JPH0574630B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] This invention relates to an acrylic water-dispersed pressure-sensitive adhesive composition. [Prior Art] In recent years, acrylic pressure-sensitive adhesives have become widely used in place of conventional natural rubber-based and synthetic rubber-based pressure-sensitive adhesives due to their excellent adhesive properties and durability. Among these types of adhesives, water-dispersible adhesives that do not use organic solvents have recently been researched and developed from the viewpoint of resource saving and environmental hygiene. Such water-dispersed adhesives are generally prepared by emulsion polymerization. That is, it is prepared by emulsion polymerization of a (meth)acrylic acid alkyl ester in an aqueous medium together with a modifying monomer such as acrylic acid, styrene, or vinyl acetate as necessary, and the room temperature obtained by this method is It is known that acrylic polymers with adhesive properties have a relatively large molecular weight compared to polymers obtained by solution polymerization, and therefore can provide relatively high cohesive strength as pressure-sensitive adhesives. [Problems to be Solved by the Invention] However, the above-mentioned conventional acrylic water-dispersed adhesives are still not satisfactory when applied to applications where a high degree of cohesion is desired.
Furthermore, when the bonded area was exposed to relatively high temperatures, the cohesive force was significantly reduced, making it almost unusable. Therefore, in order to further improve the cohesive force of this type of adhesive, external crosslinking agents similar to those used in organic solvent types, such as melamine compounds, epoxy compounds, metal salts, etc., are added to the polymer emulsion after emulsion polymerization. Attempts have been made. However, with such improvement measures, the crosslinking is generally non-uniform and the cohesive force is not very large compared to the degree of crosslinking, and even if the cohesive force can be increased, the adhesive force will decrease accordingly. However, it has been difficult to obtain a pressure-sensitive adhesive composition having high adhesive strength and high cohesive strength. Furthermore, when the cohesive force is increased by the above-mentioned means, there is a problem that in addition to a decrease in adhesive force, the adhesive also has poor repulsion resistance.
In other words, for example, when a metal plate or plastic plate is bonded to an adherend with a curved surface in a bent state, a restoring force acts on the bent metal or plastic plate, so it is difficult to resist this restoring force. This repulsion resistance is expressed by the balance between adhesive strength and cohesive force. It was impossible to satisfy them. As described above, the conventional acrylic water-dispersed adhesive has the problem that it is difficult to obtain one that highly satisfies both adhesive strength and cohesive force, and also has excellent repulsion resistance. Moreover, this type of adhesive uses an emulsifier to stabilize the polymer particles during emulsion polymerization, and this emulsifier is mixed into the adhesive composition, resulting in poor moisture and water resistance. There was also the problem that the adhesive properties were adversely affected. Therefore, the present invention provides a water-dispersed pressure-sensitive adhesive composition that does not contain any emulsifiers that cause the deterioration of the adhesive properties, and that has high adhesive strength and high adhesive strength even without the addition of an external crosslinking agent. Demonstrates cohesive power,
Furthermore, it is an object of the present invention to provide an acrylic water-dispersed pressure-sensitive adhesive composition that has excellent rebound resistance. [Means for Solving the Problems] As a result of intensive studies to achieve the above object, the inventors developed a new method instead of conventional emulsion polymerization.
We adopted two-stage polymerization in an aqueous medium that eliminated the use of emulsifiers by using a specific polymerization initiator, and also carried out a specific crosslinking treatment between the first and second stage polymerizations. When applied, it is an acrylic material that does not contain emulsifiers, has excellent moisture and water resistance, and also exhibits high adhesion and cohesive strength without the need for external crosslinking agents, as well as excellent repulsion resistance. The present invention was completed based on the knowledge that a water-dispersed pressure-sensitive adhesive composition can be obtained. That is, the present invention provides an alkyl ester of (meth)acrylic acid having 1 to 14 carbon atoms alone or an unsaturated monomer having no functional group in the molecule that can be copolymerized with this ester. The first-stage polymerization monomer, whose homopolymer or copolymer exhibits pressure-sensitive adhesive properties and whose glass transition temperature is 250°K or lower, is prepared in an aqueous medium in the presence of persulfate without using an emulsifier. Add an oil-soluble peroxide to the polymer emulsion obtained by polymerization and crosslink it by heating to achieve a gel fraction of 20 to 60% by weight.
