WO2024016912A1 - Polyolefin-based resin foamed sheet, adhesive tape, and electronic product - Google Patents

Abstract

The present invention provides a polyolefin-based resin foamed sheet, an adhesive tape, and an electronic product. The polyolefin-based resin foamed sheet of the present invention has a residual amount of an azo foaming agent of less than 2000 ppm, a percentage of close area of 93% or above, and an elongation at break of 140-650% at 80°C in the MD direction. The buffer waterproof sealing adhesive tape for an electronic product of the present invention is composed of the polyolefin-based resin foamed sheet and an adhesive layer, which is coated on one side or both sides of the polyolefin-based resin foamed sheet. The polyolefin-based resin foamed sheet of the present invention has good buffer performance and water-blocking performance, and can play the role of support, buffer and protection in a display assembly, such that the overall thickness of the display assembly is reduced, and the display assembly has good waterproof and sealing performance.

Description

Polyolefin-based resin foam sheets, tapes and electronic products Technical field

The present invention relates to the field of polyolefin foam materials, especially the field of buffer sealing materials for electronic display products, focusing on sealing tapes that are particularly suitable for waterproofing electronic equipment due to their high water resistance properties.

Background technique

As people have higher and higher requirements for electronic products, the electronic components in them are becoming increasingly sophisticated. Collision or moisture will affect the use experience of electronic products. Therefore, higher requirements are placed on the cushioning and sealing performance of electronic products. In addition, the demand for portability and thinness also requires lightweight performance of cushioning sealing materials.

Polyolefin-based resin foam sheets are lightweight and have excellent cushioning properties, and are widely used in electronic devices such as smartphones, personal computers, and electronic paper. Polyolefin-based resin foam sheets are usually placed between electronic components and frame structural members to play a buffering and sealing role.

Azo foaming agents such as azodicarbonamide (AC foaming agent) are usually used in the foaming process of polyolefin resins. However, azo foaming agents will leave foaming agent residue after foaming. On the one hand, the residual azo foaming agent has a negative impact on the mechanical properties of the polyolefin resin foam sheet, and on the other hand, it may cause serious harm to the environment. Therefore, the residual amount of azo foaming agents in electronic products is clearly limited.

Existing ideas for removing or reducing the residual amount of azo blowing agents include:

(1) Reduce the usage of foaming agent. However, it is well known that reducing the usage of foaming agent will reduce the foaming ratio and worsen the compressive strength, ultimately leading to poor cushioning properties of the foam material.

(2) Increase the foaming temperature. However, too high a foaming temperature will lead to an excessively high porosity, ultimately resulting in a deterioration of the material's positive water-blocking effect.

Therefore, another existing idea is to reduce the usage of azo foaming agents and increase the foaming temperature, so as to improve the foaming efficiency of the foaming agent and reduce the residue of the azo foaming agent. quantity. However, this idea has the following problems: on the one hand, too high a foaming temperature will still lead to too high aperture rate, which will eventually lead to a deterioration of the forward water blocking effect of the material; on the other hand, it will reduce the amount of foaming agent used. It will also lead to poor cell density and compressive strength, ultimately leading to poor cushioning performance of the foam material.

It can be seen that for foaming materials, the azo residual rate, porosity, melt strength and positive water blocking effect interact or even balance each other. In the existing technology, it is difficult to reduce the residual amount of azo foaming agent in the foam material to less than 0.1% by mass while ensuring that the material has a good positive water blocking effect.

Contents of the invention

The object of the present invention is to provide a polyolefin resin foam sheet that has a residual amount of azo foaming agent as low as 2000 ppm or less, and at the same time has a high closed cell ratio and can Provides better water blocking effect and excellent cushioning performance.

The inventors of the present invention conducted in-depth research to solve the above problems and found that when a pre-foamed polyolefin resin system containing an azo foaming agent is foamed, the foaming temperature can be increased by reducing the amount of foaming agent used. , and by controlling various parameters of the pre-expanded polyolefin resin system, the resulting polyolefin resin sheet has a specific high-temperature elongation at break, thereby reducing the azo foaming agent in the sheet The remaining amount increases the closed cell ratio and gives the sheet excellent buffering properties. Applying such polyolefin-based resin sheets to display components can reduce the overall thickness of the display component to achieve an ultra-thin display screen effect, and has good waterproof and sealing properties. Based on this concept, the present invention was completed.

The specific solutions of the present invention are as follows:

In a first aspect, the present invention discloses a display assembly, including 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. The display panel and the middle frame There is a buffer waterproof sealing tape between them and/or between the middle frame and the back cover.

The buffer waterproof sealing tape is composed of a polyolefin resin foam sheet and an adhesive layer coated on one or both sides of the sheet. The polyolefin-based resin foam sheet has a plurality of cells formed inside, and the cells are formed by using an azo foaming agent to pre-expand the polyolefin-based resin system formed from a polyolefin-based matrix resin. It is formed during the foaming process; the residual azo foaming agent of the polyolefin resin foam sheet is less than 2000ppm, the closed cell rate is more than 93%, and the 80°C breaking elongation in the MD direction is 140 -650%, preferably 150-610%.

Among them, the specific test method for the 80°C breaking elongation in the MD direction is: heat treat the sample at 80°C for 1 hour, and place the heat-treated sample at a temperature of 23°C ± 2°C and a relative humidity of 50% ± 10 Condition (i.e., place) in % standard environment for at least 4 hours. According to ASTM D882, the foam sheet is made into a standard specimen with a length of 160mm and a width of 25mm. The length direction of the standard sample is parallel to the MD direction of the foam sheet. Use a tensile testing machine to stretch the standard sample at a constant speed of 300 mm/min at 80°C, and record the elongation when the sample breaks.

In the present invention, it should be noted that the pre-expanded polyolefin resin system described in this application refers to the polyolefin resin system after cross-linking and before foaming.

In one of the solutions, the azo foaming agent is selected from one or more of azodicarbonamide, diisopropyl azodicarboxylate, azodicarboxylic acid metal salt, and azobisisobutyronitrile. species, more preferably azodicarbonamide. The initial content of the azo foaming agent (that is, the usage amount of the azo foaming agent) is 0.5-8wt% based on the total weight of the pre-foamed polyolefin resin system raw material composition; preferably 0.5-6wt %; further preferably 1-4wt%.

In another solution, the enthalpy change value of the pre-expanded polyolefin resin system in the range of 160-200°C It is 4-65J/g, preferably 10-50J/g, more preferably 20-35J/g.

In another aspect, the polyolefin-based matrix resin is selected from one or more polymer resins selected from the group consisting of polyethylene resin, polypropylene resin, and ethylene-vinyl acetate copolymer. The melt flow rate (MFR) of the polyolefin-based matrix resin and the polyolefin-based matrix resin blend composed of multiple types of the polymer resin at 190°C and a load of 2.16kg is 0.2-30g/ 10 minutes, more preferably 0.3-20g/10 minutes, even more preferably 0.5-5g/10 minutes.

In another embodiment, during the foaming process of the pre-expanded polyolefin resin system, the foaming temperature is 240-390°C, more preferably 260-380°C, and particularly preferably 280-350°C.

In another aspect, the polyolefin resin foam sheet has an expansion ratio of 1.1-18cm ³ /g, preferably 1.2-15cm ³ /g.

