Laminated Foam Structures With Enhanced Properties

The invention relates to polymer foams. In particular, the invention relates to polymer foams having low densities with enhanced physical properties.

Foam structures are useful in the areas of packaging, athletics, water sports, and construction. In general, the foams are low density polymeric materials with good physical properties that are capable of supporting loads without adverse deformation. In general, the physical properties required by these applications suggest the use of high density foams. It is generally required that the foams have good proportional limit, compressive properties, shear properties, fatigue properties, and buckling limits, as defined, for example, in "Machinery's Handbook," E. Oberg, et al., Green, Ed., Industrial Press Inc., New York, 1992, pages 166, 168 and 253.

Physically-blown foams, particularly foams with enhanced physical properties are useful, for example, in packaging, automotive, construction, contact sports, water sports, exercise, and appliance applications. It is important to maintain good foam properties (e.g., cushioning and resistance to creasing) at low foam densities.

The invention features foam structures that are laminated and have enhanced physical properties, making them useful in the areas of packaging, athletics, water sports, and construction. In general, these structures include a core of a low density foam and one or more skins of relatively high density foam covering the core. The skins provide improved physical properties to the foam structures by, for example, improving the flexural strength, resistance to bending (or crimping), and resulting damage from bending in the laminated foam structure. The foam structures have improved stiffness, resist creasing, and more effectively dissipate loading forces of the foam. The outer surface of the foam structures is smooth and flat relative to the surface of the low density case. The low density core provides a relatively low-weight product that uses relatively small amounts of polymer material. Examples of physically-blown foams are described in U.S. application Ser. No. 08/638,122, filed Apr. 26, 1996 and entitled "Cross-Linked Low-Density Polymer Foam", which is incorporated herein by reference.

In one aspect, the invention features a laminated foam structure that includes a first foam article laminated to a first surface of a second foam article and a third foam article laminated to a second surface of the second foam article. The first foam article and the third foam article each have an average foam density that is at least 1.5 times greater than the average foam density of the second foam article and a volume that is at least 1.5 times smaller than the second foam article. The flexural stiffness of the laminated foam structure is 2-20 times higher than the flexural stiffness of the second foam article. Flexural stiffness can be determined directly from beam bending tests.

In preferred embodiments, the second foam article is a laminated foam article including at least two foams. Each of the two foams have an average foam density of less than 4 pounds per cubic foot. Preferably, each of the two foams has a thickness between about 1/4 and 1 inches. It is preferred that the first foam article and the third foam article each have an average foam density of between about 4 and 15 pounds per cubic foot. Preferably, each of the first foam article and the third foam article has a thickness between about 1/16 and 5/16 inches.

Preferably, the second foam article has an average foam density of between 1 and 3 pounds per cubic foot, the first foam article has an average foam density of between about 4 and 12 pounds per cubic foot, and the third foam article has an average foam density of between about 4 and 12 pounds per cubic foot. The first foam article and the third foam article each can be laminated foam articles including two foams each having an average foam density of greater than 4 pounds per cubic foot.

In preferred embodiments, the second foam article can further include a foam layer having an average foam density greater than about 4 pounds per cubic foot. Preferably, the foam layer has an average foam density between 4 and 15 pounds per cubic foot and a thickness between about 1/16 and 1/2 inches.

In preferred embodiments, the structure has a total thickness between about 3/4 and 12 inches.

In another aspect, the invention features a laminated foam structure including a first skin laminated to a first surface of a core, and a second skin laminated to a second surface of the core. The core includes a first foam having an average foam density of between about 1 and 4 pounds per cubic foot, the first skin includes a second foam and the second skin includes a third foam each having an average foam density of between about 4 and 15 pounds per cubic foot and a thickness less than 1/2 inches, and the laminated foam structure has a total thickness of less than about 12 inches. The flexural stiffness of the laminated foam structure is 2-20 times higher than the flexural stiffness of the core.

In yet another aspect, the invention features a body board that includes a laminated foam structure. The laminated foam structure includes a first skin laminated to a first surface of a core, and a second skin laminated to a second surface of the core. The core includes a first foam having an average foam density of between about 1 and 4 pounds per cubic foot, the first skin includes a second foam and the second skin includes a third foam each having an average foam density of between about 4 and 15 pounds per cubic foot and a thickness less than 1/2 inches, and the laminated foam structure has a total thickness of less than about 3 inches. The higher densities of the first and second skins can make the structure more resistant to mechanical damage from impact, shear, and abrasive loads due to the higher polymer and lower air content of the higher density foam.

