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Low permeability elastomeric-metal phosphate nanocomposites

Low permeability elastomeric-metal phosphate nanocomposites description/claims

This invention relates to low permeability elastomeric-metal phosphate nanocomposite compositions, more particularly to isobutylene-based elastomers containing platy nano-fillers of alpha-zirconium phosphate, and especially to halogenated isobutylene elastomers, such as halogenated poly(isobutylene-co-p-methylstyrene) elastomers, particularly brominated poly(isobutylene-co-p-methylstyrene) elastomers (BIMSM), filled with alpha-zirconium phosphate exchanged with monoamines, diamines, tertiary amines, polyamides, or a combination thereof, with or without a dispersion aid such as low molecular weight polar modified polymer.

Polymers with a saturated hydrocarbon backbone are well known to possess good environmental and aging resistance which makes them highly desirable in a variety of applications. In comparison with many other common elastomers, polyisobutylene and its copolymers are notable for their low permeability to small-molecule diffusants as a result of their efficient intermolecular packing. This efficient packing in isobutylene polymers leads to their low fractional free volumes and low diffusion coefficients for penetrants. In combination with the low solubilities of small-molecule diffusants in isobutylene polymers, the low diffusion coefficients lead to the observation of low permeability values in isobutylene polymers relative to other elastomers. This low degree of permeability to gases accounts for the largest uses of butyl and halobutyl rubbers, namely tire inner tubes and tire innerliners. Among all commercially available isobutylene elastomers, brominated poly(isobutylene-co-paramethylstyrene), also referred to as BIMSM, has the lowest permeability.

Nanocomposites are polymer systems containing inorganic particles with at least one dimension in the nanometer range. Some examples of these are disclosed in U.S. Pat. No. 6,060,549, U.S. Pat. No. 6,103,817, U.S. Pat. No. 5,973,053, U.S. Pat. No. 5,936,023, U.S. Pat. No. 5,883,173, and U.S. Pat. No. 5,576,372. Common types of inorganic particles used in nanocomposites are phyllosilicates, an inorganic substance from the general class of so called “nano-clays” or “clays” generally provided in an intercalated form wherein platelets or leaves of the clay are arranged in a stack in the individual clay particles with interleaf spacing usually maintained by the insertion of another compound or chemical species between the adjacent lamellae. Ideally, intercalation should take place in the nanocomposite, wherein the polymer inserts into the space or gallery between the clay surfaces. Ultimately, it is desirable to have exfoliation, wherein the polymer is fully dispersed with the individual nanometer-size clay platelets. Due to the general enhancement in air barrier qualities of various polymer blends when clays are present, there is a desire for a nanocomposite with low air permeability; especially a vulcanized elastomer nanocomposite such as used in the manufacture of tires.

It is known from U.S. Pat. No. 5,576,373 and U.S. Pat. No. 5,665,183 to introduce organosilicates into styrene-butadiene rubber (SBR) to lower permeability values. High loadings in the SBR of montmorillonite silicates intercalated with an amine-terminated butadiene-acrylonitrile oligomer were reported to reduce the permeability of the SBR, but the permeability values of these filled rubbers were still significantly higher than those of halobutyl or BIMSM rubbers. Direct blending of BIMSM of low or high molecular weights with dimethyldialkyl ammonium-exchanged montmorillonite silicates in U.S. Pat. No. 5,807,629 and U.S. Pat. No. 6,034,164 (Elspass et al.) provided BIMSM-layered silicate nanocomposites with significantly lower permeability values. However, organosilicates in both the reactive oligomer/SBR blends and in the BIMSM nanocomposites were not exfoliated.

The extents of dispersion, exfoliation, and orientation of platy nano-fillers such as organosilicates, mica, hydrotalcite, graphitic carbon, etc., could strongly influence the permeability of the resulting polymer nanocomposites. The barrier property of a polymer in theory is significantly improved, by an order of magnitude, with the dispersion of just a few volume percent of exfoliated high aspect-ratio platy fillers, due simply to the increased diffusion path lengths resulting from long detours around the platelets. Nielsen, J. Macromol. Sci. (Chem.), vol. A1, p. 929 (1967), discloses a simple model to determine the reduction in permeability in a polymer by accounting for the increase in tortuousity from impenetrable, planarly oriented platy fillers. Gusev et al., Adv. Mater., vol. 13, p. 1641 (2001), discloses a simple stretched exponential function relating the reduction of permeability to aspect ratio times volume fraction of the platy filler, that correlates well with permeability values numerically simulated by direct three-dimensional finite element permeability calculations.

