Food packaging material and method for manufacturing the same

Abstract

To provide a food packaging material and a method for manufacturing the same.SOLUTION: This food packaging material comprises: a cast polypropylene layer; a heat-proof surface layer; and an intermediate layer laid between the heat-proof surface layer and the cast polypropylene layer. A material for the cast polypropylene layer contains a first propylene copolymer, a material for the heat-proof surface layer contains a propylene homopolymer and a petroleum resin, and a material for the intermediate layer contains a second propylene copolymer. A melting point of the heat-proof surface layer is higher than that of the cast polypropylene layer, and the melting point of the cast polypropylene layer is higher than that of the intermediate layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a food packaging material and a method for manufacturing the same, and more particularly to a food packaging material suitable for heat-seal packaging or vacuum packaging and a method for manufacturing the same. [Background technology]

[0002] Most food packaging materials currently on the market are made up of multiple materials, such as polyester layers, aluminum foil layers, and polypropylene layers. Because food packaging materials cannot be directly recycled due to the differences in materials, they tend to generate large amounts of disposable waste, raising environmental concerns.

[0003] When combining different types of materials, it is usually necessary to use adhesives. The process of applying adhesives to the layers not only complicates the manufacturing process, but also causes environmental pollution due to the solvents used in the adhesives. In the long term, such food packaging materials are likely to be unfriendly to environmental protection.

[0004] On the other hand, food packaging materials made from a single material have the advantage of being directly recyclable and reusable, but their physical properties are usually inferior to those of food packaging materials made from a composite of multiple materials.

[0005] Therefore, it is an important task for this business to manufacture food packaging materials made of a single material while maintaining good physical properties through improvements in materials and structure. Summary of the Invention [Problem to be solved by the invention]

[0006] The technical problem that the present invention aims to solve is to provide a food packaging material and a method for manufacturing the same in response to the shortcomings of the prior art. [Means for solving the problem]

[0007] To solve the above technical problems, one of the technical solutions adopted by the present invention is to provide a food packaging material. The food packaging material includes a cast polypropylene layer, a heat-resistant surface layer, and an intermediate layer disposed between the heat-resistant surface layer and the cast polypropylene layer. The material of the cast polypropylene layer includes a first propylene copolymer, the material of the heat-resistant surface layer includes a propylene homopolymer and a petroleum resin, and the material of the intermediate layer includes a second propylene copolymer. The melting point of the heat-resistant surface layer is higher than that of the cast polypropylene layer, and the melting point of the cast polypropylene layer is higher than that of the intermediate layer.

[0008] In one embodiment, the melting point of the propylene homopolymer is 160°C to 170°C.

[0009] In one embodiment, the melting point of the first propylene copolymer is 145°C to 159°C.

[0010] In one embodiment, the melting point of the second propylene copolymer is 125°C to 140°C.

[0011] In one embodiment, in the heat-resistant surface layer, when the total weight of the propylene homopolymer, petroleum resin, and inorganic filler is 100% by weight, the content of the propylene homopolymer is 87% by weight to 94% by weight, and the content of the petroleum resin is 1.5% by weight to 5% by weight.

[0012] In one embodiment, when the total weight of the first propylene copolymer is 100% by weight, the first propylene copolymer is obtained by polymerizing 90% by weight to 99.99% by weight of propylene monomer and 0.01% by weight to 10% by weight of ethylene monomer.

[0013] In one embodiment, when the total weight of the second propylene copolymer is 100% by weight, the second propylene copolymer is obtained by polymerizing 70% by weight to 85% by weight of propylene monomer and 15% by weight to 30% by weight of ethylene monomer.

[0014] In one embodiment, the petroleum resin is a hydrogenated petroleum resin having 5 or 10 carbon atoms.

[0015] In one embodiment, the petroleum resin is an aromatic copolymer-based hydrogenated petroleum resin formed by reacting a hydrogenated petroleum resin having 5 or 10 carbon atoms with an aromatic.

[0016] In one embodiment, the hydrogenated petroleum resin having 5 or 10 carbon atoms is a hydrogenated piperylene resin or a hydrogenated dicyclopentadiene resin.

[0017] In one embodiment, the thickness of the intermediate layer is 18 μm to 20 μm, the thickness of the cast polypropylene layer is 50 μm to 70 μm, and the thickness of the heat-resistant surface layer is 20 μm to 30 μm.

