Surface protective film and method for manufacturing the same

Abstract

The present application relates to the technical field of optical protective film, and particularly relates to a surface protective film and a preparation method thereof, wherein the surface protective film comprises a substrate layer, a core layer and a self-adhesive layer which are stacked in sequence, the core layer comprises a propylene-ethylene-butylene terpolymer polypropylene matrix material and long-chain polystyrene segment grafted modified nanometer cross-linked polymethylsiloxane microspheres, the Tg of the core layer is 0-10 DEG C, and no independent Tg peak appears in the temperature region higher than 150 DEG C in the tan delta curve of DMA analysis, the generation of crystal points is inhibited while high haze is ensured, so that the surface protective film with low crystal points and high haze is obtained.

Description

Technical Field

[0001] This invention relates to the field of optical protective film technology, and more specifically to a surface protective film and its preparation method. Background Technology

[0002] Brightness enhancement film (also known as prism sheet / BEF) is a key optical film used in display technology to improve the performance of backlight modules. As display technology evolves towards ultra-high definition and flexibility, the intricate microstructure of the brightness enhancement film places higher demands on the protective materials. Cast polypropylene surface protective film, as the core protective material in the manufacturing and transportation of brightness enhancement films, needs to meet the following requirements: ① high haze to match the optical characteristics of the brightness enhancement film; ② low and stable adhesion to ensure no residue or damage during peeling; ③ low crystal point to prevent damage to the intricate microstructure of the brightness enhancement film surface during lamination.

[0003] For example, CN105835490A discloses a method for manufacturing a high-haze, high-adhesion protective film. The protective film is mainly manufactured by a three-layer co-extrusion process, wherein layer A is a blend of polypropylene and polyethylene, layer B is a blend of polypropylene and polyethylene, and layer C is a polystyrene-based elastomer. This protective film uses polypropylene as the base material, relying on the roughness of the raw material itself and the diffuse reflection caused by the incompatibility between polyethylene and polypropylene to form haze, thus preventing the precipitation of small molecules. Layer C is an adhesive layer, blended using a unique formula, adhering to the film through the electrostatic attraction of the raw materials.

[0004] However, the above scheme uses diffuse reflection caused by the incompatibility of polyethylene and polypropylene to form haze, which cannot avoid the risk of uncontrollable spherulite growth size leading to the formation of large hard crystal points. This can damage the delicate microstructure of the surface when the film is laminated to the brightening film, and cannot simultaneously guarantee the high haze and low crystal point performance of the protective film. Summary of the Invention

[0005] To address the aforementioned problems with surface protective films, this invention provides a surface protective film and its preparation method, which simultaneously possesses the characteristics of low crystal point and high haze.

[0006] The present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a surface protective film comprising a substrate layer, a core layer, and a self-adhesive layer stacked sequentially, wherein the core layer comprises propylene-ethylene-butene terpolymer polypropylene and long-chain polystyrene segment graft-modified nano-crosslinked polymethylsiloxane microspheres; the core layer has a Tg of 0-10°C, and no independent Tg peak appears in the temperature region above 150°C in the tanδ curve of DMA analysis.

[0008] In the above-described scheme of the present invention, the core layer uses ternary copolymer polypropylene as the matrix resin. The introduction of ethylene and butene comonomers reduces the crystallinity and melting point of the material, enabling it to form a uniform and fine spherulite structure during processing. This provides the protective film with a high haze base while avoiding the risk of crystal point defects caused by excessively large crystals. The long-chain polystyrene segment graft-modified nano-crosslinked polymethylsiloxane microspheres have high interfacial compatibility with the matrix resin, and the core layer has a Tg of 0-10℃. Furthermore, no independent Tg peak appears in the tanδ curve above 150℃ in the DMA analysis, which can further improve the interfacial compatibility between the microspheres and the ternary copolymer polypropylene matrix resin. This results in good dispersibility of the microspheres when the two are blended, achieving high haze while suppressing the formation of large particle crystal points caused by microsphere agglomeration. The combination of the two yields a surface protective film with low crystal point and high haze.

