JPH0574632B2 - - 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 that have 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, although the above-mentioned conventional acrylic water-dispersed pressure-sensitive adhesives are still satisfactory when applied to applications where a high degree of cohesive strength 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 these improvement measures, it is difficult to select the type and amount of crosslinking agent to be used, and there are productivity problems such as the thermal energy for crosslinking which cannot be ignored. Due to the non-uniformity, the cohesive force is not so large compared to the degree of crosslinking, and even if the cohesive force can be increased, the adhesive force will decrease as a result, resulting in high adhesive force and high cohesive force. It has been difficult to obtain pressure-sensitive adhesive compositions having such properties. 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 force and cohesive force and also has excellent repulsion resistance. Moreover, this kind of adhesive is
Since an emulsifier is used to stabilize polymer particles during emulsion polymerization, this emulsifier mixes into the adhesive composition, resulting in poor moisture resistance and water resistance, which adversely affects adhesive properties. It also had problems. 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 first, polymer particles with a gel fraction of 0% by weight were produced in the first stage of polymerization. When a polymer emulsion containing a specific acrylic monomer is produced and a second stage polymerization is performed by further adding a specific acrylic monomer to this, polymer particles with a gel fraction within a specific range are produced. Since this water-dispersed composition does not contain an emulsifier, it has excellent moisture and water resistance.Moreover, it exhibits high adhesive strength and cohesive strength even without the addition of an external cross-linking agent, and has excellent rebound resistance. It was discovered that the present invention could be used as an acrylic water-dispersed pressure-sensitive adhesive composition, leading to the completion of this invention. 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. A (meth)acrylic acid alkyl ester with an alkyl group having 1 to 14 carbon atoms and a carboxyl group copolymerizable therewith are added to a polymer emulsion containing polymer particles with a gel fraction of 0% by weight obtained by polymerization. For the second stage of polymerization, the copolymer consisting of a copolymerizable unsaturated monomer containing an ethylenically unsaturated monomer as an essential component can have a glass transition point of 250°K or less, exhibiting pressure-sensitive adhesive properties. The present invention relates to a water-dispersible pressure-sensitive adhesive composition based on a polymer emulsion containing polymer particles having a gel fraction of 20 to 80% by weight, obtained by adding and polymerizing a monomer. 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 a polymer emulsion containing polymer particles having a gel fraction of 0% by weight is produced using a monomer for polymerization in the first stage. By further adding an acrylic monomer mixture containing a carboxyl group-containing ethylenically unsaturated monomer as a part of the component and post-polymerizing it, the gel fraction becomes 20 to 80% by weight. In other words, the polymer particles produced by this method contain a non-crosslinked soft polymer component inside and a hard crosslinked polymer component around it. It is presumed that the particles have a structure similar to that of polymer particles, and the gel fraction of the entire polymer constituting the particles is regulated within the specific range described above. The resulting adhesive composition exhibits high adhesive strength and high cohesive strength even without the addition of an external crosslinking agent, and also has excellent rebound resistance. 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 the present invention, a first-stage polymerization monomer is used and polymerized in an aqueous medium in the presence of a persulfate without using an emulsifier, thereby forming a gel. A polymer emulsion containing a fraction of polymer particles of 0% by weight 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. It consists of the body. 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 particularly preferred examples include ethyl acrylate, butyl acrylate, and diacrylic acid. -Ethylhexyl, isononyl acrylate, isodecyl acrylate, ethyl methacrylate, butyl methacrylate, lauryl methacrylate, and the like. In addition, as unsaturated monomers that do not have functional groups in their molecules that can be copolymerized,
