2.3. Preparation of thermoplastic resin from native and hmt rice flours
A 23 experiment – which included moisture content (MC), 20% and 30%; heating temperature, 100 °C and 120 °C; and time, 4 and 16 h – was used in the study. Only waxy rice flour (WRF) was heat-moisture treated using the mentioned conditions. The hmt flour was mixed thoroughly with native normal rice flour at a weight ratio of 70:30 using a ribbon mixer (Reliance Tech-Service, Bangkok, Thailand) for 15 min. As a control, native waxy flour was mixed with native normal flour at a similar weight ratio. Glycerol was later added to the flour mixture at a ratio of 23:77 (wet basis), and the sample was mixed further for 15 min. Afterwards, all samples were kept in polypropylene bags overnight at ambient temperature. Each sample was manually fed into a co-rotating twin-screw extruder (LTE-20-40; Labtech Engineering Co. Ltd., Samut Prakan, Thailand) with L/D = 40 and a screw speed of 170 rpm. The temperature profile along the extruder barrel ranged from 110 °C to 125 °C (from feed zone to die). The extrudates were cut by a pelletizer (LZ-120, Labtech Engineering), yielding 3-mm pellets. Thermoplastic flour (TPF) resins were oven-dried at 60 °C overnight. The dried pellets were stored in polypropylene bags at ambient temperature in desiccators containing silica gels.