JP4778719B2 - Copolymer latex for can sealant - Google Patents

Description

  The present invention relates to a copolymer latex for can sealing materials, and more particularly to a copolymer latex for can sealing materials suitable for producing a can sealing material excellent in viscosity stability and lining workability.

  Conventionally, when manufacturing canned foods, the can body is filled with the contents, then the can end (can end) is covered, and then double winding is performed between the can body flange and the peripheral edge of the can end. Sealing is performed. At this time, in order to completely seal the double-clamping portion between the can body and the can lid, a method is used in which a sealing material is lined in a groove provided at the peripheral edge of the can end and then this is wound. ing.

  This sealing material is usually prepared mainly with rubber. As this rubber, a solid rubber called a cold rubber was generally used. However, (1) Since solid rubber contains impurities such as a polymerization terminator used in the production process, these impurities must be removed. The can sealing material is used in the form of an aqueous copolymer latex or a solution. Therefore, it is necessary to disperse the solid rubber in water, or dissolve or disperse it in a solvent, which is inferior in productivity when producing a can sealing material. In addition, (2) solid rubber has a drawback that molecules are cut during mastication and processing, resulting in deterioration of mechanical and chemical characteristics, and as a result, the sealing performance of the can sealing material is deteriorated.

  As a conventional technique for solving these drawbacks, a copolymer latex suitable as a constituent material of a can sealing material, in which the toluene insoluble content and Mooney viscosity of the copolymer constituting the copolymer are defined within a predetermined numerical range. Is disclosed (for example, see Patent Document 1). If this copolymer latex is used, a can sealing material excellent in sealing property, winding property, hot water squeeze-out property, and flavor can be obtained.

However, even the copolymer latex disclosed in Patent Document 1 has a problem that its viscosity gradually decreases with time. Therefore, there is a case where it becomes difficult for the can sealing material to exhibit sufficient sealing performance for a long period of time, and thus there is a need to improve the viscosity stability. Moreover, in recent years, in order to improve the productivity of manufacturing sealed cans, there is a tendency to further increase the traveling speed of the winding machine in the can winding process. However, when such a high-speed winding system is adopted, the workability (lining workability) is not always good with the conventional can sealing material. Therefore, there is a need to improve the lining workability of the can sealing material in order to further improve the productivity of manufacturing the sealed can.
Japanese Patent No. 2619301

  The present invention has been made in view of such problems of the prior art, and the object of the present invention is to provide a can sealing suitable for the production of a can sealing material having excellent viscosity stability and lining workability. It is to provide a copolymer latex for materials.

  As a result of intensive studies to achieve the above-mentioned problems, the present inventors have performed a treatment for reducing and removing persulfate used as an initiator in the polymerization after the polymerization and remaining in the components. It has been found that the above-mentioned problems can be achieved by reducing the concentration of the persulfate, and the present invention has been completed.

  That is, according to the present invention, the following copolymer latex for can sealing materials is provided.

[1] A copolymer latex for can sealing materials produced by polymerization using persulfate as an initiator, and after polymerization, a reducing agent is added to form the residual persulfate (residual excess). A copolymer latex for a can sealing material, further comprising a reduction treatment of sulfate), wherein the concentration of the residual persulfate after the reduction treatment is 50 ppm or less.

[ 2 ] The copolymer latex for can sealing material according to [ 1 ], wherein the reducing agent is sodium bisulfite.

[ 3 ] A monomer component mainly composed of (a) a conjugated diene monomer, (b) an aromatic vinyl compound, and (c) another copolymerizable monomer used as necessary. Is produced by emulsion polymerization at a polymerization conversion of 50 to 90% by mass, and the copolymer constituting it is 10 to 70% insoluble in toluene, Mooney viscosity (ML _{1 + 4} , 100 The copolymer latex for can sealing material according to the above [1] or [2] , wherein the (° C.) is 60 to 150.

The copolymer latex for can sealing material of the present invention is obtained by polymerizing and then adding a reducing agent to further reduce the residual persulfate. The density is below a predetermined density. Therefore, there is an effect that it is suitable for production of a can sealing material excellent in viscosity stability and lining workability.

  BEST MODE FOR CARRYING OUT THE INVENTION The best mode for carrying out the present invention will be described below, but the present invention is not limited to the following embodiment, and is based on the ordinary knowledge of those skilled in the art without departing from the gist of the present invention. It should be understood that modifications and improvements as appropriate to the following embodiments also fall within the scope of the present invention.

