WO2009006168A1

WO2009006168A1 - Flame-resistant acrylic resin composition and pressure-sensitive adhesive sheet using same

Info

Publication number: WO2009006168A1
Application number: PCT/US2008/068257
Authority: WO
Inventors: Masaki Yoda
Original assignee: 3M Innovative Properties Company
Priority date: 2007-06-29
Filing date: 2008-06-26
Publication date: 2009-01-08
Also published as: JP2009007535A; JP5207674B2

Abstract

A flame-resistant acrylic resin composition comprising: (A) 100 parts by mass of a (meth)acrylic polymer; (B) 30 to 450 parts by mass of aluminum hydroxide; and (C) 5 to 50 parts by mass of expandable graphite, wherein the (meth)acrylic polymer comprises 60 to 97 mass% alkyl(meth)acrylic monomer units with a homopolymer glass transition temperature of -40°C or less and 3 to 40 mass% basic (meth)acrylic monomer units with a homopolymer glass transition temperature of 10°C or more is described. Pressure-sensitive adhesive sheets comprising such compositions, and methods of making such compositions are also described.

Description

FLAME-RESISTANT ACRYLIC RESIN COMPOSITION AND PRESSURE- SENSITIVE ADHESIVE SHEET USING SAME

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to Japanese Patent Application No. 2007-172706, filed June 29, 2007, the disclosure of which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

[0002] The present disclosure relates to a flame-resistant acrylic resin composition, and to a pressure-sensitive adhesive sheet using that composition.

BACKGROUND

[0003] Many of the materials used in television sets and other electrical goods and electronics need to be flame -resistant, and a high level of flame resistance is required of the pressure-sensitive adhesive sheets used as joining materials in such electrical and electronic goods. The most common way of achieving flame resistance is by compounding a flame retardant into the pressure-sensitive adhesive sheet. Halogen-based flame retardants in particular confer a particular high level of flame resistance, but they are avoided because they can emit toxic gasses when burned.

[0004] In recent years, it has become common to use halogen-free flame retardants in place of halogen flame-retardants in pressure-sensitive adhesive sheets. Metal hydrate compounds are known as halogen- free flame retardants for use in pressure-sensitive adhesive sheets (see Japanese Patent Application Laid-open No. 2005-54006).

[0005] Halogen- free flame retardants are also used to improve flame resistance in sheets that do not require adhesive properties. For example, a heat-conductive sheet is known which is composed of a composition comprising (A) a (meth)acrylic polymer, (B) a halogen- free flame retardant selected from a group consisting of an organic phosphorus compound, a triazine skeleton-containing compound, expandable graphite and polyphenylene ether and (C) a metal hydrate compound, wherein the metal hydrate compound represents 40 to 90 vol% based on the total volume of the composition (see Japanese Patent Application Laid-open No. 2005-226007), as well as a flame -resistant sheet comprising a base consisting primarily of an ethylene vinyl acetate copolymer which is filled with aluminum hydroxide and expandable graphite with an expansion initiation temperature of 200 to 300⁰C (see Japanese Patent Application Laid-open No. 2006-193590).

[0006] However, such halogen- free flame retardants have poor flame-retardant properties in comparison with halogen-based flame retardants. Moreover, when large amounts of a halogen- free flame retardant are added the adhesiveness of the sheet is adversely affected. Consequently, it is currently impossible to achieve both high flame resistance and adequate adhesiveness in a pressure-sensitive adhesive sheet using a halogen-free flame retardant. High flame resistance at the V-O level (UL-94) has also not been achieved in the field of acrylic pressure-sensitive adhesive sheets when using halogen-free flame retardants, and there is demand for a flame-resistant pressure-sensitive adhesive sheet having high flame resistance (UL-94 V-O level) and adhesive strength which is also durable at high temperatures.

SUMMARY

[0007] In some embodiments, the present disclosure provides a flame-resistant acrylic resin composition comprising: (A) 100 parts by mass of a (meth)acrylic polymer; (B) 30 to 450 parts by mass of aluminum hydroxide; and (C) 5 to 50 parts by mass of expandable graphite, wherein the (meth)acrylic polymer comprises 60 to 97 mass% alkyl (meth)acrylic monomer units with a homopolymer glass transition temperature of -40⁰C or less and 3 to 40 mass% basic (meth)acrylic monomer units with a homopolymer glass transition temperature of 10⁰C or more.

[0008] In some embodiments, the present disclosure provides a flame-resistant acrylic resin composition which is a heat- or ultraviolet-cured product of a mixture comprising: (a) 100 parts by mass of a (meth)acrylic monomer or its partially polymerized polymer comprising 60 to 97 mass% alkyl (meth)acrylic monomer units with a homopolymer glass transition temperature of -40⁰C or less and 3 to 40 mass% basic (meth)acrylic monomer units with a homopolymer glass transition temperature of 10⁰C or more; (b) 30 to 450 parts by mass of aluminum hydroxide; and (c) 5 to 50 parts by mass of expandable graphite. [0009] In some embodiments, the present disclosure provides a sheet comprising said flame-resistant acrylic resin composition and a pressure-sensitive adhesive sheet having an adhesive layer on at least one side of said sheet.

[0010] According to the present disclosure, a sheet of a flame-resistant acrylic resin composition can be obtained with flame resistance on the level of UL-94 V-O. By providing this sheet of flame -resistant resin composition with an adhesive layer, moreover, it is possible to obtain a multilayer sheet that is both highly adhesive and flame resistant on the level of UL-94 V-O, or in other words a flame-resistant pressure-sensitive adhesive sheet. In addition, the flame-resistant pressure-sensitive adhesive sheet is highly durable at high temperatures.