In another aspect, the 25% compressive strength of the polyolefin resin foam sheet is 35-680KPa, preferably 50-450KPa.

In a second aspect, the invention also discloses a polyolefin-based resin foam sheet, which has low azo residue and high forward water blocking properties. This polyolefin-based resin foam sheet can be used as a cushioning and waterproof sealing tape for electronic products.

In the present invention, the positive water blocking performance refers to the water blocking performance of the surface of the foam sheet. "Forward" refers to the surface of the foam sheet.

The polyolefin-based resin foam sheet has a plurality of cells inside, and the cells are foamed by using an azo-based foaming agent to foam a pre-expanded polyolefin-based resin system composed of a polyolefin-based matrix resin. Formed by the foaming process. Based on the total weight of the polyolefin resin foam sheet, the residual amount of azo foaming agent is less than 2000 ppm, preferably less than 1500 ppm, further preferably less than 1000 ppm, and the closed cell rate is 93% or more, preferably 95% or more, More preferably, it is 97% or more; the 80°C breaking elongation in the MD direction of the polyolefin-based resin foam sheet is 140-650%, preferably 150-610%.

In one of the solutions, the azo foaming agent is selected from one or more of azodiformamide, diisopropyl azodicarboxylate, azodicarboxylic acid metal salt, and azobisisobutyronitrile. species, more preferably azodicarbonamide. The initial content of the azo foaming agent is 0.5-8wt% based on the total weight of the pre-foamed polyolefin resin system raw material composition; preferably 0.5-6wt%; further preferably 1-4wt%.

In another solution, the enthalpy change value of the pre-expanded polyolefin resin system in the range of 160-200°C is 4-65J/g, preferably 10-50J/g, and further preferably 20-35J/g. .

In another aspect, the polyolefin-based matrix resin is selected from one or more polymer resins selected from the group consisting of polyethylene resin, polypropylene resin, and ethylene-vinyl acetate copolymer. A polyolefin-based matrix resin composed of one of the polymer resins and a polyolefin-based matrix resin blend composed of a plurality of the polymer resins are both in The melt flow rate (MFR) at 190° C. under a load of 2.16 kg is 0.2-30 g/10 minutes, more preferably 0.3-20 g/10 minutes, even more preferably 0.5-5 g/10 minutes.

In another embodiment, during the foaming process of the pre-expanded polyolefin resin system, the foaming temperature is 240-390°C, more preferably 260-380°C, and particularly preferably 280-350°C.

In another aspect, the polyolefin resin foam sheet has an expansion ratio of 1.1-18cm ³ /g, preferably 1.2-15cm ³ /g.

In another aspect, the 25% compressive strength of the polyolefin resin foam sheet is 35-680KPa, preferably 50-450KPa.

Similarly, the present invention also discloses a polyolefin-based resin foam sheet with low azo residue and high forward water blocking performance.

In a third aspect, the present invention also discloses a method for preparing polyolefin resin foam sheet, which will include 0.5-8wt% based on the total weight of the raw material composition; preferably 0.5-6wt%; further preferably 1-4wt % of the raw material composition of azo foaming agent, matrix resin and cross-linking auxiliary agent is mixed to obtain a mixture. After blending the mixture, a polyolefin-based matrix resin blend is obtained, and the polyolefin-based matrix resin blend is obtained. After cross-linking, a pre-foamed polyolefin resin system is obtained, which is then foamed at 240-390°C, more preferably at 260-380°C, particularly preferably at 280-350°C, and is obtained by stretching and setting. foam sheet;

Among them, during the foaming process, the enthalpy change value of the pre-foamed polyolefin resin system in the range of 160-200°C is 4-65J/g, preferably 10-50J/g, and further preferably 20-35J/g. g;

The 80°C breaking elongation in the MD direction of the polyolefin-based resin foam sheet is preferably 140-650%, preferably 150-610%.

In the present invention, the specific test method for the 80°C breaking elongation in the MD direction is: after heat treatment of the sample at 80°C for 1 hour, the heat-treated sample is placed at a temperature of 23°C ± 2°C and a relative humidity of 50%. Condition (i.e., place) in a standard environment of ±10% for at least 4 hours. According to ASTM D882, the foam sheet is made into a standard specimen with a length of 160mm and a width of 25mm. The length direction of the standard sample is parallel to the MD direction of the foam sheet. Use a tensile testing machine to stretch the standard sample at a constant speed of 300 mm/min at 80°C, and record the elongation when the sample breaks;

The heat enthalpy change value was scanned and measured using Mettler Toledo's DSC3 instrument. The test conditions were: heating from 25°C to 100°C, the heating rate was 10°C/min, the _N flow rate was 50.0ml/min; and the temperature was maintained at 100°C. 10min, _N2 flow rate is 50ml/min; heating from 100℃ to 200℃, heating rate is 10℃/min, _N2 flow rate is 50ml/min. Among them, the sample for scanning measurement is a sample cut into thin slices, and its weight is 0.8-1.2mg.

In a fourth aspect, the present invention also discloses a cushioning and waterproof sealing tape for electronic products, which is composed of the above-mentioned polyolefin resin foam sheet and an adhesive coated on one or both sides of the sheet.

In a fifth aspect, the present invention also discloses the waterproof application of the buffer waterproof sealing tape for electronic products in electronic products, specifically including in smart mobile communication equipment, notebook computers, liquid crystal displays, e-books, tablet terminals, game equipment, and cameras. , wearable electronic devices, and applications in OLED displays.

According to a specific embodiment of the present invention, the electronic product includes a display assembly. The display assembly at least includes a display panel, a middle frame and a back cover. The middle frame is used to support the display panel, and the back cover is fastened to the middle frame. frame.

When performing waterproof applications, the buffered waterproof sealing tape for electronic products of the present invention is pasted between the display panel and the middle frame and/or between the middle frame and the middle frame through steps including die cutting, cutting, gluing, installation, sealing and shaping. Between the back covers, the middle frame is used to support the display panel, and the back cover is fastened to the middle frame to provide buffering and sealing effects.

The invention has the following beneficial effects:

The polyolefin-based resin foam sheet obtained according to the present invention has a high 80°C breaking elongation in the MD direction. Even if the usage amount of the foaming agent is reduced and the foaming temperature is increased, the obtained foam sheet still has a closed cell ratio. The foaming ratio is high and the foaming ratio is maintained within a reasonable range, and the resulting buffered waterproof sealing tape for electronic products has ideal effects in three aspects: low azo foaming agent residual rate, water resistance and buffering performance. The obtained buffered waterproof sealing tape for electronic products can play the role of supporting, buffering and protecting the display component, thereby reducing the overall thickness of the display component and having good waterproof sealing performance.

Description of drawings

Figure 1 is an SEM photograph showing a cross section along the MD direction of the polyolefin-based resin foam sheet of Example 1;

Figure 2 is a schematic structural diagram of a display component of the present invention.

Explanation of reference signs
1-Display panel; 2-middle frame; 3-back cover; 4-buffer waterproof sealing tape.

Detailed ways

In order to better explain the present invention, in the following, the present invention will be described in detail with reference to the embodiments, and the main content of the present invention will be further clarified with reference to specific examples. However, the content of the present invention is not limited to the following specific implementation modes and examples.

