Coextruded crosslinked polyolefin foam with a polyamide cap layer

Abstract

The present invention provides a physically crosslinked, closed-cell, open-cell, multilayer foam structure comprising a foam layer comprising polypropylene, polyethylene, or a combination of polypropylene and polyethylene, and a polyamide cap layer, obtained by coextruding a multilayer structure comprising at least one foam composition layer and at least one cap composition layer, irradiating the coextruded structure with ionizing radiation, and continuously foaming the irradiated structure.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to and the benefit of U.S. patent application Ser. No. 16 / 836,229, filed March 31, 2020, and U.S. patent application Ser. No. 16 / 836,389, filed March 31, 2020, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a multilayer polyolefin foam / polyamide cap structure and a method for making said structure, and more particularly to a method for making a coextruded crosslinked multilayer polyolefin foam / polyamide cap structure. [Background technology]

[0003] Crosslinked polyolefin foams can be used in a variety of commercial applications, including, but not limited to, decorative components in vehicle interiors, such as door panels. To prepare polyolefin foams for use in vehicle door panels, the foam layer is typically first laminated to a film, fabric, or foil to create a bilaminate. This flexible bilaminate must then be combined with a rigid substrate to form a panel. The automotive industry employs a variety of manufacturing methods to combine the flexible bilaminate into panels, including thermoforming techniques such as negative vacuum forming (NVF) and positive vacuum forming (PVF), compression molding, and low pressure molding (LPM). In low-pressure molding, the bilaminate is placed in a mold with the film, fabric, or foil facing the "A" side of the mold. The mold is closed and polypropylene is injected into the "B" side of the cavity, filling the mold and forming the panel. In commercial production processes, polypropylene is typically an ultra-high melt flow (50-125 grams per 10 minutes at 230°C), impact-modified homopolymer or random copolymer injected at approximately 200°C. LPM design and implementation presents challenges. Because injection temperatures can be significantly higher than the foam's melt temperature, the hot polypropylene at and around the injection point can shear or degrade the foam. In some situations, visual defects such as "orange peel" can be observed from the "A" side at and around the injection point. In other situations, the foam around the injection point can shear completely, leaving a visual depression in the film, fabric, or foil at the injection site.

[0004] To mitigate these issues, manufacturers have implemented various techniques, including injecting polypropylene into the trim section of the bilaminate. While this technique is generally effective at resolving the foam shear and degradation issues within the mold cavity, it increases the cost of producing panels. Injecting into the trim section requires an additional length of trim section along at least one side of the mold. The trim section along the injection side then also contains injected polypropylene. The cost of adding this wasteful trim section, which contains both the bilaminate and injected polypropylene, can be substantial. Furthermore, this trim section cannot be easily recycled, resulting in additional waste and additional costs. Another technique to reduce foam shear and degradation defects in LPM is the use of flexible trilaminates. The trilaminate can be an LPM bilaminate with a flexible homopolymer-based thermoplastic polyolefin (TPO) or thermoplastic elastomer (TPE) layer laminated to the "B" side of the bilaminate. The TPO or TPE layer acts as a protective and / or sacrificial skin layer between the foam and the injected polypropylene. However, using a trilaminate in LPM also presents challenges. The thickness of the TPO or TPE layer can be significant relative to the overall thickness of the bilaminate, increasing material costs. Creating the trilaminate requires a second lamination step, further increasing the cost of the trilaminate. Finally, the protective TPO or TPE layer is also susceptible to shear and degradation at the injection site. Manufacturers of vehicle door panels using LPM technology typically continue to inject polypropylene into the trim of the trilaminate. While the amount of trim required for this construction is reduced with the bilaminate, it still requires more trim than direct injection into the mold cavity. The cost of the additional trim, including both the trilaminate and the injected polypropylene, is significant. The difficulty of recycling the trim adds to the cost of this manufacturing technique. Summary of the Invention

[0005] It has been discovered that physically crosslinked, closed-cell polyolefin foams having at least one polyamide cap layer can be produced in a continuous process. The multilayer structure can be laminated to a film, fabric, or foil to create a bilaminate. The bilaminate can then be used in LPM applications to overcome the problems associated with conventional LPM bilaminates and trilaminates used to produce vehicle interior trim parts.

[0006] In some embodiments, the polyamide layer can act as a more effective protective layer for the injected polypropylene (compared to a TPO or TPE layer). Common commercially available polyamides vary widely in melting point, but most have melting points higher than homopolymer polypropylene. Therefore, it is possible to select a polyamide with a melting point that not only exceeds the melting point of the injected polypropylene, but also exceeds the injection temperature. The polyamide's high melting point allows the polyamide to remain intact and provide a barrier that prevents it from melting or shearing upon contact with the injected polypropylene and at the injection temperature. Furthermore, trimming regions need not be injected, further reducing trimming waste costs. Because the polyamide cap layer is non-sacrificial, when properly selected for LPM applications, it can be substantially thinner than the TPO or TPE layer, thereby further reducing material costs. Maleic anhydride-grafted polypropylene can be a suitable compatibilizer for polyolefins and polyamides. Many maleic anhydride-grafted impact-modified polypropylene homopolymers and maleic anhydride-grafted random polypropylene copolymers are commercially available. This allows them to be a direct replacement for injection-molded impact-modified homopolymers or random copolymers in conventional LPM manufacturing processes with minimal adjustments.

[0007] In some embodiments, a multilayer foam structure is provided, the multilayer foam structure comprising: a foam layer comprising polypropylene, polyethylene, or a combination of polypropylene and polyethylene; and a cap layer on the side of the foam layer comprising polyamide and polypropylene, polyethylene, or a combination of polypropylene and polyethylene. In some embodiments, the foam layer and the cap layer may be coextruded. In some embodiments, the foam layer may comprise at least 70% by weight of polypropylene, polyethylene, or a combination of polypropylene and polyethylene. In some embodiments, the cap layer may comprise at least 40% by weight of polyamide. In some embodiments, the cap layer may comprise up to 50% by weight of polypropylene, polyethylene, or a combination of polypropylene and polyethylene; and at least 40% by weight of polyamide. In some embodiments, the cap layer may have a thickness of less than 1 mm. In some embodiments, the foam layer may comprise a crosslinking accelerator in an amount of 0.5 to 5% by weight. In some embodiments, the foam layer may comprise an additive in an amount of 1 to 20% by weight. In some embodiments, the cap layer may comprise an additive in an amount of 1 to 10% by weight. In some embodiments, the polypropylene may have a melt flow index of 0.1 to 25 grams per 10 minutes at 230° C. In some embodiments, the polyethylene may have a melt flow index of 0.1 to 25 grams per 10 minutes at 190° C. In some embodiments, the multilayer foam structure has a density of 20 to 250 kg / m 3 In some embodiments, the degree of crosslinking of the multilayer foam structure may be 20-75%. In some embodiments, the average closed cell size of the multilayer foam structure may be 0.05-1.0 mm. In some embodiments, the thickness of the multilayer foam structure may be 0.2-50 mm. In some embodiments, a laminate is provided, the laminate comprising a multi-layer foam structure including a foam layer comprising polypropylene, polyethylene, or a combination of polypropylene and polyethylene; a cap layer on a side of the foam layer comprising polyamide and polypropylene, polyethylene, or a combination of polypropylene and polyethylene; and a laminate layer on a side of the foam layer opposite the cap layer. In some embodiments, the foam layer and the cap layer may be coextruded. In some embodiments, the laminate layer may be a flexible film, fabric, or foil.

[0008] As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms unless the context clearly dictates otherwise. Additionally, as used herein, the term "and / or" is understood to refer to and encompass any and all possible combinations of one or more of the associated listed items. Furthermore, as used herein, the terms "include," "including," "comprise," and / or "comprising" specify the presence of stated features, integers, steps, operations, elements, components, and / or units, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, units, and / or groups thereof. It is understood that aspects and embodiments described herein include "consisting of" and / or "consisting essentially of" that aspect and embodiment. For all methods, systems, compositions, and devices described herein, the methods, systems, compositions, and devices can include the listed components or steps or "consist of" or "consist essentially of" the listed components or steps. When a system, composition, or device is described as "consisting essentially of" the listed components, the system, composition, or device either includes the listed components and may include other components that do not substantially affect the performance of the system, composition, or device, but is completely free of other components other than the specifically listed components that substantially affect the performance of the system, composition, or device; or it is free of additional components in sufficient concentrations or amounts to substantially affect the performance of the system, composition, or device. When a method is described as "consisting essentially of" the listed steps, the method includes the listed steps and may include other steps that do not materially affect the results of the method, but the method does not include any other steps other than the specifically listed steps that materially affect the results of the method.

