Method for producing a coextruded crosslinked polyolefin foam having 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 and benefit to U.S. Patent Application No. 16 / 836,229, filed on March 31, 2020, and U.S. Patent Application No. 16 / 836,389, filed on March 31, 2020, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a multi - layer polyolefin foam / polyamide cap structure and a method for manufacturing the same. More specifically, it relates to a method for manufacturing a co - extruded cross - linked polyolefin multi - layer foam / polyamide cap structure.

Background Art

[0003] Cross - linked polyolefin foams can be used in various commercial applications, such as, but not limited to, decorative parts for vehicle interiors such as door panels. To prepare a polyolefin foam for use in a vehicle door panel, the foam layer is usually first laminated to a film, cloth, or foil to create a double - laminate. Next, this flexible double - laminate needs to be combined with a rigid substrate to form a panel. In the automotive industry, various manufacturing methods are employed to combine a flexible double - laminate with a panel. These methods include thermoforming techniques such as negative vacuum forming (NVF) and positive vacuum forming (PVF), compression molding, low - pressure molding (LPM), etc. In low - pressure molding, the double - laminate is placed in a mold. In this mold, the film, cloth, or foil faces the "A" side of the mold. The mold is closed, and polypropylene is injected onto the "B" side of the cavity to fill the mold and form a panel. In a commercial production process, as the polypropylene, a very high melt - flow (50 - 125 grams per 10 minutes at 230 °C) impact - modified homopolymer or random copolymer is generally injected at about 200 °C. There are problems in the design and implementation of LPM. Since the injection temperature can be significantly higher than the melting temperature of the foam, the high-temperature polypropylene at the injection location and its surrounding areas may shear or degrade the foam. In certain situations, visual defects such as "orange peel" may be observed on the "A" surface at the injection location and its surrounding areas. In another situation, the foam around the injection location may be completely sheared, leaving a visual indentation on the film, cloth, or foil at the injection site.

[0004] To mitigate these problems, manufacturers have introduced various techniques. One of them is the technique of injecting polypropylene at the discarded part of the double laminate. This technique is generally effective in solving the problems of shear and degradation of the foam within the mold cavity, but it increases the cost of manufacturing the panel. When injecting at the discarded part, it is necessary to make the discarded part longer along at least one side of the mold. Subsequently, the discarded part along the injection side will also include the injected polypropylene. The cost of adding a discarded part that includes both the double laminate and the injected polypropylene can be substantial. Also, since this discarded part cannot be easily recycled, there are additional costs due to the resulting additional waste. In addition, as another technique for suppressing defects such as shear and degradation of bubbles in LPM, there is also the technique of using a flexible triple laminate. The triple laminate can also be an LPM double laminate with a thermoplastic polyolefin (TPO) or thermoplastic elastomer (TPE) layer based on a flexible homopolymer laminated on the "B" surface of the double laminate. The TPO or TPE layer functions as a protective film layer and / or sacrificial film layer between the foam and the injected polypropylene. However, using a triple laminate in LPM also causes problems. The thickness of the TPO or TPE layer becomes large relative to the thickness of the entire double laminate, and the material cost may increase. To create a triple laminate, a second lamination process is required, further increasing the cost of the triple laminate. Finally, the protective TPO or TPE layer is also easily sheared and deteriorated at the injection site. Manufacturers of vehicle door panels using LPM technology generally continue to inject polypropylene at the cut-off portion of the triple laminate. The amount of cut-off sites required for this configuration is less than that using a double laminate, but still more cut-off sites are required than when injecting directly into the mold cavity. The wasted cost (including both the additional cut-off site triple laminate and the injected polypropylene) is significant. Since it is difficult to recycle the cut-off sites, this manufacturing technology incurs additional costs.

SUMMARY OF THE INVENTION

[0005] It has been found that a physically crosslinked closed-cell polyolefin foam having at least one polyamide cap layer can be produced in a continuous process. The multi-layer structure can be laminated to a film, cloth, or foil to create a double laminate. Then, using the double laminate in LPM applications can overcome the problems associated with conventional LPM double laminates and triple laminates used to manufacture vehicle interior decorative parts.

[0006] In some embodiments, the polyamide layer can act as a more effective protective layer for the injection polypropylene (compared to the TPO or TPE layer). Although the melting points of common commercially available polyamides vary widely, most of their melting points are higher than that of the homopolymer polypropylene. Therefore, it is possible to select a polyamide whose melting point not only exceeds the melting point of the polypropylene to be injected but also exceeds the injection temperature. When the melting point of the polyamide is high, the polyamide provides a barrier that remains intact and does not melt or shear even at the injection temperature when in contact with the injected polypropylene. Further, injection into the scrap area becomes unnecessary, and the cost of scrap site disposal can be further reduced. Since the polyamide cap layer is a non-sacrificial layer, it can be made substantially thinner than the TPO or TPE layer when appropriately selected for performing the LPM method, thereby further reducing the material cost. Polypropylene grafted with maleic anhydride can be a suitable compatibilizer between the polyolefin and the polyamide. Many maleic anhydride graft impact-modified polypropylene homopolymers and maleic anhydride graft (grafter) polypropylene random copolymers are commercially available. This makes it possible to directly replace the injection impact-modified homopolymer or random copolymer with minimal adjustment in the conventional LPM manufacturing method.

[0007] In some embodiments, a method of forming a multilayer foam structure is provided, the method comprising the following steps: co-extruding a first layer comprising polypropylene, polyethylene, or a combination of polypropylene and polyethylene, and a chemical blowing agent, and a second layer on the side of the first layer, the second layer comprising a combination of polyamide, polypropylene, and polyethylene, or a combination of polypropylene and polyethylene; irradiating the co-extruded layers with ionizing radiation; and foaming the irradiated co-extruded layers. In some embodiments, the first layer may comprise at least 70% by mass of polypropylene, polyethylene, or a combination of polypropylene and polyethylene. In some embodiments, the second layer may comprise at least 40% by mass of polyamide. In some embodiments, the second layer may comprise up to 50% by mass of polypropylene, polyethylene, or a combination of polypropylene and polyethylene; and at least 40% by mass of polyamide. In some embodiments, the second layer may be less than 1 mm. In some embodiments, the first layer may comprise a crosslinking accelerator in an amount of 0.5 to 5% by mass. In some embodiments, the first layer may comprise an additive in an amount of 1 to 20% by mass. In some embodiments, the second layer may comprise an additive in an amount of 1 to 10% by mass. In some embodiments, the melt flow index of the polypropylene may be from 0.1 to 25 grams per 10 minutes at 230°C. In some embodiments, the melt flow index of the polyethylene may be from 0.1 to 25 grams per 10 minutes at 190°C. In some embodiments, the amount of the chemical blowing agent in the first layer may be 3 to 15% by mass. In some embodiments, the chemical blowing agent may comprise azodicarbonamide. In some embodiments, the ionizing radiation may be selected from the group consisting of alpha rays, beta (electron) rays, X-rays, gamma rays, and neutron rays. In some embodiments, the co-extruded structure may be irradiated separately up to 4 times. In some embodiments, the ionizing radiation may be an electron beam having an accelerating voltage of 200 to 1500 kV. In some embodiments, the absorbed electron beam dose may be from 10 to 500 kGy. In some embodiments, the ionizing radiation may crosslink the extruded structure to a crosslinking degree of 20 to 75%.In some embodiments, the foaming process may include heating the irradiated structure with molten salt and a radiant heater or a hot air oven. In some embodiments, the density of the multilayer foamed structure is 20 to 250 kg / m³. 3 It may be. In some embodiments, the average independent bubble diameter of the multilayer foamed structure may be 0.05 to 1.0 mm. In some embodiments, the thickness of the multilayer foamed structure may be 0.2 to 50 mm.

