Method for producing cross-linked vinyl chloride resin un-foamed sheet and method for producing cross-linked vinyl chloride resin foamed sheet

Abstract

To provide: a method for producing a cross-linked vinyl chloride resin un-foamed sheet which enables obtaining a cross-linked vinyl chloride resin foamed sheet having a high expansion ratio and excellent surface flatness; and a method for producing a cross-linked vinyl chloride resin foamed sheet.SOLUTION: Provided is a method for producing a cross-linked vinyl chloride un-foamed resin sheet, comprising: a sheet-forming step of molding a vinyl chloride resin composition at a first temperature, the composition containing a vinyl chloride-based copolymer having hydroxyl groups, a cross-linking agent having at least two functional groups capable of reacting with the hydroxyl groups in one molecule, and a foaming agent to produce a first vinyl chloride resin un-foamed sheet having a cross-linking degree of 5% or more and less than 20%; and a heat treatment step of heat-treating the first vinyl chloride resin un-foamed sheet at a second temperature to produce a cross-linked vinyl chloride resin un-foamed sheet having a cross-linking degree of 20 to 60%. Further provided is a method for producing a cross-linked vinyl chloride resin foamed sheet by heating and foaming the cross-linked vinyl chloride resin un-foamed sheet at a temperature equal to or higher than Td to obtain a cross-linked vinyl chloride resin foamed sheet.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a vinyl chloride resin crosslinked unfoamed sheet and a method for producing a vinyl chloride resin crosslinked foam sheet. More specifically, the present invention relates to a method for producing a vinyl chloride resin crosslinked unfoamed sheet and a method for producing a vinyl chloride resin crosslinked foam sheet, which can provide a vinyl chloride resin crosslinked foam sheet with excellent surface smoothness. [Background technology]

[0002] Vinyl chloride resins are inexpensive, and molded articles thereof have excellent mechanical properties, weather resistance, oil resistance, etc.; therefore, crosslinked vinyl chloride resin foams are widely used in various fields, such as automobiles, machinery, electricity, medicine, and construction, as materials. Crosslinked vinyl chloride resin foams are typically produced by crosslinking vinyl chloride resins and then foaming them. For example, Patent Documents 1 and 2 describe a method for producing crosslinked vinyl chloride foams by irradiating a composition containing a vinyl chloride resin and a blowing agent with ionizing radiation to crosslink the composition, followed by heating and foaming. Patent Document 3 proposes a method for producing polyvinyl chloride foams, in which a vinyl chloride resin composition is obtained by emulsion copolymerizing a vinyl chloride monomer with a vinyl monomer having a hydroxyl group, a carboxyl group, an epoxy group, or an alkoxy group, and blending the resulting vinyl chloride copolymer, primarily composed of vinyl chloride, with a plasticizer, a crosslinking agent, and a blowing agent; the vinyl chloride resin composition is melted and molded in a calender or extruder at a temperature lower than the melting temperature and higher than the gelling temperature of the composition; and the molded product is then heat-treated at a temperature higher than the melting temperature to foam. Patent Document 4 describes a method for producing a polyvinyl chloride foam, in which a vinyl chloride composition comprising a vinyl chloride emulsion polymer having a hydroxyl group or a carboxyl group in the molecule, a plasticizer, a crosslinking agent, a heat stabilizer, and a blowing agent is melt-kneaded and crosslinked to obtain a molded product having a gel content of 10 to 60%, and the molded product is then heated to a temperature above the melting point to foam the molded product. Patent Document 5 proposes a method for producing a vinyl chloride resin foam, in which a vinyl chloride resin containing a chemical blowing agent and a quaternary ammonium salt as a crosslinking catalyst is foamed after crosslinking is completed or simultaneously with the progress of crosslinking. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 48-4863 [Patent Document 2] Japanese Patent Application Publication No. 57-180645 [Patent Document 4] Japanese Patent Application Publication No. 52-10370 [Patent Document 3] Japanese Patent Application Publication No. 50-110463 [Patent Document 5] Japanese Patent Publication No. 59-223732 Summary of the Invention [Problem to be solved by the invention]

[0004] However, when vinyl chloride resin crosslinked foam sheets are produced by the manufacturing methods described in Patent Documents 1 to 5, the foaming rate may be low or the surface smoothness may be poor.

[0005] In order to solve the above problems, the present invention provides a method for producing a crosslinked vinyl chloride resin unfoamed sheet, which can produce a crosslinked vinyl chloride resin foamed sheet having a high expansion ratio and excellent surface smoothness, and a method for producing a crosslinked vinyl chloride resin foamed sheet. [Means for solving the problem]

[0006] The present invention relates to a method for producing a crosslinked vinyl chloride resin unfoamed sheet, the method comprising: a sheet-forming step of molding a vinyl chloride resin composition containing a vinyl chloride copolymer having a hydroxyl group in its molecule, a crosslinking agent having at least two functional groups in each molecule that can react with a hydroxyl group, and a foaming agent at a first temperature of at least [Tg + 50°C] and at most [Td - 30°C], where Tg is the glass transition temperature of the vinyl chloride copolymer and Td is the thermal decomposition temperature of the foaming agent, to produce a first vinyl chloride resin unfoamed sheet having a crosslinking degree of 5% or more and less than 20%; and a heat-treatment step of heat-treating the first vinyl chloride resin unfoamed sheet at a second temperature higher than the first temperature and at most [Td - 20°C] to produce a crosslinked vinyl chloride resin unfoamed sheet having a crosslinking degree of 20 to 60%.

[0007] The present invention also relates to a method for producing a cross-linked vinyl chloride resin foam sheet, which comprises heating and foaming the cross-linked vinyl chloride resin unfoamed sheet obtained by the method for producing a cross-linked vinyl chloride resin unfoamed sheet at a temperature equal to or higher than Td to obtain a cross-linked vinyl chloride resin foam sheet. [Effects of the Invention]

[0008] According to the present invention, a cross-linked vinyl chloride resin foam sheet having a high expansion ratio and excellent surface smoothness can be obtained. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present inventors have conducted extensive research to solve the above-mentioned conventional problems. As a result, they have found that, when preparing a vinyl chloride resin crosslinked unfoamed sheet using a vinyl chloride resin composition (hereinafter also simply referred to as a resin composition) containing a vinyl chloride copolymer having a hydroxyl group in the molecule, a crosslinking agent having at least two functional groups in each molecule that can react with a hydroxyl group, and a foaming agent, the following steps are performed: a first vinyl chloride resin crosslinked unfoamed sheet having a low degree of crosslinking is prepared in a first sheet-forming step; and the first vinyl chloride resin crosslinked unfoamed sheet is heat-treated in a second heat-treatment step (a step before the foaming step) to prepare a vinyl chloride resin crosslinked unfoamed sheet having a degree of crosslinking adjusted to 20 to 60%. When the vinyl chloride resin crosslinked unfoamed sheet (hereinafter also simply referred to as a crosslinked unfoamed sheet) is heated and foamed, a vinyl chloride resin crosslinked foamed sheet (hereinafter also simply referred to as a crosslinked foamed sheet) having a high expansion ratio and excellent surface smoothness can be obtained. More specifically, in a vinyl chloride resin composition containing a vinyl chloride copolymer having a hydroxyl group in its molecule and a crosslinking agent having at least two functional groups per molecule that can react with hydroxyl groups, the degree of crosslinking of the vinyl chloride copolymer is usually adjusted by the processing temperature, but as the degree of crosslinking increases, the melt viscosity of the vinyl chloride resin composition increases rapidly, which can lead to a decrease in the discharge rate, uneven thickness of the sheet, deterioration of surface properties, etc., and can also cause foaming due to heat generation associated with the viscosity increase, resulting in decreased productivity and quality. In particular, when sheet molding is performed by extrusion molding or calendar molding, a decrease in the discharge rate, uneven thickness of the sheet, deterioration of surface properties, etc., can also cause foaming due to heat generation associated with the viscosity increase, resulting in decreased productivity and quality. Therefore, by producing the first vinyl chloride resin cross-linked unfoamed sheet under conditions that result in a low degree of cross-linking, productivity and quality during sheet molding can be ensured, and by heat-treating the first vinyl chloride resin cross-linked unfoamed sheet in a heat treatment step as a pre-foaming step, it is possible to adjust the degree of cross-linking to 20 to 60% without applying shear or mixing to the resin sheet, and a vinyl chloride resin cross-linked foamed sheet with a high expansion ratio and excellent surface smoothness can be obtained.

