Preparation And Foaming Investigation Of PPC Composites

Biodegradable polymeric foams has become one of the hot research spots for its excellent properties and inherent advantages of environment protection. Among them, PPC has attracted great attention from scientific and industry area due to its good biodegradability and the ability to reduce the greenhouse effect particularity. However, its poor machinability prevent its wide applications.To address these problems, in this paper we prepare PPC/PTFE standard foamed samples by using self-designed autoclave and molds to investigate its supercritical CO₂ foaming properties and mechanical properties. It is found that PTFE was in situ fibrillated into the nanofibrillar network within the PPC matrix. With this special phase morphology, the average cell diameter of the PPC/PTFE foams was decrease into 2.29 μm, while the cell density was 10 times higher than that of neat PPC foams. The tensile strength and impact strength of the PPC/PTFE foams increased by 21% and 27% respectively. Thus, the influence of the PTFE nanofibrillar network on PPC foams was fully verified.On this basis, in view of the defects of extrusion PPC foams, we compounded polystyrene（PS） and polytetrafluoroethylene（PTFE） with poly（propylene carbonate）（PPC） to prepare multiphase composites that possess special properties and improve the extrusion foaming ability of PPC. It was found that PS was immiscible with PPC and formed dispersion phase, and PTFE were in-situ fibrillated into nanofibrillar network within PPC/PS matrix. The PPC/PS/PTFE composites had 12 °C higher glass transition temperature, 392% higher storage modulus in glassy state characterized by dynamic mechanical analysis, and 1676% higher initial viscosity than neat PPC. Moreover, the physical network formed by PTFE nanofibrils effectively prevented the shear-thinning behavior of the polymer matrix. The extrusion foaming process revealed significant influence of PTFE on the cell morphology. The cell diameter of PPC/PS/PTFE foams was decrease into 2.29 μm and The cell density was four orders of magnitude higher than PPC foams. The compressive modulus and strength of the foamed PPC/PS/PTFE composites were 58 and 34 times higher than them of PPC foams.At last, for the great demand of the fully biodegradable microcellular foaming plastics, we prepared PPC/PBS/PTFE biodegradable multiphase blends that possess special properties. The SEM images showed that PBS was immiscible with PPC and formed dispersion phase, while PTFE were in-situ fibrillated into nanofibrillar network within PPC/PBS matrix. This special phase morphology resulted in significant enhancement to the properties of PPC. As for PPC/PBS/PTFE（70-30-3） sample, a 15 °C increase in glass transition temperature and 851% higher storage modulus detected by dynamic mechanical analysis and 17 times enhancement in viscosity. Moreover, the investigation of extrusion foaming behavior found that the physical network formed by PTFE nanofibrils effectively prevented cell collapse and cell rupture during extrusion foaming and 90 °C was the optimal foaming temperature. For PPC/PBS/PTFE（70-30-3） foams, the average cell diameter was 15.3 μm while the cell density with a uniform cell structure was two orders of magnitude higher than neat PPC foams. The compressive modulus and strength of the PPC/PBS/PTFE（70-30-3） foams were 30 and 20 times higher than that of neat PPC foams, respectively. respectively.These great improvement of the foams was attribute to the synergistic effect of the enhanced CO₂ affinity, the heterogeneous nucleation effect, and the changes in melt viscosity. Therefore, in this study, we not only purposed a method to prepare PPC composites with excellent thermal, rheological and dynamic mechanical properties, which was never achieved in other publications, but also solved the foaming problems of PPC in a continuous method and provided high performance PPC composite biodegradable foams, which can be used in many applications such as biomaterials or packaging fields.