Coextruded cross-linked polyolefin foam with a polyamide cover layer

Abstract

Described herein are physically cross-linked, closed-cell, continuous, multilayer foam structures comprising: a foam layer comprising polypropylene, polyethylene, or a combination of polypropylene and polyethylene, and a polyamide cover layer. The multilayer foam structure can be obtained by coextruding a multilayer structure comprising at least one foam composition layer and at least one cover composition layer, irradiating the coextruded structure with ionizing radiation, and continuously foaming the irradiated structure.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to and the benefit of U.S. application Ser. No. 16 / 836,229, filed on March 31, 2020, and U.S. application Ser. No. 16 / 836,389, filed on March 31, 2020, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to multilayer polyolefin foam / polyamide cover structures and methods of making these structures. More particularly, it relates to methods of making coextruded cross-linked polyolefin multilayer foam / polyamide cover structures. Background Art

[0004] Crosslinked polyolefin foams can be used in a variety of commercial applications, including but not limited to decorative components for vehicle interiors, such as door panels. To prepare polyolefin foam for vehicle door panels, the foam layer is typically first laminated to a film, fabric, or foil to create a bilaminate. This flexible bilaminate then needs to be combined with a rigid substrate to create the panel.

[0005] Different production methods are used in the automotive industry to combine flexible bi-laminates with panels. These methods include thermoforming techniques such as negative vacuum forming (NVF) and positive vacuum forming (PVF), compression molding, and low pressure molding (LPM).

[0006] In low-pressure molding, the bilayer composite is placed in a mold with the film, fabric, or foil facing the "A" surface of the mold. The mold is closed, and polypropylene is injected into the "B" side of the mold cavity—this fills the mold to form the panel. In commercial production processes, polypropylene is typically an impact-modified homopolymer or random copolymer injected at a very high melt flow rate (50-125 g / 10 min at 230°C) at about 200°C.

[0007] The problem arises from the design and implementation of LPM. Hot polypropylene in and around the injection point can shear away or degrade the foam because the injection temperature can be much higher than the foam's melting temperature. In one case, an "orange peel"-type visual defect can be observed on the "A" surface in and around the injection point. In another case, the foam around the injection location can be completely sheared away, leaving a visible depression of film, fabric, or foil at the injection site.

[0008] Manufacturers have implemented different techniques to help reduce these problems. One technique is to inject the polypropylene into the scrap portion of the two-layer composite. While this is generally effective in addressing the foam shear and degradation issues within the mold cavity, it increases the manufacturing cost of the panel. Injecting at the scrap requires that the scrap be long along at least one side of the mold. The scrap along the injected side will therefore also contain the injected polypropylene. The cost of wasting additional scrap (which contains both the two-layer composite and the injected polypropylene) can be high. In addition, the scrap is not easily recyclable - which further increases the cost of this additional scrap.

[0009] Another technique for reducing foam shear and degradation defects in LPMs is the use of flexible trilaminates. Trilaminates can be similar to LPM bilaminates, with a flexible homopolymer-based TPO or TPE layer laminated to the "B" side of the bilaminate. The TPO or TPE layer acts as a protective and / or sacrificial skin between the foam and the polypropylene to be injected.

[0010] However, problems also arise from the three-layer composite used for LPM. The thickness of the TPO or TPE layer will be mainly related to the thickness of the overall two-layer composite, which increases the material cost. In order to produce the three-layer composite, a second lamination step is required, which further increases the cost of the three-layer composite. Finally, the protective TPO or TPE layer is also prone to shearing and degradation at the injection point. Vehicle door panel manufacturers using LPM technology typically inject polypropylene continuously into the waste portion of the three-layer composite. Although the amount of waste required for this setup is less than using a two-layer composite, it still requires more waste than direct injection into the mold cavity. The cost of wasting additional waste (which includes both the three-layer composite and the injected polypropylene) is very high. The difficulty of recycling the waste further increases the cost of this manufacturing technology. Summary of the Invention

[0011] We have discovered that a physically cross-linked, closed-cell polyolefin foam having at least one polyamide cover layer can be produced in a continuous process. This multilayer structure can be laminated to a film, fabric, or foil to create a bi-laminate. This bi-laminate can then be used in LPM applications, overcoming the problems associated with using conventional LPM bi- and tri-laminates to produce vehicle interior components.

[0012] In some embodiments, a polyamide layer can serve as a more effective protective layer for the polypropylene to be injected (compared to a TPO or TPE layer). The melting temperatures of commercially available polyamides vary widely, but most exhibit a melting temperature higher than that of homopolymer polypropylene. Therefore, a polyamide can be selected that is not only higher than the melting temperature of the polypropylene to be injected, but also higher than the injection temperature. The high melting temperature of the polyamide ensures that the polyamide remains intact, providing a barrier layer that does not melt or peel when in contact with the injected polypropylene—even at the injection point. In addition, injection in the waste area becomes unnecessary, further reducing the cost of waste. When appropriately selected for use in the LPM process as a non-sacrificial layer, the polyamide cover layer can be significantly thinner than the TPO or TPE layer, thereby further reducing material costs.

[0013] Maleic anhydride-grafted polypropylene (PA) is a suitable compatibilizer for polyolefins and polyamides. Maleic anhydride-grafted impact-modified PA homopolymers and PA random copolymers are commercially available in large quantities. This allows for direct substitution of the injected PA homopolymer or random copolymer in conventional LPM manufacturing processes with minimal adjustments.

[0014] In some embodiments, a multilayer foam structure is provided, comprising: a foam layer comprising polypropylene, polyethylene, or a combination of polypropylene and polyethylene; and a cover layer on one side of the foam layer, the cover layer comprising polyamide; and polypropylene, polyethylene, or a combination of polypropylene and polyethylene. In some embodiments, the foam layer and the cover layer may be co-extruded. In some embodiments, the foam layer may comprise at least 70 wt% polypropylene, polyethylene, or a combination of polypropylene and polyethylene. In some embodiments, the cover layer may comprise at least 40 wt% polyamide. In some embodiments, the cover layer may comprise up to 50 wt% polypropylene, polyethylene, or a combination of polypropylene and polyethylene; and at least 40 wt% polyamide. In some embodiments, the cover layer may have a thickness of less than 1 mm. In some embodiments, the foam layer may comprise a crosslinking promoter in an amount of 0.5-5 wt%. In some embodiments, the foam layer may comprise an additive in an amount of 1-20 wt%. In some embodiments, the cover layer may comprise an additive in an amount of 1-10 wt%. In some embodiments, the polypropylene may have a melt flow index of 0.1-25 g / 10 min at 230°C. In some embodiments, the melt flow index of the polyethylene at 190° C. may be 0.1 to 25 g / 10 min. In some embodiments, the density of the multilayer foam structure may be 20 to 250 kg / m 3In some embodiments, the multilayer foam structure may have a degree of crosslinking of 20-75%. In some embodiments, the multilayer foam structure may have an average closed cell size of 0.05-1.0 mm. In some embodiments, the multilayer foam structure may have a thickness of 0.2-50 mm.

[0015] In some embodiments, a laminate is provided, comprising: a multilayer foam structure comprising: a foam layer comprising polypropylene, polyethylene, or a combination of polypropylene and polyethylene; a cover layer on one side of the foam layer, the cover layer comprising polyamide; and polypropylene, polyethylene, or a combination of polypropylene and polyethylene; and a laminate layer on a side of the foam layer opposite the cover layer. In some embodiments, the foam layer and cover layer can be coextruded. In some embodiments, the laminate layer can be a flexible film, fabric, or foil.

[0016] As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that, as used herein, the term "and / or" refers to and includes any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms "comprises," "comprising," and / or "containing," when used herein, specify the presence of stated 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.

[0017] It is to be understood that the aspects and embodiments described herein include “consisting of” and / or “consisting essentially of” the aspects and embodiments. For all methods, systems, compositions, and apparatus described herein, such methods, systems, compositions, and apparatus may include, or may “consist of” or “consist essentially of” the listed components or steps. When a system, composition, or apparatus is described as “consisting essentially of” the listed components / components, the system, composition, or apparatus contains the listed components / components and may include other components / components that do not significantly affect the performance of the system, composition, or apparatus, but does not contain any other components / components other than those explicitly listed that significantly affect the performance of the system, composition, or apparatus, nor does it contain additional components / components in concentrations or amounts sufficient to significantly affect the performance of the system, composition, or apparatus. When a method is described as “consisting essentially of” the listed steps, the method includes the listed steps and may include other steps that do not significantly affect the result of the method, but the method does not include any other steps other than those explicitly listed that significantly affect the result of the method.