[Polyolefin base resin and blend of polyolefin base resin]

The type of polyolefin resin constituting the foam sheet is a conventional foamable base material in this field, and can be selected from polyethylene resin, polypropylene resin, ethylene-vinyl acetate copolymer, etc. Based on mechanical properties and cost considerations, polyethylene is preferred vinyl resin.

The selection of polyethylene resin is not particularly limited, including but not limited to low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, etc. In addition, the polyethylene resin can also be an ethylene-α-olefin copolymer with ethylene as the main component, wherein the α-olefin is selected from α-olefins with 2 to 12 carbon atoms, such as 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, etc. Only one type of α-olefin may be used, or two or more types may be used. The above polyethylene-based resins may be used alone or in combination of two or more types. Low density polyethylene is preferred. The density of low-density polyethylene is preferably 0.910-0.925g/cm ³ , more preferably 0.912-0.922g/cm ³ .

In order to increase the resilience and shape following properties of the polyolefin resin foam sheet, rubber and/or thermoplastic elastomer may be further added to the polyolefin base resin. The rubber and/or thermoplastic elastomer has a glass transition temperature below 20°C, and specific examples include: natural rubber, polyisobutylene, isoprene rubber, butyl rubber, chloroprene rubber, nitrile rubber and other natural or synthetic Rubber; olefin elastomers such as ethylene-vinyl acetate copolymer, polybutylene, polyisobutylene and chlorinated polyethylene; styrene-isoprene-styrene copolymer (SIS), styrene-butadiene- Styrenic elastomers such as styrene copolymer (SBS), styrene-isoprene-butadiene-styrene copolymer (SIBS), and their hydrogenated polymers; thermoplastic polyester elastomers; thermoplastic polyurethane Elastomers; and thermoplastic acrylic elastomers. Only one type of these rubbers and thermoplastic elastomers may be used, or two or more types may be used. In addition, the number of types of the components may be only one type, or may be two or more types. Based on the total weight of the polyolefin-based matrix resin, the rubber and/or thermoplastic elastomer content is 0-55 wt%, preferably 2-50 wt%.

Before forming the mixture, other functional additives can be added according to actual needs to further improve the various properties of the polyolefin foam sheet. The functional additives can be listed as: antioxidants, antibacterial agents, colorants, antistatic agents and fillers.

After mixing the raw material composition consisting of the polyolefin-based matrix resin and other raw material components, a mixture is obtained, and the mixture is blended to obtain a polyolefin-based matrix resin blend. The blending can be performed by kneading or the like.

It should be noted that no matter how the type of base material is selected, whether an elastomer material is added, or whether other functional additives are added, a polyolefin-based matrix resin composed of one type of polymer resin and a variety of polymer resins are The melt flow rate (MFR) of the polyolefin-based matrix resin blend at 190°C and a load of 2.16kg needs to be controlled at 0.2-30g/10 minutes, more preferably 0.3-20g/10 minutes, and even more preferably 0.5-5g/10 minutes. When the melt flow rate of the polyolefin-based matrix resin blend blended with elastomers, foaming agents and functional additives falls within this range, the pre-foamed polyolefin-based resin system can be guaranteed to be foamed during foaming. The melt strength is so high that it can contain gas only when foaming at high temperatures and will not break or open holes due to excessive temperature.

Here, the MFR of the polyolefin-based matrix resin blend was measured in accordance with GB/T 3682.1.

[cross-linking]

In the present invention, after the blending of the polyolefin-based matrix resin and other additives or raw material components is completed, a gelation reaction process, that is, a cross-linking reaction, is performed. Cross-linking can be performed by known techniques. Common methods include radiation cross-linking or chemical cross-linking, with radiation cross-linking being preferred.

Radiation cross-linking is cross-linking by irradiating a resin sheet with ionizing radiation such as electron rays, alpha rays, beta rays, and gamma rays. The irradiation dose of the ionizing radiation may be adjusted so that the degree of cross-linking of the obtained foam sheet falls within the desired range. However, the irradiation dose is preferably 10 to 30 Mrad, and more preferably 10 to 25 Mrad.

Chemical crosslinking is achieved by blending an organic peroxide into a pre-expanded polyolefin resin system raw material composition and decomposing the organic peroxide by heating. Radiation cross-linking and chemical cross-linking can also be used together.

The cross-linking degree of the polyolefin foam sheet of the present invention is 15-70%, preferably 18-50%, and more preferably 20-40%. If the degree of cross-linking is within this range, the melt strength of the final molded pre-expanded polyolefin-based resin will also be controlled, and at the same time, the pore size of the subsequent foamed cells can be ensured to be uniform.

The inventor found that when the melt strength of the pre-expanded polyolefin resin system after radiation cross-linking is controlled at a reasonable melt strength, it can enclose the gas during high-temperature foaming without causing the temperature to be too high. causing the opening to rupture. There are many factors that affect the melt strength of pre-expanded polyolefin resin systems, such as resin type, copolymerization method, molecular weight and molecular weight distribution, resin crystallinity and cross-linking degree. In the present invention, by controlling the melt flow rate of the polyolefin-based matrix resin blend and the cross-linking degree of the cross-linked resin, the melt strength of the pre-foamed polyolefin-based resin system is controlled at a reasonable melt strength. body strength.

[Azo foaming agent]

Azo foaming agents have the advantage of uniform and controllable foaming. In addition, for polyolefin resin foaming, from the comprehensive perspective of obtaining fine cells, economy, environmental protection and safety, azo foaming agents are better than inorganic foaming agents and nitroso foaming agents. , sulfonyl hydrazide foaming agents and physical foaming agents. Specific examples of azo foaming agents include: azodiformamide, diisopropyl azodicarboxylate, azodicarboxylic acid metal salts (barium azodicarboxylate, etc.), azobisisobutyronitrile, etc., and more preferably It is azodicarbonamide.

[foaming]

Since this application wants to solve the technical problem of keeping the residual amount of azo blowing agents in the final product as low as possible, two factors are mainly considered here:

1. Content of azo foaming agent.

When foaming, reduce the amount of azo foaming agent used. The ideal state is that the azo foaming agent is fully utilized during the foaming process and meets the predetermined expansion ratio of the polyolefin resin foam sheet. The specific content is selected as follows: the content (usage amount) of the azo foaming agent is 0.5-8wt% based on the total weight of the pre-foamed polyolefin resin system raw material composition; preferably 0.5-6wt%; further preferably 1-4wt%. When the content of the azo foaming agent is less than 0.5wt%, the foaming ratio of the polyolefin resin foam sheet cannot meet the minimum predetermined requirements; when the content of the azo foaming agent is higher than 8wt% , will lead to an inevitable increase in the residual amount of azo foaming agents in the finished polyolefin resin foam sheet.

2. Foaming temperature.

The foaming temperature determines the foaming efficiency of the azo foaming agent and the melt strength of the pre-foamed polyolefin resin system during foaming. The ideal state is that the azo foaming agent is completely decomposed at this temperature, and the gas generated by the decomposition is completely released to obtain a higher foaming ratio. When the content of azo foaming agent is small, in order to obtain a higher foaming ratio, the foaming temperature needs to be increased. For polyolefin-based resin foam sheets, especially for polyethylene-based resin foam sheets, the foaming temperature is specifically selected to be 240-390°C, more preferably 260-380°C, particularly preferably 280-280°C. 350℃. When the temperature is higher than 390°C, the melt strength of the polyolefin resin is too low when foaming, and cell collapse is prone to occur, and even decomposition of the matrix resin occurs; when the temperature is lower than 240°C, incomplete foaming is prone to occur , there is too much residual azo foaming agent, and the foaming ratio cannot meet the expectations. However, when the foaming temperature is only controlled within the above range, the problem of high porosity cannot be avoided, which will adversely affect the water blocking effect.