[0009] In the present disclosure, "substantially free" of a particular component, composition, compound, or ingredient in various embodiments means that the particular component, composition, compound, or ingredient is present at less than about 5%, 2%, 1%, 0.5%, 0.1%, 0.05%, 0.025%, or 0.01% by weight. Preferably, "substantially free" of a particular component, composition, compound, or ingredient means that the particular component, composition, compound, or ingredient is present at less than about 1% by weight. Other advantages will become readily apparent to those skilled in the art from the following detailed description.The examples and descriptions herein are to be regarded as illustrative in nature and not as limiting. Various embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional image at 100x magnification showing a microtome section of unfoamed Example 1B. [Figure 2] 1B is a 20x magnification image tilted 45° from the cap surface showing foamed Example 1B. [Figure 3] 10 is a cross-sectional image at 100x magnification showing a microtome section of unfoamed Example 2A. [Figure 4] 1 is a 20x magnification image tilted 45° from the cap surface showing foamed Example 2A. [Figure 5] 10 is a cross-sectional image at 100x magnification showing a microtome section of unfoamed Example 2D. [Figure 6] 10 is a 20x magnification image tilted 45° from the cap surface showing foamed Example 2D. [Figure 7] 10 is a cross-sectional image at 100x magnification showing a microtome section of unfoamed Example 2G. [Figure 8] 1 is a 20x magnification image tilted 45° from the cap surface showing foamed Example 2G. DETAILED DESCRIPTION OF THE INVENTION

[0011] A method for producing a crosslinked, closed-cell, co-extruded multi-layer foam structure comprising at least a foam layer comprising polypropylene, polyethylene, or a combination thereof, and at least a cap layer comprising a polyamide is described. The method for producing the crosslinked, closed-cell, co-extruded multi-layer foam structure layers may comprise the steps of (a) coextrusion, (b) irradiation, and (c) foaming. Coextrusion is the simultaneous extrusion of multiple layers of material. This type of extrusion utilizes two or more extruders to deliver a steady volumetric throughput of material to an extrusion head (die) that can extrude the material into the desired shape. In a coextrusion process, the foam composition can be fed into multiple extruders to form an unfoamed multilayer structure. For example, "A" foam composition can be fed into one extruder and "B" foam composition can be fed into a second extruder. The method for feeding the ingredients into the extruders can be based on the design of the extruders and available material processing equipment. If necessary, the ingredients of the foam composition can be premixed to facilitate dispersion. A Henschel mixer can be used for such premixing. All ingredients can be premixed and fed through a single port in the extruder. Alternatively, each ingredient can be fed individually through its own designated port. For example, if the crosslinking accelerator or any other additives are liquid, the accelerator and / or additives can be added through the extruder feed gate (or gates) or through the extruder's flow port (if equipped) instead of being premixed with the solid ingredients. Premixing and port feeding of individual ingredients can also be combined.

[0012] Each extruder delivers a steady amount of each composition to one or more manifolds, which then feed the sheet die to create the coextruded, unfoamed multilayer sheet. Two common methods for coextruding materials include (1) a feedblock manifold; and (2) a multi-manifold within the die. A feedblock manifold contains the following elements: (a) multiple inlet ports for the top, middle, and bottom layers; (b) a streamlined melt stacking area that channels the separate streams into a single, laminated melt stream within the feedblock; (c) an adapter plate between the feedblock and the sheet die; and / or (d) a sheet die (similar to a monolayer die). In a sheet die, the laminated melt streams enter the center of the die, spread along the manifold, and exit the die outlet as a distinct, multilayered extrudate. A multi-manifold die has the following elements: (a) it is similar to a single-layer die except that it has multiple feed channels; (b) each melt channel has its own choker bar for flow control; and / or (c) the melt streams converge inside the die near the outlet, resulting in a distinctly multi-layered extrudate. The thickness of the layers can be determined by the design of the manifold(s) and die. For example, an 80 / 20 feedblock manifold can deliver compositions at a ratio of about 4:1 when the speed and size of each extruder are appropriately matched. This ratio can be altered, for example, by changing: (a) the relative extrusion speed between one extruder and another; (b) the relative size of each extruder; and / or (c) the composition (i.e., viscosity) of the individual layers. The overall thickness of the multilayer sheet can be controlled by the overall die gap, but the overall thickness of the multilayer sheet can be further adjusted, for example, by stretching (i.e., "drawing") the molten multilayer extrudate and / or by flattening the molten multilayer extrudate through a nip.

[0013] The multilayer structure can include at least two layers formed from different compositions. In some embodiments, the multilayer structure includes at least one layer formed from a foamed composition and at least one layer formed from a non-foamed cap composition. In some embodiments, the structure can be a B / A layered structure, a B / A / B layered structure, a B / A / C layered structure, or can have multiple other layers. In some embodiments, the multilayer structure can include additional layers, such as a tie layer, a film layer, and / or an additional foam layer. The cap composition fed to the extruder can include at least one polyamide and polypropylene, polyethylene, or a combination thereof. The foam composition fed to the extruder can include polypropylene, polyethylene, or a combination thereof. Polyamides are polymers containing amide groups (-CONH-) as repeating moieties in the chain. Polyamides include, but are not limited to, aliphatic polyamides formed either by the condensation reaction of two difunctional monomers or by the ring-opening addition polymerization of cyclic compounds. Polyamides can be homopolymers, copolymers, terpolymers, or blends. Importantly, semicrystalline polyamides or blends of polyamides are preferred over amorphous polyamides or blends of polyamides. Commercially available aliphatic polyamide homopolymers include, but are not limited to, types 6, 11, 12, 46, 410, 56, 510, 511, 512, 513, 514, 66, 69, 610, 612, 613, 1010, 1012, and 1212. Commercially available aliphatic polyamide copolymers include, but are not limited to, 6 / 66, 6 / 69, 610 / 66, and 56 / 12 types. Commercially available aliphatic polyamide terpolymers include, but are not limited to, 6 / 66 / 12 types.

[0014] Polypropylenes include, but are not limited to, polypropylene, impact modified polypropylene, polypropylene-ethylene copolymer, impact modified polypropylene-ethylene copolymer, metallocene polypropylene, metallocene polypropylene-ethylene copolymer, metallocene polypropylene olefin block copolymer (block sequence controlled), polypropylene-based polyolefin plastomer, polypropylene-based polyolefin elastomer-plastomer, polypropylene-based polyolefin elastomer, polypropylene-based thermoplastic polyolefin, and polypropylene-based thermoplastic elastomer blends. Polypropylene can also be of a high melt strength type. Additionally, polypropylene can be grafted with maleic anhydride. Polyethylenes include, but are not limited to, low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE) (homopolymer, copolymer with butene, hexene, or octene, terpolymer with butene and / or hexene and / or octene), very low-density polyethylene (VLDPE) (homopolymer, copolymer with butene, hexene, or octene, terpolymer with butene and / or hexene and / or octene), linear very low-density polyethylene (VLLDPE) (homopolymer, copolymer with butene, hexene, or octene, terpolymer with butene and / or hexene and / or octene), high-density polyethylene (HDPE), polyethylene-propylene copolymer, metallocene polyethylene, metallocene ethylene-propylene copolymer, and metallocene polyethylene olefin block copolymer (with controlled block sequence), any of which may contain a grafted compatibilizer or copolymer with acetate and / or ester groups. These polyethylenes may also be grafted with maleic anhydride. These polyethylenes may be copolymers and terpolymers having acetate and / or ester groups, and may also be ionomers of copolymers and terpolymers having acetate and / or ester groups.

[0015] The non-foamed cap composition fed to the extruder can also include a polyamide, with the amount of polyamide being at least about 40% by weight, preferably at least about 50% by weight, more preferably at least about 60% by weight, and even more preferably at least about 70% by weight. In some embodiments, the amount of polyamide in the non-foamed cap composition fed to the extruder is about 40% by weight or more, about 50% by weight or more, about 60% by weight or more, or about 70% by weight or more. In some embodiments, the amount of polyamide in the non-foamed cap composition fed to the extruder can be about 95% by weight or less, about 90% by weight or less, about 85% by weight or less, or about 80% by weight or less. In some embodiments, the amount of polyamide in the non-foamed cap composition fed to the extruder can be about 40-95% by weight, about 50-90% by weight, about 60-85% by weight, or about 70-80% by weight. In some embodiments, the amount of polyethylene, polypropylene, or a combination thereof in the non-foamed cap composition fed to the extruder can be about 5% by weight or more, about 10% by weight or more, or about 20% by weight or more. In some embodiments, the amount of polyethylene, polypropylene, or a combination thereof in the non-foamed cap composition fed to the extruder can be about 50% by weight or less, about 40% by weight or less, about 35% by weight or less, or 30% by weight or less. In some embodiments, the amount of polyethylene, polypropylene, or a combination thereof in the non-foamed cap composition fed to the extruder can be 5-50% by weight, 10-40% by weight, or 20-30% by weight. The foam composition fed to the extruder can comprise at least about 75% by weight of polypropylene, polyethylene, or a combination thereof, preferably at least about 80% by weight, more preferably at least about 85% by weight, and even more preferably at least about 90% by weight. In some embodiments, the foam composition fed to the extruder may comprise at least about 70%, about 80%, or about 85% by weight of polypropylene, polyethylene, or a combination thereof. In some embodiments, the foam composition fed to the extruder may comprise at most about 98%, about 95%, or about 90% by weight of polypropylene, polyethylene, or a combination thereof. In some embodiments, the foam composition fed to the extruder may comprise about 70-98%, about 80-95%, or about 85-90% by weight of polypropylene, polyethylene, or a combination thereof.