[0018] In the present invention, "substantially free" of a specific component, a specific composition, a specific compound, or a specific ingredient means, in various embodiments, that less than about 5%, less than about 2%, less than about 1%, less than about 0.5%, less than about 0.1%, less than about 0.05%, less than about 0.025%, or less than about 0.01% by weight of the specific component, specific composition, specific compound, or specific ingredient is present. Preferably, "substantially free" of a specific component, specific composition, specific compound, or specific ingredient means that less than about 1% by weight of the specific component, specific composition, specific compound, or specific ingredient is present.

[0019] Additional advantages will become apparent to those skilled in the art from the detailed description that follows.The examples and descriptions herein are to be considered as illustrative rather than restrictive. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] By way of example only, various embodiments are described with reference to the accompanying drawings, in which:

[0021] Figure 1 is a cross-sectional image of an unfoamed microtome slice of Example 1B at 100× magnification;

[0022] Figure 2 is an image of foamed Example 1B at 20× magnification and 45° relative to the cover surface;

[0023] Figure 3 is a cross-sectional image of an unfoamed microtome slice of Example 2A at 100× magnification;

[0024] Figure 4 is an image of foamed Example 2A at 20× magnification and 45° relative to the cover surface;

[0025] Figure 5 is a cross-sectional image of an unfoamed microtome slice of Example 2D at 100× magnification;

[0026] Figure 6 is an image of Example 2D of the foam at 20× magnification and 45° relative to the cover surface;

[0027] Figure 7 is a cross-sectional image of an unfoamed microtome slice of Example 2G at 100× magnification; and

[0028] Figure 8 This is an image of foam Example 2G at 20× magnification and 45° relative to the cover surface. DETAILED DESCRIPTION

[0029] A method for producing a cross-linked, closed-cell, coextruded, multilayer foam structure comprising at least one foam layer comprising polypropylene, polyethylene, or a combination thereof and at least one cover layer comprising a polyamide is described. The method for producing the cross-linked, closed-cell, coextruded, multilayer foam structure layer may comprise the steps of (a) coextrusion, (b) irradiation, and (c) foaming.

[0030] Coextrusion is the simultaneous extrusion of multiple layers of material. This type of extrusion uses two or more extruders to deliver a steady volume throughput of material to an extrusion head (die) where the material can be extruded in the desired form.

[0031] In the coextrusion step, the foam composition can be fed into multiple extruders to form an unfoamed multilayer structure. For example, "A" foam composition can be fed into one extruder, and "B" foam composition can be fed into a second extruder. The method of feeding the ingredients into the extruder can be based on the design of the extruder and the available material handling equipment. If necessary, the ingredients of the foam composition can be premixed to promote their dispersion. A Henshelmixer can be used for such premixing. All ingredients can be premixed and fed through a single port in the extruder. The ingredients can also be fed individually through separate designated ports for each ingredient. For example, if the crosslinking promoter or any other additive is a liquid, the promoter and / or additive can be added through one or more feed ports on the extruder or through a vent opening on the extruder (if equipped with a vent) to replace the premixing of solid ingredients. A combination of premixing and single ingredient port feeding can also be used.

[0032] Each extruder can feed a steady amount of each composition into one or more manifolds and then to a sheeting die to produce an unfoamed coextruded multilayer sheet. There are two common approaches to coextruded materials: (1) a feedblock manifold; and (2) multiple manifolds within a die. Elements of a feedblock manifold may include: (a) inlet ports for the top, middle, and bottom layers; (b) a streamlined melt lamination region that converges the separate flow streams into a single laminated melt stream in the feedblock; (c) an adapter plate between the feedblock and the sheeting die; and / or (d) a sheeting die (similar to a single-layer die) where the laminated melt streams enter the center of the die and spread along the manifolds to exit the die outlet as distinct multilayer extrudates. The elements of a multi-manifold die can be: (a) similar to a single-layer die, except that there is more than one feed channel; (b) each melt channel has its own choker for flow control; and / or (c) the melt streams converge within the die near the exit and are formed into distinct multi-layer extrudates.

[0033] Layer thickness can depend on the design of the manifold and die. For example, an 80 / 20 feedblock manifold can deliver a composition ratio of approximately 4:1 when the speed and size of each extruder are matched accordingly. This ratio can be adjusted by varying, for example: (a) the relative extrusion speed of one extruder to the other; (b) the relative size of each extruder; and / or (c) the composition (i.e., viscosity) of the individual layers.

[0034] The total thickness of the multilayer sheet can be controlled by the overall die gap. However, the total thickness of the multilayer sheet can be further adjusted, for example, by stretching (ie, "pulling") the molten multilayer extrudate and / or flattening the molten multilayer extrudate by pinching.

[0035] The multilayer structure may comprise at least two layers consisting of different compositions. In some embodiments, the multilayer structure comprises at least one layer consisting of a foam composition and at least one layer consisting of a non-foam covering 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 may have a plurality of other layers. In some embodiments, the multilayer structure may comprise additional layers such as a tie layer, a film layer, and / or an additional foam layer.

[0036] The covering composition fed into the extruder may comprise at least one polyamide and polypropylene, polyethylene, or a combination thereof.The foam composition fed into the extruder may comprise polypropylene, polyethylene, or a combination thereof.

[0037] Polyamide is a polymer containing an amide group (-CONH-) as a repeating part of the chain. Polyamide includes but is not limited to aliphatic polyamides generated by the condensation reaction of two difunctional monomers or by the ring-opening addition polymerization of a cyclic compound. Polyamide can be a homopolymer, a copolymer, a terpolymer or a blend. Importantly, semi-crystalline polyamides or polyamide blends are superior to amorphous polyamides or polyamide blends. 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, 6131010, 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 types 6 / 66 / 12.

[0038] Polypropylene includes, but is not limited to, polypropylene, impact-modified polypropylene, polypropylene-ethylene copolymers, impact-modified polypropylene-ethylene copolymers, metallocene polypropylene, metallocene polypropylene-ethylene copolymers, metallocene polypropylene olefin block copolymers (with controlled block sequence), polyolefin plastomers based on polypropylene, polyolefin elasto-plastomers based on polypropylene, polyolefin elastomers based on polypropylene, thermoplastic polyolefins based on polypropylene, and thermoplastic elastomer blends based on polypropylene. The polypropylene can be a high melt strength type. In addition, the polypropylene can be grafted with maleic anhydride.

[0039] Polyethylenes include, but are not limited to, LDPE, LLDPE (homopolymer, copolymer with butene or hexene or octene, terpolymer with butene and / or hexene and / or octene), VLDPE (homopolymer, copolymer with butene or hexene or octene, terpolymer with butene and / or hexene and / or octene), VLLDPE (homopolymer, copolymer with butene or hexene or octene, terpolymer with butene and / or hexene and / or octene), HDPE, polyethylene-propylene copolymers, metallocene polyethylene, metallocene ethylene-propylene copolymers, and metallocene polyethylene olefin block copolymers (with controlled block sequence), any of which may contain grafted compatibilizers or copolymers containing acetate and / or ester groups. These polyethylenes may be grafted with maleic anhydride. These polyethylenes may also be copolymers and terpolymers containing acetate and / or ester groups, and may be copolymers and terpolymer ionomers containing acetate and / or ester groups.

[0040] The non-foamed covering composition fed to the extruder may comprise at least about 40 wt% polyamide, preferably at least about 50 wt% polyamide, more preferably at least about 60 wt% polyamide, even more preferably at least about 70 wt% polyamide.

[0041] In some embodiments, the polyamide in the non-foamed cover composition fed to the extruder may be greater than or equal to about 40 wt% polyamide, 50 wt% polyamide, 60 wt% polyamide, or 70 wt% polyamide. In some embodiments, the polyamide in the non-foamed cover composition fed to the extruder may be less than or equal to about 95 wt% polyamide, 90 wt% polyamide, 85 wt% polyamide, or 80 wt% polyamide. In some embodiments, the polyamide in the non-foamed cover composition fed to the extruder may be about 40-95 wt% polyamide, 50-90 wt% polyamide, 60-85 wt% polyamide, or 70-80 wt% polyamide.

