Coextruded cross-linked multilayer polyolefin foam structure having a cross-linked polyolefin cover layer, and method of making the same

Abstract

Disclosed herein is a physically crosslinked, closed-cell, continuous multilayer foam structure comprising: a coextruded foam layer comprising at least one of polypropylene and polyethylene and a crosslinked, coextruded cover layer comprising at least one of polypropylene and polyethylene. 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 No. 16 / 832,684, filed March 27, 2020, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to multilayer polyolefin foam structures and methods of making the same. More particularly, the present invention relates to coextruded crosslinked polyolefin multilayer foam structures having a crosslinked polyolefin cover layer. Background Art

[0004] Polyolefin foams can be used in a variety of applications. For example, they can be used as decorative components in vehicle interiors, such as instrument panels. Instrument panels can include a multilayer foam / covering structure positioned between a rigid substrate and a flexible film or foil. The foam layer of this structure can be adhered to the substrate, and the covering layer can be adhered to the film or foil. Furthermore, the instrument panel can include an airbag mounted on the backside of the panel.

[0005] There are different instrument panel designs for accommodating airbags and desired airbag configurations for safety and aesthetics when activated. The instrument panel can be designed to break open in a specific pattern when the airbag is inflated. These patterns can be different and are not limited. For example, the patterns can be a "U" shape, an "H" shape, or another pattern. In addition, the substrate or both the substrate and foam layer can be laser scored on the instrument panel during the manufacturing process to promote the panel to break open in a specific pattern. Other designs can be non-scored (i.e., neither the substrate nor the foam is perforated or cut to help promote the desired panel break pattern).

[0006] Regardless of the design, it is preferable that the airbag penetrate the instrument panel cleanly when deployed. Excessive splitting within or between any substrate / multi-layer foam-cover structure / film or foil is undesirable because it: (a) increases airbag rupture time; and (b) increases instrument panel cracking, which can cause instrument panel fragments to break off. One goal of the instrument panel cover layer can be to reduce the amount of cracking that can occur between the foam layer and the flexible film or foil. The cover layer increases the force required to peel the flexible film or foil from the foam. Summary of the Invention

[0007] Applicants have discovered that physically crosslinked, closed-cell polyolefin foams having at least one physically crosslinked polyolefin cover layer can be produced in a continuous process. This discovery can provide a method for producing more desirable multilayer polyolefin foam structures. For example, in the case of a vehicle instrument panel, the peel strength between the crosslinked cover layer and the uncrosslinked cover layer can be improved, thereby further reducing the possibility of rupture between the foam and the film or foil that could occur if a deploying airbag penetrates the instrument panel.

[0008] In some embodiments, a method of forming a multilayer structure comprises: coextruding a foam layer and a film layer on one side of the foam layer, the foam layer comprising at least one of polypropylene and polyethylene; and a chemical foaming agent; a crosslinking agent; and the film layer comprising at least 90 wt% of at least one of polypropylene and polyethylene; and 0.1-5 wt% of the crosslinking agent. In some embodiments, the foam layer comprises polypropylene having a melt flow index of 0.1-25 g / 10 min at 230°C. In some embodiments, the foam layer comprises polyethylene having a melt flow index of 0.1-25 g / 10 min at 190°C. In some embodiments, the foam layer comprises 0.5-5 wt% of the crosslinking agent. In some embodiments, the chemical foaming agent comprises azodicarbonamide. In some embodiments, the foam layer comprises polypropylene and polyethylene. In some embodiments, the foam layer comprises at least 75 wt% of at least one of polypropylene and polyethylene. In some embodiments, the foam layer comprises 3-15 wt% of the chemical foaming agent.

