Open cell foamed articles including silane-grafted polyolefin resins

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OPEN CELL FOAMED ARTICLES
INCLUDING SILANE-GRAFTED
POLYOLEFIN RESINS

CROSS REFERENCE TO RELATED
APPLICATION

This application is a continuation-in-part of U.S. Ser. No. 08/749,740, filed Nov. 15, 1996, now pending.

BACKGROUND OF THE INVENTION

The invention relates to open cell foamed articles.

Open cell foams emulate the open or interconnected cell structure of the a marine sea sponge. Open cell foams can be used in many different applications. Open cell foams made from polymer resins can be flexible and elastomeric. In contrast, brittle open cell foams can be made from ceramic or glass structures.

The open cell structure permits the flow of a liquid or gas medium through the interconnecting cellular structure of the foam without destroying the foam structure. The liquid can be, for example, water or oil, or the gas can be air or nitrogen. The characteristics of a foam can be modified for specific applications. For example, the foam can act as an absorbent for a specific liquid (e.g., as a sponge). Open cell foams can act as a filtering mechanism, permitting the flow of a liquid or gas through the structure while separating out materials from that flow and retaining the materials in the foam structure. It is possible to impregnate the structure with an additive which can later be mechanically squeezed out for application onto another surface. Open cell foams can also be used in cushioning applications.

Synthetic materials have been developed for producing flexible open cell foams. Natural rubber latex foams provide soft materials for body contact and the application of cosmetics. Polyvinyl chloride (PVC) plastisol foams can have a soft, durable feel that simulates leather to the touch. Open cell foams based on polyurethanes have been made suitable for many applications, such as cushions for packaging, automotive applications, home bedding, filters (e.g., for air conditioners), applicators (e.g., for shoe polish), or sound attenuating panels (e.g., for rooms or speakers). Open cell foams based on ethyl vinyl acetate (EVA) can be made in a melt process to lower foam densities than earlier foams. The EVA-based foams have a high percentage of noninterconnecting cells yielding a primarily open cell foam. The EVA-based open cell foams can be soft and pliable.

SUMMARY OF THE INVENTION

In general, the invention features open cell foamed articles including silane-grafted single-site initiated polyolefin resins. An open cell foam is a foam where there is an interconnection between cells in the foam. There can be greater than about 10 percent open cells (i.e., between 10 and 50 percent) in an open cell foam article, preferably greater than 40 percent, more preferably greater than 80 percent, and most preferably greater than 90 percent. The amount of open cells in a foam can be increased by crushing the foam. A crushed open cell foam can have between 50 and about 98 percent open cells. In contrast, a closed cell foam has a predominance of closed cells.

One way to determine the open cell content of a foam is by measuring the amount of water that is absorbed into the foam when the foam is immersed in water. Another method is the gas-volume method using a pycnometer, such as a Quantachrome Model 1000 pycnometer, which measures the percentage of open cells according to method ASTM D-2858.

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In one aspect, the invention features an open cell foamed article including a silane-grafted single-site initiated polyolefin resin. The article can include greater than 5 weight percent of the single-site initiated polyolefin resin, prefer

5 ably greater than 40 weight percent, and more preferably greater than 75 weight percent.

The single-site initiated polyolefin resin is a polyethylene, a copolymer of ethylene and a C3-C20 alpha-olefin, or a copolymer of ethylene, a C3-C20 alpha-olefin and a

1° C4-C20 diene. For example, the single-site initiated polyolefin resin can be a polyethylene, polypropylene, polystyrene, or ethylene-propylene-diene monomer (EPDM) terpolymer. The single-site initiated polyolefin resin can have a density between about 0.83 and about 0.96

15 g cm-3, a molecular weight distribution between about 1.5 and about 3.5, a melt index in the range of about 0.5 dg/min to about 100 dg/min, and a composition distribution breadth index greater than about 45 percent.