[0008] As used herein, the singular forms "a", "an", and "the" are intended to include the plural forms as well, unless the context clearly dictates otherwise. Also, the term "and / or" as used herein refers to any and all possible combinations of one or more of the associated listed items, and it is understood to be inclusive. Further, as used herein, the terms "include", "including", "comprise", and / or "comprising" identify the presence of the described features, integers, steps, operations, elements, components, and / or units, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, units, and / or groups thereof. It should be understood that the aspects and embodiments described herein include those that "consist of" and / or "consist essentially of" such aspects and embodiments. For all of the methods, systems, compositions, and apparatuses described herein, the methods, systems, compositions, and apparatuses can include the recited components or steps, or "consist of" or "consist essentially of" the recited components or steps. When a system, composition, or apparatus is described as "consisting essentially of" the recited components, the system, composition, or apparatus includes the recited components and may include other components that do not substantially affect the performance of the system, composition, or apparatus, but does not include any other components that substantially affect the performance of the system, composition, or apparatus other than those explicitly recited; or, alternatively, does not include any additional components in a concentration or amount sufficient to substantially affect the performance of the system, composition, or apparatus. When a method is described as "consisting essentially of" the recited steps, the method includes the recited steps and may include other steps that do not substantially affect the result of the method, but the method does not include any other steps that substantially affect the result of the method other than those explicitly recited.

[0009] In the present disclosure, "substantially free of" a particular component, particular composition, particular compound, or particular starting material in various embodiments means that the particular component, particular composition, particular compound, or particular starting material is present in less than about 5 wt%, less than about 2 wt%, less than about 1 wt%, less than about 0.5 wt%, less than about 0.1 wt%, less than about 0.05 wt%, less than about 0.025 wt%, or less than about 0.01 wt%. Preferably, "substantially free of" a particular component, particular composition, particular compound, or particular starting material means that the particular component, particular composition, particular compound, or particular starting material is present in less than about 1 wt%. Other advantages will be readily apparent to those skilled in the art from the following detailed description. The examples and descriptions herein are to be considered illustrative in nature and not restrictive. With reference to the accompanying drawings, various embodiments are described by way of example only.

Brief Description of the Drawings

[0010]

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Mode for Carrying Out the Invention

[0011] A method for manufacturing a crosslinked closed-cell coextruded multilayer foam structure including at least a foam layer containing polypropylene, polyethylene, or a combination thereof, and at least a cap layer containing polyamide will be described. The method for manufacturing the crosslinked closed-cell coextruded multilayer foam structure layer may include steps of (a) coextrusion, (b) irradiation, and (c) foaming. Coextrusion is simultaneously extruding a plurality of layers of materials. This type of extrusion uses two or more extruders to send a material with a stable volumetric throughput to an extrusion head (die) capable of extruding the material into a desired shape. In the co-extrusion process, it is possible to supply the foam composition to a plurality of extruders to form an unfoamed multi-layer structure. For example, it is possible to supply the "A" foam composition to one extruder and the "B" foam composition to the second extruder. The method of supplying the raw materials to the extruder can be based on the design of the extruder and the available material processing equipment. In order to facilitate dispersion, if necessary, the raw materials of the foam composition may be premixed. For such premixing, a Henschel mixer can be used. All the raw materials can be premixed and supplied from one port of the extruder. Also, each raw material can be supplied individually from a specified port. For example, if the crosslinking accelerator or any other additive is liquid, the accelerator and / or additive can be added from the supply gate (or gates) of the extruder or the flow port of the extruder (if it has a flow hole) instead of premixing with the solid raw materials. It is also possible to combine premixing and port supply of individual raw materials.

[0012] Each extruder can send a constant amount of each composition to one or more manifolds and then to a sheet extrusion die to create an unfoamed co-extruded multi-layer sheet. Two common methods for co-extruding materials are: (1) a feed block manifold; and (2) a multi-manifold within the die. The feed block manifold includes the following elements: (a) a plurality of inlet ports for the upper, middle, and lower layers; (b) a streamlined melt lamination region that feeds separate streams into one laminated melt stream within the feed block; (c) an adapter plate between the feed block and the sheet extrusion die; and / or (d) a sheet extrusion die (similar to a single-layer die). In the sheet extrusion die, the laminated melt stream enters the center of the die, spreads along the manifold, and flows out of the die outlet as a clearly multi-layered extrudate. The multi-manifold die includes the following elements: (a) similar to a single-layer die except that there are a plurality of supply channels; (b) each melt channel has its own choke bar for flow control; and / or (c) the melt stream converges inside the die near the outlet and appears as a clearly multi-layered extrudate. The thickness of the layer can be determined by the design of the manifold(s) and the die. For example, an 80 / 20 supply block manifold can deliver the composition at a ratio of approximately 4:1 when the speeds and dimensions of each extruder are appropriately matched. This ratio can be changed, for example, by: (a) the relative extrusion speed between one extruder and another; (b) the relative dimensions of each extruder; and / or (c) changing the composition (i.e., viscosity) of the individual layers. The thickness of the entire multilayer sheet can be controlled by the overall die gap. However, the thickness of the entire multilayer sheet can be further adjusted, for example, by stretching (i.e., "drawing") the melt multilayer extrudate and / or flattening the melt 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 a plurality of other layers. In some embodiments, the multilayer structure can include additional layers such as a bonding layer, a film layer, and / or an additional foam layer. The cap composition supplied to the extruder can include at least one polyamide and polypropylene, polyethylene, or a combination thereof. The foam composition supplied to the extruder can include polypropylene, polyethylene, or a combination thereof. A polyamide is a polymer containing an amide group (—CONH—) as a repeating unit of the chain. The polyamide includes, but is not limited to, aliphatic polyamides produced by either the condensation reaction of two bifunctional monomers or the ring-opening addition polymerization of a cyclic compound. The polyamide can be a homopolymer, copolymer, terpolymer, or mixture. Importantly, semi-crystalline polyamides or polyamide mixtures are preferred over amorphous polyamides or polyamide mixtures. 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, types 6 / 66, 6 / 69, 610 / 66, and 56 / 12. Commercially available aliphatic polyamide terpolymers include, but are not limited to, type 6 / 66 / 12.

