Cell Structure Control And Property Study On Polypropylene Foams

Microcellular Polypropylene（PP）foam has outstanding mechanical properties,thermal stability and dimensional stability,and has broad development prospects and market demand in home,packaging,transportation,construction and so on.Comparing with amorphous polymers,low melt strength,slow crystallization rate and narrow foaming range of PP,make it difficult for foaming.Carbon dioxide（CO₂）,it is Nonflammable and has relatively high solubility in polymers,which makes it the most potential the physical foaming agent.When CO₂ is employed for PP foaming,the crystallization behavior and the melt strength of PP can be improved,leading to the wide foaming zone.In order to study the properties of PP foams with different cellular structures,various kinds of PP foams were prepared.In this paper,a self-made supercritical molding machine and autoclave press were used to prepare PP foams by rapid depressurization method.Firstly,the influence of process conditions on the morphology of the foam was studied.In a long enough saturation time,the temperature,pressure,saturation time and pressure drop time were adjusted.When the pressure is constant,the pore size firstly increases with the increase of saturation temperature,and then decreases.On the other hand,the relative density decreases firstly and then increases.As the pressure increases when the temperature is constant,and the average pore size also showed an increasing trend which varies in the range of 10¹20μm.Furthermore,when we increase pressure drop time,the cell size becomes larger and the distribution is not uniform.Then,two kinds of microcellular materials,similar cell size with variou relative density and similar relative density with diverse cell size are selected.Cell size and relative density as the start point to analyze their mechanical properties.The tensile fracture strength and modulus of polypropylene microcellular foamed materials with similar density tend to decrease with the increase of cell size.When the diameter is below 100μm,the larger pore size has a higher elongation.When the cell size is larger than 100μm,the elongation decreases sharply.Finally,the effects of supercritical CO₂ foaming process on the crystalline structure,morphology,thermal properties and foaming properties of polypropylene were studied.In terms of saturation time and pressure relief time,the crystallization behavior of polypropylene during each foaming process was studied respectively.The results showed that,when the saturation temperature of polypropylene was above 145 ℃ and lower than the melting temperature of plasticized PP,and the CO2 pressure is 20 MPa,the foamed PP hasβ-crystal formation.However,there is noneβ-crystal formation in polypropylene at different CO₂ saturation pressure at the same crystallization temperature or at different crystallization temperature at normal pressure.This indicates that the process of pressure relief is an important factor affecting the transformation of PP crystal form.Under certain conditions,β-crystal can be formed in a short pressure drop time,and theβ-crystal disappears after prolonging the pressure relief time.Therefore,the key to the formation ofβ-crystal is the high saturation pressure and short pressure drop time.