Scorch resistant, crosslinkable composition containing mixture of copper and metal dithiocarbamates

A crosslinkable composition of a polymeric thermoplastic and/or elastomeric material which is susceptible to scorching when processed at elevated temperatures, prior to crosslinking, in the presence of a free radical initiator, is protected against such scorching by the incorporation therein of a mixture of at least two metal salts of disubstituted dithiocarbamic acid, wherein one metal salt is based on copper.

BACKGROUND OF THE INVENTION 
This invention relates to the prevention of scorching prior to crosslinking 
of a peroxide crosslinkable thermoplastic and/or elastomeric polymer 
composition. 
A major difficulty in using organic peroxides or azo compounds in 
crosslinking elastomeric and thermoplastic materials applications is that 
they are susceptible to premature crosslinking (i.e., scorch) during 
compounding and/or processing prior to the actual curing phase. With 
conventional methods of compounding, such as milling, banbury, or 
extrusion, scorch occurs when the time-temperature relationship results in 
a condition where the peroxide or azo initiator undergoes thermal 
decomposition, initiating the crosslinking reaction whereby gel particles 
may be formed. Excessive scorch may cause the loss of the entire batch, or 
plugging of equipment. 
One method of avoiding scorch is to use an initiator that is characterized 
by having a high 10 hr. half-life temperaure for the given application. 
The disadvantage to this approach is that one subsequently obtains a 
longer cure time, which results in lower throughput. High cure 
temperatures can be used but this results in higher energy costs, also a 
disadvantage. 
Another method for avoiding scorch is to lower the compounding and/or 
processing temperature to improve the scorch safety margin of the 
crosslinking agent. However, depending upon the polymer and/or process 
involved, this option may also be limited in scope. In addition, curing at 
the lower temperature results in lower throughput. 
Prior to the present invention certain additives were incorporated into 
compositions which reduced the tendency for scorching. For example, 
British Pat. No. 1,535,039 discloses the use of organic hydroperoxides as 
scorch inhibitors for peroxide-cured ethylene polymer compositions. U.S. 
Pat. No. 3,751,378 discloses the use of N-nitroso diphenylamine or 
N,N'-dinitroso-para-phenyldiphenylamine as retarders incorporated in a 
polyfunctional acrylate crosslinking monomer for providing long mooney 
scorch times in various elastomer formulations. U.S. Pat. No. 3,202,648 
discloses the use of nitrates such as isoamyl nitrite, tert-decyl nitrite 
and others as scorch inhibitors for polyethylene. U.S. Pat. No. 3,954,907 
discloses the use of monomeric vinyl compounds such as 
.alpha.-methylstyrene and n-butyl methacrylate as scorch inhibitors for 
peroxide-cured ethylene polymers. U.S. Pat. No. 3,335,124 describes the 
use of various antioxidants and vulcanization accelerators as scorch 
inhibitors for peroxide-cured polyethylene. 
With these prior art methods, the cure time can also be altered and/or the 
crosslink density of the cured composition can also be altered. This can 
lead to a change in productivity and/or product performance. 
The present invention overcomes the prior art disadvantages in that an 
improvement in scorch at compounding temperatures is achieved without 
significant impact on the final cure time or crosslink density. This is 
achieved by incorporation of low additive levels, which limits the effect 
on properties. In addition, significant scorch protection is achieved, 
since the metal salt combination results in a synergistic effect on scorch 
time at these low levels. 
SUMMARY OF THE INVENTION 
The present invention is directed to a novel peroxide composition 
comprising an organic peroxide free radical initiator and a mixture of at 
least two metal salts of disubstituted dithiocarbamic acid wherein one 
metal salt is based on copper and the weight ratio of non-copper salt(s) 
to copper based salt is 1:1 to 30:1 and the weight ratio of organic 
peroxide to metal salts is 100:1 to 5:1. 
This invention also is directed to a scorch resistant, crosslinkable 
composition comprising a polymeric thermoplastic and/or elastomeric 
material, an organic peroxide free radical initiator, and a mixture of at 
least two metal salts of disubstituted dithiocarbamic acid, wherein one 
metal salt is based on copper and the weight ratio of non-copper salt(s) 
to copper based salt is 1:1 to 30:1, and the weight ratio of organic 
peroxide to metal salts is 100:1 to 5:1. 
