Primer composition for silicone pressure-sensitive adhesives

Details Primer composition for silicone pressure-sensitive adhesives Primer composition for silicone pressure-sensitive adhesives

2001-03-15

2003-05-06

Dawson, Robert (Department: 1712)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S476000, C525S477000, C524S588000, C528S015000, C528S026000, C528S026500, C528S031000, C528S032000

Reexamination Certificate

active

06559246

ABSTRACT:

This invention relates to a primer composition for silicone pressure-sensitive adhesives, and more particularly, to a primer composition for use in a pressure-sensitive adhesive tape for providing a firm bond between a plastic film and a silicone pressure-sensitive adhesive layer thereon.
BACKGROUND OF THE INVENTION
Prior art heat resistant pressure-sensitive adhesive tape typically includes a plastic film backing such as polyester film or polyimide film on which a heat resistant, silicone pressure-sensitive adhesive is applied to form an adhesive layer. The pressure-sensitive adhesive tape is generally wound into a roll form. When the adhesive tape is forcedly unraveled on use, the silicone pressure-sensitive adhesive layer can sometimes be stripped from the backing. Also when the adhesive tape is cut, it sometimes happens that only the backing is torn off, but the silicone pressure-sensitive adhesive layer remains uncut and becomes stretched with strings being drawn. Sometimes the adhesive layer is partially separated apart from the backing.
Such problems arise on use of the pressure-sensitive adhesive tape because the adhesion between the plastic film backing and the silicone pressure-sensitive adhesive is not fully tight. One common solution employed heretofore is to apply a carbon functional silane-based primer composition to a film backing as an adhesion improver, then applying a silicone pressure-sensitive adhesive thereon, for thereby improving the adhesion between the film backing and the silicone pressure-sensitive adhesive. However, the adhesion achieved thereby was not satisfactory.
A number of primer compositions using organopolysiloxane have been proposed for helping silicone pressure-sensitive adhesives firmly bond to plastic films. For example, JP-B 54-44017 discloses a primer composition comprising a diorganopolysiloxane having hydroxyl groups at both ends of its molecular chain, an organohydrogenpolysiloxane, and a tin salt of organic carboxylic acid. JP-B 6-39584 discloses a primer composition comprising a diorganopolysiloxane having hydroxyl groups at both ends of its molecular chain, an organohydrogenpolysiloxane, a vinyl and/or acrylic copolymer containing hydrolyzable silyl groups, and a tin salt of organic carboxylic acid.
These primer compositions, however, sometimes fail to provide a satisfactory bond. For example, in the event of addition curing type silicone pressure-sensitive adhesives which are recently on use and to be cured at lower temperatures, it is more difficult to provide a firm bond between the pressure-sensitive adhesive and the backing. This is true especially when the backing is of polyimide film having better heat resistance. It is thus desired to have a primer composition capable of forming a tight bond therebetween.
Therefore, an object of the invention is to provide a primer composition capable of forming a tight bond between various film backings and silicone pressure-sensitive adhesives.
SUMMARY OF THE INVENTION
The invention pertains to a primer composition comprising an alkenyl group-containing organopolysiloxane, an organohydrogenpolysiloxane, a platinum base curing catalyst, and a retarder. It has been found that the objects are achieved when the alkenyl group-containing organopolysiloxane is a diorganopolysiloxane having an average degree of polymerization of 500 to 2,000 and alkenyl groups at both ends of its molecular chain in an amount of 0.001 to 0.005 mol per 100 g of the diorganopolysiloxane. When various plastic film backings are treated with this primer composition, and silicone pressure-sensitive adhesives are applied thereto, very tight bonds are established between the backings and the adhesives. This is true even when the plastic film backings are polyimide films which are believed difficult to establish a tight bond with silicone pressure-sensitive adhesives, and also when the adhesives are addition curing type silicone pressure-sensitive adhesives. As a consequence, satisfactorily heat resistant pressure-sensitive adhesive tapes are obtained.
