Heat-resistant and compression-resistant co-extrusion composite film and preparation method of heat-resistant and compression-resistant film

Abstract

The invention relates to a heat-resistant and pressure-resistant co-extrusion composite film which comprises an outer layer, a first bonding layer, a first reinforcing layer, a central layer, a second reinforcing layer, a second bonding layer and an inner layer, wherein the outer layer is a high-crystallinity polypropylene layer; the first bonding layer is a maleic anhydride grafted polypropylene layer; the first reinforcing layer is a nano-filler modified polypropylene layer; the central layer is a high-elasticity polyolefin elastomer layer; the second reinforcing layer is a nano-filler modified polypropylene layer; the second bonding layer is a maleic anhydride grafted polyolefin layer; the inner layer is a polyethylene layer; the central layer accounts for 25-35% of the total thickness of the composite film, and gradient compatible transition areas are formed on the interface between the central layer and the first reinforcing layer and the interface between the central layer and the second reinforcing layer. The invention further discloses a preparation method of the heat-resistant and compression-resistant film. A high-elasticity polyolefin elastomer is arranged in the center of a multi-layer co-extrusion film to serve as a stress buffer layer, an interface gradient compatible structure is constructed, and the problems that heat resistance and toughness of a traditional composite film are difficult to consider at the same time, and compression resistance is insufficient are solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high polymer composite materials, in particular to a heat-resistant and pressure-resistant co-extrusion composite film and a preparation method of the heat-resistant and pressure-resistant film. BACKGROUND

[0002] Traditional high-performance co-extrusion films generally use a PE / PA (polyamide) seven-layer system (such as a medical packaging film of BASF), which utilizes the high rigidity, high heat resistance (heat distortion temperature > 180℃) and gas barrier property of PA. However, PA is a polar engineering plastic, and has poor compatibility with polyolefins. Special sorting equipment is required for recycling, and the performance decay rate of the recycled material is > 50%, which does not meet the mandatory requirement of "100% mechanically recyclable" in the EU "Packaging and Packaging Waste Regulations". Although some manufacturers have attempted to add a compatibilizer to achieve PE / PA blending recycling, the cost has increased by more than 30%, and the tensile strength retention rate after recycling is still less than 60%, which is difficult to commercialize and popularize.

[0003] In order to avoid the recycling problem of PA, the industry has shifted to an all-polyolefin system, such as CN102975451B (2015) which discloses a polypropylene / polyethylene seven-layer co-extrusion medical packaging film. The patent uses a structure of "heat-sealing layer (mLLDPE / EMA) / PP layer / MAH grafted PP adhesive layer / PP core layer / MAH grafted PP / PP+plastic body blending layer / PP+plastic body blending layer", which achieves recyclability without PA, but the core layer is single PP material, which is too rigid, resulting in an impact strength of < 120J / m² at -20℃ or below, which cannot meet the low-temperature transportation requirements. If the POE elastomer content is simply increased to improve toughness, the compressive strength at 130℃ heat distortion temperature will decrease by more than 40%, which cannot withstand the stacking pressure.

[0004] The existing technologies all place the POE elastomer in the surface layer or the subsurface layer as a heat-sealing or toughening functional layer, and fail to use it as a structural load-bearing layer, which results in the inability of the POE stress buffer function and the PP heat-resistant rigidity to effectively play a role. SUMMARY

[0005] In view of the defects of the existing technologies, the technical problem to be solved by the present application is that the POE elastomer is mostly placed in the surface layer or the subsurface layer as a heat-sealing or toughening functional layer, and fails to play a role as a structural load-bearing layer, which results in the inability of the POE stress buffer function and the PP heat-resistant rigidity to effectively play a role.

[0006] A heat-resistant and pressure-resistant co-extrusion composite film, comprising an outer layer, a first adhesive layer, a first reinforcing layer, a center layer, a second reinforcing layer, a second adhesive layer and an inner layer.