DETAILED DESCRIPTION

[0011] Aspects of the present disclosure are explained in more detail below based on preferred embodiments. In these Specifications the term "(meth)acrylic" signifies acrylic or methacrylic, and a "(meth)acrylic monomer" may be acrylic monomer such as acrylic acid or an acrylic acid ester or a methacrylic monomer such as methacrylic acid or a methacrylic acid ester. A "(meth)acrylic polymer is a polymer made up of such a (meth)acrylic monomer.

[0012] Flame-retardant acrylic resin composition

[0013] As discussed above, the flame-retardant acrylic resin composition comprises specific amounts of three components: (A) a (meth)acrylic polymer comprising the specific alkyl (meth)acrylic monomer units and (meth)acrylic monomer units, (B) aluminum hydroxide and (C) expandable graphite. Thus, flame resistance on the level of UL-94 V-O can be obtained by selecting from the halogen- free flame retardants the two halogen-free flame retardants aluminum hydroxide and expandable graphite, and combining these having the specific amounts with the (meth)acrylic polymer comprising the above two specific monomer units. Moreover, in some embodiments, a sheet of the flame-resistant acrylic resin composition containing specific amounts of these three specific components may have both the flexibility and strength (balance) required for use as a pressure-sensitive adhesive sheet, as well as excellent durability at high temperatures. Consequently, by providing this sheet of flame -resistant acrylic resin composition with an adhesive layer, it is possible to obtain a flame -resistant pressure-sensitive adhesive sheet with flame resistance on the level of UL-94 V-O, good adhesiveness with an adherend, and excellent durability at high temperatures. The various components are explained below.

[0014] Aluminum hydroxide

[0015] Aluminum hydroxide contributes flame resistance to the acrylic resin composition. Unlike halogen-based flame retardants, aluminum hydroxide does not produce toxic substances when burned, among other environmental advantages. Also, aluminum hydroxide has better quality stability and is more resistant to blooming than phosphoric acid esters, aluminum polyphosphate, red phosphorus and other halogen-free flame retardants, so that the functional stability of the resulting flame -resistant resin composition does not degrade with the passage of time. A silane, titanate or fatty acid or other filler-disperser can also be included in order to improve the filling properties of the aluminum hydroxide in the acrylic components. For filling purposes, moreover, it is possible to use aluminum hydroxide that has been surface treated in advance with, for example, a silane, titanate, fatty acid or the like.

[0016] The minimum amount of aluminum hydroxide in the acrylic resin composition differs depending on the thickness of the acrylic resin composition sheet, but for purposes of flame retardancy it is normally 30 parts by mass per 100 parts by mass of component (A) ((meth)acrylic polymer). The thinner the sheet, the more aluminum hydroxide is required. Specifically, in the case of an 0.3 mm thick sheet the minimum amount of aluminum hydroxide is not less than 250 parts by mass (or preferably 300 parts by mass) per 100 parts by mass of component (A) ((meth)acrylic polymer). In the case of a 1.0 mm thick sheet, it is 90 parts by mass or preferably 120 or more parts by mass per 100 parts by mass of component (A) ((meth)acrylic polymer). In the case of a 2.0 mm thick sheet, it is 30 parts by mass or preferably 60 or more parts by mass per 100 parts by mass of component (A) ((meth)acrylic polymer). The maximum amount of aluminum hydroxide is 450 parts by mass considering the properties (flexibility, strength, etc.) when the resulting acrylic resin composition is made into a sheet. Preferably, the maximum amount is 400 parts by mass or 350 parts by mass per 100 parts by mass of component (A) ((meth)acrylic polymer).

[0017] In order to impart still greater flame resistance, the mean particle size of the aluminum hydroxide used in the acrylic resin composition is in the range of preferably 0.1 to 10 μm or more preferably 0.5 to 2 μm. The mean particle size here is the volume-average particle diameter, which can be measured for example by a device such as a Microtrac particle size analyzer (Nikkiso). In some embodiments, the composition may contain two or more kinds of aluminum hydroxide, from the standpoint of sheet hardness when the acrylic resin composition is made into a sheet.

[0018] Examples of aluminum hydroxide that can be used in the present invention include Nippon Light Metal Co. BFO 13 and BF083.

[0019] Expandable graphite

[0020] The acrylic resin composition contains expandable graphite in addition to aluminum hydroxide. Expandable graphite is a halogen- free flame retardant which forms a solid phase with chemicals inserted between layers of natural scaly graphite. When the graphite is combusted, the chemicals therein produce gas, resulting in expansion of the scaly graphite which blocks flame and heat (that is, the layers of expanded carbon function as heat- insulting layers, interrupting the transmittal of heat), producing a flame-retardant effect. Generally, high flame-resistance is conferred on the acrylic resin composition through the combined use of expandable graphite and the aforementioned aluminum hydroxide.

[0021] Expanded graphite can be distinguished into two types as follows. Unneutralized expandable graphite is characterized by a pH value from 2 to 4. Neutralized expandable graphite is characterized by a pH value from 6 to 8. Generally, the pH value is measured using a slurry prepared by adding 1 g of the expandable graphite to 100 ml of distilled water and mixing for 20 minutes. Then, the pH of the slurry is then measured by a pH meter.

