CN117903533A

CN117903533A - Warp-resistant foam and preparation method thereof

Info

Publication number: CN117903533A
Application number: CN202410075752.8A
Authority: CN
Inventors: 李沛; 魏琼
Original assignee: Hubei Xiangyuan New Material Technology Inc
Current assignee: Hubei Xiangyuan New Material Technology Inc
Priority date: 2024-01-18
Filing date: 2024-01-18
Publication date: 2024-04-19

Abstract

The invention belongs to the technical field of high polymer foaming materials, and particularly relates to warp-resistant foam and a preparation method thereof. The warp-resistant foam is formed by foaming polyolefin resin, comprises a surface layer and a foam hole layer, wherein the total thickness of the foam is more than or equal to 2mm, the thickness of the surface layer is less than 1/20 of the total thickness of the foam, after the foam is wound and unwound, the average foam pore diameter change rate in the winding direction of the foam is less than 10%, the warp height of the foam is less than 1mm, and the warp-resistant performance is excellent. Meanwhile, the warp-resistant foam prepared by the invention has 25% compression strength of 0.1-0.5 MPa and tensile strength of 2-5 MPa, and has buffer performance and certain structural strength. The warp-resistant foam can be directly wound and unwound to carry out subsequent back glue, die cutting and mounting operations, and is suitable for buffer materials or structural device adhesive tape base materials at different positions in new energy automobiles.

Description

Warp-resistant foam and preparation method thereof

Technical Field

The invention belongs to the technical field of high polymer foaming materials, and particularly relates to warp-resistant foam and a preparation method thereof.

Background

Along with the rapid development of new energy automobiles, the requirements for foam at different positions in the automobiles are increasingly increased, and meanwhile, the requirements for various properties of the foam are also higher. The production of common polyolefin foam mainly comprises the steps of cutting into foam with corresponding thickness after mould pressing foaming, or extruding, foaming and rolling to obtain the required foam. The foam is applied to the battery of the new energy automobile at present, and is applied to different structural parts and other positions, and besides certain buffering performance, certain strength is also required, so that the structural parts are prevented from being misplaced and other abnormal in the subsequent automobile use process. The foam applied to the places is mostly more than 2mm thick, coiled foam is used, the foam is easy to warp after unreeling, the subsequent back glue, die cutting and installation are inconvenient, the foam is required to be used after certain flatness is recovered through the subsequent heat treatment and other processes, and the rate cost is increased. Another proposal uses MPP microporous foaming material which is die-cut into sheet after mould pressing foaming, the surface of the foam is smoother, but the compliance to the uneven part is poorer, and the foam has higher cost. The competition among different new energy automobile manufacturers is increasingly strong, the demand for reducing the cost is urgent, therefore, the anti-warp coiled foam needs to be developed, the continuous die cutting production and installation are convenient to adapt, the efficiency is improved, and the cost is reduced.

Disclosure of Invention

Aiming at the defects of the prior art, the invention aims to provide the warp-resistant foam and the preparation method thereof, so as to solve the technical problems that the prior coiled foam with the thickness of more than 2mm, particularly the coiled foam with the thickness of 2-10 mm is easy to warp, is inconvenient for subsequent back glue, die cutting and installation, needs to reduce the warp height of the foam through procedures such as heat treatment and the like, and has high cost for preparing the warp-resistant foam.

In order to achieve the above object, the present invention provides a warp-resistant foam formed by foaming a polyolefin-based resin, comprising a skin layer and a cell layer; the total thickness of the foam is more than or equal to 2mm, the thickness of the surface layer of the foam is less than 1/20 of the total thickness of the foam, after the foam is wound and unwound, the average foam pore diameter change rate in the winding direction of the foam is less than 10%, and the warp height of the foam is less than 1mm.

Preferably, the total thickness of the foam is 2 mm-10 mm, and the density is 0.04g/cm ³～0.2g/cm³.

Preferably, the average foam pore size of the foam is 100-400 μm.

Preferably, the foam has a compressive strength of 0.1MPa to 0.5MPa at a compression deformation rate of 25%.

Preferably, the tensile strength of the foam is 2MPa to 5MPa.

Preferably, the foam comprises 60 to 80 parts by weight of polyolefin resin, 5 to 30 parts by weight of copolymer elastomer, 5 to 15 parts by weight of foaming agent and 1 to 5 parts by weight of auxiliary agent.

Preferably, the polyolefin resin includes at least one of polyethylene-based resin and polypropylene-based resin.

Preferably, the copolymer elastomer comprises an olefinic thermoplastic elastomer.

Further preferably, the olefinic thermoplastic elastomer includes at least one of an ethylene/a-olefin block copolymer elastomer and an ethylene/a-olefin random copolymer elastomer.

Preferably, the blowing agent comprises one or more of azodicarbonamide, OBSH blowing agent, N' -dinitroso pentamethylene tetramine, 4-oxo-bis-benzenesulfonyl hydrazide, inorganic carbonates and bicarbonates.

Preferably, the auxiliary agent includes one or more of an antioxidant, a heat stabilizer, a sensitizer, a dispersant, a flame retardant, a pigment, an antistatic agent, and a heat conductive particle.

