CN115083290B - Polyolefin resin foamed sheet, adhesive tape, display module, and electronic product - Google Patents

Polyolefin resin foamed sheet, adhesive tape, display module, and electronic product Download PDF

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Publication number
CN115083290B
CN115083290B CN202210851073.6A CN202210851073A CN115083290B CN 115083290 B CN115083290 B CN 115083290B CN 202210851073 A CN202210851073 A CN 202210851073A CN 115083290 B CN115083290 B CN 115083290B
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polyolefin resin
foamed sheet
foaming
sample
polyolefin
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CN115083290A (en
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魏琼
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Hubei Sunriver High Tech Co ltd
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Hubei Sunriver High Tech Co ltd
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Priority to PCT/CN2023/100613 priority patent/WO2024016912A1/en
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    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09J—ADHESIVES; NON-MECHANICAL ASPECTS OF ADHESIVE PROCESSES IN GENERAL; ADHESIVE PROCESSES NOT PROVIDED FOR ELSEWHERE; USE OF MATERIALS AS ADHESIVES
    • C09J7/00—Adhesives in the form of films or foils
    • C09J7/20—Adhesives in the form of films or foils characterised by their carriers
    • C09J7/22—Plastics; Metallised plastics
    • C09J7/26—Porous or cellular plastics
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/0061—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof characterized by the use of several polymeric components
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/04—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent
    • C08J9/06—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent by a chemical blowing agent
    • C08J9/10—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof using blowing gases generated by a previously added blowing agent by a chemical blowing agent developing nitrogen, the blowing agent being a compound containing a nitrogen-to-nitrogen bond
    • C08J9/102—Azo-compounds
    • C08J9/103—Azodicarbonamide
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09F—DISPLAYING; ADVERTISING; SIGNS; LABELS OR NAME-PLATES; SEALS
    • G09F9/00—Indicating arrangements for variable information in which the information is built-up on a support by selection or combination of individual elements
    • G09F9/30—Indicating arrangements for variable information in which the information is built-up on a support by selection or combination of individual elements in which the desired character or characters are formed by combining individual elements
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2203/00—Foams characterized by the expanding agent
    • C08J2203/04—N2 releasing, ex azodicarbonamide or nitroso compound
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2323/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
    • C08J2323/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment
    • C08J2323/04—Homopolymers or copolymers of ethene
    • C08J2323/06—Polyethene
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2423/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
    • C08J2423/02—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers not modified by chemical after treatment
    • C08J2423/04—Homopolymers or copolymers of ethene
    • C08J2423/08—Copolymers of ethene
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09J—ADHESIVES; NON-MECHANICAL ASPECTS OF ADHESIVE PROCESSES IN GENERAL; ADHESIVE PROCESSES NOT PROVIDED FOR ELSEWHERE; USE OF MATERIALS AS ADHESIVES
    • C09J2203/00—Applications of adhesives in processes or use of adhesives in the form of films or foils
    • C09J2203/318—Applications of adhesives in processes or use of adhesives in the form of films or foils for the production of liquid crystal displays
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09J—ADHESIVES; NON-MECHANICAL ASPECTS OF ADHESIVE PROCESSES IN GENERAL; ADHESIVE PROCESSES NOT PROVIDED FOR ELSEWHERE; USE OF MATERIALS AS ADHESIVES
    • C09J2203/00—Applications of adhesives in processes or use of adhesives in the form of films or foils
    • C09J2203/326—Applications of adhesives in processes or use of adhesives in the form of films or foils for bonding electronic components such as wafers, chips or semiconductors

Abstract

The invention provides a polyolefin resin foamed sheet, an adhesive tape, a display module and an electronic product. The display assembly comprises a display panel, a middle frame and a rear cover, wherein the middle frame is used for supporting the display panel, the rear cover is buckled on the middle frame, a buffering waterproof sealing adhesive tape is arranged between the display panel and the middle frame and/or between the middle frame and the rear cover, the buffering waterproof sealing adhesive tape comprises a polyolefin resin foaming sheet, the residual amount of an azo foaming agent of the polyolefin resin foaming sheet is less than 2000ppm, the closed pore rate is more than 93%, the elongation at break at 80 ℃ in the MD direction is 140-650%, the display assembly has excellent buffering performance and water resistance, and the display assembly can play a role in supporting, buffering and protecting, so that the whole thickness of the display assembly is reduced, and the display assembly has good waterproof sealing performance.

Description

Polyolefin resin foam sheet, adhesive tape, display module, and electronic product
Technical Field
The invention relates to the field of electronic display products, in particular to the field of buffering sealing materials of electronic display products, and particularly relates to a sealing adhesive tape which is particularly suitable for waterproofing electronic equipment due to the high water resistance of the sealing adhesive tape.
Background
With the increasing demand of people for electronic products, electronic components therein are becoming more and more precise, and the use experience of electronic products is affected by both collision and moisture, so that higher requirements are put forward on the cushioning and sealing performance of electronic products, and the cushioning sealing material is required to have light weight performance in addition to the requirements of portability and lightness and thinness.
The polyolefin resin foamed sheet has characteristics such as light weight and excellent cushioning property, and is widely used in electronic devices such as smart phones, personal computers, and electronic papers, and is generally provided between an electronic component and a frame structure to perform a cushioning and sealing function.
However, the polyolefin-based resin is foamed by using an azo-based foaming agent such as azodicarbonamide (AC foaming agent), but the foaming agent remains after foaming. On the other hand, the residual azo-based foaming agent adversely affects the mechanical properties of the polyolefin resin foamed sheet, and on the other hand, may cause serious environmental damage, and therefore the residual amount thereof in the electronic product is clearly limited.
The conventional idea of removing the azo foaming agent residue comprises the following steps:
(1) It is known that the reduction in the amount of the blowing agent decreases the expansion ratio and deteriorates the compressive strength, and finally deteriorates the cushioning properties of the foam.
(2) The foaming temperature is increased, and an excessively high foaming temperature leads to an excessively high open pore ratio, and finally leads to a poor forward water permeability effect of the material.
Therefore, there is a thought that the amount of the foaming agent used is reduced, and the foaming is performed at a higher temperature, so that the foaming efficiency of the foaming agent is improved, thereby reducing the residue. However, too high a foaming temperature still results in too high open cell content and ultimately poor water blocking effect of the material, while reducing the amount of foaming agent used also results in poor cell density and compressive strength effect and ultimately poor cushioning properties of the foamed material.
Therefore, for the foaming material, the azo residue rate, the open pore ratio, the melt strength and the forward water permeability effect are mutually influenced, and the existing foaming material in the prior art is difficult to ensure that the material still has relatively good forward water permeability effect under the condition that the azo residue is reduced to be less than 0.1%.
Disclosure of Invention
The invention aims to provide a display assembly which has good water-blocking performance, wherein a specific buffering waterproof sealing adhesive tape is arranged between a display panel and a middle frame and/or between the middle frame and a rear cover, the specific buffering waterproof sealing adhesive tape consists of a polyolefin resin foaming sheet and an adhesive coated on the surface of one side or two sides of the polyolefin resin foaming sheet, the residue of an azo foaming agent of the polyolefin resin foaming sheet is as low as less than 2000ppm, and meanwhile, the material has high closed-cell rate, so that a good water-blocking effect can be provided, and an excellent buffering effect is achieved.
As a result of intensive studies to solve the above problems, the present inventors have found that when a pre-foamed polyolefin resin system containing an azo foaming agent is foamed, the amount of the foaming agent used is reduced, the foaming temperature is raised, and various parameter indexes of the pre-foamed polyolefin resin system are controlled, so that the obtained polyolefin resin sheet has a specific high-temperature elongation at break, the azo foaming agent remains low, the closed cell ratio is high, and the cushioning properties are excellent, and further, when the pre-foamed polyolefin resin system is applied to a display module, the thickness of the entire display module can be reduced, the ultra-thin display panel effect can be realized, and the display module has excellent waterproof sealing properties. Based on this concept, the present invention has been completed.
