EP0011643A4

EP0011643A4 - Cross-linked pressure-sensitive tape adhesive of a copolymer of alkyl acrylate and copolymerizable acid.

Info

Publication number: EP0011643A4
Application number: EP19790900506
Authority: EP
Inventors: James Henry Diamond
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1978-05-01
Filing date: 1979-12-04
Publication date: 1982-01-26
Also published as: EP0011643A1; WO1979001013A1

Abstract

Pressure-sensitive tape adhesive comprising a copolymer of alkyl acrylate and copolymerizable acid such as acrylic acid, which adhesive includes a small amount of Cr3+ ion which crosslinks the acid groups to afford solvent-resistance while retaining good pressure-sensitive adhesive qualities. The adhesive has excellent electrical insulating properties.

Description

CROSS-LINKED PRESSURE-SENSITIVE TAPE ADHESIVE OF A COPOLYMER OF ALKYL ACRYLATE AND COPOLYMERIZABLE ACID Technical Field The invention concerns a pressure-sensitive adhesive tape, the adhesive layer of which is a copolymer of alkyl acrylate and copolymerizable acid. The tape is normally produced in the form of a wide sheet which may be slit to narrow widths and wound upon itself for convenient storage and shipment.

Background Art U. S. Patent No. Re.24,906 concerns pressure-sensitive tape adhesive consisting essentially of a copolymer of alkyl acrylate and a minor proportion of copolymerizable monomer such as acrylic acid. That acrylate copolymer provides excellent adhesion and holding power and experiences no observable deterioration even after years of storage in tape form.

For uses requiring improved internal strength at high temperatures and/or resistance to organic solvents, the acrylate copolymer adhesive may be crosslinked. U. S. Patent No. 2,973,286 teaches crosslinking it using an organic peroxide prior to coating and applying somewhat greater heat than would be necessary merely to dry the coating. However, the crosslinking is difficult to control. U. S. Patent No. 2,925,174 teaches crosslinking the acrylate copolymer by reaction with a polyfunctional compound such as a polyamine, a polyol or a polyepoxide. The resultant tapes tend to be deficient in immediate adhesion, usually called "wet-grab", or if tailored to provide good wet-grab, to be deficient in internal strength at ambient temperatures.

U. S. Patent No. 3,284, 423 incorporates a third copolymerizable monomer, glycidyl acrylate or methacrylate, to crosslink the acrylate copolymer. However, the gradual crosslinking of the adhesive during storage of the tape gradually reduces its tackiness so that after some months, the tape may no longer be useful for its intended purposes. U. S. Patent No. 3,740,366 concerns a class of pressure-sensitive adhesives which is much broader than and includes the acrylate copolymer of Re. 24,906. It says that if such adhesives are crosslinked by a compatible polyvalent metal compound, the shear strength is improved without loss of tackiness and that the adhesive coating becomes resistant to alkaline solutions. Among the large number of polyvalent metal compounds suggested in the patent is chromium acetate.

While the pressure-sensitive tape adhesives of U. S. patents No. 2,925,174; 3,284,423 and 2,973,286 provide good electrical insulating properties, those of the working examples of No. 3,740,366 do not because they are coated from emulsions which necessarily contain dispersing agents such as the sodium salts of the higher fatty acid sulfates. Not only are those dispersing agents harmful to electrical insulating properties, but they also tend to corrode metal conductors to be insulated. The polymerization initiators used in No. 3,740,366 such as ammonium or potassium persulfate are likewise harmful to electrical insulating properties.

No. 3,740,366 does suggest that the polymers could be prepared in organic solutions. However, almost all of the named polyvalent metal compounds would tend to cause a solution of an acrylate copolymer to get before it could be coated out, unless the solution were so dilute as to be commercially impractical. To be commercially practical, a tape-coating solution should contain at least 15% solids and preferably more than 20%.

