Injection molding thermoset interpolymers of ethylene-propylene and product thereof

An injection-moldable, peroxide crosslinkable elastomeric composition comprises a mixture of a crystalline interpolymer comprising ethylene and propylene; a low density polyethylene; one or more multifunctional vinylic or allylic monomers; a medium to high structure form of carbon; and an organic peroxide. The composition is particularly useful for the fabrication of injection molded structures which are required to flex on impact and return to their original shape when the distorting force is released. Typical structures are automotive fender extensions, grilles and front and rear fascia.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a thermosetting elastomeric composition and a 
process for its production. More particularly, this invention is concerned 
with an injection-moldable elastomeric composition comprising a mixture of 
a crystalline interpolymer comprising ethylene and propylene; a low 
density polyethylene; one or more multifunctional vinylic or allylic 
monomers which copolymerizes in the presence of peroxide and which 
functions as a co-curing agent, an amount of carbon sufficient to render 
the composition, when crosslinked, electrostatically conductive; and a 
peroxide crosslinking agent. Most particularly, this invention relates to 
the said composition, a method for its production and to cross-linked 
structures made from the composition which are fully cured and thermoset. 
Recently the need for improved fuel economy in the operation of automobiles 
has led to smaller cars and automotive manufacturers lowering the weight 
of automobiles by replacing steel, particularly in the body, with light 
metal alloys and polymeric compositions. Currently automotive companies 
are developing elastomeric structures (fascia) for the front and rear ends 
of automobiles to replace the present steel fender extensions, radiator 
grilles and the like. These structures are required to flex on impact in 
concert with energy absorbing devices and return, undamaged, to their 
original shape when the distorting forces are released. In addition, the 
structure must readily accept paint and the finished surface must have 
adequate weather resistance and show a minimum of marking or marring on 
impact and recovery. 
Suitable and commercially acceptable compositions for the fabrication of 
these structures must not only be capable of being amenable to mass 
production methods, as by conventional injection molding techniques, but 
the finished product must possess the necessary physical properties of 
high flexural stiffness, high tensile strength, hardness, ability to 
recover rapidly to its original state when deformed and to be mar and tear 
resistant. In addition, since fascia structures are usually painted 
electrostatically and the paint cured in an oven, the structure must be 
electrically conductive and must not undergo deformation when passing 
through the paint oven. 
2. Prior Art 
Currently two types of polymeric compositions are being used for the 
fabrication of fascia structures. In the one, disclosed in U.S. Pat. No. 
3,915,928, the composition comprises an injection moldable mixture of a 
crystalline copolymer of ethylene and propylene or a terpolymer of 
ethylene, propylene and a non-conjugated diene; carbon, from 5 to 30 
weight percent of chopped glass fibers and sulfur based vulcanizing 
agents. Parts injection molded from this composition show "trail" lines 
due to alignment of the glass fibers and the parts must be routinely 
sanded prior to painting in order to produce acceptable automotive fascia. 
In the other, in which the end product is a microcellular polyurethane, 
described in a paper by Prepelka and Wharton, "Reaction Injection Molding 
in the Automotive Industry," Journal of Cellular Plastics, p. 87, 
March/April 1975, the cost of the components comprising the composition is 
higher than the cost of hydrocarbon based elastomers and production of the 
polyurethane structures requires specialized metering and mixing equipment 
and presses. 
A variety of compositions and processes have been described in the patent 
literature for the post vulcanization of preformed elastomers. 
U.S. Pat. No. 3,198,868 discloses a process for vulcanizing preshaped 
articles formed from vulcanizing materials comprising blends of amorphous 
copolymers of ethylene and higher .alpha.-olefins with polyethylene 
wherein the shaped article is impregnated with a solution of an organic 
peroxide, dried and vulcanized by the application of heat. 
U.S. Pat. No. 3,256,366 discloses a process for the preparation and 
vulcanization of a mixture of a copolymer of ethylene and propylene 
containing 40 to 60 mol percent of ethylene (30.8 to 50.0 weight percent 
of ethylene) with either low density or high density polyethylene 
comprising: mixing the polymers at a temperature above 125.degree. C.; 
adding a peroxide to the mixture at a temperature in the range of about 
60.degree. C. to about 90.degree. C.; shaping the mixture to a 
conformation; and heating the shaped article to vulcanize it at a 
temperature in the range of about 150.degree. C. to 180.degree. C. 