Based on the control of the foaming agent content and foaming temperature, the foaming ratio of the polyolefin resin foam sheet of the present invention falls within the ideal range, which is 1.1-18cm ³ /g, preferably 1.2-15cm ³ /g . If the expansion ratio is less than 1.1cm ³ /g, the flexibility of the foam sheet cannot be guaranteed. If the expansion ratio is more than 18cm ³ /g, the mechanical strength of the foam sheet will be affected. Under the condition of the same foaming ratio, the usage amount of azo foaming agent in the present invention is lower than that in the prior art, and the foaming temperature is higher than that in the prior art, thereby reducing the The residual amount of azo blowing agent in the final product.

The use of foaming aids is helpful to adjust the decomposition temperature and decomposition speed of the foaming agent. Foaming aids suitable for azo foaming agents include urea, phosphate esters, organic acids, and metal salt foaming agents. The auxiliary agent is preferably a metal salt foaming auxiliary, and further preferably is a metal zinc salt foaming auxiliary such as zinc oxide, stearic acid, and zinc stearate.

The inventor found that when the pre-expanded polyolefin resin is selected at a reasonable melt strength, by selecting appropriate foaming agents and foaming auxiliaries, the pre-expanded polyolefin resin system can be maintained in the range of 160-200°C. The enthalpy change value within is 4-65J/g, preferably 10-50J/g, and further preferably 20-35J/g, which can reduce the usage of foaming agent and achieve good foaming rate at high temperatures. Control to prevent the foaming rate from being too high, causing the cells to break and form openings during the foaming process. The selection of foaming agents and foaming assistants has an impact on the change of heat enthalpy value in the 160-200°C range. In addition to the selection of the type and content of azo foaming agents and foaming assistants, including the particle size of the foaming agent. Diameter size, surface morphology, and crystal form will all affect its thermal enthalpy value. No matter how the various factors are combined, those skilled in the art will have good results by controlling the change in enthalpy value to fall within this range.

The heat enthalpy change value was scanned and measured using Mettler Toledo's DSC3 instrument. The test conditions were: heating from 25°C to 100°C, the heating rate was 10°C/min, the _N flow rate was 50.0ml/min; and the temperature was maintained at 100°C. 10min, _N2 flow rate is 50ml/min; heating from 100℃ to 200℃, heating rate is 10℃/min, _N2 flow rate is 50ml/min. Among them, the sample for scanning measurement is a sample cut into thin slices or crumbs, and its weight is 0.8-1.2mg.

[Stretch and shape]

Stretching and shaping can occur during the foaming process, or the polyolefin foam sheet can be reheated after cooling in a molten or softened state, or both processes can be performed. You can perform both synchronous and asynchronous bidirectional stretching.

The stretching ratio along the MD direction is 1.1 times to 3.0 times, preferably 1.2 times to 2.0 times, and the stretching ratio along the TD direction is 1.0 times to 3.0 times, preferably 1.1 times to 2.0 times. When the stretching ratio satisfies the above range, the polyolefin-based foam sheet can be prevented from breaking during stretching, the flexibility and tensile strength of the foam sheet are good, and the quality of the foam sheet becomes more uniform. A polyolefin-based foam sheet having a 25% compressive strength of 35-680KPa, preferably a 25% compressive strength of 50-450KPa can be finally obtained.

Here, "MD" refers to the machine direction (Machine Direction), which is the direction consistent with the extrusion direction of the polyolefin-based resin foam sheet. In addition, "TD" refers to the transverse direction, which is a direction orthogonal to MD and parallel to the foam sheet.

[Preparation method of polyolefin resin foam sheet]

The preparation method of the polyolefin-based foam sheet of the present invention sequentially includes the following steps: blending and extrusion, cross-linking, foaming, and stretching and shaping.

Specifically, the preparation method of the polyolefin foam sheet of the present invention includes the following steps:

Blending and extrusion, mixing a raw material composition including 0.5-8 wt% of azo foaming agent, polyolefin matrix resin and cross-linking auxiliary based on the total weight of the pre-foamed polyolefin resin system raw material composition. The mixture is blended using a mixer to obtain a polyolefin-based matrix resin blend, and then the blend is extruded into strip-shaped resin sheets.

Cross-linking, cross-linking strip-shaped resin sheets to obtain a pre-foamed polyolefin resin system,

Foaming, foaming the pre-foamed polyolefin resin system at 240-390°C, and

Stretching and shaping: The foamed strip-shaped resin sheet is stretched and shaped to finally obtain the polyolefin-based foamed sheet of the present invention.

[High-temperature mechanical properties of polyolefin-based resin foam sheets]

The inventor has analyzed the azo foaming agent content, foaming temperature, enthalpy change value of the pre-foamed polyolefin resin system in the 160-200°C range, and the melt flow of the polyolefin matrix resin blend. Through comprehensive limitations of many factors such as rate (MFR), cross-linking degree, foaming ratio, and stretch ratio, it was finally found that when polyolefin-based resin foam sheets When the 80°C breaking elongation in the MD direction is 140-650%, preferably 150-610%, it can ensure complete sealing when the amount of azo blowing agent is too small and the foaming temperature is too high. The cell structure, on the other hand, ensures that the cell diameter is within a reasonable range to achieve a better foaming ratio, thereby obtaining a polyolefin-based resin with ideal cushioning performance, low azo residual rate, high closed cell rate and excellent water barrier properties. Bubble sheet. Specifically, the residual amount of the azo foaming agent is less than 2000 ppm, preferably less than 1500 ppm, more preferably less than 1000 ppm, and the closed cell rate is 93% or more, preferably 95% or more, further preferably 97% or more. According to the following examples, it can be seen that from the perspective of improving the water blocking effect, reducing the azo residual rate and improving the buffering performance, the 80°C breaking elongation in the MD direction is preferably 140-650%. If it is greater than 650%, the closed cell ratio of the foam sheet will be reduced, thereby reducing the water blocking effect; if it is less than 140%, the foam sheet will not be fully foamed, resulting in low sheet magnification, poor buffering effect, and high azo residual rate. .

Here, the specific test method for the 80°C breaking elongation in the MD direction is: heat-treat the sample at 80°C for 1 hour, and place the heat-treated sample at a temperature of 23°C ± 2°C and a relative humidity of 50% ± 10 % of the standard environment for state adjustment at least 4h. According to ASTM D882, the foam sheet is made into a standard specimen with a length of 160 mm and a width of 25 mm. The length direction of the standard specimen is parallel to the MD direction of the foam sheet. Use a tensile testing machine to stretch the standard sample at a constant speed of 300 mm/min at 80°C, and record the elongation when the sample breaks.

The polyolefin resin foam sheet of the present invention can also add other functional additives during the foaming and molding process according to the application scenario. The type and content of the additives must ensure that the elongation at break of the molded polyolefin resin foam sheet at 80°C falls within the above range.

Other functional additives include but are not limited to: processing aids, flame retardants, antistatic agents, etc.