A cross-linked polymer emulsion containing polymer particles, and further a (meth)acrylic acid alkyl ester having an alkyl group of 1 to 14 carbon atoms alone or an unsaturated monomer copolymerizable with this ester and this ester. Based on a polymer emulsion obtained by polymerizing with the addition of a second-stage polymerization monomer whose homopolymer or copolymer consists of This invention relates to a water-dispersed pressure-sensitive adhesive composition. In this way, in this invention, polymerization in an aqueous medium is carried out without using an emulsifier by specifically using a persulfate as a polymerization initiator, and the above persulfate is used for its decomposition. This produces ionic end groups, which greatly contribute to polymerization stability and further to the stability of the emulsion after polymerization. Therefore, the pressure-sensitive adhesive composition based on the polymer emulsion obtained by this method has excellent moisture resistance and water resistance because it does not contain an emulsifier. The stability of the emulsion is comparable to that of the seed composition. In addition, in this invention, the polymerization in an aqueous medium as described above is divided into two stages, and the first stage and second stage are
The acrylic polymer emulsion obtained in the first stage is subjected to the specific crosslinking treatment between the two stages to form an emulsion containing specific polymer particles with a controlled gel fraction. Since an acrylic monomer with excellent pressure-sensitive adhesive properties is added to the stage and post-polymerized, the polymer particles produced by this method have a cross-linked structure inside and a cross-linked structure around them. It is estimated that the particles have a structure in which an acrylic pressure-sensitive adhesive polymer is partially formed, and this particle structure allows the gel fraction of the entire polymer constituting the particles to be within an appropriate range. Adhesive compositions based on emulsions containing these polymer particles exhibit high adhesive strength and high cohesive strength without the need to incorporate external crosslinking agents, and also have excellent repulsion resistance. It becomes something. In this specification, the gel fraction of polymer particles is an indicator of how much the polymer constituting the polymer particles is involved in crosslinking, and this is an indicator of the solvent insolubility of the polymer. It is expressed by measuring minutes (% by weight). Specifically, it is actually measured by forming a polymer film from an emulsion containing polymer particles, immersing it in a solvent to elute the polymer that does not participate in crosslinking, and measuring the remaining solvent-insoluble content. . This measurement is as shown in Examples below. Furthermore, in this specification, (meth)acrylic acid refers to acrylic acid and/or methacrylic acid, (meth)acrylic acid alkyl ester refers to acrylic acid alkyl ester and/or methacrylic acid alkyl ester, and (meth)acrylic acid alkyl ester refers to acrylic acid alkyl ester and/or methacrylic acid alkyl ester. )
Acrylate means acrylate and/or methacrylate, respectively. [Structure and operation of the invention] In this invention, first, a monomer for the first stage of polymerization is used, and this is polymerized in an aqueous medium in the presence of a persulfate without using an emulsifier, thereby achieving crosslinking. A polymer emulsion containing polymer particles having no or few bonds is produced. The above monomer for polymerization is an unsaturated monomer having no functional group in the molecule that can be copolymerized with (meth)acrylic acid alkyl ester having an alkyl group of 1 to 14 carbon atoms alone or with this ester. As the (meth)acrylic acid alkyl ester, from the viewpoint of adhesive properties, those in which the alkyl group has 1 to 14 carbon atoms are used, and a particularly preferred example is ethyl acrylate. , butyl acrylate, 2-ethylhexyl acrylate, isononyl arylate, isodecyl acrylate, ethyl methacrylate, butyl methacrylate, lauryl methacrylate, and the like. In addition, examples of copolymerizable unsaturated monomers that do not have a functional group in the molecule include acrylonitrile, methacrylonitrile, vinyl acetate, styrene or its derivatives,
These monomers mainly contribute as components that increase the cohesive force of the polymer constituting the polymer particles. The first-stage polymerization monomer consisting of such (meth)acrylic acid alkyl ester alone or the above-mentioned copolymerizable unsaturated monomer is a homopolymer or copolymer that exhibits pressure-sensitive adhesive properties. It is necessary that the composition has a glass transition point of 250°K or less as shown below. With such a composition, it is possible to prepare an adhesive composition that has both high cohesive strength and high adhesive strength. For this reason, the copolymerizable unsaturated monomer is preferably kept in a small amount relative to the (meth)acrylic acid alkyl ester, and generally, the copolymerizable unsaturated monomer is The content of the copolymerizable unsaturated monomer is usually 10% by weight or less. In the first stage of polymerization, a persulfate salt as a polymerization initiator is added and dissolved in water, heated to a temperature of 60 to 90°C, and the above-mentioned first step is added to the aqueous medium with stirring. The monomer for polymerization in the first stage is added dropwise at a predetermined dropping rate, and the reaction is allowed to occur for a predetermined time while maintaining the above temperature. Examples of persulfates used here include potassium persulfate, ammonium persulfate, and sodium persulfate. This persulfate has a function as a polymerization initiator, and the ionic terminal group produced by its decomposition contributes to the polymerization stability in an aqueous medium and the stability of the emulsion after polymerization. In addition, this persulfate usually plays a role as a polymerization initiator in the second stage of polymerization in the subsequent step, and also plays a role in polymerization stability and emulsion stability. Therefore, it is desirable to set the amount of the persulfate used within an appropriate range from the above-mentioned viewpoints, that is, from the viewpoints of the molecular weight, gel fraction, and stability of the produced polymer. In general, persulfate is 0.1 to 100 parts by weight of the monomer for the first stage polymerization.