[0009] Preferably, the amount of microspheres added is 3wt%-8wt% relative to the total mass of the core layer, and more preferably 4wt%-6wt%.

[0010] Preferably, the microspheres have a surface roughness Ra ≤ 5 nm, an average sphericity ≥ 0.95, an average particle size D50 of 2-3 μm, a particle size distribution D90 / D10 < 1.5, and a nanoindentation hardness ≤ 0.5 Gpa. They exhibit extremely high surface smoothness, shape regularity, particle size distribution concentration, and softness, further improving the dispersibility of the microspheres in the ternary copolymer polypropylene matrix resin and suppressing the formation of crystal points while improving haze.

[0011] Preferably, the microspheres are obtained by activating nano-crosslinked polymethylsiloxane microspheres to introduce carbon-carbon double bonds, and then contacting the activated nano-crosslinked polymethylsiloxane microspheres with styrene monomers to undergo a graft polymerization reaction under the action of an initiator.

[0012] Preferably, the microspheres are prepared by reactive extrusion of nano-crosslinked polymethylsiloxane microspheres, styrene monomer, and initiator.

[0013] Preferably, the grafting rate of the microspheres is 1.0%-3.0% (by mass). Within this range, the polystyrene segments and the ternary copolymer polypropylene molecular chains are fully entangled, forming a suitable and stable interface layer. This allows the microspheres to be independently and uniformly dispersed, resulting in effective and stable light scattering, further ensuring the high haze value of the surface protective film. Furthermore, a grafting rate within this range can also prevent poor product quality stability due to processing issues, and the risk of uneven microsphere particle size caused by increased side reactions during the grafting process, further improving the controllability of the number and size of crystal points in the protective film.

[0014] Preferably, the self-adhesive layer comprises hydrogenated SIS and hydrogenated petroleum resin, which has high resistance to heat and oxygen aging, improves the stability of the self-adhesive layer melt, and reduces the possibility of gel crystals forming due to molecular chain crosslinking during processing.

[0015] Preferably, the haze of the surface protective film is 66%±3%; the number of crystal points with a size φ≤0.1mm is ≤30pcs / m. 2 The number of crystal points with a diameter of 0.1mm < φ ≤ 0.15mm ≤ 2 pcs / 100 m 2 Number of crystal points with a size φ > 0.15mm = 0 pcs / m 2 .

[0016] Preferably, the initial adhesion of the surface protective film is 3 gf / 25mm - 6 gf / 25mm, the aging adhesion is 3 gf / 25mm - 6 gf / 25mm, and the adhesion change rate over time is <10%.

[0017] A second aspect of the present invention provides a method for preparing the above-mentioned surface protective film, comprising the steps of melting and mixing the raw materials of the self-adhesive layer, the core layer and the substrate layer respectively to form a self-adhesive layer melt, a core layer melt and a substrate layer melt, and the steps of extruding and casting each of the melts respectively; wherein the self-adhesive layer melt and the core layer melt are both filtered through a filter before extrusion.

[0018] By filtering the self-adhesive layer melt, large crystal particles in the self-adhesive layer melt are intercepted; by filtering the core layer melt, on the one hand, the nano-crosslinked polymethylsiloxane microspheres are subjected to final dispersive shearing, which further improves the dispersibility of the microspheres and reduces the probability of large crystal particles being generated, and on the other hand, large crystal particles in the core layer melt are intercepted.

[0019] By implementing the above technical solution, compared with the prior art, the present invention has the following beneficial effects:

[0020] The surface protective film of the present invention is designed with a core layer comprising a propylene-ethylene-butene ternary copolymer polypropylene matrix resin and long-chain polystyrene segment grafted modified nano-crosslinked polymethylsiloxane microspheres, and the core layer has a Tg of 0-10℃. In the tanδ curve of DMA analysis, no independent Tg peak appears in the temperature region above 150℃. This ensures high haze while suppressing the generation of crystal points, so as to obtain a surface protective film with low crystal point and high haze. Detailed Implementation

[0021] The present invention will be further described in detail below through specific embodiments.