For rubber compounding applications, small sub-micron fillers such as carbon black and silica are used for fatigue resistance, fracture toughness and tensile strength. Filler particles larger than a micron act tend to concentrate stress and initiate defects. Thus, platy nanofillers added to reduce permeability are desirable in elastomers. To maximize the effect of aspect ratio on permeability reduction, it is useful to maximize the degree of exfoliation and dispersion of the platelets, which are generally supplied in the form of an intercalated stack of the platelets. However, in isobutylene polymers, dispersion and exfoliation of platy nanofillers requires sufficient favorable enthalpic contributions to overcome entropic penalties. As a practical matter, it has thus proven to be very difficult to disperse ionic nanofillers such as clay into generally inert, nonpolar, hydrocarbon elastomers. The prior art has, with limited success, attempted to improve dispersion by modification of the clay particles, by modification of the rubbery polymers, by the use of dispersion aids, and by the use of various blending processes.

The “inertness” of saturated hydrocarbon polymers such as BIMSM, their low reactivity and incompatibility with most other materials, and the difficulties in adhering them to, or using them in conjunction with most other materials has restricted their use in many areas. Chemical modification of the elastomers, modification of the blend component, and the use of additional compatibilizing blend components, have been attempted. U.S. Pat. No. 5,162,445 discloses a method to improve polymer blend compatibility or blend co-curability by copolymerizing an unsaturated comonomer and/or a comonomer having reactive functionality with isobutylene. U.S. Pat. No. 5,548,029 discloses graft copolymers of isobutylene-p-methylstyrene copolymers to compatibilize blends of saturated and unsaturated elastomers.

The preparation of nanocomposites uses a number of methods to generate exfoliated clays. Swelling agents and processes for intercalating layered silicates are disclosed in U.S. Pat. Nos. 4,472,538, 4,810,734, 4,889,885 as well as WO92/02582. For example, cationic surfactants are employed with anionic montmorillonites or other phyllosilicates to facilitate dispersion, and anionic surfactants with cationic hydrotalcites. One of the most common methods relies upon the use of organically modified montmorillonite clays. Organoclays are typically produced through solution based ion-exchange reactions that replace sodium ions that exist on the surface of sodium montmorillonite with organic molecules such as alkyl or aryl ammonium compounds and are typically known in the industry as swelling or exfoliating agents. See, e.g., U.S. Pat. No. 5,807,629, WO02/100935, and WO02/100936. Other background references include U.S. Pat. No. 5,576,373, U.S. Pat. No. 5,665,183, U.S. Pat. No. 5,807,629, U.S. Pat. No. 5,936,023, U.S. Pat. No. 6,121,361, WO94/22680, WO01/85831, and WO04/058874.

Another method to improve the organoclay performance has been to use functionalized polymers to treat the clay. This approach uses materials that are soluble in water or materials that can be incorporated into the polymerization reaction. This approach has been used to prepare nylon-clay nanocomposites, using for example, oligomeric and monomeric caprolactam as the modifier. Polyolefin-clay nanocomposites, such as polypropylene nanocomposites, have utilized maleic anhydride grafted polypropylenes to achieve some success in the formation of nanocomposites.

For example, it is known to utilize exfoliated-clay filled nylon as a high impact plastic matrix, such as disclosed in U.S. Pat. No. 6,060,549 (Li et al.). In particular, Li et al. discloses a blend of a thermoplastic resin such as nylon; a copolymer of a C4 to C7 isoolefin, a para-methylstyrene and a para-(halomethylstyrene); and exfoliated clays that are used as a high impact material. Further, Japanese Unexamined Application P2000-160024 (Yuichi et al.) discloses a thermoplastic elastomer composition which can be used as an air barrier, including a blend similar to that disclosed in Li et al.

The preparation of BIMSM-clay nanocomposites from melt-blending, solution blending and an emulsion process are disclosed in commonly assigned U.S. application Ser. No. 11/183,361, Split-Stream Process for Making Nanocomposites, by W. Weng et al., filed Jul. 18, 2005; and commonly assigned U.S. application Ser. No. 11/184,000, Functionalized Isobutylene Polymer-Inorganic Clay Nanocomposites and Organic-Aqueous Emulsion Process, by W. Weng et al., filed Jul. 18, 2005.

