TECHNOLOGY.
U. S. FIBER CONSUMPTION (Sales) MILLIONS OF P O U N D S
FRED
FORDEMWALT,
American Cyanamid Co. Bound Brook, N. J.
The new synthetics may have stolen the headlines, but cellulose is still king. Almost 9 0 % of total U. S. fiber consumption in 1952 was cellulose, the overwhelming majority of which must be d y e d , pointing up the importance of the . . .
20 1920 1922 Î924 1926 1928 1930 1932 1934 1936 1938 1940 1942 1944 1946 1948 1950 1952
Dyeing of Cellulose τ F YOU can't dye it, you can't sell it; -*• this is the importance of the dyeing of cellulose. Although in recent years one has heard less about dyes and dye ing problems (because of the promi nence of some newer fields—such as the biological and atomic sciences), re search has not slackened. Excepting a few cases, dye research has been shifted largely from the universities to the industrial laboratories, however, and most of the results have been pub lished only in patent specifications. T h e new synthetic fibers have at times almost stolen the show, or at least the headlines, from cellulosics. But the XJ. S. consumed more than 5.5 billion pounds of cellulosic fibers in 1952— 8 8 . 5 % of the total fiber consumption. Cellulose is still king! Approximately 6&% of all cotton textiles for all pur poses was colored before it reached the consumer. None of this includes tonnages of cellulose consumed in the *24
paper trade, much of which must also be dyed. Continued research in the dyes field has been prompted by more than just the amount of business involved. Con tinued demand for higher degrees of fastness along with simplified applica tion methods adaptable to high speed continuous processes has driven dye chemists to make newer and better colors and to know more about what takes place when cellulose and color are brought together. The synthetic organic chemistry of dyes is as broad and as old as organic chemistry. But chemistry of the dye ing processes has received less atten tion a n d is much less understood. I n this area the physical chemical factors are of more significance t h a n the or ganic reactions. Dyeing of cellulose is old as an art but so new as a science that it assumes many of the aspects of a virgin field. C H E M I C A L
Cellulose is α Complex
Substance
A better understanding of dyeing re quires a more complete knowledge of the substrate. T h e basic structure of cellulose is that of a linear polymer of cellobiose. The greatest degree of polymerization is encountered in Ramie and flax. From these, molecular weight decreases in order through cot ton, wood cellulose, and rayon. Esti mated degrees of polymerization vary widely from as high as 20,000 glucose units for native cotton t o as low as 450 units i n some viscose materials. Native cellulose fibers consist of highly oriented "crystalline" regions along with randomly arranged "amor phous" material. The latter is first attacked in most treatments and is most substantive to dyes. Mercerization with caustic soda and treatment with low molecular weight amines modify the crystals regions and decrease amount of crystallinity. Both of these AND
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treatments improve affinity for certain dyes. Macro structure of cellulose varies with source and condition of growth. Immature cotton fibers, for example, are generally thin walled and dyeing characteristics of these are so different from those of fibers which are mature that severe dyeing problems sometimes arise when large amounts of immature cotton are present. The greatest chemical activity of cellulose resides in its O H groups. These make possible a variety of treatments to produce different modifications such as etherification and esterification. Such chemical changes alter its characteristics so that the new material bears little resemblance to native cellulose in its response to dyes and dyeing processes. Different Kinds of Dyes A r e Used on Cellulose
Most classes of dyes have some application to cellulosic products. For coloring of paper, choice of dyes is often less critical than for cotton, since many impurities, as well as additives, are frequently present to increase the take-up of dyes. Generally speaking, direct, vat, sulfur, azoic, and soluble vat dyes are employed for coloring cotton and rayon. Acid dyes are applicable only to modified material such as aminized cellulose. Cellulose esters require a special class of dyes, the disperse or acetate types. Basic dyes, as well as several natural coloring matters, are of but limited interest today, although they were at one time used quite extensively. In coloring paper, in addition to dyes suitable for cotton, both basic and acid dyes are widely used. The greater range from which to select paper colors arises primarily from the fact that often it makes little difference whether it is the cellulose, the lignin impurities, the size, or the filler which becomes colored. In any case the result is a colored sheet. In addition to all the previously mentioned soluble types, pigments are extensively used for coloring both fabrics and paper. When pigments are applied to fabrics, some kind of binding material, such as a synthetic resin, is required to obtain adhesion to the cellulose fibers. For paper, however, an intimate incorporation of colored particles into the fiber mass, along with the normal sizing agents, is sufficient. Pigments are also being used in increasing amounts in coloring of textile fibers by the spun-dyeing processes. Direct dyes are most simply applied, usually by immersing the material to b e colored, in a hot dye solution for sufficient time to permit transfer of dye from V O L U M E
3 2,
NO.
