High temperature, oil resistant thermoplastic vulcanizates made from polar plastics and acrylate or ethylene-acrylate elastomers

A polar thermoplastic and acrylate or ethylene acrylate based polar rubbers form a useful thermoplastic vulcanizate when the rubber is selectively cured with curatives which do not affect the polar thermoplastic and do not generate volatile organics. Examples of thermoplastics include aromatic polyesters, polycarbonates, poly(phenylene oxide), or combinations thereof. Examples of curatives include compounds with a multifunctional oxazoline, oxazine or imidazoline, which can react with functional groups such as carboxylic acid groups on the rubber.

FIELD OF INVENTION
 The present invention relates to thermoplastic vulcanizates (TPVs)
 containing high melting point thermoplastics such as aromatic/aliphatic
 polyesters and a vulcanized polar rubber interdispersed therein or
 dispersed therein. The use of preferred curatives, e.g. bisoxazolines,
 have the advantage of selectively crosslinking the rubber phase without
 affecting the thermoplastic and without causing the evolution of volatile
 organics.
 BACKGROUND OF THE INVENTION
 Heretofore, crystalline polyolefins such as polyethylene and polypropylene
 when utilized as the thermoplastic component in thermoplastic vulcanizates
 have an upper continuous use temperature below 130.degree. C. due to the
 relatively low melting point of the crystalline thermoplastic phase (e.g.
 commercially available polypropylene has a melting temperature of
 165.degree. C.). Accordingly, these hydrocarbon based compositions have
 limited use in under the hood applications of a vehicle where high
 temperatures are encountered and where low hydrocarbon oil swell is
 desirable. Blends of polar thermoplastics and polar rubbers in
 uncrosslinked form tend to have undesirable high compression set and poor
 mechanical strength. Conventional curatives for the polar rubbers
 generally have undesirable effects on a thermoplastic vulcanizate formed
 from a polyester and an acrylate rubber, such as degrading the
 thermoplastic or the release of undesirable volatile components during the
 curing process.
 SUMMARY OF THE INVENTION
 The thermoplastic vulcanizate compositions of the present invention contain
 a polar thermoplastic and a crosslinked polar rubber. Preferred polar
 thermoplastics include polyesters, polycarbonates, polyphenylene oxide, or
 combinations thereof Preferred rubbers include acrylate (acrylic) rubbers,
 and ethylene-acrylate rubbers with pendant or terminal carboxylic acid or
 carboxylic anhydride functionality. Preferred curatives include
 multi-functional oxazolines and oxazine compounds.
 DETAILED DESCRIPTION
 Polyesters are preferred as the thermoplastic phase of a thermoplastic
 vulcanizate (TPV). The polyesters of the thermoplastic defined herein are
 condensation polymers as contrasted to the acrylate rubbers which are
 formed by addition polymerization. The various polyesters can be either
 aromatic or aliphatic or combinations thereof and are generally directly
 or indirectly derived from the reactions of diols such as glycols
 (including aliphatic, cycloaliphatic and aryl or combinations thereof)
 having a total of from 2 to 10, 12 or 15 carbon atoms and desirably from
 about 2 to about 4 or 6 carbon atoms with either aliphatic acids having a
 total of from about 3 to about 20 carbon atoms and desirably from about 3
 to about 15 or aromatic acids having a total of from about 8 to about 15
 carbon atoms or combinations thereof. The use of aromatic diacids
 generally result in desirable higher softening temperatures. Acids
 generally refer to dicarboxylic acids, their anhydrides, or their dialkyl
 esters. Generally, aromatic polyesters are preferred such as poly(ethylene
 terephthalate) (PET). poly(propylene terephthalate) (PPT),
 poly(trimethylene terephthalate) (PTT), poly(butylene terephthalate)
 (PBT), poly(ethylene isophthalate), poly(butylene naphthalate) and the
 like, as well as an end-capped epoxy derivative thereof e.g., a
 monofunctional epoxy poly(butyleneterephthalate). Desirably at least 70 or
 80, more desirably at least 90 mole percent of the polyester, based on the
 dicarboxylic acid component, is terephthalic acid radicals. Desirably at
 least 70 or 80, more desirably at least 90 mole percent of the polyester,
 based on the diol component, is derived from ethylene glycol,
 1,3-propylene glycol or 1,4-butylene glycol or combinations thereof.