The specific scheme is as follows:
in a first aspect, the invention discloses a display assembly, which comprises a display panel, a middle frame and a rear cover, wherein the middle frame is used for supporting the display panel, the rear cover is buckled with the middle frame, and a buffering waterproof sealing adhesive tape is arranged between the display panel and the middle frame and/or between the middle frame and the rear cover.
The buffering waterproof sealing adhesive tape consists of a polyolefin resin foaming sheet and an adhesive coated on one side or two sides of the polyolefin resin foaming sheet, wherein the polyolefin resin foaming sheet foams a pre-foaming polyolefin resin system formed by polyolefin matrix resin by using an azo foaming agent to finally form a plurality of bubbles inside; the amount of residual azo blowing agent is less than 2000ppm, the closed cell ratio is 93% or more, and the elongation at break at 80 ℃ in the MD direction is 140 to 650%, preferably 150 to 610%.
The specific test method for the elongation at break at 80 ℃ comprises the following steps: the sample is placed for 1h at 80 ℃, the state of the sample after heat treatment is regulated for at least 4h under the standard environment with the temperature of 23 ℃ plus or minus 2 ℃ and the relative humidity of 50 percent plus or minus 10 percent, the foamed sheet is made into a standard sample with the length of 160mm and the width of 25mm according to ASTM D882, the length direction of the sample is parallel to the MD direction of the foamed sheet, and the elongation of the sample at break is recorded by stretching the sample at a constant speed of 300mm/min by using a tensile testing machine under the condition of 80 ℃.
In one embodiment, the azo foaming agent is selected from the group consisting of azodicarbonamide, diisopropyl azodicarboxylate, metal azodicarbonamide, azobisisobutyronitrile, and more preferably azodicarbonamide, and the initial content of the azo foaming agent is 0.5 to 8wt% based on the total weight of the pre-foamed polyolefin resin system raw material composition; preferably 0.5 to 6wt%; further preferably 1 to 4wt%;
in another embodiment, the change value of enthalpy of the pre-expanded polyolefin-based resin system is 4 to 65J/g, preferably 10 to 50J/g, and more preferably 20 to 35J/g, within a temperature range of 160 to 200 ℃;
in another embodiment, the polyolefin-based matrix resin is selected from one or more of polyethylene resin, polypropylene resin, and ethylene-vinyl acetate copolymer, and the Melt Flow Rate (MFR) of the blended polyolefin-based matrix resin at 190 ℃ under a load of 2.16kg is 0.2 to 30g/10 min, more preferably 0.3 to 20g/10 min, and still more preferably 0.5 to 5g/10 min;
in another embodiment, the foaming temperature is selected to be 240 to 390 ℃, more preferably 260 to 380 ℃, and particularly preferably 280 to 350 ℃ during the foaming of the pre-foamed polyolefin resin system;
in another embodiment, the polyolefin resin foamed sheet has a foaming ratio of 1.1 to 18cm 3 A/g, preferably 1.2-15cm 3 /g；
In another embodiment, the polyolefin-based resin foamed sheet has a 25% compressive strength of 35 to 680KPa, preferably 50 to 450KPa.
In a second aspect, the invention also discloses a polyolefin resin foamed sheet for the electronic product buffering waterproof sealing adhesive tape, which has low azo residue and high forward water resistance.
A polyolefin resin foamed sheet obtained by foaming a pre-foamed polyolefin resin system comprising a polyolefin base resin with an azo foaming agent to finally form a polyolefin resin foamed sheet having a large number of cells therein, wherein the azo foaming agent has a residual content of less than 2000ppm, preferably less than 1500ppm, more preferably less than 1000ppm, and a closed cell ratio of 93% or more, preferably 95% or more, more preferably 97% or more, based on the total weight of the polyolefin resin foamed sheet; the polyolefin resin foamed sheet has an elongation at break at 80 ℃ in the MD direction of 140 to 650%, preferably 150 to 610%;
in one embodiment, the azo foaming agent is selected from the group consisting of azodicarbonamide, diisopropyl azodicarboxylate, metal azodicarbonamide, azobisisobutyronitrile, and more preferably azodicarbonamide, and the initial content of the azo foaming agent is 0.5 to 8wt% based on the total weight of the pre-foamed polyolefin resin system raw material composition; preferably 0.5 to 6wt%; further preferably 1 to 4wt%;
in another embodiment, the change value of enthalpy of the pre-expanded polyolefin-based resin system is 4 to 65J/g, preferably 10 to 50J/g, and more preferably 20 to 35J/g, within a temperature range of 160 to 200 ℃;
in another embodiment, the polyolefin-based matrix resin is selected from one or more of polyethylene resin, polypropylene resin, and ethylene-vinyl acetate copolymer, and the Melt Flow Rate (MFR) of the blended polyolefin-based matrix resin at 190 ℃ under a load of 2.16kg is 0.2 to 30g/10 min, more preferably 0.3 to 20g/10 min, and still more preferably 0.5 to 5g/10 min;
in another embodiment, the foaming temperature is selected to be 240 to 390 ℃, more preferably 260 to 380 ℃, and particularly preferably 280 to 350 ℃ during the foaming of the pre-foamed polyolefin resin system;
in another embodiment, the polyolefin resin foamed sheet has a foaming ratio of 1.1 to 18cm 3 A/g, preferably 1.2-15cm 3 /g；
In another embodiment, the polyolefin-based resin foamed sheet has a 25% compressive strength of 35 to 680KPa, preferably 50 to 450KPa.
Also disclosed is a polyolefin resin foamed sheet having a low azo residue and a high positive water-blocking property.
In a third aspect, the invention also discloses a method for preparing a polyolefin resin foaming sheet for the buffering waterproof sealing adhesive tape of electronic products, which comprises the following steps of 0.5-8wt% based on the total weight of a pre-foaming polyolefin resin system raw material composition; preferably 0.5 to 6wt%; more preferably 1-4wt% of azo foaming agent, matrix resin and crosslinking assistant, mixing the raw material mixture, adding the mixture into a high-speed stirrer, mixing to obtain a mixture, banburying and crosslinking to obtain a pre-foamed polyolefin resin system, foaming at 240-390 ℃, more preferably 260-380 ℃, particularly preferably 280-350 ℃, and stretching and shaping to obtain the foamed sheet;
wherein, in the foaming process, the enthalpy change value of the pre-foaming polyolefin resin system in the range of 160-200 ℃ is 4-65J/g, preferably 10-50J/g, and more preferably 20-35J/g;
the polyolefin resin foamed sheet preferably has an elongation at break at 80 ℃ in the MD direction of 140 to 650%, preferably 150 to 610%.
In the invention, the specific test method of the elongation at break at 80 ℃ comprises the following steps: the sample is placed for 1h at 80 ℃, and the state of the heat-treated sample is adjusted for at least 4h under the standard environment with the temperature of 23 ℃ plus or minus 2 ℃ and the relative humidity of 50 percent plus or minus 10 percent. The foamed sheet was prepared into a standard specimen having a length of 160mm and a width of 25mm in accordance with ASTM D882, the lengthwise direction of the specimen being parallel to the MD direction of the foamed sheet. Using a tensile testing machine, and under the condition of 80 ℃, stretching the sample at a constant speed of 300mm/min to record the elongation of the sample at break;
and scanning and measuring the enthalpy change value of the sample by adopting a DSC3 instrument of Mettler Toledo, wherein the test conditions are as follows: heating to 100 ℃ at 25 ℃, wherein the heating rate is 10 ℃/min, and N 2 The flow rate is 50.0ml/min; keeping the temperature at 100 ℃ for 10min 2 The flow rate is 50ml/min; heating to 200 ℃ at 100 ℃, wherein the heating rate is 10 ℃/min, and N 2 The flow rate was 50ml/min.
