JP7585566B2 - Protective cover and method for manufacturing the same - Google Patents

Description

This disclosure relates to a protective cover and a method for manufacturing the protective cover.

Bellows are widely used in protective covers, joints, pumps, and other applications that require flexibility. In recent years, fluororesins such as polytetrafluoroethylene (PTFE) have come to be used as bellows materials because of their thermal stability, chemical resistance, non-stickiness, and lubricity.

In addition to cutting, bending molding has been proposed as a method for manufacturing such fluororesin bellows in order to improve workability and precision (see Patent Document 1).

Japanese Patent Application Publication No. 57-188320

A protective cover according to one aspect of the present disclosure is a stretchable cylindrical protective cover that is mainly composed of an ethylene-tetrafluoroethylene copolymer, that is arranged in an elongated state, and that has a bellows portion that is compressible in the axial direction, the bellows portion having alternating successive peaks and valleys in the axial direction, the average thickness H1 of the bottoms of the valleys being greater than the average thickness H2 of the tops of the peaks, and the ratio H2/H1 of the average thickness H1 of the bottoms of the valleys being 0.05 or more and 1.0 or less.

A method for manufacturing a protective cover according to another aspect of the present disclosure is a method for manufacturing a stretchable cylindrical protective cover, comprising a step of blow molding a resin composition mainly composed of an ethylene-tetrafluoroethylene copolymer into a bellows shape, the bellows portion formed after the blow molding step has peaks and valleys that are alternately continuous in the axial direction, the average thickness H1 of the bottoms of the valleys is greater than the average thickness H2 of the tops of the peaks, and the ratio H2/H1 of the average thickness H1 of the bottoms of the valleys to the average thickness H2 of the tops of the peaks is 0.05 or more and 1.0 or less.

FIG. 1 is a schematic front view showing a protective cover according to one embodiment of the present disclosure. FIG. 2 is a schematic side view illustrating a protective cover according to one embodiment of the present disclosure. FIG. 3 is a schematic cross-sectional view taken along line AA of FIG. 2, showing a protective cover according to one embodiment of the present disclosure. FIG. 4 is a schematic partial cross-sectional view showing peaks and valleys of a protective cover according to one embodiment of the present disclosure. FIG. 5 is a graph showing the relationship between the compression ratio and the maximum compressive stress in the examples. FIG. 6 is a graph showing the relationship between compression ratio and maximum compressive stress for some examples.

[Problem to be solved by the invention]
In the above-mentioned conventional technology, the bellows are sized in a contracted state, so in applications where the bellows are held in an extended state, repulsive stress is likely to occur during the holding (extension). As a result, the bellows itself may be continuously subjected to stress over a long period of use, leading to the accumulation of fatigue and elongation creep, which may cause a deterioration in performance. Furthermore, when the bellows is used as a protective cover, the repulsive stress may also act on the components housed in the protective cover, causing a load to be applied.

This disclosure was made based on these circumstances, and aims to provide a protective cover that is excellent in reducing compressive stress and can improve durability.

[Effects of the present disclosure]
The protective cover of the present disclosure has an excellent effect of reducing compressive stress and can improve durability.

[Description of the embodiments of the present disclosure]
First, the embodiments of the present disclosure will be listed and described.

A protective cover according to one aspect of the present disclosure is a stretchable cylindrical protective cover that is mainly composed of an ethylene-tetrafluoroethylene copolymer, that is arranged in an elongated state, and that has a bellows portion that is compressible in the axial direction, the bellows portion having alternating successive peaks and valleys in the axial direction, the average thickness H1 of the bottoms of the valleys being greater than the average thickness H2 of the tops of the peaks, and the ratio H2/H1 of the average thickness H1 of the bottoms of the valleys being 0.05 or more and 1.0 or less.

