Microparticle Retention Aid Systems in Mechanical Pulp Suspensions

Today's papermakers face the ever-increasing challenge of producing good quality paper while keeping prices low. This is achieved by using cheaper raw materials such as mechanical pulps and mineral fillers. To increase productivity, machine speeds have also increased. To meet these challenges, it is crucial that the papermakers are able to effectively retain mineral fillers and fiber fines while simultaneously improving dewatering rate. Microparticle retention aid systems, typically utilizing anionic colloidal silica and/or clay-based particles in combination with cationic polymers, have been developed to help deal with these challenges. Generally, microparticle retention aid systems have shown better retention and dewatering performance compared to the classical single or dual polymer retention aid systems. In the past decade, many variations of the microparticle system have been developed to further improve its performance. With respect to the silica-based particles, these include modifications to their physical and chemical structures. In the case of the clay-based particles, various methods have been devised to keep the suspension dispersed, either by mixing the clay-based particles with silica-based particles or synthetic metal silicates. Cationic microparticle systems are also available. Examples include cationic bentonite and fibrous alumina, which are used in combination with cationic or non-ionic polymers. A patent literature search on microparticle retention systems was conducted. It covered the period from 1990 to 2005. The various inventions are described in this paper.