In a fourth aspect, the invention also discloses a buffering waterproof sealing adhesive tape for electronic products, which consists of the polyolefin resin foaming sheet and an adhesive coated on one side or two sides of the polyolefin resin foaming sheet.
In a fifth aspect, the invention further discloses a waterproof application of the electronic product buffer waterproof sealing tape in an electronic product, specifically including an application in an intelligent mobile communication device, a notebook computer, a liquid crystal display, an electronic book, a tablet terminal, a game device, a camera, a wearable electronic device, and an OLED display.
When the electronic product buffering waterproof sealing adhesive tape is applied to a waterproof layer, the electronic product buffering waterproof sealing adhesive tape is attached to a display panel, a middle frame and a rear cover through steps of die cutting, gluing, attaching, sealing, shaping and the like, wherein the middle frame is used for supporting the display panel, and the rear cover is buckled with the middle frame to achieve buffering and sealing effects.
The invention has the beneficial effects that:
the polyolefin resin foamed sheet obtained according to the invention has high elongation at break at 80 ℃ in the MD direction, even if the use amount of the foaming agent is reduced and the foaming temperature is raised, the obtained foamed sheet still has high closed cell rate and the foaming multiplying power is maintained in a reasonable range, and the obtained electronic product buffering waterproof sealing adhesive tape has ideal effects in the aspects of low azo foaming agent residual rate, water resistance and buffering performance, can play the roles of supporting, buffering and protecting in a display assembly, further reduces the whole thickness of the display assembly and has good waterproof sealing performance.
Drawings
Fig. 1 is an SEM image showing a cross section of a polyolefin-based resin foamed sheet of example 1 in the MD direction;
FIG. 2 is a schematic view of a display module according to the present invention;
description of the reference numerals: 1-a display panel; 2-middle frame; 3-rear cover; 4-buffering waterproof sealing adhesive tape.
Detailed Description
In order to better explain the invention, refer to the implementation of the invention detailed description, and combine the specific examples to further clarify the main content of the invention, but the content of the invention is not limited to the following examples only.
[ polyolefin base resin ]
The polyolefin resin constituting the foamed sheet is a foamed base material which is conventional in the art, and may be selected from polyethylene resin, polypropylene resin, ethylene-vinyl acetate copolymer, and the like, and is preferably polyethylene resin in view of mechanical properties and cost.
The selection of the polyethylene resin is not particularly limited, and includes, but is not limited to, low density polyethylene, medium density polyethylene, high density polyethylene, linear low density polyethylene, and the like, and optionally, ethylene may be selectedAn ethylene-alpha-olefin copolymer as a main component, wherein the alpha-olefin is selected from alpha-olefins each having 2 to 12 carbon atoms, such as propylene (propylene), 1-butene, 1-pentene, 3-methyl-1-butene, 1-hexene, 3-methyl-1-pentene, 4-methyl-1-pentene, 3-ethyl-1-pentene, 1-octene, 1-decene, and 1-undecene. The number of kinds of such α -olefins may be only 1, or may be 2 or more. The polyethylene resins may be selected from the above resins and used alone or in combination of two or more. Preferably low density polyethylene. The density of the low-density polyethylene is preferably 0.910 to 0.925g/cm 3 More preferably 0.912 to 0.922g/cm 3 。
In order to increase the resilience and shape-following property of the polyolefin resin foamed sheet, a rubber and/or a thermoplastic elastomer having a glass transition temperature of 20 ℃ or less may be further added to the polyolefin base resin, and specifically listed are: natural or synthetic rubbers such as natural rubber, polyisobutylene, isoprene rubber, butyl rubber, chloroprene rubber, and nitrile rubber; olefin elastomers such as ethylene-vinyl acetate copolymers, polybutenes, polyisobutylenes, and chlorinated polyethylenes; styrene-based elastomers such as styrene-isoprene-styrene copolymers (SIS), styrene-butadiene-styrene copolymers (SBS), styrene-isoprene-butadiene-styrene copolymers (SIBS), and hydrogenated polymers thereof; a thermoplastic polyester elastomer; a thermoplastic polyurethane elastomer; and a thermoplastic acrylic elastomer. The number of types of these rubbers and thermoplastic elastomers may be only 1, or may be 2 or more. In addition, the number of kinds of the components may be only 1, or may be 2 or more. The content of the rubber and/or thermoplastic elastomer is 0 to 55% by weight, preferably 2 to 50% by weight, based on the total weight of the polyolefin-based matrix resin.
Before forming a mixture, other functional additives can be added according to actual requirements, so that various properties of the polyolefin foamed sheet can be further improved, and the following properties can be listed: antioxidants, antimicrobials, colorants, antistatic agents, and fillers.
It is to be noted that, regardless of the selection of the type of the base material, whether or not the elastomer material is added, and whether or not the other functional auxiliary is added, the Melt Flow Rate (MFR) of the polyolefin-based base resin after blending at 190 ℃ under a load of 2.16kg is controlled to be 0.2 to 30g/10 min, more preferably 0.3 to 20g/10 min, and still more preferably 0.5 to 5g/10 min. When the melt flow rate of the polyolefin-based matrix resin blended with the elastomer, the foaming agent and the functional assistant falls within the range, the melt strength of the pre-foamed polyolefin-based resin system during foaming can be ensured, and the pre-foamed polyolefin-based resin system can enclose gas during high-temperature foaming and can not crack and open pores due to overhigh temperature.
[ Cross-linking ]
In the present invention, after the completion of the mixing of the polyolefin-based matrix resin and other additives, the gelation reaction process, i.e., the crosslinking reaction, may be performed by a known technique disclosed in the prior art, and may be performed by a common method such as radiation crosslinking, or by chemical crosslinking, preferably radiation crosslinking.
The radiation crosslinking is performed by irradiating the resin sheet with ionizing radiation such as electron beam, α -ray, β -ray, and γ -ray. The irradiation amount of the ionizing radiation may be adjusted so that the degree of crosslinking of the resulting foam sheet is within the desired range, but is preferably 10 to 30Mrad, and more preferably 10 to 25Mrad.
The chemical crosslinking is carried out by mixing an organic peroxide into a raw material composition of a prefoamed polyolefin resin system and heating the mixture to decompose the organic peroxide, and it can be used as a co-crosslinking agent for radiation crosslinking.
The polyolefin-based foamed sheet of the present invention has a crosslinking degree of 15 to 70%, preferably 18 to 50%, more preferably 20 to 40%, and within this range, the melt strength of the finally molded pre-foamed polyolefin-based resin is also controlled, and the cell diameter of the cells to be subsequently foamed is made uniform.
The invention finds that the melt strength of the pre-foamed polyolefin resin system after irradiation crosslinking is controlled to be reasonable, the pre-foamed polyolefin resin system can contain gas during high-temperature foaming, and the pre-foamed polyolefin resin system can not be cracked and opened due to overhigh temperature. The melt strength of the pre-foamed polyolefin resin system is influenced by many factors, for example, depending on the kind of resin, copolymerization condition, molecular weight and molecular weight distribution, resin crystallinity and degree of crosslinking, and in the present invention, the melt strength of the pre-foamed polyolefin resin system is controlled to a reasonable melt strength by controlling the melt flow rate and degree of crosslinking of the polyolefin resin substrate blend.
[ azo foaming agent ]
The azo compound has uniform and controllable foaming, is better than inorganic foaming agents, nitroso foaming agents, sulfonyl hydrazine foaming agents and physical foaming agents when used for foaming polyolefin resin from the comprehensive aspects of obtaining fine bubbles, economy, environmental protection and safety, and is specifically listed as follows: azodicarbonamide, diisopropyl azodicarboxylate, metal azodicarbonamide (barium azodicarboxylate, etc.), azobisisobutyronitrile, and the like, and azodicarbonamide is more preferable.