[0022] In some embodiments, unneutralized expandable graphite may be preferred because less residual monomer may be obtained. Generally, less residual monomer results in less odor from the flame -resistant acrylic resin. In addition, during mixing with a material such as a polymer or monomer, the crystal layers of neutralized expandable graphite delaminate more easily than the crystal layers of unneutralized expandable graphite. The delaminated graphite fraction is very fine and intercepts UV light; thus, the use of neutralized expandable graphite in an UV-curing pressure-sensitive adhesive sheet sometimes may cause insufficient curing in the production. Therefore, the use of the unneutralized expandable graphite may be preferred in an UV-curing pressure-sensitive adhesive sheet. [0023] From the standpoint of flame resistance, an expandable graphite with an expansion initiation temperature of 180 to 300⁰C may be preferably used. In some embodiments, multiple types of expandable graphite having different expansion initiation temperatures and particle diameters may also be used in combination.

[0024] The amount of expandable graphite in the acrylic resin composition is 5 or more parts by mass per 100 parts by mass of component (A) ((meth)acrylic polymer). Below 5 parts mass, the flame resistance improving effect in combination with aluminum hydroxide is inadequate. In some embodiments, 10 or more parts by mass or preferably 15 or more parts by mass per 100 parts by mass of component (A) ((meth)acrylic polymer) is more desirable. The amount of expandable graphite is also no more than 50 parts by mass per 100 parts by mass of component (A) ((meth)acrylic polymer). This is based on the adhesiveness of the adhesive layer when a sheet the resulting acrylic resin composition is used as a pressure- sensitive adhesive sheet. That is, if the amount of expandable graphite exceeds 50 parts by mass the expandable graphite particles may fall off the surface of the sheet when the resulting acrylic resin composition is made into a sheet, detracting from the adhesive force between the sheet and the adhesive layer. In some embodiments, 40 parts by mass or less or preferably even 35 parts by mass or less per 100 parts by mass of component (A) ((meth)acrylic polymer) may be desirable.

[0025] An example of an expandable graphite that can be used is Tosoh Corp. GREP-EG.

[0026] (A) (Meth)acrylic polymer

[0027] Because the flame-resistant acrylic resin composition includes the aforementioned two types of halogen- free flame retardants, it proportion of halogen- free flame retardants (particulate components) in the resin composition is high. Thus, the monomers making up the (meth) acrylic polymer are selected and the monomer composition is specified so as to achieve a good balance of sheet flexibility and strength when the resin composition is made into a sheet even when the proportion of halogen-free flame retardant components in the resin composition is high. In this way, a sheet of the flame -resistant acrylic resin composition can be used as the core material of a pressure-sensitive adhesive sheet. [0028] That is, the (meth)acrylic polymer in the flame-resistant acrylic resin composition is composed of (A-I) alkyl(meth)acrylic monomer units with a homopolymer glass transition temperature of -40⁰C or less and (A-2) basic (meth)acrylic monomer units with a homopolymer glass transition temperature of 10⁰C or more, wherein component (A-I) comprises 60 to 97% by mass and component (A-2) comprises 3 to 40% by mass of the (meth)acrylic polymer. In some embodiments, monomer units other than (A-I) and (A-2) may be included in the (meth)acrylic polymer to the extent that they do not detract from the desired performance. In some embodiments, the amount of the monomer units other than (A- 1) and (A-2) may be not more than 30% by mass (when obtaining a thick sheet) or not more than 10% by mass (when obtaining a thin sheet).

[0029] (A-I) Alkyl(meth)acrylic monomer with a homopolymer glass transition temperature of -40⁰C or less

[0030] This alkyl(meth)acrylic monomer is selected with a homopolymer glass transition temperature of -40⁰C or less in order to achieve sheet flexibility when the resulting acrylic resin composition is made into a sheet. In some embodiments, the homopolymer glass transition temperature may be -80⁰C or higher. A common example of such an alkyl(meth)acrylic monomer with a homopolymer glass transition temperature of -40⁰C or less is a monomer with 6 to 12 carbon atoms in the alkyl part. The glass transition temperature will tend to be higher if the number of carbon atoms in the alkyl part of the monomer is higher or lower than this. In some embodiments, n-butyl acrylate, 2-ethylhexyl acrylate or isooctyl acrylate can be used as this monomer.

[0031] A smaller number of carbon atoms in the alkyl part is desirable from the standpoint of flame retardancy, but as discussed above, flexibility declines when the number of carbon atoms in the alkyl part is reduced. Consequently, the number of carbon atoms in the alkyl part of the alkyl(meth)acrylic monomer can be selected based on both the flexibility and flame resistance of the sheet, so that an alkyl(meth)acrylic monomer with few carbon atoms (carbon atoms: 6) in the alkyl part can be used for applications requiring more flame resistance than flexibility, while an alkyl(meth)acrylic monomer with more carbon atoms (carbon atoms: 8-12) in the alkyl part can be used for applications requiring more flexibility than flame resistance. [0032] These alkyl(meth)acrylic monomer units comprise 60 to 97% by mass of the (meth)acrylic polymer. Outside this range, the sheet may be insufficiently flexible or lack strength (cohesion) when the resulting resin composition is made into a sheet, or there may not be sufficient adhesive force between the sheet and the adhesive layer. In some embodiments, it is desirable that the amount of these alkyl(meth)acrylic monomer units be in the range of 60 to 90% by mass or even 60 to 85% by mass.

[0033] (A-2) Basic (meth)acrylic monomer with homopolymer glass transition temperature of 10⁰C or more

[0034] This (meth)acrylic monomer with a homopolymer glass transition temperature of not less than 10⁰C is selected so that in combination with the aforementioned alkyl(meth)acrylic monomer, it provides a sheet with improved sheet strength (cohesion) and a balance of flexibility and strength when the resulting acrylic resin composition is made into a sheet. In practice, the homopolymer glass transition temperature of this (meth)acrylic monomer is not more than 170⁰C (preferably 160⁰C).

