Performance of Micronized Biowax Powders Replacing PTFE Fillers in Bio-Based Epoxy Resin Coatings

In view of sustainable-by-design issues, there is an urgent need for replacing harmful coating ingredients with more ecological, non-toxic alternatives from bio-based sources. In particular, fluorine derivatives such as polytetrafluoroethylene (PTFE) powders are frequently applied as coating additives because of their versatile role in rendering hydrophobicity and lubrication. In this research, a screening study is presented regarding the performance of alternative micronized biowax powders, produced from various natural origins, when used as functional additives in protective epoxy coatings for wood. The micronized wax powders from bio-based sources (carnauba wax, rice bran wax, amide biowax) and reference fossil sources (PE wax/PTFE, PE wax, PTFE), of large (8 to 11 µm) and small sizes (4 to 6 µm), were added into fully bio-based epoxy clear coat formulations based on epoxidized flaxseed oil and proprietary acid hardener. Within concentration ranges of 0.5 to 10 wt.-%, it was observed that rice bran micropowders present higher hardness, scratch resistance, abrasion resistance, and hydrophobicity when compared to the results for PTFE. Moreover, the proprietary mixtures of biowax combined with PTFE micropowders provide synergistic effects, with PTFE mostly dominating in regards to the mechanical and physical properties. However, the granulometry of the micronized wax powders is a crucial parameter, as the smallest biowax particle sizes are the most effective. Based on further analysis of the sliding interface, a more ductile surface film forms for the coatings with rice bran and carnauba wax micropowders, while the amide wax is more brittle in parallel with the synthetic waxes and PTFE. Infrared spectroscopy confirms a favorable distribution of biowax micropowders at the coating surface in parallel with the formation of a protective surface film and protection of the epoxy matrix after abrasive wear. This study confirms that alternatives to PTFE for the mechanical protection, gloss, and hydrophobicity of wood coatings should be critically selected among the available grades of micronized waxes, depending on the targeted properties.