[0007] The outer layer is a high-crystallinity polypropylene layer.

[0008] the first adhesive layer is a maleic anhydride grafted polypropylene layer;

[0009] the first reinforcing layer is a nanofiller modified polypropylene layer;

[0010] the center layer is a high-elasticity polyolefin elastomer layer;

[0011] the second reinforcing layer is a nanofiller modified polypropylene layer;

[0012] the second adhesive layer is a maleic anhydride grafted polyolefin layer;

[0013] the inner layer is a polyethylene layer;

[0014] the center layer accounts for 25-35% of the total thickness of the composite film, and a gradient compatible transition zone is formed between the center layer and the first and second reinforcing layers.

[0015] The present application builds a sandwich structure of "rigid outer layer-elastic core-rigid inner layer" by defining that the high-elasticity polyolefin elastomer (POE) in the seven-layer structure is located in the center layer and accounts for 25-35% of the total thickness, and a gradient compatible transition zone is formed between the layer and the two side reinforcing layers. This design changes the elastomer which is only used as a heat sealing function layer in the traditional into the stress buffer core of the whole film, which can efficiently absorb and disperse stress when bearing pressure, thereby maintaining the high heat resistance brought by the high crystallinity polypropylene of the outer layer while significantly improving the pressure resistance and low temperature toughness of the composite film; the formation of the gradient compatible transition zone eliminates the stress concentration points caused by the material abrupt interface in the traditional co-extruded film by promoting the mutual diffusion and entanglement of the molecular chains at the interface, fundamentally enhances the interlayer bonding force, prevents delamination in high temperature or repeated stress environment, and realizes the synergistic optimization of rigidity, elasticity and interface stability.

[0016] As a preferred, the center layer contains 85-95 parts by weight of ethylene-octene copolymer and 5-15 parts by weight of high density polyethylene, the high density polyethylene has a density of 0.941-0.965 g / cm³ and a melt index of 0.5-2.0 g / 10 min. Pure POE has excellent elasticity but low melt strength, which easily leads to unstable film bubble or even rupture in the film blowing process, while the addition of a specific proportion of high density polyethylene (density 0.941-0.965 g / cm³, melt index 0.5-2.0) can significantly improve the strength and processing stability of the blended melt, ensure the film forming process smoothly, and the rigid component of HDPE and the elastic network of POE complement each other, so that the center layer has better anti-creept ability while maintaining high elasticity and excellent recovery ability, thereby ensuring the stability of the thickness and shape of the composite film in the long-term pressure environment.

[0017] As preferred, the width of the gradient-compatible transition zone is 2-5 μm, and the mass ratio of the polyolefin elastomer to the polypropylene in the transition zone gradually changes from 95:5 to 5:95 in a continuous gradient from the center layer to the reinforcing layer. This specific width range ensures that the interface region has sufficient scale to achieve molecular-level interpenetration and interweaving, thereby obtaining a firm bond, while avoiding the blurring of the independent roles of the functional layers due to a too wide transition zone; and the continuous composition gradient design enables the modulus and performance of the material to be smoothly transitioned at the interface, completely eliminating the stress concentration phenomenon caused by performance mutations, so that the stress can be smoothly transmitted along the gradient interface when the composite film is subjected to impact or thermal cycling, thereby improving the anti-delamination capability and overall durability.

[0018] As preferred, the nano-filler in the first reinforcing layer and the second reinforcing layer is an organically modified nano-clay, the modifier thereof is a quaternary ammonium salt compound, and the content of the nano-filler in the reinforcing layer is 3-8 parts by weight. The organic modification significantly improves the dispersibility and compatibility of the nano-clay in the polypropylene matrix, avoiding the problem of easy agglomeration of unmodified fillers, so that the reinforcing effect can be more effectively exerted; within this content range, the nano-clay can sufficiently improve the rigidity, heat resistance and barrier properties of the reinforcing layer, while having minimal negative impact on the light transmission and toughness of the material due to good dispersion and moderate content, thereby achieving the best balance between the reinforcing effect and comprehensive performance.