[0016] The ability to create a wide variety of multilayer structures and foam articles using the disclosed compositions allows for a wide range of polyamides, polypropylenes, and polyethylenes to be used in the compositions to meet a variety of in-process manufacturing and commercial end-use requirements. Non-limiting examples of "polypropylene" include isotactic homopolypropylene. Commercially available examples include, but are not limited to, Braskem FF018F, Total Petrochemicals 3271, and Phillips 66 COPYLENE™ CH020. Non-limiting examples of "impact-modified polypropylene" include homopolypropylenes containing ethylene-propylene (EP) copolymer rubber. The rubber can be amorphous or semi-crystalline, but not in sufficient amounts to impart any plastomeric or elastomeric properties to the material. Some non-limiting examples of commercially available "impact-modified polypropylene" include Braskem TI4003F and TI4015F, and LyondellBasell Pro-fax® 8623 and Pro-fax® SB786. "Polypropylene-ethylene copolymer" is polypropylene with random ethylene units. Some non-limiting examples of commercially available "polypropylene-ethylene copolymer" include 6232, 7250FL, and Z9421 from Total Petrochemicals, 6D20 and DS6D81 from Braskem, and PRO-FAX® RP311H and ADSYL® 7415XCP from LyondellBasell.

[0017] "Impact-modified polypropylene-ethylene copolymer" is polypropylene with random ethylene units and ethylene-propylene (EP) copolymer rubber. The rubber can be amorphous or semi-crystalline, but not in sufficient amounts to impart any plastomeric or elastoplastomeric properties to the material. A non-limiting example of a commercially available impact-modified polypropylene-ethylene copolymer is PRISMA® 6910 manufactured by Braskem. "Metallocene polypropylene" includes metallocene syndiotactic homopolypropylene, metallocene atactic homopolypropylene, and metallocene isotactic homopolypropylene. Non-limiting examples of "metallocene polypropylene" include those commercially available under the trade names METOCENE® manufactured by LyondellBasell and ACHIEVE™ manufactured by ExxonMobil. Metallocene polypropylenes include, but are not limited to, grades M3551, M3282MZ, M7672, 1251, 1471, 1571, and 1751 commercially available from Total Petrochemicals. "Metallocene polypropylene-ethylene copolymers" are metallocene syndiotactic, metallocene atactic, and metallocene isotactic polypropylenes with random ethylene units. Commercially available examples include, but are not limited to, Lumicene® MR10MX0 and Lumicene® MR60MC2 manufactured by Total Petrochemicals, Purell® SM170G manufactured by LyondellBasell, and the Wintech® product line manufactured by Japan Polypropylene Corporation. A "metallocene polypropylene olefin block copolymer" is a polypropylene that has a controlled block sequence, i.e., a crystalline hard "block" and an amorphous soft "block" that are arranged in an alternating fashion. Examples of "metallocene polypropylene olefin block copolymers" include, but are not limited to, the INTUNE® product line manufactured by Dow Chemical.

[0018] "Polypropylene-based polyolefin plastomers" (POPs) and "polypropylene-based polyolefin elastoplastomers" are both metallocene and non-metallocene propylene-based copolymers with plastomeric and elastoplastomeric properties. Non-limiting examples include those commercially available under the trade names VERSIFY® (metallocene) from Dow Chemical, VISTAMAXX® (metallocene) from ExxonMobil, and KOATTRO™ (non-metallocene) from LyondellBasell (a butene-1-based line of plastomeric polymers, with certain grades based on butene-1 homopolymer materials and other grades based on polypropylene-butene-1 copolymer materials). "Polypropylene-based polyolefin elastomers" (POEs) are both metallocene and non-metallocene propylene-based copolymers with elastomeric properties. Non-limiting examples of propylene-based polyolefin elastomers include polymers available under the tradenames VERSIFY® (metallocene) manufactured by Dow Chemical and VISTAMAXX® (metallocene) manufactured by ExxonMobil. "Polypropylene-based thermoplastic polyolefins" (TPOs) include polypropylene, polypropylene-ethylene copolymers, metallocene homopolypropylenes, and metallocene polypropylene-ethylene copolymers, which contain a sufficient amount of ethylene-propylene copolymer rubber to impart plastomeric, elastomeric, or elastomeric properties to the thermoplastic polyolefin blend (TPO). Non-limiting examples of TPO polymers include those commercially available under the trade names THERMORlJN® and ZELAS® manufactured by Mitsubishi Chemical Corporation, ADFLEX® and SOFTELL® manufactured by LyondellBasell, TELCAR® manufactured by Teknor Apex, and WELLNEX® manufactured by Japan Polypropylene Corporation. TPOs can be produced via multi-stage polymerization (e.g., ZELAS®, ADFLEX®, SOFTELL®, WELLNEX®) or by blending (e.g., THERMORlJN®, TELCAR®).

[0019] "Polypropylene-based thermoplastic elastomer blends" (TPEs) include polypropylene, polypropylene-ethylene copolymers, metallocene homopropylenes, and metallocene polypropylene-ethylene copolymers, containing a diblock or multiblock thermoplastic rubber modifier (such as SEBS, SEPS, SEEPS, SEP, SERC, CEBC, or HSB) in an amount sufficient to impart plastomeric, elastoplastomer, or elastomeric properties to the thermoplastic elastomer blends (TPEs). Non-limiting examples of polypropylene-based thermoplastic elastomer blends include those commercially available under the trade names GLS™ DYNAFLEX™ and GLS™ VERSAFLEX™ manufactured by Polyone, MONPRENE™ manufactured by Teknor Apex, and DURAGRIP™ manufactured by LyondellBasell. Additionally, any of the above polypropylenes may be of the high melt strength (HMS) type. Polypropylene manufacturers employ various methods to toughen the polymer in the melt phase. For example, polypropylenes exhibiting long chain branching (LCB) can be identified as high melt strength polypropylenes. Non-limiting examples of high melt strength polypropylenes include polymers commercially available under the trade names DAPLOY® manufactured by Borealis, AMPPLEO® manufactured by Braskem, and Waymax® manufactured by Japan Polypropylene. Any polypropylene, and more generally blends of TPO and TPE, may optionally be oil extended, for example with mineral oil, Chevron PARALUX® processing oil, etc., to further soften the blend, enhance the tactile properties of the blend, or improve the processability of the blend. "LDPE" and "LLDPE" refer to low density polyethylene and linear low density polyethylene, respectively. Non-limiting examples of LDPE include products from at least Dow (e.g., 640I) and Nova (e.g., Novapol® LF-0219-A). Non-limiting examples of LLDPE include products from at least ExxonMobil (e.g., LLP8501.67) and Dow (e.g., DFDA-7059NT7). Commercially available LLDPE polymers are typically copolymers or terpolymers containing the alpha olefins butene and / or hexene and / or octene.

[0020] "VLDPE" and "VLLDPE" refer to very low density polyethylene and linear very low density polyethylene, which are typically copolymers or terpolymers containing butene and / or hexene and / or octene alpha-olefins. Non-limiting examples of VLDPE and VLLDPE include those commercially available under the tradenames FLEXOMER® manufactured by Dow Chemical and certain grades of STAMYLEX® manufactured by Borealis. "Metallocene polyethylene" refers to metallocene-based polyethylene with non-elastomeric to elastomeric properties. Non-limiting examples of metallocene polyethylene include those commercially available under the trade names ENGAGE™ manufactured by Dow Chemical, ENABLE™ and EXCEED™ manufactured by ExxonMobil, and QUEO™ manufactured by Borealis. A "metallocene polyethylene olefin block copolymer" is a polyethylene that has a controlled block arrangement, i.e., a crystalline hard "block" alternating with an amorphous soft "block." Examples of "metallocene polyethylene-olefin block copolymers" include, but are not limited to, the INFUSE™ product line manufactured by Dow Chemical. All of the above polyethylenes may be grafted with maleic anhydride. Non-limiting examples of commercially available products include Admer® NF539A from Mitsui Chemicals, Inc., DuPont™ BYNEL® 4104 from Dow, and OREVAC® 18360 from Arkema. Note that many of the commercially available anhydride-grafted polyethylenes also contain rubber.

