CN115011024B

CN115011024B - Polyolefin resin foam sheet and method for producing same

Info

Publication number: CN115011024B
Application number: CN202210872303.7A
Authority: CN
Inventors: 魏琼
Original assignee: Basek Adhesive Science & Technology Suzhou Co ltd
Current assignee: Basek Adhesive Science & Technology Suzhou Co ltd
Priority date: 2022-02-23
Filing date: 2022-02-23
Publication date: 2024-01-19
Anticipated expiration: 2042-02-23
Also published as: CN114230902A; CN115011024A; CN114230902B

Abstract

The invention relates to a polyolefin resin foam sheet, the thickness is 0.04mm-2mm, the foam sheet comprises at least one crust layer, the average thickness of single crust layer is 0.5% -30% of the total thickness, the crust layer is defined as extending continuously inwards from the vertical outer surface of the foam sheet to a certain thickness, no bubble exists in the thickness or the bubble volume with the bubble diameter of less than 10 mu m is 98% or more of the total bubble volume of the layer, the foam sheet is favorable for forward water blocking, meanwhile, high buffer performance is considered, and ideal effects of low azo foaming agent residue rate, forward water blocking performance and buffer performance are achieved.

Description

Polyolefin resin foam sheet and method for producing same

Technical Field

The present invention relates to a polyolefin resin foam sheet, and more particularly to a sealing tape for waterproofing electronic devices.

Background

The polyolefin resin foam sheet can still have high impact absorption performance under a thinner condition, so that the polyolefin resin foam sheet is widely applied to electronic equipment such as smart phones, personal computers, electronic papers and the like, and is arranged between an electronic component and a frame structural member to play a role in buffer sealing.

In the prior art, an azo-based foaming agent such as azodicarbonamide (AC foaming agent) is used to foam a polyolefin-based resin substrate, but the foaming agent remains after foaming. The residual azo-based foaming agent has a negative effect on the mechanical properties of the polyolefin-based resin foam sheet, and may have a serious hazard to the environment, and the european union REACH regulation (regulations on registration, evaluation, approval and limitation of chemicals) clearly limits the residual amount thereof. Therefore, the content of the residual azo-based foaming agent in the final polyolefin-based resin foam sheet is reduced as much as possible. It is common practice to reduce the amount of foaming agent used and to perform foaming at a higher temperature to increase the foaming efficiency of the foaming agent and thereby reduce its residue.

However, in the case of a polyolefin foam sheet, too high a foaming temperature results in too high an open cell ratio, which eventually results in deterioration of the forward water blocking effect of the material, and reducing the amount of the foaming agent used results in deterioration of the cell density and the compression strength effect, which eventually results in deterioration of the cushioning property of the foam material.

Therefore, for the foaming material, the residual rate, the aperture ratio and the forward water blocking effect of the azo foaming agent are mutually influenced, and the current foaming material in the prior art can still have relatively good forward water blocking effect under the condition that the azo residual is reduced to below 0.1wt%.

Referring to prior art 1: CN108822378A

Referring to prior art 2: CN108342023A

Disclosure of Invention

The purpose of the present invention is to provide a polyolefin resin foam sheet which is capable of exhibiting a relatively good positive water blocking effect even when the residual azo-based foaming agent is reduced to 0.1wt% or less, without intentionally increasing the closed cell ratio (independent cell ratio), and which maintains a relatively large open cell ratio.

In order to achieve the above object, the present invention provides a foamed sheet of a polyolefin-based resin having a thickness of 0.04mm to 2mm, the foamed sheet comprising at least one skin layer, the average thickness of the individual skin layers being 0.5% to 30%, preferably 0.8% to 20%, further preferably 1% to 15% of the total thickness, the skin layers being defined as extending continuously inward from the vertical outer surface of the foamed sheet to a thickness in which no bubbles are present or in which the volume of bubbles having a bubble diameter of 10 μm or less is 98% or more of the total bubble volume of the layer.

The polyolefin resin foam sheet is foamed with an azo-based foaming agent, and the residual ratio of the azo-based foaming agent in the final molded foam sheet is less than 0.2wt%, preferably less than 0.15wt%, and more preferably less than 0.1wt%.

The closed porosity of the polyolefin resin foaming sheet is more than 90%, namely the highest open porosity can reach 10%, wherein in one scheme, the closed porosity of the foaming sheet is 90-95%;

the TD average foam pore diameter in the crust layer is 0-3 mu m, the MD average foam pore diameter is 0-5 mu m, the TD average foam pore diameter of the whole foaming sheet is 25-250 mu m, and the MD average foam pore diameter is 30-300 mu m;

the polyolefin resin foaming sheet has a compression strength of 30-700KPa at 25% and a tensile strength of 0.5-25MPa;

the polyolefin resin is a polyethylene resin.

The invention also discloses a method for preparing the polyolefin resin foaming sheet, which comprises the steps of mixing 2-10wt% of azo foaming agent, matrix resin and other auxiliary agent raw materials, adding the mixture into a high-speed mixer, mixing the mixture to obtain a foaming master slice after banburying and crosslinking, then foaming the foaming master slice at 200-380 ℃, cooling and pressurizing the foaming sheet through fluid in the foaming process and/or pressurizing and cooling the surface through an extrusion roller after foaming is finished, obtaining a crust layer, and obtaining the foaming sheet through stretching and shaping.