This invention also comprehends a process of crosslinking the above 
mentioned polymeric material composition at temperatures in the range of 
200.degree. to 600.degree. F. whereby the composition possesses superior 
scorch safety under the operating conditions. 
DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the present invention, it has now been unexpectedly 
discovered that superior scorch resistance of peroxide containing 
thermoplastic and/or elastomeric compositions can be achieved by 
incorporating into said composition a mixture of two or more metal salts 
of disubstituted dithiocarbamic acid, whereby one is a copper salt. 
POLYMERIC COMPOUNDS 
The polymeric compounds encompassed in the present invention are those 
defined as natural or synthetic materials which are thermoplastic or 
elastomeric in nature, and which can be crosslinked through the action of 
a free radical crosslinking agent. Reference can be made to Rubber World, 
"Elastomer Crosslinking with Diperoxyketals", October, 1983, pp. 26-32, 
and Rubber and Plastics News, "Organic Peroxides for Rubber Crosslinking", 
Sept. 29, 1980, pp. 46-50, as to the crosslinking action and crosslinkable 
polymers. For the process of this invention, these polymeric compounds 
include materials, such as ethylene-propylene terpolymers, 
ethylene-propylene co-polymers, 1,4-polybutadiene, chlorinated 
polyethylene, low density polyethylene (including linear low density 
polyethylene), high density polyethylene, silicone rubber, nitrile rubber, 
neoprene, fluoroelastomers, and ethylene-vinyl acetate. 
Polymers that contain sulfur, such as chloro-sulfonated polyethylene are 
not included within the scope of this invention. 
In addition, blends of two or more polymeric materials can be used in this 
invention. 
INITIATORS 
In accordance with the present invention, compounds such as organic 
peroxides and/or azo initiators, which upon thermal decomposition generate 
free radicals that facilitate the crosslinking reaction may be employed. 
Of the free radical initiators used as crosslinking agents, the dialkyl 
peroxide and diperoxyketal initiators are preferred. A detailed 
description of these compounds can be found in the Encyclopedia of 
Chemical Technology, 3rd edition, vol. 17, pp. 27-90. 
In the group of dialkyl peroxides, the preferred initiators are dicumyl 
peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, 
2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 
2,5-dimethyl-2,5-di(t-amylperoxy)hexane, 
2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3,2,5-dimethyl-2,5-di(t-amylperox 
y)hexyne-3, .alpha.,.alpha.-di[(t-butylperoxy)-isopropyl]benzene, di-t-amyl 
peroxide, and 1,3,5-tri-[(t-butylperoxy)isopropyl]benzene. 
In the group of diperoxyketal initiators, the preferred initiators are 
1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane, 
1,1-di(t-butylperoxy)cyclohexane, n-butyl 4,4-di(t-butylperoxy)valerate, 
ethyl 3,3-di(t-butylperoxy)butyrate, ethyl 3,3-di(t-amylperoxy)butyrate, 
2,2-di(t-amylperoxy)propane, 
3,6,6,9,9-pentamethyl-3-n-butyl-1,2,4,5-tetraoxacyclononane, and 
3,6,6,9,9-pentamethyl-3-ethoxycarbonylmethyl-1,2,4,5-tetraoxacyclononane. 
Other peroxides and/or azo initiators may also be used to provide a 
crosslinked polymer matrix. Mixtures of two or more free radical 
initiators may also be used together as the initiator within the scope of 
this invention. 
The amount of crosslinking agent present in the crosslinkable composition 
of this invention is sufficient to afford the desired degree of 
crosslinking. The amount of peroxide can range from 0.1 to 10 parts by 
weight for each 100 parts by weight of polymeric compound. Preferably, 0.5 
to 5.0 parts initiator will be used. 
SCORCH INHIBITORS 
The metal salts of disubstituted dithiocarbamic acid, which are suitable in 
the practice of this invention, can be represented by the structure, 
##STR1## 
wherein X is a metal such as nickel, cobalt, iron, chromium, tin, zinc, 
copper, lead, bismuth, cadmium, selenium, or tellurium; n is an integer of 
1 to 6 having a value equal to the valence of the metal. R.sub.1 and 
R.sub.2 are independently alkyl of 1 to 7 carbon atoms. 
