White halogen-free flame-retardant epoxy resin composition and covering film prepared using the same

Abstract

The present invention provides a white halogen-free flame-retardant epoxy resin composition and a covering film prepared using the same. The white halogen-free flame-retardant epoxy resin composition is based on halogen-free polyurethane modified epoxy resin, halogen-free phenol-aralkyl epoxy resin and halogen-free phosphorus-containing epoxy resin, supplemented with toughening agent, phosphorus-containing flame retardant, amine curing agent, curing accelerator, white pigment, silane coupling agent and organic solvent as auxiliary agents, and the obtained covering film has excellent performance. On the one hand, the epoxy resin composition of the present invention is halogen-free, and the adhesive is made into white by white pigment, which plays a certain covering effect, and the thickness of the white coating and polyimide film layer can be reduced; on the other hand, due to the use of polyimide support film, the covering film maintains good physical properties, and the flame retardant passes the requirements of UL94V-0 grade, the surface resistance and volume resistivity meet the requirements, the peel strength is high, the dimensional stability is good, and the processing performance is excellent.

Description

Technical Field

[0001] The invention relates to the technical field of epoxy resins, in particular to a white halogen-free flame-retardant epoxy resin composition and a covering film prepared using the same. Background Art

[0002] At present, there are two main methods for manufacturing white cover films. One is to use a white polyimide film, apply a layer of adhesive on its surface, and then compound it with a release material; the other is to use an ordinary yellow polyimide film, apply a layer of white ink on the surface of the film, and then apply a layer of adhesive on the other side of the film and compound it with a release material. Both of the above methods have the problem of poor covering power after the thickness becomes thinner, which cannot meet the requirements of thinning electronic devices, and there are not many reports on white cover films with flame retardant properties.

[0003] For most of the cover film materials used for flexible circuit boards (FPC) on the market, in order to ensure the flame retardant level, halogen-containing epoxy resins and additives are used, but they will produce dioxins and other carcinogens when incinerated. With the implementation of environmental protection laws in countries such as the European Union, electronic products are required not to contain environmental management substances such as halogens, lead, mercury, tin, hexavalent chromium, polybrominated biphenyls, polybrominated diphenyl ethers, etc. In order to meet the above requirements, the industry is actively developing halogen-free materials. At present, the halogen-free cover films on the market are mainly phosphorus-containing flame retardant systems, which are flame-retardant by adding phosphorus-containing flame retardants and a certain amount of inorganic fillers such as aluminum hydroxide. However, the addition of a large amount of flame retardants will reduce the mechanical properties of the cover film, increase water absorption, and there is a problem of filler migration to the surface, which affects subsequent use.

[0004] At present, the main method to solve the problem of poor covering property after the thickness of the white covering film becomes thinner is to remove the polyimide film layer, stack the white coating and the adhesive layer to make a white pure adhesive film material. Patent Nos. CN 105592623 A, CN 102079956 A, CN 206051923 U, CN 106590458 A, CN 211210030 U and CN 201509360 U all use the direct stacking of white coating and adhesive layer. Although this method can reduce the thickness of the white covering film, it is difficult for the pure adhesive film to have flame retardant properties, and the lack of polyimide film as a supporting carrier makes the physical properties of the covering film such as strength poor, making it difficult to meet the increasingly stringent process requirements of fine circuits, semiconductors and new energy industries. Patents No. CN201563288 U, CN 102501499 A, CN 215831849 U, CN 113801596A, CN 115073790A, CN201563288 U, 205439460 U and CN 205051961 U prepare white covering films by the conventional method of coating a white coating and an adhesive layer on both sides of a polyimide film. Although this method can ensure the strength and hiding power of the covering film, it cannot meet the requirements of thinning electronic devices, and it never mentions making the adhesive layer white. Summary of the invention

[0005] The present invention mainly provides a white halogen-free flame-retardant epoxy resin composition and a covering film prepared using the same. On the one hand, by making the adhesive white, it can play a certain covering role and reduce the thickness of the white coating and the polyimide film layer; on the other hand, due to the presence of the polyimide support film, the physical properties of the covering film are better and can meet the flame retardant requirements.

