Optimization of a thermal process for the production of superabsorbent materials based on a soy protein isolate

Superabsorbent materials have found several applications in fields related to diapers, hygienic tissues or controlled-release in agriculture. In order to produce more environmentally-friendly materials, some efforts have been focused on grafting copolymerization of acrylic derivatives onto a natural compound (e.g. protein, poly-saccharide). This manuscript deals with the production of biodegradable superabsorbent materials produced from a Soy Protein Isolate (SPI), in which their water absorbance capacity is modulated only through the modification of the thermal processing. Thus, SPI was blended with a plasticizer (glycerol), using a 1/1 (w/w) SPI-glycerol ratio, and then injection moulded at different mould temperatures (T-mould). Subsequently, a dehydrothermal (DHT) treatment was carried out, consisting of the storage of bioplastic produced at 50 degrees C in an oven for a certain period of time (t(DHT)). The present study demonstrated that thermal conditions (T-mould, t(DHT)) plays a crucial role in the formation of superabsorbent materials from protein sources like SPI. Mould temperatures as low as 70 degrees C resulted in materials with a water uptake which defines their superabsorbent character, as long as the t(DHT) selected is shorter than 10 h. Longer t(DHT) resulted in a tighter structure with generally higher viscoelastic properties, which was less able to swell and absorb water. A longer moulding stage also lead to a decrease in the water uptake. Thus, thermal processing parameters have been proven to modulate the super-absorbent characteristics of the materials studied, not being necessary any chemical modification of the protein source.