Chloroprene copolymer latex and method for producing the same

Abstract

The present invention provides a chloroprene copolymer latex that offers desirable flexibility and stress relaxation properties when used as gloves, and that can be vulcanized under mild conditions. [Solution] The present invention relates to chloroprene copolymer latex, chloroprene copolymer latex composition, chloroprene copolymer rubber molded product, immersion product, and a method for producing chloroprene copolymer latex, wherein the method for producing chloroprene copolymer latex comprises the step of emulsion polymerization of 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2) in the presence of sulfur (A-3).

Description

[Technical Field]

[0001] The present invention relates to a latex mainly composed of a copolymer of 2-chloro-1,3-butadiene (chloroprene) and 2-methyl-1,3-butadiene, a method for producing the same, and a molded article using the composition thereof, particularly to a dipped product. [Background Art]

[0002] Isoprene rubber (IR) and chloroprene rubber (CR) are synthetic rubbers having flexibility equivalent to that of natural rubber. For this reason, in recent years, as an anti-allergy measure, isoprene rubber or chloroprene rubber has been used in place of natural rubber as a material for dip-molding composition products (dipped products), especially surgical gloves. Chloroprene rubber can be produced at lower cost than isoprene rubber, but it has a problem of low production efficiency because vulcanization treatment at high temperature is required to obtain the target strength. In addition, compared with isoprene rubber, chloroprene rubber has greater stress relaxation when a stress is applied (a phenomenon in which the stress decreases over time when a constant strain is applied). Therefore, gloves produced using chloroprene rubber have problems of low followability to the movement of the user's fingers and reduced tactile sensation.

[0003] Techniques for improving the flexibility of chloroprene rubber have been disclosed. For example, Patent Document 1 describes that a flexible vulcanized rubber product can be produced by mixing a chloroprene polymer having a 1 wt% toluene insoluble content of 70 wt% or more and a chloroprene polymer having a 1 wt% toluene insoluble content of 10 wt% or less. Further, Patent Document 2 discloses a technique related to a latex containing a copolymer of chloroprene and 2,3-dichloro-1,3-butadiene. Regarding the stress relaxation rate, Patent Document 3 discloses a styrene-butadiene rubber having a stress relaxation rate of 50 to 70% after 6 minutes from removal of a 100% elongation tensile stress. [Prior Art Documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2019-143002 [Patent Document 2] Japanese Patent Publication No. 2019-044116 [Patent Document 3] Japanese Patent Publication No. 2001-131812 [Overview of the initiative] [Problems that the invention aims to solve]

[0005] However, no chloroprene rubber is known that can provide the desired flexibility and stress relaxation properties when used as gloves, and that can be vulcanized under mild conditions. For example, the technologies described in Patent Documents 1 and 2 do not adequately consider stress relaxation, so there is room to reduce the stress relaxation of the molded product in order to improve the tactile feel of gloves manufactured as vulcanized rubber products (molded products). Furthermore, the technology described in Patent Document 3 has the problem that it does not produce rubber products with sufficient flexibility for use as surgical gloves.

[0006] The object of the present invention is to provide a chloroprene copolymer latex for obtaining a molded product with desirable flexibility and stress relaxation properties when used as a glove, by a mild vulcanization treatment. [Means for solving the problem]

[0007] As a result of diligent research to solve the above problems, the inventors of the present invention have found that the above problems can be solved by a chloroprene copolymer latex obtained by emulsion polymerization of 2-chloro-1,3-butadiene (chloroprene) (A-1) and 2-methyl-1,3-butadiene (A-2) in the presence of sulfur (A-3), and have completed the present invention. In other words, the present invention relates to the following chloroprene copolymer latex compositions [1] to

[14] , and molded articles and immersed products obtained by curing these compositions.

[0008] [1] A method for producing a chloroprene copolymer latex (A), comprising the step of emulsion polymerization of 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2) in the presence of sulfur (A-3).

[0009] [2] A method for producing chloroprene copolymer latex (A) according to [1], wherein the proportion of 2-methyl-1,3-butadiene (A-2) in the total monomer components used in the emulsion polymerization step is 2 to 60 mol%, and the polymerization conversion rate of the total monomers is 61 to 90% by mass.

[0010] [3] A method for producing chloroprene copolymer latex (A) according to [1] or [2], wherein the amount of sulfur (A-3) used during emulsion polymerization is 0.1 to 1.0 parts by mass when the total amount of 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2) is 100 parts by mass.

[0011] [4] A latex of chloroprene copolymer, A chloroprene copolymer latex (A) obtained by emulsion polymerization of 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2) in the presence of sulfur (A-3).

[0012] [5] The chloroprene copolymer latex (A) according to [4], wherein the tetrahydrofuran insoluble fraction of the chloroprene copolymer at 25°C is 5 to 80% by mass.

[0013] [6] The chloroprene copolymer latex (A) according to [4] or [5], wherein the weight-average molecular weight of the component soluble in tetrahydrofuran at 25°C in the chloroprene copolymer is 400,000 or more.

[0014] [7] The chloroprene copolymer latex (A) according to any one of [4] to [6], wherein when the total amount of 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2) is 100 parts by mass, the amount of the sulfur (A-3) used in emulsion polymerization is 0.1 to 1.0 parts by mass.

[0015] [8] The chloroprene copolymer latex (A) according to any one of [4] to [7], wherein the proportion of monomer units derived from 2-methyl-1,3-butadiene (A-2) in all monomer units constituting the chloroprene copolymer is 10 to 50 mol%.

[0016] [9] A composition comprising 100 parts by mass in total of the chloroprene copolymer latex (A) according to any one of [4] to [8] and optionally a synthetic rubber (F), 0.1 to 20.0 parts by mass of a metal oxide (B), 0.1 to 10.0 parts by mass of a vulcanization accelerator (C), 0.1 to 10.0 parts by mass of sulfur (D), and 0.1 to 10.0 parts by mass of an antioxidant (E) A chloroprene copolymer latex composition comprising the above components.

[0017]

[10] The chloroprene copolymer latex composition according to [9], comprising a synthetic rubber (F), wherein when the total of the solid content of the chloroprene copolymer latex (A) and the synthetic rubber (F) is 100% by mass, the proportion of the synthetic rubber (F) is 1 to 50% by mass.

[0018]

[11] A chloroprene copolymer rubber molded product obtained by curing the chloroprene copolymer latex composition according to [9] or

[10] .

[12] A dipped product obtained by molding and curing the chloroprene copolymer latex composition according to [9] or

[10] by a dipping method.

[0019]

[13] The dipped product according to

[12] , which is a glove. The dipped product according to

[13] , which is a medical disposable glove.

Effects of the Invention

[0020] Provided is a chloroprene copolymer latex for obtaining a molded article having flexibility and stress relaxation properties desirable for use as a glove through vulcanization treatment under mild conditions. That is, by vulcanizing the chloroprene copolymer latex composition of the present invention under mild conditions, a molded article (chloroprene copolymer rubber molded article) that exhibits low stress relaxation, has a high elastic modulus retention rate, and provides an excellent tactile sensation can be obtained. Further, since the tensile strength of the molded article is improved by using sulfur (A-3) in the polymerization step of the chloroprene copolymer, the molded article according to the present invention can be suitably used for dipped products, particularly medical disposable gloves.

Mode for Carrying Out the Invention

[0021] Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the configurations of the following embodiments. In the claims and the description of the present specification, the symbol "~" defining a numerical range means not less than the numerical value of the lower limit and not more than the numerical value of the upper limit.

[0022] [Chloroprene Copolymer Latex (A)] Chloroprene copolymer latex (A) is obtained by emulsion polymerization of 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2) in the presence of sulfur (A-3). In chloroprene copolymer latex (A), fine particles of the chloroprene copolymer produced in the emulsion polymerization step are dispersed in a dispersion medium such as water.

