Patent Application
20110014409
1. Field of the Invention
The present invention relates to a pressure-sensitive adhesive (PSA) sheet imposing a small burden on the environment and superior adhesion properties.
The present application claims priority on the basis of Japanese Patent Application No. 2009-167843, filed on Jul. 16, 2009, the entire content of which is incorporated herein by reference.
2. Description of the Related Art
PSA sheets generally have a form in which a PSA layer is protected by a liner able to release from the PSA layer (release liner). The surface of the release liner that contacts the PSA layer is normally a release surface to which is imparted a layer composed of a release agent (release layer). A silicone-based release agent is an example of a release agent having superior handling ease and heat resistance. Technologies relating to a silicone-based release agent or release liner formed using such a release agent are described in Japanese Patent Application Publication Nos. H10-237393, H6-297645 and 2006-291121.
There has recently been a growing desire to reduce the amounts of volatile organic compounds (VOC) such as toluene emitted from PSA sheets from the viewpoints of consideration of the environment and improvement of the work environment. Consequently, when forming a PSA layer of a PSA sheet, a non-toluene-based PSA composition, having for a base thereof a polymer synthesized in an aqueous solvent or non-toluene-based organic solvent such as ethyl acetate (hereinafter, aqueous solvents and non-toluene-based organic solvents may be collectively referred to as “non-toluene-based solvents”), tends to be used preferably. However, PSA sheets formed using such non-toluene-based PSA compositions tend to have lower adhesion performance (such as curved surface adhesion) than conventional PSA sheets using PSA compositions having for a base thereof polymers synthesized in toluene.
An object of the present invention is to provide a PSA sheet that reduces the amount of VOC emission and demonstrates superior adhesion properties by using a non-toluene-based PSA composition.
The inventors of the present invention found that, by limiting to equal to or less than a prescribed value the amount of silicone that transfers from a release layer to a PSA layer, as determined by a prescribed evaluation method, in a PSA sheet provided with a release liner using a silicone-based release agent, satisfactory adhesion properties (such as curved surface adhesion) can be realized even with a PSA sheet using a non-toluene-based PSA composition, thereby leading to completion of the present invention.
According to the present invention, a PSA sheet is provided that is provided with a release liner having, on at least a first side thereof, a release layer composed of a silicone-based release agent, and a PSA layer provided on the release layer. Here, the release layer has an amount of silicone that transfers to Single-Sided Adhesive Tape No. 31 B manufactured by Nitto Denko Corp. of 10 kcps or less per unit surface area equivalent to a circle having a diameter of 30 mm when determined as X-ray intensity of silicon (Si) by X-ray fluorescence analysis. The PSA layer contains a PSA component that is synthesized in an aqueous solvent or ethyl acetate.
According to a PSA sheet employing this configuration, since the amount of toluene emission and the total amount of VOC emission (the total amount of VOC, also referred to as TVOC) can be reduced, the PSA sheet is preferable in terms of environmental health. In addition, since a PSA layer is used for the PSA layer that contains a PSA component synthesized in an aqueous solvent or non-toluene-based organic solvent, the PSA sheet also offers the advantage of imposing a small burden on the natural environment and being preferable in terms of the work environment during the production thereof. Moreover, since the transferred amount of silicone as evaluated (determined) according to the method described above is low, the effect of transfer of silicone from the release layer to the PSA layer on adhesion performance can be held to a low level, thereby making it possible to realize superior adhesion properties.
The transferred amount of silicone of the release layer per unit surface area equivalent to a circle having a diameter of 30 mm employs a value quantified in accordance with the following silicone transfer measurement method.
[Silicone Transfer Measurement Method]
A test piece is prepared by laminating an adhesive surface of Single-Sided Adhesive Tape No. 31B manufactured by Nitto Denko Corp. to the release surface (release layer) of a release liner to be measured. This test piece is then placed in a dryer at 70° C. for 24 hours after imparting a load of 5 kg followed by removing the load, taking out of the dryer and holding for an additional 2 hours at 23° C. The release liner is then peeled from the test piece and the amount F of Si (kcps) present per unit surface area equivalent to a circle having a diameter of 30 mm on the exposed adhesive surface is measured by X-ray fluorescence analysis. The amount I of Si (kcps) present per unit surface area equivalent to a circle having a diameter of 30 mm on an adhesive surface of the above-mentioned adhesive tape is measured by X-ray fluorescence analysis as a blank. The value obtained by subtracting I from F is the transferred amount of silicone of the PSA layer.
An example of a preferable silicone-based release agent for the PSA sheet disclosed herein is solvent-free silicone. Another preferable example is heat-curable silicone.
In a preferable aspect of the PSA sheet disclosed herein, the total amount of toluene emitted from the PSA sheet when the sheet is held for 30 minutes at 80° C. (toluene emission) is 20 μg or less per gram of the PSA sheet, and the amount of TVOC emitted from the sheet (TVOC emission) is 300 μg or less per gram of the PSA sheet. A PSA sheet in which toluene emission and TVOC emission are reduced in this manner can be preferably used in products used in enclosed spaces in the manner of automobile or residential interior materials or in applications for joining or fixing members of products that can reach a high temperature at the time of use.
In another preferable aspect, the PSA layer is formed from a PSA composition containing an acrylic polymer obtained by polymerizing a monomer starting material containing an acrylic monomer represented by the general formula: CH2═C(R1)COOR2. Here, R1 represents a hydrogen atom or methyl group, and R2 represents an alkyl group having 2 to 14 carbon atoms. According to this PSA composition, a PSA sheet can be provided that realizes even more superior PSA properties.
In still another preferable aspect, the PSA sheet is configured as a double-sided PSA sheet. Namely, in this aspect, a substrate of which both sides are non-releasing is laminated onto the PSA layer on the release layer, and a PSA layer is further laminated onto the substrate. As has been previously described, since the PSA sheet disclosed herein has reduced emission of toluene and TVOC and demonstrates superior adhesion properties, according to this configuration, it is preferable in terms of environmental health, and can be preferably used as joining means of various types of parts that demonstrates superior adhesion.
The following provides an explanation of preferred embodiments of the present invention. Matters other than those specifically mentioned in this description that are required for carrying out the invention can be understood to be design matters carried out by persons with ordinary skill in the art based on the prior art in the relevant field. The present invention can be carried out based on the contents disclosed in this description and matters of technical common sense in the relevant field. In addition, in the following explanations, the same reference numerals are used to indicate those members or sites that demonstrate similar actions, and duplicate explanations thereof may be omitted or simplified.
The PSA sheet according to the present invention is provided with a release liner having, on at least a first side thereof, a release layer composed of a silicone-based release agent, and a PSA layer that is provided on the release layer and formed from a non-toluene-based PSA composition. The concept of this PSA sheet can include that referred to as PSA tape, PSA labels or PSA film and the like. Although the PSA layer is typically formed continuously, it is not limited thereto, but rather may be formed in a regular or random pattern such as dots or stripes. In addition, the PSA sheet disclosed herein can be processed into various forms such as rolls or individual sheets.
The PSA sheet disclosed herein may have a cross-sectional structure schematically shown in, for example,
The release liner of the PSA sheet disclosed herein includes a substrate and a release layer at least imparted to a first side thereof (releasable film). Here, the release layer is formed using a silicone-based release agent so that the transferred amount of silicone as measured according to the above-mentioned method is 10 kcps or less. Examples of silicone-based release agents capable of realizing this transferred amount of silicone include heat-curable silicone-based release agents and ionizing radiation-curable silicone-based release agents that are cured by imparting heat or ionizing radiation (such as ultraviolet rays, α rays, β rays, γ rays, neutron beam or electron beam) after coating. One type of these silicone-based release agents can be used alone or two or more types can be used in combination. Heat-curable silicone-based release agents are used preferably from viewpoints such as economy and simplicity of the device required for coating.
In addition, these release agents may be solvent-free types that do not contain solvent or solvent types in which the release agent is dissolved or dispersed in an organic solvent. In addition, solvent-free release agents may also be used for which viscosity has been adjusted to facilitate application (and typically coating) by mixing with a suitable amount of a solvent having comparatively low surface tension. From the viewpoints of environmental health during release layer formation and further reducing the TVOC emission, a solvent-free silicone-based release agent that is substantially free of organic solvent and can be coated without modification is used preferably.
The above-mentioned heat-curable silicone-based release agents ordinarily contain organohydrogenpolysiloxane and organopolysiloxane having unsaturated aliphatic groups, and may be of the solvent-free type or solvent type. A thermal addition reaction-curable silicone-based release agent that is cured by undergoing crosslinking by a thermal addition reaction is used particularly preferably.