[0010] In this specification, when a numerical range is indicated with "to", the numerical range includes both end values ​​(upper and lower limits). For example, a numerical range of "A to B" includes both end values ​​A and B, and is the same range as "greater than or equal to A and less than or equal to B". Any number within that range and any range included within that range are specifically disclosed. In addition, when multiple numerical ranges are described in this specification, they are intended to include numerical ranges obtained by appropriately combining the upper and lower limits of different numerical ranges. In addition, when multiple upper and lower limits are described separately for a numerical range, they are intended to include numerical ranges obtained by appropriately combining the respective upper and lower limits.

[0011] <Method of manufacturing a vinyl chloride resin crosslinked unfoamed sheet> (Vinyl chloride resin composition) The resin composition contains a vinyl chloride copolymer having a hydroxyl group in the molecule (hereinafter also simply referred to as a vinyl chloride copolymer), a crosslinking agent having at least two functional groups in one molecule that can react with a hydroxyl group (hereinafter also simply referred to as a crosslinking agent), and a foaming agent.

[0012] The vinyl chloride copolymer is not particularly limited as long as it has a hydroxyl group in the molecule. For example, a vinyl chloride copolymer obtained by copolymerizing vinyl chloride with a hydroxyl group-containing monomer can be used.

[0013] The hydroxyl group-containing monomer is not particularly limited, but examples thereof include (meth)acrylic acid hydroxyalkyl esters. The (meth)acrylic acid hydroxyalkyl ester is not particularly limited, but examples thereof include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, and 8-hydroxyoctyl (meth)acrylate. Among these, from the viewpoint of cost, 2-hydroxyethyl (meth)acrylate and / or 2-hydroxypropyl (meth)acrylate are preferred. The hydroxyl group-containing monomers may be used alone or in combination of two or more. In this specification, (meth)acrylic acid includes one or more selected from the group consisting of methacrylic acid and acrylic acid, and (meth)acrylate includes one or more selected from the group consisting of methacrylate and acrylate.

[0014] For example, from the viewpoint of good processability, the vinyl chloride copolymer may contain 80 to 99.5 wt% vinyl chloride units and 0.5 to 10 wt% hydroxyl-containing monomer units; 85 to 99.2 wt% vinyl chloride units and 0.8 to 9 wt% hydroxyl-containing monomer units; 88 to 99 wt% vinyl chloride units and 1.0 to 7.5 wt% hydroxyl-containing monomer units; or 90 to 98.5 wt% vinyl chloride units and 1.5 to 6.5 wt% hydroxyl-containing monomer units. In this specification, the term "monomer units" in a vinyl chloride copolymer refers to repeating units derived from a monomer. The content of monomer units, such as hydroxyl-containing monomer units, in a vinyl chloride copolymer can be measured by Fourier transform infrared spectroscopy (FT / IR), specifically as described in the Examples.

[0015] The vinyl chloride copolymer may be copolymerized with vinyl chloride and a hydroxyl group-containing monomer, optionally with a third monomer. Examples of the third monomer include vinyl ester monomers such as vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, and vinyl stearate; olefin compounds such as ethylene, propylene, and butene; vinyl ether compounds having alkyl groups such as methyl vinyl ether, ethyl vinyl ether, octyl vinyl ether, and lauryl vinyl ether; vinylidene halide compounds such as vinylidene chloride; unsaturated carboxylic acids and their anhydrides such as acrylic acid, methacrylic acid, fumaric acid, maleic acid, itaconic acid, maleic anhydride, and itaconic anhydride; unsaturated carboxylic acid ester compounds such as methyl acrylate, ethyl acrylate, monomethyl maleate, dimethyl maleate, and butylbenzyl maleate; aromatic vinyl compounds such as styrene, α-methylstyrene, and divinylbenzene; unsaturated nitrile compounds such as acrylonitrile; and crosslinkable monomers such as diallyl phthalate. These may be used alone or in combination of two or more. The vinyl chloride copolymer may contain 0 to 15 wt %, 0 to 10 wt %, 0 to 5 wt %, or 0 to 3 wt % of a third monomer unit.

[0016] The K value of the vinyl chloride copolymer is not particularly limited, and may be, for example, 58 to 77, or 66 to 75. In this specification, the K value of a vinyl chloride resin such as a vinyl chloride copolymer can be measured in accordance with JIS K 7367-2:1999.

[0017] The glass transition temperature Tg of the vinyl chloride copolymer is not particularly limited, and may be, for example, 65 to 80° C. or 70 to 78° C. In this specification, the glass transition temperature Tg of a vinyl chloride resin such as a vinyl chloride copolymer can be measured in accordance with JIS K 7121:2012.

[0018] The vinyl chloride copolymer can be produced by any of known polymerization methods, such as suspension polymerization, microsuspension polymerization, emulsion polymerization, and bulk polymerization.

[0019] From the viewpoint of obtaining a flexible, highly expanded molded article, the resin composition preferably contains 10 to 80% by weight, more preferably 30 to 75% by weight, and even more preferably 50 to 70% by weight of the vinyl chloride copolymer. The resin composition may contain, as necessary, other vinyl chloride resins in addition to the vinyl chloride copolymer having a hydroxyl group in the molecule, within a range that does not impair the object of the present invention. The other vinyl chloride resins may be homopolymers of vinyl chloride or copolymers of vinyl chloride and monomers other than the above-mentioned hydroxyl group-containing monomers. The amount of the other vinyl chloride resins may be 0 to 100 parts by weight, or 0 to 50 parts by weight, per 100 parts by weight of the vinyl chloride copolymer.

[0020] The crosslinking agent is not particularly limited as long as it has at least two functional groups per molecule that can react with a hydroxyl group. Examples of functional groups that can react with a hydroxyl group include an isocyanate group, an epoxy group, an acid anhydride group, an alkoxysilyl group, a silanol group, a hydrosilyl group, and an N-methylol group. Among these, one or more selected from the group consisting of an isocyanate group, an epoxy group, an acid anhydride group, and an alkoxysilyl group are preferred, and an isocyanate group is more preferred.

[0021] The crosslinking agent is preferably an isocyanate-based crosslinking agent having at least two isocyanate groups per molecule, such as 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 1,3-xylene diisocyanate, 1,4-xylene diisocyanate, diphenylmethane-4,4'-diisocyanate, diphenylmethane-2,4'-diisocyanate, 3-methyldiphenylmethane diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane-4,4'-diisocyanate, dicyclohexylmethane-2,4'-diisocyanate, and lysine isocyanate. The isocyanate crosslinking agent may be a reaction product (urethane prepolymer or adduct) of the diisocyanate compound with a polyol, a homopolymer or copolymer of the diisocyanate compound, etc. Examples of polyols that react with the diisocyanate compound include polyfunctional alcohols such as ethylene glycol, propylene glycol, glycerin, and trimethylolpropane.