Accordingly, the invention provides a primer composition for silicone pressure-sensitive adhesives, comprising
(A) a diorganopolysiloxane having alkenyl groups at both ends of its molecular chain and an average degree of polymerization of 500 to 2,000, the amount of alkenyl groups being 0.001 to 0.005 mol per 100 g of the diorganopolysiloxane,
(B) an organohydrogenpolysiloxane having at least two hydrogen atoms directly attached to silicon atoms in a molecule,
(C) a platinum base curing catalyst, and
(D) a retarder.
In one preferred embodiment, the primer composition further includes
(E) an organopolysiloxane having at least one epoxy group-containing organic group, at least one hydrogen atom, and at least one alkoxysilyl group-containing alkyl group, each directly attached to a silicon atom, in a molecule and/or
(F) at least one additive selected from among a fatty acid, an acid anhydride resulting from intermolecular condensation thereof, and an intramolecular acid anhydride thereof.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the primer composition for silicone pressure-sensitive adhesives according to the invention, component (A) is a diorganopolysiloxane having alkenyl groups at both ends of its molecular chain and an average degree of polymerization of 500 to 2,000, the amount of alkenyl groups being 0.001 to 0.005 mol per 100 g of the diorganopolysiloxane. The preferred diorganopolysiloxane is of the following general formula (1).
Herein R
1
is alkenyl, R
2
is a substituted or unsubstituted monovalent hydrocarbon group, “a” is an integer of 1 to 3, and n is from 500 to 2,000.
Preferably R
1
stands for alkenyl groups of 2 to 8 carbon atoms, such as vinyl, allyl and butenyl, with vinyl being most preferred. R
2
stands for substituted or unsubstituted monovalent hydrocarbon groups of 1 to 16 carbon atoms, especially 1 to 8 carbon atoms, for example, alkyl groups such as methyl, ethyl, propyl, butyl and octyl, cycloalkyl groups such as cyclohexyl, alkenyl groups such as vinyl and allyl, aryl groups such as phenyl and tolyl, and aralkyl groups such as benzyl and phenethyl, and substituted groups in which some or all of the hydrogen atoms on the foregoing groups are replaced by halogen atoms, cyano groups or the like, for example, haloalkyl groups such as chloropropyl and trifluoropropyl. Of these, methyl and phenyl are preferred for the industrial use.
The diorganopolysiloxane having alkenyl groups at both ends of its molecular chain represented by formula (1) has an average degree of polymerization of 500 to 2,000 and alkenyl groups in an amount of 0.001 to 0.005 mol per 100 g of the diorganopolysiloxane. Preferably, the average degree of polymerization is from 1,000 to 2,000 and the amount of alkenyl groups is from 0.001 to 0.0025 mol per 100 g of the diorganopolysiloxane. With an average degree of polymerization of less than 500, the primer composition in the cured state is likely to strip from the film surface, failing to exert its effects. An average degree of polymerization of at least 500 prevents stripping and ensures primer effects. With an average degree of polymerization of 1000 to 2,000, the primer composition forms a more tight bond. An average degree of polymerization of more than 2,000 aggravates the adhesion between the pressure-sensitive adhesive and the backing. If the molar amount of alkenyl groups is less than 0.001 mol per 100 g of the diorganopolysiloxane, the primer composition will not cure. In excess of 0.005 mol/100 g, the primer effects are restrained.
The organohydrogenpolysiloxane (B) serves as a crosslinking agent to component (A) such that hydrogen atoms directly attached to silicon atoms (SiH groups) in component (B) undergo hydrosilylation reaction with alkenyl groups in component (A) for curing. The organohydrogenpolysiloxane is not critical as long as it has at least two hydrogen atoms directly attached to silicon atoms (SiH groups) per molecule. Illustrative examples are represented by the following general formulas (2) to (4).
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