In preferred embodiments, the first foam includes at least two laminated foam articles. The second foam can include at least two laminated foam articles or the third foam can include at least two laminated foam articles. Preferably, each of the foam articles has an average foam density of between 1 and 4 pounds per cubic foot and a thickness of between 1/4 and 1 inches. It is preferred that the first foam further include a foam layer having an average foam density greater than about 4 pounds per cubic foot and a thickness less than 1/2 inch.

In another aspect, the invention features a method of increasing the flexural strength of a core foam structure including the steps of laminating a first skin to a first surface of the structure, and laminating a second skin to a second surface of the core foam structure. The first skin includes a first foam having an average density that is at least 1.5 times greater than the average density of the core foam structure and a thickness that is at least 1.5 times smaller than the thickness of the core foam structure. The second skin includes a second foam having an average density that is at least 1.5 times greater than the average density of the core foam structure and a thickness that is at least 1.5 times smaller than the thickness of the core foam structure.

In preferred embodiments, the core foam structure is a laminated foam article including at least two foams each having an average foam density of less than 4 pounds per cubic foot. In other preferred embodiments, the first skin and the second skin each have an average foam density of between about 4 and 15 pounds per cubic foot. Preferably, the core foam structure has an average foam density of between 1 and 3 pounds per cubic foot, the first skin has an average foam density of between about 4 and 12 pounds per cubic foot, and the second skin has an average foam density of between about 4 and 12 pounds per cubic foot. The first skin and the second skin each can be laminated foam articles including two foams each having an average foam density of greater than 4 pounds per cubic foot.

In other preferred embodiments, the method further includes the step of including a foam layer having an average foam density greater than about 4 pounds per cubic foot in the core foam structure.

In other preferred embodiments, the foam includes a polyolefin. The polyolefin can be a polyethylene or polypropylene. Preferably, the foam further includes a single-site initiated polyolefin resin. In preferred embodiments, at least a portion of the foam is cross-linked.

The foam structures can include a variety of polyolefins in the composition, including single-site initiated polyolefin resins. Polyethylenes include ethylene-containing polyolefins. Single-site initiated polyolefin resins include polyolefins prepared from a single-site initiator that has controlled molecular weights and molecular weight distributions. The polyolefin can be polyethylene, polypropylene, or a copolymer of ethylene and alpha-unsaturated olefin monomers.

Copolymers include polymers resulting from the polymerization of two or more monomeric species, including 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 C.sub.3 -C.sub.20 alpha-olefins, and/or diolefins.

The average foam densities can be measured according to ASTM-3575, for example.

The foams in the laminated foam structures of the invention can be cross-linked. Cross-linking can occur by high energy irradiation, most preferably electron beam irradiation, peroxide treatment, or silane-grafting and cross-linking by treatment with water. Silane-grafting generally involves attaching one or more silicon-containing monomer or polymer to the original polymer chains. The use of silane-grafting for cross-linking in polymer foams is described, for example, in U.S. application Ser. No. 08/308,801, filed Sep. 19, 1994 and entitled "Cross-Linked Foam Structures of Essentially Linear Polyolefins and Process for Manufacture," which is incorporated herein by reference, and in U.S. application Ser. No. 08/638,122. The preferred foam structures contain silane-grafted cross-linked resins.

The foams of the laminated foam structures are generally closed-cell foams. The term "closed-cell," as used herein, means that predominantly, greater than approximately 70% of the foam cell volumes have cell walls isolating them from the external atmosphere. One way to determine this is by measuring the amount of water that is absorbed into the foam when the foam is immersed in water.

The invention can have one or more of the following advantages. The laminate structures include a core of a low density foam and one or more skins of relatively high density foam covering the core which improves, for example, the flexural strength, resistance to bending, and resulting damage from bending in the laminated foam structure. Because the skin is thin relative to the core, the overall weight of the laminated structure is increased little relative to the increase obtained in the physical properties of the structure. Additional improvement in the physical foam properties can result when a high density layer is added in the center of the low density core. The high density layer can help further dissipate loading forces.