The silane-grafted single-site initiated polyolefin resin

20 can have a silane-graft content of between 0.001 and 4 percent, preferably about 0.1 and 2 percent (e.g., about 1 percent). The silane can include a vinyl silane having 2 or 3 hydrolyzable groups (e.g., vinyl triethoxysilane). The silane can also include an alkyl trialkoxy silane, where the alkyl is

25 a CI to C20 group and the alkoxy is a CI to C10 group. The open cell foamed article can include a partially cross-linked polyolefin blend including the single-site initiated polyolefin resin and a copolymer including ethylene and propylene, an ethylene-propylene-diene monomer

30 terpolymer, an ethylene-vinyl acetate copolymer, an efhylene-maleic anhydride copolymer, an ethylene-ethyl acrylate copolymer, a low density polyethylene, a linear low density polyethylene, a medium density polyethylene, a high density polyethylene, or a polypropylene. The polyolefin

35 blend can be partially silane-grafted.

In another aspect, the invention features a method of making an open cell foamed article. The method includes the steps of providing a mixture including silane-grafted singlesite initiated polyolefin resin and a foaming agent, partially cross-linking the mixture, and expanding the mixture to form an open cell foamed article.

The step of expanding the mixture can include free expansion, extruding, or compression molding the mixture

45 at increased temperature. Compression molding can include the steps of pressing the polymer mixture using a high tonnage press at a temperature of between 240° and 480° F. (e.g., between 275° and 320° F.) and a pressure of between 50 and 5000 psi (e.g., between 250 and 2500 psi) for

5Q between 20 and 90 minutes followed by heating the polymer mixture at a temperature between 300° and 380° F.

The method can include the step of grafting the polyolefin blend with a silane. The step of cross-linking the polymer blend can include hydrolyzing the silane. The step of cross

55 linking the polymer blend can further include cross-linking with a peroxide.

The method can include the step of crushing the foamed article after the expanding step. The crushing step increases the percentage of open cells in the foamed article (i.e., to

go greater than 50 percent, preferably greater than 80 percent). The method can also include the step of pinning the foamed article to further increase the percentage of open cells.

The preferred foam has an average foam density between 1.0 and 25.0 pounds per cubic foot, preferably 1.5 and 3.0

65 pounds per cubic foot.

The mixture to be foamed can include other resins, cross-linking agents (e.g., less than 1.2 percent dicumyl peroxide), activators (e.g., between 0.1 and 3.5 percent), foaming agents (e.g., between 2 and 30 percent azodicarbonamide), particulate fillers (e.g., less than 95 percent, preferably less than 30 percent calcium carbonate), fibrous fillers, antioxidants, ultraviolet stabilizers, thermal stabilizers, pigments and colorants, cell-growth nucleants such as talc, cell-structure stabilizers such as fatty acids or amides, property-modifiers, processing aids, additives, fire retardants, antistatic components, antimicrobial components, or catalysts to accelerate cross-linking and other reactions.

A low-density polyethylene, or LDPE, is a polymer of ethylene with a density between 0.915 and 0.930 g cm-3. Since LDPE is prepared under, for example, free-radical conditions and high pressures, it is highly branched. Highly branched polymers are polymers that have approximately one to two short chain branches for every one hundred carbon atoms in the polymer backbone. Ashort-chain branch is a branch of a polymer backbone of 6 carbon atoms or less which can be quantified by 13C NMR spectroscopic methods. See, for example, Randall, Rev. Macromol. Chem. Phys., C29 (2 & 3), p. 285-297, incorporated herein by reference.

A copolymer is a polymer resulting from the polymerization of two or more monomeric species and includes terpolymers (e.g., resulting from the polymerization of three monomeric species), sesquipolymers, and greater combinations of monomeric species. Copolymers are generally polymers of ethylene with a C3-C20 alpha-olefin.

The densities, or specific gravities, of the polymer resins can be measured using ASTM D-792 methods. The cushioning properties of the open cell foamed articles can be measured according to ASTM D-3573 (Condition CC) or ASTM D-1596.

