Reactive extrusion is a process that involves utilizing the combined influence of temperature, shear, and pressure to effect a chemical reaction in an extruder. This technique has been explored as a means to develop continuous modification processes for starch. The product characteristics obtained from reactive extrusion are dependent on process and system parameters, such as temperature, shear rate, and thermomechanical parameters, which define the extent and time of mixing, residence or reaction time, and pressure.
Simultaneous physical and chemical modification of granular starch occurs during reactive extrusion, leading to a loss of granular/molecular order via high mechanical input and derivatizing in the molten state. This process is different from using an extruder to prepare pregelatinized starch products or to blend starches with other substances.
The advantages of using reactive extrusion for starch modification include the continuous nature of the process, a wide range of possible processing conditions, enhanced contact between the reactants, more complete reactions (high yields), relatively short reaction times, simultaneous depolymerization of starch polymers, and the lack of process effluents. However, the primary challenges encountered with reactive extrusion include reaction control under conditions of high viscosity, elevated temperature, and short residence time.
Various esters, ethers, cross-linking, oxidation, and hydrolysis have been performed via reactive extrusion. However, reactive extrusion has some possible disadvantages, including the difficulty associated with removing catalysts and unreacted reagents, the limitation that only relatively fast reactions are possible, depolymerization of starch polymers, challenges in scale-up, melt viscosity limitations, and potential damage to the screws and barrel by chemicals.
In conclusion, reactive extrusion is a promising technique for starch modification. It offers a continuous process with a wide range of processing conditions, leading to complete reactions and simultaneous depolymerization of starch polymers. However, it comes with some challenges, including the difficulty associated with removing catalysts and unreacted reagents, the limitation that only relatively fast reactions are possible, depolymerization of starch polymers, and melt viscosity limitations.