Investigation into the Cause of the Mill 'Summer Effect' Reducing Deinked Newspaper Brightness, 1999 Recycling Symposium Proceedings

John K. Borchardt, Kirk H. Raney, Anne T. Coleman, Paul L. Tortorici, Kim C. Millspaugh and P. Gregory Shpakoff

Shell Chemical Company

Personnel working in newspaper deinking mills in the U.S. and Canada have reported the summer effect: a decrease in deinked pulp brightness, during the hot summer months. Three possible causes of the summer effect are: process water temperature, newspaper storage temperature prior to deinking, and newspaper storage time.

Statistically designed experiments in our laboratory have enabled us to identify the primary cause of the summer effect as being newspaper storage (aging) temperature. Even at fairly short storage times (less than a month), the summer effect can still be observed at high warehouse temperatures. Both in the presence and absence of the deinking surfactant, increased aging temperature significantly reduced brightness after pulping. The newspaper aging time also had an effect but it was less pronounced. Increased pulping time significantly reduced pulp brightness. Increasing process water hardness had a

modestly negative effect on brightness after pulping.

While use of a deinking surfactant increased pulp brightness, the presence of the surfactant did not reduce the brightness sensitivity to newspaper aging temperature. Both in the presence and absence of surfactant, the process variables having the greatest effect on pulp brightness after washing were newspaper aging temperature and process water hardness. Pulping time, newspaper aging time, and washer screen size had little effect on pulp brightness after washing.

The adverse effect of newspaper aging temperature on pulp brightness after washing is not due to formation of an excessive amount of small ink particles in paper aged at higher temperature. Observed results can be rationalized in terms of higher aging temperatures causing stronger ink adherence to the

cellulose fibers. The most obvious reasons for stronger ink adherence are deeper penetration of the ink into

the fibers, oxidation leading to some chemical bonding of the ink vehicle or binder resin to the fibers, and heat promoted evaporation of the ink vehicle. The greater amount of ink adhering to the fibers results in lower brightness both after pulping and after washing at the higher newspaper aging temperature.