 Trifunctional and higher functionality acids and polyols desirably can be
 in the polyester in small amounts, such as less than 1, 5, or 10 mole
 percent of the acid or polyol component. U.S. Pat. No. 3,692,744 discloses
 such polyesters.
 Various polycarbonates can also be utilized as the thermoplastic and the
 same are esters of carbonic acid and the above diols. A suitable
 polycarbonate is the ester of carbonic acid and bisphenol A, i.e.,
 poly(carbonyldioxy-1,4-phenyleneisopropylidene-1,4-phenylene). The various
 ester polymers can also include polyester block copolymers such as those
 containing at least one block of a polyester with a softening temperatures
 above 100.degree. C. and at least one rubbery block with a softening
 temperature below 100.degree. C. and desirably below 75.degree. C. Such
 rubbery blocks include a polyether derived from at least one glycol having
 from 2 to 6 carbon atoms, e.g., polyethylene glycol, or from at least one
 alkylene oxide having from 2 to 6 carbon atoms. The rubbery block can also
 be another polyester that has a softening point below 100.degree. C. or
 more desirably below 75.degree. C. A preferred block polyester is
 poly(butylene terephthalate)-b-(tetramethylene glycol) which is available
 as Hytrel.TM. from DuPont. A preferred polyester block copolymer described
 in U.S. Pat. No. 4,981,908 is herein incorporated by reference. That block
 copolymer has polyester blocks with a softening temperature above
 100.degree. C. and polyester blocks with a softening temperature of less
 than 100 or less than 75.degree. C. in the weight ratios of from 1:4 to
 1:0.1 respectively. Coupling species including ester linkages and others,
 e.g. urethane, epoxy, etc. The block copolymers desirably have a number
 average molecular weight of at least 5000. The dicarboxylic acids
 desirably have molecular weights of 300 or less for the molecule less the
 carboxylic acid groups. The glycols of the ester desirably have a
 molecular weight of 300 or less for the molecule less the hydroxyl groups.
 The molecular weight of the various polyesters is such that it is a
 suitable engineering plastic. Accordingly, the weight average molecular
 weight of the various polyesters desirably range from about 40,000 to
 about 100,000 with from about 90,000 to about 100,000 being preferred.
 Polyphenylene oxides can be used as part or all of the thermoplastic phase.
 They are commercially available and generally have a molecular weight such
 that they also have a glass transition temperature of at least 150.degree.
 C., desirably at least 1 75.degree. C., and preferably 210.degree. C.
 The acrylic rubbers (acrylate) useful as the rubber phase of the
 thermoplastic vulcanizate are polymerized from monomers comprising alkyl
 acrylates wherein the alkyl portion of the ester has from 1 to 10 or 12
 carbon atoms, with from 1 to 4 carbon atoms being preferred. The total
 carbon atoms of each alkyl acrylate may range from 4 to 13 or 15 carbon
 atoms and include alkyl substituted, e.g. alkyl alkylacrylates such as
 methyl methacrylate in small amounts, i.e., desirably less than 5, 10 or
 15 mole percent. Desirably the monomers include unsaturated mono or
 polycarboxylic acids or anhydrides thereof having from about 2 to about 15
 carbon atoms. Monomers such as methyl methacrylate form thermoplastic
 rather than rubbery polymers when present in high amounts. Specific
 examples of rubbery acrylic polymers include polymers of methyl acrylate,
 butyl acrylate, butyl acrylate, ethylhexyl acrylate, and the like. The
 acrylic polymers generally include repeat units with pendant or terminal
 functionality (e.g., pendant carboxylic groups to facilitate crosslinking
 with oxazoline curatives). These polymers desirably have from about 1 or 2
 to about 10 mole percent, more desirably from about 2 or 3 to about 8 mole
 percent repeat units with at least one carboxylic acid or anhydride of a
 dicarboxylic acid. If the polymers are only copolymers of acrylate and
 acid or anhydride monomers they desirably have from about 90 to about 98
 mole percent repeat units from acrylates, more desirably from about 92 to
 about 97 or 98 mole percent.