Examples include acrylonitrile, methacrylonitrile, vinyl acetate, styrene or derivatives thereof, and 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 above copolymerizable unsaturated monomer is preferably kept in a small amount relative to the (meth)acrylic acid alkyl ester, and generally the above copolymerizable unsaturated monomer is contained in the monomers for the first stage polymerization. The content of the polymerizable 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 acts as a polymerization initiator and affects the molecular weight and gel fraction of the produced polymer, and the ionic end groups produced by its decomposition improve polymerization stability in aqueous medium. It contributes to 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. The polymer emulsion obtained in this way has a non-crosslinked structure with a gel fraction of 0% by weight of the polymer particles contained therein, and forms soft polymer particles for the second stage of polymerization. It acts as the core of the hard polymer component formed in the process and contributes to maintaining adhesive strength. Note that the solid content concentration of the polymer emulsion obtained in this first stage polymerization, that is, the concentration of polymer particles, is not particularly limited, but the amount of the monomer used for the second stage polymerization is In consideration of the above, it is generally preferable to adjust the amount to about 20 to 50% by weight so as to ensure the polymerization stability of the first and second stages and the stability of the emulsion. In this invention, a monomer for second stage polymerization is further added to the first stage polymer emulsion obtained as described above, and the gel fraction of the polymer particles is increased to 20. Produces a polymer emulsion of ~80% by weight. The amount of monomers for the second stage polymerization used here is a (meth)acrylic acid alkyl ester with an alkyl group having 1 to 14 carbon atoms and a carboxyl group-containing ethylenically unsaturated monomer that can be copolymerized with this. It consists of a mixture with a copolymerizable unsaturated monomer containing as an essential component. As the (meth)acrylic acid alkyl ester, the same monomers as those in the first stage are used. Examples of the carboxyl group-containing ethylenically unsaturated monomer include (meth)acrylic acid, crotonic acid, itaconic acid, maleic acid, and fumaric acid, and among these, acrylic acid is particularly preferred. Further, examples of copolymerizable unsaturated monomers other than those mentioned above include vinyl acetate, styrene, or derivatives thereof. Among the above copolymerizable unsaturated monomers, the carboxyl group-containing ethylenically unsaturated monomer is an essential component for setting the gel fraction of polymer particles within a specific range. If the number of polymers increases too much, it may become difficult to set the above-mentioned gel fraction, or polymerization stability may be impaired. Therefore, the amount used should be determined from the above-mentioned viewpoint, and generally it is 0.1/10 in the monomer for the second stage polymerization.
% by weight, preferably in the range from 0.5 to 7.5% by weight. Furthermore, when copolymerizable unsaturated monomers other than these are used together, the total amount of this and the carboxyl group-containing ethylenically unsaturated monomer may be within the above range. Note that the reason why polymer particles can have a crosslinked structure when the above carboxyl group-containing ethylenically unsaturated monomer is used is not necessarily clear at present. It is speculated that when the above monomers are used, emulsion particles (polymer particles) are formed during the polymerization process.
It is thought that a chain transfer reaction by radicals occurs within the molecule, and this is involved in the formation of crosslinks. Such a monomer for the second stage polymerization has a glass transition point such that the copolymer exhibits pressure-sensitive adhesive properties, as in the case of the monomer for the first stage polymerization.
It is necessary to have a composition that provides a temperature of 250°K or less, and those that provide a polymer with a temperature higher than the above-mentioned glass transition point are likely to cause problems in terms of adhesive strength and are therefore unsuitable. The amount of the monomer used in the second stage polymerization depends on the composition of the monomer used in the first stage polymerization.
In addition, to ensure that the final solid content concentration is at an appropriate ratio,
It can be determined as appropriate, and usually the monomer for second stage polymerization is 10 to 500 parts by weight, preferably 20 to 250 parts by weight, per 100 parts by weight of the monomer for first stage polymerization. It is best to set the ratio so that The second-stage polymerization is carried out by dropping the above monomer for second-stage polymerization into the polymer emulsion produced in the first stage at a predetermined dropping rate while stirring. The polymerization temperature at this time is the same as in the first stage.
The temperature may be in the range of 60 to 90°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 has a gel fraction of the polymer particles contained therein of 20 to 20.
It is characterized by being set in the range of 80% by weight. If the gel fraction is less than 20% by weight, the cohesive force will be low, and if it exceeds 80% by weight, the adhesive strength will be reduced or the rebound resistance will be impaired, so both are unsuitable. In addition, the solid content concentration of this polymer emulsion,
In other words, the concentration of the above polymer particles is generally determined from the viewpoint of emulsion stability and viscosity characteristics.