[0018] Another technical solution adopted by the present invention to solve the above technical problems is to provide a method for manufacturing a food packaging material. The method for manufacturing a food packaging material includes a casting process to form a cast polypropylene layer, a biaxial stretching process to form a laminate structure, and a process to manufacture the food packaging material by placing the cast polypropylene layer on the laminate structure. The laminate structure includes an intermediate layer and a heat-resistant surface layer, and the cast polypropylene layer is in contact with the intermediate layer. The material of the cast polypropylene layer includes a first propylene copolymer. The material of the intermediate layer includes a second propylene copolymer. The material of the heat-resistant surface layer includes a propylene homopolymer, a petroleum resin, and an inorganic filler. The melting point of the heat-resistant surface layer is higher than that of the cast polypropylene layer, and the melting point of the cast polypropylene layer is higher than that of the intermediate layer. [Effects of the Invention]

[0019] One of the advantageous effects of the present invention is that the food packaging material and its manufacturing method according to the present invention have the following technical features: "the material of the heat-resistant surface layer comprises propylene homopolymer, petroleum resin, and inorganic filler" and "the melting point of the heat-resistant surface layer is higher than that of the cast polypropylene layer, which in turn is higher than that of the intermediate layer." Due to these features, the food packaging material can be conveniently directly recyclable and has good physical properties. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic side view of a food packaging material according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] To better understand the features and technical contents of the present invention, please refer to the following detailed description of the present invention and the accompanying drawings, which are provided for reference and explanation only and do not limit the scope of the present invention.

[0022] The following describes "food packaging materials and manufacturing methods thereof" according to certain specific embodiments of the present invention, and those skilled in the art will be able to understand the advantages and effects of the present invention based on the content disclosed herein. The present invention can be implemented or applied in other different specific embodiments, and various modifications and changes can be made to the details herein based on different perspectives and applications without departing from the concept of the present invention. It should be noted in advance that the accompanying drawings of the present invention are for simple schematic illustrations and are not drawn to actual size. The technical content of the present invention will be described in more detail based on the following embodiments, but the disclosed content does not limit the scope of protection of the present invention. Furthermore, the term "or" used in this specification may include any one or more combinations of the relevant listed items depending on the actual situation.

[0023] In order to overcome the drawback that conventional food packaging materials cannot be directly recycled due to the complexity of the types of materials, the present invention has developed a food packaging material made of a single material.

[0024] Although the food packaging material of the present invention uses polypropylene as its main component and has the characteristics of being a single material, the food packaging material of the present invention still has significantly excellent physical properties (high tensile strength, low shrinkage rate, high heat resistance).

[0025] To achieve good physical properties, the food packaging material of the present invention has a multi-layer structure, with two layers with higher melting points sandwiching a layer with a lower melting point, a design concept that improves heat resistance while maintaining the flexibility of the food packaging material.

[0026] The two outer layers also have different melting points (the difference between their melting points may be 5°C to 25°C). In this way, the food packaging material is used as a heat-sealing material by performing a heat-sealing process at an appropriate temperature. Therefore, the food packaging material of the present invention has a wider range of packaging applications and can replace conventional food packaging materials that cannot be directly recycled.

[0027] In order to adjust the layers to have different properties based on the same material, in the present invention, the types and compounding ratios of monomers are adjusted or other components are blended to make the layers have different melting point ranges, thereby obtaining ideal properties.

[0028] As shown in Figure 1, the food packaging material 1 of the present invention has a three-layer structure. The food packaging material 1 comprises a cast polypropylene layer 10, a heat-resistant surface layer 20, and an intermediate layer 30. The intermediate layer 30 is disposed between the cast polypropylene layer 10 and the heat-resistant surface layer 20.

[0029] In one exemplary embodiment, the cast polypropylene layer 10 is fixed onto the intermediate layer 30 via an adhesive, and the heat-resistant surface layer 20 and the intermediate layer 30 are integrally molded, but the present invention is not limited thereto. Specific manufacturing processes for the cast polypropylene layer 10, the heat-resistant surface layer 20, and the intermediate layer 30 will be described later.

