JP3283682B2 - Method of peeling and regenerating coating film of coated plastic molded article - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a simple and effective method for peeling and regenerating a coated plastic molded article, and more particularly to a method for peeling a coated film from a plastic molded article under dry conditions. The present invention relates to a plastic recycling treatment technique in which a coating film is not mixed and a regenerated product with little deterioration in physical properties is obtained at low cost and low pollution by actively separating and removing the coating film.

[0002]

2. Description of the Related Art Conventionally, plastics have been burned in an incinerator after use, or buried in soil together with other waste. However, when the plastic is burned, the global environment is deteriorated due to the warming of the earth due to an increase in the concentration of carbon dioxide, etc., and even if buried in soil without burning, it is not decomposed by microorganisms and remains as it is. The amount of landfills was increasing year by year. In recent years, the recycling of plastics has been called out for the purpose of reusing resources and preserving the global environment such as global warming due to an increase in the concentration of carbon dioxide, and this problem has become increasingly important. However, in order to recycle and reuse used plastics, various difficult problems must be solved. One of the problems is that plastics used as final products sometimes use only a single component in the material, but the final product is often obtained by combining different materials. . If the final product thus composited is reused after use without separating dissimilar materials, the regenerated product will lose the original excellent physical properties of each plastic material itself. As a result, the required physical properties such as strength could not be obtained, or only those whose appearance and other design properties were significantly impaired could be obtained. For example, the final product obtained by coating a plastic is crushed as it is after use, pelletized again by an extruder, and molded again by a molding machine, and the regenerated product obtained is coated on the surface of the plastic. The film is not finely dispersed and remains in the regenerated product as it is. Therefore, the coating film which is not finely dispersed prevents kneading and integration in the regenerated resin and significantly reduces the physical properties of the regenerated product, so that a practical product cannot be obtained. Therefore, the following various techniques (1) to (4) can be considered as a technique for peeling and removing a coating film from a final product obtained by coating a plastic, but all of these techniques have problems in practicality.

As a method of removing a coating film from a coated plastic product, first, a method of melting a plastic in an extruder and separating a material of a coating film portion that does not melt by a screen such as a wire mesh may be considered. However, in such a method, the extrusion pressure decreases due to a remarkable increase in the extrusion pressure due to the clogging of the screen, and the production amount is remarkably reduced. In addition, the effect of removing the material of the coating film portion is poor, and a good regenerated product cannot be obtained. Therefore, in order to cover the decrease in production efficiency due to such blockage in the screen, it is conceivable to equip an automated filter exchanger or to increase the filtration area, but any method is satisfactory. It is not a technology that can be done. In order to remove a coating film from a painted plastic product, JP-A-5-93157 discloses a method in which a large amount of a strong alkali is used to hydrolyze the coating film so that the coating film is not mixed into the product. However, since separate facilities for wastewater treatment and the like are required, the treatment cost is rather high, which is not a practical technique. In order to remove the coating film from the painted plastic product, it is considered that a method of swelling the coating film with an organic solvent and then removing it by peeling is also an effective method. Since a large amount of cost is required, the cost is increased, and secondary pollution due to the use of an organic solvent is also a concern. As a method of removing a coating film from a painted plastic product, Japanese Patent Application Laid-Open No. 2-273207 proposes a method of blasting a coating film on a plastic surface using a soft abrasive, peeling the film, and regenerating the film. ing. But,
This method not only reduces the processing efficiency when the coating surface has a complicated shape, but also it is difficult to completely remove the coating film. Further, this technique is not suitable for continuously processing products having different shapes. As a method of removing a coating film from a painted plastic product, Japanese Patent Application No. 3-192431 discloses a method in which a painted plastic product is treated with water or an organic solvent such as alcohol by using a resin having a melting point equal to or less than the melting point of the resin. This is a method in which the coating material is hydrolyzed by heating to a temperature of, and then melted and kneaded to disperse the decomposed coating film in the resin. Treatment or drying step is required, processing cost increases, and the decomposed coating film is finally dispersed in the resin,
The physical properties of the recycled product are likely to decrease.

[0004]

As described above, an unsolved problem is how low-cost and high-quality plastic molded articles can be recycled from the final product of the painted plastic molded articles. Was.