Since the present application intends to solve the technical problem of the lowest possible residual amount of azo blowing agents in the final product, two factors are mainly considered here:
1. content of azo foaming agent. When foaming, the using amount of the azo foaming agent is reduced in terms of selecting the content of the azo foaming agent, the azo foaming agent is ideally fully utilized to meet the preset foaming multiplying power of the polyolefin resin foaming sheet, and the specific content is selected to be 0.5-8wt% of the initial content of the azo foaming agent based on the total weight of the pre-foamed polyolefin resin system raw material composition; preferably 0.5 to 6wt%; further preferably 1 to 4wt%. When the content of the azo foaming agent is less than 0.5wt%, the foaming ratio of the polyolefin resin foamed sheet cannot meet the minimum predetermined requirement; when the content of the azo-based blowing agent is more than 8% by weight, the residual amount of the azo-based blowing agent in the finished polyolefin resin foamed sheet inevitably increases.
2. The foaming temperature. The foaming temperature determines the foaming efficiency of the azo foaming agent and the melt strength of the polyolefin resin during foaming, ideally, the azo foaming agent is completely foamed and escaped at the temperature, and when the content of the azo foaming agent is less, the foaming temperature needs to be increased in order to obtain a higher foaming ratio, for the polyolefin resin foamed sheet, especially for the polyethylene resin foamed sheet, the foaming temperature is specifically selected to be 240-390 ℃, more preferably 260-380 ℃, and particularly preferably 280-350 ℃, and when the temperature is higher than 390 ℃, the melt strength during foaming of the polyolefin resin is too low, so that the cells are easy to collapse, and even the decomposition of the matrix resin is easy to occur; when the temperature is less than 240 ℃, there are cases where the azo type foaming agent remains excessively due to incomplete foaming and the expansion ratio cannot satisfy expectations. However, when the foaming temperature is within the above range, the problem of high open cell content is still unavoidable, and the water blocking effect is adversely affected.
The polyolefin resin foamed sheet of the present invention has a foaming ratio falling within an ideal range of 1.1 to 18cm based on the content of the foaming agent and the control of the foaming temperature 3 A/g, preferably 1.2-15cm 3 (ii) in terms of/g. If the expansion ratio is 1.1cm 3 When the foaming ratio is 18cm or less, the flexibility of the foamed sheet cannot be ensured 3 If the ratio is more than g, the mechanical strength of the foamed sheet is affected. Under the condition of the same foaming multiplying power, the content of the azo foaming agent used by the invention is lower than the using amount in the prior art, and the foaming temperature is higher than the foaming temperature in the prior art, so that the residual quantity of the azo foaming agent in a final product is reduced.
The foaming auxiliary agent is used for adjusting the decomposition temperature and the decomposition speed of the foaming agent, and the foaming auxiliary agent matched with the azo foaming agent comprises urea, phosphate, organic acid and metal salt foaming auxiliary agent, preferably metal salt foaming auxiliary agent, and more preferably metal zinc salt foaming auxiliary agent such as zinc oxide, stearic acid, zinc stearate and the like.
The inventor finds in the research process that when the pre-foamed polyolefin resin is selected to have reasonable melt strength, the enthalpy change value of the pre-foamed polyolefin resin system in the range of 160-200 ℃ is 4-65J/g, preferably 10-50J/g, and more preferably 20-35J/g by selecting proper foaming agent and foaming auxiliary agent, the content of the foaming agent can be reduced, the foaming rate can be well controlled at high temperature, and the phenomenon that cells are broken to form open cells in the foaming process due to overhigh foaming rate can be avoided. The selection of the foaming agent and the foaming auxiliary agent influences the change of the enthalpy value within the range of 160-200 ℃, except for selecting the types and the contents of the azo foaming agent and the foaming auxiliary agent, the enthalpy value can be influenced by the particle size, the surface appearance and the crystal form, and the technicians in the field have good effects by controlling the change of the enthalpy value within the range regardless of the collocation of all factors.
And scanning and measuring the enthalpy change value of the sample by adopting a DSC3 instrument of Mettler Toledo, wherein the test conditions are as follows: heating to 100 ℃ at 25 ℃, wherein the heating rate is 10 ℃/min, and N 2 The flow rate is 50.0ml/min; keeping the temperature at 100 ℃ for 10min 2 The flow rate is 50ml/min; heating to 200 ℃ at 100 ℃, wherein the heating rate is 10 ℃/min, and N 2 The flow rate was 50ml/min.
[ drawing and sizing ]
The stretch-setting may be performed during the foaming process, or after cooling, the polyolefin-based foamed sheet may be heated again in a molten or softened state, or both. When the stretching ratio in the MD direction is 1.1 to 3.0 times, preferably 1.2 to 2.0 times, and the stretching ratio in the TD direction is 1.0 to 3.0 times, preferably 1.1 to 2.0 times, the polyolefin foamed sheet can be prevented from being broken in stretching, and the flexibility and the tensile strength of the foamed sheet are good, the quality of the foamed sheet becomes further uniform, and finally the polyolefin foamed sheet has a 25% compressive strength of 35 to 680KPa, preferably a 25% compressive strength of 50 to 450KPa.
Here, "MD" refers to the Machine Direction (Machine Direction) and is a Direction that coincides with the extrusion Direction of the polyolefin resin foam sheet or the like. "TD" refers to a Transverse Direction (Transverse Direction) and is a Direction perpendicular to the MD and parallel to the foam sheet.
[ high temperature mechanical Properties of polyolefin resin foam sheet ]
The polyolefin foamed sheet of the present invention is obtained by comprehensively defining the content of the azo foaming agent, the foaming temperature, the enthalpy change value of the pre-foamed polyolefin resin system in the range of 160-200 ℃, the Melt Flow Rate (MFR) of the polyolefin matrix resin after blending, the crosslinking degree, the foaming ratio, the stretching ratio and other factors, and finally, it is found that when the elongation at break of the final polyolefin resin foamed sheet in the MD direction at 80 ℃ is 140-650%, preferably 150-610%, under the conditions of low usage of the azo foaming agent and high foaming temperature, a complete closed cell structure can be ensured on the one hand, and on the other hand, the cell diameter is in a reasonable range, and the foaming ratio is better, so that the polyolefin resin foamed sheet with ideal cushioning property, low azo residue rate, high closed cell rate and excellent water blocking property can be obtained, such that the residual content of the azo foaming agent is less than 2000ppm, preferably less than 1500ppm, further preferably less than 1000ppm, and the closed cell rate is 93%, preferably 95% or more preferably 97% or more. As can be seen from the following examples, from the viewpoint of improving the water-blocking effect, reducing the azo residual ratio, and improving the cushioning property, the elongation at break at 80 ℃ in the MD direction is preferably 140 to 650%, and if the elongation is more than 650%, the closed cell ratio of the foamed sheet is reduced, and the water-blocking effect is further reduced; if the amount is less than 140%, the foamed sheet is insufficiently foamed, resulting in a low sheet magnification, poor cushioning effect and a high azo residual ratio.
Here, the specific test method of the elongation at break at 80 ℃ is: the sample is placed for 1h at 80 ℃, the state of the sample after heat treatment is regulated for at least 4h under the standard environment with the temperature of 23 ℃ plus or minus 2 ℃ and the relative humidity of 50 percent plus or minus 10 percent, the foamed sheet is made into the standard sample with the length of 160mm and the width of 25mm according to ASTM D882, the length direction of the sample is parallel to the MD direction of the foamed sheet, and the elongation of the sample at break is recorded by stretching the sample at a constant speed of 300mm/min by using a tensile testing machine under the condition of 80 ℃.
