ANTISTATIC PRESSURE-SENSITIVE ADHESIVE COMPOSITION Detailed Description of the Invention Technical Field of the Invention The present invention pertains to a pressure-sensitive adhesive composition having ion conductivity and antistatic function.

Prior Art A static electric charge on insulating components at times leads to problems such as adsorption of dust and destruction of electrical circuits. In order to prevent the above-mentioned static electric charge, protection of components with a sheet coated with an electrically conductive pressure-sensitive adhesive composition has been used in practice. For electrically conductive pressure-sensitive adhesive compositions, those containing a metal filler or carbon particles having electrons as the electrical carrier are known. An electrically conductive pressure-sensitive adhesive composition having ions as the electrical carrier are also known.

In general, when ions are the electrical carrier, the material is referred to as a solid electrolyte and is often used as a structural component of cells, as disclosed in Japanese Kokai Patent Application No. Sho 60 [1985]-47372, etc. According to the this publication, solid electrolytes are structured of a polymer compound having a polyethylene oxide as a side chain and an ionic compound. The above-mentioned solid electrolyte is used as a solid electrolyte for electrochemical devices such as primary cells, secondary cells and electrochromic elements, and tackiness is absent.

A solid electrolyte having tackiness, in other words, an ionic conductive pressure-sensitive adhesive composition, is disclosed in Japanese Kokai Patent Application No. Hei 1-266161. The above-mentioned pressure-sensitive adhesive composition includes a copolymer of a polyurethane polyol prepolymer and a polyurethane polyisocyanate prepolymer and an ionic compound. The above- mentioned copolymer is a polyurethane with increased polymer molecular weight achieved through growth at the urethane bond from a urethane oligomer through a polyaddition reaction. The above-mentioned reaction is a sequential polymerization reaction. In general, a tin catalyst, etc. is used, and the reaction commonly requires a long time.

Furthermore, ionic conductive pressure-sensitive adhesive sheets are disclosed in publications such as Tokuhyo Patent Application Hei 9 [1997]-501009, the specification of United States Patent No. 5,378,405, and Japanese Kokai Patent Application No. Hei 9 1997-208910. The pressure-sensitive adhesive sheets described in the above-mentioned publications include adhesive fine particles. An ionic conductive material, such as nonyl phenoxy polyethylene glycol acrylate containing an average of at least eight ethylene oxide units and forming a complex with a metal ion, is grafted onto the surface of the above-mentioned adhesive fine particle.

In general, the above-mentioned adhesive fine particles are produced by means of suspension polymerization or emulsion polymerization. In the above- mentioned polymerization method, it is necessary to disperse the reaction material in a polar solvent such as water. In general, a surfactant is used for dispersing.

However, even when a surfactant is used, the amount of acrylate monomer having a high water-solubility such as acrylic acid is limited. Furthermore, with passage of time the residual surfactant included in the pressure-sensitive adhesive sheet migrates toward the surface of the bonding object and is concentrated. In many cases, the above-mentioned migration and concentration lead to problems such as a change in adhesion or glue residue that contaminates the surface of the bonding object. When the surfactant is concentrated at the surface of the bonding object, water is likely to be absorbed in that area. The absorbed water in turn reduces the surface resistance of the pressure-sensitive adhesive sheet and antistatic properties can be achieved, but the above-mentioned absorption depends strongly on moisture; thus, the surface resistivity is likely to be affected by moisture. In other words, antistatic properties are likely to be influenced by the external environment.

Thus, the objective of the present invention is to produce an ion-conductive pressure-sensitive adhesive composition that does not require a solvent or surfactant at the time of production and, as a result, is not influenced by the external environment.

Solvents and surfactants are not required at the time of production of the pressure-sensitive adhesive composition of the present invention; thus, an ion- conductive pressure-sensitive adhesive having antistatic properties that are not influenced by the external environment can be produced. Furthermore, the pressure-

sensitive adhesive sheet of the present invention exhibits very high light transmittance.

Summary of the Invention According to the present invention, an antistatic pressure-sensitive adhesive composition is provided comprising (1) copolymer produced upon performing irradiation polymerization of a polymeric mixture containing (a) a urethane acrylate, (b) a (meth) acrylate having a polyalkylene oxide chain, and (c) an adhesion-imparting monomer, and (2) an ionic compound.

