HYDROSILYLATION CURED THERMOPLASTIC ELASTOMERS

Abstract: A light-stabilized thermoplastic elastomer comprising a blend of thermoplastic resin and unsaturated rubber, which rubber has been dynamically vulcanized by hydrosilylation in the presence of a hindered amine light stabilizer compound which is free of sterically unhindered amine functionality.

Description: HYDROSILYLATION CURED THERMOPLASTIC ELASTOMERS BACKGROUND OF THE INVENTION Field of the Invention This invention relates to thermoplastic elastomer compositions prepared using hydrosilylation crosslinking of the elastomer component of the composition. A thermoplastic elastomer is generally defined as a polymer or blend of polymers that can be processed and recycled in the same way as a conventional thermoplastic material, yet has properties and functional performance similar to that of vulcanized rubber at service temperatures. Blends or alloys of plastic and elastomeric rubber have become increasingly important in the production of high performance thermoplastic elastomers, particularly for the replacement of thermoset rubbers in various applications. High performance thermoplastic elastomers in which a highly vulcanized rubbery polymer is intimately dispersed in a thermoplastic matrix are generally known as thermoplastic vulcanizates. Description of the Related Art Polymer blends which have a combination of both thermoplastic and elastic properties are generally obtained by combining a thermoplastic resin with an elastomeric composition in a way such that the elastomer component is intimately and uniformly dispersed as a discrete particulate phase within a continuous phase of the thermoplastic. Early work with vulcanized rubber components is found in U.S. Pat. No. 3,037.954 which discloses both static vulcanization of the rubber, as well as the technique of dynamic vulcanization wherein a vulcanizable elastomer is dispersed into a molten resinous thermoplastic polymer and the elastomer is cured while continuously mixing and shearing the blend. The resulting composition is a micro-gel dispersion of cured elastomer in an uncured matrix of thermoplastic polymer. In U.S. Pat. No. Re. 32,028 polymer blends comprising an olefin thermoplastic resin and an olefin copolymer are described, wherein the rubber is dynamically vulcanized to a state of partial cure. The resulting compositions are reprocessible. U.S. Pat. Nos. 4,130,534 and 4,130,535 further disclose thermoplastic vulcanizates comprising butyl rubber and polyolefin resin, and olefin rubber and polyolefin resin, respectively. The compositions are prepared by dynamic vulcanization and the rubber component is cured to the extent that it is essentially insoluble in conventional solvents. A range of crosslinking. or curing, agents for the vulcanization of the rubber are described in the early art. including peroxides, sulfurs. phenolic resins, radiation, and the like. U.S. Pat. No. 4,803.244 generally discusses the use of multifunctional organosilicon compounds in conjunction with a catalyst as an agent for crosslinking the rubber component of a thermoplastic elastomer by hydrosilylation. Hydrosilylation involves the addition of a silicon hydride across a multiple bond, often with a transition metal catalyst. This patent describes a rhodium catalyzed hydrosilylation of EPDM rubber in a blend with polypropylene to produce thermoplastic elastomers having a gel content of up to 34% (after correction for the plastic phase). This degree of vulcanization was achieved cύy with a high level of catalyst. A further modification of hydrosilylation crosslinking of the rubber in a thermoplastic elastomer composition is disclosed in European Patent Application No. 651 ,009. A compatibilizing agent containing in the same molecule a component having an affinity for the rubber and a component having an affinity for the thermoplastic resin is incorporated into the composition, and is said to improve adhesion between the rubber and resin in order to prevent agglomeration. U.S. Pat. No. 5.672,660 discloses the preparation of thermoplastic elastomers using hydrosilylation crosslinking of the rubber component, wherein very low amounts of platinum catalyst are used in conjunction with specific diene containing rubbers, and further discloses the desirability of conducting the reaction in a medium which is free of materials with Lewis base behavior. International (PCT) Application WO 96/24632 describes the preparation of thermoplastic elastomers prepared using a phenolic curative, and stabilized with a hydrolysis-insensitive HALS compound. SUMMARY OF THE INVENTION The present invention is based on the discovery that selected hindered amine light stabilizers (HALS) can be incorporated into a one pass dynamic vulcanization process, using a platinum-catalyzed hydrosilylation cure, to prepare a thermoplastic elastomer from a blend of thermoplastic