High-strength polypropylene-based film for composite current collectors and method of making

Abstract

The application relates to the technical field of polypropylene, and discloses a high-strength polypropylene-based film for a composite current collector and a preparation method.The polypropylene, polyethylene terephthalate, 4-methyl acryloyloxy salicylic acid grafted polypropylene and antioxidant are mixed, melted, extruded, cast into a sheet through a screw extrusion casting machine, stretched through a two-way stretching machine, heat set and the like to obtain the high-strength polypropylene-based film.The compatibility of the polypropylene and the PET is improved, and the tensile strength and the elongation at break of the polypropylene-based film are obviously increased.The 4-methyl acryloyloxy salicylic acid grafted polypropylene contains a large number of hydroxyl and carboxyl hydrophilic groups, and when the 4-methyl acryloyloxy salicylic acid grafted polypropylene is added into the polypropylene, the water contact angle of the film can be reduced, the surface hydrophilicity is improved, the adhesion between the polypropylene film and a conductive layer such as copper and the bonding strength are improved, and a brand-new strategy is provided for preparing a high-performance composite current collector.

Description

Technical Field

[0001] This invention relates to the field of polypropylene technology, specifically to a high-strength polypropylene base film for composite current collectors and its preparation method. Background Technology

[0002] Current collectors are core components in battery systems, primarily used to carry the active materials of the electrodes and collect current. Compared to traditional copper foil current collectors, metal-plastic composite current collectors are lightweight, corrosion-resistant, and possess excellent electrical insulation and shock absorption properties, making them widely used in lithium-ion batteries, the automotive industry, and aerospace. Currently, the plastic film matrix of composite current collectors is mainly composed of polypropylene, polyethylene, and polyethylene terephthalate.

[0003] Polypropylene (PP) is chemically stable and inexpensive, making it widely used in composite current collectors for lithium-ion batteries. However, PP films have low mechanical strength and lack polar functional groups on their surface. When laminated with metal layers such as copper, the adhesion between the two is low, the bonding strength is poor, and the adhesion performance is unfavorable, hindering the fabrication of high-performance current collectors. Chemical modification of the PP film is usually required. The journal *Salt Lake Research*, 2025, 03, 28, reported on the use of dopamine to modify PP, enhancing the hydrophilicity of the membrane surface and improving the interfacial bonding between the PP substrate and the copper layer. This solved the problem of poor adhesion between the PP substrate and the metal layer, and the prepared current collector exhibited good electrochemical performance. However, this journal article did not address the issue of low mechanical strength of the PP film. Summary of the Invention

[0004] This invention solves the problem of low mechanical strength of high-strength polypropylene membrane materials used in composite current collectors.

[0005] The technical solution of this invention is: a method for preparing a polypropylene-based film: (1) Polypropylene was added to xylene, heated and stirred, and then 4-methacryloyloxysalicylic acid and initiator were added. The reaction was carried out in a nitrogen atmosphere, cooled and filtered, and the product was added to acetone for precipitation. After filtration, the precipitate was dried to obtain 4-methacryloyloxysalicylic acid grafted polypropylene.

[0006] (2) Polypropylene, polyethylene terephthalate, 4-methacryloyloxysalicylic acid grafted polypropylene and antioxidant are mixed in a mixer, and then melt-extruded and cast into sheets by a screw extrusion casting machine. The sheets are then stretched, heat-set, corona-treated and traction-wound by a biaxial stretching machine to obtain a high-strength polypropylene base film.

[0007] Furthermore, in (1), the mass ratio of polypropylene, 4-methacryloyloxysalicylic acid and initiator is 100:(15-35):(0.8-2).

[0008] Furthermore, in (1), the reaction temperature is 120-135℃ and the reaction time is 1-3h.

[0009] Furthermore, in (1), the initiator is azobisisobutyronitrile or diisopropylbenzene peroxide.

