Preparation of high-solid PLA waterborne dispersions with PEG-PLA-PEG block copolymer as surfactant and their use as hydrophobic coating on paper

Paper-based packaging is experiencing a resurgence due to its inherent biodegradability and recyclability. To meet the barrier properties required for certain applications, a coating is necessary. This coating must enhance functionality without compromising the environmental sustainability of the substrate. With this goal in mind, we prepared waterborne dispersions of biodegradable poly(lactic acid) (PLA) using a PEG-PLA-PEG triblock copolymer as the main surfactant. We achieved formulations with good stability over a 6-month period and high solids content (∼40 wt%). The waterborne dispersions underwent analysis by dynamic light scattering (DLS), size exclusion chromatography (SEC), gravimetric tests, and rotational rheology with and without xanthan gum (0.2–0.8 wt%) as a thickener. Subsequently, the thickened dispersions were coated at 60 °C onto a paper substrate. SEM analyses revealed the formation of a polymer layer on the paper surface with thickness and morphology dependent on the processing conditions. Partial interpenetration between the coating and the paper fibers was observed, resulting in excellent adhesion between the layers. The coated paper exhibited good barriers to liquid and water vapor, with Cobb60 < 5 g/m2 and water vapor transmission rate (WVTR) < 100 g/(m2∙day) for coating weights ≤15 g/m2, comparable to the performance of solvent-based PLA paper coatings. The surface energy of the coating was approximately 50 dyne/cm, higher than that of neat PLA, making it suitable for printing with common inks. Furthermore, the coated paper can be fully pulped in water, indicating that it can still be recycled in the paper stream, albeit with potentially increased processing time due to the coating weight.