[0030] The cast polypropylene layer 10 is the inner surface of the food packaging material 1 and is the main surface that comes into contact with food. In a preferred embodiment, the cast polypropylene layer 10 is formed from a single component and does not contain any other materials or additives. In some applications, the cast polypropylene layer 10 also serves as a heat-sealing layer. That is, the melting point of the cast polypropylene layer 10 is lower than the melting point of the heat-resistant surface layer 20, so that the heat-resistant surface layer 20 can maintain its original shape and condition when the cast polypropylene layer 10 is subjected to a heat-sealing process.

[0031] The material of the cast polypropylene layer 10 includes a first propylene copolymer. The melting point of the first propylene copolymer is 145°C to 159°C, preferably 147°C to 155°C. The melting point of the first propylene copolymer may be 148°C, 149°C, 150°C, 151°C, 152°C, 153°C, or 154°C. Experimentally measured, the melt flow index (MI) of the first propylene copolymer is 5g / 10min to 10g / 10min (measured at 230°C and 2.16kg).

[0032] The cast polypropylene layer 10 is formed by casting and is not subjected to a stretching process, so that the cast polypropylene layer 10 can have relatively excellent heat resistance and transparency.

[0033] The first propylene copolymer is obtained by copolymerizing propylene monomer and ethylene monomer. When the total weight of the first propylene copolymer is 100 wt%, the first propylene copolymer is obtained by polymerizing 90 wt% to 99.99 wt% of propylene monomer and 0.01 wt% to 10 wt% of ethylene monomer. For example, the propylene monomer content may be 92 wt%, 94 wt%, 96 wt%, or 98 wt%, and the ethylene monomer content may be 1 wt%, 2 wt%, 4 wt%, 6 wt%, 8 wt%, or 9 wt%. During the polymerization process, ethylene monomer randomly mixes with the propylene monomer, which may reduce the crystallinity and melting point of the first propylene copolymer. However, adding a small amount of ethylene monomer improves the toughness, impact resistance, oxidation (aging) resistance, and long-term heat resistance of the first propylene copolymer.

[0034] The heat-resistant surface layer 20 is the outer surface of the food packaging material 1 and is exposed to the external environment for a long period of time. The heat-resistant surface layer 20 is also the most heat-resistant layer in the food packaging material 1 and has the highest melting point. Therefore, when the food packaging material 1 is subjected to a heat-sealing process, the heat-sealing temperature must not exceed the melting point of the heat-resistant surface layer 20.

[0035] The material of the heat-resistant surface layer 20 includes a propylene homopolymer, a petroleum resin, and an inorganic filler, where the propylene homopolymer is the main component (i.e., the content is greater than 70% by weight).

[0036] Compared to propylene copolymers, propylene homopolymers have a relatively regular molecular arrangement and therefore have a relatively high melting point. Specifically, the melting point of the propylene homopolymer is 160°C to 170°C, preferably 165°C to 168°C. For example, the melting point of the first propylene copolymer may be 166°C or 167°C. Experimental measurements have shown that the melt flow index of the propylene homopolymer is 2g / 10min to 3g / 10min (measurement conditions: 230°C, 2.16kg).

[0037] The addition of the petroleum resin improves the crystallinity of the amorphous phase of the propylene homopolymer, and further improves the melting point and rigidity of the propylene homopolymer. Preferably, the petroleum resin is a hydrogenated petroleum resin (also called a hydrogenated resin). The hydrogenated petroleum resin not only imparts appropriate crystallinity to the propylene homopolymer, but also serves to thicken and reinforce the propylene homopolymer, thereby improving the mechanical and optical properties of the propylene homopolymer and imparting desirable heat resistance to the heat-resistant surface layer 20.

[0038] For example, the petroleum resin may be a hydrogenated petroleum resin having 5 or 10 carbon atoms. For example, the petroleum resin may be a hydrogenated piperylene resin or a hydrogenated dicyclopentadiene (DCPD) resin. In an embodiment, the hydrogenated petroleum resin having 5 or 10 carbon atoms may be further reacted with an aromatic to produce an aromatic copolymer-based hydrogenated petroleum resin. The aromatic copolymer-based hydrogenated petroleum resin can impart superior heat resistance to the heat-resistant surface layer 20.