One embodiment of the copolymer latex for can sealing material of the present invention is produced by polymerization using a persulfate as an initiator. After polymerization, a reducing agent is added to the residual latex. Further reduction treatment of sulfate (residual persulfate) is performed, and the concentration of residual persulfate after the reduction treatment is 50 ppm or less. The details will be described below.

  The copolymer latex for can sealing material of the present embodiment is produced by polymerization using persulfate as an initiator. After polymerization, the persulfate (residual excess) remaining in the component is used. (Sulphate) reduction treatment. Here, the “reduction treatment” is a treatment for reducing / removing the persulfate remaining immediately after polymerization (copolymer latex precursor) by chemically treating the persulfate, for example, using a reducing agent. Say. More specifically, it can be carried out by adding a reducing agent to the copolymer latex precursor and mixing it for an appropriate time. Examples of the reducing agent include sodium bisulfite, sodium sulfite, sodium pyrobisulfite, and ferrous sulfate. Among these, sodium bisulfite is preferably used because of its high effect of reducing the persulfate remaining in the copolymer latex precursor.

  The concentration of residual persulfate after the reduction treatment of the copolymer latex for can sealing material of this embodiment is 50 ppm or less. When the concentration of the residual persulfate is 50 ppm or less, the viscosity is hardly lowered even when a long period of time elapses, and an effect of excellent viscosity stability is obtained. Therefore, it becomes possible to obtain a can sealing material excellent in reliability over a long period of time. Moreover, since the copolymer latex for can sealing materials of this embodiment is hard to reduce in viscosity, a can sealing material excellent in lining workability can be provided. For this reason, if the copolymer latex for can sealing materials of this embodiment is used, the productivity improvement of sealed can manufacture can also be aimed at.

  In general, when persulfate is used as an initiator, the amount of this persulfate remaining in the copolymer latex decreases with time, but the “residual persulfate concentration (amount)” referred to in this specification. "" Means the concentration (amount) measured 48 hours after the completion of polymerization. From the viewpoint of providing a can sealing material that is more excellent in viscosity stability and lining workability, the concentration of residual persulfate after the reduction treatment is preferably 20 ppm or less, and more preferably 10 ppm or less. The lower limit value of the residual persulfate concentration after the reduction treatment of the copolymer latex for can sealing material of the present embodiment is not particularly limited, but is most preferably 0 ppm. Here, “the concentration of residual persulfate is 0 ppm” in this specification means that no persulfate is detected even by a potentiometric titration method (detection limit value = 0.1 ppm) in accordance with JIS K0113 (1997). Say.

The copolymer latex for can sealing material of the present embodiment is limited to the types and blending ratios of the monomer components constituting the copolymer latex as long as it is produced by polymerization using persulfate as an initiator. There is no. However, the copolymer latex for a can sealing material of the present embodiment includes (a) a conjugated diene monomer (hereinafter also simply referred to as “(a) component”), (b) an aromatic vinyl compound (hereinafter simply referred to as “a”). A monomer comprising “(b) component”) and (c) other copolymerizable monomer (hereinafter also simply referred to as “(c) component”) used as necessary. The body component is produced by emulsion polymerization at a polymerization conversion of 50 to 90% by mass, and the copolymer constituting it is 10 to 70% insoluble in toluene, Mooney viscosity (ML _{1 + 4} , 100 [deg.] C.) is preferably from 60 to 150 because a sealing material excellent in sealing property, winding property, hot water squeeze-out property, and flavor can be obtained. Hereinafter, the physical properties of each monomer component and copolymer will be described.

  Specific examples of the component (a) include 1,3-butadiene, isoprene, 2-chloro-1,3-butadiene and the like. Among these, 1,3-butadiene is particularly preferable. Moreover, these (a) components can be used individually or in mixture of 2 or more types.

  Specific examples of the component (b) include styrene, α-methylstyrene, p-methylstyrene, vinyltoluene, chlorostyrene, sodium styrenesulfonate, and the like. Of these, styrene is particularly preferable. Moreover, these (b) components can be used individually or in mixture of 2 or more types.

