STARCH IN RUBBER Zinc Starch Xanthate in Latex Masterbatching R .
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B U C H A N A N ,
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W E I S L O G E L ,
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RUSSELL,
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R l S T
Northern Regional Research Laboratory, U. S . Department of Agriculture, Peoria, Ill. 61604
Starch xanthate was evaluated as a modifier of latex coagulation, a rubber reinforcing agent, and a n accelerator of vulcanization. Mixtures of starch xanthate with degree of substitution (D.S.) 0.07 and elastomer latices were coprecipitated with zinc sulfate. The coprecipitated curd, from mixtures containing more than 6 parts starch xanthate per 100 parts of rubber ( p h r ) , had as its continuous phase the insoluble zinc starch xanthate. The curd w a s easily dewatered and dried. Milling of dried coprecipitate reversed starch and rubber phases and gave a uniform dispersion of fine particles of starch xanthate in the rubber. The zinc starch xanthate in these masterbatches accelerated sulfur vulcanization in proportion to the level of incorporation. Tensile properties of vulcanizates showed that zinc starch xanthate served as a reinforcing agent for a variety of elastomers. It was comparable to SRF black in reinforcing natural rubber and SBR up to 25 phr loading. In nitrile rubber it w a s comparable to HAF black up to 25 phr loading.
THEpotential market for starch xanthate in papermaking (Hamerstrand et al., 1967; Russell et al., 1962) has led to the development of rapid, continuous processes for its production a t low cost (Lancaster et a1 , 1966; Swanson et al , 1964). Alkyl xanthates are used as accelerators in vulcanization; therefore, such polyxanthates as starch xanthates should also be useful in rubber compounding. Even though starch and elastomer are incompatible, fine particle dispersions of starch xanthates in rubber may be prepared. Such dispersions are analogous to reactive resin-rubber blends in that xanthate groups are chemically reactive toward elastomer molecules during vulcanization. As reinforcing hard resins (Burke, 1965), starch xanthate when dispersed as colloidal particles increases the strength of elastomers. Starch xanthate masterbatches were prepared by a coprecipitation procedure similar to the process for making lignin-reinforced rubber (Keilen and Pollak, 1947). This process is compatible with current industrial latex masterbatching. Starch xanthate solutions were mixed with rubber latices, then starch and elastomer were quantitatively coprecipitated by addition of dilute zinc sulfate solution. The zinc ion simultaneously coagulated latex and formed zinc starch xanthate to give a curd in which zinc starch xanthate was the continuous phase. The curd was recovered by filtration and dried to a characteristic hard crumb. Subsequent milling reversed the starch and rubber phases, consolidating the crumb to a rubber mass containing a fine particle dispersion of zinc starch xanthate. These starch-rubber masterbatches were formulated by a sulfur accelerator recipe and press cured to give vulcanizates showing reinforcement of tensile properties. These rein-
forced vulcqnizates had much better tensile properties than rubbers containing either unmodified starch or starch xanthide incorporated as dry powder by mill mixing. Starch xanthide is oxidatively crosslinked starch xanthate (Russell et a1 , 1962). Experimental
Coprecipitation. Latices were types 2000 and 2105 SBR, Chemigum 236 NBR, Chemigum 550 carboxylic, and ammoniated centrifuged natural rubber. Type 2000 SBR is hot polymerized, 5OL' styrene, small particle sized latex of 4 l C r solids content. Type 2105 SBR is cold polymerized, 30' c styrene, large-particle latex of 60' c solids content. Chemigum 236 is a fine particle K B R latex of 45', acrylonitrile content and 40% solids. Chemigum 550 is a carboxylic modified NBR of 50% solids content. Sodium starch xanthate degree of substitution (D.S.) was 0.07 + 0.02 and the sodium hydroxide to starch mole ratio was 0.5 (Swanson et a1 , 1964). Since starch xanthate decomposes to varying degrees during incorporation in rubber, starch xanthate content in all systems is based on the amount of free starch that would be regenerated on complete removal of xanthate groups. Aqueous stock solutions of xanthate were made up to approximately 10' c concentration and were stored a t 2 ' C . to avoid deterioration. Only solutions less than 15 days old were used. The stock solutions are similar t o viscose in that they are quite alkaline and on acidification give off hydrogen sulfide and carbon disulfide; therefore, they should be handled accordingly. Usual batch size was based on 1000 grams total latex solids. A starch xanthate solution was mixed with latex and diluted with sufficientwater to reduce viscosity enough VOL. 7 N O . 2 JUNE 1 9 6 8
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Figure 1. incorporation of zinc starch xanthate into rubber by latex masterbatching