[Adhesive layer]

The obtained polyolefin-based resin foam sheet needs to be glued before lamination. After glueing, a buffer waterproof sealing tape for electronic products is formed. That is, an adhesive layer is formed on the surface of the sheet. Various existing gluing methods can be applied, such as the method of directly applying the adhesive composition to the foam sheet (direct method), applying the above-mentioned adhesive composition to an appropriate peeling surface, A method such as forming an adhesive layer on the peeling surface, bonding the adhesive layer to a foam sheet, and transferring the adhesive layer (transfer method). For coating, known or conventional coating methods such as a gravure roll coater, a reverse roll coater, a contact roll coater, a dip roll coater, a bar coater, a blade coater, and a spray coater can be used. machine. The thickness of the adhesive layer is controlled at 1 μm to 50 μm, preferably 2-20 μm, and more preferably 3-10 μm.

The type of adhesive constituting the adhesive layer is not particularly limited, and specific examples include: acrylic adhesives, rubber adhesives (natural rubber adhesives, synthetic rubber adhesives, mixtures thereof, etc.), silicone adhesives. One of various well-known adhesives such as mixture, polyester-based adhesive, urethane-based adhesive, polyether-based adhesive, polyamide-based adhesive, fluorine-based adhesive, etc. Or an adhesive layer composed of two or more adhesives. From the viewpoint of transparency and weather resistance, it is preferable to use an acrylic adhesive to form the adhesive layer.

[Practical application]

The polyolefin-based resin foam sheet of the present invention forms a buffer waterproof sealing tape for electronic products, and is installed between the printed circuit board and the cover plate of the electronic product, or between the image display component and the display glass plate, especially between the carrying unit and the display glass plate. The limited space between the casing and the display panel is sealed and shaped to play a buffering and waterproof role.

The electronic products include: intelligent mobile communication equipment, notebook computers, liquid crystal displays, OLED displays, e-books, tablet terminals, game devices, cameras, and wearable electronic devices.

As shown in Figure 2, a buffer waterproof sealing tape 4 for electronic products is formed from the polyolefin-based resin foam sheet of the present invention, and is particularly used in display components. The display assembly includes 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 has a blocking part 21 and a bearing part 22 vertically connected to the blocking part 21. The display panel 21 is disposed on the bearing part 22 and is connected and fixed with the display panel 21 through a buffer waterproof sealing tape 4. The back cover 3 and the middle frame 2 can also be sealed and connected through buffer waterproof sealing tape. In addition, other electronic components in the display assembly, such as PCB boards, batteries and other electronic components, are arranged in a receiving space formed by the display panel 1, the middle frame 2 and the back cover 3. A piece of buffering and waterproof sealing tape can also be provided on the side of the back cover facing the display panel to buffer and protect the electronic components. Since the buffered waterproof sealing tape 4 has excellent buffering and water-blocking properties, it can play a supporting, buffering and protective role in the display component. In addition, it can also reduce the overall thickness of the display component and has good Waterproof sealing performance.

The present invention utilizes the excellent water-blocking performance of the buffer waterproof sealing tape 4 to achieve a good sealing and protection effect on the display assembly, and utilizes the excellent buffering effect of the buffer waterproof sealing tape 4 to achieve slimming of the display panel.

Example

Hereinafter, several embodiments related to the present invention will be described, but there is no intention to limit the present invention to these embodiments.

In the following examples and comparative examples, the equipment used includes:

Mixing machine: Plasti-Corder Lab-Station LS3-100 manufactured by Brabender Company of Germany;

Stretching machine: DMTFilm series manufactured by DMT Company (USA);

Extruder: JWS120 type manufactured by Suzhou Jinwei Machinery Manufacturing Co., Ltd.

Example 1

Mix 100 parts by weight of LDPE (trade name: Sinopec 2426H), 5 parts by weight of azodicarbonamide and 0.2 parts by weight of zinc oxide in a high-speed mixer, and then blend in a mixer at 130°C to obtain MFR It is a polyolefin resin base material blend of 1.87g/10min. The polyolefin resin base blend is then extruded into strip-shaped resin sheets. Then, the strip-shaped resin sheet was irradiated on both sides, the irradiation energy was 1.5Mev, and the irradiation dose was 25Mrad. The resin sheet is cross-linked. The cross-linked resin sheet is continuously fed into a heating furnace at 345°C. The heating furnace is heated by an infrared heater to foam the resin sheet. Then, it was stretched in a stretching machine at 120° C. 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 to obtain a polyolefin-based foam sheet. The thickness of the obtained polyolefin-based foam sheet was 0.3 mm, and the average MD bubble diameter measured from the SEM photo was 120 μm, as shown in Figure 1.

Example 2

Mix 100 parts by weight of LDPE (trade name: Sinopec 2426H), 6 parts by weight of azodicarbonamide and 0.2 parts by weight of zinc oxide in a high-speed mixer, and then blend in a mixer at 130°C to obtain MFR It is a polyolefin resin base material blend of 0.86g/10min. The polyolefin resin base material blend is then extruded into strip-shaped resin sheets using an extruder. Then, the strip-shaped resin sheet is irradiated on both sides, the irradiation energy is 1.5 Mev, and the irradiation dose is 30 Mrad, so that the resin sheet is cross-linked. The cross-linked resin sheet is continuously fed into a heating furnace at 290°C. The heating furnace is heated by an infrared heater to foam the resin sheet. Then, it was stretched in a stretching machine at 120° C. at a stretching ratio of 1.5 times in the MD direction and at a stretching ratio of 1.5 times in the TD direction to obtain a polyolefin-based foam sheet. The thickness of the obtained polyolefin-based foam sheet was 0.05 mm, and the average MD cell diameter measured from the SEM photograph was 90 μm.

Example 3

Mix 80 parts by weight of LDPE (trade name: Sinopec 2426H), 20 parts by weight of LLDPE, 5 parts by weight of azodicarbonamide and 0.2 parts by weight of zinc oxide in a high-speed mixer, and then mix it in a mixer at 130°C. Blending to obtain a polyolefin resin base material blend with an MFR of 1.6g/10min. The polyolefin resin base material blend is then extruded into strip-shaped resin sheets using an extruder. Then, the strip-shaped resin sheet is irradiated on both sides, the irradiation energy is 1.5Mev, and the irradiation dose is 28 Mrad, so that the resin sheet is cross-linked. The cross-linked resin sheet is continuously fed into a heating furnace at 360°C. The heating furnace is heated by an infrared heater to foam the resin sheet. Then, it was stretched in a stretching machine at 120° C. at a stretching ratio of 1.3 times in the MD direction and at a stretching ratio of 1.4 times in the TD direction to obtain a polyolefin-based foam sheet. The thickness of the obtained polyolefin-based foam sheet was 0.08 mm, and the average MD cell diameter measured from the SEM photo was 80 μm.

Example 4

Mix 80 parts by weight of LDPE (trade name: Sinopec 2426H), 20 parts by weight of LLDPE, 4 parts by weight of azodicarbonamide and 0.2 parts by weight of zinc oxide in a high-speed mixer, and then mix it in a mixer at 130°C. Blending to obtain a polyolefin resin base blend with an MFR of 2.77g/10min. The polyolefin resin base material blend is then extruded into strip-shaped resin sheets using an extruder. Then, the strip-shaped resin sheet is irradiated on both sides, the irradiation energy is 1.5 Mev, and the irradiation dose is 20 Mrad, so that the resin sheet is cross-linked. The cross-linked resin sheet is continuously fed into the 300°C In the heating furnace, the heating furnace uses an infrared heater to heat the resin sheet to foam. Then, it was stretched in a stretching machine at 120° C. at a stretching ratio of 1.1 times in the MD direction and at a stretching ratio of 1 times in the TD direction to obtain a polyolefin-based foam sheet. The thickness of the obtained polyolefin-based foam sheet was 0.2 mm, and the average MD cell diameter measured from the SEM photo was 110 μm.

