Heat resistant ethylene-propylene rubber and insulated conductor product thereof

A crosslink curable ethylene-propylene rubber composition with improved resistance to heat, the cured rubber composition and electrical conductors insulated with said cured rubber composition. The improved rubber composition comprises a combination of ethylene-propylene rubber, chlorosulfonated polyethylene, zinc oxide, talc and carbon black in particular proportions.

BACKGROUND OF THE INVENTION 
Loss of elasticity, or embrittlement, upon exposure to above ambient 
temperatures has been a longstanding impediment of many common elastomers. 
The deteriorating effect of heat upon elastomers has prompted continuing 
efforts and the use of a variety of remedial measures to improve their 
resistance to heat, such as the development and use of antioxidants or 
agents which block the action of oxygen or free radical forming 
ingredients, and new compound formulations. 
The characteristic loss of elasticity or embrittlement of elastomers is a 
particularly significant and critical problem in the electrical insulating 
field wherein conductors insulated with heat degradable elastomers are 
utilized in areas which are exposed to high temperatures, such as leads or 
wires in or about motors and transformers, or within apparatus containing 
heating elements or which are otherwise disposed in locations encountering 
high temperatures. 
SUMMARY OF THE INVENTION 
This invention comprises a novel rubber composition of a specific 
combination of compounded ingredients and proportions thereof which has 
significantly improved resistance to heat, or heat aging, along with 
satisfactory physical and electrical properties. The heat resistant rubber 
composition of this invention is composed of an essential combination of 
ethylene-propylene rubber, chlorosulfonated polyethylene, zinc oxide, talc 
and carbon black in particular proportions, and can include optional 
components which enhance the overall attributes of the rubber composition. 
The invention additionally includes electrical conductors insulated with 
the highly heat resistant ethylene-propylene rubber compound. 
OBJECTS OF THE INVENTION 
It is a primary object of this invention to provide a novel rubber 
composition having increased resistance to high temperatures. 
It is a further object of this invention to provide a novel curable rubber 
composition and products thereof which withstand high temperatures and 
exhibit stable chemical, physical and electrical properties over extended 
periods of exposure to elevated temperatures. 
It is an additional object of this invention to provide a crosslink cured 
ethylene-propylene rubber composition that is resistant to embrittlement 
and loss of elasticity upon prolonged heating and possesses physical and 
electrical attributes which render it advantageously suitable for use as 
an electrical insulating material for conductors such as wire and cable. 
It is also a primary object of this invention to provide a new and improved 
heat resistant electrical conductor product insulated with a cured 
elastomer composition having a high tolerance to elevated temperature and 
apt physical and electrical properties whereby the insulating material 
substantially maintains its original elastic, flexible and dielectric 
characteristics through its service life regardless of temperature 
conditions. 
It is a specific object of this invention to provide an ethylene-propylene 
rubber composition possessing apt electrical properties, abrasion and tear 
resistance, flexibility and elasticity, and which retains these attributes 
notwithstanding high temperature conditions, and an improved heat stable 
electrical insulation thereof.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
This invention specifically consists of a novel combination of compounded 
ingredients and relative proportions thereof which in total produce an 
elastomeric composition having outstanding stability and resistance to 
deterioration upon subjection to elevated temperatures over extended 
periods. 
The rubber composition of this invention comprises the combination, in 
approximate parts by weight, consisting essentially of: 
______________________________________ 
Ethylene-propylene rubber 
100 
Chlorosulfonated polyethylene 
3 - 10 
Zinc Oxide 15 - 30 
Talc 75 - 200 
Vinyl silane 1- 3 
Carbon black 10 - 30 
Antimony oxide 0 - 10 
Antioxidant 0.5 - 6 
Peroxide curing agent 2 - 8 
Curing Coagent 0 - 3 
______________________________________ 
The ethylene-propylene rubber component comprises ethylene-propylene 
copolymers and terpolymers of typical commercially available compositions 
constituting about 25 to about 75 parts by weight of ethylene monomer 
copolymerized with about 75 to about 25 parts by weight of propylene 
monomer. Terpolymers of ethylene-propylene include those commercial 
rubbers produced by the copolymerization of ethylene and propylene 
together with minor proportions of dienses such as ethylidene norbornene, 
dicyclopentadiene and 1,4-hexadiene. 