Other functional additives can be added into the polyolefin resin foamed sheet according to application scenes in the foaming and forming processes, and the addition type and content of the additives need to ensure that the breaking elongation of the formed polyolefin resin foamed sheet at the melt strength of 80 ℃ falls within the range.
Other functional adjuvants include, but are not limited to: processing aids, flame retardants, antistatic agents, and the like.
[ adhesive layer ]
The obtained polyolefin resin foaming sheet needs to be glued before being attached to form the buffering waterproof sealing adhesive tape for the electronic product, which comprises the following components in percentage by weight: the pressure-sensitive adhesive layer is formed on the surface of the sheet, and various methods known in the art, for example, a method in which a pressure-sensitive adhesive composition is directly applied to a foamed sheet (direct method), a method in which a pressure-sensitive adhesive layer is formed on an appropriate release surface by applying the pressure-sensitive adhesive composition to the release surface, and the pressure-sensitive adhesive layer is bonded to the foamed sheet and transferred (transfer method), and the like can be applied. The coating can be performed using a known or conventional coater such as a gravure roll coater, a reverse roll coater, a kiss roll coater, a dip roll coater, a bar coater, a knife coater, or a spray coater. The thickness is controlled to be 1 μm to 50 μm, preferably 2 to 20 μm, and more preferably 3 to 10 μm.
The type of the adhesive constituting the adhesive layer is not particularly limited, and specifically includes: an adhesive layer composed of one or more of known various adhesives such as an acrylic adhesive, a rubber adhesive (natural rubber, synthetic rubber, a mixture thereof, and the like), a silicone adhesive, a polyester adhesive, a urethane adhesive, a polyether adhesive, a polyamide adhesive, and a fluorine adhesive. From the viewpoint of transparency and weather resistance, the pressure-sensitive adhesive layer is preferably formed using an acrylic pressure-sensitive adhesive.
[ practical application ]
The polyolefin resin foaming sheet forms the electronic product buffering waterproof sealing adhesive tape, is attached between a printed circuit board and a cover plate of an electronic product or between an image display part and a display glass plate, and particularly plays a role in buffering and waterproofing in a limiting space of a bearing unit, a shell and a display panel through sealing and shaping.
The electronic product comprises: smart mobile communication devices, notebook computers, liquid crystal displays, OLED displays, electronic books, tablet terminals, gaming devices, cameras, wearable electronic devices.
As shown in fig. 2, the polyolefin resin foamed sheet of the present invention is used to form a buffering waterproof sealing tape for electronic products, and is particularly applied to a display module, wherein the display module comprises a display panel 1, a middle frame 2 and a back cover 3, the middle frame 2 is used to support the display panel 1, and the back cover 3 is fastened to the middle frame 3. The middle frame 2 is provided with a bearing part 22 with a blocking part 21 vertically connected with the blocking part 21, the display panel 21 is arranged on the bearing part 22 and is fixedly connected with the display panel 21 through a buffering waterproof sealing adhesive tape 4, the rear cover 3 and the middle frame 2 can also be hermetically connected through the buffering waterproof sealing adhesive tape, in addition, other electronic elements in the display assembly, such as a PCB (printed circuit board), a battery and other electronic elements, are arranged in an accommodating space formed by the display panel 1, the middle frame 2 and the rear cover 3, and one side of the rear cover facing the display panel can also be provided with a piece of buffering waterproof sealing adhesive tape to play a role in buffering and protecting the electronic elements. The buffering waterproof sealing adhesive tape 4 has excellent buffering performance and water resistance performance, so that the buffering waterproof sealing adhesive tape can play a role in supporting, buffering and protecting a display assembly, further the whole thickness of the display assembly is reduced, and the buffering waterproof sealing adhesive tape has good waterproof sealing performance.
According to the invention, the excellent water resistance of the buffering waterproof sealing adhesive tape 4 can realize a good sealing protection effect on the display assembly, and the excellent buffering effect of the buffering waterproof sealing adhesive tape 4 can realize the thinning of the display panel.
The present invention will be described below with reference to some examples, but the present invention is not intended to be limited to these examples.
Example 1:
100 parts by weight of LDPE (trade name: zhongpetrochemical 2426H), 5 parts by weight of azodicarbonamide and 0.2 part by weight of zinc oxide were mixed in a high-speed mixer, the MFR of the polyolefin resin base material blend was 1.87g/10min, then kneading was performed at 130 ℃, and then extruded into a strip-like sheet, then double-sided irradiation was performed on the strip-like resin sheet, the irradiation energy was 1.5Mev, the irradiation dose was 25Mrad, the resin sheet was crosslinked, the crosslinked resin sheet was continuously fed into a heating furnace at 345 ℃, the heating furnace was heated by an infrared heater to foam the resin sheet, then stretching was performed at 120 ℃ at a stretching ratio of 1.8 times in the MD direction and at a stretching ratio of 1.8 times in the TD direction, and further a polyolefin-based foamed sheet was obtained, the thickness was 0.8 mm, and the average MD cell diameter was 120. Mu.m as measured by the SEM chart, as shown in FIG. 1.
Example 2
100 parts by weight of LDPE (trade name: meditanidine 2426H), 6 parts by weight of azodicarbonamide, and 0.2 part by weight of zinc oxide were mixed in a high-speed mixer, the MFR of the polyolefin resin base material blend was 0.86g/10min, then kneading was performed at 130 ℃, then extrusion was performed to obtain a strip-like sheet, then both-side irradiation was performed to the strip-like resin sheet with irradiation energy of 1.5Mev and irradiation dose of 30Mrad to crosslink the resin sheet, the crosslinked resin sheet was continuously fed into a heating furnace at 290 ℃, the heating furnace was heated by an infrared heater to foam the resin sheet, then stretching was performed at 120 ℃ at a stretching ratio of 1.5 times in the MD direction and stretching was performed at a stretching ratio of 1.5 times in the TD direction to obtain a polyolefin-based foamed sheet, the thickness was 0.05mm, and the average MD cell diameter was 90 μm as measured by SEM.
Example 3
80 parts by weight of LDPE (trade name: mimeti No. 2426H) and 20 parts by weight of LLDPE,5 parts by weight of azodicarbonamide and 0.2 part by weight of zinc oxide were mixed in a high-speed mixer, followed by kneading at 130 ℃ with MFR of 1.6g/10min for polyolefin resin base material blending, followed by extrusion into a strip-shaped sheet, followed by double-side irradiation of the strip-shaped resin sheet with irradiation energy of 1.5Mev and irradiation dose of 28Mrad to crosslink the resin sheet, the crosslinked resin sheet was continuously fed into a heating furnace of 360 ℃ to foam the resin sheet by heating with an infrared heater, and then stretched at 120 ℃ at a stretch ratio of 1.3 times in the MD direction and at a stretch ratio of 1.4 times in the TD direction to obtain a polyolefin-based foamed sheet having a thickness of 0.08mm and an average MD cell diameter of 80 μm as measured by SEM.
Example 4
80 parts by weight of LDPE (trade name: zhongpetrochemical 2426H) and 20 parts by weight of LLDPE,4 parts by weight of azodicarbonamide and 0.2 part by weight of zinc oxide were mixed in a high-speed mixer, and then kneaded at 130 ℃ to give a polyolefin resin base material having an MFR of 2.77g/10min, and then extruded into a strip-shaped sheet, and then the strip-shaped sheet was subjected to double-sided irradiation with an irradiation energy of 1.5Mev and an irradiation dose of 20Mrad to crosslink the resin sheet, and the crosslinked resin sheet was continuously fed into a heating furnace at 300 ℃ to be heated by an infrared heater to foam the resin sheet, and then stretched at 120 ℃ at a stretch ratio of 1.1 in the MD direction and at a stretch ratio of 1 in the TD direction to obtain a polyolefin-based foamed sheet having a thickness of 0.2mm and an average bubble diameter of 110 μm as measured by the MD diagram.
