The processing steps of the 7:3 mixture of PLA and PBS
The mixing process of PLA and PBS is accomplished by utilizing the ‘twin-screw extrusion’ method, of which the main experimental apparatus is the twin-screw extruder. The fundamental operating principle of the apparatus is shown in the following figure 1.
Basically, the working process of the equipment can be divided into three stages. Initially, raw materials (PLA and PBS) from the feeding hopper are injected into the machine barrel, along the direction of the screw groove forward, which is known as the ‘feeding conveyor’ process. In this stage, as indicated in the figure, the clean mixed polymers which are mainly composed of PLA are made into flakes and injected into the barrel together with some reinforced agent such as PBS by the auger feeder. Generally, the ratio of PLA and PBS is 7:3, in order to give a maximum shock strength of the mixture.[1] Meanwhile, the liquid compatibilizer used to bind two incompatible polymers together is added through a metering pump. Secondly, because of the strong mixing and shearing effect of the mixture between the screw and the barrel, and the external heating of the barrel, the mixture heats up and begins to melt until it melts completely. This process is called ‘compression and molten’ stage. In this stage, solid polymers are gradually compacted due to resistance from the head of the barrel, and then the ‘PLA and PBS’ mixture is melt blended and goes into the ‘vacuum sizing tank’. The third stage is referred to as ‘homogenization’ stage, in which the polymer mixture is further homogenized, and finally, the product is extruded uniformly from the head passage at a fixed quantity and pressure, as a result of increasing the temperature and pressure.[2] The vacuum pumps in the equipment are used to equalize and control the pressure of the blend system.
Figure1：The twin-screw extrusion apparatus
The twin-screw extrusion granulation temperature sections
The 10 different extrusion granulation temperature sections for twin-screw extrusion have been measured through relevant experiments, the results are as follows(See Table 1):
Table 1:The twin-screw extrusion granulation temperature sections( centigrade degree)
From this table, it can be seen that the temperature of zone 1 to zone 10 shows a decreasing trend after gradually increases towards a maximum value of about 200 centigrade degrees. The reason for this tendency is that in the first stage of processing, the temperature should not be too high otherwise the extruder torque distributor gear will be warped by heat, thus accelerating wear, and the temperature increases because heat is needed to transfer the solid mixture to the molten state and tightly wrap it around the spiral groove surface[3]. Then the temperature should keep constant for a moment to conserve energy before it decreases. Finally, the drop in temperature is caused by the requirement for crystallization of the mixture. That is the total process through zone 1 to 10.
References
[1]Xu W H, Yang Z T, Yin X C, Xu B P, et al. Study on the mechanical properties of toughened PLA/PBS blend under tensile deformation [J]. China plastics,2016,30(01):34-38.
[2]Deng J H, Tan X H, Li Y X. Performance analysis and application of twin screw extruder [J]. Food and machinery,2004(06):41-43.
[3]Liu H Y, OuYang B L. Design of temperature control system for twin screw extruder [J]. Journal of northeast agricultural university,2004(05):594-596.