JPH0574631B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] 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, 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 of crosslinking agent and the amount used, and there are problems in terms of productivity, such as the thermal energy required for crosslinking, which cannot be ignored. As the cohesive force becomes more uniform, 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. It has been difficult to obtain such pressure-sensitive adhesive compositions. 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.
By using a specific polymerization initiator, a two-stage polymerization in an aqueous medium is adopted that eliminates the use of emulsifiers, and in the first stage of polymerization, some components are ethylenically unsaturated monomers containing carboxyl groups. A polymer emulsion in which the gel fraction of the polymer particles falls within a specific range is produced using a specific acrylic monomer mixture, which is a specific acrylic monomer. When polymerization is carried out in the second step, it does not contain an emulsifier, so it has excellent moisture and water resistance, and also exhibits high adhesion and cohesive strength without the need for external crosslinking agents. The inventors discovered that an acrylic water-dispersed pressure-sensitive adhesive composition with excellent properties could be obtained, and thus completed the present invention. That is, the present invention provides a copolymer comprising a (meth)acrylic acid alkyl ester having an alkyl acid having 1 to 14 carbon atoms and an ethylenically unsaturated monomer containing a carboxyl group, which has a glass transition point at which the copolymer exhibits pressure-sensitive adhesive properties. A polymer containing polymer particles with a gel fraction of 20 to 60% by weight obtained by polymerizing a monomer mixture that can have a temperature of 250°K or less in an aqueous medium in the presence of a persulfate without using an emulsifier. In the emulsion,
A (meth)acrylic acid alkyl ester having an alkyl group of 1 to 14 carbon atoms alone, or a homopolymer or copolymer thereof consisting of this ester and an unsaturated monomer having no functional group in the molecule that can be copolymerized with the ester The glass transition point that indicates pressure-sensitive adhesion is
The present invention relates to a water-dispersed pressure-sensitive adhesive composition based on a polymer emulsion obtained by polymerization with the addition of a monomer for post-polymerization that can have a temperature of 250°K or less. 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 an ecrylic monomer mixture containing a cahoxyl group-containing ethylenically unsaturated monomer is used as a monomer for the first stage of polymerization. By using this method, an emulsion containing specific polymer particles with a controlled Kel fraction is produced, and a second stage acrylic monomer with excellent pressure-sensitive adhesive properties is added to this emulsion. Since post-polymerization is performed, in other words, the polymer particles produced by this method have a crosslinked structure inside and a structure made of acrylic pressure-sensitive adhesive polymer as the surrounding part. It is estimated that the gel fraction of the entire polymer constituting the particles is regulated within a specific range due to the particle structure. It exhibits high adhesive strength and high cohesive strength even without the addition of agents, and has excellent repulsion 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 polymer particles whose fraction is regulated within a specific range is produced. The above monomer for polymerization is a monomer mixture consisting of a (meth)acrylic acid alkyl ester whose alkyl group has 1 to 14 carbon atoms and an ethylenically unsaturated monomer containing a carboxyl group. ) As the acrylic acid alkyl ester, those having an alkyl group of 1 to 14 carbon atoms are used from the viewpoint of adhesive properties, and particularly preferred examples include ethyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate. , isononyl allylate, isodecyl acrylate, ethyl methacrylate, butyl methacrylate, lauryl methacrylate, and the like. Further, as the carboxyl group-containing ethylenically unsaturated monomer, (meth)acrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, etc. are used, and among these, acrylic acid is particularly preferred. This carboxyl group-containing ethylenically unsaturated monomer is an essential component for forming crosslinks in polymer particles.Why are crosslinks formed when such monomers are used? is not necessarily clear at present. It is speculated that when the above monomer is used, a chain transfer reaction by radicals occurs within the emulsion particles (polymer particles) during the polymerization process, and this is thought to be involved in the formation of crosslinks. Such a monomer mixture consisting of a (meth)acrylic acid alkyl ester and a carboxyl group-containing ethylenically unsaturated monomer has a glass transition point of 250°K or lower at which the copolymer exhibits pressure-sensitive adhesive properties. It is necessary that the composition be such that With such a composition, it is possible to prepare an adhesive composition that has both high cohesive strength and high adhesive strength. The proportion of the carboxyl group-containing ethylenically unsaturated monomer in the monomer mixture is generally about 0.1 to 10% by weight, preferably about 0.5 to 7% by weight. If this ratio is too small, the gel fraction of the polymer particles cannot be set within the desired range, and if it is too large, the polymerization stability will deteriorate significantly, and the gel fraction of the polymer particles will become too high, resulting in poor adhesive strength. Also, problems tend to occur in terms of repulsion resistance. In the first stage of polymerization, persulfate as a polymerization initiator is added and dissolved in water, heated to a temperature of 60 to 90°C, and the above monomers are added to the aqueous medium with stirring. The mixture may be added dropwise at a predetermined dropping rate and allowed to react for a predetermined period of 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. Generally, the proportion of persulfate is preferably 0.1 to 5 parts by weight per 100 parts by weight of the monomer mixture used in the first stage polymerization. The polymer emulsion obtained in this way has a gel fraction of the polymer particles contained therein of 20 to 20.
