Sulfur-containing amine compositions and agents for vulcanizing rubber

A vulcanizable composition comprises sulfur-vulcanizable rubber, sulfur or a sulfur-donor, a zinc compound, and a sulfur-containing amine of the formula EQU R.sup.1 --S.sub.n --R--NR.sup.2 R.sup.3 wherein R is a divalent radical comprising a chain of at least 2 C atoms, R.sup.1 is an organic radical, and R.sup.2 and R.sup.3 are each H or an organic radical, provided that NR.sup.2 R.sup.3 is not NH(C.sub.1-6 alkyl), and n is an integer, or a salt thereof, provided that the amine is not 3-(methylthio)propylamine. The amine is preferably cystamine.

FIELD OF THE INVENTION 
This invention relates to vulcanizable compositions and to agents for 
vulcanizing rubber. 
BACKGROUND OF THE INVENTION 
In recent years, environmental and legislative pressures have made it 
necessary to replace many of the active agents that have been widely used 
in rubber technology. One particular area of concern has been the use of 
compounds, usually based on secondary amines, that are extremely effective 
accelerators but which can react to give toxic nitrosamines. Various 
compounds have been proposed, to avoid this problem. For example, the use 
of dialkyldithiocarbamates and xanthates is described in EP-A-0184301, 
tetrabenzylthiuram sulfides in EP-A-0283552, EP-A-0284649 and 
EP-A-0284650, and the combination in EP-A-0413504. 
Although these compounds are effective, the degree of cure that they 
provide is usually lower than that provided by conventional secondary 
amine accelerators. One such accelerator is 2-mercaptoimidazoline (also 
known as ethylene thiourea) that is both nitrosatable and carcinogenic. 
Chem. Abs. 80 (1974) 16145p discloses that diaminodisulfides, e.g. 
cystamine, provide increased scorch times when used in halogen-containing 
poller rubbers. The particular example of a composition of this type 
comprises 100 parts polyepichlorohydrin rubber, 5 parts Pb.sub.3 O.sub.4, 
50 parts carbon black, 1.5 parts 2-mercapto-imidazoline and 1 part 
2,2'-diaminodiphenyldisulfide. 
SUMMARY OF THE INVENTION 
It has now been found that diaminoalkyldisulfides such as cystamine, and 
other compounds having both an amine group and at least one sulfur atom, 
are useful primary and, more especially, secondary accelerators in 
sulfur-vulcanizable systems. As amines, such compounds do increase the 
rate of cure of rubber, but the surprising discovery behind the present 
invention is that the cured rubber is stable, i.e. it does not suffer 
either reversion or post-vulcanization once the cure has reached its 
plateau. According to the present invention, therefore, a vulcanizing 
agent for a sulfur-vulcanizable rubber (i.e. not a halogen-containing 
polymer rubber) comprises sulfur, zinc oxide or another zinc compound, and 
a sulfur-containing amine of the formula 
EQU R.sup.1 --S.sub.n --R--NR.sup.2 R.sup.3 
wherein R is a divalent radical comprising a chain of at least 2 C atoms, 
R.sup.1 is an organic radical, and R.sup.2 and R.sup.3 are each H or an 
organic radical, provided that NR.sup.2 R.sup.3 is not NH(C.sub.1-6 
alkyl), and n is an integer, or a salt thereof. Perhaps because of its 
tendency to decompose into the relatively stable compounds methylmercaptan 
and allylamine at vulcanizing temperatures, however, 
3-(methylthio)propylamine is unsuitable for use in the invention; its use 
causes reversion. 
DESCRIPTION OF THE INVENTION 
The sulfur-containing amine used in the present invention apparently has 
both stabilising/anti-oxidant properties as well as acting as an 
accelerator. Particularly if anti-oxidant properties are required, 
monosulfides (n=1) may be preferred. Especially stable compounds of this 
type are those in which R.sup.1 is a long-chain alkyl group, e.g. of up to 
20 C atoms. Thus, for example, the amine may be 2-(decylthio)ethylamine. 
In order to obtain the accelerating effect associated with the amine group, 
it is often preferred that R.sup.1 should include another NR.sup.2 R.sup.3 
group. Preferred diamines of this type have the formula S.sub.n (RNR.sup.2 
R.sup.3).sub.2. While the amount of the amine that is used in the present 
invention may be 0.1 to 2, and preferably 0.2 to 1, parts by weight by 100 
parts rubber, these ranges are particulary suitable for diamines; 
monoamines may be used in correspondingly larger amounts. 
The divalent group R is preferably a straight-chain alkylene radical, e.g. 
C.sub.2-8 alkylene, and preferably --CH.sub.2 CH.sub.2 --. R may also be 
aromatic; compounds such as 2,2'-diaminophenyldisulfide have good 
anti-oxidant properties. 
While n may be, for example, up to 4, e.g. 1 as indicated above, it is 
preferably 2. Accordingly, preferred compounds for use in the invention 
are diaminoalkyldisulfides, of which cystamine is the simplest example. 
The amines used in the invention may be secondary amines, with the given 
proviso. An example of a suitable secondary amine is 
di2-(N-(2-benzthiazyl)amino)ethyl!disulfide. When R.sup.2 is H, R may be, 
for example, --CSSM (M=Na or 1/2 Zn), --SHet (Het=2-benzthiazyl or 
2-thiazolinyl), or benzyl. 