Examples of the metal salt of disubstituted dithiocarbamic acid include 
bismuth dimethyldithiocarbamate, cadmium diamyldithiocarbamate, cadmium 
diethyldithiocarbamate, copper dimethyldithiocarbamate, lead 
diamyldithiocarbamate, lead dimethyldithiocarbamate, selenium 
dimethyldithiocarbamate, tellurium diethyldithiocarbamate, zinc 
diamyldithiocarbamate, zinc diethyldithiocarbamate, zinc 
dimethyldithiocarbamate, and selenium dimethyldithiocarbamate. 
Two or more of the above metal salts may be used, however, one metal salt 
must be based on copper. Preferred mixtures include copper 
dimethyldithiocarbamate in combination with one of the following: zinc 
dimethyldithiocarbamate, zinc diethyldithiocarbamate, cadmium 
diethyldithiocarbamate, cadmium diamyldithiocarbamate, bismuth 
dimethyldithiocarbamate, nickel dimethyldithiocarbamate, selenium 
diethyldithiocarbamate and lead dimethyldithiocarbamate. Most preferably 
are the mixtures of copper dimethyldithiocarbamate and zinc 
dimethyldithiocarbamate or zinc diethyldithiocarbamate. 
The level of metal salt is in an amount that is sufficient to achieve the 
desired balance in cure characteristics. Use levels can range from 0.001 
to 2.0 parts by weight per hundred parts by weight of polymer, preferably 
0.01 to 0.5 parts by weight. The weight ratio of non-copper based salts to 
the copper based metal salt can range from preferably a ratio of 1:1 to 
30:1. The weight ratio of organic peroxide to the metal salt blend can 
range from 100:1 to 5:1, preferably from 50:1 to 10:1. 
In the novel peroxide compositions of the present invention of a mixture of 
an organic peroxide and metal salt blend wherein two or more metal salts 
of disubstituted dithiocarbamic acid are used where one metal salt must be 
based on copper, the components of organic peroxide and metal salt blend 
of the novel composition are used at a weight ratio of organic peroxide to 
metal salt blend from 100:1 to 5:1, preferably at a weight ratio of 50:1 
to 10:1. These novel compositions can also contain one or more fillers (or 
carriers). Some of the commonly used fillers are calcium carbonate, 
calcium silicate, silica and various grades of clay. Also, polymeric 
carriers such as EPDM and EPM may be used in this novel composition. 
Incorporation of a filler is preferred wherein the organic peroxide in its 
natural state is a liquid or a semi-crystalline material. Generally 
accepted industry practice is to incorporate 40 to 50% organic peroxide 
onto a filler such as those previously mentioned. Incorporation of the 
metal salt into such extended peroxides is also within the scope of this 
invention. 
The crosslinkable composition of the present invention may also contain 
fillers such as carbon black, titanium dioxide, the alkaline earth metal 
carbonates, and co-agents such as triallylcyanurate, triallylisocyanurate, 
liquid 1,2-polybutadienes, various methacrylates and acrylate compounds, 
but not sulfur. 
Metal oxides such as zinc oxide and magnesium oxide can also be included in 
the composition. The composition may also contain antioxidants, 
stabilizers, plasticizers, and processing oils. 
PROCESS CONDITIONS 
The crosslinkable composition is heat cured to a time sufficient to obtain 
the desired degree of crosslinking. The heat curing has a temperature-time 
relationship which is primarily dependent on the polymeric compound and 
the peroxide or azo initiator present, but is also affected by the 
formulation as a whole. It is customary to use a time equal to about 6-8 
half-lives of the initiator. 
The crosslinking may be carried out at a temperature of 
200.degree.-600.degree. F. or more. The cure time is inversely related to 
temperature. The preferred initiators heat cure at temperature-time 
relations of about 300.degree.-500.degree. F. and 0.5 to 30 minutes. 
The heat curing may be carried out in any of the manners now used by the 
industry. These might be mold cures, or oil bath cures, where oil does not 
harm the polymeric compound; or oven cures, steam cures, or hot metal salt 
bath cures.