CN112778565B

CN112778565B - Impact-resistant ultrathin polyolefin foamed sheet and preparation method and application thereof

Info

Publication number: CN112778565B
Application number: CN202011609066.2A
Authority: CN
Inventors: 李沛; 魏琼
Original assignee: Guangde Xiangyuan New Material Technology Co ltd
Current assignee: Guangde Xiangyuan New Material Technology Co ltd
Priority date: 2020-12-30
Filing date: 2020-12-30
Publication date: 2023-03-31
Anticipated expiration: 2040-12-30
Also published as: CN112778565A

Abstract

The invention discloses an impact-resistant ultrathin polyolefin foamed sheet, which is characterized in that the polyolefin foamed sheet is formed by foaming polyolefin resin and has an independent cell structure; the foaming process uses fumed silica modified foaming agent particles, and the particle size of the modified foaming agent particles is less than 10 mu m; the cross-linking degree of the foaming sheet is 30-60%, the thickness of the foaming sheet is 0.03-0.5 mm, the average cell diameter of each direction of the foaming sheet is 20-100 mu m, the ratio of the average cell diameter in the MD direction to the cell diameter in the TD direction is 0.8-1.2, the foaming multiplying power is 1.5-6 times, the 25% compression stress is less than or equal to 0.1MPa, and the impact force absorption is more than or equal to 60%. The invention also provides a preparation method and application of the impact-resistant ultrathin polyolefin foamed sheet, the process is simple, the foamed sheet is suitable for industrial mass production, and the prepared foamed sheet has the advantages of impact resistance, good waterproofness, softness, thinness and the like compared with the common thin film material, and is suitable for being used as a sealing buffer material or an adhesive tape base material of electronic products.

Description

Impact-resistant ultrathin polyolefin foamed sheet and preparation method and application thereof

Technical Field

The invention belongs to the technical field of high polymer materials, and particularly relates to an impact-resistant ultrathin polyolefin foamed sheet and a preparation method and application thereof.

Background

In recent years, the design of electronic devices such as smart phones, notebook computers, liquid crystal televisions and the like is gradually developing towards the trend of lightness, thinness and flexibility, and corresponding structural materials, packaging materials and functional materials must also develop along with the product trend. The resin foamed sheet has the advantages of light weight, thin thickness, heat insulation, moisture resistance and good physical properties, and is commonly used as a sealing material or a buffer material in electronic equipment, and the adhesive tape based on the resin foamed sheet can also play a good buffer protection role in the bonding of parts in the electronic equipment. Compared with the foaming sheet material adopting a chemical crosslinking foaming mode, the irradiation crosslinking is carried out at normal temperature, no chemical crosslinking agent is left, and the irradiation crosslinking foaming sheet material can meet various environmental protection indexes of electronic products.

However, in the production process of the common resin foamed sheet, due to the physical properties of the common resin foamed sheet, it is difficult to directly obtain a thin foamed sheet with a thickness of less than 0.6mm, the industry generally adopts a die cutting and slicing mode for processing, however, for the thin sheet with a thickness of less than 0.6mm, the uniformity of the processed plane thickness is difficult to guarantee, which has a great influence on the stability of electronic equipment; in addition, the thin foamed sheet is required to have excellent impact resistance, and has high requirements on a resin system and a cell structure of the foamed sheet.

When the smart phone pursues the lightness and thinness, the smart phone has the requirement for attractive design, so that the screen of the smart phone is larger, the back shell is made of a multi-purpose glass material, and the double-sided glass material of the smart phone is easy to damage due to falling impact, and therefore, the smart phone has higher requirements on the impact resistance of the buffer material inside the smart phone.

Disclosure of Invention

Aiming at the defects or improvement requirements of the prior art, the invention provides the impact-resistant ultrathin polyolefin foamed sheet, and the preparation method and the application thereof, and the impact-resistant ultrathin polyolefin foamed sheet has the advantages of impact resistance, good waterproofness, softness, thinness and the like compared with the common film material, and is more suitable for being used as a sealing buffer material or an adhesive tape base material of an electronic product.