[0014] Examples of polypropylene 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 (with controlled block arrangement), polypropylene-based polyolefin plastomer, polypropylene-based polyolefin elast-plastomer, polypropylene-based polyolefin elastomer, polypropylene-based thermoplastic polyolefin, and polypropylene-based thermoplastic elastomer mixture. Polypropylene can also be of the high melt strength type. Furthermore, polypropylene may be grafted with maleic anhydride. Examples of the polyethylene 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 arrangement). Any of these may include a grafted compatibilizer or a copolymer having an acetic acid group and / or an ester group. These polyethylenes may be grafted with maleic anhydride. These polyethylenes may be copolymers and terpolymers having an acetic acid group and / or an ester group, or ionomers of copolymers and terpolymers having an acetic acid group and / or an ester group.

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

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

[0017] "Impact-modified polypropylene-ethylene copolymer" is polypropylene having random ethylene units and ethylene-propylene (EP) copolymer rubber. The rubber can be amorphous or semi-crystalline, but is not present in an amount sufficient to impart any plastomeric or elastoplastic properties to the material. A non-limiting example of a commercially available impact-modified polypropylene-ethylene copolymer is PRISMA® 6910 manufactured by Braskem. Examples of "metallocene polypropylene" include 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. Examples of metallocene polypropylene include, but are not limited to, commercially available products from Total Petrochemicals, grades M3551, M3282MZ, M7672, 1251, 1471, 1571, 1751. "Metallocene polypropylene-ethylene copolymer" is a metallocene syndiotactic, metallocene atactic, and metallocene isotactic polypropylene having random ethylene units. Examples of commercially available products include, but are not limited to, Lumicene® MR10MX0 and Lumicene® MR60MC2 manufactured by Total Petrochemicals, Purell® SM170G manufactured by LyondellBasell, and the Wintec® product line manufactured by Japan Polypropylene. "Metallocene polypropylene olefin block copolymer" is a polypropylene in which crystalline hard "blocks" and amorphous soft "blocks" are artificially arranged alternately, that is, a polypropylene in which the block arrangement is controlled. Examples of "metallocene polypropylene olefin block copolymer" include, but are not limited to, the INTUNE® product line manufactured by Dow Chemical.

[0018] "Polypropylene-based polyolefin plastomers" (POP) and "polypropylene-based polyolefin elastoplastomers" are both metallocene and non-metallocene propylene-based copolymers having plastomer and elastoplastomer 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 (butene-1 line of plastomer polymers, butene-1 homopolymer-based materials as a specific grade, and polypropylene-butene-1 copolymer-based materials as other grades). "Polypropylene-based polyolefin elastomers" (POE) are either metallocene propylene-based copolymers or non-metallocene propylene-based copolymers having elastomer properties. Non-limiting examples of propylene-based polyolefin elastomers include polymers commercially available under the trade names VERSIFY® (metallocene) from Dow Chemical and VISTAMAXX® (metallocene) from ExxonMobil. Examples of "polypropylene-based thermoplastic polyolefins" (TPO) include polypropylene, polypropylene-ethylene copolymers, metallocene homopolypropylene, and metallocene polypropylene-ethylene copolymers, which contain an amount of ethylene-propylene copolymer rubber sufficient to impart plastomer, elastoplastic, or elastomeric properties to the thermoplastic polyolefin mixture (TPO). Non-limiting examples of TPO polymers include polymers commercially available under the trade names THERMORlJN (registered trademark) and Zelas (registered trademark) manufactured by Mitsubishi Chemical Corporation, ADFLEX (registered trademark) and SOFTELL (registered trademark) manufactured by LyondellBasell, TELCAR (registered trademark) manufactured by Teknor Apex, and Wellnex (registered trademark) manufactured by Japan Polypropylene Corporation. TPO can be produced via multi-stage polymerization (e.g., Zelas (registered trademark), ADFLEX (registered trademark), SOFTELL (registered trademark), Wellnex (registered trademark)) or by mixing (e.g., THERMORlJN (registered trademark), TELCAR (registered trademark)).

[0019] Examples of "polypropylene-based thermoplastic elastomer mixtures" (TPE) include polypropylene, polypropylene-ethylene copolymers, metallocene homopolypropylene, and metallocene polypropylene-ethylene copolymers, which contain an amount of diblock or multiblock thermoplastic rubber modifier (SEBS, SEPS, SEEPS, SEP, SERC, CEBC, HSB, etc.) sufficient to impart plastomer, elastoplastic, or elastomeric properties to the thermoplastic elastomer mixture (TPE). Non-limiting examples of polypropylene-based thermoplastic elastomer blend polymers include polymer mixtures commercially available under the trade names GLS (trademark) DYNAFLEX (registered trademark) and GLS (trademark) VERSAFLEX (registered trademark) manufactured by Polyone, MONPRENE (registered trademark) manufactured by Teknor Apex, and DURAGRIP (registered trademark) manufactured by LyondellBasell. Also, any of the above polypropylenes may be of the high melt strength (HMS) type. Polypropylene manufacturers employ various methods to strengthen the polymer in the melt phase. For example, polypropylene exhibiting long chain branching (LCB) can be identified as high melt strength polypropylene. Non-limiting examples of high melt strength polypropylenes include polymers commercially available under the trade names DAPLOY® by Borealis, AMPPLEO® by Braskem, and WAYMAX® by Japan Polypropylene Corporation. Any polypropylene, and more generally a mixture of TPO and TPE, may optionally be oil-extended, for example, with mineral oil, PARALUX® process oil by Chevron, etc., to further soften the mixture, enhance the tactile properties of the mixture, or improve the processability of the mixture. "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 α-olefins of butene and / or hexene and / or octene.

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

[0021] These polyethylenes may also be copolymers and terpolymers having acetate groups 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® manufactured by Dow; EVATANE®, LOTADER®, and LOTRYL® manufactured by Arkema; and ESCORENE®, ESCOR®, and OPTEMA® manufactured by ExxonMobil. The polypropylene and polyethylene described above can be functionalized. The functionalized polypropylene and polyethylene contain grafted monomers. Typically, the monomers are grafted onto polypropylene or polyethylene by a free radical reaction. Monomers suitable 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-methylpropanesulfonic 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 polypropylene includes that functionalized with maleic anhydride. Non-limiting examples include Admer® QF and QB series manufactured by Mitsui Chemicals, PLEXAR® 6000 series manufactured by LyondellBasell, DuPont™ BYNEL® 5000 series manufactured by Dow, and OREVAC® PP series manufactured by Arkema. The most commonly commercially available functionalized polyethylene is also the one functionalized with maleic anhydride. Non-limiting examples include Admer® NF and SE series manufactured by Mitsui Chemicals, PLEXAR® 1000, 2000 and 3000 series manufactured by LyondellBasell, DuPont™ BYNEL® 2100, 3000, 3800, 3900, 4000 series manufactured by Dow, and several of the OREVAC® PE, T, and LOTADER® series manufactured by Arkema. The most common method in various industries for compatibilizing polyamide with polypropylene or polyethylene is to use maleic anhydride grafted polypropylene or polyethylene. For example, in flexible food packaging methods, the adhesion of a polyamide film to polypropylene can be enabled by inserting a bonding layer of maleic anhydride grafted polypropylene between the films. It should be noted that polyethylene functionalized with other graft monomers is also commercially available. Non-limiting examples include DuPont™ BYNEL® 1100, 2200, and 3100 series manufactured by Dow, and the LOTADER® AX series manufactured by Arkema. It should also be noted that polymers other than polypropylene and polyethylene functionalized with maleic anhydride are also commercially available. For example, the ROYALTUF® series manufactured by Addivant are a series of ethylene-propylene-diene rubbers (EPDM) functionalized with maleic anhydride. In another example, the KRATON® FG series manufactured by Kraton are a series of SEBS polymers functionalized with maleic anhydride.

