An expandable composite paper and a method of making the same

Abstract

The application belongs to the technical field of functional composite materials, and particularly relates to an inflatable composite paper and a preparation method thereof. The inflatable composite paper comprises an intermediate layer, inflatable adhesive layers arranged on two sides of the intermediate layer, transition adhesive layers connected with the inflatable adhesive layers, and outer isolation layers. The transition adhesive layers are adhesives and are arranged between the inflatable adhesive layers and the outermost isolation layers. The composite paper is mainly applied to filling narrow gaps in electrically insulated spaces and fixing elements in narrow spaces. The composite paper is subjected to surface pretreatment of the outermost isolation layers, and is coated with low-viscosity adhesive material, so that the surface of one side of the isolation layer composite forms a smooth structure, the problem that layers are prone to delamination after expansion of the existing inflatable composite material is solved, and better stiffness and tensile strength can be maintained.

Description

Technical Field

[0001] This invention specifically relates to an expandable composite paper and its preparation method, belonging to the field of functional composite material technology. Background Technology

[0002] In recent years, expandable composite paper materials have gradually become a research hotspot due to their ability to expand under heat. There are many applications for thermally expandable composite paper. It can be used for fixing and insulating the stator slots of new energy vehicle motors, reducing the subsequent coating process, reducing production costs, and ensuring slot fullness. It can be used for sealing gaps in the construction and fire protection fields. It can be used in the manufacturing process of flexible circuit boards to reduce stress concentration and effectively release thermal stress. It can also be used in semiconductor packaging materials to play a role in thermal conduction and buffering.

[0003] Currently, there are two main types of expandable composite paper products on the market. One type involves directly coating the plastic film with expandable slurry. This type of product has a high surface roughness and a high coefficient of friction, which can easily lead to difficulties in insertion into narrow gaps. Furthermore, the expanded microspheres are exposed to the outside and are prone to falling off after being heated and expanded, affecting the filling effect. The other type involves constructing a "sandwich" structure, where an expandable adhesive is coated on the outside of the plastic film, and then an isolation layer is bonded to the outside of the adhesive.

[0004] Currently, the outermost insulating layer of expandable composite paper used in semiconductor packaging materials is generally aramid paper or polyester nonwoven fabric. This product has a smooth surface and good stiffness, which can compensate for the difficulty of insertion caused by the rough surface when directly coating expandable paste on the outside of plastic film. In addition, the insulating layer can protect the expanded microspheres from falling off due to external friction. However, since the outer insulating layer is made of aramid paper or nonwoven fabric, which is composed of fibers and has micropores, and the main component of the expandable adhesive is epoxy resin, the epoxy resin has poor adhesion after being expanded and further cured by heat. The effective bonding area with the outer insulating layer is reduced, which easily leads to delamination and causes safety hazards.

[0005] In summary, the existing technology has obvious shortcomings. For components that fill narrow gaps in electrical insulation space and fix narrow spaces, it is of great significance to develop an expandable composite paper that enhances the adhesion between the insulating layer and the expandable adhesive layer, prevents delamination, and ensures that the expandable adhesive layer maintains its original structure after thermal expansion. Summary of the Invention

[0006] This invention addresses the shortcomings of existing technologies by providing an expandable composite paper and its preparation method. This expandable composite paper is primarily used for filling narrow gaps in electrical insulation and fixing components in confined spaces. It solves the problem of delamination between the insulating layer and the expandable adhesive layer in existing products of the same type. The composite paper pre-treats the outermost insulating layer and coats it with a low-viscosity adhesive material to form a transition adhesive layer. This results in a smooth surface structure for the outermost insulating layer, allowing for better and tighter bonding with the expandable adhesive layer. This solves the problem of easy delamination between layers in existing expandable composite materials after expansion, while maintaining better stiffness and tensile strength. In the application of flat wire motors in new energy vehicles, it can reduce stator production steps, ensure full motor slot filling, and improve the overall safety and stability of the motor.

[0007] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: an expandable composite paper, the composite paper comprising an intermediate layer, expandable adhesive layers disposed on both sides of the intermediate layer, a transition adhesive layer connected to the expandable adhesive layers, and an outer isolation layer;

[0008] The transition adhesive layer is an adhesive and is located between the expandable adhesive layer and the outer isolation layer;

[0009] The intermediate layer is a plastic film;

[0010] The expandable adhesive layer is provided by an expandable adhesive comprising thermally expandable microspheres and a thermosetting resin;

[0011] The outer isolation layer is made of non-woven material.

[0012] Furthermore, the mass content of the thermally expandable microspheres is 10-30% of the total mass of the expandable adhesive; preferably, the content is 15-20%.

[0013] Furthermore, the thickness of the plastic film is 50~125μm; the thickness of the expandable adhesive layer before expansion is 2~10μm; the thickness of the transition adhesive layer is 1~5μm; and the thickness of the isolation layer is 25~85μm.

[0014] Furthermore, the thickness of the plastic film is 75 μm; the thickness of the expandable adhesive layer before expansion is 3~5 μm; the thickness of the transition adhesive layer is 2~3 μm; and the thickness of the isolation layer is 50 μm.