[0021] These polyethylenes may also be copolymers and terpolymers containing acetate and / or ester groups. Comonomer groups include, but are not limited to, vinyl acetate, methyl acrylate, ethyl acrylate, butyl acrylate, glycidyl methacrylate, and acrylic acid. Non-limiting examples include those commercially available under the trade names DuPont™ BYNEL™, DuPont™ ELVAX™, and DuPont™ ELVALOY™ from Dow; EVATANE™, LOTADER™, and LOTRYL™ from Arkema; ​​and ESCORENE™, ESCOR™, and OPTEMA™ from ExxonMobil. The polypropylenes and polyethylenes listed above can be functionalized. The functionalized polypropylenes and polyethylenes contain grafted monomers. Typically, the monomers are grafted onto the polypropylene or polyethylene by a free radical reaction. Suitable monomers for preparing functionalized polypropylene and polyethylene include, for example, olefinically unsaturated monocarboxylic acids such as acrylic acid or methacrylic acid and the corresponding tert-butyl esters, such as tert-butyl (meth)acrylate, olefinically unsaturated dicarboxylic acids such as fumaric acid, maleic acid, and itaconic acid and the corresponding mono- and / or di-tert-butyl esters, such as mono- or di-tert-butyl fumarate and mono- or di-tert-butyl maleate, olefinically unsaturated dicarboxylic anhydrides such as maleic anhydride, sulfo- or sulfonyl-containing olefinically unsaturated monomers such as p-styrenesulfonic acid, 2-(meth)acrylamido-2-methylpropenesulfonic acid or 2-sulfonyl-(meth)acrylate, oxazolinyl-containing olefinically unsaturated monomers such as vinyloxazoline and vinyloxazoline derivatives, and epoxy-containing olefinically unsaturated monomers such as glycidyl (meth)acrylate or allyl glycidyl ether.

[0022] The most commonly commercially available functionalized polypropylenes include those functionalized with maleic anhydride, non-limiting examples of which include the Mitsui Chemicals Admer® QF and QB series, the LyondellBasell PLEXAR® 6000 series, the Dow DuPont™ BYNEL® 5000 series, and the Arkema OREVAC® PP series. The most commonly commercially available functionalized polyethylenes are also those functionalized with maleic anhydride. Non-limiting examples include the Mitsui Chemicals ADMER® NF and SE series, the LyondellBasell PLEXAR® 1000, 2000, and 3000 series, the Dow DuPont™ BYNEL® 2100, 3000, 3800, 3900, and 4000 series, and some of the Arkema OREVAC® PE, T, and LOTADER® series. The most common method used in various industries to compatibilize polyamides with polypropylene or polyethylene is the use of maleic anhydride grafted polypropylene or polyethylene. For example, in flexible food packaging applications, adhesion of polyamide films to polypropylene is achieved by inserting a tie layer of maleic anhydride grafted polypropylene between the films. It should be noted that polyethylenes functionalized with other grafting monomers are also commercially available, non-limiting examples of which include the DuPont™ BYNEL® 1100, 2200, and 3100 series from Dow and the LOTADER® AX series from Arkema. It should also be noted that polymers other than polypropylene and polyethylene that are functionalized with maleic anhydride are commercially available. For example, Addivant's ROYALTUF® series is a series of ethylene-propylene-diene rubbers (EPDMs) functionalized with maleic anhydride. Another example is Kraton's KRATON® FG series, a series of SEBS polymers functionalized with maleic anhydride.

[0023] The cap layer can include at least one extrusion-grade or general-purpose polyamide. Extrusion and general-purpose polyamides are characterized by a viscosity ranging from approximately high to approximately medium. High- to medium-viscosity polyamides are more likely to match the melt flow characteristics of the foamable layer, resulting in greater thickness uniformity across each coextruded layer from the center to the edge of the die. Most types of polyamides are hygroscopic, and moisture in polyamides can affect melt flow and resistance to flow at a given shear rate. Because moisture affects flow characteristics, polyamides typically adhere to alternative standards for melt flow rate and melt volume rate. ISO 307 and ASTM D789 are two standards used to quantify the viscosity of polyamides. ISO 307 allows polyamides to be dissolved in a dilute solution in a specific solvent and the viscosity number measured. ASTM D789 allows polyamides to be dissolved in a concentrated solution in a specific solvent and the relative viscosity measured. The corresponding standards, ISO 16396-1 and ASTM D6779, provide guidance for commercial polyamide manufacturers to identify their products in a standardized system. These nomenclature systems help polyamide manufacturers identify grades appropriately for extrusion (e.g., cast film, sheet), injection molding, blow molding, etc. It is important to note that viscosity or relative viscosity numbers are not published by most polyamide manufacturers. Rather, polyamide resins are typically marketed under general viscosity categories (e.g., ultra-low, low, medium (or standard), medium-high, high) and for recommended processing applications (e.g., general extrusion, injection, compounding, monofilament, melt spinning, industrial yarn, etc.).

[0024] Any foamable layer and / or cap layer composition provided herein can contain at least one polypropylene having a melt flow index of about 0.1 to about 25 grams per 10 minutes at 230°C. Any foamable layer and / or cap layer composition provided herein can also contain at least one polyethylene having a melt flow index of about 0.1 to about 25 grams per 10 minutes at 190°C. In some embodiments, the polypropylene(s) and / or polyethylene(s) preferably have a melt flow index of about 0.3 to about 20 grams per 10 minutes at 230°C and 190°C, respectively, and more preferably about 0.5 to about 15 grams per 10 minutes at 230°C and 190°C, respectively. The "Melt Flow Index" (MFI) value of a polymer is defined and measured according to ASTM D1238 at 230°C for polypropylene and polypropylene-based materials, and at 190°C for polyethylene and polyethylene-based materials, using a 2.16 kg plunger for 10 minutes. For resins with relatively high melt flow, the test time may be shortened. MFI is a measure of polymer flowability and is an indicator of the molecular weight and processability of a polymeric material. A high MFI value indicates a low viscosity. If the MFI value is too high, extrusion according to the present disclosure cannot be performed satisfactorily. Problems associated with too high an MFI value include low extrusion pressure, problems setting the thickness profile, uneven cooling profiles due to low melt viscosity, poor melt strength, and / or mechanical problems. Conversely, a low MFI value indicates a high viscosity. If the MFI value is too low, high pressures will occur during melt processing, sheet quality and profile problems will occur, and high extrusion temperatures will be required with the risk of blowing agent decomposition or activation.

[0025] The MFI range is also important for the foaming process because it can reflect the viscosity of the material, which affects foaming. Without being bound by theory, it is believed that there are several reasons why certain MFI values ​​are more effective. Materials with lower MFIs have longer molecular chains, which increases the energy required for chain flow when stress is applied, potentially improving some physical properties. Furthermore, longer molecular weight (MW) chains tend to crystallize, resulting in more crystalline structures and stronger intermolecular bonds. However, if the MFI is too low, the viscosity will be too high. Meanwhile, polymers with higher MFI values ​​have shorter chains. Therefore, for a given volume, materials with higher MFI values ​​have more chain ends at the microscopic level that can rotate, compared to polymers with lower MFI values. The space required for such rotation (e.g., rotation above the polymer's Tg, or glass transition temperature) creates free volume. This increases free volume, facilitating flow under stress. In addition to the polymer, the composition fed to the extruder may also contain additives compatible with the production of the disclosed multilayer structures. Typical additives include, but are not limited to, organic peroxides, antioxidants, lubricants, processing aids, heat stabilizers, colorants, flame retardants, antistatic agents, nucleating agents, plasticizers, antimicrobial agents, mildew inhibitors, light stabilizers, ultraviolet (UV) absorbers, antiblocking agents, fillers, deodorizers, odor absorbers, anti-fog agents, volatile organic compound (VOC) absorbers, semi-volatile organic compound (SVOC) absorbers, thickeners, cell dimensional stabilizers, metal deactivators, and combinations thereof.

[0026] In some embodiments, the amount of additive(s) other than the chemical blowing agent(s) and crosslinking accelerator(s) in the foam layer composition can be about 20 PPR% or less, about 15 PPR% or less, about 10 PPR% or less, or about 8 PPR% or less of the composition. In some embodiments, the amount of additive(s) other than the chemical blowing agent(s) and crosslinking accelerator(s) in the foam layer composition can be about 1 PPR% or more, about 2 PPR% or more, about 4 PPR% or more, or about 6 PPR% or more of the composition. In some embodiments, the amount of additive(s) other than the chemical blowing agent(s) and crosslinking accelerator(s) in the foam layer composition can be about 1-20 PPR%, about 2-15 PPR%, about 4-10 PPR%, or about 6-8 PPR% of the composition. In some embodiments, the amount of additive(s) other than the chemical blowing agent(s) and crosslinking promoter(s) in the foam layer composition can be about 1-20%, about 2-15%, about 3-10%, about 4-8%, or about 5-7% by weight of the foam layer composition. In some embodiments, the amount of additive(s) in the cap layer composition can be about 20 PPR% or less, about 15 PPR% or less, about 10 PPR% or less, about 7 PPR% or less, about 5 PPR% or less, or about 3 PPR% or less of the composition. In some embodiments, the amount of additive(s) in the cap layer composition can be about 0.5 PPR% or more, about 1 PPR% or more, about 2 PPR% or more, or about 3 PPR% or more of the composition. In some embodiments, the amount of additive(s) in the cap layer composition can be about 0.5-20 PPR%, about 1-10 PPR%, or about 2-7 PPR% of the composition. In some embodiments, the amount of additive(s) in the cap layer composition can be about 0.5-20%, about 1-10%, or about 2-6% by weight of the cap layer composition.