[0019] As preferred, the outer layer is a β-crystal polypropylene with a β-crystal content of not less than 40%, and the β-crystal polypropylene is prepared by modification with a β-nucleating agent, and the β-nucleating agent is N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide. A high content of β-crystals endows the polypropylene with much better impact toughness than ordinary α-crystals, because it can absorb a large amount of energy through the transformation from β-crystals to α-crystals when impacted; at the same time, the β-crystal polypropylene still maintains the inherent high heat resistance and rigidity of polypropylene. This design enables the outer layer to remain stable in shape when subjected to a high-temperature environment (such as sterilization conditions), and is not prone to brittle fracture when subjected to low-temperature impact, thereby successfully solving the technical contradiction between high heat resistance and high toughness that the traditional polypropylene film cannot reconcile.

[0020] A preparation method of a heat-resistant and pressure-resistant film, for preparing the heat-resistant and pressure-resistant co-extrusion composite film according to any one of claims 1-5, comprising the following specific steps:

[0021] (1) The outer layer high-crystallinity polypropylene, the first adhesive layer maleic anhydride grafted polypropylene, the first reinforcing layer nano-filler modified polypropylene, the center layer polyolefin elastomer, the second reinforcing layer nano-filler modified polypropylene, the second adhesive layer maleic anhydride grafted polyolefin and the inner layer polyethylene are respectively put into seven extruders;

[0022] (2) the outer layer polypropylene extruder temperature is set to 210-220°C, the first adhesive layer extruder temperature is set to 200-210°C, the first reinforcing layer extruder temperature is set to 205-215°C, the center layer polyolefin elastomer extruder temperature is set to 185-195°C, the second reinforcing layer extruder temperature is set to 205-215°C, the second adhesive layer extruder temperature is set to 190-200°C, and the inner layer polyethylene extruder temperature is set to 180-190°C;

[0023] (3) the seven-layer melt is converged through a co-extrusion die head, the die head temperature is maintained at 200±5°C, the blow ratio is 2.5-3.5:1, and the draw ratio is 4-6:1;

[0024] (4) the extruded film bubble is cooled and shaped by a downward blowing water cooling method, cooling air with a temperature of 15-25°C is introduced into the film bubble, and a four-zone gradient cooling system is used to cool the outside of the film bubble;

[0025] (5) the cooled film is drawn to a winding device by a traction roller, and the winding tension is controlled at 50-80N.

[0026] The differentiated extrusion temperature setting (210-220°C for the outer layer PP, 185-195°C for the center layer POE, and 180-190°C for the inner layer PE) utilizes the thermodynamic properties of each layer of material, ensuring sufficient plasticization while preventing the degradation of heat-sensitive components; the downward blowing water cooling combined with the inside-outside collaborative cooling method ensures the stability of the film bubble and high cooling efficiency, which is conducive to obtaining a film with uniform thickness and low internal stress.

[0027] As a preferred, the four-zone gradient cooling system includes four continuous cooling zones: the first cooling zone temperature is 10-15°C, the second cooling zone temperature is 20-25°C, the third cooling zone temperature is 35-45°C, and the fourth cooling zone temperature is 25-30°C. This gradient cooling strategy precisely regulates the crystallization characteristics of different functional layers: the first zone rapidly cools the outer layer to lock in high β crystal content; the second zone moderately cools the reinforcing layer to facilitate the ordered distribution of nano fillers; the third zone slowly cools the center elastic layer to avoid loss of elasticity due to quenching; and the fourth zone balances the cooling of the inner layer to ensure its heat sealing performance. This differentiated cooling process optimizes the microstructure of each layer, synergistically improving the overall performance of the composite film.