  Specific examples of the component (c) include vinyl cyanide monomers such as acrylonitrile and methacrylonitrile, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutyl. Methacrylate, amyl acrylate, amyl methacrylate, isoamyl acrylate, isoamyl methacrylate, hexyl acrylate, hexyl methacrylate, n-octyl acrylate, n-octyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, isononyl acrylate, isononyl methacrylate, lauryl Acrylic acid alkyl esters such as acrylate and lauryl methacrylate or Crylic acid alkyl esters, itaconic acid, acrylic acid, methacrylic acid, fumaric acid and other ethylenically unsaturated carboxylic acid monomers, acrylamide, methacrylamide and other amide monomers, glycidyl acrylate, glycidyl methacrylate and other glycidyl monomers Acrylic acid hydroxyalkyl ester monomers or methacrylic acid hydroxyalkyl ester monomers such as mer, β-hydroxyethyl acrylate, β-hydroxyethyl methacrylate, β-hydroxypropyl acrylate, β-hydroxypropyl methacrylate, etc. it can. Among these, an ethylenically unsaturated carboxylic acid monomer and fumaric acid are particularly preferable. Moreover, these (c) components can be used individually or in mixture of 2 or more types.

  The composition ratio of the components (a) to (c) in the monomer components is determined by the toluene insoluble content of the copolymer constituting the copolymer latex for can sealing materials obtained by emulsion polymerization of these monomer components. , Mooney viscosity, and glass transition temperature are taken into consideration. Preferably (a) component 30-80 mass%, (b) component 20-70 mass%, and (c) component 0-40 mass%, More preferably, (a) component 40-70 mass%, ( b) Component 30 to 60% by mass, (c) Component 0 to 30% by mass (provided that (a) + (b) + (c) = 100% by mass).

  When the component (a) is less than 30% by mass, the sealing property tends to be lowered. On the other hand, when the component (a) is more than 80% by mass, the squeeze resistance tends to decrease. Further, when the component (b) is less than 20% by mass, the squeeze resistance tends to decrease. On the other hand, when the component (b) is more than 70% by mass, the sealing property tends to be lowered. Furthermore, when the component (c) is more than 40% by mass, the sealing property tends to be lowered.

  Emulsion polymerization can be performed under normal emulsion polymerization conditions. For example, in water in an amount corresponding to 90 to 300% by mass of the monomer component, in the presence of an emulsifier, a molecular weight regulator, a polymerization initiator, and other electrolytes and pH regulators as necessary, the monomer What is necessary is just to emulsion-polymerize a component. The emulsion polymerization temperature is usually 5 to 85 ° C and preferably 35 to 80 ° C. The conjugated diene-based (co) polymer latex can generally be classified into two types, a cold rubber (polymerization temperature of about 10 ° C. or less) and a hot rubber (polymerization temperature of 35 to 85 ° C.) depending on the emulsion polymerization temperature. In the present embodiment, any copolymer latex may be used. However, a relatively large amount of a polymerization inhibitor is contained in the cold rubber, which may be mixed into the composition constituting the can sealing material and affect the flavor of the canned contents. Therefore, in this embodiment, it is preferable to use a hot rubber obtained by performing emulsion polymerization at a temperature of 35 to 80 ° C.

  Emulsifiers used in emulsion polymerization include anionic emulsifiers such as sodium dodecylbenzenesulfonate, sodium lauryl sulfate, sodium diphenyl ether disulfonate, sodium dialkyl ester sulfonate succinate, potassium rosinate and disproportionated potassium rosinate, polyoxy Nonionic emulsifiers such as ethylene alkyl ester and polyoxyethylene alkyl allyl ether can be used alone or in combination of two or more. Sodium dodecylbenzenesulfonate, disproportionated potassium rosinate, and purified disproportionated potassium rosinate are particularly preferred.

  The amount of the emulsifier used is preferably 0.1 to 7% by mass, and more preferably 0.5 to 5% by mass with respect to the monomer component. When the amount of the emulsifier used is less than 0.1% by mass relative to the monomer component, the polymerization stability tends to deteriorate and a coagulated product tends to be generated. On the other hand, if it exceeds 7% by mass, the sealing property of the can sealing material obtained as the final product tends to be lowered.