[0027] Regardless of how the ingredients are fed to the extruder, the shear and mixing within the extruder can be sufficient to produce a homogeneous layer. Co-rotating and counter-rotating twin screw extruders can provide sufficient shear and mixing through the extruder barrel to extrude a layer with uniform properties. Specific energy is a measure of how much work is required to extrude a layer of material and how intensive the extrusion process is. Specific energy is defined as the energy applied to the material being processed through the extruder, normalized per kilogram. Specific energy is quantified in kilowatts (KW) of energy applied per kilogram of total material fed per hour. Specific energy is calculated according to the following formula: Specific energy = KW (applied) / feed rate (kg / hr) In the above formula, KW (applied) = (KW (motor rating) x (torque % from maximum allowable in decimal) x revolutions per minute (RPM) (actual RPM) x 0.97 (gearbox efficiency) / maximum RPM (extruder output potential)

[0028] Specific energy can be used to quantify the amount of shear and mixing of ingredients within an extruder. Extruders used to form the multi-layer structures disclosed herein can generate a specific energy of at least about 0.100 kW hr / kg, preferably at least about 0.125 kW hr / kg, and more preferably at least about 0.150 kW hr / kg. Any foamable layer can contain a chemical blowing agent (CFA). The extrusion temperature of any foamable layer can be 0-10°C lower, preferably 10°C or more lower, than the thermal decomposition onset temperature of the chemical blowing agent. If the extrusion temperature exceeds the thermal decomposition temperature of the blowing agent, the blowing agent will decompose, resulting in undesirable "pre-expansion." The extrusion temperature of any cap layer can be 0-10°C lower, preferably 10°C or more lower, than the thermal decomposition onset temperature of the chemical blowing agent in any foamable layer adjacent to the cap layer. If the extrusion temperature of the cap layer exceeds the thermal decomposition temperature of the blowing agent in the adjacent layer, the blowing agent in the adjacent layer will decompose, potentially resulting in undesirable "pre-expansion." The foam layer composition can include a variety of different chemical blowing agents. Examples of chemical blowing agents include, but are not limited to, azo compounds, hydrazine compounds, carbazides, tetrazoles, nitroso compounds, and carbonates. Chemical blowing agents may be used singly or in any combination. In some embodiments, azodicarbonamide (ADCA) is one chemical blowing agent that can be used. An example of an ADCA chemical blowing agent is UNIFOAM® TC-18I manufactured by PT. Lautan Otsuka Chemical Co., Ltd. ADCA typically undergoes thermal decomposition at temperatures of about 200-240°C. To prevent thermal decomposition of ADCA in the extruder, the extrusion temperature can be maintained below 200°C.

[0029] The amount of chemical blowing agent in the foam layer composition can be about 30 PPR or less, about 20 PPR or less, about 15 PPR or less, about 10 PPR or less, or about 8 PPR or less of the composition. In some embodiments, the amount of chemical blowing agent in the foam layer composition can be about 1 PPR or more, about 2 PPR or more, about 3 PPR or more, about 4 PPR or more, or about 5 PPR or more of the composition. In some embodiments, the amount of chemical blowing agent in the foam layer composition can be about 1-30 PPR, about 2-20 PPR, about 3-15 PPR, about 4-10 PPR, or about 5-8 PPR. In some embodiments, the amount of chemical blowing agent in the foam layer composition can be about 1-30 PPR, about 2-20 PPR, about 3-15 PPR, about 4-10 PPR, or about 5-7 PPR by weight of the foam layer composition. The amount of chemical blowing agent can depend on the thickness of the unfoamed sheet, the desired foam thickness, the desired foam density, the material being extruded, the crosslinking rate, the type of chemical blowing agent (the amount of gas generated by different blowing agents can vary considerably), etc. It should be noted that the amounts of chemical blowing agents listed above may be specific to ADCA only. Other blowing agents will produce different volumetric amounts of gas per mass of CFA and can be considered accordingly. For example, comparing ADCA to the chemical blowing agent p-toluenesulfonylsemicarbazide (TSS), if the foamable layer contains 40 PPR% ADCA, approximately 63 PPR% TSS would be required to generate approximately the same amount of gas during the foaming process. When the difference between the decomposition temperature of the thermally decomposable blowing agent and the melting point of the polymer with the highest melting point is large, a blowing agent decomposition catalyst may be used. Examples of catalysts include, but are not limited to, zinc oxide, magnesium oxide, calcium stearate, glycerin, and urea. The lower extrusion temperature limit can be the same as the melting point of the polymer with the highest melting point. If the extrusion temperature is lower than the melting point of the polymer with the highest melting point, undesirable "unmelted" properties will appear. During foaming, extruded layers extruded at temperatures lower than this lower temperature limit may exhibit non-uniform thickness, non-uniform cell structure, pockets of cell collapse, and other undesirable properties.

[0030] Whether the blowing agent is a physical blowing agent, a chemical blowing agent, or a combination of both, typical extrusion foaming produces a polymer sheet with both major surfaces that are significantly rougher than comparable structures produced by the disclosed methods. The surface profile of multilayer (or single-layer) foam sheets can be critical in many applications, so extruded foam sheets may not be used for these applications. These applications may require a smooth foam surface to achieve desired properties, such as ease of lamination to films, fabrics, fiber layers, and leather; percentage of contact area in the laminate; and visual aesthetics. PCT Publication WO2016109544, incorporated herein by reference in its entirety, provides examples illustrating the difference in surface roughness between extruded foamed polymer sheets and comparable foamed polymer sheets produced by the disclosed methods. The surface of extruded foam articles may generally be rougher with larger cell size (compared to foams produced in accordance with the present disclosure). Although cell size and cell size distribution may not be critical for most commercial applications, because surface roughness is a function of cell size, large cell foams may be less desirable than small cell foams in applications requiring a smooth foam surface. The thickness of the unfoamed coextruded multilayer structure can be about 0.1 to about 30 mm, about 0.2 to about 25 mm, about 0.3 to about 20 mm, or about 0.4 to about 15 mm. In some embodiments, the thickness of any individual cap layer can be at least about 0.02 mm, at least about 0.05 mm, at least about 0.1 mm, at least about 0.15 mm, or at least about 0.2 mm. In some embodiments, the thickness of any individual cap layer can be about 1.0 mm or less, about 0.7 mm or less, or about 0.4 mm or less. In some embodiments, the thickness of any individual cap layer can be about 0.01 to 1.0 mm or 0.02 to 0.7 mm. In some embodiments, the thickness of the unfoamed cap is not limited and can be as thin as about 0.1 μm relative to the overall unfoamed coextruded multilayer sheet, or can be a thickness typical of very thin tie layers used in multilayer flexible packaging and barrier films.

[0031] In some embodiments, the thickness of the foam layer of the unfoamed coextruded multilayer structure can be about 0.1 to 5 mm, about 0.5 to 4 mm, about 1 to 3 mm, or about 1 to 2 mm. In some embodiments, the thickness of the foam layer of the unfoamed coextruded multilayer structure can be about 5 mm or less, about 3 mm or less, about 2 mm or less, about 1.5 mm or less, about 1 mm or less, or about 0.5 mm or less. In some embodiments, the thickness of the foam layer of the unfoamed coextruded multilayer structure can be about 0.1 mm or more, about 0.5 mm or more, about 1 mm or more, about 1.5 mm or more, about 2 mm or more, or about 3 mm or more. In some embodiments, the overall thickness of an unfoamed coextruded multilayer structure can be measured using a stem-type thickness gauge mounted on a flat base. The tip of the gauge can be fitted with a hemispherical contact point with a radius of 1.6 mm. The stem is then raised to place the unfoamed structure on the base. During measurement, a force of 100-150 gf can be applied to the structure at the contact point.

[0032] In some embodiments, the thickness of the cap layer of an unfoamed coextruded multilayer structure can be measured using a microscope. To measure the thickness of the cap layer, a small sample of the structure can be cut from a continuous sheet, and the cross section of the sample can be sliced ​​into thin sections using a microtome. The sections can be placed on a microscope for viewing. Measurements can be made with either a digital microscope or a conventional microscope. Common commercially available digital microscopes can be equipped with various software features to facilitate thickness measurements. Conventional commercially available microscopes can be equipped with lenses with measurement scales to facilitate thickness measurements. The cap can be thin and flexible when melted so as not to significantly impede the expansion of the foamable layer(s) during the foaming process. Among the many physical properties that can impede the expansion of other layer(s), include the cap's thickness, flexibility, and melt strength. The thickness, flexibility, melt strength, and crosslinking rate of the foamable layer(s), as well as the final thickness and density of the foamed layer, also play a role in whether the cap will inhibit the expansion of the foamable layer(s). As a general guideline for the maximum cap thickness, the maximum cap thickness should be no more than about 20%, no more than about 15%, no more than about 10%, or no more than about 5% of the total coextruded unfoamed sheet. If the cap's thickness exceeds about 20% of the total coextruded unfoamed sheet, problems with bowing, buckling, and bending of the multilayer sheet may occur when the multilayer sheet is heated and foamed.