Examples of such thermal addition reaction-curable silicone-based release agents that can be used include a release agent containing: a polysiloxane having hydrogen atoms (H) bonded to silicone atoms (Si) in a molecule thereof (Si—H group-containing polysiloxane); and, a polysiloxane containing functional groups (Si—H group-reactive functional groups) having reactivity with Si—H bonds (Si—H covalent bonds) in a molecule thereof (Si—H group-reactive polysiloxane). This release agent can be cured by crosslinking Si—H groups with Si—H group-reactive functional groups by an addition reaction.
In the above-mentioned Si—H group-containing polysiloxane, Si bonded to H may be Si present in the main chain or Si present in a side chain. Polysiloxanes containing two or more Si—H groups in a molecule thereof are preferable. Examples of polysiloxanes having two or more Si—H groups in a molecule thereof include dimethylhydrogensiloxane-based polymers such as poly(dimethylsiloxane-methylsiloxane).
On the other hand, polysiloxanes of a form in which an Si—H group-reactive functional group or side chain containing such functional group is bonded to Si that forms the main chain (backbone) of the siloxane-based polymer (such as Si on the end of the main chain or Si within the main chain) can be used for the above-mentioned Si—H group-reactive polysiloxane. In particular, polysiloxanes in which a Si—H group-reactive functional group is bonded directly to Si of the main chain are preferable. In addition, polysiloxanes containing two or more Si—H group-reactive functional groups in a molecule thereof are preferable. Examples of Si—H group-reactive functional groups include alkenyl groups such as a vinyl group or hexenyl group.
Examples of siloxane-based polymers that form the above-mentioned main chain portion include polydialkylsiloxanes such as polydimethylsiloxane, polydiethylsiloxane or polymethylethylsiloxane (in which the two alkyl groups may be the same or different), and polymers obtained by polymerizing a plurality of Si-containing monomers such as polyalkylarylsiloxane or poly(dimethylsiloxane-methylsiloxane). A particularly preferable example of a main chain polymer is polydimethylsiloxane.
A thermal addition reaction-curable silicone release agent that contains a polysiloxane containing two or more Si—H groups in a molecule thereof and a polysiloxane containing two or more Si—H group-reactive functional groups in a molecule thereof is used particularly preferably.
Although there are no particular limitations on the mixing ratio of Si—H group-containing polysiloxane and Si—H group-reactive polysiloxane contained in the above-mentioned release agent provided it is within a range in which the release agent is adequately cured and the above-mentioned transferred amount of silicone can be realized, it is preferably selected such that the number of moles X of Si of Si—H groups and the number of moles Y of Si—H group-reactive functional groups is such that X≧Y, and normally is such that X:Y is about 1:1 to 2:1 (and more preferably 1.2:1 to 1.6:1).
In addition, a catalyst for accelerating the crosslinking reaction may be added to the heat-curable silicone-based release agent as described above. Examples of such catalysts include platinum-based catalysts such as platinum microparticles, chloroplatinic acid and derivatives thereof. Although there are no particular limitations on the amount of catalyst added, it is selected from, for example, a preferable range of 0.1 to 1000 ppm (and more preferably 1 to 100 ppm) based on the Si—H group-reactive polysiloxane.
Mixtures obtained by suitably preparing or acquiring the components described above or commercially available products containing the components described above can be used for the heat-curable silicone-based release agent. In addition to the components described above, known, commonly used additives may also be suitably added as necessary, examples of which include fillers, antistatic agents, antioxidants, ultraviolet absorbers, plasticizers or colorants (such as dyes or pigments).
On the other hand, a solvent-free type or solvent type can be used for the above-mentioned ionizing radiation-curable silicone-based release agent. UV-curable silicone-based release agents that are cured by undergoing a crosslinking reaction by irradiation with ultraviolet (UV) light are used particularly preferably.
Examples of UV-curable silicone-based release agents that can be used include release agents that are cured by undergoing a chemical reaction such as cationic polymerization, radical polymerization, radical addition polymerization or hydrosilylation by being irradiated with UV light. UV-curable silicone-based release agents that are cured by cationic polymerization are used particularly preferably.
As for such cationic-polymerizable UV-curable silicone-based release agent, can be used release agents that contain an epoxy group-containing polysiloxane, of a form in which at least two epoxy groups are bonded to Si that forms the main chain (backbone) of a siloxane-based polymer (such as Si on the end of the main chain or Si within the main chain) and/or to Si contained in a side chain either directly or through a divalent group (including groups such as a methylene group, ethylene group or other alkylene group and an ethyleneoxy group, propyleneoxy group or other alkyleneoxy group). The type of bonding of the at least two epoxy groups to Si may be the same or different. In other words, polysiloxanes containing two or more side chains that contain one type or two or more types of epoxy groups are used. Examples of epoxy group-containing side chains include glycidyl, glycidoxy (glydicyloxy), 3,4-epoxycyclohexyl and 2,3-epoxycyclopentyl groups. The epoxy group-containing polysiloxane may be linear, branched or consist of a mixture thereof.
An epoxy group-containing polysiloxane as described above may be suitably prepared in accordance with conventional known methods, or a commercially available product containing such an epoxy group-containing polysiloxane can be used for the UV-curable silicone-based release agent. An example of a synthesis method that can be used to prepare the epoxy group-containing polysiloxane consists of carrying out an addition reaction of an olefin-based epoxy monomer such as 4-vinylcyclohexene oxide, allyl glycidyl ether or 7-epoxy-1-octen on polymethylhydrogensiloxane as a base polymer using a platinum-based catalyst.
The above-mentioned cationic-polymerizable UV-curable silicone-based release agent may have a composition that contains one type or two or more types of an onium salt-based UV cleavage-type initiator (onium salt-based photopolymerization initiator) as a UV cleavage-type initiator (photopolymerization initiator) in addition to the polysiloxane described above. Examples of an onium salt-based UV cleavage-type initiator that can be used include onium salt-based UV cleavage-type initiators described in, for example, Japanese Patent Application Publication Nos. H6-32873, 2000-281965, 1111-228702 and Japanese Examined Patent Application Publication No. H8-26120. Specific examples include diaryliodinium salts, triarylsulfonium salts, triarylselenonium salts, tetraarylphosphonium salts and aryldiazonium salts. Diaryliodinium salts are used particularly preferably.
Examples of diaryliodinium salts include salts represented by the general formula: [Y2I]+X−. Similarly, examples of triarylsulfonium salts, triarylselenonium salts, tetraarylphosphonium salts and aryldiazonium salts include salts represented by general formulas [Y3S]+X−, [Y3Se]+X−, [Y4P]+X− and [YN2]+X−, respectively. Here, Y represents an optionally substituted aryl group, I represents an iodine atom and X− represents a non-nucleophilic and non-basic anion. In addition, S, Se, P and N represent a sulfur atom, selenium atom, phosphorous atom and nitrogen atom, respectively.
Specific examples of the above-mentioned anion (X−) include SbF6−, SbCl6−, BF4−, [B(C6F5)4]−, [B(C6H4CF3)4]−, [(C6F5)2BF2]−, [C6F5BF3]−, [B(C6H3F2)4]−, AsF6−, PF6−, HSO4− and ClO4−. In particular, anions containing elementary antimony (Sb) and anions containing elementary boron (B) are preferable. Particularly preferable examples of onium salts include Sb-containing diaryliodinium salts and B-containing diaryliodinium salts.
Although there are no particular limitations on the amount of UV cleavage-type initiator contained in the above-mentioned cationic-polymerizable UV-curable silicone-based release agent provided it is within a range that allows the initiator to function as a catalyst, it is preferably, for example, about 0.1 to 8 parts by weight (more preferably 0.3 to 5 parts by weight and even more preferably 0.5 to 3 parts by weight) based on 100 parts by weight of the epoxy group-containing polysiloxane.
A UV-curable silicone-based release agent obtained by suitably preparing or acquiring and mixing the components described above, or a commercially available product containing the components described above can be used for the UV-curable silicone-based release agent. In addition, other known, commonly used additives may also be suitably added as necessary in addition to the above-mentioned components, examples of which include fillers, antistatic agents, antioxidants, UV absorbers, plasticizers or colorants (such as dyes or pigments).
There are no particular limitations on the material of the substrate (substrate for the release liner) that holds the release layer composed of the above-mentioned silicone-based release agent. For example, a single-layer material or multi-layer material formed from plastic, paper or various types of fibers and the like can be used as the substrate.
