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INDUSTRIAL AND ENGINEERING CHEMISTRY ACKNOWLEDGMENT
The authors wish to express their thanks to the Goodyear Tire and Rubber Company and H. J. Osterhof for permission to publish this work, to G. H. Gates, R. D. Juve, and Edward Cousins for formulating the compounds used, and to H. E. Rutledge for the combined sulfur determinations. This investigation was carried out under the sponsorship of the Office of Rubber Reserve, Reconstruction Finance Corporation, in connection with the government synthetic rubber program.
Vol. 42, No. 3
(4)
Fisher, J. C., Hollomon, J. H., and Turnbull, D., Zbid., 19, 775
(5) (6)
Forman, D. B.,IID.
[email protected], 36, 738 (1944). Gehman, S. D., Woodford, D. E., and Wilkinson, C. S., Jr., Ibid.,
(1948). 39, 1108 (1947).
(7) Gregory, J. B., Pockel, I., and Stiff, J. F., I n d i a Rubber Wodd, 177, 611 (1948). (8) Kauzmann, W., Chem. Rev’., 43, 219 (1948). (9) Morris, R. E., Hollister, J. W., and Mallard, P. A., I n d i a Rubber W o r l d , 112, 455 (1945). (10) Wood, L. A,, in “Advances in Colloid Science,” ed. by Mark and
Whitby, Vol. 2, p. 57,
New
York, Interscience Publishers,
1946.
LITERATURE CITED
(11)
(1) Am. SOC. Testing Materials, Philadelphia, Pa., “Standards on Rubber Products,” p. 527, 1949. (2) Beatty, J. R., and navies, J. bf., J . Applied P h y s . , 20, 533 (1949). (3) Conant, F. S., and Liska, J. W., Ibid., 15, 767 (1944).
Wood, L. A , , and Bekkedahl, N., J . Applied Phus., 17, 362 (1946).
‘
RECEIVEDNovember 3, 1949. Presented before the Division of Rubber SOCIETY, AtChemistry at the 116th Meeting of the AIERICAK CHEMICAL lantic City, N. J. Contribution 170 from the research laboratory of the Goodyear Tire and Rubber Company.
Stabilization of Zein Filaments CURING WITH FORMALDEHYDE IN ACIDIC NONAQUEOUS MEDIUMS C. BRADFORD CROSTON 1Vorthern Regional Research Laboratory, Peoria, I l l . Essentially anhydrous curing mixtures consisting of an inert solvent, aldehyde, and strong acid have a remarkable stabilizing effect on zein fihers. The cure is very rapid at 100” C. and produces irreversible formaldehyde crosslinks. The stabilization is most evident in the reduction of shrinkage which is reflected in unusual strengths of the fiber after boiling in acid d g e baths.
T
HE usual aldehyde cure of protein bodies is carried out in aqueous systems and, though subject to several objections, persists for the stabilization of all azlon fibers. The most serious objection is that the formaldehyde crosslinks formed by the reaction are not stable in boiling acid baths such as are used to dye wool-Le., most of the formaldehyde is reversibly bound and the protein reverts to its uncured condition when boiled in acids. With zein fibers (I), rapid curing in aqueous mediums must be done a t elevated temperatures and requires holding the fiber a t high tension (requiring special equipment) so that it will not undergo excessive shrinkage. The aqueous cure is harsh, causing a reduction in strength and an embrittlement of the fiber to such an extent that it previously seemed advisable to use an acetylating treatment to moderate the undesirable effects ( 2 ) . British patents (6) cover the stabilization of peanut protein fiber by the use of aqueous formaldehyde cures containing very high concentrations of mineral acids. Evans and Croston ( I ) have recently shown that increased acid concentrations in zein fiber curing baths promote the formation of acid-stable linkages. The use of such highly acid cures requires multiple curing baths of increasing acid concentrations. The aldehyde cure described here differs from the above-mentioned methods mainly in that it is conducted under essentially nonaqueous conditions, Several of the objections to the aqueous cure are avoided. The nonaqueous cure has been used and studied most extensively for its remarkable stabilization of zein fibers in both acidic and alkaline mediums. The treatment is also effective in markedly modifying certain properties, such as solubility, of powdered zein and other proteins. This stabilization of zein fiber, particularly to shrinkage in boiling acid solutions, occurs presumably through the formation of irreversible crossbonds between the protein molecules. Xglon,