The invention also discloses a waterproof application of the polyolefin resin foam sheet in electronic products.

The electronic product comprises: intelligent mobile communication equipment, notebook computers, electronic books, tablet terminals, game equipment and cameras; the polyolefin resin foaming sheet is applied between a printed circuit board and a cover plate of an electronic product or between an image display part and a display glass plate through the steps of die cutting, rubberizing, attaching, sealing and shaping.

Wherein the polyolefin resin foam sheet material disclosed by the invention is adhered to the inner wall of the cover plate of the intelligent mobile communication equipment.

The invention has the beneficial effects that:

the applicant has found through a great deal of researches that, for the polyolefin resin foamed sheet, when the resin raw material containing the foaming agent is foamed, the use amount of the foaming agent is reduced, the foaming temperature is increased, and when the thickness of the crust layer is controlled within a specific range through secondary cooling and rolling, the forward water blocking is facilitated, and meanwhile, the high buffering performance is considered, so that the ideal effects of low azo foaming agent residual rate, forward water blocking and buffering performance are achieved.

Drawings

FIG. 1 is a 50-fold SEM image of a cross-section of a polyolefin resin base material of example 3 of the invention.

FIG. 2 is a 100-times SEM image of a cross-section of a polyolefin resin base material of example 3 of the invention.

FIG. 3 is a 200-times SEM image of a cross-section of a polyolefin resin base material of example 3 of the invention.

Detailed Description

For a better explanation of the present invention, the main content of the present invention is further elucidated with reference to the embodiments of the present invention, and is further elucidated with reference to the specific examples, but the content of the present invention is not limited to the following examples.

[ polyolefin resin substrate ]

The polyolefin resin constituting the foamed sheet may be selected from polyethylene resins, polypropylene resins, ethylene-vinyl acetate copolymers, and the like, and among them, polyethylene resins are preferable. When the polyethylene resin is selected for azo-based foaming, the foamed sheet surface thereof is liable to form a skin layer, and it is excellent in cushioning and waterproofing properties under a sheet.

The polyethylene resin as the base material is not particularly limited, and for example, low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, ethylene- α -olefin copolymer containing ethylene as a main component, and the like may be used alone or in combination of two or more. Examples of such alpha-olefins include alpha-olefins each having 2 to 12 carbon atoms such as propylene (propylene), 1-butene, 1-pentene, 3-methyl-1-butene, 1-hexene, 3-methyl-1-pentene, 4-methyl-1-pentene, 3-ethyl-1-pentene, 1-octene, 1-decene, and 1-undecene. The number of kinds of such alpha-olefins may be only 1, or may be 2 or more.

Among the polyethylene resins, low-density polyethylene is preferable. The density of the low-density polyethylene is preferably 0.910 to 0.927g/cm ³ More preferably 0.912-0.925g/cm ³ 。

The MFR of the polyethylene resin is preferably 0.5 to 50g/10 min, more preferably 1.5 to 30g/10 min, and still more preferably 2 to 15g/10 min, from the viewpoint of processing and skin thickness control.

MFR test the test was performed using the GB/T3682.1-2018 standard method using a melt flow rate meter, with a test temperature of 190℃and a nominal load of 2.16kg.

In order to increase the resilience and shape following property of the polyolefin resin foamed sheet, a rubber and/or a thermoplastic elastomer having a glass transition temperature of 20 ℃ or less, specifically: natural or synthetic rubber such as natural rubber, polyisobutylene, isoprene rubber, butyl rubber, chloroprene rubber, and nitrile rubber; olefin-based elastomers such as ethylene-vinyl acetate copolymers, polybutenes, polyisobutenes, and chlorinated polyethylene; styrene-based elastomers such as styrene-butadiene-styrene copolymer (SBS), styrene-isoprene-styrene copolymer (SIS), styrene-isoprene-butadiene-styrene copolymer (SIBS), and hydrogenated polymers thereof; thermoplastic polyester elastomers; thermoplastic polyurethane elastomers; and thermoplastic acrylic elastomers. The number of kinds of these rubbers and thermoplastic elastomers may be only 1, or may be 2 or more. In addition, the number of kinds of the components may be only 1, or may be 2 or more. The rubber and/or thermoplastic elastomer content is 0 to 55wt%, preferably 2 to 50wt%, based on the total weight of the polyolefin resin substrate.

Before forming the mixture, other auxiliary agents can be added to further improve various properties of the polyolefin foam sheet, which can be listed as: antioxidants, antimicrobial agents, colorants, antistatic agents, and fillers.

[ Cross-linking ]

The present invention is to carry out a gelation reaction, that is, a crosslinking reaction after the completion of the mixing of the polyolefin-based substrate with other additives, and crosslinking can be carried out by a known technique disclosed in the prior art, for example, by using radiation crosslinking, or by chemical crosslinking, preferably by using radiation crosslinking.

The radiation crosslinking is performed by irradiating the resin sheet with ionizing radiation such as electron beam, α ray, β ray, and γ ray. The irradiation amount of the ionizing radiation may be adjusted so that the crosslinking degree of the obtained foam sheet falls within the desired range, but is preferably 5 to 15Mrad, more preferably 6 to 13Mrad.

The chemical crosslinking is carried out by compounding an organic peroxide in the foamable composition in advance and decomposing the organic peroxide by heating, and it can be used as a co-crosslinking agent for radiation crosslinking.