The pressure-sensitive adhesive composition of the present invention includes a copolymer produced by radiation polymerization. The pressure-sensitive adhesive composition is produced without a solvent and in a nonaqueous system; thus, surfactants and water that lead to an adhesive residue and changes in the surface resistivity of the pressure-sensitive adhesive composition are essentially absent.

Embodiment of the Invention The copolymer comprises a urethane acrylate in the pressure-sensitive adhesive composition of the present invention, which imparts flexibility and rigidity to the pressure-sensitive adhesive composition. A urethane oligomer having in the molecular structure a soft polyol segment with a molecular weight of at least 1000 is suitable. For the above-mentioned urethane oligomer acrylates, polyester urethane acrylates, and polyether urethane acrylates can be mentioned. The amount of urethane acrylate included in the copolymer is preferably in the range of 10-50 wt%.

When the urethane acrylate is too low, the cohesion is not adequate; on the other hand, when the amount exceeds 50 wt%, the level of tackiness is lowered, which is not desirable. Some urethane acrylates yellow and some are non-yellowing; in this case, it is preferred to use the non-yellowing type so that a transparent, colorless pressure-sensitive adhesive composition can be produced. Also, in order to promote mobility of the ion, it is desirable that the glass transition temperature of the copolymer included in the pressure-sensitive adhesive composition be low. From the

standpoint of tackiness, a composition with a glass transition temperature of 0°C or below is especially desirable.

Commercially available suitable urethane acrylates include UV-3000B, UV- 3200B, and UV-3300B produced by Nippon Synthetic Chemical Co., Ltd., CN-964, NC-965, CN-966, CN-984, CN-986, and the like, produced by Sartomer Co. Among urethane acrylates, the glass transition temperature of UV-3000B is especially low, 39°C, which is preferred (Meth) acrylates having a polyalkylene oxide chain impart ionic conductivity to the pressure-sensitive adhesive composition. The polyalkylene oxide chain forms a complex with a cation having a relatively small ionic radius such as lithium ions, potassium ions, and sodium ions, and imparts ionic conductivity to the pressure- sensitive adhesive composition of the present invention.

Examples of (meth) acrylates having polyalkylene oxide chains, include polyethylene glycol (meth) acrylate, polypropylene glycol (meth) acrylate, and polytetramethylene glycol (meth) acrylate, and the like. The molecular weight of the above-mentioned (meth) acrylates is not especially limited, but from the standpoint of commercial availability, AM-90G, which is a methoxypolyethylene glycol acrylate produced by Shin Nakamura Chemical Co., is especially suitable. The mixing ratio of the (meth) acrylate monomer having a polyalkylene oxide chain included in the copolymer is preferably 50 wt% or below. When the mixing ratio exceeds 50 wt%, the cohesion of the composition drops to an undesirable level.

The adhesion of the pressure-sensitive adhesive composition can be controlled by the adhesion-imparting monomer according to the mixing ratio. For the above- mentioned monomer, hydroxypropyl (meth) acrylate, hydroxyethyl (meth) acrylate, hydroxybutyl (meth) acrylate, acrylic acid, etc. are useful. Hydroxypropyl acrylate has excellent solubility with urethane acrylates having high viscosity; thus, the material can be used as a diluent and can be used effectively for mixing or coating during the course of production of the pressure-sensitive adhesive. Acrylic acid increases the adhesion with the substrate; thus, the use of an acrylic acid is effective when an especially high adhesion is required. Furthermore, the amount used (mixing ratio) is limited when a conventional suspension polymerization reaction wherein the monomer is dispersed in water is used since the solubility is high in those monomers, but any mixing ratio can be used in the solvent-free suspension polymerization

method in the present invention. The amount of monomer that imparts adhesion to be included in the copolymer is determined by the adhesion required.

In general, 50 wt% or less is desirable for hydroxyalkyl (meth) acrylates such as hydroxypropyl (meth) acrylate, and 30 wt% or less is desirable when acrylic acid is used. The reason is that when the amount of the acrylic acid is increased, the glass transition temperature of the composition increases and degree of electrical conductivity increases.

As a monomer for the copolymer included in the pressure-sensitive adhesive composition of the present invention, comonomers that are copolymerizable with the above-mentioned monomers may be included. For example, when dilution of the pressure-sensitive adhesive composition system is desired with little change in the electrical properties or adhesive properties, it is possible to use an inexpensive acrylate such as isooctyl acrylate (IOA) as a comonomer. The amount of comonomer used, when desireable, is 50 wt% or below.