[0023] [Chloroprene Copolymer] The chloroprene copolymer contained in the chloroprene copolymer latex (A) of the present invention comprises structures (monomer units) derived from at least 2-chloro-1,3-butadiene (chloroprene) (A-1) and 2-methyl-1,3-butadiene (A-2).

[0024] The monomer units constituting the chloroprene copolymer may consist only of 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2). When the total monomer units constituting the chloroprene copolymer are set to 100 mol%, the proportion of monomer units derived from 2-chloro-1,3-butadiene (A-1) is preferably 50 to 90 mol%, more preferably 70 to 90 mol%, and even more preferably 75 to 90 mol%. When the total monomer units constituting the chloroprene copolymer are set to 100 mol%, the proportion of monomer units derived from 2-methyl-1,3-butadiene (A-2) is preferably 10 to 50 mol%, more preferably 10 to 30 mol%, and even more preferably 10 to 25 mol%. Note that, as described later, sulfur-derived structures are not counted as monomer units.

[0025] A higher proportion of monomer units derived from 2-chloro-1,3-butadiene (A-1) in the chloroprene copolymer is preferable because it tends to accelerate the polymerization reaction. A lower proportion of monomer units derived from 2-chloro-1,3-butadiene (A-1) in the chloroprene copolymer is preferable because it results in a molded product obtained by vulcanizing the chloroprene copolymer latex (A) with high flexibility. If the proportion of monomer units derived from 2-methyl-1,3-butadiene (A-2) in the chloroprene copolymer is 10 mol% or more, the resulting molded product will exhibit sufficient strength even when the composition containing the chloroprene copolymer is vulcanized under relatively mild conditions such as 110°C. If the proportion of monomer units derived from 2-methyl-1,3-butadiene (A-2) in the chloroprene copolymer is 50 mol% or less, the polymerization reaction in the emulsion copolymerization reaction can proceed relatively quickly.

[0026] Since chloroprene copolymers are polymerized in the presence of sulfur (A-3), it is thought that the chloroprene copolymers contain at least some of the sulfur (A-3) that was present with the monomers during polymerization. Note that sulfur (A-3) is not included in monomer (A-4), which will be discussed later. It is presumed that sulfur (A-3) acts as a chain transfer agent and does not become a monomer unit constituting the chloroprene copolymer. The type of sulfur (A-3) is not particularly limited, but powdered sulfur, precipitated sulfur, colloidal sulfur, surface-treated sulfur, insoluble sulfur, etc., can be used. Sulfur (A-3) may contain S8 cyclic molecules or chain-like molecules. Sulfur (A-3) may be used alone or in combination of two or more types. Sulfur (A-3) is preferably powdered sulfur.

[0027] Since sulfur (A-3) acts as a chain transfer agent, the greater the amount of sulfur (A-3) present during emulsion polymerization, the lower the degree of polymerization of the chloroprene copolymer tends to be. Furthermore, it is presumed that at least a portion of the sulfur (A-3) does not undergo a chain transfer reaction and remains in its original form within the latex particles of the chloroprene copolymer, forming a cross-linked structure when the chloroprene copolymer is vulcanized. For this reason, compared to molded articles obtained from latex containing chloroprene copolymer polymerized under conditions where sulfur (A-3) is absent, molded articles obtained from the chloroprene copolymer latex composition of the present invention are expected to exhibit improved tensile strength and reduced stress relaxation. It is also presumed that the sulfur atoms contained in the sulfur (A-3) that reacted as a chain transfer agent are bonded to the chloroprene copolymer, but it is impossible to specify and structurally describe the state of each bond in the polymer, including the side chains, and it is presumed that the structure of the chloroprene copolymer also fluctuates depending on conditions such as temperature or polymerization time during emulsion polymerization. The structure obtained by the reaction of sulfur with chloroprene copolymer is not particularly limited, but -CH-SH-, -S-, -SS-, -SC(=S)-, -SC(=S)O-, etc., are conceivable. That is, the sulfur atom may be bonded to the end of the main chain, or it may form a cross-linking structure.

[0028] The amount of sulfur (A-3) used in the production of chloroprene copolymer latex (A) is preferably 0.1 to 1.0 parts by mass, more preferably 0.1 to 0.8 parts by mass, and even more preferably 0.1 to 0.5 parts by mass, when the total amount of 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2) is 100 parts by mass. If the amount of sulfur (A-3) used in the production of chloroprene copolymer latex (A) is 0.1 parts by mass or more, the crosslinking reactivity of the chloroprene copolymer latex composition described later is excellent, and a molded product with excellent mechanical properties can be obtained by vulcanization treatment under milder conditions than conventional methods, which is preferable. Furthermore, if the amount of sulfur (A-3) is 1.0 part by mass or less, the decrease in flexibility and yellowing of the molded product due to sulfur (A-3) can be suppressed, which is also preferable.

[0029] The tetrahydrofuran (THF) insoluble fraction in chloroprene copolymer at 25°C is typically 5 to 80% by mass, preferably 5 to 75% by mass, and more preferably 10 to 75% by mass. The tetrahydrofuran insoluble fraction is formed when polymer chains within the chloroprene copolymer particles are gelled by three-dimensional crosslinking. This tetrahydrofuran insoluble fraction can be measured by the method used in the examples described later. When the tetrahydrofuran insoluble fraction in the chloroprene copolymer is 5% by mass or more, a molded product with relatively little stress relaxation can be obtained, resulting in a good tactile feel for gloves made from chloroprene copolymer latex. When the tetrahydrofuran insoluble fraction in the chloroprene copolymer is 80% by mass or less, a molded product with excellent flexibility and tensile strength can be obtained. The tetrahydrofuran insoluble fraction in the chloroprene copolymer can be controlled by adjusting the polymerization conversion rate, the amount of sulfur (A-3), or the amount of chain transfer agent (but not sulfur (A-3)) used during the production of the chloroprene copolymer. The polymerization conversion rate is controlled by the polymerization time and polymerization temperature of the chloroprene copolymer. The polymerization conversion rate tends to increase with longer polymerization times, and also with higher polymerization temperatures. For example, increasing the polymerization conversion rate tends to increase the tetrahydrofuran insoluble fraction in the chloroprene copolymer. Also, increasing the amount of sulfur (A-3) present during the emulsion polymerization of the chloroprene copolymer tends to decrease the tetrahydrofuran insoluble fraction in the chloroprene copolymer. Furthermore, increasing the amount of chain transfer agent tends to decrease the tetrahydrofuran insoluble fraction in the chloroprene copolymer.

[0030] The chloroprene copolymer may contain, in addition to structures (monomer units) derived from 2-chloro-1,3-butadiene (A-1), structures (monomer units) derived from 2-methyl-1,3-butadiene (A-2), and structures derived from sulfur (A-3), monomer units derived from monomer (A-4) to the extent that it does not impair the objectives of the present invention. Here, monomer (A-4) is a monomer other than 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2), which is copolymerizable with at least one of 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2). Monomer (A-4) may also be a monomer copolymerizable with both 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2). Examples of monomers (A-4) include butadiene, 2,3-dichloro-1,3-butadiene, styrene, acrylonitrile, acrylic acid and its esters, methacrylic acid and its esters, etc. The chloroprene copolymer may contain two or more monomers (A-4) as needed.

