CROSSLINKED, COEXTRUDED MULTILAYER POLYOLEFIN FOAM STRUCTURES, POLYOLEFIN CAP LAYERS AND METHODS OF MANUFACTURING THE SAME.

Abstract

This document discloses continuous, physically crosslinked, closed-cell, multilayer foam structures comprising a coextruded foam layer containing at least one polypropylene and one polyethylene, and a crosslinked coextruded coating layer containing at least one polypropylene and one polyethylene. The multilayer foam structure can be obtained by coextruding a multilayer structure comprising at least one foam layer composition and at least one coating layer composition, irradiating the coextruded structure with ionizing radiation, and continuously foaming the irradiated structure.

Description

CROSSLINKED, COEXTRUDED MULTILAYER POLYOLEFIN FOAM STRUCTURES, POLYOLEFIN CAP LAYERS AND METHODS OF MANUFACTURING THEM Cross-reference to related application This application claims priority and benefit for United States application number 16 / 832,684, filed on March 27, 2020, the full content of which is incorporated herein by reference. Field of dissemination This disclosure relates to multilayer polyolefin foam structures and methods for manufacturing them. More specifically, this disclosure relates to coextruded, crosslinked, multilayer polyolefin foam structures with crosslinked polyolefin overlay layers. Background of the invention Polyolefin foams can be used in a variety of applications. For example, polyolefin foams can be used as a trim component in a vehicle's interior, such as an instrument panel. The instrument panel may include a multi-layer foam / coating structure where the foam / coating structure is sandwiched between a rigid substrate and a flexible film or sheet. The foam layer of the structure may adhere to the substrate, and the coating layer may adhere to the film or sheet. Additionally, the instrument panel may have an airbag installed on its back. There are several instrument panel designs to accommodate an airbag and, when activated, a safe and aesthetically pleasing airbag deployment. The instrument panel can be designed to open in a specific pattern as the airbag expands. These patterns can vary and are not limited. For example, these patterns can be U-shaped, H-shaped, or another pattern. Additionally, laser scoring of the substrate or laser scoring of both the substrate and the foam layer can be performed on the instrument panel during manufacturing to facilitate the panel opening in a particular pattern. Other designs may not have scoring (i.e., neither the substrate nor the foam is perforated or cut to facilitate a desired panel rupture pattern). Regardless of the design, when an airbag deploys, it is preferable that the airbag pass through the instrument panel seamlessly. Excessive splitting within or between any of the multi-layer foam coating structures / films or sheets / substrate can be undesirable due to: (a) increased airbag rupture time; and (b) increased instrument panel fracturing, which can cause instrument panel fragments to splinter. One purpose of the coating layer on an instrument panel is to reduce the splitting that can occur between the foam layer and the flexible film or sheet. The coating layer increases the force required to separate the flexible film or sheet from the foam. ινΐΛ / a / zuzz / uii yúo Brief description of the disclosure The applicants have discovered that it is possible to produce a physically crosslinked, closed-cell polyolefin foam with at least one physically crosslinked polyolefin coating layer in a continuous process. This discovery may provide a method for producing more desirable multilayer polyolefin foam structures. For example, in the case of a vehicle instrument panel, the peel resistance between a crosslinked coating layer and a non-crosslinked coating layer can be improved, thereby further reducing the separation that can occur between the foam and the film or foil as a deployable airbag passes through the instrument panel. In some embodiments, a method for forming a multilayer structure includes the coextrusion of a foam layer comprising at least one polypropylene and one polyethylene; a chemical foaming agent; a crosslinking agent; and a film layer on one side of the foam layer. The film layer comprises at least 90 wt% of at least one polypropylene and one polyethylene; and 0.1 wt% to 5 wt% of a crosslinking agent. In some embodiments, the foam layer comprises polypropylene with a melt flow rate of 0.1 grams to 25 grams per 10 minutes at 230 °C. In some embodiments, the foam layer comprises polyethylene with a melt flow rate of 0.1 grams to 25 grams per 10 minutes at 190 °C. In some embodiments, the foam layer comprises 0.5 wt% to 5 wt% of a crosslinking agent. In some embodiments, the chemical foaming agent comprises azodicarbonamide.In some embodiments, the foam layer comprises polypropylene and polyethylene. In some embodiments, the foam layer comprises at least 75% by weight of at least one of polypropylene and one of polyethylene. In some embodiments, the foam layer comprises 3% to 15% by weight of the chemical foaming agent. In some embodiments, a method for forming a multilayer foam structure includes coextruding: a foam layer comprising at least one polypropylene and one polyethylene, a chemical foaming agent, and a crosslinking agent; and a film layer on one side of the foam layer, the film layer comprising at least 90 wt% of at least one polypropylene and one polyethylene, and 0.1 wt% to 5 wt% of a crosslinking agent; irradiating the coextruded layers with ionizing radiation; and foaming the irradiated and coextruded layers. In some embodiments, the ionizing radiation is selected from the group consisting of alpha, beta (electrons), X-rays, gamma rays, and neutrons. In some embodiments, the coextruded structure is irradiated up to four times separately. In some embodiments, the ionizing radiation is an electron beam with an accelerating voltage of 200 kV to 1500 kV. In some modalities, an absorbed electron beam dosage is 10 kGy to 500 kGy.In some embodiments, ionizing radiation crosslinks the coextruded structure to a degree of crosslinking of 20% to 75%. In some embodiments, foaming involves heating the irradiated structure with molten salt. In some embodiments, the multilayer foam structure has a density of 20 kg / m³ to 250 kg / m³. In some embodiments, the multilayer foam structure has a thickness of 0.2 mm to 50 mm. In some embodiments, the foam layer comprises polypropylene and polyethylene. In some embodiments, the foam layer comprises at least 75% by weight of at least one of polypropylene and one of polyethylene. In some embodiments, the foam layer comprises 3% to 15% by weight of the chemical foaming agent. As used here, the singular forms of “a,” “an,” “the,” and “the” are intended to include the plural forms, unless the context indicates otherwise. It should be understood that the term “and / or,” as used here, refers to and encompasses any and all possible combinations of one or more of the listed associated items. It should also be understood that the terms “includes,” “comprises,” and / or “comprises,” when used, specify the presence of features, integrators, stages, operations, elements, components, and / or units, but do not preclude the presence or addition of one or more of the features, integrators, stages, operations, elements, components, units, and / or groups thereof. It is understood that the aspects and modalities described herein include those that are "consisting of" and / or "essentially consisting of" aspects and modalities. For all methods, systems, compositions, and devices described herein, the methods, systems, compositions, and devices may either comprise the listed components or steps, or may "consist of" or "essentially consist 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 contains the listed components and may contain other components that do not substantially affect the performance of the system, composition, or device, but also does not contain any other components that substantially affect the performance of the system, composition, or device other than those components expressly listed; or does not contain a sufficient concentration or quantity of the additional components 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 contains the listed steps, and may contain other steps that do not substantially affect the result of the method, but the method does not contain any other steps that substantially affect the result of the method other than the steps expressly listed. In disclosure, “substantially free from” a specific component, specific composition, specific compound, or specific ingredient in various forms means that less than approximately 5%, less than approximately 2%, less than approximately 1%, less than approximately 0.5%, less than approximately 0.1%, less than approximately 0.05%, less than approximately 0.025%, or less than approximately 0.01% of the specific component, specific composition, specific compound, or specific ingredient is present by weight. Preferably, “substantially free from” a specific component, specific composition, specific compound, or specific ingredient indicates that less than approximately 1% of the specific component, specific composition, specific compound, or specific ingredient is present by weight. The additional advantages will be readily apparent to those skilled in the art from the following detailed description. The examples and descriptions herein are intended to be illustrative and not restrictive. Brief description of the figures Several modalities are described with reference to the attached figures, in which: Figure 1 is a table of the various components and component descriptions used in the examples disclosed in this document. Figure 2A provides a table of the formulations for the examples disclosed herein, as well as the coextrusion, irradiation, and other properties of the multilayer structures of Examples 1 and 2 disclosed herein. Figure 2B is a continuation of the table in Figure 2A. Figure 2C is a continuation of the table in Figures 2A and 2B. Figure 3 is an image of example 1 with a magnification of 30X and 45° from the