Dissolved substances from coated broke

A paper machine made 50 g/m2 industrial toweling grades from a furnish using different grades of recovered paper. An acid-curing urea-formaldehyde (UF) wet-strength resin was used, and this necessitated a wet-end pH of about 4, which was achieved by addition of sulfuric acid. Any calcium carbonate in the furnish thus had to be dissolved, leading to elevated levels of dissolved calcium and sulfate ions.

Over the course of the sampling period (shown in top graph), the carbonate content of furnish varied considerably, leading to a substantial increase in conductivity and in dissolved organics as measured by chemical oxygen demand (COD). The single pass retention (SPR) of the total furnish and fines fraction varied in an unpredictable way despite the use of a polyacrylamide retention aid (Bottom graph).

The main adverse consequence of the buildup of dissolved solids was a severe drop in the total retention (TR) of the UF wet strength. This necessitated a doubling of the resin dose to maintain the toweling's wet-strength specification. This decline in resin retention was caused by a number of effects stemming from the increase in dissolved solids, including the following:

Blocking of adsorption sites by calcium ions

Reduced resin cationicity from sulfate and anionic organics

Drop in fines single pass retention.