Spare a thought for the paper recycler.

The evolution of printing over recent years has resulted in new printing processes as well as the development of the old. The associated ink developments have presented and continue to present challenges to the recycling technologist because the quality of the recycled product depends largely

on the success of the deinking, that is, removal of old ink from the pulp. This in turn is influenced largely by the structure and composition of the surface coating applied. New formulations often pose fresh challenges to the deinking technologist. The original problem of deinking fibre printed with simple mineral oil.-based news inks has drastically increased with the introduction of non impact inks, water-based gravure and flexo, conventional UV-cured inks and coatings, cationic UV-cured inks and coatings including water-based flexo, gravure and screen process, heal set and cold-set web offset news inks. Modern recycling plants must be able to deink all types of inks and each category has its own deinking peculiarities. Mixed office waste may increasingly be expected to contain new inks including high density laser and photocopy, digital offset, coloured laser and photocopy, water-based gravure, UV-cured, thermal printing, and inkjet inks both solid and liquid.

The nature of the ink contaminants that may be present in fibre furnishes can be grouped into four categories:

(i) News and magazine inks. The problems presented by these have been largely resolved in conventional recycling plants. tim most recent innovation being cold-set colour news inks.

(ii) Radiation curing inks and coatings arising from magazine covers and packaging products. The problems created by these arise from their tough film-forming characteristics and they require special treatment. (iii) Non impact inks. These are predominantly fused toner systems which include digital offset inks and which. like UV-curing inks, are difficult to break down.

(iv) Water-based inks. Paradoxically, although considered to be environmentally friendly during application. these are difficult to deink and were once considered a 'looming menace' for the paper recycler.

Some ink components relevant to the deinking process are summarised in table 1.

The nature of the problem

In conventional deinking ink is detached from the paper surface by mechanically repulping under alkaline conditions (pH 9.5 to 11.0), in the presence of surfactants. The sodium hydroxide used swells the cellulose fibres and aided by the deinking surfactants, the ink separates. Resin-based binders may undergo saponification or hydrolysis which assists ink removal. The chemical technology of deinking has been fully reviewed elsewhere (1). Under mechanical agitation in the pulper, the ink detaches and breaks up into particles of varying size. The ease of detachment depends upon the chemical and physical properties of the binder and in some polymerising inks becomes more difficult with aging. The work of adhesion between binder and base paper, and binder and deinking surfactant is an important factor. The particle size ranges produced are influenced additionally by the printing process and the base paper (Table 2).