[0005]

Means for Solving the Problems [Summary of the Invention] The inventors of the present invention have conducted intensive studies in view of the above-mentioned problems, and as a result, have been found to use a metal roll to apply a shear force to a coating film of a plastic molded article having the coating film formed thereon. The present invention has been completed based on the finding that the coating film can be peeled off by applying stress, and the coated plastic molded article can be regenerated. That is, the method of peeling the coating film of the coated plastic molded article of the present invention, the plastic molded article coated on part or all of the surface, without cutting or without cutting,
Under the temperature condition of less than 70 ° C., they are arranged in parallel to each other
High peripheral speed between two rolls with different peripheral speeds
Place the side where the coating film is formed on the roll and place the plastic
Through the mold, compress it, and determine the difference between the peripheral speeds of both rolls.
Accordingly , the present invention is characterized in that the coating film is peeled by applying a shear stress to the coating film portion . The method for regenerating a coating film of a coated plastic molded article according to the other invention of the present invention includes the steps of:
Without cutting or cleavage, at a temperature of less than 70 ° C., phase
Of two rolls with different peripheral speeds
In the meantime, the side where the coating film is formed on the roll with a high peripheral speed
Placing, passing through the plastic molding, compressing,
Shear stress is applied to the coating due to the difference in peripheral speed between the two rolls.
To peel off the coating, and then melt-mold again with or without blending other optional components with the peeled plastic.

[I] Detailed description of the invention [I] Method of treating coated plastic molded article (1) Raw material (a) Painted plastic molded article Used in the method of peeling off and regenerating the coating film of the coated plastic molded article of the present invention. A plastic molded article whose surface is partially or wholly coated is a plastic product that has been partially painted, regardless of the outer surface or inner surface of the molded article substrate. Means a product used for its original purpose, or a product manufactured for the purpose of use but which has become defective during manufacture or for any other reason. The above plastic products are made of thermoplastic resin by various molding methods such as injection molding, extrusion molding, blow molding, transfer molding, press molding, etc., automobile exterior parts such as automobile bumpers, wheel caps, side moldings, instrument panels. , Levers, lining and other interior parts, vacuum cleaners,
Washing machine, refrigerator, lighting equipment, molded products molded into various shapes such as electrical appliances such as audio equipment, in the present invention, regardless of the outer surface, the inner surface of these molded body base material, at least a portion thereof It is intended for all products that are painted.

(B) Molded body substrate Specific examples of the molded body base made of the thermoplastic resin include ethylene-based resins such as high-density polyethylene, medium-density polyethylene, low-density polyethylene, and linear low-density polyethylene. , Propylene homopolymers, propylene resins such as propylene-ethylene block or random copolymers, olefin resins, thermoplastic polyester resins such as polyethylene terephthalate and polybutylene terephthalate, polyamide, polycarbonate, polyphenylene ether, poly Molded using engineering resins such as oxymethylene, styrene resins such as acrylonitrile / styrene copolymer or acrylonitrile / styrene / butadiene copolymer, or composites (including alloys) of these resins But Kill.
Among these molded base materials made of thermoplastic resins, olefin resins represented by ethylene resins and propylene resins and composite materials using the same are preferable, and propylene resins are particularly preferable. Among these molded body substrates, components other than the thermoplastic resin, for example, talc,
Inorganic fillers such as glass fiber, titanium oxide and carbon, natural rubber, ethylene-propylene copolymer rubber, styrene-butadiene rubber, styrene-isoprene rubber, acrylonitrile-butadiene rubber, isoprene rubber, urethane rubber, styrene-diene block copolymer It may also contain a compounding material such as a rubber component such as a combined hydrogenated product and an organic pigment such as quinacridone and ferrocyan blue.

(C) Coating primer The coated plastic molded article used in the method for peeling off and regenerating the coating film of the coated plastic molded article of the present invention is obtained by directly applying a coating to the plastic molded article substrate without using a primer. It may be painted, but before painting the plastic molded body substrate, a primer may be applied in advance to form a primer layer and then painted. Examples of the primer include modified propylene resins such as chlorinated polypropylene resin (chlorinated PP) and maleated polypropylene (maleated PP), and styrene-ethylene-butylene-styrene polymer (SE).
BS polymer) or styrene / ethylene / propylene /
Styrene polymers (SEPS polymers) and the like can be mentioned. Among these, it is preferable to use a chlorinated polypropylene resin. These primers are generally formed with a primer layer having a thickness of 3 to 30 μm, preferably 5 to 15 μm.

