WO2002055622A2

WO2002055622A2 - Pressure sensitive adhesive and pressure sensitive adhesive sheet using the same

Info

Publication number: WO2002055622A2
Application number: PCT/US2001/046401
Authority: WO
Inventors: Koji Imai
Original assignee: 3M Innovative Properties Company
Priority date: 2000-12-15
Filing date: 2001-12-04
Publication date: 2002-07-18
Also published as: WO2002055622A3; AU2002241582A1; JP2002194312A

Abstract

A pressure sensitive adhesive which has excellent adhesion to a coating film of an acidic rain-resistant coating composition which scarcely contains or does not entirely contain a melamine resin, and also has high humidity resistance. An adhesive obtained by polymerizing a polymerizable composition comprising a first (meth)acrylate having a C4-12 alkyl group and a second (meth)acrylate having a cyclic imide group.

Description

Pressure Sensitive Adhesive and Pressure Sensitive Adhesive Sheet using the Same

Field of the Invention

The present invention relates to an adhesive (hereinafter also referred to as a "pressure-sensitive adhesive") and, more particularly, to a humidity-resistant pressure sensitive adhesive having excellent adhesion to a coating film of an automotive coating composition which contains a melamine resin in a reduced amount or does not entirely contain a melamine resin, and a pressure sensitive adhesive sheet using the adhesive.

Background of the Invention

As well known, a pressure sensitive adhesive has widely been used in the field of industry because it is a viscoelastic material having both wettability and cohesive strength and can simply provide an adhesive strength capable of fitting to practical use under a very small pressure.

In the automotive industry, for example, vehicles are decorated or protected by attaching parts such as exterior parts using a pressure sensitive adhesive tape obtained by coating one or both surfaces of a substrate with a pressure sensitive adhesive, or a double- coated pressure sensitive adhesive tape with no substrate. In the automotive industry, the water resistance and weathering resistance such as a severe temperature change are requested to the adhesive. A typical adhesive having excellent water resistance and weathering resistance is an acrylic adhesive. Particularly, an acrylic acid-containing adhesive also has excellent adhesion to a coating film of a crosslinking melamine resin coating material made from melamine-alkyd or acrylic - melamine, which is generally used for coating of vehicles.

This melamine resin coating material is liable to be decomposed comparatively by an acidic rain caused by recent change of global environment. Therefore, an acidic rain- resistant coating material has recently been used. A typical acidic rain-resistant coating material contains a melamine resin as a crosslinking agent in a reduced amount or does not substantially contain it, but contains an auxiliary crosslinking agent such as acid anhydride/epoxy or epoxy/silanol hydroxyl group in place of the melamine resin, as disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 7-265784 and 7- 275792.

However, the acrylic acid-containing adhesive (acrylic adhesive) described above generally has poor pressure-sensitive adhesion to a coating film of the acidic rain-resistant coating composition. As disclosed in Japanese Unexamined Patent Publication No. 6-

200225, such a pressure-sensitive adhesion can be temporarily improved by using an acrylic adhesive which contains a polymer containing an amide-containing monomer such as cyclic (meth)acrylamide or N-vinyl (meth)acrylamide. However, the amide-containing monomer is very hydrophilic and therefore tends to lower the humidity resistance of the acrylic adhesive. The humidity resistance is required in case where parts such as exterior parts of the vehicle are attached using a pressure sensitive adhesive tape coated with a pressure sensitive adhesive, or a double-coated pressure sensitive adhesive tape. That is, it is requested to be a pressure sensitive adhesive which has enough humidity resistance to prevent falling of parts or partial peeling as a result of hydrophilization of the adhesive (e.g. reduction in adhesive strength due to interlaminar peeling between a coating film and an adhesive) caused by penetration of water when the vehicle is left to stand in an ambient environment in the high-temperature and high-humidity season such as rainy season, or washed.

Summary of the Invention

An object of the present invention is to solve one or more of the previously described problems. Thus, an additional object of the present invention can be to provide a humidity-resistant adhesive having satisfactory, and preferably excellent, adhesion (pressure-sensitive adhesion) to a coating film of an acidic rain-resistant coating composition, such as that described above. Another object of the present invention can be to provide an adhesive sheet using such an adhesive.