coating surface and 45° from the machine direction (MD); Figure 4 is an image of example 2 with a magnification of 30X and 45° from the coating surface and 45° from the machine direction (MD). Detailed description of the disclosure Methods are described for producing crosslinked, closed-cell, coextruded, multilayer foam structures comprising a foam layer with at least one polypropylene and one polyethylene component and a crosslinked coating layer with at least one polypropylene and one polyethylene component. The methods for producing crosslinked, closed-cell, coextruded, multilayer foam structures may include the steps of (a) coextrusion, (b) irradiation, and (c) foaming. Coextrusion is the extrusion of multiple layers of material simultaneously. This type of extrusion can use two or more extruders to distribute a constant volumetric flow of material to an extrusion head (die) that can extrude the materials into the desired shape. In the coextrusion step, foam compositions can be fed into multiple extruders to form a multi-layered, non-foamed structure. For example, foam composition A can be fed into one extruder, and non-foam composition B can be fed into a second extruder. The method of feeding ingredients into the extruders can be based on the extruder design and the available material handling equipment. Premixing the foam composition ingredients can be performed, if necessary, to facilitate dispersion. A Henshel mixer can be used for this premixing. All ingredients can be premixed and fed through a single orifice in the extruder. Alternatively, ingredients can be fed individually through separate designated orifices for each ingredient.For example, if the crosslinking promoter or any other additive is a liquid, the promoter and / or additives can be added through a feed gate (or gates) in the extruder or through a vent opening in the extruder (if equipped with venting) instead of being premixed with solid ingredients. Combinations of premixing and port feeding of individual ingredients can also be used. ivix / a / zuzz / uii yoo Each extruder can supply a constant amount of each composition into one or more manifolds followed by a sheet die to create a coextruded multilayer sheet without foaming. There are two common methods for coextruding materials: (1) feed block manifolds; and (2) multiple manifolds within the die. The elements of a feed block manifold may include: (a) inlet holes for top, middle, and bottom layers; (b) an optimized melt lamination area that channels separate flow streams into a laminated melt stream within the feed block; (c) an adapter plate between the feed block and the sheet die; and / or (d) a sheet die (similar to a monolayer die), where the laminated melt stream enters the center of the die and disperses along the manifold, flowing out of the die outlet as a distinct multilayer extruded product.The elements of a multi-collector die may be: (a) similar to a single-layer die, except that there is more than one feed channel; (b) that each melt channel has its own choke bar for flow control; and / or (c) that the melt streams converge within the die near the outlet and emerge as a distinct multi-layer extruded product. Layer thicknesses can be determined by the design of the manifolds and die. For example, an 80 / 20 feed block manifold can distribute compositions in approximately a 4:1 ratio when the speed and size of each extruder are matched accordingly. This ratio can be altered by changing, for example: (a) the relative extrusion speed between extruders; (b) the relative size of each extruder; and / or (c) the composition (i.e., viscosity) of the individual layers. The overall thickness of the multi-layer sheet can be controlled by the overall die clearance. However, the overall thickness of the multi-layer sheet can also be adjusted, for example, by stretching (i.e., pulling) the molten multi-layer extruded product and / or by flattening the molten multi-layer extruded product through a clamping point. Multilayer structures may include at least two layers composed of different materials. In some embodiments, multilayer structures include at least one layer composed of a foam composition and at least one layer composed of a non-foaming coating composition. In some embodiments, the structure may be a B / A layered structure, a B / A / B layered structure, a B / A / C layered structure, or it may have multiple other layers. In some embodiments, a non-foaming coating composition may include a crosslinking promoter. Furthermore, multilayer structures may include additional layers such as bonding layers, film layers, and / or additional foam layers, among others. A foamable composition and a non-foamable layer composition fed into the extruder may include at least one polypropylene, at least one polyethylene, and / or a combination thereof. Polypropylene includes, but is not limited to, polypropylene, impact-modified polypropylene, polypropylene-ethylene copolymer, impact-modified polypropylene-ethylene copolymer, metallocene polypropylene, metallocene-ethylene polypropylene copolymer, metallocene polypropylene olefin block copolymer (with a controlled block sequence), polypropylene-based polyolefin plastomer, polypropylene-based polyolefin elasto-plastomer, polypropylene-based polyolefin elastomer, polypropylene-based thermoplastic polyolefin blend, and polypropylene-based thermoplastic elastomeric blend. The polypropylene may be of a high melt strength type. In addition, the polypropylenes may be grafted with maleic anhydride. Polyethylene includes, but is not limited to, low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE) (homopolymer, copolymer with butene or hexene or octene, terpolymer with butene and / or hexene and / or octene), very low-density polyethylene (VLDPE).(homopolymer, copolymer with butene or hexene or octene, terpolymer with butene and / or hexene and / or octene), very low-density linear polyethylene (VLLDPE) (homopolymer, copolymer with butene or hexene or octene, terpolymer with butene and / or hexene and / or octene), high-density polyethylene (HDPE), polyethylene-propylene copolymer, metallocene polyethylene, metallocene-propylene ethylene copolymer, and metallocene polyethylene olefin block copolymer (with a controlled block sequence), any of which may contain compatibilizers or grafted copolymers containing acetate and / or ester groups. These polyethylenes may be grafted with maleic anhydride. These polyethylenes can also be copolymers and terpolymers containing acetate and / or ester groups and can be ionomers of copolymers and terpolymers containing acetate and / or ester groups. A foam composition and a non-foam coating composition introduced into the extruder may include at least 25% by weight of polypropylene, polyethylene or a combination thereof; at least 50% by weight of polypropylene, polyethylene or a combination thereof; at least 75% by weight of polypropylene, polyethylene or a combination thereof; at least approximately 85% by weight of polypropylene, polyethylene or a combination thereof; at least 90% by weight of polypropylene, polyethylene or a combination thereof; at least 95% by weight of polypropylene, polyethylene or a combination thereof; or at least approximately 98% by weight of polypropylene, polyethylene or a combination thereof. Since a wide range of multi-layer structures and foam articles can be created with the described compositions, a wide range of polypropylenes and polyethylenes can be employed in the compositions to meet various in-process manufacturing requirements and commercial end-use requirements. A non-limiting example of polypropylene is an isotactic homopolypropylene. Commercially available examples include, but are not limited to, Braskem's FF018F, Total Petrochemicals' 3271, and Phillips 66's COPYLENE™ CH020. A non-limiting example of an impact-modified polypropylene is a homopolypropylene with ethylene-propylene (EP) copolymer rubber. The rubber may be amorphous or semicrystalline but is not present in sufficient quantity to impart any plastomeric or elastomeric properties to the material. Some non-limiting examples of commercially available impact-modified polypropylene are Braskem's TI4003F and TI4015F, and Lyondell's PRO-FAXMR8623 and PRO-FAXMRSB786. “Polypropylene-ethylene copolymer” is polypropylene with random ethylene units. A few non-limiting examples of commercially available polypropylene-ethylene copolymers are 6232, 7250FL, and Z9421 from Total Petrochemicals, 6D20 and DS6D81 from Braskem, and PRO-FAXMRRP311H and ADSYLMR7415 XCP from LyondeIBasell. “Impact-modified polypropylene-ethylene copolymer” is polypropylene with random ethylene units and ethylene-propylene (EP) copolymer rubber. The rubber may be amorphous or semicrystalline but is not present in sufficient quantity to impart any plastomeric or elastoplastomeric properties to the material. A non-limiting example of a commercially available impact-modified polypropylene-ethylene copolymer is Braskem's PRISMAMR6910. “Metalocene polypropylene” refers to 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® from LyondeIBasell and ACHIEVE® from ExxonMobil. Metallocene polypropylenes are also commercially available from Total Petrochemicals and include, but are not limited to, grades M3551, M3282MZ, M7672, 1251, 1471, 1571, and 1751. “Metallocene-ethylene polypropylene copolymer is syndiotactic metallocene, atactic metallocene, and isotactic metallocene polypropylene with random ethylene units. Commercially available examples include, but are not limited to, Lumicene® MR10MX0 and Lumicene® MR60MC2 from Total Petrochemicals, Purell® SM170G from Lyondell Basell, and the WINTEC® product line from Japan Polypropylene Corporation.” “Metalocene polypropylene olefin block copolymer is a polypropylene with alternating crystallizable hard blocks and amorphous soft blocks that are not randomly distributed, i.e., with a controlled block sequence. An example of a “metallocene polypropylene olefin block copolymer” includes, but is not limited to, the INTUNEMR product line from Dow Chemical Company. “Polypropylene-based polyolefin plastomer” (POP) and “polypropylene-based polyolefin