It is desirable that the content be set in the range of 50 to 70% 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 the composition contains a colorant and a filler as necessary. Conventionally known additives such as , anti-aging agents, and tackifiers can be appropriately blended. The amount to be added may be a normal amount. Also,
The above composition exhibits high adhesive strength and high cohesive strength by itself, but if it is desired to further increase the cohesive strength, various conventionally known external crosslinking agents may be blended within a range that does not impair the characteristics of the present invention. There is no problem. [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 feature 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 consisting of 40 parts of n-butyl acrylate and 60 parts of 2-ethylhexyl acrylate was carried out. A monomer (copolymer glass transition point: 209°K) was continuously added dropwise over 5 hours while maintaining the above temperature to obtain a first stage polymer emulsion. The solid content concentration of this emulsion was 49.0%; the gel fraction of the polymer particles was 0%. Next, the above polymer emulsion was heated to 80°C under a nitrogen stream, and then a second stage polymerization consisting of 10 parts of ethyl acrylate, 38 parts of 2-ethylhexyl acrylate, and 1.5 parts of acrylic acid was added. Polymer monomer (copolymer glass transition point 214°K) was continuously added dropwise over a period of 3 hours while maintaining the above temperature, and after dropping, the temperature was further maintained at 80°C for 2 hours to carry out the second stage polymerization. I went. The solid content concentration of the polymer emulsion thus obtained was 59.8%, and the gel fraction of the polymer particles was
47.5%, and this emulsion was directly used 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 exactly the same manner as in Example 1, except that the use of acrylic acid in the second-stage polymerization monomer was omitted. Note that the gel fraction of the polymer particles of the emulsion obtained in the second stage polymerization was 10.5%. Comparative Example 2 As second-stage polymerization monomers, 10 parts of ethyl acrylate and 39.45 parts of 2-ethylhexyl acrylate were used.
A water-dispersed pressure-sensitive adhesive composition was obtained in exactly the same manner as in Example 1, except that a composition containing 0.05 parts of acrylic acid and 0.05 parts of acrylic acid was used. In addition, the gel fraction of the polymer particles of the emulsion obtained in the second stage polymerization is
It was 1.25%. Examples 2 to 5 The same procedure as in Example 1 was carried out except that the monomers shown in Table 1 below were used as the monomers for first-stage polymerization and the monomers for second-stage polymerization. A water-dispersed pressure-sensitive adhesive composition of this invention was obtained. The final solid content concentration of this composition and the gel fraction of the first and second stage polymer particles are also listed in Table 1. In addition, the glass transition point in Table 1 means the glass transition point of a copolymer consisting of the first-stage 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 5 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
The gel fraction obtained by polymerizing the first stage polymerization monomer, which can be at 250°K or less, in the presence of persulfate in an aqueous medium without using an emulsifier, is 0% by weight.
In a polymer emulsion containing polymer particles, a (meth)acrylic acid alkyl ester with an alkyl group having 1 to 14 carbon atoms and a carboxyl group-containing ethylenically unsaturated monomer copolymerizable with the same are essential components. The copolymer consisting of a copolymerizable unsaturated monomer containing
A water-dispersed texture based on a polymer emulsion containing heavy particles with a gel fraction of 20 to 80% by weight, obtained by polymerization with the addition of a second-stage polymerization monomer that can be at a temperature of 250°K or less. Pressure adhesive composition. 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. The water-dispersed pressure-sensitive adhesive composition according to claim 1 or 2, wherein the carboxyl group-containing ethylenically unsaturated monomer accounts for 0.1 to 10% by weight of the second-stage polymerization monomer. thing. 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 The polymerization temperature is 60 to 90°C to obtain a polymer emulsion containing polymer particles with a gel fraction of 0% by weight.
The water-dispersed pressure-sensitive adhesive composition according to any one of claims 1 to 4. 6. In a polymer emulsion containing polymer particles with a gel fraction of 20 to 80% by weight, the concentration of solid content consisting of the particles is 50 to 70% by weight in the emulsion.
The water-dispersed pressure-sensitive adhesive composition according to any one of claims 1 to 5.