Example 5

Mix 65 parts by weight of LDPE (trade name: Sinopec 2426H), 35 parts by weight of LLDPE, 4 parts by weight of azodicarbonamide and 0.2 parts by weight of zinc oxide in a high-speed mixer, and then mix it in a mixer at 130°C. Blending to obtain a polyolefin resin base material blend with an MFR of 3.85g/10min. The polyolefin resin base material blend is then extruded into strip-shaped resin sheets using an extruder. Then, the strip-shaped resin sheet is irradiated on both sides, the irradiation energy is 1.5 Mev, and the irradiation dose is 15 Mrad, so that the resin sheet is cross-linked. The cross-linked resin sheet is continuously fed into a heating furnace at 290°C. The heating furnace is heated by an infrared heater to foam the resin sheet. Then, it was stretched in a stretching machine at 120°C at a stretching ratio of 1.2 times in the MD direction and at a stretching ratio of 1.1 times in the TD direction to obtain a polyolefin-based foam sheet. The thickness of the obtained polyolefin-based foam sheet was 0.12 mm, and the average MD cell diameter measured from the SEM photo was 110 μm.

Example 6

Mix 80 parts by weight of LDPE (trade name: Sinopec 2426H), 20 parts by weight of POE, 5 parts by weight of azodicarbonamide and 0.2 parts by weight of zinc oxide in a high-speed mixer, and then mix it in a mixer at 130°C. Blending to obtain a polyolefin resin base material blend with an MFR of 5g/10min. The polyolefin resin base material blend is then extruded into strip-shaped resin sheets using an extruder. Then, the strip-shaped resin sheet is irradiated on both sides, the irradiation energy is 1.5 Mev, and the irradiation dose is 10 Mrad, so that the resin sheet is cross-linked. The cross-linked resin sheet is continuously fed into a heating furnace at 280°C. The heating furnace is heated by an infrared heater to foam the resin sheet. Then, the film was stretched in a stretching machine at 120° C. at a draw ratio of 2 times in the MD direction and at a draw ratio of 2 times in the TD direction to obtain a polyolefin-based foam sheet. The thickness of the obtained polyolefin-based foam sheet was 0.45 mm, and the average MD cell diameter measured from the SEM photo was 180 μm.

Example 7

Mix 85 parts by weight of LDPE (trade name: Sinopec 2426H), 15 parts by weight of POE, 6 parts by weight of azodicarbonamide and 0.2 parts by weight of zinc oxide in a high-speed mixer, and then mix it in a mixer at 130°C. Blending to obtain a polyolefin resin base blend with an MFR of 3.11g/10min. The polyolefin resin base material blend is then extruded into strip-shaped resin sheets using an extruder. Then, the strip-shaped resin sheet is irradiated on both sides, the irradiation energy is 1.5 Mev, and the irradiation dose is 15 Mrad, so that the resin sheet is cross-linked. The cross-linked resin sheet is continuously fed into a heating furnace at 290°C. The heating furnace is heated by an infrared heater to foam the resin sheet. Then in the stretching machine along the MD direction at 120°C The film was stretched in the TD direction at a draw ratio of 1.8 times and in the TD direction at a draw ratio of 1.5 times to obtain a polyolefin-based foam sheet. The thickness of the obtained polyolefin-based foam sheet was 0.15 mm, and the average MD cell diameter measured from the SEM photo was 110 μm.

Example 8

Mix 70 parts by weight of LDPE (trade name: Sinopec 2426H), 30 parts by weight of POE, 3 parts by weight of azodicarbonamide and 0.2 parts by weight of zinc oxide in a high-speed mixer, and then mix it in a mixer at 130°C. Blending to obtain a polyolefin resin base blend with an MFR of 1.83g/10min. The polyolefin resin base material blend is then extruded into strip-shaped resin sheets using an extruder. Then, the strip-shaped resin sheet is irradiated on both sides, the irradiation energy is 1.5Mev, and the irradiation dose is 26 Mrad, so that the resin sheet is cross-linked. The cross-linked resin sheet is continuously fed into a heating furnace at 310°C. The heating furnace is heated by an infrared heater to foam the resin sheet. Then, it was stretched in a stretching machine at 120° C. at a stretching ratio of 2.8 times in the MD direction and at a stretching ratio of 3 times in the TD direction to obtain a polyolefin-based foam sheet. The thickness of the obtained polyolefin-based foam sheet was 0.2 mm, and the average MD cell diameter measured from the SEM photo was 110 μm.

Example 9

Mix 80 parts by weight of LDPE (trade name: Sinopec 2426H), 20 parts by weight of EVA, 4 parts by weight of azodicarbonamide and 0.2 parts by weight of zinc oxide in a high-speed mixer, and then mix it in a mixer at 130°C. Blending to obtain a polyolefin resin base blend with an MFR of 4.72g/10min. The polyolefin resin base material blend is then extruded into strip-shaped resin sheets using an extruder. Then, the strip-shaped resin sheet is irradiated on both sides, the irradiation energy is 1.5 Mev, and the irradiation dose is 13 Mrad, so that the resin sheet is cross-linked. The cross-linked resin sheet is continuously fed into a heating furnace at 280°C. The heating furnace is heated by an infrared heater to foam the resin sheet. Then, it was stretched in a stretching machine at 120° C. 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-based foam sheet. The thickness of the obtained polyolefin-based foam sheet was 0.32 mm, and the average MD cell diameter measured from the SEM photo was 125 μm.

Example 10

Mix 70 parts by weight of LDPE (trade name: Sinopec 2426H), 30 parts by weight of EVA, 4 parts by weight of azodicarbonamide and 0.2 parts by weight of zinc oxide in a high-speed mixer, and then mix it in a mixer at 130°C. Blending to obtain a polyolefin resin base blend with an MFR of 4.76g/10min. The polyolefin resin base material blend is then extruded into strip-shaped resin sheets using an extruder. Then, the strip-shaped resin sheet is irradiated on both sides, the irradiation energy is 1.5 Mev, and the irradiation dose is 13 Mrad, so that the resin sheet is cross-linked. The cross-linked resin sheet is continuously fed into a heating furnace at 280°C. The heating furnace is heated by an infrared heater to foam the resin sheet. Then, it is stretched in a stretching machine at 120°C at a stretching ratio of 2 times in the MD direction and 2.5 times in the TD direction to obtain polyolefin. It is a foam sheet. The thickness of the obtained polyolefin-based foam sheet was 0.44 mm, and the average MD cell diameter measured from the SEM photo was 140 μm.

