Interleaving paper for stretching carbon fiber base paper as well as preparation method and application of interleaving paper

Abstract

The invention belongs to the technical field of paper-based materials, and provides lining paper for stretching carbon fiber base paper as well as a preparation method and application of the lining paper. The lining paper for stretching the carbon fiber base paper comprises a paper base body, at least one side of the paper base body is provided with a coating, the coating comprises a bottom coating and a top coating, the bottom coating is in direct contact with the paper base body, the paper base body with a specific longitudinal tensile index and a specific longitudinal wet strength retention rate is adopted, the top coating is prepared by adopting top coating paint, and the paper base body is prepared by adopting the top coating paint. A first polymer emulsion and waxy nanoparticles are adopted as main components of the top coating, the MFFT of the first polymer emulsion is 30-80 DEG C, and the Tg of the first polymer emulsion is 50-80 DEG C. After the lining paper and the cured carbon fiber body paper are subjected to hot-pressing compounding, the whole is stretched, safe and efficient orientation of the carbon fiber body paper is achieved, meanwhile, after the stretching orientation process is finished, the lining paper and the carbon fiber body paper can be completely stripped, and the lining paper can be recycled.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of paper-based materials, and more particularly to a backing paper for carbon fiber base paper stretching, a preparation method and application thereof. BACKGROUND

[0002] Carbon fiber base paper is a non-woven material made of chopped carbon fibers through a wet-forming process, and is widely used in fuel lithium ion battery separator reinforcement layer, electromagnetic shielding material, friction material and carbon / carbon composite material precursor, etc. However, due to the brittleness of carbon fibers and weak inter-fiber bonding, the mechanical properties of the traditional wet-formed carbon fiber base paper are isotropic and the strength is low in the machine direction (MD), which limits its use in high stress application scenarios.

[0003] Mechanical draw orientation (MDO) is a technology that re-arranges fibers in the longitudinal direction, improves orientation and strength by applying a longitudinal stretching force to the material in the running direction (i.e. the running direction from unwinding-processing zone-winding) through the application of tension in the machine direction (i.e. the tension is regulated by the speed difference between the front and rear traction rollers), which has been widely used in polypropylene, polyethylene terephthalate (PET) and other polymer films. However, if the MDO process is directly applied to carbon fiber base paper, there are major technical obstacles, because the brittleness of carbon fiber base paper increases after drying and curing, and it is prone to breakage during stretching, so it is difficult to achieve effective orientation through MDO.

[0004] Therefore, there is an urgent need to develop a method that can realize the stretching and orientation of carbon fiber base paper and avoid breakage. SUMMARY

[0005] The present application aims to at least solve one of the technical problems existing in the prior art. To this end, the present application provides a backing paper for carbon fiber base paper stretching, a preparation method and application thereof. The backing paper provided by the present application can be used as a temporary support material in the mechanical draw orientation process of carbon fiber base paper, can bear and transfer the stretching force, realize the safe and efficient orientation and stretching of carbon fiber base paper, avoid the breakage of carbon fiber base paper, and at the same time, the backing paper and the carbon fiber base paper can be completely peeled off after the stretching and orientation process, and the backing paper can be recycled.

[0006] The first aspect of the present application provides a backing paper for carbon fiber base paper stretching.

[0007] Specifically, a backing paper for carbon fiber base paper stretching, comprising a paper base body, at least one side of the paper base body is provided with a coating layer, the coating layer comprises a bottom coating layer and a top coating layer, the bottom coating layer is in direct contact with the paper base body; The longitudinal tensile index of the paper base body is ≥65 N·m / g, and the longitudinal wet strength retention rate of the paper base body is ≥30%. The top coating is prepared by using a top coating paint, and the top coating paint comprises the following components: a first polymer emulsion and waxy nanoparticles. The minimum film formation temperature (MFFT) of the first polymer emulsion is 30-80℃, and the glass transition temperature (Tg) of the first polymer emulsion is 50-80℃.

[0008] The present application provides a base paper with a coating, the top coating is prepared by using a first polymer emulsion and waxy nanoparticles, which are two low surface energy materials, and the base paper is further used for stretching of carbon fiber raw paper, the top coating has van der Waals force with the carbon fiber raw paper, can maintain the interfacial adhesion after high temperature compounding, but the van der Waals force is weak, and in the curing process of the first polymer emulsion, part of the waxy nanoparticles is embedded in the polymer matrix, and part of the waxy nanoparticles protrudes from the surface, thereby forming a micron-level undulating hierarchical rough structure (caused by emulsion drying shrinkage and waxy nanoparticles), the above two reasons reduce the actual contact area of the top coating and the carbon fiber raw paper, so that the interfacial adhesion is not too high; in addition, the top coating has super-hydrophobic properties, making it difficult for pollutants (dust, carbon dust, trace resin, etc.) to adhere (if the coating is contaminated, the surface performance can be restored by washing with water or a weak solvent); the first polymer emulsion with specific MFFT and Tg makes the top coating only slightly soften but not melt and flow after hot pressing, maintaining the structural integrity of the top coating and avoiding mechanical anchoring with the carbon fiber raw paper. Therefore, the top coating can adhere to the carbon fiber raw paper, but the adhesion is not too strong, so the base paper can not only be compounded with the carbon fiber raw paper for stretching, but also can be separated from the carbon fiber raw paper after stretching, as a super-hydrophobic, easy-to-peel, low-adhesion coating, ensuring the integrity of the base paper and the carbon fiber raw paper after peeling, improving the yield of the carbon fiber raw paper, and the peeled base paper can be recycled and reused. The first polymer emulsion as a film-forming matrix can also provide good adhesion to the primer layer, anchor the waxy nanoparticles, and prevent them from falling off, forming a continuous, transparent, and flexible polymer film after drying and curing; the waxy nanoparticles themselves have extremely low surface energy and easy-to-peel properties, and part of them is embedded in and protrudes from the polymer surface during film formation, forming a micron-nanometer hierarchical rough structure (super-hydrophobic, similar to lotus effect), further enhancing the easy-to-peel properties of the top coating; its super-hydrophobic performance also ensures that the base paper can be repeatedly used after simple cleaning or dust removal. The present application uses a high-strength, high-wet-strength paper base, which resists water molecule invasion by virtue of its covalent cross-linking network, maintaining the integrity of the fiber network structure, which not only prevents the base paper from being damaged during recycling, but also ensures the long-term adhesion stability of the primer layer and the paper base interface to meet the needs of the base paper and the carbon fiber raw paper for stretching together.