[0042] In some embodiments, the amount of polyethylene, polypropylene, or a combination thereof in the non-foamed cover composition fed to the extruder may be greater than or equal to about 5 wt%, 10 wt%, or 20 wt% polyethylene, polypropylene, or a combination thereof. In some embodiments, the amount of polyethylene, polypropylene, or a combination thereof in the non-foamed cover composition fed to the extruder may be less than or equal to about 50 wt%, 40 wt%, 35 wt%, or 30 wt% polyethylene, polypropylene, or a combination thereof. In some embodiments, the amount of polyethylene, polypropylene, or a combination thereof in the non-foamed cover composition fed to the extruder may be 5-50 wt%, 10-40 wt%, or 20-30 wt% polyethylene, polypropylene, or a combination thereof.

[0043] The foam composition fed to the extruder may comprise at least about 75 wt% polypropylene, polyethylene, or a combination thereof, preferably at least about 80 wt%, more preferably at least about 85 wt%, even more preferably at least about 90 wt%.

[0044] In some embodiments, the foam composition fed into the extruder may be at least about 70 wt%, 80 wt%, or 85 wt% polypropylene, polyethylene, or a combination thereof. In some embodiments, the foam composition fed into the extruder may be a maximum of about 98 wt%, 95 wt%, or 90 wt% polypropylene, polyethylene, or a combination thereof. In some embodiments, the foam composition fed into the extruder may be about 70-98 wt%, 80-95 wt%, or 85-90 wt% polypropylene, polyethylene, or a combination thereof.

[0045] Because a wide range of multilayer structures and foamed articles can be produced using the disclosed compositions, a wide range of polyamides, polypropylenes, and polyethylenes can be used in the compositions to meet different ongoing manufacturing requirements and commercial end-use requirements.

[0046] A non-limiting example of "polypropylene" is isotactic homopolypropylene. Commercially available examples include, but are not limited to, Braskem's FF018F, Total Petrochemicals' 3271, and Phillips 66's COPYLENE TM CH020.

[0047] A non-limiting example of an "impact modified polypropylene" is homopolypropylene and ethylene-propylene (EP) copolymer rubber. The rubber may be amorphous or semi-crystalline, but the amount is insufficient to impart any plastomer or elastomeric properties to the material. Several non-limiting examples of commercially available "impact modified polypropylene" are Braskem's TI4003F and TI4015F and LyondellBasell's Pro-fax 8623 and Pro-fax SB786.

[0048] "Polypropylene-ethylene copolymer" is a polypropylene having random ethylene units. Several non-limiting examples of commercially available "polypropylene-ethylene copolymers" are 6232, 7250FL and Z9421 from Total Petrochemicals, 6D20 and DS 6D81 from Braskem and PRO-FAX from LyondellBasell. RP 311H and ADSYL 7415XCP.

[0049] "Impact modified polypropylene-ethylene copolymer" is a polypropylene having random ethylene units and an ethylene-propylene (EP) copolymer rubber. The rubber may be amorphous or semi-crystalline, but the amount is insufficient to impart any plastomeric or elastomeric plastomeric properties to the material. A non-limiting example of a commercially available impact modified polypropylene-ethylene copolymer is Braskem's PRISMA 6910.

[0050] "Metallocene polypropylene" is metallocene syndiotactic homopolypropylene, metallocene atactic homopolypropylene, and metallocene isotactic homopolypropylene. A non-limiting example of "metallocene polypropylene" is the metallocene polypropylene sold under the trade name METOCENE® by LyondellBasell. and ExxonMobil's trademark ACHIEVE TM Commercially available ones. Metallocene polypropylenes are also commercially available from Total Petrochemicals, including but not limited to grades M3551, M3282MZ, M7672, 1251, 1471, 1571 and 1751.

[0051] "Metallocene polypropylene-ethylene copolymers" are metallocene syndiotactic, metallocene atactic, and metallocene isotactic polypropylenes with random ethylene units. Commercially available examples include, but are not limited to, Lumicene from Total Petrochemicals. MR10MX0 and Lumicene MR60MC2, Purell by LyondellBasell SM170G and WINTEC from Japan Polypropylene Corporation product line.

[0052] A "metallocene polypropylene olefin block copolymer" is a polypropylene having alternating crystallizable hard "blocks" and amorphous soft "blocks", the former being not randomly distributed, i.e. having a controlled block order. An example of a "metallocene polypropylene olefin block copolymer" includes, but is not limited to, INTUNE® from Dow Chemical Company. product line.

[0053] "Polyolefin plastomers based on polypropylene" (POP) and "polyolefin elastomeric plastomers based on polypropylene" are copolymers based on both metallocene and non-metallocene propylene, having both plastomeric and elastomeric properties. Non-limiting examples are sold under the trade name VERSIFY® by Dow Chemical Company. (metallocene), ExxonMobil's trade name VISTAMAXX (metallocene) and LyondellBasell's trade name KOATTRO TM (Non-metallocene) (Butene-1 based series of plastomer polymers - some grades are butene-1 homopolymer based materials, others are polypropylene-butene-1 copolymer based materials) those commercially available.

[0054] "Polyolefin elastomers based on polypropylene" (POE) are copolymers based on both metallocene and non-metallocene propylene having elastomeric properties. A non-limiting example of a propylene based polyolefin elastomer is the one sold under the trade name VERSIFY (metallocene) and ExxonMobil's trade name VISTAMAXX (Metallocene) polymers such as those commercially available.

[0055] "Polypropylene-based thermoplastic polyolefins" (TPOs) are polypropylene, polypropylene-ethylene copolymers, metallocene homopolypropylenes, and metallocene polypropylene-ethylene copolymers having an ethylene-propylene copolymer rubber in an amount sufficient to impart plastomeric, elastomeric, or elastomeric properties to the thermoplastic polyolefin blend (TPO). Non-limiting examples of TPO polymers are those sold under the trade name THERMOR1JN by Mitsubishi Chemical Corporation. and ZELAS , a trademark of LyondellBasell called ADFLEX and SOFTELL , a trade name of Teknor Apex Company, TELCAR and Japan Polypropylene Company's trademark WELNEXTM TPO can be produced via multi-stage polymerization (e.g. ZELAS ,ADFLEX , SOFTELL and WELNEX ) or produced by blending (e.g. THERMORUN and TELCAR ).

[0056] "Polypropylene-based thermoplastic elastomer blends" (TPEs) are polypropylene, polypropylene-ethylene copolymers, metallocene homopolypropylene, and metallocene polypropylene-ethylene copolymers having a diblock or multiblock thermoplastic rubber modifier (SEBS, SEPS, SEEPS, SEP, SERC, CEBC, HSB, etc.) in an amount sufficient to impart plastomeric, elastomeric, or elastomeric properties to the thermoplastic elastomer blend (TPE). Non-limiting examples of polypropylene-based thermoplastic elastomer blend polymers are those sold under the trade name GLS® by Polyone Corporation. TM DYNAFLEX and GLS TM VERSAFLEX , a trade name of TeknorApex Company, MONPRENE and LyondellBasell's trademark DURAGRIP Blends of those polymers are commercially available.

[0057] Any of the above polypropylenes may also be of the high melt strength (HMS) type. Polypropylene manufacturers use different methods to strengthen the polymer in the melt phase. For example, polypropylene that exhibits long chain branching (LCB) may be identified as a high melt strength polypropylene. A non-limiting example of a high melt strength polypropylene is sold under the trade name DAPLOY by Borealis. , Braskem's trademark AMPPLEO and Japan Polypropylene Corporation's trademark WAYMAX Those polymers are commercially available.

[0058] Any polypropylene, but more commonly TPO and TPE blends, may optionally be treated with, for example, mineral oil, Chevron's PARALUX Processing oil or the like is used to extend the oil to further soften the blend, enhance the tactile properties of the blend, or improve the processability of the blend.