[0010] Furthermore, in (2), the mass ratio of polypropylene, polyethylene terephthalate, 4-methacryloyloxysalicylic acid grafted polypropylene, and antioxidant is 100:(10-30):(8-25):(0.1-0.3).

[0011] Furthermore, in (2), the polypropylene is composed of low melt index polypropylene and high melt index polypropylene with a mass ratio of (30-90):(10-70). Low melt index polypropylene has a high molecular weight and good mechanical properties, but low fluidity and poor processing performance; high melt index polypropylene has high fluidity and good processing performance. By compounding low melt index and high melt index polypropylene, it is possible to improve the processing fluidity of polypropylene while maintaining good mechanical properties.

[0012] Furthermore, in (2), the temperature of zones 1-5 of the screw extrusion casting machine is 160-265℃, and the screw speed is 30-80r / min.

[0013] Furthermore, in (2), the longitudinal stretching temperature is 140-150℃, the longitudinal stretching ratio is 7.5-8.5 times, and the longitudinal heat setting temperature is 145-155℃.

[0014] Furthermore, in (2), the transverse stretching temperature is 155-165℃, the transverse stretching ratio is 7.5-8.5 times, and the transverse heat setting temperature is 160-170℃.

[0015] Furthermore, the corona treatment power is 10-20 W / min. 2 .

[0016] Furthermore, the tension during traction winding is 20-30 N / m, and the pressure is 100-200 N / m.

[0017] Furthermore, during the traction and winding process, the fresh air supply temperature of the oven is 105-130℃, the fresh air supply velocity is 1000-2000 kg / h, and the oven cooling velocity is 2000-4000 kg / h.

[0018] Furthermore, high-strength polypropylene-based membranes are used in composite current collectors.

[0019] The beneficial technical effects of this invention are as follows: Polypropylene is solution grafted with 4-methacryloyloxysalicylic acid to obtain 4-methacryloyloxysalicylic acid-grafted polypropylene. This grafted polypropylene is then blended with low melt flow index polypropylene, high melt flow index polypropylene, and polyethylene terephthalate (PET), extruded, cast, and stretched to form a high-strength polypropylene-based film. The grafted polypropylene contains benzene rings and ester groups similar to PET, which helps to reduce the solubility parameter difference between the grafted polypropylene and PET. This allows the grafted polypropylene to compatibilize with both polypropylene and PET, improving their compatibility and significantly increasing the tensile strength and elongation at break of the polypropylene-based film.

[0020] The 4-methacryloyloxysalicylic acid-grafted polypropylene of this invention contains a large number of hydrophilic hydroxyl and carboxyl groups. When added to polypropylene, it can reduce the water contact angle of the membrane and improve the surface hydrophilicity. The carboxyl and hydroxyl groups, as anchoring sites for metal active sites, are beneficial to improving the adhesion and bonding strength between the polypropylene membrane and conductive layers such as copper, providing a novel strategy for preparing high-performance composite current collectors. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention; that is, the described embodiments are merely some embodiments of the invention, and not all embodiments.

[0022] Following the method disclosed in paragraph 0080 of patent EP0732322B1, entitled "Acetylsalicylic acid derivatives having avinyl group," 4-methacryloyloxysalicylic acid is prepared using 2,4-dihydroxybenzoic acid and acryloyl chloride as reactants. The structural formula is as follows: .

[0023] The following low melt flow index polypropylene is Daehan Oil & Chemical 5014L BCF; the high melt flow index polypropylene is Borealis HC300BF. The polyethylene terephthalate is Dongguan Xinrui XH4201WUPYLA.

[0024] Example 1 (1) Add 200g of low melt index polypropylene to 0.8L xylene, heat to 125℃, stir for 30min, add 30g (135.1mmol) of 4-methacryloyloxysalicylic acid and 1.6g of azobisisobutyronitrile, stir and react for 1h under nitrogen atmosphere, cool and filter, add acetone to precipitate, filter and dry the precipitate to obtain 4-methacryloyloxysalicylic acid grafted polypropylene.