In addition to improving the overall structural properties of the foam structures, the laminated structures can also have an improved smoother surface on the laminate. Because the skin generally has a higher average density than the core, it is generally a tougher material than the core. The "toughened" surface of the structure makes it more durable as well.

The foams including silane-grafted single-site initiated polyolefin resins generally have lower foam densities while retaining good strength and other physical foam properties. See, for example, U.S. application Ser. No. 08/638,122. In general, by lowering the average density and improving the physical properties of the laminated foam structures, laminated structures that contain less material are obtained. This decreases the cost of the materials and decreases wasted material compared to non-laminated structures.

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

FIG. 1 is a drawing depicting a laminated foam structure having a low density core and two high density skins.

FIG. 2 is a drawing depicting a laminated foam structure having a low density core with a high density foam layer and two high density skins.

FIG. 3 is a drawing depicting a laminated foam structure having a low density core and two laminated high density skins.

FIG. 4 is a graph depicting the bending stress curves for a low density foam core and a laminated foam structure having one higher density skin layer on each surface.

FIG. 5 is a graph depicting the bending stress curves for a low density foam core and a laminated foam structure having two higher density skin layers on each surface.

The laminated polymeric foam structures include a core of a low density foam and one or more skins of high density foam relative to the core that are laminated to the core. The skin covers a surface of the core. In general, the core has a skin on at least one surface of the core and can have a second skin laminated to another surface of the core. In general, each of the skins and the core can be a laminated foam structure. The laminated structure can be produced using any conventional lamination technique, including heat, film, or adhesive lamination. The laminated construction improves the mechanical properties of the structure such as proportional limit, compressive properties, shear properties, fatigue, and buckling.

Preferably, the foam articles are foam sheets or planks which can be prepared as described, for example, in U.S. application Ser. No. 08/638,122. Foam articles with a broad range in physical properties, including a broad range of average foam densities, can be prepared by the methods described therein. Particularly preferred laminated foam structures are described and illustrated in FIGS. 1, 2, and 3.

Referring to FIG. 1, the laminated foam structure 1 has a core 4 laminated to a first skin 6 on one surface of core 4. The core 4 is also laminated to a second skin 8 on a second surface of core 4. Skin 6 and skin 8 are generally foams having average densities of between 4 and 12 pounds per cubic foot and thicknesses of between 1/16 and 1/8 inches. Core 4 is a laminated foam with multiple layers. In these preferred embodiments, core 4 has four layers including foam 10, foam 12, foam 16, and foam 18. The foams 10, 12, 16, and 18 in core 4 each have average foam densities of between 1.2 and 2.5 pounds per cubic foot and thicknesses of between 3/8 and 5/8 inches. Core 4, and its constituent foams 10, 12, 16 and 18, have average foam densities that are low relative to the first skin 6 and second skin 8. The structure 1 has a total thickness 20 which is generally between 3/4 and 8 inches. Foams according to FIG. 2 can be used in packaging applications.

Referring to FIG. 2, the laminated foam structure 2 has a core consisting of a low density sub-core 4a and a low density sub-core 4b laminated, respectively, to each surface of a relatively high density foam layer 7. The foam layer 7 is a foam having an average density of between 4 and 12 pounds per cubic foot and a thickness of between 1/16 and 1/8 inches. The core is laminated to a first skin 6 on one surface of the core and to a second skin 8 on a second surface of the core. Skin 6 and skin 8 are generally foams having average densities of between 4 and 12 pounds per cubic foot and thicknesses of between 1/16 and 1/8 inches. Each of the sub-cores 4a and 4b is a laminated foam with multiple layers. In this preferred embodiment, sub-core 4a has three layers including foam 10, foam 12, and foam 13 and sub-core 4b has three layers including foam 14, foam 16, and foam 18. The foams 10, 12, 13, 14, 16, and 18 in the core each have average foam densities of between 1.2 and 2.5 pounds per cubic foot and thicknesses of between 3/8 and 5/8 inches. Each sub-core 4a and 4b, their constituent foams, and the core including foam layer 7 have average foam densities that are low relative to the first skin 6 and second skin 8. The structure 2 has a total thickness 20 which is generally between 3/4 and 8 inches or greater. Foams according to FIG. 2 can be used in packaging applications.