elastoplastomer” are metallocene- and non-metallocene-propylene-based copolymers with plastomeric and elastoplastomeric properties. Non-limiting examples include those commercially available under the trade names VERSIFYMR (metallocene) from Dow Chemical Company, VISTAMAXXMR (metallocene) from ExxonMobil, and KOATTROMR (non-metallocene) from LyondellBasell (a line of plastomeric polymers based on 1-butene; some grades are 1-butene-based homopolymers, and others are 1-butene-polypropylene-based copolymers). “Polypropylene-based polyolefin elastomer” (POE) refers to both metallocene and non-metallocene propylene-based copolymers with elastomeric properties. Non-limiting examples of propylene-based polyolefin elastomers include commercially available polymers such as VERSIFYMR (metallocene) from Dow Chemical Company and VISTAMAXXMR (metallocene) from ExxonMobil. Thermoplastic polyolefin blend based on polypropylene (TPO) is polypropylene, polypropylene-ethylene copolymer, metallocene homopolypropylene, and copolymer of MA / a / ZUZZ / Ul 1 »30 metallocene-ethylene polypropylene, which have ethylene-propylene copolymer rubber in sufficiently large quantities to give the thermoplastic polyolefin (TPO) blend plastomeric, elastoplastomeric, or elastomeric properties. Non-limiting examples of TPO polymers are those commercially available under the trade names THERMORIJN® and ZELAS® from Mitsubishi Chemical Corporation, ADFLEX® and SOFTELL® from LyondeIBasell, TELCAR® from Teknor Apex Company, and WELNEX™ from Japan Polypropylene Company. TPO can be produced by multi-stage polymerization (e.g., ZELAS, ADFLEX®, SOFTELL®, and WELNEX®) or by blending (e.g., THERMORUN® and TELCAR®). “Polypropylene-based thermoplastic elastomer blend” (TPE) is polypropylene, polypropylene-ethylene copolymer, metallocene homopolypropylene, and metallocene-ethylene polypropylene copolymer, having diblock or multiblock thermoplastic rubber modifiers (SEBS, SEPS, SEEPS, SEP, SERC, CEBC, HSB, and the like) in sufficiently large quantities to impart plastomeric, elastoplastomeric, or elastomeric properties to the thermoplastic elastomer blend (TPE). Non-limiting examples of polypropylene-based thermoplastic elastomer blend polymers are those commercially available polymer blends under the trade names GLSMRDYNAFLEXMR and GLSMVERSAFLEXMR from PolyOne Corporation, MONPRENEMR from Teknor Apex Company, and DURAGRIPMR from Lyondel IBasell. Any of the polypropylenes described above can also be a high-melt strength (HMS) type. Polypropylene manufacturers employ various methods to strengthen the polymer at the melt stage. For example, polypropylene exhibiting long-chain branching (LCB) can be identified as a high-melt strength polypropylene. Non-limiting examples of high-melt strength polypropylene include commercially available polymers such as DAPLOY® from Borealis, AMPPLEO® from Braskem, and WAYMAX® from Japan Polypropylene Corporation. Any polypropylene, but more commonly blends of TPO and TPE, can be optionally extended with oil, e.g. mineral oil, Chevron's PARALUX® process oils, etc., to further soften the blend, improve the haptic property of the blend, or improve the processability of the blend. “LDPE” and “LLDPE” are low-density polyethylene and linear low-density polyethylene, respectively. Non-limiting examples of LDPE include at least those provided by Dow (e.g., 640I) and Nova (e.g., Novapol® LF-0219-A). Non-limiting examples of LLDPE include at least those provided by ExxonMobil® (e.g., LLP8501.67) and Dow (e.g., DFDA7059 NT 7). Commercial LLDPE polymers are typically copolymers or terpolymers containing α-olefins of butene and / or hexene and / or octene. VLDPE and VLLDPE are very low-density polyethylene and linear very low-density polyethylene, and commonly copolymers or terpolymers containing α-olefins of butene and / or hexene and / or octene. Non-limiting examples of VLDPE and VLLDPE are commercially available under the trade name FLEXOMER® from Dow Chemical Company and specific grades of STAMYLEX®. Borealis. “Metallocene polyethylene” is metallocene-based polyethylene with properties ranging from non-elastic to elastomeric. Non-limiting examples of metallocene polyethylene are commercially available under the trade names ENGAGE® by Dow Chemical Company, ENABLE® and EXCEED® by ExxonMobil®, and QUEO® by Borealis. Metallocene polyethylene block copolymer is a polyethylene with alternating crystallizable hard blocks and amorphous soft blocks that are not randomly distributed, i.e., with a controlled block sequence. An example of a metallocene polyethylene block copolymer includes, but is not limited to, the INFUSE® product line from Dow Chemical Company. All of the polyethylenes listed above can be grafted with maleic anhydride. Commercially available, non-limiting examples include ADMER® NF539A from Mitsui Chemicals, DuPont™ BYNEL® 4104 from Dow, and OREVAC® 18360 from Arkema. It should be noted that many commercially available polyethylenes grafted with maleic anhydride also contain rubber. These polyethylenes can also be copolymers and terpolymers containing acetate and / or ester groups. Comonomeric groups include, but are not limited to, vinyl acetate, methyl acrylate, ethyl acrylate, butyl acrylate, glycidyl methacrylate, and acrylic acid. Non-limiting examples are 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. Functionalized polypropylenes and polyethylenes may include a grafted monomer. Typically, the monomer is grafted onto the polypropylene or polyethylene by a free-radical reaction. Suitable monomers for the preparation of functionalized polypropylenes and polyethylenes include, for example, olefinic unsaturated monocarboxylic acids, such as acrylic acid or methacrylic acid, and the corresponding tert-butyl esters, such as tert-butyl (meth)acrylate; olefinic 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; olefinic unsaturated dicarboxylic anhydrides, such as maleic anhydride; and olefinic unsaturated monomers containing sulfo or sulfonyl, such as...p-Styrenesulfonic acid, 2(meth)acrylamido-2-methylpropenesulfonic acid or 2-sulfonyl-(meth)acrylate, olefinic unsaturated monomers containing oxazolinyl, for example, vinylxylazines and vinylxoline derivatives, and olefinic unsaturated monomers containing epoxy, for example, glycidyl (meth)acrylate or allyl-glycidyl ether. The most commonly available commercially functionalized polypropylenes are those functionalized with maleic anhydride. Non-limiting examples include the ADMERMRQF and QB series from Mitsui Chemicals, the PLEXARMR6000 series from Lyondell / IBasell, the DuPont BYNELMR5000 series from Dow Chemical Company, and the OREVACMRPP series from Arkema. The most commonly available commercially functionalized polyethylenes are also MA / a / ZUZZ / UI 1 those functionalized with maleic anhydride. Non-limiting examples are the ADMERMRNF and SE series from Mitsui Chemicals, the PLEXARMR1000, 2000 and 3000 series from LyondelIBasell, the DuPontMRseries BYNELMR2100, 3000, 3800, 3900, 4000 from Dow, and the OREVAC MR PE, T series and some of the LOTADERMR series from Arkema. Polyethylenes functionalized with other grafted monomers are also commercially available. Non-limiting examples include DuPontMRseries BYNELMR1100, 2200, and 3100 from Dow Chemical Company and the LOTADERMRAX series from Arkema. It should be noted that polymers other than polypropylene and polyethylene functionalized with maleic anhydride are also commercially available. For example, Addivant's ROYALTLJFMR series are a series of ethylene propylene diene monomer (EPDM) rubbers functionalized with maleic anhydride. As another example, Kraton's KRATONMRFG series are a series of SEBS polymers functionalized with maleic anhydride. The composition of any foamable layer and any coating layer provided herein may contain at least one polypropylene having a melt flow index of approximately 0.1 grams to approximately 25 grams per 10 minutes at 230°C and / or at least one polyethylene having a melt flow index of approximately 0.1 grams to approximately 25 grams per 10 minutes at 190°C. In some embodiments, the melt flow index of the polypropylenes and / or polyethylenes is preferably approximately 0.3 grams to approximately 20 grams per 10 minutes at 230°C and 190°C, respectively, and more preferably approximately 0.5 grams to approximately 15 grams per 10 minutes at 230°C and 190°C, respectively.The melt flow index (MFI) value for a polymer is defined and measured according to ASTM D1238 at 230 °C for polypropylenes and polypropylene-based materials and at 190 °C for polyethylenes and polyethylene-based materials using a 2.16 kg plunger for 10 minutes. The test time may be reduced for resins with relatively high melt flow. The MFI can provide a measure of a polymer's flow characteristics and is an indication of the molecular weight and processability of a polymer material. High MFI values ​​correspond to low viscosities. If the MFI values ​​are too high, extrusion according to this disclosure cannot be carried out satisfactorily. Problems associated with excessively high MFI values ​​include low pressures during extrusion, difficulties setting the thickness profile, uneven cooling profile due to low melt viscosity, poor melt strength, and / or machine problems. Conversely, low MFI values ​​correspond to high viscosities.MFI values ​​that are too low can cause high pressures during melt processing, sheet quality and profile problems, and higher extrusion temperatures that lead to a risk of decomposition and activation of the foaming agent. The aforementioned MFI ranges are also important for foaming processes because they can reflect the material's viscosity, which affects foam formation. While not limited by any single theory, it is believed that there are several reasons why particular MFI values ​​are more effective. A material with a lower MFI can improve certain physical properties as the molecular chain length increases, creating more energy required for the chains to flow under stress. Furthermore, the longer the molecular chain (MW), the more crystal entities the chain can crystallize, thus providing greater strength through intermolecular