 The carboxylic acid cure site in the rubber may alternatively be generated
 by heat during the rubber and plastic melt blending process. For example,
 tert-butyl acrylate or tert-butoxycarbonyl acrylate that is copolymerized
 into the ethylene-acrylate or acrylate rubber can decompose to a repeat
 unit as from acrylic acid and a free isobutylene molecule (and carbon
 dioxide in the case of the tert-butoxycarbonyl group), thus generating the
 desired carboxylic acid cure sites. The tert-butyl acrylate. tert-butyl
 fumarate and/or tert-butoxycarbonyl acrylate are desirably present as
 repeat units in the amounts set forth above for carboxyl and/or anhydride
 groups. A limited amount of unmasked acid cure sites in the rubber, or
 acids such as camphorsulfonic acid or methanesulfonic acid may be used to
 catalyze decomposition of the pendent tert-butyl groups in such a rubber.
 The use of the masked cure sites described above may be useful in cases
 where the rubber does not form a good blend with the plastic due to acid
 catalyzed decomposition and/or crosslinking reactions of the rubber. When
 the cure sites in the rubber are masked, the desired cure sites are
 generated only after an intimate rubber and plastic blend has been formed,
 thus precluding a premature cure of the rubber portion of the TPV. This
 technology could therefore offer a process advantage in TPV production.
 The rubber, plastic, and curative could be melt mixed simultaneously,
 instead of the normal procedure of adding the curative to the rubber and
 plastic melt blend. The presence of the masked cure site would prevent
 rubber crosslinking prior to suitable rubber and plastic blend formation.
 Although pendant functionality and curatives are listed as separate
 components it is anticipated that alternatively the curatives could be
 pendantly or terminally attached to the acrylic rubber or
 ethylene-acrylate rubber prior blending the thermoplastic and rubber to
 form the TPV.
 Other suitable acrylic rubbers include copolymers of ethylene and the
 above-noted alkyl acrylates wherein the amount of ethylene is desirably
 high, e.g. from about 10 to about 90 mole percent, desirably from about 30
 to about 70 mole percent, and preferably from about 50 to about 70 mole
 percent of the repeat groups based upon the total number of moles of
 repeat groups in the copolymer. Thus the alkyl acrylates in the copolymer
 are desirably from about 10 to about 90 mole percent, more desirably from
 about 30 to about 70 mole percent, and preferably from about 30 to about
 50 mole percent of the ethylene-acrylate copolymers.
 Other acrylic copolymers include polymers from three or more different
 monomers such as ethylene-acrylate-carboxylic acid polymers, or
 ethylene-acrylate-maleic anhydride polymers, wherein the unsaturated acids
 have from 2 to 15 carbon atoms and desirably from 2 to 10 carbon atoms.