Examples of the above-mentioned plastic substrates that can be used include film-like substrates composed of polyolefins such as polyethylene (PE) or polypropylene (PP), polyesters such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN) or polybutylene terephthalate (PBT), polyamides (so-called nylons) or cellulose (such as cellophanes). The plastic films may be of the non-oriented type or oriented type (including uniaxially and biaxially oriented types).
Examples of paper substrates that can be used include those using Washi paper, ordinary paper, wood-free paper, glassine paper, kraft paper, full pack paper, crepe paper, clay coated paper, top coated paper and synthetic paper. Although there are no particular limitations on the grammage of the paper substrate, normally that having a grammage of about 50 to 100 g/m2 is used suitably.
Examples of various types of fibrous substrates include woven and non-woven fabric obtained by spinning one type of a mixture of various types of fibrous substances (that may be natural fibers, semi-synthetic fibers or synthetic fibers, examples of which include cotton fiber, staple fiber, manila hemp, pulp, rayon, acetate fiber, polyester fiber, polyvinyl alcohol fiber, polyamide fiber and polyolefin fiber).
Examples of substrates composed of other materials include rubber sheets such as natural rubber or butyl rubber, foamed sheets composed of foamed materials such as polyurethane foam or polychloroprene rubber foam, metal foil such as aluminum foil or copper foil, and composites thereof.
A release liner in which polyethylene has been laminated on at least the front surface (surface on the side of the PSA layer) of paper (preferably wood-free paper or glassine paper), and in which release treatment (silicone treatment) using a silicone-based release agent has been carried out on the surface thereof, can be preferably used for the release liner in the technology disclosed herein.
Various types of surface modification treatment such as corona discharge treatment, plasma treatment or application of a primer or various types of surface processing such as embossing may be carried out as necessary on the surface of the release liner substrate provided with a release layer. In addition, various types of additives such as fillers (including inorganic and organic fillers), anti-aging agents, antioxidants, ultraviolet absorbers, antistatic agents, lubricants, plasticizers or colorants (including pigments and dyes) may also be incorporated as necessary.
The thickness of the release liner is preferably about 50 to 200 μm (and more preferably 60 to 160 μm).
A conventionally known method can be employed as a method for imparting the release layer to the release liner. For example, the release layer can be formed by coating and drying a silicone-based release agent as described above onto a substrate using various types of coaters. Examples of coaters that can be suitably selected include a direct gravure coater, offset gravure coater, roll coater, bar coater and die coater.
Although there are no particular limitations on the thickness of the release layer, the coated thickness can be, for example, about 0.03 to 5 μm (and preferably 0.05 to 3 μm). If the thickness of the release layer is less than the above-mentioned range, adequate release properties are less likely to be obtained. If the thickness of the release layer exceeds the above-mentioned range, there may be cases in which the transferred amount of silicone increases due to the presence of residual uncured substances.
Although the coated amount of the release agent can be suitably selected corresponding to the type of adhesive used, type of liner substrate or type of release agent and the like, the coated amount can be, based on the solid content, for example, about 0.01 to 10 g/m2 (preferably 0.05 to 5 g/m2, more preferably 0.5 to 5 g/m2 and even more preferably 0.5 to 4 g/m2).
The release layer is dried after being imparted to the substrate. There are no particular limitations on drying conditions, and drying conditions suitably for the release agent used can be suitably selected. Typically, the release layer is dried at a temperature of about 80 to 150° C. In the case of using a heat-curable release agent, for example, the drying step and the curing step can be carried out simultaneously by drying while heating. In addition, the release agent may be cured by heating after air-drying. In addition, in the case of using an ionizing radiation-curable silicone release agent as well, the drying step and the curing step can be allowed to proceed simultaneously by simultaneously carrying out heating and ionizing radiation. In addition, the curing step can also be carried out after carrying out the drying step. These steps can be carried out by suitably selecting and employing a drying method and curing method suitable for the release agent used from conventionally known methods. Conditions relating to formation of the release layer can be suitably set so that the target transferred amount of silicone is realized.
Thus, matters disclosed according to this description include a production method of a PSA sheet that includes:
the formation of a release layer composed of a silicone-based release agent on at least a first side of a release liner substrate, wherein, the release layer is formed such that the amount of silicone that transfers to Single-Sided Adhesive Tape No. 31B manufactured by Nitto Denko Corp. per unit surface area equivalent to a circle having a diameter of 30 mm is 10 kcps or less when determined as X-ray intensity of silicon by X-ray fluorescence analysis; and
A PSA layer containing a PSA component synthesized in an aqueous solvent or ethyl acetate is provided on the release layer.
The transferred amount of silicone in the technology disclosed herein can be measured by fluorescence X-ray analysis according to the previously described method. Fluorescence X-ray analysis can be carried out using an XRF analyzer. A commercially available product can be preferably used for the XRF analyzer. Although a crystal spectrometer can also be suitably selected and used, Si-Kα, for example, can be used preferably. In addition, although the output setting and the like can be suitably selected corresponding to the apparatus used, adequate sensitivity can normally be obtained at an output of about 50 kV and 70 mA, for example.
The PSA layer laminated onto the above-mentioned release layer is formed from a PSA composition of a form in which a PSA component is dispersed or dissolved in an aqueous solvent or non-toluene-based organic solvent such as ethyl acetate (namely, a non-toluene-based solvent, and typically a solvent that is substantially free of toluene). Here, the term aqueous solvent refers includes water and mixed solvents consisting mainly of water. A composition that contains a PSA component in an aqueous solvent is referred to as an aqueous PSA composition. Among these, a composition of a form in which the PSA component is dispersed in an aqueous solvent is referred to as a water-dispersed (emulsion type) PSA composition. A composition of a form in which the PSA component is dissolved in an aqueous solvent is referred to as an aqueous solution-type PSA composition. In addition, a composition that contains the PSA component in ethyl acetate is referred to as an ethyl acetate solvent-type PSA composition. The PSA component in an ethyl acetate solvent-type PSA composition it typically dissolved in ethyl acetate. This aqueous PSA composition and ethyl acetate solvent-type PSA composition are typical examples of the “non-toluene-based PSA composition” in the present invention, and in the present description, a PSA sheet formed using such a PSA composition is also referred to as a “low-TVOC PSA sheet”.
The above-mentioned PSA component can use as a base polymer thereof various types of known polymers such as acrylic, rubber, polyester, urethane, polyether, silicone, polyamide and fluorine-based polymers. Here, a base polymer refers to a polymer that serves as the basic component of the PSA, and typically is the main component among polymer components contained in the PSA. A preferable example of a PSA composition in the technology disclosed herein is a non-toluene-based PSA composition in which the main component among the polymer components contained in the PSA is an acrylic polymer. In particular, an emulsion-type composition in which an acrylic polymer is dispersed in water (water-dispersed acrylic PSA composition) and a composition in which an acrylic polymer is dissolved in ethyl acetate (ethyl acetate solvent-type acrylic PSA composition) are preferable.
The above-mentioned water-dispersed type acrylic PSA composition contains an aqueous dispersion of an acrylic polymer (water-dispersed acrylic polymer). In addition, the above-mentioned ethyl acetate solvent-type acrylic PSA composition contains an ethyl acetate solution of an acrylic polymer. In the technology disclosed herein, the acrylic polymer is used as a base polymer of the PSA that composes the PSA layer (basic component of the adhesive). For example, 50% by weight or more of the PSA is preferably the above-mentioned acrylic polymer. An acrylic polymer having an alkyl (meth)acrylate for the main constituent monomer component (monomer main component, or in other words, the component that accounts for 50% by weight or more of the total amount of monomer that composes the acrylic polymer) can be preferably employed for the acrylic polymer.
In the present description, “(meth)acrylate” refers to acrylate and methacrylate, inclusively. Similarly, “(meth)aryloyl” refers to acryloyl and methacryloyl, inclusively, while “(meth)acrylic” refers to acrylic and methacrylic, inclusively.
A compound represented by the following formula (1) can be preferably used as alkyl (meth)acrylate.
CH2═C(R1)COOR2 (1)
Here, R1 in the above formula (1) represents a hydrogen atom or methyl group. In addition, R2 represents an alkyl group having 1 to 20 carbon atoms. Specific examples of R2 include alkyl groups such as a methyl group, ethyl group, propyl group, isopropyl group, butyl group, isobutyl group, s-butyl group, t-butyl group, pentyl group, isoamyl group, neopentyl group, hexyl group, heptyl group, octyl group, isooctyl group, 2-ethylhexyl group, nonyl group, isononyl group, decyl group, isodecyl group, undecyl group, dodecyl group, tridecyl group, tetradecyl group, pentadecyl group, hexadecyl group, heptadecyl group, octadecyl group, nonadecyl group or eicosyl group. From the viewpoint of the storage modulus of the PSA, an alkyl (meth)acrylates in which R2 is an alkyl group having 2 to 14 carbon atoms (this range of the number of carbon atoms may be hereinafter abbreviated as “C2-14”) are preferable, while alkyl (meth)acrylates in which R2 is a C2-10 alkyl group are more preferable.
