High-barrier polyether ether ketone composite material and forming method thereof

Abstract

This invention discloses a high-barrier polyether ether ketone (PEEK) composite material and its molding method. The method involves introducing active epoxy groups into the PEEK molecular chain through reactive processing, then using an amino / anhydride compound as a chain extender. The reaction between the active epoxy groups and the chain extender constructs a large number of branched structures in the PEEK. This is then melt-blended with a heat-resistant hyperbranched polymer to increase the molecular entanglement density of the PEEK, resulting in an ultra-high molecular entanglement PEEK composite material. Furthermore, the composite material undergoes compression-pause-compression cyclic hot pressing. Through cyclic compression and relaxation, the free volume voids between the amorphous molecular chains are gradually extruded, promoting molecular chain rearrangement. The ultra-high molecular entanglement network then fixes the compacted molecular structure resulting from compression, ultimately yielding a high-barrier PEEK composite material. This material shows promising application prospects in high-end fields such as aerospace fuel pipelines, hydrogen energy storage and transportation containers, and high-vacuum electronic packaging.

Description

Technical Field

[0001] This invention belongs to the field of engineering plastics processing, specifically relating to a high-barrier polyether ether ketone composite material and its molding method. Background Technology

[0002] Polyetheretherketone (PEEK) belongs to the polyaryl etherketone polymer family and is a semi-crystalline special engineering plastic with high thermal stability, excellent chemical resistance, and mechanical properties. It is widely used in electronics, biomedicine, automotive, precision machinery, aerospace, and defense industries. As modern industry develops towards high-end, precision, and multi-functional applications, higher demands are placed on the comprehensive performance of PEEK materials, especially in terms of gas and liquid barrier properties. Currently available commercially available PEEK materials are insufficient to meet the application requirements of some special scenarios.

[0003] Currently, methods such as blending with high-barrier polymers and filling with inorganic particles are commonly used to improve the barrier properties of PEEK. For example, in the preparation of sulfonated polyether ether ketone / polyethylene composite film and its use in the storage and preservation of lettuce, Modern Food Science and Technology, 2020, 36(10): 157-164, a barrier film was prepared by combining PEEK with polyethylene. Compared with pure PEEK film, the water vapor permeability coefficient of the composite film decreased by about two orders of magnitude. In Sulfonated polyether ether ketone / modified graphene containing crosslinkable groups, Journal of Northeast Normal University (Natural Science Edition), 2021, 53(01): 99-104, graphene oxide nanoparticles were introduced into the PEEK matrix through casting. A certain amount of graphene oxide can effectively improve the gas barrier properties of PEEK, and its methanol diffusion coefficient is reduced. However, due to the high rigidity of PEEK molecular chains, it has poor compatibility with most high-barrier polymers and is prone to phase separation during blending, resulting in a significant decrease in the mechanical and processing properties of the material. In addition, the high surface energy of nanoparticles makes them prone to aggregation in the PEEK matrix, which in turn makes the barrier performance less than expected and limits its practical application. Summary of the Invention

[0004] Purpose of the invention: The first purpose of the present invention is to provide a high-barrier polyether ether ketone composite material, and the second purpose of the present invention is to provide a molding method for the above-mentioned high-barrier polyether ether ketone composite material.

[0005] Technical solution: The high-barrier polyether ether ketone composite material of the present invention comprises the following components by weight: 100 parts of branched polyether ether ketone constructed from active epoxy groups and chain extenders, and 10-30 parts of heat-resistant hyperbranched polymer.

[0006] Furthermore, the heat-resistant hyperbranched polymer includes hyperbranched polyphenylene sulfide and heat-resistant aromatic hyperbranched polyester. The thermal decomposition temperature of the heat-resistant hyperbranched polymer is higher than the melting point of polyether ether ketone, which enables it to remain stable at the melt processing temperature of polyether ether ketone, thereby achieving more complete entanglement with the polyether ether ketone molecular chain during melt blending.