The proportion is preferably 5 parts by weight. In the polymer emulsion thus obtained, the polymer particles contained therein have no or almost no crosslinks, and the solid content concentration, that is, the concentration of the polymer particles, is as follows:
Considering the amount of monomer used for the second stage polymerization, it is generally about 20 to 50% by weight to ensure the stability of the polymerization in the first and second stages and the stability of the emulsion. It is better to adjust it so that In this invention, by adding an oil-soluble peroxide to the first stage polymer emulsion and crosslinking it by heating, a crosslinked polymer with a gel fraction of 20 to 60% by weight of the polymer particles is produced. Produces a coalesced emulsion. After setting the gel fraction of the polymer particles in this manner, by performing the second stage of polymerization described below, it is possible to obtain the high adhesion and cohesive strength that is the objective of this invention, as well as the repulsion resistance. For the first time, it is possible to obtain an adhesive composition with excellent properties. That is, if the above-mentioned gel fraction is less than 20% by weight, the cohesive force will decrease, and if it exceeds 60% by weight, the adhesive strength will decrease or the rebound resistance will be significantly impaired. As the oil-soluble peroxide used to achieve such a gel fraction, there are various organic peroxides such as benzoyl peroxide and lauroyl peroxide. The amount of peroxide used may be generally about 0.1 to 5 parts by weight based on 100 parts by weight of the polymerization monomer used to obtain the first stage polymer emulsion. When adding peroxide, if it is in liquid form, it may be added as is or after being diluted with an appropriate organic solvent, and if it is in powder form, it may be added after being dissolved in an appropriate organic solvent. Various organic solvents can be used as long as they have an affinity for the polymer particles and do not interfere with the crosslinking reaction or the stability of the emulsion. Specific examples include toluene, Examples include xylene and ethyl acetate. In this invention, when the above-mentioned oil-soluble peroxide is added to the first stage polymer emulsion cooled to a predetermined temperature, this peroxide diffuses and impregnates into the polymer particles. The crosslinking treatment is carried out by impregnating as described above, heating to a temperature of 60 to 80°C, and stirring for 2 to 6 hours. At this time, by appropriately adjusting the amount of oil-soluble peroxide and the above treatment conditions, the gel fraction of the polymer particles can be easily set within the above range. In this invention, a second stage polymerization monomer is further added to the crosslinked polymer emulsion containing polymer particles having a gel fraction of 20 to 60% by weight, which is obtained as described above. The stage polymerization produces a polymer emulsion containing polymer particles with a suitably crosslinked structure. The above-mentioned second-stage polymerization monomer is composed of a (meth)acrylic acid alkyl ester having an alkyl group of 1 to 14 carbon atoms alone, or this ester and an unsaturated monomer copolymerizable therewith. It is what it is.
As (meth)acrylic acid alkyl ester,
The same monomers as those in the first stage are used. In addition, unsaturated monomers that can be copolymerized with this include (meth)acrylic acid, itaconic acid, fumaric acid, maleic acid, 2-hydroxyethyl (meth)acrylate, vinyl acetate, acrylonitrile, methacrylonitrile, Examples include styrene or its derivatives. Such a monomer for second stage polymerization has a glass transition point of 250 such that its homopolymer or copolymer exhibits pressure-sensitive adhesive properties, as in the case of the monomer for first stage polymerization. The composition must be such that the temperature is below °K, and those that provide a polymer with a temperature higher than the above-mentioned glass transition point tend to cause problems in terms of adhesive strength. Therefore, even when using an unsaturated monomer copolymerizable with the (meth)acrylic acid alkyl ester, the amount of the latter monomer used is 10% of the monomers for the second stage polymerization. The proportion used should be such that the above glass transition point is satisfied within a range of % by weight or less. The amount of the monomer used for this second stage polymerization depends on the composition of the monomer for the first stage polymerization, the gel fraction of the polymer particles, etc., and also depending on the final solid content concentration. The ratio is determined as appropriate, and usually the monomer for the second stage polymerization is 10 to 500 parts by weight to 100 parts by weight of the monomer for the first stage polymerization. It is best to set the ratio as follows. The second stage polymerization is carried out by dropping the monomer for the second stage polymerization into the crosslinked polymer emulsion at a predetermined dropping rate while stirring. The polymerization temperature at this time is 60 to 90, as in the first stage.