  In the emulsion polymerization, it is preferable to use a molecular weight regulator. Examples of molecular weight regulators include mercaptans such as t-dodecyl mercaptan, n-dodecyl mercaptan, octyl mercaptan, xanthogen disulfides such as dimethylxanthogen disulfide, diethylenexanthogen disulfide, diisopropylxanthogen disulfide, tetramethylthiuram disulfide, tetraethylenethiuram disulfide, Thiuram disulfides such as tetrabutylthiuram disulfide, halogenated hydrocarbons such as carbon tetrachloride and ethylene bromide, hydrocarbons such as α-methylstyrene dimer and pentaphenylethane, acrolein, methacrolein, allyl alcohol, 2- Examples thereof include ethylhexyl thioglycolate, terpinolene, α-terpinene, γ-terpinene, and dipentene. Among these, mercaptans are particularly preferable. These molecular weight regulators can be used alone or in combination of two or more. In addition, it is preferable to set it as 0.01-3 mass% with respect to a monomer component, and, as for the usage-amount of a molecular weight regulator, it is still more preferable to set it as 0.01-2 mass%.

  In emulsion polymerization, a persulfate such as potassium persulfate, sodium persulfate, or ammonium persulfate is used as a polymerization initiator. These persulfates can be used alone or in combination of two or more. The amount of the polymerization initiator used is preferably 0.03 to 2% by mass and more preferably 0.05 to 1% by mass with respect to the monomer component. In order to promote emulsion polymerization, reducing agents such as sodium pyrobisulfite, sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, glucose, formaldehyde sulfoxylate, L-ascorbic acid, glycine, alanine, ethylenediamine A chelating agent such as sodium acetate can also be used in combination.

  In performing emulsion polymerization, a batch addition method, a continuous addition method, or the like can be employed as a method for adding the monomer component into the polymerization system. The polymerization conversion rate in the emulsion polymerization is preferably 50 to 90% by mass.

  The toluene insoluble content of the copolymer constituting the copolymer latex for can sealing material of the present embodiment is preferably 10 to 70%, and more preferably 10 to 60%. When the toluene insoluble content of the copolymer is more than 70%, the adhesiveness of the copolymer becomes insufficient, and the sealing property of the can sealing material obtained as the final product tends to be lowered. On the other hand, when the toluene insoluble content of the copolymer is less than 10%, the squeeze resistance tends to be inferior.

  The “toluene insoluble matter” in the present specification refers to a value measured and calculated by the following method. First, the pH of the copolymer latex obtained by emulsion polymerization is adjusted to about 8, and then cast and dried on a glass plate to form a film having a thickness of 0.3 mm. Next, 0.3 g of the formed film is placed in 100 ml of toluene and allowed to stand for about 16 hours, and then shaken by hand. Furthermore, after leaving still for 4 hours and dissolving a film, the obtained solution is filtered with a 120 mesh metal-mesh, and a filtrate is extract | collected using a whole pipette. The collected filtrate is evaporated and dried using a hot plate to measure the mass of toluene insoluble matter. From the measured mass, the ratio of the toluene insoluble to the mass of the copolymer can be calculated as “toluene insoluble”.

  Toluene-insoluble content is adjusted by changing the amount of mercaptans used relative to the monomer component within the range of 0.01 to 1.5% by mass, for example, when mercaptans are used as molecular weight regulators. Can do. In order to produce a copolymer latex having a good flavor, the amount of mercaptans used relative to the monomer component is preferably within the range of 0.01 to 1% by mass. The toluene-insoluble content of the copolymer can also be adjusted by selecting various conditions such as the amount of the polymerization initiator used, the polymerization temperature, the polymerization conversion rate, or the composition of the monomer components. By combining the selection of these conditions with the selection of the type and amount of the molecular weight regulator, a copolymer having a desired toluene-insoluble content can be produced.

Further, the Mooney viscosity (ML _{1 + 4} , 100 ° C.) of the copolymer constituting the copolymer latex for can sealing material of the present embodiment is preferably 60 to 150, and preferably 65 to 130. Further preferred. If the Mooney viscosity (ML _{1 + 4} , 100 ° C.) of the copolymer is less than 60, the tensile strength of the can sealing material obtained as the final product tends to be small and tearing tends to occur. When tearing occurs, the torn part is squeezed out of the tightening part and mixed in the contents of the can, or the sealing property after long-term storage tends to decrease due to creep. On the other hand, when the Mooney viscosity (ML _{1 + 4} , 100 ° C.) of the copolymer exceeds 150, the sealing property of the can sealing material obtained as the final product tends to be lowered. The Mooney viscosity (ML _{1 + 4} , 100 ° C.) of the copolymer can be adjusted by adjusting the monomer composition, the type and amount of the molecular weight regulator, the polymerization temperature, the polymerization conversion rate, and the like.