[0031] When the chloroprene copolymer contains monomer (A-4) units, the upper limit of the monomer (A-4) content is preferably 10 mol parts or less, more preferably 8 mol parts or less, and even more preferably 5 mol parts or less, when the total amount of monomer units derived from 2-chloro-1,3-butadiene (A-1) and monomer units derived from 2-methyl-1,3-butadiene (A-2) in the chloroprene copolymer is 100 mol parts. Furthermore, when the chloroprene copolymer contains monomer (A-4) units, the lower limit of the monomer (A-4) content is preferably 0.01 mol parts or more, more preferably 0.1 mol parts or more, and even more preferably 0.5 mol parts or more, when the total amount of monomer units derived from 2-chloro-1,3-butadiene (A-1) and monomer units derived from 2-methyl-1,3-butadiene (A-2) in the chloroprene copolymer is 100 mol parts. When the total amount of monomer units derived from 2-chloro-1,3-butadiene (A-1) and monomer units derived from 2-methyl-1,3-butadiene (A-2) in the chloroprene copolymer is 10 mol parts or less, the tensile strength and elongation of the molded product are good.

[0032] The weight-average molecular weight of the tetrahydrofuran-soluble component in the chloroprene copolymer, measured by the method or conditions used in the examples described later, is preferably 400,000 or more, more preferably 500,000 or more, and even more preferably 550,000 or more. If the weight-average molecular weight of the tetrahydrofuran-soluble component in the chloroprene copolymer is 400,000 or more, a molded article with good mechanical properties can be obtained. The weight-average molecular weight of the tetrahydrofuran-soluble component in the chloroprene copolymer is preferably 3,000,000 or less, more preferably 2,000,000 or less, and even more preferably 900,000 or less. If the weight-average molecular weight of the tetrahydrofuran-soluble component in the chloroprene copolymer is 3,000,000 or less, a molded article with good flexibility and tensile strength can be obtained.

[0033] [Method for producing chloroprene copolymer latex (A)] A simple and industrially advantageous method for producing chloroprene copolymer latex (A) is to radically polymerize 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2) in an aqueous emulsion.

[0034] By emulsion polymerization of 2-chloro-1,3-butadiene (A-1), 2-methyl-1,3-butadiene (A-2), and optionally a monomer (A-4) in the presence of sulfur (A-3) in a dispersion medium such as water, a copolymer latex (A) in which chloroprene copolymer particles are dispersed in a dispersion medium such as water is obtained. The polymerization temperature during emulsion polymerization is preferably 20 to 35°C, and the polymerization time is preferably 5 to 8 hours. When the polymerization temperature and polymerization time during emulsion polymerization are within the above range, the desired polymerization conversion rate is obtained, which is therefore preferable.

[0035] The 2-methyl-1,3-butadiene content in the chloroprene copolymer can be adjusted by the ratio of 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2) used during polymerization, the polymerization conversion rate, and other factors.

[0036] The higher the proportion of 2-methyl-1,3-butadiene (A-2) in the total monomers at the start of polymerization, the higher the content of monomer units derived from 2-methyl-1,3-butadiene (A-2) in the final chloroprene copolymer. However, because 2-methyl-1,3-butadiene (A-2) is less reactive at the start of emulsion polymerization compared to 2-chloro-1,3-butadiene (A-1), a higher proportion of 2-methyl-1,3-butadiene tends to slow down polymerization and prolong the reaction time.

[0037] 2-methyl-1,3-butadiene (A-2) is more readily incorporated into the polymer as the polymerization of the chloroprene copolymer progresses. Therefore, by increasing the polymerization conversion rate during the polymerization of the chloroprene copolymer, the content of 2-methyl-1,3-butadiene (A-2) in the final chloroprene copolymer can be increased. Conversely, if the polymerization conversion rate is low, a large amount of residual monomer remains, requiring the effort of removing the residual monomer and degrading the mechanical properties of the molded product.

[0038] From the above, in order to efficiently obtain the chloroprene copolymer in the present invention, it is preferable that the amount of 2-methyl-1,3-butadiene (A-2) in the total monomer components used during polymerization be 2 to 60 mol%, more preferably 10 to 50 mol%, and even more preferably 15 to 35 mol%. Furthermore, it is preferable that the polymerization conversion rate of the total monomer be 61% by mass or more, more preferably 75% by mass or more, and even more preferably 80% by mass or more. It is preferable that the polymerization conversion rate of the total monomer be 90% by mass or less, and even more preferably 85% by mass or less. If the polymerization conversion rate of the total monomer is 90% by mass or less, the quality of the chloroprene copolymer obtained by polymerization is good, and the physical properties of the molded product obtained from the chloroprene copolymer latex (A) are also good.

[0039] Anionic surfactants are preferred as emulsifiers for emulsion polymerization. Examples of anionic surfactants include rosinate soap, sodium salt of naphthalene sulfonic acid condensate, sodium salt of dodecylbenzenesulfonic acid, and sodium salt of dodecyl sulfate. Ordinary rosinate soap can be used due to the ease of the coagulation process. Sodium and / or potassium salts of disproportionated rosinate can be used in particular from the viewpoint of color stability. Furthermore, a salt of rosinate soap and dimer acid may be used from the viewpoint of particle stability in latex.

[0040] The amount of emulsifier used is preferably 0.5 to 20.0 parts by mass, more preferably 1.0 to 10.0 parts by mass, and even more preferably 1.5 to 5.0 parts by mass, when the total amount of 2-chloro-1,3-butadiene (A-1), 2-methyl-1,3-butadiene (A-2), and monomer (A-4) is 100 parts by mass. If the amount of emulsifier used is 0.5 parts by mass or more, emulsification failure is less likely to occur and the heat generated by polymerization can be controlled. Also, if the amount of emulsifier used is 0.5 parts by mass or more, problems such as the formation of aggregates and poor product appearance will not occur. On the other hand, if the amount of emulsifier used is 20.0 parts by mass or less, emulsifiers such as rosinic acid will not remain in the chloroprene copolymer, so stickiness of the chloroprene copolymer is less likely to occur. Therefore, if the amount of emulsifier used is 20.0 parts by mass or less, problems with processability and handling due to adhesion to the mold (former) during molding of the chloroprene copolymer latex composition, and adhesion during use of the molded product will not occur, and deterioration of the color tone of the molded product will not occur.

[0041] Conventional radical polymerization initiators can be used as polymerization initiators. In the case of emulsion polymerization, for example, organic or inorganic peroxides such as benzoyl peroxide, potassium persulfate, ammonium persulfate, cumene hydroperoxide, t-butyl hydroperoxide, or azo compounds such as azobisisobutyronitrile can be used. A single polymerization initiator may be used alone, or two or more may be used in combination.

[0042] In the polymerization of the chloroprene copolymer, a chain transfer agent (but not sulfur (A-3)) can be used to adjust the tetrahydrofuran insoluble fraction. The amount of chain transfer agent used is preferably 0.01 to 15.0 parts by mass, more preferably 0.05 to 10.0 parts by mass, and even more preferably 0.1 to 1.0 parts by mass, when the total of 2-chloro-1,3-butadiene (A-1), 2-methyl-1,3-butadiene (A-2), and monomer (A-4) is 100 parts by mass. While not particularly limited, known chain transfer agents such as alkyl mercaptans (e.g., n-dodecyl mercaptan, n-decyl mercaptan, octyl mercaptan, or tert-dodecyl mercaptan), dialkyl xanthogen disulfides (e.g., diisopropyl xanthogen disulfide or diethyl xanthogen disulfide), and iodoform can be used. More preferably, alkyl mercaptans are used, and even more preferably, n-dodecyl mercaptan.

[0043] In the polymerization of chloroprene copolymers, a co-catalyst may be used in conjunction with the polymerization initiator, if desired. The co-catalysts that can be used with the polymerization initiator are not particularly limited, and general co-catalysts can be used. Examples include anthraquinone sulfonates, potassium sulfite, sodium disulfite, sodium sulfite, tetraethylenepentamine, N,N-dimethyl-p-toluidine, and copper sulfate. One co-catalyst may be used alone, or two or more may be used in combination.