 Such ethylene-acrylate-maleic anhydride terpolymer rubbers are available
 from DuPont. More specifically, such polymers from three or more different
 monomers generally contain from about 35 to about 90 mole percent and
 desirably from about 48 or 60 to about 80 mole percent of ethylene repeat
 groups, generally from about 0.5 to about 10 mole percent and desirably
 from about 1 or 2 to about 8 mole percent of carboxylic acid repeat and/or
 anhydride groups (e.g. from an unsaturated carboxylic acid), and generally
 from about 9.5 or 10 to about 60 or 65 mole percent and desirably from
 about 18 or 19 to about 50 mole percent of alkyl acrylate repeat groups
 based upon the total number of repeat groups in the terpolymer. The acid
 repeat groups are preferably carboxylic acid groups derived from
 unsaturated mono or polycarboxylic acids or anhydrides of unsaturated
 polycarboxylic acids, which repeat groups have been copolymerized into the
 acrylic rubber. A specific commercially available compound is Vamac GLS,
 manufactured by DuPont, which generally has about 68 mole percent
 ethylene, about 30 mole percent of methyl acrylate, and about 2 mole
 percent of anhydride functionality.
 While peroxides are not used for curing the acrylate rubbers during mixing
 for TPV synthesis, they may be used to partially crosslink the acrylate
 rubber before it is mixed with the thermoplastic phase. Desirably
 substantially all the peroxide is decomposed prior to or early in the
 mixing with the thermoplastic in order to prevent thermoplastic
 degradation.
 The amount of the polar rubber utilized in the present invention generally
 ranges from about 25 or 50 to about 400 parts by weight, desirably from
 about 100 or 200 to about 375 parts by weight, and preferably from about
 250 to about 360 parts by weight for every 100 parts by weight of the one
 or more polar thermoplastic polymers.
 The one or more thermoplastic polymer is desirably from about 15 to about
 70 parts by weight and more desirably from about 25 to about 65 parts by
 weight per 100 parts by weight total for said thermoplastic polymers and
 rubber. The rubber is desirably from about 30 to about 85 parts by weight,
 more desirably from about 35 to about 75 parts by weight per 100 parts by
 weight total of said thermoplastic polymers and said rubber.
 In the practice of the present invention, it is not an objective to affect
 in any way the integrity of the engineering thermoplastic polymer. This
 polymer forms the matrix within which the crosslinked rubber particles are
 carried. The crosslinking should therefore not be carried out in a way
 which causes degradation of the thermoplastic (e.g. by chain
 fragmentation), or crosslinking of the thermoplastic (either with itself
 or with the rubber), or grafting of the rubber and thermoplastic. Rather
 the objective is to cause the rubber to selectively crosslink, i.e. to
 form attachments between chains of the rubber molecules themselves,
 without involving the thermoplastic matrix.
 As amine curatives can degrade many of the polar thermoplastics
 (polyesters, polycarbonates), it is desirable to exclude amine curatives.
 Further, the reaction of an amine curative with the anhydride cure site on
 the rubber would release one molecule of water, which would require
 devolatilization and the water released could cause hydrolysis of the
 thermoplastic matrix.
 The use of multifunctional oxazoline, oxazine and/or imidazoline curatives
 are preferred in this invention due to the lack of interaction with or
 degradation of the polar thermoplastics and the absence of by-products due
 to the addition curing reaction with the rubber.
 Another important aspect of the present invention is the utilization of
 addition type curatives which, unlike free radical curatives, do not break
 down the thermoplastic phase and desirably do not form volatile
 by-products. A highly preferred addition curative or crosslinking agent
 are the various multifunctional oxazolines including polyvalent oxazolines
 such as 2,2'-bis(2-oxazoline),
 2,2'-hexamethylenedicarbamoylbis(2-oxazoline), and
 1,3-phenylene-2,2'-bis(2-oxazoline); multifunctional oxazines including
 bis-5,6-dihydro-4H-1,3-oxazine; multifunctional imidazolines and
 polycarbodiimides.