[0025] (2) 900g of low melt index polypropylene, 100g of high melt index polypropylene, 100g of polyethylene terephthalate, 80g of 4-methacryloyloxysalicylic acid grafted polypropylene, and 1.8g of antioxidant 168 were mixed in a mixer and then melt-extruded through a screw extruder. The temperatures of zones 1-5 were 160℃, 220℃, 250℃, 265℃, and 260℃, and the screw speed was 30r / min. After extrusion, the film was cast and stretched and heat-set by a biaxial stretching machine. The longitudinal stretching temperature was 140℃, the longitudinal stretching ratio was 8 times, and the longitudinal heat-setting temperature was 145℃. The transverse stretching temperature was 160℃, the transverse stretching ratio was 8 times, and the transverse heat-setting temperature was 170℃. Then, corona treatment and traction winding were performed to obtain a high-strength polypropylene base film.

[0026] Example 2 (1) Add 200g of low melt index polypropylene to 1L xylene, heat to 120℃, stir for 40min, add 42g of 4-methacryloyloxysalicylic acid and 2.3g of azobisisobutyronitrile, stir and react for 3h in a nitrogen atmosphere, cool and filter, add acetone to precipitate, filter and dry the precipitate to obtain 4-methacryloyloxysalicylic acid grafted polypropylene.

[0027] (2) 700g of low melt index polypropylene, 300g of high melt index polypropylene, 160g of polyethylene terephthalate, 130g of 4-methacryloyloxysalicylic acid grafted polypropylene, and 3g of antioxidant 168 were mixed in a mixer and then melt-extruded through a screw extruder. The temperatures of zones 1-5 were 160℃, 220℃, 250℃, 265℃, and 260℃, and the screw speed was 50r / min. After extrusion, the film was cast and stretched and heat-set by a biaxial stretching machine. The longitudinal stretching temperature was 150℃, the longitudinal stretching ratio was 8.1 times, and the longitudinal heat-setting temperature was 155℃. The transverse stretching temperature was 155℃, the transverse stretching ratio was 8.5 times, and the transverse heat-setting temperature was 165℃. Then, corona treatment and traction winding were performed to obtain a high-strength polypropylene base film.

[0028] Example 3 (1) Add 200g of high melt index polypropylene to 1L xylene, heat to 135℃, stir for 30min, add 56g of 4-methacryloyloxysalicylic acid and 3.1g of dicumyl peroxide, stir and react for 2h in a nitrogen atmosphere, cool and filter, add the product to acetone for precipitation, filter and dry the precipitate to obtain 4-methacryloyloxysalicylic acid grafted polypropylene.

[0029] (2) Mix 500g of low melt index polypropylene, 500g of high melt index polypropylene, 230g of polyethylene terephthalate, 190g of 4-methacryloyloxysalicylic acid grafted polypropylene, and 1g of antioxidant 168 in a mixer, and then melt extrude the mixture through a screw extruder. The temperatures of zones 1-5 are 160℃, 220℃, 250℃, 265℃, and 260℃, and the screw speed is 80r / min. After extrusion, cast the film and stretch and heat-set it through a biaxial stretching machine. The longitudinal stretching temperature is 140℃, the longitudinal stretching ratio is 8.5 times, and the longitudinal heat-setting temperature is 150℃. The transverse stretching temperature is 165℃, the transverse stretching ratio is 8.5 times, and the transverse heat-setting temperature is 170℃. Then, corona treatment and traction winding are performed to obtain a high-strength polypropylene base film.

[0030] Example 4 (1) Add 200g of high melt index polypropylene to 1.2L xylene, heat to 125℃, stir for 30min, add 70g of 4-methacryloyloxysalicylic acid and 4g of azobisisobutyronitrile, stir and react for 3h in a nitrogen atmosphere, cool and filter, add acetone to precipitate, filter and dry the precipitate to obtain 4-methacryloyloxysalicylic acid grafted polypropylene.