It is important that the content is set within a range of 60% by weight. This setting can be easily achieved mainly by adjusting the type and amount of the carboxyl group-containing ethylenically unsaturated monomer, and in addition, by appropriately selecting the amount of persulfate used and polymerization conditions as described above. It can be done. By setting the gel fraction of the polymer particles within the above range, it is possible to obtain an adhesive composition that has high adhesive strength, high cohesive strength, and excellent repulsion resistance. When the fraction is less than 20% by weight, the cohesive force decreases, and when it exceeds 60% by weight, the adhesive strength decreases and it becomes difficult to obtain an adhesive composition with excellent rebound resistance. 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 post-polymerization is further added to the polymer emulsion containing polymer particles having a gel fraction of 20 to 60% by weight, obtained as described above, to carry out the second stage polymerization. This produces a polymer emulsion containing polymer particles with a suitably crosslinked structure. The above monomer for post-polymerization may be a (meth)acrylic acid alkyl ester having an alkyl group of 1 to 14 carbon atoms alone or an unsaturated monomer having no functional group in the molecule that can be copolymerized with this ester. It consists of a quantity. As the (meth)acrylic acid alkyl ester, the same monomers as those in the first stage are used. Further, examples of unsaturated monomers having no functional group in the molecule that can be copolymerized include acrylonitrile, methallylonitrile, vinyl acetate, styrene, and derivatives thereof. These copolymerizable monomers are used as necessary components for modifying adhesive properties. Such a monomer for post-polymerization has a glass transition point of 250°K or less at which the homopolymer or copolymer exhibits pressure-sensitive adhesive properties, as in the case of the first-stage monomer mixture. The composition must be such that a polymer having a higher glass transition point than the above is likely to cause problems in terms of adhesive strength. Therefore, even if an unsaturated monomer having no functional group in the molecule that can be copolymerized with the (meth)acrylic acid alkyl ester is used, the amount of the latter monomer to be used is limited to the monomer for post-polymerization. 10 in mass
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 for subsequent polymerization to be used depends on the composition of the monomer mixture used in the first stage polymerization and the gel fraction of the polymer particles composed of this mixture.
In addition, to ensure that the final solid content concentration is at an appropriate ratio,
It is determined as appropriate, and usually the amount of the monomer for post-polymerization is 10 to 500 parts by weight, preferably 20 to 250 parts by weight, based on 100 parts by weight of the monomer mixture in the first stage. It is better to use a percentage. The second stage polymerization is carried out by dropping the above monomer for post-polymerization into the polymer emulsion produced in the first stage 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 . 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 manner generally has a concentration of polymer particles having a moderately crosslinked structure, that is, a solid content concentration of 50%, from the viewpoint of stability and viscosity characteristics of the emulsion.
It is desirable that the content be set in the range of ~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 if necessary, a colorant and a filler are added to the composition. Conventionally known additives such as , anti-aging agents, and tackifiers 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 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, a monomer mixture (copolymer glass transition point 225°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 49.8%,
The gel fraction of the polymer particles was 38.5%. Next, the above polymer emulsion was heated to 80°C under a nitrogen stream, and then 9.5 parts of isobutyl acrylate and 40 parts of 2-ethylhexyl acrylate were added.