To achieve the above objects, according to one aspect of the present invention, there is provided an impact-resistant ultra-thin polyolefin foamed sheet, which is foamed from a polyolefin resin and has an independent cell structure; in the foaming process, fumed silica modified foaming agent particles are used, and the particle size of the modified foaming agent particles is less than 10 mu m; the impact force absorption of the polyolefin foaming sheet is more than or equal to 60 percent.

As a further improvement of the invention, the thickness of the polyolefin foamed sheet is 0.03-0.5 mm, and the thickness deviation is +/-0.01 mm.

As a further improvement of the invention, the polyolefin foamed sheet is characterized in that the average cell diameter of each direction of the polyolefin foamed sheet is between 20 and 100 mu m, and the ratio of the average cell diameter in the MD direction to the cell diameter in the TD direction is 0.8 to 1.2.

In a further improvement of the present invention, the polyolefin foamed sheet has a foaming ratio of 1.5 to 6 times.

As a further improvement of the invention, the 25% compressive stress of the polyolefin foamed sheet is less than or equal to 0.1MPa.

As a further improvement of the invention, the crosslinking degree of the polyolefin foaming sheet ranges from 30% to 60%.

In a further improvement of the present invention, the polyolefin resin is a copolymer of polyethylene and an olefinic thermoplastic elastomer.

As a further development of the invention, the blowing agent particles comprise an organic blowing agent or an inorganic blowing agent, in particular azodicarbonamide, inorganic carbonates or bicarbonates.

According to another aspect of the present invention, there is provided a method for preparing the impact-resistant ultra-thin polyolefin foamed sheet, comprising the steps of:

s1, uniformly mixing foaming agent particles and fumed silica according to a certain proportion to prepare modified foaming agent particles;

s2, respectively mixing and banburying the modified foaming agent particles and the auxiliary agent with polyolefin resin, wherein the banburying temperature is 80-180 ℃, the pressure is 1-2 MPa, and the time is 5-25 min; the mixture is banburied into a ball, vertically added into a hopper of a single screw or a double screw, the processing temperature is 100-180 ℃, and the corresponding resin master batch is obtained through the working procedures of extrusion, bracing, grain cutting and drying;

s3, uniformly mixing the resin master batches obtained in the step S2, and extruding the mixture into sheets to obtain foamed sheet substrates;

s4, performing irradiation crosslinking on the foamed sheet substrate in the step S3 to obtain a foamed sheet master slice, wherein the irradiation dose is 22-30 Mrad;

s5, freely foaming the foaming sheet master slice in a vertical foaming furnace at the foaming temperature of 180-280 ℃ to obtain a foaming sheet;

and S6, stretching and extending the foamed sheet in the step S5 in a mode of at least one-time double-sided heating and longitudinal stretching, wherein the stretching ratio is 2-4, and thus the ultrathin polyolefin foamed sheet is obtained.

According to another aspect of the present invention, there is provided a use of the impact-resistant ultrathin polyolefin foam sheet as a sealing buffer material or an adhesive tape substrate for electronic products.

In general, compared with the prior art, the above technical solution contemplated by the present invention can achieve the following beneficial effects:

according to the impact-resistant ultrathin polyolefin foaming sheet, the foaming process adopts the foaming agent particles modified by using fumed silica, the particle size of the modified foaming agent particles is less than 10 micrometers, the foaming agent particles can be more uniformly dispersed in a polymer component and are not easy to agglomerate, more and relatively uniform foam holes can be obtained, and when the foaming sheet is impacted and extruded, each closed-cell bubble has independent rebound potential energy, so that the foaming sheet can absorb and offset more impact energy; and by further controlling the thickness, the average cell diameter, the foaming multiplying power, the crosslinking degree and the like of the polyolefin foamed sheet, the polyolefin foamed sheet has the advantages of impact resistance, good waterproofness, softness, lightness and thinness and the like compared with common film materials, and is more suitable for being used as a sealing buffer material or an adhesive tape base material of electronic products.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