[0015] Furthermore, the thermally expandable microspheres are high molecular weight polymers with an expansion temperature range of 100~160℃, a diameter of 2~20μm before expansion, and an expansion ratio of 30~50 times.

[0016] Furthermore, the thermosetting resin includes a modified epoxy resin, a curing agent, and a thermoplastic component.

[0017] Furthermore, the modified epoxy resin is at least one of bisphenol epoxy resin, fatty acid backbone epoxy resin, glycidyl ether epoxy resin, phenolic modified epoxy resin, and cyclic epoxy resin; the thermoplastic component is phenoxy resin.

[0018] Furthermore, the transition adhesive layer is at least one of epoxy adhesives, acrylate adhesives, and polyurethane adhesives.

[0019] Furthermore, the plastic film is any one of polyethylene terephthalate (PET) film, polyethylene naphthalate (PEN) film, polyimide (PI) film, polyphenylene sulfide (PPS) film, and polyether ether ketone (PEEK) film.

[0020] Furthermore, the insulating layer is at least one of aramid paper, kraft paper, barley paper, and polyester nonwoven fabric.

[0021] This invention also discloses a method for preparing expandable composite paper, the method comprising the following steps:

[0022] S1. Unwind the two rolls of separator layer, coat them with the transition adhesive layer, dry them, and rewind them for later use.

[0023] S2. Unwind the plastic film of the middle layer, coat both sides with expandable adhesive to form an expandable adhesive layer, press the expandable adhesive layer with the isolation layer prepared in step S1 at room temperature, and then roll it up and cure to obtain expandable composite paper.

[0024] Furthermore, in step S2, after the intermediate layer is coated with expandable adhesive on both sides, it passes through an oven tunnel at 65~85℃ and stays in the oven for 30~70 seconds.

[0025] Furthermore, the time interval between pressing the expandable adhesive layer and the treated isolation layer at room temperature in step S2 should not exceed 5 minutes.

[0026] Furthermore, in step (2), the pressing pressure is 1~4MPa.

[0027] The beneficial effects of this invention are:

[0028] (1) The expandable composite paper of the present invention pre-treats the outermost isolation layer and coats it with a low-viscosity adhesive material to form a transition adhesive layer, so that the surface of the isolation layer composite side forms a smooth structure. The adhesive of the expandable adhesive layer has better compatibility with the smooth transition adhesive layer, and the interlayer bonding is stronger. At the same time, the transition adhesive layer is coated with a low-viscosity adhesive, which will penetrate into the outermost non-woven isolation layer during the coating process, and has a larger bonding area with the fibers, making the bonding with the isolation layer stronger, thus solving the problem that existing expandable composite materials are prone to delamination between layers after expansion.

[0029] (2) The method for preparing expandable composite paper described in this invention not only solves the problem that expandable composite paper prepared by previous methods will delaminate after being heated and expanded during use due to insufficient adhesion at the bonding points and fiber shedding; but also increases the stiffness, tensile strength and elongation of expandable composite paper through the mutual interaction between layers, making the overall mechanical properties of composite paper more beneficial and improving the overall safety and stability of motor.

[0030] The expandable composite paper of this invention pre-treats the outermost insulating layer by coating its surface with a low-viscosity adhesive material to form a transitional bonding layer. This transitional bonding layer fills the pores and gaps between the fibers on the surface of the nonwoven material, creating a smooth surface structure and increasing the effective bonding area. This results in a tighter bond between the outermost insulating layer and the expandable adhesive layer. When heated, the expandable adhesive layer further enhances the adhesion between the expandable adhesive layer and the outer insulating layer, preventing delamination after the composite paper expands. The bonding between the transitional adhesive layer and the nonwoven material surface improves the stiffness of the outermost insulating layer. Combined with the stronger adhesion between the insulating layer and the expandable adhesive layer, the combined effect of the layers after curing increases the overall stiffness, tensile strength, and elongation of the material.

[0031] (3) The expandable composite paper described in this invention is mainly used to fill narrow gaps in electrical insulation space and to fix components in narrow spaces. Taking the stator slot insulation of a flat wire motor in a new energy vehicle as an example, the expandable composite paper described in this invention can be heated to expand after being inserted into the copper wire slot of the stator of the flat wire motor, thereby fixing the flat copper wire. This effectively saves the process of fixing with paint dripping in the prior art, simplifies the motor manufacturing process, improves production efficiency, and reduces production costs. At the same time, compared with fixing with paint dripping, the expandable composite paper can better fix the copper wire, prevent copper wire displacement and vibration, and improve the overall performance, safety and stability of the motor. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the structure of the expandable composite paper described in this invention. Detailed Implementation

[0033] The specific embodiments of the present invention will be described in detail below. The present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed.

[0034] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used is for describing particular embodiments only and is not intended to limit the invention.

[0035] like Figure 1 As shown, an expandable composite paper includes an intermediate layer 1, expandable adhesive layers 2 disposed on both sides of the intermediate layer, a transition adhesive layer 3 connected to the expandable adhesive layers, and an outer isolation layer 4; the transition adhesive layer 3 is an adhesive and is located between the expandable adhesive layer 2 and the outermost isolation layer 4; the intermediate layer 1 is a plastic film; the expandable adhesive layer 2 is provided by an expandable adhesive, which includes thermally expandable microspheres and thermosetting resin; the outer isolation layer 4 is a nonwoven material.