In one aspect of the present invention, a pressure sensitive adhesive is provided that is obtained by polymerizing a polymerizable composition comprising a first (meth)acrylate having a C .ι₂ alkyl group and a second (meth)acrylate having a cyclic imide group. Such an adhesive can have excellent adhesion to a coating film of an acidic rain- resistant coating composition which scarcely contains or does not entirely contain a melamine resin, and also has high humidity resistance.

Brief Description of the Drawings

Fig. 1 is a sectional view showing an aspect of a double-coated pressure sensitive adhesive sheet according to the present invention.

Fig. 2 is a sectional view showing an aspect of the single-coated pressure sensitive adhesive sheet with a base according to the present invention. Fig. 3 is a perspective view showing a specimen in the measurement of the adhesive strength of the adhesive sheet.

Detailed Description of the Invention

The present invention will be explained by way of preferred embodiment, but it is easily appreciated by a person with an ordinary skilled in the art that the present invention is not limited thereto.

The pressure sensitive adhesive of the present invention can be obtained by polymerizing a polymerizable composition comprising a first (meth)acrylate having a C .ι₂ alkyl group and a second (meth)acrylate having a cyclic imide group. In general, the polymerizable composition can further contain a polymerization initiator to the first

(meth)acrylate and the second (meth)acrylate.

The respective components of this polymerizable composition will now be described.

(1) First (meth)acrylate

The first (meth)acrylate is required to provide the adhesive of the present invention with the adhesion, particularly to provide the adherend (e.g. coating) with the wettability by the adhesive.

The first (meth)acrylate is a (meth)acrylate having a C .₁₂ alkyl group, and is preferably n-butyl (meth)acrylate, isobutyl (meth)acrylate, isooctyl (meth)acrylate, n-octyl

(meth)acrylate, 2-ethylhexyl (meth)acrylate, isononyl (meth)acrylate, or a mixture thereof. The first (meth)acrylate is usually contained in the polymerizable composition in the amount within a range from about 50 to 98% by weight. When the amount is less than about 50% by weight, the pressure sensitive adhesion tends to be impaired. On the other hand, when the amount exceeds about 98% by weight, the cohesive strength tends to be lowered. In consideration of balance between the adhesion and cohesive strength, the first

(meth)acrylate is preferably contained in the polymerizable composition in the amount within a range from 60 to 95% by weight.

(2) Second (metbpacrylate In the present invention, the second (meth)acrylate is a (meth)acrylate having a cyclic imide group. The second (meth)acrylate acts to provide an adhesion of a pressure sensitive adhesive to an adherend (substrate) and a cohesive strength of a pressure sensitive adhesive. The second (meth)acrylate provides a hydrophobic nature compared to the case where a conventional amide-containing monomer is used, and thus it provides a humidity resistance to the pressure sensitive adhesive. Examples of the second

(meth)acrylate include imide (meth)acrylate having a phthalimide skeleton, imide (meth)acrylate having a hexahydrophthalimide skeleton, imide (meth)acrylate having succinic imide skeleton, imide (meth)acrylate having a tetrahydrophthalimide skeleton with a double bond at the 4- and 5-positions, and imide (meth)acrylate having a tetrahydrophthalimide skeleton with a double bond at the 1- and 2-positions. More specifically, the compounds described above are represented by the following formulas (I) to (V):

(wherein 1 represents an integer of 2 to 3; m represents an integer of 1 to 3; Rl, R2 and R3 represent H or CH₃ and Rl, R2 and R3 in a molecule may be the same or different; and R4 to R7 represent H or C_nH_2n+1 (n is an integer of 1 to 6) and R4 to R7 in a molecule may be the same or different).

For example, the imide (meth)acrylate is commercially available from Toagosei Chemical Industry Co., Ltd. under the trade name of Aronix. Any of these imide (meth)acrylates is liquid and soluble in the polymerizable composition, which can be easily homogeneous. Specifically, the imide (meth)acrylate is usually contained in the polymerizable composition in the amount within a range from 1 to 50% by weight, and preferably from 2 to 40% by weight in consideration of balance between the adhesion and cohesive strength, and is compatible with the first (meth)acrylate, thereby making it possible to provide a uniform polymerizable composition. Such a uniform polymerizable composition is very advantageous in that an adhesive having good pressure-sensitive adhesion properties and breaking strength can be obtained.