The invention also provides a preparation method of the foam, which comprises the following steps:

s1, melting, mixing and banburying polyolefin resin, a copolymer elastomer, a foaming agent and an auxiliary agent according to a proportion to prepare resin master batches with uniform particles;

S2, extruding and molding the resin master batch to obtain a pre-foaming substrate;

S3, carrying out irradiation crosslinking treatment on the pre-foaming substrate to obtain a pre-foaming master;

s4, performing foaming treatment on the pre-foaming master slice to obtain a foaming sheet;

s5, progressively cooling the foaming sheet material by at least two sections of cooling rollers, and then rolling to obtain the warp-resistant foam material.

Preferably, in step S2, the temperature of the extrusion molding is lower than the decomposition temperature of the foaming agent.

Preferably, in step S3, the dose of irradiation is 1Mrad to 10Mrad.

Preferably, in step S4, the temperature of the foaming treatment is higher than the decomposition temperature of the foaming agent.

Preferably, in step S5, the temperature of the foamed sheet is reduced progressively by two cooling rolls, wherein the temperature of the first cooling roll is 30-60 ℃ lower than the foaming temperature, and the temperature of the second cooling roll is 80-120 ℃ lower than the foaming temperature.

The invention also provides a buffer material or a structural device adhesive tape base material suitable for different positions in the new energy automobile, which comprises the foam.

In general, the above technical solutions conceived by the present invention have the following beneficial effects compared with the prior art:

(1) The warp-resistant foam is formed by foaming polyolefin resin, comprises a surface layer and a foam hole layer, wherein the total thickness of the foam is more than or equal to 2mm, the thickness of the surface layer is less than 1/20 of the total thickness of the foam, after the foam is wound and unwound, the average foam pore diameter change rate in the winding direction of the foam is less than 10%, the warp height of the foam is less than 1mm, and the warp-resistant performance is excellent. Meanwhile, the warp-resistant foam prepared by the method has 25% compression strength of 0.1-0.5 MPa and tensile strength of 2-5 MPa, and has a certain structural strength while having a buffering performance.

(2) According to the preparation method of the warp-resistant foam, the formula and the process are adjusted to regulate and control the thickness, the density, the average pore diameter change, the tensile strength, the compressive strength, the warp height and the like of the surface layer of the foam, so that the prepared warp-resistant foam has excellent warp-resistant performance, buffering performance and structural strength. The warp-resistant foam can be directly received and unreeled for subsequent operations such as back adhesive, die cutting, mounting and the like, is suitable for continuous die cutting production and mounting, improves the efficiency, reduces the cost, and is suitable for buffer materials or structural device adhesive tape base materials at different positions in new energy automobiles. Compared with the prior art, the warp-resistant foam can be prepared by reducing the warp degree of the foam without the procedures of heating treatment and the like.

Drawings

FIG. 1 is an SEM image of warp-resistant foam prepared according to example 1 of the invention.

Detailed Description

The present invention will be described in further detail with reference to the following examples in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about". Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that may vary depending upon the desired properties to be obtained. The use of numerical ranges by endpoints includes all numbers subsumed within that range as well as any range within that range, e.g., 1 to 5 includes 1, 1.5, 2, 3.5, 4.6, 5, and the like.

The warp-resistant foam comprises a skin layer and a foam hole layer, wherein the skin layer is a resin layer with a certain thickness, the cross section of the foam extends inwards from the outer surface continuously, and the resin layer is a resin layer without foam holes or a resin layer with low foaming degree (the average foam pore diameter of the layer is less than 10% of the average foam pore diameter of the foam). The remainder of the warp-resistant foam described above is defined as the "cell layer". Specifically, when the cross-sectional morphology of the foam prepared in example 1 of the present invention was observed using SEM, it was found that the foam skin layer had a lower foaming layer (as shown in fig. 1), even achieving no cells.

The term "MD" refers to the machine direction (Machine Direction) which coincides with the direction of extrusion of foam.

When the thickness of the foam is smaller than 2mm, the phenomenon of warping generally does not occur when the foam is wound and unwound, when the thickness of the foam is larger than 2mm, the phenomenon of warping of the foam also occurs easily even if the technological parameters of winding and unwinding and rubberizing equipment are regulated, and the phenomenon of warping is caused because after the thickness of the foam is increased, the foam outside the foam is influenced by stretching force when the foam is wound and bent, and if the bending stretching force exceeds the yield force of resin, the foam cannot recover better, and permanent stretching deformation of the foam can be generated to a certain extent, so that the phenomenon of warping is caused. Aiming at the problems, the invention provides the warp-resistant foam, the total thickness of the foam is 2-10 mm, the thickness of the surface layer is smaller than 1/20 of the total thickness of the foam, the foam is tightly wound on a 6-inch tube core, the foam is unwound after standing for 7 days under standard laboratory conditions, the average cell diameter change rate of the foam in the MD direction (along the winding direction of the foam) is less than 10%, the warp heights are less than 1mm, and the warp-resistant performance is excellent. Meanwhile, the warp-resistant foam prepared by the method has 25% compression strength of 0.1-0.5 MPa and tensile strength of 2-5 MPa, and has a certain structural strength while having a buffering performance. The warp-resistant foam can be directly received and unreeled for subsequent operations such as back adhesive, die cutting, mounting and the like, is suitable for continuous die cutting production and mounting, improves the efficiency, reduces the cost, and is suitable for buffer materials or structural device adhesive tape base materials at different positions in new energy automobiles.

