Crosslinked EPDM/thermoplastic Elastomer Blend

Blending of EPDM with unsaturated rubbers has been practiced for several years to improve ozone resistance of the rubber. However, such blends are sensitive to recipe and processing conditions, with the result that in many instances the resulting blends still have relatively poor ozone resistance.

It is an object of this invention to provide an improved thermoplastic elastomer composition;

It is yet a further object of this invention to provide an EPDM/thermoplastic elastomer blend having improved stability; and

It is yet a further object of this invention to provide an EPDM/thermoplastic elastomer blend which is stable over a wide range of processing conditions.

In accordance with this invention, a crosslinked EPDM polymer/thermoplastic elastomer blend is provided.

Surprisingly, it has been found that crosslinking an EPDM polymer prior to blending with a thermoplastic elastomer results in a composition which is more stable than that obtained with uncrosslinked EPDM polymer.

The major component of the blends of this invention is a thermoplastic elastomer. A thermoplastic elastomer is a composition which exhibits thermoplastic and elastomeric behavior. Such compositions are generally linear or branched teleblock copolymers of conjugated dienes and monovinyl-substituted aromatic compounds. Because of the diene component, these materials are elastomeric. However, because of the terminal resinous blocks, the materials exhibit these elastomeric qualities without chemical crosslinking and can be remelted in the same manner as ordinary thermoplastic resinous compositions. Because they possess good green tensile strength, they do not have to be vulcanized, and thus remain thermoplastic and can be molded into useful rubbery objects by conventional means or used in the thermoplastics art. Scraps of such materials such as flashing, sprues, etc., can be subsequently remolded if desired to greatly reduce waste and cost. However, as noted hereinabove, such materials, like other rubber materials, are known to be susceptible to attack by ozone, resulting in embrittlement and cracking of the molded objects.

The compositions of this invention exhibit desirable thermoplastic and elastomeric properties characteristic of the above-described well known polymers without the known disadvantages of severe degradation by ozone.

The conjugated dienes useful in the preparation of the linear and branched teleblock copolymers of this invention are generally those containing 4 to 8 carbon atoms per molecule and preferably those containing 4 or 5 carbon atoms per molecule. Specific examples of useful conjugated dienes include 1,3-butadiene, isoprene, 1,3-pentadiene, 1,3-hexadiene, 1,3-octadiene and the like.

Monovinyl-substituted aromatic compounds which are employed with the above-described conjugated dienes in the linear and branched teleblock copolymer of this invention include those containing 8 to 12 and preferably 8 or 9 carbon atoms per molecule. Examples of specific monovinyl-substituted aromatic compounds include styrene, .alpha.-methylstyrene, p-vinyltoluene, p-t-butylstyrene, and the like.

The linear and branched teleblock copolymers of this invention can be prepared by techniques well known in the art. Such techniques are disclosed in U.S. Pat. Nos. 3,251,905, 3,281,383 and 3,639,521, the disclosures of which are hereby incorporated by reference. Typical of such techniques are the sequential polymerization of monomer increments initiated by organomonolithium compounds followed by optional coupling with polyfunctional coupling agents and the sequential polymerization of monomer increments initiated by organopolylithium compounds.

The teleblock copolymers of this invention correspond to the formulae I or II ##STR1## wherein A represents a block of polymerized monovinyl-substituted aromatic units, B represents a block of polymerized conjugated diene units, Y is a residual unit from a polyfunctional coupling agent or a polyfunctional initiating species and n has a value from 2 to 6.

The useful teleblock copolymers will generally contain from 55 to 95, and preferably from 65 to 85 weight percent polymerized conjugated diene units (block B in formula I or II) with the balance being polymerized monovinyl-substituted aromatic units (block A in formulae I or II). Weight average molecular weights of the useful copolymers are generally in the range of 50,000 to 750,000, and preferably in the range of 100,000 to 400,000.