  In addition, it is preferable that the glass transition temperature of the copolymer which comprises the copolymer latex for can sealing materials of this embodiment is -20 degrees C or less, and it is still more preferable that it is -30--80 degreeC. When the glass transition temperature of the copolymer is higher than -20 ° C, the sealing property of the can sealing material obtained as the final product tends to be lowered.

  Further, the copolymer latex for can sealing material of the present embodiment is a so-called core shell obtained by a two-stage polymerization method in which the first monomer component is emulsion-polymerized and then the second monomer component is emulsion-polymerized. A type of copolymer latex is preferable because a can sealing material having better sealing properties and winding properties can be obtained. Here, the glass transition temperature of the polymer (first polymer) obtained by polymerizing the first monomer component is preferably −50 to 0 ° C., and −40 to −10 ° C. More preferably. The glass transition temperature of the first polymer is preferably higher by 10 ° C. than the glass transition temperature of the polymer obtained by polymerizing the second monomer component (second polymer), It is preferably 20 ° C. or higher.

  When the glass transition temperature of the first polymer is less than −50 ° C., the sealing property of the can sealing material obtained as the final product tends to be lowered. On the other hand, if the glass transition temperature of the first polymer is more than 0 ° C., the modulus of the resulting can sealing material tends to be high, and the winding property tends to be lowered. Further, if the difference between the glass transition temperature of the first polymer and the glass transition temperature of the second polymer is less than 10 ° C., the sealing property of the can sealing material obtained as the final product tends to be lowered. . Further, the glass transition temperature of the second polymer is preferably −20 ° C. or less, and more preferably −80 to −30 ° C. When the glass transition temperature of the second polymer is higher than −20 ° C., the sealing property and the winding property of the can sealing material obtained as the final product tend to be lowered. Details of the method for preparing such a core-shell type copolymer latex are described in, for example, Japanese Patent Publication No. 5-1813.

  “Glass transition temperature” as used herein refers to a value measured and calculated by the following method. First, about 5 g of copolymer latex is thinly drawn on a glass plate and dried at 25 ° C. for 7 days to obtain a dry film. About the obtained dried film, for example, a differential scanning calorimeter (DSC) manufactured by Rigaku Corporation can be used to measure the glass transition temperature (measuring conditions: heating rate = 20 ° C./min, nitrogen). Sample amount = 20 mg under atmosphere). In addition, adjustment of the glass transition temperature of a copolymer can be performed by changing a monomer composition, for example.

  In the copolymer latex for can sealing material of the present embodiment, for example, an inorganic pigment such as heavy calcium carbonate, light calcium carbonate, clay, titanium oxide, zinc oxide, rosin or the like used in the production of general can sealing materials Can sealing materials can be produced by blending additives such as esters, terpene resins and other tackifiers, other thickeners, anti-aging agents, and preservatives. As a preferable blending example as a compound for a can sealing material, 100 parts by mass of a copolymer latex (in terms of solid content), 50 to 200 parts by mass of an inorganic pigment, 50 to 150 parts by mass of a tackifier, and other additives 1-5 mass parts, etc. can be mentioned. In addition, it is preferable that the solid content concentration of the can sealing material using the copolymer latex of the present embodiment is adjusted to about 35 to 70% by mass.

  A compound for a can sealing material can be prepared according to a conventional method. For example, it is prepared by preparing aqueous dispersions or aqueous solutions of various components including copolymer latex, which are blended in a compound for a can sealing material, and blending them so as to have a predetermined blending ratio. can do. Here, the average particle diameter of the inorganic pigment in the aqueous dispersion of the inorganic pigment (pigment and inorganic filler) blended in the compound for the can sealing material is preferably 0.3 to 8 μm.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited to these Examples. In the examples and comparative examples, “parts” and “%” are based on mass unless otherwise specified. Moreover, the measuring method and evaluation method of various physical property values are shown below.

  [Toluene-insoluble matter]: After adjusting the pH of the copolymer latex to about 8, it is cast and dried on a glass plate to form a film having a thickness of 0.3 mm. Next, 0.3 g of the formed film is placed in 100 ml of toluene and allowed to stand for about 16 hours, and then shaken by hand. Furthermore, after leaving still for 4 hours and dissolving a film, the obtained solution is filtered with a 120 mesh metal-mesh, and a filtrate is extract | collected using a whole pipette. The collected filtrate is evaporated and dried using a hot plate to measure the mass of toluene insoluble matter. From the measured mass, the ratio of the toluene insoluble content to the copolymer mass is calculated as “toluene insoluble content (%)”.