[0006] In order to achieve the above object, the present invention provides a white halogen-free flame-retardant epoxy resin composition, characterized in that, calculated by solid weight, it comprises the following components in parts by weight:

[0007]

[0008]

[0009] (1) Halogen-free polyurethane modified epoxy resin

[0010] The modification method of the halogen-free polyurethane modified epoxy resin can be to synthesize a polyurethane prepolymer by polyhydroxy compounds and polyisocyanates, and then graft the prepolymer with epoxy resin to synthesize the halogen-free polyurethane modified epoxy resin; or to adopt bulk synchronous polymerization to directly synthesize the halogen-free polyurethane modified epoxy resin; no matter which of the above methods is used to prepare the halogen-free polyurethane modified epoxy resin, as long as it can meet the performance requirements of use, it can be used in the formulation of the present invention.

[0011] Halogen-free polyurethane modified epoxy resin and toughening agent can be used together to fully exert their toughening effects by adjusting the formula system. In addition, since the molecular weight of halogen-free polyurethane modified epoxy resin is not large, it can also increase the cross-linking density of the cured product to a certain extent, thereby increasing the cohesive strength of the adhesive layer and finally increasing the peel strength of the covering film. Currently available halogen-free polyurethane modified epoxy resins include HyPox UA 10 and HyPox UA 11 (manufactured by CVC Thermosetting Special Materials Company, USA).

[0012] (2) Halogen-free phenol-aralkyl epoxy resin

[0013] Halogen-free phenol-aralkyl epoxy resin contains the following typical structure,

[0014]

[0015] The structure contains a large amount of aralkyl groups, and has the following properties: excellent heat resistance, high Tg, high Td, good processability, low hygroscopicity and low CTE, low dielectric constant (Dk) and dielectric loss factor (Df), and intrinsic flame retardancy. In addition, it also includes derivatives modified on the basis of halogen-free phenol-aralkyl epoxy resin: biphenyl-type phenolic epoxy resin or other epoxy resins with similar structures. No matter which of the above resins can meet the performance requirements of use, they can be used in the formulation of the present invention.

[0016] Since halogen-free phenol-aralkyl epoxy resins are mainly phenol-aralkyl epoxy resins containing a large amount of aralkyl groups and prepared with new phenolic resins (Xylok resins) as the skeleton, they have excellent heat resistance and intrinsic flame retardant properties, and such resins have good compatibility with other resins and toughening agents in the formula. Such halogen-free phenol-aralkyl epoxy resins include NC-3500 manufactured by Nippon Kayaku Co., Ltd. and EPICLON HP-4710 manufactured by DIC Corporation.

[0017] (3) Halogen-free phosphorus-containing epoxy resin

[0018] Phosphorus-containing epoxy resin refers to the use of reactive phosphorus compounds to chemically bond to the epoxy resin molecular chain to synthesize an intrinsically flame-retardant epoxy resin. The reactive phosphorus compound may include phosphophanate compounds having a typical structure as shown below:

[0019]

[0020] 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and its derivatives: 10-(2,5-dihydroxyphenyl)-10-hydrogen-9-oxa-10-phosphaphenanthrene-10-oxide, or other reactive organic phosphorus and inorganic phosphorus compounds. As long as the performance requirements of use are met, any modified phosphorus-containing epoxy resin can be used in the formulation of the present invention.

[0021] Optionally, the phosphorus content of the phosphorus-containing epoxy resin is 0.5%-16%.

[0022] Specifically, commercially available phosphorus-containing epoxy resins include XEN-0140, SEN-6085 (SHIN-A T&C, South Korea), and the like.

[0023] (4) Toughening agent

[0024] Toughener can be synthetic rubbers such as nitrile rubber, styrene butadiene rubber, butadiene rubber, ethylene propylene rubber, etc., or can be polyurethane toughener and elastomer, or one or more of the above-mentioned tougheners. Commercially available nitrile rubber includes CTBN1300×8 (manufactured by CVC Thermosetting Special Materials Company of the United States), styrene butadiene rubber includes Buna VSL 5025-2 HM (manufactured by Lanxess), butadiene rubber includes BR-9000 (manufactured by Sinopec), ethylene propylene rubber includes EPDM6950 (manufactured by Lanxess), and polyurethane toughener and elastomer include Desmocap11A (manufactured by Covestro).

[0025] (5) Phosphorus-containing flame retardants

[0026] Phosphorus-containing flame retardants include, but are not limited to, PX-200 (manufactured by Daihachi Chemical Industry Co., Ltd., Japan), OP930 (manufactured by Clariant, Germany), and the like.