Talc, of course, consists of a well known but distinctive mineral form of 
hydrated magnesium silicate. It is preferred that the talc component of 
this invention be of the plate type in its physical form. 
Antioxidants for use in the composition of this invention should be of the 
amine, hydroquinoline, or other heterocyclic types. Amine type 
antioxidants include, for example, di-B-naphthyl-p-phenylenediamine, and 
hydroquinoline type comprise the polymerized 
1,2-dihydro-2,2,4-trimethylquinolines. Other types of heterocyclic 
antioxidants include, for example, zinc salt of 2-mercaptobenzimidazole. 
Peroxide crosslink curing agents for the ethylene-propylene rubber compound 
of this invention comprise the free radical forming organic peroxides such 
as tertiary peroxides characterized by at least one unit of the structure 
##STR1## 
which is activated by its decomposition at temperatures in excess of about 
295.degree. F. The use of such peroxides in crosslinking polymers is 
described in detail in U.S. Pat. Nos. 2,888,424; 3,079,370; and 3,214,422. 
A commonly used and preferred curing agent for this invention is dicumyl 
peroxide. Other useful peroxide curing agents include the tertiary 
diperoxides such as 2,5-dimethyl-2,5 (t-butyl peroxy) hexane, and 
2,5-dimethyl-2,5 di (t-butyl peroxy) hexane-3, and the like diperoxy 
compounds. 
The use of a peroxide curing coagent in the crosslinking of the novel 
composition of this invention is strongly preferred to increase the 
efficiency of the cure in accordance with the technology in this art. Apt 
curing coagents include, for example, trimethylol propane trimethacrylate 
and other esters of methacrylic acid such as ethylene glycol 
dimethacrylate and 1,3 butyl glycol dimethacrylate. Additional examples of 
coagents are set forth in an article entitled "Evaluation Of Cross-Linking 
Coagents In Ethylene-Propylene Rubber" by Lenas, I&EC Product Research & 
Development, Volume 2, No. 3, September, 1963, pages 202-208. 
A processing aid is generally desirably included in the rubber composition 
of this invention to facilitate the compositions preparation and handling 
such as the mixing or compounding of its ingredients and/or its forming 
and consolidation into a product shape by molding or extrusion. Processing 
aids include oils, waxes and jellies derived from petroleum or hydrocarbon 
sources and they serve to unite the ingredients into a coherent and 
uniform workable plastic mass and/or to provide a lubricant therein. 
The following comprise examples illustrating specific embodiments of this 
invention and demonstrating their improved resistance to heat in relation 
to a control comprising a prior art heat resistant composition. 
The compounds of the control and each example of this invention were all 
prepared in an identical manner, comprising first admixing all components, 
except the peroxide curing agent and curing coagent, in a Banbury for 
about 12 minutes while heating to about 250.degree. F. After cooling to 
room temperature, the curing coagent was added to the admixture on a two 
roll rubber mill followed by the addition of the peroxide and dispersed 
through the other ingredients. 
The compositions of the control and the examples of this invention were as 
follows in approximate parts by weight: 
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CONTROL EXAMPLES 
A B C I II III 
__________________________________________________________________________ 
Ethylene-propylene-diene 
terpolymer (du Pont- 
Nordel 1145) 100 100 100 100 100 100 
Chlorosulfonated polyethylene 
(du Pont-Hypalon 40) 
-- -- -- 5 5 5 
Zinc Oxide 5 5 20 20 20 20 
Talc - hydrated magnesium 
silicate (Sierra Talc - 
Mistron Vapor) 179 179 179 179 179 179 
Vinyl Silane 3 3 3 3 3 3 
Carbon Black 20 20 20 20 20 20 
Antimony Trioxide -- -- -- -- 5 5 
Antioxidants - Reaction product 
of acetone & diphenylamine 
(Uniroyal - B.L.E-25) 
2 -- -- -- -- -- 
Zinc Salt of 2-mercaptobenzi- 
midazole(Mobay Chemical-Z.M.B.) 
