A TPU car cover with low-temperature self-healing function and its preparation method

Abstract

This invention discloses a TPU car cover with low-temperature self-healing function, comprising a TPU substrate layer, a low-temperature self-healing functional layer, and a protective layer sequentially laminated together. The TPU substrate layer is a thermoplastic polyurethane layer modified with flexible segments of polydimethylsiloxane (PDMS), with a PDMS mass percentage of 4%~8%. The low-temperature self-healing functional layer is composed of a multi-hydrogen bond cross-linked network constructed from a chain extender containing catechol groups and hydrogenated hydroxyl-terminated polybutadiene (HHPB) in a mass ratio of (3~8):(3~8). The protective layer is a fluorocarbon resin layer. This invention, through the synergistic design of the PDMS-modified TPU substrate and the HHPB / catechin functional layer, enables the car cover to achieve a scratch repair rate of over 80% within 60 minutes at -20℃, while maintaining high elasticity, abrasion resistance, and weather resistance, making it particularly suitable for automotive paint protection in cold regions.

Description

Technical Field

[0001] This invention relates to the field of automotive paint protection film technology, specifically to a TPU car wrap with low-temperature self-healing function and its preparation method. Background Technology

[0002] Thermoplastic polyurethane (TPU) paint protection film (commonly known as car cover) has become the mainstream product for automotive paint protection due to its excellent elasticity, abrasion resistance, and chemical corrosion resistance. In recent years, self-healing functions have been introduced into car cover design, mainly including two types: heat-healing and room-temperature self-healing. These self-healing mechanisms usually rely on the rearrangement of polymer molecular chain segments or the reversible breaking and recombination of dynamic chemical bonds (such as hydrogen bonds and dynamic covalent bonds) to achieve scratch repair.

[0003] However, existing self-healing car wraps have significant shortcomings in low-temperature environments. Due to the limited mobility of polymer molecular chains at low temperatures, the ability of dynamic bonds to rearrange weakens, leading to a significant decrease in self-healing efficiency or even complete failure. Furthermore, low temperatures can cause the TPU substrate to harden and become brittle, further reducing the mechanical properties and user experience of the car wrap. Therefore, the application of existing technologies in cold regions or winter climates is significantly limited.

[0004] For example, Chinese patent CN115785834B discloses a TPU high and low temperature film with repair function, but its self-healing film layer is a conventional polyurethane self-healing coating, without addressing optimization in low-temperature environments. Chinese patent CN202110359872 discloses a self-healing anti-yellowing paint surface protective film, but its self-healing coating is a thermosetting functionalized polyurethane acrylate resin, which also fails to address the low-temperature self-healing issue. In academic research, although polyurethane materials based on hydrogenated hydroxyl-terminated polybutadiene (HHPB) and catechol groups have been reported to possess self-healing properties (see *China Adhesives*, Vol. 31, No. 10, 2022, hereinafter referred to as "Document A"), this research mainly focuses on the adhesive performance and thermally activated self-healing of the material in humid environments. Its self-healing performance requires heating conditions (viscosity transition temperature 81℃) to be effectively realized, without addressing self-healing applications in low-temperature environments or multilayer film structure design.

[0005] Therefore, developing a TPU car cover that can efficiently self-heal and maintain good mechanical properties in low-temperature environments has significant market value and technical significance. Summary of the Invention

[0006] The present invention aims to solve the technical defect of existing TPU paint protective films that have a significantly reduced or even completely failed self-healing ability in low-temperature environments.

[0007] This invention is not limited to simply coating a conventional self-healing layer onto the TPU surface, but rather involves a collaborative design at both the substrate and functional layer levels. On one hand, by optimizing the molecular structure of the TPU substrate (introducing flexible PDMS segments), its glass transition temperature is lowered, allowing the substrate to maintain sufficient molecular segment mobility at low temperatures. On the other hand, a specially designed functional layer with low-temperature self-healing capabilities is employed. This functional layer utilizes a material system containing a multi-hydrogen bond cross-linking network (a specific ratio of HHPB and catechol groups), enabling reversible breakage and recombination of dynamic bonds under low-temperature conditions, thereby achieving rapid healing of the damaged interface. Simultaneously, this functional layer exhibits good compatibility and interfacial bonding strength with the TPU substrate, and a weather-resistant and wear-resistant protective layer can be laminated onto its surface, forming a multi-level structure.