[0027] (6) Amine curing agent

[0028] The amine curing agent is one or more of 4,4'-diaminodiphenylmethane, meta-phenylenediamine, meta-phenylenediamine or dicyandiamide.

[0029] (7) Curing accelerator

[0030] The curing accelerator is one or more of N-(2-hydroxyphenyl)-N',N'-dimethylurea, N-(2-hydroxy-4-nitrobenzene)-N',N'-dimethylurea, N-(4-chlorophenyl)-N',N'-dimethylurea, N-(5-chloro-2-hydroxybenzene)-N',N'-dimethylurea, 2,4,6-tris(dimethylaminomethyl)phenol, and boron trifluoride.

[0031] (8) White pigment

[0032] The white pigment is preferably titanium dioxide, preferably 2-6 parts. The present invention mainly uses white pigment to achieve the purpose of coloring and covering. Commercially available white pigments include TIPAQUE R-930 (Ishihara Sangyo Co., Ltd., Japan), Ti-Pure TM (Chuntai TM )R-960 (manufactured by The Chemours Chemical (Shanghai) Co., Ltd.), etc.

[0033] (9) Silane coupling agent

[0034] The silane coupling agent is KH550.

[0035] (10) Organic solvents

[0036] The organic solvent is one or more of toluene, acetone, butanone, cyclohexanone, toluene, ethylene glycol methyl ether, propylene glycol methyl ether, ethyl acetate, propylene glycol methyl ether acetate, N',N'-dimethylformamide, and dimethyl sulfoxide.

[0037] In addition, the present invention also provides a covering film prepared from a white halogen-free flame-retardant epoxy resin composition, characterized in that it includes: a polyimide film, a white halogen-free flame-retardant epoxy resin composition layer coated on the polyimide film, and a release paper attached to the white halogen-free flame-retardant epoxy resin composition layer.

[0038] The polyimide film is amber (or light brown) and transparent, with a thickness of 5-50 μm; the white halogen-free flame-retardant epoxy resin composition layer is white and opaque, with a coating thickness of 8-35 μm; and the release paper has a thickness of 50-150 μm.

[0039] The components in the white halogen-free flame-retardant epoxy resin composition are dissolved and mixed by an organic solvent to form a white suspension slurry, and the white suspension slurry is coated on the polyimide film using a coating device. The polyimide film coated with the white suspension slurry is passed through an oven, heated at 70-180°C for 3 to 7 minutes to form a layer of the composition in a solvent-free adhesive film state, and then thermally laminated with release paper at 60-100°C and 0.05-2.0MPa, and rolled up to obtain a covering film.

[0040] The white suspension slurry is prepared by uniformly mixing the resin component, inorganic solid component, curing agent, auxiliary agent and organic solvent in the white halogen-free flame-retardant epoxy resin composition using grinding equipment such as a sand mill and dispersing equipment such as a high-speed stirrer.

[0041] The rotational viscosity of the white suspension slurry is preferably 1500-3000 cps, which can obtain good processability and ensure that no appearance defects occur during the coating process.

[0042] Compared with the prior art, the present invention has the following advantages: on the one hand, the epoxy resin composition of the present invention does not contain halogen, and the adhesive is made white by using white pigment, which plays a certain covering role and can reduce the thickness of the white coating and the polyimide film layer; on the other hand, due to the use of the polyimide support film, the covering film maintains good physical properties, and the flame retardancy passes the requirements of UL94 V-0 grade, the surface resistance and volume resistivity meet the requirements, the peel strength is high, the dimensional stability is good, and the processing performance is excellent. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a schematic diagram of the structure of the covering film provided by the present invention. DETAILED DESCRIPTION