[0009] In some embodiments, a method of forming a multilayer foam structure comprises coextruding a foam layer and a film layer on one side of the foam layer, the foam layer comprising at least one of polypropylene and polyethylene, a chemical foaming agent and a crosslinking agent; and the film layer comprising at least 90 wt% of at least one of polypropylene and polyethylene, and 0.1-5 wt% of a crosslinking agent; irradiating the coextruded layer with ionizing radiation; and foaming the irradiated coextruded layer. In some embodiments, the ionizing radiation is selected from α, β (electrons), x-rays, γ and neutrons. In some embodiments, the coextruded structure is irradiated up to four separate times. In some embodiments, the ionizing radiation is an electron beam with an acceleration voltage of 200-1500 kV. In some embodiments, the absorbed electron beam dose is 10-500 kGy. In some embodiments, the ionizing radiation crosslinks the coextruded structure to a degree of crosslinking of 20-75%. In some embodiments, foaming comprises heating the irradiated structure with a molten salt. In some embodiments, the density of the multilayer foam structure is 20-250 kg / m 3In some embodiments, the thickness of the multilayer foam structure is 0.2-50 mm. In some embodiments, the foam layer comprises polypropylene and polyethylene. In some embodiments, the foam layer comprises at least 75 wt% of at least one of polypropylene and polyethylene. In some embodiments, the foam layer comprises 3-15 wt% of a chemical foaming agent.

[0010] 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 the recited 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.

[0011] 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.

[0012] 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.

[0013] 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 limiting. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Various embodiments are described with reference to the accompanying drawings, in which:

[0015] Figure 1 is a table of the various components used in the examples disclosed herein and descriptions of those components.

[0016] Figure 2A Tables are provided for the formulations of the examples disclosed herein and for the coextrusion, irradiation, and other properties of the multilayer structures of Examples 1 and 2 disclosed herein.

[0017] Figure 2B yes Figure 2A Continuation of the form.

[0018] Figure 2C yes Figures 2A-2B Continuation of the form.

[0019] Figure 3 is an image of Example 1 at 30× magnification and 45° relative to the cover surface and 45° relative to the machine direction (“MD”);

[0020] Figure 4 is an image of Example 2 at 30× magnification and 45° relative to the cover surface and 45° relative to the machine direction (“MD”). DETAILED DESCRIPTION

[0021] A method for producing a crosslinked, closed-cell, coextruded, multilayer foam structure comprising a foam layer comprising at least one of polypropylene and polyethylene and a crosslinked cover layer comprising at least one of polypropylene and polyethylene is described. The method for producing the crosslinked, closed-cell, coextruded, multilayer foam structure may include the steps of (a) coextrusion, (b) irradiation, and (c) foaming.

[0022] 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.

[0023] 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" non-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.

[0024] 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 the 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 within 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) in which 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.

[0025] The 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.

[0026] 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.

[0027] 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 non-foam covering composition may comprise a crosslinking accelerator. In addition, the multilayer structure may comprise additional layers such as a tie layer, a film layer, and / or additional foam layers.

[0028] The foam composition and the non-foam covering composition that are fed into the forcing machine can comprise at least one polypropylene, at least one polyethylene or its combination.Polypropylene includes but is 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 (having controlled block sequence), based on polypropylene polyolefin plastomer, based on polypropylene polyolefin elastic-plastomer, based on polypropylene polyolefin elastomer, based on polypropylene thermoplastic polyolefin, with based on polypropylene thermoplastic elastomer blend.Polypropylene can be the high melt strength type.In addition, polypropylene can be grafted with maleic anhydride.

[0029] 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.

[0030] The foam composition and the non-foam covering composition fed into the extruder can contain at least about 25 wt% polypropylene, polyethylene, or a combination thereof; at least about 50 wt% polypropylene, polyethylene, or a combination thereof; at least about 75 wt% polypropylene, polyethylene, or a combination thereof; at least about 85 wt% polypropylene, polyethylene, or a combination thereof; at least about 90 wt% polypropylene, polyethylene, or a combination thereof; at least about 95 wt% polypropylene, polyethylene, or a combination thereof; or at least about 98 wt% polypropylene, polyethylene, or a combination thereof.

[0031] Because a wide range of multilayer structures and foam articles can be produced using the disclosed compositions, a wide range of polypropylenes and polyethylenes can be used in the compositions to meet different ongoing manufacturing requirements and commercial end-use requirements.