[0023] The cap layer can comprise at least one extrusion grade or general purpose grade of polyamide. The extrusion and general purpose polyamides are characterized by being from approximately high viscosity to approximately medium viscosity. The high to medium viscosity polyamides tend to match the melt fluidity of the foaming layer, and as a result, the thickness uniformity in each coextruded layer from the center to the edge of the die is higher. Most types of polyamide are hygroscopic, and the moisture in the polyamide can affect the melt flow and the resistance to flow at a given shear rate. Since moisture affects the flow characteristics, alternative specifications for melt flow rate and melt volume rate are usually applied to polyamides. ISO 307 and ASTM D789 are the two above-mentioned standards adopted to quantify the viscosity of polyamides. In ISO 307, the polyamide can be dissolved in a solution diluted with a specific solvent, and the viscosity number value can be measured. In ASTM D789, the polyamide can be dissolved in a solution concentrated with a specific solvent, and the relative viscosity can be measured. The corresponding standards ISO 16396-1 and ASTM D6779 provide guidance for commercial polyamide manufacturers to specify products in a standard system. These nomenclatures are useful for polyamide manufacturers to specify grades as appropriate for extrusion (cast film, sheet, etc.), injection molding, blow molding, etc. It is important to note that viscosity numbers and relative viscosity numbers are not published by most polyamide manufacturers. Rather, polyamide resins are usually commercially available under general viscosity categories (ultra-low, low, medium (or standard), medium-high, high, etc.) for recommended processing applications (general extrusion, injection, compounding, monofilament, melt spinning, industrial yarn, etc.).

[0024] The composition of any foaming layer and / or any cap layer 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. The composition of any foaming layer and / or any cap layer 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 melt flow index of the polypropylene(s) and / or polyethylene(s) is preferably 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 in accordance with ASTM D1238 using a 2.16 kg plunger over 10 minutes at 230°C for polypropylene and polypropylene-based materials and at 190°C for polyethylene and polyethylene-based materials. In the case of resins with a relatively high melt flow, the test time may be shortened. MFI is a measure of the fluidity of a polymer and an indicator of the molecular weight and processability of a polymer material. A high MFI value means a low viscosity. If the MFI value is too high, extrusion according to the present disclosure cannot be performed satisfactorily. Problems associated with an MFI value that is too high include low pressure during extrusion, problems with setting the thickness profile, non-uniform cooling profiles due to low melt viscosity, insufficient melt strength, and / or mechanical problems. Conversely, a low MFI value means a high viscosity. If the MFI value is too low, high pressure during melt processing, problems with sheet quality and profile, and high extrusion temperatures associated with the risk of decomposition and activation of the foaming agent occur.

[0025] Also, since the above MFI range can reflect the viscosity of materials that affect foaming, it is important for the foaming process. Without being bound by theory, there are several reasons why a specific MFI value is more effective. Materials with a low MFI have a long molecular chain length, and when stress is applied, the energy required for chain flow increases, which may improve some physical properties. Also, the longer the molecular chain (MW), the more the chains crystallize and the more crystals are formed, and the strength increases due to intermolecular bonding. However, if the MFI is too low, the viscosity becomes too high. On the other hand, the chains of polymers with a high MFI value are short. Therefore, in a material with a high MFI value for a given volume, there are more chain ends at the microscopic level compared to polymers with a low MFI value, and they can rotate. The free volume is formed by the space required for such rotation (for example, rotation occurring at a temperature exceeding the Tg or glass transition temperature of the polymer). This can increase the free volume and facilitate flow under stress. In addition to the polymer, the composition supplied to the extruder may contain additives suitable for the production of the disclosed multilayer structure. Common additives include, but are not limited to, organic peroxides, antioxidants, lubricants, processing aids, heat stabilizers, colorants, flame retardants, antistatic agents, nucleating agents, plasticizers, antibacterial agents, fungicides, light stabilizers, ultraviolet (UV) absorbers, antiblocking agents, fillers, deodorants, odor adsorbents, antifogging agents, volatile organic compound (VOC) adsorbents, semi-volatile organic compound (SVOC) adsorbents, thickeners, bubble size stabilizers, metal deactivators, and combinations thereof.

[0026] In some embodiments, the amount of additives (if any) other than the chemical foaming 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 additives (if any) other than the chemical foaming 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 additives (if any) other than the chemical foaming 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 additives (if any) other than the chemical foaming agent(s) and crosslinking accelerator(s) in the foam layer composition can be about 1 - 20% by mass, about 2 - 15% by mass, about 3 - 10% by mass, about 4 - 8% by mass, or about 5 - 7% by mass of the foam layer composition. In some embodiments, the amount of additives (if any) 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 additives (if any) 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 additives (if any) 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 additives (if any) in the cap layer composition can be about 0.5 - 20% by mass, about 1 - 10% by mass, or about 2 - 6% by mass of the cap layer composition.

[0027] Regardless of how the raw materials are fed into the extruder, the shear force 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 force and mixing through the extruder barrel and are capable of extruding a layer with uniform properties. Specific energy is an indicator that shows how much work is required to extrude the raw materials of the layer and how intensively the extrusion process is carried out. Specific energy is defined as the energy applied to the material processed by the extruder and is normalized per kilogram. Specific energy quantifies the energy applied per kilogram of the total material supplied per hour in kilowatts (kW). Specific energy is calculated according to the following formula: Specific energy = KW (applied) / Feed rate (kg / hour) In the above formula, KW (applied) = (KW (motor rating) × (torque % from the allowable maximum value expressed as a decimal) × revolutions per minute (RPM) (actual operating RPM) × 0.97 (gearbox efficiency) / maximum RPM (possible output of the extruder)

[0028] Specific energy can be used to quantify the amount of shear and mixing of the raw materials within the extruder. The extruder used to form the multilayer 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, 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 to 10 °C lower than the thermal decomposition start temperature of the chemical blowing agent, preferably 10 °C or more lower. If the extrusion temperature exceeds the thermal decomposition temperature of the blowing agent, the blowing agent decomposes and undesirable "pre-foaming" occurs. The extrusion temperature of any cap layer can be 0 to 10 °C lower than the thermal decomposition start temperature of the chemical blowing agent in any foamable layer adjacent to the cap layer, preferably 10 °C or more lower. 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 may decompose and undesirable "pre-foaming" may also occur. The foam layer composition can contain various different chemical blowing agents. Examples of chemical blowing agents include, but are not limited to, azo compounds, hydrazine compounds, carbohydrazide, tetrazole, nitroso compounds, and carbonates. Also, the chemical blowing agent may be used alone or in any combination of two or more. In some embodiments, azodicarbonamide (ADCA) is one type of chemical blowing agent that can be used. An example of an ADCA chemical blowing agent is UNIFOAM® TC-18I manufactured by P.T. LAUTAN OTSUKA CHEMICAL. ADCA usually undergoes thermal decomposition at a temperature of about 200 to 240°C. In order to prevent ADCA from thermally decomposing in the extruder, it is possible to maintain the extrusion temperature at 200°C or lower.