[0028] As preferred, the internal center layer flow channel of the co-extrusion die is provided with a helical turbulence element, the pitch of the helical turbulence element is 20-30 mm, and the helical angle is 30-45 degrees. The element can effectively break the laminar flow state of the melt, increase the mutual disturbance and premixing of the center layer elastomer and the melt of the adjacent reinforcing layer before merging, thereby greatly promoting the entanglement and diffusion of the molecular chains at the interface, providing a key process implementation means for forming the gradient compatible transition zone defined in claim 3, and ultimately improving the interlayer bonding strength.

[0029] As preferred, the area tension of the outer layer is 75-85 N, the area tension of the center layer is 35-45 N, and the area tension of the inner layer is 55-65 N, and the pulling speed is 120-150 m / min. This differentiated tension control takes into account the differences in the rigidity and elasticity of each layer of material: the higher outer layer tension ensures sufficient stretching and molecular orientation of the rigid polypropylene layer, improving the strength; the lower center layer tension protects the elastomer network from being stretched too much and destroying its resilience; the moderate inner layer tension ensures the flatness of the film and the subsequent processability. This technology avoids internal defects caused by the deformation of rigid and elastic layers that is not coordinated under uniform tension, making the film forming more uniform and stable.

[0030] As preferred, the helical turbulence element rotates at a speed of 50-150 rpm during the extrusion process, and the ratio of the rotation speed to the center layer polyolefin elastomer extrusion rate is kept at 0.8-1.2:100. The dynamically rotating turbulence element can produce stronger and more uniform mixing shear than the static structure, further strengthening the interface fusion effect; controlling the ratio of rotation speed to extrusion rate within a reasonable range can accurately control the intensity of interface mixing, maximizing the promotion of interlayer compatibility while avoiding excessive shear that can cause the melt temperature to be too high or the pressure to fluctuate, ensuring process stability and repeatability.

[0031] In summary, by placing a high-elasticity polyolefin elastomer in the center of a multi-layer co-extruded film as a stress buffer layer and constructing an interface gradient compatible structure, the technical problems of traditional composite films, such as difficulty in balancing heat resistance and toughness, and insufficient compression resistance, are solved. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solutions and advantages of the present application more clear and explicit, the present application will be further described in detail below with specific examples.

[0033] Example 1: Standard Heat-Resistant and Compression-Resistant Composite Film

[0034] 1. Raw material formula (weight parts)

[0035] Outer layer (high crystallinity β-modified PP layer): Borealis HE348R homopolymer PP 98.5 parts, β-nucleating agent N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide 0.3 parts, antioxidant 1010 / 168 (1:1) 1.0 part, antistatic agent 0.2 part.

[0036] First / second adhesive layer: Sumitomo Chemical HI1001 maleic anhydride grafted PP (grafting rate 1.2%) 100 parts.

[0037] First / second reinforcing layer: Borealis HE348B homopolymer PP 85 parts, organically modified nanoclay (Zhejiang Fenghong DK-4, quaternary ammonium salt modified) 5 parts, Dow INFUSE 9507 POE (octene content 45 wt%) 8 parts, antioxidant 2 parts.

[0038] Center layer (high-elasticity POE layer): Dow INFUSE 9507 POE 90 parts, Borealis HE1325 HDPE (density 0.952 g / cm³, melt index 0.8 g / 10 min) 10 parts, antioxidant 1010 / 168 (1:1) 1 part.

[0039] Inner layer: ExxonMobil Exceed 2064 mLLDPE 98 parts, anti-blocking agent 2 parts.

[0040] 2. Seven-layer thickness distribution

[0041] Total thickness 126 μm, thickness of each layer: outer layer 20 μm → adhesive layer 8 μm → reinforcing layer 15 μm → center layer 40 μm → reinforcing layer 15 μm → adhesive layer 8 μm → inner layer 20 μm. The center layer accounts for 31.7% (within the range of 25-35%).