[0039] The inorganic filler is added to prevent the layer from blocking during the manufacturing process, which may affect quality. The inorganic filler may be silicon dioxide, for example, silicon dioxide having a size of 2 μm to 15 μm. In one exemplary embodiment, inorganic fillers having two particle sizes may be used in combination, for example, silicon dioxide having a particle size of 1 μm to 3 μm and silicon dioxide having a particle size of 3.1 μm to 5 μm.

[0040] In one exemplary embodiment, when the total weight of the propylene homopolymer, petroleum resin, and inorganic filler is 100% by weight, the content of the propylene homopolymer is 87% to 94% by weight, the content of the petroleum resin is 1.5% to 5% by weight, and the content of the inorganic filler is 0.03% to 0.15% by weight. For example, the content of the propylene homopolymer may be 88%, 89%, 90%, 91%, 92%, or 93% by weight.

[0041] Intermediate layer 30 serves as a foldable, flexible layer in food packaging material 1, improving the convenience of food packaging material 1. Intermediate layer 30 is disposed between cast polypropylene layer 10 and heat-resistant surface layer 20, and is the layer with the lowest melting point in food packaging material 1. That is, the melting point of heat-resistant surface layer 20 is higher than that of cast polypropylene layer 10, which in turn is higher than that of intermediate layer 30.

[0042] The material of the intermediate layer 30 includes a second propylene copolymer and may optionally include an inorganic filler, where the second propylene copolymer is the major component (i.e., the content is greater than 70% by weight).

[0043] The melting point of the second propylene copolymer is 125°C to 140°C, preferably 130°C to 133°C. The melting point of the second propylene copolymer may be 131°C or 132°C. Experimental measurements have shown that the melt flow index of the first propylene copolymer is 5g / 10min to 7g / 10min (measured at 230°C and 2.16kg).

[0044] The second propylene copolymer is obtained by copolymerizing propylene monomer and ethylene monomer. When the total weight of the second propylene copolymer is 100 wt%, the second propylene copolymer is obtained by polymerizing 70 wt% to 85 wt% of propylene monomer and 15 wt% to 30 wt% of ethylene monomer. For example, the propylene monomer content may be 72 wt%, 74 wt%, 76 wt%, 78 wt%, 80 wt%, 82 wt%, or 84 wt%, and the ethylene monomer content may be 16 wt%, 18 wt%, 20 wt%, 22 wt%, 24 wt%, 26 wt%, or 28 wt%.

[0045] In short, by adjusting the type and structure of microscopic molecules, different melting points can be imparted to the polypropylene material, thereby controlling the properties of the layers. The melting point of the propylene homopolymer is higher than that of the first propylene copolymer, which in turn is higher than that of the second propylene copolymer. Therefore, the melting point of the heat-resistant surface layer 20 is higher than that of the cast polypropylene layer 10, which in turn is higher than that of the intermediate layer 30.

[0046] For ease of use and packaging functionality, the thickness of cast polypropylene layer 10 is adjusted to be greater than the thickness of heat-resistant surface layer 20, which in turn is adjusted to be greater than the thickness of intermediate layer 30. In a preferred embodiment, the thickness of the intermediate layer is 18 μm to 20 μm, the thickness of the cast polypropylene layer is 50 μm to 70 μm, and the thickness of the heat-resistant surface layer is 20 μm to 30 μm.

[0047] The method for producing a food packaging material according to the present invention comprises the following steps. First, a casting process is carried out using the first propylene copolymer to produce a cast polypropylene layer (cast polypropylene film, CPP film). In the casting process, the first propylene copolymer is melted in an extruder and then extruded through a T-shaped die. The molten first propylene copolymer is formed into a sheet on a smoothly rotating roll, and when cooled and hardened, a cast polypropylene layer is obtained.

[0048] The propylene homopolymer, petroleum resin, and inorganic filler are mixed to form a heat-resistant surface layer material. The second propylene copolymer and inorganic filler are mixed to form an intermediate layer material. Subsequently, the heat-resistant surface layer material and the intermediate layer material are each placed in the feed port of a biaxial stretching machine, and melt extruded and stretched to obtain a laminated structure. The laminated structure is formed by integrally molding the heat-resistant surface layer and the intermediate layer. That is, both the heat-resistant surface layer and the intermediate layer are biaxially oriented polypropylene film (BOPP film). In actual products, no clear interface is observed between the heat-resistant surface layer and the intermediate layer, but the two are certainly different in terms of their components.