 The multifunctional (polyvalent) oxazolines and oxazines generally have the
 formula
 ##STR1##
 wherein R is an aliphatic or aromatic hydrocarbon group such as alkylene or
 arylene having 1 to 24 carbon atoms optionally substituted with one or
 more lower alkyl groups having 1 to 6 carbon atoms or substituted with an
 aryl group having 6 to 19 carbon atoms; m is 1 or 2; n is 0 or 1, when n
 equals 1 then X and Y independently are hydrogens or a 2-oxazoline group
 or 1,3-oxazine group, and when n equals 0 then R is not present and no X
 and Y are present, further each oxazoline or oxazine group of the above
 formula may optionally be substituted with an alkyl of 1 to 6 carbon
 atoms. Further descriptions of said polyvalent oxazolines are given in
 U.S. Pat. No. 4,806,588 herein incorporated by reference. The
 multifunctional 1,3-oxazine compounds are known to the art.
 The multifunctional imidazolines have the formula
 ##STR2##
 where R and n are defined as above for the multifunctional (polyvalent)
 oxazolines and X and Y are hydrogens or imidazoline groups. A preferred
 multifunctional imidazoline is bismidazoline.
 The polycarbodiimide is desirably an oligomeric polycarbodiimide such as
 shown below
 ##STR3##
 where R.sub.1, R.sub.2, and R.sub.3 are individually alkyl of 1 to 6 carbon
 atoms or aryl groups of 6 to 12 carbon atoms and p is from 12 to 42.
 The amount of curative will vary depending on type used as well as the
 degree of cure desired, as is well recognized in the art. The various
 additional catalysts or combinations thereof are generally utilized in an
 amount of from about 1 to 12, desirably from 2 to 10, and preferably from
 about 2.5 to about 7 parts by weight for every 100 parts by weight of the
 rubber. The addition curatives effect crosslinking by reacting with
 functional groups such as the carboxylic acid groups present in the
 acrylic or ethylene-acrylate rubbers.
 In addition to the thermoplastic polymer (polyester or phenylene oxide),
 and the rubber (acrylate or ethylene acrylate), the processing agents and
 the curative components, the compositions of the present invention can
 include various conventional additives such as reinforcing and
 non-reinforcing fillers, extenders, antioxidants, stabilizers,
 plasticizers, rubber processing oil, extender oils, lubricants,
 antiblocking agents, antistatic agents, waxes, foaming agents, pigments,
 flame retardants and other additives known in the rubber compounding art.
 Such additives can generally comprise up to about 60 weight percent of the
 total composition, and can be in the plastic phase, the rubber phase or
 both. Fillers and extenders which can be utilized include conventional
 inorganics such as calcium carbonate, clays, silica, talc, titanium
 dioxide, carbon black and the like. The rubber processing oils generally
 are paraffinic, naphthenic or aromatic oils derived from petroleum
 fractions. The plasticizer may also be a low molecular weight polyester or
 a sulfonamide. The amount of plasticizer may range from 0 to 100 or 200
 parts by weight per 100 parts by weight rubber.
 Regardless of the thermoplastic, rubber, and other components of the
 present invention, the rubber component(s) is desirably partially or fully
 vulcanized (crosslinked) in one or more steps in preparing the
 thermoplastic vulcanizate. The word "vulcanizate" as used herein does not
 require sulfidic crosslinks. The terms "fully vulcanized" and "completely
 vulcanized" as used in the specification and claims means that the rubber
 component to be vulcanized has been cured 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 vulcanizate composition. The degree of cure can be described
 in terms of gel content or, conversely, extractable components. Desirably
 in one embodiment, at least 90 percent, more desirably at least 95 percent
 and preferably at least 98 weight percent of the rubber is
 non-extractable, after vulcanization, with a solvent that readily
 dissolves the uncrosslinked rubber. Alternatively this can be expressed as
 at least 90, 95 or 98 weight percent of said rubber is crosslinked and/or
 said values are the gel content of the rubber. 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. 5,100,947 and
 5,157,081, both of which are fully incorporated herein by this reference.