The polyolefin foam sheet of the present invention has a crosslinking degree of 15 to 70%, preferably 18 to 65%, more preferably 20 to 55%, and in this range, the softness and impact strength can be desirably achieved while ensuring uniformity of cell size in the subsequent foaming.

[ azo-based blowing agent ]

The azo compound is uniformly and controllably foamed, and from the comprehensive point of view of obtaining micro bubbles, economy, environmental protection and safety, the foaming agent for polyolefin resin is better than inorganic foaming agent, nitroso foaming agent, sulfonyl hydrazide foaming agent and physical foaming agent, specifically: azodicarbonamide, diisopropyl azodicarbonate, metal azodicarbonate (such as barium azodicarbonate), azobisisobutyronitrile, and the like, and azodicarbonamide is more preferable.

Since the present application intends to solve the technical problem that the residual amount of the azo-based foaming agent in the final product is as low as possible, two factors are mainly considered here:

1. azo foaming agent content. In the foaming, the amount of the azo-based foaming agent to be used is reduced in the selection of the content thereof, and it is preferable that the azo-based foaming agent is used just completely, and the predetermined expansion ratio of the polyolefin resin foam sheet is satisfied, and the specific content is selected so that the azo-based foaming agent is 2 to 10% by weight, more preferably 2.5 to 9% by weight, and particularly preferably 3 to 8% by weight based on the content of the matrix resin before foaming. When the content of the azo-based foaming agent is less than 2wt%, the foaming ratio of the polyolefin resin foamed sheet does not reach the minimum predetermined requirement; when the content of the azo-based foaming agent is more than 10% by weight, an unavoidable increase in the residual amount of the azo-based foaming agent in the final product of the polyolefin resin foam sheet is caused.

2. Foaming temperature. The foaming temperature determines the foaming efficiency of the azo-based foaming agent and the melt strength of the polyolefin-based resin at the time of foaming, and desirably, the azo-based foaming agent is completely foamed and escapes at the temperature, and when the content of the azo-based foaming agent is small, the foaming temperature needs to be raised in order to obtain a higher foaming ratio, and the foaming temperature is specifically selected to be 200-380 ℃, more preferably 220-365 ℃, particularly preferably 250-300 ℃, and when the temperature is higher than 380 ℃, the melt strength of the polyolefin-based resin at the time of foaming is too low, so that the occurrence of cell collapse is easy, and even the decomposition of the matrix resin occurs; when the temperature is less than 200 ℃, incomplete foaming tends to occur, resulting in excessive residual azo-based foaming agent, and the foaming ratio cannot meet the expected conditions. However, when the foaming temperature is within the above range, the problem of a high aperture ratio is still unavoidable, and the positive water blocking effect is adversely affected.

Based on the control of the content of the foaming agent and the foaming temperature, the foaming multiplying power of the polyolefin resin foaming sheet disclosed by the invention falls into an ideal range and is 1.2-20cm ³ Preferably 1.5-15 cm/g ³ Per g, particularly preferably from 2 to 10cm ³ And/g. If the foaming ratio is 1.2cm ³ If the ratio of the ratio to the total weight of the polymer/g is not more than 20cm, the flexibility of the foamed sheet cannot be ensured ³ If the ratio is not less than/g, the mechanical strength of the foamed sheet is affected. Under the condition of the same foaming multiplying power, the content of the azo foaming agent used in the invention is lower than the use amount in the prior art, and the foaming temperature is higher than the foaming temperature in the prior art, so that the residual quantity of the azo foaming agent in the final product is reduced.

[ crust layer ]

In order to solve the problem that the foaming temperature is too high, which results in high aperture ratio and poor forward water blocking effect, the inventor finds out that the forward water blocking effect can be effectively improved by controlling the thickness of the crust layer after a great deal of research so as to achieve the effect of combining the residual amount of the azo foaming agent and the forward water blocking.

The forward water blocking effect refers to the condition of water penetration in the direction perpendicular to the skinning surface inside the foaming material, if the foaming material is not easy to penetrate, the forward water blocking effect is good, the forward water blocking effect is judged by measuring the hydrostatic pressure value when the first water drop just appears, and the higher the hydrostatic pressure is, the better the forward water blocking effect is proved.

Specifically, the skin layer is defined as continuously extending inward from the vertical outer surface of the foamed sheet to a thickness in which no bubbles are present or in which the volume of bubbles having a bubble diameter of 10 μm or less is 98% or more of the total bubble volume of the layer. In one embodiment, the crust layer is a dense layer with no bubbles visible under 200 x SEM; in another embodiment, bubbles are present in the skin layer from the surface, but are less than 10 μm; in another embodiment, the skin layer is a dense layer without bubbles visible under 200 times SEM, and gradually transitions to a layer with bubbles less than 10 μm, forming the skin layer as a whole. In general, if bubbles are present in the skin layer, the volume of bubbles having a cell diameter of 10 μm or less accounts for 98% or more of the total bubble volume of the layer.

The thickness of the sealing tape for waterproofing electronic equipment, which is applied to the polyolefin resin foamed sheet, is limited to 0.04mm to 2mm, preferably 0.1mm to 1mm, particularly preferably 0.2mm to 0.5mm, and the characteristics of thinness and cushioning sealing performance of the polyolefin resin foamed sheet can be fully exhibited in this range.