A single-site initiated polyolefin resin is a polyolefin prepared from a single-site initiator that has controlled molecular weights and molecular weight distributions. The polyolefin can be polyethylene or a copolymer of ethylene and alpha-unsaturated olefin monomers. One class of a single-site initiators of particular interest are the metallocene initiators which are described, for example, in J. M. Canich, U.S. Pat. No. 5,026,798, in J. Ewen, et al., U.S. Pat. No. 4,937,299, in J. Stevens, et al., U.S. Pat. No. 5,064,802, and in J. Stevens, et al., U.S. Pat. No. 5,132,380, each of which are incorporated herein by reference. These initiators, particularly those based on group 4 transition metals, such as zirconium, titanium and hafnium, are extremely high activity ethylene polymerization initiators.

The single-site initiators are versatile. The polymerization conditions such as a initiator composition and reactor conditions can be modified to provide polyolefins with controlled molecular weights (e.g., in a range from 200 g mol-1 to about 1 million or higher g mol-1) and controlled molecular weight distributions (e.g., M^/M^ in a range from nearly 1 to greater than 8, where M„ is the weight average molecular weight and M„ is the number average molecular weight). Molecular weights and molecular weight distributions of polymers can be determined, for example, by gel permeation chromatography.

The polyolefins provided by these initiators can contain uniformly distributed, highly controlled short chain branching sites. Certain polyolefins can have less than about one long-chain branch for every ten thousand carbon atoms in the backbone of the polymer. As described above, one method of determining branching is 13C NMR spectroscopy.

When the single-site initiated polyolefins are copolymers, the composition distribution breadth index (CDBI) is gen

erally greater than 50% and most preferably above 70%. The CDBI is a measurement of the uniformity of distribution of comonomers among the individual polymer chains having a comonomer content within 50% of the median bulk molar comonomer content. The CDBI of a copolymer can be determined by temperature rising elution fractionation (TREF), as described in, for example, Wild et al., J. Poly. Sci., Poly. Phys. Phys. Ed., Vol. 20, p. 441 (1982).

Melt index (MI) of a polymer resin is a measurement of processability under low shear rate conditions. The MI can be determined by ASTM D-1238 Condition E (190° C./2.16 kg). The MI of the polyolefins is generally between about 0.2 dg/min and about 100 dg/min, preferably, between about 1 dg/min and about 10 dg/min, and most preferably between about 2 dg/min and about 8 dg/min. The melt index of the polymer resins can be measured using ASTM D-1238.

Silane-grafting is attaching one or more siliconcontaining monomer or polymer to the original polymer chains. The grafting is generally accomplished by forming active grafting sites on the original polymer chains in the presence of silicon-containing monomers, which can further polymerize as branches from the original polymer chains. Active grafting sites can be generated, for example, by free radicals or anions.

A slow silane is a silane cross-linking agent that hydrolyzes (i.e., cross-links) more slowly than vinyl trimethoxy silane VTMOS (e.g., vinyl triethoxy silane VTEOS). It can take a longer time to cure a slow silane-grafted material than a VTMOS-grafted material.

The silane-grafted polyolefinic materials can be crosslinked, optionally with a peroxide co-cure, to produce an open cell foam article. The rates of reaction for the silane cross-linking mechanism are controlled to permit regulation of the foaming reaction in order to produce open cell material. If a peroxide co-cure is used, a variety of common organic peroxides can be used to further cross-link the polymers. By controlling the cross-linking reaction rates, up to 100% of the single site initiated polyolefin resin, or blends with other polyolefinic materials can be used to make an open cell foamed articles.