Referring to FIG. 3, the laminated foam structure 3 has a core 4 laminated to a first skin 6 on one surface of core 4. The core 4 is also laminated to a second skin 8 on a second surface of core 4. Core 4 is a laminated foam with multiple layers. In this embodiment, core 4 has four layers including foam 10, foam 12, foam 16, and foam 18 each have average foam densities of between 1.2 and 2.5 pounds per cubic foot and thicknesses of between 3/8 and 1/2 inches. Core 4, and its constituent foams 10, 12, 16 and 18, have average foam densities that are low relative to the first skin 6 and second skin 8. Skin 6 is a laminated foam including outer foam 22 and inner foam 24. Skin 8 is a laminated foam including outer foam 26 and inner foam 28. Foams 22, 24, 26, and 28 each have average densities of between 4 and 12 pounds per cubic foot and thicknesses of between 1/16 and 1/8 inches, with the outer foams 22 and 26 having average densities lower than the inner foams 24 and 28. In preferred embodiments, each of outer foams 22 and 26 has an average density of 6 pounds per cubic foot and a thickness of between 1/16 and 1/8 inches and each of inner foams 24 and 28 has an average density of 8 pounds per cubic foot and a thickness of about 1/8 inch. Core 4 is a laminated foam with multiple layers. In these preferred embodiments, core 4, and its constituent foams 10, 12, 16 and 18, have average foam densities that are low relative to the first skin 6 and second skin 8. The structure 3 has a total thickness 30 generally between 2 and 2 1/4 inches. Foams according to FIG. 3 can be used in water sports as body boards or kick boards, in exercise equipment (e.g., as gym mats), and in construction applications as eaves fillers.

The preferred foams are polyethylene foams that are described, for example, in U.S. application Ser. No. 08/638,122. The preferred skins are foams that have an average foam densities of between about 4 and 15 pounds per cubic foot (pcf), preferably between about 4 and 12 pcf, and thicknesses between 1/16 and 3/8 inches. Most preferably, the skins have average foam densities of about 8 pcf and thicknesses of 1/8 inch. The preferred core is a foam with an average foam density of less than 4 pcf, preferably between about 1.2 and 2.5 pcf. The core is a foam laminate with multiple foam layers each having thicknesses between about 3/8 and 5/8 inches. The laminated core preferably has between 2 and 20 foam layers. The total thickness of the core layer is determined by the overall thickness requirement of the application of the laminated foam structure. The total thickness of the laminated foam structure is, most preferably, between about 3/4 and 12 inches. The laminated core can include a high density foam layer, having an average foam density between about 4 and 12 pcf and thicknesses between 1/16 and 3/8 inches.

In embodiments in which the skin is a laminate, the outer layer of the skin preferably has a higher density than the adjacent foam layer. In the laminated skin, the preferred skin preferably has an average foam density of 8 pcf and a thickness of 1/8 inch which is laminated to an outer skin of having an average foam density of 6 pcf and a thickness of about 1/16 inch. The laminated skin structure gives better resistance to creasing in flexure, which is important, for example, in the water sports applications such as body boards.

The laminated foam structures and their potential applications are varied. For example, a laminated foam structure with a total thickness between about 3/4 to 8 inches is useful in packaging. The low density foam core contributes to a low weight of the total package while the high density foam of the skin provides aesthetic improvement and improved load spreading properties. The skin has a higher foam density, generally as a result of smaller cell size. As a result, the surface of the skin is denser and smoother than the surface of the low density core and has the appearance of a highly cross-linked surface.

The embodiments depicted in FIG. 1 or FIG. 2 are examples of structures that can be used in packaging applications. In another example, a 2 inch laminated foam structure can be used in exercise equipment such as gym mats, where the high density skin gives improved load spreading and resistance to damage resulting from heavy use. In another example, a 1 inch laminated foam structure can be used as a construction eaves filler, where the high density skin gives the structure improved compression resistance, a more durable surface, and improved die cutting characteristics. In another example, a 2 inch laminated foam structure can be used in the water sports industry for making body boards, where the improved flexural strength gives resistance to bending and creasing, as well as fatigue resistance. The embodiments depicted in FIG. 3 are examples of structures that can be used in water sports, exercise, and construction applications. In each of the preceding examples, the laminated foam structures generally provides for overall weight reductions over an extruded plank construction, increased compression resistance, and improved load spreading behavior.