The EPDM terpolymers suitable for lightly crosslinking and blending with the above-described teleblock copolymers to provide the inventive ozone-resistant compositions are well known in the art as rubbery, vulcanizable terpolymers of ethylene, propylene and a nonconjugated diene monomer. The molar ratio of ethylene to propylene will generally be in the range of about 95:5 to about 55:45, and preferably in the range of about 90:10 to about 60:40. The diene comonomer will generally be present in an amount in the range of 0.5 to about 10, preferably, in the range of about 0.75 to about 5, and more preferably in the range of about 1 to about 2.5 mol percent, based on the total mols of propylene and ethylene. The diene comonomer can be any monomer which has two nonconjugated olefinic double bonds and is copolymerizable with ethylene and propylene. The hydrocarbyl diolefins having 5 to 12 carbon atoms are presently preferred. Examples of suitable diene comonomers include dicyclopentadiene, 1,4-hexadiene, norbornadiene, 5-methylene-2-norbornene, 5-ethylidene-2-norbornene, 1,5-cyclooctadiene, and the like, and mixtures thereof. The terpolymer can be produced by any technique known to the art.

The above-described EPDM terpolymers can be lightly crosslinked by mixing in the presence of well known free radical sources at temperatures sufficient to induce decomposition of said free radical sources into free radicals, for instance. Many such sources are commercially available under a wide variety of tradenames. Specific examples of several commercially available free radical sources are dicumyl peroxide, di-t-butyl peroxide, t-butyl hydroperoxide, 2,5-bis(t-butylperoxy)-2,5-dimethylhexane, mixtures thereof, and the like. It is also within the scope of this invention to employ appropriate azo compounds as radical sources.

The above-described free radical sources can be mixed with the above-described EPDM terpolymers in any mixers which are well known and widely employed in the rubber art. The temperature of said mixing is selected according to the decomposition temperatures of the free radical sources and convenient processing temperatures for the EPDM. This mixing is generally conducted at temperatures in the range of 100.degree. to 250.degree. C. for times which are dependent upon the decomposition half-life of the free radical sources at the temperatures employed. It is desirable to continue the mixing operation until substantially all, e.g., at least 95%, of the radical source has decomposed. Such mixing times are generally in the range of one minute to one hour.

Though the above described hot-mixing operation for crosslinking the EPDM terpolymer is preferred, it is also within the scope of this invention to employ other methods of crosslinking such as treating a solution of EPDM in any appropriate solvent with a free radical source and elevating the temperature of the stirred solution to a temperature at which the free radical source decomposes and, after a suitable reaction period, isolating the lightly crosslinked EPDM by coagulation or stripping of solvent. It may also be desirable to initiate the decomposition of the free radical source photolytically using any appropriate source of radiation.

The desired degree of crosslinking to be imparted to the EPDM rubber during the above-described mixing operation can best be defined in terms of the increase in Mooney viscosity (e.g., ML-4 measured at 115.6.degree. C. according to ASTM Procedure D-1646-63) of the polymer measured after the mixing operation compared to the original EPDM terpolymer before the mixing operation. In order to be useful in this invention the crosslinked EPDM terpolymer must exhibit a Mooney viscosity at least two Mooney points above that of a comparable EPDM compound mixed in the absence of free radical source. The preferred degree of crosslinking will result in a crosslinked polymer exhibiting a Mooney viscosity of less than 185 ML-4 (115.6.degree. C.) and at least 15 Mooney points above that of the uncrosslinked EPDM containing the same amount of compounding ingredients and hot mixed in the same manner.

The above-described linear and branched teleblock copolymers and the above-described crosslinked EPDM terpolymers are mixed using any appropriate means known in the art, such as solution blending or hot-mixing.

The compositions of this invention contain from 10 to 40 percent by weight of crosslinked EPDM and from 60 to 90 percent by weight teleblock copolymer, i.e., 10 to 40 weight percent based on the total weight of the EPDM polymer and the thermoplastic elastomer neglecting other ingredients which may be present. It is preferred, however, to employ 20 to 30 weight percent crosslinked EPDM.

It may be desirable to employ other ingredients well known in the rubber art in the compositions of this invention depending upon the intended application for the composition. Such ingredients include fillers, reinforcing agents, plasticizers, extenders, pigments, stabilizers, processing aids and the like.

The compositions of this invention possess high green tensile strength and ozone resistance which make them suitable for many uses without vulcanization. They have applications as molded articles such as shoe soles, toys, containers, pipes. They can also be employed in the manufacture of tires, gaskets and many other rubbery type articles. They can also be employed in pressure sensitive adhesives, as well as in sealing and caulking compositions.

