Theory And Technology Of The Fabrication Of Expanded Polypropylene (EPP) Beads Using An Autoclave System

In essence, bead foaming technology comprises two main processes: foamed beadfabrication and steam-cheating molding. Bead foaming technology is an innovativetechnology in the plastic foam industry that allows the manufacture of low-density polymericfoam products of a variety of shapes and densities, which is not achievable with conventionalfoam extrusion or injection mold processes. Owing to their high expansion ratio andclosed-cell morphologies, articles of molded foam beads are very light-in-weight and possesssuperior impact and thermal insulation properties. Because of these excellent properties,molded foam bead articles have been widely adopted in a broad spectrum of industries,including the automotive, transportation, construction, chemical engineering, sports,recreation, etc. An example of bead foams is expanded polypropylene （EPP）. Products madewith EPP have high service temperature and excellent mechanical properties. Because of itslow cost-to-performance ratio, the market of EPP products has been consistently expanding,in particular, in the automotive industry. Despite its potential, research about themanufacturing technology of EPP, in particular, in the aspects of foaming mechanism andprocessing control of bead manufacturing is still very limited.The goal of this research is to elucidate factors during the fabrication of EPP that wouldlead to the production of its double-peak melting/crystallization characteristic, which willaffect the sintering behaviour of the beads in the subsequent steam-chest molding process andthe quality of the final foamed bead products. A lab-scale autoclave EPP foaming system wasdesigned and constructed to facilitate the study of the influence of various critical processingparameters on EPP fabrication. This autoclave system adopted a modular die design to allowthe study of the influence of pressure drop and pressure drop rate during the depressurizationprocess of EPP manufacturing. Through the designed lab-scale autoclave EPP foaming system,this research investigated the effect and correlation of a variety of processing parameters onthe manipulation and control of the foaming properties and melting/crystallization behaviourof the EPP beads, thus explores the control mechanism of the EPP fabrication process. Thishelps to provide reference for further study of the theory and practice of EPP fabricationtechnology.The double-peak melting/crystallization characteristic of EPP is a requisite for properbead sintering in the steam-chest molding process of bead foaming technology. This researchexamined the foaming behaviour of four different types of PP （branched, linear, blockcopolymer, and random copolymer PP） using a batch foaming visualization system in order to identify suitable EPP materials. As illustrated from the foam visualization experiments, themolecular chain structure significantly affected the foaming behaviour of PP. The high degreeof molecular chain entanglement of branched PP helped cell stabilization during foamexpansion. Linear PP had low melt strength, and hence, the foamability of this PP wasrelatively poor. The high crystallinity of block copolymer PP had hindered the diffusion anddissolution of carbon dioxide （CO₂）. Random copolymer PP had relatively low degree ofmolecular chain regularity and crystallinity, allowing CO₂to dissolve into the matrix easily.Amongst the four types of PP examined, only random copolymer PP had yielded awell-distributed double-peak melting/crystallization characteristic after the foaming process.Based on the results, random copolymer PP is a suitable candidate for EPP fabrication.Similar to other foaming techniques, cell nucleation and cell growth are two competingprocesses in EPP bead fabrication. Through the use of dies of different diameters （D） andlengths （L）, this work investigated the correlation between pressure drop, pressure drop rate,cell nucleation rate and cell density. Through the study of the flow and depressurizationcharacteristic in the autoclave system, the model regarding the flow of PP pellets within thedie was established, and hence the effects of die geometry, volumetric flow rate and systemproperties on the pressure drop and pressure drop rate were analyzed. The requirements ofachieving high pressure drop and pressure drop rate for the production of EPP with high celldensity and expansion ratios were also identified.With the designed lab-scale autoclave EPP foaming system and using CO₂as theblowing agent, EPP beads of different cell densities, expansion ratios, and double-peakmelting/crystallization characteristic were attained through varying processing parameters. Inthis work, the EPP beads manufactured under the optimized processing condition possessedan expansion ratio of15-fold, a cell density of3.47×10⁸cells/cm3, bi-cellular foam structure,and melting/crystallization behaviour that equivalent to the EPP produced by JSP. Throughthe study of the effects of saturation pressure, saturation temperature, saturation time anddepressurization conditions （i.e., die geometry） on the EPP fabrication, the following resultscan be drawn: The expansion ratio and cell density increased as the saturation pressure. As thesaturation temperature increased, the expansion ratio increased while the cell densitydecreased. As the saturation pressure increased, the temperature difference of the double-peakdecreased, while the area ratio between low-melting-peak and high-melting-peak increased.As the saturation temperature increased, the area ratio between low-melting-peak andhigh-melting-peak increased, and the temperature difference of the double-peak increasedwhen the saturation pressure was high. However, as the saturation pressure decreased, the influence of the saturation temperature on the temperature difference of the double-peak wasnot obvous. As the saturation time increased, the area ratio between low-melting-peak andhigh-melting-peak decreased while the temperature difference of the double-peak changedlittle. When the saturation pressure, saturation temperature and saturation time were fixed, byincreasing the length of the dies with the same diameter （i.e., increasing the pressure drop）,both the expansion ratio and the area ratio between low-melting-peak and high-melting-peakdecreased, while the cell density and the temperature difference of the double-peak changedlittle. By decreasing the pressure drop rate of the dies with the same pressure drop, both theexpansion ratio and cell density decreased, the area ratio between low-melting-peak andhigh-melting-peak sometimes increased and sometimes decreased, and the temperaturedifference of the double-peak changed little.