Asessing the effect of emulsion viscosity on the adesive properties of model waterbone acrylic pressure sensitive adhesives

Abstract

In recent years, increasing social and political awareness, coupled with the tightening of worldwide environmental legislation, has forced coating industries to decrease levels of pollutant substances released to the atmosphere. Solvents from the coatings industry are considered to be volatile organic substances. For this reason there is an increasing tendency to move away from conventional solvent-borne coatings to those that use water as the fluid vehicle [1]. One of the more important film-forming pigment binders used in water borne coatings is that class of resins known as acrylics. Acrylic polymers are widely used as pressure-sensitive adhesives (PSAs). An emulsion is a mixture of two immiscible liquids, one the dispersed phase, existing as discrete droplets dispersed throughout the other, the continuous phase. In the case of emulsion polymerization the continuous phase is comprised mainly of water and is therefore termed the aqueous phase. Emulsion polymer is actually a dispersion of solids, or semi-solids, polymer particles in a continuous aqueous phase. Acrylic polymer emulsions give excellent weather resistance, water and alkali resistant films due to the main polymer chain’s carbon-carbon bonds. However, the elasticity and abrasion resistance of acrylic polymer emulsions are inferior [2]. It has long been recognized that the reaction pathway in emulsion. Polymerization plays a decisive role in determining the particle morphology and consequently the emulsion polymers properties. Zhao and his co workers carried out a study of structured polymer latex particles which were prepared by a swelling emulsion polymerization process, in which the initial particles are first swollen by ethylenically unsaturated monomers and the polymerization of the latter is then carried out [3]. Latex particle size distribution (PSD) is probably the most important variable in designing low viscosity, concentrated aqueous polymer dispersions. Asua and colleagues prepared the adhesive and rheological properties of model acrylic pressuresensitive adhesives (PSA) films prepared from high solid emulsions with different particles sizes and distributions have been investigated with a customized probe tack apparatus. The results showed that a broad PSD is favored over a narrow a PSD and that the present of large particles is recommended [4]. The molecular weight directly influences the final viscosity of the resins. The viscosity of the polymer is further strongly influenced by the rigidity of the sequences. At identical molecular weights and at equal concentrations, a solution of polymethyl methacrylate for example will be much more viscous than a solution of polybutyl acrylate. Another factor strongly influencing viscosity is the formation of associations. In non-polar solvents, the carboxyl groups have a strong tendency to form associations between one another via hydrogen bonding. These hydrogen bonds markedly increase the viscosity. The degree of polymerization is proportional to the concentration of monomers and inversely proportional to the initiator concentration. As a general rule, the lower the concentration of monomer, the lower the molecular weight of the polymer. The higher concentration of initiator, the lower the final molecular weight obtained [5].