The invention provides an impact-resistant ultrathin polyolefin foamed sheet, which is suitable for being used as a sealing buffer material or an adhesive tape base material of an electronic product, and is a sheet with an independent foam cell structure formed by foaming polyolefin resin, wherein fumed silica modified foaming agent particles are selected in the foaming process, and the particle size of the modified foaming agent particles is less than 10 mu m. The fumed silica to blowing agent ratio is preferably 1. The foaming agent particles are easy to absorb moisture, and are easy to adsorb and agglomerate under the electrostatic action, when the small-particle-size foaming agent is directly mixed with a polymer component, a large amount of agglomeration is easy to cause, when the polymer is foamed, undispersed particles are easy to generate holes and large air pockets, and relatively uniform foam holes cannot be obtained, so that the foam performance is influenced; according to the invention, after the foaming agent is uniformly mixed and modified by fumed silica according to a certain proportion, the foaming agent particles can be more uniformly dispersed in the polymer component, and are not easy to agglomerate, so that more and relatively uniform cells can be obtained; the foaming agent with small particle size can be screened by particle size grading equipment, and foaming agent particles with small particle size can obtain cells with small pore size, so that an independent cell structure can be realized, the standard deviation of the cell pore size in each direction can be reduced, and the cell size is more uniform; when the foamed sheet is impacted and extruded, each closed cell bubble has independent resilience potential energy, so that the foamed sheet can absorb and offset more impact energy; meanwhile, the waterproof and dustproof material also has better waterproof and dustproof performances.

The thickness of the polyolefin foaming sheet is 0.03-0.5 mm, and the thickness deviation is +/-0.01 mm; the buffering performance of the material is positively correlated with the thickness, when the thickness is less than 0.03mm, the buffering performance of the material cannot be ensured, and the number of foam holes in the thickness direction is too small, so that the material is easy to damage in appearance and poor in durability; the thickness is more than 0.5mm, so the light-weight and thin-type electronic equipment is not suitable for the trend of light weight and thin-type electronic equipment at present; the thickness deviation is controlled to be +/-0.01 mm, the foam is uniform in thickness, the mounting requirement of the foam in the electronic equipment can be met, and when the foam is impacted, the uniform dispersion of impact stress is facilitated, and the effect of impact absorption is better achieved.

The polyolefin foamed sheet has the crosslinking degree of 30-60 percent and less than 30 percent, the polymer polymerization degree is lower, the foam strength is lower, the post-processing stretching process is not facilitated, the crosslinking degree is more than 60 percent, the polymer polymerization degree is too high, the foam hardness is increased and is not soft, and the improvement of the impact resistance of the foam is not facilitated.

The polyolefin foamed sheet has an average bubble pore diameter of 20-100 microns and a ratio of the average bubble pore diameter in the MD direction to the cell pore diameter in the TD direction of 0.8-1.2; wherein "MD" means a Machine Direction (Machine Direction) and a Direction coinciding with the extrusion Direction of the polyolefin resin foam sheet or the like; "TD" refers to the Transverse Direction (Transverse Direction) and to the Direction perpendicular to the MD and parallel to the foam sheet.

The smaller average cell diameter can enable a larger number of cells to exist in a thinner or narrower foamed sheet, so that the cell number density is improved, when the foamed sheet is impacted, the superimposed resilience of more cells enables the foamed sheet to have better impact resistance, and simultaneously, the breakage of the cells can be prevented, so that the durability is improved; and the preferable ratio of the MD-direction average cell diameter to the TD-direction cell diameter ensures that the deviation of the MD-direction cell diameter and the TD-direction cell diameter is smaller and the cells tend to round cells after being stretched, so that the thickness of the cell walls can be better kept relatively uniform, and the cells are prevented from being broken due to uneven stress.

The foaming multiplying power of the polyolefin foaming sheet is 1.5-6 times, and the 25 percent compressive stress is less than or equal to 0.1MPa; the foaming sheet is softer due to lower compressive stress, large continuous stress can not be generated on devices in the electronic equipment while the foaming sheet is attached to and protected by uneven planes in the electronic equipment, the durability and the stability of the electronic equipment are facilitated, and the impact force absorption is more than or equal to 60%.