under somewhat similar anhydrous conditions, reacts with formaldehyde, adding methylol groups to the amide nitrogen of the chains and methylene links between the amide nitrogens (8, 6). These links, however, are readily broken by acid hydrolysis. CURING CONDITIONS AND RESULTS
This new cure improves zein fiber from any stage of the former process ( 9 ) which included an acetylation and aqueous formaldehyde postcure of the raw fiber. The cure is most advantageously applied, however, to the raw fiber (that which has been stretched and dried after the initial mild aqueous precure). At this stage, the fiber has undergone a minimum of processing, is a t its maximum strength, and has excellent resilience, although it exhibits considerable shrinkage in water. The experimental procedure consisted of introducing small loose skeins of the air-dry fiber into the curing mixtures which were at refluxing temperatures or, for the higher boiling mixtures, a t 100” to 110”C. The treating time was arbitrarily set a t 15 minutes when it was not a variable factor. Because the ultimate water shrinkage was measured in most experiments, the treated fiber was washed several times in dioxane only. An aqueous sodium bicarbonate wash seemed to be adequate and tended to preshrink the treated fiber which always retained a residual shrinkage of a few per cent. The curing mixtures consisted of an aldehyde-yielding agent and a relatively strong acid in an inert organic solvent. Formulas for four mixtures which produced satisfactory cures are given in Table I. Formaldehyde, the cheapest and most effective aldehyde, was used most frequently and was introduced into the mixture as formalin, paraformaldehyde, trioxane, or acetals yielding formaldehyde in acid mediums. Acids used successfully were sulfuric, hydrochloric, trichloroacetic, and dichloroacetic acids. Acids weaker than the last named do not function properly. The acid convenient11 promotes decomposition of the aldehyde-yielding agent, but its main function is as a catalyst for the protein-aldehyde reaction, and it is essential even when formalin is used. A wide variety of organic solvents, such as dioxane, toluene, Stoddards solvent, and tetrachloroethane can be used. The solvent must dissolve nt least traces of the acid and
March 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
10-
TEMPERATURE OF CURE,'C.
Figure 1. Effect of Temperature of Cure 1 (15 Minutes) on Shrinkage of Fiber in Boiling O.lY0 Acetic Acid
-,
liberated aldehyde and also be relatively inert to the fiber. An important function of the solvent, however, may be to swell the fiber slightly, thus increasing the rate of cure. Discretion must be used in selecting the components of a curing mixture so that they are compatible and are not injurious to the zein fiber. Compositions must be selected which do not dissolve or excessively swell the fiber as happens when the concentration of water (above 2% in dioxane), acids, or trioxane is too high. Dioxane is convenient to use experimentally because it is a good solvent for the other components, except paraformaldehyde which was used in a suspended state. Acetals, such as dioxolane, function as the organic medium and also as the source of formaldehyde. When hydrogen chloride was used, the generated gas was usually bubbled through the treating mixture, thas conveniently keeping the concentration constant. The effect of the curing temperature on the rate of cure, as measured by the ultimate shrinkage of the treated fiber in boiling 0.1% acetic acid, is noted in Figure 1. In this particular case, the treating mixture was No. 1 of Table I and the treating time was 15 minutes. The other cures were a t least as effective. Temperatures around 100" C. were very effective for producing fibers having low shrinkage. The successful use of high temperatures may be the reason for the new cure being so advantageous as it is impossible to use such high temperatures with aqueous cures. Figure 2 is a time curve showing the rapid shrinkage control obtained in the same mixture at the boiling point. Even though the concentrations of formaldehyde and acid in this mix-