Examples of alkyl (meth)acrylates having a C2-14 alkyl group as described above include ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate and tetradecyl (meth)acrylate. Particularly preferable examples of alkyl (meth)acrylates include butyl acrylate (BA) and 2-ethylhexyl acrylate (2-EHA).
In a preferable aspect, about 50% by weight or more of the total amount of alkyl (meth)acrylate used to synthesize the acrylic polymer (more preferably 70% by weight or more and, for example, about 90% by weight or more) is an alkyl (meth)acrylate in which R2 in the above-mentioned formula (1) represents a C2-14 (preferably C2-10 and more preferably C4-8) alkyl group. According to such a monomer composition, an acrylic polymer is easily obtained in which the storage modulus in the vicinity of room temperature becomes a preferable range for use as a PSA. Substantially all of the alkyl (meth)acrylate used may be C2-14 alkyl (meth)acrylate.
The alkyl (meth)acrylate that composes the acrylic polymer in the technology disclosed herein may be BA alone, 2EHA alone, or a combination of two types of alkyl (meth)acrylates consisting of BA and 2EHA. In the case of using a combination of BA and 2EHA for the alkyl (meth)acrylate, there are no particular limitations on the ratio at which they are used. For example, a ratio in which roughly 10% by weight or more (for example, about 10 to 40% by weight) of the total amount of BA and 2EHA is 2EHA can be used preferably.
Another monomer capable of polymerizing with the alkyl (meth)acrylate (which may also be referred to as a “copolymerizable monomer component”) may also be used as a monomer component that composes the acrylic polymer within a range in which the alkyl (meth)acrylate is still the main component. The ratio of the alkyl (meth)acrylate to the total amount of monomer components that compose the acrylic polymer can be about 80% by weight or more (and typically, 80 to 99.9% by weight), and is preferably 85% by weight or more (for example, 85 to 99.9% by weight). The ratio of the alkyl (meth)acrylate may also be 90% by weight or more (for example, 90 to 99% by weight).
The above-mentioned copolymerizable monomer component can be useful for introducing crosslinking sites into the acrylic polymer or for enhancing the cohesive strength of the acrylic polymer. This copolymerizable monomer can be used alone or two or more types can be used in combination.
More specifically, monomer components containing various types of functional groups can be used as a copolymerizable monomer component for introducing crosslinking sites into the acrylic polymer (and typically, monomer components containing a heat-crosslinkable functional group for introducing crosslinking sites that are crosslinked by heat into the acrylic polymer). The use of a monomer component containing such functional groups makes it possible to improve adhesive strength to an adherend. There are no particular limitations on the monomer component containing such a functional group provided it can be copolymerized with the alkyl (meth)acrylate and is able to provide functional groups that serve as crosslinking sites. For example, a functional group-containing monomer such as a carboxyl group-containing monomer, hydroxyl group-containing monomer, amide group-containing monomer, amino group-containing monomer, epoxy-group containing monomer, cyano group-containing monomer, keto group-containing monomer, monomer having a nitrogen atom-containing ring or alkoxysilyl group-containing monomer can be used alone or as a combination of two or more types thereof.
Examples of carboxyl group-containing monomers include ethylenic unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid or erotic acid, ethylenic unsaturated dicarboxylic acids such as maleic acid, itaconic acid or citraconic acid, and anhydrides thereof (such as maleic anhydride or itaconic anhydride).
Examples of hydroxyl group-containing monomers include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate or 2-hydroxybutyl (meth)acrylate, and unsaturated alcohols such as vinyl alcohol or allyl alcohol.
Examples of amide group-containing monomers include (meth)acrylamide, N,N-dimethyl (meth)acrylamide, N-butyl (meth)acrylamide, N-methylol (meth)acrylamide, N-methylolpropane (meth)acrylamide, N-methoxymethyl (meth)acrylamide and N-butoxymethyl (meth)acrylamide.
Examples of amino group-containing monomers include aminoethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate and t-butylaminoethyl (meth)acrylate.
Examples of epoxy group-containing monomers include glycidyl (meth)acrylate, methylglycidyl (meth)acrylate and allyl glycidyl ether.
Examples of cyano group-containing monomers include acrylonitrile and methacrylonitrile.
Examples of keto group-containing monomers include diacetone (meth)acrylamide, diacetone (meth)acrylate, vinyl methyl ketone, vinyl ethyl ketone, allyl acetoacetate and vinyl acetoacetate.
Examples of monomers having a nitrogen atom-containing ring include N-vinyl-2-pyrrolidone, N-methylvinylpyrrolidone, N-vinylpyridine, N-vinylpiperidone, N-vinylpyrimidine, N-vinylpiperazine, N-vinylpyrazine, N-vinylpyrrole, N-vinylimidazole, N-vinyloxazole, N-vinylmorpholine, N-vinylcaprolactam and N-(meth)acryloylmorpholine.
Examples of alkoxysilyl group-containing monomers include 3-(meth)acryloxypropyl trimethoxysilane, 3-(meth)acryloxypropyl triethoxysilane, 3-acryloxypropyl triethoxysilane, 3-(meth)acryloxypropylmethyl dimethoxysilane and 3-(meth)acryloxypropylmethyl diethoxysilane.
Among these functional group-containing monomers, one type or two or more types of selected from carboxyl group-containing monomers or acid anhydrides thereof can be used preferably. Substantially all of the functional group-containing monomer component may be carboxyl group-containing monomer. Particularly preferable examples of carboxyl group-containing monomers include acrylic acid and methacrylic acid. One of these may be used alone or acrylic acid and methacrylic acid may be used in combination at an arbitrary ratio thereof.
The above-mentioned functional group-containing monomer component is preferably used within the range of, for example, about 10 parts by weight or less (for example, about 0.1 to 10 parts by weight and preferably about 1 to 5 parts by weight) to 100 parts by weight of alkyl (meth)acrylate. If the amount of the functional group-containing monomer component used is excessively large, cohesive strength becomes excessively high which can tend to cause a decrease in adhesion properties (such as adhesive strength).
Other copolymerizable components can be used in addition to the functional group-containing monomer described above in order to enhance cohesive strength of the acrylic polymer. Examples of this copolymerizable component include vinyl ester-based monomers such as vinyl acetate or vinyl propionate, aromatic vinyl compounds such as styrene, substituted styrene (such as α-methyl styrene) or vinyl toluene, non-aromatic ring-containing (meth)acrylates such as cycloalkyl (meth)acrylates (such as cyclohexyl (meth)acrylate or cyclopentyl di(meth)acrylate) or isobornyl (meth)acrylate, aromatic ring-containing (meth)acrylates such as aryl (meth)acrylates (such as phenyl (meth)acrylate, aryloxyalkyl (meth)acrylates (such as phenoxymethyl (meth)acrylate) or arylalkyl (meth)acrylates (such as benzyl (meth)acrylate), olefin-based monomers such as ethylene, propylene, isoprene, butadiene or isobutylene, chlorine-containing monomers such as vinyl chloride or vinylidene chloride, isocyanate group-containing monomers such as 2-(meth)acryloyloxyethyl isocyanate, alkoxy group-containing monomers such as methoxyethyl (meth)acrylate or ethoxyethyl (meth)acrylate, and vinyl ether-based monomers such as methyl vinyl ether or ethyl vinyl ether.
Other examples of copolymerizable monomer components include monomers having a plurality of functional groups in a molecule thereof. Examples of such functional group-containing monomers include 1,6-hexanediol di(meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, (poly)ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, glycerin di(meth)acrylate, epoxy acrylate, polyester acrylate, urethane acrylate, divinyl benzene, butyl di(meth)acrylate and hexyl di(meth)acrylate.
A known, commonly used method can be used to obtain an acrylic polymer by polymerizing such monomers. For example, an aqueous dispersion of an acrylic polymer can be preferably prepared by dispersing a monomer starting material in an aqueous medium followed by emulsion polymerization. Similarly, an ethyl acetate solution of an acrylic polymer can be preferably prepared by dissolving a monomer starting material in ethyl acetate followed by polymerization.