resin and unsaturated rubber. The resulting thermoplastic elastomers can be fully or partially cured, with desirable tensile and elasticity properties as well as improved resistance to degradation by ultraviolet (UV) light. The preferred structure of HALS compounds for use in the invention is one in which the sterically unhindered amine functionality is minimized. The compositions of the present invention have utility as replacements for thermoset rubber compounds in a variety of applications, particularly where molding or extrusion is involved and the combination of thermoplastic and elastomeric properties, as well as UV stability, provide an advantage. Typical uses include molded articles for automobile underhood o parts, engineering and construction materials, mechanical rubber goods, industrial parts such as hose, tubing and gaskets, electrical applications and household goods. DESCRIPTION OF THE PREFERRED EMBODIMENTS Thermoplastic elastomer compositions may generally be prepared by blending a thermoplastic resin and a rubber, then melting the thermoplastic component and mixing the melt until the blend is homogeneous. If a composition of vulcanized rubber in a thermoplastic matrix is desired, crosslinking agents (also referred to as curatives or vulcanizing agents) are added to the blend and crosslinking occurs during the mixing. This latter process is described as dynamic vulcanization. A wide range of thermoplastic resins and rubbers and/or their mixtures have been used in the preparation of thermoplastic elastomers, including polypropylene. HDPE. LDPE.VLDPE. LLDPE, cyclic olefin homopolymers or copolymers as well as olefinic block copolymers, polystyrene, polyphenylene sulfide. polyphenylene oxide and ethylene propylene copolymer (EP) thermoplastics, with rubbers such as ethylene propylene diene (EPDM), butyl, halobutyl, acrylonitrile butadiene (NBR), styrene butadiene (SBR) and natural (NR) as the elastomers. Hydrosilylation Agents Hydrosilylation has been disclosed as a crosslinking method. In this method a silicon hydride having at least two SiH groups in the molecule is reacted with the carbon-carbon multiple bonds of the unsaturated (i.e. containing at least one carbon-carbon double bond) rubber component of the thermoplastic elastomer, in the presence of the thermoplastic resin and a hydrosilylation catalyst. Silicon hydrides useful in the process of the invention include methylhydrogen polysiloxanes, methylhydrogen dimethyl-siloxane copolymers. alkylated methyl hydrogen polysiloxanes, bis(dimefhylsilyl)alkanes and bis(dimethylsilyl) benzene. The amount of silicon hydride compound useful in the process of the present invention can range from about 0.1 to about 10.0 mole equivalents of SiH per carbon-carbon double bond in the rubber, and preferably is in the range of about 0.5 to about 5.0 mole equivalents of SiH per carbon-carbon double bond in the rubber component of the thermoplastic elastomer. Thermoplastic Resins Thermoplastic resins useful in the compositions produced by the invention include crystalline polyolefin homopolymers and copolymers. They are desirably prepared from monoolefin monomers having 2 to 20 carbon atoms, such as ethylene, propylene. 1-butene, 1-pentene and the like, as well as copolymers derived from linear and cyclic olefins, with propylene being preferred. As used in the specification and claims the term polypropylene includes homopolymers of propylene as well as reactor copolymers of polypropylene which can contain about 1 to about 20 wt% of ethylene or an olefin comonomer of 4 to 20 carbon atoms, and mixtures thereof. The polypropylene can be crystalline isotactic or syndiotactic. and may be prepared by Ziegler-Natta or metallocene catalysis. Other thermoplastic resins which are substantially inert to the rubber, the silicon hydride and the hydrosilylation catalyst would also be suitable. Blends of thermoplastic resins may also be used. The amount of thermoplastic resin found to provide useful compositions is generally from about 5 to about 90 weight percent, based on the weight of the rubber and resin. Preferably, the thermoplastic resin content will range from about 20 to about 80 percent by weight of the total polymer. Rubbers Unsaturated rubbers useful to prepare thermoplastic elastomers according to the invention include monoolefin copolymer rubbers comprising non-polar, rubbery copolymers of two or more -monoolefins, preferably copolymerized with at least one polyene. usually a diene. However, unsaturated monoolefin rubber such as EPDM rubber is more suitable. EPDM is a polymer of ethylene, propylene and one or more non-conjugated diene or non-conjugated dienes, and the monomer components may be polymerized using Ziegler-Natta or metallocene catalyzed reactions, among others. Satisfactory non-conjugated dienes include 5-ethylidene-2-norbornene (ENB); 1 ,4-hexadiene (HD); 