The protective cover is mainly composed of an ethylene-tetrafluoroethylene copolymer, and the ratio H2/H1 of the average thickness H2 of the tops of the peaks to the average thickness H1 of the bottoms of the valleys is 0.05 or more and 1.0 or less, so that the protective cover has an excellent effect of reducing compressive stress and reduces the load on the protective cover. The protective cover has excellent compressibility, so that the amount of displacement of the protective cover can be improved. Furthermore, the average thickness H1 of the bottoms of the valleys is greater than the average thickness H2 of the tops of the peaks, so that deformation of the shape during compression can be suppressed, and the elasticity can be improved. In addition, since the bellows portion is arranged in an extended state, the generation of repulsive stress is suppressed, and the effect of suppressing deterioration of performance can be improved. Therefore, even in a state where expansion and contraction are repeated, excessive load is not applied, and the durability of the protective cover can be improved.

It is preferable that the maximum compressive stress [MPa] at a compression ratio of 0.6, which is the ratio of the average compression amount [mm] to the axial length [mm] of the bellows portion, is 30 MPa or more and 70 MPa or less. When the maximum compressive stress at a ratio of the average compression amount to the axial length of the bellows portion of 0.6 is in the above range, the compressive stress reduction effect is further enhanced, and the load on the protective cover can be further reduced.

In a graph in which the compression ratio is plotted on the horizontal axis and the maximum compressive stress is plotted on the vertical axis, it is preferable that there is no inflection point where the gradient of the graph increases sharply in the range of the compression ratio of 0.55 or less. In the graph, the absence of an inflection point where the gradient of the graph increases sharply in the range of the compression ratio of 0.55 or less enhances the compressive stress reduction effect, thereby further reducing the load on the protective cover.

A method for manufacturing a protective cover according to another aspect of the present disclosure is a method for manufacturing a stretchable cylindrical protective cover, comprising a step of blow molding a resin composition mainly composed of an ethylene-tetrafluoroethylene copolymer into a bellows shape, the bellows portion formed after the blow molding step has peaks and valleys that are alternately continuous in the axial direction, the average thickness H1 of the bottoms of the valleys is greater than the average thickness H2 of the tops of the peaks, and the ratio H2/H1 of the average thickness H1 of the bottoms of the valleys to the average thickness H2 of the tops of the peaks is 0.05 or more and 1.0 or less.

The manufacturing method for the protective cover includes a step of blow molding a resin composition mainly composed of an ethylene-tetrafluoroethylene copolymer into a bellows shape, so that a protective cover in an extended state can be easily and reliably manufactured. In particular, a protective cover that can follow a protected object having multiple movements such as expansion/contraction and rotation, such as a semiconductor manufacturing device, can be integrally molded, so that the number of parts can be reduced. In addition, the average thickness H1 of the bottom of the valleys in the bellows portion formed after the blow molding step is greater than the average thickness H2 of the top of the peaks, and the ratio of the average thickness of the top of the peaks to the average thickness of the bottom of the valleys is 0.05 or more and 1 or less, so that a protective cover that has excellent compressive stress reduction effect and compressibility and can improve durability can be obtained.

In this disclosure, "main component" refers to the component with the highest content, for example, a component with a content of 50% by mass or more. "Compression amount" refers to the thickness compressed in the axial direction. "Average compression amount" refers to the average compression amount measured at 10 test pieces. "Average thickness" refers to the average thickness (wall thickness) measured at any 10 points. "Compressive stress" refers to the compressive stress per unit area, and is measured in accordance with JIS-K7181 (2011) "Plastics - Determination of compression characteristics."

[Details of the embodiment of the present disclosure]
Preferred embodiments of the present disclosure will now be described with reference to the drawings.

<Protective cover>
The protective cover is a protective cover for suppressing damage, breakage, deterioration, etc. of the elastic member to be protected, and the elastic member to be protected is housed in the protective cover. In other words, the protective cover is provided so as to cover the elastic member to be protected. FIG. 1 is a schematic front view of the protective cover, FIG. 2 is a schematic side view, and FIG. 3 is a schematic cross-sectional view along the line A-A in FIG. 2. As shown in FIGS. 1 to 3, the protective cover 1 is a cylindrical protective cover that is flexible and made of an ethylene-tetrafluoroethylene copolymer as a main component. The protective cover 1 is cylindrical and formed into a hollow shape as shown in FIG. 2. The protective cover 1 has a bellows portion 2 that is arranged in an extended state and is compressible in the axial direction. Therefore, the protective cover 1 can expand and contract in response to the movement of the elastic member to be protected. The protective cover 1 has a base portion 4 formed continuously at both ends of the bellows portion 2. Note that L in FIG. 1 indicates the length in the axial direction, that is, the length of the bellows portion 2.