[0022] The crosslinking agent is preferably a blocked isocyanate crosslinking agent in which all or part of the terminal isocyanate groups of a compound (isocyanate crosslinking agent) having an isocyanate group at its terminal are blocked with a blocking agent. Examples of the blocking agent include oximes such as acetoxime and methyl ethyl ketoxime; active methylene compounds such as dimethyl malonate, diethyl malonate, methyl acetoacetate, ethyl acetoacetate, and acetylacetone; alcohols such as methanol, ethanol, 2-propanol, n-butanol, sec-butanol, and 2-ethyl-1-hexanol; and phenols such as phenol, cresol, and ethylphenol.

[0023] The crosslinking agent may be used alone or in combination of two or more. The amount of the crosslinking agent is not particularly limited. However, from the viewpoints of crosslinking efficiency and thermal stability of the crosslinked foam sheet, the amount is preferably 0.5 to 10 parts by weight, more preferably 1 to 8 parts by weight, and even more preferably 1.5 to 6 parts by weight, per 100 parts by weight of the vinyl chloride copolymer. The resin composition may contain up to 3 parts by weight of another crosslinking agent in addition to the crosslinking agent having at least two functional groups reactive with hydroxyl groups per molecule, within a range that does not impair the object of the present invention. When the other crosslinking agent is contained, the total amount of the crosslinking agent having at least two functional groups reactive with hydroxyl groups per molecule and the other crosslinking agent is preferably 0.5 to 10 parts by weight per 100 parts by weight of the vinyl chloride copolymer.

[0024] The foaming agent is not particularly limited, and chemical foaming agents such as organic foaming agents and inorganic foaming agents can be used. Examples of organic foaming agents include azo foaming agents, hydrazide foaming agents, and nitroso foaming agents. Examples of azo foaming agents include azodicarbonamide (ADCA), azobisisobutyronitrile (AIBN), barium azodicarboxylate, and diazoaminobenzene. Examples of hydrazide foaming agents include p,p'-oxybis(benzenesulfonylhydrazide) (OBSH), paratoluenesulfonylhydrazide (TSH), and hydrazodicarbonamide (HDCA). Examples of nitroso foaming agents include dinitrosopentamethylenetetramine (DPT). Examples of inorganic foaming agents include carbonates and nitrites. Examples of carbonates include sodium carbonate and ammonium carbonate. Examples of nitrites include ammonium nitrite. The foaming agent may be used alone or in combination of two or more. The amount of the foaming agent is not particularly limited and can be appropriately set based on the target expansion ratio. For example, from the viewpoint of foamability and cell uniformity, the amount is preferably 5 to 30 parts by weight, more preferably 8 to 25 parts by weight, and even more preferably 10 to 20 parts by weight, per 100 parts by weight of the vinyl chloride copolymer.

[0025] The resin composition may further contain a plasticizer. The plasticizer is not particularly limited, and plasticizers for vinyl chloride resins can be used as appropriate. Specific examples include phthalate ester-based plasticizers such as di-2-ethylhexyl phthalate (DOP), di-normal octyl phthalate, dibutyl phthalate, diisononyl phthalate (DINP), and butyl benzyl phthalate; phosphate ester-based plasticizers such as tricresyl phosphate and tri-2-ethylhexyl phosphate; adipate ester-based plasticizers such as di-2-ethylhexyl adipate; sebacate ester-based plasticizers such as di-2-ethylhexyl sebacate; and azelaate ester-based plasticizers such as di-2-ethylhexyl azelate. Examples of suitable plasticizers include trimellitic ester plasticizers such as tri-2-ethylhexyl trimellitate, polyester plasticizers, benzoic ester plasticizers such as di-2-ethylhexyl benzoate, diethylene glycol dibenzoate, and 2,2,4-trimethyl-1,3-pentanediol isobutyrate benzoate, citrate ester plasticizers such as acetyl tributyl citrate, glycolic ester plasticizers, chlorinated paraffin plasticizers, chlorinated fatty acid ester plasticizers, epoxy plasticizers, and texanol isobutyrate. Among these, phthalic ester plasticizers are preferred, and one or more selected from the group consisting of di-2-ethylhexyl phthalate and diisononyl phthalate are more preferred.

[0026] The plasticizer may be used alone or in combination of two or more. The amount of the plasticizer is not particularly limited, but is preferably 20 to 80 parts by weight, more preferably 30 to 70 parts by weight, and even more preferably 35 to 60 parts by weight, per 100 parts by weight of the vinyl chloride copolymer. When the amount of plasticizer is 20 parts by weight or more, cells do not coalesce and the desired expansion ratio is easily obtained, and when the amount of plasticizer is 80 parts by weight or less, the strength of the cross-linked foam molded article is improved.

[0027] The resin composition may contain a stabilizer, if necessary, within the scope of the present invention. Examples of the stabilizer include organic tin stabilizers such as dimethyltin mercapto, dibutyltin mercapto, dioctyltin mercapto, dibutyltin maleate, dioctyltin maleate, and dibutyltin laurate; lead-based stabilizers such as lead stearate, dibasic lead phosphite, and tribasic lead sulfate; zinc-based stabilizers such as calcium-zinc-based stabilizers and barium-zinc-based (Ba-Zn-based) stabilizers; epoxy-based stabilizers such as epoxidized soybean oil, epoxidized linseed oil, epoxidized tetrahydrophthalate, and epoxidized polybutadiene; and phosphate esters. The stabilizers may be used alone or in combination of two or more. The amount of stabilizer may be 0 to 20 parts by weight or 1 to 10 parts by weight per 100 parts by weight of the vinyl chloride copolymer.

[0028] The resin composition may contain a processing aid, if necessary, within the scope of the present invention. Examples of the processing aid include acrylic processing aids. Examples of the acrylic processing aid include (meth)acrylate polymers and styrene-acrylonitrile copolymers. Examples of the (meth)acrylate polymer include copolymers of (meth)acrylate with one or more copolymerization components selected from the group consisting of butyl (meth)acrylate, isobutyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, styrene, vinyl acetate, and acrylonitrile. Commercially available (meth)acrylate polymers, such as Kaneka's "Kane Ace PA20," "Kane Ace PA40," and "Kane Ace PA60," can also be used. The amount of the processing aid may be 0 to 10 parts by weight or 0.5 to 8 parts by weight per 100 parts by weight of the vinyl chloride copolymer.

[0029] The resin composition may contain a rubber component as needed within the scope of the present invention. As the rubber component, natural rubber and synthetic rubber can be used without any particular limitation. Examples of synthetic rubbers include acrylic rubbers such as butyl acrylate rubber, ethyl acrylate rubber, and octyl acrylate rubber, nitrile rubbers such as butadiene-acrylonitrile copolymers, chloroprene rubber, butadiene rubber, isoprene rubber, isobutylene rubber, styrene-butadiene rubber, methyl methacrylate-butyl acrylate block copolymers, styrene-isobutylene block copolymers, styrene-butadiene block copolymers, hydrogenated styrene-butadiene block copolymers, ethylene-propylene copolymers (EPR), hydrogenated ethylene-butadiene copolymers (EPDM), ethylene-vinyl acetate copolymers, ethylene-vinyl acetate-carbon monoxide copolymers, polyurethane, chlorosulfonated polyethylene, silicone rubbers (millable type, room-temperature vulcanization type, etc.), butyl rubber, fluororubber, olefin-based thermoplastic elastomers, styrene-based thermoplastic elastomers, urethane-based thermoplastic elastomers, polyamide-based thermoplastic elastomers, polyester-based thermoplastic elastomers, and fluorine-based thermoplastic elastomers. The rubber component may be used alone or in combination of two or more thereof. The amount of the rubber component may be 0 to 20 parts by weight, or 1 to 10 parts by weight, based on 100 parts by weight of the vinyl chloride copolymer.