[0035] For purposes of combination with expandable graphite, a (meth)acrylic monomer is selected that exhibits basic properties. That is, as discussed above, gas derived from chemicals in expandable graphite is generated due to the heat during combustion, resulting in expansion of the scaly graphite and consequent flame resistance. These chemicals are generally nitric acid, sulfuric acid and other inorganic acids or organic acids, and such acids (strong acids) adversely affect the durability of the resulting acrylic resin composition or acrylic resin composition sheet under high temperature conditions (due to deterioration of the resin components and the like). Even when the resulting acrylic resin composition or a sheet thereof is stored at room temperature, these chemicals can seep out from the expandable graphite, hardening the sheet. For this reason, a monomer exhibiting basic properties is selected as the (meth)acrylic monomer in order to neutralize acids, and when such a basic (meth)acrylic monomer is used, the acrylic resin composition or acrylic resin composition sheet will have good durability at high temperatures.

[0036] Specific examples of (meth)acrylic monomers that can be used include acrylamides such as N,N-dimethylacrylamide, N,N-diethylacrylamide, N₅N- dimethylaminopropylacrylamide, acryloylmorpholine and the like. [0037] This (meth)acrylic monomer constitutes 3 to 40% by mass of the (meth)acrylic polymer. Outside this range, sheet flexibility is insufficient or strength (cohesion) declines when the resulting acrylic resin composition is made into a sheet, there is insufficient adhesive force between the sheet and the adhesive layer, and even in the case of a pressure- sensitive adhesive sheet with an adhesive layer, it may not be possible to adequately fix the object of adhesion. In some embodiments, the amount of the (meth) acrylic monomer may be in the range of 5 to 40% by mass or even 10 to 40% by mass.

[0038] Acrylic Resin Composition Preparation Method

[0039] There are no particular limits on the method for preparing the acrylic resin composition. Therefore:

(i) (meth)acrylic monomers comprising component (A-I) and component

(A-2) can be first polymerized to obtain a (meth)acrylic polymer as component (A), which is then mixed and kneaded with component (B) (aluminum hydroxide) and component (C) (expandable graphite) to obtain the acrylic resin composition, or else

(ii) (meth)acrylic monomers or a its partially polymerized polymer thereof comprising component (A-I) and component (A-2) can be mixed with component (B) (aluminum hydroxide) and component (C) (expandable graphite), and a heat polymerization initiator or photopolymerization initiator can be then added to this mixture to cure it and obtain the acrylic resin composition. Curing can be accomplished by various methods, such as for example heat polymerization, ultraviolet polymerization, electron beam polymerization, gamma-ray irradiation, ionizing beam irradiation and the like.

[0040] Thus, in some embodiments of case (ii) the acrylic resin composition is the heat- or ultraviolet-cured product of a mixture comprising (a) 100 parts by mass of a (meth)acrylic monomer or its partially polymerized polymer composed of 60 to 97 mass% alkyl(meth)acrylic monomer units with a homopolymer glass transition temperature of -40⁰C or less and 3 to 40 mass% basic (meth)acrylic monomer units with a homopolymer glass transition temperature of 10⁰C or more, (b) 30 to 450 parts by mass of aluminum hydroxide and (c) 5 to 50 parts by mass of expandable graphite.

[0041] In preparation method (ii), if the acrylic monomers have low viscosity, there is a risk that the aluminum hydroxide and other components may precipitate when mixed with the monomers. In such cases, the acrylic monomers are preferably partially polymerized in advance to increase viscosity. Such partially polymerized polymerization is preferably performed to about 100 to 10,000 centipoise (cP). This partially polymerized polymerization can be performed by a variety of methods, including heat polymerization, ultraviolet polymerization, electron beam polymerization, gamma-beam irradiation, ionizing beam irradiation and the like. A heat polymerization initiator or photopolymerization initiator is common used in such its partially polymerized polymerization. The heat polymerization initiators and photopolymerization initiators described below can be used as heat polymerization initiators and photopolymerization initiators. In some embodiments, each of the component (A-I) or (A-2) may be partially polymerized, the component (A-I) and the component (A-2) may be partially polymerized, respectively, or the components (A-I) and (A-2) may be copolymerized to make a partially polymerized polymer.

[0042] Examples of heat polymerization initiators include diacylperoxides, peroxyketals, ketonperoxides, hydroperoxides, dialkylperoxides, peroxyesters, peroxydicarbonates and other organic peroxide free radical initiators. Specific examples include lauroyl peroxide, benzoyl peroxide, cyclohexanone peroxide, l,l-bis(t-butylperoxy)3,3,5- trimethylcyclohexane, t-butyl hydroperoxide and the like. A persulfate/bisulfate combination can also be used.

[0043] Examples of photopolymerization initiators include benzoin ethyl ether, benzoin isopropyl ether and other benzoyl ethers, anisoin ethyl ether and anisoin isopropyl ether, Michler's ketone (4,4'-tetramethyl diaminobenzophenone), 2,2-dimethoxy-2- phenylacetophenone (such as KB-I from Sartomer and Irgacure™ 651 from Ciba Specialty Chemical), and 2,2-diethoxyacetophenone and other substituted acetophenones. Other examples include 2-methyl-2-hydroxypropiophenone and other substituted alpha-ketones, 2- naphthalensulfonyl chloride and other aromatic sulfonyl chlorides, and l-phenon-1,1- propandion-2-(o-ethoxycarbonyl)oxime and other photoactive oxime compounds. A combination of any of the heat polymerization initiators or photopolymerization initiators described above can also be used. The amount of the initiator for its partially polymerized polymerization is not particularly limited but is normally 0.001 to 1 parts by mass per 100 parts by mass of acrylic monomers.