[0032] 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.

[0033] 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 8623 and SB786.

[0034] "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 DS6D81 from Braskem and DS6D81 from LyondellBasell. RP311H and 7415XCP.

[0035] "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 6910.

[0036] "Metallocene polypropylene" is metallocene syndiotactic homopolypropylene, metallocene atactic homopolypropylene, and metallocene isotactic homopolypropylene. Non-limiting examples of "metallocene polypropylene" are those manufactured under the trade name of LyondellBasell. and ExxonMobil's trademark ACHIEVE TM Metallocene polypropylenes are also commercially available from Total Petrochemicals, including but not limited to grades M3551, M3282MZ, M7672, 1251, 1471, 1571, and 1751.

[0037] "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, Total Petrochemicals' MR10MX0 and MR60MC2, LyondellBasell SM170G and Japan Polypropylene Corporation product line.

[0038] A "metallocene polypropylene olefin block copolymer" is a polypropylene 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 polypropylene olefin block copolymer" includes, but is not limited to, Dow Chemical Company's product line.

[0039] "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 those sold under the trade names of Dow Chemical Company. (metallocene), a trademark of ExxonMobil (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.

[0040] "Polyolefin elastomers based on polypropylene" (POE) are copolymers based on both metallocene and non-metallocene propylene having elastomeric properties. Non-limiting examples of propylene based polyolefin elastomers are those sold under the trade name of Dow Chemical Company. (metallocene) and ExxonMobil trademarks (Metallocene) polymers such as those commercially available.

[0041] "Polypropylene-based thermoplastic polyolefins" (TPOs) are polypropylenes, 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 names of Mitsubishi Chemical Corporation. and LyondellBasell's trademark and Trade name of Teknor Apex Company and Japan Polypropylene Company's trademark WELNEX TM Those polymers that are commercially available. TPO can be produced via multi-stage polymerization (e.g. and ) or produced by blending (e.g. and ).

[0042] "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 and GLS TM Trade name of Teknor Apex Company and LyondellBasell's trademarks Blends of those polymers are commercially available.

[0043] 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 the polypropylene sold under the trade name Borealis. Braskem's trademark and Japan Polypropylene Corporation's trademark Those polymers are commercially available.

[0044] Any polypropylene, but more commonly TPO and TPE blends, may optionally be treated with mineral oil, Chevron's 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.

[0045] "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., 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.

[0046] "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 trade names of Dow Chemical Company. and specific levels of Borealis Commercially available.

[0047] "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 Commercially available.

[0048] 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.

[0049] All of the above polyethylenes can be grafted with maleic anhydride. Non-limiting commercially available examples are Mitsui Chemicals' NF539A, DuPont by Dow TM 4104 and Arkema 18360. It should be noted that many commercially available anhydride grafted polyethylenes also contain rubber.

[0050] 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 DuPont TM and DuPont TM Arkema's trademark and ExxonMobil's trademark and Commercially available.

[0051] The polypropylene and polyethylene listed above can be functionalized. Functionalized polypropylene and polyethylene comprise grafted monomers. Usually, monomer has 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 ester, 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 ester, 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.

[0052] The most commonly used commercially available functionalized polypropylene is polypropylene functionalized with maleic anhydride. A non-limiting example is Mitsui Chemicals' QF and QB series, LyondellBasell 6000 Series, DuPont by Dow TM 5000 Series and Arkema PP series.

[0053] The most commonly used commercially available functionalized polyethylenes are also those functionalized with maleic anhydride. A non-limiting example is Mitsui Chemicals' NF and SE series, LyondellBasell 1000, 2000 and 3000 Series, DuPont by Dow TM 2100, 3000, 3800, 3900, 4000 series, and Arkema PE, T, and some series.

[0054] Polyethylene functionalized with other grafting monomers is also commercially available. Non-limiting examples include Dow, DuPont, TM 1100, 2200 and 3100 series and Arkema AX series.