Specifically, the warp-resistant foam provided by the invention is formed by foaming polyolefin resin and comprises a skin layer and a cell layer; the total thickness of the foam is more than or equal to 2mm, the thickness of the surface layer is less than 1/20 of the total thickness of the foam, after the foam is wound and unwound, the average foam pore diameter change rate in the winding direction of the foam is less than 10%, and the warp height of the foam is less than 1mm. This is because, when the thickness of the foam skin layer is greater than 1/20 of the total foam thickness, foam foaming is incomplete, the difference between the foam skin layer thickness and the foam cell layer thickness is too large, when the foam is bent and rolled and placed, the plastic deformation accumulated by creep of the rolled outer foam skin resin is more than that of the inner cells and the rolled inner foam, and after unreeling, obvious warping is generated due to different plastic deformation of the inner side and the outer side of the foam. When the foam is rolled and bent, the foam cells in the MD direction at the outer side of the foam rolling can generate tensile strain, the foam cells in the MD direction at the inner side of the foam rolling can generate compressive strain, and when the average cell diameter change rate of the foam cells in the MD direction after unreeling is greater than 10%, the difference between the cell diameters at the outer side and the inner side of the foam rolling is overlarge, so that the foam is warped.

In some embodiments, the foam has a total thickness of 2mm to 10mm and a density of 0.04g/cm ³～0.2g/cm³. The foam prepared by the invention is mainly used for solving the technical problem that the foam with the thickness of 2 mm-10 mm is easy to warp when being applied to a new energy automobile. When the foam is applied to different positions of a new energy automobile, the foam needs to have good damping performance and good strength, so that a part of parts can be supported. When the density of the foam is lower than 0.04g/cm ³, the foam has larger foaming multiplying power and lower strength, is insufficient to play a supporting role in the application of the new energy battery part, and when the density is higher than 0.2g/cm ³, the foam has lower foaming multiplying power and lower damping performance.

In some embodiments, the foam has an average foam pore size of 100 μm to 400 μm. When the average foam pore diameter of the foam is smaller than 100 mu m, for thicker foam, the larger the number of foam pore layers in the thickness direction is, the larger the size change rate of the foam pores generated by stretching and compressing the outside and the inside of the outside foam pores is, the easier the foam pore wall resin reaches the stretching/compression yield strain, so that the foam pore recovery performance of the foam after unreeling is reduced, and the warping height is increased; when the average foam pore diameter of the foam is larger than 400 mu m, the foam pore strength of the foam is reduced, the strength requirement of application requirements is not easily met, and the application requirements of the foam used as buffer materials or structural device adhesive tape base materials at different positions in new energy automobiles are not met.

In some embodiments, the foam has a compressive strength of 0.1MPa to 0.5MPa at a compression deformation rate of 25% and a tensile strength of 2MPa to 5MPa. When the compressive strength of the foam at the compression deformation rate of 25% is less than 0.1MPa or the tensile strength is less than 2MPa, the foam is too soft and cannot play a supporting role. When the compression strength of the foam at the compression deformation rate of 25% is greater than 0.5MPa or the tensile strength of the foam is greater than 5MPa, the foam strength is increased, but the buffer performance is reduced, and the compression/tensile yield strain of the foam is reduced, so that after the foam is unreeled, the foam hole recovery performance is reduced, the deformation difference of the foam at the inner side and the outer side is large, and the warping height is increased.

In some embodiments, the foam includes 60 to 80 parts by weight of a polyolefin resin, 5 to 30 parts by weight of a copolymer elastomer, 5 to 15 parts by weight of a foaming agent, and 1 to 5 auxiliary agents.

In a preferred embodiment, the polyolefin resin includes at least one of polyethylene-based resin and polypropylene-based resin. The present invention is not limited to the specific type of the polyethylene resin or the polypropylene resin, and any polyethylene resin or polypropylene resin reported in the prior art may be used. In the present invention, the polyethylene-based resin includes, but is not limited to, one or more of LDPE, HDPE, MDPE, LLDPE, VLDPE; the polypropylene resin includes, but is not limited to, one or more of homo-polypropylene (PP-H), block-copolymerized polypropylene (PP-B) and random (random) copolymerized polypropylene (PP-R).

In a more preferred embodiment, the polyolefin resin has a melt index of 1.0g/10min to 3.0g/10min. Experiments show that when the melt index of the polyolefin resin is lower than 1.0g/10min, the processing difficulty is high and the foam is difficult to extrude when the pre-foaming substrate is prepared, so that large cells, even broken cells and the like easily appear in the prepared foam; when the melt index of the polyolefin resin is higher than 3.0g/10min, the prepared foam is too soft to achieve the above-mentioned required compressive strength and tensile strength and cannot play a supporting role.

In the foam, when the weight part of the copolymer elastomer is less than 5, the optimization performance of the copolymer elastomer on the resin system is not obvious, and when the weight part of the copolymer elastomer is more than 30, on one hand, the cost is increased, on the other hand, the molecular entanglement effect of the resin system is excessively remarkable, the elongation at break is increased very rapidly, the size of the foam holes is increased obviously in the foaming process, the strength of the foam is reduced, and the strength requirement of application requirements cannot be met.