[0009] Preferably, the first polymer emulsion is at least one of a fluorine-free modified acrylate emulsion, a modified vinyl acetate-acrylate copolymer emulsion, an anionic waterborne polyurethane (PU) emulsion, and a non-ionic waterborne polyurethane (PU) emulsion.

[0010] Preferably, the waxy nanoparticles are waxy nanocrystals. The waxy nanocrystals are nanoparticles formed by nano-emulsification and crystallization of waxes of natural or bio-based origin.

[0011] Preferably, the waxy nanoparticles are at least one of palm wax nanoparticles, rice bran wax nanoparticles, sugarcane wax nanoparticles, beeswax nanoparticles, lotus leaf wax nanoparticles, lanolin wax nanoparticles, and polyethylene wax nanoparticles.

[0012] Preferably, the average particle size of the waxy nanoparticles is 80-250 nm.

[0013] Preferably, the top coating includes 75-90 wt% of the first polymer emulsion and 10-25 wt% of the waxy nanoparticles.

[0014] Preferably, the paper substrate has a basis weight of 50-80 g / m 2 , and / or a longitudinal MIT folding endurance (MIT method) of the paper substrate is ≥700, and / or a transverse tear index of the paper substrate is ≥10 mN•m 2 / g.

[0015] Preferably, the paper substrate is one of unbleached kraft paper, bleached kraft paper, and resin-reinforced wet-strength paper.

[0016] Preferably, the paper substrate is one of unbleached kraft paper, bleached kraft paper, and resin-reinforced wet-strength paper.

[0017] Preferably, the long-fiber chemical wood pulp accounts for 50-100% of the total dry fiber mass of the paper substrate.

[0018] Preferably, the base coating is prepared using a base coating material including a second polymer emulsion, a binder containing hydroxyl groups, a thickening agent, and a filler.

[0019] The second polymer emulsion plays a major role in film formation and adhesion, forming a continuous phase on the surface of the paper substrate. The adhesion agent containing hydroxyl groups not only plays an auxiliary adhesion role, but also forms strong hydrogen bonds with the fibers of the paper substrate, improving the adhesion of the primer layer to the paper substrate, improving the film formation density of the second polymer emulsion, reducing coating voids, and having good compatibility with starch to form a semi-interpenetrating network structure, enhancing the toughness of the coating. The thickening agent, on the one hand, improves the coating leveling property and permeability resistance (prevents excessive penetration of the coating into the paper substrate), and on the other hand, plays an adhesion role, and cooperates with the adhesion agent to improve the interfacial bonding strength. The filler improves the dimensional stability and thermal stability of the coating, and also improves the surface flatness of the coating.

[0020] Further preferably, the primer coating includes the following components by weight percentage: second polymer emulsion 50-70 wt%, adhesion agent containing hydroxyl groups 5-15 wt%, thickening agent 10-20 wt%, and filler 3-10 wt%.

[0021] Preferably, the second polymer emulsion is at least one of a fluorine-free modified acrylate emulsion, an ethylene-vinyl acetate copolymer emulsion (EVA), a modified vinyl acetate-acrylate copolymer emulsion, an anionic water-based polyurethane emulsion (PU), a non-ionic water-based polyurethane emulsion (PU), and a polyhydroxy aliphatic acid ester (PHA, a biobased degradable material).

[0022] Preferably, the adhesion agent containing hydroxyl groups is at least one of polyvinyl alcohol (PVA), hydroxyethyl cellulose (HEC), xanthan gum, and soybean protein isolate.

[0023] Preferably, the thickening agent is at least one of starch, oxidized starch, acetic acid ester starch, hydroxypropyl starch, and cationic starch.

[0024] Preferably, the filler is a sheet-shaped barrier filler. The arrangement of sheet layers can hinder heat conduction and improve overall thermal dimensional stability.

[0025] Further preferably, the filler is a layered silicate mineral with a sheet-shaped crystal structure.

[0026] More preferably, the filler is uncalcined sheet-shaped kaolinite, sheet-shaped sodium-based montmorillonite, organically modified sheet-shaped montmorillonite, sheet-shaped talc powder (such as sheet-shaped talc powder with an aspect ratio ≥ 10), and sheet-shaped mica (such as synthetic mica, also known as fluorophlogopite, such as sheet-shaped mica with an average particle size of 5-50 μm and a thickness ≤ 1 μm). The above sheet-shaped fillers can be arranged in the primer coating in the plane direction, forming a "labyrinth effect" to effectively hinder the heat conduction path and improve the thermal dimensional stability.

[0027] Preferably, the paper substrate is provided with a coating on both sides. Double-sided coating of the backing paper can ensure that the backing paper can be repeatedly used after cleaning or dust removal.

[0028] Preferably, the thermal shrinkage of the backing paper at 160℃ is ≤1.5%. So that it can keep synchronous deformation with the carbon fiber raw paper during the stretching process, avoiding stress concentration leading to breakage or uneven orientation.

[0029] Further preferably, the thermal shrinkage of the backing paper at 160℃ is 0.5-1%.

[0030] The second aspect of the present application provides a preparation method of a backing paper for carbon fiber raw paper stretching.

[0031] The preparation method of a backing paper for carbon fiber raw paper stretching comprises the following steps: (1) making the primer coating on one surface of the paper base; (2) then coating the top coating material on the surface of the primer coating, drying to obtain the top coating, and preparing the backing paper.

