Determination of Repulpability of Talc-Filled Biopolymer Dispersion Coatings and Optimization of Repulped Reject for Improved Material Efficiency by Tailoring Coatings, PaperCon2017

traditional synthetic-polymer-based dispersion coatings are generally considered repulpable, but the repulpability of composite-type, highly bio-based dispersion coatings have been studied less. In this study it was found that the amount of reject after sieving of repulped biopolymer-coated paperboard was up to 50%. This indicates that the material loss can be significant, and new approaches are thus needed for utilizing the vast amount of reject. In order to clarify the effect of coating composition on the amount of reject, paperboard coated with either hydroxypropylated starch (HPS) or hydroxypropyl-cellulose (HPC) and different proportions of talc and styrene-butadiene latex binder were prepared and analysed. It was revealed that repulpability of paperboards coated with HPC-based dispersions was poorer than that of HPS-coated ones. Interestingly, the presence of talc improved the repulpability of HPC-coated boards, but similar effect was not detected from samples whose main coating component was HPS, which was ascribed to poorer dissolving of HPS at low temperatures that assisted in keeping the talc particles bound in starch. Therefore, we studied the possibility to re-utilize dispersion-coated reject as a raw material in paperboard production. The formation of the handsheets containing reject was only slightly impaired by a 5% addition level, but increasing the proportion of reject to 15% led to a 20% increase in formation due to large flocks originated from the coated broke. Chemical properties of reject was also discussed in terms of e.g. zeta-potential. Physical properties were also determined. Reject addition decreased the air permeance of the handsheets moderately regardless of whether the source of the reject was coated or uncoated board. Moreover, the results suggested that HPC-containing reject may slightly increase bulk. The results implied that the repulpability of dispersion-coated paperboard is highly dependent on the pulpability of the base board, but there are no particular hindrances to re-utilize small amounts of dispersion-coated board reject in board manufacturing in terms of physical properties of the paperboard. 

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