[0044] Example 1

[0045] Halogen-free polyurethane modified epoxy resin (product model HyPox UA 10, epoxy equivalent 215g / eq, manufactured by CVC Thermosetting Special Materials Company, USA) 20 parts by weight; halogen-free phenol-aralkyl epoxy resin (product model NC-3500, epoxy equivalent 210g / eq, manufactured by Nippon Kayaku Co., Ltd.) 10 parts by weight; halogen-free phosphorus-containing epoxy resin (product model XEN-0140, epoxy equivalent 430g / eq, manufactured by SHIN-A T&C, South Korea) 10 parts by weight; toughener 1 (product model CTBN 1300×8, manufactured by CVC Thermosetting Special Materials Company, USA) 20 parts by weight; toughening agent 2 (Desmocap11A, manufactured by Covestro) 10 parts by weight; phosphorus-containing flame retardant (OP930, manufactured by Clariant, Germany) 20 parts by weight; dicyandiamide (DICY) 3.85 parts by weight; N-(2-hydroxy-4-nitrobenzene)-N',N'-dimethylurea 0.15 parts by weight; white pigment (TIPAQUE R-930, Ishihara Sangyo Co., Ltd., Japan) 5.5 parts by weight; silane coupling agent KH550 0.5 parts by weight; the rotational viscosity of the glue solution is adjusted to 1500-3000cps with solvents (butanone and ethyl acetate) to prepare a white halogen-free flame retardant epoxy resin composition. The composition was evenly coated on a polyimide film with a thickness of 12.5 μm, with a coating thickness of 15 μm, and then placed in an oven at 170° C. for 4 minutes, and then laminated with a release paper through a hot roller and rolled up to obtain a covering film.

[0046] Example 2

[0047] Halogen-free polyurethane modified epoxy resin (product model HyPox UA 11, epoxy equivalent 215g / eq, manufactured by CVC Thermosetting Special Materials Company, USA) 25 parts by weight; halogen-free phenol-aralkyl epoxy resin (product model NC-3500, epoxy equivalent 210g / eq, manufactured by Nippon Kayaku Co., Ltd.) 10 parts by weight; halogen-free phosphorus-containing epoxy resin (product model XEN-0140, epoxy equivalent 430g / eq, manufactured by SHIN-AT&C, South Korea) 10 parts by weight; toughener 1 (product model CTBN 1300×8, manufactured by CVC Thermosetting Special Materials Company, USA) 17 parts by weight; toughening agent 2 (Desmocap11A, manufactured by Covestro) 8 parts by weight; phosphorus-containing flame retardant (OP930, manufactured by Clariant, Germany) 20 parts by weight; dicyandiamide (DICY) 3.85 parts by weight; N-(2-hydroxy-4-nitrobenzene)-N',N'-dimethylurea 0.15 parts by weight; white pigment (TIPAQUE R-930, Ishihara Sangyo Co., Ltd., Japan) 5.5 parts by weight; silane coupling agent KH550 0.5 parts by weight; the rotational viscosity of the glue solution is adjusted to 1500-3000cps with solvents (butanone and ethyl acetate) to prepare a white halogen-free flame retardant epoxy resin composition. The composition was evenly coated on a polyimide film with a thickness of 12.5 μm, with a coating thickness of 15 μm, and then placed in an oven at 170° C. for 4 minutes, and then laminated with a release paper through a hot roller and rolled up to obtain a covering film.

[0048] Example 3

[0049] Halogen-free polyurethane modified epoxy resin (product model HyPox UA 10, epoxy equivalent 215g / eq, manufactured by CVC Thermosetting Special Materials Company, USA) 20 parts by weight; halogen-free phenol-aralkyl epoxy resin (product model EPICLON HP-4710, epoxy equivalent 170g / eq, manufactured by DIC Corporation of Japan) 9.9 parts by weight; halogen-free phosphorus-containing epoxy resin (product model XEN-0140, epoxy equivalent 430g / eq, manufactured by SHIN-A, South Korea) T&C manufactured) 10 parts by weight; toughener 1 (product model CTBN1300×8, manufactured by CVC Thermosetting Special Materials Company of the United States) 20 parts by weight; toughener 2 (Desmocap11A, manufactured by Covestro) 10 parts by weight; phosphorus-containing flame retardant (OP930, manufactured by Clariant of Germany) 20 parts by weight; dicyandiamide (DICY) 3.95 parts by weight; N-(2-hydroxy-4-nitrobenzene)-N',N'-dimethylurea 0.15 parts by weight; white pigment (TIPAQUE R-930, Ishihara Sangyo Co., Ltd. of Japan) 5.5 parts by weight; silane coupling agent KH550 0.5 parts by weight; the rotational viscosity of the glue solution is adjusted to 1500-3000cps with a solvent (butanone and ethyl acetate) to prepare a white halogen-free flame retardant epoxy resin composition. The composition was evenly coated on a polyimide film with a thickness of 12.5 μm, with a coating thickness of 15 μm, and then placed in an oven at 170° C. for 4 minutes, and then laminated with a release paper through a hot roller and rolled up to obtain a covering film.