In the protective cover 1, the bellows portion 2 has peaks 22 and valleys 21 that are alternately arranged in the axial direction of the protective cover 1. The bellows portion 2 is formed by axially arranging valleys 21 with smaller diameters and peaks 22 with larger diameters. FIG. 4 is a schematic partial cross-sectional view showing the peaks 22 and valleys 21 of the protective cover 1. As shown in FIG. 4, the peaks 22 have peaks 26 on their outer periphery, and the valleys 21 have bottoms 25 on their outer periphery, with the peaks 26 and bottoms 25 being continuously formed by inclined surfaces. In this disclosure, the average thickness of the bottoms 25 of the valleys 21 is represented by H1, and the average thickness of the peaks 26 of the peaks 22 is represented by H2.

By closely contacting the base 4 formed continuously at both ends of the bellows portion 2 with the elastic member to be protected, it is possible to suppress the adhesion of chemicals, solvents, dust, etc. from the outside to the elastic member to be protected, and the intrusion of foreign matter, etc., can be suppressed. The structure of the base 4 is not particularly limited, and can be designed according to the structure of the elastic member to be protected. The base 4 may also function as a fastening member for fixing to the object to be protected. Furthermore, the protective cover 1 does not need to have a base 4. Furthermore, the protective cover 1 may be used in combination with an engagement member for attaching to the elastic member to be protected.

The objects to be protected by the protective cover 1 are not particularly limited, but examples include power cylinders that are driven to expand and contract by hydraulic pressure, air pressure, water pressure, electricity, etc., valve members such as piston valves, and piston parts of cleaning devices that perform reciprocating motion. The protective cover 1 has a bellows portion 2, so it can expand and contract in response to the movements at various operating angles caused by the expansion and contraction, reciprocating, etc. of these objects to be protected.

The protective cover is a blow molded product mainly composed of ethylene-tetrafluoroethylene copolymer. The ethylene-tetrafluoroethylene copolymer (ETFE) is a fluororesin formed by polymerizing ethylene (C ₂ H ₄ ) and tetrafluoroethylene (C ₂ F ₄ ). The protective cover can be molded as a single piece and has excellent compressibility because it is mainly composed of ethylene-tetrafluoroethylene copolymer. Therefore, the protective cover has an excellent effect of reducing compressive stress and can improve durability. Furthermore, it has corrosion resistance and chemical resistance.

The lower limit of the average thickness of the protective cover 1 is 0.05 mm, preferably 0.075 mm, and more preferably 0.1 mm. On the other hand, the upper limit of the average thickness is 10 mm, preferably 5 mm, and more preferably 1 mm. When the average thickness of the protective cover 1 is equal to or greater than the lower limit, the strength of the protective cover 1 can be improved. On the other hand, when the average thickness of the protective cover 1 is equal to or less than the upper limit, the effect of reducing compressive stress is excellent.

In this protective cover 1, the average thickness H1 of the bottoms 25 of the valleys 21 is greater than the average thickness H2 of the tops 26 of the peaks 22. By making the average thickness H1 of the bottoms of the valleys greater than the average thickness H2 of the tops of the peaks, deformation of the shape during compression can be suppressed, improving stretchability.