[0044] Partial acrylic polymerization can also be performed with chain transfer agent in order to control the molecular weight and content of the polymer in the resulting its partially polymerized polymer. Examples of such chain transfer agents include mercaptanes, disulfites and combinations of these. When used, the chain transfer agent is normally used in the amount of 0.01 to 1.0 parts by mass or preferably 0.02 to 0.5 parts by mass per 100 parts by mass of (meth)acrylic monomers.

[0045] Preparation method (ii) may be preferred from the standpoint of productivity and flame resistance of the resulting sheet. In the case of heat polymerization, the heat polymerization initiator described with reference to its partially polymerized polymerization can be used in the same amount. In the case of ultraviolet polymerization or other photopolymerization, the photopolymerization initiator described with reference to its partially polymerized polymerization can be used in the same amount. In the case of electron beam polymerization or other polymerization with particle energy beams, a polymerization initiator is not normally required. In the case of heat polymerization, the precursor composition is subjected to a polymerization reaction by heating at about 50⁰C to 200⁰C.

[0046] In some embodiments, it is preferable from the standpoint of ease of sheet control and energy efficiency that the acrylic resin composition be obtained by ultraviolet curing when the acrylic resin composition is to be made into a sheet. When curing the mixture of components by ultraviolet polymerization, they are first deaerated together in a planetary mixer or the like, and the mixture is then exposed to ultraviolet radiation to obtain an acrylic resin composition.

[0047] When an acrylic resin composition containing a large quantity of halogen-free flame retardants (particulate components) is obtained by ultraviolet curing, low-molecular- weight acrylic polymer components are likely to be generated on the surface of the sheet when the resulting resin composition is made into a sheet, detracting from flame resistance and adhesiveness. Consequently, heat polymerization is preferred in such cases. Moreover, it has been found that even when an acrylic resin composition containing a large quantity of halogen-free flame retardants (particulate components) is ultraviolet cured, generation of low-molecular- weight acrylic polymer components on the surface of the sheet can be inhibited by including an ultraviolet absorbent in the acrylic resin composition. The amount of the ultraviolet absorbent included in the ultraviolet cured product should be 0.1 to 8 parts by mass or preferably 0.1 to 3 parts by mass per 100 parts per mass of acrylic monomer or its partially polymerized polymer in order to prevent decreased cohesion of the sheet surface and allow the composition to cure adequately.

[0048] A crosslinking agent can be used to increase the strength when the resulting acrylic resin composition is worked into a sheet for example. A crosslinking agent that is activated by heat can be used as the crosslinking agent. Examples include lower-alkoxylated amino formaldehyde condensation products with one to four carbon atoms in the alkyl group, hexamethoxymethylmelamine (such as American Cyanamide Cymell™ 303) or tetramethoxymethyl urea (such as American Cyanamide Beetle™ 65) or tetrabutoxymethyl urea (Beetle™ 85). Other useful crosslinking agents include 1,6-hexanediol diacrylate, tripropylene glycol diacrylate and other polyfunctional acrylates. The crosslinking agent is used in the amount of ordinarily 0.001 to 5 parts by mass or preferably 0.01 to 1 part by mass per 100 parts by mass of monomers. Alternatively, a combination of the crosslinking agents described above can be used.

[0049] In addition to the components described above, pigments, antioxidants, heat stabilizers and other components may be added to the acrylic resin composition according to the desired purpose and application to the extent that they do not detract from the desired properties. However, adding a plasticizer in an effort to improve flexibility may result in poorer adhesiveness of the sheet itself when the acrylic resin composition is made into a sheet, or detract from adhesion with the adhesive layer. Generally, adequate sheet flexibility can be obtained without the use of a plasticizer if the acrylic resin compositions of the present disclosure are compounded in the specific way described above.

[0050] Sheet

[0051] In some embodiments, the acrylic resin composition described above can be used in the form of a sheet. This sheet is extremely flame resistant, and because it maintains a balance between flexibility and strength, it has excellent adhesiveness with the adhesive layers used in pressure-sensitive adhesive sheets.

[0052] The contents of the (meth)alkyl polymer, aluminum hydroxide and expandable graphite in the sheet can be determined appropriately within the range described above according to the object and thickness of the sheet. Because thinner sheets are less flame resistant, it is desirable to increase the compounded amounts of aluminum hydroxide and/or expandable graphite in the case of a thin sheet.

[0053] The sheet thickness is not particularly limited but is generally 0.3 to 2 mm. In conventional halogen-based flame-resistant sheets it has been difficult to achieve flame resistance on the level of UL-94 V-O with thin sheets. According to the present disclosure, because the sheet is made using the acrylic resin composition having the specific formulation as discussed above, a sheet having flame resistance on the level of UL-94 V-O can be achieved with a smaller thickness.

[0054] An example of a sheet preparation method corresponding to the acrylic resin composition preparation method of (i) above is a method in which (meth)acrylic monomers consisting of component (A-I) and component (A-2) are polymerized to obtain a (meth)acrylic polymer as component (A), and component (B) (aluminum hydroxide) and component (C) (expandable graphite) are then mixed and kneaded to obtain the acrylic resin composition which is made into a sheet by convention sheet molding methods.