[0033] It is important to distinguish between "physical" and "chemical" crosslinking. Chemical crosslinking involves the use of crosslinking accelerators but without the use of ionizing radiation. Chemical crosslinking typically involves the use of peroxides, silanes, or vinylsilanes. In peroxide crosslinking processes, crosslinking typically occurs in the extrusion die. In silane and vinylsilane crosslinking processes, crosslinking typically occurs in a secondary operation after extrusion, where crosslinking of the extruded material can be accelerated with heat and moisture. Regardless of the chemical crosslinking method, chemically crosslinked foam sheets typically exhibit significantly rougher primary surfaces than comparable structures produced by the disclosed methods. Because the surface profile of multilayer (or single-layer) foam sheets can be critical in many applications, chemically crosslinked foam sheets may not be used for these applications. These applications may require a smooth foam surface to achieve desired properties, such as ease of lamination to films, fabrics, fiber layers, and leather; percentage of contact area in the laminate; and visual aesthetics. PCT Publication WO2016109544 provides examples showing the difference in surface roughness between a chemically crosslinked expanded polymer sheet and an equivalent expanded polymer sheet produced by the disclosed method. The surface of chemically crosslinked foam articles may generally be rougher with larger cell size (compared to foams produced in accordance with the present disclosure). Although cell size and cell size distribution are not critical for most commercial applications, because surface roughness is a function of cell size, large cell foams may be less desirable than small cell foams in applications requiring a smooth foam surface.

[0034] Examples of ionizing radiation include, but are not limited to, alpha rays, beta (electron) rays, X-rays, gamma rays, and neutron rays. In particular, electron beams with uniform energy can be used to prepare crosslinked polyolefin foam / crosslinked polyolefin cap structures. The exposure time, number of irradiations, and acceleration voltage during electron beam irradiation can be widely varied depending on the desired degree of crosslinking and the thickness of the multilayer structure. However, the ionizing radiation dose can generally be in the range of about 10 to about 500 kGy, about 20 to about 300 kGy, or about 20 to about 200 kGy. If the exposure dose is too low, cell stability may not be maintained during foaming. If the exposure dose is too high, the resulting multilayer foam structure may have insufficient moldability. Moldability is a desirable property when using a multilayer foam sheet for thermoforming. Furthermore, since unfoamed sheets may soften due to heat generation during exposure to electron beam irradiation, excessive exposure may result in deformation of the structure. Furthermore, the polymer component may deteriorate due to excessive polymer chain scission.

[0035] The coextruded unfoamed multilayer sheet may be irradiated up to four times, preferably up to two times, and more preferably only once. Irradiation more than about four times can degrade the polymeric components, resulting in, for example, a loss of uniform cell formation in the resulting foam layer(s) upon foaming. For extruded structures with thicknesses greater than about 4 mm, it may be preferable to irradiate each major surface of the multilayer profile with ionizing radiation to achieve a more uniform degree of crosslinking in the major surface(s) and inner layers. Electron beam irradiation has the advantage that it can effectively crosslink coextruded sheets of various thicknesses by controlling the electron acceleration voltage. The acceleration voltage can typically range from about 200 to about 1500 kV, about 400 to about 1200 kV, or about 600 to about 1000 kV. Acceleration voltages below about 200 kV may prevent radiation from reaching the inner portion of the coextruded sheet. This can result in coarse cells in the inner portion, potentially resulting in uneven foaming. Acceleration voltages that are too low for a given thickness profile can cause arcing, resulting in the formation of "pinholes" or "tunnels" in the foam structure. Acceleration voltages above about 1500 kV may result in polymer degradation. In some embodiments, the radiation source may be directed toward layer B of the coextruded unfoamed multilayer sheet during irradiation. In some embodiments, the radiation source may be directed toward layer A of the coextruded unfoamed multilayer sheet during irradiation.

[0036] Regardless of the type of ionizing radiation selected, crosslinking can be carried out so that the composition of the extruded structure is about 20 to about 75% or about 30 to about 60% crosslinked, as measured by the Toray gel fraction method. According to the Toray gel fraction method, tetralin solvent is used to dissolve the non-crosslinked polyolefin components in the composition. Typically, non-crosslinked polyolefin materials are dissolved in tetralin, and the degree of crosslinking is expressed as the mass percentage of crosslinked material in the total composition. The equipment used to determine the percentage of polymer crosslinking includes: 100 mesh (wire diameter: 1.143 mm (0.0045 in)); Type 304 stainless steel bags; numbered wires and clips; Miyamoto thermostatic oil bath apparatus; analytical balance; fume hood; gas burner; high-temperature oven; anti-static gun; and three 3.5-liter wide-mouth stainless steel containers with lids. The reagents and materials used include tetralin polymer solvent, acetone, and silicone oil. Specifically, an empty wire mesh bag is weighed and its mass is recorded. For each sample, weigh out 100 mg ± 5 mg of sample and transfer it to a wire mesh bag. Record the mass of the wire mesh bag and the sample, usually in the form of thinly sliced ​​foam pieces. Attach each bag to the corresponding wire and clip. Once the solvent temperature reaches 130°C, immerse the bundle (bag and sample) in the solvent. Shake the sample up and down about 5 or 6 times to release air bubbles and thoroughly wet the sample. Attach the sample to a stirrer and stir for 3 hours until the solvent dissolves the foam. Then, cool the sample in a fume hood. Shake the sample up and down about 7 or 8 times in a container containing primarily acetone to wash the sample. Wash the sample twice with a second acetone wash. Wash the washed sample once more in a third container containing fresh acetone, as described above. Next, hang the sample in the fume hood and allow it to evaporate for about 1 to 5 minutes. Next, dry the sample in a drying oven at 120°C for about 1 hour. Allow the sample to cool for a minimum of about 15 minutes. Weigh the wire mesh bag on an analytical balance and record the mass. The degree of crosslinking is then calculated using the formula 100 x (CA) / (BA).where A represents the mass of the empty wire mesh bag; B represents the mass of the wire bag plus the foam sample before immersion in tetralin; and C represents the mass of the wire bag plus the dissolved sample after immersion in tetralin.

[0037] It is important to note that polyamides are not soluble in tetralin, therefore the gel percentage calculated by the above method includes the crosslinked polyolefin component(s) plus the polyamide component(s). Suitable crosslinking promoters include, but are not limited to, commercially available di-, tri-, tetra-, penta-, and higher functional monomers. Such crosslinking monomers are available in liquid, solid, pellet, and powder forms. Examples of crosslinking accelerators include, but are not limited to, acrylates or methacrylates such as 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, trimethylolpropane trimethacrylate, tetramethylolmethane triacrylate, 1,9-nonanediol dimethacrylate, and 1,10-decanediol dimethacrylate; allyl esters of carboxylic acids (e.g., trimellitic acid triallyl ester, pyromellitic acid triallyl ester, and oxalic acid diallyl ester); allyl esters of cyanuric acid or isocyanuric acid such as triallyl cyanurate and triallyl isocyanurate; maleimide compounds such as N-phenylmaleimide and N,N'-m-phenylenebismaleimide; compounds having at least two triple bonds such as dipropargyl phthalate and dipropargyl maleate; and divinylbenzene. These crosslinking accelerators may be used singly or in any combination of two or more. In the present disclosure, divinylbenzene (DVB), a difunctional liquid crosslinking monomer, can be used as a crosslinking promoter in the present disclosure. An example of a suitable commercially available DVB is DVB HP manufactured by Dow.