A polymerization initiator can be suitably selected from known, commonly used polymerization initiators for use as a polymerization initiator used during polymerization corresponding to the type of polymerization method. For example, an azo polymerization initiator can be preferably used in an emulsion polymerization method. Specific examples of azo polymerization initiators include 2,2′-azobisisobutyronitrile (AIBN), 2,2′-azobis(2-methylpropionamidine) disulfate, 2,2′-azobis(2-amidinopropane) dihydrochloride, 2,2′-azobis[2-(5-methyl-2-imidazolin-2-yl)propane]dihydrochloride, 2,2′-azobis(N,N′-dimethyleneisobutylamidine), 2,2′-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]hydrate, 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2′-azobis(2,4-dimethylvaleronitrile), 2,2′-azobis(2-methylbutyronitrile), 1,1′-azobis(cyclohexane-1-carbonitrile), 2,2′-azobis(2,4,4-trimethylpentane) and dimethyl-2,2′-azobis(2-methylpropionate).
Other examples of polymerization initiators include persulfates such as potassium persulfate or ammonium persulfate, peroxide-based initiators such as benzoyl peroxide, t-butyl hydroperoxide, di-t-butyl peroxide, t-butylperoxybenzoate, dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,5,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)cyclodecane or hydrogen peroxide, substituted ethane-based initiators such as phenyl-substituted ethane, and aromatic carbonyl compounds. Still other examples of polymerization initiators include redox initiators using a combination of peroxide and reducing agent. Examples of redox initiators include combinations of a peroxide and ascorbic acid (such as a combination of aqueous hydrogen peroxide and ascorbic acid), combinations of hydrogen peroxide and ferric salt (such as a combination of aqueous hydrogen peroxide and ferric salt), and combinations of persulfate and sodium bisulfite.
These polymerization initiators can be used alone or two or more types can be used in combination. The amount of polymerization initiator used is a normally used amount, and can be selected from a range of, for example, about 0.005 to 1 part by weight (typically, 0.01 to 1 part by weight) to 100 parts by weight of all monomer components. If the amount of polymerization initiator used is excessively large or excessively small, there are cases in which it may be difficult to obtain desired adhesion performance.
A chain transfer agent (which is also understood as a molecular weight regulator or polymerization regulator) can also be used as necessary during polymerization. A known or commonly used chain transfer agent can be used for the chain transfer agent, examples of which include mercaptans such as dodecylmercaptan(dodecanethiol), lauryl mercaptan, glycidyl mercaptan, 2-mercaptoethanol, mercaptoacetic acid, 2-ethylhexyl thioglycolate or 2,3-dimethylmercapto-1-propanol as well as α-methylstyrene dimer. These chain transfer agents can be used alone or two or more types can be used in combination. The amount of chain transfer agent used may be about the same as that used normally, and can be selected from, for example, a range of about 0.001 to 0.5 parts by weight to 100 parts by weight of monomer starting material.
In a preferable aspect for polymerizing the monomer starting material in the technology disclosed herein, a polymerization reaction is carried out on the monomer starting material by supplying the monomer starting material and a dispersion medium or solvent to a reaction vessel having a polymerization initiator, and maintaining the temperature of the system at a polymerization temperature higher than room temperature (preferably about 40 to 80° C., and for example, about 50 to 80° C.). Subsequently, the contents of the reaction vessel (reaction liquid) are typically cooled to room temperature. For example, the entire amount of monomer starting material may be supplied all at once to a reaction vessel containing the entire amount of polymerization initiator, may be supplied continuously over a prescribed amount of time (continuous supply), or may be supplied by dividing into small increments at prescribed intervals (such as about 5 to 60 minutes) (incremental supply). In the case of carrying out emulsion polymerization in an aqueous dispersion, typically all or a portion of the monomer (and typically all of the monomer) is first mixed with water (and typically, using a suitable amount of an emulsifier to be described later along with water), and the emulsified emulsion (monomer emulsion) is continuously supplied to a reaction vessel. In the case of carrying out polymerization in ethyl acetate solution as well, the monomer starting material can be supplied all at once, continuously or incrementally either directly or after diluting with a suitable solvent (typically, ethyl acetate).
In the case of supplying the monomer starting material continuously, the duration of the continuous supply is suitably about 1 to 8 hours, and preferably about 2 to 6 hours (for example, about 3 to 5 hours). In addition, in the case of supplying the monomer starting material incrementally, the duration of the time when the first monomer starting material fraction is supplied to the time the last monomer starting material fraction is supplied is suitably about 1 to 8 hours, and preferably about 2 to 6 hours (for example, about 3 to 5 hours).
Alternatively, at least a portion of the monomer starting material may be supplied to a reaction vessel containing a portion of the polymerization initiator, polymerization of the monomer starting material may be initialized, and then the remainder of the polymerization initiator may be supplied continuously over a prescribed amount of time or supplied incrementally at prescribed intervals.
Polymerization time (referring to the time during which polymerization of the monomer starting material is carried out, and can be understood to be the amount of time during which the system is maintained at the polymerization temperature following initiation of the polymerization reaction) can be suitably set corresponding to the type of polymerization initiator used, polymerization temperature, the forms in which the polymerization initiator and monomer starting material are supplied and the like. For example, the polymerization time can be about 2 to 12 hours, and from the viewpoint of productivity and the like, normally the polymerization time is preferably about 4 to 8 hours. In addition, the contents of the reaction vessel (reaction liquid) are preferably maintained at the polymerization temperature (to carry out so-called aging) during a prescribed amount of time after the entire amount of the monomer starting material has finished being supplied to the reaction vessel. As a result of carrying out this aging, the amount of monomer remaining in the reaction liquid is reduced, allowing the obtaining of an acrylic polymer capable of forming a PSA layer having a lower TVOC content. The duration of this aging can be, for example, about 30 minutes to 4 hours, and from the viewpoint of productivity and the like, normally the aging time is preferably about 1 to 3 hours. The polymerization time referred to here includes this aging time. Thus, polymerization time in the case of, for example, maintaining a reaction vessel containing the entire amount of polymerization initiator at the polymerization temperature, continuously supplying the entire amount of monomer starting material over the course of 4 hours, and then carrying out aging of the reaction liquid by continuing to maintain the polymerization temperature for 2 hours after having finished supplying the monomer starting material, is 6 hours. The polymerization temperature may be constant throughout the entire duration of the polymerization time, or the polymerization temperature may differ between one time period and another time period. For example, after supplying the monomer starting material at a prescribed polymerization temperature, aging may be carried out at a higher polymerization temperature.
In a preferable aspect of the case of emulsion polymerization of a monomer starting material, additional polymerization initiator is supplied to the contents of the reaction vessel (reaction liquid) after allowing an amount of time to pass after finishing supplying the monomer starting material to the reaction vessel. By supplying additional polymerization initiator (to be referred to as “additional initiator”) in this manner, the amount of monomer remaining in the reaction liquid can be effectively reduced (typically, by promoting polymerization of the residual monomer). As a result thereof, an acrylic polymer can be obtained that allows the formation of a PSA layer having a lower TVOC content. The additional initiator used may be the same as or different from the polymerization initiator previously used in the polymerization reaction of the monomer starting material (namely, the polymerization initiator introduced into the reaction vessel prior to finishing supplying the monomer starting material; to be referred to as the “primary initiator”). A polymerization initiator (initiator selected from, for example, an azo polymerization initiator, persulfate or peroxide polymerization initiator) having a half-life temperature lower than that of the primary initiator (initiator selected from, for example, an azo polymerization initiator, persulfate or peroxide polymerization initiator) can be used preferably for the additional initiator that enables reduction of the amount of residual monomer to be carried out efficiently. In addition, a redox-type polymerization initiator is also preferably used for the additional initiator. For example, a redox-type polymerization initiator obtained by combining a peroxide (such as hydrogen peroxide) and ascorbic acid, a peroxide (such as hydrogen peroxide) and ferric salt or a persulfate (such as ammonium persulfate) and sodium bisulfite and the like can be used preferably. In particular, the combination of a peroxide (typically used in the form of aqueous hydrogen peroxide having a concentration of about 1 to 35%) and ascorbic acid is preferable since there is no formation of excessive residue following the redox reaction. There are no particular limitations on the amount of additional initiator used, and the amount of used can be selected from a range of, for example, about 0.005 to 1 part by weight to 100 parts by weight of monomer starting material.