The single crust layer has a certain thickness, and then a compact layer is formed on the surface layer of the polyolefin resin foaming sheet, so that the positive water blocking effect is achieved. However, if the thickness of the skin layer is too thick, the mechanical properties, particularly cushioning properties, of the foamed sheet are affected. Thus, the average thickness of the individual skin layers is from 0.5% to 30%, preferably from 0.8% to 20%, and more preferably from 1% to 15% of the total thickness.

To obtain the crust layer, a foamed master is obtained after banburying and radiation crosslinking, howeverAnd then foaming is carried out at 200-380 ℃, cooling and pressurizing are carried out on the foaming sheet material through fluid in the foaming process, or the surface is cooled through pressurizing and pressurizing by a compression roller after the foaming is finished, or the two steps of fluid and the compression roller are carried out sequentially, so that the surface crust can be formed. Wherein the fluid is air, nitrogen, condensed water or supercritical CO at a temperature lower than the surface of the foamed sheet ₂ The surface temperature of the extrusion roller is lower than the surface temperature of the foaming sheet material by more than 100 ℃.

Thickness measurement of the skin layer thickness measurement can be performed by cross-sectional SEM images.

[ stretch Forming ]

Stretching and shaping may occur during foaming, or after cooling, the polyolefin-based foamed sheet may be heated again in a molten or softened state, or both. When the stretching ratio in the MD direction is 1.2 to 5.0 times, preferably 1.3 to 3.0 times, and the stretching ratio in the TD direction is 1.1 to 4.0 times, preferably 1.3 to 2.5 times, the polyolefin foam sheet can be prevented from breaking during stretching, and the foamed sheet is excellent in softness and tensile strength, and the quality of the foamed sheet becomes further uniform.

After completion of the stretching and shaping, the polyolefin resin foamed sheet has a TD average cell diameter of 0 to 3. Mu.m, more preferably 0 to 2. Mu.m, a MD average cell diameter of 0 to 5. Mu.m, still more preferably 0 to 4. Mu.m, a TD average cell diameter of 30 to 400. Mu.m, more preferably 50 to 300. Mu.m, a MD average cell diameter of 35 to 500. Mu.m, still more preferably 60 to 400. Mu.m, a TD average cell diameter of 25 to 250. Mu.m, still more preferably 50 to 180. Mu.m, and a MD average cell diameter of 30 to 300. Mu.m, more preferably 55 to 200. Mu.m in the skin layer.

The final polyolefin resin foamed sheet has a 25% compression strength of 30 to 700KPa, a tensile strength of 0.5 to 25MPa, preferably a 25% compression strength of 50 to 600KPa, a tensile strength of 1 to 20MPa, more preferably a 25% compression strength of 80 to 500KPa, and a tensile strength of 2 to 15MPa.

[ adhesive layer ]

The obtained polyolefin resin foam sheet needs to be sized before bonding, namely: the pressure-sensitive adhesive layer is formed on the surface of the sheet, and various methods in the prior art, such as a method of directly applying the pressure-sensitive adhesive composition to the foam substrate (direct method), a method of applying the pressure-sensitive adhesive composition to an appropriate release surface, forming the pressure-sensitive adhesive layer on the release surface, and bonding the pressure-sensitive adhesive layer to the foam substrate to transfer the pressure-sensitive adhesive layer (transfer method), can be applied. The coating may be performed using a known or conventional coater such as a gravure roll coater, a reverse roll coater, a contact roll coater, a dip roll coater, a bar coater, a blade coater, or a spray coater. The thickness is controlled to be 1 μm to 50. Mu.m, preferably 2 to 20. Mu.m, more preferably 3 to 10. Mu.m.

The kind of the adhesive constituting the adhesive layer is not particularly limited, and specific examples are: an adhesive layer composed of one or two or more of known various adhesives such as an acrylic adhesive, a rubber adhesive (natural rubber, synthetic rubber, a mixture thereof, and the like), a silicone adhesive, a polyester adhesive, a urethane adhesive, a polyether adhesive, a polyamide adhesive, and a fluorine adhesive. From the viewpoint of transparency and weather resistance, an acrylic adhesive is preferably used to form the adhesive layer.

[ practical application ]

The polyolefin resin foaming sheet material can be attached to the inner wall of the cover plate of an electronic product after forming a waterproof adhesive tape, and is formed between a printed circuit board and the cover plate of the electronic product or between an image display part and a display glass plate, and then the waterproof adhesive tape is sealed and shaped to play a role in buffering and waterproofing.

The electronic product comprises: smart mobile communication devices, notebook computers, electronic books, tablet terminals, gaming devices, cameras, and wearable electronic devices, among which smart mobile communication devices having higher requirements for thickness and waterproof sealing performance are preferred.

Examples

The present invention will be described in more detail with reference to the following examples, which are not intended to limit the scope of the invention.