[0032] After coating and curing, the surface of the top coating forms a micro / nano hierarchical rough structure, and the static contact angle with water is ≥150°.

[0033] Preferably, in step (1), making the primer coating on one surface of the paper base comprises the following steps: coating the primer coating material on one surface of the paper base, and drying to obtain the primer coating.

[0034] Further preferably, in step (1), making the primer coating on one surface of the paper base comprises the following steps: coating the primer coating material on one surface of the paper base, and drying at 120-140℃ to obtain the primer coating.

[0035] Preferably, the preparation method of the primer coating material comprises the following steps: mixing the components of the primer coating material to prepare the primer coating material.

[0036] Preferably, the preparation method of the top coating material comprises the following steps: mixing the components of the top coating material to prepare the top coating material.

[0037] Preferably, the coating method of the primer coating material and the top coating material is independently selected from one of air knife coating, anilox coating, micro gravure coating, and slot coating.

[0038] Preferably, the total dry coating amount of the coating layer is 4.0-6.0 g / m 2 .

[0039] Preferably, the primer coating accounts for 30-40% of the total coating amount of the coating layer.

[0040] Preferably, the top coating layer accounts for 60-70% of the total coating amount of the coating layer.

[0041] Preferably, in step (2), the top coating paint is coated on the surface of the bottom coating layer, and the top coating layer is obtained after drying at 140-160°C, thereby preparing the backing paper.

[0042] Preferably, in step (2), after the top coating layer is obtained, a back bottom coating layer is further prepared on the other surface of the paper substrate, and then the top coating paint is coated on the surface of the back bottom coating layer, and a back top coating layer is obtained after drying, thereby the paper substrate is provided with coating layers on both sides.

[0043] The third aspect of the present application provides a use of a backing paper for stretching of carbon fiber raw paper.

[0044] The use of a backing paper for stretching of carbon fiber raw paper in the preparation of carbon fiber raw paper.

[0045] The fourth aspect of the present application provides a stretching method of carbon fiber raw paper.

[0046] The stretching method of carbon fiber raw paper comprises the following steps: (1) preheating the backing paper, then bonding the cured carbon fiber raw paper and the coating layer of the backing paper, and then hot-pressing and compounding to obtain a compound; (2) stretching the compound; (3) peeling off the backing paper to obtain the treated carbon fiber raw paper.

[0047] Preferably, in step (1), before use, the backing paper is subjected to multi-stage heat setting treatment, and the multi-stage heat setting treatment comprises the following steps: preheating and setting the backing paper at 90-110°C for 50-120 ms, and then main setting at 140-160°C for 50-120 ms. The backing paper is subjected to preheating and setting and multi-stage heat setting treatment to release stress and stabilize size.

[0048] Preferably, in step (1), the temperature of the hot-pressing and compounding is 140-180°C, and / or the linear pressure of the hot-pressing and compounding is 2-5 kN / m, and / or the time of the hot-pressing and compounding is 40-100 ms.

[0049] Preferably, in step (2), the stretching is longitudinal stretching or transverse stretching.

[0050] Further preferably, in step (2), the stretching is longitudinal stretching.

[0051] Preferably, in step (2), the longitudinal stretching is performed in a longitudinal stretching device.

[0052] Preferably, the composite is stretched to 1.05-1.30 times of the initial length. The carbon fiber base paper is progressively stretched at high temperature to achieve the orientation enhancement of the carbon fiber base paper along the machine direction.

[0053] Further preferably, the composite is stretched to 1.10-1.25 times of the initial length.

[0054] More preferably, the composite is stretched to 1.15-1.25 times of the initial length.

[0055] Preferably, the stretching is performed by a stretching device, the longitudinal stretching device has at least three groups of traction rollers, the surface linear speed of each traction roller is sequentially increased, and the speed ratio between adjacent traction rollers is 1.02-2.0. The stretching device helps to achieve the progressive micro-stretching of the high-brittle carbon fiber base paper, avoiding the paper breakage problem caused by traditional single-stage large-speed-ratio stretching.

[0056] Further preferably, the stretching is performed by a stretching device, the stretching device has at least three groups of traction rollers, the surface linear speed of each traction roller is sequentially increased, and the speed ratio between adjacent traction rollers is 1.2-1.25.

[0057] Preferably, in step (3), the peeling is performed in a peeling roller group, the peeling roller group includes carbon fiber base paper traction rollers and backing paper traction rollers (used for traction of the stretched carbon fiber base paper and backing paper respectively), the surface linear speed of the carbon fiber base paper traction rollers is 1.05-1.25 times of the surface linear speed of the backing paper traction rollers. The speed difference between the rollers helps to completely peel the carbon fiber base paper from the backing paper, obtain the longitudinally oriented enhanced carbon fiber base paper, and recycle the complete backing paper for reuse.

[0058] Preferably, in step (3), after the peeling, the backing paper further includes post-processing, the post-processing includes sequentially performing impurity removal and drying. After the post-processing, the backing paper can be reused for the next stretching cycle.

[0059] Preferably, the impurity removal is performed by electrostatic dust removal or water washing.

[0060] Preferably, in step (3), after the peeling, the backing paper has a tensile strength retention rate > 90% and a surface contact angle > 130°. The backing paper can still be reused in cycles.

[0061] Preferably, after step (3) is completed, the processed carbon fiber base paper is further subjected to steps (1)-(3) for at least 5 times. The backing paper can be reused for at least 5 times.