As a means used for curing of the pressure-sensitive adhesive composition of the present invention, an actinic radiation such as ultraviolet, visible, or electron beam can be mentioned. Furthermore, a radical polymerization initiator may be included, as needed. It is desirable to use a non-yellowing type photopolymerization initiator, and, for example, DarocurtD 1173, Irgacure 184, etc. produced by Ciba Geigy Co. are especially suitable. In general, the amount of photopolymerization initiator used is 1 part by weight for 100 parts by weight of the monomer.

The ionic compound included in the pressure-sensitive adhesive composition of the present invention forms a complex with the polyalkylene chain and imparts ionic conductivity. For the above-mentioned ionic compound, a compound with a cation having a relatively small ionic radius, such as a lithium ion, potassium ion, or sodium ion, for example, perchlorates of lithium, potassium, or sodium, or inorganic salts (inorganic salt) thereof, such as chlorides, bromides, iodides, and thiocyanides can be mentioned. Lithium has the smallest ionic radius, so it can impart a high degree of charge mobility to the pressure-sensitive adhesive composition. On the other hand, when the absence of a metal ion is desired for the pressure-sensitive adhesive composition, an organic salt such as perchlorotetrabutyl ammonium salt can be used effectively. The amount of ionic compound included in the pressure-sensitive adhesive composition is preferably 10 wt% or below.

In order to increase the conductivity, a compound having a high dielectric constant as in the case of a propylene carbonate can be included in the pressure- sensitive adhesive composition of the present invention. The above-mentioned compound having a high dielectric constant supplements the ionic conductivity of the polyalkylene glycol chain included in the copolymer. Compared with (meth) acrylate having a polyalkylene chain such as the above-mentioned polyethylene glycol acrylate, propylene carbonate exhibits a higher dielectric constant and is capable of dissolving the above-mentioned inorganic salt in an organic salt; thus, the compound can be used effectively.

As long as adverse effects on the present invention are absent, other additives can be included in the pressure-sensitive adhesive composition of the present invention, such as plasticizers, tackifiers, antioxidants, and the like.

Unlike adhesive fine particles produced by methods such as emulsion polymerization or suspension polymerization, the copolymer included in the pressure- sensitive adhesive composition of the present invention is produced by curing by means of exposure to ultraviolet or an electron beam; thus, water and solvents are not required. In the present invention, chain polymerization based on a radical activation source is used and compared with the above-mentioned sequential polymerization, the reaction rate is very high. Among all types of acrylates, the curing rate of urethane acrylate is very high; thus, the material can be used effectively for increased productivity of the pressure-sensitive adhesive composition.

The pressure-sensitive adhesive composition of the present invention can be produced as follows. An ionic compound such as lithium perchlorate is added as needed to a polymeric mixture containing a urethane acrylate such as a polyester urethane acrylate, a (meth) acrylate monomer having a polyalkylene chain such as methoxy polyethylene glycol acrylate, an adhesion imparting monomer such as hydroxypropyl acrylate, and a comonomer such as isooctyl acrylate, and mixing is performed; then, a photopolymerization initiator is added as needed, and stirring is provided until the materials are thoroughly dissolved. Then when radiation such as ultraviolet or an electron beam is applied to the mixture produced as described above, curing occurs, and a pressure-sensitive adhesive can be produced. In general, the exposure level of the radiation is in the range of 100-1000 mJ/cm2.

Furthermore, the pressure-sensitive adhesive composition of the present invention is formed on a substrate and used as a pressure-sensitive adhesive sheet.

Useful examples of substrates include polyesters such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), polyethylene (PE), polypropylene (PP), and the like. The thickness of the substrate is not especially limited, it is typically in the range of 10-200 gm.

The above-mentioned pressure-sensitive adhesive sheet can be produced as shown below. The mixture produced as described above is coated onto a base such as a PET sheet and a base sheet such as a PET sheet treated with a release agent such as silicone is deposited on it so as to produce a sandwich structure. An adjustment is made for the thickness of the pressure-sensitive adhesive to form the desired thickness using a knife coater, etc. Radiation such as ultraviolet and electron beam is applied. and curing is achieved so as to produce a pressure-sensitive adhesive sheet.