Disclosure of Invention

The present invention concerns what is believed to be the first crosslinked acrylate copolymer of the type disclosed in Re. 24,906, the crosslinking of which is easy to control and provides all of the following properties: good solvent resistance good wet-grab good internal strength at high temperatures unchanging properties during prolonged storage good electrical insulating properties. Like the crosslinked pressure-sensitive adhesives of patent Nos. 2,973,286 and 2,925,174, the tape adhesive of the present invention comprises an acrylate copolymer of monomers

(a) acrylic acid ester of non-tertiary alcohol, the molecules of which have from 1-14 carbon atoms, the average being about 4-12 carbon atoms, at least a major proportion of said molecules having a carbon-to-carbon chain of at least 4 carbon atoms terminating at the hydroxyl oxygen atoms, said chain containing at least about 1/2 the total number of carbon atoms in the alcohol molecule, said acrylic acid ester being further characterized as being per se polymerizable to a sticky, stretchable elastic adhesive polymer mass, and (b) at least one copolymerizable monoethylenic monomer selected from the group consisting of acrylic acid, methacrylic acid and itaconic acid, the total copolymerizable monomer (b) comprising about one-half to ten percent by weight of the total of said monomers (a) and (b). Best results have been attained using acrylate copolymers having inherent viscosities within the range of 1.2 to 1.6, although values of 1.0 or somewhat lower should also be useful. The pressure-sensitive tape adhesive of the present invention includes Cr³⁺ ion in an amount up to 0.2 percent by weight of the acrylate copolymer. To provide significant improvements in internal strength and solvent resistance, at least 0.002 weight percent of Cr ³⁺ ion should be present, and preferably at least 0.005 percent when the acid content of the acrylate copolymer is above three percent. Above about 0.05 percent Cr ³⁺ ion, the adhesive may have less wet-grab than is desirable for most uses. The adhesive layer of a tape of the present invention should be free from emulsifying agents and hence coated from solution, since it is not known how to obtain uniform coatings from emulsions without using dispersing agents which would be harmful to electrical insulating properties. In order to combine the Cr ³⁺ ion with the acrylate copolymer, it should be added to the coating solution as a compound which is soluble in the acrylate copolymer. Such compounds which have been used in the practice of this invention include chromium octoate, chromium acetate, chromium oleate, chromium neodecanoate and chromium naphthenate. While the manner in which the Cr ³⁺ ion achieves the crosslinking is not clearly understood, it is believed that it forms an ionic or salt bridge between acid groups. The crosslinking appears to proceed to virtual completion at times and temperatures that would have been used to drive off the solvent in the absence of any crosslinking agent. The degree of insolubility attained is surprisingly high in view of the fact that the number of Cr³⁺ ions may be only about 1/100 of the number of carboxylic acid groups in the copolymer. While there is only minor benefit in doing so, the novel adhesive may be combined with a small amount of a highly reactive polyfunctional epoxide. However, to avoid noticeable loss of wet-grab, the amount of the epoxide should be less than that used in U. S. Patent No. 2,925,174, i.e., less than 0.4 equivalent of the epoxide and preferably less than 0.1 equivalent of the epoxide per acid equivalent of the acrylate copolymer. It is believed that the Cr³⁺ ion catalyzes reaction between the epoxy groups and the acid groups of the acrylate copolymer in addition to bridging others of the acid groups. When the adhesive includes epoxide, some crosslinking continues during storage at room temperature, whereas in the absence of epoxide, no continuing crosslinking has been noticeable. As in 2,925,174, the polymerizable mixture of nontertiary alkyl ester monomer (a) and the copolymerizable acid monomer (b) may include small amounts of other copolymerizable monomers such as ethyl acrylate, vinyl chloride, vinyl acetate, various methacrylic esters, butadiene, isoprene, diallyl phthalate and acrylamide in such limited amounts as do not produce drastically altered properties. In preparing the acrylate copolymer, best results for the present invention are obtained if the amount of the copolymerizable acid monomer (b) is within the range of 1 to 6 percent of the total weight of the copolymerizable monomers. Below 1% , the resultant adhesive tends to have lower internal strength whereas above 6%, it tends to have reduced wet-grab.