[0036] The expandable composite paper described in this embodiment of the invention comprises a seven-layer structure, but the technology of the present invention is not limited to a seven-layer structure, and multiple layers can be composited as needed.

[0037] Specifically, the mass content of the thermally expandable microspheres is 10-30% of the total mass of the expandable adhesive; preferably, the content is 15-20%.

[0038] Specifically, the thickness of the plastic film is 50~125μm; the thickness of the expandable adhesive layer before expansion is 2~10μm; the thickness of the transition adhesive layer is 1~5μm; and the thickness of the isolation layer is 25~85μm.

[0039] More specifically, the thickness of the plastic film is 75 μm; the thickness of the expandable adhesive layer before expansion is 3~5 μm; the thickness of the transition adhesive layer is 2~3 μm; and the thickness of the isolation layer is 50 μm.

[0040] Specifically, the thermally expandable microspheres are high molecular weight polymers with an expansion temperature range of 100~160℃, a diameter of 2~20μm before expansion, and an expansion ratio of 30~50 times.

[0041] Specifically, the thermosetting resin includes a modified epoxy resin, a curing agent, a thermoplastic component, and additives. In actual production applications, other necessary additives can be added to the thermosetting resin according to usage requirements.

[0042] Specifically, the modified epoxy resin is at least one of bisphenol epoxy resin, fatty acid backbone epoxy resin, glycidyl ether epoxy resin, phenolic modified epoxy resin, and cyclic epoxy resin; the thermoplastic component is phenoxy resin. Adding phenoxy resin as a thermoplastic component can enhance the film-forming properties of the thermally expanding epoxy adhesive.

[0043] Specifically, the adhesive used in the transition bonding layer has good compatibility and adhesion with the expandable bonding layer and the outer isolation layer, and the transition bonding layer is at least one of epoxy adhesive, acrylic adhesive and polyurethane adhesive.

[0044] Specifically, the plastic film is any one of polyethylene terephthalate (PET) film, polyethylene naphthalate (PEN) film, polyimide (PI) film, polyphenylene sulfide (PPS) film, and polyether ether ketone (PEEK) film.

[0045] Specifically, the outer insulating layer of the present invention has a smooth surface and good tear resistance and bending resistance. The insulating layer is at least one of aramid paper, kraft paper, barley paper and polyester nonwoven fabric.

[0046] The preparation of an expandable composite paper according to an embodiment of the present invention includes the following steps:

[0047] S1. Unwind the two rolls of separator layer, coat them with the transition adhesive layer, dry them in the drying tunnel, and rewind them for later use.

[0048] S2. Unwind the plastic film of the middle layer, coat both sides with expandable adhesive, and pass it through the oven tunnel at 65~85℃ for 30~70 seconds to remove the solvent from the expandable adhesive, forming an expandable bonding layer. In the semi-cured state, press it with the isolation layer treated in step S1 at room temperature with a pressing pressure of 1~4MPa. After winding and curing, expandable composite paper is obtained.

[0049] More specifically, the time interval between pressing the expandable adhesive layer in step S2 and the isolation layer treated in step S1 at room temperature should not exceed 5 minutes. After 5 minutes, the expandable adhesive layer is prone to curing, resulting in the adhesive surface not sticking and affecting the composite effect.

[0050] The raw materials involved in the specific embodiments of the present invention are as follows:

[0051] Aramid paper: Minstar YT564.

[0052] Acrylic adhesive: Adhesive peak 5020.

[0053] Polyurethane adhesive: 3M DP6310NS.

[0054] Epoxy adhesive: Hans MegaGlue.

[0055] Polyethylene terephthalate (PET) film: Hostaphan.

[0056] Polyethylene naphthalate (PEN) film: Teonex Q51.

[0057] Polyimide (PI) film: Kapton.

[0058] Polyphenylene sulfide (PPS) film: Toray Torelina.

[0059] Example 1

[0060] The preparation of an expandable composite paper specifically includes the following steps:

[0061] S1. Two rolls of 50μm thick aramid paper are used as separators. They are then coated with 2μm thick acrylic glue to form a transition bonding layer. After drying in the drying tunnel, they are rolled up for later use.

[0062] S2. Unwind a 75μm thick PEN plastic film with an intermediate layer, and coat both sides with a 3μm thick expandable adhesive, wherein the mass content of expandable microspheres is 20% of the total mass of the expandable adhesive; pass it through a 70℃ oven tunnel for 50 seconds to remove the solvent from the expandable adhesive, forming an expandable bonding layer, and then press it with the isolation layer treated in S1 at room temperature while it is in a semi-cured state. After winding and curing, expandable composite paper is obtained.

[0063] Example 2

[0064] The preparation of an expandable composite paper specifically includes the following steps:

[0065] S1. Two rolls of 50μm thick aramid paper are used as separators. They are then coated with 3μm thick polyurethane adhesive to form a transition bonding layer. After drying in the drying tunnel, they are rolled up for later use.