These imide (meth)acrylates can predominantly provide the adhesive of the present invention with the cohesive strength. Furthermore, the compounds of the formulas (I) to

(V) among these imide (meth)acrylates, i.e. second (meth)acrylates, maintain the concentration of a highly polar imide moiety in a molecule within a desired range by an integer of m in the above formulas.

In the case described above, it is made possible to form a pressure sensitive adhesive containing a polymer having usually a molecular weight of 100,000 or more, preferably 500,000 or more, by rapidly polymerizing the second (meth)acrylate with the first (meth)acrylate due to light such as ultraviolet light. Such an adhesive has a desired pressure-sensitive adhesion due to the wettability and cohesive strength described above. In case where the imide (meth)acrylates are imide (meth)acrylates wherein Rl to R7 of the formulas (I) to (V) are hydrogen atoms and 1 of the formula (I) and m of the formulas (II) to (N) are respectively 2 and 1, the polymerizability with the first (meth)acrylate and adhesion of the pressure sensitive adhesive are further excellent, thus making it possible to provide the adhesive with an adhesive strength within a range from 20 Ν/12 mm to 3 N/12 mm. As used herein, the term "adhesive strength" is defined by a 180° peel test which is carried out in accordance with ASTM D3330 87.

As described above, the second (meth)acrylate is hydrophobic compared to a conventional amide-containing monomer, and thus provide humidity resistance to the adhesive. Since the second (meth)acrylates of the formulas (I) to (V) among the second (meth)acrylates can maintain the concentration of a highly polar imide moiety within a desired range, the hydrophobicity is excellent and the adhesive of the present invention can also be provided with particularly excellent humidity resistance. Accordingly, in case where parts such as exterior parts of the vehicle are attached using a pressure sensitive adhesive tape or a double-coated pressure sensitive adhesive tape coated with the pressure sensitive adhesive of the present invention, the adhesive is not hydrophilized by penetration of water when the vehicle is left to stand in an ambient environment at the high-temperature and high-humidity season such as rainy season. Therefore, falling of parts or partial peeling does not occur. Accordingly, the adhesive of the present invention has excellent weathering resistance, thus making it possible to maintain the adhesion (e.g. interlaminar adhesion strength) to an acidic rain-resistant coating composition for a long period.

(3) Photoporvmerization initiator

If necessary, photopolymerization initiators may be usually added to the polymerizable composition in the amount within a range from 0.01 to 5.0 parts by weight, and preferably from 0.01 to 2.0 parts by weight, based on 100 parts by weight of the composition, thereby to accelerate the photopolymerization and to improve the productivity of the adhesive of the present invention described hereinafter.

In the present invention, examples of the polymerization initiator include benzoin and benzoin ethers such as benzoin methyl ether and benzoin isopropyl ether; acetophenones such as acetophenone, 2,2-dimethoxy-2-phenyl-acetophenone, 2,2- diethoxy-2-acetophenone, 1,1-dichloroacetophenone, 1-hydroxyacetophenone, 1- hydroxycyclohexyl phenyl ketone, and 2-methyl-l-[4-(methylthio)phenyl]-2-morpholino- propan-1-one; anthraquiones such as 2-methylanthraquinone, 2-ethylanthraquinone, 2- tertiary-butylanthraquinone, 1-chloroanthraquinone, and 2-aminoanthraquinone; thioxanthones such as 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, 2- chlorodioxanthone, and 2,4-diisopropylthioxanthone; and ketals such as acetophenone dimethyl ketal, benzyl dimethyl ketal, and benzoinphenone, benzylmetlylsulfonium salts such as p-phenylbenzylmetlylsulfonium salts, triarylsulfonyl salts such as triphenylsulfonium salts, diphenyl-4-thiophenoxy sulfonium salts and disulfonium salts having bis-[4-diphenylsulfonio)phenyl]sulfide skeleton with a counter anion for the sulfonium salts being SbFβ^", AsFe^", PF₆ ^" and BF₆ ^".