[0031] (2) Mix 300g of low melt index polypropylene, 700g of high melt index polypropylene, 300g of polyethylene terephthalate, 250g of 4-methacryloyloxysalicylic acid grafted polypropylene, and 1g of antioxidant 168 in a mixer, and then melt extrude the mixture through a screw extruder. The temperatures of zones 1-5 are 160℃, 220℃, 250℃, 265℃, and 260℃, and the screw speed is 50r / min. After extrusion, cast the film and stretch and heat set it through a biaxial stretching machine. The longitudinal stretching temperature is 145℃, the longitudinal stretching ratio is 7.5 times, and the longitudinal heat setting temperature is 155℃. The transverse stretching temperature is 155℃, the transverse stretching ratio is 7.5 times, and the transverse heat setting temperature is 160℃. Then, corona treatment and traction winding are performed to obtain a high-strength polypropylene base film.

[0032] Comparative Example 1 differs from Example 1 in that low melt index polypropylene is used instead of 4-methacryloyloxysalicylic acid grafted polypropylene.

[0033] (1) 980g of low melt index polypropylene, 100g of high melt index polypropylene, 100g of polyethylene terephthalate and 1.8g of antioxidant 168 were mixed in a mixer and then melt-extruded through a screw extruder. The temperatures of zones 1-5 were 160℃, 220℃, 250℃, 265℃ and 260℃, and the screw speed was 30r / min. After extrusion, the film was cast and stretched and heat-set by a biaxial stretching machine. The longitudinal stretching temperature was 140℃ and the longitudinal stretching ratio was 8 times. The longitudinal heat-setting temperature was 145℃. The transverse stretching temperature was 160℃ and the transverse stretching ratio was 8 times. The transverse heat-setting temperature was 170℃. Then, the film was corona treated and wound up to obtain a polypropylene base film.

[0034] Comparative Example 2 differs from Example 1 in that an equimolar amount of hydroxyethyl methacrylate is used instead of 4-methacryloyloxysalicylic acid.

[0035] (1) Add 200g of low melt index polypropylene to 0.8L xylene, heat to 125℃, stir for 30min, add 17.56g (135.1mmol) hydroxyethyl methacrylate and 1.6g azobisisobutyronitrile, stir and react for 1h in a nitrogen atmosphere, cool and filter, add acetone to precipitate, filter and dry the precipitate to obtain hydroxyethyl methacrylate grafted polypropylene.

[0036] (2) Mix 900g of low melt index polypropylene, 100g of high melt index polypropylene, 100g of polyethylene terephthalate, 80g of hydroxyethyl methacrylate-grafted polypropylene, and 1.8g of antioxidant 168 in a mixer, and then melt extrude the mixture through a screw extruder. The temperatures of zones 1-5 are 160℃, 220℃, 250℃, 265℃, and 260℃, and the screw speed is 30r / min. After extrusion, cast the film and stretch and heat set it through a biaxial stretching machine. The longitudinal stretching temperature is 140℃, the longitudinal stretching ratio is 8 times, and the longitudinal heat setting temperature is 145℃. The transverse stretching temperature is 160℃, the transverse stretching ratio is 8 times, and the transverse heat setting temperature is 170℃. Then, corona treatment and traction winding are performed to obtain a polypropylene base film.

[0037] Comparative Example 3 differs from Example 1 in that it uses an equimolar amount of hydroquinone monomethacrylate (structural formula: (CAS No. 31480-93-0) replaces 4-methacryloyloxysalicylic acid.

[0038] (1) Add 200g of low melt index polypropylene to 0.8L xylene, heat to 125℃, stir for 30min, add 24.05g (135.1mmol) hydroquinone monomethacrylate and 1.6g azobisisobutyronitrile, stir and react for 1h under nitrogen atmosphere, cool and filter, add acetone to precipitate, filter and dry the precipitate to obtain hydroquinone monomethacrylate grafted polypropylene.