It is also possible to use tertiary amines, i.e. in which R.sup.2 and 
R.sup.3 are each organic radicals, e.g. C.sub.1-8 alkyl. One such compound 
is tetramethylcystamine, of which a given amount may be less effective 
than cystamine, but which has the advantage over cystamine of being more 
compatible with rubber. 
As for cystamine, it is generally preferred that R.sup.2 and R.sup.3 are 
each H, i.e. the amine is a primary amine. Further examples of amines that 
can be used in the invention are 3,3'-dithiobispropylamine and also salts 
of the amines. Suitable salts are those formed with stearic or lauric acid 
or carbon dioxide. 
Cystamine itself is a liquid at ambient temperatures. It may be used as 
such, especially with latex rubber, but, for ease of handling, it may be 
encapsulated. Advantages associated with the use of cystamine in 
particular are that it gives no objectionable odours and provides few 
problems during mill mixing. It acts as an effective post-vulcanization 
stabiliser in addition to accelerators, giving high rates and states of 
cure of vulcanizates, without reversion, and hence good rheological and 
physical properties. Further, since cystamine is a primary amine, its use 
does not produce stable nitrosamines. 
If the given amine, e.g. cystamine is used, in accordance with the 
invention, in order to boost the effect of a primary accelerator, that 
accelerator can be relatively non-toxic. In particular, primary 
accelerators can be used that do not react to give toxic nitrosamines. 
The cystamine or other sulfur-containing amine is effective in a wide range 
of elastomers, except chloroprene and other halogen-containing polymer 
rubbers. Examples of rubbers in which it can be used are natural, nitrile, 
butyl, Hypalon, SBR and EPDM rubbers. The rubber may be in dry or latex 
form. Nitrile rubbers exemplify rubbers that are generally resistant to 
reversion; here and in other cases, reversion resistance may not be a 
primary object of the present invention, but advantages are then still 
associated with, for example, reduced toxicity. 
The sulfur-containing amine functions as a secondary accelerator for a 
variety of primary accelerators such as dithiocarbamates, thiurams, 
thiazoles and sulfenamides. Thus, for example, compounds such as 
tetrabenzylthiuram disulfide or tetrasulfide, which have been proposed for 
use in low-nitrosamine curing systems but which are only moderate 
accelerators, can be boosted so that cures are achieved which match those 
obtained using accelerators based on amines which can give rise to 
nitrosamines and are therefore listed as dangerous. The general formulae 
and specific examples of other such accelerators whose performance can be 
boosted in accordance with the invention are given in EP-A-0184301, 
EP-A-0413504 and U.S. patent application Ser. No. 08/120,041, filed Sep. 
10, 1993, the contents of which are incorporated herein. 
As indicated above, the vulcanizing agent should include a zinc compound. 
This may be zinc oxide or a zinc complex such as 
diammine-diisocyanatozinc. Alternatively, or in addition, zinc may be 
provided as a zinc compound that can act as a primary accelerator. In 
particular, zinc-containing accelerators such as the zinc salt of 
2-mercaptobenzthiazole, zinc dibenzyldithiocarbamate or zinc 
diisononyldithiocarbamate, or an analogue thereof, may be used; another 
such compound is zinc dibutyldithiophosphate, although it is an advantage 
of the present invention that, if desired, the use of 
phosphorus-containing compounds can be avoided. 
Without wishing to be bound by theory, the present invention may depend on 
the interaction of the sulfur-containing amine and sulfur. A complex or 
reaction product of the two may be used, as may a complex of the amine and 
the zinc compound. Alternatively, the amine may be complexed with any 
other component of the composition, as in the case of a 
2-mercaptobenzthiazole:cystamine complex. The sulfur may be provided as 
elemental sulfur or in an available form, i.e. in a sulfur donor such as a 
thiuram or dithiodimorpholine. 
The following Examples 1-5, 7, 10, 12, 14, 16, 21, 24-26, 28, 30 and 32 
illustrate compositions of the invention and their use. Examples 6, 8, 9, 
11, 13, 15, 17-20, 22, 23, 27, 29 and 31 are given for the purposes of 
comparison. 
In all formulations, amounts are given in parts by weight (unless otherwise 
specified). The vulcanization temperature was 160.degree. C. (unless 
otherwise specified). The following abbreviations apply: 
ZBeD=zinc dibenzyldithiocarbamate 
ZDNC=zinc diisononyldithiocarbamate 
MBT=2-mercaptobenzthiazole 
DPG=1,3-diphenylguanidine 
P25=dipentamethylenethiuram tetrasulfide 
PPD=piperidinium pentamethylene dithiocarbamate 
DDZ=diamminediisocyanatozinc 
SMR=standard Malaysian rubber 
SBR=styrene-butadiene rubber 
NR=natural rubber 
NR05=100 parts NR, 5 parts ZnO, 1 part stearic acid, 1 part antioxidant 
WSP, 50 parts precipitated coated calcium carbonate, 5 parts SRF black 
Results are expressed in terms of RH (cure rate), MH (maximum torque) and 
reversion. Rheometer curves give RH (%) as that part of the rheometer 
trace where the graph is climbing; MH indicates the maximum cure (kg.cm). 
Reversion (%) is calculated from (1-MH.sup.1 /MH.sup.2).times.100, where 
MH.sup.1 and MH.sup.2 are respectively the initial and final maximum 
torque values.