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X-Rav Studv of Reactions J
J
Orientation of Crystalline Phases on G. L. CLARK AND R. L. LETOURNEAU University of Illinois, Urbana, 111.
Chemical reactions between accelerator and accelerator activator are shown by the x-ray method to take place in rubber stocks at the curing temperature; e . g., tetramethylthiuram disulfide reacts with zinc oxide to form zinc dimethyldithiocarbamate. These crystalline accelerator salts are highly oriented in the rubber matrix on stretching. This signifies a strong bond between the rubber crystallites and the grains of accelerator salt. Perhaps this explains some of the desirable physical
J. M. BALL 11. T. Vanderbilt Company, East Norwalk, Conn.
BOUT a year ago, in the course of a systematic survey
A
of the x-ray diffraction patterns of a number of routine rubber samples with different accelerators present, certain similarities were noted in the diffraction effects from the accelerators in these stocks. Close examination of these effects showed that they were identical. The stocks contained the accelerators Tuads, Monex, and Zimate which are chemically similar (Table I) ; the first is the disulfide, the second, the monosulfide, and the third, the zinc salt. Thus, the diffraction patterns indicate that, on curing, the disulfide and
the monosulfide in the presence of sulfur react with zinc oxide to form the zinc salt. This is not a new concept. The zinc salts have been extracted with solvents from stocks similar to the above, and it has been supposed that such reactions occur. However, the extraction method leaves some doubt as to whether the salt was originally present in the rubber matrix or whether it was formed in the solvent on extraction, BETWEEK ACCELERBTOR AND ACTIVATOR TABLE I. REACTIONS Every crystalline material has its (CHB)ZNC-S-CN(CHB)Z ZnO 4- S (CHI)ZNC-S-Z~-S--CN(CH~)S own characteristic x-ray diffraction patI1 11 I1 11 tern which is different from that of s s S S every other crystalline material, just Tetramethylthiuram Zinc dimethyldithiocarbamate monosulfide (Monex) (methyl Zimate) as the fingerprints of each person are different from that of every other per(CH3)zNC--S-S-CN(CH~) zZnO- (CHa)zNC-S-Zn-S-CN(CH3) z son. Thus, with x-ray “fingerprint” /I I/ /I I/ patterns it is possible to identify posiS S S S Tetramethylthiuram disulfide Zinc dimethyldithiocarbamate tively the presence of these salts in (methyl Tuads) (methyl Zimate) the rubber matrix and show that they must have been formed in the rub(CIH~)~NC--S-S-CN(CZH~)Z ZnO (C2Ht)2NC-S-Zn-S-CN(CzHa)2 ber mix on curing. Further, it was I1 I1 I1 I1 noted that this crystalline salt is S S S S strongly oriented in the rubber matrix Tetraethylthiuram disulfide Zinc diethyldithiocarbamate (ethyl Tuads) (ethyl Zimate) on stretching. It seemed advisable, then, to investigate other similar re(CHa)2NC-S-S-CN(CHa)z PbO ,( C H I ) Z N C - - S - P ~ S - C ~ ( C H I ) ~ actions by the x-ray method and to il il /I /I investigate the significance of the strong S S S S orientation of these reaction products in Tetramethylthiuram disulfide Lead dimethyldithiocarbamate (ethyl Tuads) (Ledate) the rubber matrix.
Experimental Procedure C-S-Zn-S-C Benzothiazyl disulfide (Altax)
Zinc salt of mercaptobensothiazole (Zenite)
C-S-Zn-S-C Meroaptobenzothiacole (Captax)
Zinc salt of mcrcaptobenzothiazole (Zenite)
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The rubber stoclrs listed in Table I were prepared (in the Vanderbilt laboratory) and tested by the x-ray method (at the University of Illinois). Series A consists of stocks made up according to standard compounding practice. Series B is prepared with larger quantities of accelerator than are usually employed so that the reaction products will be larger in amount and thus more easily detected in the rubber matrix
Involving Accelerators
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