The polyolefin resin of the present invention is a copolymer of polyethylene and an olefinic thermoplastic elastomer. The thermoplastic elastomer component is added, so that the tensile property of the foamed sheet can be improved, the post-processing is facilitated, and the ultra-thinning is carried out; and the toughness of the cell wall resin can be increased, and the impact resistance of independent cells is improved. Wherein the polyethylene resin comprises one or more of LDPE, HDPE, MDPE, LLDPE and VLDPE; the olefinic thermoplastic elastomer is selected from one of the following classes: one is modified polypropylene (PP) or Polyethylene (PE) blends of ethylene propylene diene monomer (EPR), ethylene Propylene Diene Monomer (EPDM), butadiene Rubber (BR), natural Rubber (NR), isobutylene rubber (IBR), nitrile rubber (NBR) and the like; one is a polypropylene (PP) or Polyethylene (PE) blend modified by ethylene-vinyl acetate copolymer (EVA) or ethylene acrylic acid copolymer (EAA).

In a preferred embodiment of the present invention, the blowing agent particles are an organic blowing agent or an inorganic blowing agent such as azodicarbonamide, inorganic carbonate or bicarbonate.

In the process of preparing the foamed sheet, any one or more of functional additives such as an antioxidant, a heat stabilizer, a sensitizer, a dispersant, a flame retardant, a pigment, an antistatic agent, heat conducting particles and the like can be added on the premise of not influencing the performance of the foamed sheet. Wherein, the antioxidant is used for improving the oxidation resistance of the foaming body or the foaming sheet in the processing process and the using process and prolonging the service life; the dispersing agent promotes the mixing uniformity of the raw materials, the flame retardant can improve the flame retardant property of the foamed sheet, and the antistatic agent increases the antistatic property of the foamed sheet.

The preparation method of the impact-resistant ultrathin polyolefin foamed sheet comprises the following steps:

(1) Modification of blowing agents

Mixing the foaming agent particles with fumed silica according to a certain proportion, and fully and uniformly mixing in a stirring device to prepare modified foaming agent particles; the proportion of fumed silica to the foaming agent particles is preferably 1;

(2) Granulating

Mixing and banburying the modified foaming agent particles and other auxiliary agents in the step (1) with polyolefin resin respectively, wherein the temperature of a banburying bin is 80-180 ℃, the pressure is 1-2 MPa, and the banburying time is 5-25 min, adding the resin banburied into a mass into a hopper of a single screw or a double screw, wherein the processing temperature in a cylinder of the screw is 100-180 ℃, and finally obtaining foaming agent resin master batches and an antioxidant or other resin master batches through the processes of extrusion, bracing, granulating and drying;

(3) Extruded sheet

Uniformly mixing the foaming agent resin master batch obtained in the step (2) with other resin master batches, and extruding into sheets by using a double-screw extruder to obtain a foamed sheet substrate;

(4) Irradiation crosslinking:

irradiating the foamed sheet substrate obtained in the step (3) by high-speed electron beams to enable free radicals generated by resin chain segments to generate crosslinking through chain segment recombination to obtain a foamed sheet master slice, wherein the irradiation dose is 22-30 Mrad;

(5) Foaming

Freely foaming the foaming sheet master slice obtained in the step (4) through a vertical foaming furnace at the foaming temperature of 180-280 ℃ to obtain a foaming sheet;

(6) Post-processing

And (3) stretching and extending the foamed sheet obtained in the step (5) in a mode of at least one-time double-sided heating and longitudinal stretching, wherein the stretching ratio is 2-4, and finally obtaining the ultrathin polyolefin foamed sheet.

The test method adopted by the invention for the foamed sheet is as follows:

(1) Powder particle size: 0.1g of the powder was weighed, pure water was used as a dispersion medium, the powder was put into a BeTTERSIZE2600 wet laser particle size distribution instrument, the particle size was measured, and D50 data was selected as the average particle size of the powder.

(2) Impact resistance of foamed sheet: a 10cm by 10cm sample of foamed sheet was prepared and placed on the bottom stress-sensing device platform in a standard laboratory environment. Using a stress induction device with 4.3g of acrylic small balls falling to the bottom freely from the height of 30mm, reading the maximum stress before and after placing the foam, and the impact absorption = (maximum stress value before placing the foam-maximum stress value after placing the foam)/maximum stress value before placing the foam is 100%. The model of the bottom stress induction device is SHOWA-1340A in Japan.