Comparative example 1

Mix 100 parts by weight of LDPE (trade name: Sinopec 2426H), 6 parts by weight of azodicarbonamide and 0.2 parts by weight of zinc oxide in a high-speed mixer, and then blend in a mixer at 130°C to obtain MFR The polyolefin resin base material blend is 0.45g/10min, and then the polyolefin resin base material blend is extruded into a strip-shaped resin sheet using an extruder. Then, the strip-shaped resin sheet is irradiated on both sides, the irradiation energy is 1.5Mev, and the irradiation dose is 18 Mrad, so that the resin sheet is cross-linked. The cross-linked resin sheet is continuously fed into a heating furnace at 260°C. The heating furnace is heated by an infrared heater to foam the resin sheet. Then, it was stretched in a stretching machine at 120° C. at a stretching ratio of 2.1 times in the MD direction and at a stretching ratio of 2 times in the TD direction to obtain a polyolefin-based foam sheet. The thickness of the obtained polyolefin-based foam sheet was 0.25 mm, and the average MD cell diameter measured from the SEM photo was 78 μm.

Comparative example 2

Mix 80 parts by weight of LDPE (trade name: Sinopec 2426H), 20 parts by weight of LLDPE, 5 parts by weight of azodicarbonamide and 0.2 parts by weight of zinc oxide in a high-speed mixer, and then mix it in a mixer at 130°C. Blending, polyolefin resin base material blend with MFR 5.5g/10min. The polyolefin resin base material blend is then extruded into strip-shaped resin sheets using an extruder. Then, the strip-shaped resin sheet is irradiated on both sides with an irradiation energy of 1.5 Mev and a radiation dose of 20 Mrad to cross-link the resin sheet. The cross-linked resin sheet is continuously fed into a heating furnace at 380°C. The heating furnace is heated by an infrared heater to foam the resin sheet. Then, it was stretched in a stretching machine at 120° C. at a stretching ratio of 1.5 times in the MD direction and at a stretching ratio of 1.6 times in the TD direction to obtain a polyolefin-based foam sheet. The thickness of the obtained polyolefin-based foam sheet was 0.3 mm, and the average MD cell diameter measured from the SEM photo was 150 μm.

Test Methods:

1-Forward water blocking effect test

Cut the polyolefin foam sheet into a sample with a size of 70×70mm, and put it into the test tank with one side facing the test tank. The test tank is filled with distilled water. A support mesh is used on the other side of the sample to support the malleable elastic material. Place the test tank horizontally on the test bench (FX 300-IV Hydro Tester), and slowly inject 60 mL of purified water (funnel or syringe available) into the test tank from the upper inlet. When clamping the specimen, make sure that distilled or deionized water does not penetrate the specimen due to pressure before starting the test. Apply continuously increasing water pressure to the sample at a water pressure rise rate of 6.0kpa/min±0.3kpa/min until the pressure is stopped at 120kpa, and observe the water permeability phenomenon. The sample with water permeability during this process is called N, and the sample without water permeability is called Y.

2-Determination of azo residual rate

Cut the polyolefin foam sheet into samples with a size of at least 70×70mm, and measure according to the SVHC test of the SGS general standard.

3-Determination of closed cell ratio

Measured according to ASTM D2856 (1998) standard, calculate the closed cell ratio F1 and closed cell ratio F2 of the sample:

Opening ratio F1 (%) = 100 × (W2-W1)/V2

Closed cell ratio F2 (%) = 100-F1

Determination of 4-25% compressive strength

Conduct a compressive stress test in accordance with ISO3386-1. Materials with a thickness of 10 mm or less are laminated to a thickness of 10 mm or more. The compression speed is set to be as close to 50% of the material thickness per minute as possible, and the compressive stress at 25% deformation is measured.

5-Determination of foaming ratio

Measure the size of the sample in accordance with the provisions of GB/T6342-1996, in centimeters (cm). Measure at least three locations for each dimension, and for plate-like hard materials, measure five locations for each dimension in the middle. Calculate the average value of each dimension separately and calculate the sample volume V.

Weigh the sample as M, accurate to 0.5%, and the unit is grams (g).

Calculate the foaming ratio according to the following formula and take the average value.

Foaming ratio=V/M

6-High temperature (80℃) elongation at break

The sample is subjected to heat treatment at 80°C for 1 hour, and the heat-treated sample is conditioned (i.e., placed) for at least 4 hours in a standard environment with a temperature of 23°C ± 2°C and a relative humidity of 50% ± 10%. ASTM D882-2009 makes the foam sheet into a standard specimen with a length of 160 mm and a width of 25 mm. The length direction of the specimen is parallel to the MD direction of the foam sheet. Use a tensile testing machine to test the standard specimen at 80°C. Stretch at a constant speed of 300mm/min, and record the elongation at break of the sample.

The above state adjustment process is also referred to as "annealing" below.

7-Elongation at break at room temperature

At a temperature of 23°C, the normal temperature elongation of the sample in the MD direction is measured in accordance with ASTM D882-2009.

8-Determination of cross-linking degree

a. Take 100mg of sample from the foam sheet and accurately weigh the sample’s weight A (mg);

b. Wrap the sample with a 200-mesh metal mesh, immerse the sample wrapped in the metal mesh into xylene at 120°C, and let it stand for 24 hours. Through the filtration function of the metal mesh, the insoluble matter can be collected in the metal mesh; after vacuum drying, the weight of the insoluble matter B (mg) is accurately weighed;

c. Calculate the degree of cross-linking (mass %):

Cross-linking degree (mass %) = 100% × (B/A).

9-Thickness and cell diameter measurement

SEM (scanning electron microscope) observation was performed on the TD direction and MD direction of the sample, and the average pore diameter in different directions was measured.

Among them, the measurement method of the average pore diameter of the sample in the MD direction is: cut the foam sheet into a square sample with a side length of 50 mm as the sample for measurement; soak the sample in liquid nitrogen for 1 minute, and use a blade to cut it along the MD direction. Cut the sample in the direction of The average pore diameter in the MD direction. The measurement method of the average pore diameter of the sample in the TD direction is basically the same as the above method, except that the sample is cut along the TD direction with a blade.

The thickness of the sample is measured according to the method recorded in "6 Dimensions Measurement" in GB/T 40872-2021.

The structural indicators and performance indicators of the examples and comparative examples are shown in Table 1 and Table 2:

Table 1 - Various structural indicators of Examples 1-10 and Comparative Examples 1-2

Table 2 - Various performance indicators of Examples 1-10 and Comparative Examples 1-2

It can be seen from Tables 1 and 2 that when the 80°C breaking elongation (%) after annealing in the MD direction does not fall within the specified range, at least one performance index, namely azo residual rate and waterproof performance, cannot reach the ideal range. For example, the 80°C breaking elongation after annealing in the MD direction of Comparative Example 1 is too low, and its azo residual rate is as high as 2200 ppm. It can be seen that compared with Example 1, the 80°C breaking elongation after annealing in the MD direction of Comparative Example 1 is too low and the foaming is insufficient. Although the water blocking effect is good, the foamed sheet has a low magnification and a poor buffering effect. The reason for the high residual rate of azo may be that its high cross-linking degree prevents complete foaming, and the foamed material is highly brittle and has low elongation at break; the elongation at break at 80°C after annealing in the MD direction of Comparative Example 2 If it is too high, although in order to ensure a low azo residual rate, the foaming temperature and the heat enthalpy change value are increased, it will lead to a reduction in the closed cell rate and poor water blocking effect.