[0022] It should be noted that the following embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or make equivalent substitutions for some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

[0023] In this specific embodiment, a surface protective film is provided, comprising a substrate layer, a core layer, and a self-adhesive layer in sequence. The composition of each layer is as follows:

[0024] Substrate layer: 100% homopolymer polypropylene or random copolymer polypropylene. The homopolymer polypropylene is Shanghai Jinshan F800E, and the random copolymer polypropylene is Hanwha Total RF450.

[0025] Core layer: 92%-97% propylene-ethylene-butene terpolymer polypropylene matrix resin and 3%-8% modified nano-crosslinked polymethylsiloxane microspheres with long-chain polystyrene segments grafted onto the surface. The core layer has a Tg of 0-10℃, and no independent Tg peaks appeared in the temperature region above 150℃ in the tanδ curve of DMA analysis.

[0026] Self-adhesive layer: 90% hydrogenated SIS, 10% hydrogenated petroleum resin. The hydrogenated SIS is Kronen D1114, and the hydrogenated petroleum resin is GOYENCHEM-HR20.

[0027] Preparation of modified nano-crosslinked polymethylsiloxane microspheres:

[0028] 1. Prepared via a wet process, as follows:

[0029] S1. Microsphere Pretreatment and Activation: Disperse the nano-crosslinked polymethylsiloxane microspheres in an appropriate amount of toluene or xylene to form a uniform suspension. Add 3%-5% (relative to the mass of the microspheres) of γ-methacryloyloxypropyltrimethoxysilane (KH-570) to the suspension and reflux at 70-80℃ for 4-6 hours.

[0030] S2. Graft polymerization reaction: The microspheres after the above reaction are centrifuged and washed several times with toluene to remove unreacted KH-570, yielding surface-activated microspheres. The activated microspheres are redispersed in a reactor containing toluene, and nitrogen gas is introduced to purge oxygen. 20%-30% of styrene (St) monomer and 1%-2% of azobisisobutyronitrile (AIBN) initiator, equivalent to the mass of the microspheres, are added. Under nitrogen protection, the reaction is stirred at 75-85℃ for 6-8 hours.

[0031] S3. Post-treatment: After the reaction is complete, the above product is poured into a large amount of methanol or ethanol to precipitate. The product is then continuously extracted with acetone using a Soxhlet extractor for 24 hours to completely remove excess polystyrene (polystyrene not grafted onto the microspheres).

[0032] S4. The product from which excess polystyrene has been removed is dried in a vacuum oven at 60°C to constant weight to obtain nano-crosslinked polymethylsiloxane microspheres with surface-grafted polystyrene segments.

[0033] Microspheres 1 and 2 were obtained through the above wet process, wherein the grafting rate of microsphere 1 was 2% and the grafting rate of microsphere 2 was 3%.

[0034] 2. Prepared via a dry process, as follows:

[0035] S1. Raw material pretreatment and premixing

[0036] The nano-crosslinked polymethylsiloxane microspheres, 20% of the mass of styrene monomer relative to the mass of the microspheres, 2.0% of the mass of dicumyl peroxide (DCP) initiator relative to the mass of styrene monomer, and 0.1% of the mass of hydroquinone polymerization inhibitor relative to the mass of styrene monomer were premixed using a high-speed mixer.

[0037] S2. The premixed material is fed into a twin-screw extruder, and the extruded strips are cooled in a water tank and then pelletized to produce nano-crosslinked polymethylsiloxane microspheres with surface-grafted polystyrene segments.

[0038] Screw process parameter settings:

[0039] Temperature zone control:

[0040] Feeding area: The temperature is 80-100℃ to ensure stable material conveying and prevent premature volatilization or reaction of styrene.

[0041] Melting reaction zone: The temperature is 140-160℃, and sufficient mechanical shear force is provided in this region. This region is the main area where initiator decomposition and grafting reactions occur. The mechanical shear force helps to break the weak aggregation of microspheres and provides activation energy to their surface.

[0042] Homogenization extrusion zone: temperature 130-150℃, to stabilize the melt and reduce monomer volatilization.

[0043] Screw configuration: High-shear kneading block elements are configured in the molten reaction zone to provide sufficient shear energy and mixing effect.