The preferable timing at which the additional initiator is supplied (added) to the reaction liquid can also vary according to the type of additional initiator. The additional initiator is preferably supplied after allowing a certain period of time (typically, about 10 minutes to 4 hours, and for example, about 1 to 3 hours) has allowed to elapse after finishing supplying the monomer starting material to the reaction vessel and before the reaction liquid has finished cooling. For example, in the case of an additional initiator of the type that reduces the amount of residual monomer by forming radicals as a result of decomposing by heat (such as an azo polymerization initiator, persulfate or peroxide polymerization initiator), the additional initiator is preferably added at an intermediate point during the above-mentioned aging period (such as at the point about 30 to 95% of the aging period has elapsed). In addition, in the case of using a redox-type additional initiator, an aspect can be preferably employed in which the additional initiator is added at roughly the time of completion of the aging period (for example, an aspect in which cooling is started at about the same time as addition of the additional initiator), or at an intermediate point during cooling of the reaction liquid. As a result thereof, the amount of residual monomer can be effectively reduced while minimizing effects on productivity. Addition of the additional initiator may be carried out all at once, continuously or incrementally. In consideration of simplicity of the procedure, an aspect in which the additional initiator is added all at once can be preferably employed.
In the case of the above-mentioned emulsion polymerization, since the amount of residual monomer can be efficiently reduced by using the additional initiator as described above, an emulsion of an acrylic polymer in which the amount of residual monomer has been adequately reduced can be obtained without significantly extending polymerization time (even in the case where the polymerization time is, for example, about 8 hours or less). According to a PSA composition obtained by incorporating a tackifier resin to such an acrylic polymer emulsion, a PSA layer can be formed in which toluene emission and TVOC emission are significantly reduced. Being able to realize a lower VOC emission of a PSA without requiring a significantly long polymerization time in this manner is preferable from the viewpoint of improving productivity of a PSA composition and a PSA sheet produced using that composition.
According to this emulsion polymerization, a polymer liquid (acrylic polymer emulsion) is obtained in the form of an emulsion in which an acrylic polymer is dispersed in water. The above-mentioned polymer liquid or that in which the polymer liquid has undergone suitable post-treatment can be preferably used for the water-dispersed acrylic polymer in the technology disclosed herein. Alternatively, an acrylic polymer may also be synthesized by a polymerization method other than emulsion polymerization (such as solution polymerization, photopolymerization or bulk polymerization), and a water-dispersed acrylic polymer may be used that is prepared by dispersing that polymer in water.
In preparing a water-dispersed acrylic polymer, an emulsifier can be used as necessary. Anionic, nonionic or cationic emulsifiers can be used for the emulsifier. An anionic or nonionic emulsifier is normally used preferably. These emulsifiers can preferably be used when, for example, emulsion polymerizing the monomer component or when dispersing an acrylic polymer obtained by another method in water. There are no particular limitations on the amount of emulsifier used provided it is an amount that allows the acrylic polymer to be prepared in the form of an emulsion. For example, the amount of emulsifier used is suitably selected from, for example, a range of about 0.2 to 10 parts by weight (and preferably about 0.5 to 5 parts by weight) based on the solid content thereof to 100 parts by weight of the acrylic polymer.
The PSA composition in the technology disclosed herein can further contain a tackifier resin in addition to the above-mentioned acrylic polymer. There are no particular limitations on the tackifier resin, and various types of tackifier resins can be used, such as rosin-based, terpene-based, hydrocarbon-based, epoxy-based, polyamide-based, elastomer-based, phenol-based or ketone-based resins. Such tackifier resins can be used alone or two or more types can be used in combination.
More specifically, examples of rosin-based tackifier resins include non-modified rosins (raw rosins) such as gum rosin, wood rosin, tall oil rosin, modified rosins obtained by modifying these non-modified rosins by hydrogenation, disproportionation or polymerization and the like (such as hydrogenated rosins, disproportionated rosins, polymerized rosins or other chemically modified rosins), and other various types of rosin derivatives. Examples of rosin derivatives include rosin esters such as non-modified rosins esterified by alcohol (namely, rosin esterification products) or modified rosins (such as hydrogenated rosins, disproportionated rosins or polymerized rosins) esterified by alcohol (namely, modified rosin esterification products), unsaturated fatty acid-modified rosins in which a non-modified rosin or modified rosin (such as hydrogenated rosins, disproportionated rosins or polymerized rosins) has been modified with an unsaturated fatty acid, unsaturated fatty acid-modified rosin esters in which a rosin ester has been modified with an unsaturated fatty acid, rosin alcohols in which a carboxyl group in a non-modified rosin, modified rosin (such as a hydrogenated rosin, disproportionated rosin or polymerized rosin), unsaturated fatty acid-modified rosin or unsaturated fatty acid-modified rosin ester has been reduced, metal salts of rosins such as non-modified rosins, modified rosins or various types of rosin derivatives (and particularly, rosin esters), and rosin phenol resins obtained by adding phenol to a rosin (such as a non-modified rosin, modified rosin or various types of rosin derivatives) using an acid catalyst followed by thermal polymerization.
Examples of terpene-based tackifier resins include terpene-based resins such as α-pinene polymers, β-pinene polymers or dipentene polymers, and modified terpene-based resins obtained by modifying these terpene-based resins (such as by phenol modification, aromatic modification, hydrogenation modification or hydrocarbon modification). Examples of the above-mentioned modified terpene resins include terpene-phenol-based resins, styrene-modified terpene-based resins, aromatic-modified terpene-based resins and hydrogenated terpene-based resins.
Examples of hydrocarbon-based tackifier resins include various types of hydrocarbon-based resins such as aliphatic-based hydrocarbon resins, aromatic-based hydrocarbon resins, aliphatic-based cyclic hydrocarbon resins, aliphatic-aromatic-based petroleum resins (such as styrene-olefin-based copolymers), aliphatic-alicyclic-based petroleum resins, hydrogenated hydrocarbon resins, coumarone-based hydrocarbon resins and coumarone-indene-based resins. Examples of aliphatic-based hydrocarbon resins include polymers of one type or two or more types of aliphatic hydrocarbons selected from olefins and dienes having about 4 to 5 carbon atoms. Examples of the above-mentioned olefins include 1-butene, isobutylene and 1-pentene. Examples of the above-mentioned dienes include butadiene, 1,3-pentadiene and isoprene. Examples of aromatic-based hydrocarbon resins include polymers of vinyl group-containing aromatic-based hydrocarbons having about 8 to 10 carbon atoms (such as styrene, vinyltoluene, α-methylstyrene, indene and methylindene). Examples of aliphatic-based cyclic hydrocarbon resins include alicyclic hydrocarbon-based resins obtained by polymerization following cyclic dimerization of so-called “C4 petroleum fractions” or “C5 petroleum fractions”, and polymers of cyclic diene compounds (such as cyclopentadiene, dicyclopentadiene, ethylidene norbornene or dipentene) or hydrogenation products thereof, and alicyclic hydrocarbon-based resins obtained by hydrogenating an aromatic ring of an aromatic-based hydrocarbon resin or aliphatic-aromatic-based petroleum resin.
In the case of adding to an aqueous dispersion of an acrylic polymer, the tackifier resin can preferably be used in the form of an emulsion in which the resin is dispersed in water (tackifier resin emulsion). In addition, in the case of adding to an ethyl acetate solution of an acrylic polymer, the tackifier resin can preferably be used as is or in the form of a solution in which the resin is dissolved in ethyl acetate or an organic solvent compatible with ethyl acetate. In the case of using a dispersion medium or a solvent when adding the tackifier resin, the dispersion medium or solvent preferably does not substantially contain at least an aromatic hydrocarbon-based solvent (and more preferably, does not substantially contain an aromatic hydrocarbon-based solvent or other organic solvent). As a result thereof, an adhesion layer can be provided that has a lower emission of toluene.
The above-mentioned tackifier resin emulsion can be prepared using an emulsifier as necessary. One type or two or more types of emulsifiers suitably selected from the same emulsifiers as those able to be used to prepare the acrylic polymer emulsion can be used for the emulsifier. There are no particular limitations on the amount of emulsifier used provided it allows the preparation of an tackifier resin in the form of an emulsion, and the amount used can be selected from, for example, a range of about 0.2 to 10 parts by weight (and preferably about 0.5 to 5 parts by weight) based on the solid content thereof to 100 parts by weight of the tackifier resin.
In the technology disclosed herein, there are no particular limitations on the amount of the tackifier resin incorporated in the acrylic polymer, and can be suitably set corresponding to the target adhesion performance (such as rough surface adhesion or adhesion to lowly polar materials such as polyolefins). The tackifier resin (based on solid content) can be used at a ratio of about 5 to 100 parts by weight (preferably about 10 to 80 parts by weight, more preferably about 15 to 60 parts by weight, and for example, about 20 to 40 parts by weight) to 100 parts by weight of the acrylic polymer. If the amount of the tackifier resin used is excessively low, there are cases in which it may be difficult to adequately demonstrate the effect of addition of the tackifier resin on improving adhesion (such as adhesion to rough surfaces or poorly adhesive adhered objects such as polyolefin members). In addition, if the amount of the tackifier resin used is excessively high, there is increased susceptibility to a lack of compatibility with the acrylic polymer, or low-temperature properties tend to decrease. In addition, it is also preferable to avoid the use of an excessive amount of the tackifier resin in order to significantly decrease the toluene emission and TVOC emission of the PSA layer (and ultimately, the PSA sheet).