Example 5
65 parts by weight of LDPE (trade name: zhongpetrochemical 2426H) and 35 parts by weight of LLDPE,4 parts by weight of azodicarbonamide and 0.2 part by weight of zinc oxide were mixed in a high-speed mixer, and then kneaded at 130 ℃ to give a polyolefin resin base material having an MFR of 3.85g/10min, and then extruded into a strip-shaped sheet, and then the strip-shaped sheet was subjected to double-sided irradiation with an irradiation energy of 1.5Mev and an irradiation dose of 15Mrad to crosslink the resin sheet, and the crosslinked resin sheet was continuously fed into a heating furnace at 290 ℃ to foam the resin sheet by heating with an infrared heater, and then stretched at 120 ℃ at a stretch ratio of 1.2 times in the MD direction and at a stretch ratio of 1.1 times in the TD direction to obtain a polyolefin-based foamed sheet having a thickness of 0.12mm and an average cell diameter of 110 μm as measured by SEM.
Example 6
80 parts by weight of LDPE (trade name: mitsubishi 2426H) and 20 parts by weight of POE,5 parts by weight of azodicarbonamide and 0.2 part by weight of zinc oxide were mixed in a high-speed mixer, and then kneaded at 130 ℃ to give a polyolefin resin base material having an MFR of 5g/10min, and then extruded into a strip-shaped sheet, and then the strip-shaped sheet was subjected to double-sided irradiation with an irradiation energy of 1.5Mev and an irradiation dose of 10Mrad to crosslink the resin sheet, and the crosslinked resin sheet was continuously fed into a 280 ℃ heating furnace, and the resin sheet was foamed by heating with an infrared heater in a heating furnace, and then stretched at 120 ℃ at a stretch ratio of 2 times in the MD direction and at a stretch ratio of 2 times in the TD direction to obtain a polyolefin-based foamed sheet having a thickness of 0.45mm and an average MD cell diameter of 180 μm as measured by an SEM chart.
Example 7
85 parts by weight of LDPE (trade name: mitsubishi 2426H) and 15 parts by weight of POE,6 parts by weight of azodicarbonamide and 0.2 part by weight of zinc oxide were mixed in a high-speed mixer, and then kneaded at 130 ℃ to give a polyolefin resin base material having an MFR of 3.11g/10min, and then extruded into a strip-shaped sheet, and then the strip-shaped sheet was double-side irradiated with 1.5Mev at an irradiation dose of 15Mrad to crosslink the resin sheet, and the crosslinked resin sheet was continuously fed into a heating furnace at 290 ℃ to foam the resin sheet by heating with an infrared heater, and then stretched at 120 ℃ at a stretch ratio of 1.8 times in the MD direction and at a stretch ratio of 1.5 times in the TD direction to obtain a polyolefin-based foamed sheet having a thickness of 0.15mm and an average cell diameter of 110 μm as measured by an SEM chart.
Example 8
70 parts by weight of LDPE (trade name: mitsubishi 2426H) and 30 parts by weight of POE,3 parts by weight of azodicarbonamide and 0.2 part by weight of zinc oxide were mixed in a high-speed mixer, and then kneaded at 130 ℃ to give a polyolefin resin base material having an MFR of 1.83g/10min, and then extruded into a strip-shaped sheet, and then the strip-shaped sheet was double-side irradiated with 1.5Mev at an irradiation dose of 26Mrad to crosslink the resin sheet, and the crosslinked resin sheet was continuously fed into a heating furnace at 310 ℃, and the heating furnace was heated by an infrared heater to foam the resin sheet, and then stretched at 120 ℃ at a stretch ratio of 2.8 times in the MD direction and at a stretch ratio of 3 times in the TD direction to obtain a polyolefin-based foamed sheet having a thickness of 0.2mm and an average bubble diameter of 110 μm as measured by the SEM.
Example 9
80 parts by weight of LDPE (trade name: zhongpetrochemical 2426H) and 20 parts by weight of EVA,4 parts by weight of azodicarbonamide and 0.2 part by weight of zinc oxide were mixed in a high-speed mixer, and then kneaded at 130 ℃ to give a polyolefin resin base material having a blend MFR of 4.72g/10min, and then extruded into a strip-shaped sheet, and then the strip-shaped sheet was subjected to double-sided irradiation with an irradiation energy of 1.5Mev and an irradiation dose of 13Mrad to crosslink the resin sheet, and the crosslinked resin sheet was continuously fed into a 280 ℃ heating furnace, and the heating furnace was heated by an infrared heater to foam the resin sheet, and then stretched at 120 ℃ at a stretching ratio of 2.5 times in the MD direction and at a stretching ratio of 3 times in the TD direction to obtain a polyolefin foamed sheet having a thickness of 0.32mm and an average bubble diameter of 125 μm as measured by the SEM.
Example 10
70 parts by weight of LDPE (trade name: mimeti chemical 2426H) and 30 parts by weight of EVA,4 parts by weight of azodicarbonamide, and 0.2 part by weight of zinc oxide were mixed in a high-speed mixer, followed by kneading at 130 ℃ to give a polyolefin resin base material having a MFR of 4.76g/10min, followed by extrusion into a strip-shaped sheet, followed by double-side irradiation of the strip-shaped resin sheet with an irradiation energy of 1.5Mev and an irradiation dose of 13Mrad to crosslink the resin sheet, the crosslinked resin sheet was continuously fed into a heating furnace at 280 ℃, the heating furnace was heated by an infrared heater to foam the resin sheet, and then the sheet was stretched at 120 ℃ at a stretch ratio of 2 times in the MD direction and at a stretch ratio of 2.5 times in the TD direction to give a polyolefin-based foamed sheet having a thickness of 0.44mm, and an average bubble diameter of 140 μm as measured by the MD chart.
Comparative example 1
100 parts by weight of LDPE (trade name: meditanidine 2426H), 6 parts by weight of azodicarbonamide, and 0.2 part by weight of zinc oxide were mixed in a high-speed mixer, and then kneaded at 130 ℃ to give a polyolefin resin base material having a MFR of 0.45g/10min, and then extruded into a strip-like sheet, and then the strip-like resin sheet was irradiated on both sides with an irradiation energy of 1.5Mev and an irradiation dose of 18Mrad to crosslink the resin sheet, and the crosslinked resin sheet was continuously fed into a heating furnace at 260 ℃ to foam the resin sheet by heating with an infrared heater, and then stretched at 120 ℃ at a stretch ratio of 2.1 times in the MD direction and at a stretch ratio of 2 times in the TD direction to obtain a polyolefin-based foamed sheet having a thickness of 0.25mm and an average MD cell diameter of 78 μm as measured by SEM.
Comparative example 2
80 parts by weight of LDPE (trade name: zhongpetrochemical 2426H) and 20 parts by weight of LLDPE,5 parts by weight of azodicarbonamide, and 0.2 part by weight of zinc oxide were mixed in a high-speed mixer, followed by kneading at 130 ℃ with a MFR of 5.5g/10min for polyolefin resin base material blending, followed by extrusion into a strip-shaped sheet, followed by double-side irradiation of the strip-shaped resin sheet with an irradiation energy of 1.5Mev and an irradiation dose of 20Mrad to crosslink the resin sheet, the crosslinked resin sheet was continuously fed into a heating furnace of 380 ℃ to foam the resin sheet by heating with an infrared heater, and then drawn at 120 ℃ at a draw ratio of 1.5 times in the MD direction and drawn at a draw ratio of 1.6 times in the TD direction to obtain a polyolefin-based foamed sheet having a thickness of 0.3mm and an average MD cell diameter of 150 μm as measured by SEM.