[0044] Vacuum devolatilization: A vacuum port is installed at the end of the extruder to extract unreacted monomers and byproducts, ensuring the purity of the final product.

[0045] Microspheres 3 and 4 were obtained through the above dry process, wherein the grafting rate of microsphere 3 was 2% and the grafting rate of microsphere 4 was 1%.

[0046] Characterization and verification of the above microspheres:

[0047] Fourier transform infrared spectroscopy (FTIR): In the spectrum of the grafted microspheres, clearly visible at ~700 cm⁻¹. -1 and ~760 cm -1 The two strong absorption peaks are characteristic peaks of the out-of-plane bending vibration of the monosubstituted CH bond in the benzene ring. Their co-occurrence is a "fingerprint feature" of polystyrene.

[0048] Meanwhile, at ~3020 cm -1 A relatively weak absorption peak is visible, attributed to the stretching vibration of the CH bond on the benzene ring. In contrast, these three characteristic peaks are completely absent in the spectrum of the unmodified microspheres. This definitively proves that polystyrene segments have been successfully grafted onto the microsphere surface.

[0049] Thermogravimetric analysis (TGA): Tested under a nitrogen atmosphere, the grafted microspheres will show obvious weight loss steps corresponding to the decomposition of polystyrene segments at ~400℃. The grafting rate can be calculated from the percentage of weight loss.

[0050] The microspheres and key performance parameters used in the specific embodiments of the present invention are shown in Table 1.

[0051] Table 1. Microspheres and key performance parameters used in specific embodiments of the present invention

[0052]

[0053] The method for preparing the above-mentioned surface protective film includes:

[0054] S1. The raw materials of the self-adhesive layer, core layer and substrate layer are respectively fed into three extruders for melting and mixing to form a self-adhesive layer melt, a core layer melt and a substrate layer melt. The melting temperature of the self-adhesive layer raw material is 180-190℃, and the melting temperature of the core layer and substrate layer raw materials is 220-230℃.

[0055] S2. The self-adhesive layer melt and the core layer melt are filtered by 20μm and 30μm precision filters, respectively, and the filter pressure difference is controlled below 20MPa;

[0056] S3. The substrate layer melt, as well as the filtered self-adhesive layer melt and core layer melt, are stably transported to the co-extrusion distributor via a gear pump, extruded at 195±5℃, formed through a T-die, and rapidly cooled and shaped on a quenching roller at 30-35℃. The film is then pulled and wound up by guide rollers in a cleanroom environment to obtain a surface protective film.

[0057] The raw material composition of Examples 1-6 and Comparative Examples 1-4 is shown in Table 2.

[0058] Table 2 Raw material composition of Examples 1-6 and Comparative Examples 1-4

[0059]

[0060] The surface protective films prepared in each embodiment and comparative example were subjected to the following performance tests to verify their effectiveness. All tests were conducted under a standard environment of 23±2℃ and 50±10% relative humidity, after which the samples were conditioned for at least 24 hours before testing.

[0061] 1. Haze

[0062] Test method: GBT 2410-2008 Determination of light transmittance and haze of transparent plastics; Test equipment: fog imager, NDH8000.

[0063] 2. Crystal points

[0064] Test method: Refer to SEMI G74 (applicable to optical thin film defect detection), and conduct the test in a Class 10,000 cleanroom or clean bench background in a dark room with a light source illuminance of 4500±500 Lux. Sample requirements: The sampling area should be at least 1m². 2 (Multiple segments can be continuously detected to accumulate the area).

[0065] 3. Adhesion

[0066] Initial adhesion test method: The prepared surface protective film is cut into strips of 25mm*20cm and attached to an optical substrate PET with Ra of 0.015-0.040μm. The strips are pressed back and forth three times at 300mm / min using a 2kg rubber roller (Kejian electric roller press instrument) to form a sample. After being placed at room temperature of 23℃ for 20min, a 180° peel test is performed using a Kejian peel tester (KJ-1065A-H) at a speed of 300mm / min. The standard adopted is GB / T 2792-2014.

