International audienceThe North American market for biodegradable plastics in 2005 was estimated to be around 60 million pounds (27 kt). Starch-based polymer blends were expected to account for 30 million pounds (14 kt) with significantly lower growth rates than other biodegradable polymers such as polyesters. The main hurdle in the growth of starch-based products is the thermodynamic immiscibility and non-wetting of starch with other polymers which leads to serious deterioration of mechanical properties at >25–30 wt% starch. Higher amounts of starch in the blends entail adding suitable functional groups on starch and other polymers in the blend to make them more compatible. The primary challenge is to develop fast reaction chemistries that can be transformed into viable processes and integrated into existing process lines with economically viable formulations. This article briefly reviews some of the most promising chemistries available for the reactive extrusion of starch-based polymer blends (biodegradable/non-biodegradable)

Biodegradable starch-polyester polymer composites are useful in many applications ranging from numerous packaging end-uses to tissue engineering. However the amount of starch that can form composites with polyesters without significant property deterioration is typically less than 25% because of thermodynamic immiscibility between the two polymers. We have developed a reactive extrusion process in which high amounts of starch (approx. 40 wt%) can be blended with a biodegradable polyester (polycaprolactone, PCL) resulting in tough nanocomposite blends with elongational properties approaching that of 100% PCL. We hypothesize that starch was oxidized and then crosslinked with PCL in the presence of an oxidizing/ crosslinking agent and modified montmorillonite (MMT) organoclay, thus compatibilizing the two polymers. Starch, PCL, plasticizer, MMT organoclay, oxidizing/crosslinking agent and catalysts were extruded in a co-rotating twinscrew extruder and injection molded at 120°C. Elongational properties of reactively extruded starch-PCL nanocomposite blends approached that of 100% PCL at 3 and 6 wt% organoclay. Strength and modulus remained the same as starch-PCL composites prepared from simple physical mixing without any crosslinking. X-ray diffraction results showed mainly intercalated flocculated behavior of clay at 1,3,6, and 9wt% organoclay. Scanning electron microscopy (SEM) showed that there was improved starch-PCL interfacial adhesion in reactively extruded blends with crosslinking than in starch-PCL composites without crosslinking. Dynamic mechanical analysis showed changes in primary ␣-transition temperatures for both the starch and PCL fractions, reflecting crosslinking changes in the nanocomposite blends at different organoclay contents. Also starch-polytetramethylene adipate-co-terephthalate (PAT) blends prepared by the above reactive extrusion process showed the same trend of elongational properties approaching that of 100% PAT. The reactive extrusion concept can be extended to other starch-PCL like polymer blends with polymers like polyvinyl alcohol on one side and polybutylene succinate, polyhydroxy butyrate-valerate and polylactic acid on the other to create cheap, novel and compatible biodegradable polymer blends with increased toughness.

Rheological behavior of reactively extruded starch–PCL nanocomposite blends was evaluated in an off‐line capillary rheometer. Power law models for blends with different nanoclay volume fractions were developed using appropriate correction factors. Consistency coefficients K for blends containing starch were significantly higher than 100% PCL. Starch–PCL nanocomposite blends showed shear‐thinning behavior with higher pseudoplasticity than did 100% PCL. Viscosities of nanocomposite blends were significantly lower than that of 100% PCL and nonreactive starch–PCL composites synthesized from simple extrusion mixing. Power law coefficients developed in this study will be used to evaluate rheology‐dependent parameters during scaling up the reactive extrusion process from a batch micro‐extruder to a high output continuous twin‐screw extruder. POLYM. ENG. SCI. 46:650–658, 2006. © 2006 Society of Plastics Engineers.