[0055] It is noted that polymers other than polypropylene and polyethylene functionalized with maleic anhydride are also commercially available. For example, Addivant's series is a series of maleic anhydride functionalized EPDM rubbers. In another example, Kraton's The FG series is a series of SEBS polymers functionalized with maleic anhydride.

[0056] The composition of any foamable layer and any cover layer provided herein may contain at least one polypropylene having a melt flow index of about 0.1 to about 25 g / 10 min at 230° C. and / or 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, 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.

[0057] The MFI value provides a measure of the flow properties 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.

[0058] 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 generate 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.

[0059] 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.

[0060] In some embodiments, the amount of additives other than chemical blowing agents and cross-linking accelerators in the foam layer composition and / or the non-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 chemical blowing agents and cross-linking accelerators in the foam layer composition and / or the non-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 chemical blowing agents and cross-linking accelerators in the foam layer composition and / or the non-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 promoter in the foam layer composition and / or the non-foam layer composition can 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.

[0061] In some embodiments, the amount of additives other than the chemical blowing agent and the cross-linking accelerator in the cover layer 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 additives other than the chemical blowing agent and the cross-linking accelerator in the cover layer composition may be greater than or equal to about 0.5 PPR%, 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 additives other than the chemical blowing agent and the cross-linking accelerator in the cover layer 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 additives other than the chemical blowing agent and the cross-linking accelerator in the cover layer composition may be about 0.5-20 wt%, about 1-10 wt%, or about 2-6 wt% of the cover layer composition.

[0062] 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.

[0063] 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:

[0064] in

[0065]

[0066] 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 0.050 kW·h / kg, preferably at least about 0.100 kW·h / kg, and more preferably at least about 0.150 kW·h / kg.

[0067] Any foamable layer may contain a chemical foaming agent (CFA). The extrusion temperature of any foamable layer may be at least 10°C lower than the thermal decomposition onset temperature of the chemical foaming agent. If the extrusion temperature exceeds the thermal decomposition temperature of the foaming agent, the foaming agent will decompose, which produces undesirable "pre-foaming". The extrusion temperature of any cover layer may be at least 10°C lower than the thermal decomposition onset temperature of the chemical foaming agent in any foamable layer adjacent to the cover layer. If the extrusion temperature of the cover 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 produces undesirable "pre-foaming".

[0068] The foam layer composition may include a plurality 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 may be used alone or in any combination. One chemical foaming agent that may be used in some embodiments is azodicarbonamide (ADCA). An example of an ADCA chemical foaming agent is manufactured by PT Lauten Otsuka Chemical. TC-18I. Thermal decomposition of ADCA typically occurs at a temperature of about 190-230° C. To prevent thermal decomposition of ADCA in the extruder, the extrusion temperature can be maintained at or below 190° C.

[0069] 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, and / or the type of chemical blowing agent (different blowing agents will produce significantly different amounts of gas), etc.

[0070] 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.

[0071] 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 may 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 may 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" may occur. Upon foaming, an extruded layer extruded below this lower limit temperature may exhibit uneven thickness, uneven cell structure, cell collapse, and other undesirable properties.

[0072] 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.

[0073] 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.

[0074] The thickness of the unfoamed coextruded multilayer structure can be about 0.1 to about 30 mm, about 0.2 to about 25 mm, about 0.3 to about 20 mm, or about 0.4 to about 15 mm. The thickness of any individual A or B layer can be 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. The thickness of any individual A or B layer can be less than or equal to about 0.2 mm, about 0.15 mm, or about 0.10 mm. In some embodiments, the cover layer of the unfoamed coextruded multilayer structure can be about 0.1 to 300 microns, about 25 to 200 microns, or about 30 to 175 microns. In some embodiments, the cover layer of the unfoamed coextruded multilayer structure may have a thickness of less than 300 microns, less than 250 microns, less than 200 microns, less than 175 microns, less than 150 microns, less than 125 microns, less than 100 microns, less than 90 microns, less than 80 microns, less than 70 microns, less than 60 microns, less than 50 microns, less than 40 microns, less than 30 microns, less than 20 microns, less than 10 microns, less than 5 microns, or less than 1 micron. In some embodiments, the cover layer of the unfoamed coextruded multilayer structure may have a thickness greater than 1 micron, greater than 5 microns, greater than 10 microns, greater than 20 microns, greater than 30 microns, greater than 40 microns, greater than 50 microns, greater than 60 microns, greater than 70 microns, greater than 80 microns, greater than 90 microns, greater than 100 microns, greater than 125 microns, greater than 150 microns, greater than 175 microns, greater than 200 microns, or greater than 250 microns. There is no limit to how thin the unfoamed cover thickness can be, it will be relative to 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. In some embodiments, the thickness of the foam layer of the unfoamed coextruded multilayer structure can be about 0.1-5 mm, about 0.5-3 mm, about 1-2 mm, or about 1-1.5 mm. In some embodiments, the thickness of the foam layer of the unfoamed coextruded multilayer structure can 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 can 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.