[0044] Generally, in emulsion polymerization, a polymerization inhibitor is added to stop the polymerization reaction when a predetermined polymerization conversion rate is reached, in order to obtain a polymer having a desired molecular weight and molecular weight distribution. A polymerization inhibitor may also be used in embodiments of the present invention. The type of polymerization inhibitor is not particularly limited, and commonly used polymerization inhibitors such as phenothiazine, para-t-butylcatechol, hydroquinone, hydroquinone monomethyl ether, and diethylhydroxylamine can be used. One polymerization inhibitor may be used alone, or two or more may be used in combination.

[0045] Furthermore, to the extent that the objectives of the present invention are not impaired, stabilizers such as acid acceptors and / or antioxidants may be added to the chloroprene copolymer latex (A).

[0046] Furthermore, a thickening agent may be added to the chloroprene copolymer latex (A) to the extent that the objective of the present invention is not impaired. The usable thickening agent is not particularly limited, and general thickening agents can be used. For example, methylcellulose, urethane-modified polyethers, acrylic polymers, etc., can be used. One type of thickening agent may be used alone, or two or more types may be used in combination.

[0047] [Chloroprene copolymer latex composition] The chloroprene copolymer latex composition contains the solid content of the chloroprene copolymer latex (A) obtained by the polymerization method described above, a metal oxide (B), a vulcanization accelerator (C), sulfur (D), and an antioxidant (E). Here, the solid content of the chloroprene copolymer latex (A) is the component obtained by drying the chloroprene copolymer latex (A) by standing it in an oven at 141°C for 30 minutes, and is the component obtained by removing water or other dispersion media from the chloroprene copolymer latex (A). The chloroprene copolymer latex composition may also contain water or other dispersion media in the chloroprene copolymer latex (A). When synthetic rubber (F), described later, is not included, the chloroprene copolymer latex composition contains, with 100 parts by mass of solids of chloroprene copolymer latex (A), 0.1 to 20.0 parts by mass of metal oxide (B), 0.1 to 10.0 parts by mass of vulcanization accelerator (C), 0.1 to 10.0 parts by mass of sulfur (D), and 0.1 to 10.0 parts by mass of antioxidant (E). By compounding with this composition, a rubber molded product (e.g., a film) with improved stress relaxation resistance to tensile stress can be obtained from the chloroprene copolymer latex composition. Of the raw materials used in compounding, water-insoluble components and components that destabilize the colloidal state of chloroprene copolymer latex are first prepared as aqueous dispersions and then added to the chloroprene copolymer latex.

[0048] The type of metal oxide (B) is not particularly limited; for example, zinc oxide, lead oxide, and trilead tetroxide can be used, with zinc oxide being particularly preferred. One type of metal oxide (B) may be used alone, or two or more types may be used in combination.

[0049] The amount of metal oxide (B) contained in the chloroprene copolymer latex composition according to this embodiment is usually 0.1 to 20.0 parts by mass, preferably 0.5 to 15.0 parts by mass, and more preferably 0.5 to 10.0 parts by mass, when the amount of solids of the chloroprene copolymer latex (A) is 100 parts by mass. If the amount of metal oxide (B) is 0.1 parts by mass or more, the chloroprene copolymer can be cured by vulcanization treatment, and if it is 0.5 parts by mass or more, the chloroprene copolymer can be vulcanized more efficiently. If the amount of metal oxide (B) is 20.0 parts by mass or less, a good crosslinked structure is obtained by vulcanization treatment, and scorching is less likely to occur. In addition, since the colloidal state of the chloroprene copolymer latex composition is stabilized, problems such as sedimentation are less likely to occur.

[0050] The type of vulcanization accelerator (C) is not particularly limited, and any commonly used in the vulcanization treatment of isoprene-based polymer latex or chloroprene-based polymer latex can be used. Examples include thiram-based, dithiocarbamate-based, thiourea-based, guanidine-based, and thiazole-based vulcanization accelerators. Examples of thiuram-based vulcanization accelerators include tetraethyl thiuram disulfide and tetrabutyl thiuram disulfide. Examples of dithiocarbamate-based vulcanization accelerators include sodium dibutyldithiocarbamate, zinc dibutyldithiocarbamate, and zinc diethylthiodicarbamate. Examples of thiourea-based vulcanization accelerators include ethylene thiourea, diethyl thiourea, trimethyl thiourea, and N,N'-diphenylthiourea (DPTU). Examples of guanidine-based vulcanization accelerators include diphenylguanidine (DPG) and dioltotoluylguanidine. Examples of thiazole-based vulcanization accelerators include 2-mercaptobenzothiazole, di-2-benzothiazolyl disulfide, and zinc 2-mercaptobenzothiazole. Vulcanization accelerator (C) may be used alone or in combination of two or more types.

[0051] The amount of vulcanization accelerator (C) contained in the chloroprene copolymer latex composition according to this embodiment is usually 0.1 to 10.0 parts by mass, preferably 0.5 to 5.0 parts by mass, and more preferably 0.5 to 3.0 parts by mass, when the amount of solids in the chloroprene copolymer latex (A) is 100 parts by mass. If the amount of vulcanization accelerator (C) is within this range, an appropriate vulcanization rate can be obtained, making it less likely for a deficiency in the crosslinked structure due to insufficient vulcanization treatment to occur, and also making scorching less likely. Furthermore, the vulcanization density (density of the crosslinked structure due to vulcanization treatment) of the molded product obtained from the chloroprene copolymer latex composition according to this embodiment will also be appropriate, so by setting the amount of vulcanization accelerator (C) within the above range, appropriate mechanical properties can be imparted to the molded product.

[0052] Unlike sulfur (A-3), sulfur (D) is added to chloroprene copolymer latex (A) when producing a chloroprene copolymer latex composition after emulsion copolymerization. The type of sulfur (D) is not particularly limited, but in addition to powdered sulfur, precipitated sulfur, colloidal sulfur, surface-treated sulfur, and insoluble sulfur, sulfur-containing compounds such as polysulfides and high-molecular-weight polysulfides (excluding the above-mentioned vulcanization accelerators) can be used. Sulfur (D) may be used alone or in combination of two or more types. The amount of sulfur(D) contained in the chloroprene copolymer latex composition according to this embodiment is usually 0.1 to 10.0 parts by mass, preferably 0.2 to 7.0 parts by mass, and more preferably 0.8 to 5.0 parts by mass, when the amount of solids in the chloroprene copolymer latex (A) is 100 parts by mass. If the amount of sulfur(D) is within this range, an appropriate vulcanization rate can be obtained, and it is less likely that a deficiency in the crosslinking structure due to insufficient vulcanization treatment will occur, and scorching will also be less likely to occur. In addition, the colloidal state of the chloroprene copolymer latex composition is stabilized, so problems such as sedimentation are less likely to occur.

[0053] The type of antioxidant (E) is not particularly limited, but if it is desirable for the molded product to have high heat resistance, it is preferable to use in combination an antioxidant that prevents aging due to heat and an antioxidant that prevents aging due to ozone.

[0054] Examples of antioxidants that prevent aging due to heat include diphenylamine-based antioxidants such as octylated diphenylamine, p-(p-toluenesulfonylamide)diphenylamine, and 4,4'-bis(α,α-dimethylbenzyl)diphenylamine. Such antioxidants can impart heat resistance to molded products and also provide stain resistance (such as suppression of discoloration).

[0055] Examples of antioxidants that prevent aging caused by ozone include N,N'-diphenyl-p-phenylenediamine (DPPD) and N-isopropyl-N'-phenyl-p-phenylenediamine (IPPD).

[0056] When the chloroprene copolymer rubber molded product according to this embodiment is used as a disposable medical glove, appearance (especially color) and hygiene are important, so it is preferable to use a hindered phenol antioxidant as the antioxidant (E). Examples of hindered phenol antioxidants include 2,2'-methylenebis-(4-ethyl-6-t-butylphenol) and 4,4'-methylenebis-(2,6-di-t-butylphenol).

