When precipitated or synthetized calcium carbonate (PCC) is prepared in the presence of cellulosic fibers, a large fraction of the PCC attaches partially onto and even into the fibers, forming a PCC-fiber composite. As the composite properties in, for example, a papermaking process are affected by the PCC morphology, the PCC-fiber composite properties can be engineered and optimized via PCC morphology control. This may be of special interest for micro- and nanofibrillated celluloses because when the composite is formed from these materials, it may be possible to deal with problems associated with the effects of filler on the mechanical properties of paper, filler retention, dewatering behavior and redispersion of micro- and nanofibrillated cellulose.

The aim of this study was to develop methods to control the morphology and growth of the mineral phase and the enrichment of PCC onto different fiber size fractions and onto different parts of the fiber. The PCC-fiber composite was prepared on different fibers (microcellulose, microfibrillated cellulose and bleached chemithermomechanical pulp) by carbonization of a suspension of lime (lime milk) in the presence of a fiber suspension in an open batch reactor with carbon dioxide (CO2). PCC-chemithermomechanical pulp composite was fractionated in order to determine the affinity of the mineral enrichment in the various fractions and to further characterize the properties of the fractions and composite using e.g. laboratory handsheets.

The morphology of the precipitate was successfully controlled by altering CO2 feed and fiber consistency, linked with the targeted (PCC) ash content of the composite material via the initial calcium hydroxide (Ca(OH)2) concentration. The differences in morphology were ascribed to differences in the ratio of calcium to carbonate ions ([Ca2+]/[CO3 2-]). Although the PCC showed an affinity to all the fibers investigated, it was found to enrich onto the finer fiber fractions and onto cellulose-rich areas of individual fibers. The anionic surface charge of the fiber fraction before precipitation showed a good correlation with the ash content after precipitation. Whether or not this indicated that the fiber chemistry influences the PCC affinity is further discussed. In the laboratory handsheets, the nano-PCC on the fiber surfaces had a debonding effect, but refining the CTMP prior to the precipitation or strengthening agents could be used to restore the strength of the sheets.