[0075] The overall thickness of an unfoamed, coextruded, multilayer structure is measured using a valve-stem-style 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 valve stem is raised, and the unfoamed structure is placed on the substrate. During the measurement, a force of 100-150 gf can be applied to the structure at the contact point.

[0076] The thickness of the cover layer of an unfoamed, coextruded multilayer structure is measured microscopically. To measure the cover layer thickness, a small sample of the structure can be cut from a continuous sheet and a cross-section of the sample sliced ​​into thin sections using a microtome. One section can be placed under a microscope for observation. Measurements can be performed using either a digital or conventional microscope. Typical commercial digital microscopes have various software features to facilitate thickness measurements. Conventional commercial microscopes have a lens with a measuring scale to facilitate thickness measurements.

[0077] 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, melt strength, and cross-linking percentage, can hinder the foaming expansion of other layers. Similarly, the thickness, flexibility, melt strength, and cross-linking percentage of the foamable layer, as well as the final thickness and density of the foam layer, are also factors that determine whether the cover inhibits the expansion of the foamable layer. A general guideline for maximum cover thickness is that it should be no more 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.

[0078] It is important to distinguish between "physical" crosslinking and "chemical" crosslinking. In chemical crosslinking, crosslinking is produced with a crosslinking accelerator, but no ionizing radiation is used. 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 the 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 equivalent structures produced by the disclosed methods. 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 obtain 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, which is incorporated herein by reference in its entirety, includes examples showing the difference in surface roughness between a chemically cross-linked foamed polymer sheet and an equivalent foamed polymer sheet produced by the disclosed method.

[0079] 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.

[0080] 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 foamed 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 when the exposure is too high. In addition, the polymer components can degrade due to excessive polymer chain scission.

[0081] 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 degrade, resulting in a lack of uniform pores during foaming, such as in the resulting foam layer. 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 of the primary surfaces and the inner layers.

[0082] 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.

[0083] Regardless of the type of ionizing radiation selected, crosslinking can be performed so that the composition of the extruded structure, foam layer, and / or non-foam layer is 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 components of the composition. In principle, the non-crosslinked material dissolves 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 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 foam sample before immersion in tetralin; and C = mesh bag + weight of dissolved sample after immersion in tetralin.

[0084] Suitable crosslinking promoters include, but are not limited to, commercially available difunctional, trifunctional, tetrafunctional, pentafunctional and higher functionality monomers. Such crosslinking monomers are used in liquid, solid, granular and powdered 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. In some embodiments, divinylbenzene (DVB) is a difunctional liquid crosslinking monomer that can be used as a crosslinking promoter in the present invention. For example, a suitable commercially available DVB can include Dow's DVB HP.

[0085] The amount of crosslinking agent / 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 crosslinking 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 crosslinking accelerator in the foam layer composition may be about 0.1-5 PPR%, about 0.5-3 PPR%, about 1-3 PPR%, or about 2-3 PPR% of the composition. In some embodiments, the amount of crosslinking accelerator in the foam layer composition may be about 0.5-5 wt %, or about 1-3 wt % of the foam layer composition.