[0030] The resin composition may contain other additives, such as fillers, antioxidants, reinforcing agents, ultraviolet absorbers, antistatic agents, lubricants, pigments, surface treatment agents, thixotropic agents, antifungal agents, and detergents, as needed, within the scope of the present invention. The amount of the other additives to be added may be 0 to 10 parts by weight based on 100 parts by weight of the vinyl chloride copolymer.

[0031] The resin composition can be obtained by stirring and mixing (compounding) the vinyl chloride copolymer, crosslinking agent, and blowing agent described above using a known method. For the stirring and mixing, a mixer such as a Hemmel mixer, or a kneader such as a roll compaction machine, gear pelletizer, Banbury mixer, or extruder can be used. The stirring and mixing must be performed under conditions that do not cause the blowing agent to foam. For example, when the glass transition temperature of the vinyl chloride copolymer is Tg and the thermal decomposition temperature of the blowing agent is Td, the mixture is preferably stirred and mixed at a surface temperature in the range of [Tg + 10°C] or higher and [Td - 50°C] or lower, or [Tg + 20°C] or higher and [Td - 60°C] or lower.

[0032] (Sheet forming process) In the sheet-molding process, the resin composition is molded at a first temperature of [Tg + 50°C] or higher and [Td - 30°C] or lower, where Tg is the glass transition temperature of the vinyl chloride copolymer and Td is the thermal decomposition temperature of the foaming agent, to produce a first vinyl chloride resin unfoamed sheet (also referred to as a primary raw sheet) having a crosslinking degree of 5% or higher but less than 20%. In this specification, the "first temperature" refers to the maximum surface temperature of the first vinyl chloride resin unfoamed sheet during the sheet-molding process. The first temperature is preferably in the range of [Tg + 60°C] or higher and [Td - 35°C] or lower. This ensures productivity and quality during sheet molding, and allows the crosslinking degree to be adjusted to 5% or higher but less than 20% while suppressing foaming. Furthermore, in the subsequent heat treatment process, i.e., the process prior to the foaming process, the crosslinking degree can be adjusted to 20-60% without applying shear or mixing to the sheet. In this specification, the glass transition temperature (Tg) of the vinyl chloride copolymer and the thermal decomposition temperature of the foaming agent can be measured as described in the Examples. In this specification, the degree of crosslinking (also referred to as crosslinked gel fraction) of a sheet such as a first vinyl chloride resin unfoamed sheet can be measured as described in the Examples.

[0033] The method for forming the sheet is not particularly limited, and may be any of extrusion molding using an extruder, calender molding using a calender, and press molding using a roll.

[0034] In the extrusion molding, the extruder may be a single-screw extruder or a twin-screw extruder. In the extrusion molding, from the viewpoint of easily adjusting the degree of crosslinking of the first vinyl chloride resin unfoamed sheet (extrusion-molded sheet) to 5% or more and less than 20%, the first temperature is preferably [Tg+50°C] or more and [Td-30°C] or less, more preferably [Tg+60°C] or more and [Td-35°C] or less, and even more preferably [Tg+70°C] or more and [Td-40°C] or less. The temperatures of the barrel and head of the extruder are preferably 10 to 20°C lower than the first temperature, and the temperature of the die of the extruder is preferably 20 to 30°C lower than the first temperature. From the viewpoint of easily adjusting the degree of crosslinking of the first vinyl chloride resin unfoamed sheet (extrusion-molded sheet) to 5% or more and less than 20%, the discharge rate of the vinyl chloride resin composition from the extruder is preferably 3 to 100 kg / hr, more preferably 5 to 80 kg / hr, and even more preferably 10 to 50 kg / hr.

[0035] In calender molding, from the viewpoint of easily adjusting the degree of crosslinking of the first vinyl chloride resin unfoamed sheet (calender sheet) to 5% or more and less than 20%, the first temperature is preferably [Tg + 50°C] or more and [Td - 30°C] or less, more preferably [Tg + 60°C] or more and [Td - 35°C] or less, and even more preferably [Tg + 70°C] or more and [Td - 40°C] or less. In calender molding, from the viewpoint of easily adjusting the degree of crosslinking of the first vinyl chloride resin unfoamed sheet (calender sheet) to 5% or more and less than 20%, the take-up speed of the first vinyl chloride resin unfoamed sheet (calender sheet) is preferably 0.5 to 20 m / min, more preferably 0.7 to 18 m / min, and even more preferably 1.0 to 15 m / min.

[0036] In calender molding, from the viewpoint of easily adjusting the degree of crosslinking of the first vinyl chloride resin unfoamed sheet (calender sheet) to 5% or more and less than 20%, the temperature of the kneaded vinyl chloride resin composition during pre-kneading is preferably 10 to 30° C. lower than the first temperature. For pre-kneading, for example, a kneading machine such as a roll compaction machine, a gear pelletizer, a Banbury mixer, or various extruders can be used.

[0037] The thickness of the first vinyl chloride resin unfoamed sheet is not particularly limited and may be appropriately determined depending on the application, etc., but may be, for example, 0.3 to 10 mm, 0.4 to 5 mm, or 0.5 to 3 mm.

[0038] (Heat treatment process) In the heat treatment step, the first vinyl chloride resin unfoamed sheet is heat-treated at a second temperature higher than the first temperature and equal to or lower than [Td-20°C] to obtain a vinyl chloride resin crosslinked unfoamed sheet (also referred to as a secondary raw sheet) with a crosslinking degree of 20 to 60%. The first vinyl chloride resin unfoamed sheet may be heat-treated immediately after the sheet molding step. However, if necessary, the first vinyl chloride resin unfoamed sheet obtained in the sheet molding step may be cooled to room temperature, or may be cooled to room temperature and then stored at room temperature for a predetermined period before being heat-treated. In this specification, the second temperature refers to the maximum surface temperature of the vinyl chloride resin crosslinked unfoamed sheet during the heat treatment step. The second temperature is preferably at least 5°C higher than the first temperature and equal to or lower than [Td-20°C], and more preferably at least 10°C higher than the first temperature and equal to or lower than [Td-25°C]. This makes it possible to obtain a vinyl chloride resin crosslinked unfoamed sheet having a crosslinking degree adjusted to 20 to 60% while suppressing foaming without applying shear or mixing to the first vinyl chloride resin unfoamed sheet, and by foaming the vinyl chloride resin crosslinked unfoamed sheet, a vinyl chloride resin crosslinked foamed sheet with a high expansion ratio and excellent surface smoothness can be obtained. The heat treatment time is not particularly limited as long as it can adjust the crosslinking degree of the vinyl chloride resin crosslinked unfoamed sheet to 20 to 60%, but may be, for example, 3 to 40 minutes, 5 to 30 minutes, or 7 to 20 minutes from the viewpoint of obtaining a smooth foam surface.

[0039] The heat treatment step is not particularly limited, but can be carried out using, for example, a heating device that uses dry air as a medium, such as a hot air circulation oven or a hot air heating furnace. More specifically, the first vinyl chloride resin unfoamed sheet can be placed on a metal mesh placed in the heating device. The set temperature of the heating device can be, for example, 0 to 10°C higher, or 3 to 8°C higher, than the second temperature. The mesh size of the metal mesh is not particularly limited, but can be, for example, 30 to 360 mesh or 100 to 200 mesh. The material of the metal mesh is not particularly limited, but can be, for example, stainless steel wire, annealed iron wire, or galvanized iron wire.