British Pat. No. 1,294,665 discloses cross-linked, electrically conductive, 
heat-shrinkable polymer compositions having volume resistivities below 
1000 ohms-centimeter which comprise mixtures of: at least 40 parts of an 
electrically conductive filler; at least 20 parts of a natural or 
synthetic rubber; and at least 10 parts of a normally solid, heat-flowable 
homo- or copolymer of ethylene. 
SUMMARY OF THE INVENTION 
This invention is concerned with a method for the preparation and the 
thermoset, cross-linked product of a composition consisting essentially of 
(a) an elastomeric polymer selected from the group consisting of copolymers 
of ethylene and propylene containing 62 to 80 weight percent ethylene and 
possessing a crystalline content in the range of about 10 to about 25 
weight percent, and terpolymers of ethylene, propylene and a C.sub.6 
-C.sub.10 non-conjugated diolefin containing from about 72 to about 80 
weight percent of ethylene and possessing a crystalline content in the 
range of about 15 to about 25 weight percent; 
(b) 50 to 150 parts per hundred parts by weight of elastomeric polymer, 
preferably 80 to 120 parts, of a low-density polyethylene having a melt 
index in the range of 2 to 40, preferably 12 to 20; 
(c) 50 to 150 parts per hundred parts by weight of elastomeric polymer, 
preferably 80 to 120 parts, of a medium to high structure form of carbon, 
such as carbon black; 
(d) 0.5 to 5 parts per hundred parts by weight of elastomeric polymer, 
preferably 1 to 3 parts, of one or more polyfunctional vinylic or allylic 
monomers; and 
(e) 1 to 10 parts per hundred parts by weight of elastomeric polymer, 
preferably 2 to 6 parts of an organic peroxide comprising one or more 
peroxide moieties in the molecule having the following structure: 
##STR1## 
wherein R and R' are independently selected from the group consisting of 
C.sub.1 to C.sub.8 alkyl radicals and provided said peroxide has a 
half-life at 130.degree. C. in excess of 5 hours and less than 1 minute at 
230.degree. C. when tested in low-density polyethylene. 
The above composition, when compounded by the method of this invention as 
disclosed hereinbelow, possesses a rheology which permits the unvulcanized 
compound to flow through narrow orifices over relatively large distances 
at temperatures and under pressures that will not prematurely vulcanize 
the compound when large structures are fabricated by injection molding. 
The inclusion of carbon in an amount sufficient to give the vulcanized 
composition a volume resistivity in the range of 10.sup.3 to 10.sup.8 
ohm-cm permits the vulcanized composition to be painted electrostatically. 
The presence of carbon black is considered essential for reinforcement 
purposes allowing the product to be removed from the hot mold without 
tearing. 
The physical properties of the cross-linked composition makes the 
composition particularly useful in the automotive field for the 
fabrication of fascia, fender extensions and grilles.

DESCRIPTION OF PREFERRED EMBODIMENTS 
A. Polymers 
Copolymers of ethylene and propylene containing from about 62 to about 80 
weight percent of ethylene, preferably 65 to 76 weight percent of ethylene 
and possessing a crystalline content in the range of about 10 to about 25 
weight percent; and terpolymers of ethylene, propylene and a C.sub.6 to 
C.sub.10 non-conjugated diolefin containing from about 72 to about 80 
weight percent of ethylene and a crystalline content in the range of about 
15 to about 25 weight percent, having a range of molecular weights and 
Mooney viscosities suitable for the practice of this invention may be 
readily prepared using soluble Ziegler-Natta catalyst combinations well 
known in the art. 
Suitable copolymers have a Mooney Viscosity, ML(1+8) at 127.degree. C. in 
the range of about 10 to about 40, preferably 13 to 27. 
Suitable terpolymers have from about 0.5 to 5 weight percent of a C.sub.6 
to C.sub.10 non-conjugated diolefin, nonlimiting examples of which 
include: 5-ethylidene-2-norbornene, 1,4-hexadiene, and dicyclopentadiene. 
These terpolymers have Mooney viscosities, ML(1+8) at 127.degree. C. in 
the range of 10 to 40. 