[0039] (2) 900g of low melt index polypropylene, 100g of high melt index polypropylene, 100g of polyethylene terephthalate, 80g of hydroquinone monomethacrylate grafted polypropylene, and 1.8g of antioxidant 168 were mixed in a mixer and then melt-extruded through a screw extruder. The temperatures of zones 1-5 were 160℃, 220℃, 250℃, 265℃, and 260℃, and the screw speed was 30r / min. After extrusion, the film was cast and stretched and heat-set by a biaxial stretching machine. The longitudinal stretching temperature was 140℃, the longitudinal stretching ratio was 8 times, and the longitudinal heat-setting temperature was 145℃. The transverse stretching temperature was 160℃, the transverse stretching ratio was 8 times, and the transverse heat-setting temperature was 170℃. Then, the film was corona-treated and wound up to obtain a polypropylene base film.

[0040] Comparative Example 4 differs from Example 1 in that 4-vinylbenzoic acid (CAS No. 1075-49-6) is used in place of 4-methacryloyloxysalicylic acid in an equimolar amount.

[0041] (1) Add 200g of low melt index polypropylene to 0.8L xylene, heat to 125℃, stir for 30min, add 20g (135.1mmol) of 4-vinylbenzoic acid and 1.6g of azobisisobutyronitrile, stir and react for 1h under nitrogen atmosphere, cool and filter, add acetone to precipitate, filter and dry the precipitate to obtain 4-vinylbenzoic acid grafted polypropylene.

[0042] (2) Mix 900g of low melt index polypropylene, 100g of high melt index polypropylene, 100g of polyethylene terephthalate, 80g of 4-vinylbenzoic acid grafted polypropylene, and 1.8g of antioxidant 168 in a mixer, and then melt extrude the mixture through a screw extruder. The temperatures of zones 1-5 are 160℃, 220℃, 250℃, 265℃, and 260℃, and the screw speed is 30r / min. After extrusion, cast the film and stretch and heat set it through a biaxial stretching machine. The longitudinal stretching temperature is 140℃, the longitudinal stretching ratio is 8 times, and the longitudinal heat setting temperature is 145℃. The transverse stretching temperature is 160℃, the transverse stretching ratio is 8 times, and the transverse heat setting temperature is 170℃. Then, corona treatment and traction winding are performed to obtain a polypropylene base film.

[0043] The tensile strength of the polypropylene-based film was tested according to GB / T 1040.3-2006 standard. The water contact angle was tested according to GB / T 30693-2014 standard.

[0044] Table 1 Performance Tests

[0045] The poor compatibility between polypropylene and polyethylene terephthalate (PET) in Comparative Example 1 resulted in low tensile strength and elongation at break in both the transverse and longitudinal directions of the polypropylene-based film. Furthermore, it exhibited a large water contact angle, low surface hydrophilicity, and lacked polar functional groups, making it unsuitable for loading conductive layers such as copper onto the film surface and hindering its application in composite current collectors. In the polypropylene-based films of each embodiment, 4-methacryloyloxysalicylic acid-grafted polypropylene was added. This grafted polypropylene contains benzene rings and ester groups similar to those in PET, which helps reduce the solubility difference between the grafted polypropylene and PET. This allows the grafted polypropylene to compatibilize both polypropylene and PET, improving their compatibility. The tensile strength and elongation at break of the polypropylene-based film significantly increased. Moreover, the grafted polypropylene contains a large number of hydrophilic hydroxyl and carboxyl groups, which can reduce the water contact angle of the polypropylene-based film and improve surface hydrophilicity. The carboxyl and hydroxyl groups act as anchoring sites for metal active sites, which helps improve the adhesion and bonding strength between the polypropylene film and conductive layers such as copper.