[0057] The amount of antioxidant (E) contained in the chloroprene copolymer latex composition according to this embodiment is usually 0.1 to 10.0 parts by mass, preferably 0.5 to 5.5 parts by mass, and more preferably 2.0 to 4.8 parts by mass, when the amount of solids of the chloroprene copolymer latex (A) is 100 parts by mass. If the amount of antioxidant (E) is within this range, a sufficient antioxidant effect can be obtained without hindering the vulcanization process, and deterioration of color tone is unlikely to occur.

[0058] The chloroprene copolymer latex composition may contain a synthetic rubber (F) that is miscible with the chloroprene copolymer latex (A). The inclusion of synthetic rubber (F) in the chloroprene copolymer latex composition is preferable because it allows for the imparting of other rubber properties to the molded product that are not present in the chloroprene copolymer alone. The miscible synthetic rubber (F) can be selected from isoprene rubber, chloroprene rubber (excluding the chloroprene copolymer contained in the chloroprene copolymer latex (A)), acrylonitrile butadiene rubber, butadiene rubber, and the like. From the viewpoint of compatibility with the chloroprene copolymer latex (A), isoprene rubber and chloroprene rubber are preferred as synthetic rubber (F). If necessary, two or more types of synthetic rubber (F) may be used in the chloroprene copolymer latex composition.

[0059] The synthetic rubber (F) in the chloroprene copolymer latex composition may be blended in a range that does not hinder the objectives of the present invention. When the chloroprene copolymer latex composition contains a mixable synthetic rubber (F), the upper limit of the proportion of synthetic rubber (F) is preferably 50% by mass or less, more preferably 25% by mass or less, and even more preferably 10% by mass or less, when the total of the solid content of the chloroprene copolymer latex (A) and the synthetic rubber (F) is taken as 100% by mass. Furthermore, when the chloroprene copolymer latex composition contains synthetic rubber (F), the lower limit of the proportion of synthetic rubber (F) is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more. When the proportion of synthetic rubber (F) is 50% by mass or less, the maturation time and vulcanization time of the chloroprene copolymer latex composition are not prolonged. When the proportion of synthetic rubber (F) is 1% by mass or more, it is preferable for expressing the characteristics of other synthetic rubbers (F). The amount of synthetic rubber (F) added to the chloroprene copolymer latex composition is preferably 100 parts by mass or less, more preferably 33 parts by mass or less, and even more preferably 10 parts by mass or less, when the solid content of the chloroprene copolymer latex (A) is 100 parts by mass. The amount of synthetic rubber (F) added is preferably 1 part by mass or more, more preferably 3.1 parts by mass or more, and even more preferably 5.3 parts by mass or more, when the solid content of the chloroprene copolymer latex (A) is 100 parts by mass.

[0060] When a chloroprene copolymer latex composition contains chloroprene copolymer latex (A) and synthetic rubber (F), if the total solid content of chloroprene copolymer latex (A) and synthetic rubber (F) is 100 parts by mass, the chloroprene copolymer latex composition may contain 0.1 to 20.0 parts by mass of metal oxide (B), 0.1 to 10.0 parts by mass of vulcanization accelerator (C), 0.1 to 10.0 parts by mass of sulfur (D), and 0.1 to 10.0 parts by mass of antioxidant (E). Furthermore, the synthetic rubber (F) may be a latex in which fine particles of synthetic rubber (F) are dispersed. When using synthetic rubber (F) latex, if the total solid content of the chloroprene copolymer latex (A) and the synthetic rubber (F) latex is 100 parts by mass, the chloroprene copolymer latex composition may contain 0.1 to 20.0 parts by mass of metal oxide (B), 0.1 to 10.0 parts by mass of vulcanization accelerator (C), 0.1 to 10.0 parts by mass of sulfur (D), and 0.1 to 10.0 parts by mass of antioxidant (E).

[0061] In addition to chloroprene copolymer latex (A), metal oxide (B), vulcanization accelerator (C), sulfur (D), antioxidant (E), and synthetic rubber (F), other additives may be added to the chloroprene copolymer latex composition according to this embodiment, as long as the objectives of the present invention are not impaired. Examples of additives that can be added include pH adjusters, fillers, pigments, colorants, defoamers, and thickeners.

[0062] [Chloroprene copolymer rubber molded product] A chloroprene copolymer rubber molded product can be obtained by molding and curing the chloroprene copolymer latex composition according to the present invention. For example, a dipped product can be obtained by molding the chloroprene copolymer latex composition according to the embodiment by a dipping process. Prior to the immersion process, the chloroprene copolymer latex composition according to this embodiment can be aged under predetermined conditions. The aging temperature is 15 to 40°C, and the aging time is 15 to 72 hours. For example, aging at 23°C for 20 hours can be used. The aging process begins when the chloroprene copolymer latex (A) is mixed with all of the metal oxide (B), vulcanization accelerator (C), sulfur (D), and antioxidant (E). After maturation, a film-like molded product is obtained by performing the following steps in this order: immersion and solidification treatment, drying, and vulcanization (hardening).

[0063] The immersion and solidification treatment can be carried out by immersing a plate or mold coated with a solidifying agent in a chloroprene copolymer latex composition for a predetermined period of time, thereby depositing solid components in the chloroprene copolymer latex composition, such as chloroprene copolymer, onto the surface of the plate or mold. This is presumed to be because, in the chloroprene copolymer latex (A), fine particles covered with a film such as an emulsifier having surfactant properties are formed, but the film of the fine particles breaks down due to the action of the solidifying agent adhering to the surface of the plate or mold, causing the chloroprene copolymer and other components in the fine particles to adhere to the surface of the plate or mold. As the solidifying agent, metal salts can be used, for example, metal nitrates can be used. To avoid problems with the appearance of the molded product, such as the formation of blisters and pinholes, a drying process (rough drying process) may be performed before the vulcanization process at a relatively low temperature of 70°C to 100°C.

[0064] The vulcanization temperature in the vulcanization process can be set to 110 to 140°C. For example, the solid portion of chloroprene copolymer latex deposited by immersion and solidification treatment can be vulcanized at 110°C under air. The vulcanization time within the above vulcanization temperature range can be, for example, 15 minutes to 90 minutes, but it is preferable to perform sufficient vulcanization within a range that does not worsen the tensile strength and tensile elongation of the molded product. A chloroprene copolymer rubber molded product can be obtained by vulcanizing the composition deposited on the surface of a plate or mold under the above conditions.

[0065] When vulcanization is performed, it is presumed that crosslinking structures are formed in the chloroprene copolymer deposited on the surface of the plate or mold due to both sulfur (A-3) and sulfur (D) that remain unreacted during polymerization. Here, since sulfur (A-3) was contained in the microparticles before the breakdown of the microparticles, sulfur (A-3) tends to accumulate between the chloroprene copolymers contained within the same microparticles, and as a result, it is presumed that the formation of crosslinking structures between chloroprene copolymers contained within the same microparticles is easily promoted by sulfur (A-3). On the other hand, sulfur (D) is added during the manufacturing stage of the chloroprene copolymer latex composition and does not enter into the microparticles of the chloroprene copolymer. Therefore, sulfur (D) tends to accumulate between the chloroprene copolymers that existed between different microparticles before the breakdown of the microparticles, and as a result, it is presumed that the formation of crosslinking structures between molecules of chloroprene copolymers that existed between different microparticles before the breakdown of the microparticles is easily promoted by sulfur (D). Furthermore, depending on the structure derived from sulfur (A-3) or the distribution of sulfur (D), cross-linked structures may be formed within a single chloroprene polymer molecule. It is presumed that the cross-linked structures formed by sulfur (A-3) and sulfur (D) are particularly abundant at the positions where monomer units derived from 2-methyl-1,3-butadiene (A-2) are present within the chloroprene polymer molecule.