Ethylene content of the polymers may be readily determined by the method of 
Gardner, Cozewith and VerStrate: Rubber Chem. & Tech. 44, 1015 (1971). 
Crystallinity of the polymers may be determined by the method of VerStrate 
& Wilchinsky: J. Polymer Sci. A-2, 9, 127 (1971). 
Low-density polyethylene having a density of 0.93 g/cm.sup.3 or less and a 
melt index in the range of about 2 to about 40, preferably 12 to 20 is 
preferred for mixing with the polymers comprising ethylene and propylene. 
All of the above polymers are produced commercially and are available in 
tonnage quantities. 
B. Carbon Black 
Carbon blacks suitable for the practice of this invention include medium to 
high structure blacks which not only add reinforcement to the cross-linked 
structure but when used in an amount equal to about 20 to 40 weight 
percent based on the total composition yield a cross-linked structure 
having adequate electrical conductivity for painting by electrostatic 
means. 
The carbon blacks may be further defined as those having a nitrogen surface 
area of about 30 to 100 square meters per gram and a DBP absorption (ASTM 
D-2414) of about 60-125. Specific examples are the ASTM D-2516 grades of 
carbon black such as N-326, N-330, N-339, N-347, N-351, N-440, N-539, 
N-550, N-660, N-650, N-762 and N-765. 
C. Polyfunctional Vinylic and Allylic Monomers 
Polyfunctional vinyl and allylic monomers have been found to be a critical 
ingredient in the compositions of the present invention. The presence of 
these monomers in the composition is essential to provide curing of the 
composition so that the injection molded article prepared in accordance 
with the present invention passes the Heat Sag test which is an indication 
of the extent of crosslinking that has been achieved. 
The monomers useful in the present invention are those polyfunctional 
vinylic and allylic monomers containing two or more polymerizable groups, 
at least one of which is a vinyl or allyl functional group. 
Illustrations of such vinylic and allylic monomers useful in the present 
invention are polyfunctional monomers containing two or more vinyl groups 
such as divinylbenzene, trivinylbenzene, 2,3-divinylpyridine, divinyl 
sulfone and 2,5-divinyl-6-methylpyridine, polyfunctional acrylate monomers 
such as ethylene glycol dimethacrylate, trimethylol propane 
trimethacrylate, 1,2-propanediol dimethacrylate, polyfunctional allyl 
monomers such as diallyl cyanurate, triallyl cyanurate, diallyl maleate, 
diallyl phthalate. 
Particularly preferred monomers for use in the present invention are 
ethylene glycol dimethacrylate, trimethylol propane trimethacrylate, 
divinylbenzene and triallyl cyanurate, with the latter being especially 
effective. 
D. Peroxides 
The choice of the peroxide is critical to both the physical properties and 
the paintability of structures molded from the composition. In order for a 
particular peroxide to be suitable for the practice of this invention it 
must not undergo appreciable decomposition with attendant free-radical 
formation at the temperature at which it is blended into the composition 
and at the temperature at which the composition is injection molded, but 
the decomposition end-products of the peroxide must be compatible with the 
crosslinked elastomeric structure. Preferably, the peroxide should 
decompose at the lowest possible temperature above the flux temperature of 
the compound. 
Acceptable peroxides for use in the present invention are defined in terms 
of half-life at two temperature ranges. It has been determined in 
accordance with the present invention that the peroxide must have a 
half-life, when measured in low density polyethylene, which is greater 
than 5 hours at 130.degree. C. and less than 1 minute at 230.degree. C. An 
organic peroxide curing agent which exhibits this half-life will provide 
the proper balance of inhibition of curing during processing temperature 
and the desirable crosslinking promotion under injection molding 
conditions and produce articles which do not exhibit "bloom." 
2,5-dimethyl-2,5-di(t-butyl peroxy)hexane: 
##STR2## 
meets the half-life requirements noted hereinabove, has the required 
stability at blending temperatures and effects a cure of the molded 
structure at a temperature and time which is acceptable under current 
production schedules and can produce bloom-free articles and therefore 
represents a preferred peroxide curing agent. Its precise half-life 
measurements are reported in Table I. 