bonds. However, at excessively low MFI values, the viscosity becomes too high. Conversely, polymers with higher MFI values ​​have shorter chains.Therefore, in a given volume of a material with higher MFI values, there are more chain ends at a microscopic level compared to polymers with lower MFI values. These chain ends can be rotated, creating free volume due to the space required for this rotation (e.g., rotation occurring above the polymer's glass transition temperature, Tg). This can increase the free volume and allow for easy flow under tensile forces. In addition to polymers, the compositions fed into extruders may also contain additives compatible with the production of the disclosed multilayer structures. Common additives include, but are not limited to, organic peroxides, antioxidants, lubricants, processing aids, heat stabilizers, colorants, flame retardants, antistatic agents, nucleating agents, plasticizers, antimicrobials, fungicides, light stabilizers, UV absorbers, antiblocking agents, fillers, deodorants, odor adsorbents, antifogging agents, volatile organic compound (VOC) adsorbents, semi-volatile organic compound (SVOC) adsorbents, thickening agents, cell size stabilizers, metal deactivators, and combinations thereof. In some embodiments, the amount of additives other than chemical foaming agents and crosslinking promoters in a foam layer composition and / or non-foam layer composition may be less than or equal to approximately 20% PPR, approximately 15% PPR, approximately 10% PPR, or approximately 8% PPR of the composition. In some embodiments, the amount of additives other than chemical foaming agents and crosslinking promoters in a foam layer composition and / or non-foam layer composition may be greater than or equal to approximately 1% PPR, approximately 2% PPR, approximately 4% PPR, or approximately 6% PPR of the composition.In some embodiments, the amount of additives other than chemical foaming agents and crosslinking promoters in a foam layer composition and / or non-foam layer composition may be approximately 1% to 20% PPR, approximately 2% to 15% PPR, approximately 4% to 10% PPR, or approximately 6% to 8% PPR of the composition. In some embodiments, the amount of additives other than chemical foaming agents and crosslinking promoters in a foam layer composition and / or non-foam layer composition may be approximately 1% to 20% by weight, approximately 2% to 15% by weight, approximately 3% to 10% by weight, approximately 4% to 8% by weight, or approximately 5% to 7% by weight of the foam layer composition. In some embodiments, the amount of additives other than chemical foaming agents and crosslinking promoters in a coating layer composition may be less than or equal to approximately 20% PPR, approximately 15% PPR, approximately 10% PPR, approximately 7% PPR, approximately 5% PPR, or approximately 3% PPR of the composition. In some embodiments, the amount of additives other than chemical foaming agents and crosslinking promoters in a coating layer composition may be greater than or equal to approximately 0.5% PPR, approximately 1% PPR, approximately 2% PPR, approximately 3% PPR, approximately 4% PPR, or approximately 5% PPR of the composition. In some embodiments, the amount of additives other than chemical foaming agents and crosslinking promoters in a coating layer composition may be approximately 0.5% PPR to 20% PPR, approximately 1% PPR to 10% PPR, or approximately 2% PPR to 7% PPR of the composition. In some embodiments, the amount of additives other than chemical foaming agents and crosslinking promoters in a coating layer composition may be approximately 0.5% by weight to 20% by weight, approximately 1% by weight to 10% by weight, or approximately 2% by weight to 6% by weight of the coating layer composition. Regardless of how the ingredients are fed into the extruders, the shear force and mixing within an extruder can be sufficient to produce a homogeneous layer. Co-rotating and counter-rotating twin-screw extruders can provide sufficient shear force and mixing through the extruder barrel to extrude a layer with uniform properties. Specific energy is an indicator of how much work is being applied during the extrusion of ingredients for a layer and how intense the extrusion process is. Specific energy is defined as the energy applied to a material being processed by the extruder, normalized to a per-kilogram basis. Specific energy is quantified in kilowatts of energy applied per kilogram of total material fed per hour. Specific energy is calculated according to the formula: Specific energy =-----—----—¡^ , where feed rate n KW (applied) = KW (motor power) * (% torque from the maximum allowed in decimal form) * RPM (current stroke RPM) * 0.97 (gearbox efficiency) Max RPM (extruder capacity) Specific energy can be used to quantify the amount of cutting and mixing of the ingredients within the extruder. Extruders used to form the multi-layer structures described herein may be capable of producing a specific energy of at least approximately 0.050 kWh / kg, preferably at least approximately 0.120 kWh / kg, and more preferably at least approximately 0.100 kWh / kg. Any foamable layer may contain a chemical foaming agent (CFA). The extrusion temperature for any foamable layer must be at least 10°C below the thermal decomposition onset temperature of the chemical foaming agent. If the extrusion temperature exceeds the thermal decomposition temperature of the foaming agent, the foaming agent will decompose, resulting in undesirable foam preforming. The extrusion temperature for any coating layer must be at least 10°C below the thermal decomposition onset temperature of the chemical foaming agent in any foamable layer adjacent to the coating layer.If the extrusion temperature of the coating layer exceeds the thermal decomposition temperature of the foaming agent in the adjacent layer, then the foaming agent in the adjacent layer may decompose, which also results in unwanted foam pre-formation. The foam layer composition can include a variety of different chemical foaming agents. Examples of chemical foaming agents include, but are not limited to, azo compounds, hydrazine compounds, carbazides, tetrazoles, nitroso compounds, and carbonates. Furthermore, a chemical foaming agent can be used alone or in any combination. One chemical foaming agent that can be used in some applications is azodicarbonamide (ADCA). An example of an ADCA foaming agent is UNIFOAM® TC-181 manufactured by PT Lauten Otsuka Chemical. Thermal decomposition of ADCA typically occurs at temperatures between approximately 190°C and 230°C. To prevent ADCA from thermally decomposing in the extruder, the extrusion temperature can be maintained at or below 190°C. The amount of chemical foaming agent in a foam layer composition may be less than or equal to approximately 30% PPR, approximately 20% PPR, approximately 15% PPR, approximately 10% PPR, or approximately 8% PPR of the composition. In some embodiments, the amount of chemical foaming agent in a foam layer composition may be greater than or equal to approximately 1% PPR, approximately 2% PPR, approximately 3% PPR, approximately 4% PPR, or approximately 5% PPR of the composition. In some embodiments, the amount of chemical foaming agent in a foam layer composition may be from approximately 1% PPR to 30% PPR, approximately 2% PPR to 20% PPR, approximately 3% PPR to 15% PPR, approximately 4% PPR to 10% PPR, or approximately 5% PPR to 8% PPR of the composition.In some applications, the amount of chemical foaming agent in a foam layer composition can range from approximately 1% to 30% by weight, approximately 2% to 20% by weight, approximately 3% to 15% by weight, approximately 4% to 10% by weight, or approximately 5% to 7% by weight of the foam layer composition. The amount of chemical foaming agent may depend on the thickness of the unfoamed sheet, the desired foam thickness, the desired foam density, the materials being extruded, the crosslinking percentage, and / or the type of chemical foaming agent (different foaming agents can generate significantly different amounts of gas), among other factors. It should be noted that the quantities of foaming agent listed above may be specific to ADCA only. Other foaming agents may produce varying amounts of volumetric gas per mass of CFA and should be considered accordingly. For example, when comparing ADCA to the foaming agent p-toluenesulfonyl semicarbazide (TSS), if a foamable layer contains 40 PPR of ADCA, approximately 63 PPR of TSS would be required to generate roughly the same amount of gas during the foaming step. If the difference between the decomposition temperature of the thermally decomposing foaming agent and the melting point of the polymer with the highest melting point is significant, a catalyst can be used to decompose the foaming agent. Examples of catalysts include, but are not limited to, zinc oxide, magnesium oxide, calcium stearate, glycerin, and urea. The lower temperature limit for extrusion can be that of the polymer with the highest melting point. If the extrusion temperature falls below the melting temperature of the polymer with the highest melting point, unwanted unmelted masses will appear. After foaming, the extruded layer produced below this lower temperature limit may exhibit uneven thickness, a non-uniform cell structure, cell collapse pockets, and other undesirable characteristics. Regardless of whether the foaming agents are physical, chemical, or a combination, conventional extrusion foaming produces polymer sheets where both primary surfaces are significantly rougher than equivalent structures produced by the described method. The surface profile of a multi-layer (as well as single-layer) foam sheet can be critical in many applications, and therefore, extrusion foam sheets cannot be used for these applications. These applications may require a smooth foam surface to achieve desired properties, such as ease of lamination to a film, fabric, fiber layer, or leather; percentage of contact area in lamination; visual aesthetics; and so on.The PCT publication WO 2016109544, which is hereby incorporated in full by reference, includes examples