In the absence of additives, adhesive layers of the invention are highly transparent. Inert fillers and pigments may be added for such purposes as to provide color and/or opacity, or to increase firmness. If the tape is to be exposed to certain solvents, the backing and primer coating should also be resistant to those solvents. Backing members which are resistant to most common solvents include biaxially-oriented polyethylene terephthalate, glass cloth, polytetrafluoroethylene, polyimide and metal foils.

The need for low-adhesion backsizes or low-adhesion liners with the novel pressure-sensitive adhesive tapes to allow the tape to be unwound more readily from roll form depends to a large extent on the specific adhesive formulation, since relatively larger percentages of the copolymerizable acid monomer (b), e.g., about 6-10 percent of the acrylate copolymer, provide adhesives which tend to become tenaciously bonded to another surface on prolonged contact therewith, while acrylate copolymer adhesives produced with smaller percentages of the monomer (b) are more easily removed.

Tapes of the present invention were tested as follows : Solubility A strip of the tape is immersed in a bottle of acetone. After 3 days at room temperature, the tape is removed and placed in an oven at 65°C overnight, cooled and weighed to determine the loss. In some cases, the same test is performed after holding a roll of the tape in an oven for 96 hours at 65°C.

Electrical Corrosion Test Cellulose acetate cloth is wrapped around a nonconductive panel and two #32-gauge (0.2-mm diameter) copper wires are wrapped over the cloth so as to be spaced 0.63 cm. The tape to be tested is adhered by its own adhesive to the cloth over the wires and rolled down firmly with a rubber roller. The test specimen is suspended in a sealed container maintained for 20 hours at 49°C and 95% relative humidity while 250 volts DC is applied across the wires. The breaking strength of the positive wire is measured and compared either to an untested piece of the wire or to the negative wire. The latter is preferred because the negative wire Is never corroded by the test and tends to have the same breaking strength as the positive wire before testing, assuming the two specimens were adjacent in the original spool.

Shear Strength at 65°C

The test tape is slit to 1.27-cm width and adhered at the edge of a stainless steel plate by its own adhesive coating to provide a bonded area 1.27 cm square. The tape is pressed into contact with the plate under the weight of 4 passes of a 2040-gram hard-rubber roller and held in an oven at 65°C for 10 minutes before testing at that temperature. With the steel plate in the vertical position, a 500-gram mass is suspended from the adhesive tape and the time to failure is noted. When the test is discontinued without failure, the time until discontinuance is reported with a "greater than" ( > ) sign.

Peelback Adhesion Test At 21°C the test tape is adhered by its own adhesive to a stainless steel plate using four passes of a 2040gram hard-rubber roller and allowed to dwell for 10 minutes. The free end of the tape is then peeled back at an angle of l80° at the rate of 30 cm/minute.

Best Mode for Carrying Out the Invention In the following examples, all proportions are by weight, as are proportions disclosed above except where noted.

Example 1 A copolymer of 98 parts of isooctyl acrylate and 2 parts acrylic acid was prepared as described in Re.24, 906 using ethyl acetate and heptane in 1:2 ratio as solvents and azobisisobutyronitrile as catalyst. The inherent viscosity of the copolymer was 1.51. The product solution contained 24.4% solids and had a coatable viscosity of 5500 cps at 25°C.

