Moisture sorption and plasticization of bloodmeal-based thermoplastics

Abstract

Sorption characteristics, thermo-mechanical and mechanical properties of bloodmeal-based thermoplastics have been investigated between water activities (a _w) of 0.2 and 0.8, using water and tri-ethylene glycol (TEG) as plasticizers. Three different mass ratios of TEG to water were used, 1:1, 1:2 and 5:6 with a total plasticizer content of 60 parts per hundred parts bloodmeal. It was found that the equilibrium moisture content and mechanical properties were highly dependent on relative humidity suggesting that material properties may vary during use. The BET and Flory–Huggins equations gave the best fit for desorption and adsorption, respectively, but a significant difference was observed between adsorption and desorption behaviour below a water activity of 0.6, which was thought to be due to changes in intermolecular interactions. The monolayer adsorption capacity (0.05 g/g) was unaffected by the TEG content, using the BET sorption isotherm. The water activity required to form a monolayer (a _wl) was also independent of the amount of TEG, but was different for adsorption and desorption (about 0.5 and 0.2, respectively). Increasing TEG did not have a strong influence on the equilibrium moisture content, especially at low water activity. Dynamic mechanical analysis revealed that the glass transition temperature decreased almost linearly with increasing water activity, ranging between 3 and 85 °C, however, above a water activity of 0.6 a second transition was observed, most likely due to phase separation. Depending of TEG content, tensile strength increased from about 10 to 15 MPa at a water activity of 0.4, where after a drastic decrease was observed. A similar trend was observed for elongation at break. At low water activity (below 0.4) elongation was less than 3%, increasing between 30 and 50% at higher water activities. It was concluded that 10–15 wt% represented a critical point above which mechanical properties becomes very sensitive to the relative humidity of the environment.