[0007] The molding method of the above-mentioned high-barrier polyetheretherketone composite material includes the following steps: (1) A mixed dispersion of a modifier containing active epoxy groups and polyether ether ketone resin is dispersed in an organic solvent. After degassing, the mixture is subjected to ultraviolet irradiation reaction. After the reaction, polyether ether ketone-g-modifier is obtained. (2) The polyether ether ketone-g-modifier, chain extender, catalyst and antioxidant are mixed and then kneaded to obtain branched polyether ether ketone; (3) After pulverizing the branched polyether ether ketone, branched polyether ether ketone particles are obtained, and then blended with heat-resistant hyperbranched polymer to obtain ultra-high molecular weight entangled polyether ether ketone composite material. (4) The ultra-high molecular weight entangled polyether ether ketone composite material is hot-pressed, and the resulting preform is then subjected to a cycle of compression-pause-compression hot pressing. After the process is completed, a high-barrier polyether ether ketone composite material is obtained.

[0008] Further, in step (1), the mass ratio of the modifier to the polyetheretherketone resin is 5~20:100; the organic solvent is acetone; the process parameters for the degassing treatment are: nitrogen gas is introduced for 10~15 min; the ultraviolet wavelength of the ultraviolet irradiation reaction is 254~365 nm and the ultraviolet light intensity is 1~30 mW / cm². 2 The irradiation time is 15~120 min; the modifier containing active epoxy groups is one or more of glycidyl acrylate, allyl glycidyl ether, allyl bisphenol A diglycidyl ether, 1,2-epoxy-4-vinylcyclohexane, 4-vinylbenzyl glycidyl ether, and 4-vinylbenzyl glycidyl ether.

[0009] Further, in step (2), the mass ratio of the polyether ether ketone-g-modifier, chain extender, catalyst and antioxidant is 100:2~20:0.2~3:0.1~3; the process parameters for the internal mixing are: melt mixing in an internal mixer for 10~30 min, processing temperature of 350~400 ℃, and rotation speed of 30~100 rpm.

[0010] Further, the chain extender includes an amino compound or an anhydride compound, wherein the anhydride compound is one or more of pyromellitic dianhydride, benzophenone dianhydride, biphenyl dianhydride, diphenyl ether dianhydride, hexahydrophthalic anhydride, and methyl nadic anhydride; and the amino compound is one or more of isophorone diamine, diaminodiphenyl sulfone, melamine, 1,3-cyclohexanedimethylamine, polyethyleneimine, and polyazelanoic anhydride.

[0011] Further, the catalyst is one or more of 2-ethyl-4-methylimidazole, 1-methylimidazole, 2-phenylimidazole, 2,4,6-tris(dimethylaminomethyl)phenol, benzyldimethylamine, tetrabutylammonium bromide, tetraethylammonium chloride, zinc octanoate, aluminum acetylacetonate, and tetrabutyl titanate.

[0012] Further, the antioxidant is one or more of the following: β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 2,6-di-tert-butyl-p-cresol, tris(2,4-di-tert-butylphenyl) phosphite, bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite, dilauryl thiodipropionate, and N-isopropyl-N'-phenyl-p-phenylenediamine.

[0013] Further, in step (3), the particle size of the branched polyether ether ketone particles is 2~6 mm; the process parameters for blending are: melt blending by a twin-screw extruder, the screw speed is 50~200 rpm, and the barrel temperature is 360~390 ℃.

[0014] Further, in step (4), the hot pressing process parameters are: hot pressing temperature of 360~400 ℃, pressure of 10~30 MPa, and hot pressing time of 10~20 min; the cyclic hot pressing process parameters are: hot pressing temperature of 360~400 ℃, pressure of 10~30 MPa, compression time of 15~20 s, pause time of 5~10 s, and number of cycles of 50~500 times; the thickness of the obtained preform is 3~5 mm; and the thickness of the obtained high barrier polyether ether ketone composite material is 0.3~0.8 mm.

[0015] Invention Principle: In this invention, reactive epoxy groups are introduced into the PEEK molecular chain through reactive processing. Then, an amino / anhydride compound is used as a chain extender. By utilizing the reaction between the reactive epoxy groups and the chain extender, a large number of branched structures are constructed in PEEK. This structure is then melt-blended with a heat-resistant hyperbranched polymer to increase the molecular entanglement density of PEEK, resulting in an ultra-high molecular entanglement PEEK composite material. Based on this, the composite material is subjected to compression-pause-compression cyclic hot pressing. Through cyclic compression and relaxation, the free volume voids between the molecular chains in the amorphous region are gradually squeezed out, and molecular chain rearrangement is promoted. Then, the ultra-high molecular entanglement network is used to fix the dense molecular structure brought about by compression, ultimately obtaining a high-barrier PEEK composite material.