The test method comprises the following steps:
1-Water-blocking Effect test
The polyolefin foam pieces were cut into samples of 70X 70mm in size, which were placed into a tank facing a test tank for filling with distilled water. A support web was used on the other side of the sample to support the extensible elastic material. The test chamber was horizontally placed on a test stand (FX 300-IV Hydro Tester), and 60mL of purified water (which may be used as a funnel or a syringe) was slowly injected into the penetration test chamber from an upper inlet. The sample is held in place to ensure that the distilled or deionized water does not penetrate the sample under pressure before the test is initiated. And applying continuously increasing water pressure to the sample at a water pressure rising rate of 6.0kpa/min +/-0.3 kpa/min until the pressure stops being increased by 120kpa, and observing the water permeation phenomenon, wherein N is the water permeation phenomenon in the process, and Y is the water permeation phenomenon which does not occur.
2-azo residue ratio measurement
The polyolefin foam pieces were cut into samples having dimensions of at least 70 x 70mm, as determined by the SGS standard SVHC test.
3-determination of the closed porosity
The closed cell ratio F1 and F2 of the test specimens were calculated, as measured according to ASTM D2856 (1998):
open pore ratio F1 (%) = 100 × (W2-W1)/V2
Closed cell fraction F2 (%) = 100-F1
Measurement of 4-25% compressive Strength
The compressive stress test was carried out in accordance with ISO3386-1, in which materials having a thickness of 10mm or less were laminated to 10mm or more, the compression rate was set to a rate as close as possible to 50% of the thickness of the material per minute, and the compressive stress at 25% deformation was measured.
Measurement of 5-expansion ratio
The dimensions of the samples were measured in centimeters (cm) as specified in GB/T6342-1996. At least three positions are measured per dimension, and for plate-like hard materials five positions are measured per dimension in the middle. The average value of each size is calculated separately and the sample volume V is calculated.
The sample was weighed as M to the nearest 0.5% in grams (g).
The expansion ratio was calculated from the following formula, and the average value was taken.
Expansion ratio = V/M
6-elongation at Break at high temperature
The test piece was annealed at 80 ℃ for 30min, and after cooling for 30min, the elongation at break in the MD direction was measured according to the method of ASTM D882-2009.
7-elongation at Break at Normal temperature
The test specimens were tested for room temperature elongation in the MD at a temperature of 23 ℃ according to ASTM D882-2009.
Determination of the degree of 8-crosslinking
a. Taking a sample of 100mg from the foamed sheet and accurately weighing the sample by weight A (mg);
b. a sample was wrapped with a 200-mesh metal mesh, and the metal mesh-wrapped sample was immersed in xylene at 120 ℃ and allowed to stand for 24 hours. Insoluble substances can be collected in the metal mesh by the filtering action of the metal mesh; accurately weighing the weight B (mg) of insoluble substances after vacuum drying;
c. calculating the crosslinking degree (mass%):
degree of crosslinking (% by mass) = 100% × (B/a).
9-thickness and cell diameter measurement
SEM observations were made for TD, MD and ZD directions of the samples, and the thickness and average pore diameter in different directions were measured.
The structural and performance indices of the examples and comparative examples are shown in tables 1 and 2:
TABLE 1- -structural indices of examples 1-10 and comparative examples 1-2
Examples Total thickness of (mm) A second resin Polyolefin-based matrix resin Post-lipid-blending MFR ( g/10min ） Degree of crosslinking (%) Average MD cell diameter (μm) Closed porosity （ % ） Expansion ratio ( cm3/g ） Residual ratio of azo （ ppm ）
Example 1 0.3 / 1.87 35 120 97 3.2 1500
Example 2 0.05 / 0.86 58 90 99 1.4 1900
Example 3 0.08 LLDPE 1.6 50 80 98 1.7 940
Example 4 0.2 LLDPE 2.77 36 110 95 2.9 710
Example 5 0.12 LLDPE 3.85 27 110 96 2.7 220
Example 6 0.45 POE 5 45 180 96 15 390
Example 7 0.15 POE 3.11 33 110 97 2.2 700
Example 8 0.2 POE 1.83 24 110 99 2 1800
Example 9 0.32 EVA 4.72 55 125 98 4.5 1460
Example 10 0.44 EVA 4.76 21 140 97 10 1100
Comparative example 1 0.25 / 0.45 40 78 99 1 2200
Comparative example 2 0.3 LLDPE 5.5 25 150 90 3.5 180
TABLE 2-Performance indices of examples 1 to 10 and comparative examples 1 to 2
Examples Enthalpy change value (cal &) g) 25% compressive strength (KPa) Expansion ratio (cm 3 /g) Elongation at break at 80 ℃ after annealing Length MD (%) Elongation at break at ordinary temperature MD （ % ） Water blocking test
Example 1 31.75 150 3.2 273.24 257.44 Y
Example 2 4.14 680 1.4 154.03 322.39 Y
Example 3 4.23 500 1.7 247.17 330.99 Y
Example 4 17.51 130 2.9 495.07 347.85 Y
Example 5 17.14 100 2.7 550.32 331.55 Y
Example 6 49.07 35 15 492.40 224.12 Y
Example 7 57.50 350 2,2 345.30 307.87 Y
Example 8 17.51 700 2 143.30 110.23 Y
Example 9 31.75 180 4.5 219.46 121.92 Y
Example 10 41.49 50 10 361.30 177.98 Y
Comparative example 1 3.24 740 1.1 120 250 Y
Comparative example 2 68.53 180 3.5 680 264 N
It can be seen that when the elongation at break at 80 ℃ MD (%) after annealing does not fall within the above-mentioned limited range, at least one of the azo residue and the water repellency exhibited cannot be within the desired range, for example, the elongation at break at 80 ℃ MD after annealing of comparative example 1 is too low, and the azo residue thereof is as high as 2200ppm, and thus, as compared with example 1, the elongation at break at 80 ℃ after annealing (longitudinal direction) of comparative example 1 is too low, foaming is insufficient, although the water repellency effect is good, the expansion ratio of the foamed sheet is low, the cushioning effect is poor, and the azo residue is high, which may be due to its high degree of crosslinking, resulting in incomplete foaming, and the foamed material is high in brittleness and low in elongation at break; comparative example 2 elongation at break (longitudinal direction) at 80 ℃ after annealing was too high, in order to ensure low azo residue, increase foaming temperature, increase enthalpy change value, etc., but resulted in decreased closed cell ratio and poor water blocking effect. Further, it was also found through the test of the ordinary temperature elongation at break of the experimental examples and comparative examples that if the examples and comparative examples were distinguished by the ordinary temperature elongation at break MD, it was not regularly repeated, and thus it was seen that only when the 80 ℃ elongation at break MD (%) after annealing was controlled within the range defined in the present invention while the thickness was also within the defined range, as shown in examples 1 to 10, balanced and desirable effects were exhibited in the azo residual ratio, mechanical properties, and water blocking effect.
The present invention can be practiced in other forms than those described above without departing from the scope of the present invention. The embodiments disclosed in the present application are one example, and are not limited to these.
Industrial applicability of the invention
The polyolefin resin foamed sheet is sized to obtain the electronic product buffering waterproof sealing adhesive tape, and can be used for waterproof buffering in various electronic products, such as intelligent mobile communication equipment, notebook computers, liquid crystal displays, electronic books, tablet terminals, game equipment, cameras, wearable electronic equipment and the like.