(3) Thickness and thickness deviation of foamed sheet: measured according to the method described in ASTM D3574 standard, the area of the base is 650mm or more ² The thickness gauge special for the soft foam is used for measuring the thickness, the thickness is measured on a foaming sheet sample of 10cm to 10cm by a 9-point method (the center point of the sample, the center point of the four sides close to the sample and the four corner points close to the sample), and the thickness average value and the thickness deviation (the deviation refers to the difference between the measured value and the measured average value and can be used for measuring the precision of the measured result).

(4) Expansion ratio of foamed sheet: a 10cm to 10cm foamed sheet standard sample was selected, the thickness was measured by the method described in ASTM D3574 standard, and the mass was weighed to calculate the reciprocal of the density.

(5) 25% compressive stress of foamed sheet: measured according to the method described in ASTM D3574 standard, and tested using an electronic tensile machine. Under the standard laboratory environment, sample specification 50mm to 50mm, highly stack to 10mm, the tester inductor is raised to about 10mm height and is fixed, begins compression test after zero clearing displacement stress, and compression speed 5mm/min records the stress that produces when 25% compression set.

(6) Cell diameter and cell number density: in a standard laboratory environment, for each direction in which the average bubble pore size is to be determined, a 10mm x 10mm sample is cut from the sample to be measured. Randomly cutting the test piece from the trial, wherein the thickness of the test piece is smaller than the diameter of a single cell, so as to ensure that an image is not covered due to overlapping of cell walls, the optimal slice thickness is determined according to the average cell size of the foaming material, and the smaller cell diameter is taken as the slice thickness; inserting the thin sheet into a slide with scales, adjusting the coordinates of the scales to enable the zero point of the scale to be positioned at the top of the measuring area, and reloading the slide; inserting the slide into a projector, and adjusting the focal length to enable the image to be clear on a wall or a screen; determining the number of the cells or the cell walls within the range of 0.5mm of the length of the ruler to obtain the cell number density, and then dividing the straight line length by the number of the cells to obtain the average cell chord length t.

(7) And (3) testing the crosslinking degree: soaking a copper net of 200 meshes or more in absolute ethyl alcohol for 2 hours in advance, drying the copper net in a blast drying oven at 75 ℃ for about 30 minutes, weighing a dry object to be tested, wherein the mass is m1, wrapping the first layer with filter paper, then wrapping the first layer with a steel net, binding the first layer with a staple or wrapping the first layer with a steel wire, and weighing the total mass as m2. Selecting a proper volume of xylene by using a Soxhlet extractor, wherein the purity of the xylene is analytically pure and the content of the xylene is about 80 percent, soaking the wrapped sample in the xylene, heating the sample by using a temperature-control electric heating jacket, adjusting a knob, and selecting a proper temperature to ensure that the xylene is moderately boiled within 10 minutes, wherein the extraction time is 24 hours. And (4) finishing extraction, turning off a power supply, and drying the wrapped steel mesh in a constant-temperature drying box at 75 ℃ for 6h. After the drying was completed, the power was turned off, cooled to room temperature in a drying tower, and weighed to m3. And (3) calculating the crosslinking degree: a = [ (m 3-m 2)/m 1 ]. 100%.

To better illustrate the impact-resistant ultra-thin polyolefin foamed sheet of the present invention and the method for preparing the same, the following examples and comparative examples are given.

Example 1:

(1) Modification of a foaming agent: and (2) fully and uniformly mixing azodicarbonamide and gas-phase silica in a stirring device according to the weight ratio of 19.