[0016] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) By reactive processing, the molecular branching method is used to increase the molecular entanglement of PEEK and melt-blend it with heat-resistant hyperbranched polymer to obtain ultra-high molecular entanglement PEEK composite material; on the one hand, ultra-high molecular entanglement makes the molecular chains stack more tightly, which can improve the barrier performance of PEEK to a certain extent; on the other hand, molecular entanglement forms a restricted physical cross-linking network in the amorphous region, which can restrict the slippage and relaxation of molecular chains in the subsequent compression process, providing a mechanical basis for maintaining a dense molecular structure; (2) Compression-pause-compression cycle hot pressing can gradually squeeze out the free volume voids between molecular chains and promote chain rearrangement by step compression and relaxation, while avoiding material damage, thereby significantly improving the densification of the aggregated structure of PEEK; the combination of ultra-high molecular entanglement structure and compression-pause-compression cycle hot pressing can use the physical cross-linking network formed by ultra-high molecular entanglement to fix the molecular chain densification effect brought about by compression, and finally achieve a significant improvement in barrier performance. Detailed Implementation

[0017] The present invention will now be further described with reference to specific embodiments.

[0018] Example 1: The high-barrier polyether ether ketone composite material provided in this example comprises the following components by weight: 100 parts of branched polyether ether ketone constructed from active epoxy groups and chain extenders, and 10 parts of hyperbranched polyphenylene sulfide. The molding method includes the following steps (the raw material parts mentioned below are by weight): Five parts glycidyl acrylate and 100 parts PEEK resin were uniformly dispersed in acetone, and nitrogen gas was introduced into the mixture for 10 min for degassing. Subsequently, the mixture was reacted under ultraviolet light irradiation for 60 min (UV wavelength 325 nm, UV intensity 15 mW / cm²). 2After irradiation, the sample was removed and immediately rinsed repeatedly with a large amount of acetone to remove unreacted monomers and homopolymers physically adsorbed on the surface, thus obtaining PEEK-g-glycidyl acrylate. (2) Mix 100 parts of PEEK-g-glycidyl acrylate, 5 parts of pyromellitic dianhydride, 0.2 parts of 2-ethyl-4-methylimidazolium, and 0.1 parts of β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate evenly, add them to a mixer and melt-blend for 10 min at a processing temperature of 400 ℃ and a rotation speed of 30 rpm to obtain branched PEEK; (3) Use a high-speed crusher to crush branched PEEK into particles (particle size about 2 mm), mix 100 parts of branched PEEK particles and 10 parts of hyperbranched polyphenylene sulfide evenly, and use a twin-screw extruder for melt blending, with a screw speed of 50 rpm and a barrel temperature of 390 ℃, to obtain ultra-high molecular weight entangled PEEK composite material. (4) The ultra-high molecular weight entangled PEEK composite material was pressed into a blank (thickness of 3 mm) by hot pressing. The hot pressing temperature was 400 ℃, the pressure was 10 MPa, and the hot pressing time was 10 min. Then, the blank was subjected to compression-pause-compression cycle hot pressing. The hot pressing temperature was 400 ℃, the pressure was 10 MPa, the compression time was 15 s, the pause time was 5 s, and after 80 compression-pause-compression cycles, a high barrier PEEK composite material (thickness of 0.8 mm) was obtained.