The lower limit of the ratio H2/H1 of the average thickness H2 of the tops 26 of the peaks 22 to the average thickness H1 of the bottoms 25 of the valleys 21 is 0.05, and preferably 0.2. On the other hand, the upper limit of the ratio H2/H1 of the average thickness H2 of the tops 26 of the peaks 22 to the average thickness H1 of the bottoms 25 of the valleys 21 is 1, and preferably 0.6. If the ratio H2/H1 is less than the lower limit, there is a risk of buckling or breakage due to insufficient thickness of the peaks. On the other hand, if the ratio H2/H1 exceeds the upper limit, there is a risk of the compressive stress becoming too large, which increases the load on the protective cover.

The lower limit of the axial length L [mm] of the bellows portion is preferably 50 mm, and more preferably 100 mm. On the other hand, the upper limit of the axial length L of the bellows portion is preferably 1000 mm, and more preferably 500 mm. By having the axial length L in the above range, it becomes easier to more appropriately form the valleys of the bellows portion.

When the ratio of the average compression amount [mm] to the axial length [mm] of the bellows portion is 0.6, the lower limit of the maximum compressive stress is preferably 30 MPa, and more preferably 40 MPa. On the other hand, the upper limit of the maximum compressive stress is preferably 70 MPa, and more preferably 60 MPa. If the maximum compressive stress is less than the lower limit, the strength of the protective cover 1 may be insufficient and buckling may easily occur. On the other hand, if the maximum compressive stress exceeds the upper limit, the load on the protective cover 1 may be too high, and the durability and amount of displacement may not be sufficient.

In a graph with the compression ratio on the horizontal axis and the maximum compressive stress on the vertical axis, the compression ratio range in which there is no inflection point where the gradient of the graph increases sharply is preferably 0.55 or less, and more preferably 0.65 or less. The absence of an inflection point where the gradient of the graph increases sharply in the compression ratio range increases the compressive stress reduction effect, thereby further reducing the load on the protective cover.

The lower limit of the content of the ethylene-tetrafluoroethylene copolymer in the protective cover is preferably 50% by mass, more preferably 75% by mass, even more preferably 98% by mass, and particularly preferably 100% by mass, i.e., the molded product after molding is composed only of the ethylene-tetrafluoroethylene copolymer. If the content of the ethylene-tetrafluoroethylene copolymer is less than the above lower limit, there is a risk that the protective cover will not be properly molded.

The resin component of the protective cover may contain polymerization units derived from other copolymerizable monomers than the ethylene-tetrafluoroethylene copolymer, as long as the effect of the present invention is not impaired. For example, it may contain polymerization units of copolymerizable monomers such as perfluoro(alkyl vinyl ether), hexafluoropropylene, (perfluoroalkyl)ethylene, and chlorotrifluoroethylene. The upper limit of the content of the polymerization units derived from the other copolymerizable monomers is, for example, 3 mol% relative to the total polymerization units constituting the ethylene-tetrafluoroethylene copolymer.

The protective cover may contain components other than ethylene-tetrafluoroethylene copolymer as long as the purpose of this disclosure is not impaired.

(Sliding agent)
The addition of a sliding agent reduces the coefficient of friction and improves slidability. Examples of the sliding agent include oils such as lubricating oil (machine oil) and solid lubricants. The amount of the sliding agent is preferably in the range of 1 part by mass to 10 parts by mass per 100 parts by mass of the resin.

The above-mentioned lubricating oils include paraffinic and naphthenic mineral oils such as spindle oil, refrigeration oil, dynamo oil, turbine oil, machine oil, cylinder oil, and gear oil, as well as grease, hydrocarbons, esters, polyglycols, polyphenylene ethers, silicones, and halocarbon synthetic oils.

The solid lubricant may be polytetrafluoroethylene particles, molybdenum disulfide, graphite, silicone rubber, polyethylene, etc. As for polyethylene, ultra-high molecular weight polyethylene with a molecular weight of 2 million or more and a particle diameter of about 3 μm to 40 μm is preferable.

(Reinforcement)
The addition of a reinforcing material can improve mechanical strength, creep resistance, etc. Examples of reinforcing materials include glass fillers such as glass fiber (glass fiber) and spherical glass, carbon fiber, calcium carbonate, talc, silica, alumina, aluminum hydroxide, inorganic whiskers such as basic magnesium sulfate whiskers, zinc oxide whiskers, and potassium titanate whiskers, and inorganic fillers such as montmorillonite and synthetic smectite. The blending amount of the reinforcing material is preferably 5 parts by mass to 100 parts by mass per 100 parts by mass of the resin.

