Hydrophobic development and mechanical properties of cellulose substrates supercritically impregnated with food-grade waxes

Supercritical impregnation may be used to impart specific functional properties into porous substrates such as wood, textiles, or paper. In the current study, food-grade beeswax (BW), carnauba wax (CW) and vegetable wax (VW) were impregnated into paper substrates to improve their hydrophobicity and mechanical strength. The contact angle of impregnated and annealed samples was approximately 110–120° when annealed at 140 °C, and 130° when annealed at 160 °C. SEM analyses revealed that dual micro- and nano-scale roughness was generated in impregnated paper substrates that also underwent annealing. FTIR analysis showed evidence of H-bonding between the waxes and cellulose, but this was more dominant with BW/CW compared with VW due to the different chemical structures of the waxes. Annealed samples showed lower intensity FTIR peaks, tentatively confirming a phase transition of the waxes as a result of the annealing. A reduced tan delta signal up to the secondary alpha transition temperature for paper was observed with BW/CW impregnated samples, indicating the formation of additional chemical bonds between cellulose and wax. Compared with untreated paper substrates, the sharp decrease in storage modulus during degradation occurred at temperatures up to 10 °C higher for wax-impregnated papers, and up to 40 °C lower for baseline papers impregnated and annealed without wax. It is believed that the H-bonds between the waxes and cellulose were able to withstand higher temperatures in the degradation region, thus offsetting the effects of sample preparation.