[0066] As described above, when the chloroprene copolymer latex composition according to the present invention is subjected to vulcanization treatment, it is presumed that the number of crosslinked structures between the chloroprene copolymers contained in the same fine particles in the chloroprene copolymer latex (A) will be greater than when a chloroprene copolymer manufactured without using sulfur (A-3) is crosslinked. As a result, it is thought that a chloroprene copolymer rubber molded product with high tensile strength, excellent flexibility, and low stress relaxation can be obtained. In other words, when the chloroprene copolymer latex composition according to the present invention is subjected to immersion and solidification treatment, it is presumed that the unreacted sulfur (A-3) contained in the fine particles of chloroprene copolymer latex can be dispersed in the solid content deposited on the surface of the plate or mold. Furthermore, as a result, it is thought that when sulfur (A-3) is used in the production of chloroprene copolymer latex, the distribution of sulfur in the deposited solid content becomes finer compared to when sulfur (A-3) is not used, resulting in a molded product that is more flexible and has lower stress relaxation than conventional molded products. In the embodiments described later, the 100% modulus of elasticity is used as an indicator of flexibility, with a smaller value indicating higher flexibility. Furthermore, the retention rate of the 100% modulus of elasticity is used as an indicator of stress relaxation, with a value closer to 100% indicating less stress relaxation.

[0067] [Medical disposable gloves] Chloroprene copolymer rubber molded products can be suitably used, in particular, as disposable medical gloves. The 100% modulus of elasticity, 500% modulus of elasticity, tensile strength, 100% modulus retention rate, and tensile elongation of chloroprene copolymer rubber molded products can be measured by the methods used in the examples described later.

[0068] If the 100% modulus of elasticity of a chloroprene copolymer rubber molded product is 1.1 MPa or less, it has sufficient flexibility for use as a disposable medical glove. The lower limit of the 100% modulus of elasticity of a chloroprene copolymer rubber molded product may be, for example, 0.6 MPa or more. If the 500% modulus of elasticity of a chloroprene copolymer rubber molded product is between 1.0 and 2.5 MPa, the feel of medical disposable gloves will be soft, making them less tiring for the user even when worn for extended periods. A tensile strength of 20 MPa or higher is preferable for medical disposable gloves, as it reduces the likelihood of breakage. The upper limit of the tensile strength of the chloroprene copolymer rubber molded product may be, for example, 35 MPa or less. If the 100% elastic modulus retention rate is 90% or higher, the glove will have a high degree of adaptability to the stretching and contracting caused by the user's hand movements, and the user will be able to experience a superior feel. The upper limit of the 100% elastic modulus retention rate may be, for example, 95% or less. A tensile elongation of 800% or more in a chloroprene copolymer rubber molded product is preferable because it reduces the likelihood of breakage in medical disposable gloves. The upper limit of the tensile elongation of a chloroprene copolymer rubber molded product may be, for example, 1500% or less. [Examples]

[0069] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0070] <Method for calculating polymerization conversion rate> The emulsion of the chloroprene copolymer after polymerization initiation was collected and dried in an oven at 141°C for 30 minutes to obtain a dry product. The dry product obtained by the drying process contains both polymer and non-polymer solids. Therefore, the mass of the non-polymer solids was calculated from the amount of polymerization raw materials used, based on the various components used in emulsion polymerization that do not volatilize at 141°C. Furthermore, the "amount of chloroprene copolymer produced" was calculated by subtracting the mass of the non-polymer solids from the mass of the dry product obtained by drying the emulsion after polymerization initiation, and the polymerization conversion rate was calculated using formula (1). The calculated polymerization conversion rates are shown in Table 1. Polymerization conversion rate [mass%] = [(amount of chloroprene copolymer produced) / (total amount of monomers used)] × 100 ... (1) In formula (1), "total monomer charge mass" is the sum of the amount of 2-chloro-1,3-butadiene (A-1), 2-methyl-1,3-butadiene (A-2), and optionally monomer (A-4) contained in the emulsion collected to obtain the dry product.

[0071] [Method for measuring the physical properties of chloroprene copolymer latex (A)] The various physical properties of the obtained chloroprene copolymer latex (A) were evaluated using the following method.

[0072] <Insoluble fraction of tetrahydrofuran in chloroprene copolymer> The tetrahydrofuran insoluble fraction of the chloroprene copolymer was measured as follows: 1 g of chloroprene copolymer latex (A) was added dropwise to 100 mL of tetrahydrofuran at 25°C and shaken for 10 hours in a Yamato Scientific Co., Ltd. shaker (SA300). The mixture of chloroprene copolymer latex (A) and tetrahydrofuran after shaking was subjected to centrifugal sedimentation separation using a centrifugal sedimentation separator (Kokusan Co., Ltd., H-9R) to obtain the supernatant dissolved phase. The obtained dissolved phase was heated to 100°C, and the tetrahydrofuran was evaporated over 1 hour. The mass of the dry substance was then measured. This yielded the mass of the chloroprene copolymer that was dissolved in the dissolved phase.

[0073] By substituting the mass of the chloroprene copolymer in 1 g of chloroprene copolymer latex (A) and the mass of the dissolved portion mentioned above into equation (2), the content of tetrahydrofuran-insoluble components in the chloroprene copolymer that do not dissolve in tetrahydrofuran at 25°C (tetrahydrofuran-insoluble fraction) was calculated. The tetrahydrofuran-insoluble fractions are shown in Table 1. Tetrahydrofuran insoluble fraction (mass%) ={1-[(mass of dissolved substance) / (mass of chloroprene copolymer in 1g of chloroprene copolymer latex (A))]}×100 ···(2) In formula (2), the mass of chloroprene copolymer in 1 g of chloroprene copolymer latex (A) was considered to be the mass of solids obtained by drying 1 g of chloroprene copolymer latex (A). When drying chloroprene copolymer latex (A), it was dried by standing it in an oven at 141°C for 30 minutes.

[0074] <Weight average molecular weight (Mw)> The following describes an example of how to determine the weight-average molecular weight (Mw) of the tetrahydrofuran-soluble component in a chloroprene copolymer at 25°C. The sample was prepared by similar procedures to those used for measuring the tetrahydrofuran-insoluble component, by separating the dissolved phase from the supernatant after centrifugal sedimentation, and then diluting it with tetrahydrofuran. The weight-average molecular weight (Mw) was determined by measuring the molecular weight of the obtained sample in polystyrene equivalents using GPC (gel permeation chromatography).

[0075] The GPC measurement conditions were as follows: Shimadzu Corporation LC-20AD GPC analyzer, Shimadzu Corporation RID-10A (differential refractive index detector) detector, Agilent Technologies PLgel 10μm MiniMIX-B column, tetrahydrofuran eluent (Kanto Chemical Co., Ltd., for HPLC), column temperature: 40°C, efflux rate: 0.4 ml / min.

