Extrusion Foaming Behavior Of Different Polypropylene Foaming Systems

Polyolefin foams have been widely used in the fileds of packaging, automotive, building, industry, sports/leisure, etc. It is relatively difficult to extrusion foaming standard isotactic polypropylene (PP) because of its low melt strength. Thus enhancing its foamability, i.e. melt elasticity (melt strength) , is the key point while extrusion foaming PP.Being different from methods of crosslinking, grafting, and lowering the melt temperature, two new effective solutions to increase the melt strength of PP are put forward, including melt-blending method and melt-intercalating method. Effects of rheological and crystallization behavior of the system as well as formula and processing parameters for the system on its extrusion foaming are studied. It is aimed at developing innovative preparation ways for high melt strength PP so as to lower the cost of raw materials for PP foaming system, and finding out an effective approach to control foaming process and improve quality of the resultant foam. The main work is as following:1. In order to improve the melt strength, a blend of linear PP (LPP ) /long chain branching PP (LCBPP) is prepared by the new melt-blending method. Based on rheological behavior and mechanical properties as well as Dio-Edwards' Snake-tube theory for macromolecular movement, it is found that the optimal ratio of LPP/LCBPP suitable for extrusion foaming is 80/20.2. Crystallization behavior of LPP/LCBPP blends are studied by Differential Scanning Calorimetry (DSC) . It is shown that LCBPP has obvious effect on the crystallization behavior and crystal morphology of LPP/LCBPP blend. The initial crystallization temperature (T_o) and peak crystallization temperature (T_p) of LCBPP are both 10℃higher than those of LPP. The crystallization behavior of LPP/LCBPP blend is similar to that of LCBPP. Heterogeneous nucleation prevails in the crystallization of the blend. No obvious change happened to the crystal morphology during the crystallization. Nucleating agents in the foaming system have little influence on the crystallization behavior of the blend with the presence of LCBPP. At certain cooling rates, the nucleating agents, talc, can not move T_o and T_p of the blend.Study on the kinetics of non-isothermal crystallization of the LPP/LCBPP blend shows that the Avrami index is not impacted by cooling rates, which proves the heterogeneous nucleation of long chain branches quantitatively. Equations of Ozawa and Mo can be applied to explore the non-isothermal crystallization of the blend.Based on the above results, some ideas are put forward: ( 1 ) heterogeneous nucleation during the LPP/LCBPP crystallization ascribes to the long chain molecular structure, can be thought to a typical "self-nucleation" behavior according to the theory of polymer crystallization self-nucleation. (2) Because the long chain branches in LCBPP function as the crystallization nucleating agent for the blend and the precedent crystallization crystals act temporarily as physical crosslink points, the blend exhibits good melt elasticity, which enables the LPP/LCBPP blend a suitable extrusion foaming system.3. Extrusion foaming behavior of LPP/LCBPP is investigated. LPP/LCBPP is extrusion foamed continuously on a single screw extruder. According to the optimization of kinds and loadings of nucleating agents and foaming agents, a PP foam is obtained with appropriate bulk density, even cell distribution and low open cells as well as cell density of the order of 10~6 cell/cm~3 and cell size of 100μm or so.4. Melt-intercalation of PP into layers of nano-clay forms a sandwich structure, which can be regarded as a temporary physical cross-link point macroscopically and functions similarly to chemical cross-linking point in rubber elasticity. This increases the melt elasticity and foamability of PP. Melt-intercalation may be a cost-effect technology for the preparation of high melt strength PP. The composite might be a good system for the extrusion foaming of PP.5. It is also found that the intercalation effect of LCBPP is superior to that of LPP. Nano-clay in the foaming system has no obvious influence on the crystallization behavior and crystal morphology of LCBPP/nano-clay and (LPP/LCBPP)/nano-clay composite.6. Work on the extrusion foaming behavior of LPP/nano-clay composite indicates that nano-clay layers can prevent gas from escaping during foaming, thus avoiding cell collapse, increasing expansion factor.It can be concluded that: (1) incorporation nano-caly into the PP foaming system can not only prevent the gas from escaping during the cell growth, but also improve the melt strength of linear PP. (2) The LPP/nano-clay composite might be a good system for the extrusion foaming of PP.