Pressure-sensitive adhesives and self-adhesive films using...

Details Pressure-sensitive adhesives and self-adhesive films using... Pressure-sensitive adhesives and self-adhesive films using...

1999-04-21

2001-04-10

Zirker, Daniel (Department: 1771)

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

Synthetic resins

Ethylenically unsaturated reactant admixed with either...

C525S035000, C428S3550AC

Reexamination Certificate

active

06214931

ABSTRACT:

The present invention relates to pressure-sensitive adhesives, i.e. adhesives whose adhesion to a given substrate is obtained after simple application of light pressure. More particularly, the present invention relates to such pressure-sensitive adhesives, which are in aqueous emulsion form and have improved remanence of the adhesive properties when the said adhesives are applied to various substrates, in particular films of polyvinyl chloride, polyethylene, polypropylene, polyethylene terephthalate, paper, glass, etc. The present invention also relates to self-adhesive films and to other similar articles using the said adhesives.
Pressure-sensitive adhesives intended for plasticized polyvinyl chloride were originally obtained by radical polymerization in organic solvents such as ethyl acetate, toluene or hexane, or alternatively in a combination of several of these solvents. In view of the nature of the parameters governing radical copolymerization in solvents, leading to low molecular masses, these products necessarily had to undergo considerable post-crosslinking in order to have an equilibrium between the various adhesive properties, which are compatible with the requirements inherent to the very nature of plasticized polyvinyl chloride. This means, for example, that the internal cohesion of the product must be sufficient to be able to compensate for the adverse changes in the properties due to the migration of the polyvinyl chloride plasticizers.
For obvious reasons of environmental protection, the pressure-sensitive adhesives obtained as an aqueous emulsion are nowadays preferred to those obtained in solution in an organic solvent. Thus, U.S. Pat. No. 3,983,297 discloses a pressure-sensitive adhesive comprising, as main constituent, a copolymer prepared as an emulsion comprising (1) 94 to 98 mol % of at least one alkyl acrylate, where appropriate as a mixture with methyl methacrylate or vinyl acetate, (2) 2 to 6 mol % of acrylic acid and/or methacrylic acid, and (3) 0.002 to 0.05 mol % of a copolymerizable polyfunctional compound. Similarly, U.S. Pat. No. 3,998,997 describes a pressure-sensitive adhesive obtained by emulsion polymerization of a monomer system comprising (1) 50 to 60 parts by weight of 2-ethylhexyl acrylate, (2) 35 to 45 parts by weight of ethyl acrylate, (3) 1 to 3 parts by weight of acrylic acid, (4) 2 to 5 parts by weight of diacetophenone acrylamide or diacetone acrylamide and, where appropriate, hydroxypropyl (meth)acrylate, and/or maleic anhydride, and/or glycidyl (meth)acrylate and/or a polyfunctional vinylic crosslinking agent.
Japanese patent application published under No. 2-003,404 teaches the emulsion polymerization of a monomer mixture comprising:
(A) from 60 to 99% by weight of a (C
4
-C
8
)alkyl acrylate,
(B) from 0.2 to 10% by weight of a functional monomer chose from &agr;,&bgr;-unsaturated carboxylic acids and their anhydrides, 2-hydroxyalkyl (meth)acrylates and (methylol) (meth)acrylamides, and
(C) from 0 to 30% of another vinyl monomer,
in the presence of a water-soluble compound chosen from dihydrofuran, dihydropyran and methylcyclohexenedicarboxylic and norbornenedicarboxylic acids, in order to obtain a copolymer dispersion having a glass transition temperature of from −10 to −90° C.
In the case of the pressure-sensitive adhesives obtained in solution form, it is possible to obtain an acceptable compromise between the adhesion and the adhesive power (tack), on the one hand, and the cohesion, on the other hand, by crosslinking the product using, for example, metal chelates. This technique offers fairly wide flexibility and allows the properties of the product to be adjusted fairly easily to the desired level.
In the case of the pressure-sensitive adhesives obtained in emulsion form, the problem is considerably more complex since there is greater dispersity in the molar mass distribution; furthermore, the equilibrium of the product is also disrupted by the presence of a series of polymerization auxiliaries such as emulsifiers, which are external to the polymer chains, and which can behave, for example, as plasticizers. It is accepted in particular that the presence of long acrylic chains, which are characteristic of emulsion polymerization, is responsible for the lack of adhesion of these products.
If the length of the acrylic chains is reduced (for example by using a transfer agent during the emulsion polymerization), the adhesion is indeed improved (peel strength), but, in this case, the cohesion is lost (shear strength), since it is generally accepted that long chains are needed in order to obtain acceptable cohesion.
Many means have been mentioned in the literature to bring the cohesion of the pressure-sensitive adhesives obtained in emulsion form to the desired level while at the same time maintaining aggressiveness of contact (as defined below), at least equal to 7 N/25 mm approximately. One of the means most commonly used is the copolymerization of functionalized monomers such as N-methylolacrylamide. This monomer effectively makes it possible to increase the internal cohesion of the product, but at the expense of the other properties. Furthermore, these properties are greatly influenced by the heat treatment to which the copolymer has been subjected.
In addition, the presence in N-methylolacrylamide of certain impurities such as acrylamide or formaldehyde can also produce harmful effects, such as adverse increases in viscosity. Other means for increasing the cohesion, such as the use of functional silanes bearing a double bond (for example propyltrimethoxysilane methacrylate or vinyltriethoxysilane methacrylate), give rise to problems of degradation of the properties over time or alternatively can only be used in the context of a two-component system, which imposes practical constraints on the user that are often unacceptable.
Lastly, aqueous dispersions, used in particular in heat-resistant adhesive compositions or contact adhesives, are known from patent application EP-A-620,234, these dispersions being prepared by emulsion polymerization of a monomer, a homopolymer of which has a glass transition temperature (Tg) of greater than 30° C. and has only one ethylenic unsaturation in the molecule, and of a flexible monomer, a homopolymer of which has a glass transition temperature of less than 10° C. and at most has one ethylenic unsaturation in the molecule, the proportions of the two monomers being such that the glass transition temperature of the copolymer is between 5 and 25° C., in the presence of methacrylic anhydride as crosslinking monomer.
These aqueous dispersions, the water content of which is from 45 to 55% by weight, do not, however, afford pressure-sensitive adhesives which have the desired compromise of adhesion and cohesion properties. The state of the art can thus be summarized in the field of the pressure-sensitive acrylic adhesives obtained in emulsion form by the fact that, on account of the antagonistic effects exerted by long-chain acrylics on these two properties, it is difficult to obtain a good compromise between adhesion and cohesion in aqueous emulsion form.
The problem for the invention to solve thus consists in developing a pressure-sensitive adhesive which can be applied in particular to polyvinyl chloride, polyethylene, polypropylene, polyethylene terephthalate, paper, glass, etc. and which has both a cohesion which is sufficient to combat the effects of migration of the plasticizers, and an initial adhesion and tack which are sufficiently high, in order to retain an acceptable level of properties after ageing of the product. It is generally accepted that a product has correct accelerated ageing if the value of the adhesive properties, measured after the film has remained for 7 days in an oven at 60° C., remains equal to or greater than 60% of the values measured initially.
This problem is solved by the present invention in the context of a pressure-sensitive adhesive one-component system consisting of an aqueous dispersion of an acrylic ester copolymer obtained in the presence of a cros

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