[0009] paint The paint used to form the molded body base portion or painted on all painted plastic molding of the surface, generally widely used organic solvent-based paints, water-soluble resin Examples include paints, water-dispersible resin paints, and aqueous emulsion paints. Specific examples of these paints include paints in which the resin component of the paint is acrylic, epoxy, polyester, or alkyd, and the crosslinking component is urethane or melamine.
Among these paints, it is preferable to use an acrylic or polyester resin component and a melamine-urethane (isocyanate cross-linkable) paint as a crosslinking component. These paints may be of any thickness generally used, for example, having a thickness of 20 to 200 μm, preferably 30 to 100 μm, in an amount of 10% by weight or less, preferably 7% by weight or less based on the molded substrate. It was used.

(2) Appropriate shape In order to peel off the coating film of the coated plastic molded body, the coating film can be peeled off without cutting the coated plastic molded body. It is necessary to apply a stress equal to or greater than the adhesion strength to the coating film, and when applying a stress to the coating film with a stress generator of a roll described below, in order to efficiently transmit the stress and perform the peeling more sufficiently, By cutting in advance, the shape of the coated plastic molded article is determined to have a coating film surface area of generally 0.25 cm ² or more, preferably 1 cm ² or more, particularly preferably 100 to 300 cm ^2.
It should be of the size of. In a coated plastic molded article having a size less than the above range, even if a stress is applied to the coated plastic molded article, it is difficult to apply a stress greater than the adhesion strength to the coating film, or the peeling cannot be sufficiently performed,
Unable to fix painted plastic moldings and coatings sufficiently,
Since it is difficult to apply a stress greater than the adhesion strength to the coating film, the coating cannot be sufficiently peeled off. In the case of a large-sized product, only a portion that is smooth and close to a flat surface may be cut, and the cut portion may be selectively used to be used in the method for peeling a coating film of a coated plastic molded article of the present invention. As a device for forming the above-mentioned coated plastic molded body into an optimal shape when applying a stress, a crusher, a shredder, a shear cutter, a saw, a water jet, or the like is used.

(3) Heat treatment In the method of peeling a coating film of a coated plastic molded article of the present invention, heat treatment is not necessarily required. The heat treatment uses an electric hot air circulating drier, a burner type hot air circulating drier, a high-frequency heater, or the like, and heats the surface of the coated plastic molded body at a temperature of less than 70 ° C. under dry conditions. The lower the heating temperature, the simpler the apparatus and process, and the more advantageous the coating film processing cost. Therefore, the heating temperature is set within a temperature range of less than 70 ° C.

(4) Roll Apparatus In the method for peeling a coating film of a coated plastic molded article of the present invention, in order to peel the coating film, it is necessary to apply a peeling stress equal to or more than the adhesion strength to the coating film. As a device for applying a stress to the coating film of such a coated plastic molded body, a device using a roll as shown below is used.

A roll in which two rolls are arranged in parallel to each other
Roller device Two rolls having the same diameter or different diameters are arranged in parallel to each other, and a gap is formed between the two rolls. The width of the gap is such that when passing through the coated plastic molded body, the plastic molded body is compressed, and the coating surface side of the plastic molded body is formed on the coating film portion by the speed difference between the outer peripheral portions of both rolls. It is formed to such an extent that a shear stress (striping stress due to displacement) can be applied. In particular, when the diameter of the roll is the same, a shear stress is applied to the coating film by changing the number of revolutions. However, when the diameter of the roll is different, the roll can be rotated at the same number of revolutions with the same number of revolutions. As the material of the roll, a metal material is suitable for effectively generating a shear stress in the coating film, but it is not necessarily required to be a metal. As a method of effectively generating shear stress in the coating film, the roll surface may be roughened with a matte surface, textured or engraved, and the friction coefficient may be increased, but the coating film may be peeled even with a mirror surface. is there. When the peripheral speed of the roll rotation is high, the side on which the coating film is formed is arranged, and when the peripheral speed is low, the molding substrate side is arranged so that shear stress is easily applied to the coating film. . The greater the rotation ratio or the peripheral speed ratio of the roll, the greater the shear stress applied to the coating film, and the more easily the coating film peels off. In general, the roll is rotated in the same direction as the traveling direction of the coated plastic molded body, but one of the rolls may be rotated in the opposite direction. The narrower the gap between the rolls, the higher the compressibility of the coated plastic molded article and the greater the shear stress, so that the coating film is easily peeled. However, if the compression ratio is too high, the bite into the roll becomes worse, and the pushing force must be increased, or the resin part of the plastic molded body delaminates, and the peeled coating film enters into the resin. Or get mixed in. When the roll is a mirror surface and the roll rotation ratio is 1: 4, the compression ratio on the roll is generally 40-8.
It is carried out at 0%, preferably 50-70%. This device is capable of adjusting the gap between metal rolls and individually adjusting the number of rotations of the rolls, and is provided with a roll capable of heating the roll surface. The roll need not be heated, but may be heated in order to keep the peeling conditions constant. As a heating method in that case, any heating method using electricity, steam, oil, or the like may be adopted.