[0040] <Method of manufacturing a vinyl chloride resin cross-linked foam sheet> The method for producing a vinyl chloride resin crosslinked foam sheet includes a foaming step of heating and foaming the vinyl chloride resin crosslinked unfoamed sheet obtained by the method for producing a vinyl chloride resin crosslinked unfoamed sheet at a temperature equal to or higher than Td to obtain a vinyl chloride resin crosslinked foam sheet. That is, the method for producing a vinyl chloride resin crosslinked foam sheet includes a foaming step in addition to the sheet molding step and heat treatment step described above. The vinyl chloride resin material, the sheet molding step, and the heat treatment step are as described above, so redundant explanations will be omitted.

[0041] (Foaming process) In the foaming step, the crosslinked unfoamed sheet is heated and foamed at a temperature equal to or higher than Td to obtain a crosslinked foamed sheet. The heating and foaming temperature may be equal to or higher than [Td + 5°C] and equal to or lower than [Td + 100°C], or may be equal to or higher than [Td + 10°C] and equal to or lower than [Td + 80°C]. By heating and foaming the crosslinked unfoamed sheet produced in the sheet molding step and heat treatment step in the foaming step, a crosslinked foamed sheet having a high expansion ratio and excellent surface smoothness can be obtained.

[0042] The expansion ratio of the crosslinked foam sheet is not particularly limited and may be determined depending on the application and purpose. However, from the viewpoints of cushioning properties and light weight, for example, it is preferably 10 times or more, more preferably 13 times or more, and even more preferably 16 times or more. From the viewpoint of cell density, the expansion ratio is preferably 60 times or less, more preferably 50 times or less, and even more preferably 40 times or less. Specifically, the expansion ratio of the vinyl chloride resin crosslinked foam sheet is preferably 10 to 60 times, more preferably 13 to 50 times, and even more preferably 16 to 40 times. In this specification, the expansion ratio can be measured as described in the examples.

[0043] The degree of crosslinking of the crosslinked foam sheet is not particularly limited, but is preferably 55% or more, and may be 60 to 98%, from the viewpoints of elasticity, strength, heat resistance, and the like.

[0044] The thickness of the crosslinked foam sheet is not particularly limited and may be appropriately determined depending on the application, etc., and may be, for example, 0.5 to 50 mm, 1 to 30 mm, or 2 to 10 mm. [Example]

[0045] The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples.

[0046] First, the measurement and evaluation methods used in the examples and comparative examples will be described.

[0047] (glass transition temperature) The glass transition temperature of vinyl chloride resins such as vinyl chloride copolymers was measured in accordance with JIS K 7121:2012.

[0048] (pyrolysis temperature) Approximately 5 mg of the target blowing agent was precisely weighed, and the weight change and heat quantity were measured at a heating rate of 5°C / min using a differential thermal thermogravimetry analyzer (STA7200, manufactured by Hitachi Science Co., Ltd.), and the temperature at which the exothermic peak was observed was taken as the thermal decomposition temperature.

[0049] <Method for measuring the content of hydroxyl group-containing monomer units in vinyl chloride resins> Approximately 0.1 g of the target vinyl chloride resin was placed in a 30 mL flask and dissolved in 10 mL of tetrahydrofuran to obtain a sample solution. Using a Fourier transform infrared spectrophotometer (JASCO Corporation, FT / IR-4700), the sample solution was dropped onto a KBr plate, dried, and a coating film was formed. The infrared absorption spectrum of the sample was measured. -1 Around 1740cm -1 The content of the hydroxyl group-containing monomer unit was calculated from the peak intensity ratio of the maximum intensity in the vicinity of the peak intensity using a separately measured calibration curve.

[0050] (Expansion ratio) After accurately weighing the weight (W0) of the sheet, the sheet was submerged in a measuring cylinder filled with 50 mL of methanol, and the increased volume (L0) was measured. The expansion ratio was calculated based on the following formula. Sheet density (g / cm 3 )=W0(g) / L0(cm 3 ) Expansion ratio (times) = (density of unfoamed sheet) / (density of foamed sheet)

[0051] (degree of crosslinking) 100 mL of tetrahydrofuran (THF) was placed in an Erlenmeyer flask, and 0.3 g of sample (W10) obtained by cutting the sheet into 3 mm squares was weighed and placed in the flask. The flask was then covered with a lid and left to dissolve at room temperature for 24 hours. One sheet of glass fiber filter paper "GF-75 Φ110 mm" manufactured by Advantech Co., Ltd. was taken out and the weight of the filter paper (W11) was weighed. Using this filter paper, a THF solution containing the sample was filtered under reduced pressure, and the filtered filter paper, together with any insoluble residue remaining on the filter paper, was dried in an oven at 40°C for 3 hours. The weight (W12) of the filter paper including the insoluble residue after drying was precisely weighed. The degree of crosslinking of the sheet (crosslinked gel fraction) was calculated using the following formula: Crosslinking degree [%]=[(W12-W11) / W10]×100

[0052] (crosslinked gel density) When measuring the degree of crosslinking, the density of the crosslinked gel was visually evaluated from the state of the insoluble residue on the glass fiber filter paper. A: The insoluble residue retains the shape of the swollen sample. B: The insoluble residue is gel-like and the sample boundary cannot be identified. C: No visible insoluble residue

[0053] (Surface smoothness) A: The surface is smooth B: There are some unevenness on the surface C: The entire surface is uneven and wrinkled

[0054] (mesh peelability) The state of peeling from a metal mesh (200 mesh, material: stainless steel 304) or a Teflon (registered trademark) mesh (200 mesh) when the crosslinked foam sheet was recovered was evaluated according to the following criteria. A: The cross-linked foam sheet is not adhered to the mesh. B: Part of the cross-linked foam sheet is fused to the mesh C: The entire cross-linked foam sheet is fused to the mesh.

[0055] Example 1 <Preparation of Resin Composition> 100 parts by weight of a 2-hydroxypropyl acrylate (HPA)-containing vinyl chloride copolymer (K ​​value: 72, glass transition temperature Tg: 74°C) containing 2.0 wt% HPA-derived units, 6 parts by weight of an acrylic processing aid (Kaneka Corporation, "PA60"), 40 parts by weight of diisononyl phthalate (J-Plus Corporation), 6 parts by weight of a Ba-Zn liquid stabilizer (ADEKA Corporation, "AC-255"), 2 parts by weight of a blocked isocyanate (Asahi Kasei Corporation, "E404-B80B"), and 15 parts by weight of ADCA (Eiwa Chemical Industry Co., Ltd., "AC#R", thermal decomposition temperature Td: 200°C) were mixed in a 300L Henschel mixer. When the surface temperature of the mixture reached 110°C, the mixture was transferred to a cooling tank and stirred while water-cooling the jacket. When the surface temperature of the mixture reached 50°C, the mixture was discharged from the tank to obtain a powdered resin composition. <Production of crosslinked unfoamed sheet> The powdered resin composition was molded into a sheet using a 50 mm diameter single screw extruder under the following conditions. A single-screw extruder (screw length L / screw diameter D = 27) with a full-flight screw (compression ratio 2.5) was used. A metal mesh (200 mesh, material: stainless steel 304) was inserted at the tip, and a 300 mm wide T-die was installed. The powdered resin composition was mixed and extruded, and the mixed material was extruded horizontally to a thickness of 0.5 mm using a take-up machine to produce a primary raw sheet. The set temperatures from the barrel to the head were 120 °C, the die was 110 °C, and the extrusion rate was 10 kg / hr. The primary raw sheet (surface temperature: 130 °C) was continuously obtained. The first temperature was 130 °C. The degree of crosslinking of the resulting primary raw sheet was 15.5%. Next, the obtained primary raw sheet was placed on a metal mesh (200 mesh, material: stainless steel 304) placed in a hot air circulation oven set at 150°C and left to dwell for 10 minutes to produce a secondary raw sheet. The second temperature was 150°C. The crosslinking degree of the obtained secondary raw sheet was 25.4%. <Preparation of crosslinked foam sheet> The obtained secondary raw sheet was heated for 3 minutes in a hot air circulating oven set at 230°C while still placed on the metal mesh to obtain a crosslinked foam sheet. The resulting crosslinked foam sheet had a degree of crosslinking of 60.7% and an expansion ratio of 22.1 times, was not fused to the mesh, and had a smooth surface.