[0019] Example 2: The high-barrier polyether ether ketone composite material provided in this example comprises the following components by weight: 100 parts of branched polyether ether ketone constructed from active epoxy groups and chain extenders, and 15 parts of hyperbranched polyphenylene sulfide. The molding method includes the following steps (the raw material parts mentioned below are by weight): (1) 10 parts of allyl glycidyl ether and 100 parts of PEEK resin were uniformly dispersed in acetone, and nitrogen gas was introduced into the mixed dispersion for 12 min for degassing treatment; then, the mixture was reacted under ultraviolet light irradiation for 90 min (ultraviolet light wavelength of 365 nm and ultraviolet light intensity of 25 mW / cm). 2 After irradiation, the sample was removed and immediately rinsed repeatedly with a large amount of acetone to remove unreacted monomers and homopolymers physically adsorbed on the surface, thus obtaining PEEK-g-propyl glycidyl ether. (2) Mix 100 parts of PEEK-g-propyl glycidyl ether, 10 parts of isophorone diamine, 0.5 parts of 1-methylimidazole, and 0.5 parts of bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite evenly, add it to a mixer and melt-blend for 20 min at a processing temperature of 380 ℃ and a rotation speed of 45 rpm to obtain branched PEEK; (3) Use a high-speed crusher to crush branched PEEK into particles (particle size about 5 mm), mix 100 parts of branched PEEK particles and 15 parts of heat-resistant aromatic hyperbranched polyester evenly, and use a twin-screw extruder for melt blending, with a screw speed of 80 rpm and a barrel temperature of 380 ℃, to obtain ultra-high molecular weight entangled PEEK composite material. (4) The ultra-high molecular weight entangled PEEK composite material was pressed into a blank (thickness of 4 mm) by hot pressing. The hot pressing temperature was 360 ℃, the pressure was 30 MPa, and the hot pressing time was 20 min. Then, the blank was subjected to compression-pause-compression cycle hot pressing. The hot pressing temperature was 380 ℃, the pressure was 20 MPa, the compression time was 18 s, the pause time was 6 s, and after 200 compression-pause-compression cycles, a high barrier PEEK composite material (thickness of 0.6 mm) was obtained.

[0020] Example 3: The high-barrier polyether ether ketone composite material provided in this example comprises the following components by weight: 100 parts of branched polyether ether ketone constructed from active epoxy groups and chain extenders, and 25 parts of hyperbranched polyphenylene sulfide. The molding method includes the following steps (the raw material parts mentioned below are by weight): (1) 15 parts of allyl bisphenol A diglycidyl ether and 100 parts of PEEK resin were uniformly dispersed in acetone, and nitrogen gas was introduced into the mixed dispersion for 15 min for degassing treatment; then, the mixture was reacted under ultraviolet light irradiation for 45 min (UV wavelength of 254 nm and UV intensity of 10 mW / cm). 2 After irradiation, the sample was removed and immediately rinsed repeatedly with a large amount of acetone to remove unreacted monomers and homopolymers physically adsorbed on the surface, thus obtaining PEEK-g-allyl bisphenol A diglycidyl ether. (2) Mix 100 parts of PEEK-g-allylbisphenol A diglycidyl ether, 15 parts of biphenyl tetracarboxylic dianhydride, 0.6 parts of tetrabutylammonium bromide and 1 part of 2,6-di-tert-butyl-p-cresol evenly, add them to a mixer and melt-blend for 25 min at a processing temperature of 390 ℃ and a rotation speed of 60 rpm to obtain branched PEEK; (3) Use a high-speed crusher to crush branched PEEK into particles (particle size of about 4 mm), mix 100 parts of long-chain branched PEEK particles and 25 parts of hyperbranched polyphenylene sulfide evenly, and use a twin-screw extruder for melt blending, with a screw speed of 120 rpm and a barrel temperature of 390 ℃, to obtain ultra-high molecular weight entangled PEEK composite material. (4) The ultra-high molecular weight entangled PEEK composite material was pressed into a blank (thickness of 4.5 mm) by hot pressing. The hot pressing temperature was 390 ℃, the pressure was 30 MPa, and the hot pressing time was 20 min. Then, the blank was subjected to compression-pause-compression cycle hot pressing. The hot pressing temperature was 385 ℃, the pressure was 28 MPa, the compression time was 18 s, the pause time was 7 s, and after 400 compression-pause-compression cycles, a high barrier PEEK composite material (thickness of 0.4 mm) was obtained.

[0021] Comparative Example 1: Compared with Example 3, the difference of Comparative Example 1 is that in step (1), the amount of allyl bisphenol A diglycidyl ether used is 1 part, while the other components, preparation steps and parameters are the same.

[0022] Comparative Example 2: Compared with Example 3, the difference of Comparative Example 2 is that in step (2), the amount of biphenyl tetracarboxylic dianhydride used is 0.5 parts, while the other components, preparation steps and parameters are the same.