In some embodiments, the copolymer elastomer comprises an olefinic thermoplastic elastomer; in a preferred embodiment, the olefinic thermoplastic elastomer comprises at least one of an ethylene/a-olefin block copolymer elastomer (OBC) and an ethylene/a-olefin random copolymer elastomer (POE). The ethylene/a-olefin block copolymer elastomer is a block copolymer elastomer formed by polymerizing ethylene and a-olefin. The above-mentioned ethylene/a-olefin random copolymer elastomer means a random copolymer elastomer formed by polymerizing ethylene and a-olefin.

The elongation at break of the copolymer elastomer is related to the glass transition temperature, and the lower the glass transition temperature is, the larger the elongation at break of the copolymer elastomer is, so that the toughness of the polyolefin resin can be improved, and the recovery performance of the resin on the inner side and the outer side after the foam is wound and unwound can be improved. In a specific embodiment of the present invention, the copolymer elastomer has a glass transition temperature (Tg) of-70 ℃ to-40 ℃ and a molecular weight PDI of 1.5 to 3.

The copolymer elastomer has no unsaturated double bond in the molecular structure, a physical cross-linking point structure is formed between the ethylene chain segments partially crystallized in the POE chain segment and a special hard segment-soft segment block structure in the OBC chain segment, so that the copolymer elastomer has very narrow molecular weight distribution and short branched chain structure (even short branched chain distribution), and the soft chain of octene is curled and crystallized ethylene chain is used as the physical cross-linking point, thus providing excellent physical and mechanical properties such as high elasticity, high strength, high elongation and the like. The narrow molecular weight distribution makes the resin material not easy to flex in the winding process after extrusion and foaming, the molecular weight distribution of the copolymer elastomer is narrow, the side octyl in the molecular structure is longer than the side ethyl, and a junction point can be formed in the molecular structure, so that the components can be connected and buffered, the impact energy can be dispersed when the system is impacted, and the buffer performance of foam can be improved. When the foam winding resin system is under tension, the network-shaped structure formed by the bonding points can be greatly deformed, so that the yield and elongation of the resin are increased, and after the foam is unreeled, the recovery performance of the foam holes is increased, and the warping is reduced. In an embodiment of the present invention, the copolymer elastomer has a molecular weight PDI of 1.5 to 3.

The foaming agent is a substance added to form cells of the resin material, and can release gases such as carbon dioxide, nitrogen and the like after being heated and decomposed, and the cells are formed in the polymer composition, or the foaming agent forms the cells in the polymer composition through the physical form change of a certain substance, namely, the expansion of compressed gas, the volatilization of liquid or the dissolution of solid. The type of the foaming agent is not particularly limited, but a foaming agent having a low cost and a good foaming effect is preferable. Such as, but not limited to, one or more of azodicarbonamide, OBSH blowing agent, N' -dinitroso pentamethylene tetramine, 4-oxo-bis-benzenesulfonyl hydrazide, inorganic carbonates and bicarbonates. In the foam provided by the invention, the weight part of the foaming agent is 5-15, when the weight part of the foaming agent is less than 5, the foam cell density is smaller, the foam density is increased, the foam strength is increased, the buffering performance is reduced, the buffering requirement in application is not met, the foam cell is subjected to the increase of the external tensile stress and the internal compressive stress caused by bending in a rolling state, the tensile/compressive yield strain of the foam cell wall resin is easier, the foam cell recovery performance of the foam cell after unreeling is reduced, and the warping height is increased. When the weight part of the foaming agent is more than 15, the foam density is larger, the foam density is reduced, the foam strength is reduced, and the strength requirement of application requirements is not easy to meet.

In order to meet the requirements of different application environments, the foam prepared by the invention also comprises 1 to 5 parts by weight of auxiliary agents so as to endow the foam with various properties. Such as, but not limited to, one or more of antioxidants, heat stabilizers, sensitizers, dispersants, flame retardants, pigments, antistatic agents, and thermally conductive particles. Specifically, the antioxidant has the effects of improving the oxidation resistance of the foam or the foam sheet in the processing process and the using process and prolonging the service life; the dispersing agent can promote the mixing uniformity of the raw materials, the flame retardant can improve the flame retardant property of the foaming sheet, and the antistatic agent can increase the antistatic property of the foaming sheet.

In some embodiments, in step S2, the temperature of the extrusion molding is lower than the decomposition temperature of the foaming agent, so as to avoid premature decomposition of the foaming agent, thereby affecting the performance of the foam. In the specific embodiment of the invention, the temperature of the extrusion molding is 140-180 ℃. In practical applications, it is also within the scope of the present invention to adaptively increase or decrease the extrusion molding temperature depending on the specific type of blowing agent selected.

In some embodiments, step S3 is specifically: and irradiating the pre-foaming substrate by high-speed electron beams to ensure that free radicals generated by the resin chain segments are crosslinked through chain segment recombination, so that the pre-foaming substrate has foamability, and the pre-foaming master slice is obtained. In actual production, a person skilled in the art may select an appropriate irradiation dose to perform the irradiation crosslinking treatment according to the kind of the selected polyolefin resin, the kind and property of the copolymer elasticity, the weight fraction of the foaming agent, etc., thereby preparing the above warp-resistant foam. In the specific embodiment of the invention, the irradiation dose is 1 Mrad-10 Mrad. When the irradiation dose is too small, the crosslinking degree of the resin is insufficient, the size of the foam holes is larger in the foaming process of the foam, and the foaming strength is reduced; when the irradiation dose is too large, the resin crosslinking degree is too large, the foam hole size of the foam is smaller, the tensile/compressive yield strain of the foam hole wall resin is easier to reach, the recovery performance of the foam holes after unreeling of the foam is reduced, and the warping height of the foam is increased.