It has been common practice to rate peroxides in terms of half-life time 
(50% decomposition) at a particular temperature. Nearly all of the data 
reported in the literature have been based on determinations made in 
solution in benzene with results which differ materially when the data are 
obtained for example for the case where the peroxide has been blended with 
a thermoplastic such as low-density polyethylene (LDPE). 
Table I gives the results obtained with "Luperco 101-XL," a commercial 
grade of 2,5-dimethyl-2,5-di(t-butyl peroxy) hexane containing 45% active 
ingredient, the 55% inactive portion being an inert silica support, when 
tested in benzene and LDPE. 
TABLE I 
______________________________________ 
HALF-LIFE TIME TEMPERATURE OF LUPERCO 101-XL 
Temperature for 50% Decomposition 
Medium 1 Minute 10 Hours 
______________________________________ 
In Benzene, .degree.C. 
175.degree. 119.degree. 
In LDPE, .degree.C. 
192.degree. 131.degree. 
______________________________________ 
Peroxides such as ter-butyl peroxypivalate, dicumyl peroxide and 
2,4-dichlorobenzoyl peroxide have been evaluated but have been found not 
to have the requisite curing characteristics to produce products in 
accordance with the present invention. 
E. Process 
In accordance with the present invention, the process for producing an 
injection molded composition, which, when crosslinked, meets the 
requirements for automotive fascia, namely: process rheology, physical 
properties and amenability to electrostatic painting comprises: (a) mixing 
the copolymer or terpolymer interpolymer comprising ethylene and propylene 
with the low-density polyethylene and carbon in an internal mixer, such as 
a Banbury mixer at a temperature above the crystalline melting point of 
the polyethylene; (b) cooling the mixture to a temperature below 
130.degree. C.; (c) adding the peroxide and the polyfunctional vinylic or 
allylic co-curing agent and thoroughly mixing and fluxing the composition 
while maintaining the temperature below about 130.degree. C., the 
composition being flowable below 130.degree. C.; (d) introducing the mixed 
and fluxed composition into the mold of an injection molding machine; (e) 
curing said composition to the crosslinked, thermoset state in the mold of 
said injection molding machine at a temperature of about 175.degree. C. to 
about 230.degree. C. for about 1 minute to about 10 minutes; and (f) 
obtaining from said mold a cured thermoset composition having a flexural 
modulus of about 20,000 to 30,000 psi at room temperature. 
A further embodiment of the present invention are the thermoset products 
prepared in accordance with the foregoing process. 
F. Properties 
Compositions suitable for the production of automotive fascia by injection 
molding techniques must possess a rheology which will permit the 
fabrication of structures which may be as large as 170 cm by 80 cm by 1 
cm. When attempts are made to injection mold elastomers, very high 
pressures must be used, as contrasted to thermoplastics, since as a 
general rule elastomers have a much higher viscosity than thermoplastics 
at the same temperature. The difficulties in the use of elastomers for the 
production of fascia are made more severe since fascia structures are 
required to have high flexural modulus. To achieve high flexural modulus 
with most elastomers usually requires that the elastomers be compounded 
with large amounts of reinforcing fillers. The addition of fillers 
increases the viscosity of the compounded elastomer so that the use of 
injection molding for fabrication requires impractically high injection 
pressures. Use of fillers which do not appreciably increase the viscosity 
of the compounded stock, yields structures which do not meet the required 
physical properties. Attempts to obtain the necessary stiffness by the 
incorporation of a substantial quantity of glass fiber has not been too 
satisfactory since the molded parts usually show the flow pattern of the 
glass fiber on its surface and the part requires extensive sanding and 
buffing before painting. Also, glass fibers can adversely affect the mold 
itself by causing abrasion of the mold surface. 
A major object of this invention is the production of an elastomeric 
composition, and a process for its preparation, which possesses a rheology 
suitable for the fabrication of automotive fascia by injection molding and 
after crosslinking has a flexural modulus in the range of about 20,000 to 
30,000 psi at room temperature and a conductivity suitable for painting by 
electrostatic means. 
We have now found that homogeneous blends of elastomeric polymers 
comprising ethylene and propylene; low-density polyethylene, carbon black, 
a multi-functional vinylic or allylic monomer; and a peroxide possess a 
viscosity which permits the compound to be injection molded through small 
orifices into a mold cavity at reasonable temperatures and pressures, and 
when crosslinked by the application of heat yields smooth structures 
requiring no prefinishing before painting, possesses adequate flexural 
strength and is readily painted by electrostatic means. 