[0062] Compared with the prior art, the present application has the following beneficial effects: The present invention provides a backing paper for stretching carbon fiber base paper, comprising a paper matrix, at least one side of which is coated with a coating including a base coating and a top coating. The base coating is in direct contact with the paper matrix and uses a paper matrix with a specific longitudinal tensile index and longitudinal wet strength retention rate. The top coating is prepared by a top coating material, using a first polymer emulsion and waxy nanoparticles as the main components of the top coating material. The first polymer emulsion has a minimum film-forming temperature (MFFT) of 30-80°C and a glass transition temperature (Tg) of 50-80°C. The backing paper has a longitudinal tensile index of 68-82 N·m / g, a longitudinal wet strength retention rate of 32-45%, a heat shrinkage rate of 0.5-1% at 160°C, and exhibits superhydrophobicity. Furthermore, the liner paper of this invention is used as a temporary traction support. After being hot-pressed and laminated with the cured carbon fiber base paper, the entire assembly is stretched. The liner paper drives the carbon fiber base paper to undergo synchronous longitudinal stretching. The liner paper and the cured carbon fiber base paper achieve precise synchronous deformation, promoting uniform fiber orientation. During the MDO stretching process of the liner paper and the cured carbon fiber base paper, the liner paper does not break or wrinkle, achieving complete and damage-free peeling between the carbon fiber base paper and the liner paper. The liner paper can also be recycled. This solves the problem of easy loosening of the fiber structure and reduction of physical strength of the carbon fiber base paper during the MDO stretching process. Attached Figure Description

[0063] Figure 1 This is a schematic diagram of the device structure for longitudinal stretching of carbon fiber base paper using the liner paper as an aid in Application Example 1 of the present invention. Detailed Implementation

[0064] To enable those skilled in the art to more clearly understand the technical solutions described in this invention, the following embodiments are provided for illustration. It should be noted that the following embodiments do not constitute a limitation on the scope of protection claimed by this invention.

[0065] Unless otherwise specified, the raw materials, reagents or devices used in the following examples are available from conventional commercial sources or can be obtained by existing known methods.

[0066] The sources of the main raw materials involved in the embodiments and comparative examples of this invention are as follows: Paper matrix: Bleached sulfate softwood pulp kraft paper (with added low-chlorinated polyamide epichlorohydrin resin PAE), i.e., bleached kraft paper. The oven-dry weight of long-fiber chemical wood pulp accounts for 100% of the total oven-dry fiber weight of the paper matrix. Brand: Mondi, Model: Advantage Protect White, Basis weight 80 g / m³ 2 The longitudinal wet strength retention rate of the paper matrix was 34%, and the following performance tests were conducted, with results shown in the table below: Table 1. Performance test results of bleached sulfate softwood pulp kraft paper matrix

[0067] From the above, the paper base adopted by the application has the characteristics of high strength, high wet strength, folding resistance, and low high-temperature thermal shrinkage.

[0068] Cured carbon fiber base paper: The purchased carbon fiber base paper (produced by Guangdong Guanhao New Material Research and Development Co., Ltd., model TZ2560, basis weight 25±1 gsm, thickness 210±15 μm. The "carbon fiber base paper" is a semi-finished product of carbon fiber paper, which has not been treated by carbonization and graphitization, and is a porous network structure prepared only from organic binder + chopped carbon fibers), is cured in an oven at a temperature of 160°C and a speed of 8 m / min to improve the strength of the carbon fiber base paper.

[0069] Silicone release paper: produced by Japan Lindi Ke Co., Ltd., model RS-80HS, basis weight 80 g / m 2 Silicone paper for high-speed coating.

[0070] PET release film: fluorine-containing acrylate release coating is coated on PET to form a release layer, produced by Japan Shin Nittsu Chemical Co., Ltd., model X-71-101.

[0071] Organically modified montmorillonite: manufacturer BYK, model Cloisite® 15A / 30B.

[0072] Fluorine-free modified acrylate emulsion: methyl methacrylate-butyl acrylate-acrylic acid terpolymer, manufacturer Zhanxin Resin, model NeoCryl® XK-90, Tg=72°C, MFFT=68°C.

[0073] Polyvinyl alcohol: manufacturer Kao Lai, model POVAL 217, alcoholysis degree 87-89 mol%.

[0074] Cationic pregelatinized starch: manufacturer Shandong Fuyang Biology, model FY-CPS-80.

[0075] Palm wax nanocrystal dispersion: manufacturer Lubrizol, model Solsperse™ W-2200 / W-2300 series.

[0076] Example 1 A backing paper for stretching of carbon fiber base paper, comprising in sequence a coating layer, a paper base and a back coating layer arranged in layers.

[0077] The preparation method of the above-mentioned backing paper for stretching of carbon fiber base paper, comprising the following steps: (1) Preparation of the primer coating: 60wt% of the fluorine-free modified acrylate emulsion (methyl methacrylate-butyl acrylate-acrylic acid terpolymer) was added to water, and 15wt% of polyvinyl alcohol (PVA, a binder containing hydroxyl groups, prepared as a 10wt% aqueous solution in advance), 10wt% of cationic gelatinized starch liquid (prepared as a 5wt% gelatinized liquid in advance, a thickening agent), and 8wt% of organically modified montmorillonite (a sheet-shaped filler, with a single-layer sheet thickness of 1.0±0.2nm, dispersed into a suspension by ultrasonic treatment with water in advance) were sequentially added to the primer coating under stirring, and the stirring was continued for 60min until the mixture was uniform, and then the mixture was filtered through a 100-mesh filter bag to obtain the primer coating with a solid content of 30wt%.

[0078] (2) Preparation of the top coating: 80wt% of the fluorine-free modified acrylate emulsion (first polymer emulsion, methyl methacrylate-butyl acrylate-acrylic acid terpolymer, Tg=72℃, MFFT=68℃) was added to water, and 20wt% of the palm wax nanocrystal dispersion (waxy nanoparticles, with an average particle size of 150±30nm and a Zeta potential of -28mV) was added to the top coating under high-speed stirring, and the stirring was continued for 30min until the dispersion was uniform, and then the dispersion was filtered through a 100-mesh filter bag to obtain the top coating with a solid content of 35wt%.