[0076] <Monomer unit content in chloroprene copolymer> The content of the 2-methyl-1,3-butadiene (A-2) derived component in the chloroprene copolymer is: 1 The composition was determined by 1H-NMR analysis. The obtained chloroprene copolymer latex was solidified with methanol, dried, and then deuterated chloroform was added to the resulting solid. After filtering out substances insoluble in deuterated chloroform, the obtained solution was collected. 1 1H-NMR analysis was performed. 1 For 1H-NMR analysis, a JEOL Ltd. JNM-AL400 was used as the measuring instrument, and tetramethylsilane was used as the chemical shift reference. 1 From the peaks derived from 2-chloro-1,3-butadiene (A-1) (5.4 ppm) and 2-methyl-1,3-butadiene (A-2) (5.1 ppm) in the 1H-NMR spectrum, the content of the component derived from 2-methyl-1,3-butadiene (A-2) was calculated using equation (3). Content (mol%) of 2-methyl-1,3-butadiene (A-2) derived component = (Peak area at 5.1 ppm) / (Peak area at 5.1 ppm + Peak area at 5.4 ppm) × 100 ... (3)

[0077] Furthermore, even if monomer (A-4) is present, if monomer (A-4) does not have peaks that overlap with the 5.1 ppm and 5.4 ppm peaks, formula (3) can be used to determine the proportion of 2-methyl-1,3-butadiene (A-2) in the total of 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2). When determining the content of monomer (A-4), the peak area of ​​the peak derived from monomer (A-4) that does not overlap with either the peaks of 2-chloro-1,3-butadiene (A-1) or 2-methyl-1,3-butadiene (A-2) is used. Using the same formula as in formula (3), the proportion of monomer (A-4) in the total of 2-chloro-1,3-butadiene (A-1) and monomer (A-4) is calculated. Similarly, the proportion of monomer (A-4) can be determined when the total amount of monomer units derived from 2-chloro-1,3-butadiene (A-1) and monomer units derived from 2-methyl-1,3-butadiene (A-2) is set to 100 mol%.

[0078] If monomer (A-4) has peaks that overlap with the 5.1 ppm peak and the 5.4 ppm peak, 1 H- 1By using multidimensional NMR measurement results such as H COSY (COrrelation Spectroscop Y), peaks originating from 2-chloro-1,3-butadiene (A-1), 2-methyl-1,3-butadiene (A-2), and monomer (A-4) are identified, and the proportion of each substance can be determined by performing similar calculations using the peak areas.

[0079] [Example 1] (1) Preparation of chloroprene copolymer latex (A) In a 5L reactor, 1200g of 2-chloro-1,3-butadiene (A-1), 300g of 2-methyl-1,3-butadiene (A-2), 3.8g of sulfur (Tsurumi Chemical Industries, Ltd., Kinka brand finely powdered sulfur) (A-3), 1360g of pure water, 65g of disproportionated rosinic acid (Arakawa Chemical Industries, Ltd., R-600), 2.3g of dimer acid (Harima Chemicals Group Co., Ltd., Haridaimer 200), 54.0g of sodium hydroxide, 21.1g of sodium salt of β-naphthalene sulfonic acid formalin condensate, and 11.7mg of copper sulfate were charged. The starting materials charged in the reactor were emulsified, and rosinic acid was converted into rosinic acid soap.

[0080] Furthermore, 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2) were added as raw material monomers for the chloroprene copolymer, and pure water was added as a dispersion medium for emulsion polymerization. In addition, rosin acid, dimer acid, and sodium hydroxide were added as raw materials for emulsifiers, and the sodium salt of β-naphthalene sulfonic acid formalin condensate was added as an emulsifier. Copper sulfate was added as a co-catalyst for emulsion polymerization.

[0081] To the emulsion obtained by emulsifying the starting materials, 4 g of potassium persulfate was added as a polymerization initiator, and emulsion polymerization was carried out at 30°C under a nitrogen gas atmosphere. Polymerization was stopped when the polymerization conversion rate of all monomers reached 84% by mass (after 7.6 hours). The method for calculating the polymerization conversion rate will be described later. Subsequently, unreacted 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2) were removed by steam distillation to obtain chloroprene copolymer latex (A1).

[0082] (2) Preparation of chloroprene copolymer latex composition To the chloroprene copolymer latex (A1) obtained in (1) above, zinc oxide (B), a vulcanization accelerator (C), sulfur (D), and an antioxidant (E) were added. The amounts of these added were as follows, per 100 parts by mass of solids in the chloroprene copolymer latex (A1): 0.5 parts by mass of zinc oxide (AZ-SW, manufactured by Osaki Industries Co., Ltd.), 0.5 parts by mass of zinc dibutyldithiocarbamate (Noxella® BZ, manufactured by Ouchi Shinko Chemical Industry Co., Ltd.), 0.5 parts by mass of zinc 2-mercaptobenzothiazole (Noxella® MZ, manufactured by Ouchi Shinko Chemical Industry Co., Ltd.), 0.25 parts by mass of diphenylguanidine (Noxella® D, manufactured by Ouchi Shinko Chemical Industry Co., Ltd.), 1.5 parts by mass of sulfur (S-50, manufactured by Nippon Color Industry Co., Ltd.), and 2.0 parts by mass of a phenolic antioxidant (K-840, manufactured by Chukyo Oils Co., Ltd.). The above amounts of chloroprene copolymer latex (A1), zinc oxide (B), vulcanization accelerator (C), sulfur (D), and antioxidant (E) were placed in a container equipped with a stirring device. The mixture was stirred for 20 minutes to obtain a chloroprene copolymer latex composition. The chloroprene copolymer latex composition, after stirring, was allowed to mature by standing at 23°C for 20 hours.

[0083] Furthermore, since zinc oxide (AZ-SW), sulfur (S-50), and phenolic antioxidant (K-840) are in the form of dispersions in which the active ingredients zinc oxide (B), sulfur (D), and antioxidant (E) are dispersed in a liquid medium, the amount of zinc oxide (AZ-SW), sulfur (S-50), and phenolic antioxidant (K-840) charged as described above refers only to the amount of the active ingredients among the charged zinc oxide (AZ-SW), sulfur (S-50), and phenolic antioxidant (K-840).

[0084] (3) Film preparation Using the chloroprene copolymer latex composition obtained in (2) above, a chloroprene copolymer film was formed by immersion processing. A ceramic plate measuring 200 mm in length, 100 mm in width, and 5 mm in thickness was prepared as a mold for the chloroprene copolymer film. This mold was immersed in a 30% by mass calcium nitrate aqueous solution, then removed and dried in a 40°C oven for 10 minutes, thereby adhering the calcium nitrate, which acts as a coagulant, to the surface of the mold.

[0085] Furthermore, the dried mold was immersed in the chloroprene copolymer latex composition obtained in (2) above, and the solid components of the chloroprene copolymer latex composition were deposited on the surface of the mold. After removing the mold from the chloroprene copolymer latex composition, it was dried in a 70°C oven for 30 minutes. Next, the mold with solid material deposited on its surface was heated in an oven at 110°C for 30 minutes to vulcanize and cure the solid material of the chloroprene copolymer latex composition deposited on the surface of the mold. After cooling in the atmosphere, the cured molded product was cut from the surface of the mold into the desired shape and size to obtain a film as a vulcanized chloroprene copolymer rubber molded product.

[0086] (4) Method for measuring the physical properties of molded products The film was cut to match the size of a No. 6 dumbbell as specified in JIS K6251:2017 to obtain test specimens. The thickness of these test specimens was 0.15 to 0.25 mm.

[0087] <Tensile strength, tensile elongation, elastic modulus> Tensile tests were performed at 23°C according to the method specified in JIS K6251:2017 to measure tensile strength, tensile elongation, modulus of elasticity at 100% elongation (100% modulus), and modulus of elasticity at 500% elongation (500% modulus). The various physical properties of the measured films are summarized in Table 1.

[0088] <Retention rate of elastic modulus> The test specimens obtained by the same method as described above were stretched at 23°C at a tensile speed of 200 mm / min until the elongation reached 100%, and held for 2 minutes. The ratio of the initial stress to the stress after 2 minutes was calculated using equation (4) and was defined as the elastic modulus retention rate. 100% Elastic Modulus Retention Rate (%) = (Elongation rate after 2 minutes, modulus of elasticity at 100% elongation) / (Initial elongation rate, modulus of elasticity at 100% elongation) × 100 ... (4) Table 1 summarizes the various physical properties of the film measured as described above.