Generally, short fiber fillers are the most effective inorganic fillers for increasing the elastic modulus, but chopped strand glass fiber with an average fiber length of about 1 to 3 mm is preferable. Also, because glass fiber is made of glass, it can increase the transparency of the molded product. Furthermore, if the glass fiber is surface-treated with a surface treatment agent, the affinity between the polyolefin resin and the glass fiber increases, further improving transparency.

Examples of the above surface treatment agent include silane coupling agents and titanium coupling agents that have alkyl chains containing amino, glycidyl, mercapto, vinyl, acryloxy, and methacryloxy groups. Examples of crosslinking assistants that have functional groups that react with the surface treatment agent include those that have amino, glycidyl, hydroxyl, isocyanate, carboxyl, and carbodiimide groups.

(Antioxidants)
The addition of an antioxidant can improve stability. The amount of the antioxidant is preferably 0.0005 to 0.5 parts by mass, and more preferably 0.001 to 0.1 parts by mass, per 100 parts by mass of the resin.

Examples of antioxidants include one or a combination of two or more selected from the group consisting of 2,2'-methylene-bis[6-(1-methylcyclohexyl-p-cresol)], 2,2'-methylene-bis(4-ethyl-6-tert-butylphenol), 2,2'-methylene-bis(4-methyl-6-tert-butylphenol), 4,4'-butylidenebis(3-methyl-6-tert-butylphenol) and 2,6-di-tert-butyl-p-cresol.

(Crosslinking assistant)
The crosslinking aid is blended to promote crosslinking of the resin by irradiation with ionizing radiation. The blending amount of the crosslinking aid varies depending on the type of the crosslinking aid, but is usually preferably 1 part by mass to 20 parts by mass, more preferably 2 parts by mass to 15 parts by mass, per 100 parts by mass of the resin.

Examples of crosslinking aids include oximes such as p-quinone dioxime and p,p'-dibenzoylquinone dioxime; acrylates or methacrylates such as ethylene dimethacrylate, polyethylene glycol dimethacrylate, trimethylolpropane trimethacrylate, cyclohexyl methacrylate, acrylic acid/zinc oxide mixture, allyl methacrylate, and trimethacrylic isocyanurate (hereinafter also referred to as TMIC); vinyl monomers such as divinylbenzene, vinyl toluene, and vinyl pyridine; allyl compounds such as hexamethylene diallyl nadimide, diaryl itaconate, diallyl phthalate, diallyl isophthalate, diallyl monoglycidyl isocyanurate, triallyl cyanurate, and triallyl isocyanurate (hereinafter also referred to as TAIC); and maleimide compounds such as N,N'-m-phenylene bismaleimide and N,N'-(4,4'-methylene diphenylene) dimaleimide. These crosslinking agents may be used alone or in combination.

(Polyfunctional Monomer)
A polyfunctional monomer is a monomer having a molecular weight of 1000 or less and having at least two carbon-carbon double bonds in the molecule. By using a polyfunctional monomer having a molecular weight of 1000 or less, a molded product having excellent heat resistance while maintaining transparency can be obtained, and these properties can be achieved simultaneously. In addition, by using a polyfunctional monomer having a molecular weight of 1000 or less, the monomer has a viscosity that allows easy kneading with a resin, and is often less colored by the polyfunctional monomer itself.

Examples of the polyfunctional monomers include 1,6-hexanediol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, ethylene oxide modified trimethylolpropane tri(meth)acrylate, propylene oxide modified trimethylolpropane tri(meth)acrylate, ethylene oxide modified bisphenol A di(meth)acrylate, diethylene glycol di(meth)acrylate, dipentaerythritol hexaacrylate, dipentaerythritol monohydroxypentaacrylate, caprolactone modified dipentaerythritol hexaacrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, polyethylene glycol di(meth)acrylate, tris(acryloxyethyl)isocyanurate, tris(methacryloxyethyl)isocyanurate, and the like. Among these are tris(acryloxyethyl) isocyanurate, tris(methacryloxyethyl) isocyanurate, trimethylolpropane tri(meth)acrylate, etc.