The foams are generally foamed polymers and polymer blends. Examples of suitable polymers include single-site initiated polyolefins, low density polyethylene (LDPE), high density polyethylene (HDPE), linear low density polyethylene (LLDPE), ethylene-propylene rubber, ethylene-propylene-diene monomer terpolymer (EPDM), polystyrene, polyvinylchloride (PVC), polyamides, polyacrylates, celluloses, polyesters, polyhalocarbons, and copolymers of ethylene with propylene, isobutene, butene, hexene, octene, vinyl acetate, vinyl chloride, vinyl propionate, vinyl isobutyrate, vinyl alcohol, allyl alcohol, allyl acetate, allyl acetone, allyl benzene, allyl ether, ethyl acrylate, methyl acrylate, acrylic acid, or methacrylic acid. The polymer blends can also include rubber materials such as polychloroprene, polybutadiene, polyisoprene, polyisobutylene, nitrilebutadiene rubber, styrene-butadiene rubber, chlorinated polyethylene, chlorosulfonated polyethylene, epichlorohydrin rubber, polyacrylates, butyl rubber, or halobutyl rubber. The rubber material can be peroxide-cured or vulcanized. Preferred resins include single-site initiated polyolefins, LDPE, LLDPE, polypropylene, polystyrene, or ethylene copolymers such as ethylene-vinyl acetate copolymer (EVA), or ethylene-ethyl acrylate copolymer (EEA).

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 C.sub.3 -C.sub.20 olefin comonomers. Preferably, the alpha-unsaturated 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-1-pentene, 4-methyl-1-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 C.sub.4 -C.sub.20 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 such dienes include, but are not limited to, 1,3-butadiene, 1,4-hexadiene, 1,6-octadiene, 5-methyl-1,4-hexadiene, 3,7-dimethyl-1,6-octadiene, 3,7-dimethyl-1,7-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.

Single-site initiated polyolefin resins can be prepared using single-site initiators. 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.sup.-1 to about 1 million or higher g mol.sup.-1) and controlled molecular weight distributions (e.g., M.sub.w /M.sub.n in a range from nearly 1 to greater than 8, where M.sub.w is the weight average molecular weight and M.sub.n is the number average molecular weight). Molecular weights and molecular weight distributions of polymers can be determined, for example, by gel permeation chromatography.

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

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 Exact.TM., and include Exact.TM. 3022, Exact.TM. 3024, Exact.TM. 3025, Exact.TM. 3027, Exact.TM. 3028, Exact.TM. 3031, Exact.TM. 3034, Exact.TM. 3035, Exact.TM. 3037, Exact.TM. 4003, Exact.TM. 4024, Exact.TM. 4041, Exact.TM. 4049, Exact.TM. 4050, Exact.TM. 4051, Exact.TM. 5008, and Exact.TM. 8002. Other single-site initiated resins are available commercially from Dow Plastics, Midland, Mich. (or DuPont/Dow), under the tradenames Engage.TM. and Affinity.TM., and include CL8001, CL8002, EG8100, EG8150, PL1840, PL1845 (or DuPont/Dow 8445), EG8200, EG8180, GF1550, KC8852, FW1650, PL1880, HF1030, PT1409, CL8003, and D8130 (or XU583-00-01). Most preferably, the single-site initiated polyolefin resins are selected from the group consisting of Exact.TM. 3024, Exact.TM. 3031, Exact.TM. 4049, PL1845, EG8200, and EG8180.

The preferred foams include polyethylene, such as, for example, single-site initiated polyethylenes or LDPE. LDPE resins are described, for example, in "Petrothene.RTM. 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 535I, Dow 683, Dow 760C, Dow 768I, Dow 537I, Novacor LF219A, Novacor LC05173, Novacor LC0522A, Mobil LMA003, 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.