illustrating the difference in surface roughness between extruded foamed polymer sheets and equivalent foamed polymer sheets produced by the described method. The rougher surfaces of extruded foam articles in general can be caused by larger cell sizes (when compared to foams produced in accordance with this disclosure). Although cell size and cell size distribution may not be as critical in most commercial applications, because surface roughness is a function of cell size, foams with larger cells may be less desirable than foams with smaller cells for applications requiring a smooth foam surface. The thickness of the non-foamed, coextruded multi-layer structure can be from approximately 0.1 mm to approximately 30 mm, from approximately 0.2 mm to approximately 25 mm, from approximately 0.3 mm to approximately 20 mm, or from approximately 0.4 mm to approximately 15 mm. Any individual layer A or B can have a thickness of at least approximately 0.05 mm, at least approximately 0.1 mm, at least approximately 0.15 mm, or at least approximately 0.2 mm. Any individual layer A or B can have a thickness of less than or equal to approximately 0.2 mm, approximately 0.15 mm, or approximately 0.10 mm. In some embodiments, a coating layer of the non-foamed, coextruded multi-layer structure can have a thickness of approximately 0.1 micrometers to 300 micrometers, approximately 25 micrometers to 200 micrometers, or approximately 30 micrometers to 175 micrometers.In some forms, a coating layer of the non-foamed coextruded multi-layer structure may have a thickness of less than 300 micrometers, less than 250 micrometers, less than 200 micrometers, less than 175 micrometers, less than 150 micrometers, less than 125 micrometers, less than 100 micrometers, less than 90 micrometers, less than 80 micrometers, less than 70 micrometers, less than 60 micrometers, less than 50 micrometers, less than 40 micrometers, less than 30 micrometers, less than 20 micrometers, less than 10 micrometers, less than 5 micrometers, or less than 1 micrometer.In some forms, a coating layer of the non-foamed coextruded multi-layer structure may have a thickness of more than 1 micrometer, more than 5 micrometers, more than 10 micrometers, more than 20 micrometers, more than 30 micrometers, more than 40 micrometers, more than 50 micrometers, more than 60 micrometers, more than 70 micrometers, more than 80 micrometers, more than 90 micrometers, more than 100 micrometers, more than 125 micrometers, more than 150 micrometers, more than 175 micrometers, more than 200 micrometers, or more than 250 micrometers. The thickness of the non-foamed coating is not limited in how thin it can be relative to the general non-foamed coextruded multi-layer sheet, and can be as thin as approximately 0.1 pm, or the usual thickness of a very thin bonding layer used in barrier films and multi-layer flexible packaging.In some embodiments, a foam layer of the unfoamed coextruded multilayer structure may have a thickness of approximately 0.1 mm to 5 mm, approximately 0.5 mm to 3 mm, approximately 1 mm to 2 mm, or approximately 1 mm to 1.5 mm. In some embodiments, a foam layer of the unfoamed coextruded multilayer structure may have a thickness less than or equal to approximately 5 mm, approximately 3 mm, approximately 2 mm, approximately 1.5 mm, approximately 1 mm, or approximately 0.5 mm. In some embodiments, a foam layer of the unfoamed coextruded multilayer structure may have a thickness greater than or equal to approximately 0.1 mm, approximately 0.5 mm, approximately 1 mm, approximately 1.5 mm, approximately 2 mm, or approximately 3 mm. The total thickness of the coextruded, non-foaming, multi-layer structure is measured using a stem-style thickness gauge mounted on a flat base. The gauge tip is equipped with a hemispherical contact point with a radius of 1.6 mm. The stem is raised, and the non-foaming structure is placed on the base. A force of 100 gf to 150 gf is applied to the structure at the contact point during the measurement. The thickness of the coating layer of the coextruded, non-foaming, multilayer structure is measured using a microscope. To measure the coating thickness, a small sample is cut from the continuous sheet structure, and the cross-section of the sample is sliced ​​into thin sections using a microtome. One section is placed under the viewing microscope. A measurement can be performed with either a digital or traditional microscope. A common commercial digital microscope may have various software features to facilitate thickness measurement. A traditional commercial microscope may have a lens with measuring scales to facilitate thickness measurement. The coating can be thin and easily flexible when melted so as not to significantly impede the expansion of the foaming layers during the foaming step. The coating's thickness, flexibility, melt strength, and crosslinking percentage are among many physical properties that can hinder the foaming expansion of the other layers. Similarly, the thickness, flexibility, melt strength, and crosslinking percentage of the foamable layers, as well as the final thickness and density of the foamed layers, are also factors in whether the coating inhibits the expansion of the foamable layers. A general guideline for the maximum coating thickness is that it should not exceed approximately 20%, approximately 15%, approximately 10%, or approximately 5% of the overall coextruded non-foamed sheet.If the coating thickness is greater than approximately 20% of the general coextruded non-foamed sheet, problems with curling, buckling, and folding of the multi-layer sheet upon itself may occur as the multi-layer sheet heats up and foams. It is important to distinguish between physical crosslinking and chemical crosslinking. In chemical crosslinking, crosslinks are generated using crosslinking promoters, but without the use of ionizing radiation. Chemical crosslinking typically involves the use of peroxides, silanes, or vinylsilane. In peroxide crosslinking processes, crosslinking usually occurs in the extrusion die. For silane and vinylsilane crosslinking processes, crosslinking usually occurs after extrusion in a secondary operation where the crosslinking of the extruded material is accelerated with heat and moisture. Regardless of the chemical crosslinking method, chemically crosslinked foam sheets typically exhibit primary surfaces that are significantly rougher than equivalent structures produced by the described method.The surface profile of a multi-layer (as well as a single-layer) foam sheet can be critical in many applications, and therefore chemically crosslinked foam sheets cannot be used for these applications. These applications may require a smooth foam surface to achieve desired properties, such as ease of lamination into a film, fabric, fiber layer, or leather; percentage of contact area in lamination; visual aesthetics; and so on. PCT publication WO 2016109544, which is incorporated herein by reference in its entirety, includes examples illustrating the difference in surface roughness between chemically crosslinked foamed polymer sheets and equivalent foamed polymer sheets produced by the disclosed method. The rougher surfaces of chemically crosslinked foamed articles in general can be caused by larger cell sizes (when compared to foams produced according to this description). Although cell size and size distribution are not critical in most commercial applications because surface roughness is a function of cell size, foams with larger cells may be less desirable than foams with smaller cells for applications requiring a smooth foam surface. Examples of ionizing radiation include, but are not limited to, alpha, beta (electrons), x-rays, gamma rays, and neutrons. Among these, an electron beam with uniform energy can be used to prepare the crosslinked polyolefin foam / crosslinked polyolefin coating structure. The exposure time, irradiation frequency, and accelerating voltage after electron beam irradiation can vary widely depending on the degree of crosslinking and the thickness of the multilayer structure. However, ionizing radiation generally ranges from approximately 10 kGy to approximately 500 kGy, approximately 20 kGy to approximately 300 kGy, or approximately 20 kGy to approximately 200 kGy. If the exposure is too low, cell stability may not be maintained after foaming.If the exposure is too high, the moldability of the resulting multilayer foam structure may be poor. Moldability is a desirable property when the multilayer foam sheet is used in thermoforming applications. Furthermore, the unfoamed sheet can soften due to exothermic heat release after exposure to electron beam radiation, such that the structure may deform when the exposure is too high. Additionally, the polymeric components may also degrade due to excessive polymer chain cleavage. The coextruded, non-foamed, multi-layer sheet can be irradiated up to four separate times, preferably no more than twice, and preferably only once. If the irradiation frequency exceeds approximately four times, the polymer components may suffer degradation such that, after foaming, for example, uniform cells are not created in the resulting foaming layers. When the thickness of the extruded structure is greater than approximately 4 mm, it may be preferable to irradiate each primary surface of the multi-layer profile with ionized radiation to make the degree of crosslinking between the primary surfaces and the inner layer more uniform. Electron beam irradiation offers the advantage that coextruded sheets of varying thicknesses can be effectively crosslinked by controlling the electron beam's accelerating voltage. The accelerating voltage can generally range from approximately 200 kV to approximately 1500 kV, approximately 400 kV to approximately 1200 kV, or approximately 600 kV to approximately 1000 kV. If the accelerating voltage is lower than approximately 200 kV, the radiation may not reach the inner portion of the coextruded sheets. As a result, the cells in the inner portion may be thick and uneven during foaming. Furthermore, an accelerating voltage that is too low for a given thickness profile can cause arcing, resulting