(4) Others

As far as the effect and object of the present invention are not adversely affected, the polymerizable composition may further contain various components. For example, the polymerizable composition may contain a bifunctional or polyfunctional acrylate such as 1,6-hexanediol diacrylate, which exhibits a function capable of providing the adhesive with improved cohesive strength, or monomer or oligomer of an unsaturated carboxylic acid such as acrylic or methacrylic acid, which exhibits a function capable of providing improved adhesion with a coating film, as an acrylic monomer or oligomer capable of polymerizing with the first (meth)acrylate and second (meth)acrylate described above.

In addition to the components described above, heat aging inhibitors, ozone deterioration inhibitors, softeners, surfactants, plasticizers, lubricants, colorants, antistatic agents, antifungal agents, ultraviolet absorbers, and flame retardants can also be mixed.

Further, the polymerizable composition can be filled with microsphere fillers or have an inert gas such as nitrogen gas or carbonate gas or foaming agent dispersed to form a foamed pressure sensitive adhesive, thereby enhancing a cushionability.

The substrate (adherend), to which the adhesive of the present invention is applied, is not specifically limited, but its utility is particularly remarkable when applied to a coating film of an acidic rain-resistant coating composition. That is, it exhibits the pressure-sensitive adhesion to the coating film of the coating composition for an automobile, which contains in an auxiliary manner or does not entirely contain a melamine resin, and exhibits an excellent humidity resistance. The substrate, to which the adhesive of the present invention is advantageously applied, is a coating film of a coating composition wherein an acid/epoxy curing agent comprising an epoxide having a low number-average molecular weight, an acid anhydride and an onium salt curing agent, or an epoxy/silanol/hydroxyl group-based crosslinking agent having an alkoxysilane, an epoxy and a hydroxyl group as a functional group is used in place of a melamine resin as a crosslinking agent. Specifically, such a coating composition is described as a second clear coat in Japanese Unexamined Patent Publication (Kokai) No. 7-265784.

As described above, the adhesive of the present invention is often used in the form of a sheet. Such an adhesive in the form of a sheet can be prepared, for example, by the following conventional technique using the polymerizable composition described above. First, the components described above are charged in a transparent container to prepare a polymerizable composition. Then, an air in the transparent container is purged with a nitrogen gas. Thereafter, the polymerizable composition is polymerized by irradiating with ultraviolet light (usually within a range from 300 to 400 nm) through the transparent container to obtain a partially polymerized solution having a predetermined viscosity. To this partially polymerized solution, a photopolymerization initiator and, if necessary, other acrylic monomer or acrylic oligomer are added, followed by mixing with stirring and degassing under reduced pressure, to obtain a transparent adhesive precursor. A double-coated pressure adhesive sheet, and a single-coated pressure sensitive adhesive sheet with a base can be prepared from this adhesive precursor. In Fig. 1, a sectional view showing an aspect of the double-coated pressure sensitive adhesive sheet 1 according to the present invention is shown. This sheet is prepared in the following procedure. The adhesive precursor is interposed between a pair of transparent supporting films (release film 4), the surface of which to be contacted with the adhesive precursor is release-treated, and then formed into adhesive precursor sheet. The transparent supporting film is not specifically limited, but is usually made of polyethylene terephthalate (PET). Then, the adhesive precursor is photopolymerized by irradiating with the ultraviolet light described above having a predetermined energy dose, for example, 0.1 mW/cm² to 100 mW/cm through two transparent supporting films in one step or in stepwisely separated steps to obtain an double-coated adhesive sheet 1 wherein both surfaces of an adhesive 3 are interposed between release films 4.

A sectional view showing an aspect of the single-coated pressure sensitive adhesive sheet 2 with a base according to the present invention is shown in Fig. 2. The adhesive sheet can be prepared in the same procedure as in case of the above double- coated adhesive sheet, but a non-release plastic film (e.g. poly( vinyl chloride) film, polyurethane film, polyethylene terephthalate film, etc.) is used as a transparent supporting film (base 5) at one surface, thus obtaining a single-coated adhesive sheet 2 with a base, comprising a non-release base 5 on one surface of the adhesive 3 and a release film 4 on the opposite surface of the adhesive 3. To improve the adhesion between the base 5 and the adhesive 3, the base 5 may be treated with a primer.

