ARS | Publication request: WATER SORPTION AND DIFFUSION IN STARCH/PHBV COMPOSITES

Submitted to: Corn Utilization Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: June 3, 1998
Publication Date: N/A

Technical Abstract: Starch-based composite materials have received considerable attention as replacements for non-biodegradable thermoplastics in disposable packing applications. Many of these applications require contact with contents which contain water. Starch is a hygroscopic material and absorbs water, which acts as a plasticizer. It is therefore important to determine water sorption and diffusion processes in starch-based materials to provide data useful for designing biodegradable packaging. The kinetics of water sorption in materials based on starch and poly(hydroxybutyrate-co-valerate) or PHBV, have been determined. Injection molded disks were prepared with starch contents of 0, 12.5, 25, 37.5, and 50 weight per cent. Three levels of PHBV plasticizer were used: 0, 5, and 10 parts per 100 parts PHBV. Water sorption was determined by immersing disks in deionized water (with 0.2% NaN3) and measuring weight gain until constant weight was reached. Sorption kinetics were analyzed using the plane sheet solution of Fickian diffusion. All materials displayed Fickian behavior in the initial stages of sorption. Measured values of D were in the range (0.5-4.0) x 10**-9 cm **2/sec. As starch content increased from 0 to 25%, values of D decreased by more than half. Above 25% starch, D slowly increased but remained lower than the value for PHBV alone. Water sorption could be described by a single value of the diffusion coefficient when the starch content was 25% or less. At greater starch contents, a single value of D was not adequate to describe sorption, indicating an increase in D with water content. This behavior led to shorter equilibration times for higher starch contents, despite their lower initial D values. Diffusion coefficients increased with increasing PHBV plasticizer content.