5-methylene-2-norbornene (MNB); 1 ,6-octadiene; 5-methyl-l,4-hexadiene; 3,7-dimethyl-l,6-octadiene; 1.3-cyclopentadiene; 1.4-cyclohexadiene; dicyclopentadiene (DCPD); 5-vinyl-2-norbornene (VNB) and the like, or a combination thereof. Blends of any of the above rubbers may also be employed, rather than a single olefinic rubber. In preparing the compositions of the invention, the amount of rubber generally ranges from about 95 to about 10 weight percent, based on the weight of the rubber and thermoplastic resin. Preferably, the rubber content will be in the range of from about 80 to about 20 weight percent of total polymer. Hydrosilylation Catalysts It has previously been understood that any catalyst, or catalyst precursor capable of generating a catalyst in situ, which will catalyze the hydrosilylation reaction with the carbon-carbon bonds of the rubber can be used. Such catalysts have included transition metals of Group VIII such as palladium, rhodium, platinum and the like, including complexes of these metals. Chloroplatinic acid has been disclosed as a useful catalyst in U.S. Pat. No. 4,803,244 and European Application No. 651.009. which further disclose that the catalyst may be used at concentrations of 5 to 10,000 parts per million by weight and 100 to 200.000 parts per million by weight based on the weight of rubber, respectively. Platinum-containing catalysts which are useful in the process of the invention are described, for example, in U.S. Pat. No. 4.578.497; U.S. Pat. No. 3,220,972; and U.S. Patent No. 2,823,218 all of which are incorporated herein by this reference. These catalysts include chloroplatinic acid, chloroplatinic acid hexahydrate. complexes of chloroplatinic acid with sym-divinyltetramethyldisiloxane. dichloro-bis(triphenylphosphine) platinum (II), cis-dichloro-bis(acetonitrile) platinum (II), dicarbonyldichloroplatinum (II), platinum chloride and platinum oxide. Zero valent platinum metal complexes such as Karstedt's catalyst are particularly preferred, as described in U.S. Pat. No. 3.775.452; U.S. Pat. No. 3,814.730; and U.S. Pat. No. 4,288,345 all of which are incorporated herein by this reference. Additives The thermoplastic elastomer may contain conventional additives, which can be introduced into the composition in the thermoplastic resin, the rubber, or in the blend either before, during or after the hydrosilylation and curing. Examples of such additives are antioxidants, processing aids, reinforcing and nonreinforcing fillers, pigments, waxes, rubber processing oil, extender oils, antiblocking agents, antistatic agents, plasticizers (including esters), foaming agents, flame retardants and other processing aids known to the rubber compounding art. Such additives may comprise from about 0.1 to about 300 percent by weight based on the weight of the final thermoplastic elastomer product. Fillers and extenders which can be utilized include conventional inorganics such as calcium carbonate, clays, silica, talc, titanium dioxide, carbon black and the like. Additives, fillers or other compounds which may interfere with the hydrosilylation should be added after curing reaches the desired level. HALS Hindered amine light stabilizers are regularly compounded into materials requiring improved UV resistance. UV protection is provided by the amine functionality of the stabilizer, which is easily oxidized to form nitroxyl amines. In the case of transition metal catalyzed hydrosilylation, such reactions are sensitive to the presence of Lewis bases. It was thought that these reaction systems should be essentially free of compounds such as amines, sulfides and phosphines. Interference with hydrosilylation reactions by these compounds is believed to be from bonds formed between the non-bonded pairs of electrons donated by the Lewis base and the transition metal center. Since this bond with the metal center is stronger than those that characterize the bonds of "good" ligands. the activity of the catalyst is reduced. However, it has been discovered that careful selection of the HALS to be employed makes it possible to include such stabilizers in the dynamic vulcanization reaction, even when hydrosilylation is used as the crosslinking (curing) process. The preferred HALS structures are those substantially free of sterically unhindered amine functionality. HALS compounds having the following structures were tested. (1) (») (III) (IV) (V) R R N (CH 2 ) 2 -N- -(CH 2 ) 2 -N- "(CH 2 ) 3 -N- H H (VI) (VII) (VIH) I Chimassorb® 944 (Ciba) II Uvinul® 4050H (BASF) III FS042 (Ciba) IV Cyasorb® 3346 (Cytec) V Tinuvin® 770 (Ciba) VI Chimassorb 1 19 (Ciba) VII Tinuvin 123 (Ciba) VIII Cyasorb 3835 (Cytec) Processing The rubber component of the thermoplastic elastomer is generally present as small, i.e. micro-size, particles within a continuous thermoplastic resin matrix, although a co-continuous morphology or a phase inversion is also possibl . depending upon the amount of rubber relative to plastic and the degree of cure of the rubber. The rubber is desirably at least partially crosslinked, and preferably is completely or fully crosslinked. It is preferred that the rubber be crosslinked by the process of dynamic vulcanization. As used in the specification and claims, the term "dynamic vulcanization" means a vulcanization or curing process for a rubber blended with a thermoplastic resin, wherein the rubber is vulcanized under conditions of shear at a temperature at which the mixture will flow. The rubber is thus simultaneously crosslinked and dispersed as fine particles within the thermoplastic resin matrix, although as noted above other morphologies may exist. Dynamic vulcanization is effected by mixing the thermoplastic elastomer components at elevated temperatures in conventional mixing equipment such as roll mills. Banbury mixers, Brabender mixers, continuous mixers, mixing extruders and the like. The unique characteristic of dynamically cured compositions is that, notwithstanding the fact that the rubber component is partially or fully cured, the compositions can be processed and reprocessed by conventional plastic processing techniques such as extrusion, injection molding and compression molding. Scrap or flashing can be salvaged and reprocessed. The terms "fully vulcanized" and "fully cured" or "fully crosslinked" as used in the specification and claims means that the rubber component to be vulcanized has been cured or crosslinked to a state in which the elastomeric properties of the crosslinked rubber are similar to those of the rubber in its conventional vulcanized state, apart from the thermoplastic elastomer composition. The degree of cure can be described in terms of gel content, or conversely, extractable components. Gel content reported as percent gel (based on the weight of crosslinkable rubber) is determined by a procedure which comprises determining the amount of insoluble polymer by soaking the specimen for 48 hours in organic solvent at room temperature, weighing the dried residue and making suitable corrections based upon knowledge of the composition. Thus, corrected initial and final weights are obtained by subtracting from the initial weight the weight of soluble components, other than rubber to be vulcanized, such as extender oils, plasticizers and components of the composition soluble in organic solvent, as well as that rubber component of the product which is not intended to be cured. Any insoluble polyolefins, pigments, fillers, and the like are subtracted from both the initial and final weights. The rubber component can be described as fully cured when less than about 5%. and preferably less than 3%, of the rubber which is capable of being cured by hydrosilylation is extractable from the thermoplastic elastomer product by a solvent for that rubber. Alternatively the degree of cure may be expressed in terms of crosslink density. All of these descriptions are well known in the art, for example in U.S. Pat. Nos. 4.593.062. 5,100,947 and 5,157,081, all of which are fully incorporated herein by this reference. The following general procedure was used in the preparation of thermoplastic elastomers by the process of the invention, as set forth in the examples. The thermoplastic resin and oil extended rubber were placed in a heated internal mixer, with the hydrosilylation agent, hydrosilylation catalyst and HALS compound. The hydrosilylation agent and catalyst can be incorporated into the composition by any suitable technique, for example by injection as solutions in oil or as neat components, although a dilute catalyst solution is preferred. Additives such as antioxidants, ultraviolet stabilizers and fillers may also be added as a slurry in oil. Masterbatches of the components may also be prepared to facilitate the blending process. The mixture was heated to a temperature sufficient to melt the thermoplastic component, and the mixture was masticated, with added processing oil if desired, until a maximum of mixing torque indicated that vulcanization had occurred. Mixing was continued until the desired degree of vulcanization was achieved. The invention will be better understood by reference to the following examples which serve to illustrate but not limit the present process. In the examples, the following test methods were used to determine the properties of the thermoplastic elastomer products. Hardness (Shore A/D) - ASTM D 2240 Ultimate tensile strength (UTS - psi) - ASTM D 412 Ultimate elongation (UE - %) - ASTM D 412 Modulus at 100/300% elongation (M 1 or M3 - psi) - ASTM D412 Tension set (TS - %) - ASTM D 412 Oil swell (OS - %) - ASTM D 471 EXAMPLES Compositions were prepared by the method of the invention as generally described above, using polypropylene resin and EPDM rubber containing 5-vinyl, 2-norbornene as the diene component. The