[0086] The amount of crosslinking agent / accelerator in the cover 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 crosslinking accelerator in the cover 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 crosslinking accelerator in the cover composition may be about 0.1-5 PPR%, about 0.5-3 PPR%, or about 1-2 PPR% of the composition. In some embodiments, the amount of crosslinking accelerator in the cover composition may be about 0.1-5 wt%, about 0.5-3, about 1-2 wt%, or about 1-1.5 wt% of the cover composition.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] Foaming can generally be performed by heating the cross-linked multilayer sheet with molten salt, radiant heater, vertical or horizontal hot air oven, 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 via molten salt, radiant heater, vertical or horizontal hot air oven, microwave energy, or a combination of these methods. Optionally, the cross-linked 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.

[0091] The overall thickness of a multilayer foam sheet is measured according to JIS K6767. 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 particularly 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 will have various software components to facilitate thickness measurements. Conventional commercial microscopes will have a lens with a measuring scale to facilitate thickness measurements.

[0092] 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.

[0093] The multi-layer foam sheet may not be limited to a cross-sectional density of about 250 kg / m 3 It is also possible to produce a cross-sectional density of about 350kg / m 3 , about 450kg / m 3 , or about 550kg / m 3 However, it is preferred that the density of the foam sheet is less than about 250 kg / m 3 , as greater density is often cost-prohibitive when compared to other materials available for a given application.

[0094] 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 3If 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.

[0095] The overall thickness of multilayer polyolefin foam / polyolefin cover sheet can be about 0.2mm-about 50mm, about 0.4mm-about 40mm, about 0.6mm-about 30mm, or about 0.8mm-about 20mm.If thickness is less than about 0.2mm, then foaming can not be effective, and this is due to the large gas loss from primary surface.If thickness is greater than about 50mm, then the expansion during the foaming step can become more and more difficult to control.Therefore, production has multilayer polyolefin foam / polyolefin cover sheet of uniform cross-sectional density and thickness and can be more and more difficult.In some embodiments, the thickness of the cover layer of the coextruded multilayer structure of foaming can be about 0.1-100 micron, about 1-80 micron, or about 5-60 micron.In some embodiments, the thickness of the foam layer of the coextruded multilayer structure of foaming can be about 0.5-5mm, about 1-4mm, or about 2-3mm.

[0096] 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.

[0097] The thickness of the cover layer decreases during foaming of the multilayer sheet. This is likely due to the expansion of the foamable layer and the resulting stretching of 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.

[0098] The disclosed multilayer foam structure can be used in a variety of applications. One such application is thermoformed articles. To thermoform the multilayer foam structure, the foam can be heated to the melting point of the polyolefin blend for all layers in the multilayer foam structure. If any layer has immiscible polymers, the multilayer foam structure can exhibit more than one melting point. In this case, the multilayer foam structure is typically thermoformed when the foam is heated to a temperature between the lowest melting point and the highest melting point of the multilayer foam composition. In addition, the multilayer foam structure can be thermoformed onto a substrate such as hard polypropylene, ABS, or a wood fiber composite. The substrate itself can also be thermoformed simultaneously with the multilayer foam structure. Preferably, the substrate can be applied to the foam layer of the multilayer foam structure.

[0099] An example of a thermoformed article is an automotive vent 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 cross-linked cover layer on the outside of the multi-layer vent duct protects the duct from punctures and cuts during installation and over the life of the vehicle. Therefore, a robust multi-layer foam structure is suitable for automotive vent ducts.

[0100] In some embodiments, the multilayer foam structure is a laminate containing a multilayer foam and a laminate layer. Preferably, the laminate layer can be applied to one side (i.e., the surface) of the cover layer of the multilayer foam. 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, cloth and other fabrics; leather and / or fiber layers such as nonwovens. Such layers can be manufactured using conventional techniques well known to those skilled in the art. Importantly, the multilayer foam of the present invention can be laminated with these materials on one or both sides and can contain multiple other layers.

[0101] 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.