Example 1:

100 parts by weight of LDPE (trade name: medium petrifaction 2426H) having an MFR of 2g/10min, 5.6 parts by weight of azodicarbonamide, and 0.5 part by weight of an antioxidant were mixed in a high-speed mixer, followed by kneading at 130℃and then extrusion into a strip-shaped sheet, followed by irradiation of both sides of the strip-shaped resin sheet with an electron beam of an acceleration voltage of 500kV for 10Mrad to crosslink the resin sheet, the crosslinked resin sheet was continuously fed into a heating furnace at 350℃and heated by an infrared heater to foam the resin sheet, the foaming resin sheet was purged with room temperature air at the end of the foaming furnace for 5 minutes so that the surface temperature of the sheet was reduced to 135℃or lower, and then the thickness was controlled by an extrusion roll having a roll surface temperature of 20℃while forming a crust layer. Finally, the sheet was reheated to 120℃and stretched at a stretching ratio of 3 times in the MD and at a stretching ratio of 3 times in the TD to obtain a polyolefin foam sheet having a thickness of 0.083mm.

Example 2

100 parts by weight of LDPE (trade name: medium petrifaction 2426H) having an MFR of 2g/10min, 3.1 parts by weight of azodicarbonamide, and 0.5 part by weight of an antioxidant were mixed in a high-speed mixer, followed by kneading at 130℃and then extrusion into a strip-shaped sheet, followed by irradiation of both sides of the strip-shaped resin sheet with an electron beam of an acceleration voltage of 500kV for 10Mrad to crosslink the resin sheet, the crosslinked resin sheet was continuously fed into a heating furnace at 215℃and heated by an infrared heater to foam the resin sheet, the foaming resin sheet was purged with room temperature air at the end of the foaming furnace for 5 minutes so that the surface temperature of the sheet was reduced to 135℃or lower, and then the thickness was controlled by an extrusion roll having a roll surface temperature of 20℃while forming a crust layer. Finally, the sheet was reheated to 120℃and stretched at a stretch ratio of 1.3 times in the MD and at a stretch ratio of 1.3 times in the TD, whereby a polyolefin foam sheet was obtained having a thickness of 0.205mm.

Example 3

100 parts by weight of LDPE (trade name: medium petrifaction 2426H) having an MFR of 2g/10min, 6.2 parts by weight of azodicarbonamide, and 0.5 part by weight of an antioxidant were mixed in a high-speed mixer, followed by kneading at 130℃and then extrusion into a strip-shaped sheet, followed by irradiation of both sides of the strip-shaped resin sheet with an electron beam of an acceleration voltage of 500kV for 10Mrad to crosslink the resin sheet, the crosslinked resin sheet was continuously fed into a heating furnace of 300℃and heated by an infrared heater to foam the resin sheet, the foaming resin sheet was purged with room temperature air at the end of the foaming furnace for 5 minutes so that the surface temperature of the sheet was reduced to 135℃or lower, and then the thickness was controlled by an extrusion roll having a roll surface temperature of 20℃while forming a crust layer. Finally, the sheet was reheated to 120℃and stretched at a stretching ratio of 2.5 times in the MD and at a stretching ratio of 2 times in the TD, whereby a polyolefin foam sheet was obtained having a thickness of 0.498mm.

Example 4

100 parts by weight of LDPE (trade name: medium petrifaction 2426H) having an MFR of 2g/10min, 7.1 parts by weight of azodicarbonamide, and 0.5 part by weight of an antioxidant were mixed in a high-speed mixer, followed by kneading at 130℃and then extrusion into a strip-shaped sheet, followed by irradiation of both sides of the strip-shaped resin sheet with an electron beam of an acceleration voltage of 500kV for 10Mrad to crosslink the resin sheet, the crosslinked resin sheet was continuously fed into a heating furnace at 320℃and heated by an infrared heater to foam the resin sheet, the foaming resin sheet was purged with room temperature air at the end of the foaming furnace for 5 minutes so that the surface temperature of the sheet was reduced to 135℃or lower, and then the thickness was controlled by an extrusion roll having a roll surface temperature of 20℃while forming a crust layer. Finally, the sheet was reheated to 120℃and stretched at a stretch ratio of 1.5 times in the MD and at a stretch ratio of 1.5 times in the TD, whereby a polyolefin foam sheet was obtained having a thickness of 0.785mm.

Example 5

100 parts by weight of LDPE (trade name: medium petrifaction 2426H) having an MFR of 2g/10min, 7.8 parts by weight of azodicarbonamide, 0.5 part by weight of an antioxidant were mixed in a high-speed mixer, followed by kneading at 130℃and then extrusion into a strip-shaped sheet, followed by irradiation of both sides of the strip-shaped resin sheet with an electron beam of an acceleration voltage of 500kV for 10Mrad to crosslink the resin sheet, the crosslinked resin sheet was continuously fed into a heating furnace of 300℃and heated by an infrared heater to foam the resin sheet, the foaming resin sheet was purged with room temperature air at the end of the foaming furnace for 5 minutes so that the surface temperature of the sheet was reduced to 135℃or lower, and then the thickness was controlled by an extrusion roll having a roll surface temperature of 20℃while forming a crust layer. Finally, the sheet was reheated to 120℃and stretched at a stretch ratio of 1.5 times in the MD and at a stretch ratio of 1.5 times in the TD, whereby a polyolefin foam sheet was obtained having a thickness of 0.996mm.