The above-mentioned shear stress (peeling stress due to displacement) refers to a difference in compression ratio, a difference in rotational peripheral speed of a roll, and a difference in roughness of a roll surface due to a difference in a gap between rolls and a thickness of a coated plastic molded body. , Roll heating temperature,
Changes due to differences in roll equipment such as roll force, type of coating film, adhesion strength between coating film and plastic molded product, and differences in physical properties such as elasticity and size of painted plastic molded product, etc. Therefore, the value cannot be determined unequivocally. However, if the peeling of the coating film cannot be performed at once, it is sufficient to pass the roll again to apply shear stress. In addition, a roll device in which a plurality of pairs of the above-mentioned rolls are combined in parallel with each other,
It is also possible to form a roll device in which a plurality of roll devices are alternately combined in parallel.

[II] Regeneration of plastic molded article (1) Small-piece treatment First, the coating base material from which the coating film has been peeled off and removed is first cut into small pieces suitable for the subsequent molding step.

(2) Mixing The molded body substrate from which the coating film has been peeled off is melted again by an extruder.
Pelletized and recycled, it is also possible to mix unused plastic and other additional components. Unused plastics may be the same or different types of plastics from recycled products. Further, examples of other additional components to be blended in the reprocessed product include the following. That is, hindered phenol-based, sulfur-based, phosphorus-based antioxidants, hindered amine-based, triazole-based, benzophenone-based, benzoate-based, nickel-based,
Salicylic acid-based light stabilizers, antistatic agents, lubricants, molecular weight regulators such as peroxides, metal deactivators, organic and inorganic nucleating agents, neutralizing agents, antacids, antibacterial agents, fluorescent Examples include a whitening agent, a filler, and a flame retardant auxiliary.

(3) Forming The molded body substrate from which the coating film has been peeled is melted again by an extruder, and if necessary, pelletized and recycled. The extruder used for the pelletization is not particularly limited, and a generally used single-screw or twin-screw extruder can be used. The extruder is used to remove dust, metal, and the like that could not be removed in the coating film peeling step or the like.
It can also be removed using a wire mesh of up to 200 μm or a filter. However, re-pelletization by an extruder is not essential, and the final product can be directly formed from a regenerated product. As a molding method, various molding methods such as injection molding, extrusion molding, blow molding, transfer molding, and press molding can be adopted. The molding conditions are appropriately set according to the properties of the plastic, the shape of the molded product, and the like.

(4) Coating The recycled molded product of the molded article base material may be used as a plastic molded article which has been regenerated without coating, or may be any of the organic solvent-based paints, water-soluble resin paints, water-dispersible resin paints, and the like. A coating such as an aqueous emulsion coating may be applied to part or all of the surface of the molded article base material after or without the application of the primer to obtain a coated plastic molded article. These paints are generally used in a range of 10% by weight or less, preferably 7% by weight or less based on the base material of the molded article. In the case of applying the above-mentioned coating, similarly to the method performed in the method of peeling the coating film of the plastic molded article of the present invention, it is necessary to apply a primer before applying the coating to the plastic molded body base material Is preferred.