In some embodiments, in step S4, the foaming temperature is higher than the decomposition temperature of the foaming agent, so that the foaming agent in the pre-foamed master sheet is fully foamed to form a polyolefin foam with an independent closed pore structure, thereby obtaining a foamed sheet. In some embodiments, the foaming process is performed at a temperature of 180℃to 280 ℃. It is understood that the temperature of the foaming process is adaptively adjusted according to the specific type, weight portion and the like of the selected foaming agent, and the foaming process is also within the protection scope of the invention.

In some embodiments, in step S5, the above-mentioned foamed sheet is cooled progressively by two sections of cooling rollers, so as to make the decomposition of the foaming agent in the foamed sheet more complete, and make the surface layer of the foamed cotton thin, so that the foamed cotton with thinner surface layer thickness is obtained, and the warping height of the foamed cotton after winding is reduced. In some embodiments, the temperature of the first cooling roller is 30-60 ℃ lower than the foaming temperature, and the temperature of the second cooling roller is 80-120 ℃ lower than the foaming temperature.

The following describes the above technical scheme in detail with reference to specific embodiments.

In the embodiment of the invention, all the reagents are commercial products unless specified otherwise.

Example 1

(1) Granulating: polyolefin resin (CO-polypropylene CO-PP, brand J340, melt index 1.8g/10 min), olefin block copolymer OBC elastomer (glass transition temperature Tg is-65 ℃, copolymer elastomer molecular weight dispersity PDI is 2.41), foaming agent azodicarbonamide and antioxidant 1010 are mixed and banburying according to the weight ratio of 75:15:8:2, the banburying bin temperature is 140 ℃, the pressure is 2MPa, the banburying time is 15min, the banburying agglomerated resin is added into a hopper of a double screw, the processing temperature in the barrel of the screw is 135 ℃, and the resin master batch with uniform particles is obtained through the processes of extrusion, bracing, granulating and drying.

(2) Extruding: extruding the resin master batch obtained in the step (1) into sheets by using a double-screw extruder, and extruding the sheets by adopting a calendaring cooling mode to obtain the pre-foaming substrate.

(3) And (3) irradiation: and (3) irradiating the pre-foaming substrate obtained in the step (2) by high-speed electron beams, and performing electron irradiation crosslinking, wherein the irradiation dose is 4Mrad, so as to obtain the pre-foaming master slice.

(4) Foaming: and (3) freely foaming the pre-foaming master slice obtained in the step (3) through a vertical foaming furnace, wherein the foaming temperature is 245 ℃.

(5) And (3) cooling and rolling: and (3) cooling the foam obtained in the step (4) by two sections of cooling rollers, and then rolling to obtain the warp-resistant foam P1. Wherein the temperature of the first section of cooling roller is 190 ℃, and the temperature of the second section of cooling roller is 140 ℃.

Example 2

(1) Granulating: polyolefin resin (medium density polyethylene MDPE, melt index 2.9g/10 min), olefin block copolymer OBC elastomer (same as in example 1), foaming agent azodicarbonamide and antioxidant 1010 in the weight ratio of 73:7:10:2 are mixed and banked, the banked temperature is 140 ℃, the pressure is 2MPa, the banked time is 15min, the banked resin is added into a hopper of a double screw, the processing temperature in the screw barrel is 135 ℃, and the resin master batch with uniform particles is obtained through the steps of extrusion, bracing, granulating and drying.

(3) And (3) irradiation: and (3) irradiating the pre-foaming substrate obtained in the step (2) by high-speed electron beams, and performing electron irradiation crosslinking, wherein the irradiation dose is 7Mrad, so as to obtain the pre-foaming master slice.

(4) Foaming: and (3) freely foaming the pre-foaming master slice obtained in the step (3) through a vertical foaming furnace, wherein the foaming temperature is 235 ℃.

(5) And (3) cooling and rolling: and (3) cooling the foam obtained in the step (4) by two sections of cooling rollers, and then rolling to obtain the warp-resistant foam P1. Wherein the temperature of the first section of cooling roller is 200 ℃, and the temperature of the second section of cooling roller is 160 ℃.

Example 3

(1) Granulating: polyolefin resin (same as in example 2), an ethylene/a-olefin random copolymer POE elastomer (glass transition temperature Tg is-43 ℃, molecular weight dispersity PDI of the copolymer elastomer is 1.82), an foaming agent azodicarbonamide and an antioxidant 1010 are mixed and banburying according to the weight ratio of 61:27:11:1, the banburying bin temperature is 140 ℃, the pressure is 2MPa, the banburying time is 15min, the banburying agglomerated resin is added into a hopper of a double screw, the processing temperature in the barrel of the screw is 135 ℃, and the resin master batch with uniform particles is obtained through the steps of extrusion, bracing, granulating and drying.

(3) And (3) irradiation: and (3) irradiating the pre-foaming substrate obtained in the step (2) by high-speed electron beams, and performing electron irradiation crosslinking, wherein the irradiation dose is 8Mrad, so as to obtain a pre-foaming master slice.