-- 2 2 2 2 2 
Polymerized 1,2-dihydro-2,2,4- 
trimethylquinoline 
(R.T. Vanderbilt - Agerite MA) 
-- 2 2 2 2 2 
Diisodecyl phthalate 
-- -- -- -- -- 0.5 
Processing lubricant oil 
(Sun Oil - Sunpar 2280) 
54 54 54 54 54 54 
Dicumyl peroxide curing agent 
(Hercules - Di Cup R) 
4.19 
4.19 
4.19 
4.19 
4.19 
4.19 
Curing Coagent - trimethylol pro- 
pane trimethacrylate 
(Sartomer Resin - SR-350) 
2 2 2 2 2 2 
__________________________________________________________________________ 
Samples of each composition of the controls and of the examples of this 
invention were prepared and crosslink cured by molding identical 
quantities for 3 minutes at 200.degree. F. -220.degree. F. in a press and 
then curing in an open steam vulcanizer for 1 minute at 250 pounds per 
square inch gauge steam pressure. 
The tensile strength and percent elongation were determined for the 
original crosslink cured composition of each control and example and for 
each of said compositions after an accelerated heat aging for 13 days at a 
temperature of 175.degree. C. The measured properties were as follows: 
__________________________________________________________________________ 
CONTROLS EXAMPLES 
A B C I II III 
__________________________________________________________________________ 
Original Properties 
Tensile Strength, lbs/in.sup.2 
1002 1111 1005 1009 1001 1011 
Elongation, percent 
262 213 227 271 233 280 
13 Days In Air Oven At 175.degree. C 
Tensile strength, lbs/in.sup.2 
Brittle 
Brittle 
371 756 1121 786 
Percent retention 
Brittle 
Brittle 
36.9 74.9 112 77.7 
Elongation, percent 
Brittle 
Brittle 
22 57 58 58 
Percent retention 
Brittle 
Brittle 
9.7 21 26 20.7 
__________________________________________________________________________ 
Additional samples of unaged compositions of Control A and Examples II and 
III were subjected to long term aging tests under the following conditions 
and their resultant tensile strength and elongation properties were 
determined as follows: 
______________________________________ 
CONTROL EXAMPLES 
Long Term Aging A II III 
______________________________________ 
30 Days In Air Oven at 136.degree. C 
Tensile, lbs/in.sup.2 
1112 1141 1202 
Percent retention 111 114 119 
Elongation, percent 
193 223 217 
Percent retention 74 95.8 77.5 
60 Days In Air Oven at 136.degree. C 
Tensile lbs/in.sup.2 
1048 1166 1228 
Percent retention 104.6 116.5 121.5 
Elongation, percent 
133 212 205 
Percent retention 50.8 91.1 73.2 
90 Days In Air Oven at 136.degree. C 
Tensile, lbs/in.sup.2 
953 1119 1201 
Percent retention 95.1 111.8 118.8 
Elongation, percent 
102 177 187 
Percent retention 38.9 76 66.8 
______________________________________ 
Referring to the drawing, there is shown a typical construction for an 
insulated electrical wire or cable product 10, comprising a metallic 
conductive element 12 and an overlying body of cured elastomeric 
insulation 14 extending thereabout or covering the conductor. In the 
drawing, the product 10 is illustrated as a short section with the 
insulation 14 removed from the end portion of the conductor 12. According 
to one embodiment of this invention, the novel heat resistant 
ethylene-propylene rubber composition thereof can be used to provide or 
form the insulation 14 on a conductor 12 of wire or cable product 10. It 
is to be understood from the foregoing, however, that the insulation can 
comprise a coating on any portion of a conductive element and that the 
insulation need not completely enclose the element where such is not 
necessary for a desired insulative effect. 
Although the invention has been described with reference to certain 
specific embodiments thereof, numerous modifications are possible and it 
is desired to cover all modifications falling within the spirit and scope 
of this invention.