483
ture were very low (0.2% and 0.05%, respectively) in a n attempt to slow down the reaction, shrinkage control was complete in 15 minutes. Even lower concentrations were effective, but the reserve supply became too low for practical significance. A number of experiments, however, verified the fact that both the aldehyde and the acid were necessary for the cure. Sometimes they occurred in sufficient amounts as contaminants, such LM formaldehyde from acetals in technical dioxane, or from the methylol groups of the precured fiber. The control of shrinkage of zein fiber is of utmost importance. A fiber having high shrinkage will necessarily have poor properties after being preshrunk. However, good control of shrinkage is not necessarily indicative of other desirable properties because of a possible degrading effect of the cure on the protein structure. I n addition, the fiber may shrink excessively while being cured although this shrinkage is not included in the above measurements. The measurement of final strengths of azlon fibers that have been relaxed in boiling acid and/or alkali is an excellent single property measurement of the usefulness of the fiber. Values so low that they have no significance are often obtained. Raw zein fiber having a dry and wet tenacity, respectively, of 1.58 and 0.6 grams per denier shrinks about 70% and dries to a horny maw. The same fiber, however, after being cured by the above method and then boiled in blank acid dye baths, still had tenacities as high as 1.4 and 0.7 gram per denier. The elastic properties remained desirably close to those of the raw fiber. When the new cure is applied to raw zein fiber, the acetylation step is circumvented. If acetylation is desired, the treatment can be combined with the nonaqueous cure by replacing all or preferably part of the organic solvent with acetic anhydride. For example, paraformaldehyde and trichloroacetic acid in a 1 to 3 ratio of acetic anhydride to toluene a t 110" C. simultaneously acetylated and stabilized with formaldehyde, resulting in an excellent fiber with dry and wet tenacities of 1.2 and 0.6 gram per denier. DISCUSSION OF REACTION MECHANISM
The mechanism of the effective curing reaction is not known, but it is felt the reaction is of sufficient importance to be reported. 45
1
I
I
I
I
I
I
I
I
I
1
I?
TABLE I. KONAQUEOUS CURINGMIXTURES (Parts b y weight) Formula
Solvent Acid 1 100 dioxane 0.05 sulfuric 2 91 toluene Hydrogen chloridea 3 91 tetrachloroethane Hydrogen chloridea 4 97 Stoddards solvent 1 trichloroacetic a Saturated with bubbling HCl.
NO.
Aldehyde 0 . 6 formaldehyde (40%) 9 paraformaldehyde 9 paraformaldehyde 2 paraformaldehyde
Figure 2. Effect of Time of Cure 1 (at Boil) on Shrinkage of Fiber in Boiling 0.1%Acetic
Acid
INDUSTRIAL AND ENGINEERING CHEMISTRY
484
The low shrinkage of treated zein fibers in boiling acids and the resulting high strengths indicate the formation during the cure of irreversible crosslinks between the protein molecules. Proteinformaldehyde reactions formerly postulated do not account for this irreversible stabilization of zein fiber. Fraenkel-Conrat and Olcott ( 4 ) reported irreversible crosslinking between protein groups by a Mannich type of reaction. The absence of amino groups in zein excludes this possibility unless it is assumed that ammonia of hydrolysis is first converted to dimethylolamine which then condenses with two active =C-groups. Such a series of reactions Seems to be improbal,le because of the rapidity of the cure. The well-known phenol-formaldehyde reaction readily occurs under conditions of the cure. Though usually not considrred as a possible crosslinking mechanism for proteins, the reaction would explain the tying of two protein chains together n-ith an acid resistant methylene linkage between their tyrosine residues.