(5) And (3) cooling and rolling: and (3) cooling the foam obtained in the step (4) by two sections of cooling rollers, and then rolling to obtain the warp-resistant foam P1. Wherein the temperature of the first section of cooling roller is 180 ℃, and the temperature of the second section of cooling roller is 130 ℃.

Example 4

(1) Granulating: the copolymer elastomer was a mixture of an Olefin Block Copolymer (OBC) elastomer (same as example 1) and an ethylene/a-olefin random copolymer (POE) elastomer (same as example 3), the mass ratio of OBC elastomer to POE elastomer being 2:1. Polyolefin resin (same as in example 1), copolymer elastomer, foaming agent azodicarbonamide and antioxidant 1010 in the weight ratio of 63:20:15:2 are mixed and banked, the banked temperature is 140 ℃, the pressure is 2MPa, the banked time is 15min, the banked resin is added into a hopper of a double screw, the processing temperature in the screw barrel is 135 ℃, and the resin master batch with uniform particles is obtained through the processes of extrusion, bracing, granulating and drying.

(5) And (3) cooling and rolling: and (3) cooling the foam obtained in the step (4) by two sections of cooling rollers, and then rolling to obtain the warp-resistant foam P1. Wherein the temperature of the first section of cooling roller is 190 ℃, and the temperature of the second section of cooling roller is 130 ℃.

Example 5

(1) Granulating: the copolymer elastomer is ethylene/a-olefin random copolymer POE elastomer (the glass transition temperature Tg is-54 ℃, the molecular weight dispersity PDI of the copolymer elastomer is 1.65), and the foaming agent is OBSH foaming agent. Polyolefin resin (same as in example 2), copolymer elastomer, foaming agent OBSH and antioxidant 1010 are mixed and banburying according to the weight ratio of 80:5:13:2, wherein the banburying bin temperature is 140 ℃, the pressure is 2MPa, the banburying time is 15min, banburying agglomerated resin is added into a hopper of a double screw, the processing temperature in a screw machine barrel is 135 ℃, and the resin master batch with uniform particles is obtained through the processes of extrusion, bracing, granulating and drying.

(3) And (3) irradiation: and (3) irradiating the pre-foaming substrate obtained in the step (2) by high-speed electron beams, and performing electron irradiation crosslinking, wherein the irradiation dose is 10Mrad, so as to obtain the pre-foaming master slice.

(4) Foaming: and (3) freely foaming the pre-foaming master slice obtained in the step (3) through a vertical foaming furnace, wherein the foaming temperature is 180 ℃.

(5) And (3) cooling and rolling: and (3) cooling the foam obtained in the step (4) by two sections of cooling rollers, and then rolling to obtain the warp-resistant foam P1. Wherein the temperature of the first section of cooling roller is 135 ℃, and the temperature of the second section of cooling roller is 100 ℃.

The prepared warp-resistant foam P5 was subjected to performance test, and the results are shown in Table 1.

Comparative example 1

The polyolefin resin of example 1, azodicarbonamide as a blowing agent, and antioxidant 1010 were mixed and banburying at a weight ratio of 90:8:2, without adding the copolymer elastomer, at a dose of 11Mrad, and other conditions or parameters were consistent with example 1. The prepared foam D1 was subjected to performance test, and the results are shown in Table 1.

Comparative example 2

The polyolefin resin, the olefin block copolymer OBC elastomer, the blowing agent azodicarbonamide and the antioxidant 1010 of example 1 were mixed and banburying at a weight ratio of 58:32:8:2, with a radiation dose of 11Mrad, and other conditions or parameters consistent with example 1. The prepared foam D2 was subjected to performance test, and the results are shown in Table 1.

Comparative example 3

The copolymer elastomer in example 1 was replaced with an ethylene/a-olefin random copolymer POE elastomer (glass transition temperature Tg of-38 ℃ C., copolymer elastomer molecular weight dispersity PDI of 3.11), then a polyolefin resin, a copolymer elastomer, a blowing agent azodicarbonamide, and an antioxidant 1010 were mixed and banned in a weight ratio of 63:21:15:1, the irradiation dose was 11Mrad, the foaming temperature was 235 ℃ C., the first-stage chill roll temperature 160 ℃ C., the second-stage chill roll temperature 100 ℃ C., and other conditions or parameters were consistent with those in example 1. The prepared foam D3 was subjected to performance test, and the results are shown in Table 1.

Comparative example 4

The irradiation dose in example 1 was set to 1.5Mrad, and a length of chill roll was used for the cool-down winding step, the chill roll temperature being 120 ℃, and other conditions or parameters were consistent with example 1. The prepared foam D4 was subjected to performance test, and the results are shown in Table 1.

Comparative example 5

The copolymer elastomer of example 2 was replaced with a metallocene ethylene propylene diene monomer (epdm) having a glass transition temperature Tg of-43 ℃ and a copolymer elastomer molecular weight dispersion PDI of 2.51, with other conditions or parameters consistent with example 2. The performance of the foam D5 obtained was tested and the results are shown in Table 1.

The performance test methods of the foam prepared in examples and comparative examples are as follows.

(1) And (3) testing the thickness of the foam epidermis: when the cross-sectional morphology of the foam is tested by using SEM, the foaming effect of the foam skin layer is low, and then the foam-free resin layer or the foaming strength is low, and the thickness of the skin layer is measured by using SEM marks.

(2) And (3) testing the thickness of foam: foam thickness was measured based on ASTM D3574 standard.