thermoplastic (41 parts) and rubber (100 parts) were melt mixed in a Brabender mixer at 180°C until the polypropylene was melted. Silicone hydride (alkylated methyl hydrogen polysiloxane) [2 phr] was added dropwise to the melt mix, followed by addition of an oil solution containing 0.75 ppm platinum [platinate (II) hexachloro, dihydrogen reaction product with 2,4,6,8-tetraethenyl-2,4,6,8-tetramethyl cyclotetrasiloxane]. The HALS compound was added to the blend neat after silicone hydride addition, in a ratio of 1.5 gram of HALS to 60 grams of plastic/rubber blend. The rubber was dynamically vulcanized by mixing the blend until the maximum torque was reached. The product was removed from the mixer, then returned to the mixer and masticated at 180°C for an additional minute. Plaques were prepared by compression molding the products of the dynamic vulcanization at 200°C to a thickness of 60 mil and cooling under pressure, and the physical properties were determined using these plaques. All of the products were elastomeric, as defined by ASTM D1566, i.e. all had tension set values of less than 50%. The compositions and their properties are set forth in Table I TABLE I iend HALS Hardness UTS (psi) Ml (psi) M3 (psi) UE (%) OS (%) 1 None 58 822 450 232 96 2 I 54 653 264 566 394 168 3 II 51 500 250 410 535 240 4 III+VI 50 430 215 400 400 220 5 IV 57 916 352 770 396 113 6 V 57 1031 428 911 368 100 7 VI 55 940 340 710 460 116 8 VII 59 650 648 168 82 9 VIII 61 822 478 230 96 Shore A hardness 2 125° C for 24 hours It can be seen from the data set forth in Table I that a HALS having no sterically unhindered amine functionality (e.g. structures V and VIII) gives thermoplastic elastomer products with properties essentially the same as the control blend 1 (i.e. without HALS). Compositions prepared using HALS having amine functionality capable of reacting with platinum (e.g. structures I, II and III) have poor properties compared to the control. While the best mode and preferred embodiment of the invention have been set forth in accord with the Patent Statutes, the scope of the invention is not limited thereto, but rather is defined by the attached claims.

Claims: WHAT IS CLAIMED IS: 1. A light-stabilized thermoplastic elastomer composition comprising a blend of (a) about 5 to about 95 percent by weight of a thermoplastic resin, and (b) about 95 to about 5 percent by weight of an unsaturated rubber which has been dynamically vulcanized by hydrosilylation in the presence of said thermoplastic resin and a hindered amine light stabilizer compound which is substantially free of sterically unhindered amine functionality. 2. The composition of claim 1 wherein the thermoplastic resin is selected from the group consisting of ethylene, propylene, olefinic copolymers and mixtures thereof. 3. The composition of claim 1 wherein the unsaturated rubber is EPDM rubber. 4. The composition of claim 1 wherein the dynamic vulcanization is conducted in a single pass through a mixer. 5. The composition of claim 1 wherein the hydrosilylation is conducted using a silicon hydride compound present from about 0.1 to about 10 mole equivalents of silicon hydride per carbon-carbon double bond in the rubber. 6. The composition of claim 1 wherein the hydrosilylation is catalyzed by platinum 7. The composition of claim 1 wherein the rubber is fully cured, and the composition has a tension set of less than about 50%. 8. In a process for the preparation of a light-stabilized thermoplastic elastomer by the crosslinking of an unsaturated rubber using dynamic vulcanization in the presence of a thermoplastic resin and a hydrosilylation agent, the improvement which comprises incorporating into the vulcanization a hindered amine light stabilizer compound which is substantially free of sterically unhindered amine functionality. 9. The process of claim 8 wherein the thermoplastic resin is selected from the group consisting of ethylene, propylene, olefinic copolymers and mixtures thereof, and the unsaturated rubber is EPDM rubber. 10. The process of claim 8 wherein the dynamic vulcanization is conducted in a single pass through a mixer. 11. The process of claim 8 wherein the hydrosilylation is conducted using a silicon hydride compound present from about 0.1 to about 10 mole equivalents of silicon hydride per carbon-carbon double bond in the rubber. 12. The process of claim 8 wherein the hydrosilylation is catalyzed by platinum. 13. The process of claim 8 wherein the rubber is fully cured.

Inventors: GILBERTSON GARY W, MEDSKER ROBERT E, ZHAO JIANQUN, GILBERTSON, GARY, W., MEDSKER, ROBERT, E., ZHAO, JIANQUN
Applicants: ADVANCED ELASTOMER SYSTEMS, GILBERTSON GARY W, MEDSKER ROBERT E, ZHAO JIANQUN, ADVANCED ELASTOMER SYSTEMS, L.P., GILBERTSON, GARY, W., MEDSKER, ROBERT, E., ZHAO, JIANQUN
Application year: 1998
Main classification: C08L83
IPC codes: C08F8, C08L21, C08L23, C08L83, C08L101