It may be within the range of °C. Incidentally, during this second stage polymerization, it is not necessary to intentionally add a persulfate as a polymerization initiator. However, they may be added if specifically desired. The polymer emulsion obtained in this way generally has a solid content of 50 to 70% from the viewpoint of emulsion stability and viscosity characteristics.
It is desirable to set it within a range of % by weight. The water-dispersed pressure-sensitive adhesive composition of the present invention is based on the polymer emulsion obtained in the second stage polymerization, and if necessary, a colorant and a filler are added to the composition. Conventionally known additives such as anti-aging agents, tackifiers and the like can be appropriately blended. The amount to be added may be a normal amount. Furthermore, although the above composition exhibits high adhesive strength and high cohesive strength by itself, if it is desired to further increase the cohesive strength, various conventionally known external crosslinking agents may be used within a range that does not impair the characteristics of the present invention. There is no problem in combining them. [Effects of the Invention] As described above, since the water-dispersed pressure-sensitive adhesive composition of the present invention does not contain an emulsifier, there is no deterioration in adhesive properties due to a decrease in moisture resistance and water resistance caused by an emulsifier. Moreover, it exhibits high adhesive strength and high cohesive strength even without the addition of an external crosslinking agent, shows no decline in cohesive strength especially at high temperatures, and has excellent rebound resistance. This characteristic makes it extremely useful not only for general pressure-sensitive adhesive tapes, sheets, labels, etc., but also for applications that particularly require rebound resistance. [Examples] Below, examples of the present invention will be described in more detail. In addition, in the following, "part" means part by weight, and "%" means weight %, respectively. Moreover, adhesive force, cohesive force, repulsion resistance, and gel fraction were measured by the following methods. <Adhesive strength> A pressure-sensitive adhesive composition was applied to both sides of a 25 μm thick polyester film so that the thickness after drying was 50 μm on one side, and dried at 100°C for 3 minutes to make a double-sided adhesive tape, and JIS Z- 1528, the 180 degree peeling adhesive strength (g/20 mm width) was measured. <Cohesive force> Make double-sided adhesive tape similar to the adhesive force test,
This was bonded to two Bakelite plates with an adhesive area of 25 mm x 25 mm, and a load of 1 kg was applied at 40°C and 80°C, and the time (minutes) until the Bakelite plate fell was measured. <Repulsion resistance> A pressure-sensitive adhesive composition was applied to one side of a 0.3 mm thick aluminum plate so that the thickness after drying would be 50 μm, and after drying at 100°C for 3 minutes, a 10 mm x 80 mm
A test piece was made by cutting it to a size of (mm) was measured. <Gel fraction> Coat the polymer emulsion on one side of a 25 μm thick polyester film so that the thickness after drying is 50 μm, dry it at 100℃ for 3 minutes, and then
A test piece was prepared by cutting it into a size of 50 mm, and this test piece was immersed in heated acetone for 24 hours, and the gel fraction was determined by the following method. Gel fraction (%) = At-P/Ao-P×100 At: Dry weight of test piece after immersion Ao: Weight of test piece before immersion P: Weight of polyester film constituting the test piece Example 1 Temperature In a 500 ml reactor equipped with a meter, stirrer, nitrogen inlet tube and reflux condenser, add 0.5
After dissolving 100 parts of distilled water and heating it to 80°C under a nitrogen stream, the first stage polymerization unit consisting of 95 parts of n-butyl acrylate and 5 parts of ethyl acrylate was added. copolymer (glass transition temperature of copolymer)
220°K) was continuously added dropwise over a period of 5 hours while maintaining the above temperature, and after the addition, the temperature was further maintained at 80°C for 2 hours to obtain a first stage polymer emulsion. The gel fraction of the polymer particles contained in this emulsion was 0%. This copolymer emulsion was cooled to 40°C,
While maintaining this temperature, dissolve 1 part of benzoyl peroxide in 10 parts of toluene and add the solution dropwise.