[0056] Example 2 <Preparation of crosslinked unfoamed sheet> A primary raw sheet (surface temperature: 160°C) was obtained in the same manner as in Example 1, except that the barrel and head temperatures of the extruder were set to 140°C and the die temperature was set to 130°C. The crosslinking degree of the obtained primary raw sheet was 17.4%. The first temperature was 160°C. The obtained primary raw sheet was placed on a metal mesh (200 mesh, material: stainless steel 304) placed in a hot air circulating oven set at 170°C and left there for 5 minutes to produce a secondary raw sheet. The second temperature was 170°C. The crosslinking degree of the obtained secondary raw sheet was 45.0%. <Preparation of crosslinked foam sheet> A foamed sheet was produced under the same conditions as in Example 1 using the obtained secondary raw sheet. The resulting crosslinked foam sheet had a degree of crosslinking of 66.3% and an expansion ratio of 23.4 times, was not fused to the mesh, and had a smooth surface.

[0057] Example 3 <Preparation of Resin Composition> In the same manner as in Example 1, a powdery resin composition was obtained. <Preparation of crosslinked unfoamed sheet> A powdered resin composition was pre-mixed in a Banbury mixer. When the surface temperature of the kneaded mixture reached 120°C, a portion of the roll sheet was continuously fed onto the top of four inverted L-shaped calendar rolls while retaining the mixture on a charge roll. A 0.7 mm thick calendar sheet (primary raw sheet) was obtained under the sheet production conditions of a calendar sheet surface temperature of 150°C and a take-up speed of 12.0 m / min. The first temperature was 150°C. The degree of crosslinking of the resulting primary raw sheet was 6.0%. The obtained primary raw sheet was placed on a metal mesh (150 mesh, material: stainless steel 304) in a hot air heating furnace (set temperature: 165°C) set so that the surface temperature of the secondary raw sheet was 160°C, and left there for 15 minutes to produce a secondary raw sheet. The second temperature was 160°C. The crosslinked gel content of the obtained secondary raw sheet was 47.6%. <Preparation of crosslinked foam sheet> A crosslinked foam sheet was produced under the same conditions as in Example 1 using the obtained secondary raw sheet. The resulting crosslinked foam sheet had a degree of crosslinking of 78.6% and an expansion ratio of 20.4 times, was not fused to the mesh, and had a smooth surface.

[0058] Example 4 <Preparation of crosslinked unfoamed sheet> A calendered sheet (primary raw sheet) having a thickness of 0.7 mm was obtained in the same manner as in Example 3, except that the surface temperature of the calendered sheet was 160°C under the sheet production conditions. The crosslinking degree of the obtained primary raw sheet was 10.7%. The first temperature was 160°C. The obtained primary raw sheet was placed on a metal mesh (150 mesh, material: stainless steel 304) in a hot air heating furnace (set temperature: 173°C) set so that the surface temperature of the secondary raw sheet was 170°C, and left there for 7 minutes to produce a secondary raw sheet. The second temperature was 170°C. The crosslinked gel content of the obtained secondary raw sheet was 51.1%. <Preparation of crosslinked foam sheet> A crosslinked foam sheet was produced under the same conditions as in Example 1 using the obtained secondary raw sheet. The resulting crosslinked foam sheet had a degree of crosslinking of 70.4% and an expansion ratio of 21.3 times, was not fused to the mesh, and had a smooth surface.

[0059] (Comparative Example 1) <Production of crosslinked unfoamed sheet> A primary raw sheet was produced in the same manner as in Example 1. The degree of crosslinking of the obtained primary raw sheet was 15.5%. <Preparation of crosslinked foam sheet> The obtained primary raw sheet was placed on a metal mesh (150 mesh, material: stainless steel 304), and a crosslinked foam sheet was produced under the same conditions as in Example 1. The resulting crosslinked foam sheet had a degree of crosslinking of 50.7% and an expansion ratio of 6.0 times, and was fused to the mesh. The surface was wrinkled overall and not smooth.

[0060] (Comparative Example 2) <Preparation of Resin Composition> In the same manner as in Example 1, a powdery resin composition was obtained. <Production of crosslinked unfoamed sheet> An attempt was made to continuously produce a primary raw sheet in the same manner as in Example 1, except that the set temperatures from the barrel to the head were 160°C and the die was 140°C. However, after 0.5 hours, when the temperature of the primary raw sheet, i.e., the first temperature, exceeded 170°C, the foaming agent decomposed, and the continuous production of the primary raw sheet was interrupted. The degree of crosslinking of the obtained primary raw sheet was 45.0%. <Preparation of crosslinked foam sheet> The obtained primary raw sheet was placed on a metal mesh (150 mesh, material: stainless steel 304), and a crosslinked foam sheet was produced under the same conditions as in Example 1. The resulting crosslinked foam sheet had a degree of crosslinking of 84.3% and an expansion ratio of 1.9 times. The sheet was not fused to the mesh, but the surface was not smooth.

[0061] (Comparative Example 3) <Preparation of Resin Composition> In the same manner as in Example 1, a powdery resin composition was obtained. <Production of crosslinked unfoamed sheet> A primary raw sheet was produced in the same manner as in Example 1, except that the set temperatures from the barrel to the head were 125°C, the die was 120°C, and the discharge rate was 16 kg / hr. The crosslinking degree of the obtained primary raw sheet was 22.6%. <Preparation of crosslinked foam sheet> The obtained primary raw sheet was placed on a metal mesh (150 mesh, material: stainless steel 304), and a crosslinked foam sheet was produced under the same conditions as in Example 1. The resulting crosslinked foam sheet had a degree of crosslinking of 55.6% and an expansion ratio of 18.8 times, but was partially fused to the mesh and had an uneven surface.

[0062] Comparative Example 4 <Production of crosslinked unfoamed sheet> A primary raw sheet was produced in the same manner as in Comparative Example 2. The degree of crosslinking of the obtained primary raw sheet was 22.6%. The obtained primary raw sheet was placed on a metal mesh (150 mesh, material: stainless steel 304) in a hot air heating furnace (set temperature: 152°C) set so that the surface temperature of the secondary raw sheet was 150°C, and left there for 15 minutes to produce a secondary raw sheet. The second temperature was 150°C. The crosslinking degree of the obtained secondary raw sheet was 48.6%. <Preparation of crosslinked foam sheet> A crosslinked foam sheet was produced under the same conditions as in Example 1 using the obtained secondary raw sheet. The resulting crosslinked foam sheet had a degree of crosslinking of 82.3% and an expansion ratio of 5.8 times. Although it was not fused to the mesh, the surface was uneven due to partial swelling.

[0063] (Comparative Example 5) <Production of crosslinked unfoamed sheet> A primary raw sheet was produced in the same manner as in Example 3. The degree of crosslinking of the obtained primary raw sheet was 6.0%. <Preparation of crosslinked foam sheet> The obtained primary raw sheet was placed on a metal mesh (150 mesh, material: stainless steel 304), and a crosslinked foam sheet was produced under the same conditions as in Example 3. The resulting crosslinked foam sheet had a degree of crosslinking of 45.2% and an expansion ratio of 12.0 times, but was fused to the mesh and had an uneven surface with swelling occurring all over.

