Emulsion adhesives are often inferior to solvent based adhesives. For example at the same peel level emulsion adhesives tend to have more than then ten times lower shear strength. It is an object of the invention to solve this problem with emulsion adhesives so they can compete with solvent systems.

Previously to solve this problem formulators have attempted to increase Tg of the polymer or use covalent bond cross-linking within the emulsion. However increasing Tg only increases the cohesiveness within the individual particles, with little effect on the macro structure of the whole adhesive film, it also tends to decrease peel. Using covalent cross- linkers adversely effects emulsion stability and requires very high temperatures to cure. It to reduces peel strength.

The applicant has discovered that a latex contains hydrogen bonding donors such as carboxy and/or amino groups and hydrogen bonding acceptors such as carboxy can be made in a conventional fashion and when wet behaves conventionally. However when the latex dries to form a film a very effective H-bonding is created to forms links between particles that dramatically enhances film cohesive strength.

Therefore broadly in accordance with the invention there is provided a pressure sensitive adhesive comprising an acrylic latex of dispersed polymer in an aqueous phase comprising at least one H donor optionally selected from carboxylic acid and/or amino groups with at least one free hydrogen and at least one H acceptor optionally a carboxy group, characterised in that the H donor and H acceptor are located in the latex on different polymer particles to be capable of forming interparticle H bonds there-between and such that when the latex is applied to an substrate to form an adhesive film thereon the film has enhanced shear strength for a given peel.

Without wishing to be bound by any mechanism it is believed that the invention shifts attention from intra-particle properties to focuses on inter-particle interactions that to a large extent govern a films cohesive strength. H-bonding interactions do not adversely affect emulsion stability and do not require any curing conditions.

The emulsion polymers of the present invention have at a similar peel level several times higher shear than a good solvent system. The latices can be made normally and no extra cost.

A hydrogen bond ("H-bond") occurs when a hydrogen atom is linked, preferably co- linearly, in between two strongly electronegative atoms. An H-bond may be formed from one electronegative atom that has a hydrogen atom covalently attached thereto (this electronegative atom is denoted the"H-donor") and another electronegative atom (denoted the"H-acceptor"). In the formed H-bond the H-donor has a large partial negative charge so in that the hydrogen atom is closer and more tightly bound to the H- donor atom. The H-acceptor though less electronegative than the H-donor is still sufficiently electronegative to have a partial negative charge thereon to attract the hydrogen and thus form the H-bond.

In PSAs of the present invention preferred H-bonds are those formed where: the H-donor atom is a fluorine, nitrogen or oxygen atom, more preferably nitrogen or oxygen, for example those N or O comprised in an amino group with at least one free hydrogen or a hydroxy group, and/or the H-acceptor atom is a nitrogen or oxygen atom, for example O that comprised in a free carboxyl group.

Conveniently PSA of the present invention comprise a species formed by at least two H bonds which together with the groups to which they are attached form a ring, for example a carboxy group bound to an amide group, H-bonded as shown in Figure 1: In accordance with another aspect of the present invention there is provided a pressure sensitive adhesive composition that comprises at least one <-RNCONH group hydrogen bonded to at least one carboxylic acid group.

Many other variations embodiments of the invention will be apparent to those skilled in the art and such variations are contemplated within the broad scope of the present invention.

Further aspects of the invention and preferred features thereof are given in the claims herein.

Example The present invention will now be described in detail with reference to the following non limiting example which are by way of illustration only.

A acrylic latex was prepared from polymers containing a cyclic amide groups and a carboxylic acid group as described and H bonding was observed between particles.

The latex was used as a PSA by applying as a film to an adherend and it was observed that the shear was increased from (kg/1/4 in2) 1.5 to over 200 hours.