[0066] S2. Unwind a PI plastic film with a 75μm thick intermediate layer, and coat both sides with an expandable adhesive with a thickness of 5μm, wherein the mass content of the expandable microspheres is 20% of the total mass of the expandable adhesive; pass it through a 65℃ oven tunnel for 60 seconds to remove the solvent from the expandable adhesive, forming an expandable bonding layer, and then press it with the isolation layer treated in S1 at room temperature while it is in a semi-cured state. After winding and curing, expandable composite paper is obtained.

[0067] Example 3

[0068] The preparation of an expandable composite paper specifically includes the following steps:

[0069] S1. Two rolls of 50μm thick polyester nonwoven fabric are used as the isolation layer. They are coated with 2μm thick polyurethane adhesive to form a transition bonding layer. They are then dried in the drying tunnel and rolled up for later use.

[0070] S2. Unwind a PET plastic film with a 75μm thick interlayer, and coat both sides with an expandable adhesive with a thickness of 3μm, wherein the mass content of expandable microspheres is 20% of the total mass of the expandable adhesive; pass it through an 85℃ oven tunnel for 30 seconds to remove the solvent from the expandable adhesive, forming an expandable bonding layer, and then press it with the isolation layer treated in S1 at room temperature while it is in a semi-cured state. After winding and curing, expandable composite paper is obtained.

[0071] Example 4

[0072] The preparation of an expandable composite paper specifically includes the following steps:

[0073] S1. Two rolls of 50μm thick aramid paper are used as separators. They are then coated with 3μm thick acrylic glue to form a transition bonding layer. After drying in the drying tunnel, they are rolled up for later use.

[0074] S2. Unwind a 75μm thick PEN plastic film with an intermediate layer, and coat both sides with an 8μm thick expandable adhesive, wherein the mass content of expandable microspheres is 30% of the total mass of the expandable adhesive; pass it through a 65℃ oven tunnel for 70 seconds to remove the solvent from the expandable adhesive, forming an expandable bonding layer, and heat-press it with the isolation layer treated in S1 in a semi-cured state, and then roll it up and cure to obtain expandable composite paper.

[0075] Example 5

[0076] The preparation of an expandable composite paper specifically includes the following steps:

[0077] S1. Two rolls of aramid paper with a thickness of 25μm are used as the separator. They are then coated with acrylic glue with a thickness of 1μm to form a transition bonding layer. After drying in the drying tunnel, they are rolled up for later use.

[0078] S2. Unwind a 50μm thick PPS plastic film with an intermediate layer, and coat both sides with a 2μm thick expandable adhesive, wherein the mass content of expandable microspheres is 15% of the total mass of the expandable adhesive; pass it through a 65℃ oven tunnel for 50 seconds to remove the solvent from the expandable adhesive, forming an expandable bonding layer, and then press it with the isolation layer treated in S1 at room temperature while it is in a semi-cured state. After winding and curing, expandable composite paper is obtained.

[0079] Example 6

[0080] The preparation of an expandable composite paper specifically includes the following steps:

[0081] S1. Two rolls of aramid paper with a thickness of 85μm are used as the separator. They are then coated with epoxy adhesive with a thickness of 5μm to form a transition bonding layer. After drying in the drying tunnel, they are rolled up for later use.

[0082] S2. Unwind a 125μm thick PEEK plastic film with an intermediate layer, and coat both sides with a 10μm thick expandable adhesive, wherein the mass content of expandable microspheres is 10% of the total mass of the expandable adhesive; pass it through a 65℃ oven tunnel for 70 seconds to remove the solvent from the expandable adhesive and form an expandable bonding layer, and then press it with the isolation layer treated in S1 at room temperature while it is in a semi-cured state. After winding and curing, expandable composite paper is obtained.

[0083] Comparative Example 1

[0084] The preparation of an expandable composite paper, in Comparative Example 1, adopts the same method as in Example 1, except that the outermost aramid paper of Comparative Example 1 is not coated with acrylic adhesive to form a transition bonding layer.

[0085] Specifically, the following steps are included:

[0086] S1. Unwind a 75μm thick PEN plastic film with an intermediate layer, and coat both sides with a 3μm thick expandable adhesive, wherein the mass content of expandable microspheres is 20% of the total mass of the expandable adhesive; pass it through a 70℃ oven tunnel for 50 seconds to remove the solvent from the expandable adhesive, forming an expandable bonding layer, and then press it with aramid paper at room temperature in a semi-cured state. After winding and curing, expandable composite paper is obtained.

[0087] Comparative Example 2

[0088] The preparation of an expandable composite paper, in Comparative Example 2, adopts the same method as in Example 1, except that in step S2 of Comparative Example 2, the two sides of the intermediate layer are coated with epoxy resin adhesive without the addition of expandable microspheres.

[0089] Specifically, the following steps are included:

[0090] S1. Two rolls of 50μm thick aramid paper are used as separators. They are then coated with acrylic glue to form a transition bonding layer. After drying in the drying tunnel, they are rolled up for later use.

[0091] S2. Unwind a 75μm thick PEN plastic film with an intermediate layer, coat both sides with epoxy resin adhesive without added expanding microspheres, pass it through a 70℃ oven tunnel, and bake it in the oven for 50 seconds to remove the solvent from the expandable adhesive, forming an expandable bonding layer. Then, in a semi-cured state, press it with the isolation layer treated in S1 at room temperature, and after winding and curing, obtain expandable composite paper.