in perforations or tunneling in the foamed structure.On the other hand, if the accelerating voltage is greater than approximately 1500 kV, then the polymers may degrade. In some configurations, the radiation source may be directed towards layer B of the unfoamed coextruded multilayer sheet during irradiation. In some configurations, the radiation source may be directed towards layer A of the unfoamed coextruded multilayer sheet during irradiation. Regardless of the type of ionizing radiation selected, crosslinking is performed such that the composition of the extruded structure, a foam layer, and / or a non-foam layer is crosslinked between 20% and 75% or approximately 30% to approximately 60%, as measured by the Toray Gel Fraction Percentage Method. According to the Toray Gel Fraction Percentage Method, tetralin solvent is used to dissolve non-crosslinked components in a composition. In principle, the non-crosslinked material is dissolved in tetralin, and the degree of crosslinking is expressed as the weight percentage of crosslinked material in the entire composition. The apparatus used to determine the polymer crosslinking percentage includes: 100 mesh (0.11 mm (0.0.045 inch diameter wire); type 304 stainless steel bags; numbered wires and clamps; a Miyamoto thermostatic oil bath apparatus; an analytical balance; a fume hood; a gas burner; a high-temperature oven; an anti-static gun; and three 3.5-liter wide stainless steel containers with lids. Reagents and materials used include high molecular weight tetralin solvent, acetone, and silicone oil. Specifically, an empty wire mesh bag is weighed and the weight recorded. For each sample, 100 milligrams ± 5 milligrams of sample are weighed and transferred to the wire mesh bag. The weight of the wire mesh bag and the sample, usually in the form of thinly sliced ​​foam scraps, is recorded. Each bag is attached to the corresponding wire and clamp.When the solvent temperature reaches 130 °C, the packaging (bag and sample) is immersed in the solvent. The samples are agitated up and down approximately 5 or 6 times to loosen any air bubbles and ensure they are thoroughly wetted. The samples are then attached to a stirrer and agitated for three (3) hours to allow the solvent to dissolve any foam. The samples are then cooled in a fume hood. They are washed by agitating them up and down approximately 7 or 8 times in a container of primary acetone. The samples are washed a second time in a second acetone wash. The washed samples are then washed once more in a third container of fresh acetone, as before. The samples are then hung in a fume hood to evaporate the acetone for approximately 1 to 5 minutes. Finally, the samples are dried in a drying oven for approximately 1 hour at 120 °C.The samples are cooled for a minimum of approximately 15 minutes. The wire mesh bag is weighed on an analytical balance and the weight is recorded. The crosslinking is then calculated using the formula 100*(CA) / (BA), where A = weight of empty wire mesh bag; B = weight of wire bag + foam sample before immersion in tetralin; and C = weight of wire bag + dissolved sample after immersion in tetralin. Suitable crosslinking promoters include, but are not limited to, commercially available difunctional, trifunctional, tetrafunctional, pentafunctional, and higher-functionality monomers. These crosslinking monomers are available in liquid, solid, granular, and powder forms.Examples include, but are not limited to, acrylates or methacrylates such as 1,6-hexanediol diacrylate, 1,6-hexanediol triacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, trimethylolpropane trimethacrylate, tetramethylolmethane triacrylate, 1,9-nonanediol dimethacrylate, and 1,10-decanediol dimethacrylate; allyl esters of carboxylic acids (such as trimellitic acid trialyl ester, pyromellitic acid trialyl ester, and oxalic acid diallyl ester); allyl esters of cyanulic acid or isocyanulic acid such as trialyl cyanurate and trialyl isocyanurate; Maleimide compounds such as N-phenylmaleimide and N,N'-m-phenylene bismaleimide; compounds having at least two triple bonds such as italic acid dipropagyl of maleic acid; and divinylbenzene. Additionally, these crosslinking promoters can be used alone or in any combination.In some embodiments, divinylbenzene (DVB), a difunctional liquid crosslinking monomer, may be used as a crosslinking promoter in this disclosure. For example, a suitable commercially available DVB may include DVB HP by Dow Chemical Company. The amount of crosslinking agent / promoter in a foam layer composition may be less than or equal to approximately 5% PPR, approximately 4% PPR, approximately 3% PPR, approximately 2.5% PPR, approximately 2% PPR, approximately 1.5% PPR, or approximately 1% PPR of the composition. In some embodiments, the amount of crosslinking promoter in a foam layer composition may be greater than or equal to approximately 0.5% PPR, approximately 1% PPR, approximately 1.5% PPR, approximately 2% PPR, approximately 2.5% PPR, approximately 3% PPR, or approximately 4% PPR of the composition. In some embodiments, the amount of crosslinking promoter in a foam layer composition may be from approximately 0.1% PPR to 5% PPR, approximately 0.5% PPR to 3% PPR, approximately 1% PPR to 3% PPR, or approximately 2% PPR to 3% PPR of the composition.In some forms, the amount of crosslinking promoter in a foam layer composition can be approximately 0.5% by weight to 5% by weight or approximately 1% by weight to 3% by weight of the foam layer composition. The amount of crosslinking agent / promoter in a coating layer composition may be less than or equal to approximately 5% PPR, approximately 4% PPR, approximately 3% PPR, approximately 2.5% PPR, approximately 2% PPR, approximately 1.5% PPR, or approximately 1% PPR of the composition. In some embodiments, the amount of crosslinking promoter in a coating layer composition may be greater than or equal to approximately 0.5% PPR, approximately 1% PPR, approximately 1.5% PPR, approximately 2% PPR, approximately 2.5% PPR, approximately 3% PPR, or approximately 4% PPR of the composition. In some embodiments, the amount of crosslinking promoter in a coating layer composition may be from approximately 0.1% PPR to 5% PPR, approximately 0.5% PPR to 3% PPR, or approximately 1% PPR to 2% PPR of the composition.In some forms, the amount of crosslinking promoter in a coating layer composition can be approximately 0.1 wt to 5 wt, approximately 0.5 wt to 3 wt, approximately 1 wt to 2 wt, or approximately 1 wt to 1.5 wt of the coating layer composition. It should be noted that the quantities of crosslinking promoter listed above may be specific to DVB only. Other crosslinking promoters may be more or less efficient at crosslinking than DVB. Therefore, the quantity required for another crosslinking promoter should be considered accordingly. Crosslinking promoters can vary in crosslinking efficiency based on the ionizing radiation dose, the polymers being crosslinked, the chemical structure of the monomer, the number of functional groups in the monomer, and whether the monomer is a liquid or a powder. Appropriate crosslinks can be generated using a variety of techniques and can form intermolecularly, between different polymer molecules, and intramolecularly between portions of a single polymer molecule. These techniques include, but are not limited to, providing crosslinking promoters that are cleaved from a polymer chain and providing polymer chains that incorporate a crosslinking promoter containing a functional group that can form a crosslink or be activated to form a crosslink. ινΐΛ / a / zuzz / uiiy jo After irradiating the coextruded sheet, foaming can be achieved by heating the crosslinked multilayer sheet to a temperature higher than the decomposition temperature of the thermally decomposing blowing agent. Foaming can be carried out at approximately 200°C to 260°C or approximately 220°C to 240°C in a continuous process. A continuous foaming process may be preferable to a batch process for the production of continuous foam sheet. Foaming can typically be achieved by heating the cross-linked multi-layer sheet with molten salt, radiant heaters, a vertical or horizontal hot air oven, microwave energy, or a combination of these methods. Foaming can also be carried out in an impregnation process using, for example, nitrogen in an autoclave, followed by free foaming using molten salt, radiant heaters, a vertical or horizontal hot air oven, microwave energy, or a combination of these methods. Optionally, the cross-linked multi-layer sheet can be softened by preheating before foaming. This can help stabilize the expansion of the structure after foaming, particularly with thick, rigid sheets. The overall thickness of the multilayer foam sheet is measured according to JIS K6767. The thickness of the coating layer of the multilayer foam sheet is measured using a microscope. To measure the coating layer, a small sample is taken from the foam structure of the continuous foam sheet. The sample is cut with an extra-sharp blade, and the cross-section is observed along the cut with a microscope. A measurement can be performed with either a digital or a traditional microscope. A common commercial digital microscope may have various software features to facilitate thickness measurement. A traditional commercial microscope may have a lens with measuring scales to facilitate thickness measurement. The density of multi-layer foam sheets can be defined and measured using sectional or overall density, rather than core density, as measured by JIS K6767. Multi-layer foam sheets produced using the method described above can produce foams with sectional or overall densities of approximately 20 kg / m³ to 250 kg / m³, approximately 30 kg / m³ to 200 kg / m³, or approximately 50 kg / m³ to 150 kg / m³. The sectional density can be controlled by the amount of blowing agent and the thickness of the extruded structure. If the density of the multi-layer foam sheet is less than approximately 20 kg / m³, the sheet may not foam efficiently due to the large amount of chemical blowing agent required to achieve the desired density.Furthermore, if