Physical stabilization of starch-allylurea blends by EB-grafting: a compositional and structural study.

A structural and compositional study of thermoplastic blends prepared from native potato starch (NPS) and various amounts of allylurea (AU) was performed to gain a better understanding of possible radiochemical routes to physically stable materials. The blends, mixed at ca. 130 degrees C, were studied in the form of 150 microns-thick films. Upon aging at room temperature, the samples obtained from these blends exhibit macroscopic phase separation under the form of allylurea blooming at their surface. The maximal compatibility of allylurea in amorphized potato starch was assessed by gravimetry and compared to that of urea in mixtures with NPS prepared in similar conditions. Physical aging of the unstable blends was monitored for various initial AU contents. Electron beam (EB) processing of fresh films with AU content above the solubility limit was shown to prevent phase separation, essentially as a consequence of radiation grafting onto starch of the unsaturated additive. X-ray diffractometry was performed to control (i) the effective amorphization of starch upon mixing, (ii) the recrystallization of the incompatible AU fraction from untreated blends in the tetragonal form, and (iii) the retardation or the suppression of this phenomenon after EB processing. The physical stability of the blends treated with a sufficient radiation dose (400-800 kGy) was confirmed by dynamic thermomechanical analysis of samples submitted to various hygrometric conditioning.