[0008] The present invention aims to achieve the following effects: in low-temperature environments of -20℃ and below, the car cover can still efficiently self-repair surface scratches (the repair rate can reach more than 80%); while maintaining the original high elasticity, high wear resistance and excellent weather resistance of the TPU substrate, it significantly improves the reliability and service life of the car cover under cold conditions; and finally expands the application area and seasonal range of the car cover product, meeting the demand of the high-end automobile, new energy vehicle and other markets for all-weather, high-performance paint protection film, thereby enhancing the product's technological added value and market competitiveness.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] A TPU car cover with low-temperature self-healing function, comprising sequentially laminated:

[0011] The TPU substrate layer is a thermoplastic polyurethane layer modified with flexible segments of polydimethylsiloxane (PDMS), wherein the mass percentage of PDMS is 4% to 8%.

[0012] A low-temperature self-healing functional layer, wherein the functional layer is composed of a multiple hydrogen-bonded cross-linked network constructed from a chain extender containing catechol groups and hydrogenated hydroxyl-terminated polybutadiene (HHPB), wherein the mass ratio of the chain extender containing catechol groups to HHPB is (3~8):(3~8); and

[0013] The protective layer is a fluorocarbon resin layer.

[0014] Furthermore, the thickness of the low-temperature self-healing functional layer is 30~60 μm.

[0015] Furthermore, the thickness of the protective layer is 15~30 μm.

[0016] Furthermore, a pressure-sensitive adhesive layer and a release film layer are sequentially laminated beneath the TPU substrate layer.

[0017] Furthermore, the protective layer further contains nano-silica particles.

[0018] The present invention also provides a method for preparing the above-mentioned TPU car cover, comprising the following steps:

[0019] (1) Plasma surface treatment was performed on the PDMS modified TPU substrate for 2 to 5 minutes and 50 to 100W.

[0020] (2) Dissolve the chain extender containing catechin groups and HHPB in a solvent at a mass ratio of (3~8): (3~8), stir evenly, coat it on the surface of the TPU substrate treated in step (1), and dry it at 80~120℃ to form a low-temperature self-healing functional layer with a thickness of 30~60 μm.

[0021] (3) A fluorocarbon resin is coated on the functional layer and hot-pressed at a temperature of 120~150℃ and a pressure of 5~10 MPa to form a protective layer;

[0022] (4) Curing at room temperature for 25 to 35 days yields TPU car wrap.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. Significantly Enhanced Low-Temperature Self-Healing Performance: Through the synergistic effect of HHPB's extremely low glass transition temperature (Tg approximately -80℃ to -60℃) and the dynamic hydrogen bond network of catechol groups, the functional layer maintains sufficient chain segment mobility and dynamic bond reversibility even at -20℃, achieving highly efficient scratch repair. Experiments show that the scratch repair rate reaches over 80% within 60 minutes at -20℃, far superior to traditional car wraps (less than 10%) and direct film formation of material A (approximately 23%).

[0025] 2. Preservation of substrate's low-temperature mechanical properties: By modifying TPU with PDMS flexible segments, the glass transition temperature of the substrate is reduced (Tg drops from approximately -30℃ to below -50℃), enabling the substrate to maintain high elasticity and flexibility at low temperatures, avoiding hardening and brittleness, and forming a good mechanical match with the functional layer.

[0026] 3. Dual-layer synergistic effect: The modified TPU substrate provides low-temperature mechanical support for the functional layer, while the functional layer supplements the surface self-healing function of the substrate. The two exhibit a synergistic effect in low-temperature environments, enabling the overall car cover to achieve a tensile modulus repair efficiency of over 90% at -25℃.

[0027] 4. Excellent overall performance: The fluorocarbon resin protective layer gives the car cover excellent weather resistance (gloss retention rate ≥90% after 1000 hours of xenon lamp aging) and abrasion resistance (Taber test mass loss ≤0.02g), while maintaining the original high transparency of TPU (visible light transmittance ≥90%).