[0055] An example of a sheet preparation method corresponding to the acrylic resin composition preparation method of (ii) above is a sheet- forming method in which (meth)acrylic monomers or partially polymerized (meth)acrylic monomers consisting of component (A-I) and component (A-2) are mixed with component (B) (aluminum hydroxide) and component (C) (expandable graphite) to obtain a mixture which is then polymerized so as to form a sheet. In this case, the sheet is preferably obtained by applying or coating the mixture on a release liner or other support surface and calendar molding or press molding to form a sheet which is then polymerized. In this case, the sheet should be formed in a nitrogen or other inactive atmosphere so as to prevent oxygen from impeding polymerization. [0056] Pressure-sensitive adhesive sheet

[0057] In some embodiments, the aforementioned sheet of acrylic resin composition can be provided with an adhesive layer on at least one side and used as a pressure-sensitive adhesive sheet.

[0058] Adhesive layer

[0059] The pressure-sensitive adhesive sheet is one in which an adhesive layer is held on a least one side of the aforementioned sheet of acrylic resin composition. The adhesive layer can be selected according to the object of adhesion. Adhesive layers can also be provided on both surfaces (top and bottom) of the sheet of acrylic resin composition according to the object and application, and in this case the adhesive layers on the top and bottom surfaces of the sheet may be different from one another.

[0060] There are no particular limits on the method of laminating the adhesive layer on the sheet of acrylic resin composition, and it can be laminated in the form of a transfer tape or an adhesive agent can be directly applied to the sheet. Each adhesive layer may be a continuous layer or a discontinuous layer. A primer layer may also be provided as necessary between the adhesive layer and the sheet of acrylic resin composition.

[0061] The adhesive layer is preferably composed of a (meth) acrylic polymer containing an acidic (meth)acrylic monomer unit. Because the sheet of acrylic resin composition contains a basic (meth)acrylic monomer, affinity between basic and acidic monomers is achieved by including acidic (meth)acrylic monomer units, resulting in good binding properties and adhesiveness between adjacent sheets of acrylic resin composition and adhesive layers. Consequently, while in conventional pressure-sensitive adhesive sheets having multilayer adhesive layers the layers are normally attached by means of a thermoplastic resin or other primer layers, in some embodiments, with the pressure-sensitive adhesive sheets according to the present disclosure having one or multiple adhesive layers there is sufficient adhesive force between the layers without the use of a primer layer, and interlayer damage during use is prevented. Specific examples of acidic (meth)acrylic monomers include (meth)acrylic acid, fumaric acid, itaconic acid, beta-carboxyethyl acrylate and the like. [0062] The thickness of the adhesive layer (including the thickness of a primer layer when such a layer is present) is generally 10 to 60 micrometers (μm), but depending on the thickness of the sheet of acrylic resin composition as discussed above, it can be set appropriately in order to maintain adequate adhesiveness and flame resistance on the level of UL-94 V-O. However, adhesiveness will be insufficient if the adhesive layer is thinner than 10 μm, while flame resistance may decline if it exceeds 60 μm. A flame-retardant component such as aluminum hydroxide may also be included in the adhesive layer as necessary, in which case the added amount is preferably 10 wt% or less of the adhesive layer as a whole. This is because adequate adhesive strength is not obtained at 10 wt% or more.

[0063] The adhesive layer may also be a foam. In such cases, the pressure-sensitive adhesive sheet adheres more faithfully to the object of adhesion. This is particularly advantageous when the object of adhesion is irregular.

[0064] The surface of the adhesive layer on the pressure-sensitive adhesive sheet may also be covered with a peelable liner for purposes of protection during storage and transport following manufacture.

[0065] As discussed above, in some embodiments, the pressure-sensitive adhesive sheet or in other words the acrylic flame-resistant pressure-sensitive adhesive sheet is a pressure- sensitive adhesive sheet having both flame resistance on the level of UL-94 V-O and excellent adhesiveness with the object of adhesion, and also having excellent durability at high temperatures.

[0066] As used herein, "excellent adhesiveness with the object of adhesion" means that the 90° peel strength of the pressure-sensitive sheet with a stainless steel plate is 10 N/cm or more in the case of an 0.3 mm thick sheet and 20 N/cm or more in the case of a 1.0 mm thick sheet.

[0067] In the present disclosure, UL-94 (combustibility test for plastic materials used in devices and electrical machine parts) is evaluated as followed. A 13 mm x 125 mm pressure- sensitive sheet sample is held vertically by a clamp at one end. Cotton is placed 30 cm below the sample. A burner flame is applied for 10 seconds to the free end of the sample, and once the flame has disappeared after this first application, the burner flame is applied for a further 10 seconds as the second applications. Two sets of tests are performed using five samples. The following five items are recorded for each sample:

(1) time that flame persists after first application of burner flame;

(2) time that flame persists after second application of burner flame;

(3) time of glow combustion after second application of burner flame;

(4) whether flame drip ignites the cotton placed below the sample; and

(5) whether the sample is burned up to the holding clamp.

[0068] Those samples that are found to fulfill conditions (a) to (e) as a result of testing are considered to have "V-O level" flame resistance:

(a) total flame persistence time for all samples is 10 seconds or less;

(b) total flame persistence time for both sets of five samples is 50 seconds or less;

(c) flame persistence and glow combustion times for each sample after second burner flame application are 30 seconds or less;

(d) flame drip from sample does not ignite cotton;

(e) glow combustion or flame persistence combustion from all samples does not extent to the holding clamp.

[0069] Use

[0070] As discussed above, if the pressure-sensitive adhesive sheet is a pressure-sensitive adhesive sheet having both flame resistance on the UL-97 V-O level and excellent adhesiveness towards the object of adhesion, and also having excellent durability at high temperatures, it can be used not only for joining parts in electrical and electronic products but also for such applications as building materials, automobiles, airplanes, ships and the like.

[0071] Examples [0072] The present disclosure is explained in more detail below using examples, but is not limited by these examples.