The following inventive Run 1 and comparative Runs 2 through 6 illustrate the usefulness of this invention in preparing a composition based on a radial (branched) teleblock copolymer of butadiene and styrene and containing a lightly crosslinked EPDM terpolymer which composition exhibits desirable physical properties and ozone resistance.

The inventive and comparative runs of this example were based on the following recipe:

The compositions prepared in Runs 1 to 6 were prepared as follows:

EPDM, peroxide and Carbowax were combined in a Midget Banbury mixer and mixed for 5 minutes at 160.degree.-182.degree. C., then sheeted on a roll mill. The resulting lightly crosslinked EPDM was subsequently combined with the butadiene/styrene copolymer and other ingredients in a Midget Banbury (total mixing time--4.5 min., dump temperature--166.degree. C.). The resulting mixture was sheeted on a roll mill.

All ingredients (peroxide was not included in this run) were combined in a Midget Banbury (total mixing time--4.5 min., dump temperature--155.degree. C.). The composition was sheeted on a roll mill.

EPDM was heated and mixed in a Midget Banbury to 121.degree. C. Peroxide and Carbowax were added and mixing continued for 45 sec. after which the uncrosslinked EPDM mixture was sheeted on a roll mill. The resulting EPDM mixture was combined with the other ingredients in a Midget Banbury (total mixing time--5.5 minutes, dump temperature--166.degree. C.). The composition was sheeted on a roll mill.

Butadiene/styrene copolymer, EPDM, peroxide and Carbowax were combined in a Midget Banbury, mixed for 5 minutes at 160.degree.-182.degree. C. and then sheeted on a roll mill. The resulting crosslinked composition was combined with the other ingredients in a Midget Banbury (total mixing time 3.5 min., dump temperature--149.degree. C.). The composition was sheeted on a roll mill.

All ingredients were combined in a Midget Banbury (total mixing time--3.75 min., dump temperature--160.degree. C.). The composition was sheeted on a roll mill.

All ingredients except peroxide were combined in a Midget Banbury. After homogenization of ingredients, peroxide was added (total mixing time--4 min., dump temperature--155.degree. C.). The composition was sheeted on a roll mill.

The above-described compositions were molded at 171.degree. C. into test specimens, the properties of which are recorded in Table I.

The data in Table I demonstrate the improvement in ozone resistance of the inventive Run 1 compared to comparative Run 2 (which is typical of the properties obtained in the prior art using EPDM). Comparative Runs 3-6 show that a good balance of properties (high tensile and good ozone resistance) is dependent on using the sequence of this invention, that is, crosslinking the EPDM and then blending it with the thermoplastic elastomer. Comparative Run 4 which exhibited the same ozone resistance as inventive Run 1 was markedly deficient in tensile strength. Compositions prepared in Runs 3 and 5 would have very limited utility due to the observed curing during testing which would seriously affect processibility of the composition. Thus comparative Run 2 shows the poorer results obtained without a crosslinking agent and comparative Runs 3-6 show that the crosslinking which is not carried out in accordance with this invention is not effective.

The following runs illustrate the preparation of lightly crosslinked EPDM polymers useful in this invention and the amount of crosslinking (as measured by increase in Mooney viscosity over that of the original EPDM) by particular amounts of peroxide.

Blends of 100 parts by weight EPDM (see Example I, Recipe, footnote 2), 2 parts by weight Carbowax 4000 and variable amounts of peroxide (see Example I, Recipe, footnote 3) were mixed in a Midget Banbury for 5 minutes at 160.degree.-182.degree. C. then sheeted on a roll mill. Mooney viscosities (ASTM D-1646-63) were measured at 115.6.degree. C. (240.degree. F.). Results are tabulated in Table II.

The data in Table II shows that lightly crosslinked EPDM useful in the blends of this invention were obtained by hot-mixing EPDM in the presence of from 0.2 to 0.8 phr commercial crosslinking agent (50% by weight active material), i.e., 0.1 to 0.4 actual peroxide.

While this invention has been described in detail for the purpose of illustration, it is not to be construed as limited thereby but is intended to cover all changes and modifications within the spirit and scope thereof .