In addition, it can also be found from the normal temperature elongation at break test results of the Examples and Comparative Examples that there is no rule to follow if the Examples and Comparative Examples are distinguished by the normal temperature elongation at break in the MD direction. Only when the 80°C breaking elongation (%) after annealing in the MD direction is controlled within the range limited by the present invention, as shown in Examples 1-10, the azo residual rate, mechanical properties, and water blocking effect are shown. Produce balanced and ideal results.

At the same time, those skilled in the art can understand that, generally speaking, the higher the thickness of the foam sheet, the better its water-blocking performance. It can be seen from the thickness data in Table 1 that the thickness of the foam sheet of the present invention is not high, that is to say, the foam sheet of the present invention achieves a better water blocking effect when it is thinner. This makes the foam sheet of the present invention very suitable as a buffer sealing material for electronic products, which is in line with the development trend of thinner and lighter electronic products.

The present invention can be implemented in forms other than those described above within the scope of the gist thereof. The embodiment disclosed in this application is an example and is not limited to these.

Industrial applicability

After the polyolefin-based resin foam sheet of the present invention is glued, a cushioning and waterproof sealing tape for electronic products is obtained, which can be used for waterproof cushioning in various electronic products, such as smart mobile communication equipment, notebook computers, liquid crystal displays, electronic products, etc. Books, tablet terminals, game devices, cameras, wearable electronic devices, etc.

Claims (15)

1. A polyolefin-based resin foam sheet, the polyolefin-based resin foam sheet has a plurality of cells inside, and the cells are formed from a polyolefin-based matrix resin using an azo foaming agent The pre-foamed polyolefin resin system is formed through the foaming process and is characterized by:

   Based on the total weight of the polyolefin resin foam sheet, the residual azo foaming agent is less than 2000ppm, and the closed cell rate is more than 93%;

   The 80°C breaking elongation in the MD direction of the polyolefin resin foam sheet is 140-650%;

   Among them, the specific test method for the 80°C breaking elongation in the MD direction is: heat treat the sample at 80°C for 1 hour, and place the heat-treated sample at a temperature of 23°C ± 2°C and a relative humidity of 50% ± 10 % of the standard environment for at least 4 hours of condition adjustment, according to ASTM D882-2009, the foam sheet is made into a 160 mm long and 25 mm wide standard specimen, the length direction of the specimen is parallel to the MD direction of the foam sheet, and a tensile testing machine is used. Under the conditions of 80°C, the standard sample was stretched at a constant speed of 300 mm/min, and the elongation at break of the sample was recorded.
2. The polyolefin resin foam sheet according to claim 1, characterized in that, based on the total weight of the polyolefin resin foam sheet, the residual amount of azo foaming agent is less than 1500 ppm, and the closed cell ratio is 95 %above.
3. The polyolefin-based resin foamed sheet according to claim 1, wherein the 80° C. breaking elongation in the MD direction of the polyolefin-based resin foamed sheet is 150-610%.
4. The polyolefin resin foam sheet according to claim 1, wherein the azo foaming agent is selected from the group consisting of azodicarbonamide, diisopropyl azodicarboxylate, and metal azodicarboxylate. One or more of salt and azobisisobutyronitrile.
5. The polyolefin resin foam sheet according to claim 4, wherein the azo foaming agent is selected from azodicarbonamide.
6. The polyolefin-based resin foam sheet according to claim 1, characterized in that:

   During the foaming process, the enthalpy change value of the pre-foamed polyolefin resin system in the range of 160-200°C is 4-65J/g;

   The enthalpy change value is measured using Mettler Toledo's DSC3 instrument to scan and measure the sample. The test conditions are: heating from 25°C to 100°C, the heating rate is 10°C/min, and the _N flow rate is 50.0ml/min; at 100 Keep the temperature at ℃ for 10 minutes, and the N ₂ flow rate is 50 ml/min; heat up from 100 ℃ to 200 ℃, the heating rate is 10 ℃/min, and the N ₂ flow rate is 50 ml/min.
7. The polyolefin-based resin foam sheet according to claim 1, wherein the polyolefin-based matrix resin is selected from one or more of polyethylene resin, polypropylene resin, and ethylene-vinyl acetate copolymer. polymer tree fat,

   The polyolefin-based matrix resin composed of one of the above-mentioned polymer resins and the polyolefin-based matrix resin blend composed of multiple kinds of the above-mentioned polymer resins are both melted at 190°C and under a load of 2.16kg The flow rate is 0.2-30g/10 minutes.
8. The polyolefin resin foam sheet according to claim 1, wherein during the foaming process, the foaming temperature is 240-390°C.
9. The polyolefin resin foam sheet according to claim 1, wherein the polyolefin resin foam sheet has an expansion ratio of 1.1-18 cm ³ /g.
10. The polyolefin resin foam sheet according to claim 1, wherein the 25% compressive strength of the polyolefin resin foam sheet is 35-680 KPa.
11. A method for preparing the polyolefin resin foam sheet according to any one of claims 1 to 10, characterized in that it includes the following steps:

    First, a raw material composition including 0.5-8 wt% of azo foaming agent, polyolefin matrix resin and cross-linking assistant based on the total weight of the raw material composition is mixed to obtain a mixture. After blending the mixture, a polyethylene foaming agent is obtained. Olefin base resin blend,

    Then, the polyolefin-based matrix resin blend is cross-linked to obtain a pre-expanded polyolefin-based resin system.

    Then, the pre-expanded polyolefin resin system is foamed at 240-390°C.

    Then or while foaming, obtain the foamed sheet by stretching and shaping;

    Among them, during the foaming process, the enthalpy change value of the pre-foamed polyolefin resin system in the range of 160-200°C is 4-65J/g.

    The enthalpy change value is measured using Mettler Toledo's DSC3 instrument to scan and measure the sample. The test conditions are: heating from 25°C to 100°C, the heating rate is 10°C/min, and the _N flow rate is 50.0ml/min; at 100°C Keep it warm for 10 minutes, _N2 flow rate is 50ml/min; heat up from 100℃ to 200℃, heating rate is 10℃/min, _N2 flow rate is 50ml/min; pre-foamed polyolefin resin system at 190℃, 2.16kg The melt flow rate under load is 0.2-30g/10 minutes;

    The 80°C breaking elongation in the MD direction of the polyolefin resin foam sheet is 140-650%. The specific testing method for the 80°C breaking elongation in the MD direction is as described in claim 1 .
12. A buffer waterproof sealing tape for electronic products, consisting of the polyolefin resin foam sheet according to any one of claims 1 to 10 and an adhesive layer coated on one or both sides of the sheet.
13. According to the waterproof application of the buffer waterproof sealing tape for electronic products in electronic products according to claim 12.
14. The waterproof application of buffer waterproof sealing tape for electronic products in electronic products according to claim 13, characterized in that the electronic products include: intelligent mobile communication equipment, notebook computers, liquid crystal displays, e-books, tablet terminals, Gaming devices, cameras, wearable electronics, OLED displays.
15. The waterproof application of buffer waterproof sealing tape for electronic products in electronic products according to claim 13, characterized in that the electronic product includes a display component, and the display component at least includes a display panel, a middle frame and a back cover, so The middle frame is used to support the display panel, and the back cover is fastened to the middle frame.

    The buffer waterproof sealing tape is attached between the display panel and the middle frame and/or between the middle frame and the back cover through steps including die cutting, cutting, gluing, installation, sealing, and shaping.