for the direct colors is not recommended in the coloring of ground wood furnishes, wherein certain particular considerations enter. The phenomenon of color absorption in the form of dye is not one in which mere surface deposition of color is the entire, or even the predominant, component, but involves a decided degree of penetration of color within the fiber substance, whereas the dye lakes are deposited upon the fiber surface. I n the particular instance of ground wood, it is found that the opacity of the stock operates to mask and obscure such portions of the dye as have become deposited within the fiber. With yellow dyes the effect observed is not pronounced, but the impairment in tinctorial strength and brightness of direct dyes, in general, is very material. I n dyeing Pontamine Sky Blue 5BX, for example, in 6-lb. shades upon bleached sulfite and ground wood furnishes, it is found that the depth of shade developed on the bleached sulfite stock is several times greater than upon the ground wood, although there is no material difference in the amount of dye actually absorbed. It is possible to obtain shades of better brightness and of considerably greater depth in the case of ground wood by dyeing in the form of lake. I n the coloring of ground wood furnishes, therefore, it is advantageous, in general, to obtain color retention in the form of some suitable lake with the entire avoidance of absorption of the dye itself, as may easily be effected by the addition of a suitable amount of calcium chloride, or other metallic salt, prior to the introduction of the dye. The dyeings obtained thereby are brighter and stronger, in a degree which varies with the color and absorption characteristics of individual dyes and, in some measure, upon the amount of color employed, but which is generally considerable.
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THE JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY MIXEDFURNISHES
I n the dyeing of mixed furnishes containing ground wood, wherein difficulty is encountered in effecting uniformity of coloring in consequence of the decided difference in depth of shade obtained with ground wood and with the other ingredient materials under identical dyeing conditions, the general manipulation of the customary practice should be followed. The ground wood should be introduced into the beater and receive a preliminary and partial coloring before the other stork and tlhe remainder of the color are introduced, or the greatly preferable course of a dyeing in separate beaters with a subsequent combining of the stocks followed when the necessary equipment is available. In either case, however, the general provisions which have been outlined as essential to the effective coloring of the separate stocks should be followed. It will not be found possible to obtain complete effectiveness in coloring if it is necessary to carry out the dyeing in a single beater. The best results possible in that case are obtained by adding the ground wood, a suitable amount of mineral salt, a portion of the color, the remaining stock, the remainder of the color, salt, and a second suitable amount of mineral salt, size, and alum in the order given. The operation is not essentially more complicated than the usual practice in such cases and will give decidedly better dyeings in respect to strength and brightness. The degree of uniformity of coloring effected will be dependent, as it is in the more usual practice, upon the correct determination of the proportion of the color which is to be added at the beginning of the dyeing. These methods which have been advocated for the application of the direct dyes are found to have an influence which is very slightly favorable, rather than otherwise, upon the stability of the dyed product to photochemical action. On the other hand, the employment of basic colors in conjunction with direct colors affords, a t best, products of only moderate stability to light, and may result, under unfavorable circumstances, in products of but slight fastness, in color losses, and in variations in shade. The lake compounds of direct and basic dyes are not of superior characteristics when formed under the most favorable conditions, while compounds are formed, in other circumstances, which are undesirable in shade and inferior in brilliance, are soluble under beater conditions, and possess an affinity for cellulose which is of a low order. The brightness of the direct dyes is amply sufficient in the great majority of fields of application provided the methods by which they are applied are such as are suitable.
COPPERTREATMENT Although it is well known that the Sky Blues may be improved in fastness to light by a treatment with copper sulfate, it does not appear to be recognized widely that the treatment is of any decided practical value and may be extended to any considerable variety of dyes. The relative fastness of the copper lakes and the degree in which the fastness of the dye is effected by conversion into the lake, vary considerably with different dyes. The influence of conversion into the lake upon the shade is equally variable. In general, the copper lakes are even duller than the aluminium lakes, but in numerous instances they are moderately bright and attractive in shade, while with certain direct yellows they are superior in desirability of shade to other metallic lakes of the dyes in question and but little duller than the latter. By the judicious employment of copper treatments in dyeing various individual dyes and dye combinations it is possible to obtain a great variety of shades of satisfactory brightness and excellent permanence. The following dyes are improved in fastness in a material degree by conversion into the copper lakes:
Vol. 14, No. 10
Pontamine Sky Blue 5BX Pontamine Sky Blue 6BX Pontamine Blue A X Pontamine Copper Blue 2RX Pontamine Brown C C Pontamine Scarlet 4BS Pontamine Scarlet 8BS
With the exception of Pontamine Sky Blue 5BX, of which the lake is deficient in brightness, these dyes give lakes of satisfactory brightness and tinctorial power as well as of excellent fastness. The lake shade of Pontamine Blue AX is a neutral blue; that of Pontamine Sky Blue 6BX a very greenish blue, and that of Pontamine Copper Blue 2RX a very reddish blue, whereby a very extensive range of blue shades is possible. A second list of dyes which are improved in fastness by copper sulfate in a degree which is moderate, although not sufficient to warrant the treatment in their cases, but which may be employed to advantage in shading the dyes of the first list, would include: Pontamine Pontamine Pontamine Pontamine
Diazo Black BH Conc. Blue BBF Brown D3G Fast Orange S
A third list of dyes which are not improved in fastness by a copper treatment but which retain their original excellent fastness and are only very slightly impaired in brightness thereby, and are, accordingly, also suitable for shading the dyes of the first list, would include: Pontamine Brown R Pontamine Yellow CH Du Pont Brilliant Paper Yellow Conc. Pontamine Light Yellow 5GX
I n the case of the last dye, the loss in brightness upon conversion into the lake is slightly greater than with the other dyes of the list but the fastness is excellent and the greenish cast of the shade thus obtained renders the dye particularly well adapted for employment with Pontamine Sky Blue 6BX in the production of fast greens. Certain other direct dyes of excellent fastness, among which may be mentioned Pontamine Violet N, Pontamine Fast Pink BL, and Pontamine Faat Red F, give copper lakes which are of such inferior shades as to preclude the possibility of the employment of the dyes in conjunction with any copper treatment. I n the application of the copper treatment it is advisable to add the color to the beater first and to postpone the addition of the copper sulfate until after the color has been allowed to run for a short period, rather than to precede the addition of the color by that of the copper salt. The dyeings resulting from the former practice are somewhat stronger and brighter, as would be expected, and they are also distinctly faster to light. This may probably be explained upon the hypothesis that the lakes, although unquestionably materially faster than the corresponding dyes, are deposited entirely upon the fiber surface, in which position they are fully exposed to photochemical attack, whereas such portions of the color as have penetrated within the fiber substance are afforded some measure of protection from the influence of light through the absorption of active rays by the supernatant material and surface deposit of lake. With the possible exception of ground wood, it is advantageous to permit a measurable degree of absorption of dye before effecting the precipitation of the lake, By the employment of the copper treatment with the dyes specified, it is possible to obtain a considerable range of attractive shades of excellent fastness which may hardly be duplicated by other means.