The choice of the olefinic homopolymer thermoplastic that is blended with 
the ethylene-propylene copolymer or terpolymer is critical. The 
homopolymer must be compatible with the elastomer while at the same time 
flux at a temperature below about 130.degree. C. which is the maximum safe 
processing temperature that can be maintained in the barrel of the 
injection molding machine. Both high density polyethylene and 
polypropylene are not suitable since both require higher processing 
temperatures which could cause serious scorching problems in the barrel. 
When scorching occurs, the compound undergoes a significant increase in 
viscosity and loses its ability to flow through the mold. In addition, 
polypropylene, unlike low-density polyethylene, undergoes chain scission 
in the presence of peroxides, rather than forming crosslinks. Other 
non-olefinic thermoplastics do not have sufficient compatibility with EPM 
or EPDM elastomers and are therefore not suitable for blending. 
While the physical properties desired in automotive fascia have not been 
finalized by the manufacturers, the best estimate of the property 
requirements from published information is as follows: 
TABLE II 
______________________________________ 
PHYSICAL PROPERTIES OF AUTOMOTIVE FASCIA 
Property Requirement 
______________________________________ 
Tensile at Failure, psi (ASTM D-638) 
1,500 Minimum 
Ultimate Elongation, % (ASTM D-638) 
150 Minimum 
Tear Strength, ppi (ASTM D-624) 
300 Minimum 
Flexural Modulus, psi (ASTM D-790) 
at -28.degree. C. 100,000 Maximum 
at 23.degree. C. 20-30,000 
at 70.degree. C. 7,000 Minimum 
Flexural Set, Chevrolet CTZ-ZZOO3.sup.a 
15 Maximum 
Degree after 5 minutes 
Heat Sag, Chevrolet CTZ-ZZOO6.sup.b 
4 Maximum 
Cm at 121.degree. C. 
______________________________________ 
Notes: 
.sup.a The Chevrolet Flexural Recovery of Elastomeric Materials Test 
CTZZZOO3 measures the ability of an elastomeric material to recover after 
being bent 180 degrees around a 0.50" mandrel at room temperature. Good 
recovery of fascia structures after impact is essential. An injection 
molded sample measuring 5" .times. 1/2" .times. 1/8" is bent 180 degrees 
and the angle of recovery measured after 5 minutes. A specimen that 
returns to its original position has a flexural set of 0 degrees, while a 
specimen that recovers only halfway has a flexural set of 90 degrees. 
.sup.b The Chevrolet High Temperature Sag of Elastomeric Materials Test 
measures the sag of an injection molded specimen measuring 6" .times. 1" 
.times. 1/8" clamped with a 4 inch overhang and heated at a specified 
temperature in a circulating hotair oven for 1 hour. 
This invention will be further understood by reference to the following 
examples which include but are non-limiting to preferred embodiments of 
the instant invention. Parts reported are by weight. 
EXAMPLE 1 
Fifty parts of an ethylene-propylene and copolymer which comprised 65 
percent by weight of ethylene, had a crystalline content of 11.5 weight 
percent, a Mn of 35,000 and a Mooney Viscosity, ML (1+8) of 27.degree. at 
127.degree. C. was masterbatched in a Banbury mixer at 180.degree. C. for 
5 minutes with 50 parts of a low-density polyethylene having a melt index 
of 21 and 50 parts of a general purpose furnace black N-660 and 0.2 parts 
of zinc stearate as a lubricant. The mixture was cooled and fluxed at a 
temperature of about 100.degree. C. with 5 parts of a 45 percent active 
2,5-dimethyl-2,5-di(t-butyl peroxy) hexane and 2 parts of a 75 percent 
active triallyl cyanurate. 
Using the above compound, test specimens were injection molded in an 
injection molding machine equipped with a reciprocating screw, a 5 ounce 
capacity and a 100 ton clamp. Conditions during molding were as follows: 
______________________________________ 
Cylinder Temperature: 
Rear 90.degree. C. 
Center 100.degree. C. 
Front 110.degree. C. 
Nozzle Temperature 110.degree. C. 
Mold Temperature 205.degree. C. 