[0042] 3. Preparation process parameters

[0043] Extrusion temperature (seven single-screw extruders):

[0044] Outer layer: barrel 210-220 °C, neck 215 °C, die 210 °C;

[0045] Adhesive layer: barrel 200-210 °C, neck 205 °C, die 200 °C;

[0046] Reinforcing layer: barrel 205-215 °C, neck 210 °C, die 205 °C;

[0047] Center layer: barrel 185-195 °C, neck 190 °C, die 195 °C (20 °C lower than the PP layer to prevent POE thermal degradation);

[0048] Inner layer: Cylinder 180-190℃, neck 185℃, die 180℃

[0049] Co-extrusion die: 7-layer co-extrusion blow die (diameter 200mm), helical element installed in the center layer channel (lead 25mm, helix angle 40°), element rotation speed 100rpm (ratio to center layer extrusion rate 1:100).

[0050] Blowing and drawing: blowing ratio 3.0:1, drawing ratio 5.0:1, drawing speed 135m / min.

[0051] Four-zone gradient cooling:

[0052] First cooling zone (corresponding to outer layer): 12℃ (fast cooling, lock β-crystalline form);

[0053] Second cooling zone (corresponding to reinforcing layer): 22℃ (medium-speed cooling, optimize nano-clay orientation);

[0054] Third cooling zone (corresponding to center layer): 40℃ (slow cooling, protect POE elastic network);

[0055] Fourth cooling zone (corresponding to inner layer): 28℃ (equilibrium cooling, ensure heat sealing performance);

[0056] Tension control: outer layer area tension 80N, center layer area tension 40N, inner layer area tension 60N, winding taper decrement rate 6N / 1000m.

[0057] 4. Product performance actual measurement data

[0058] 70℃ / 0.5MPa / 24h thickness permanent deformation rate: 2.8% (sample size 100mm×100mm, load 500N).

[0059] -40℃ impact strength: 45.2 kJ / m² (notched sample, cantilever beam method).

[0060] Interlayer peeling strength: 1.35 N / mm (room temperature), 1.22 N / mm (after 70℃ / 95%RH×24h).

[0061] β-crystalline form content: β-crystalline form in outer layer PP accounted for 47% (WAXD test)

[0062] Gradient transition zone width: 3.2μm.

[0063] 100% recycled regranulation performance: tensile strength retention rate 92%, impact strength retention rate 88% (cycle 5 times).

[0064] Transparency: haze 9.2%, light transmittance 87%.

[0065] 5. Technical effect analysis

[0066] In this embodiment, the central POE layer forms a continuous elastic network that absorbs energy by deforming in micro-zones under pressure; the 40 μm thickness ensures that the stress buffering effect is sufficient. The gradient transition zone improves the interfacial shear strength by 120%, avoiding delamination under cyclic loading. The four-zone cooling precisely matches the crystallization kinetics of each layer, with a β-crystal lock-in rate of >85%.

[0067] Example 2: High rigidity type (suitable for heavy instrument packaging).

[0068] Differences from Example 1:

[0069] 1. Formulation adjustment

[0070] Enhanced layer: The content of nano-clay is increased to 8 parts, and a double quaternary ammonium salt modified clay (Zhejiang Fenghong DK-6) is used to improve dispersibility.

[0071] Central layer: The POE / HDPE ratio is adjusted to 85:15 (POE 85 parts, HDPE 15 parts), and the HDPE melt index is reduced to 0.5 g / 10 min.

[0072] Outer layer: The β-nucleating agent is increased to 0.5 parts, increasing the β-crystal content to 55%.

[0073] 2. Thickness distribution

[0074] Total thickness 140 μm, central layer 45 μm (32.1% of the total), and each enhanced layer 18 μm.

[0075] 3. Process adjustment

[0076] Outer layer extrusion temperature increased to 215-225°C.

[0077] Third cooling zone temperature reduced to 35°C (enhanced layer cooling rate increased).