[0023] Comparative Example 3: Compared with Example 3, the difference of Comparative Example 3 is that hyperbranched polyphenylene sulfide was not added in step (3), while the other components, preparation steps and parameters were the same.

[0024] Comparative Example 4: Compared with Example 3, the difference of Comparative Example 4 is that in step (4), the number of cycles of compression-pause-compression hot pressing is 20, while the other components, preparation steps and parameters are the same.

[0025] Comparative Example 5: Compared with Example 3, the difference of Comparative Example 5 is that in step (4), the ultra-high molecular entanglement PEEK composite material sample is not subjected to compression-pause-compression cycle hot pressing molding, but only the blank thickness is compressed to 0.4 mm by pressing molding, and the other components, preparation steps and parameters are the same.

[0026] The polyetheretherketone composite sheets prepared in Examples 1-3 and Comparative Examples 1-5 were tested for water vapor and oxygen permeability according to GB / T 1037-2021. The test results are shown in Table 1.

[0027] Table 1. Water vapor and oxygen permeability of polyetheretherketone composites

[0028]

[0029] As shown in Table 1, for Examples 1-3, through molecular branching modification and compression-pause-compression cyclic hot pressing, the free volume between molecular chains in the amorphous region of the PEEK composite material was significantly reduced, and the compacted state brought about by compression was fixed by the ultra-high molecular entanglement network, greatly improving its barrier properties; the water vapor and oxygen permeability coefficients of the composite material were 1.3~2.1×10⁻⁶, respectively. -14 cm 3 .cm / (cm 2 .s.Pa) and 1.1~1.8×10 −15 cm 3 .cm / (cm 2 .s.Pa).

[0030] For Comparative Examples 1 and 2, the addition of 1 part allyl bisphenol A diglycidyl ether and 0.5 parts biphenyl tetracarboxylic dianhydride, respectively, significantly reduced the degree of molecular branching in the composite material, resulting in a lower molecular entanglement density. This made it impossible to completely fix the compacted state caused by compression, leading to a decrease in the water vapor and oxygen permeability coefficients of the composite material to 7.5–8.8 × 10⁻⁶, respectively. -14 cm 3 .cm / (cm 2 .s.Pa) and 6.7~7.3×10 −15 cm 3 .cm / (cm 2 .s.Pa).

[0031] For Comparative Example 3, the absence of branched polyphenylene sulfide resulted in a reduction in the molecular entanglement of the composite material, causing the water vapor and oxygen permeability coefficients of the PEEK composite material to decrease to 6.9 × 10⁻⁶. -14 cm 3 .cm / (cm 2 .s.Pa) and 6.2×10 −15 cm 3 .cm / (cm 2 .s.Pa).

[0032] For Comparative Example 4, the compression-pause-compression cycle hot pressing only had 20 cycles, which was insufficient to effectively reduce the free volume between molecular chains in the amorphous region. The water vapor and oxygen permeability coefficients of the composite material were only 5.3 × 10⁻⁶. -13 cm 3 .cm / (cm2.s.Pa) and 4.2×10 −14 cm 3 .cm / (cm 2 .s.Pa).

[0033] For Comparative Example 5, without subjecting the ultra-high molecular weight entangled PEEK composite material sample to compression-pause-compression cycle hot pressing, and only compressing the preform thickness to 0.4 mm through pressing, the free volume between molecular chains in the amorphous region of the composite material did not decrease, resulting in a significant reduction in its water vapor and oxygen permeability coefficients to 4.3 × 10⁻⁶. -12 cm 3 .cm / (cm 2 .s.Pa) and 3.8×10 −13 cm 3 .cm / (cm 2 .s.Pa).

Claims

1. A high-barrier polyetheretherketone composite material, characterized in that, By weight, it includes the following components: 100 parts of branched polyether ether ketone constructed from active epoxy groups and chain extenders, and 10-30 parts of heat-resistant hyperbranched polymer.

2. The high-barrier polyetheretherketone composite material according to claim 1, characterized in that, The heat-resistant hyperbranched polymers include hyperbranched polyphenylene sulfide and heat-resistant aromatic hyperbranched polyester.