[0079] (3) A paper base (bleached sulfate softwood pulp kraft paper (with low-chlorine polyamide epichlorohydrin resin PAE added), i.e., bleached kraft paper, produced by mondigroup, with a model of Advantage Protect White) was used for continuous operation of a multi-functional coating machine on a whole roll of the paper base (bleached kraft paper, with a basis weight of 80g / m 2 , a longitudinal tensile index of 82N·m / g, and a longitudinal wet strength retention rate of the paper base of 34%), and the paper base was coated on both sides (the structures, thicknesses, and components of the two coating layers obtained were completely the same, one of which was used for compounding with carbon fiber base paper, and the other of which was used for keeping clean), specifically: the primer coating was coated on one surface of the paper base by micro-gravure coating, at a coating machine speed of 12±5m / min and an absolute dry coating amount of 1.5g / m 2 , and then the primer coating was cured in a hot air oven at 130℃ to obtain a primer coating layer; and then the top coating was coated on the surface of the primer coating layer, at an absolute dry coating amount of 3.5g / m 2 , and then the top coating was cured in a hot air oven at 150℃ to obtain a top coating layer. (4) Then, repeat the step (3) on the other surface of the paper substrate, i.e. first apply the primer coating, dry to obtain the back primer layer (the preparation method of the primer layer is the same as step (3)), then apply the top coating, dry to obtain the back top coating (the preparation method of the top coating is the same as step (3)), and finally obtain the backing paper. The finished product is wound and packaged for use.

[0080] Example 2 A backing paper for carbon fiber raw paper stretching, which is only different from Example 1 in that it does not contain a back coating layer, i.e. step (4) is not performed.

[0081] Example 3 A backing paper for carbon fiber raw paper stretching, which is only different from Example 1 in that in step (2), the waxy nano-particles are replaced with an equal weight of rice bran wax nano-particles (manufacturer: Shandong Longli Biological Technology Co., Ltd., model: LL-RBW Nano-30).

[0082] Example 4 A backing paper for carbon fiber raw paper stretching, which is only different from Example 1 in that in step (2), the first polymer emulsion is replaced with an equal weight of modified vinyl acetate-acrylate copolymer emulsion (manufacturer: Wacker Chemie, model VINNAPAS® EP 750, Tg = 60°C, MFFT = 45°C).

[0083] Example 5 A backing paper for carbon fiber raw paper stretching, which is only different from Example 1 in that in step (2), the first polymer emulsion is replaced with an equal weight of anionic water-based polyurethane (PU) emulsion (manufacturer: Covestro, model Dispercoll® U 54, Tg = 65°C, MFFT = 40°C).

[0084] Example 6 A backing paper for carbon fiber raw paper stretching, which is only different from Example 1 in that in step (3), the paper substrate is replaced with a resin-reinforced wet-strength paper (manufacturer: Sappi, model GuardKote® WS-80, longitudinal tensile index 74 N·m / g, longitudinal wet strength retention rate of the paper substrate 45%) of the same thickness and basis weight.

[0085] Example 7 A backing paper for carbon fiber raw paper stretching, which is only different from Example 1 in that in step (3), the paper substrate is replaced with an unbleached kraft paper (manufacturer: Hebei Jinhuang Paper Co., Ltd., model JH-UWK80, longitudinal tensile index 68 N·m / g, longitudinal wet strength retention rate of the paper substrate 32%) of the same thickness and basis weight.

[0086] Comparative Example 1 A backing paper for carbon fiber raw paper stretching, which is only different from Example 1 in that it does not contain a top coating and a back top coating.

[0087] Comparative Example 2 A backing paper for carbon fiber raw paper stretching, which is only different from Example 1 in that it does not contain a bottom coating and a back bottom coating.

[0088] Comparative Example 3 A backing paper for carbon fiber raw paper stretching, which is only different from Example 1 in that the paper base is replaced by a non-wet-strength-treated kraft paper (manufacturer: Jiulong Paper Industry, model: ND-Kraft 80) with the same basis weight and thickness, a longitudinal tensile index of 50 N·m / g, and a longitudinal wet strength retention rate of 8%.

[0089] Comparative Example 4 A backing paper for carbon fiber raw paper stretching, which is only different from Example 1 in that the first polymer emulsion is replaced by a fluorine-free acrylate copolymer emulsion (manufacturer: BASF, model: Acronal® S 720) with a Tg of 40°C.

[0090] Comparative Example 5 A backing paper for carbon fiber raw paper stretching, which is only different from Example 1 in that the first polymer emulsion is replaced by a fluorine-free acrylate copolymer emulsion (manufacturer: Zannan Resin, model: NeoCryl® XK-14) with a Tg of 86°C. Comparative Example 6 A backing paper, which is a commercially available silicone oil release paper.

[0091] Comparative Example 7 A backing paper, which is a commercially available PET release film (fluorine-containing coating).

[0092] Product Effectiveness Test I. Performance Test of the Coating of the Backing Paper The surface contact angle (water) and the interfacial peeling force of the coating of the backing paper prepared in Example 1 were tested. The interfacial peeling force refers to the 180° peeling strength of the coating of the backing paper and the cured carbon fiber raw paper after the coating of the backing paper of Example 1 was bonded to the cured carbon fiber raw paper, hot-pressed at 160°C, and cooled.

[0093] Table 2. Performance test results of the coating of Example 1

[0094] As can be seen from the above table, the surface contact angle (water) of the backing paper of Example 1 is 152°, which has hydrophobicity; the interfacial peeling force is 0.22 N / cm, and there is a certain adhesion between the backing paper and the cured carbon fiber raw paper, so that subsequent synchronous stretching can be ensured.

[0095] II. Test of base paper and its application performance 1. Performance test of base paper The base paper of each example and comparative example was tested according to the following methods.

[0096] (1) Base paper longitudinal tensile index: GB / T 12914-2018 "Paper and board - Determination of tensile strength - Constant rate of elongation method (20 mm / min)".