[0078] Pulling speed reduced to 120 m / min.

[0079] Tension: outer layer 85 N, central layer 35 N, inner layer 55 N.

[0080] 4. Performance data

[0081] 70°C / 0.5 MPa deformation rate: 2.5%.

[0082] -40°C impact strength: 42.0 kJ / m² (slightly decreased, but still >40 kJ / m²).

[0083] 130°C bending modulus: 1850 MPa (increased by 15% compared to Example 1).

[0084] Interlayer peel strength: 1.28 N / mm.

[0085] Transparency: Haze 11.5% (increased with nano-clay).

[0086] Compressive strength: 24h residual strain under 0.5MPa load only 2.5%, stack height up to 3m.

[0087] Example 3: High-elasticity type (suitable for low-temperature cold-chain packaging).

[0088] Differences from Example 1:

[0089] 1. Formulation adjustment

[0090] Center layer: POE / HDPE ratio adjusted to 95:5 (POE 95 parts), HDPE density reduced to 0.941 g / cm³.

[0091] Reinforced layer: Nano-clay reduced to 3 parts, POE increased to 12 parts.

[0092] Outer layer: Cancel β-nucleating agent, use α-crystal PP (HE125MO) instead, improve rigidity.

[0093] 2. Thickness distribution

[0094] Total thickness 114μm, center layer 38μm (33.3%).

[0095] 3. Process adjustment

[0096] Center layer extrusion temperature reduced to 180-190℃ (reduce POE degradation).

[0097] Third cooling zone temperature increased to 45℃ (slow down center layer cooling).

[0098] Spiral turbulence element speed reduced to 60rpm.

[0099] Tension: Outer layer 75N, center layer 35N, inner layer 55N.

[0100] 4. Performance data

[0101] 70℃ / 0.5MPa deformation rate: 3.1%.

[0102] -40℃ impact strength: 48.5 kJ / m² (significantly improved).

[0103] -60℃ ultimate impact strength: 38.2 kJ / m².

[0104] Interlayer peel strength: 1.18 N / mm.

[0105] Elongation at break: 580%.

[0106] 100 times repeated folding: no cracks (suitable for repeated bending scenarios).

[0107] Comparative Example 1: Traditional PE / PA seven-layer film (commercially available BASF medical packaging film)

[0108] Structure: PE / adhesive layer / PA / PP / PA / adhesive layer / PE.

[0109] Total thickness: 125 pm, each PA layer 15 pm.

[0110] Performance: 70°C / 0.5 MPa deformation rate: 14.5%.

[0111] -40°C impact strength: 11.5 kJ / m² (brittle fracture).

[0112] Interlayer peeling strength: 0.58 N / mm (room temperature), decreased to 0.25 N / mm after 70°C / 95%RH (severe delamination).

[0113] After 100% recycling: phase separation, tensile strength retention <40%.

[0114] Conclusion: The high rigidity of the PA layer leads to low-temperature brittleness, the large interface polarity difference leads to hot and humid delamination, and the recycling performance is poor.

[0115] Comparative Example 2: Traditional all-polyolefin film with POE placed in the inner layer

[0116] Structure: outer layer PP / adhesive layer / reinforced layer (nano-PP) / central PP layer / reinforced layer (nano-PP) / adhesive layer / POE inner layer (thickness 20 pm).

[0117] Central layer: single PP material, thickness 40 pm.

[0118] Performance: 70°C / 0.5 MPa deformation rate: 11.8% (central PP layer has no elastic buffer).

[0119] -40°C impact strength: 21.0 kJ / m² (only the inner layer POE toughens, with limited effect).

[0120] Interlayer peeling strength: 0.65 N / mm.

[0121] Conclusion: POE does not participate in structural bearing and cannot synergistically improve heat resistance and pressure resistance, and the performance improvement is not significant.