[0049] Next, the food packaging material of the present invention can be produced by placing a cast polypropylene layer in the laminate structure and contacting the cast polypropylene layer with the intermediate layer. Because both the cast polypropylene layer and the laminate structure are molded, they can be adhered and fixed with a polyolefin adhesive, but the present invention is not limited to this.

[0050] In one exemplary embodiment, the polyolefin adhesive is prepared from polyolefin colloid particles, a modifier, a curing agent, and a mixed solvent. The polyolefin particles may be a propylene random copolymer. The modifier may be selected from the group consisting of maleic anhydride, methyl tetrahydrophthalic anhydride (MTHPA), 3,4,5,6-tetrahydrophthalic anhydride (3,4,5,6-tetrahydrophthalic anhydride), 1,2,3,6-tetrahydrophthalic anhydride (THPA), methylhexahydrophthalic anhydride (MHHPA), methyl nadic anhydride (MNA), and 2,3-naphthalenedicarboxylic anhydride. The curing agent may be a polyisocyanate-based curing agent, such as Desmodur® ultra N3300, Desmodur® ultra N3600, or a combination thereof. The mixed solvent contains a nonpolar solvent and a polar solvent, and the mass ratio of the nonpolar solvent to the polar solvent (nonpolar solvent:polar solvent) is 4:1 to 3:2. The nonpolar solvent may be methylcyclohexane, cyclohexane, n-hexane, or a combination thereof. The polar solvent may be methyl ethyl ketone, ethyl acetate, methyl isobutyl ketone, n-propyl acetate, or a combination thereof. Preferably, the nonpolar solvent is methylcyclohexane, and the polar solvent is methyl ethyl ketone, ethyl acetate, or a mixture thereof.

[0051] It is worth noting that the order in which the cast polypropylene layer and the laminate structure are formed is not limited to the above description, and the laminate structure may be formed first, followed by the cast polypropylene layer. [Example]

[0052] In order to measure the properties of the food packaging material of the present invention, food packaging materials of Example 1 and Comparative Examples 1 and 2 were manufactured according to the above steps, and the composition of the materials forming each layer is as shown in Table 1 below. The difference between the Examples and Comparative Examples is the composition of the components in the heat-resistant surface layer. Specifically, in Comparative Example 1, no rigidity improver pellets were added, and in Comparative Example 2, no rigidity improver pellets were added and propylene homopolymer was not used as the main component.

[0053] Additionally, to ensure uniform mixing of the materials, the petroleum resin (rigidity improver pellets) and inorganic filler (anti-blocking agent pellets) may be added in pellet form. Specifically, the content of hydrogenated petroleum resin (hydrogenated petroleum resin formed by the reaction of dicyclopentadiene with aromatics) in the rigidity improver pellets is 30% to 50% by weight, and the content of silicon dioxide in the anti-blocking agent pellets is 3% to 5% by weight. In the examples described below, the content of hydrogenated petroleum resin (hydrogenated petroleum resin formed by the reaction of dicyclopentadiene with aromatics) in the rigidity improver pellets is 50% by weight, and the content of silicon dioxide in the anti-blocking agent pellets is 5% by weight. In Table 1, the amounts of pellets added are shown, and the specific contents of petroleum resin or inorganic filler can be calculated.

[0054] To demonstrate the excellent properties of the food packaging material according to the present invention, the tensile strength of the food packaging material was measured according to the standard test method of ASTM D-638. After the food packaging material was left standing at temperatures of 120°C and 150°C for 3 minutes, the shrinkage rate relative to its original size was measured. Two sheets of food packaging material were heat-sealed at 180°C and 0.2 MPa for 1 second, and then peeled at a speed of 300 m / min using a tensile tester to measure the heat seal strength. The temperature at which the food packaging material began to block due to heat exposure was recorded to evaluate its heat resistance. The measurement results are shown in Table 1.

[0055] [Table 1]

[0056] As can be seen from the results in Table 1, the food packaging material of the present invention has relatively excellent tensile strength, making it possible to replace currently commercially available food packaging materials, and it also has the advantage of being directly recyclable. Furthermore, the food packaging material of the present invention has a relatively low shrinkage rate and favorable heat resistance (relatively high blocking temperature), maintaining its original size even at a temperature of 120°C and only slightly shrinking at a temperature of 150°C. Because of these excellent properties, the food packaging material of the present invention can be used in a wider range of applications, particularly in heat-seal packaging and vacuum packaging.