[0067] The aging adhesion test method is as follows: the prepared surface protective film is cut into strips of 25mm*20cm and attached to an optical substrate PET with Ra of 0.015-0.040μm. The strips are then pressed back and forth 3 times at 300mm / min using a 2kg rubber roller (Kejian electric roller press instrument) to form a sample. The attached sample is placed in a constant temperature oven at 50±2℃ and left to stand for 72 hours. After being taken out, it is cooled for 1 hour under standard conditions before the above peel test is performed.

[0068] The rate of change of adhesive force over time = (aging adhesive force - initial adhesive force) / less than initial adhesive force * 100.

[0069] The DMA test method is as follows: The test is conducted using a dynamic thermomechanical analyzer (TA Instruments Q800) according to the method provided in GB / T40396-2021. This standard is applicable to the determination of the glass transition temperature of polymer-based composite materials and provides a normative basis for this test.

[0070] 1. Test mode: Use single cantilever beam mode or tensile mode. Ensure optimal matching between test conditions and material morphology.

[0071] 2. Sample specifications: The core material is made into regular strips with a length of 10~30mm, a width of 2~10mm, and a thickness of 1~3mm, ensuring that the sample surface is flat and smooth and free of visible defects.

[0072] 3. Temperature program: Under nitrogen protection, the temperature was scanned from -50°C to 200°C at a heating rate of 3°C / min to observe the complete transition range.

[0073] 4. Dynamic parameters: The test frequency is set to 1Hz, and the strain amplitude is controlled within the linear viscoelastic region of the material, typically 0.01% - 0.05%.

[0074] In the tanδ-temperature curve obtained by dynamic mechanical analysis (DMA), a virtual baseline is drawn based on the curve trend from 150℃ to the test endpoint. This baseline is typically a straight line with a smooth, slowly rising trend. A condition is considered to be 'no independent Tg peak' if all of the following conditions are met:

[0075] Peak height threshold: Within the temperature range from 150℃ to the test endpoint, there are no peaks or bulges with tanδ values ​​exceeding this virtual baseline of 0.02. This threshold (Δtanδ < 0.02) ensures that any minute fluctuations that may originate from instrument noise or trace volatilization are not considered as an independent phase transition.

[0076] Peak shape requirements: No tanδ peaks with typical glass transition characteristics that can be clearly identified as polymer chain segment motion appear in this region. That is, there is no complete peak shape that rises and falls, and whose peak height and half-width are sufficient to identify the Tg of a polymer phase.

[0077] The test results are shown in Table 3.

[0078] Table 3 Performance test results of the surface protective films obtained in Examples 1-6 and Comparative Examples 1-4

[0079]

[0080] As shown in Table 3, the core samples from Examples 1-6 of this invention exhibited only one glass transition peak in their tanδ curves during dynamic mechanical analysis (DMA). Furthermore, within the temperature range of 150℃ to 200℃, the tanδ curves were smooth, without any independent peaks exceeding 0.02 mm above the baseline. The surface protective films obtained in Examples 1-6 had a crystal point count ≤ 1 pcs / 100 μm with a diameter of 0.1 < φ ≤ 0.15 mm. 2 φ≤0.1mm, number of crystal points <20pcs / m 2 For crystals with a diameter > 0.15mm, the number of crystal points is 0 pcs / m. 2 It meets the product requirements for surface protective film. Furthermore, both the initial adhesion and the adhesion after aging are between 3-6 gf / 25mm, the adhesion change rate over time is <10%, and the haze also meets the requirements for high-haze products.

[0081] In contrast, Comparative Example 1, which uses an existing surface protective film formulation, has lower haze, significantly more crystal points, and higher adhesion after aging compared to Example 1, with a large rate of change in adhesion over time.

[0082] Compared to Example 1, Comparative Example 2 did not add modified nano-crosslinked polymethylsiloxane microspheres to the core layer. The results showed that although the haze of the obtained surface protective film was comparable to that of Example 1, an independent Tg peak appeared in the high-temperature region above 150°C, so the number of crystal points was significantly greater than that of Example 1.