[0028] 5. Controllable preparation process: Plasma surface treatment improves the interlayer bonding strength, and the coating method and hot pressing composite process are mature and suitable for large-scale production. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the layered structure of a TPU car cover with low-temperature self-healing function according to the present invention.

[0030] In the diagram: 1-Top protective layer (fluorocarbon resin layer), 2-Low temperature self-healing functional layer (containing catechol groups and HHPB layer), 3-TPU substrate layer, 4-Pressure-sensitive adhesive layer, 5-Release film layer. Detailed Implementation

[0031] The present invention will now be described in detail with reference to specific embodiments. These embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0032] Test Method Description

[0033] (1) Low-temperature self-healing rate test: A V-shaped scratch with a width of 50 μm and an initial depth of 20 μm was prepared on the surface of the car cover using a micron scratch tester (model: CSM Revetest). The sample was placed in a -20℃ constant temperature chamber (accuracy ±0.5℃) for 60 minutes. After removal, the residual depth of the scratch was measured using a white light interferometer (model: Bruker Contour GT). Repair rate = (initial depth - repaired depth) / initial depth × 100%. Each group of samples was tested 5 times and the average value was taken.

[0034] (2) Tensile modulus repair efficiency test: The method described in the literature [Advanced Materials, 2018, 30(45):1804025] was followed. The car cover sample was cut into dumbbell-shaped standard strips (ISO 37-2 type). The strips were stretched to 50% strain at a rate of 50 mm / min on a universal testing machine and then unloaded. The strips were left to stand for 30 minutes. The middle of the strips was completely cut off with a blade, and the cut surfaces were tightly joined together and healed in a -25℃ constant temperature chamber for 24 hours. After healing, the strips were stretched again under the same conditions, and the modulus recovery rate was recorded. Modulus repair efficiency = Modulus after healing / Original modulus × 100%.

[0035] (3) Weather resistance test: In accordance with GB / T 16422.2-2014 standard, a xenon lamp aging test chamber was used with an irradiance of 0.51W / m²@340nm, a black standard temperature of 65℃, a relative humidity of 50%, and continuous illumination for 1000 hours. The gloss (60°) retention rate before and after the test was measured.

[0036] (4) Abrasion resistance test: According to ASTM D1044 standard, a Taber abrasion tester with CS-17 grinding wheel was used, with a load of 1kg, a rotation speed of 60 rpm, and a running speed of 500 revolutions. The mass loss was then measured.

[0037] (5) Visible light transmittance test: According to GB / T 2410-2008 standard, a haze transmittance tester was used.

[0038] Example 1

[0039] 1. TPU Substrate Layer: 4 wt% of amino-terminated polydimethylsiloxane (PDMS, molecular weight approximately 2000, purchased from Sigma-Aldrich) was premixed into aliphatic TPU (Bayer Desmopan 5377A) raw material. After granulation by twin-screw extrusion, the modified TPU substrate with a thickness of 150 μm was obtained by casting. The substrate was then subjected to plasma surface treatment (atmospheric plasma, power 80W, time 3 minutes).

[0040] 2. Preparation of Low-Temperature Self-Healing Functional Layer: A chain extender containing catechol groups (a Schiff base product synthesized from 3,4-dihydroxybenzaldehyde and serine in an equimolar ratio, with a molecular weight of approximately 310) and hydrogenated hydroxyl-terminated polybutadiene (HHPB, with a molecular weight of approximately 2300 and a hydroxyl value of 0.80 mmol / g, purchased from Idemitsu, Japan) were mixed in ethyl acetate solvent at a mass ratio of 5:3 (i.e., 5 parts chain extender and 3 parts HHPB). The mixture was stirred until homogeneous, with a solid content of 20 wt%. This solution was coated onto the surface of a plasma-treated TPU substrate and dried at 100°C to obtain a functional layer with a thickness of approximately 35 μm.

[0041] 3. Preparation of protective layer: Fluorocarbon resin (FEVE resin, model Lumiflon LF-200) is coated on the functional layer and hot-pressed at 130℃ and 8 MPa to obtain a protective layer with a thickness of 20 μm.