In general, fine adhesive particles produced by emulsion polymerization or suspension polymerization such as the particles described in Tokuhyo Patent Application Hei 9 [1997]-501009 have a particle diameter of approximately 1 um; thus, the particles act to scatter light, and the pressure-sensitive adhesive sheet itself becomes semitransparent. It is difficult to find surface flaws in semitransparent pressure-sensitive adhesive sheets, but flaws can be detected upon bonding the pressure-sensitive adhesive sheet onto a component for the first time. However, scattering hardly occurs in the pressure-sensitive adhesive composition of the present invention, and a pressure-sensitive adhesive sheet having high transmittance in the visible light region is made possible. The light transmittance of the pressure-sensitive adhesive composition of the present invention to white light is 90% or higher for a pressure-sensitive adhesive sheet with a thickness of 50 u. m.

The present invention is explained in further detail below with application examples.

Application Example 1 Production of an antistatic pressure-sensitive adhesive composition was carried out as follows. First, mixing was performed for 20 parts of UV-3000B, a urethane acrylate produced by Nippon Synthetic Chemical Industries, and 20 parts by weight of AM-90G, methoxy polyethylene glycol acrylate produced by Shin

Nakamura Chemical Co. until complete dissolving was achieved. Subsequently, 35 parts by weight of hydroxypropyl acrylate, 15 parts by weight of isooctyl acrylate, and 10 parts by weight of AM-90G solution of 0.4 wt% lithium perchlorate and 1 part by weight of Darocur 1173 (registered trademark) were added, and stirring was provided until complete dissolving was achieved.

Subsequently, the sample solution produced as described above was added dropwise to a substrate made of PET with a thickness of 50 pm, and a substrate made of a silicone-treated PET was further applied. An adjustment was made by a knife coater so that the thickness of the pressure-sensitive adhesive produced was 50 zm.

The sample produced was quickly passed through an ultraviolet radiator to apply ultraviolet at an exposure level of approximately 800 mJ/cm2 for curing to produce a pressure-sensitive adhesive sheet.

Evaluations were made of the surface resistivity, adhesion, and transmittance of the pressure-sensitive adhesive sheet produced. The results obtained are shown in Table 1. In this case, the measurement of the surface resistivity was done according to standard test method S 11.11 of the EOS/ESD Association. The measurement of the adhesion was done based on the 180° peel test. Furthermore, measurement of the optical light transmittance was done by a hazemeter (product of Nippon Denshoku Ind., NDH-SENSOR). As shown in the table, the surface resistivity was 1. lx101° Q/sq., and the value is adequately low for prevention of a static electric charge. Furthermore, an adhesive residue was not observed, and the light transmittance was 90% or higher, and excellent transparency was confirmed.

Table I Pressure-sensitive adhesive characteristics Surface resistivity (Q/D) 1.1 x 10l° Adhesion (g/25 mm) 49 Light transmittance (%) 90% or higher Glue residue None Note 1) The measurement was made with the PET substrate in place using white light.

Application Example 2 The production of an antistatic pressure-sensitive adhesive composition was carried out as described below. First, mixing was performed for 20 parts of the

urethane acrylate UV-3000B, and 20 parts by weight of the methoxy polyethylene glycol acrylate AM-90G until complete dissolving was achieved. Subsequently, acrylic acid (AA) and isooctyl acrylate in the amounts shown in Table II below, 10 parts by weight of 0.4 wt% lithium perchlorate and 1 part by weight of Darocure (registered trademark) 1173 were added and stirring was performed until complete dissolving was achieved. Curing was performed for the mixture produced above as in the case of Application Example I above.

As shown above, (1) propylene carbonate was used in combination with methoxy polyethylene glycol acrylate as an ionic conductive medium, (2) acrylic acid (AA) was used instead of hydroxypropyl acrylate, and (3) the mixing ratio of acrylic acid/isooctyl acrylate was changed, and an evaluation was performed for the effectiveness of the adhesion.

Table II shows the results obtained when the pressure-sensitive adhesive sheets produced were applied to a glass base and the peel test was performed. From the results obtained, it is clear that the adhesion has a close relationship with the amount of acrylic acid used. Furthermore, adequately low surface resistivity was achieved in all systems.

Table II AA/IOA (parts by weight) 5/45 10/40 20/30 Adhesion (g/25 mm) 12 41 590 Adhesiveresidue None None None Surface resistivity (Q/0) 5.7 x 108 1. 1 x 109 4. 0 x 109