[0059] "LDPE" and "LLDPE" are low density polyethylene and linear low density polyethylene, respectively. Non-limiting examples of LDPE include polyethylene manufactured from at least Dow (e.g., 640I) and Nova (e.g., Novapol Non-limiting examples of LLDPE include those provided by at least ExxonMobil (e.g., LLP8501.67) and Dow (e.g., DFDA-7059NT 7). Commercially available LLDPE polymers are typically copolymers or terpolymers of α-olefins containing butene and / or hexene and / or octene.

[0060] "VLDPE" and "VLLDPE" are very low density polyethylene and very linear density low density polyethylene, typically copolymers or terpolymers of alpha-olefins containing butene and / or hexene and / or octene. Non-limiting examples of VLDPE and VLLDPE are sold under the trade name FLEXOMER® by Dow Chemical Company. and specific grades of STAMYLEX from Borealis Commercially available.

[0061] "Metallocene polyethylene" is a metallocene-based polyethylene having properties ranging from inelastic to elastomeric. A non-limiting example of a metallocene polyethylene is sold under the trade name ENGAGE TM ExxonMobil's ENABLE TM and EXCEED TM and Borealis' QUEO Commercially available.

[0062] A "metallocene polyethylene olefin block copolymer" is a polyethylene having alternating crystallizable hard "blocks" and amorphous soft "blocks", the blocks being not randomly distributed, i.e., having a controlled block order. An example of a "metallocene polyethylene olefin block copolymer" includes, but is not limited to, INFUSE from Dow Chemical Company. TM product line.

[0063] All of the above polyethylenes can be grafted with maleic anhydride. A non-limiting commercially available example is ADMER from Mitsui Chemicals. NF539A, DuPont by Dow TM BYNEL 4104 and Arkema's OREVAC 18360. It should be noted that many commercially available anhydride grafted polyethylenes also contain rubber.

[0064] These polyethylenes may also be copolymers and terpolymers containing acetate and / or ester groups. Comonomer groups include, but are not limited to, vinyl acetate, methyl acrylate, ethyl acrylate, butyl acrylate, glycidyl methacrylate, and acrylic acid. Non-limiting examples include those sold under the trade name DuPont® by Dow. TM BYNEL , DuPont TM ELVAX and DuPont TM ELVALOY Arkema's trademark EVATANE ,LOTADER and LOTRYL ; ExxonMobil's trademark ESCORENE ,ESCOR and OPTEMA Commercially available.

[0065] The polypropylene and polyethylene listed above can be functionalized. Functionalized polypropylene and polyethylene include grafted monomers. Usually, monomers have been grafted to polypropylene or polyethylene by free radical reaction. Suitable monomers for the preparation of functionalized polypropylene and polyethylene are for example ethylenically unsaturated monocarboxylic acids such as acrylic acid or methacrylic acid, and corresponding tert-butyl esters, for example tert-butyl (methyl) acrylate, ethylenically unsaturated dicarboxylic acids such as fumaric acid, maleic acid and itaconic acid and corresponding mono- and / or di-tert-butyl esters, for example fumaric acid mono- or di-tert-butyl ester and maleic acid mono- or di-tert-butyl ester, ethylenically unsaturated dicarboxylic anhydride such as maleic anhydride, ethylenically unsaturated monomers containing sulfonic group or sulfonyl are for example p-styrenesulfonic acid, 2-(methyl) acrylamido-2-methylpropylenesulfonic acid or 2-sulfonyl-(methyl) acrylate, ethylenically unsaturated monomers containing oxazolinyl such as vinyloxazoline and vinyloxazoline derivatives, and ethylenically unsaturated monomers containing epoxy such as glycidyl (methyl) acrylate or allyl glycidyl ether.

[0066] The most commonly used commercially available functionalized polypropylene is polypropylene functionalized with maleic anhydride. A non-limiting example is ADMER from Mitsui Chemicals. QF and QB series, PLEXAR by LyondellBasell 6000 Series, DuPont by Dow TM BYNEL 5000 Series and Arkema's OREVAC PP series.

[0067] The most commonly used commercially available functionalized polyethylenes are also those functionalized with maleic anhydride. A non-limiting example is ADMER from Mitsui Chemicals. NF and SE series, LyondellBasell's PLEXAR 1000, 2000 and 3000 Series, DuPont by Dow TM BYNEL 2100, 3000, 3800, 3900, 4000 series, and Arkema's OREVAC PE, T, and some LOTADER series.

[0068] The most popular method for compatibilizing polyamide with polypropylene and polyethylene in various industries is to use maleic anhydride grafted polypropylene or polyethylene. For example, in flexible food packaging, polyamide film can be adhered to polypropylene film by applying a tie layer of maleic anhydride grafted polypropylene between the films.

[0069] It is noted that polyethylene functionalized with other grafting monomers is also commercially available. Non-limiting examples include Dow, DuPont TM BYNEL 1100, 2200 and 3100 series and Arkema's LOTADER AX series.

[0070] It is also noted that polymers other than polypropylene and polyethylene functionalized with maleic anhydride are also commercially available. For example, Addivant's ROYALTUF The series is a series of maleic anhydride functionalized EPDM rubbers. In another example, Kraton's KRATON The FG series is a series of SEBS polymers functionalized with maleic anhydride.

[0071] The cover layer may comprise at least one extrusion-grade or general-purpose polyamide. Extrusion and general-purpose polyamides can be characterized as having a viscosity ranging from approximately high to approximately medium. High- to medium-viscosity polyamides are more likely to conform to the melt flow characteristics of the foamable layer, resulting in greater thickness uniformity from the center to the edge of the die within each coextruded layer. Most polyamide types are hygroscopic, and the moisture content of the polyamide affects its melt flow and its resistance to flow at a given shear rate. Because moisture content affects flow characteristics, polyamides are often subject to alternating standards for melt flow rate and melt volume rate. ISO 307 and ASTM D789 are two such standards used to quantify the viscosity of polyamides. In ISO 307, viscosity values can be determined by dissolving polyamides into dilute solutions using certain specified solvents. In ASTM D789, relative viscosity can be determined by dissolving polyamides into concentrated solutions using certain specified solvents. The corresponding standards, ISO 16396-1 and ASTM D6779, provide guidance for polyamide manufacturers in specifying their products within a standardized framework. These nomenclatures help polyamide manufacturers identify grades suitable for extrusion (cast film, sheet, etc.), injection molding, blow molding, etc. It is important to note that most polyamide manufacturers do not publish viscosity and relative viscosity values. Instead, polyamide resins are typically sold with generic viscosity grades (very low, low, medium (or standard), medium-high, high, etc.) and recommended processing applications (general purpose extrusion, injection molding, compounding, filamentization, melt spinning, industrial yarn, etc.).

[0072] The composition of any foamable layer and / or any cover layer provided herein may comprise at least one polypropylene having a melt flow index of about 0.1 to about 25 g / 10 min at 230°C. The composition of any foamable layer and / or any cover layer provided herein may also comprise at least one polyethylene having a melt flow index of about 0.1 to about 25 g / 10 min at 190°C. In some embodiments, the melt flow index of the polypropylene and / or polyethylene is preferably about 0.3 to about 20 g / 10 min at 230°C and 190°C, respectively, and more preferably about 0.5 to about 15 g / 10 min at 230°C and 190°C, respectively. The "melt flow index" (MFI) value of a polymer is defined and measured according to ASTM D1238, at 230°C for polypropylene and polypropylene-based materials, and at 190°C for polyethylene and polyethylene-based materials, and using a 2.16 kg piston for 10 minutes. For relatively high melt flow rate resins, the test time can be reduced.

[0073] The MFI value provides a measure of the flow characteristics of a polymer and is an indicator of the molecular weight and processability of a polymer material. A high MFI value corresponds to low viscosity. If the MFI value is too high, extrusion according to the present invention cannot be performed satisfactorily. Problems associated with excessively high MFI values include low pressure during extrusion, problems with setting the thickness profile, uneven cooling curves due to low melt viscosity, poor melt strength, and / or machine problems. Conversely, a low MFI value corresponds to high viscosity. Excessively low MFI values can lead to high pressure during melt processing, sheet quality and profile problems, and higher extrusion temperatures, which pose the risk of decomposition and activation of the blowing agent.