10
the solution to the cellulose. Common salt is added to improve the exhaustion. These dyes do not have any common chemical characteristics to explain their applicability. Continued research has resulted in development of many different chemical types. As a rule, however, they are sodium salts of sulfonated colored organic compounds and are thus water soluble. For this reason, they are subject to partial removal whenever the dyed m a t e r i a l s washed. The vats, sulfurs, soluble vats, and azoics are all insoluble colored materials which penetrate into the cellulose fiber in an intermediate soluble state and are converted to an insoluble form and fixed there b y some step essential to the dyeing process. With vat and sulfur dyes this involves application of the reduced or leuco form of the color followed by oxidation. Insoluble azoic dyes are formed by coupling soluble components inside the fiber. The soluble vats penetrate the cellulose as soluble esters of the reduced dye and are then hydrolyzed and oxidized to become the insoluble colors inside the fibers. All these insoluble coloring materials are highly resistant to removal by washing. Dyes Must Be Permanent
Whether or not any coloring material will ever become a commercial dye depends upon many factors. Not counting the important economic questions, these fall into two groups involving quality of final dyeing and application properties of the dye. Although there is no such thing as a perfect material, a dye must be satisfactory in both these regards to be of commercial interest. Standards for both are constantly becoming more difficult to meet.
With regard to quality, the color of a properly dyed cellulosic material will not fade excessively on exposure to sunlight or be severely affected by exposure to chemical furnes,Q will not lose color w h e n washed in hot soap and water even when chlorine is present, will not bleed in contact with perspiration, sublime under h o t pressing, or rub off on garments with which it comes in contact. Tests for all these qualities have been thoroughly standardized by the dyes and textile industries and constitute a routine part of all d y e evaluations. Individual colors frequently exhibit outstanding fastness in some respects b u t deficiencies in others. Thus, each dye may b e eminently suited for certain purposes but unfit for others. W h e n a dye is used for a job for which it is not suited, a just basis for customer complaint exists. Fastness properties within a given class of dyes vary widely between individual members. There is no class of colors which contains only universally fast dyes. For example, most sulfur dyes are resistant to all wet treatments, but lose color badly -when chlorine is present in t h e wash. Resin-bonded pigment colorings commonly h a v e outstanding resistance to light fading and household laundering but tend to rub off, particularly in heavy shades. The disperse dyes o n acetate n e e d to b e selected carefully to avoid fading due to atmospheric fumes. T h e best all-around fastness on cotton a n d viscose is obtained by the anthraquinone vat dyes. Application Must Be Commercial
But no matter h o w good a dye may be from the standpoint of brilliance and permanence, it will not become a commercial item unless it can b e a p -
: S-A symposium presented by the Division .cflCeltuIpse Chemistr#a*itne^ £|24th/nationa! i meeting, ofuthe^Amerîcan .ChelnîcahScîcîety^ Chicago^ introductory Remarks. %f=red Fordemwâlt^yAmenccTn- Cyanamidtt \r Bound Brook;:N.J.U - .„ - . / , - Λ * ^ / ' Λ Ο * 5 * * · ? ^ ' "- - ^ Cellulose, The M a t e r i a l > s , a Substrate foraDyes; .Waiter MS? Washington] *D.< C i * V ^ V f ^ V j n If Scott,:Department ofagriculture, » Dyes' foVCelluIose:^ Dona Id & Marnonl *Amefican'Amlfae?P/oducts,t Xt New Xork,, and D^LiRandall, [General %nWrieîariâXFIfkii, EasionffaT* Methods and Machines Used in the Dyeing of C e l l w l o s e ^ p r y / l l v i Clark;American Cyànomîd'; Bound BrBoQNSJTÇz*. ' * ' V ^ * ' * ^ Printing of Cellulose Fabrics. Silvert N.^ Glarum,:Clbà Co.fN/w^ Î?xYorki andF.% Jacobs, consultant, Utile Nick/Long*Μαη4;'Ν$Ύΐ'*\γ -PhysîcaTChemistry of Cellulose Dyeing. Emery,!· Valko, consult^ * - onf, Mountain*Lakes, N. J . t* ** *^ί*, ν** £* ^ , Γ ^ ^ # J , • *£> Requirements and Evaluation Methods in^thfe Dyeing tof Cellus lose. Jules U b a à h e f M e//o7£/^^^ ReseârchfTitts^
: Coloring of P ô | i f . 1 f l ^ ^ Û p ^ D i f ' l F ^ S q A W ^ < ^ ^ ^ ® ^ ? ^
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8,
1954
925
plied b y modern methods. It must be applicable in mill equipment to give uniform producible level dyeings. Commercial machines are designed for simplicity and mass production. Complicated or trick methods have no place in modern dyeing technology except for occasional small volume specialties. This fact, although at times difficult for the chemist to accept, is an a priori principle in research for new and better colors.