[Mooney viscosity (ML _{1 + 4} , 100 ° C.)]: Measured in accordance with JIS K6300 using an L rotor under conditions of preheating 1 minute, rotor operating time 4 minutes, temperature 100 ° C.

  [Glass Transition Temperature]: About 5 g of copolymer latex is thinly stretched on a glass plate and dried at 25 ° C. for 7 days to obtain a dry film. About the obtained dried film, using a differential scanning calorimeter DSC (manufactured by Rigaku Denki Co., Ltd.), the glass transition temperature (° C.) under the conditions of a heating rate = 20 ° C./min, a nitrogen atmosphere, and a sample amount = 20 mg. ).

  [Residual persulfate (ammonium persulfate) concentration]: Measured by a potentiometric titration method based on JIS K0113 (1997). The detection limit value (lower limit value) is 0.1 ppm.

  [BH Viscosity]: The liquid temperature of the compound was adjusted to 25 ° C., and the viscosity (mPa · s) at the time of rotor # 4 and rotation of 20 was measured with a BH viscometer (manufactured by Tokyo Keiki Co., Ltd.).

  [AP viscosity]: A compound with a liquid temperature adjusted to 25 ° C. is placed in a container with a capacity of 100 ml in which a nozzle having a nozzle diameter of 0.7 mm is disposed, and the compound is pushed out from the nozzle by applying an air pressure of 0.098 MPa. It was. The time from the start of the pneumatic load until the compound was pushed out was measured as “AP viscosity (seconds)”.

[Viscosity stability]: Viscosity stability was evaluated according to the following criteria.
○: AP viscosity change is within 3 seconds after 1 month.
Δ: AP viscosity change is within 15 seconds after 1 week, and further, the difference between the viscosity after 1 week and the viscosity after 1 month is within 5 seconds.
X: Other than the above (◯, Δ).

[Lining workability]: Using a high-speed lining machine, a compound is applied from a nozzle to a 2-inch aluminum can lid so that the film volume is 50 mm ^3, and the lining workability is achieved according to the following criteria. evaluated. In addition, the lining workability | operativity was confirmed by performing the continuous lining for 24 hours using the compound which passed 1 week or more after manufacture.
○: After setting to the prescribed coating amount, no adjustment was required, continuous lining was possible within ± 5% of the coating amount, and no cover contamination such as protrusion of the compound occurred.
Δ: After setting to the prescribed coating amount, without adjustment, the coating amount exceeded ± 5%, and continuous lining was possible within ± 10%, and no cover stains such as protrusion of the compound occurred.
X: After setting to the prescribed coating amount, continuous lining was impossible within ± 10% of the coating amount without adjustment.

Example 1
6 parts 1,3-butadiene and 8 parts styrene as monomer components, 0.3 part t-dodecyl mercaptan as molecular weight regulator, 3.0 parts potassium rosinate as emulsifier, 0.1 part ammonium persulfate as polymerization initiator And 130 parts of water were charged into a stainless steel reactor having an internal volume of 100 liters, and emulsion polymerization was carried out while stirring at a polymerization temperature of 50 to 80 ° C. The polymerization conversion rate at this time was 85%. Subsequently, 54 parts of 1,3-butadiene and 32 parts of styrene were continuously added to carry out emulsion polymerization, and the polymerization was stopped when the polymerization conversion rate was 85%, and sodium bisulfite was added. 0.15 parts was added to perform a reduction treatment to reduce residual persulfate (ammonium persulfate). Residual monomer components were removed by steaming and concentrated to a total solid content of 50% to obtain a copolymer latex (Example 1).

The copolymer constituting the obtained copolymer latex has a residual ammonium persulfate concentration of 0 ppm (less than the detection limit (0.1 ppm)), a toluene insoluble content of 50%, and a Mooney viscosity (ML _{1 + 4} , 100 ° C.). 80 and the glass transition temperature was −50 ° C. Table 1 shows the formulation of the copolymer latex and various physical property values.