[0074] The above MFI ranges are also important for foaming processes because they reflect the viscosity of the material, which affects foaming. Without being bound by any theory, it is believed that certain MFI values are more effective for several reasons. Lower MFI materials can improve certain physical properties because larger molecular chain lengths require more energy to flow when stress is applied. Similarly, the longer the molecular chain (MW), the more crystalline entities the chain can crystallize into, thereby providing greater strength through intermolecular bonding. However, at too low an MFI, viscosity becomes too high. On the other hand, polymers with higher MFI values have shorter chains. Therefore, in a given volume of material with a higher MFI value, there are more chain ends at the microscopic level relative to a lower MFI polymer, which can rotate and create free volume. This is due to the space required for such rotation (e.g., rotation occurring above the polymer's Tg or glass transition temperature). This increases free volume and allows for easier flow under stress.

[0075] In addition to the polymer, the composition fed to the extruder may also contain additives compatible with 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, antimicrobial agents, fungicides, light stabilizers, UV absorbers, anti-caking agents, fillers, deodorants, odor absorbers, anti-fog agents, volatile organic compound (VOC) absorbers, semi-volatile organic compound (SVOC) adsorbents, thickeners, cell size stabilizers, metal passivators, and combinations thereof.

[0076] In some embodiments, the amount of additives other than the chemical blowing agent and the cross-linking accelerator in the foam layer composition may be less than or equal to about 20 PPR%, about 15 PPR%, about 10 PPR%, or about 8 PPR% of the composition. In some embodiments, the amount of additives other than the chemical blowing agent and the cross-linking accelerator in the foam layer composition may be greater than or equal to about 1 PPR%, about 2 PPR%, about 4 PPR%, or about 6 PPR% of the composition. In some embodiments, the amount of additives other than the chemical blowing agent and the cross-linking accelerator in the foam layer composition may 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 other than the chemical blowing agent and the cross-linking accelerator in the foam layer composition may be about 1-20 wt%, about 2-15 wt%, about 3-10 wt%, about 4-8 wt%, or about 5-7 wt% of the foam layer composition.

[0077] In some embodiments, the amount of the additive in the cover composition may be less than or equal to about 20 PPR%, about 15 PPR%, about 10 PPR%, about 7 PPR%, about 5 PPR%, or about 3 PPR% of the composition. In some embodiments, the amount of the additive in the cover composition may be greater than or equal to about 0.5 PPR%, about 1 PPR%, about 2 PPR%, or about 3 PPR% of the composition. In some embodiments, the amount of the additive in the cover composition may be about 0.5-20 PPR%, about 1-10 PPR%, or about 2-7 PPR% of the composition. In some embodiments, the amount of the additive in the cover composition may be about 0.5-20 wt%, about 1-10 wt%, or about 2-6 wt% of the cover composition.

[0078] Regardless of how the ingredients are fed into the extruder, the shear and mixing within the extruder will be sufficient to produce a uniform layer. Co-rotating and counter-rotating twin-screw extruders can provide sufficient shear and mixing through the extruder barrel to extrude a layer of uniform properties.

[0079] Specific energy is an indicator of how much work was applied during the extrusion of the layer's components and how aggressive the extrusion process was. Specific energy is defined as the energy applied to the material being processed through the extruder, normalized to a per kilogram basis. Specific energy is quantified in kilowatts of applied energy per kilogram of total feed material per hour. Specific energy is calculated using the following formula:

[0080]

[0081]

[0082] Specific energy can be used to quantify the amount of shear and mixing of the ingredients within the extruder. The extruder used to form the multilayer structures disclosed herein can be capable of generating a specific energy of at least about 0.100 kW·h / kg, preferably at least about 0.125 kW·h / kg, and more preferably at least about 0.150 kW·h / kg.

[0083] Any foamable layer may contain a chemical foaming agent (CFA). The extrusion temperature of any foamable layer may be 0-10°C lower than the thermal decomposition start temperature of the chemical foaming agent, preferably more than 10°C lower. If the extrusion temperature exceeds the thermal decomposition temperature of the foaming agent, the foaming agent will decompose, which produces undesirable "pre-foaming". The extrusion temperature of any covering layer may be 0-10°C lower than the thermal decomposition start temperature of the chemical foaming agent in any foamable layer adjacent to the covering layer, preferably more than 10°C lower. If the extrusion temperature of the covering layer exceeds the thermal decomposition temperature of the foaming agent in the adjacent layer, the foaming agent in the adjacent layer will decompose, which also forms undesirable "pre-foaming".

[0084] 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, carbohydrazides, tetrazoles, nitroso compounds, and carbonates. In addition, chemical foaming agents can be used alone or in any combination. One chemical foaming agent that can be used in some embodiments is azodicarbonamide (ADCA). An example of an ADCA chemical foaming agent is UNIFOAM manufactured by PT Lauten Otsuka Chemical. TC-18I. Thermal decomposition of ADCA typically occurs at a temperature of about 200-240° C. To prevent thermal decomposition of ADCA in the extruder, the extrusion temperature can be maintained at or below 200° C.

[0085] The amount of chemical blowing agent in the foam layer composition may be less than or equal to about 30 PPR%, about 20 PPR%, about 15 PPR%, about 10 PPR%, or about 8 PPR% of the composition. In some embodiments, the amount of chemical blowing agent in the foam layer composition may be greater than or equal to about 1 PPR%, about 2 PPR%, about 3 PPR%, about 4 PPR%, or about 5 PPR% of the composition. In some embodiments, the amount of chemical blowing agent in the foam layer composition may be about 1-30 PPR%, about 2-20 PPR%, about 3-15 PPR%, about 4-10 PPR%, or about 5-8 PPR% of the composition. In some embodiments, the amount of chemical blowing agent in the foam layer composition may be about 1-30 wt%, about 2-20 wt%, 3-15 wt%, about 4-10 wt%, or about 5-7 wt% of the foam layer composition. The amount of chemical blowing agent may depend on the unfoamed sheet thickness, the desired foam thickness, the desired foam density, the material to be extruded, the cross-linking percentage, the type of chemical blowing agent (different blowing agents will produce significantly different amounts of gas), etc.

[0086] It should be noted that the amounts of chemical blowing agents listed above are specific to ADCA only. Other blowing agents can produce different volumes of gas per CFA mass and should be considered. For example, when comparing ADCA to the chemical blowing agent p-toluenesulfonyl semicarbazide (TSS), if the foamable layer contains 40 PPR% ADCA, approximately 63 PPR% TSS would be required to produce approximately the same amount of gas during the foaming step.

[0087] If 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 high, a catalyst for the decomposition of the blowing agent can be used. Exemplary catalysts include, but are not limited to, zinc oxide, magnesium oxide, calcium stearate, glycerol, and urea. The lower limit temperature for extrusion can be the melting point of the polymer with the highest melting point. If the extrusion temperature is lowered below the melting temperature of the polymer with the highest melting point, an undesirable "unmelt" will occur. During foaming, an extruded layer extruded below this lower limit temperature will exhibit uneven thickness, uneven cell structure, cell collapse, and other undesirable properties.

[0088] Regardless of whether the blowing agent is a physical blowing agent, a chemical blowing agent, or a combination thereof, extrusion foaming typically produces a polymer sheet in which both primary surfaces are significantly rougher than equivalent structures produced in the disclosed methods. The surface profile of multilayer (as well as single layer) foam sheets can be critical in many applications, and therefore extrusion foamed sheets cannot be used in these applications. These applications can require a smooth foam surface to obtain desired properties such as ease of lamination to films, fabrics, fiber layers, and leather; percentage of contact area in lamination; visual aesthetics, etc. PCT Publication WO2016109544, which is incorporated herein by reference in its entirety, includes examples showing the difference in surface roughness between extrusion foamed polymer sheets and equivalent foamed polymer sheets produced by the disclosed methods.

[0089] The rougher surface of an extrusion foamed article may generally be caused by larger cell sizes (when compared to foams produced according to the present invention). Although cell size and cell size distribution may not be critical in most commercial applications, since surface roughness is a function of cell size, foams with larger cells may not be as desirable as foams with smaller cells for applications requiring a smooth foam surface.