The single-site initiated polyolefin resin-based open cell foams can be used in place of EVA, urethane, PVC, or other types of open cell foams. By using silane-grafted polyolefinic materials, in particular those of the single site initiated type, to produce an open cell foam having no toxic residues, permitting their use in medical applications, for example. Specifically, residual materials ordinarily found in urethane, PVC, and EVA can irritate human skin. The amounts of these irritating materials can be reduced or eliminated in open cell foams when single site initiated polyolefin resins are included in the foams. In addition, EVA residuals can interfere with active additives (e.g., nerve gas deactivating chemicals in a gas mask filter). In the single site initiated polyolefin resin open cell foam there are no unwanted materials to be removed from the foamed material for particular applications (and, therefore, no need to remove such materials in separate processing steps), since the polyolefin is essentially inert. By minimizing the amount of additives in the foam, the foams can be produced with material and economic savings. The open cell foams can be used, for example, as air or water filtering media without imparting possible allergenic or potentially toxic components (or otherwise hazardous materials) into a downstream flow of air or water. The open cell foams can also have improved weatherability and durability by resisting drying and cracking.

Other features and advantages of the invention will be apparent from the following detailed description thereof, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram depicting a foam crushing apparatus.

FIG. 2 is a graph depicting a cushioning curve for an open cell foam article.

FIG. 3 is a graph depicting a cushioning response curve for an open cell foam article.

DETAILED DESCRIPTION

An open cell foamed article can be prepared from compositions including a single-site initiated polyolefin resin. The preferred level of single-site initiated polyolefin resin included in the foam, as a percentage of total polymeric resin, preferably is greater than 5 percent, more preferably between about 20 and 80 percent, and most preferably between about 40 and 60 percent.

The single-site initiated polyolefin resins are derived from ethylene polymerized with at least one comonomer selected from the group consisting of at least one alpha-unsaturated C3-C20 olefin comonomers. Preferably, the alphaunsaturated olefins contain between 3 and 16 carbon atoms, most preferably between 3 and 8 carbon atoms. Examples of such alpha-unsaturated olefin comonomers used as copolymers with ethylene include, but are not limited to, propylene, isobutylene, 1-butene, 1-hexene, 3-methyl-l-pentene,

4- methyl-l-pentene, 1-octene, 1-decene, 1-dodecene, styrene, halo- or alkyl-substituted styrene, tetrafluoroethylene, vinylcyclohexene, and vinylbenzocyclobutane.

The comonomer content of the polyolefin resins is generally between about 1 mole percent and about 32 mole percent, preferably between about 2 mole percent and about 26 mole percent, and most preferably between about 6 mole percent and about 25 mole percent.

The copolymer can include one or more C4—C20 polyene monomers. Preferably, the polyene is a straight-chain, branched chain or cyclic hydrocarbon diene, most preferably having between 6 and 15 carbon atoms. It is also preferred that the diene be non-conjugated. Examples of suitable dienes include, but are not limited to, 1,3-butadiene, 1,4hexadiene, 1,6-octadiene, 5-methyl-l,4-hexadiene, 3,7dimethyl-l,6-octadiene, 3,7-dimethyl-l,7-octadiene,

5- ethylidene-2-norbornene, and dicyclopentadiene. Especially preferred is 1,4-hexadiene.

The preferred single-site initiated polyolefin resins include either ethylene/alpha-unsaturated olefin copolymers or ethylene/alpha-unsaturated olefin/diene terpolymers.

Preferred single-site initiated polyolefin resins are described, for example, in S.-Y. Lai, et al., U.S. Pat. Nos. 5,272,236, 5,278,272, and 5,380,810, in L. Spenadel, et al, U.S. Pat. No. 5,246,783, in C. R. Davey, et al, U.S. Pat. No. 5,322,728, in W. J. Hodgson, Jr., U.S. Pat. No. 5,206,075, and in F. C. Stehling, et al, WO 90/03414, each of which is incorporated herein by reference. The resins contain varying amounts of short-chain and long-chain branching, which depend, in part, on the processing conditions.