[0064] (Comparative Example 6) <Production of crosslinked unfoamed sheet> A primary raw sheet was produced in the same manner as in Example 3. The degree of crosslinking of the obtained primary raw sheet was 6.0%. The obtained primary raw sheet was placed on a metal mesh (150 mesh, material: stainless steel 304) in a hot air heating furnace (set temperature: 173°C) set so that the surface temperature of the secondary raw sheet was 170°C, and left there for 7 minutes to produce a secondary raw sheet. The second temperature was 170°C. The crosslinking degree of the obtained secondary raw sheet was 18.3%. <Preparation of crosslinked foam sheet> A crosslinked foam sheet was produced under the same conditions as in Example 3 using the obtained secondary raw sheet. The resulting crosslinked foam sheet had a degree of crosslinking of 56.8% and an expansion ratio of 20.8 times, but was fused to the mesh and had wrinkles on the surface, making it less smooth.

[0065] (Comparative Example 7) <Preparation of crosslinked unfoamed sheet> A calendered sheet (primary raw sheet) having a thickness of 0.7 mm was obtained in the same manner as in Example 3, except that the surface temperature of the calendered sheet was set to 165°C under the sheet production conditions. The first temperature was 165°C. During the sheet production, the surface of the rolled sheet while it was retained on the charge roll suddenly shrunk, causing unevenness, and streaky flow marks were visually observed in the sheet flow direction of the primary raw sheet. The degree of crosslinking of the obtained primary raw sheet was 45.7%. <Preparation of crosslinked foam sheet> The obtained primary raw sheet was placed on a metal mesh (150 mesh, material: stainless steel 304) and a crosslinked foam sheet was produced under the same conditions as in Example 3. The resulting crosslinked foam sheet had a degree of crosslinking of 80.8% and an expansion ratio of 22.0 times, and was not fused to the mesh, but had noticeable surface irregularities along the flow marks and was not smooth.

[0066] (Comparative Example 8) <Preparation of Resin Composition> In the same manner as in Example 1, a powdery resin composition was obtained. <Preparation of crosslinked unfoamed sheet> A powdered resin composition was pre-mixed in a Banbury mixer. When the surface temperature of the kneaded mixture reached 110°C, a portion of the roll sheet was continuously fed onto the top of four inverted L-shaped calendar rolls while retaining the mixture on a charge roll. A 0.7 mm thick calendar sheet (primary raw sheet) was obtained under the sheet production conditions of a calendar sheet surface temperature of 140°C and a take-up speed of 12.0 m / min. The first temperature was 140°C. The degree of crosslinking of the resulting primary raw sheet was 4.1%. The obtained primary raw sheet was placed on a metal mesh in a hot air heating oven (set temperature: 173°C) set so that the surface temperature of the secondary raw sheet was 170°C, and left there for 20 minutes to produce a secondary raw sheet. The second temperature was 170°C. The crosslinked gel content of the obtained secondary raw sheet was 48.4%. <Preparation of crosslinked foam sheet> A crosslinked foam sheet was produced under the same conditions as in Example 3 using the obtained secondary raw sheet. The resulting crosslinked foam sheet had a degree of crosslinking of 52.2% and an expansion ratio of 15.8 times, and was partially fused to the mesh, and the surface was wrinkled and not smooth.

[0067] Comparative Example 9 <Preparation of Resin Composition> A powdery resin composition was prepared in the same manner as in Example 1, except that a vinyl chloride homopolymer ("S1003" manufactured by Kaneka Corporation) was used instead of the HPA-containing vinyl chloride copolymer, and 0.8 parts by weight of trimethylolpropane diallyl ether ("Neoallyl T-20" manufactured by Osaka Soda Co., Ltd.) was used instead of the blocked isocyanate. <Preparation of crosslinked foam sheet> The resulting resin composition was placed in a 6-inch diameter twin roll (14 rpm, 0.7 mm gap) set at 150°C and kneaded for 5 minutes while folding the sheet over every 30 seconds to produce a roll sheet. The crosslinking degree of the resulting roll sheet was 0%. The obtained roll sheet was cut into 10 cm squares, placed on a Teflon (registered trademark) mesh (200 mesh), and placed in an electron beam irradiator. The sheet was irradiated with electron beams at an acceleration voltage of 600 kV and a dose of 90 kGy to produce a crosslinked foam sheet. The resulting crosslinked foam sheet had a degree of crosslinking of 17.4% and an expansion ratio of 19.4 times. It was fused to the Teflon (registered trademark) mesh, which made peeling difficult, and the surface was uneven and not smooth.

[0068] (Comparative Example 10) <Preparation of Resin Composition> A powdery resin composition was prepared in the same manner as in Comparative Example 9, except that 2.8 parts by weight of polyethylene glycol diacrylate (NK Ester A-600 manufactured by Shin-Nakamura Chemical Co., Ltd.) was used instead of trimethylolpropane diallyl ether. <Preparation of crosslinked foam sheet> Using the obtained resin composition, a roll sheet and a crosslinked foam sheet were produced in the same manner as in Comparative Example 9. The degree of crosslinking of the obtained roll sheet was 0%. The resulting crosslinked foam sheet had a degree of crosslinking of 55.4% and an expansion ratio of 24.1 times. It was fused to the Teflon (registered trademark) mesh, was difficult to peel off, and had many wrinkles on the surface, making it not smooth.

[0069] The crosslinking degree and crosslinked gel density of the crosslinked unfoamed sheets and crosslinked foamed sheets of the Examples and Comparative Examples were measured and evaluated as described above. The expansion ratio, surface smoothness, and mesh releasability of the crosslinked foamed sheets of the Examples and Comparative Examples were measured and evaluated as described above. The results are shown in Tables 1 and 2 below. Tables 1 and 2 below also show the preparation conditions.

[0070] [Table 1]

[0071] [Table 2]

[0072] As can be seen from the results in Tables 1 and 2 above, in Examples 1 to 4, in which a vinyl chloride resin composition containing a vinyl chloride copolymer having a hydroxyl group in the molecule, a crosslinking agent having at least two functional groups in each molecule that can react with a hydroxyl group, and a foaming agent was used to produce a first vinyl chloride resin unfoamed sheet having a crosslinking degree of 5% or more but less than 20% in a sheet molding process, and a crosslinked unfoamed sheet having a crosslinking degree of 20 to 60% was produced in a heat treatment process prior to the foaming process, when the crosslinked unfoamed sheet was heated and foamed, a crosslinked foamed sheet with a high expansion ratio, surface smoothness, and good mesh peelability could be obtained.

[0073] On the other hand, in Comparative Examples 1 and 5, in which a crosslinked unfoamed sheet having a crosslinking degree of 5% or more but less than 20% obtained in the sheet molding process was directly heated and foamed, the crosslinked foamed sheets had a low expansion ratio and also poor surface smoothness and mesh releasability. In Comparative Examples 2, 3, and 7, in which a crosslinked unfoamed sheet having a crosslinking degree of 20% or more obtained in the sheet molding process was directly heated and foamed, the crosslinked foamed sheets had poor surface smoothness. In Comparative Example 4, in which the first vinyl chloride resin unfoamed sheet obtained in the sheet molding process had a crosslinking degree of 20% or more, the crosslinking degree of the crosslinked unfoamed sheet was adjusted to 48.6% in the heat treatment step prior to the foaming step, but the crosslinked foamed sheet obtained by heat foaming the crosslinked unfoamed sheet had poor surface smoothness. In Comparative Example 6, in which the degree of crosslinking of the first vinyl chloride resin unfoamed sheet obtained in the sheet molding step was 5% or more but less than 20%, and the degree of crosslinking of the crosslinked unfoamed sheet was adjusted to 18.3% in the heat treatment step before the foaming step, the crosslinked foamed sheet obtained by heat-foaming the crosslinked unfoamed sheet had poor surface smoothness and mesh releasability. In Comparative Example 8, in which the degree of crosslinking of the first vinyl chloride resin unfoamed sheet obtained in the sheet molding step was less than 5%, and the degree of crosslinking of the crosslinked unfoamed sheet was adjusted to 20 to 60% in the heat treatment step before the foaming step, the crosslinked foamed sheet obtained by heat-foaming the crosslinked unfoamed sheet had poor surface smoothness and poor mesh releasability.