U.S. Pat. No. 6,841,642 to Kaszas (WO02/16452) discloses a polymer formed by reaction of a mixture of isobutylene, isoprene, divinylbenzene and a chain transfer agent. Incorporation of DVB to the isobutylene-isoprene copolymer with the chain transfer agent is said to provide a butyl polymer having an improved (higher degree of) filler dispersion.

U.S. Pat. No. 6,548,585 (Ozawa et al.) discloses refrigerant hoses made with an inner tube composition of a brominated copolymer rubber such as BIMSM with an inorganic lamellar compound such as graphite, zirconium phosphate, calcogenides, talc, kaolinite, benotnite, montmorillonite, mica, chlorite, etc.

Other general background references that have suggested general polymer compositions containing zirconium phosphate include U.S. Pat. No. 6,872,687 (Shimada et al.); Publication US20020031716 (Nagata et al.); and Publication US20060046199 (Mitsumoto et al.).

Publication US2005026238 (Dupuy et al.) and Publication US20040033186 (Bougelot et al.) disclose zirconium phosphate intercalated with amines, and thermoplastic compositions comprising mixtures of the zirconium phosphate in a thermoplastic polymer.

The present invention provides elastomeric nanocomposites made with metal phosphate nanofillers dispersed in an isobutylene-based rubber, such as, by way of non-limiting example, amine- or amide-modified alpha-zirconium phosphate in a halogenated poly(isobutylene-co-p-methylstyrene) elastomer, preferably brominated poly(isobutylene-co-p-methylstyrene) elastomer (BIMSM). Elastomeric nanocomposites incorporating the modified metal phosphates have unexpectedly improved intercalation, exfoliation and/or dispersion compared to the nanoclays and other prior art nanofillers, and further have improved barrier and other desirable properties.

Although not intending to be bound by any one theory, the unique ion exchange characteristics of the metal phosphate nanofillers may explain their improved performance. The ion exchange behavior of platy crystalline alpha-zirconium phosphate depends on its crystallinity. The phosphate proton can be removed or exchanged with cations. If amines or amides are used to intercalate alpha-zirconium phosphate, hydrogen bonds form between amine or amide and phosphate. See Clearfield et al., J. Inorg. Nucl. Chem., vol. 41, p. 871 (1979). The hydrogen bonding of the amine or amide intercalants facilitates more favorable interactions for isobutylene-based elastomers to intercalate and/or to exfoliate zirconium phosphate. In addition, zirconium phosphate can be crystallized in a commercial process, not mined and processed like montmorillonites or other phyllosilicates, and hence zirconium phosphate can have a more uniform and monodisperse particle size and aspect ratio.

In one embodiment, the nanocomposite broadly comprises an isobutylene-based elastomer filled with platy nanofiller comprising ion-exchangeable phosphate, wherein phosphate protons are exchanged with a compound capable of hydrogen bonding with the phosphate.

In various embodiments, the nanofiller is in the form of platelets having an aspect ratio of 200 or more, 375 or more, or 500 or more. The phosphate can be a metal phosphate such as titanium phosphate, alpha-zirconium phosphate, or the like. The nanofiller can be used in one embodiment at from 0.1 to 30 parts by weight per 100 parts by weight rubber or elastomer (phr). In embodiments, the nanofiller is intercalated, exfoliated, or preferably is a mixture of intercalated nanofiller and partially exfoliated nanofiller in the elastomer. Embodiments of the extent of exfoliation include exfoliation of from 0.0001 to 10 volume percent of the nanofiller, from 0.005 to 5 volume percent, or from 0.01 to 0.5 volume percent of the nanofiller.

In an embodiment of the invention, the nanofiller is surface exchanged with an amine, preferably in an amount effective to intercalate the nanofiller. In embodiments, the nanofiller is amine-exchanged in an amount from at least 25 but less than 100 percent of exchange capacity, or at from 25 to 50 percent of exchange capacity. In different embodiments, the amine is a monoamine or diamine, and the amine groups can be primary amines, secondary amines, tertiary amines, quaternary amines, or a combination thereof. The amine can be embodied as a short chain alkylamine or diamine wherein the alkyl group has from 1 to 12 or 13, 1 to 10, 2 to 8, 2 to 6, or 2 to 4 carbon atoms; or as a long chain alkylamine or diamine having 14 or more carbon atoms, e.g. 15 to 50, 16 to 30, 16 to 30, or 18 to 24 carbon atoms. The amine can be exchanged in either an overlapping configuration between the opposing platelet surfaces, or in a double layer configuration.

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