[0083] Compared to Example 1, Comparative Example 3 used nano-crosslinked polymethylsiloxane microspheres in the core layer, but without graft modification. Dynamic mechanical analysis (DMA) was performed on the core layer sample. In addition to the main transformation peak, a distinct independent tanδ peak appeared at 165°C. The unmodified microspheres had poor interfacial compatibility with the matrix resin. Therefore, the results showed that although the haze of the obtained surface protective film was comparable to that of Example 1, the number of crystal points was significantly higher than that of the surface protective film obtained in Example 1, and the adhesion after aging was also higher, with a large rate of change in adhesion over time.

[0084] Compared to Example 1, in Comparative Example 4, homopolymer polypropylene was used instead of propylene-ethylene-butene terpolymer polypropylene matrix resin in the core layer. The results showed that the haze of the obtained surface protective film was comparable to that of Example 1, but the number of crystal points was significantly greater than that of the surface protective film obtained in Example 1.

[0085] The above results demonstrate that, in the technical solution of this invention, the core layer is a combination of propylene-ethylene-butene ternary copolymer polypropylene matrix resin and modified nano-crosslinked polymethylsiloxane microspheres grafted with long-chain polystyrene segments on the surface. The core layer Tg is controlled at 0-10℃, and no independent Tg peak appears in the temperature region above 150℃ in the tanδ curve of DMA analysis. This plays an important role in achieving low crystal point, high haze, and suitable adhesion of the surface protective film. Both are indispensable and work together to achieve the comprehensive performance of the surface protective film.

Claims

1. A surface protective film comprising a substrate layer, a core layer, and a self-adhesive layer stacked sequentially, characterized in that, The core layer comprises propylene-ethylene-butene ternary copolymer polypropylene and long-chain polystyrene segment grafted modified nano-crosslinked polymethylsiloxane microspheres; the core layer has a Tg of 0-10℃, and no independent Tg peak appears in the temperature region above 150℃ in the tanδ curve of DMA analysis.

2. The surface protective film according to claim 1, characterized in that, The amount of microspheres added is 3wt%-8wt% relative to the total mass of the core layer.

3. The surface protective film according to claim 1, characterized in that, The microspheres have a surface roughness Ra ≤ 5 nm, an average sphericity ≥ 0.95, an average particle size D50 of 2-3 μm, a particle size distribution D90 / D10 < 1.5, and a nanoindentation hardness ≤ 0.5 Gpa.

4. The surface protective film according to claim 1, characterized in that, The microspheres are obtained by activating nano-crosslinked polymethylsiloxane microspheres to introduce carbon-carbon double bonds, and then contacting the activated nano-crosslinked polymethylsiloxane microspheres with styrene monomers to undergo a graft polymerization reaction under the action of an initiator.

5. A surface protective film according to claim 1, characterized in that, The microspheres are prepared by reactive extrusion of nano-crosslinked polymethylsiloxane microspheres, styrene monomer, and initiator.

6. The surface protective film according to claim 1, characterized in that, The grafting rate of the microspheres is 1.0% - 3.0%.

7. A surface protective film according to claim 1, characterized in that, The self-adhesive layer comprises hydrogenated SIS and hydrogenated petroleum resin.

8. A surface protective film according to claim 1, characterized in that, The haze of the protective film is 66%±3%; the number of crystal points with a size φ≤0.1mm is ≤30pcs / m 2 Number of crystal points with a diameter of 0.1mm < φ ≤ 0.15mm ≤ 2 pcs / 100m 2 The number of crystal points with a size φ > 0.15 mm is 0 pcs / m 2 .

9. A surface protective film according to claim 1, characterized in that, The initial adhesion of the protective film is 3 gf / 25 mm - 6 gf / 25 mm, the aging adhesion is 3 gf / 25 mm - 6 gf / 25 mm, and the adhesion change rate over time is <10%.

10. A method for preparing a surface protective film according to any one of claims 1-9, characterized in that it is prepared by melting the raw materials of the self-adhesive layer, the core layer, and the substrate layer respectively to form a self-adhesive layer melt, a core layer melt, and a substrate layer melt, and then extruding and casting them, wherein... Both the self-adhesive layer melt and the core layer melt are filtration-processed before extrusion.

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