N-Substituted Alkvlsulfenvl Dithiocarbarnates as Vulcanization Accelerators J
J
J. F. SVETLIIC, H. E. RAILSBACII, C. C. BIdRD, . i X D R. P. IAOUTH_4N Research Division, Phillips Petroleum C o . , Bartlesville, Okla.
REAT advances have been made in recent years in rubber processing and vulcanizing equipment and the present trend is toward faster processing and shorter curing cycles to increase factory output. The use of high-speed machinery results in greater heat generation during mixing and final processing, which increases the tendency of stocks to "scorch" (vulcanize prematurely). The use of larger quantities of accelerators or accelerator activators to shorten curing cycles aggravates the problem, because fast-curing stocks also tend to scorch more rapidly. Numerous investigators have studied the problem in a n effort to devise means of obtaining fast-curing stocks which possess sufficient scorch resistance to withstand high-temperature processing. Alliger and Harrison ( Z ) , Holbrook and Hunter (S), and Wolf and deHilster (10)have investigated delayed-action accelerators which afford processing safety for stocks reinforced with furnace black. Throdahl and Harman (8) and Wolf and associates (9) investigated other curing agents which could be used t o replace part or all of the elemental sulfur and thereby alleviate the scorch problem during processing. The sulfenamide-type accelerators, which are widely used today, impart excellent physical properties and moderate scorch resistance. However, a major need exists for a powerful delayed-action accelerator to satisfy both of the basic requirements of the industry: greater processing safety to withstand high-temperature processing without scorching and powerful accelerating activity to be suitable for applications requiring short curing cycles. N,N-Disubstituted alkylsulfenyl dithiocarbamates have been found to be powerful accelerators which afford improved processing safety. The activity and scorch resistance can be altered considerably by modifying the molecular structure. The most powerful accelerator with the required scorch resistance yet found is N,N-dimethyl-tert-butylsulfenyl dithiocarbamate.
mary amine, *~-isopropyl-tert-but3-lsulfenq.l dithiocarbamate (iPrtBuSD), was not so powerful an accelerator as those made from secondary amines. A comparison of N,N-dimethyl-teributylsulfenyl dithiocarbamate ( MentBuSD), N,N-dimethyl-nbutylsulfenyl dithiocarbamate (MennBuSD), and N,N-dimethyl methylsullenyl dithiocarbamate (Me&IeSD) shows that when a normal alkyl group was substituted for the tert-butyl group the tendency to scorch (a, 6) was increased. These materials were powerful accelerators, as can be seen in the data on compression set (Y), niodulue, and tensile stlength. As the data in Table I sho.ived that the dimethylamine derivatives functioned as accelerators, it was decided to investigate other secondary amines. The tert-but,yl group was also found to be desirable for imparting good scorch resistance; hence, this portion of the molecule was retained in common in the other derivatives studied.
Table I.
Effect of Chemical Structure on Accelerator Activity and Rubber Properties COXPOUNDINQ RECIPE No. 1 smoked sheets Philblack Oa Zinc oxide Stearic acid Flexamineb Pine t a r Sulfur Accelerator Cure. 30 minuter a t 307' F.
100 60 4
3 1 3 2
Variable
Corn300% Scorch pression Modulus Tensile Elongaat Set, Lb./Sq.' Lb./Sqj tion, 250' F . , Tspe phrc % Inch Inch ?& Llin 770 0.6 30,5 iPrtBuSD 2230 535 45 1280 0.3 12.8 Me2tBuSD 3130 520 23 i 1410 0.3 12.6 MeznBuSD 3280 540 13.5 l5,6 1390 3350 540 11 3IezXleSD 0.2 1480 0.4 17.0 3430 525 17 CHBTSd ... 34.9 480 1710 575 45 Trade-mark of Phillips Petroleum Co. b Physical mixture containing 65% of complex diarylamine-ketone reaction product and 35% of N,N'-diphenyl-p-phenylenediamine. c Parts per 100 parts of ruhber. d Cyclohexylbenzothiazylsulfenamide.
_- Accelerator
...
SYNTHESIS O F N-SUBSTITUTED ALKYLSULFENYL DITHIOCARBAMATES
The alkylsulfenyl dithiocarbamateA may be prepared by various methods (4, 6),and may be represented by the following general formula:
Table 11. Effect of Molecular Weight of ~'V,~V-Disubstituted-tert-butylsulfen~lDithiocarbamates on Ruhher PropertiesQ Molecular Weight 209.40
Cornpression Set,
300% 1IoduSrorch Ius, Tensile. Elonat Lb /Sq. Lb /Sq gation, 250" F., Inch Inch YO hlin. 1280 3130 620 23,s
Accelerator Type phr r0 3feztBuS.D 0.3 12.8 IvIorphollne 0.4 251.43 17.8 1240 tBuSD (Dodecy1)z tBuSD 0.5 517.97 16.9 1230 4 Compounding recipe given in Table I .
R1 and R2 may both be alkyl groups or one may be a hydrogen. It is also possible to use cyclic amines like morpholine: 2H*-cHTNH
EFFECT OF STRUCTURE AND MOLECULAR WEIGHT ON ACCELERATOR ACTIVITY
2860
515
25
3280
570
33.5
The data in Table 11, using the compounding recipe given in Table I, show t h a t N,N-dimethyl-tert-butylsulfenyldithiocarbamate was more powerful than morpholine-tert-butylsulfenyl dithiocarbamate (Morpholine 1BuSD) and N,N-didodecyl-tertbutylsulfenyl dithiocarbamate [(Dodecyl)a tBuSD] . The accelerating pdwer decreased with increasing molecular aeight and
Numerous derivatives have been prepared to establish the effect of different primary and secondary amines, R1 and R*,and of varying the alkylsulfenyl group, Ra, T h e data in Table I show that the derivative made from a pri-
352
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1955
larger quantities of the materials of higher molecular weight were required to obtain similar curing rates. Because N,N-dimethyl-tert-butylsulfenyldithiocarbamate was found to possess the best balance of accelerating activity and scorch resistance, it was decided to utilize this material in more extensive compounding studies.
I
STORAGE STABILITY
I loooo 5
I
300% MODULUS
DAYS 2OAGED 0 AT 212F.Y
It is desirable that accelerators designed for general purpose use undergo little or no deterioration during storage. To illus-
ofAge Accelerator, Years 6 1