[0090] 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 cover 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 cover layer can be less than or equal to about 1.0 mm, about 0.7 mm, or about 0.4 mm. In some embodiments, the thickness of any individual cover layer can be from about 0.01 to 1.0 mm, or from 0.02 to 0.7 mm. In some embodiments, there is no limit to how thin the unfoamed cover layer can be, and it can be associated with the overall unfoamed coextruded multilayer sheet and can be as thin as about 0.1 μm, or the typical thickness of very thin tie layers used in multilayer flexible packaging and barrier films.

[0091] In some embodiments, the thickness of the foam layer of the unfoamed coextruded multilayer structure may be about 0.1-5 mm, about 0.5-4 mm, about 1-3 mm, or about 1-2 mm. In some embodiments, the thickness of the foam layer of the unfoamed coextruded multilayer structure may be less than or equal to about 5 mm, about 3 mm, about 2 mm, about 1.5 mm, about 1 mm, or about 0.5 mm. In some embodiments, the thickness of the foam layer of the unfoamed coextruded multilayer structure may be greater than or equal to about 0.1 mm, about 0.5 mm, about 1 mm, about 1.5 mm, about 2 mm, or about 3 mm.

[0092] In some embodiments, the overall thickness of an unfoamed coextruded multilayer structure can be measured using a stem-type thickness gauge attached to a flat substrate. The tip of the gauge can be fitted with a hemispherical contact point with a 1.6 mm radius. The stem is raised and the unfoamed structure is placed on the substrate. A force of 100-150 gf can be applied to the contact point of the structure during the measurement.

[0093] In some embodiments, the thickness of the cover layer of the unfoamed coextruded multilayer structure can be measured using a microscope. In order to measure the cover layer thickness, a small sample of the structure can be cut from a continuous sheet and the cross section of the sample can be sliced into thin sections using a microtome. A section can be placed under microscopic observation. Measurements can be made using a digital or traditional microscope. Typical commercial digital microscopes have different software components to facilitate thickness measurement. Traditional commercial microscopes have lenses for measuring scales to facilitate thickness measurement.

[0094] The cover can be thin and flex easily when melted so as not to significantly hinder the expansion of the foamable layer during the foaming step. Many physical properties of the cover, such as thickness, flexibility, and melt strength, can hinder the foaming expansion of other layers. The thickness, flexibility, melt strength, and crosslinking percentage of the foamable layer, as well as the final thickness and density of the foam layer, are also factors in determining whether the cover inhibits the expansion of the foamable layer. A general guideline for maximum cover thickness is that it should be no greater than about 20%, about 15%, about 10%, or about 5% of the entire coextruded unfoamed sheet. If the cover thickness is greater than about 20% of the entire coextruded unfoamed sheet, problems associated with curling, bending, and folding of the multilayer sheet itself may occur when the multilayer sheet is heated and foamed.

[0095] It is important to distinguish between "physical" and "chemical" crosslinking. In chemical crosslinking, crosslinking is produced using a crosslinking accelerator, but without the use of ionizing radiation. Chemical crosslinking typically involves the use of peroxides, silanes, or vinyl silanes. In peroxide crosslinking methods, crosslinking typically occurs in the extrusion die. For silane and vinyl silane crosslinking methods, crosslinking typically occurs in a secondary operation after extrusion, where crosslinking of the extruded material can be accelerated with heat and moisture. Regardless of the chemical crosslinking method, chemically crosslinked foam sheets typically exhibit a primary surface that is significantly rougher than an equivalent structure produced by the disclosed method. The surface profile of multilayer (as well as single-layer) foam sheets can be critical in many applications, and therefore chemically crosslinked foam sheets cannot be used in these applications. These applications may require a smooth foam surface to achieve desired properties such as ease of lamination to films, fabrics, fiber layers, and leather; percentage of contact area during lamination; visual aesthetics, etc. PCT Publication WO2016109544 includes examples that show the difference in surface roughness between chemically crosslinked foamed polymer sheets and equivalent foamed polymer sheets produced by the disclosed method.

[0096] The rougher surface of chemically cross-linked foamed articles will generally result from larger cell sizes (when compared to foams produced according to the present invention). While cell size and cell size distribution may not be critical in most commercial applications, since surface roughness is a function of cell size, foams with larger cells may not be as desirable as foams with smaller cells for applications requiring a smooth foam surface.

[0097] Examples of ionizing radiation include, but are not limited to, α, β (electrons), x-rays, gamma, and neutrons. Among these, electron beams with uniform energy can be used to prepare crosslinked polyolefin foams / crosslinked polyolefin covering structures. The exposure time, irradiation frequency, and accelerating voltage of electron beam irradiation can vary widely, depending on the desired degree of crosslinking and the thickness of the multilayer structure. However, the ionizing radiation can typically be about 10 to about 500 kGy, about 20 to about 300 kGy, or about 20 to about 200 kGy. If the exposure is too low, cell stability cannot be maintained during foaming. If the exposure is too high, the moldability of the resulting multilayer foam structure will be poor. Moldability is a desirable property when multilayer foam sheets are used in thermoforming applications. Similarly, unfoamed sheets can soften due to the release of heat when exposed to electron beam radiation, causing the structure to deform if the exposure is too high. In addition, the polymer components can degrade due to excessive polymer chain scission.

[0098] The coextruded, unfoamed multilayer sheet can be irradiated up to four times, preferably no more than two times, and more preferably only once. If the irradiation frequency is greater than about four times, the polymer components may be subject to degradation, so that uniform cells will not be produced in the foamed layer, for example, when foamed. When the thickness of the extruded structure is greater than about 4 mm, it is preferred to irradiate each primary surface of the multilayer profile with ionizing radiation to achieve a more uniform degree of crosslinking between the primary surface and the inner layer.

[0099] One advantage of electron beam irradiation is that coextruded sheets of varying thicknesses can be effectively crosslinked by controlling the electron acceleration voltage. The acceleration voltage can typically be between about 200 and about 1500 kV, about 400 and about 1200 kV, or about 600 and about 1000 kV. If the acceleration voltage is less than about 200 kV, the radiation cannot reach the interior of the coextruded sheet. As a result, the internal cells will be rough and uneven during foaming. Furthermore, for a given thickness profile, an acceleration voltage that is too low can cause arcing, resulting in the formation of "pinholes" or "tunnels" in the foamed structure. On the other hand, if the acceleration voltage is greater than about 1500 kV, the polymer will degrade. In some embodiments, the radiation source can be directed toward layer B of the coextruded, unfoamed multilayer sheet during irradiation. In some embodiments, the radiation source can be directed toward layer A of the coextruded, unfoamed multilayer sheet during irradiation.

[0100] Regardless of the type of ionizing radiation selected, crosslinking can be performed so that the composition of the extruded structure can be crosslinked to about 20 to about 75%, or about 30 to about 60%, as measured by the "Toray Gel Fraction Percentage Method." According to the "Toray Gel Fraction Percentage Method," tetralin solvent is used to dissolve the non-crosslinked polyolefin component of the composition. In principle, the non-crosslinked polyolefin material is dissolved in tetralin, and the degree of crosslinking is expressed as the weight percentage of the crosslinked material in the entire composition. The equipment used to determine the percentage of polymer crosslinking includes: a 100 mesh screen (0.0045 inch wire diameter); a 304 type stainless steel bag; numbered wires and clamps; a Miyamoto constant temperature oil bath apparatus; an analytical balance; a fume hood; a gas burner; a high temperature oven; an antistatic 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, the empty wire mesh bag is weighed and the weight is recorded. For each sample, 100 mg ± 5 mg of sample was weighed and transferred to a wire mesh bag. The weight of the wire mesh bag and sample (usually in the form of thinly sliced foam cutouts) was recorded. Each bag was attached to a corresponding numbered wire and clamp. When the solvent temperature reached 130°C, the bundle (bag and sample) was immersed in the solvent. The sample was shaken up and down about 5 or 6 times to release any bubbles and completely wet the sample. The sample was attached to a stirrer and stirred for three (3) hours to allow the solvent to dissolve the foam. The sample was then cooled in a fume hood. The sample was washed by shaking up and down in a first container of acetone about 7 or 8 times. The sample was washed a second time in a second acetone wash. The washed sample was washed again in a third container of fresh acetone as above. The sample was then hung in a fume hood to allow the acetone to evaporate for about 1 to about 5 minutes. The sample was then dried in a drying oven at 120°C for about 1 hour. The sample was allowed to cool for at least about 15 minutes. The wire mesh bag was weighed on an analytical balance and the weight was recorded. Crosslinking was then calculated using the formula 100 x (CA) / (BA), where A = empty mesh bag weight; B = mesh bag + weight of foamed sample before immersion in tetralin; and C = mesh bag + weight of dissolved sample after immersion in tetralin.