As the polyfunctional monomer, a commercially available polyfunctional monomer can also be used. However, since commercially available polyfunctional monomers may contain stabilizers and the like to an extent that affects the effects of the present disclosure, it is preferable to conduct a simple preliminary test on the effects of the present disclosure before use to confirm that the effects of the present disclosure are not affected. As the polyfunctional monomer, one containing 1000 ppm or less of stabilizer is usually used, and the smaller the amount, the more preferable it is to prevent the effects of the present disclosure from being affected.

The amount of polyfunctional monomer is preferably 0.05 parts by mass or more and 20 parts by mass or less per 100 parts by mass of resin. If it is less than 0.05 parts by mass, the efficiency of irradiation crosslinking of the resin will be low, and sufficient heat resistance and light resistance stability may not be obtained. On the other hand, if it exceeds 20 parts by mass, handling during kneading becomes difficult, and there is a risk that the additive will bleed out from the molded product. In addition, there is a risk of transparency decreasing due to self-polymerization of the additive itself.

In addition to the above components, other components that can be added without impairing the purpose of this disclosure include, for example, ultraviolet absorbers, weather stabilizers, copper damage inhibitors, flame retardants, colorants, etc.

This protective cover has an excellent effect of reducing compressive stress, and the load on the protective cover is reduced. Furthermore, because it has excellent compressibility, the amount of displacement of the protective cover can be improved. In addition, because the bellows portion is arranged in an extended state, the generation of repulsive stress is suppressed, and the effect of suppressing deterioration of performance can be improved. Therefore, even in a state where expansion and contraction are repeated, excessive load is not applied, and the durability of the protective cover can be improved.

<Method of manufacturing protective cover>
The method for producing a protective cover is a method for producing a stretchable cylindrical protective cover, and includes a step of blow molding a resin composition containing an ethylene-tetrafluoroethylene copolymer as a main component into a bellows shape. The method for producing a protective cover may further include a step of irradiating with an electron beam.

(Blow molding process)
In the blow molding step, as described above, a resin composition mainly composed of an ethylene-tetrafluoroethylene copolymer is blow molded into a bellows shape. Since the manufacturing method for the protective cover includes a blow molding step, a protective cover in an extended state can be easily and reliably manufactured. In particular, since a protective cover that can follow multiple movements such as expansion and contraction and rotation, such as a semiconductor manufacturing device that is the elastic member to be protected, can be integrally molded, the number of parts can be reduced. Furthermore, since the manufacturing method for the protective cover uses an ethylene-tetrafluoroethylene copolymer as a main component, a protective cover that is excellent in the effect of reducing compressive stress and can improve durability can be manufactured.

(Step of irradiating with electron beam)
In the electron beam irradiation step, the molded body obtained in the blow molding step as described above is irradiated with an electron beam.

The electron beam is irradiated onto the ethylene-tetrafluoroethylene copolymer that constitutes the molded body. This electron beam irradiation promotes cross-linking of the ethylene-tetrafluoroethylene copolymer, improving the heat resistance, shape memory effect, chemical resistance, creep resistance and abrasion resistance of the resulting protective cover.

The atmospheric temperature during the electron beam irradiation process can be set to above room temperature and below the melting point. This can further improve the manufacturing efficiency of the method for manufacturing the protective cover.

In addition, in the above-mentioned electron beam irradiation process, the electron beam can be irradiated in the atmosphere. Therefore, since no equipment or energy is required to adjust the atmosphere, manufacturing efficiency can be further improved.