[0029] The amount of the 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 the 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 the chemical blowing agent in the foam layer composition can be about 1 to 30 PPR%, about 2 to 20 PPR%, about 3 to 15 PPR%, about 4 to 10 PPR%, or about 5 to 8 PPR% of the composition. In some embodiments, the amount of the chemical blowing agent in the foam layer composition can be about 1 to 30% by mass, about 2 to 20% by mass, 3 to 15% by mass, about 4 to 10% by mass, or about 5 to 7% by mass of the foam layer composition. The amount of the chemical blowing agent can depend on the thickness of the unfoamed sheet, the desired foam thickness, the desired foam density, the material to be 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 amount of the chemical blowing agent described above can be specialized only for ADCA. Other blowing agents produce various volumes of gas per mass of CFA, and accordingly can be considered. For example, when comparing ADCA with the chemical blowing agent p-toluenesulfonyl semicarbazide (TSS), when the foaming layer contains 40 PPR% of ADCA, about 63 PPR% of TSS is 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 catalyst for decomposing the blowing agent may be used. Examples of the catalyst include, but are not limited to, zinc oxide, magnesium oxide, calcium stearate, glycerin, and urea. The lower limit temperature of extrusion can be made the same as the melting point of the polymer with the highest melting point. When the extrusion temperature is lower than the melting point of the polymer with the highest melting point, undesirable "unmelted" appears. During foaming, the extruded layer extruded at a temperature lower than this lower limit temperature may exhibit non-uniform thickness, non-uniform cell structure, pockets of cell collapse, and other undesirable characteristics.

[0030] Regardless of whether the blowing agent is a physical blowing agent, a chemical blowing agent, or a combination of both, typical extrusion foaming forms a polymer sheet, and both main surfaces of this sheet become significantly rougher than equivalent structures produced by the disclosed method. Since the surface profile of a multi-layer (single-layer) foam sheet may have significant meaning in many applications, extrusion foam sheets may not be used in these applications. These applications may require a smooth foam surface to obtain desired properties such as ease of lamination to films, fabrics, fiber layers, and leather; the ratio of the contact area in lamination; visual aesthetics; etc. PCT Publication No. WO2016109544, which is incorporated herein by reference in its entirety, describes an example showing the difference in surface roughness between an extrusion foam polymer sheet and an equivalent foam polymer sheet produced by the disclosed method. The surface of an extruded foam article may generally be rougher with larger cell dimensions (compared to the foam produced according to the present disclosure). Cell dimensions and cell dimension distribution may not be of great significance in most commercial applications, but since surface roughness is a function of cell size, in applications that require a smooth foam surface, a foam with larger cells may not be as desirable as a foam with smaller cells. The thickness of the unfoamed coextruded multilayer structure can be from about 0.1 to about 30 mm, from about 0.2 to about 25 mm, from about 0.3 to about 20 mm, or from 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 from about 0.01 to 1.0 mm or from 0.02 to 0.7 mm. In some embodiments, the thickness of the unfoamed cap is not limited and can be as thin as desired relative to the entire unfoamed coextruded multilayer sheet, as thin as about 0.1 μm, or can be a typical thickness of a very thin bonding layer used in multilayer flexible packaging materials and barrier films.

[0031] In some embodiments, the thickness of the foam layer of the unfoamed coextruded multilayer structure can be from about 0.1 to 5 mm, from about 0.5 to 4 mm, from about 1 to 3 mm, or from 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 the unfoamed coextruded multilayer structure can be measured using a stem-type thickness gauge mounted on a flat base. A hemispherical contact point with a radius of 1.6 mm can be provided at the tip of the gauge. Lift the stem and 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 the 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 out from the continuous sheet, and the cross-section of the sample can be sliced into thin sections by a microtome. The sections can be placed on an observation microscope. The measurement can be performed with a digital microscope or a conventional microscope. A general commercially available digital microscope can be equipped with various software functions to facilitate thickness measurement. A conventional commercially available microscope can be equipped with a lens with a measurement scale to facilitate thickness measurement. The cap can be thin and bendable when melted so as not to significantly impede the expansion of the foamable layer(s) during the foaming process. Among many physical properties that may prevent the foaming expansion of other layer(s), the thickness, flexibility, and melt strength of the cap are included. The thickness, flexibility, melt strength, and crosslinking ratio of the foamable layer(s), as well as the final thickness and density of the foam layer, are also factors related to whether the cap inhibits the expansion of the foamable layer(s). As a general guideline for the maximum thickness of the cap, the cap maximum thickness should be about 20% or less, about 15% or less, about 10% or less, or about 5% or less of the entire coextruded unfoamed sheet. When the thickness of the cap exceeds about 20% of the entire coextruded unfoamed sheet, problems such as the multilayer sheet bending, buckling, and warping may occur when the multilayer sheet is heated and foamed.

[0033] It is important to distinguish between "physical" crosslinking and "chemical" crosslinking. In chemical crosslinking, crosslinking promoters are used to form crosslinks without using ionizing radiation. Peroxides, silanes, or vinyl silanes are typically used for chemical crosslinking. In the peroxide crosslinking process, crosslinking usually occurs in an extrusion die. In the crosslinking process of silanes or vinyl silanes, crosslinking generally occurs in a secondary operation after extrusion, and in the secondary operation, it is possible to promote the crosslinking of the extruded material with heat and moisture. Regardless of the chemical crosslinking method, chemically crosslinked foam sheets typically exhibit a significantly rougher primary surface than equivalent structures produced by the disclosed methods. Since the surface profile of multi-layer (single-layer) foam sheets can have significant implications in many applications, chemically crosslinked foam sheets may not be used in these applications. These applications may require a smooth foam surface to obtain desired properties such as ease of lamination to films, fabrics, fiber layers, and leather; the ratio of the contact area in lamination; visual aesthetics; etc. WO2016109544 of the PCT publication describes an example showing the difference in surface roughness between a chemically crosslinked foam polymer sheet and an equivalent foam polymer sheet produced by the disclosed method. The surface of a chemically crosslinked foamed article may generally be rougher with larger bubble dimensions (compared to the foam produced in accordance with the present disclosure). Bubble size and bubble size distribution do not have much significance in most commercial applications, but since surface roughness is a function of bubble size, in applications that require a smooth foam surface, foams with larger bubbles may not be more desirable than foams with smaller bubbles.