(3) Foam density testing: a10 cm by 10cm standard sample of the foamed sheet was selected, the thickness was measured by the method described in ASTM D3574, and the mass was weighed to calculate the density.

(4) Foam MD average foam pore size test: in a standard laboratory environment, for each direction in which the pore size of the bubbles is to be determined, the sample is cut with a microtome, the thickness of which should be less than the diameter of a single cell. The sample is inserted into a slide with scales, then projected onto a screen by a projector, and the focal length is adjusted to make the image of the sample imaged clearly on a wall or a screen. Determining the number of cells or pore walls within the length range of 30mm of the scale, and dividing the linear length by the number of cells to obtain an average cell diameter; if the test piece length is less than 30mm, the number of cells is determined at the maximum length.

(5) Winding and unwinding experiments of foam: cutting foam into a size of 20cm multiplied by 30cm, tightly winding and attaching the foam on a 6-inch tube core along the winding direction of the foam, fixing the winding edge position of the foam by using adhesive tapes, keeping the winding state of the foam, standing for 7 days under standard laboratory conditions, tearing off the adhesive tapes, taking down a sample, and placing the foam on a plane table board.

(6) Testing the average foam aperture change rate of foam: the foam is subjected to winding and unwinding experiments, then the average cell sizes in the MD direction before and after winding and unwinding of the foam are respectively measured according to the method (4), and the average foam pore diameter change rate is calculated to be = [ (average foam pore diameter in the MD direction after winding and unwinding of the foam, average foam pore diameter in the MD direction before winding/average foam pore diameter in the MD direction before winding of the foam ] ×100%.

(7) 25% Compressive stress of foam: and testing by adopting a universal testing machine, under the environment of a standard laboratory, the specification of a sample is 50mm multiplied by 50mm, the height is overlapped to 10mm, the sample is placed on a pressure plate, the height of the upper pressure plate is regulated until the upper surface of the sample is just contacted, the compression test is started after the displacement stress is cleared, the compression speed is 5mm/min, and the stress generated during 25% compression deformation is recorded.

(8) Foam tensile strength: the electronic tensile machine test was used, as determined based on the method described in ASTM D3574.

(9) And (3) testing the warping height of foam: testing the warping height of foam after winding and unwinding experiments, putting the foam on a horizontal plane, detecting the initial height d ₀ by using a Sanfeng 547-301 thickness gauge, and measuring the equipment precision: less than or equal to 0.01mm; then, carrying out winding and unwinding experiments on the foam according to the method (5), horizontally placing the foam on a horizontal plane, and detecting the height d _max from the highest point of the foam sample to the horizontal plane; foam warp height = d _max-d₀.

Table 1 results of performance test of foam obtained in examples and comparative examples

As can be seen from Table 1, the total thickness of the warp-resistant foam P1-P5 prepared in examples 1-5 of the invention is 2 mm-10 mm, the thickness of the foam surface layer is less than 1/20 of the total thickness of the foam, the foam P1-P5 is tightly wound on a 6-inch tube core, the foam is unwound after standing for 7 days under standard laboratory conditions, the average cell diameter change rate of the foam in MD direction (along the winding direction of the foam) is less than 10%, the warp height is less than 1mm, and the warp-resistant performance is excellent. When the total thickness of the foam is 2 mm-10 mm, the thickness of the foam epidermis is greater than 1/20 of the total thickness of the foam, the average foam aperture change rate in the MD direction of the foam (along the rolling direction of the foam) is more than 10%, the warping height is more than 1.4mm, and the foam is seriously warped. The analysis reasons may be that when the thickness of the foam is greater than 2mm, the foam outside the foam is affected by the tensile force and is easy to warp when the foam is wound and bent. At this time, if the thickness of the foam skin layer is greater than 1/20 of the total thickness of the foam, the foam foaming is incomplete, the difference between the thickness of the foam skin layer and the thickness of the foam cell layer is too large, when the foam is bent and rolled and placed, the deformation of the rolled outer foam skin resin due to creep accumulated is more than that of the inner cell and the rolled inner foam, and after unreeling, obvious warping is generated due to different plastic deformation of the inner side and the outer side of the foam. Specifically, when the foam is bent in a winding manner, tensile strain can be generated in the foam winding outer side MD direction cells, compressive strain can be generated in the foam winding inner side MD direction cells, and when the change rate of the foam diameter in the foam winding MD direction after unreeling is greater than 10%, the pore diameter difference between the foam winding outer side and the foam inner side is too large, so that the foam is warped.