[0102] In some embodiments, the foam / cover / laminate structure can be used as part of a vehicle instrument panel, wherein the foam / cover / laminate structure has the foam side adhered to a rigid board substrate, and wherein an airbag is mounted on the back side of the panel.

[0103] In other embodiments, the multilayer foam structures or laminates can be used in automotive interior parts such as door panels, door rollers, door inserts, door fillers, trunk fillers, armrests, center consoles, seat cushions, seat backs, headrests, seat back panels, knee bolsters, or headrests. These multilayer foam structures or laminates can also be used in furniture (e.g., commercial, office, and residential furniture) such as chair cushions, chair backs, sofa cushions, sofa trims, recliner cushions, recliner trims, couch cushions, couch trims, sleeper cushions, or sleeper trims. These multilayer foam laminates or structures can also be used in walls such as molded walls, removable walls, wall panels, molded panels, office system panels, room dividers, or portable partitions. The multilayer foam laminates or structures can also be used in storage boxes (e.g., commercial, office, and residential), which can be mobile or fixed. In addition, the multi-layer foam laminates and structures can also be used in coverings such as chair cushion covers, seat back covers, armrest covers, sofa covers, sofa cushion covers, recliner cushion covers, recliner covers, ottoman cushion covers, ottoman covers, sleeper cushion covers, berth covers, wall coverings and building coverings.

[0104] 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.

[0105] Example

[0106] Figure 1 A table of the raw materials used in the following examples is provided. Specifically, Figure 1 A table of the various components used in the following examples and descriptions of those components is provided. FIG. 2 provides a table of the formulations for Examples 1 and 2 and the coextrusion, irradiation, and other properties of the multilayer structures of Examples 1 and 2.

[0107] Figure 3 is an image of Example 1 at 30× magnification and 45° relative to the cover surface and 45° relative to the machine direction (“MD”). Figure 4 is an image of Example 2 at 30× magnification and 45° relative to the cover surface and 45° relative to the machine direction (“MD”).

[0108] 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 ranges, even if the specification does not verbatim state the exact range limitations, because the present invention can be practiced throughout the disclosed numerical ranges.

[0109] 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 method of forming a multilayer foam structure, comprising: Coextrusion: A foam layer comprising: at least one of polypropylene and polyethylene; Chemical foaming agents; cross-linking agent; and a film layer on one side of the foam layer, the film layer comprising: at least 90 wt% of at least one of polypropylene and polyethylene; and 1-1.5 wt% of a cross-linking agent; irradiating the coextruded layer with ionizing radiation; and foaming the irradiated coextruded layer, wherein the thickness of the film layer of the multi-layer foam structure is 15-40 microns, and The density of the multi-layer foam structure is 50-250 kg / m 3 .

2. The method according to claim 1, wherein the foam layer comprises polypropylene having a melt flow index at 230°C of 0.1 to 25 g / 10 min.

3. The method according to any one of claims 1-2, wherein the foam layer comprises polyethylene having a melt flow index at 190°C of 0.1-25 g / 10 min.

4. The method according to any one of claims 1 to 3, wherein the foam layer comprises 0.5 to 5 wt% of a cross-linking agent.

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

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

7. The method of any one of claims 1-6, wherein the foam layer comprises at least 75 wt% of at least one of polypropylene and polyethylene.

8. The method according to any one of claims 1 to 7, wherein the foam layer comprises 3 to 15 wt% of a chemical blowing agent.

9. The method of any one of claims 1 to 8, wherein the ionizing radiation is selected from alpha, beta (electrons), x-rays, gamma or neutrons.

10. The method according to any one of claims 1 to 9, wherein the coextruded structure is irradiated up to four separate times.

11. The method according to claim 9, wherein the ionizing radiation is an electron beam having an acceleration voltage of 200-1500 kV.

12. The method according to claim 11, wherein the absorbed electron dose is 10-500 kGy.

13. The method of any one of claims 1-12, wherein the ionizing radiation crosslinks the extruded structure to a degree of crosslinking of 20-75%.

14. The method of any one of claims 1 to 13, wherein foaming comprises heating the irradiated structure with a molten salt.

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

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