A crosslinking agent may also be incorporated in the above-mentioned PSA composition as necessary. There are no particular limitations on the type of crosslinking agent, and a known or commonly used crosslinking agent can be suitably selected and used (such as isocyanate-based crosslinking agents, epoxy-based crosslinking agents, oxazoline-based crosslinking agents, aziridine-based crosslinking agents, melamine-based crosslinking agents, peroxide-based crosslinking agents, urea-based crosslinking agents, metal alkoxide-based crosslinking agents, metal chelate-based crosslinking agents, metal salt-based crosslinking agents, carbodiimide-based crosslinking agents or amine-based crosslinking agents). Both oil-soluble crosslinking agents and water-soluble crosslinking agents can be used. The crosslinking agent can be used alone or two or more types can be used in combination. There are no particular limitations on the amount of crosslinking agent used, and the amount used can be selected from, for example, a range of about 20 parts by weight or less (for example, about 0.005 to 20 parts by weight and preferably about 0.01 to 10 parts by weight) to 100 parts by weight of the acrylic polymer. The PSA may also be crosslinking by irradiating with an active energy beam such as an electron beam or ultraviolet light instead of using such a crosslinking agent or in addition to using the crosslinking agent.
The above-mentioned PSA composition can also contain an acid or base (such as aqueous ammonia) that is used for the purpose of adjusting pH and the like as necessary. Examples of other arbitrary components able to be contained in the composition include viscosity adjusting agents (such as thickeners), leveling agents, release adjusting agents, plasticizers, softeners, fillers, colorants (such as pigments or dyes), surfactants, antistatic agents, preservatives, aging preventers, ultraviolet absorbers, antioxidants, photo stabilizers and various other types of common additives used in the field of aqueous PSA compositions.
The PSA layer in the technology disclosed herein can be produced by various methods. For example, in the case of a double-sided PSA sheet provided with an adhesive substrate, a PSA layer can be formed on each surface of the substrate by applying a method selected from a method in which a PSA composition is directly applied to each of first and second surfaces of a non-releasing substrate followed by drying or curing to form a PSA layer (PSA film) (direct method), and a method in which a PSA layer formed on a release surface of a release liner is laminated to the substrate followed by transferring the PSA film to the substrate (transfer method). In addition, in the case of double-sided PSA sheet not provided with a substrate, a PSA layer can be formed on the release surface by, for example, directly applying a PSA composition to the release surface of a release liner followed by drying or curing. In addition, in the case of a single-sided PSA sheet of a form in which a PSA layer is provided on a non-releasing surface of a non-releasing substrate, the above-mentioned direct method or transfer method can be preferably employed to form the PSA layer.
When applying (and typically, coating) a PSA composition, a commonly used coater (such as a gravure roll coater, reverse roll coater, kiss roll coater, dip roll coater, bar coater, knife coater or spray coater) can be used. There are no particular limitations on the thickness of the PSA layer, and can be, for example, about 2 to 200 μm (and preferably about 5 to 100 μm).
In the PSA sheet disclosed herein, a substrate similar to the substrate for the previously described release liner can be used in a state in which the release layer is not imparted for the non-releasing substrate that supports the PSA layer. Preferable examples thereof include paper and fabric (such as a non-woven fabric). Surface modification processing or incorporation of additives and the like can be carried out in the same manner as for the release liner as necessary. Although there are no particular limitations on the thickness of the non-releasing substrate, generally a thickness of about 10 to 500 μm is suitable.
The PSA sheet disclosed herein preferably has a toluene emission when the sheet is heated for 30 minutes at 80° C. (to be simply referred to as “toluene emission”) of 20 μg or less per gram of PSA layer (to be indicated as “20 μg/g”). Although there are no particular limitations on the lower limit of the toluene emission in consideration of such factors as adhesion properties and production efficiency, the lower limit is normally 0.5 μg or more and typically 1 μg/g or more. A value obtained by the toluene emission measurement method described below can be employed to determine toluene emission.
[Toluene Emission Measurement Method]
A sample containing a PSA sheet of a prescribed size (for example, having a surface area of 5 cm2) is placed in a vial and sealed. The vial is then heated for 30 minutes at 80° C. and 1.0 mL of heated gas is injected into a gas chromatograph measuring apparatus (GC measuring apparatus) using a headspace autosampler to measure the amount of toluene. The amount of toluene generated (emitted) per gram of PSA layer contained in the sample is then calculated from this measurement result (μg/g).
The value obtained by subtracting the weight of the substrate per sample surface area from the weight of the PSA sheet excluding the release liner can be used for the weight of the PSA layer that serves as a reference for calculating the toluene emission per gram of the PSA layer.
In a preferable aspect of the PSA sheet disclosed herein, the amount of TVOC emission when the sheet is heated for 30 minutes at 80° C. (to also be simply referred to as “TVOC emission”) is about 300 μg or less per gram of PSA layer. Although there are no particular limitations on the lower limit of the TVOC emission, in consideration of such factors as adhesion properties and production efficiency, the lower limit is normally 5 μg/g or more and typically 10 μg/g or more. The value obtained according to the TVOC measurement method indicated below is used for the amount of TVOC emission.
[TVOC Emission Measurement Method]
A sample similar to that used in the toluene emission measurement method described above is placed in a vial, heated for 30 minutes at 80° C., and 1.0 mL of heated gas is injected into a GC measuring apparatus using a head space autosampler. Peak assignment and quantification are then carried out on volatile substances predicted to be emitted from the material used to produce the PSA layer on the basis of the resulting gas chromatogram (including monomers used to synthesize the acrylic polymer and solvent used to product an tackifier resin emulsion to be described later) using standards, while other peaks (which are difficult to assign) are quantified by converting to toluene, followed by determining TVOC emission per gram of PSA layer contained in the sample (μg/g).
A value calculated in the same manner as for measurement of toluene emission can be used for the weight of the PSA layer that serves as a reference for calculating the TVOC emission per gram of PSA layer.
Measurement conditions of gas chromatography used in each of the measurement methods of toluene emission and TVOC emission described above are as indicated below.
Column: DB-FFAP, 1.0 μm (0.535 mmφ×30 m)
Carrier gas: He, 5.0 mL/min
Column head pressure: 23 kPa (40° C.)
Injection port: Split (split ratio: 12:1, temperature: 250° C.)
Column temperature: 40° C. (0 min)−<+10° C./min>−250° C. (9 min)
(after raising the temperature from 40° C. to 250° C. at the rate of 10° C./min, the temperature is held at 250° C. for 9 minutes)
Detector: FID (temperature: 250° C.)
Although the following provides an explanation of several examples relating to the present invention, the present invention is not intended to be limited to the indicated examples. In the following explanations, the terms “parts” and “percent (%)” are based on weight unless specifically indicated otherwise.
35 parts of ion exchange water were placed in a reaction vessel equipped with a stirrer, thermometer, reflux condenser, dropping device and nitrogen gas feed tube followed by stirring for 1 hour or more at 60° C. while introducing nitrogen gas. 0.1 parts of 2,2′-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]hydrate (Wako Pure Chemical Industries, trade name “VA-057”) were added thereto as a polymerization initiator.
A monomer starting material was prepared by adding 90 parts of n-butylacrylate (BA), 10 parts of 2-ethylhexyl acrylate (2-EHA), 4 parts of acrylic acid (AA), 2 parts of sodium polyoxyethylene lauryl sulfate (emulsifier) and 0.05 parts of dodecanethiol (chain transfer agent) were added to 40 parts of ion exchange water and emulsifying.
This monomer starting material emulsion was gradually dropped into the reaction liquid held at 60° C. over the course of 4 hours followed by emulsification polymerization. Following completion of dropping of the monomer starting material, stirring was continued for an additional 2 hours at 60° C. followed by discontinuing heating. Next, 0.1 part of ascorbic acid and 0.1 part of 35% aqueous hydrogen peroxide (additional polymerization initiator) were added to 100 parts of the monomer followed by carrying out redox treatment. After allowing this to cool to room temperature, 10% aqueous ammonia was added to adjust to pH 7 and obtain an acrylic polymer emulsion (water-dispersed acrylic polymer).