[0101] It is important to note that polyamide is insoluble in tetralin. Therefore, the gel percentage calculated by the above method includes the crosslinked polyolefin component plus the polyamide component.

[0102] Suitable crosslinking promoters include, but are not limited to, commercially available difunctional, trifunctional, tetrafunctional, pentafunctional and higher functionality monomers. Such crosslinking monomers can be used 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 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 trimesic acid, triallyl pyromellitic acid, and 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 dipropargyl phthalic acid and dipropargyl maleic acid; and divinylbenzene. Furthermore, such crosslinking accelerators may be used alone or in any combination. Divinylbenzene (DVB) is a difunctional liquid crosslinking monomer that can be used as a crosslinking accelerator in the present invention. For example, suitable commercially available DVB can include Dow's DVB HP.

[0103] The amount of cross-linking accelerator in the foam layer composition may be less than or equal to about 5 PPR%, about 4 PPR%, about 3 PPR%, about 2.5 PPR%, about 2 PPR%, about 1.5 PPR%, or about 1 PPR% of the composition. In some embodiments, the amount of cross-linking accelerator in the foam layer composition may be greater than or equal to about 0.5 PPR%, about 1 PPR%, about 1.5 PPR%, about 2 PPR%, about 2.5 PPR%, about 3 PPR%, or about 4 PPR% of the composition. In some embodiments, the amount of cross-linking accelerator in the foam layer composition may 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 cross-linking accelerator in the foam layer composition may be about 0.5-5 wt %, or about 1-3 wt % of the foam layer composition.

[0104] It should be noted that the amounts of crosslinking accelerators listed above may be specific to DVB only. Other crosslinking accelerators may be more or less efficient at crosslinking than DVB. Therefore, the amount required for another crosslinking accelerator should be considered accordingly. The crosslinking efficiency of a crosslinking accelerator will vary based on the ionizing radiation dose, the polymer to be crosslinked, the chemical structure of the monomer, the number of functional groups on the monomer, and whether the monomer is a liquid or powder.

[0105] Crosslinks can be produced using a variety of different techniques and can be formed intermolecularly, between different polymer molecules, and intramolecularly, between portions of a single polymer molecule. Such techniques include, but are not limited to, providing a crosslinking promoter separate from the polymer chain and providing a polymer chain into which a crosslinking promoter containing functional groups capable of forming crosslinks or activated to form crosslinks has been incorporated.

[0106] After irradiating the coextruded sheet, foaming can be accomplished by heating the crosslinked multilayer sheet to a temperature above the decomposition temperature of the thermally decomposable foaming agent. Foaming can be performed in a continuous process at about 200-260° C. or about 220-240° C. A continuous foaming process may be preferred over a batch process for producing continuous foam sheets.

[0107] Foaming can typically be performed by heating the crosslinked multilayer sheet with molten salt, radiant heaters, vertical or horizontal hot air ovens, microwave energy, or a combination of these methods. Foaming can also be performed in an impregnation process using, for example, nitrogen in an autoclave, followed by free foaming with molten salt, radiant heaters, vertical or horizontal hot air ovens, microwave energy, or a combination of these methods. Optionally, the crosslinked multilayer sheet can be preheated to soften it prior to foaming. This helps stabilize the expansion of the structure during foaming, particularly when using thick and hard sheets.

[0108] The overall thickness of the multi-layer foam sheet can be measured according to JIS K6767.

[0109] The thickness of the cover layer of a multilayer foam sheet can be measured using a microscope. To measure the cover layer, a small sample of the foam structure can be taken from the continuous foam sheet. The sample can be cut with a specially sharp blade, and a cross-section of the sample can be observed along the cut surface using a microscope. Measurements can be performed using either a digital or conventional microscope. Typical commercial digital microscopes have various software features to facilitate thickness measurement. Conventional commercial microscopes have a lens with a measuring scale to facilitate thickness measurement.

[0110] The density of a multilayer foam sheet can be defined and measured using the cross-sectional or "bulk" density, rather than the "core" density, as measured by JIS K6767. The multilayer foam sheet produced using the above method can achieve a cross-sectional or "bulk" density of about 20-250 kg / m 3 , about 30-200kg / m 3 , or about 50-150kg / m 3 The cross-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 about 20 kg / m 3 , the sheet cannot be foamed effectively because a large amount of chemical foaming agent is required to obtain the density. In addition, if the density of the sheet is less than about 20 kg / m 3, the expansion of the sheet during the foaming step becomes increasingly difficult to control. In addition, if the density of the multilayer foam sheet is less than about 20 kg / m 3 , the foaming will become more prone to cell collapse. Therefore, at a density of less than about 20 kg / m 3 It is difficult to produce multi-layer foam sheets with uniform cross-sectional density and thickness.

[0111] In some embodiments, the multi-layer foam sheet may be limited to about 250 kg / m 3 Cross-sectional density of about 350kg / m 3 , about 450kg / m 3 , or about 550kg / m 3 However, the density of the foam sheet may be less than about 250 kg / m 3 , as greater density is often cost-prohibitive when compared to other materials available for a given application.

[0112] The foam layer produced using the above method can have closed cells. Preferably, at least 90% of the cells have undamaged cell walls, preferably at least 95%, and more preferably greater than 98%. The average cell size can be from about 0.05 to about 1.0 mm, preferably from about 0.1 to about 0.7 mm. If the average cell size is less than about 0.05 mm, the density of the foam structure can generally be greater than 250 kg / m 3 If the average cell size is greater than 1 mm, the foam may have an uneven surface. There is also the possibility that if the cell population in the foam does not have the preferred average cell size, the foam structure may tear undesirably. This can occur when the foam structure is stretched or a portion of it undergoes secondary processing. The cell sizes in the foam layer may have a bimodal distribution, meaning that the cell population in the core of the foam structure is relatively round, and the cell population in the skin layer closer to the surface of the foam structure is relatively flat, thin, and / or oval.

[0113] The overall thickness of the multilayer polyolefin foam / polyamide cover sheet can be from about 0.2 mm to about 50 mm, from about 0.4 mm to about 40 mm, from about 0.6 mm to about 30 mm, or from about 0.8 mm to about 20 mm. If the thickness is less than about 0.2 mm, foaming will not be effective due to the large gas loss from the primary surface. If the thickness is greater than about 50 mm, the expansion during the foaming step will become increasingly difficult to control. Therefore, it will be increasingly difficult to produce a multilayer polyolefin foam / polyolefin cover sheet with uniform cross-sectional density and thickness. In some embodiments, the thickness of the cover layer of the foamed coextruded multilayer structure can be from about 0.0001 mm to about 0.2 mm, from about 0.001 mm to about 0.15 mm, or from about 0.05 mm to about 0.1 mm. In some embodiments, the thickness of the foam layer of the foamed coextruded multilayer structure can be from about 0.5 mm to about 6 mm, from about 1 mm to about 5 mm, or from about 2 mm to about 4 mm.

[0114] In some embodiments, the desired thickness can be achieved by secondary processing such as slicing, grinding, or bonding. Slicing, grinding, or bonding can produce a thickness of about 0.1 mm to about 100 mm.

[0115] The thickness of the cover layer will decrease as the multilayer sheet expands. This is likely because the foamable layer expands and thereby stretches the cover layer. Thus, for example, if the multilayer sheet expands to twice its initial area, the cover layer thickness can be expected to decrease by approximately half. If the multilayer sheet expands to four times its initial area, the cover layer thickness can be expected to decrease to approximately one-quarter of its initial thickness.

[0116] The disclosed multilayer foam structure can be used in a variety of applications. One such application is as an article produced using LPM. The Summary of the Invention describes the multilayer foam structure as a decorative component for a vehicle interior, specifically a door panel. However, the multilayer foam structure is not limited to vehicle door panels and can also be used in other vehicle interior parts such as door rollers, door inserts, door padding, trunk padding, armrests, center consoles, seat cushions, seat backs, headrests, seat back panels, knee bolsters, or headrests.