[0073] 1. Preparation of adhesive layer

[0074] 90 parts by mass of 2-ethylhexyl acrylate its partially polymerized polymer, 10 parts by mass of acrylic acid, 0.08 parts by mass of 1,6-hexandioldiacrylate (crosslinking agent) and 0.1 parts by mass of Ciba Specialty Chemicals Irugacure™ 651 (photoinitiator) where placed all together in a planetary mixer and kneaded for 30 minutes under reduced pressure (50 mm Hg) to obtain an acrylic composition. The resulting acrylic composition was sandwiched between two polyethylene terephthalate (PET) liners treated with a silicone release agent, and calendar molded as a sheet. With the composition still between the two PET liners, the sheet was exposed on both sides to ultraviolet beams at 0.3 mW/cm² for 5 minutes and then to ultraviolet for 5 minutes at 6.0 mW/cm² to cure the composition and obtain an adhesive layer of acrylic composition with a thickness of 0.025 mm.

[0075] 2. Preparation of pressure-sensitive adhesive sheet

[0076] Example 1

[0077] The components of the composition shown in Table 1 were placed all together in a planetary mixer and kneaded for 30 minutes under reduced pressure (50 mm Hg) to obtain an acrylic resin composition. The resulting acrylic resin composition was sandwiched between two polyethylene terephthalate (PET) liners treated with a silicone release agent, and calendar molded as a sheet. With the thermally conductive composition still held between the two PET liners, the sheet was cured by exposing it on both sides to ultraviolet at 0.3 mW/cm² for 10 minutes and then to ultraviolet at 6.0 mW/cm² for 10 minutes to obtain a sheet of acrylic resin composition with a thickness of 0.95 mm (core layer). The adhesive layer prepared above was laminated on both sides of this acrylic flame-resistant sheet to prepare a pressure- sensitive adhesive sheet 1.0 mm thick. [0078] Examples 2 to 7 and 9 to 12, and Comparative Examples 1 to 6. Pressure-sensitive adhesive sheets were obtained as in Example 1 with the thicknesses shown in Table 1 using the compositions shown in Table 1.

[0079] Example 8. The components of the composition shown in Table 1 were placed all together in a planetary mixer and kneaded for 30 minutes under reduced pressure (50 mm Hg) to obtain an acrylic resin composition. The resulting thermally conductive composition was sandwiched between two polyethylene terephthalate (PET) liners treated with a silicone release agent, and calendar molded as a sheet. While still sandwiched between the PET liners, the molded sheet was cured by heating it in an oven for 15 minutes at 140⁰C to obtain a sheet of acrylic resin composition (core layer) 0.95 mm thick. The adhesive layer prepared above was laminated on both sides of this acrylic flame-resistant sheet to obtain a pressure- sensitive adhesive sheet 1.0 mm thick.

Table 1 : Compositions

C: Neutralized expandable graphite characterized by a pH value from 6 to 8

C-I : Neutralized expandable graphite characterized by a pH value from 6 to 8 C-2:Unneutralized expandable graphite characterized by a pH value from 2 to 4 In Table 1:

Alkyl (meth) acrylic monomers, partially polymerized product thereof, and (meth) acrylic monomers

[2] Aluminum hydroxide

[3] Neutralized expandable graphite (PH 6)

[4] Other component

[5] Homo-polymer has a glass transition temperature of -58 ⁰C

The partially polymerized product of 2-Ethylhexyl Acrylate was obtained as follows:

First, 100 parts by mass of 2-ethylhexyl acrylate and

0.04 parts of Irgacure 651) (photo- initiator) by mass were mixed in a glass container. [6] Then, after purging sufficiently with nitrogen to remove dissolved oxygen in the mixture, the partly polymerized reaction was obtained by irradiating with UV light in an intensity of 3mW/cm2 for several tens of seconds using a low-pressure mercury lamp in a nitrogen atmosphere. The obtained reaction had a viscosity of about 4000 mPa-s.

[7] Homo-polymer has a glass transition temperature of 89 119 ⁰C

[8] Homo-polymer has a glass transition temperature of 81 ⁰C

[9] Homo-polymer has a glass transition temperature of 145 ⁰C

[10] Homo-polymer has a glass transition temperature of 135 ⁰C

[11] Homo-polymer has a glass transition temperature of 106 ⁰C

[12] Produced by Nippon Light Metal Company, Ltd.

[13] Produced by Nippon Light Metal Company, Ltd.

[14] Produced by Tosoh Corp.

[15] Produced by Ciba Specialty Chemicals K. K.

[16] Produced by Ciba Specialty Chemicals K. K.

[17] Produced by NOF Corp.

[18] Produced by Nippon Soda Co., Ltd.

[19] Produced by Ciba Specialty Chemicals K. K.

[20] Produced by Clariant, Ltd.

[21 ] Unneutralized expandable graphite

Neutralized expandable graphite [22] Produced by AIR WATER INC. (SS-3N is produced by neutralizing SS-3.)

Neutralized expandable graphite [23] Produced by AIR WATER INC. (CA60N is produced by neutralizing CA60.)

Unneutralized expandable graphite Produced by AIR WATER INC.

Unneutralized expandable graphite Produced by AIR WATER INC. [0080] 3. Evaluation

[0081] The pressure-sensitive adhesive sheets obtained in the aforementioned examples and comparative examples were evaluated by the following methods for 90° peel adhesive strength, heat-resistant shear adhesion and flame resistance. The results are shown in Table 2. The hardness of the pressure-sensitive adhesive sheets obtained in these examples and comparative examples was also evaluated by the following methods, with the results shown in Table 3.