It was impregnated into the polymer particles for 2 hours at °C. Subsequently, the mixture was heated to 80° C. and reacted for 4 hours to obtain a crosslinked polymer emulsion containing polymer particles with a gel fraction of 37.8%. Next, the above polymer emulsion was heated to 80°C under a nitrogen stream, and then acrylic acid n
- Monomers for the second stage of polymerization consisting of 45 parts of butyl and 5 parts of acrylic acid (glass transition point of the copolymer)
228°K) was continuously added dropwise over a period of 3 hours while maintaining the above temperature, and after the dropwise addition, the temperature was further maintained at 80°C for 2 hours to perform the second stage polymerization. The solid content concentration of the polymer emulsion thus obtained was 57.5%, and this emulsion was used as it was as the water-dispersed pressure-sensitive adhesive composition of the present invention. Comparative Example 1 A water-dispersed pressure-sensitive adhesive composition was obtained in the same manner as in Example 1, except that the second stage polymerization was performed without performing the first stage polymerization crosslinking treatment. Comparative Example 2 A water-dispersed pressure-sensitive adhesive composition was obtained in the same manner as in Example 1, using 6 parts of t-butylperoxy 2-ethylhexanoate as the crosslinking treatment after the first stage polymerization. Ta. Note that the gel fraction of the aggregate particles after the above-mentioned crosslinking treatment was 72.3%. Examples 2 to 6 As a monomer for first-stage polymerization, an oil-soluble peroxide for crosslinking treatment, and a monomer for second-stage polymerization,
A water-dispersed pressure-sensitive adhesive composition of the present invention was obtained in the same manner as in Example 1, except that the composition shown in Table 1 below was used. The final solid content concentration of this composition and the gel fraction of the polymer particles after crosslinking treatment were determined as follows.
Also listed in the table. The glass transition point in Table 1 means the glass transition point of the copolymer made of the first or second stage polymerization monomer.

【table】

[Table] The results of examining the adhesive strength, cohesive force, and repulsion resistance of each of the adhesive compositions of Examples 1 to 6 and Comparative Examples 1 to 2 above were as shown in Table 2 below. .

[Table] As is clear from the above results, the water-dispersed pressure-sensitive adhesive composition of the present invention has high adhesive strength and high cohesive strength, and also has excellent repulsion resistance. Since it does not contain an emulsifier, it has excellent moisture resistance and water resistance, and together with this, it is found that it is an adhesive composition with extremely high practical value.

Claims (1)

[Scope of Claims] 1. (Meth)acrylic acid alkyl ester having an alkyl group of 1 to 14 carbon atoms alone or this ester and an unsaturated monomer having no functional group in the molecule that can be copolymerized with the ester. The glass transition point at which the homopolymer or copolymer consisting of
An oil-soluble peroxide is added to a polymer emulsion obtained by polymerizing the first-stage polymerization monomer, which can have a temperature of 250°K or less, in an aqueous medium in the presence of a persulfate without using an emulsifier. A crosslinked polymer emulsion containing polymer particles with a gel fraction of 20 to 60% by weight is obtained by crosslinking by heating, and a (meth)acrylic acid alkyl ester having an alkyl group of 1 to 14 carbon atoms is further added to the emulsion. A monomer for the second stage polymerization, in which the homopolymer or copolymer consisting alone or of this ester and an unsaturated monomer copolymerizable therewith can exhibit pressure-sensitive adhesiveness and have a glass transition point of 250°K or less. A water-dispersible pressure-sensitive adhesive composition based on a polymer emulsion obtained by polymerizing a polymer. 2. The water-dispersed pressure-sensitive adhesive composition according to claim 1, wherein the persulfate is present in an amount of 0.1 to 5 parts by weight based on 100 parts by weight of the monomer for first stage polymerization. 3 Oil-soluble peroxide is the monomer for the first stage polymerization
The water-dispersed pressure-sensitive adhesive composition according to claim 1 or 2, wherein the amount is 0.1 to 5 parts by weight per 100 parts by weight. 4. The method according to any one of claims 1 to 3, wherein the second stage polymerization monomer is 10 to 500 parts by weight based on 100 parts by weight of the first stage polymerization monomer. Water-dispersed pressure-sensitive adhesive composition. 5. Claim 1, wherein the polymerization temperature for obtaining the polymer emulsion before crosslinking treatment is 60 to 90°C.
The water-dispersed pressure-sensitive adhesive composition according to any one of items 1 to 4. 6. The water-dispersed pressure-sensitive adhesive composition according to any one of claims 1 to 5, wherein the crosslinking treatment conditions are 60 to 90°C for 2 to 6 hours. 7. In the polymer emulsion obtained by adding and polymerizing a monomer for second stage polymerization, the solid content concentration of the emulsion is 50 to 70% by weight, according to claims 1 to 6. The water-dispersible pressure-sensitive adhesive composition according to any one of the above.