 The partial or complete crosslinking can be achieved by adding one or more
 of the above-noted rubber curatives to the blend of thermoplastic (e.g.
 polyester) and rubber (e.g. acrylate or ethylene-acrylate rubber) and then
 vulcanizing the rubber to the desired degree under conventional
 vulcanizing conditions. 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 contained in a blend of at least one thermoplastic
 and at least one rubber (thermoplastic vulcanizate components), wherein
 the rubber is vulcanized under conditions of shear at a temperature above
 the melting point of the thermoplastic component. The rubber is thus
 simultaneously crosslinked and dispersed, e.g. as fine particles, within
 the thermoplastic matrix, although other morphologies may also exist.
 Dynamic vulcanization is effected by mixing the thermoplastic vulcanizate
 components at elevated temperature in conventional batch mixing equipment
 (e.g., batch or continuous; internal or exposed mixing surfaces) such as
 multiroll mills. Banbury.TM. mixers, Brabender.TM. mixers, continuous
 mixers, mixing extruders and the like. The unique characteristic of
 thermoplastic vulcanizates such as in the present invention 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. In
 the present invention the mixing temperature is desirably from about
 180.degree. C. to about 260 or 280.degree. C. and preferably from about
 200.degree. or 220.degree. C. to about 230 or 260.degree. C.
 The following general procedure can be used to form general thermoplastic
 vulcanizates and was used in the preparation of thermoplastic vulcanizates
 of the present invention as set forth in the examples.
 Thermoplastic vulcanizates (TPVs) were produced in a laboratory Brabender
 plasticorder, model EPL-V5502. The mixing bowl had a capacity of 60 mL
 with roller type rotors, which gave good mixng for samples with batch
 weight of 40-45 grams. For higher batch weight TPVs, less bulky cam rotors
 were used, which gave a bowl capacity of 85 mL. TPVs were prepared at
 240.degree. C. and 75 rpm rotor speed. The rubber was masticated first,
 followed by addition of plastic materials when the rubber temperature had
 reached about 230.degree. C. After a steady torque was obtained for 1-2
 minutes in order to ensure as complete a homogenization as possible of the
 rubber and plastic melt blend, an appropriate amount of curative was added
 to achieve the desired state of rubber cure without affecting the
 thermoplastic. The curing was continued for an appropriate time, usually
 about 8 minutes. A level torque reading was generally obtained after about
 4-5 minutes of curing. This is indicative that the desired cure level has
 been reached. The TPV obtained was sheeted when hot in a cold press, and
 subsequently compression molded at 250.degree. C. in order to produce
 plaques for physical testing. Plasticizers were added either to the rubber
 and plastic melt blend prior to cure or to the TPV after 4-5 minutes of
 curing.
 The thermoplastic vulcanizate compositions of the present invention
 generally have good tensile strength, good elongation and good compression
 set properties. Most notably, they have higher use temperature, e.g. up to
 150.degree. C. for continuous use and very low oil swell, i.e. excellent
 IRM903 oil resistance properties comparable to that of thermoset acrylate
 or ethylene-acrylate rubber. Oil swell values as measured by the percent
 of weight gain at 150.degree. C. for 72 hours is generally 35 percent or
 less, desirably 20 to 25 percent or less, and preferably 15 percent or
 less in IRM903 oil. IRM903 oil is similar in swelling capability to ASTM
 #3 oil. The oil swell values are represented as weight gain in the
 following tables.
 The thermoplastic vulcanizate compositions of the present invention can be
 used in applications wherever acrylate or ethylene-acrylate rubber is
 used. They have desirable elongations, modulus, low oil swell, low tension
 set, and low compression set. Thus, they can be utilizes as seals,
 gaskets, molded articles and the like. The thermoplastic vulcanizates can
 be mixed with other polymers such as polyamides (nylons) in broad mixing
 ratios to prepare thermoplastic vulcanizates with at least 10, 20 or 30
 weight percent polyamide polymers based on the total weight of the
 thermoplastic vulcanizate.