Aqueous poly(lactic acid) copolymer dispersions as paperboard barrier coatings

Polymer dispersions expand the possible methods for applying barrier coatings on fiber-based substrates, provide more freedom for preparing formulations, and enable thinner coatings than typically possible by extrusion. Although most of the dispersions today are based on oil-based building blocks, growing environmental concerns have increased efforts to develop biopolymer dispersions as well.
Lactic acid -based polymers are one of the most extensively studied biomass-derived polymers. Therefore, also aqueous dispersions of poly(lactic acid) (PLA) and its copolymers have received a lot of attention. However, the applications of interest have so far been mainly outside of high-volume applications.
Two new types of unsaturated low-molecular weight lactic acid copolymers were synthesized, and aqueous dispersions were produced from these copolymers with a thermomechanical method both at laboratory and pilot scale. This method is both scalable and environmentally feasible as no solvents are involved.
Laboratory coating tests were carried out with paperboard to evaluate the performance of non-crosslinked lactic acid copolymer dispersions in comparison with a set of pre-screened references. Some of the lactic acid copolymer coatings were excellent grease barriers and provided barrier also against n-hexane vapor used to mimic mineral oil migration. These coatings had a comparable or even better water vapor barrier than the references at 50% relative humidity. However, the non-crosslinked coatings were not feasible for a direct contact with water, and they did not provide oxygen barrier. It has been demonstrated earlier that a double coating of a cross-linked copolymer with a similar total coat weight can also provide oxygen barrier. Heat seal tests indicated a need to adjust both the copolymer composition and the sealing parameters, especially in the case of paperboard.