The double-coated pressure sensitive adhesive sheet thus obtained is employed as follow. The supporting film (release film) on one surface of the adhesive sheet is peeled off and then applied to plastic parts such as emblem and molding of automobiles, thereby enabling the film to form portion of the parts. In the assembly line of the automobile manufacturing process, the supporting film (release film) at the opposite side is also peeled off immediately before applying and then applied to the surface coated with an acidic rain-resistant coating composition. The pressure sensitive adhesive tape with a base (e.g. decorative film tape) is directly provided to the assembly line of the automobile manufacturing process, and then the release supporting film (release film) opposite the base (e.g. decorative film) is peeled off immediately before applying and applied to the surface coated with an acidic rain- resistant coating composition.

Examples

The present invention will be described in more detail with reference to the following examples. It will be appreciated that the present invention is not limited to the following examples.

Preparation of adhesive sheet Example 1

In this Example, the following components were charged in a transparent container to prepare a polymerizable composition.

(1) Isooctyl acrylate 65 parts by weight

(2) Cyclic imideacrylate 35 parts by weight (Aronix TO1429, manufactured by Toagosei Chemical Industry Co., Ltd.)

(3) Photopolymerization initiator 0.04 parts by weight (Irgacure™ 651 , manufactured by Ciba Specialty Chemicals Co.)

Then, an air in the transparent container was purged with a nitrogen gas. Thereafter, the polymerizable composition was irradiated with ultraviolet light, which has a wavelength within a range from 300 to 400 nm and a maximum intensity at a wavelength of about 350 nm, through the transparent container to obtain a partially polymerized solution having a viscosity of about 3000 mPa'S.

To this partially polymerized solution, the following components were added.

(4) Photopolymerization initiator 0.10 parts by weight (Irgacure™ 651, manufactured by Ciba Specialty Chemicals Co.) 1,6-hexanediol diacrylate 0.08 parts by weight After mixing with stirring and degassing under reduced pressure, a transparent adhesive precursor was obtained. The resulting adhesive precursor was interposed between two transparent supporting films and then formed into adhesive precursor sheets, each of which has a thickness of 0.2 mm and 0.5 mm. At this time, a 50 μm thick PET film, the surface of which is release-treated with silicone, was used as the transparent supporting film. Then, each adhesive precursor sheet was photopolymerized by irradiating with the ultraviolet light described above through two supporting films at an energy dose of 1000 mJ/cm² to obtain a pressure sensitive adhesive sheet.

Comparative Example 1 In the same procedure as in Example 1 , except that 31 parts by weight of N,N- dimethylacrylamide was used in place of the cyclic imide acrylate, pressure sensitive adhesive sheets were prepared in this example.

Comparative Example 2 In the same procedure as in Example 1, except that 77.5 parts by weight of butyl acrylate and 22.5 parts by weight of N-vinyl caprolactam were used in place of the isooctyl acrylate and cyclic imide acrylate, pressure sensitive adhesive sheets were prepared in this example.

Evaluation of adhesive sheet

( 1 ) Measurement of adhesive strength

(i) Preparation of specimens

Specimens were prepared from the respective pressure sensitive adhesive sheets having a thickness of 0.2 mm in the following procedure for a measurement of the adhesive strength.

First, a PET protective film having a thickness of 50 μm was prepared and one surface of PET was coated with a primer (N-200 primer, manufactured by Sumitomo 3M Co., Ltd.) in order to satisfactorily bond the PET film with the adhesive sheet, and then the solvent was evaporated by allowing to stand for about one minutes. After the supporting film was peeled off from one surface of the adhesive sheet, the adhesive film was laminated on the protective film through the primer while providing one end of the protective film with a tab portion with at least 20 mm length for tensile test. The adhesive sheet was cut while remaining the tab portion for tensile test at this state to obtain a striplike specimen having a width of 12 mm (see Fig. 3).

(ii) Measurement of adhesive strength

The adhesive strength of the adhesive sheet was measured, for example, in the following procedure.

First, the remaining supporting film was peeled off from the specimen thereby to expose the adhesive sheet. This adhesive sheet was brought into contact with an acid/epoxy-crosslinked acidic rain-resistant coated steel plate (DCT-5001, commercially available from PPG Industries), and then contact-bonded by using a roller of 2 kg. The adhesive sheet was aged in this state, together with the steel plate, by allowing to stand at room temperature (23 °C, 50 RH%) for two weeks. Then, a 180° peel test was carried out by stretching the protective film at a rate of 300 mm/minute thereby to peel off the adhesive film from the acid rain-resistant coated steel plate. The 180° peel test was also carried out after this adhesive sheet was allowed to stand under the wet heat aging conditions of 50°C and 95% RH (relative humidity) for two weeks. Furthermore, the 180° peel test was carried out by using a melamine-alkyd-coated steel plate in place of the acidic rain-resistant coated steel plate. The measurement results of the 180° peel tests are shown in Table 1.