Vol. 42, No. 3
That t,yrosine is involved in at least part of the mechanism is supported by the following facts: t h ~ * f i bcure ~ r RdTmd ~ the irltensity of color of t’he31illon test, on 2. The cure was r,ot effective on partially iodinated fiber. 3. Only a relatively small amount of tyrosine was isolated m :rf a ~ ~ ~other proteins~ such as gliadin, casein, and soybean protein which contain appreciable amounts of tyrosine were markedly modified by the cure; whereas gelatin, which contains only traces of the amino acid, was not af-
fer t,ed.
LITERATC‘RE CITED
(1) E v a n s , C. D., a n d Croston, C. B., Teztile Research J . , 19, 202-11 (1949). (2) Evans, C. D., Croaton, C. B., a n d Van E t t e n , Cecil, Ibid., 17, 562-67 (1947). (3) F o s t e r , H. D., and L a r c h a r , A. IT., U. S. P a t e n t 2,430,866 (1947). (4) . , Fraenkel-Conrat, Heinz, a n d Olcott, H. S., J . Bid. Chem., 174, 827-43 (1948). (5) Lewis, J. R., et al., U. 9. P a t e n t 2,434,247 (1948). (6) Wormell, R. L., Brit. P a t e n t s 549,642 (1942); 564.591 (1944); a n d 565,011 (1944).
RECEIVED September 19, 1940. A patent application by C. B. Croaton, C. D. Evans, I,. L. McICinney, and .J. C. Cowan covering the results of this work has been filed in tlie U. S. Patent Office and assigned to t,he Seoretary of Agriculture.
Creep Behavior of Plasticized Polyvinyl Chloride J
J
LIQUID POLYMER PLASTICIZERS M. DAUD ALI, HERMAN F. MARK, AND ROBERT B. MESROBIAN Polytechnic Institute of Brooklyn, Brooklyn, N . Y . Studies on the creep behavior of vinyl chloride resin plasticized with liquid polymers derived from common vinyl monomers are reported. The various chemical factors that influence the formation of liquid vinyl polymers are described. In general, efficient chain transfer reagents such as carbon tetrabromide may be used successfully with the acrylate nlonomers to yield liquid polymers, whereas “degradative” chain reagents of the allyl type yield liquid polymers with a variety of vinyl monomers when used in copolymerizing systems. A number of the liquid polymers are compared, by tensile creep, with commercially used plasticizers; their plasticizing efficiency, under certain conditions of temperature and concentmtion, was found to be quite satisfactory.
T
HE development of polymeric plasticizers constitutes one of the recent important advances in the field of plasticizers for
vinyl resins. As typical examples, one might consider the high molecular weight polyesters (‘7) and nitrile rubbers (9) which have attracted considerable attention as useful plasticizers because of their outstanding resistance to volatile loss and extraction and because of their low marring action. There have been no systematic attempts reported, however, on the preparation of low molecular weight polymers of the common vinyl monomers, from the point of view of employing such ma-
terials as plasticizers. In the studies reported here the established principles of chain transfer and polymerization kinetics were applied to the preparation of a series of liquid vinyl polymers and copolymers, and their relative merits as plasticizers for vinyl resins were evaluated by simple tensile creep and physical property measurements. Preparation of viscous liquids rather than low molecular weight solids was always strived for since it was felt that in this way maximum plasticizing efficiency could be attained, consistent with the requirements for low volatilitv. EXPERIMENTAL PROCEDURE
The procedure employed in the present studies for preparation of plasticized films of polyvinyl chloride (Geon 101) and vinyl chloride-acetate copolymer (VYNW) is identical to that described by Aiken, Alfrey, Janssen, and Mark ( 1 ) . Films were cast containing 25 and 45% plasticizer. The methoas of these authors were also used for measurement and calculation of tensile creep. The creep tests were carried out in the temperature range -20’ to 50” C. and the data were plotted as strain divided by stress ( y / S ) , square cm. per dyne, against logarithmic time in seconds. Care was taken that the extent of creep did not exceed 10% throughout each run in order to obviate corrections for the changes in cross-sectional area of the samples. Measurements of ultimate tensile strength and elongation a t break of the plasti-
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