[0050] Example 4

[0051] Halogen-free polyurethane modified epoxy resin (product model HyPox UA 10, epoxy equivalent 215g / eq, manufactured by CVC Thermosetting Special Materials Company, USA) 20 parts by weight; halogen-free phenol-aralkyl epoxy resin (product model NC-3500, epoxy equivalent 210g / eq, manufactured by Nippon Kayaku Co., Ltd.) 10 parts by weight; halogen-free phosphorus-containing epoxy resin (product model SEN-6085, epoxy equivalent 325g / eq, manufactured by SHIN-A T&C, South Korea) 9.85 parts by weight; toughener 1 (product model CTBN 1300×8, manufactured by CVC Thermosetting Special Materials Company, USA) 20 parts by weight; toughening agent 2 (Desmocap11A, manufactured by Covestro) 10 parts by weight; phosphorus-containing flame retardant (OP930, manufactured by Clariant, Germany) 20 parts by weight; dicyandiamide (DICY) 4.00 parts by weight; N-(2-hydroxy-4-nitrobenzene)-N',N'-dimethylurea 0.15 parts by weight; white pigment (TIPAQUE R-930, Ishihara Sangyo Co., Ltd., Japan) 5.5 parts by weight; silane coupling agent KH550 0.5 parts by weight; the rotational viscosity of the glue solution is adjusted to 1500-3000cps with solvents (butanone and ethyl acetate) to prepare a white halogen-free flame retardant epoxy resin composition. The composition was evenly coated on a polyimide film with a thickness of 12.5 μm, with a coating thickness of 15 μm, and then placed in an oven at 170° C. for 4 minutes, and then laminated with a release paper through a hot roller and rolled up to obtain a covering film.

[0052] Comparative Example 1

[0053] The base formula is calculated based on 100 parts by weight of solid content, and except for the white pigment, the addition ratios of other components are the same as those in Example 1. No white pigment is added.

[0054] Comparative Example 2

[0055] The base formula is calculated based on 100 parts by weight of solid content, and except for the halogen-free polyurethane modified epoxy resin, the addition ratios of other components are the same as those in Example 1. The halogen-free polyurethane modified epoxy resin is replaced with an equal amount of ordinary epoxy resin E51.

[0056] Comparative Example 3

[0057] The base formula is calculated based on 100 parts by weight of solid content, and except for the halogen-free phenol-aralkyl epoxy resin, the addition ratios of other components are the same as those in Example 1. The halogen-free phenol-aralkyl epoxy resin is replaced with an equal amount of ordinary epoxy resin E51.

[0058] Comparative Example 4

[0059] The base formula is calculated based on 100 parts by weight of solid content, and except for the halogen-free phosphorus-containing epoxy resin, the addition ratios of other components are the same as those in Example 1. The halogen-free phosphorus-containing epoxy resin is replaced with an equal amount of ordinary epoxy resin E51.

[0060] Performance Testing

[0061] The covering films prepared in the embodiments and comparative examples were subjected to performance tests. The specific test methods are as follows. The measured results are shown in Tables 1 and 2.

[0062] (1) Color and light transmittance: Visually inspect the sample under a standard light source.

[0063] (2) Thickness: Tested according to GB / T7125-2014 method.

[0064] (3) Peel strength: Test the peel strength between the cover film and the copper foil according to IPC-TM-650 2.4.9 method.

[0065] (4) Glue overflow: Tested according to IPC-TM-650 2.3.17 method.

[0066] (5) Solder resistance: Tested according to IPC-TM-650 2.4.13 method.

[0067] (6) Surface resistance and volume resistivity: tested according to IPC-TM-650 2.5.17 method.

[0068] (7) Flame retardancy: tested according to UL94 vertical burning method.

[0069] Table 1 - Formulation examples of white halogen-free flame-retardant epoxy resin compositions and properties of their prepared covering films

[0070]

[0071]

[0072] Table 2 - Comparative examples of formulations of white halogen-free flame-retardant epoxy resin compositions and properties of their prepared covering films

[0073]

[0074]

[0075] In order to illustrate the key components in the present invention, the key components are shown in Tables 1 and 2. From the measured results, it can be seen that the white halogen-free flame retardant epoxy resin composition of the present invention does not contain halogen, uses white pigment to achieve the purpose of whiteness and light shielding, and better solves the problem of thickness and covering properties being difficult to balance, and because a polyimide support film is used, the covering film maintains good physical properties, and the flame retardancy passes the requirements of UL94 V-0 grade, the surface resistance and volume resistivity meet the standards, the peel strength is high, the dimensional stability is good, and the processing performance is excellent.