[0097] (2) Base paper longitudinal wet strength retention rate: GB / T 465.2-2008 "Paper and board - Determination of tensile strength after immersion".

[0098] (3) Base paper 160°C heat shrinkage rate: GB / T 15059-1994 "Method for determination of expansion and shrinkage of sheet base".

[0099] (4) Base paper surface contact angle (water): GB / T 30693-2014 "Measurement of contact angle of plastics film with water".

[0100] 2. Test of base paper breaking resistance, non-destructive peeling ability and recycling performance in MDO stretching process Examples 1 base paper (plant fiber base paper), Comparative Example 6 (silicone release paper), and Comparative Example 7 (PET release film) were used as the base paper to be tested for MDO stretching test, and the test method was as follows (as shown in Figure 1 ): (1) Hot pressing: the base paper to be tested was first shaped in two preheating rollers (multi-stage heat setting treatment), i.e. the base paper was first preheated and shaped at 100°C for 60 ms, and then main shaped at 150°C for 60 ms. Then, it was laminated with a solidified carbon fiber base paper, and then hot pressed in a hot pressing roller group at 160°C and 3.5 kN / m linear pressure for 60 ms to obtain a composite; (2) MDO longitudinal stretching: the composite was stretched in a longitudinal stretching roller group, with a stretching ratio of 1:2 and a speed of 12 m / min. The longitudinal stretching device had 3 groups of traction rollers, and the surface linear speed of each traction roller increased in turn, and the speed ratio between adjacent traction rollers was 1.25. The composite was stretched to 1.15 times the initial length; (3) Peeling: After the stretching is completed, the cured carbon fiber base paper is peeled from the backing paper in the peeling roller set, the peeling is performed in the peeling roller set, the peeling roller set includes carbon fiber base paper traction rollers and backing paper traction rollers (used for traction of the stretched carbon fiber base paper and the backing paper respectively), the surface linear speed of the carbon fiber base paper traction roller is 1.20 times the surface linear speed of the backing paper traction roller; after the peeling, the carbon fiber base paper enters the carbon fiber base paper winding station, and the backing paper enters the plant fiber backing paper winding station after passing through the electrostatic dust removal or cleaning device; then, the interfacial peeling force of the backing paper and the cured carbon fiber base paper is tested, the MDO stretching process of the backing paper and the cured carbon fiber base paper is recorded, the peeling integrity of the backing paper and the cured carbon fiber base paper is recorded, and the stretching multiple of the carbon fiber base paper after the peeling is tested, and the test results are shown in Table 3. (4) Circulation: finally, the backing paper is subjected to the "hot-pressing compounding-longitudinal stretching-peeling" circulation again according to the conditions of the above steps (1)-(3) (to test whether the backing paper can be used for multiple cycles), and the number of times of circulation of the backing paper is recorded.

[0101] Among them, the method of the test items involved is as follows: (1) Interfacial peeling force of the backing paper and the cured carbon fiber base paper: GB / T 2790-1995 "Adhesives - Test methods for 180° peel strength - Flexible materials to rigid materials".

[0102] (2) MDO stretching process of the backing paper and the cured carbon fiber base paper: the breaking and wrinkling of the backing paper in the MDO longitudinal stretching process are observed and recorded.

[0103] (3) Peeling integrity of the backing paper and the cured carbon fiber base paper: the integrity and easy peeling degree of the backing paper and the cured carbon fiber base paper after the MDO longitudinal stretching and the peeling are observed and recorded.

[0104] (4) Stretching multiple of the carbon fiber base paper after the peeling: after the peeling, samples are taken along the longitudinal direction (MD) of the carbon fiber base paper, the sample size is 10 mm (width) x 150 mm (length), the long side is the stretching direction, and the number of samples is 5. Before the sample test, the sample shall be subjected to constant temperature and humidity treatment according to GB / T 10739-2023 "Paper, paperboard and pulp - Standard atmospheric conditions for sample conditioning and testing" (23℃, 50%RH, 4h). Under the state of no tension, the length L0 of the cured carbon fiber base paper and the length L of the carbon fiber base paper after the stretching and peeling and cooling are tested by using a high-precision vernier caliper respectively. The stretching multiple = (L-L0) / L0.

[0105] (5) The recyclable times of the backing paper: after peeling, if the backing paper has no breakage, no wrinkle, and complete coating, and then the tensile strength and the surface contact angle (water) of the backing paper are tested again after simple cleaning or electrostatic dust removal, if the tensile strength retention rate of the backing paper is > 90% and the surface contact angle is > 130°, it means that the backing paper can be used continuously, and the recyclable times (the MDO stretching process of the backing paper and the cured carbon fiber base paper is repeated again) of the backing paper are recorded.

[0106] The test results are shown in the following table: Table 3 Performance test results of the backing paper of each example and the comparative example

[0107] As shown in the above table, the backing paper provided by the examples 1-7 has a longitudinal tensile index of 68-82 N·m / g, a longitudinal wet strength retention rate of 32-45%, a heat shrinkage rate of the backing paper at 160℃ of 0.5-1%, and a surface contact angle (water) of the backing paper of 150-152°, showing super-hydrophobicity. In the MDO stretching process of the examples 1-7, the backing paper and the cured carbon fiber base paper are continuously operated for 120 min, the interfacial peeling force between the backing paper and the cured carbon fiber base paper is 0.2-0.23 N / cm, and the cured carbon fiber base paper and the backing paper are all free of any breakage and wrinkle, and the stretching multiple of the carbon fiber base paper is 1.2-1.25 times, indicating that the backing paper can effectively bear and uniformly transfer the tensile stress; the peeling process is smooth, easy to peel, free of jamming and tearing; the surface of the carbon fiber base paper after peeling is complete and clean, and no fiber residue or coating transfer is observed under a microscope; the surface coating of the backing paper after peeling is intact, and the recyclable times of the backing paper can reach 6-8 times, and the backing paper only needs to be simply cleaned or electrostatically dusted after experiencing ≤5 times of recycling to be reused. The above results show that the backing paper and the cured carbon fiber base paper of the present application can be losslessly peeled after stretching, and the appearance is free of cracks and delamination, and the backing paper can be recycled for more than 5 times by applying 0.5-0.8 g / m 2 top coating, and can be recycled again, thereby reducing the comprehensive cost.

