Coating: influence of latex properties on the formation of white pitch.

APPLICATION: This study increases our understanding of the behavior of synthetic binders during disintegration of coated broke and the formation of white pitch.

White pitch is a well-known problem at mills producing coated paper grades. During the past decades, several studies sought to reveal the mechanisms behind white pitch deposition. The main problem so far, in studying the formation of white pitch, has been the absence of good analytical methods for quantifying binder-rich particles in coated broke. However, the use of flow cytometry (FCM) now enables us to quantify the amount of binder-rich particles and to investigate the aggregation of such particles.

In this study, we characterized three different styrene-butadiene latexes by determining their film-tackiness at different temperatures. We used the measured tackiness to predict the formation of binder-rich particles in the coated broke as a function of disintegration temperature. The amount of binder in the coating color had a huge effect on the formation of binder-rich particles.

We also investigated the disintegration time, pH, and presence of wood pitch. We found out that the amount of binder-rich particles increased with disintegration time. Without wood pitch, pH had only a minor effect on the amount of binder-rich particles in the filtrate of coated broke. However, if wood pitch was present, binder-rich particles deposited extensively onto fibers as the pH decreased from 8.5 to 5.5. View this paper online at http://www.tappi.org/index.asp?pid=32229&ch=1

Lari Vahasalo and Bjarne Holmbom are with Abo Akademi University, Process Chemistry Centre, Porthansgatan 3, FIN-20500 Turku/Abo, Finland. Email Vahasalo at [e-mail] and Holmbom at [e-mail].