(5) Product The plastic molded article reclaimed as described above can be applied again as a variety of industrial products with or without coating. Specifically, automotive exterior parts such as automobile bumpers, wheel caps, side moldings, interior parts such as instrument panels, levers, and linings, and vacuum cleaners, washing machines, refrigerators, lighting equipment, audio equipment, and other electrical products And the like.

[0020]

EXAMPLES In order to describe the method of peeling a coating film of a coated plastic molded article of the present invention in more detail, specific examples will be described below with reference to examples and comparative examples. However,
The present invention is not limited to these examples.

Examples 1 to 6 and Comparative Examples 1 to 10 (1) Production of molded body base resin Melt flow rate (MF) with ethylene content of 5% by weight
R) is 30 g / 10 min, 68 wt% of a propylene / ethylene block copolymer, and the melt flow rate is 20 g /
10 minutes, 10% by weight of an ethylene polymer having a density of 0.96 g / cm ³ , and an ethylene content of 80% by weight and 100 ° C.
20% by weight of an ethylene / propylene copolymer rubber having a Mooney viscosity (ML _{1 + 4} ) of 70 measured by
A molded article base resin composed of a resin composition mixed with 2% by weight of talc of 2 μm was produced.

(2) Manufacture of Painted Plastic Products (a) Molding of Injection Molded Product The molded body substrate resin was molded at a molding temperature of 220 ° C. and an injection pressure of 600 using an N-100BII type injection molding machine manufactured by Nippon Steel Corporation.
Injection molded product 120mm × 12 under the condition of kg / cm ²
Plates of 0 mm and 2.0 and 3.0 mm thickness were formed. (b) Application of primer Next, the injection-molded article was subjected to steam washing with 1,1,1-trichloroethane, and chlorinated polypropylene resin (chlorinated P
A primer mainly composed of P) was applied in a thickness of 5 μm.

(C) Production of Painted Plastic Product Production of Painted Plastic Product A An injection molded product (120 mm × 1) treated with the primer
20 μm × 3.0 mm) One-component white paint of 40 μm in which the resin component of the paint is acrylic and the crosslinking component is melamine
And baked in an oven at a temperature of 100 ° C. for 60 minutes to produce a coated plastic product A. Production of painted plastic product B On the other hand, injection molded product (1
(20 mm x 120 mm x 3.0 mm), a two-component white paint with a resin component of urethane and a cross-linking component of urethane with a thickness of 40 µm was applied and baked in an oven at a temperature of 90 ° C for 30 minutes. To produce a painted plastic product B. Production of Painted Plastic Product C The injection molded product (120 mm × 120 mm × 3.0 m)
m), a plasma treatment (apparatus: microwave plasma processing apparatus (TMZ-9602B, manufactured by Toshiba Corporation)) by the method described in JP-A-59-98140, conditions: oxygen gas flow rate 400
cc / min, pressure 0.5 torr, output 400 W, processing time 10 seconds), in which the resin component of the paint is acrylic and contains urethane bonds (crosslinking component is melamine).
The one-pack white paint was applied to a thickness of 40 μm, and baked in an oven at a temperature of 120 ° C. for 30 minutes to produce a coated plastic product C. Production of Painted Plastic Product D The injection molded product (120 mm × 120 mm × 3.0 m)
m), a plasma treatment was performed in the same manner as for the coated plastic product C, and a two-component white paint having a urethane resin component was applied to the paint at a thickness of 40 μm. The coated plastic product D was manufactured by baking for minutes. Manufacture of painted plastic product E The same paint as painted plastic product A is applied to the injection molded product (1
(20 mm × 120 mm × 2.0 mm) to produce a coated plastic product E. Production of Painted Plastic Product F Using the same primer as described above, apply the primer layer to a thickness of about 20 μm, which is about twice the thickness of the primer layer.
Was manufactured. Production of Painted Plastic Product G Injection molded product (120 m
m × 120 mm × 3.0 mm) is coated with a one-part silver metallic paint having a resin component of acrylic and a melamine cross-linking component with a thickness of 40 μm and then baked in an oven at a temperature of 100 ° C. for 30 minutes. And then
After the clear was coated with the same paint to a thickness of 20 μm, it was baked in an oven at a temperature of 100 ° C. for 60 minutes to produce a coated plastic product G.