[0122] Comparative Example 3: Without four-zone gradient cooling (using conventional single-zone cooling)

[0123] Preparation condition: use the formulation of example 1, but only a single cooling water ring of 25℃ outside the bubble, no zoning.

[0124] Result: 70℃ / 0.5MPa deformation rate: 5.2% (POE layer cools too quickly, and the elastic network is not fully formed).

[0125] -40℃ impact strength: 38.0 kJ / m² (β-crystal content is only 28%, not effectively locked).

[0126] Interlayer peeling strength: 0.85 N / mm (interface does not form a gradient compatible structure).

[0127] Conclusion: single-zone cooling cannot match the crystallization kinetics of multi-layer materials, resulting in a comprehensive decline in performance.

[0128] Comparative example 4: without spiral turbulence elements (using a conventional center layer flow channel)

[0129] Preparation condition: use the formulation of example 1 and four-zone cooling, but remove the spiral turbulence elements in the die.

[0130] Result: interlayer peeling strength: 0.92 N / mm (32% lower than example 1).

[0131] Gradient transition zone width: <1μm (interface is a sudden structure).

[0132] Peeling strength after 70℃ / 95%RH aging: 0.48 N / mm (serious delamination under heat and humidity).

[0133] Conclusion: turbulence elements are the key to achieving gradient compatibility, and the interface bonding force cannot meet the durability requirements without them.

[0134] Comparative example 5: center layer thickness ratio is 15% (lower than the scope of the invention)

[0135] Preparation condition: use the formulation of example 1, the center layer thickness is reduced to 18μm (14.3% of the ratio), and each reinforced layer is thickened to 23μm.

[0136] Result: 70℃ / 0.5MPa deformation rate: 8.5% (elastic network is discontinuous, and the cushion is insufficient).

[0137] -40℃ impact strength: 32.0 kJ / m² (toughness improvement is not obvious).

[0138] Conclusion: when the center layer thickness is <25%, an effective stress cushion layer cannot be formed, and the invention effect cannot be achieved.

[0139] Summary table of example data:

[0140] Item Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Center layer thickness ratio 31.7% 32.1% 33.3% / / 31.7% 31.7% 14.3% 70°C / 0.5 MPa deformation rate 2.8% 2.5% 3.1% 14.5% 11.8% 5.2% 2.9% 8.5% -40°C impact strength 45.2 42.0 48.5 11.5 21.0 38.0 45.0 32.0 Peeling strength (N / mm) 1.35 1.28 1.18 0.58 0.65 0.85 0.92 1.30 β-crystal content 3.2 3.5 3.0 <1 <1 <1 <1 2.8 Gradient transition zone width (μm) 92% 90% 93% <40% 75% 91% 92% 91%

[0141] In summary, the above only describes the preferred embodiments of the present application, and is not intended to limit the present application. Any modification, equivalent replacement and improvement within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A heat-resistant and pressure-resistant co-extruded composite film, comprising an outer layer, a first adhesive layer, a first reinforcing layer, a central layer, a second reinforcing layer, a second adhesive layer, and an inner layer: The outer layer is a highly crystalline polypropylene layer; The first adhesive layer is a maleic anhydride-grafted polypropylene layer; The first reinforcing layer is a nanofiller-modified polypropylene layer; The central layer is a highly elastic polyolefin elastomer layer; The second reinforcing layer is a nanofiller-modified polypropylene layer; The second adhesive layer is a maleic anhydride-grafted polyolefin layer; The inner layer is a polyethylene layer; Its features are: The central layer accounts for 25-35% of the total thickness of the composite film, and a gradient compatibility transition zone is formed between the central layer and the two adjacent layers.

2. The heat-resistant and pressure-resistant co-extruded composite film according to claim 1, characterized in that: The central layer comprises 85-95 parts by weight of ethylene-octene copolymer and 5-15 parts by weight of high-density polyethylene, wherein the high-density polyethylene has a density of 0.941-0.965 g / cm³ and a melt flow index of 0.5-2.0 g / 10 min.