Some single-site initiated polyolefin resins are available commercially from Exxon Chemical Company, Houston, Tex., under the tradename ExactTM, and include ExactTM 3022, ExactTM 3024, ExactTM 3025, ExactTM 3027, ExactTM 3028, ExactTM 3031, ExactTM 3034, ExactTM 3035, ExactTM

3037, ExactTM 4003, ExactTM 4024, ExactTM 4041, ExactTM 4049, ExactTM 4050, ExactTM 4051, ExactTM 5008, and ExactTM 8002. Other single-site initiated resins are available commercially from Dow Plastics, Midland, Mich, (or DuPont/Dow), under the tradenames EngageTM and AffinityTM, and include CL8001, CL8002, EG8100, EG8150, PL1840, PL1845 (or DuPont/Dow 8445), EG8200, EG8180, GF1550, KC8852, FW1650, PL1880, HF1030, PT1409, CL8003, Dow 8452, Dow 1030, Dow 8950, Dow 8190, and D8130 (or XU583-00-01). Most preferably, the single-site initiated polyolefin resins are selected from the group consisting of ExactTM 3024, ExactTM 3031, ExactTM 4049, PL1845, EG8200, Dow 8452, Dow 1030, Dow 8950, and EG8180.

LDPE resins are described, for example, in "Petrothene® Polyolefins .. .A Processing Guide," Fifth Edition, Quantum USI Division, 1986, pages 6-16, incorporated herein by reference. Some LDPE resins are commercially available from Exxon Chemical Company, Houston, Tex., Dow Plastics, Midland, Mich., Novacor Chemicals (Canada) Limited, Mississauga, Ontario, Canada, Mobil Polymers, Norwalk, Conn., Rexene Products Company, Dallas, Tex., Quantum Chemical Company, Cincinnati, Ohio, and Westlake Polymers Corporation, Houston, Tex. Commercially available LDPE resins include Eastman 1924P, Eastman 1550F, Eastman 800A, Exxon LD 117.08, Exxon LD 113.09, Dow 5351, Dow 683, Dow 760C, Dow 7681, Dow 5371, Novacor LF219A, Novacor LC05173, Novacor LC0522A, Mobil LMA-003, Mobil LFA-003, Rexene 2018 (7018), Rexene 1023, Rexene XO 875, Rexene PE5050, Rexene PE1076, Rexene PE2030, Quantum NA953, Quantum NA951, Quantum NA285-003, Quantum NA271-009, Quantum NA324, Westlake EF606AA, Westlake EF612, and Westlake EF412AA.

Other polymers or resins can be included in the mixture to be foamed, which can alter the physical properties of the foamed article. The polymeric components can be blended before or after the grafting or cross-linking steps. Examples of the polymers and resins which can be added to the mixture include polypropylene, other single-site initiated polyolefin resins, low density polyethylene (LDPE), high density polyethylene (HDPE), linear low density polyethylene (LLDPE), ethylene-propylene rubber, ethylenepropylene-diene monomer terpolymer (EPDM), polystyrene, styrene copolymers, polyvinylchloride (PVC), polyamides, polyacrylates, celluloses, polyesters, polyhalocarbons, and copolymers of ethylene with propylene, isobutene, butene, hexene, octene, vinyl acetate (EVA), vinyl chloride, vinyl propionate, vinyl isobutyrate, vinyl alcohol, allyl alcohol, allyl acetate, allyl acetone, allyl benzene, allyl ether, ethyl acrylate (EEA), methyl acrylate, acrylic acid, or methacrylic acid. Preferred resins include other single-site initiated polyolefins, LDPE, LLDPE, polypropylene, polystyrene, or ethylene copolymers such as EVA, or EEA.

The single-site initiated polyolefin resin is silane grafted. Silane-grafting of the polyolefin resin or resin blend occurs when the polymer backbone is activated and reacts with a silane reagent to form the graft copolymer. The silane-graft can include a subsequently cross-linkable moiety in the graft chain. For example, the cross-linking can occur under warm, moist conditions when the cross-linkable moiety is hydrolyzable, optionally in the presence of a suitable catalyst. Levels of cross-linking can be adjusted by varying the amount of silane-grafting introduced to the polyolefin resin or blend. The silane-grafting can occur in a separate process, or during a continuous blending and extruding process.