As can be seen from the comparison of examples 1 and comparative examples 1 to 4, the thickness of the skin layers of the foam D1 to D4 obtained in comparative examples 1 to 4 is greater than 1/20 of the total thickness of the foam, the average foam pore diameter change rate in the MD direction (along the winding direction of the foam) of the foam is more than 10%, the warp height is more than 2mm, the warp height is obvious, and the warp resistance is poor. The analytical reasons may be that, compared with example 1, the comparative example 1 was free from the addition of the olefin block copolymer OBC elastomer, the resin strength was high, the skin layer of the foam product D1 was thick, the outer skin was affected by stretching when the foam was wound, the recovery performance of the skin layer after unwinding was poor, and the warp height was obvious. The addition amount of the olefin block copolymer OBC elastomer in comparative example 2 was excessive (32 parts by weight), the increase in cell size during foaming was remarkable, and even if the irradiation dose was increased, the cell size of the foam product D2 was still remarkably larger than that in example 1, the foam strength was lowered, too soft and easy to deform, and foam was easy to warp. The copolymer elastomer in comparative example 3 has a slightly higher glass transition temperature, a larger molecular weight dispersity, limited optimizing performance of the whole resin, and a larger change rate of cell size in MD direction of the foam finished product D3 after a winding and unwinding test, and is easy to generate warping. In comparative example 4, the foam is directly cooled and rolled after foaming, the surface layer of the foam finished product D4 is thicker, when the foam is rolled, the outer surface is affected by stretching, the recovery performance of the surface layer after unreeling is poor, and the warping height is obvious; in the embodiment 1, after foaming, the foam is cooled by using two sections of cooling rollers in a progressive manner, so that the foaming agent in the foam is decomposed more completely, the thickness of the surface layer of the obtained foam is lower, and the warping height of the foam after winding is reduced.

As can be seen from example 2 and comparative example 5, in comparative example 5, the metallocene epdm elastomer was used to modify the polyolefin resin, and the obtained foam product D5 had a higher 25% compressive strength and tensile strength, but the average foam pore diameter change rate in the MD direction and the foam warp height were both greater than those of example 1, which means that the foam obtained in comparative example 5 had a worse warp resistance than that of example 1, and the analysis reason probably was that the blending modification performance optimizing effect of the metallocene epdm elastomer and the polyolefin resin was worse than that of the olefin copolymer elastomer.

In conclusion, the foam with the total thickness of 2-10 mm and excellent warp resistance can be prepared by adjusting the formula and the process, the average cell diameter change rate of the foam in the MD direction (along the winding direction of the foam) is less than 10%, and the warp height is less than 1mm. Meanwhile, the warp-resistant foam prepared by the method has 25% compression strength of 0.1-0.5 MPa and tensile strength of 2-5 MPa, and has a certain structural strength while having a buffering performance. Compared with the prior art, the warp-resistant foam can be prepared by reducing the warp degree of the foam without the procedures of heating treatment and the like. In addition, the warp-resistant foam can be directly received and unreeled for subsequent operations such as back adhesive, die cutting, mounting and the like, is suitable for continuous die cutting production and mounting, improves the efficiency, reduces the cost, and is suitable for buffer materials or structural device adhesive tape base materials at different positions in new energy automobiles.

It will be readily appreciated by those skilled in the art that the foregoing description is merely a preferred embodiment of the invention and is not intended to limit the invention, but any modifications, equivalents, improvements or alternatives falling within the spirit and principles of the invention are intended to be included within the scope of the invention.

Claims

1. The warp-resistant foam is characterized by being formed by foaming polyolefin resin and comprising a skin layer and a cell layer; the total thickness of the foam is more than or equal to 2mm, the thickness of the surface layer is less than 1/20 of the total thickness of the foam, after the foam is wound and unwound, the average foam pore diameter change rate in the winding direction of the foam is less than 10%, and the warp height of the foam is less than 1mm.

2. The foam according to claim 1, wherein the foam has a total thickness of 2mm to 10mm and a density of 0.04g/cm ³～0.2g/cm³.

3. The foam according to claim 1, wherein the foam has an average foam pore size of 100 μm to 400 μm.

4. The foam of claim 1, wherein the foam has a compressive strength of 0.1MPa to 0.5MPa at 25% compression set.

5. The foam according to claim 1, wherein the foam has a tensile strength of 2MPa to 5MPa.

6. The foam according to any one of claims 1 to 5, comprising 60 to 80 parts by weight of polyolefin resin, 5 to 30 parts by weight of copolymer elastomer, 5 to 15 parts by weight of foaming agent and 1 to 5 parts by weight of auxiliary agent.

7. The foam of claim 6, wherein the polyolefin resin comprises at least one of a polyethylene-based resin and a polypropylene-based resin;

the copolymer elastomer includes an olefinic thermoplastic elastomer; preferably, the olefinic thermoplastic elastomer includes at least one of an ethylene/a-olefin block copolymer elastomer and an ethylene/a-olefin random copolymer elastomer;

The foaming agent comprises one or more of azodicarbonamide, OBSH foaming agent, N' -dinitroso pentamethylene tetramine, 4-oxo-bis-benzenesulfonyl hydrazide, inorganic carbonate and bicarbonate;

the auxiliary agent comprises one or more of an antioxidant, a heat stabilizer, a sensitizer, a dispersing agent, a flame retardant, a pigment, an antistatic agent and heat conducting particles.

8. The method for producing foam according to any one of claims 1 to 7, comprising the steps of:

9. The method according to claim 8, wherein in step S2, the extrusion molding temperature is lower than the decomposition temperature of the foaming agent;

in the step S3, the irradiation dose is 1 Mrad-10 Mrad;

In step S4, the temperature of the foaming treatment is higher than the decomposition temperature of the foaming agent;

in the step S5, the foaming sheet material is progressively cooled through two sections of cooling rollers, wherein the temperature of a first section of cooling roller is 30-60 ℃ lower than the foaming temperature, and the temperature of a second section of cooling roller is 80-120 ℃ lower than the foaming temperature.

10. A cushioning material or structural device adhesive tape substrate suitable for use in a new energy vehicle at different locations, comprising the foam of any one of claims 1 to 7.