The foams can be cross-linked, however, non-cross-linked foams also can be made. The foams can be cross-linked with peroxides, UV irradiation, or by silane-grafting. The use of silane-grafting for cross-linking in polymer foams is described, for example, in U.S. application Ser. No. 08/308,801, and in U.S. application Ser. No. 08/638,122.

The foam can preferably be a polymer blend including at least one silane-grafted single-site initiated polyolefin resin. The preferred level of silane-grafted single-site initiated polyolefin resin, in weight percent of the total polymeric content of the foam, is preferably between about 2 percent and about 30 percent more preferably between about 3 percent and about 18 percent. The single-site initiated polyolefin resin can be silane-grafted before blending with other polymer resins. Alternatively, the foam can be a polymer blend. The blend can be 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 polymer blend. Alternatively, cross-linking can be introduced by reaction of the polymers with peroxides. UV irradiation of the polymers can also be used to introduce cross-linking.

A cross-linking graft can include other monomers, such as di- and tri-allyl cyanurates and isocyanurates, alkyl di- and tri-acrylates and methacrylates, zinc dimethacrylates and diacrylates, styrenes, divinylbenzene, and butadiene.

The graft initiator, or peroxide cross-linking agent can be a free radical generating species, for example, a peroxide. Examples of peroxides include dicumylperoxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-di-(t-butylperoxy)cyclohexane, 2,2'-bis(t-butylperoxy)diisopropylbenzene, 4,4'-bis(t-butylperoxy)butylvalerate, t-butylperbenzoate, t-butylperterephthalate, and t-butyl peroxide. Most preferably, the peroxide is dicumylperoxide or 2,2'-bis(t-butylperoxy)diisopropylbenzene.

The silane-grafted polymer can be cross-linked by exposure to moisture to effect silanol condensation reactions of the hydrolyzable groups of the pendant silane-grafts. Cross-linking develops through hydrolysis of the silane Y groups to form silanols which condense to form siloxanes. The condensation of silanols to siloxanes is catalyzed by metal carboxylates such as, for example, dibutyl tin dilaurate or dibutyl tin maleate. The most preferred silanol condensation catalyst is dibutyl tin dilaurate.

The cross-linking of silane-grafted polymers can be induced by the presence of atmospheric moisture, steam, or hot water. Cross-linking can take place predominantly (e.g., more than 50% of the potential cross-linking) prior to expansion (or extrusion) of the foam. Alternatively, the cross-linking can take place predominantly after expansion of the foam.

Exposure of the compositions to high energy radiation to induce cross-linking can be accomplished at dosages of ionizing radiation in the range of about 0.1 to 40 Megarads, and preferably, at about 1 to 20 Megarads. The amount of cross-linking can be appropriately controlled by adjusting the dosage of high energy radiation.

Regardless of the method of cross-linking used, acceptably flexible articles, particularly foamed articles, can only be obtained in certain ranges of cross-linking density or level, which is related to the amount of silane-grafting in the blend. Too much cross-linking can render the material inelastic. In a foam, this can result in less than optimal expansion and greater than optimal density for a given level of foaming agent. Too little cross-linking can be detrimental to physical properties such as compression set properties or thermal resistance, for example. It is important to choose cross-linking levels that afford materials with particular desired properties. The silane-grafting and resulting cross-links increase the melt strength of the composition. The cross-linking levels can be determined by establishing the gel content of the of the composition, for example, by extraction with a solvent such as xylenes.

The foams can be prepared using physical or chemical foaming agents. Physical foaming agents include low molecular weight organic compounds including C.sub.1 -C.sub.6 hydrocarbons such as acetylene, propane, propene, butane, butene, butadiene, isobutane, isobutylene, cyclobutane, cyclopropane, ethane, methane, ethene, pentane, pentene, cyclopentane, pentene, pentadiene, hexane, cyclohexane, hexene, and hexadiene, C.sub.1 -C.sub.5 organohalogens, C.sub.1 -C.sub.6 alcohols, C.sub.1 -C.sub.6 ethers, C.sub.1 -C.sub.5 esters, C.sub.1 -C.sub.5 amines, ammonia, nitrogen, carbon dioxide, neon, or helium. Chemical foaming agents include, for example, azodicarbonamide, p-p'-oxybis(benzene)sulfonyl hydrazide, p-toluenesulfonyl hydrazide, p-toluenesulfonyl semicarbazide, 5-phenyltetrazole, ethyl-5-phenyltetrazole, dinitrosopentamethylenetetramine, and other azo, N-nitroso, semicarbazide, sulfonyl hydrazides, carbonate, and bicarbonate compounds that decompose when heated. The preferred foaming agents include azodicarbonamide or isobutane.