3. The heat-resistant and pressure-resistant co-extruded composite film according to claim 1 or 2, characterized in that: The width of the gradient compatibility transition zone is 2-5 μm, and the mass ratio of polyolefin elastomer to polypropylene in the transition zone changes continuously from the central layer to the reinforcing layer, gradually transitioning from 95:5 to 5:

95.

4. The heat-resistant and pressure-resistant co-extruded composite film according to claim 1, characterized in that: The nanofillers in the first and second reinforcing layers are organically modified nanoclays, and the modifiers are quaternary ammonium salt compounds. The content of the nanofillers in the reinforcing layers is 3-8 parts by weight.

5. The heat-resistant and pressure-resistant co-extruded composite film according to claim 1, characterized in that: The outer layer is β-crystalline polypropylene with a β-crystalline content of not less than 40%. The β-crystalline polypropylene is prepared by modification with a β-nucleating agent, which is N,N'-dicyclohexyl-2,6-naphthalenediamide.

6. A method for preparing a heat-resistant and pressure-resistant film, used to prepare the heat-resistant and pressure-resistant co-extruded composite film as described in any one of claims 1-5, characterized in that: The specific steps include the following: (1) The outer layer of highly crystalline polypropylene, the first adhesive layer of maleic anhydride-grafted polypropylene, the first reinforcing layer of nanofiller-modified polypropylene, the central layer of polyolefin elastomer, the second reinforcing layer of nanofiller-modified polypropylene, the second adhesive layer of maleic anhydride-grafted polyolefin, and the inner layer of polyethylene are respectively fed into seven extruders. (2) The extruder temperature for the outer polypropylene layer is set to 210-220℃, the extruder temperature for the first adhesive layer is set to 200-210℃, the extruder temperature for the first reinforcing layer is set to 205-215℃, the extruder temperature for the middle polyolefin elastomer is set to 185-195℃, the extruder temperature for the second reinforcing layer is set to 205-215℃, the extruder temperature for the second adhesive layer is set to 190-200℃, and the extruder temperature for the inner polyethylene layer is set to 180-190℃. (3) The seven layers of melt are combined through a co-extrusion die, the die temperature is maintained at 200±5℃, the blow-up ratio is 2.5-3.5:1, and the traction ratio is 4-6:1; (4) The extruded film bubble is cooled and shaped by the down-blowing water cooling method. Cooling air at a temperature of 15-25℃ is introduced into the film bubble, and a four-zone gradient cooling system is used to cool the outside of the film bubble. (5) The cooled film is pulled to the winding device by the traction roller, and the winding tension is controlled at 50-80N.

7. The method for preparing the heat-resistant and pressure-resistant membrane according to claim 6, characterized in that: The four-zone gradient cooling system includes four continuous cooling zones: the first cooling zone has a temperature of 10-15℃, the second cooling zone has a temperature of 20-25℃, the third cooling zone has a temperature of 35-45℃, and the fourth cooling zone has a temperature of 25-30℃.

8. The method for preparing the heat-resistant and pressure-resistant membrane according to claim 6, characterized in that: The co-extrusion die head has a spiral turbulence element in the central flow channel. The spiral turbulence element has a lead of 20-30 mm and a spiral angle of 30-45 degrees.

9. The method for preparing the heat-resistant and pressure-resistant membrane according to claim 6, characterized in that: The outer layer has a regional tension of 75-85N, the middle layer has a regional tension of 35-45N, the inner layer has a regional tension of 55-65N, and the traction speed is 120-150m / min.

10. The method for preparing the heat-resistant and pressure-resistant membrane according to claim 8, characterized in that: The spiral turbulence element rotates at a speed of 50-150 rpm during the extrusion process, and the ratio of the rotation speed to the extrusion rate of the central layer polyolefin elastomer is maintained at 0.8-1.2:100.