(3) Peeling of the coating film Using the coated plastic products A, B, C, D, E, and F, the coating film was peeled by a roll method without heating or heating as described below. did. (a) Heat treatment Heating of the coated plastic products A and B is performed using an electric hot-air dryer in the range of room temperature to 70 ° C.
Heated for hours. (b) Peeling of the coating film by the roll method Next, the coating film was peeled by the roll method. The roll used in the peeling of the coating film by the roll method was made of a metal whose surface was chrome-plated, and had a diameter of 152.4 mm ^φ and a length of 380 mm, and two rolls were used. The number of rotations of the roll is 6 to 24r on the substrate side.
pm and the coating film side were varied from 6 to 60 rpm, and the rotational speed ratio was in the range of substrate side: coating side = 1: 1 to 10. The gap between the rolls was varied in the range of 0.5 to 3.0 mm. The heating was performed in a range from room temperature without heating the roll to heating at 65 ° C. The results are shown in Tables 1 to 3.

(4) Recycling The plastic product from which the coating film of Example 5 obtained by the above method was peeled was pulverized by a pulverizer, put into an extruder, and again heated, melted and kneaded. A molded product of a reclaimed product was obtained by pelletizing and injection molding, and the molded product was again painted. The mechanical strength (impact strength, rigidity, elongation, etc.) of the painted product was measured. Recycled plastic products with no physical properties could be obtained. The appearance was almost the same as that of the virgin resin.

Examples 25 to 29 and Comparative Example 14 Regarding the shape of the coated plastic product, the coated plastic sheet was cut into the size shown in Table 4 by a cutting machine, and was cut by the same apparatus as in Example 5. Except under the conditions shown in Example 4, the coating film was peeled off in the same manner as in Example 5. Table 4 shows the obtained results.

Examples 30-33 and Comparative Examples 15-16 Both chrome-plated rolls of the above-mentioned roll apparatus were replaced with rolls that had been subjected to graining, and the coated plastic product A was used.
Was peeled off. Table 5 shows the obtained results.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

[Table 3]

[0031]

[Table 4]

[0032]

[Table 5]

Evaluation criteria The evaluation criteria for peeling of the coating film in Tables 1 to 5 are as follows. A: The coating film was completely peeled off. ：: About 80 to 95% of the coating film was peeled. X: The coating film could not be peeled off.

[0034]

The method of the present invention for peeling off a coating film on a coated plastic molded article is a method of applying a stress to a coated plastic molded article by heating it dry or without heating. Since the membrane can be positively separated and removed,
Because the coating film is not mixed in the obtained recycled product,
An inexpensive and low-pollutant recycled plastic product can be obtained without deteriorating the physical properties of the recycled product.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-337941 (JP, A) JP-A-5-69443 (JP, A) JP-A 7-171832 (JP, A) (58) Field (Int. Cl. ⁷ , DB name) B29B 17/00-17/02

Claims (8)

(57) [Claims] The method according to claim 1 plastic molded body painted on a part or the whole of the surface, without cutting or cleavage, at a temperature of less than 70 ° C., it different relative to the circumferential velocity which is arranged parallel to
Between two rolls with a high peripheral speed
The side on which the plastic molding is formed
And compress it.
A method for peeling a coating film of a coated plastic molded article, wherein the coating film is peeled by applying a shear stress to the coating. 2. The method according to claim 1, wherein the peeling of the coating film is performed under a dry condition. 3. The method according to claim 1, wherein the plastic is an olefin resin. 4. The method according to claim 1, wherein the surface area of the coating of the plastic molded body when applying a stress to the coating is 0.25 cm ² or more. 5. A plastic molded body painted on a part or the whole of the surface, without cutting or cleavage, at a temperature of less than 70 ° C., it different relative to the circumferential velocity which is arranged parallel to
Between two rolls with a high peripheral speed
The side on which the plastic molding is formed
And compress it.
A method for regenerating a coated plastic molded article, which comprises applying a shear stress to the resin to peel off the coating film, and then blending the peeled plastic with or without blending any other optional component. 6. The method according to claim 5, wherein the peeling of the coating film is performed in a dry manner. 7. The method according to claim 5, wherein the plastic is an olefin resin. 8. The method according to claim 5, wherein the surface area of the coating of the plastic molded body when applying a stress to the coating is 0.25 cm ² or more.