the sheet density is less than approximately 20 kg / m³, then the sheet expansion during the foaming step can become increasingly difficult to control. Additionally, if the density of the multi-layer foam sheet is less than approximately 20 kg / m³, then the foam can become increasingly prone to cell collapse. Consequently, it can be difficult to produce a multi-layer foam sheet with uniform cross-sectional density and thickness at a density lower than approximately 20 kg / m³. Multi-layer foam sheets are not limited to a section density of approximately 250 kg / m³. Foam with section densities of approximately 350 kg / m³, approximately 450 kg / m³, or approximately 550 kg / m³ can also be produced. However, a foam sheet with a density of less than approximately 250 kg / m³ may be preferable, as higher densities can generally be prohibitively expensive compared to other materials that can be used in a given application. Foam layers produced using the method described above may have closed cells. Preferably, at least 90% of the cells have undamaged cell walls, preferably at least 95%, and more preferably 98%. The average cell size may be from approximately 0.05 mm to approximately 1.0 mm, and preferably from approximately 0.1 mm to approximately 0.7 mm. If the average cell size is less than approximately 0.05 mm, then the density of the foam structure may typically be greater than 250 kg / m³. If the average cell size is larger than 1 mm, the foam may have an uneven surface. There is also a possibility that the foam structure may tear undesirably if the cell population in the foam does not have the preferred average cell size. This can occur when the foam structure is stretched or portions of it undergo secondary processing.The cell size in foam layers can have 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 the lining near the surfaces of the foam structure that are relatively flat, thin and / or oblong. The overall thickness of the multi-layer polyolefin foam / polyolefin coating sheet can range from approximately 0.2 mm to approximately 50 mm, from approximately 0.4 mm to approximately 40 mm, from approximately 0.6 mm to approximately 30 mm, or from approximately 0.8 mm to approximately 20 mm. If the thickness is less than approximately 0.2 mm, foaming may not be efficient due to significant gas loss from the primary surfaces. If the thickness is greater than approximately 50 mm, expansion during the foaming step can become increasingly difficult to control. Consequently, it can become increasingly difficult to produce a multi-layer polyolefin foam / polyolefin coating sheet with uniform cross-sectional density and thickness. In some embodiments, a coating layer of the coextruded foamed multi-layer structure can have a thickness of approximately 0.1 micrometer to 100 micrometers, approximately 1 micrometer to 80 micrometers, or approximately 5 micrometers to 60 micrometers. In some embodiments, a foam layer of the coextruded foamed multilayer structure may have a thickness of approximately 0.5 mm to 5 mm, approximately 1 mm to 4 mm, or approximately 2 mm to 3 mm. In some forms, the desired thickness can be achieved by a secondary process such as slicing, trimming, or joining. Slicing, trimming, or joining can produce a thickness range from approximately 0.1 mm to approximately 100 mm. The thickness of the coating layer can be reduced when foam forms on the multi-layer sheet. This is because the foamable layers expand, stretching the coating layers. Therefore, for example, if the multi-layer sheet expands to twice its original area, the coating thickness can be expected to be reduced by approximately half. If the multi-layer sheet expands to four times its original area, the coating thickness can be expected to be reduced to approximately one-quarter of its original thickness. The disclosed multilayer foam structures can be used in a variety of applications. One such application is thermoforming. To thermoform the multilayer foam structure, the foam can be heated to the melting point of the polyolefin blend for all layers of the multilayer foam structure. If any layer contains immiscible polymers, the multilayer foam structure may exhibit more than one melting point. In this case, the multilayer foam structure can generally be thermoformed when the foam is heated to a temperature midway between the lowest and highest melting points of the multilayer foam composition. Furthermore, the multilayer foam structure can be thermoformed onto a substrate such as a composite product made of rigid polypropylene, ABS, or wood fiber.The substrate itself can also be thermoformed at the same time as the multi-layer foam structure. Preferably, the substrate can be applied to a foam layer of the multi-layer foam structure. An example of a thermoformed item is an automotive air duct. A closed-cell foam structure can be particularly suitable for this application due to its lower weight (compared to solid plastic), its insulating properties that help maintain the temperature of the air flowing through the duct, and its resistance to vibration (compared to solid plastic). A cross-linked coating on the exterior of the multi-layered air duct can protect it from punctures and cuts during installation and throughout the vehicle's lifespan. Therefore, a rigid, multi-layered foam structure can be suitable for an automotive air duct. In some embodiments, multi-layer foam structures are laminates containing the multi-layer foam and a laminate layer. Preferably, the laminate layer can be applied to one side (i.e., surface) of a coating layer of the multi-layer foam. In these laminates, the multi-layer foam structure can be combined, for example, with a film and / or sheet. Examples of suitable materials 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 woven materials; and leather and / or fiber layers such as non-woven fabrics. These layers can be manufactured using standard techniques that are well known to those skilled in the art.Importantly, the multi-layered foam of the disclosure can be laminated on one or both sides with these materials and can include multiple other layers. In these laminates, one layer can be bonded to an adjacent layer by chemical bonds, mechanical means, or combinations thereof. Adjacent laminated layers can also be held together by any other means, including the use of attractive forces between materials with opposite electromagnetic charges or attractive forces present between materials that have a MA / a / ZUZZ / Ul 1 predominantly hydrophobic character or a predominantly hydrophilic character. In some embodiments, a foam / coating / laminate structure can be used as a component in a vehicle instrument panel where the foam / coating / laminate structure is bonded to the foam side on a hard panel substrate and where an airbag is installed on the back side of the panel. In other applications, multi-layer foam structures or laminates can be used in automotive interiors, such as door panels, sill rollers, door inserts, door fillers, trunk liners, armrests, center consoles, seat cushions, seat backs, headrests, seat back panels, knee bolsters, or headliners. These multi-layer foam laminates or structures can also be used in furniture (e.g., commercial, office, and residential furniture) such as chair cushions, chair backs, sofa cushions, sofa trims, recliner chair cushions, recliner chair moldings, divan cushions, divan moldings, car seat cushions, or car seat moldings.These multi-layer foam laminates or structures can also be used in walls such as modular walls, movable walls, wall panels, modular panels, office system panels, room dividers, or portable partitions. Multi-layer foam structures or laminates can also be used in storage enclosures (e.g., commercial, office, and residential) that can be either mobile or stationary. Furthermore, multi-layer foam structures or laminates can also be used in covers such as chair cushion covers, chair back covers, armrest covers, sofa covers, couch cushion covers, recliner chair covers, divan cushion covers, daybed covers, car seat covers, bedroom car seat covers, wall coverings, and architectural coverings. To meet the requirements of any of the above applications, the structures described in this disclosure may be subjected to various secondary processes, including, but not limited to, embossing, corona or plasma treatment, surface roughening, surface smoothing, drilling or micro-drilling, splicing, cutting, trimming, layering, bonding, and hole punching. Examples Figure 1 provides a table of the raw materials used in the following examples. Specifically, Figure 1 provides a table of the various components and descriptions of those components used in the following examples. Figure 2 provides a table of the formulations for Examples 1 and 2, as well as the coextrusion, irradiation, and other properties of the multilayer structures in Examples 1 and 2. Figure 3 is an image of Example 1 at 30X magnification and 45° from the coating surface and 45° from the machine direction (MD). Figure 4 is an image of Example 2 at 30X magnification and 45° from the coating surface and 45° from the machine direction (MD). This application discloses several numerical ranges in the text and figures. The disclosed numerical ranges inherently support any interval or value within the disclosed numerical ranges, even though a precise range limitation is not explicitly stated in the specification because this invention can be implemented across all disclosed numerical ranges. The foregoing description is presented to enable a person skilled in the art to carry out and use the invention, and is provided in the context of a particular application and its requirements. Several modifications to the preferred embodiments will be readily apparent to such persons skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Accordingly, it is not proposed that this invention be limited to the embodiments shown, but rather that the broadest scope consistent with the principles and features described herein be agreed upon. Finally, all patent and publication descriptions referred to in this application are incorporated herein by reference.