[0092] Comparative Example 3

[0093] The preparation of an expandable composite paper is carried out in the same manner as in Example 4 in Comparative Example 3, except that in step S2 of Comparative Example 3, the mass content of expandable microspheres in the coated expandable adhesive is 50%.

[0094] Specifically, the following steps are included:

[0095] S1. Two rolls of 50μm thick aramid paper are used as separators. They are then coated with 3μm thick acrylic glue to form a transition bonding layer. After drying in the drying tunnel, they are rolled up for later use.

[0096] S2. Unwind a 75μm thick PEN plastic film with an intermediate layer, and coat both sides with an 8μm thick expandable adhesive, wherein the mass content of expandable microspheres is 50% of the total mass of the expandable adhesive; pass it through a 65℃ oven tunnel for 70 seconds to remove the solvent from the expandable adhesive, forming an expandable bonding layer, and heat-press it with the isolation layer treated in S1 in a semi-cured state, and then roll it up and cure to obtain expandable composite paper.

[0097] Comparative Example 4

[0098] The preparation of an expandable composite paper specifically includes the following steps:

[0099] S1. Two rolls of 50μm thick aramid paper are used as separators. They are then coated with 2μm thick acrylic glue to form a transition bonding layer. After drying in the drying tunnel, they are rolled up for later use.

[0100] S2. Unwind a 75μm thick PEN plastic film with an intermediate layer, and coat both sides with a 3μm thick expandable adhesive, wherein the mass content of expandable microspheres is 5% of the total mass of the expandable adhesive; pass it through a 70℃ oven tunnel for 30 seconds to remove the solvent from the expandable adhesive, forming an expandable bonding layer, and then press it with the isolation layer treated in S1 at room temperature while it is in a semi-cured state. After winding and curing, expandable composite paper is obtained.

[0101] Comparative Example 5

[0102] The preparation of an expandable composite paper, in Comparative Example 5, adopts the same method as in Example 1, except that: in step S1 of Comparative Example 5, the acrylate adhesive applied exceeds the thickness set by the present invention, and the coating thickness is 10 μm. At this time, the thickness of the transition adhesive layer is not within the range set by the present invention.

[0103] Specifically, the following steps are included:

[0104] S1. Two rolls of 50μm thick aramid paper are used as separators and coated with 10μm thick acrylic glue to form a transition bonding layer. The paper is then dried in an oven.

[0105] Phenomenon: After drying, the substrate becomes wrinkled and curled, making it impossible to proceed to the next lamination step.

[0106] Comparative Example 6

[0107] The preparation of an expandable composite paper is carried out in the same manner as in Example 1, except that in step S2, the paper passes through an oven tunnel at 50°C, and the temperature of the oven tunnel is lower than that in Example 1.

[0108] Specifically, the following steps are included:

[0109] S1. Two rolls of 50μm thick aramid paper are used as separators. They are then coated with 1μm thick acrylic glue to form a transition bonding layer. After drying in the drying tunnel, they are rolled up for later use.

[0110] S2. Unwind a 75μm thick PEN plastic film with an intermediate layer, and coat both sides with a 3μm thick expandable adhesive, wherein the mass content of expandable microspheres is 20% of the total mass of the expandable adhesive; pass it through a 50℃ oven tunnel for 50 seconds to remove the solvent from the expandable adhesive, forming an expandable bonding layer, and then press it with the isolation layer treated in S1 at room temperature while it is in a semi-cured state. After winding and curing, expandable composite paper is obtained.

[0111] Phenomenon: In step S2, when the temperature of the oven passage is lower than the temperature set by the present invention, wrinkles will occur when the transition adhesive layer is pressed at room temperature in step S3, affecting the flatness between layers during the pressing process. At the same time, the expandable adhesive layer will overflow during the pressing process.

[0112] Comparative Example 7

[0113] The preparation of an expandable composite paper, in Comparative Example 7, adopts the same method as in Example 1, except that: in step S2, the time in the oven is 20 seconds, which is shorter than that in Example 1.

[0114] Specifically, the following steps are included:

[0115] S1. Two rolls of 50μm thick aramid paper are used as separators. They are then coated with 1μm thick acrylic glue to form a transition bonding layer. After drying in the drying tunnel, they are rolled up for later use.

[0116] S2. Unwind a 75μm thick PEN plastic film with an intermediate layer, and coat both sides with a 3μm thick expandable adhesive, wherein the mass content of expandable microspheres is 20% of the total mass of the expandable adhesive; pass it through a 70℃ oven tunnel for 20 seconds to remove the solvent from the expandable adhesive and form an expandable bonding layer, and then press it with the isolation layer treated in S1 at room temperature while it is in a semi-cured state. After winding and curing, expandable composite paper is obtained.

[0117] Phenomenon: In step S2, when the time in the oven is lower than the temperature set by the present invention, wrinkles will appear when the transition adhesive layer is pressed at room temperature in step S3, affecting the flatness between layers during the pressing process. At the same time, the expandable adhesive layer will overflow during the pressing process.