The foam can be partially or extensively cross-linked prior to expansion, or can be extensively cross-linked after expansion.

Additional additives in the foam composition can dramatically effect the properties of the foam. These include gas exchange additives and cell nucleating agents, such as zinc stearate and talc, respectively. The preferred gas exchange additive concentration in the foam is between 0.5 and 2.0 percent. The preferred cell nucleating agent concentration in the foam is between 0.05 and 2.0 percent. The foam can also include gas exchange additives, also known as cell-structure stabilizers, such as, for example, fatty acids, fatty acid carboxylate salts (e.g., zinc stearate), fatty acid esters (e.g. glycerol monostearate), or fatty acid amides, assist in the gas exchange process and the aging of the foams.

Other additives, alone or in combination, can be added to the foam compositions, including antioxidants (e.g., hindered phenolics such as Irganox 1010, phosphites such as Irgafos 168, or polymerized trimethyldihydroquinoline such as Agerite AK, Resin D or Flectol H), ultra-violet stabilizers, thermal stabilizers, antistatic components, flame retardants, pigments or colorants, and other processing aids.

The foam can take virtually any physical configuration, preferably the form of a sheet, plank, or other regular or irregular extruded profile. Foam sheets are extruded from circular dies and have thicknesses between about 1/32 inch and 1 inch and widths up to 82 inches. Parts of smaller size, depending on requirements of the application, can be cut from the sheets. For example, a board with typical dimensions of 20 inches by 30 inches may be cut from the larger sheets, and further shaped by molding or machining to produce a body board for water sports. Alternatively, the foams can be configured as planks, extruded from flat dies, with plank thicknesses between about 1 inch and 4.5 inches and widths between about 24 inches and 48 inches. The foam planks and sheets can be laminated by direct application of heat or adhesives to the interface between two or more planks. In preferred embodiments, it is not necessary to add an adhesive to the interface to laminate the planks or sheets.

The foam lamination can be achieved by heat treatment of the laminate interface, film lamination, or by using an adhesive. These techniques are generally well known in the sheet fabrication industries. Heat lamination is a process in which two sheets of foam or other sheet material are brought together under pressure and heat to join the materials. In practice, foam is taken from rolls of approximately 1/2 inches thickness .times.48 inches width .times.400 feet in length. The foam sheets are fed together with pressure exerted by two turning rollers. Immediately prior to the materials meeting in the nip of the rollers, heat is applied to the surfaces which are about to be pressed together. The heat can be supplied by hot air guns, gas-fired flames, infrared heaters, or a combinations thereof. Heat can be applied to both foam sheets, or only to one. The heat makes the foam surface tacky by creating local regions of melting on the surface. The foam sheets passing through the rollers nip are joined by a bond upon cooling. A similar laminate can be made by applying an adhesive to one or both sheets prior to the foam passing through the nip rollers, or by extrusion of a thin continuous layer of polymer onto one surface immediately prior to the foam passing through the nip rolls. By choosing a film material which is compatible with the substrates, a laminate is formed. Adhesives include, but are not limited to, rubber, epoxy, and acrylic adhesives. Heat and film lamination methods are preferred since those methods can avoid the use of solvents in the lamination process.

In adhesive lamination, the foam articles can be coated with an adhesive using any of a number of conventional coating techniques including reverse roll coating, knife over roll coating, or extrusion coating. Optionally, the coated substrate can be passed through an in-line dryer to remove solvent or water, or to chemically alter the coating. Machinery for coating these tapes can be purchased from equipment suppliers such as Ameriflex Group Incorporated, Black Clawson Converting Machinery Corporation, Inta-Roto, Incorporated, Klockner Er-We-Pa, and Wolverine Massachusetts Corporation.

The following specific examples are to be construed as merely illustrative, and not limitive, of the remainder of the disclosure.