[0034] Examples of ionizing radiation include, but are not limited to, alpha rays, beta (electron) rays, X-rays, gamma rays, and neutron rays. Among these, it is possible to use an electron beam having uniform energy for the preparation of the crosslinked polyolefin foam / crosslinked polyolefin cap structure. The exposure time, number of irradiations, and acceleration voltage during electron beam irradiation can be widely varied according to the target degree of crosslinking and the thickness of the multilayer structure. However, ionizing radiation 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 amount is too low, the bubble stability may not be maintained during foaming. If the exposure amount is too high, the moldability of the obtained multilayer foam structure may become insufficient. Moldability is a desirable property when using a multilayer foam sheet for thermoforming applications. In addition, since the unfoamed sheet may soften by generating heat during electron beam irradiation exposure, there is a possibility that the structure may be deformed if the exposure amount is too high. Also, the polymer component may deteriorate due to excessive polymer chain scission.

[0035] The coextruded unfoamed multilayer sheet may be irradiated up to 4 times, preferably up to 2 times, more preferably only 1 time. If the number of irradiations exceeds about 4 times, the polymer component deteriorates, and during foaming, for example, uniform bubbles are not formed in the obtained foam layer(s). When the thickness of the extruded structure exceeds about 4 mm, it may be preferable to irradiate ionizing radiation to each main surface of the multilayer profile to make the degree of crosslinking of the main surface(s) and the inner layer more uniform. Irradiation with an electron beam has the advantage that co-extruded sheets of various thicknesses can be effectively crosslinked by controlling the acceleration voltage of the electrons. The acceleration voltage can generally be in the range of about 200 to about 1500 kV, about 400 to about 1200 kV, or about 600 to about 1000 kV. If the acceleration voltage is less than about 200 kV, the radiation may not reach the inner part of the co-extruded sheet. As a result, the bubbles in the inner part may become coarse, and uneven foaming may occur. Also, an acceleration voltage that is too low for a given thickness profile may cause an arc discharge, and "pinholes" or "tunnels" may be formed in the foamed structure. On the other hand, if the acceleration voltage exceeds about 1500 kV, the polymer may deteriorate. In some embodiments, the radiation source may be directed towards the B layer of the co-extruded unfoamed multilayer sheet during irradiation. In some embodiments, the radiation source may be directed towards the A layer of the co-extruded 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 can be crosslinked by about 20% to about 75% or about 30% to about 60% according to the measurement by the "Toray gel fraction method". According to the "Toray gel fraction method", a tetralin solvent is used to dissolve the uncrosslinked polyolefin component in the composition. In principle, the uncrosslinked polyolefin material is dissolved in tetralin, and the degree of crosslinking is expressed as the mass ratio of the crosslinked product in the whole composition. The equipment used to determine the ratio of polymer crosslinking includes: 100 mesh (wire diameter: 1.143 millimeters (0.0045 inches)); type 304 stainless steel bag; numbered wire and clip; Miyamoto constant temperature oil bath device; chemical 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 high-molecular-weight solvent, acetone, and silicone oil. Specifically, an empty wire mesh bag is weighed and its mass is recorded. For each sample, a sample of 100 milligrams ± 5 milligrams is weighed and transferred to the wire mesh bag. The mass of the wire mesh bag and the sample, usually in the form of thinly sliced foam fragments, is recorded. Each bag is attached to the corresponding numbered wire and clip. When the temperature of the solvent reaches 130 °C, the bundle (bag and sample) is immersed in the solvent. The sample is shaken up and down about 5 or 6 times to release the bubbles and wet the sample sufficiently. The sample is attached to a stirrer and stirred for 3 hours, and the solvent dissolves the foam. Then, the sample is cooled in a fume hood. The sample is shaken up and down about 7 or 8 times in a container mainly containing acetone for washing. In the second acetone wash, the sample is washed twice. The washed sample is washed once more in a third container filled with fresh acetone in the same manner as above. Next, the sample is hung in a fume hood and acetone is evaporated over about 1 to about 5 minutes. Next, the sample is dried in a drying oven at 120 °C for about 1 hour. The sample is cooled for at least about 15 minutes. The wire mesh bag is weighed with a chemical balance and its mass is recorded. Next, the degree of crosslinking is calculated using the formula 100×(C - A) / (B - A).In the formula, A represents the mass of an empty wire mesh bag; B represents the mass of the wire bag + the foam sample before tetralin immersion; C represents the mass of the wire bag + the dissolved sample after tetralin immersion.

[0037] It is important to note that polyamide does not dissolve in tetralin. Therefore, the gel ratio calculated by the above method includes the crosslinked polyolefin component(s) + the polyamide component(s). Suitable crosslinking accelerators include, but are not limited to, commercially available bifunctional, trifunctional, tetrafunctional, pentafunctional, and higher-functional monomers. Such crosslinking monomers are available in liquid, solid, pellet, and powder forms. Examples include 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., triallyl trimellitate, triallyl pyromellitate, diallyl oxalate); 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 - phenylene bismaleimide; compounds having at least two triple bonds such as dipropazyl phthalate and dipropazyl maleate; and divinylbenzene, but are not limited thereto. Also, such crosslinking accelerators may be used alone or in any combination of two or more. In the present disclosure, divinylbenzene (DVB), a bifunctional liquid crosslinking monomer, can be used as the crosslinking accelerator of the present disclosure. An example of a suitable commercially available DVB is DVB HP manufactured by Dow.

[0038] The amount of the 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 the 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 the 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 the crosslinking accelerator in the foam layer composition can be about 0.5 - 5 mass%, or about 1 - 3 mass% of the foam layer composition. It should be noted that the amount of the crosslinking accelerator described above may be specific only to DVB. The crosslinking efficiency of other crosslinking accelerators can be higher or lower than that of DVB. Therefore, the required amount of another crosslinking accelerator should be considered as appropriate. The crosslinking efficiency of the crosslinking accelerator can vary depending on the amount of ionizing radiation, the polymer to be crosslinked, the chemical structure of the monomer, the number of functional groups on the monomer, and whether the monomer is liquid or powder. Crosslinking can be generated using various different techniques and can be formed between different polymer molecules within a molecule and between parts of a single polymer molecule within a molecule. Such techniques include, but are not limited to, techniques that provide a crosslinking accelerator separate from the polymer chain and techniques that provide a polymer chain incorporating a crosslinking accelerator containing functional groups capable of forming or activating to form crosslinks.