______________________________________ 
The injection molded specimens were retained in the mold for 105 seconds 
following the termination of the injection in order to effect crosslinking 
or vulcanization. The physical properties of the vulcanized compound were 
as follows: 
TABLE III 
______________________________________ 
PROPERTIES OF VULCANIZED COMPOSITION 
______________________________________ 
Shore D Hardness 
Initial 49 
15 Seconds Reading 43 
Tensile Strength, psi 
2,630 
Ultimate Elongation, % 
230 
Tear Strength, Die C, ppi 
410 
5 Minutes Flexural Set, Degrees 
12 
Droop at 121.degree. C. cm 
3.5 
Secant Flexural Modulus, psi 
20,000 
______________________________________ 
EXAMPLE 2 
The procedure of Example 1 was repeated except that the 50 parts of the EPM 
copolymer was replaced with 55 parts of an EPM copolymer containing 76 
weight percent of ethylene having a Mooney Viscosity ML (1+8) of 13 at 
127.degree. C. The physical properties of the molded composition after 
curing for 5 minutes at 180.degree. C. were as follows: 
TABLE IV 
______________________________________ 
PROPERTIES OF VULCANIZED COMPOUND 
______________________________________ 
Shore D Hardness 
Initial 48 
15 Seconds Reading 42 
Tensile Strength, psi 3,000 
Ultimate Elongation, % 
280 
Tear Strength, Die C, ppi 
400 
5 Minutes Flexural Set, Degrees 
14 
Droop at 121.degree. C. cm 
3.5 
Secant Flexural Modulus, psi 
at -29.degree. C. 100,000 
at 23.degree. C. 28,000 
at 70.degree. C. 8,000 
______________________________________ 
EXAMPLE 3 
The procedure of Example 1 was repeated with a composition which had the 
following proportions in parts by weight: 
______________________________________ 
EPM.sup.a 80 
Low Density PE (Melt Index-21) 
50 
N-660 Carbon Black 60 
Zinc Stearate 0.2 
Triallyl Cyanurate (TAC) 75% Active 
2 
Luperco 101-XL Peroxide (45% Active) 
6.5 
______________________________________ 
.sup.a Ethylenepropylene copolymer, 76 wt. % ethylene. 
Mooney Viscosity, ML (1+8) of 13 at 127.degree. C. Physical properties of 
test specimens molded from the above composition and cured for 5 minutes 
at 180.degree. C. were as follows: 
TABLE V 
______________________________________ 
PROPERTIES OF VULCANIZED COMPOUND 
______________________________________ 
Shore D Hardness 
Initial 47 
15 Seconds Reading 42 
Tensile Strength, psi 3,100 
Ultimate Elongation, % 
310 
Tear Strength, Die C, ppi 
420 
5 Minutes Flexural Set, Degrees 
12 
Droop at 121.degree. C. cm 
3.0 
Secant Flexural Modulus, psi 
at -29.degree. C. 100,000 
at 23.degree. C. 27,000 
at 70.degree. C. 7,000 
______________________________________ 
EXAMPLE 4 
Example 3 was repeated with the same polymers but with the following 
proportions: 
______________________________________ 
EPM 50 
Low Density PE 50 
N-660 40 
Zinc Stearate 0.2 
TAC (75%) 2.0 
Luperco 101-XL (45%) 5.0 
______________________________________ 
Physical properties after molding and curing for 5 minutes at 5 minutes at 
180.degree. C. were as follows: 
TABLE VI 
______________________________________ 
PROPERTIES OF VULCANIZED COMPOSITION 
______________________________________ 
Shore D Hardness 
Initial 49 
15 Seconds Reading 43 
Tensile Strength, psi 3,000 
Ultimate Elongation 310 
Tear Strength, Die C, ppi 
400 
5 Minutes Flexural Set, Degrees 
13 
Droop at 121.degree. C. cm 
4.0 
Secant Flexural Modulus, psi 
at -29.degree. C. 100,000 
at 23.degree. C. 25,000 
at 70.degree. C. 7,000 
______________________________________ 
All of the test specimens molded from the above compositions had smooth 
surfaces free from bloom and had conductivities in the range of 10.sup.3 
to 10.sup.8 ohm-cm. The results also show that the compositions meet the 
current criteria for automotive fascia.