(2) And (3) granulation: mixing and banburying the obtained modified foaming agent, low-density polyethylene (LDPE) resin and ethylene-vinyl acetate copolymer (EVA) according to the weight ratio of 10 to 55, wherein the temperature of a banburying bin is 150 ℃, the pressure is 1.5MPa, the banburying time is 15min, adding the banburied agglomerated resin into a hopper of a double screw, the processing temperature in a machine barrel of the screw is 135 ℃, and finally obtaining the resin master batch of the modified foaming agent through the procedures of extrusion, bracing, granulating and drying;

(3) Sheet extrusion: extruding the foaming agent resin master batch obtained in the step (2) and the rest resin particles in the formula (the modified foaming agent master batch: LDPE: EVA: master batch =40: 27);

(4) Irradiation crosslinking: irradiating the foamed sheet substrate obtained in the step (3) by a high-speed electron beam, and performing electron irradiation crosslinking, wherein the irradiation dose is 25Mrad, so as to obtain a foamed sheet master slice;

(5) Foaming: freely foaming the foaming sheet master slice obtained in the step (4) through a vertical foaming furnace at the foaming temperature of 250 ℃ to obtain a foaming sheet with the thickness of 0.55mm +/-0.01 mm;

(6) Post-processing: and (5) stretching and spreading the foamed sheet obtained in the step (5) by adopting a primary double-sided heating longitudinal stretching mode, wherein the stretching ratio is 3.0, and finally obtaining the ultrathin spoke polyolefin foamed sheet with the thickness of 0.2mm +/-0.01 mm.

The prepared impact-resistant ultrathin irradiation crosslinked polyolefin foamed sheet has the thickness of 0.2mm +/-0.01 mm, the crosslinking degree of 51 percent, the foaming ratio of 5 times, the average cell diameter in the MD direction of 71 microns, the average cell diameter in the TD direction of 64 microns, the ratio of the average cell diameter in the MD direction to the cell diameter in the TD direction of 1.11, the 25 percent compression stress of 0.07MPa and the impact absorption of 81 percent.

Example 2

The difference from example 1 is that Ethylene Propylene Rubber (EPR) was added as a resin modifier in order to obtain a thinner foamed sheet. The soft rubber has excellent low-temperature impact resistance, good fluidity and controllable molding shrinkage, greatly improves the tensile deformation, and can be used for making thinner foamed sheets. The same foaming agent modification process and granulation process as in example 1, except that the proportion of the formula particles in the sheet extrusion process is modified foaming agent master batch: LDPE (Low-Density polyethylene): EVA: EPR: the method comprises the following steps of (1) selecting a master batch = 38; irradiating the obtained foamed sheet substrate by high-speed electron beams, and performing electron irradiation crosslinking, wherein the irradiation dose is 25Mrad, so as to obtain a foamed sheet master slice; the foaming temperature is 250 ℃, and a foaming sheet with the thickness of 0.58mm +/-0.01 mm is obtained;

and in the post-processing step, stretching and extending the foamed sheet in a mode of 2 times of double-sided heating longitudinal stretching, wherein the first stretching ratio is 2.5, and the second stretching ratio is 2.3, and finally obtaining the ultrathin radiation crosslinking polyolefin foamed sheet with the thickness of 0.1mm +/-0.01 mm.

The prepared impact-resistant ultrathin radiation cross-linked polyolefin foamed sheet has the thickness of 0.1mm +/-0.01 mm, the cross-linking degree of 53 percent, the foaming multiplying power of 4.5 times, the MD-direction average foam pore diameter of 79 mu m, the TD-direction average foam pore diameter of 72 mu m, the ratio of the MD-direction average foam pore diameter to the TD-direction foam pore diameter of 1.1, the 25 percent compression stress of 0.05MPa and the impact absorption of 90 percent.

Example 3:

the particle size of the modified foaming agent is 4 μm in the modification process of the foaming agent based on example 1, and other components and processing techniques are the same as those of example 1.

Example 4:

based on the example 1, the weight ratio of the azodicarbonamide to the fumed silica in the foaming agent modification process is 50.

Example 5:

adjusting on the basis of example 2, wherein the weight ratio of the modified foaming agent master batch in the sheet extrusion formula is as follows: LDPE (Low-Density polyethylene): EVA: EPR: color master particle = 38.

Comparative example 1:

on the basis of the embodiment 1, the foaming agent is azodicarbonamide, the particle size is 8 mu m, no gas phase silica is modified, and the rest is the same as the embodiment 1;

comparative example 2:

the particle diameter of the modified blowing agent was 15 μm in example 1, and the procedure was otherwise the same as in example 1.