[0117] Another application is in thermoformed articles. To thermoform a multilayer foam structure, the structure can be heated to the melting point of the polyolefin foam layer and the polyamide cover layer. Because most commercially available polyamides have a melting temperature greater than the polyolefin component of the present invention, the multilayer foam structure can be heated to the melting point of the polyamide.

[0118] An example of a thermoformed article is an automotive air duct. Closed-cell foam structures are particularly well-suited for this application due to their lower weight (when compared to solid plastic), their insulating properties (which help maintain the temperature of the air flowing through the duct), and their resistance to vibration (compared to solid plastic). A polyamide cover on the outside of the multi-layer duct protects the duct from contact with liquids and grease "under the hood" and inside the vehicle, which could adversely impact the functionality of the polyolefin foam. The cover also protects the foam layer from punctures and cuts during installation and over the life of the vehicle. Therefore, a robust polyolefin foam with a polyamide cover is suitable for automotive air ducts.

[0119] In some embodiments, the multilayer foam structure can be a laminate containing a multilayer foam body and a laminate layer. Preferably, the laminate layer can be applied to the side of the foam layer opposite to the cover layer (i.e., the surface). In these laminates, the multilayer foam structure can be combined with a film and / or foil, for example. Examples of suitable materials for such layers include, but are not limited to, polyvinyl chloride (PVC); thermoplastic polyolefin (TPO); thermoplastic polyurethane (TPU); fabrics such as polyester, polypropylene, clothing and other fabrics; leather; and / or fiber layers such as nonwovens. Such layers can be manufactured using conventional techniques known to those skilled in the art. Importantly, the multilayer foam of the present invention can contain multiple other layers.

[0120] In these laminates, layers can be connected to adjacent layers by chemical bonds, mechanical means, or a combination thereof. Adjacent laminate layers can also be connected to each other by any other means, including the use of attractive forces between materials with opposite electromagnetic charges, or the attractive forces that exist between materials with either predominantly hydrophobic properties or predominantly hydrophilic properties.

[0121] In order to meet the requirements of any of the above applications, the structures disclosed in the present invention can undergo various secondary operations, including but not limited to embossing, corona or plasma treatment, surface roughening, surface smoothing, perforation or microperforation, splicing, slicing, grinding, lamination, bonding and punching.

[0122] Example

[0123] Raw materials used in the examples

[0124] Table 1 below provides a listing of the components used in the following examples and descriptions of those components.

[0125]

[0126]

[0127]

[0128]

[0129] Cross-sectional images of microtome slices of the unfoamed multilayer structures of Examples 1B, 2A, 2D, and 2G at 100× magnification can be found in Figure 1 、 3 , 5 and 7. Images of the corresponding foam structures of Examples 1B, 2A, 2D and 2G at 20× magnification can be found in Figure 2 、 4 , 6 and 8.

[0130] This application discloses several numerical ranges in the text and drawings. The disclosed numerical ranges inherently support any range or value within the disclosed numerical range (including the endpoints), even if the specification does not verbatim state the precise range limitations, because the present invention can be practiced within the entire disclosed numerical range.

[0131] The above description is presented to enable those skilled in the art to make and use the invention, and is provided in the context of a specific 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 invention. Therefore, the present invention is not intended to be limited to the embodiments shown, but rather to be consistent with the widest scope of the principles and features disclosed herein. Finally, the entire contents of the patents and publications mentioned in this application are incorporated herein by reference.

Claims

1. A multi-layer foam structure comprising: A foam layer comprising: Polypropylene, polyethylene, or a combination of polypropylene and polyethylene; and A covering layer on one side of the foam layer, the covering layer consisting of: 1-10wt% additives, greater than 70 wt% polyamide, and Less than 30 wt% polypropylene, polyethylene, or a combination of polypropylene and polyethylene.

2. The multilayer foam structure according to claim 1, wherein the foam layer and the cover layer are co-extruded.

3. The multilayer foam structure according to any one of claims 1-2, wherein the foam layer comprises at least 70 wt% polypropylene, polyethylene or a combination of polypropylene and polyethylene.

4. The multilayer foam structure according to any one of claims 1-2, wherein the thickness of the cover layer is less than 1 mm.

5. The multi-layer foam structure according to any one of claims 1-2, wherein the foam layer further comprises a cross-linking accelerator in an amount of 0.5-5.0 wt%.

6. The multilayer foam structure according to any one of claims 1-2, wherein the foam layer further comprises an additive in an amount of 1-20 wt%.

7. The multilayer foam structure according to any one of claims 1-2, wherein the polypropylene has a melt flow index at 230°C of 0.1-25 g / 10 min.

8. The multilayer foam structure according to any one of claims 1-2, wherein the polyethylene has a melt flow index at 190°C of 0.1-25 g / 10 min.

9. The multilayer foam structure according to any one of claims 1 to 2, wherein the density of the multilayer foam structure is 20-250 kg / m 3 .

10. The multilayer foam structure according to any one of claims 1-2, wherein the degree of crosslinking of the multilayer foam structure is 20-75%.

11. The multilayer foam structure according to any one of claims 1-2, wherein the average closed cell size of the multilayer foam structure is 0.05-1.0 mm.

12. The multilayer foam structure according to any one of claims 1-2, wherein the thickness of the multilayer foam structure is 0.2-50 mm.

13. The multi-layer foam structure according to claim 1, wherein the additive is at least one selected from the group consisting of: organic peroxides, antioxidants, lubricants, heat stabilizers, colorants, flame retardants, antistatic agents, nucleating agents, plasticizers, antimicrobial agents, light stabilizers, UV absorbers, anti-caking agents, deodorants, anti-fogging agents, volatile organic compound (VOC) absorbers, semi-volatile organic compound (SVOC) adsorbents, thickeners, cell size stabilizers, and metal deactivators.

14. A laminate comprising: A multi-layer foam structure comprising: a foam layer comprising polypropylene, polyethylene, or a combination of polypropylene and polyethylene; A covering layer on one side of the foam layer, the covering layer consisting of: 1-10wt% additives, greater than 70 wt% polyamide; and Less than 30 wt% polypropylene, polyethylene, or a combination of polypropylene and polyethylene; and A laminate layer is provided on the side of the foam layer opposite the cover layer.

15. The laminate of claim 14, wherein the foam layer and the cover layer are co-extruded.

16. The laminate according to any one of claims 14-15, wherein the laminate layer is a flexible film, fabric or foil.

17. The laminate according to any one of claims 14-15, wherein the laminate layer is foamed.

18. The laminate according to any one of claims 14-15, wherein the cover layer is unfoamed.

19. The laminate according to any one of claims 14-15, wherein the foam layer comprises at least 70 wt% polypropylene, polyethylene, or a combination of polypropylene and polyethylene.

20. The laminate according to any one of claims 14-15, wherein the thickness of the cover layer is less than 1 mm.

21. The laminate according to any one of claims 14-15, wherein the foam layer further comprises an additive in an amount of 1-20 wt%.

22. The laminate according to any one of claims 14-15, wherein the polypropylene has a melt flow index of 0.1-25 g / 10 min at 230°C.

23. The laminate according to any one of claims 14-15, wherein the polyethylene has a melt flow index of 0.1-25 g / 10 min at 190°C.

24. The laminate according to any one of claims 14-15, wherein the density of the multilayer foam structure is 20-250 kg / m 3 .

25. The laminate according to any one of claims 14-15, wherein the degree of cross-linking of the multilayer foam structure is 20-75%.

26. The laminate according to any one of claims 14-15, wherein the average closed cell size of the multilayer foam structure is 0.05-1.0 mm.

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

28. The laminate according to claim 14, wherein the additive is at least one selected from the group consisting of: organic peroxides, antioxidants, lubricants, heat stabilizers, colorants, flame retardants, antistatic agents, nucleating agents, plasticizers, antimicrobial agents, light stabilizers, UV absorbers, anti-caking agents, deodorants, anti-fogging agents, volatile organic compound (VOC) absorbers, semi-volatile organic compound (SVOC) adsorbents, thickeners, cell size stabilizers, and metal deactivators.

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