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 degrees when annealed at 140 degrees C, and 130 degrees when annealed at 160 degrees 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 degrees C higher for wax-impregnated papers, and up to 40 degrees 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.

Automated summary

Supercritical impregnation can be used to enhance specific functional properties in porous substrates, as demonstrated in this study by impregnating food-grade beeswax, carnauba wax, and vegetable wax into paper substrates to improve hydrophobicity and mechanical strength. The analyses revealed changes in surface roughness and chemical bonding, leading to improved properties in the impregnated and annealed samples compared to untreated substrates.