Comparative example 3:

on the basis of example 2, the post-processing procedure was changed to one transverse stretching and one longitudinal stretching, the first transverse stretching ratio was 1.5, the second longitudinal stretching ratio was 3, and the others were identical to example 2.

The data for the foam products of the examples and comparative examples are shown in the following table:

comparative example 1 the foaming agent is not modified by fumed silica, and the foaming agent particles are easy to agglomerate, so that the pore size of part of foam in the foamed cotton is increased, the average pore diameter is increased, the size deviation of the pore diameter is large, the stress cannot be uniformly dispersed when the foam is impacted, the large pores are easily broken when the foam is impacted, and the impact absorption performance of the foam is reduced.

Comparative example 2 the particle size of the foaming agent is larger, the volume of the corresponding foaming cells is increased, and the number of the cells in the foam is reduced under the condition of the same thickness, so that the cells are easier to break when the foam is impacted, and the overall impact absorption performance of the foam is reduced after the cells for dispersing stress are reduced.

Comparative example 3 adopts biaxial stretching, and stretching in the transverse direction causes the thickness of the middle area of the foam to be thinner, and the overall thickness deviation to be larger. The longitudinal stretching is continuous stretching, the stretching direction is consistent with the winding and unwinding direction of the foam, and the thickness deviation can be better controlled.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims

1. An impact-resistant ultrathin polyolefin foamed sheet is characterized in that the polyolefin foamed sheet is formed by foaming polyolefin resin and has an independent cell structure; the foaming process uses fumed silica modified foaming agent particles, and the particle size of the modified foaming agent particles is less than 10 mu m; the impact force absorption of the polyolefin foaming sheet is more than or equal to 60 percent; the thickness of the polyolefin foaming sheet is 0.03-0.5 mm;

the modified foaming agent particles are prepared by uniformly mixing foaming agent particles and fumed silica according to a certain proportion, wherein the weight ratio of the fumed silica to the foaming agent particles is 1;

the foaming agent particles comprise an organic foaming agent or an inorganic foaming agent, and specifically comprise azodicarbonamide, inorganic carbonate or bicarbonate.

2. The impact resistant ultra-thin polyolefin foamed sheet according to claim 1, wherein the polyolefin foamed sheet has a thickness deviation of ± 0.01mm.

3. The impact-resistant ultrathin polyolefin foamed sheet according to claim 1, wherein the average cell diameter per direction of the polyolefin foamed sheet is 20 to 100 μm, and the ratio of the average cell diameter in the MD direction to the cell diameter in the TD direction of the polyolefin foamed sheet is 0.8 to 1.2.

4. The impact-resistant ultrathin polyolefin foamed sheet according to any one of claims 1 to 3, wherein the expansion ratio of the polyolefin foamed sheet is 1.5 to 6 times.

5. The impact-resistant ultra-thin polyolefin foam sheet of claim 4, wherein the polyolefin foam sheet has a 25% compressive stress of 0.1MPa or less.

6. The impact-resistant ultra-thin polyolefin foam sheet of claim 1 or 5, wherein the cross-linking degree of the polyolefin foam sheet is in the range of 30% to 60%.

7. The impact-resistant ultrathin polyolefin foamed sheet according to claim 6, wherein the polyolefin resin is a copolymer of polyethylene and an olefinic thermoplastic elastomer.

8. A method for preparing an impact-resistant ultra-thin polyolefin foam sheet according to any one of claims 1-7, comprising the steps of:

s2, mixing and banburying the modified foaming agent particles and the auxiliary agent with polyolefin resin respectively at 80-180 ℃ under 1-2 MPa for 5-25 min; vertically adding the mixture which is internally mixed into a lump into a hopper of a single screw or a double screw, wherein the processing temperature is 100-180 ℃, and obtaining corresponding resin master batches through the working procedures of extrusion, bracing, granulating and drying;

and S6, stretching and extending the foamed sheet in the step S5 in a mode of at least one time of double-sided heating and longitudinal stretching, wherein the stretching ratio is 2 to 4, and thus obtaining the ultrathin polyolefin foamed sheet.

9. Use of the impact-resistant ultra-thin polyolefin foam sheet according to any one of claims 1 to 7 as a sealing cushion material or an adhesive tape substrate for electronic products.