[0118] Comparative Example 8

[0119] The preparation of an expandable composite paper is carried out in the same manner as in Example 1, except that in step S2, the paper passes through an oven tunnel at 95°C, and the temperature of the oven tunnel is higher than that in Example 1.

[0120] Specifically, the following steps are included:

[0121] S1. Two rolls of 50μm thick aramid paper are used as separators. They are then coated with 1μm thick acrylic glue to form a transition bonding layer. After drying in the drying tunnel, they are rolled up for later use.

[0122] S2. Unwind a 75μm thick PEN plastic film with an intermediate layer, and coat both sides with a 3μm thick expandable adhesive, wherein the mass content of expandable microspheres is 20% of the total mass of the expandable adhesive; pass it through a 95℃ oven tunnel for 50 seconds to remove the solvent from the expandable adhesive, forming an expandable bonding layer, and then press it with the isolation layer treated in S1 at room temperature while it is in a semi-cured state. After winding and curing, expandable composite paper is obtained.

[0123] Phenomenon: In step S3, during pressing, the expandable adhesive layer and the transition adhesive layer do not bond completely and the adhesion is not strong.

[0124] Comparative Example 9

[0125] The preparation of an expandable composite paper, in Comparative Example 9, adopts the same method as in Example 1, except that: in step S2, the time in the oven is 80 seconds, which is longer than that in Example 1.

[0126] Specifically, the following steps are included:

[0127] S1. Two rolls of 50μm thick aramid paper are used as separators. They are then coated with 1μm thick acrylic glue to form a transition bonding layer. After drying in the drying tunnel, they are rolled up for later use.

[0128] S2. Unwind a 75μm thick PEN plastic film with an intermediate layer, and coat both sides with a 3μm thick expandable adhesive, wherein the mass content of expandable microspheres is 20% of the total mass of the expandable adhesive; pass it through a 70℃ oven tunnel for 80 seconds to remove the solvent from the expandable adhesive, forming an expandable bonding layer, and then press it with the isolation layer treated in S1 at room temperature while it is in a semi-cured state. After winding and curing, expandable composite paper is obtained.

[0129] Phenomenon: During pressing, the expandable adhesive layer and the transition adhesive layer do not bond completely and the adhesion is weak.

[0130] Comparative Example 10

[0131] In the preparation of an expandable composite paper, Comparative Example 10 involves coating a transition adhesive layer onto an expandable adhesive layer and then pressing it together with an isolation layer at room temperature. The amounts of each component are the same as in Example 1.

[0132] Specifically, the following steps are included:

[0133] S1. Unwind a PEN plastic film with a 75μm thick intermediate layer, and coat both sides with an expandable adhesive with a thickness of 3μm, wherein the mass content of the expandable microspheres is 20% of the total mass of the expandable adhesive; pass it through a 70℃ oven tunnel to remove the solvent from the expandable adhesive, forming an expandable bonding layer, and allow it to fully cure.

[0134] S2. An acrylic adhesive with a thickness of 1 μm is coated on the surface of the expandable adhesive layer in step S1 to form a transition adhesive layer. The solvent is dried in an oven tunnel, and the layer is pressed with aramid paper with a thickness of 50 μm in a semi-cured state at room temperature. After winding and curing, expandable composite paper is obtained.

[0135] Phenomenon: If acrylic adhesive is applied to the expandable adhesive layer in a semi-cured state in step S1, it is difficult to ensure uniformity of the adhesive application because the semi-cured adhesive layer is still in a flowing state. This results in uneven application of the transition adhesive layer, as well as adhesive dripping and stringing. Therefore, in Comparative Example 10, after applying the expandable adhesive, the solvent in the expandable adhesive is removed in the oven tunnel, and the acrylic adhesive is applied only after the formed expandable adhesive layer has completely cured.

[0136] Performance tests were conducted on the expandable composite paper prepared in the examples and comparative examples, and the measurement methods involved referred to GB / T5591.2-2017. The expansion ratio test method was as follows: before thermal expansion, the average thickness of the sample at 5 points was recorded as h1. Then, the sample was placed in an oven at 150℃ for 5 minutes. After removal, the average thickness of the sample after heating was measured using the same method and recorded as h2. The expansion ratio (Δ) was calculated as follows: Δ = h2 / h1. The method for determining whether delamination occurred after thermal expansion was as follows: the appearance of the sample after thermal expansion was observed to check for delamination phenomena such as edge curling and cracking between layers. The performance test data of the expandable composite paper prepared in the examples and comparative examples are shown in Table 1 below.

[0137] Table 1 Performance test data of expandable composite paper prepared in the examples and comparative examples

[0138]

[0139] As can be seen from the data in Table 1, the expandable composite paper prepared by the method described in this invention in Examples 1-6 has good stiffness and tensile strength, and does not delaminate after expansion during use. In particular, in Examples 1-6, the composite paper not only does not delaminate after expansion, but also has a better expansion ratio while maintaining better stiffness and tensile strength. The method for preparing the expandable composite paper of this invention involves surface pretreatment of the outermost isolation layer, and filling the pores and gaps between fibers on the surface of the non-woven material with a transition adhesive layer, forming a smooth surface structure. This results in a tighter bond between the outermost isolation layer and the expandable adhesive layer. The expandable adhesive layer, after thermal expansion, better bonds the expandable adhesive layer and the outer isolation layer, solving the problem of delamination between layers after expansion of the composite paper. The presence of the transition adhesive layer improves the stiffness of the outermost aramid paper, and combined with the stronger adhesion between the aramid paper and the expandable adhesive layer, after curing, the combined effect of the layers increases the stiffness, elongation strength, and elongation of the entire material.

