Study On Expanded Polypropylene Beads Prepared By Autoclave Method

The parts manufactured from expanded polypropylene(EPP) beads have received a lot of attention in the automotive and packaging industries due to their unique combination of high expansion ratio and free geometries and a high dimensional accuracy. The manufacturing technology of EPP is still monopolized by minority external companies, which results in a high price of domestic products made with EPP. To study the manufacturing technology of EPP is very necessary. In order to find out foaming mechanism and processing control of EPP, a series of expanded beads were prepared through a autoclave foaming process, using CO2 as the blowing agent. The properties of expanded beads was investigated.In the study, the influences of gas saturation time and foaming temperature(Tf) and foaming pressure(Pf) on melting/crystallization behavior and foaming properties of EPP were investigated. It is found that the effects of gas saturation time on double melting peak and foaming properties of EPP are tiny when it is higher than 30 min. The double melting peaks of EPP move to higher temperature and the ratio of crystallinity between high-temperature crystal and low-temperature crystal increases with increasing Tf or Pf. Furthermore, the effect of Tf on double melting peaks of EPP is more sensitive than Pf. When Pf ranges from 2 to 4 MPa, the expansion ratio and cell density of EPP increase with increasing Pf. When Tf ranges from 133.6 to 140 ℃, the expansion ratio and cell size increase with increasing Tf, but the cell density decreases with increasing Tf. When the gas saturation time and Tf and Pf are 30 min and 133.6 and 4 MPa, respectively, ℃EPP with desired double melting peaks, uniform cell structure and smooth surface is obtained. The cell density is the largest, and the distribution of cell size becomes the narrowest.The influences of HPP contents on rheological properties and melting/crystallization behavior of LPP/HPP blends and properties of expanded LPP/HPP blends were investigated when HPP of the melting point of 166 ℃ was used to enhance the LPP of the melting point of 130 ℃. It is found that the addition of HPP enhanced the crystallization temperature and complex viscosity and melt strength of LPP/HPP blends. The ratio of crystallinity between low-temperature crystal and high-temperature crystal of expanded LPP/HPP blends decreases and total crystallinity increases slightly with increasing HPP contents. The cell density of expanded LPP/HPP blends increases, cell size and expansion ratio decrease with increasing HPP contents. When the process conditions is constant, the expansion ratio of expanded LPP/HPP blends can be efficiently adjusted through varying HPP contents.The influences of wood flour(WF) contents on rheological properties and melting/crystallization behavior of PP/WF composites and properties of its expanded beads were investigated. It is found that the addition of WF has a significant effect on the foaming behavior of PP. The complex viscosity, melting and crystallization temperature of composites were increased and loss tangent was decreased with increasing WF contents. Both expansion ratio and cell density of expanded PP/WF composites first were increased then decreased with increasing WF contents, and the cell size first was decreased then increased with increasing WF contents. Expanded PP/WF composites with mean cell diameter(67.69 μm), cell density(5.791×107 cells/cm3) and expansion ratio(10.4) is obtained at 10 wt.% WF contents.