ducing solution. D u r i n g this passage the d y e is reduced a n d fixed in the cloth. In the Pad Steam Process the fabric onto which t h e dye has been p a d d e d passes through a second p a d d e r to pick up reducing chemicals and then immediately through a steam chest. The heat from the steam brings about the chemical reduction and fixation of the d y e . T w o recent innovations in continuous piece goods dyeing are the H o t
Arrangement of chain The color may be applied to cellulose at any stage of the manufacturing process: 1. Raw stock—loose fiber as it comes from t h e gin. 2. Slubbing or roving—carded and drawn ready for spinning. 3. Yarn—skeins, ball or chain warps, packages, or beams. 4. Piece goods—woven or knitted fabric. Least critical of these is raw stock dyeing. Small variations at this stage tend to become leveled out as die fibers are blended in further processing. Most exacting is piece goods dyeing, in which no variations or imperfections can b e tolerated. There are many advantages to cloth dyeing, however, and each year millions of yards of cotton fabrics are dyed this way. A particularly desirable feature is that the decision as to w h a t the color will be can b e delayed until after the fabric has been made. This cannot b e done if the coloring is done at any earlier step.
molecules in cellulose Oil Method and the British Standfast Process. In the first, heating results from a passage through hot oil and in the second b y passing through molten metal. Neither of these has, as yet, been widely adopted in this country. P a c k a g e dyeing places a particular set of demands on t h e dyes. Yarn is w o u n d on perforated tubes to form rather compact spools. These packages a r e placed on t h e spindle of a carrier and dye dispersion or solution is p u m p e d through t h e package, alternately inside out and outside in, in a closed tank. Because of filtering action of such thick layers, dyes must have rigidly designed properties to color all portions of t h e package to exactly the same shade. W h e n fabrics are printed they are dyed only in local areas which are exactly controlled to p r o d u c e the design. T h e application of six to 12 different colors in a single pattern is c o m m o n practice. F o r printing applications t h e dye is incorporated in a carrier h a v i n g flow properties which permit a n accurate reproduction of t h e p a t t e r n . In such situations an entirely n e w set of problems and requirements is encountered. All dyes must work together on t h e same machine, carriers and dyes must not interact, a n d transfer of dye from t h e film of printed paste into t h e cellulose must proceed rapidly and uniformly to impart a sharp, clearly colored design which will n o t smear or b e removed in subs e q u e n t operations. Because of these peculiar conditions, printing compositions are often formulated differently from those t o be employed in dyeing processes, although, in general, the same dyes, particularly the vats, are used for both. H e a t treatment o r ageing of the prints is carried out without disturbing position of the dye o n the printed areas. Textile printing is in itself highly specialized a n d complicated both as an art and as a science.
Each Process Has Its O w n Requirements
To meet the present day competitive market, there is a trend toward greater speeds and continuous dyeing processes. Jig, beck, and reel dyeing, being batch processes, are slower and consequently more expensive. In certain continuous processes the dye is p a d d e d onto the fabric which is then treated with chemicals and subjected to a heat treatment, with or without intermediate drying. Since heating time is seldom more, and generally less, than one minute, owing to operating speeds, dyes are required to fix rapidly. The most common continuous processes in America are the Williams Process and t h e P a d Steam Process. Both are particularly adapted to application of vat dyes. The padding operation is the same for both. In the Williams Process for vat dyeing, the padded fabric passes through a channeled bath filled with hot re926
C H E M I C A L
In coloring paper it is common practice to add t h e coloring matter either to the pulp or to t h e finished sheet by coating or staining operations. The latter processes, except in lightweight papers, generally produce only surface coloring. I n beater dyeing processes color is distributed throughout the sheet. Variations in beater temperatures, in kinds of pulp, and in sizing and filling materials place rather difficult requirements on colors for beater addition. T h e problems and corrective solutions are unique to the t r a d e . A perennial problem arises from tendency of fine fibers, which absorb proportionally more color, to b e pulled from one side of the w e b a n d through the screen, causing one side of the sheet to be more weakly colored than t h e other. Physical Chemical Factors Are I m p o r t a n t
T h e very great n u m b e r of variables encountered in all aspects of dye application has m a d e a careful scientific study of the dyeing processes extremely difficult. But there has been a growing appreciation of t h e importance of physical chemical methods in this field and some promising avenues of research are n o w recognized. There is little opportunity for chemical reaction, in the commonly understood sense, b e t w e e n dyes and cellulose. There is m u c h t o indicate that the most p e r m a n e n t coloring results when an insoluble a n d inert colored material is deposited in the interior of fibers. This seems t o explain t h e great fastness of t h e vat, sulfur, a n d azoic dyes. Existence of discrete particles has been demonstrated in some cases. Fixation of soluble direct dyes probably depends upon such factors as h y d r o g e n bonding and Van de~ Waal's forces to obtain affinity. As more information is obtained on the influence of time, temperature, and concentration, of diffusion rates, degrees of aggregation and salt effects, of dye configurations, cellulose structure and equilibrium conditions, our knowledge and control of cellulose dyeing will expand. In the meantime the art and the science can be d e p e n d e d upon to work together for advancement of mutual interests. Importance of this advance is in n o w a y modified b y the growing field of man-made noncellulosic synthetics. T h e r e is a b u n d a n t room for these to expand, a n d their range of m a n y desirable properties makes it u r g e n t t h a t they should. A growing population will continue t o provide a d e m a n d for increasing volumes of t h e cellulosic materials for m a n y years to come, however, even if the most optimistic hopes of the creators of the synthetics are realized. A N D
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