Example 6

100 parts by weight of LDPE (trade name: medium petrochemical 2426H) having an MFR of 2g/10min, 8.2 parts by weight of azodicarbonamide, and 0.5 part by weight of an antioxidant were mixed in a high-speed mixer, followed by kneading at 130℃and then extrusion into a strip-shaped sheet, followed by irradiation of both sides of the strip-shaped resin sheet with an electron beam of an acceleration voltage of 500kV for 10Mrad to crosslink the resin sheet, the crosslinked resin sheet was continuously fed into a heating furnace at 320℃and heated by an infrared heater to foam the resin sheet, the foam resin sheet was purged with room temperature air at the end of the foaming furnace for 5 minutes so that the surface temperature of the sheet was reduced to 135℃or lower, and then the thickness was controlled by an extrusion roll having a roll surface temperature of 20℃while forming a crust layer. Finally, the sheet was reheated to 120℃and stretched at a stretch ratio of 1.5 times in the MD and at a stretch ratio of 1.5 times in the TD, whereby a polyolefin foam sheet having a thickness of 1.612mm was obtained.

Example 7

100 parts by weight of LDPE (trade name: medium petrochemical 2426H) having an MFR of 2g/10min, 8.5 parts by weight of azodicarbonamide, 0.5 part by weight of an antioxidant, were mixed in a high-speed mixer, kneaded at 130℃and then extruded into a strip-shaped sheet, and then the resin sheet was crosslinked by irradiation of both sides of the strip-shaped resin sheet with an electron beam of an acceleration voltage of 500kV of 10Mrad, and the crosslinked resin sheet was continuously fed into a heating furnace at 330℃and heated by an infrared heater to foam the resin sheet, and the foam resin sheet was purged with room temperature air at the end of the foaming furnace for 5 minutes so that the surface temperature of the sheet was reduced to 135℃or lower, and then the thickness was controlled by an extrusion roll having a roll surface temperature of 20℃while forming a crust layer. Finally, the sheet was reheated to 120℃and stretched at a stretch ratio of 1.5 times in the MD and at a stretch ratio of 1.5 times in the TD, whereby a polyolefin foam sheet having a thickness of 1.983mm was obtained.

Example 8

100 parts by weight of LDPE (trade name: medium petrochemical 2426H) having an MFR of 2g/10min, 25 parts by weight of POE (trade name: DOW OBC 9500), 5.6 parts by weight of azodicarbonamide, and 0.5 part by weight of an antioxidant were mixed in a high-speed mixer, followed by kneading at 130℃and then extrusion into a strip-shaped sheet, followed by irradiation of both sides of the strip-shaped resin sheet with an electron beam of 10Mrad having an acceleration voltage of 500kV to crosslink the resin sheet, the crosslinked resin sheet was continuously fed into a heating furnace at 310℃and heated by an infrared heater to foam the resin sheet, the foamed resin sheet was purged with room temperature air at the end of the foaming furnace for 5 minutes so that the surface temperature of the sheet was reduced to 135℃or lower, and then the thickness was controlled by an extrusion roll having a roll surface temperature of 20℃while forming a skin layer. Finally, the sheet was reheated to 120℃and stretched at a stretch ratio of 1.5 times in the MD and at a stretch ratio of 1.5 times in the TD, whereby a polyolefin foam sheet was obtained having a thickness of 0.201mm.

Example 9

100 parts by weight of LDPE (trade name: medium petrochemical 2426H) having an MFR of 2g/10min, 25 parts by weight of isoprene rubber (trade name: japanese rayleigh IR 2200), 5.6 parts by weight of azodicarbonamide, and 0.5 part by weight of an antioxidant were mixed in a high-speed mixer, followed by kneading at 130℃and then extrusion into a strip-shaped sheet, followed by irradiation of both sides of the strip-shaped resin sheet with electron rays 7Mrad having an acceleration voltage of 500kV to crosslink the resin sheet, the crosslinked resin sheet was continuously fed into a heating furnace at 300℃and heated by an infrared heater to foam the resin sheet, the foamed resin sheet was purged with room temperature air at the end of the foaming furnace for 5 minutes so that the sheet surface temperature was reduced to 135℃or lower, and then the thickness was controlled by an extrusion roll having a roll surface temperature of 20℃while forming a skin layer. Finally, the sheet was reheated to 120℃and stretched at a stretch ratio of 1.5 times in the MD and at a stretch ratio of 1.5 times in the TD, whereby a polyolefin foam sheet was obtained having a thickness of 0.176mm.

Example 10

100 parts by weight of POE (trade name: DOW OBC 9500), 5.6 parts by weight of azodicarbonamide, 0.5 part by weight of an antioxidant were mixed in a high-speed mixer, followed by kneading at 130℃and then extrusion into a strip-shaped sheet, followed by irradiation of both sides of the strip-shaped resin sheet with an electron beam of an acceleration voltage of 500kV of 14Mrad to crosslink the resin sheet, the crosslinked resin sheet was continuously fed into a heating furnace at 320℃and heated by an infrared heater to foam the resin sheet, the foamed resin sheet was purged with room temperature air at the end of the foaming furnace for 5 minutes so that the surface temperature of the sheet was reduced to 135℃or lower, and then the thickness was controlled by an extrusion roll having a roll surface temperature of 20℃while forming a skin layer. Finally, the sheet was reheated to 120℃and stretched at a stretch ratio of 1.5 times in the MD and at a stretch ratio of 1.5 times in the TD, whereby a polyolefin foam sheet was obtained having a thickness of 0.215mm.