Table 1 Measurement Results of 180° peel test

(Note)

Unit of the measured value is N/12 mm. (3) Measurement of breaking strength

The breaking strength of the adhesive sheet was measured by using the respective pressure sensitive adhesive sheets having a thickness of 0.5 mm.

First, the adhesive sheet was aged by allowing to stand at room temperature (23 °C, 50 RH%) for two weeks, and then cut into pieces having a width of 5 mm and a length of 40 mm. The supporting film was peeled off from both surfaces of the adhesive sheet and the tensile strength at break was measured at a rate of 500 mm/minute. The tensile strength at break was also measured after the adhesive sheet was allowed to stand under the wet heat aging conditions of 50°C and 95% RH (relative humidity) for two weeks. The measurement results of the breaking strength is shown in Table 2.

Table 2 Measurement Results of Breaking Strength

Unit of the measured value is N/5 mm. For each specimen, two tests were conducted and two measurement results were shown for each condition.

As is apparent from Table 1, the adhesive sheet of Example 1 has an adhesive strength to an acidic rain-resistant coated steel plate, which is the same as or larger than an adhesive strength to a melamine-alkyd-coated steel plate regardless of aging at room temperature (23°C, 50 RH%) and aging under the wet heat aging conditions. Even after aging under the wet heat aging conditions, reduction in the adhesive strength to the acidic rain-resistant coated steel plate is not shown. On the other hand, regarding the adhesive sheet of Comparative Example 1, reduction in the adhesive strength to any of the melamine-alkyd-coated steel plate and the acidic rain-resistant coated steel plate after the wet aging is shown. Furthermore, according to Table 2, the adhesive sheet of Example 1 exhibits a constant breaking strength and maintains a desired cohesive strength regardless of aging at room temperature (23°C, 50 RH%) and aging under the wet heat aging conditions. On the other hand, the adhesive sheet of Comparative Example 2 exhibits a constant breaking strength and maintains a desired cohesive strength after aging at room temperature (23 °C,

50 RH%), but the breaking strength is reduced to about half of the desired breaking strength after aging under wet heat aging conditions.

Accordingly, it has been found that the adhesive sheet of Example 1 according to the present invention has excellent humidity resistance and can maintain the desired braking strength as well as the desired adhesive strength to not only the melamine-alkyd- coated steel plate but also the acidic rain-resistant coated steel plate after or before aging.

The adhesive of the present invention has excellent adhesion to a coating film of an acidic-resistant coating composition, which scarcely contains or does not entirely contain a melamine resin, and also has high humidity resistance.

Claims

We claim:

1. A pressure sensitive adhesive obtained by polymerizing a polymerizable composition comprising a first (meth)acrylate having a C .ι₂ alkyl group and a second (meth)acrylate having a cyclic imide group.

2. The pressure sensitive adhesive according to claim 1, wherein said second (meth)acrylate is at least one imide (meth)acrylate selected from the group consisting of imide (meth)acrylates represented by the following formulas (I) to (N):

wherein 1 represents an integer of 2 to 3; m represents an integer of 1 to 3; Rl, R2 and R3 represent H or CH₃ and Rl, R2 and R3 in a molecule may be the same or different; and R4 to R7 represent H or C_nH2n₊ι (n is an integer of 1 to 6) and R4 to R7 in a molecule may be the same or different.

3. The pressure sensitive adhesive according to claim 1, wherein said first (meth)acrylate and second (meth) acrylate are contained in said polymerizable composition in the amount of 50-98% by weight and 2-50% by weight, respectively.

4. The pressure sensitive adhesive according to claim 1, wherein a polymerization initiator of said first (meth) acrylate and second (meth)acrylate is further contained in said polymerizable composition.

5. A pressure sensitive adhesive sheet comprising the pressure sensitive adhesive of any one of claims 1 to 4.