[0108] Compared with example 1, the comparative example 1 does not contain the top coating and the back top coating, resulting in a decrease in the hydrophobicity of the backing paper, and large-area wrinkle of the backing paper, and large-area adhesion and frequent tearing of the carbon fiber base paper during the stretching process, because the comparative example 1 loses the top coating, and cannot maintain extremely low interfacial adhesion and cleanability after high-temperature compounding.

[0109] Compared with example 1, the comparative example 2 does not contain the bottom coating and the back bottom coating, resulting in partial wrinkle of the backing paper and partial adhesion and occasional tearing of the carbon fiber base paper during the stretching process, because the comparative example 2 loses the barrier effect of the bottom coating, resulting in a large amount of penetration of the top coating into the paper matrix, and the inability to form a continuous super-hydrophobic and low-adhesion coating on the surface.

[0110] Compared with Example 1, the first polymer emulsion with lower Tg was used in Comparative Example 4, resulting in tackiness of the top coating layer, causing local creases of the backing paper and partial adhesion and occasional tearing of the carbon fiber base paper during stretching.

[0111] Compared with Example 1, the first polymer emulsion with higher Tg was used in Comparative Example 5, resulting in discontinuous film formation of the top coating layer and defects such as coating omission, causing large-area creases of the backing paper and large-area adhesion and frequent tearing of the carbon fiber base paper during stretching.

[0112] Compared with Comparative Examples 6 and 7, the backing paper of the present application has no breakage during MDO stretching, which is significantly better than silicone oil paper and release paper, benefiting from the mechanical stability of the paper matrix; moreover, the carbon fiber base paper and the backing paper can be completely and intactly peeled off, avoiding problems such as adhesion, tearing and residue, which is attributed to the design of the coating; the backing paper of the present application also has the advantages of recyclability and environmental protection, and the comprehensive performance is better than that of the existing disposable release material, which is suitable for continuous production process of high-performance carbon fiber base paper.

[0113] Compared with Example 2, the recyclable times of the backing paper of Example 1 containing a back coating layer are increased.

[0114] As can be seen from the above, the backing paper provided by the present application is further used for longitudinal stretching of the carbon fiber base paper, and at least has the following advantages: 1. The backing paper provided by the present application cooperates with the high-strength wet-strength paper matrix and the coating (top coating layer and bottom coating layer). First, the top coating layer: constructs a "super-hydrophobic-low-adhesion" interface to realize the intact peeling and recycling of the carbon fiber base paper. The core function of the top coating layer is to maintain extremely low interfacial adhesion after high-temperature compounding, while having cleanability. Both wax and fluorine-free acrylate are low-surface-energy materials, and the van der Waals force between them and the carbon fiber base paper is weak; during the curing process of the acrylate emulsion, part of the wax crystal is embedded in the polymer matrix, and part of the wax crystal protrudes from the surface, forming a hierarchical rough structure of micron-level undulation (caused by emulsion drying shrinkage) + nanometer-level protrusion (wax crystal). This structure reduces the actual contact area between the coating and the carbon fiber base paper (<10%) and reduces the interfacial adhesion, and on the other hand, makes the coating have super-hydrophobic properties, making it difficult for contaminants (dust, carbon dust, trace resin, etc.) to adhere; even if the coating is contaminated, it can be restored to its original state by washing with water or a weak solvent. During hot pressing at 140-180℃, the top coating layer is slightly softened but not molten and flowed (the first polymer emulsion has a specific Tg and MFFT), maintaining the structural integrity and avoiding mechanical anchoring with the carbon fiber base paper.

[0115] Secondly, the base coating layer, as a dense transition layer, serves as a bridge between the top and bottom layers. It firmly anchors to the paper substrate, providing interfacial bonding, and creates a "low-permeability barrier" for the top coating layer, ensuring its full functionality. Because the surface of the high-strength wet-strength paper substrate is a porous network structure woven from fibers, directly applying the top coating layer would cause a large amount of emulsion and wax crystals to penetrate into the interior, resulting in insufficient effective components on the surface and preventing the formation of a continuous superhydrophobic, low-adhesion interface. The second polymer emulsion in the base coating formulation, acting as the main film-forming agent, provides a continuous polymer phase, firmly anchoring the coating to the paper substrate. Starch + PVA and cellulose form multiple hydrogen bonds and a semi-interpenetrating network, giving the coating high viscosity and low permeability in macroscopic properties. After curing, it forms a "low-permeability barrier" on the paper surface, effectively blocking the large pores. The sheet-like montmorillonite, on the one hand, fills the micropores of the paper surface, further improving the density and smoothness of the base layer; on the other hand, the layered arrangement hinders heat conduction, improving overall thermal dimensional stability.