A monomer for post-polymerization (copolymer glass transition point: 225°K) consisting of
A second stage of polymerization was carried out by holding the mixture for a certain period of time. The solid content concentration of the polymer emulsion thus obtained was 59.5%, 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 exactly the same manner as in Example 1, except that 100 parts of 2-ethylhexyl acrylate was used instead of the first monomer mixture. Note that the gel fraction of the polymer particles of the emulsion obtained in the first stage polymerization was 0%. Comparative Example 2 100 parts of n-butyl acrylate was used instead of the monomer mixture in the first stage, and 40 parts of n-butyl acrylate and acrylonitrile were used as monomers for post-polymerization in the second stage. A water-dispersed pressure-sensitive adhesive composition was obtained in exactly the same manner as in Example 1, except that 9.5 parts of the water-dispersed pressure-sensitive adhesive composition was used. Note that the gel fraction of the polymer particles of the emulsion obtained in the first stage polymerization was 0%. Comparative Example 3 As the first stage monomer mixture, acrylic acid 2
- A water-dispersed pressure-sensitive adhesive composition was obtained in exactly the same manner as in Example 1, except that a monomer mixture of 99.98 parts of ethylhexyl and 0.02 parts of acrylic acid was used.
Note that the gel fraction of the polymer particles of the emulsion obtained in the first stage polymerization was 9.5%. Examples 2 to 6 A first-stage polymer emulsion was obtained in exactly the same manner as in Example 1, except that the monomer mixture shown in Table 1 below was used as the first-stage monomer mixture. . The gel fraction of the polymer particles contained in this emulsion was as shown in Table 1. In Table 1, the glass transition point means the glass transition point of a copolymer made of a monomer mixture.

【table】

[Table] Next, a second polymer emulsion was prepared in the same manner as in Example 1, except that the monomers for post-polymerization shown in Table 2 below were used for the polymer emulsion in the first stage. A stage polymer emulsion was obtained. This emulsion was directly used as the water-dispersed pressure-sensitive adhesive composition of the present invention. The solid content concentration in the second stage polymer emulsion was as shown in Table 2. Moreover, the glass transition point in Table 2 means the glass transition point of the copolymer composed of the monomer for post-polymerization.

【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 3 above were as shown in Table 3 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)

[Claims] 1. A glass transition in which a copolymer consisting of an alkyl (meth)acrylic acid ester having an alkyl group having 1 to 14 carbon atoms and an ethylenically unsaturated monomer containing a carboxyl group exhibits pressure-sensitive adhesive properties. A polymer containing polymer particles with a gel fraction of 20 to 60% by weight obtained by polymerizing a monomer mixture whose point can be below 250°K in the presence of a persulfate in an aqueous medium without using an emulsifier. The number of carbon atoms in the alkyl group in the combined emulsion is 1.
~14 (meth)acrylic acid alkyl ester alone or its homopolymer or copolymer consisting of this ester and an unsaturated monomer having no functional group in the molecule that can be copolymerized with it exhibits pressure-sensitive adhesive properties. A water-dispersible pressure-sensitive adhesive composition based on a polymer emulsion obtained by polymerization with the addition of a post-polymerization monomer that can have a glass transition point of 250°K or less. 2. In a monomer mixture for obtaining a polymer emulsion containing polymer particles having a gel fraction of 20 to 60% by weight, the carboxyl group-containing ethylenically unsaturated monomer is 0.1 to 10% by weight in the above mixture. A water-dispersible pressure-sensitive adhesive composition according to claim 1. 3 Monomer mixture for obtaining a polymer emulsion containing polymer particles with a gel fraction of 20 to 60% by weight
3. 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 per 100 parts by weight. 4 Monomer mixture for obtaining a polymer emulsion containing polymer particles with a gel fraction of 20 to 60% by weight
4. The water-dispersed pressure-sensitive adhesive composition according to claim 1, wherein the post-polymerization monomer is present in an amount of 10 to 500 parts by weight per 100 parts by weight. 5. The polymerization temperature is 60-60% to obtain a polymer emulsion containing polymer particles with a gel fraction of 20-60% by weight.
The water-dispersed pressure-sensitive adhesive composition according to any one of claims 1 to 4, wherein the temperature is 90°C. 6. Any one of claims 1 to 5, wherein in a polymer emulsion obtained by polymerizing with the addition of a monomer for post-polymerization, the solid content concentration is 50 to 70% by weight in the emulsion. The water-dispersed pressure sensitive adhesive composition described.