Claims (16)

1. A display assembly comprises a display panel, a middle frame and a rear cover, wherein the middle frame is used for supporting the display panel, the rear cover is buckled on the middle frame, and a buffering waterproof sealing adhesive tape is arranged between the display panel and the middle frame and/or between the middle frame and the rear cover; the residual content of azo foaming agent is less than 2000ppm, the closed pore rate is more than 93 percent, and the elongation at break at 80 ℃ in the MD direction is 140-650 percent; the specific test method for the elongation at break at 80 ℃ comprises the following steps: the sample is placed for 1h at 80 ℃, the state of the heat-treated sample is regulated for at least 4h under the standard environment with the temperature of 23 ℃ plus or minus 2 ℃ and the relative humidity of 50 percent plus or minus 10 percent, the foamed sheet is made into a standard sample with the length of 160mm and the width of 25mm according to ASTM D882, the length direction of the sample is parallel to the MD direction of the foamed sheet, and the sample is stretched at a constant speed of 300mm/min by using a tensile testing machine under the condition of 80 ℃ to record the elongation of the sample at break;
the crosslinking degree of the polyolefin resin foaming sheet is 15-70%.
2. A polyolefin resin foamed sheet for use in a cushioning waterproof sealing tape for electronic products, which is obtained by foaming a prefoamed polyolefin resin system comprising a polyolefin base resin with an azo foaming agent to form a polyolefin resin foamed sheet having a plurality of cells therein, the polyolefin resin foamed sheet comprising: a residual content of the azo foaming agent is less than 2000ppm and a closed cell ratio is 93% or more, based on the total weight of the polyolefin resin foamed sheet; the polyolefin resin foamed sheet has an elongation at break at 80 ℃ in the MD direction of 140 to 650%;
the specific test method for the elongation at break at 80 ℃ comprises the following steps: the sample is placed for 1h at 80 ℃, the state of the heat-treated sample is regulated for at least 4h under the standard environment with the temperature of 23 ℃ plus or minus 2 ℃ and the relative humidity of 50 percent plus or minus 10 percent, the foamed sheet is made into a standard sample with the length of 160mm and the width of 25mm according to ASTM D882, the length direction of the sample is parallel to the MD direction of the foamed sheet, and the sample is stretched at a constant speed of 300mm/min by using a tensile testing machine under the condition of 80 ℃ to record the elongation of the sample at break;
wherein the crosslinking degree of the polyolefin resin foamed sheet is 15 to 70%.
3. The polyolefin resin foamed sheet according to claim 2, wherein the polyolefin resin foamed sheet has a residual azo-based foaming agent content of less than 1500ppm and a closed cell content of 95% or more, based on the total weight of the polyolefin resin foamed sheet.
4. The foamed polyolefin resin sheet according to claim 2, wherein the foamed polyolefin resin sheet has an elongation at break at 80 ℃ in the MD direction of 150 to 610%.
5. The polyolefin resin foamed sheet according to claim 2, wherein the azo foaming agent is selected from the group consisting of azodicarbonamide, diisopropyl azodicarboxylate, metal azodicarboxylate, and azobisisobutyronitrile, and the initial content of the azo foaming agent is 0.5 to 8wt% based on the total weight of the prefoamed polyolefin resin system raw material composition.
6. The polyolefin resin foamed sheet according to claim 5, wherein the azo foaming agent is selected from azodicarbonamide.
7. The polyolefin-based resin foamed sheet according to claim 2, wherein the change in enthalpy of the pre-foamed polyolefin-based resin system during foaming is from 4 to 65J/g at a temperature of from 160 to 200 ℃;
wherein the enthalpy change value is measured by scanning a sample by adopting a DSC3 instrument of Mettler Toledo, and the test conditions are as follows: heating to 100 ℃ at 25 ℃, wherein the heating rate is 10 ℃/min, and N 2 The flow rate is 50.0ml/min; keeping the temperature at 100 ℃ for 10min 2 The flow rate is 50ml/min; heating to 200 ℃ at 100 ℃, wherein the heating rate is 10 ℃/min, and N 2 The flow rate was 50ml/min.
8. The polyolefin resin foamed sheet according to claim 2, wherein the polyolefin base resin is one or more selected from the group consisting of a polyethylene resin, a polypropylene resin, and an ethylene-vinyl acetate copolymer, and wherein the melt flow rate of the blended polyolefin base resin at 190 ℃ under a load of 2.16kg is 0.2 to 30g/10 min.
9. The polyolefin resin foamed sheet according to claim 2, wherein a foaming temperature is selected from 240 to 390 ℃ during the foaming.
10. The polyolefin resin foamed sheet according to claim 2, wherein the foaming ratio of the polyolefin resin foamed sheet is 1.1 to 18cm 3 /g。
11. The polyolefin resin foamed sheet according to claim 2, wherein the polyolefin resin foamed sheet has a 25% compressive strength of 35 to 680KPa.
12. A method for preparing polyolefin resin foamed sheet used for electronic product buffer waterproof sealing adhesive tape is characterized in that raw material mixture comprising 0.5-8wt% of azo foaming agent, matrix resin and crosslinking auxiliary agent based on the total weight of pre-foamed polyolefin resin system raw material composition is mixed and added into a high-speed mixer to be mixed to obtain mixed material, pre-foamed polyolefin resin system is obtained after banburying and crosslinking, then foaming is carried out at 240-390 ℃, and the foamed sheet is obtained through stretching and shaping;
wherein, in the foaming process, the enthalpy change value of the pre-foaming polyolefin resin system in a 160-200 ℃ interval is 4-65J/g, the enthalpy change value is scanned and measured by a DSC3 instrument of Mettler Toledo, and the test conditions are as follows: heating to 100 ℃ at 25 ℃, wherein the heating rate is 10 ℃/min, and N 2 The flow rate is 50.0ml/min; keeping the temperature at 100 ℃ for 10min 2 The flow rate is 50ml/min; heating to 200 ℃ at 100 ℃, wherein the heating rate is 10 ℃/min, and N 2 The flow rate is 50ml/min; the melt flow rate of the pre-foamed polyolefin resin system is 0.2-30g/10 min at 190 ℃ under the load of 2.16 kg;
the polyolefin resin foamed sheet has an elongation at break at 80 ℃ in the MD direction of 140 to 650%;
the specific test method for the elongation at break at 80 ℃ comprises the following steps: the sample is placed for 1h at 80 ℃, the state of the heat-treated sample is regulated for at least 4h under the standard environment with the temperature of 23 ℃ plus or minus 2 ℃ and the relative humidity of 50 percent plus or minus 10 percent, the foamed sheet is made into a standard sample with the length of 160mm and the width of 25mm according to ASTM D882, the length direction of the sample is parallel to the MD direction of the foamed sheet, and the sample is stretched at a constant speed of 300mm/min by using a tensile testing machine at 80 ℃ to record the elongation of the sample at break;
wherein the crosslinking degree of the polyolefin resin foamed sheet after crosslinking is 15 to 70%.
13. A buffering waterproof sealing tape for electronic products, which comprises the polyolefin resin foamed sheet according to any one of claims 2 to 11 and an adhesive coated on one or both surfaces thereof.
14. The electronic product buffering waterproof sealing tape of claim 13, wherein the electronic product comprises a display module, the display module at least comprises a display panel, a middle frame and a rear cover, the middle frame is used for supporting the display panel, and the rear cover is fastened to the middle frame, specifically comprising: smart mobile communication devices, notebook computers, liquid crystal displays, electronic books, tablet terminals, gaming devices, cameras, wearable electronic devices, OLED displays.
15. Use of the electronic product buffering waterproof sealing tape according to one of claims 13 to 14 for waterproofing in an electronic product.
16. The waterproof electronic product buffer sealing tape of claim 15, wherein the electronic product comprises a display module, the display module comprises at least a display panel, a middle frame and a back cover, the middle frame is used to support the display panel, the back cover is fastened to the middle frame, and the waterproof buffer sealing tape is attached between the display panel and the middle frame and/or between the middle frame and the back cover by steps including die cutting, gluing, attaching, sealing and shaping.
CN202210851073.6A 2022-07-20 2022-07-20 Polyolefin resin foamed sheet, adhesive tape, display module, and electronic product Active CN115083290B (en)

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