Лако-красочные материалы — производство

T. M. Goulding

Consultant, Johannesburg, South Africa

C. INTRODUCTION

Pressure-sensitive or permanent-tack adhesives are, as their name implies, adhesives that remain sticky even when dried or cured. This means that they are capable of bonding to surfaces simply by the application of light pressure. This makes them arguably the most convenient products available today from the end user’s viewpoint and undoubtedly, accounts for the success they enjoy. Although figures are hard to come by, a survey by Business Trend Analysts quoted in the June 1990 issue of Adhesives Age shows that pressure-sensitive adhesives grew from 38% of total adhesive sales in the United States in 1980 to 44.6% in 1988, at an annual rate of 12%, to reach a sales value of $4.9 billion in 1989.

Tack is a word used to describe various phenomena, including wet tack, which is the ability of an adhesive to form a bond while still wet; green tack, which is the ability of certain polymers, specifically rubbers, to bond to themselves for several hours after drying, even though the surfaces do not feel sticky; and pressure-sensitive tack, which is the phenomenon of importance to this section. This relates to the ability of a dried film to bond tenaciously to most surfaces under light pressure. As pressure is increased, the bond improves. The classic theory of tack is that it arises from the presence of a two-phase system in which an elastic continuous phase provides the strength while a disperse phase acts as a viscous liquid that wets and adheres to the surface. Although this appears to be the dominant mechanism in the older rubber-resin systems, however, many modern systems do not rely on this apparently incompatible two-phase system. Acrylics, for exam­ple, can produce aggressive tack from a single component. Thus tack is also believed to stem from the viscoelasticity of many polymers, allowing them to conform to the substrate to be adhered and ‘‘wet’’ it even in the dry state. It follows that a fundamental requirement for tack is a glass transition temperature substantially below the application temperature to permit the necessary degree of flow.

Pressure-sensitive adhesives fall into three broad product categories: water based, solvent based, and hot melt. Application areas tend to overlap, and all three types can be used in most of the application areas. Despite this overlap, tapes tend to be produced from solvent-based adhesives, while water-based adhesives are preferred for label stock. Hot melts are used in both applications. Pressure-sensitive tapes for a variety of uses, such as masking, packaging, and insulation, are the largest application area, followed by self-adhesive labels. Although these applications appear outwardly similar, in fact there are fundamental differences. With tapes the adhesive fills the major role, ensuring adequate adhesion and requiring special properties, which may include high dielectric strength, heat resistance, or low toxicity. In labeling applications the major demands are on the backing, which needs the right lay-flat or curl properties and ease of cutting and printing, with relatively few demands on the adhesive. For certain applications, the adhesive may have to retain flexibility and tack at temperatures down to — 20° C, or be easily removable. With tapes the adhesive is usually applied directly to the backing, while label adhesives are usually applied to the release paper and subsequently transferred to the backing. Other pressure-sensitive applications include self-adhesive floor tiles, adhesives for decor papers and flypapers, gloss lamination, disposable diapers and other personal hygiene products, and temporary assemblies.