Added to the product solution was a pigment dispersion comprising titanium dioxide plus a minor proportion of polyterephthalate aldazine which had been ball milled overnight in solvens . The resultant composition (here called "Composition A") comprised: Parts

To biaxially-oriented polyethylene terephthalate film having a thickness of 25 micrometers was applied a solution of butadiene/acrylonitrile rubber, liquid bisphenolA-type epoxy resin and polyamide resin followed by strongly irradiating the ultra-thin dried coating with ultraviolet light as disclosed in U. S. Patent No. 3,188,266 to provide an adhesion-promoting primer coating. To lengths of this primed film were applied coatings of Composition A, some containing various amounts of chromium octoate and a diepoxide, namely, 3, 4-epoxycyclohexylmethyl-( 3, 4-epoxy) cyclohexane carboxylate, as indicated in Table A. The coatings were dried in an air-circulating oven at times and temperatures sufficient merely to drive off the solvents, and the resultant tapes were wound upon themselves in roll form. The dried coating weight of the pressure-sensitive adhesive coating in each case was about 10 grains per 24 square inches (42 grams per square meter). Test results were as reported in Table A.

Example 2

Tapes similar to those of Example 1, except excluding pigment, were tested with results as shown in Table B.

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Example_3 Tapes were prepared as in Example 1 except using a copolymer of 94 parts isooctyl acrylate and 6 parts acrylic acid. Test results are reported in Table C.

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A number of the tapes of the above examples have been subjected to the Electrical Corrosion Test. In every case the breaking strength of the positive wire after the test was equal to the original breaking strength within the 2% accuracy of the test, indicating no measurable degree of corrosion.

A number of the tapes of the above examples were tested for electrical insulating resistance under ASTM D 1000/76. In every case the value exceeded 10⁶ megohms, the limit of the testing instrument.

Claims

1. A pressure-sensitive adhesive tape, the adhesive layer of which comprises a copolymer of (a) acrylic acid ester of non-tertiary alcohol, the molecules of which have from 1-14 carbon atoms, the average being about 4-12 carbon atoms, at least a major proportion of said molecules having a carbon-to-carbon chain of at least 4 carbon atoms terminating at the hydroxyl oxygen atoms, said chain containing at least about one-half the total number of carbon atoms in the molecule, said acrylic acid ester being per se polymerizable to a sticky, stretchable elastic adhesive polymer mass, and (b) at least one copolymerizable monoethylenic monomer selected from the group consisting of acrylic acid, methacrylic acid and itaconic acid, the total copolymerizable monomer (b) comprising about one-half to ten percent by weight of the total of said monomers

(a) and (b), characterized by the feature that: the adhesive layer includes Cr ³⁺ ion in an amount up to 0.2 percent of the weight of the copolymer.

2. A pressure-sensitive adhesive tape as defined in claim 1, further characterized by the feature that: the amount off Cr³⁺ ion is 0.002 to 0.05 percent of the weight of the copolymer.

3. Method of making a pressure-sensitive adhesive tape comprising the steps of (1) dissolving in solvent a copolymer of (a) acrylic acid ester of non-tertiary alcohol, the molecules of which have from 1-14 carbon atoms, the average being about 4-12 carbon atoms, at least a major proportion of said molecules having a carbon-to-carbon chain of at least 4 carbon atoms terminating at the hydroxyl oxygen atoms, said chain containing at least about 1/2 the total number of carbon atoms in the molecule, said acrylic acid ester being per se polymerizable to a sticky, stretchable elastic adhesive polymer mass, and (b) at least one copolymerizable monoethylenic monomer selected from the group consisting of acrylic acid, methacrylic acid and itaconic acid, the total copolymerizable monomer (b) comprising about one-half to ten percent by weight of the total of said monomers (a) and (b) to provide a solution of coatable viscosity, (2) coating the solution onto a solvent-resistant backing member and (3) drying the coating, characterized by the feature that in step (1) a Cr ³⁺ salt is dissolved in an amount providing up to 0.2 percent of the weight of the copolymer.

4. Method as defined in claim 3, further characterized by the feature that the Cr ³⁺ salt is chromium octoate, chromium acetate, chromium oleate, chromium neodecanoate and/or chromium naphthenate.

5. Method as defined in claims 3 or 4, further characterized by the feature that in step (1) also dissolved is a polyfunctional epoxide in an amount providing less than 0.1 epoxide equivalent per acid equivalent of the copolymer of monomers (a) and (b).