[0089] [Examples 2-8] A chloroprene copolymer latex composition was prepared in the same manner as in Example 1, except that the polymerization and compounding conditions were as shown in Table 1. Furthermore, films and test pieces were manufactured in the same manner as in Example 1, and various evaluations were performed. The results are shown in Table 1.

[0090] [Comparative Examples 1-3] Comparative Examples 1-3 are comparative examples in which sulfur (A-3) is not present during the polymerization of the chloroprene copolymer, but sulfur (D) is used when preparing the latex composition. Chloroprene copolymer latex compositions were prepared in the same manner as in Example 1, except that the polymerization conditions and blending conditions were as shown in Table 2. Furthermore, films and test pieces were manufactured in the same manner as in Example 1, and various evaluations were performed. The results are shown in Table 2.

[0091] [Comparative Example 4] Comparative Example 4 is a comparative example in which n-dodecyl mercaptan was present during the polymerization of the chloroprene copolymer, but without the presence of sulfur (A-3). In Comparative Example 4, the chloroprene copolymer was produced in the same manner as in Example 1, except that 17.1 g of potassium hydroxide was added during polymerization, the amount of sodium hydroxide added was changed to 3.9 g, the amount of sodium salt of β-naphthalene sulfonic acid formalin condensate added was changed to 3.3 g, and 1.65 g of n-dodecyl mercaptan was added. Furthermore, the chloroprene copolymer latex composition, film, and test pieces were prepared in the same manner as in Example 1, and various evaluations were performed in the same manner as in Example 1. The results are shown in Table 2.

[0092] [Comparative Examples 5-7] Comparative Examples 5-7 are comparative examples in which, instead of using 2-methyl-1,3-butadiene (A-2) as the monomer unit during the polymerization of chloroprene copolymers, 2,3-dichloro-1,3-butadiene is copolymerized with 2-chloro-1,3-butadiene (A-1). In Comparative Examples 5 and 6, sulfur (A-3) is not present during copolymerization, while in Comparative Example 7, sulfur (A-3) is present during copolymerization. A chloroprene copolymer latex composition was prepared in the same manner as in Example 1, except that the polymerization and compounding conditions were as shown in Table 2. Furthermore, films and test pieces were manufactured in the same manner as in Example 1, and various evaluations were performed. The results are shown in Table 2.

[0093] [Table 1]

[0094] [Table 2]

[0095] The chloroprene copolymer latex obtained in Examples 1-8, which was produced by emulsion polymerization of 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2) in the presence of sulfur (A-3), was vulcanized under mild conditions to obtain a chloroprene copolymer rubber molded product with a high tensile strength of 20 MPa or more. Furthermore, the chloroprene copolymer rubber molded products obtained in Examples 1-8 have a high 100% elastic modulus retention rate of 90% and low stress relaxation characteristics. For these reasons, gloves manufactured using the chloroprene copolymer rubber molded products obtained in Examples 1-8 are less prone to breakage, have high flexibility to follow the stretching and contracting caused by the user's hand movements, and have a good tactile sensation, making them preferable as disposable medical gloves. It is presumed that such molded products were obtained because, during the vulcanization process, sulfur (A-3) promoted the formation of crosslinking structures between chloroprene copolymers present within one microparticle of chloroprene copolymer latex (A), and further, sulfur (D) promoted the formation of crosslinking structures between chloroprene copolymers present in different microparticles of chloroprene copolymer latex (A).

[0096] On the other hand, in Comparative Examples 1 to 4, where copolymerization was carried out under conditions where sulfur (A-3) was absent, the tensile strength and 100% modulus retention rate did not reach the performance required for medical disposable gloves. As in Comparative Example 3, when the amount of sulfur (D) blended into the latex composition was increased, the 100% modulus retention rate improved, but the tensile strength was lower compared to the mechanical properties shown in Examples 1 to 4. In Comparative Examples 1 to 4, unlike Examples 1 to 4, it is presumed that no crosslinking structure of the chloroprene copolymer due to sulfur (A-3) was formed, and only a crosslinking structure between chloroprene copolymers due to sulfur (D) was formed. These results suggest that, when mild vulcanization conditions such as heating at 110°C for 30 minutes are used, the crosslinking structure between chloroprene copolymers due to sulfur (D) blended in Comparative Examples 1 to 4 alone is insufficient to obtain molded products with the performance required for medical disposable gloves.

[0097] From the above comparative results, it can be seen that the coexistence of sulfur (A-3) during the emulsion polymerization of chloroprene copolymer is useful in the production of chloroprene copolymer latex that has the desired flexibility and stress relaxation properties for use as gloves.

[0098] Furthermore, as shown in Examples 2, 4, 6, and 8, when sulfur (A-3) is present during the emulsion polymerization of the chloroprene copolymer, even when the amount of vulcanization accelerator (C) is reduced, a molded product with a tensile strength of 20 MPa or more and a 100% modulus retention rate of 90% or more can be obtained, making it suitable for use as a disposable medical glove. Furthermore, the chloroprene copolymer latex compositions produced in Examples 2, 4, 6, and 8 do not contain diphenylguanidine or N,N'-diphenylthiourea, which are recognized as sensitizing substances in Europe and other regions, and are therefore more effective in preventing allergies.

[0099] As shown in Comparative Examples 5 to 7, when the chloroprene copolymer contained in the latex composition does not contain monomer units derived from 2-methyl-1,3-butadiene (A-2), the tensile strength and 100% modulus retention of the resulting molded product are lower.

Claims

1. The material contains 100 parts by mass of a chloroprene copolymer latex (A) obtained by emulsion polymerization of 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2) in the presence of 0.1 to 1.0 parts by mass of sulfur (A-3) when the total amount of 2-chloro-1,3-butadiene (A-1) and 2-methyl-1,3-butadiene (A-2) is 100 parts by mass, and optionally synthetic rubber (F). 0.1 to 20.0 parts by mass of metal oxide (B) Vulcanization accelerator (C) in an amount of 0.1 to 10.0 parts by mass, Sulfur (D) in an amount of 0.1 to 10.0 parts by mass, and Antioxidant (E) 0.1 to 10.0 parts by mass It is a chloroprene copolymer latex composition, A chloroprene copolymer latex composition having a 100% modulus retention rate of 90% or more when a tensile test is performed at 23°C in accordance with JIS K6251:2017 on a chloroprene copolymer rubber molded product obtained by curing the chloroprene copolymer latex composition.

2. The chloroprene copolymer latex composition according to claim 1, wherein a chloroprene copolymer rubber molded product obtained by curing the chloroprene copolymer latex composition exhibits a tensile strength of 20 MPa or more when subjected to a tensile test at 23°C in accordance with JIS K6251:2017.

3. The chloroprene copolymer latex composition according to claim 1, wherein the tetrahydrofuran insoluble fraction of the chloroprene copolymer at 25°C is 5 to 80% by mass.

4. The chloroprene copolymer latex composition according to claim 1, wherein the weight-average molecular weight of the component soluble in tetrahydrofuran at 25°C in the chloroprene copolymer is 400,000 or more.

5. Contains synthetic rubber (F), The chloroprene copolymer latex composition according to claim 1, wherein the proportion of synthetic rubber (F) is 1 to 50% by mass when the total of the solid content of the chloroprene copolymer latex (A) and the synthetic rubber (F) is 100% by mass.

6. A chloroprene copolymer rubber molded product obtained by curing the chloroprene copolymer latex composition according to any one of claims 1 to 5.

7. A dipped product obtained by molding and curing a chloroprene copolymer latex composition according to any one of claims 1 to 5 by dipping.

8. The immersion product according to claim 7, which is a glove.

9. The immersion product according to claim 8, which is a disposable medical glove.

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