[0038] The amount of crosslinking accelerator in the foam layer composition can be about 5 PPR% or less, about 4 PPR% or less, about 3 PPR% or less, about 2.5 PPR% or less, about 2 PPR% or less, about 1.5 PPR% or less, or about 1 PPR% or less of the composition. In some embodiments, the amount of crosslinking accelerator in the foam layer composition can be about 0.5 PPR% or more, about 1 PPR% or more, about 1.5 PPR% or more, about 2 PPR% or more, about 2.5 PPR% or more, about 3 PPR% or more, or about 4 PPR% or more of the composition. In some embodiments, the amount of crosslinking accelerator in the foam layer composition can be about 0.5-5 PPR%, about 0.5-3 PPR%, about 1-3 PPR%, or about 2-3 PPR% of the composition. In some embodiments, the amount of crosslinking accelerator in the foam layer composition can be about 0.5-5% or about 1-3% by weight of the foam layer composition. It should be noted that the amounts of crosslinking promoter listed above may be specific to DVB only. The crosslinking efficiency of other crosslinking promoters may be higher or lower than that of DVB. Therefore, the required amount of another crosslinking promoter should be considered accordingly. The crosslinking efficiency of a crosslinking promoter may vary depending on the ionizing radiation dose, the polymer being crosslinked, the chemical structure of the monomer, the number of functional groups on the monomer, and whether the monomer is a liquid or powder. Crosslinks can be produced using a variety of different techniques, and can form intramolecularly between different polymer molecules and intramolecularly between portions of a single polymer molecule, including, but not limited to, providing a crosslinking promoter separate from the polymer chains, and providing polymer chains incorporating a crosslinking promoter that includes functional groups capable of forming crosslinks or that can be activated to form crosslinks.

[0039] After irradiating the coextruded sheet, the crosslinked multilayer sheet may be expanded by heating it to a temperature higher than the decomposition temperature of the thermally decomposable expanding agent. The expansion can be carried out in a continuous process at about 200 to 260°C or about 220 to 240°C. The continuous expansion method is preferable to the batch method for producing an open-cell foam sheet. Foaming is typically accomplished by heating the crosslinked multilayer sheet using molten salt, radiant heaters, vertical or horizontal hot air ovens, microwave energy, or a combination of these methods. Foaming can also be accomplished by impregnation with nitrogen, for example, in an autoclave, followed by free foaming using molten salt, radiant heaters, vertical or horizontal hot air ovens, microwave energy, or a combination of these methods. Optionally, the crosslinked multilayer sheet can be preheated to soften before foaming. This softening can help stabilize the expansion of the structure during foaming, especially in the case of thick, stiff sheets.

[0040] The thickness of the entire multilayer foam sheet can be measured in accordance with JIS K6767. The thickness of the cap layer of a multilayer foam sheet can be measured using a microscope. To measure the cap layer, a small sample of the foam structure can be taken from the open-cell foam sheet. This sample can be cut with an ultra-sharp blade, and the cross section of the sample can be observed along the cut surface under a microscope. Measurements can be performed with either a digital microscope or a conventional microscope. Common commercially available digital microscopes can be equipped with various software features to facilitate thickness measurements. Conventional commercially available microscopes can be equipped with lenses with measurement scales to facilitate thickness measurements. The density of the multilayer foam sheet can be defined and measured in terms of cross-sectional density or "whole" density rather than "core" density, as measured in accordance with JIS K6767. The multilayer foam sheet produced using the above method has a density of about 20 to 250 kg / m 3 , about 30~200kg / m 3 , or about 50 to 150 kg / m 3 It is possible to produce foams with a cross-sectional or "overall" density of about 20 kg / m. The cross-sectional density can be controlled by the amount of blowing agent and the thickness of the extruded structure. 3 If the density is less than 20 kg / m, the sheet may not be able to expand efficiently due to the large amount of chemical expansion agent required to achieve that density. 3 If the density of the multilayer foam sheet is less than about 20 kg / m, it may become increasingly difficult to control the expansion of the sheet during the foaming process.3 If the density is less than about 20 kg / m, the foam may become increasingly susceptible to cell collapse. 3 At densities below this, it may be difficult to produce a multilayer foam sheet with uniform cross-sectional density and thickness.

[0041] In some embodiments, the cross-sectional density of the multilayer foam sheet is about 250 kg / m 3 It is not limited to approximately 350 kg / m 3 , about 450kg / m 3 , or approximately 550 kg / m 3 Foams having cross-sectional densities of about 250 kg / m may also be produced. However, higher densities generally can be costly compared to other materials that may be used in a given application, so foam sheets are generally recommended to have densities of about 250 kg / m. 3 In some cases, it may be preferable to have less than The foam layer produced using the above method may be closed-cell. Preferably, at least 90%, preferably at least 95%, and more preferably more than 98% of the cells have intact cell walls. The average cell size can be about 0.05 to about 1.0 mm, preferably about 0.1 to about 0.7 mm. When the average cell size is less than about 0.05 mm, the density of the foam structure typically is less than 250 kg / m. 3 The average cell size can be higher. Average cell sizes greater than 1 mm can also result in irregularities on the surface of the foam. Also, if the average cell size of the cell population in the foam is not desirable, the foam structure can undesirably tear. This can occur when the foam structure is stretched or when a portion of it is subjected to a secondary process. The cell size distribution in the foam layer(s) exhibits a bimodal distribution, representing a population of cells in the core of the foam structure that are relatively round, and a population of cells in a skin near the surface of the foam structure that is relatively flat, thin, and / or rectangular.

[0042] The overall thickness of the multilayer polyolefin foam / polyamide cap sheet can be about 0.2 mm to about 50 mm, about 0.4 mm to about 40 mm, about 0.6 mm to about 30 mm, or about 0.8 mm to about 20 mm. Thicknesses less than about 0.2 mm can result in inefficient foaming due to high gas loss from the primary surface(s). Thicknesses greater than about 50 mm can make it increasingly difficult to control expansion during the foaming process. Thus, it can be increasingly difficult to produce a multilayer polyolefin foam / polyolefin cap sheet with uniform cross-sectional density and thickness. In some embodiments, the cap layer thickness of the foamed coextruded multilayer structure can be about 0.0001 to 0.2 mm, about 0.001 to 0.15 mm, or about 0.05 to 0.1 mm. In some embodiments, the foam layer thickness of the foamed coextruded multilayer structure can be about 0.5 to 6 mm, about 1 to 5 mm, or about 2 to 4 mm. In some embodiments, the desired thickness is achieved by a secondary process such as slicing, skiving, or gluing, which allows for thicknesses ranging from about 0.1 mm to about 100 mm. The thickness of the cap layer may decrease during foaming of the multilayer sheet. This may be due to the expansion of the foamable layer(s), resulting in elongation of the cap layer(s). Thus, for example, if the multilayer sheet expands to twice its original area, the cap thickness can be expected to decrease by approximately half. If the multilayer sheet expands to four times its original area, the cap thickness can be expected to decrease to approximately one-quarter of its original thickness.

[0043] The disclosed multilayer foam structures can be used in a variety of applications. One example of such an application is an article manufactured by LPM. The "Summary" describes the multilayer foam structure as a decorative component in a vehicle interior, specifically, a door panel. However, the multilayer foam structure is not limited to vehicle door panels; it can also be used in other vehicle interior components, such as door rolls, door inserts, door stuffers, trunk stuffers, armrests, center consoles, seat cushions, seat backs, headrests, seat back panels, knee bolsters, or headliners. Another example application is thermoforming. To thermoform a multilayer foam structure, the structure can be heated to the melting point of the polyolefin foam layer and the polyamide cap layer. Since most commercially available polyamides have a higher melting point than the polyolefin component(s) described in this disclosure, the multilayer foam structure can be heated to the melting point of the polyamide. An example of a thermoformed product is an automotive air duct. Closed-cell foam structures may be particularly suitable for this application because of their light mass (compared to solid plastics), insulating properties that help maintain the temperature of the air flowing through the duct, and vibration resistance (compared to solid plastics). A polyamide cap layer on the outside of a multilayer air duct can protect the air duct from contact with liquids and gases "under the hood" or in the vehicle interior that could adversely affect the functionality of the polyolefin foam. The cap layer can also protect the foam layer from punctures and cuts during installation and throughout the life of the vehicle. Therefore, a rigid polyolefin foam with a polyamide cap may be suitable for automotive air ducts.

[0044] In some embodiments, the multilayer foam structure can be a laminate including a multilayer foam and a laminate layer. Preferably, the laminate layer is applied to the side (i.e., the surface) of the foam layer opposite the cap layer. In these laminates, the multilayer foam structure can also be combined with, for example, a film and / or foil. Examples of materials suitable for such layers include, but are not limited to, polyvinyl chloride (PVC); thermoplastic polyolefin (TPO); thermoplastic urethane (TPU); fabrics, such as polyester, polypropylene, textiles, and other fabrics; leather; and / or fibrous layers, such as nonwoven fabrics. Such layers may be manufactured using standard techniques known to those skilled in the art. Importantly, the multilayer foams of the present disclosure may also include multiple other layers. In these laminates, one layer may be joined to an adjacent layer by chemical bonding, mechanical means, or a combination thereof. Adjacent laminate layers may also be attached to each other by any other means, including the use of attractive forces between materials with opposite electromagnetic charges or the attractive forces that exist between materials that both have either predominantly hydrophobic or predominantly hydrophilic properties. To meet the requirements of any of the above applications, the structures disclosed in this disclosure may be subjected to various secondary processes including, but not limited to, embossing, corona or plasma treatment, surface roughening, surface smoothing, perforating or micro-perforating, lap jointing, slicing, skiving, layering, gluing, and drilling. [Example]

[0045] Ingredients for the Example Table 1 below provides a list of ingredients used in the following examples and a description of those ingredients.