The lower limit of the electron beam irradiation dose in the electron beam irradiation process is preferably 220 kGy, more preferably 240 kGy. On the other hand, the upper limit of the electron beam irradiation dose is not particularly limited, but is preferably 480 kGy. If the electron beam irradiation dose is less than the lower limit, the creep resistance and abrasion resistance of the resulting protective cover may not be sufficiently improved. On the other hand, if the electron beam irradiation dose exceeds the upper limit, the cost-effectiveness of the electron beam irradiation may not be sufficiently achieved.

As described above, the bellows portion formed after the blow molding process has alternating peaks and valleys in the axial direction, and the average thickness H1 of the bottoms of the valleys is greater than the average thickness H2 of the tops of the peaks. This makes it possible to suppress deformation of the shape during compression, thereby improving elasticity.

In addition, the ratio H2/H1 of the average thickness H2 of the top of the peak to the average thickness H1 of the bottom of the valley is 0.05 or more and 1 or less. Therefore, a protective cover can be obtained that has excellent compressive stress reduction effects and compressibility, and can improve durability.

This manufacturing method for the protective cover allows for easy and reliable manufacturing of the protective cover in the extended state. In addition, it is possible to integrally mold a protective cover with a complex structure that can expand and contract in response to various movements, even for objects to be protected that have multiple movements such as expansion and contraction, reciprocating motion, etc., thereby reducing the number of parts.

[Other embodiments]
The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present disclosure is not limited to the configurations of the above-described embodiments, but is indicated by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

The present disclosure will be explained in more detail below with reference to examples, but the present disclosure is not limited to these examples.

[Bellows No. 1 to No. 10]
The bellows portions No. 1 to No. 10 were blow molded from a resin composition made of ethylene-tetrafluoroethylene copolymer (ETFE) as test specimens. The bellows portions No. 1 to No. 6, No. 9, and No. 10 had 12 peaks and 11 valleys, and had an axial length of 111 mm. The bellows portion No. 7 had 12 peaks and 11 valleys, and had an axial length of 3.33 mm. The bellows portion No. 8 had 12 peaks and 11 valleys, and had an axial length of 1332 mm. The bellows portion No. 5 was blow molded from a resin composition made of polytetrafluoroethylene (PTFE) as test specimens. Table 1 shows the length L of the bellows portion, the average thickness H1 of the bottom of the valleys, the average thickness H2 of the top of the peaks, and the ratio H2/H1 of the average thickness H2 to the average thickness H1.

[evaluation]
(Maximum compressive stress at compression ratio of 0.6)
The relationship between the average compression amount and the maximum compressive stress with respect to the axial length of the obtained bellows portions No. 1 to No. 10 was obtained by simulation. Table 2 shows specific data on the relationship between the average compression amount and the maximum compressive stress with respect to the axial length of the bellows portions No. 1 to No. 10. Table 1 shows the maximum compressive stress [MPa] at a compression ratio of 0.6, which is the ratio of the average compression amount [mm] to the axial length [mm] of the bellows portions No. 1 to No. 10. FIG. 5 shows a graph showing the relationship when the compression ratios of No. 1 to No. 10 are taken on the horizontal axis and the maximum compressive stresses are taken on the vertical axis. Among the graphs shown in FIG. 5, a graph showing the relationship between the compression ratio and the maximum compressive stress in No. 1 and No. 9 is shown in FIG. 6.

(The inflection point of the graph with the compression ratio on the horizontal axis and the maximum compressive stress on the vertical axis)
The compression ratios at which the inflection points at which the gradient suddenly increases were observed were evaluated from the graph in Figure 5, which shows the relationship between the compression ratio and the maximum compressive stress in No. 1 to No. 10. The compression ratios at the inflection points in the graph in Figure 5 for No. 1 to No. 10 are shown in Table 1.