Comparative example 1

100 parts by weight of LDPE (trade name: medium petrifaction 2426H) having an MFR of 2g/10min, 5.6 parts by weight of azodicarbonamide, and 0.5 part by weight of an antioxidant were mixed in a high-speed mixer, kneaded at 130℃and extruded into a strip-shaped sheet, and then the resin sheet was crosslinked by irradiation of both sides of the strip-shaped resin sheet with an electron beam having an acceleration voltage of 500kV of 14Mrad, and the crosslinked resin sheet was continuously fed into a heating furnace at 300℃and heated by an infrared heater to foam the resin sheet, and after the foaming was completed, the resin sheet was directly subjected to a stretching process, stretched at a stretching ratio of 5.7 times in the MD direction, and stretched at a stretching ratio of 4.4 times in the TD direction, to thereby obtain a polyolefin-based foamed sheet having a thickness of 0.094mm.

Comparative example 2

100 parts by weight of LDPE (trade name: medium petrifaction 2426H) having an MFR of 2g/10min, 5.6 parts by weight of azodicarbonamide, and 0.5 part by weight of an antioxidant were mixed in a high-speed mixer, followed by kneading at 130℃and then extrusion into a strip-shaped sheet, followed by irradiation of both sides of the strip-shaped resin sheet with an electron beam of an acceleration voltage of 500kV of 14Mrad to crosslink the resin sheet, the crosslinked resin sheet was continuously fed into a heating furnace at 215℃and heated by an infrared heater to foam the resin sheet, the foaming resin sheet was purged with room temperature air at the end of the foaming furnace for 5 minutes so that the surface temperature of the sheet was reduced to 135℃or lower, and then the thickness was controlled by an extrusion roll having a roll surface temperature of 20℃while forming a crust layer. Finally, the sheet was reheated to 120℃and stretched at a stretch ratio of 1.5 times in the MD and at a stretch ratio of 1.5 times in the TD, whereby a polyolefin foam sheet was obtained having a thickness of 0.189mm.

Comparative example 3

100 parts by weight of LDPE (trade name: medium petrifaction 2426H) having an MFR of 2g/10min, 5.6 parts by weight of azodicarbonamide, and 0.5 part by weight of an antioxidant were mixed in a high-speed mixer, followed by kneading at 130℃and then extrusion into a strip-shaped sheet, followed by irradiation of both sides of the strip-shaped resin sheet with an electron beam of an acceleration voltage of 500kV of 14Mrad to crosslink the resin sheet, the crosslinked resin sheet was continuously fed into a heating furnace at 410℃and heated by an infrared heater to foam the resin sheet, the foaming resin sheet was purged with room temperature air at the end of the foaming furnace for 5 minutes so that the surface temperature of the sheet was reduced to 135℃or lower, and then the thickness was controlled by an extrusion roll having a roll surface temperature of 20℃while forming a crust layer. Finally, the sheet was reheated to 120℃and stretched at a stretching ratio of 2 times in the MD and at a stretching ratio of 2 times in the TD, to obtain a polyolefin foam sheet having a thickness of 1.597mm.

The testing method comprises the following steps:

1-Forward Water blocking Effect test

The polyolefin foam sheet was cut into a sample of 70X 70mm in size, and placed in a test tank for filling distilled water, facing the test tank. A support mesh is used on the other side of the sample to support the malleable elastic material. The test cell was placed horizontally on a test stand (FX 300-IV Hydro Tester) and 60mL of purified water (which may be a funnel or syringe) was slowly injected into the penetration test cell from the inlet at the top. The clamping of the sample ensures that distilled or deionized water does not penetrate the sample under pressure before the test begins. The sample was subjected to a continuously increasing water pressure at a water pressure rising rate of 6.0 kpa/min.+ -. 0.3kpa/min, and the water penetration phenomenon was observed. The hydrostatic pressure value at the moment when the first water droplet on the sample just appears is recorded.

Determination of 2-azo residual Rate

The polyolefin foam sheet was cut into samples having a size of at least 70 x 70mm, as determined according to the SGS standard SVHC test.

3-determination of closed cell content

The closed cell ratio F1 and the closed cell ratio F2 of the sample are calculated according to ASTM D2856 (1998) standard:

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

Closed cell content F2 (%) =100-F1

Determination of 4-25% compression Strength

According to ISO3386-1, a material having a thickness of 10mm or less was laminated to a thickness of 10mm or more, and the compression rate was set as close as possible to 50% of the material thickness per minute, and the compression stress at 25% deformation was measured.

Measurement of 5-expansion ratio

The dimensions of the sample were measured in centimeters (cm) as specified in GB/T6342-1996. At least three positions are measured per dimension, and five positions are measured per dimension in the middle for a plate-like hard material. Each dimensional average was calculated separately and the sample volume V was calculated.

The samples were weighed as M to the nearest 0.5% in grams (g).

The foaming ratio was calculated by the following formula, and the average value thereof was taken.

Foaming ratio=v/M

6-determination of tensile Strength

The test was carried out according to GB/T6344-2008 "determination of tensile Strength and elongation at break of Soft foam Polymer Material".