[0042] 4. Curing: Curing at room temperature for 30 days yields the TPU car wrap.

[0043] Example 2

[0044] 1. TPU substrate layer: 8 wt% amino-terminated PDMS was mixed into aliphatic TPU, and the modified TPU substrate with a thickness of 180 μm was obtained by extrusion molding. Plasma treatment time was 4 minutes, power was 90 W.

[0045] 2. Preparation of low-temperature self-healing functional layer: A chain extender containing catechol groups (same as in Example 1) and HHPB were mixed in ethyl acetate at a mass ratio of 8:5 (i.e. 1.6:1), with a solid content of 25%. After coating and drying (110°C), the thickness of the functional layer was about 55 μm.

[0046] 3. Preparation of protective layer: 3 wt% of nano-silica (particle size 20 nm, purchased from Degussa) was added to fluorocarbon resin (same as in Example 1), ultrasonically dispersed and coated to a thickness of 25 μm, and hot-pressed at 140℃ and 9 MPa.

[0047] 4. Curing: Curing at room temperature for 30 days yields the TPU car wrap.

[0048] Example 3

[0049] 1. TPU substrate layer: 6 wt% amino-terminated PDMS was mixed into aliphatic TPU, and the modified TPU substrate was obtained by extrusion molding with a thickness of 200 μm. Plasma treatment time was 5 minutes, power was 70 W.

[0050] 2. Preparation of low-temperature self-healing functional layer: A chain extender containing catechol groups is mixed with HHPB in a mass ratio of 6:8 (i.e. 3:4) in methyl ethyl ketone, with a solid content of 22%. After coating and drying (90℃), the thickness of the functional layer is about 50 μm.

[0051] 3. Protective layer: Fluorocarbon resin layer (without nanofillers), 18 μm thick, hot-pressed at 130℃ and 7 MPa.

[0052] 4. Curing: Curing at room temperature for 30 days yields the TPU car wrap.

[0053] Comparative Example 1 (Traditional Car Cover)

[0054] Using the same TPU substrate as in Example 1 (without PDMS modification), without a low-temperature self-healing functional layer, only a conventional polyurethane self-healing coating (commercially available product, BOPP self-healing coating liquid) is coated on the surface of the TPU substrate, and the protective layer is the same as in Example 1. This serves as a representative of traditional car wraps.

[0055] Comparative Example 2 (Direct film formation of material A from literature)

[0056] HHPB-DS linear polyurethane was synthesized according to the method described in Reference A (China Adhesives, Vol. 31, No. 10, 2022). Specific steps: HHPB (molecular weight approximately 2300, 10 g, 4.35 mmol) and isophorone diisocyanate (IPDI, 1.93 g, 8.70 mmol) were dissolved in 50 mL of anhydrous toluene. Dibutyltin dilaurate (0.01 g) was added as a catalyst, and the reaction was carried out at 80°C for 2 hours. Then, catechol chain extender (4.35 mmol, approximately 1.35 g, at a molar ratio of 1:1) was added, and the reaction was continued for 4 hours. The product was vacuum dried at 60°C to form a film with a thickness of 150 μm. Its self-healing properties were directly tested without composite substrate and protective layer.

[0057] Comparative Example 3 (Unmodified substrate + functional layer of the present invention)

[0058] The same TPU substrate as in Example 1 was used, but without PDMS modification (i.e., ordinary TPU). The remaining steps were the same as in Example 1 (functional layer ratio 5:3, same protective layer) to verify the synergistic effect of PDMS modification.

[0059] Performance test results

[0060] Test Project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Self-healing rate at -20℃ (60 min) 85% 91% 88% 8% 23% 67% -25℃ Tensile modulus repair efficiency 94% 98% 95% 62% 45% 78% 1000h xenon lamp aging gloss retention rate 92% 93% 92% 89% 72% 91% Taber wear resistance mass loss (g) 0.020 0.018 0.021 0.022 0.035 0.020 Visible light transmittance (%) 91% 90% 91% 92% 88% 91%

[0061] Results analysis:

[0062] Examples 1-3 all achieved a self-healing rate of over 85% at -20℃, significantly better than Comparative Example 1 (traditional car wrap, 8%) and Comparative Example 2 (material A from literature, 23%). It is particularly noteworthy that Comparative Example 2 (material A from literature) only achieved a repair rate of 23% under the same testing conditions, indicating that the HHPB+catechin system in Literature A cannot achieve efficient self-healing at low temperatures without component ratio optimization and multilayer composite. In contrast, this invention, by controlling the ratio of HHPB to catechin (the mass ratios of Examples 1-3 were 5:3, 8:5, and 6:8, respectively, all falling within the range of (3~8):(3~8)) and combining it with a PDMS-modified substrate, increased the repair rate to over 85%, producing an unexpected synergistic effect.

[0063] The self-healing rate at -20℃ for Comparative Example 3 (unmodified substrate + functional layer of the present invention) was 67%, significantly lower than the 85% of Example 1, demonstrating that the PDMS-modified TPU substrate has an important synergistic promoting effect on the low-temperature self-healing of the functional layer. This is because PDMS modification lowers the glass transition temperature of the substrate, allowing it to maintain flexibility at low temperatures, thereby providing better mechanical support and deformation adaptability for the functional layer, which is beneficial for close contact of the damaged interface during the healing process.

[0064] Furthermore, the weather resistance, abrasion resistance, and transparency of Examples 1-3 are all superior to or equivalent to Comparative Example 1, and significantly superior to Comparative Example 2, demonstrating the advantages of the multi-layer structure design of the present invention in maintaining comprehensive performance.

[0065] Industrial application

[0066] The TPU car wrap provided by this invention can be mass-produced using existing coating and laminating equipment, and the raw materials are all commercially available, making costs controllable. The product is suitable for paint protection of high-end automobiles and new energy vehicles, and is especially suitable for use in cold winter regions, possessing broad market prospects.

[0067] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A TPU car cover with low-temperature self-healing function, characterized in that, Including sequentially compounded: The TPU substrate layer is a thermoplastic polyurethane layer modified with flexible segments of polydimethylsiloxane (PDMS), wherein the mass percentage of PDMS is 4% to 8%. A low-temperature self-healing functional layer, wherein the functional layer is composed of a multi-hydrogen bond cross-linking network constructed from a chain extender containing catechol groups and hydrogenated hydroxyl-terminated polybutadiene (HHPB), wherein the mass ratio of the chain extender containing catechol groups to HHPB is (3~8):(3~8). as well as The protective layer is a fluorocarbon resin layer.

2. The TPU car cover with low-temperature self-healing function according to claim 1, characterized in that, The thickness of the low-temperature self-healing functional layer is 30~60 μm.

3. The TPU car cover with low-temperature self-healing function according to claim 1, characterized in that, The thickness of the protective layer is 15~30 μm.

4. The TPU car cover with low-temperature self-healing function according to claim 1, characterized in that, Below the TPU substrate layer, a pressure-sensitive adhesive layer and a release film layer are sequentially laminated.

5. The TPU car cover with low-temperature self-healing function according to claim 1, characterized in that, The protective layer further contains nano-silica particles.

6. The TPU car cover with low-temperature self-healing function according to claim 1, characterized in that, The TPU car wrap reduces scratch depth by more than 80% within 60 minutes at -20℃.

7. The TPU car cover with low-temperature self-healing function according to claim 1, characterized in that, The TPU car cover achieves a tensile modulus repair efficiency of over 90% at -25℃.

8. A method for preparing a TPU car cover with low-temperature self-healing function as described in any one of claims 1-7, characterized in that... Includes the following steps: (1) Plasma surface treatment was performed on the PDMS modified TPU substrate for 2-5 minutes and 50-100 W. (2) Dissolve the chain extender containing catechin groups and HHPB in a solvent at a mass ratio of (3~8): (3~8), stir evenly, coat it on the surface of the TPU substrate treated in step (1), and dry it at 80~120℃ to form a low-temperature self-healing functional layer with a thickness of 30~60 μm. (3) A fluorocarbon resin is coated on the functional layer and hot-pressed at a temperature of 120~150℃ and a pressure of 5~10 MPa to form a protective layer; (4) Curing at room temperature for 25 to 35 days yields TPU car wrap.

Images