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Stamping out poor recycling: one drawback of adhesives is the way that they can cause problems for the paper recycling process. A team of researchers, led by S Jong, show how the US postal service have spearheaded a programme to fix the problem. (Resins).

Article Excerpt
There has been a growing concern that the pressure sensitive adhesives (PSAs) currently used in many products cause the `stickles' problem in the paper recycling process and are not environmentally friendly.

Most of the earlier repulpable adhesives were water-soluble PSAs that accumulated in the recycling water stream and exhibited poor moisture resistance and low adhesion performance. The US Postal Service (USPS)

took the initiative to spearhead a program to develop PSAs for use in the US postage stamp.

Extensive industrial research has been devoted to the development of a repulpable PSA. The PSA manufacturers have collaborated with paper producers, release liner companies, converters, paper recycling mills and testing labs for the development of a benign PSA.

MARKET VARIATIONS

Both emulsion and solvent-based acrylics, rubber and hot melt adhesives are among the different types of pressure sensitive adhesives currently on the market.

Acrylic PSAs are more popular because of the combination of desirable performance properties and low cost. Monomers used in the production of acrylic PSAs can be categorised into three classes. The first two classes are characterised by their respective [T.sub.g] values.

Soft monomers, characterised by low [T.sub.g] values, include n-butyl acrylate, 2-ethyl hexyl acrylate and iso-octyl acrylate. These monomers are longer chain alkyl acrylates and exhibit low water solubility (in other words high hydrophobicity).

Hard monomers, characterised by high [T.sub.g] values, include styrene, methyl methacrylate, ethyl acrylate, and methyl acrylate.

The third class is the functional monomer with various functional groups, such as carboxyl or hydroxyl. Examples of functional monomers are acrylic acid and hydroxyethyl acrylate, which are highly soluble.

The hydrophobicity of the hard monomer depends on the monomer structure. Methyl acrylate and ethyl acrylate, which are short chain alkyl acrylates, have higher water solubility. The adhesion performance of acrylic PSAs is controlled by the composition, molecular weight and crosslinking density of the polymers.

Typically, you adjust the soft monomer, hard monomer and functional monomer to obtain the desirable adhesion property balance for a specific application.

RECYCLING PROCESSES

Recycling paper mills usually have four basic unit operations: pulping, screening, centrifugation and flotation. When PSAs are present in the...