Claims

1. A method for forming a multi-layer structure, comprising: coextrusion: a foam layer comprising: at least one of polypropylene and polyethylene; a chemical foaming agent; a crosslinking agent; and a film layer on one side of the foam layer, the film layer comprising: at least 90% by weight of at least one of polypropylene and polyethylene; and 0.1% by weight to 5% by weight of a crosslinking agent.

2. The method according to claim 1, wherein the foam layer comprises polypropylene with a melt flow index of 0.1 grams to 25 grams per 10 minutes at 230 °C.

3. The method according to any of claims 1 to 2, wherein the foam layer comprises polyethylene with a melt flow index of 0.1 grams to 25 grams per 10 minutes at 190°C.

4. The method according to any of claims 1 to 3, wherein the foam layer comprises 0.5% by weight to 5% by weight of crosslinking agent.

5. The method according to any of claims 1 to 4, wherein the foaming chemical agent comprises azodicarbonamide.

6. The method according to any of claims 1 to 5, wherein the foam layer comprises polypropylene and polyethylene.

7. The method according to any of claims 1 to 6, wherein the foam layer comprises at least 75% by weight of at least one of polypropylene and polyethylene.

8. The method according to any of claims 1 to 7, wherein the foam layer comprises 3% by weight to 15% by weight of the chemical foaming agent.

9. A method for forming a multilayer foam structure comprising: coextrusion; a foam layer comprising: at least one of polypropylene and polyethylene; a chemical foaming agent; a crosslinking agent; and a film layer on one side of the foam layer, the film layer comprising: at least 90% by weight of at least one of polypropylene and polyethylene; and 0.1% by weight to 5% by weight of a crosslinking agent; irradiating the coextruded layers with ionizing radiation; and foaming the irradiated coextruded layers.

10. The method according to claim 9, wherein the ionizing radiation is selected from the group consisting of alpha, beta (electrons), X-rays, gamma, or neutrons.

11. The method according to any of claims 9 to 10, wherein the coextruded structure is irradiated up to four times separately.

12. The method according to claim 10, wherein the ionizing radiation is an electron beam with an accelerating voltage of 200 kV to 1500 kV.

13. The method according to claim 12, wherein an absorbed electron dosage is 10 kGy to 500 kGy.

14. The method according to any of claims 9 to 13, wherein the ionizing radiation cross-links the extruded structure to a degree of cross-linking of 20% to 75%.

15. The method according to any of claims 9 to 14, wherein foam formation consists of heating the irradiated structure with molten salt.

16. The method according to any of claims 9 to 15, wherein the multi-layer foam structure has a density of 20 kg / m3 to 250 kg / m3.

17. The method according to any of claims 9 to 16, wherein the multi-layer foam structure has a thickness of 0.2 mm to 50 mm.

18. The method according to any of claims 9 to 17, wherein the foam layer comprises polypropylene and polyethylene.

19. The method according to any of claims 9 to 18, wherein the foam layer comprises at least 75% by weight of at least one of polypropylene and polyethylene.

20. The method according to any of claims 9 to 19, wherein the foam layer comprises 3% by weight to 15% by weight of the chemical foaming agent.