A laminated foam structure having four low density foam layers in a core and a higher density skin on each surface of the core can be manufactured using the following steps:

Step 1

Four layers of 1/2 inch polyethylene foam with a density of 1.7 pcf are continuously laminated from roll stock using hot air injected between the layers which are then pressed together between nip rolls. Sheets are cut after lamination to make handling easier. The 1.7 pcf laminated foam core (hereafter referred to as Example 1A) had a total thickness of 1 inch.

Step 2

A layer of 8 pcf polyethylene foam that is 3/16 inches thick is laminated to one side of the foam core of Example 1A (i.e., the 1 inch thick 1.7 pcf laminated foam core) by feeding the 1 inch thick sheets into the laminator used in step 1. The 8 pcf layer is fed from roll stock. The resultant sheets are cut as in step 1.

Step 3

Step 2 is repeated to laminate a second 8 pcf polyethylene foam layer that is 3/16 inches thick to the other side of the 1.7 sheet, resulting in a laminated foam structure consisting of a core of four laminated 1.7 pcf polyethylene foams with a skin of 8 pcf polyethylene foam on each side of the core with a total thickness of 2.1 inches (i.e., an 8/1.7/8 laminate, hereafter referred to as Example 1). Example 1 has a structure similar to that shown in FIG. 1.

A foam laminate structure having four low density foam layers in a core and a two-layer higher density skin on each surface of the core can be manufactured using the following steps:

Step 1

A four-ply laminated foam core of 2 pcf, 1/2 inch thick polyethylene foam sheets is produced by the process of step 1 of Example 1 (hereafter referred to as Example 2A).

Step 2

A layer of 8 pcf polyethylene foam that is 3/16 inches thick is laminated to each side of the four-ply laminated foam core Example 2A by the process of step 2 and step 3 of Example 1 to afford an 8/2/8 laminated foam structure.

Step 3

A layer of 6 pcf polyethylene foam that is 1/8 inches thick is laminated to each side of the 8/2/8 laminated foam structure of step 2 by the process of step 2. The final laminated foam structure has a core of four laminated 2 pcf polyethylene foams with a skin of 8 pcf foam and an outer skin of 6 pcf foam on each side of the core with a total thickness of 2.25 inches (i.e., an 6/8/2/8/6 laminate, hereafter referred to as Example 2). Example 1 has a structure similar to that shown in FIG. 3.

Bending Test

The flexural stiffness, or flexural strength, of the laminated foam structures were tested by bending a 36 inch length of the laminated foam structures Example 1 (the 8/1.7/8 laminate) and Example 2 (the 6/8/2/8/6 laminate). A 36 inch length of the corresponding core structures Example 1A (the 1.7 pcf core) and Example 2A (the 2 pcf core) at the same thicknesses was bent as a comparison. Beam bending tests are used in the plastics industry as a measure or stiffness. As an example, ASTM-D790 and ASTM-D229 are used extensively to evaluate the flexural modulus (i.e., flexural strength) of solid plastics. In our method, two point supports are located 36 inches apart on a solid horizontal surface. The midpoint of the supported length is determined. Successive weights are added to the midpoint in increments of 100 to 1000 grams, and the deflection is immediately measured. Stiffer materials are more resistant to bending and produce lower deflection. Deflection is measured in inches displaced from the straight beam position. The bending stress curves for Example 1A and Example 1 are shown in FIG. 4. The bending stress curves of Example 2A and Example 2 are shown in FIG. 5.

Generally, the laminated foam structures have much higher resistance to bending than the comparative core foams. Also, the laminated foam structures (e.g., Example 1) do not crease when bent at angles up to 90 degrees. However, under the same conditions, the laminate without a higher density foam outer layer (e.g., Example 1A), creases on the surface.

Example 2 is a suitable construction for use in a body board, for example. Suitable foams balance the stiffness and density properties for particular applications. The body board product should be as light as possible, affording the greatest flotation for its size (i.e., have the lower total density). The surfaces should resist abrasion (e.g., from sand) and resist absorption of water (i.e., it should be a closed cell foam). The board should be stiff enough to resist the mechanical forces imparted to the board during use (i.e., it should resist creasing). Buckling under ordinary use is undesirable, since this results in permanent damage which cannot be repaired.

Other embodiments are within the claims.