[0082] 90° peel strength (stainless steel plate)

[0083] The resulting sheets were cut into 25 mm x 200 mm, and backed with anodized aluminum foil (130 μm). The backed samples were affixed to stainless steel plates (SUS 304), and crimped by 1 return pass of a 7 kg roller. After crimping, this was left for 72 hours at room temperature, and adhesive strength was measured by peeling at an angle of 90° by Tensilon at a rate of 300 mm/minute (n = 2, average given as measurement value). Samples with an adhesive strength below 10 N/cm failed the test.

[0084] Heat-resistant shear adhesion

[0085] The resulting sheets were cut to 25 mm x 12 mm, and both sides were affixed to SUS plates and crimped by 1 return pass with a 2 kg roller. After crimping, these were left for 72 hours at room temperature and then attached to 2 kg weights in an 80⁰C atmosphere, and the time taken for the samples to fall was measured. Those samples that did not fall after 5000 minutes or more were described as "5000+" (n = 2, average given as measurement value), while those that fell in less than 1000 minutes were evaluated as "failed".

[0086] Flame resistance

[0087] Flame resistance was evaluated according to the UL standard mentioned above (UL-94 "combustibility test for plastic materials for use in devices and electrical parts"). Those samples with flame resistance corresponding to V-O were evaluated as good, while those measured as V-I or less were evaluated as "failed". [0088] Hardness

[0089] Ten of the prepared flame-resistant pressure-sensitive adhesive sheets with a thickness of 1.0 mm were stacked to prepare a measurement sample, and the initial hardness of the sample was measured at a load of 1 kg with an Asker C durometer. The sample was also stored in a 110⁰C oven (in air), and hardness after two weeks was measured in the same way after the sample had been cooled to 25°C. The rate of change ΔH (%) in the hardness of the sample was also determined by the following formula. The smaller ΔH, the better long-term stability. ΔH (%) = ((hardness after two weeks) - (initial hardness)/(initial hardness) x 100.

Table 2: Experimental results.

Table 3 : Hardness results.

[0090] Residual monomer

[0091] A polyester mesh was dried for 24 hours in a 120⁰C oven, and the mesh was cooled down to room temperature in a desiccator. Then, a mass of the mesh (Wl) was measured. A sample of an adhesive sheet was cut to 100 mm x 100 mm and put on the mesh. The mass of the cut adhesive sheet with the mesh (W2) was measured. The adhesive sheet with the mesh was dried in a 120⁰C oven for 2 hours and was cooled to room temperature in a desiccator. Then, the mass of the dried adhesive sheet with the mesh (W3) was measured. Residual monomer (R) was calculated by using a following formula: R (wt%) = 100 - (W3-W1) x 100 / (W2-W1). The smaller the value of R, the lower the residual monomer content.

Table 4: Residual monomer results

Claims

What is claimed is:

1. A flame-resistant acrylic resin composition comprising:

(A) 100 parts by mass of a (meth)acrylic polymer;

(B) 30 to 450 parts by mass of aluminum hydroxide; and

(C) 5 to 50 parts by mass of expandable graphite, wherein the (meth)acrylic polymer comprises 60 to 97 mass% alkyl (meth)acrylic monomer units with a homopolymer glass transition temperature of -40⁰C or less and 3 to 40 mass% basic (meth)acrylic monomer units with a homopolymer glass transition temperature of 10⁰C or more.

2. The flame-resistant acrylic resin composition according to claim 1, wherein the expandable graphite comprises unneutralized expandable graphite.

3. The flame-resistant acrylic resin composition according to claim 1 or 2, further comprising 0.1 to 8 parts by mass of an ultraviolet absorbent.

4. The flame-resistant acrylic resin composition according to claim 1 or 2, further comprising at least one of a heat polymerization initiator, an ultraviolet polymerization initiator, a crosslinking agent, and a chain transfer agent.

5. The flame-resistant acrylic resin composition according to any one of claims 1 to 4, wherein the particle size of the aluminum hydroxide is 0.1 to 10 μm.

6. A sheet comprising the flame-resistant acrylic resin composition according to any one of claims 1 through 5.

7. The sheet of claim 6, further comprising an adhesive layer on at least one side of the sheet.

8. The sheet of claim 7, wherein the adhesive layer comprises a (meth) acrylic polymer containing an acidic (meth)acrylic monomer unit.

9. A method of preparing a flame -resistant acrylic resin composition comprising: (i) compolymerizing 60 to 97 mass% alkyl (meth)acrylic monomer units having a homopolymer glass transition temperature of -40⁰C or less and 3 to 40 mass% basic (meth)acrylic monomer units having a homopolymer glass transition temperature of 10⁰C or more to form a (meth)acrylic polymer;

(ii) mixing 100 parts by mass of the (meth)acrylic polymer with 30 to 450 parts by mass of aluminum hydroxide and 5 to 50 parts by mass of expandable graphite.

10. A method of preparing flame-resistant acrylic resin composition comprising:

(i) mixing 60 to 97 mass% alkyl (meth)acrylic units having a homopolymer glass transition temperature of -40⁰C or less and 3 to 40 mass% basic (meth)acrylic units having a homopolymer glass transition temperature of 10⁰C with 30 to 450 parts by mass of aluminum hydroxide and 5 to 50 parts by mass of expandable graphite;

(ii) copolymerizing the alkyl (meth)acrylic units and the basic (meth)acrylic units.

11. The method according to claim 10, wherein the alkyl (meth)acrylic units and the basic (meth)acrylic units are partially polymerized prior to mixing with the aluminum hydroxide and the expandable graphite.