to make stirring easy. Antioxidant was added, 0.8 part of65"c emulsion of styrenated phenol per 100 parts elastomer (phr). Then 2 N H,SO, was added dropwise in an amount calculated to give a final serum of p H 6.0 to 6.5 after coprecipitation. Finally, sufficient M ZnSOI solution was added t o give the masterbatch usually 3 t o 5 phr zinc calculated as oxide. Immediately upon addition of zinc. a dense curd precipitated and was recovered by filtration on a Buchner funnel. The water-swollen curd contained 50 to 90'; solids depending on starch content and was dried to less than 2 ' L moisture in a forced draft oven a t 70"C. Dried products were analyzed for moisture, starch, and zinc content before further processing. Processing of Dried Crumb. The coprecipitate was milled and compounded on a roll mill described by ASTM D l 5 64T (American Society for Testing Materiais, 1963a). The crumb was passed through the mill without banding a t 0.010- to 0.020-inch clearance to consolidate into sheets. Then the thin sheets were combined and banded on the mill with the opening adjusted to give a small "rolling bank." An additional 1 phr octylated diphenylamine antioxidant was added, and milling was continued up to 45 minutes. Roll temperature was kept below 65°C. by cooling. Milled masterbatch was rolled, wrapped in aluminum foil, and stored in the dark until needed for compounding. Figure 1 is a schematic diagram of the experimental masterbatching process. Compounding and Curing. Masterbatches were compounded with added zinc oxide to give a total 5 phr 156
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Figure 2. Effect of starch xanthate on cure of SBR 2105
zinc as zinc oxide based on dried Lrumb analysis. Two parts stearic acid per 100 parts rubber (phr), 2 phr sulfur, 1.5 phr 2,2'-dithiobisbenzothiazoleand 0.2 phr tetramethylthiuram disulfide were incorporated by mill mixing. Specimens were press cured 10, 20, and 40 minutes a t 150°C. in a n ASTM D15-64T (American Society for Testing Materials, 1963a) slab mold (6 x 6 x 0.075 inch). Physical Testing. Tensile properties by testing procedure ASTM D412-62T (American Society for Testing Materials, 1963b) were determined for best cured specimens, generally 10 minutes for natural rubber; 20 minutes for SBR,nitrile, and carboxylic rubbers. Mooney curing characteristics a t 125"C. were determined by ASTM D1646-63 procedure (American Society for Testing Materials, 1 9 6 3 ~ ) . Volume swelling and change in tensile properties resulting from 70-hour water immersion a t 23°C. were measured by ASTM D471-63T method (American Society for Testing Materials, 1963d). Results
Coprecipitation. The experimental zinc precipitation procedure was not satisfactory for controls without the starch xanthate. Zinc-coagulated latex solids formed a ball, which occluded serum and was difficult to dewater and dry. Control masterbatches had to be prepared by salt-acid coagulation. But incorporation of as little as 6 phr starch xanthate completely changed the zinc precipitation to give an easily filtered product.
Typical zinc starch xanthate-elastomer coprecipitates were white free-draining curds. Their appearance was like the insoluble curd from zinc precipitation of starch xanthate alone. Presence of latex solids was not evident, and the coprecipitated curds were nontacky when wet. Curd was easily dewatered by either pouring on a screen or filtering through wet-strength paper. Serum was sparkling clear, contained no elastomer, and only a trace of starch. Dry batch weights and analytical values showed that coprecipitation of starch and latex total solids was quantitative. Zinc was 80 to 95';; coprecipitated. Curing Characteristics. The effect zinc starch xanthate had on Mooney curing of SBR stocks is shown by Figure 2. Data are for SBR 2105 masterbatches compounded by the recipe given above containing sulfur and accelerators. Addition of zinc starch xanthate increased acceleration above that given by the thiazole-thiuram combination. Acceleration was proportional to starch xanthate loading. The shape of' Mooney viscosity-time curves was typical as described in the ASTM example. Probably, viscosity reflected increased chemical binding between starch xanthate particles and rubber molecules instead of, or in addition to, curing in the rubber phase. Variation of press cure time from 10 to 40 minutes a t l.5W C. had little effect on vulcanizate tensile properties for SRR 2103 stocks containing from 0 to 41 phr zinc starch xant,hate. Vulcanizates with no added zinc oxide were transparent and nearly colorless; whereas, those with zinc oxide added were opaque and white. Vulcanizate Tensile Properties. Zinc starch xanthate reinforced the tensile properties of several elastomers (Table I ) . Reinforcement of natural rubber and SBR 2000 was about the same as given by an equal volume loading of SRF black. Probably, the more effective reinforcement of nitrile and carboxylic rubbers occurred because their polar groups make them more compatible with starch.
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Table I. Tensile Properties of Zinc Starch Xanthate-Reinforced Rubbers
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