[0057] [Advantageous Effects of the Embodiments] One of the advantageous effects of the present invention is that the food packaging material and its manufacturing method according to the present invention have the following technical features: "the material of the heat-resistant surface layer comprises propylene homopolymer, petroleum resin, and inorganic filler" and "the melting point of the heat-resistant surface layer is higher than that of the cast polypropylene layer, which in turn is higher than that of the intermediate layer." Due to these features, the food packaging material can be conveniently directly recycled and has good physical properties.

[0058] The above disclosure is merely a preferred embodiment of the present invention, and the scope of the claims of the present invention is not limited thereto. Therefore, any equivalent technical modifications made using the specification and drawings of the present invention are included in the scope of the claims of the present invention. [Explanation of symbols]

[0059] 1...Food packaging materials 10...Cast polypropylene layer 20...Heat-resistant surface layer 30...Middle class

Claims

1. a cast polypropylene layer comprising a first propylene copolymer in its material; a heat-resistant surface layer containing propylene homopolymer and petroleum resin; a middle layer disposed between the heat-resistant surface layer and the cast polypropylene layer, the middle layer comprising a second propylene copolymer; A food packaging material, characterized in that the melting point of the heat-resistant surface layer is higher than the melting point of the cast polypropylene layer, and the melting point of the cast polypropylene layer is higher than the melting point of the intermediate layer.

2. 2. The food packaging material according to claim 1, wherein the melting point of the propylene homopolymer is 160°C to 170°C.

3. 2. The food packaging material according to claim 1, wherein the melting point of the first propylene copolymer is 145°C to 159°C.

4. 2. The food packaging material according to claim 1, wherein the melting point of the second propylene copolymer is 125°C to 140°C.

5. 2. The food packaging material according to claim 1, wherein, in the heat-resistant surface layer, when the total weight of the propylene homopolymer, the petroleum resin, and the inorganic filler is 100% by weight, the content of the propylene homopolymer is 87% by weight to 94% by weight, and the content of the petroleum resin is 1.5% by weight to 5% by weight.

6. The food packaging material according to claim 1, wherein the first propylene copolymer is obtained by polymerizing 90% by weight to 99.99% by weight of propylene monomer and 0.01% by weight to 10% by weight of ethylene monomer, assuming that the total weight of the first propylene copolymer is 100% by weight.

7. The food packaging material according to claim 1, wherein the second propylene copolymer is obtained by polymerizing 70% by weight to 85% by weight of propylene monomer and 15% by weight to 30% by weight of ethylene monomer, assuming that the total weight of the second propylene copolymer is 100% by weight.

8. 2. The food packaging material according to claim 1, wherein the petroleum resin is a hydrogenated petroleum resin having 5 or 10 carbon atoms.

9. 2. The food packaging material according to claim 1, wherein the petroleum resin is an aromatic copolymer-based hydrogenated petroleum resin formed by reacting a hydrogenated petroleum resin having 5 or 10 carbon atoms with an aromatic compound.

10. 10. The food packaging material according to claim 8, wherein the hydrogenated petroleum resin having 5 or 10 carbon atoms is a hydrogenated piperylene resin or a hydrogenated dicyclopentadiene resin.

11. 2. The food packaging material according to claim 1, wherein the thickness of the intermediate layer is 18 μm to 20 μm, the thickness of the cast polypropylene layer is 50 μm to 70 μm, and the thickness of the heat-resistant surface layer is 20 μm to 30 μm.

12. a casting step of forming a cast polypropylene layer comprising a first propylene copolymer on the material; a biaxial stretching step to form a laminate structure including an intermediate layer containing the second propylene copolymer as a material and a heat-resistant surface layer containing the propylene homopolymer and the petroleum resin as a material; and placing the cast polypropylene layer on the laminate structure to produce a food packaging material, the cast polypropylene layer is in contact with the intermediate layer; A method for manufacturing a food packaging material, characterized in that the melting point of the heat-resistant surface layer is higher than the melting point of the cast polypropylene layer, and the melting point of the cast polypropylene layer is higher than the melting point of the intermediate layer.

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