7-measurement of crosslinking degree

a. Taking 100mg of a sample from the foamed sheet, and accurately weighing the weight A (mg) of the sample;

b. the sample is wrapped by a 200-mesh metal net, and the sample wrapped by the metal net is immersed in dimethylbenzene at 120 ℃ and is kept stand for 24 hours. Insoluble substances can be collected in the metal net through the filtering action of the metal net; accurately weighing the weight B (mg) of insoluble matters after vacuum drying;

c. the degree of crosslinking (mass%) was calculated:

degree of crosslinking (mass%) =100% × (B/a).

8-thickness and cell diameter measurement

And carrying out SEM scanning on the sample to obtain a scanning electron microscope image, and measuring the thickness of the whole sample and the crust layer and the average pore diameters in different directions by using software.

The structural and performance indices of the examples and comparative examples are shown in tables 1 and 2:

TABLE 1 structural indices of examples 1-10 and comparative examples 1-3

TABLE 2 Performance indices for examples 1-10 and comparative examples 1-3

From this, it can be seen that when the ratio of the skin layers does not fall within the above-mentioned limited range, at least one performance index of the azo residual ratio, the compressive strength, the tensile strength, and the hydrostatic pressure is not within the desired range, for example, comparative example 1 has no skin layer, the water blocking performance (hydrostatic pressure) is only 1.73Kpa, comparative example 2 has a skin layer ratio too high, the azo residual ratio is high, and the 25% compressive strength is too high, and the same comparative example 3 has a skin layer ratio too low, and the tensile strength and the water blocking performance are also not desired. Only when the pore diameters of the skinning layer and the non-skinning layer are controlled within the range defined in the present invention while the thickness also falls within the defined range, as shown in examples 1 to 10, a balanced and desirable effect is exhibited in terms of azo residual rate, mechanical properties, and water blocking effect.

The present invention can be implemented in other forms than the above-described forms within a range not exceeding the gist of the present invention. The embodiments disclosed in the present application are examples, and are not limited to these. The scope of the present invention should be construed as being defined by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Industrial applicability

The polyolefin resin foam sheet of the present invention can be used for waterproof buffering in various electronic products, such as intelligent mobile communication devices, notebook computers, electronic books, tablet terminals, game devices, cameras, wearable electronic devices, and the like.

Claims

1. A foamed sheet of a polyolefin resin having a thickness of 0.04mm to 2mm, characterized in that the foamed sheet comprises at least one skin layer, the average thickness of the individual skin layers being 0.5% to 30% of the total thickness, the skin layers being defined as continuously extending inward from the vertical outer surface of the foamed sheet by a thickness in which the volume of bubbles having a bubble diameter of 10 μm or less is 98% or more of the total bubble volume of the layer,

the polyolefin resin foaming sheet is foamed by using azo foaming agent, the residual rate of the azo foaming agent in the final formed foaming sheet is lower than 0.2wt%,

the polyolefin resin foam sheet has a closed cell content of 90% or more and a crosslinking degree of 15 to 70%,

the TD average foam pore diameter in the crust layer is more than 0 and less than or equal to 3 mu m, the MD average foam pore diameter is more than 0 and less than or equal to 5 mu m, the TD average foam pore diameter of the whole foaming sheet is 25-250 mu m, and the MD average foam pore diameter is 30-300 mu m.

2. The polyolefin-based resin foamed sheet according to claim 1, wherein the average thickness of the individual skin layers is 0.8% to 20% of the total thickness.

3. The polyolefin-based resin foamed sheet according to claim 2, wherein the average thickness of the individual skin layers is 1 to 15% of the total thickness.

4. The polyolefin-based resin foamed sheet according to claim 1, wherein the residual ratio of the azo-based foaming agent in the final molded foamed sheet is less than 0.15% by weight.

5. The polyolefin resin foamed sheet according to claim 4, wherein the residual ratio of the azo-based foaming agent in the final molded foamed sheet is less than 0.1% by weight.

6. The polyolefin-based resin foamed sheet according to claim 1, wherein the polyolefin-based resin foamed sheet has a 25% compressive strength of 30 to 700KPa and a tensile strength of 0.5 to 25MPa.

7. The polyolefin resin foamed sheet according to claim 1, wherein the polyolefin resin is a polyethylene resin.

8. A method for producing the polyolefin resin foamed sheet according to claim 1, characterized in that 2 to 10% by weight of an azo-based foaming agent, a matrix resin and other auxiliary agent raw materials are mixed and added into a high-speed mixer to be mixed to obtain a mixture, a foamed master sheet is obtained after banburying and crosslinking, then foaming is performed at 200 to 380 ℃, cooling and pressurizing the foamed sheet by a fluid during foaming and/or pressurizing and cooling the surface by an extrusion roller after foaming is completed to obtain a skin layer, and the foamed sheet is obtained by stretching and shaping.

9. Use of the polyolefin resin foamed sheet according to any one of claims 1 to 7 for waterproofing in electronic products.

10. The waterproof application of claim 9, wherein the electronic product comprises: smart mobile communication devices, notebook computers, electronic books, tablet terminals, gaming devices, cameras, and wearable electronic devices.

11. The waterproofing application according to one of claims 9 and 10, characterized in that the polyolefin resin foamed sheet comprises a step of die cutting, sizing, attaching, sealing, and shaping, which is attached between the printed circuit board and the cover plate of the electronic product or between the image display member and the display glass plate.

12. An intelligent mobile communication device, wherein the polyolefin resin foamed sheet according to claim 1 is attached to an inner wall of a cover plate of the intelligent mobile communication device.