From the results of Tables 1, 2 and FIG. 5, No. 1 to No. 3, No. 7 and No. 8, which contain ETFE as the main component, have an average thickness H1 of the bottom of the valleys in the bellows portion that is greater than the average thickness H2 of the top of the peaks, and have a ratio H2/H1 of the average thickness H2 of the top of the peaks to the average thickness H1 of the bottom of the valleys that is 0.05 to 1.0, do not show a significant increase in maximum compressive stress even when the compression ratio is increased, and are excellent in the effect of reducing compressive stress and can improve elasticity. In addition, the evaluation of the maximum compressive stress at a compression ratio of 0.6 in No. 1 to No. 3, No. 7 and No. 8 was all in the range of 30 MPa to 70 MPa. Furthermore, as shown in the graph of FIG. 5, No. 1 to No. 3, No. 7 and No. For No. 8, there was no inflection point where the gradient of the graph suddenly increased in the range of compression ratios below 0.55. From this, it is believed that No. 1 to No. 3, No. 7, and No. 8 can improve durability by reducing the load on the protective cover.

On the other hand, in No. 4, No. 6 and No. 10, in which the average thickness H1 of the bottom of the valley is equal to the average thickness H2 of the top of the peak, or the average thickness H1 of the bottom of the valley is equal to the average thickness H2 of the top of the peak, and in No. 5, in which PTFE is the main component and the average thickness H1 of the bottom of the valley is equal to the average thickness H2 of the top of the peak, a significant increase in maximum compressive stress was observed as the compression ratio increased. In particular, in No. 6 and No. 10, in which the ratio H2/H1 of the average thickness H2 of the top of the peak to the average thickness H1 of the bottom of the valley is greater than 1.0, a very significant increase in maximum compressive stress was observed as the compression ratio increased. That is, as shown in the graph of FIG. 5, in No. 4 to No. 6 and No. 10, an inflection point where the gradient of the graph increases sharply appears in the range of compression ratios of 0.55 or less.
6, in No. 9, in which the ratio H2/H1 of the average thickness H2 of the tops of the peaks to the average thickness H1 of the bottoms of the valleys is less than 0.05, no increase in the maximum compressive stress was observed even when the compression ratio was increased, but buckling occurred in the valleys and peaks of the bellows portion during compression. This is considered to be because, in No. 9, a phenomenon was observed in which compression proceeded while buckling occurred, and because the load was released during buckling, the maximum compressive stress did not increase even when the compression ratio was increased, and compression proceeded at a constant maximum compressive stress.

These results show that the protective cover has excellent compressive stress reduction effects and can improve durability.

1 Protective cover 2 Bellows portion 4 Base 21 Root 22 Peak 25 Bottom 26 Top L Axial length H1 Average thickness H2 of bottom of root Average thickness H3 of top

Claims (4)

A flexible cylindrical protective cover,
The main component is an ethylene-tetrafluoroethylene copolymer,
a bellows portion disposed in a tensioned state and axially compressible;
The bellows portion has peaks and valleys that are alternately arranged in the axial direction,
the average thickness H1 of the bottom of the valley is greater than the average thickness H2 of the top of the peak,
A protective cover in which a ratio H2/H1 of an average thickness H2 of the tops of the peaks to an average thickness H1 of the bottoms of the valleys is 0.20 or greater and 0.40 or less. The protective cover according to claim 1, wherein the maximum compressive stress [MPa] until the compression ratio, which is the ratio of the average compression amount [mm] to the axial length [mm] of the bellows portion , reaches 0.6, is 30 MPa or more and 70 MPa or less. The protective cover according to claim 2, wherein in a graph in which the compression ratio is plotted on the horizontal axis and the maximum compressive stress is plotted on the vertical axis, there is no inflection point at which the gradient of the graph increases sharply in the range of the compression ratio of 0.55 or less. A method for manufacturing a stretchable tubular protective cover, comprising the steps of:
The method includes a step of blow molding a resin composition containing an ethylene-tetrafluoroethylene copolymer as a main component into a bellows shape,
The bellows portion formed after the blow molding step has peaks and valleys alternately arranged in an axial direction,
the average thickness H1 of the bottom of the valley is greater than the average thickness H2 of the top of the peak,
A method for producing a protective cover, wherein a ratio H2/H1 of an average thickness H2 of the tops of the peaks to an average thickness H1 of the bottoms of the valleys is 0.20 or greater and 0.40 or less.

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