[0140] Non-woven fabric materials are composed of fibers with tiny pores, and the expandable adhesive is mainly composed of epoxy resin. After the epoxy resin is heated and further cured, its adhesion is poor, the effective bonding area with the external isolation layer is reduced, and delamination is prone to occur, causing safety hazards.

[0141] According to the data in Table 1, a comparison of the experimental results of Comparative Example 1 and Example 1 shows that when the outermost isolation layer is not coated with acrylic adhesive to form a transitional bonding layer, the elongation and stiffness of the resulting expandable composite paper decrease significantly, and the expandable bonding layer delaminates during use. This is because the aramid paper surface is chemically inert, resulting in poor adhesion between it and the expandable bonding layer. Furthermore, because aramid paper is composed of fibers with micropores and gaps, the high viscosity of the expandable bonding layer prevents it from effectively penetrating these pores and gaps when in contact with the aramid paper. Therefore, during use, when the expandable bonding layer expands, delamination occurs due to insufficient adhesion and fiber shedding. In Example 1, the outermost aramid paper is pretreated with acrylic adhesive to form a strong transitional intermediate layer. This intermediate layer can also bond well with the expandable bonding layer, preventing delamination after expansion during use. Because acrylic adhesive has excellent fluidity and compatibility with aramid paper, it can fill the pores and gaps on the surface of aramid paper while also achieving a certain depth of penetration. The presence of this transitional adhesive layer makes the outermost aramid paper more rigid. Combined with the stronger adhesion between the aramid paper and the expandable adhesive layer, this increases the stiffness and elongation of the entire material.

[0142] According to the data in Table 1, a comparison of the experimental results of Comparative Example 2 and Example 1 shows that in Comparative Example 2, in step S2, the intermediate layer is coated with epoxy resin adhesive without the addition of expanded microspheres. The resulting composite paper does not have expandable properties. This is because the expansion properties of the composite paper are mainly achieved through expanded microspheres. If expanded microspheres are not added, it will not have expansion properties and will not meet the usage requirements.

[0143] Based on the data in Table 1, a comparison of the experimental results of Comparative Example 3 and Example 4 shows that in step S2, when the mass content of expandable microspheres in the coated expandable adhesive is 50%, the expansion ratio increases, and delamination occurs in the expanded layer after expansion, but the elongation of the expandable composite material decreases. This is because the expansion performance is mainly provided by the expandable microspheres; when the content of expandable microspheres in the expandable adhesive layer is high, the expansion ratio increases. Under external force, each microsphere is a defect point in the overall structure of the composite paper. Cracks easily generate and propagate at these relatively fragile microsphere-fiber interfaces when stressed. The more microspheres there are, the more stress concentration points there are, and the tensile strength, bursting strength, and tear strength of the paper will decrease sharply.

[0144] Based on the data in Table 1, a comparison of the experimental results of Comparative Example 4 and Example 1 shows that in step S2, when the mass content of expandable microspheres in the coated expandable adhesive layer is 5%, the expansion ratio decreases. This is because the expansion performance is mainly provided by the expandable microspheres, and when the content of expandable microspheres in the expandable adhesive layer is low, the expansion ratio decreases.

[0145] Based on the data in Table 1, a comparison of the experimental results of Comparative Example 5 and Example 1 shows that in step S1, when the coated acrylic adhesive is thicker than the thickness set in this invention, the time the isolation layer spends in the oven after coating will be prolonged in order to ensure that the adhesive layer is completely dry, thereby reducing the machine speed and affecting production efficiency. Furthermore, the transition adhesive layer cures during the drying process in the oven, and the shrinkage stress generated during the curing process is large, exceeding the tear resistance of the isolation layer substrate, causing the substrate to wrinkle and curl, making it impossible to proceed to the next step of lamination.

[0146] In the preparation processes of Comparative Examples 6 and 7, wrinkles appeared during room-temperature pressing with the transition adhesive layer in step S3, affecting the flatness between layers during pressing. Simultaneously, the expandable adhesive layer overflowed during pressing. According to the data in Table 1, comparing the experimental results of Comparative Examples 6, 7, and Example 1 shows that in step S2, when the temperature of the oven tunnel or the residence time in the oven is lower than the range set by this invention, the solvent in the adhesive layer does not evaporate completely, resulting in high adhesive fluidity. This makes it easy for wrinkles or adhesive overflow to occur during subsequent lamination processes, affecting the product appearance. This is because, in step S2, the temperature of the oven tunnel in Comparative Example 6 is lower than the temperature set by this invention, and the residence time in the oven in Comparative Example 7 is lower than the temperature set by this invention. This leads to incomplete evaporation of the solvent in the expandable adhesive layer, resulting in fluidity of the adhesive. Consequently, wrinkles and overflow of the expandable adhesive layer occur in step S3; furthermore, this leads to uneven composite paper lamination, poor uniformity, and affects downstream applications.

