Ink and dirt behavior in pulping after artificial aging of cold-set offset printed newspapers in different humidity conditions

• Effect of relative humidity (RH) on ink release from aged newspapers was studied.
• Increase in RH results in lower formation of microscopic ink particles in pulping.
• Increase in RH results in dirtier pulp after pulping.
• Increase in RH results in higher ink content bound to fibers after pulping.
• RH control during artificial aging tests and storage of recycled paper is discussed.

Although moisture has a decisive role in the permanence of cellulose and unprinted paper, its role in ink release from the surface of printed recovered papers remains unclear. This work aimed to determine whether various relative humidity levels in the artificial aging test affect ink release from the surface of the paper and its fibers when pulping cold-set offset printed newspapers in alkaline pulping conditions. The results showed that relative humidity variations from 30% to 70% negligibly affected ink release at 60 °C. However, compared with these lower humidity levels, exposure to 90% relative humidity for at least 25 h resulted in a higher ink content bound to fibers, a greater formation of visible dirt specks after pulping and a smaller formation of microscopic ink particles. In practical applications, the accurate estimation of climate effects during the storage and the transportation of recycled papers requires that humidity conditions are taken into account while performing artificial aging tests with paper products.