(Examples 2-6, Comparative Example 1)
While setting it as the mixing | blending prescription shown in Table 1, the quantity of the added sodium hydrogen sulfite is 0.12 part in Example 2: 0.10 part in Example 3: 0.08 part in Example 4: 0.08 part, Example 5: Copolymer latex (0.05%), Example 6: 0.03 part, and Comparative Example 1 except that the reduction treatment was performed as 0 part (not added). Examples 2 to 6 and Comparative Example 1) were obtained. Various physical property values of the obtained copolymer latex are shown in Table 1.

(Compound A)
Add pigment (titanium oxide, carbon) and inorganic filler (clay) to a ball mill containing water and a dispersant (sodium salt of β-naphthalenesulfonic acid formalin condensate), and rotate the ball mill at 1500 rpm for 6 hours. Thus, an aqueous dispersion of pigment and inorganic filler (average particle diameter of pigment and inorganic filler = 4 μm) was obtained. Also, an aqueous dispersion of the tackifying resin was obtained by adding and emulsifying the tackifying resin to an aqueous solution containing water, a surfactant (sodium alkylbenzenesulfonate), and a protective colloid (casein). Furthermore, an aqueous solution of a thickener / thickener was obtained by dissolving a thickener (methylcellulose) and a thickener (karaya gum) in water and an aqueous ammonia solution.

  100 parts of copolymer latex (Example 1) in dry solids, 70 parts of inorganic filler, 10 parts of pigment, 0.5 part of N, N'-di-2-naphthyl-p-phenylenediamine as an antioxidant , 50 parts hydrogenated rosin glycerin ester as tackifying resin, 3 parts dispersant / surfactant, 1 part benzoic acid as preservative, and 3 parts thickener / viscosity agent into each mixing tank did. Compound A was produced by uniformly mixing the contents of the mixing tank with a stirrer. Table 2 shows the compounding formula of the compound.

  The BH viscosity of the produced Compound A was 4700 mPa · s immediately after production, 4600 mPa · s after 1 week, and 4650 mPa · s after 1 month. The AP viscosity of Compound A thus obtained was 73 seconds immediately after production, 71 seconds after 1 week, and 73 seconds after 1 month. Furthermore, the evaluations of viscosity stability and lining workability were both “◯”. Table 1 shows the viscosity of Compound A and the evaluation results.

(Compounds B to G)
Compounds B to G were produced in the same manner as in the case of Compound A described above except that any one of the copolymer latexes of Examples 2 to 6 and Comparative Example 1 was used. Table 1 shows the viscosities and evaluation results of the produced compounds B to G.

  As is clear from the results shown in Table 1, the compound obtained using the copolymer latex of Example 1 is good with little change in BH viscosity and AP viscosity over time, viscosity stability, and The lining workability was also good. In addition, for the compounds obtained using the copolymer latex of Examples 2 to 4, the BH viscosity was good with little change over time. The AP viscosity tended to be slightly lower than that immediately after production, but there was no change thereafter. Further, the viscosity stability and lining workability were slightly inferior to those of Example 1. About the compound obtained using the copolymer latex of Examples 5 and 6, both BH viscosity and AP viscosity showed a tendency to decrease with time, and the viscosity stability and lining workability were slightly higher than those of Example 1. Somewhat inferior. On the other hand, for the compound obtained using the copolymer latex of Comparative Example 1, both the BH viscosity and the AP viscosity were greatly reduced over time. Furthermore, viscosity stability and lining workability were also significantly inferior to those of Example 1.

  The copolymer latex for can sealing material of the present invention produces a can sealing material that is lined in a groove provided at the peripheral edge of the can end in order to seal the double winding portion between the can body and the can lid. It is suitable as a material for this.

Claims (3)

A copolymer latex for can sealing material produced by polymerization using persulfate as an initiator,
After polymerization, a reducing agent is added to further reduce the residual persulfate (residual persulfate).
A copolymer latex for can sealing material, wherein the concentration of the residual persulfate after the reduction treatment is 50 ppm or less. Wherein the reducing agent is a can sealing material for copolymer latex of claim 1 is sodium bisulfite. A monomer component mainly composed of (a) a conjugated diene monomer, (b) an aromatic vinyl compound, and (c) another copolymerizable monomer used as necessary, It is produced by emulsion polymerization at a polymerization conversion rate of ˜90% by mass,
The copolymer for can sealing material according to claim 1 or 2 , wherein the copolymer constituting the copolymer has a toluene insoluble content of 10 to 70% and a Mooney viscosity (ML _{1 + 4} , 100 ° C) of 60 to 150. latex.