[0147] In the preparation processes of Comparative Examples 8 and 9, during step S3, when the expandable adhesive layer was pressed with the transition adhesive layer at room temperature, incomplete and weak adhesion occurred between the expandable adhesive layer and the transition adhesive layer. This is because the oven tunnel temperature was too high in Comparative Example 8, and the residence time in the oven was too long in Comparative Example 9, causing the expandable adhesive layer to solidify and reducing the adhesive layer's bonding ability. Consequently, incomplete and weak adhesion occurred between the expandable adhesive layer and the transition adhesive layer, affecting the composite effect.

[0148] Based on the data in Table 1, a comparison of the experimental results of Comparative Example 10 and Example 1 shows that: In Comparative Example 10, the transition adhesive layer was coated onto the expandable adhesive layer, and then directly pressed with the isolation layer at room temperature. The resulting expandable composite paper exhibited delamination during use. This is because: the lamination process sequence of Comparative Example 10 is the same as that of conventional expandable composite paper. After the transition adhesive layer is coated onto the expandable adhesive layer and the solvent is dried in an oven, the adhesive layer of the transition adhesive layer is in a semi-cured state. When laminating with the outermost isolation layer, the adhesive cannot penetrate into the interior of the isolation layer. The adhesive layer of the transition adhesive layer only bonds to the outermost fibers of the isolation layer, significantly reducing the bonding area. After thermal expansion, it still exhibits delamination due to the same problem as conventional expandable composite paper on the market. The transition adhesive layer in the conventional lamination process of Comparative Example 10 cannot achieve the effect mentioned in this patent.

[0149] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are exhaustively listed. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0150] For those skilled in the art, various modifications and improvements can be made without departing from the concept of the present invention, and these modifications and improvements are all within the scope of protection of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. An expandable composite paper, characterized in that, The composite paper includes an intermediate layer, expandable adhesive layers disposed on both sides of the intermediate layer, a transition adhesive layer connected to the expandable adhesive layers, and an outer isolation layer. The transition bonding layer is an adhesive with a thickness of 1~5μm, and is located between the expandable bonding layer and the outer isolation layer; The intermediate layer is a plastic film with a thickness of 50~125μm; The expandable adhesive layer is provided by an expandable adhesive comprising thermally expandable microspheres and a thermosetting resin, and the thickness of the expandable adhesive layer before expansion is 2~10μm; The thermosetting resin includes a modified epoxy resin, a curing agent, and a thermoplastic component; The modified epoxy resin is at least one of bisphenol epoxy resin, fatty acid backbone epoxy resin, glycidyl ether epoxy resin, phenolic modified epoxy resin, and cyclic epoxy resin. The transition bonding layer is at least one of epoxy adhesives, acrylic adhesives, and polyurethane adhesives; The outer isolation layer is a nonwoven material, which is at least one of aramid paper, kraft paper, barley paper and polyester nonwoven fabric, and the thickness of the outer isolation layer is 25~85μm; The mass content of the thermally expandable microspheres is 10-30% of the total mass of the expandable adhesive; The thermally expandable microspheres are high molecular polymers with a diameter of 2-20 μm and an expansion ratio of 30-50 times before expansion in the expansion temperature range of 100-160℃. The expandable composite paper is used to fill narrow gaps in electrical insulation spaces and to fix components in narrow spaces; The method for preparing the expandable composite paper includes the following steps: S1. Unwind the two rolls of separator layer, coat them with the transition adhesive layer, dry them, and rewind them for later use. S2. Unwind the plastic film of the middle layer, coat both sides with expandable adhesive, and pass it through the oven tunnel at 65~85℃. The time spent in the oven is 30~70 seconds to form an expandable adhesive layer. The expandable adhesive layer is then pressed with the separator layer prepared in step S1 at room temperature. After rewinding and curing, expandable composite paper is obtained.

2. The expandable composite paper according to claim 1, characterized in that, The thickness of the plastic film is 75 μm; The thickness of the expandable adhesive layer before expansion is 3~5μm; The thickness of the transition bonding layer is 2~3μm; The thickness of the outer isolation layer is 50 μm.

3. The expandable composite paper according to claim 1, characterized in that, The thermoplastic component is phenoxy resin.

4. The expandable composite paper according to claim 1, characterized in that, The plastic film is any one of polyethylene terephthalate film, polyethylene naphthalate film, polyimide film, polyphenylene sulfide film, and polyetheretherketone film.

5. A method for preparing an expandable composite paper according to any one of claims 1-4, characterized in that, The preparation method includes the following steps: S1. Unwind the two rolls of separator layer, coat them with the transition adhesive layer, dry them, and rewind them for later use. S2. Unwind the plastic film of the middle layer, coat both sides with expandable adhesive, and pass it through the oven tunnel at 65~85℃. The time spent in the oven is 30~70 seconds to form an expandable adhesive layer. The expandable adhesive layer is pressed with the isolation layer processed in step S1 at room temperature, and after winding and curing, expandable composite paper is obtained.

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