3. A molding method for the high-barrier polyetheretherketone composite material according to claim 1, characterized in that, Includes the following steps: (1) A mixed dispersion of a modifier containing active epoxy groups and polyether ether ketone resin is dispersed in an organic solvent. After degassing, the mixture is subjected to ultraviolet irradiation reaction. After the reaction, polyether ether ketone-g-modifier is obtained. (2) The polyether ether ketone-g-modifier, chain extender, catalyst and antioxidant are mixed and then kneaded to obtain branched polyether ether ketone; (3) After pulverizing the branched polyether ether ketone, branched polyether ether ketone particles are obtained, and then blended with heat-resistant hyperbranched polymer to obtain ultra-high molecular weight entangled polyether ether ketone composite material. (4) The ultra-high molecular weight entangled polyether ether ketone composite material is hot-pressed, and the resulting preform is then subjected to a cycle of compression-pause-compression hot pressing. After the process is completed, a high-barrier polyether ether ketone composite material is obtained.

4. The molding method according to claim 3, characterized in that, In step (1), the mass ratio of the modifier to the polyetheretherketone resin is 5~20:100; the organic solvent is acetone; the process parameters for the degassing treatment are: nitrogen gas is introduced for 10~15 min; the ultraviolet wavelength of the ultraviolet irradiation reaction is 254~365 nm and the ultraviolet light intensity is 1~30 mW / cm 2 The irradiation time is 15~120 min.

5. The molding method according to claim 3, characterized in that, In step (1), the modifier containing active epoxy groups is one or more of glycidyl acrylate, allyl glycidyl ether, allyl bisphenol A diglycidyl ether, 1,2-epoxy-4-vinylcyclohexane, 4-vinylbenzyl glycidyl ether, and 4-vinylbenzyl glycidyl ether.

6. The molding method according to claim 3, characterized in that, In step (2), the mass ratio of the polyether ether ketone-g-modifier, chain extender, catalyst and antioxidant is 100:2~20:0.2~3:0.1~3; the process parameters for the internal mixing are: melt mixing in an internal mixer for 10~30 min, processing temperature of 350~400 ℃, and rotation speed of 30~100 rpm.

7. The molding method according to claim 3, characterized in that, In step (2), the chain extender includes an amino compound or an acid anhydride compound, wherein the acid anhydride compound is one or more of pyromellitic dianhydride, benzophenone dianhydride, biphenyl dianhydride, diphenyl ether dianhydride, hexahydrophthalic anhydride, and methyl nadic anhydride; and the amino compound is one or more of isophorone diamine, diaminodiphenyl sulfone, melamine, 1,3-cyclohexanedimethylamine, polyethyleneimine, and polyazelanoic anhydride.

8. The molding method according to claim 3, characterized in that, In step (2), the catalyst is one or more of 2-ethyl-4-methylimidazolium, 1-methylimidazolium, 2-phenylimidazolium, 2,4,6-tris(dimethylaminomethyl)phenol, benzyl dimethylamine, tetrabutylammonium bromide, tetraethylammonium chloride, zinc octanoate, aluminum acetylacetonate, and tetrabutyl titanate; the antioxidant is one or more of β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 2,6-di-tert-butyl-p-cresol, tris(2,4-di-tert-butylphenyl)phosphite, bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite, dilauryl thiodipropionate, and N-isopropyl-N'-phenyl-p-phenylenediamine.

9. The molding method according to claim 3, characterized in that, In step (3), the particle size of the branched polyether ether ketone particles is 2~6 mm; the process parameters for blending are: melt blending by a twin-screw extruder, with a screw speed of 50~200 rpm and a barrel temperature of 360~390 ℃.

10. The molding method according to claim 3, characterized in that, In step (4), the hot pressing process parameters are: hot pressing temperature of 360~400 ℃, pressure of 10~30 MPa, and hot pressing time of 10~20 min; the cyclic hot pressing process parameters are: hot pressing temperature of 360~400 ℃, pressure of 10~30 MPa, compression time of 15~20 s, pause time of 5~10 s, and number of cycles of 50~500 times; the thickness of the obtained preform is 3~5 mm; and the thickness of the obtained high barrier polyether ether ketone composite material is 0.3~0.8 mm.

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