[0046] [Table 1] TIFF0007728279000002.tif43170

[0047] Example Conversion Process Table 2 below shows the formulation and coextrusion information for Examples 1 and 2. All examples were foamed by heating the multilayer sheet with molten salt.

[0048] [Table 2] TIFF0007728279000004.tif24978

[0049] Table 3 below shows the irradiation and properties of the multilayer structures of Examples 1 and 2. Sections of the unfoamed sheet of Example 1 were irradiated at three different doses and further designated Examples 1A-1C. Sections of the unfoamed sheet of Example 2 were irradiated at seven different doses and further designated Examples 2A-2G.

[0050] [Table 3] TIFF0007728279000006.tif54170

[0051] Cross-sectional images of microtome sections of the unfoamed multilayer structures of Examples 1B, 2A, 2D, and 2G at 100x magnification are shown in Figures 1, 3, 5, and 7. Images of the corresponding foams of Examples 1B, 2A, 2D, and 2G at 20x magnification are shown in Figures 2, 4, 6, and 8.

[0052] This application discloses several numerical ranges in the text and drawings, and because the disclosure can be practiced throughout the disclosed numerical ranges, the disclosed numerical ranges inherently correspond to any range or value within the disclosed numerical ranges, including the endpoints, even if an exact range limitation is not explicitly stated in the specification. The above description is presented to enable any person skilled in the art to make and use the disclosure, and is provided in the context of a particular application and its requirements. Various modifications to the preferred embodiment will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the disclosure. Thus, the present disclosure is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. Finally, the entire disclosures of the patents and publications referenced in this application are hereby incorporated by reference. Another aspect of the present invention may be as follows. [1] A multilayer foam structure comprising: Polypropylene, polyethylene, or a combination of polypropylene and polyethylene a foam layer comprising: Polyamide; and a cap layer on the side of the foam layer comprising polypropylene, polyethylene, or a combination of polypropylene and polyethylene. A multi-layer foam structure comprising: [2] The multilayer foam structure according to [1], wherein the foam layer and the cap layer are co-extruded. [3] The multilayer foam structure according to [1] or [2], wherein the foam layer contains at least 70% by weight of polypropylene, polyethylene, or a combination of polypropylene and polyethylene. [4] The multilayer foam structure according to any one of [1] to [3], wherein the cap layer contains at least 40% by mass of polyamide. [5] The multilayer foam structure according to any one of [1] to [4], wherein the cap layer comprises up to 50% by weight of polypropylene, polyethylene, or a combination of polypropylene and polyethylene; and at least 40% by weight of polyamide. [6] The multilayer foam structure according to any one of [1] to [5] above, wherein the cap layer has a thickness of less than 1 mm. [7] The multilayer foam structure according to any one of [1] to [6], wherein the foam layer contains a crosslinking accelerator in an amount of 0.5 to 5.0% by mass. [8] The multilayer foam structure according to any one of [1] to [7], wherein the foam layer contains an additive in an amount of 1 to 20% by mass. [9] The multilayer foam structure according to any one of [1] to [8], wherein the cap layer contains an additive in an amount of 1 to 10% by mass.

[10] The multilayer foam structure according to any one of [1] to [9], wherein the polypropylene has a melt flow index of 0.1 to 25 grams per 10 minutes at 230°C.

[11] The multilayer foam structure according to any one of [1] to

[10] , wherein the polyethylene has a melt flow index of 0.1 to 25 grams per 10 minutes at 190°C.

[12] Density: 20 to 250 kg / m 3 The multilayer foam structure according to any one of [1] to

[11] above,

[13] The multilayer foam structure according to any one of the above [1] to

[12] , having a degree of crosslinking of 20 to 75%.

[14] The multilayer foam structure according to any one of the above [1] to

[13] , wherein the average closed cell size is 0.05 to 1.0 mm.

[15] The multilayer foam structure according to any one of the above [1] to

[14] , which has a thickness of 0.2 to 50 mm.

[16] A laminate comprising: Polypropylene, polyethylene, or a combination of polypropylene and polyethylene A foam layer comprising: Polyamide, and Polypropylene, polyethylene, or a combination of polypropylene and polyethylene a cap layer on the side of the foam layer, comprising: a laminate layer on the side of the foam layer opposite the cap layer a multilayer foam structure comprising A laminate comprising:

[17] The laminate according to

[16] , wherein the foam layer and the cap layer are co-extruded.

[18] The laminate according to

[16] or

[17] , wherein the laminate layer is a flexible film, cloth, or foil.

[19] The laminate according to any one of the above

[16] to

[18] , wherein the laminate layer is unfoamed or foamed.

[20] The laminate according to any one of the above

[16] to

[19] , wherein the cap layer is unfoamed.

[21] The laminate according to any one of

[16] to

[20] , wherein the foam layer contains at least 70% by mass of polypropylene, polyethylene, or a combination of polypropylene and polyethylene.

[22] The laminate according to any one of the above

[16] to

[21] , wherein the cap layer contains at least 40% by mass of polyamide.

[23] The laminate according to any one of

[16] to

[22] , wherein the cap layer comprises up to 50% by weight of polypropylene, polyethylene, or a combination of polypropylene and polyethylene; and at least 40% by weight of polyamide.

[24] The laminate according to any one of the above

[16] to

[23] , wherein the thickness of the cap layer is less than 1 mm.

[25] The laminate according to any one of the above

[16] to

[24] , wherein the foam layer contains an additive in an amount of 1 to 20% by mass.

[26] The laminate according to any one of the above

[16] to

[25] , wherein the polypropylene has a melt flow index of 0.1 to 25 grams per 10 minutes at 230°C.

[27] The laminate according to any one of

[16] to

[26] , wherein the polyethylene has a melt flow index of 0.1 to 25 grams per 10 minutes at 190°C.

[28] The density of the multilayer foam structure is 20 to 250 kg / m 3 The laminate according to any one of the above

[16] to

[27] ,

[29] The laminate according to any one of the above

[16] to

[28] , wherein the degree of crosslinking of the multilayer foam structure is 20 to 75%.

[30] The laminate according to any one of the above

[16] to

[29] , wherein the average closed cell size of the multilayer foam structure is 0.05 to 1.0 mm.

[31] The laminate according to any one of the above

[16] to

[30] , wherein the multilayer foam structure has a thickness of 0.2 to 50 mm.

Claims

1. 1. A multi-layer foam structure comprising: Polypropylene, polyethylene, or a combination of polypropylene and polyethylene a physically crosslinked closed-cell foam layer comprising: 1 to 10% by weight of an additive; at least 40% by weight of polyamide, and Up to 50% by weight of polypropylene, polyethylene, or a combination of polypropylene and polyethylene a cap layer on the side of the foam layer consisting only of A multi-layer foam structure comprising:

2. 10. The multilayer foam structure of claim 1, wherein the foam layer comprises at least 70% by weight of polypropylene, polyethylene, or a combination of polypropylene and polyethylene.

3. 3. The multilayer foam structure of claim 1 or 2, wherein the cap layer has a thickness of less than 1 mm.

4. The multilayer foam structure according to any one of claims 1 to 3, wherein the foam layer comprises a crosslinking accelerator in an amount of 0.5 to 5.0 wt%.

5. 5. The multilayer foam structure according to any one of claims 1 to 4, wherein the foam layer comprises an additive in an amount of 1 to 20% by weight.

6. Density: 20 to 250 kg / m 3 The multilayer foam structure according to any one of claims 1 to 5, wherein

7. 7. The multilayer foam structure according to any one of claims 1 to 6, having a degree of crosslinking of 20 to 75%.

8. 8. The multilayer foam structure of any one of claims 1 to 7, having an average closed cell size of 0.05 to 1.0 mm.

9. The multilayer foam structure according to any one of claims 1 to 8, having a thickness of 0.2 to 50 mm.

10. The multilayer foam structure according to any one of claims 1 to 9, wherein the additive is at least one selected from the group consisting of organic peroxides, antioxidants, lubricants, processing aids, heat stabilizers, colorants, flame retardants, antistatic agents, nucleating agents, plasticizers, antibacterial agents, mildew inhibitors, light stabilizers, UV absorbers, antiblocking agents, fillers, deodorizers, odor adsorbents, anti-fogging agents, volatile organic compound (VOC) adsorbents, semi-volatile organic compound (SVOC) adsorbents, thickeners, bubble dimensional stabilizers, and metal deactivators.

11. 1. A laminate comprising: The multilayer foam structure according to any one of claims 1 to 10; and a third layer on the side of the foam layer opposite the cap layer; A laminate comprising:

12. 12. The laminate of claim 11, wherein the third layer is a flexible film, fabric, or foil.

13. 13. The laminate of claim 11 or 12, wherein the third layer is unfoamed or foamed.

14. The laminate according to any one of claims 11 to 13, wherein the cap layer is unfoamed.

Images