30 parts of an emulsion of a tackifier resin based on the solid content thereof per 100 parts of the acrylic polymer contained in the acrylic polymer emulsion were added to the emulsion to obtain a water-dispersed PSA composition. A product having the trade name “SK-253NS” manufactured by Harima Chemicals (aqueous emulsion of rosin-based resin having a softening point of 145° C., and manufactured without the use of substantially any organic solvent) was used for the tackifier resin.
A liner substrate was prepared in which a PE layer having a thickness of 25 μm was laminated onto one side of wood-free paper (grammage: 100 g/m2). A mixture of a non-transferring, heat-curable solvent-free silicone-based release agent and curing catalyst were coated onto the PE layer of this substrate at a coated amount of 1.1 g/m2. This was then dried and cured by holding for 1 minute at 120° C. to obtain a release liner A.
The above-mentioned water-dispersed PSA composition was then coated at a thickness of 70 μm on the release layer of the resulting release liner A to form a PSA layer. Two of these PSA layers were prepared and respectively transferred to each side of a non-woven cloth substrate (Daifuku Paper Mfg., trade name: “SP Genshi-14”, pulp-based non-woven fabric, grammage: 14 g/m2, thickness: 42 μm, bulk density: 0.33 g/cm3) to obtain a double-sided PSA sheet.
70 parts of n-butyl acrylate (BA), 27 parts of 2-ethylhexyl acrylate (2-EHA), 3 parts of acrylic acid (AA), 0.1 part of 2-hydroxyethyl acrylate and ethyl acetate were placed in a reaction vessel equipped with a condenser, nitrogen gas feed tube, thermometer and stirrer followed by introducing nitrogen gas while stirring gently to replace the inside of the vessel with nitrogen. This reaction liquid was heated to 70° C. followed by the addition of 0.2 parts of AIBN (polymerization initiator). A polymerization reaction was carried out for 8 hours while holding the system at 70° C. to obtain an ethyl acetate solution of an acrylic polymer having a weight average molecular weight of 70×104.
20 parts of an tackifier resin (Rika TAC, trade name: “RikaTAC PCJ”, polymerized rosin ester resin having a softening point of 128° C.) and 2 parts of a crosslinking agent (Nippon Polyurethane Industry, trade name: “Coronate L”, isocyanate-based crosslinking agent) were added to this solution per 100 parts of the acrylic polymer contained in the solution followed by uniformly stirring and mixing to obtain a non-toluene solvent-based PSA composition using ethyl acetate as a solvent.
A double-sided PSA sheet, in which both adhesive surfaces were protected by two release liners A, was then obtained in the same manner as Example 1 with the exception of using the resulting non-toluene solvent-based PSA composition.
A UV-curable, solvent-free silicone-based release agent was coated onto a PE layer of a substrate obtained in the same manner as Example 1 instead of the mixture of release agent and catalyst of Example 1. The amount of the release agent coated was 1.3 g/m2. After coating the release agent, the release agent was irradiated with ultraviolet light under conditions of luminance of 2 W/cm2 and a line speed of 70 m/min using a high-pressure mercury lamp for the light source to cure the release agent and obtain a release liner B.
A double-sided PSA sheet, provided with a PSA layer composed of a water-dispersed PSA composition, was obtained in the same manner as Example 1 with the exception of using the release liner B instead of the release liner A.
A double-sided PSA sheet, provided with a PSA layer composed of a non-toluene solvent-based PSA composition, was obtained in the same manner as Example 2 with the exception of using the release liner B instead of the release liner A.
A release liner C was obtained in the same manner as Example 1 with the exception of using a general heat-curable, solvent-free silicone-based release agent and a curing catalyst instead of the release agent and curing catalyst of Example 1 and making the coated amount of the release agent 1.5 g/m2.
A double-sided PSA sheet, provided with a PSA layer composed of a water-dispersed PSA composition, was obtained in the same manner as Example 1 with the exception of using the release liner C instead of the release liner A.
A double-sided PSA sheet, provided with a PSA layer composed of a non-toluene solvent-based PSA composition, was obtained in the same manner as Example 2 with the exception of using the release liner C instead of the release liner A.
The release liners A to C and the double-sided PSA sheets obtained in Examples 1 to 6 were measured and evaluated as described below. Those results are shown in Table 1 along with the types of PSA compositions used.
[Measurement of Toluene Emission]
The amount of toluene emission per gram of PSA layer was measured for each of the double-sided PSA sheets of Examples 1 to 6 according to the previously described method.
[Measurement of TVOC Emission]
The amount of TVOC emission per gram of PSA layer was measured for each of the double-sided PSA sheets of Examples 1 to 6 according to the previously described method. The same test pieces as those used to measure toluene emission were used.
A value of about 0.91 g was used for the weight of the PSA layer contained in 1 g of each double-sided PSA sheet to calculate the above-mentioned toluene and TVOC emission.
[Measurement of Release Liner Peel Strength]
Peel strength was measured for each of the release liners A, B and C. Namely, a piece of adhesive tape (Nitto Denko Corp., product no. 502, 50 mm wide acrylic double-sided adhesive tape) having a length of about 20 cm was prepared, the yellow release paper was peeled off, and a release liner was affixed to the exposed adhesive surface using a hand roller in an environment at a temperature of 23° C. and RH of 50% to produce a test piece. This test piece was then subjected to a load of 1 kg in an environment at 100° C. for 1 hour followed by holding for 1 hour in an environment at a temperature of 23° C. and RH of 50%.
The adhesive strength when the release liner was peeled for a distance of 50 mm using a tensile tester under conditions of a peeling angle of 180° and pulling speed of 300 mm/min in an environment at 23° C. and 50% RH was measured, and the maximum value thereof was defined as peel strength (N/50 mm). An auxiliary plate was used to measure peel strength.
[Measurement of Amount of Silicone Transfer]
The amount of silicone transfer was measured for each of the release liners A, B and C according to the previously described method using the Model “ZSX-100e” manufactured by Rigaku Corp. for the XRF analyzer under the conditions indicated below.
X-ray source: Vertical Rh tube
Analysis range: Within a circle having a diameter of 30 mm
Spectroscopic crystals: SI-Kα
Output: 50 kv, 70 mA
[Measurement of SUS 180° Peeling Adhesive Strength]
The first release liner was peeled from each double-sided PSA sheet followed by backing the PSA sheet with a PET film having a thickness of 25 μm by affixing the PET film thereto. The backed PSA sheet was cut to the shape of a rectangle measuring 20 mm×200 mm to produce a test piece. The second release liner was peeled from this test piece, and the exposed adhesive surface was affixed to a stainless steel (SUS: B304) plate serving as an adherend by passing a 2 kg roller back and forth over the PSA sheet affixed to the stainless steel plate. After holding for 30 minutes in an environment at 23° C. and 50% RH, SUS 180° peeling adhesive strength was measured in compliance with JIS Z 0237 using a tensile tester (Shimadzu, trade name: “Tensilon”) under conditions of a peeling angle of 180° and pulling speed of 300 mm/min in an environment at 23° C. and 50% RH.
[Measurement of PP 180° Peeling Adhesive Strength]
PP 180° peeling adhesive strength was measured in the same manner as the above-mentioned SUS 180° peeling adhesive strength with the exception of using a PP plate instead of an SUS plate for the adherend.
[Evaluation of Curved Surface Adhesion]
Each double-sided PSA sheet was cut to a width of 20 mm and length of 180 mm, the first release liner was peeled off, and an aluminum plate cut to the same size and having a thickness of 0.4 mm was affixed to the exposed first adhesive surface to produce an aluminum-backed test piece. After pressing this test piece onto a PP plate cut to a size of 30 mm×200 mm and having thickness of 2 mm using a laminator in an environment at 23° C. and 50% RH, the laminate was held for 24 hours in the same environment. Next, as shown in
As is shown in Table 1, in the PSA sheets of Examples 1 to 4, having a PSA layer composed of heat-curable, solvent-free or UV-curable, solvent-free silicone, the amounts of silicone transferred from the PSA layer were decreased to half or less the amounts of the PSA sheets of Examples 5 and 6, having a PSA layer composed of general, heat-curable, solvent-free silicone, and these PSA sheets also demonstrated superior results with respect to SUS adhesive strength, PP adhesive strength and curved surface adhesion despite containing a non-toluene-based PSA layer.
Although the above has provided a detailed explanation of specific examples of the present invention, the specific examples are merely exemplary of the present invention and do not limit the scope of the claims. Various modifications and alterations of the previously listed specific examples are included in the technology described in the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-167843 | Jul 2009 | JP | national |