[0076] The above embodiments do not impose any limitation on the content of the composition of the present invention. Any slight modification, equivalent change and modification made to the above embodiments based on the technical essence or composition ingredients or content of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A white halogen-free flame-retardant epoxy resin composition, characterized in that: Calculated by solid weight, it includes the following components in parts by weight: Halogen-free polyurethane modified epoxy resin 10-30 parts; 5-20 parts of halogen-free phenol-aralkyl epoxy resin; 5-20 parts of halogen-free phosphorus-containing epoxy resin; 15-40 parts of toughening agent; 15-30 parts of phosphorus-containing flame retardant; 1-10 parts of amine curing agent; Curing accelerator 0.01-1 part; 1-8 parts of white pigment; Silane coupling agent 0.01-1 part; Appropriate amount of organic solvent; The halogen-free polyurethane modified epoxy resin is modified by synthesizing a polyurethane prepolymer from a polyhydroxy compound and a polyisocyanate, and then grafting the prepolymer with an epoxy resin to synthesize the halogen-free polyurethane modified epoxy resin, or directly synthesizing the halogen-free polyurethane modified epoxy resin by bulk synchronous polymerization; The halogen-free phenol-aralkyl epoxy resin is or, a derivative of the above structure, or, a biphenyl-type phenolic epoxy resin; The halogen-free phosphorus-containing epoxy resin is 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, One or more of .

2. The white halogen-free flame-retardant epoxy resin composition according to claim 1, characterized in that: The phosphorus content of the halogen-free phosphorus-containing epoxy resin is 0.5%-16%.

3. The white halogen-free flame-retardant epoxy resin composition according to any one of claims 1 to 2, characterized in that: The toughening agent is one or more of nitrile rubber, styrene-butadiene rubber, butadiene rubber, ethylene-propylene rubber, and polyurethane; And / or, the amine curing agent is one or more of 4,4'-diaminodiphenylmethane, m-phenylenediamine, m-phenylenediamine or dicyandiamide; And / or, the curing accelerator is one or more of N-(2-hydroxyphenyl)-N',N'-dimethylurea, N-(2-hydroxy-4-nitrobenzene)-N',N'-dimethylurea, N-(4-chlorophenyl)-N',N'-dimethylurea, N-(5-chloro-2-hydroxybenzene)-N',N'-dimethylurea, 2,4,6-tris(dimethylaminomethyl)phenol, and boron trifluoride; And / or, the white pigment is titanium dioxide; And / or, the silane coupling agent is KH-550; And / or, the organic solvent is one or more of toluene, acetone, butanone, cyclohexanone, toluene, ethylene glycol methyl ether, propylene glycol methyl ether, ethyl acetate, propylene glycol methyl ether acetate, N',N'-dimethylformamide, and dimethyl sulfoxide.

4. The white halogen-free flame-retardant epoxy resin composition according to claim 3, characterized in that: The amount of the white pigment is 2-6 parts.

5. A covering film prepared from the white halogen-free flame-retardant epoxy resin composition according to any one of claims 1 to 4, characterized in that: include: A polyimide film, a white halogen-free flame-retardant epoxy resin composition layer coated on the polyimide film, and a release paper attached to the white halogen-free flame-retardant epoxy resin composition layer.

6. The covering film according to claim 5, characterized in that The polyimide film is amber, light brown or transparent, and has a thickness of 5-50 μm; and / or, the coating thickness of the white halogen-free flame-retardant epoxy resin composition layer is 8-35 μm; And / or, the thickness of the release paper is 50-150 μm.

7. A method for preparing a covering film as claimed in claim 5 or 6, characterized in that: The following steps are included: Step 1, dissolving and mixing the components in the white halogen-free flame-retardant epoxy resin composition with an organic solvent to form a white suspension slurry, and coating the white suspension slurry on the polyimide film using a coating device; Step 2, passing the polyimide film coated with the white suspension slurry through an oven, heating at 70-180° C. for 3 to 7 minutes to dry to form a composition layer containing a solvent-free adhesive film, and then thermally laminating the film with release paper at 60-100° C. and 0.05-2.0 MPa, and rolling the film to obtain a covering film; Wherein, the rotational viscosity of the white suspension slurry is 1500-3000 cps.

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