[0074] The present invention is not particularly limited, but may include, for example, the following embodiments.

[0075] [1] A sheet-molding step of molding a vinyl chloride resin composition containing a vinyl chloride copolymer having a hydroxyl group in the molecule, a crosslinking agent having at least two functional groups reactive with a hydroxyl group in each molecule, and a foaming agent at a first temperature of [Tg + 50°C] or higher and [Td - 30°C] or lower, where Tg is the glass transition temperature of the vinyl chloride copolymer and Td is the thermal decomposition temperature of the foaming agent, to produce a first vinyl chloride resin unfoamed sheet having a crosslinking degree of 5% or higher but lower than 20%; A method for producing a crosslinked vinyl chloride resin unfoamed sheet, comprising a heat treatment step of heat treating the first vinyl chloride resin unfoamed sheet at a second temperature that is higher than the first temperature and not higher than [Td-20°C] to produce a crosslinked vinyl chloride resin unfoamed sheet having a crosslinking degree of 20 to 60%. [2] The method for producing a vinyl chloride resin crosslinked unfoamed sheet according to [1], wherein the sheet molding step is carried out by extrusion molding or calendar molding. [3] The method for producing a crosslinked unfoamed vinyl chloride resin sheet according to [2], wherein when the sheet molding step is carried out by extrusion molding, the discharge rate of the vinyl chloride resin composition from the extruder is 3 to 100 kg / hr. [4] The method for producing a crosslinked vinyl chloride resin unfoamed sheet according to [2] or [3], wherein, when the sheet molding step is carried out by extrusion molding, the first temperature is not less than [Tg+50°C] and not more than [Td-30°C], and the second temperature is not less than [Tg+55°C] and not more than [Td-20°C]. [5] The method for producing a vinyl chloride resin crosslinked unfoamed sheet according to [2], wherein, when the sheet molding step is carried out by calendar molding, the first temperature is not less than [Tg+50°C] and not more than [Td-30°C]. [6] The method for producing a crosslinked unfoamed vinyl chloride resin sheet according to any one of [1] to [5], wherein the first unfoamed vinyl chloride resin sheet has a thickness of 0.3 to 10 mm. [7] The method for producing a crosslinked unfoamed vinyl chloride resin sheet according to any one of [1] to [6], wherein the vinyl chloride copolymer contains 0.1 to 10% by weight of units derived from a hydroxyl group-containing monomer. [8] The method for producing a crosslinked vinyl chloride resin unfoamed sheet according to any one of [1] to [7], wherein the crosslinking agent is a blocked isocyanate. [9] The method for producing an unfoamed vinyl chloride resin sheet according to any one of [1] to [8], wherein the vinyl chloride resin composition contains 10 to 80% by weight of the vinyl chloride copolymer.

[10] The method for producing an unfoamed vinyl chloride resin sheet according to any one of [1] to [9], wherein the vinyl chloride resin composition further contains a plasticizer, and the vinyl chloride resin composition contains 0.5 to 10 parts by weight of the crosslinking agent, 5 to 30 parts by weight of the foaming agent, and 20 to 80 parts by weight of the plasticizer relative to 100 parts by weight of the vinyl chloride copolymer.

[11] A method for producing a vinyl chloride resin crosslinked foam sheet, comprising: A method for producing a crosslinked vinyl chloride resin foamed sheet, comprising heating and foaming a vinyl chloride resin crosslinked unfoamed sheet obtained by the method for producing a crosslinked vinyl chloride resin unfoamed sheet according to any one of [1] to

[10] at a temperature equal to or higher than Td to obtain a crosslinked vinyl chloride resin foamed sheet.

[12] The method for producing a cross-linked vinyl chloride resin foam sheet according to

[11] , wherein the expansion ratio of the cross-linked vinyl chloride resin foam sheet is 10 to 60 times.

Claims

1. a sheet-molding step of molding a vinyl chloride resin composition containing a vinyl chloride copolymer having a hydroxyl group in the molecule, a crosslinking agent having at least two functional groups reactive with a hydroxyl group in each molecule, and a foaming agent at a first temperature of [Tg+50°C] or higher and [Td-30°C] or lower, where Tg is the glass transition temperature of the vinyl chloride copolymer and Td is the thermal decomposition temperature of the foaming agent, to produce a first vinyl chloride resin unfoamed sheet having a crosslinking degree of 5% or higher but lower than 20%; a heat treatment step of heat treating the first vinyl chloride resin unfoamed sheet at a second temperature that is higher than the first temperature and not higher than [Td-20°C] to produce a vinyl chloride resin crosslinked unfoamed sheet having a crosslinking degree of 20 to 60%.

2. The method for producing a vinyl chloride resin crosslinked unfoamed sheet according to claim 1, wherein the sheet molding step is carried out by extrusion molding or calendar molding.

3. 3. The method for producing a crosslinked unfoamed vinyl chloride resin sheet according to claim 2, wherein when the sheet molding step is carried out by extrusion molding, the discharge rate of the vinyl chloride resin composition from the extruder is 3 to 100 kg / hr.

4. 3. The method for producing a vinyl chloride resin crosslinked unfoamed sheet according to claim 2, wherein, when the sheet molding step is carried out by extrusion molding, the first temperature is not less than [Tg+50°C] and not more than [Td-30°C], and the second temperature is not less than [Tg+55°C] and not more than [Td-20°C].

5. 3. The method for producing a vinyl chloride resin crosslinked unfoamed sheet according to claim 2, wherein, when the sheet molding step is performed by calendar molding, the first temperature is [Tg+50°C] or higher and [Td-30°C] or lower.

6. 2. The method for producing a crosslinked unfoamed vinyl chloride resin sheet according to claim 1, wherein the first unfoamed vinyl chloride resin sheet has a thickness of 0.3 to 10 mm.

7. 2. The method for producing a vinyl chloride resin crosslinked unfoamed sheet according to claim 1, wherein the vinyl chloride copolymer contains 0.1 to 10% by weight of units derived from a hydroxyl group-containing monomer.

8. The method for producing a cross-linked vinyl chloride resin unfoamed sheet according to claim 1, wherein the cross-linking agent is a blocked isocyanate.

9. 2. The method for producing a vinyl chloride resin unfoamed sheet according to claim 1, wherein the vinyl chloride resin composition contains 10 to 80% by weight of the vinyl chloride copolymer.

10. 2. The method for producing a vinyl chloride resin unfoamed sheet according to claim 1, wherein the vinyl chloride resin composition further contains a plasticizer, and the vinyl chloride resin composition contains 0.5 to 10 parts by weight of the crosslinking agent, 5 to 30 parts by weight of the foaming agent, and 20 to 80 parts by weight of the plasticizer relative to 100 parts by weight of the vinyl chloride copolymer.

11. A method for producing a vinyl chloride resin cross-linked foam sheet, comprising: A method for producing a vinyl chloride resin crosslinked unfoamed sheet, comprising heating and foaming a vinyl chloride resin crosslinked unfoamed sheet obtained by the method for producing a vinyl chloride resin crosslinked unfoamed sheet according to any one of claims 1 to 10 at a temperature equal to or higher than Td to obtain a vinyl chloride resin crosslinked foamed sheet.

12. The method for producing a cross-linked vinyl chloride resin foam sheet according to claim 11, wherein the expansion ratio of the cross-linked vinyl chloride resin foam sheet is 10 to 60 times.

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