Pilot scale production of thermoplastic starch/bentonite nanocomposites and their blends with polystyrene

MARÍA PAULA GUARÁS; ANDRÉS TORRES NICOLINI; VERA ALEJANDRA ALVAREZ; LEANDRO LUDUEÑA

Lisboa

Conferencia; 18th International Conference on Materials and Engineering Technology; 2016

INSTITUTO UNIVERSITARIO DE LISBOA (IUL)

The non-biodegradability of most plastics has caused many environmental problems. This fact has carried to an increased use of biodegradable polymers for industrial applications. The development of new materials for industrial applications consists of different stages. The first one is the design of synthesis routes and new material properties at laboratory scale matching the application requirements. Once this step is completed, the next step is the scaling of processes. In recent times, the development of starch-based products has a growing interest. Nevertheless, the thermoplastic starch (TPS) has low stability in high moisture conditions and has a fragile nature. The mechanical properties of TPS can be greatly enhanced with the addition of a small amount of nanoclays (usually less than 10 wt%). In addition, blending TPS with other polymers has been used. Polystyrene (PS) is a polymer widely used in thermoformed products. TPS/PS blends could meet partial biodegradability, mechanical properties stability, and improved processability. The aim of this work is to scale up the synthesis of TPS nanocomposites and their blends with PS in order to obtain partial biodegradable products by thermoforming. The blends were prepared using Corn starch in powder form. The plasticizer used was glycerol (G, JT Baker). PS was purchased from INNOVA. Natural bentonite from Minarmco S.A. was used as nanoreinforcement. TPS/clay nanocomposites (TPS5B) were prepared by twin screw extrusion blending corn starch, glycerol, stearic acid and 5wt.% of bentonite. The twin screw extruder has a barrel diameter of D=18mm and a screw-length/barrel-diameter ratio L/D=25D. Screw speed was set at 25 rpm and the temperature profile used was 120/130/130/140/140ºC. Neat TPS matrix was prepared by the same procedure. Figure 1 shows a scheme of the set up used for the preparation of thermoplastic starch.Blends 50/50wt.% of TPS/PS and TPS5B/PS were prepared using the set up shown in Figure 1. The temperature profile used was 150/170/180/190/190 and the screw speed was set at 25 rpm. Two processing routes were used in order to analyze the effect of extrusion cycles on the TPS thermal degradation. In the first case, polystyrene pellets (PS1) were pre-mixed with TPS ones in solid state. Then, the mixture was inserted into the main feeder of the extruder. In this case TPS was synthesized and then reprocessed to prepare the TPS/PS1 blend by twin screw extrusion. In the second procedure polystyrene pellets (PS2) were inserted in the side feeder of the extruder at the same time that TPS components were inserted into the main feeder. In such a way, the same extrusion process is used for both TPS synthesis and the preparation of the TPS/PS blend. Reduced TPS thermal degradation is expected in this case. Thermoforming trials of all blends and neat matrices were carried out in a manual vacuum thermoforming machine. Thermal characterization was carried out by using a TA Thermogravimetrical Analyzer. Tensile tests were performed on an INSTRON 4467 universal testing machine at 65% relative humidity. It was found that clay promotes TPS thermal degradation and the detriment of mechanical properties. PS2 lateral feeding significantly improved mechanical properties. This was a consequence of minimizing the number of extrusion cycles. Table 1 resumes the characteristics of the obtained thermoformed products.PS/TPS 50/50wt.% blends were successfully thermoformed. Nanoclay incorporation and/or TPS reprocessing promotes TPS thermal degradation leading to thermoformed containers with strong brown color and weak and fragile mechanical properties.