[0039] After irradiating the coextruded sheet, foaming may be performed by heating the crosslinked multilayer sheet to a temperature higher than the decomposition temperature of the thermally decomposable blowing agent. Foaming can be carried out in a continuous process at about 200 - 260 °C, or about 220 - 240 °C. It can be said that the continuous foaming method is more preferable than the batch method for manufacturing the continuous foam sheet. Foaming can usually be carried out by heating the crosslinked multilayer sheet using molten salt, a radiant heater, a vertical or horizontal hot air oven, microwave energy, or a combination of these methods. Foaming can also be carried out, for example, by an impregnation method using nitrogen in an autoclave, and then freely foamed by molten salt, a radiant heater, a vertical or horizontal hot air oven, microwave energy, or a combination of these methods. Optionally, before foaming, it is possible to soften the crosslinked multilayer sheet by preheating. This softening can help to stabilize the expansion of the structure during foaming, especially in the case of thick and hard sheets.

[0040] The thickness of the entire multilayer foam sheet can be measured according to JIS K6767. The thickness of the cap layer of the 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 continuous foam sheet. This sample can be cut with an ultra-sharp blade, and the cross-section of the sample can be observed with a microscope along the cut surface. The measurement can be carried out with a digital microscope or a conventional microscope. A general commercially available digital microscope can be equipped with various software functions that facilitate thickness measurement. A conventional commercially available microscope can be equipped with a lens with a measurement scale that facilitates thickness measurement. The density of the multilayer foam sheet can be defined and measured as the cross-sectional density or "overall" density rather than the "core" density, as in the measurement according to JIS K6767. The multilayer foam sheet manufactured using the method described above can produce a foam having a cross-sectional density or "overall" density of about 20 - 250 kg / m 3 , about 30 - 200 kg / m 3 , or about 50 - 150 kg / m 3 . The cross-sectional density can be controlled by the amount of blowing agent and the thickness of the extruded structure. When the density of the multilayer foam sheet is less than about 20 kg / m 3 , the amount of chemical blowing agent required to achieve that density increases, and in some cases, the sheet may not foam efficiently. Also, when the density of the sheet is less than about 20 kg / m 3 , it may become increasingly difficult to control the expansion of the sheet during the foaming process. Furthermore, when the density of the multilayer foam sheet is about 20 kg / m3 If it is less, the foam may be more likely to cause bubble collapse. Therefore, at a density of less than about 20 kg / m 3 it may be difficult to produce a multilayer foam sheet having a 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 but is not limited thereto. Foams having a cross-sectional density of about 350 kg / m 3 , about 450 kg / m 3 , or about 550 kg / m 3 may also be produced. However, as the density increases, generally the cost may be higher compared to other materials that can be used for a given application. Therefore, in some cases, it may be preferable that the density of the foam sheet is less than about 250 kg / m 3 . The foam layer produced using the above method may be a closed-cell type. It is preferable that at least 90%, preferably at least 95%, more preferably more than 98% of the bubble walls of the bubbles are not damaged. The average bubble size can be about 0.05 to about 1.0 mm, preferably about 0.1 to about 0.7 mm. If the average bubble size is less than about 0.05 mm, the density of the foam structure can usually be higher than 250 kg / m 3 . If the average bubble size is greater than 1 mm, irregularities may form on the surface of the foam. Also, if the average bubble size of the bubble population in the foam is not desirable, the foam structure may be unnecessarily torn. This can occur when the foam structure is stretched or when a part of it is subjected to a secondary process. The bubble size in the foam layer(s) exhibits a bimodal distribution, which indicates that there is a bubble population in the core of the relatively round foam structure, as well as a bubble population in the film near the surface of the relatively flat and thin, and / or rectangular foam structure.

[0042] The total 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. If the thickness is less than about 0.2 mm, gas loss from the primary surface(s) is significant, and foaming may be inefficient. If the thickness is greater than about 50 mm, it may become increasingly difficult to control expansion during the foaming process. Thus, it may become increasingly difficult to produce a multilayer polyolefin foam / polyolefin cap sheet with uniform cross-sectional density and thickness. In some embodiments, the thickness of the cap layer of the foam coextruded multilayer structure can be about 0.0001 - 0.2 mm, about 0.001 - 0.15 mm, or about 0.05 - 0.1 mm. In some embodiments, the thickness of the foam layer of the foam coextruded multilayer structure can be about 0.5 - 6 mm, about 1 - 5 mm, or about 2 - 4 mm. In some embodiments, the desired thickness is obtained by a secondary process such as slicing, skiving, or adhesion. Slicing, skiving, or adhesion can make it possible to achieve a thickness in the range of 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 expandable layer(s) expanding and, as a result, the cap layer(s) stretching. Thus, for example, if the multilayer sheet expands to twice its original area, it can be expected that the cap thickness will be about halved. If the multilayer sheet expands to four times its original area, it can be expected that the cap thickness will decrease to about 1 / 4 of the original thickness.

[0043] The disclosed multilayer foam structure can be used in various applications. As an example of such an application, an article manufactured by LPM can be mentioned. In the "Summary of the Invention", a multilayer foam structure as a decorative component in vehicle interiors (specifically, door panels) was described. However, the multilayer foam structure can be used not only in vehicle door panels but also in other vehicle interior components such as door rolls, door inserts, door stunners, trunk stunners, armrests, center consoles, seat cushions, backrests, headrests, backrest panels, knee bolsters, or headliners. As another example of an application, thermoformed articles can be mentioned. To thermoform the multilayer foam structure, it is possible to heat the structure up to the melting points 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 the present disclosure, the multilayer foam structure can be heated up to the melting point of the polyamide. As an example of a thermoformed article, an automotive air duct can be mentioned. A closed-cell foam structure can be particularly suitable for this application. This is because it is lightweight (compared to solid plastics), has insulating properties that promote the maintenance of the temperature of the air flowing through the duct, and is resistant to vibration (compared to solid plastics). By providing a polyamide cap layer on the outside of the multilayer air duct, it becomes possible to protect the air duct from contact with liquids and gases "under the bonnet" and inside the vehicle that may adversely affect the functionality of the polyolefin foam. Also, the cap layer can protect the foam layer from puncture and cutting during installation and during the vehicle's service life. Therefore, a robust polyolefin foam with a polyamide cap can 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 can be applied to the foam layer side (i.e., the surface) opposite the cap layer. In these laminates, the multilayer foam structure can also be combined with, for example, a film and / or a foil. Examples of materials suitable for such layers include, but are not limited to, polyvinyl chloride (PVC); thermoplastic polyolefin (TPO); thermoplastic urethane (TPU); polyester, polypropylene, cloth such as fabric, and other cloths; leather; and / or fiber layers such as non-woven fabrics. Such layers may be manufactured using standard techniques well known to those skilled in the art. What is important is that the multilayer foam of the present disclosure may include a plurality of other layers. In these laminates, one layer may be joined to an adjacent layer by chemical bonding, mechanical means, or a combination thereof. Also, each adjacent laminate layer may be attached to each other by any other means. This any other means includes the use of the attraction between materials having electromagnetic charges opposite to each other, or the use of the attraction existing between materials both having dominant hydrophobic properties or dominant hydrophilic properties. To meet the requirements of any of the above applications, the structures disclosed in the present 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, splicing, slicing, skiving, layer forming, adhesion, and drilling.