[0046] In Comparative Example 2, the hydroxyethyl methacrylate-grafted polypropylene does not contain a benzene ring structure, making it difficult to compatibilize with polypropylene and PET. The compatibility between the two is poor, resulting in low tensile properties of the polypropylene-based film. Furthermore, the hydroxyethyl methacrylate-grafted polypropylene does not contain carboxyl groups, leading to a larger surface water contact angle and poor surface hydrophilicity of the polypropylene-based film.

[0047] Comparative Example 3, which is hydroquinone monomethacrylate-grafted polypropylene, does not contain carboxyl groups, and the polypropylene base film has a large surface water contact angle.

[0048] Comparative Example 4: 4-vinylbenzoic acid-grafted polypropylene does not contain ester groups, making it difficult to compatibilize polypropylene and PET. The compatibility between the two is poor, resulting in low tensile properties of the polypropylene-based film. Furthermore, 4-vinylbenzoic acid-grafted polypropylene does not contain hydroxyl groups, leading to a larger surface water contact angle and poor surface hydrophilicity of the polypropylene-based film.

[0049] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, and improvements made by those skilled in the art within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a high-strength polypropylene-based film, characterized in that, The preparation method is as follows: polypropylene, polyethylene terephthalate, 4-methacryloyloxysalicylic acid grafted polypropylene, and antioxidant are mixed in a mixer, and then melt-extruded and cast into sheets by a screw extruder. The sheets are then stretched, heat-set, corona-treated, and wound up by a biaxial stretching machine to obtain a high-strength polypropylene base film.

2. The method for preparing a high-strength polypropylene-based film according to claim 1, characterized in that, The mass ratio of polypropylene, polyethylene terephthalate, 4-methacryloyloxysalicylic acid-grafted polypropylene, and antioxidant is 100:(10-30):(8-25):(0.1-0.3).

3. The method for preparing a high-strength polypropylene-based film according to claim 1, characterized in that, The polypropylene is composed of low melt index polypropylene and high melt index polypropylene in a mass ratio of (30-90):(10-70).

4. The method for preparing a high-strength polypropylene-based film according to claim 1, characterized in that, The temperature of zones 1-5 of the screw extrusion casting machine is 160-265℃, and the screw speed is 30-80 r / min.

5. The method for preparing a high-strength polypropylene-based film according to claim 1, characterized in that, The longitudinal stretching temperature is 140-150℃, the longitudinal stretching ratio is 7.5-8.5 times, and the longitudinal heat setting temperature is 145-155℃; the transverse stretching temperature is 155-165℃, the transverse stretching ratio is 7.5-8.5 times, and the transverse heat setting temperature is 160-170℃.

6. The method for preparing a high-strength polypropylene-based film according to claim 1, characterized in that, The preparation method of 4-methacryloyloxysalicylic acid grafted polypropylene is as follows: polypropylene is added to xylene, heated to 120-135℃, stirred, and then 4-methacryloyloxysalicylic acid and initiator are added. The reaction is carried out in a nitrogen atmosphere for 1-3 hours. After cooling, the mixture is filtered, and the product is added to acetone for precipitation. After filtration, the precipitate is dried to obtain 4-methacryloyloxysalicylic acid grafted polypropylene.

7. The method for preparing a high-strength polypropylene-based film according to claim 6, characterized in that, In the preparation method of 4-methacryloyloxysalicylic acid grafted polypropylene, the mass ratio of polypropylene, 4-methacryloyloxysalicylic acid and initiator is 100:(15-35):(0.8-2).

8. The method for preparing a high-strength polypropylene-based film according to claim 6, characterized in that, The initiator is azobisisobutyronitrile or diisopropylbenzene peroxide.

9. A high-strength polypropylene base film obtained by the preparation method according to any one of claims 1-8.

10. The application of the high-strength polypropylene base film as described in claim 9 in a composite current collector.

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