[0116] Finally, the high-strength, high-wet-strength paper matrix provides macroscopic mechanical support and stress transfer, achieving the durability of the liner paper structure. Specifically, (1) in terms of structural characteristics: the paper matrix preferably uses long-fiber plant fiber paper with added environmentally friendly wet-strength resin (such as low-chlorinated polyamide epichlorohydrin resin PAE, bio-based crosslinking agent, etc.). During the forming process, the fibers interweave to form a dense three-dimensional network structure, and at the same time, the wet-strength resin undergoes a chemical in-situ reaction between the fibers to form a water-resistant covalent crosslinking network. The above structural characteristics give the liner paper the following properties: high dry physical strength (longitudinal tensile index ≥65N•m / g, transverse tear index ≥10mN•m). 2 / g), the wet strength can maintain 30-50% of the dry strength, significantly better than ordinary paper; low thermal expansion rate, ensuring a thermal shrinkage rate of <1.5% at 140-180℃; structural durability, able to withstand no less than 5 cycles of "hot pressing-longitudinal stretching-peeling-washing / dust removal-drying" without fuzzing, sudden drop in strength, or appearance damage. Mechanism of action: During hot pressing at 140-180℃ and subsequent longitudinal stretching, the paper matrix, as the main load-bearing skeleton, efficiently and evenly distributes the external tensile load to the entire liner paper plane due to its high modulus and low thermal shrinkage rate, avoiding brittle fracture of the carbon fiber base paper due to local stress concentration; the wet-strength cross-linking network significantly inhibits fiber creep at high temperatures, allowing the liner paper to maintain geometric stability and a draw ratio of 1.05-1.30 during hot pressing and stretching, thereby ensuring precise synchronous deformation of the liner paper and the cured carbon fiber base paper, promoting uniform fiber orientation. This solves the problem of fiber structure loosening and physical strength reduction in ordinary paper during stretching, while the high-strength wet-strength paper matrix resists water molecule intrusion and maintains the integrity of the fiber network structure thanks to its covalent cross-linked network. This not only prevents the liner paper from breaking during cycling, but also ensures the long-term adhesion stability of the base coating at the paper base interface.

[0117] Therefore, the backing paper provided by the present application can effectively bear tensile stress, realize efficient traction of the carbon fiber base paper in the dry longitudinal stretching process, reduce the risk of breakage of the carbon fiber base paper in the mechanical stretching orientation process, realize complete and lossless peeling of the carbon fiber base paper, and guarantee the surface quality of the product.

[0118] 2. The backing paper mainly uses fluorine-free and plant fiber raw materials, and the proportion of bio-based materials is high. Starch, PVA, PHA, wax crystals and the like are green and environmentally friendly materials, and PFAS persistent organic pollutants are avoided, thereby meeting the sustainable development trend.

[0119] 3. The backing paper can be recycled at least 5 times or more (for example, the backing paper can be directly recycled, or after being recycled for 5 times or more, the backing paper is cleaned by using water, steam or a weak polar solvent to remove surface pollutants and then reused, or the backing paper can be coated with a top coating material, dried and then reused), thereby greatly reducing material costs and environmental burden.

Claims

1. A backing sheet, characterized in that, The paper base is provided with a coating on at least one side, the coating comprises a primer layer and a top coating layer, the primer layer is in direct contact with the paper base; The paper base has a longitudinal tensile index ≥65 N·m / g, and a longitudinal wet strength retention rate ≥30%; The top coating layer is prepared by using a top coating material, the top coating material comprises the following components: A first polymer emulsion and waxy nano-particles; The first polymer emulsion has a minimum film formation temperature of 30-80℃, and a glass transition temperature of 50-80℃.

2. The backing paper according to claim 1, characterized in that, The first polymer emulsion is at least one of a fluorine-free modified acrylate emulsion, a modified vinyl acetate-acrylate copolymer emulsion, an anionic water-based polyurethane emulsion, and a non-ionic water-based polyurethane emulsion.

3. The backing paper according to claim 1, wherein The waxy nano-particles are at least one of palm wax nano-particles, rice bran wax nano-particles, sugarcane wax nano-particles, beeswax nano-particles, lotus leaf wax nano-particles, lanolin wax nano-particles, and polyethylene wax nano-particles.

4. The backing paper according to claim 1, wherein The top coating material comprises 75-90wt% of the first polymer emulsion and 10-25wt% of the waxy nano-particles.

5. The backing paper according to claim 1, wherein basis weight of 50-80 g / m 2 and / or a longitudinal MIT folding endurance of the paper base of > 700 cycles, and / or a transverse tear index of the paper base of > 10 mN-m / g. 2 ​ 6. The backing paper according to claim 1, wherein The primer layer is prepared by using a primer coating material, the primer coating material comprises the following components: a second polymer emulsion, a binder containing hydroxyl groups, a thickening agent, and a filler.

7. The backing paper according to claim 6, characterized in that The second polymer emulsion is at least one of a fluorine-free modified acrylate emulsion, a vinyl acetate-ethylene copolymer emulsion, a modified vinyl acetate-acrylate copolymer emulsion, an anionic water-based polyurethane emulsion, a non-ionic water-based polyurethane emulsion, and a polyhydroxy aliphatic ester.

8. The backing paper according to claim 1, wherein The paper base is provided with a coating on both sides.

9. A method of producing the backing paper according to any one of claims 1 to 8, characterized in that, The method comprises the following steps: (1) preparing the primer layer on one surface of the paper base; (2) then coating the top coating material on the surface of the primer layer, and drying to obtain the top coating layer, thereby obtaining the backing paper.

10. Use of the backing paper according to any one of claims 1-8 in the preparation of carbon fiber base paper.

11. A method of stretching a carbon fiber precursor sheet, characterized by, The method comprises the following steps: (1) preheating the backing paper according to any one of claims 1-8, then bonding the cured carbon fiber base paper and one side of the coating layer of the backing paper, and then hot-pressing and compounding to obtain a compound; (2) stretching the compound; (3) peeling off the backing paper to obtain the treated carbon fiber base paper.

12. The method of claim 11, wherein, In step (1), the backing paper is subjected to multi-stage heat setting treatment before use, and the multi-stage heat setting treatment comprises the following steps: preheating and setting the backing paper at 90-110℃ for 50-120ms, and then main setting at 140-160℃ for 50-120ms.

13. The method of claim 11, wherein, In step (1), the temperature of the hot-pressing and compounding is 140-180℃, and / or the linear pressure of the hot-pressing and compounding is 2-5kN / m, and / or the time of the hot-pressing and compounding is 40-100ms.

14. The method of claim 11, wherein, In step (2), the stretching is longitudinal stretching or transverse stretching.

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