Examples

[0045] Raw materials of the examples Table 1 below shows a list of the components used in the following examples and descriptions of those components.

[0046]

Table 1

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

[0048] [Table 2] TIFF0007717716000004.tif25079

[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 separate doses, and these sections were further designated as Examples 1A - 1C. Sections of the unfoamed sheet of Example 2 were irradiated at seven separate doses, and these sections were further designated as Examples 2A - 2G.

[0050] [Table 3] TIFF0007717716000006.tif54170

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

[0052] This application discloses several numerical ranges in the text and drawings. Since the present disclosure can be implemented over the entire disclosed numerical range, even if the exact range limitations are not described verbatim in the specification, the disclosed numerical ranges essentially correspond to any range or value within the disclosed numerical range including the endpoints. The foregoing description is presented to enable a person of ordinary skill 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 embodiments 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. Accordingly, the 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 referred to in this application are hereby incorporated by reference into this specification. Another aspect of the present invention may be as follows. 〔1〕A method for forming a multilayer foam structure, comprising the following steps: A first layer containing polypropylene, polyethylene, or a combination of polypropylene and polyethylene, and A chemical blowing agent And a second layer on the side of the first layer, containing Polyamide; and Polypropylene, polyethylene, or a combination of polypropylene and polyethylene Co-extruding; Irradiating the co-extruded layer with ionizing radiation; and Foaming the irradiated co-extruded layer A method comprising. 〔2〕The method according to 〔1〕 above, wherein the first layer contains at least 70% by mass of polypropylene, polyethylene, or a combination of polypropylene and polyethylene. 〔3〕The method according to 〔1〕 or 〔2〕 above, wherein the second layer contains at least 40% by mass of polyamide. 〔4〕The method according to any one of 〔1〕 to 〔3〕 above, wherein the second layer contains a maximum of 50% by mass of polypropylene, polyethylene, or a combination of polypropylene and polyethylene; and at least 40% by mass of polyamide. 〔5〕The method according to any one of 〔1〕 to 〔4〕 above, wherein the thickness of the second layer is less than 1 mm. 〔6〕The method according to any one of 〔1〕 to 〔5〕 above, wherein the first layer contains a crosslinking accelerator in an amount of 0.5 to 5% by mass. 〔7〕The method according to any one of 〔1〕 to 〔6〕 above, wherein the first layer contains an additive in an amount of 1 to 20% by mass. 〔8〕The method according to any one of 〔1〕 to 〔7〕 above, wherein the second layer contains an additive in an amount of 1 to 10% by mass. 〔9〕The method according to any one of 〔1〕 to 〔8〕 above, wherein the polypropylene has a melt flow index of 0.1 to 25 grams per 10 minutes at 230 °C. 〔10〕The method according to any one of 〔1〕 to 〔9〕 above, wherein the polyethylene has a melt flow index of 0.1 to 25 grams per 10 minutes at 190 °C. 〔11〕The method according to any one of 〔1〕 to 〔10〕 above, wherein the amount of the chemical blowing agent in the first layer is 3 to 15% by mass. 〔12〕The method according to any one of 〔1〕 to 〔11〕 above, wherein the chemical blowing agent contains azodicarbonamide. 〔13〕The method according to any one of 〔1〕~〔12〕, wherein the ionizing radiation is selected from the group consisting of alpha rays, beta (electron) rays, X-rays, gamma rays, and neutron rays. 〔14〕The method according to any one of 〔1〕~〔13〕, wherein the co-extruded structure is irradiated separately up to a maximum of 4 times. 〔15〕The method according to any one of 〔1〕~〔14〕, wherein the ionizing radiation is an electron beam having an acceleration voltage of 200~1500 kV. 〔16〕The method according to 〔15〕, wherein the dose of the absorbed electron beam is 10~500 kGy. 〔17〕The method according to any one of 〔1〕~〔16〕, wherein the ionizing radiation crosslinks the extruded structure to a crosslinking degree of 20~75%. 〔18〕The method according to any one of 〔1〕~〔17〕, wherein the foaming step includes heating the irradiated structure with a molten salt and a radiation heater or a hot air oven. 〔19〕The density of the multilayer foamed structure is 20~250 kg / m 3 The method according to any one of 〔1〕~〔18〕. 〔20〕The method according to any one of 〔1〕~〔19〕, wherein the average independent bubble size of the multilayer foamed structure is 0.05~1.0 mm. 〔21〕The method according to any one of 〔1〕~〔20〕, wherein the thickness of the multilayer foamed structure is 0.2~50 mm.

Claims

1. A method for forming a multilayer foam structure, comprising the following steps: Polypropylene, polyethylene, or a combination of polypropylene and polyethylene, a chemical blowing agent, and a crosslinking accelerator in an amount of 0.5 to 5% by mass to form a first layer, 60 to 85% by mass of polyamide; and 10 to 40% by mass of polypropylene, polyethylene, or a combination of polypropylene and polyethylene to form a second layer on the side of the first layer, and co-extruding the two layers; irradiating the co-extruded layers with ionizing radiation; and foaming the first layer of the irradiated co-extruded layers to form a multilayer foam structure.

2. The method according to claim 1, wherein the first layer comprises at least 70% by mass of polypropylene, polyethylene, or a combination of polypropylene and polyethylene.

3. The method according to claim 1 or 2, wherein the thickness of the second layer is less than 1 mm.

4. The method according to any one of claims 1 to 3, wherein the first layer comprises an additive in an amount of 1 to 20% by mass.

5. The method according to any one of claims 1 to 4, wherein the second layer comprises an additive in an amount of 1 to 10% by mass.

6. The method according to any one of claims 1 to 5, wherein the amount of the chemical blowing agent in the first layer is 3 to 15% by mass.

7. The method according to any one of claims 1 to 6, wherein the chemical blowing agent comprises azodicarbonamide.

8. The method according to any one of claims 1 to 7, wherein the ionizing radiation is selected from the group consisting of alpha rays, beta (electron) rays, X-rays, gamma rays, and neutron rays.

9. The method according to any one of claims 1 to 8, wherein the co-extruded layers are irradiated separately up to a maximum of 4 times.

10. The method according to any one of claims 1 to 9, wherein the ionizing radiation is an electron beam having an accelerating voltage of 200 to 1500 kV.

11. The method according to any one of claims 1 to 10, wherein the ionizing radiation crosslinks the co-extruded layers to a crosslinking degree of 20 to 75%.

12. The method according to any one of claims 1 to 11, wherein the foaming step comprises heating the irradiated co-extruded layers with a molten salt and a radiant heater or a hot air oven.

13. The density of the multilayer foam structure is 20 to 250 kg / m 3 The method according to any one of claims 1 to 12, which is such.

14. The method according to any one of claims 1 to 13, wherein the multilayer foam structure has an average independent bubble size of 0.05 to 1.0 mm.

15. The method according to any one of claims 1 to 14, wherein the thickness of the multilayer foam structure is 0.2 to 50 mm.

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