A study of cell nucleation in the extrusion of polypropylene foams

Abstract

An investigation has been performed of the cell nucleation and initial growth behaviors in the foam processing of polypropylene (PP) in both the linear and branched forms. These materials were foamed in extrusion with the two blowing agents, CO, and isopentane. The cell density generally increased with an increased content of the blowing agent, for both CO, and isopentane. The effect of processing pressure on the cell density was distinct when CO, was used, whereas no pressure effect was observed in the foam processing with isopentane. The cell morphologies for the two PPs were found to be significantly different. A slightly lower nuclei density was observed in the branched PP foams than in the linear PP foams. However, the phenomenon of cell coalescence was observed much less in the branched PP foams. Most cells in the branched PP foams were closed, whereas in the linear PP foams they were connected to each other. The experimental results indicated that the branched structure played an important role in determining the cell morphologies through its effects on the melt strength and/or melt elasticity.

Effect of Long-chain Branching on the Foaming of Polypropylene with Azodicarbonamide

“…Effect of Long-chain Branching on the Foaming of PP sizes in the order of 10 mm achieved by foam extrusion with higher soluble blowing agents like CO 2 and shorter dies [2,10].…”

“…The number of nuclei and the nucleation rate are mainly determined by the distribution of the nucleating particles within the polymer melt, the pressure drop at the die of the foaming apparatus and the surface tension of the polymer melt [2]. From investigations on linear and long-chain branched polyethylenes, it is known that the influence of LCB on the surface tension of the melt is very small [19].…”

“…Whereas, for the linear PP the cellular structure collapsed due to rupture of the cell walls, a foam with a homogenous cellular structure and significant lower density could be produced using the PP with the high melt strength. Park and coworkers performed intensive studies on the development of extruded PP foams with a very low density and a fine cell structure [2,[5][6][7][8]. Extremely large density reductions up to 90% could be achieved by foaming of a LCB-PP under optimized processing conditions.…”

“…The PP is a commodity polymer, has a comparably low density, relatively high service temperatures, a good impact resistance, and an excellent chemical resistance, especially in comparison to the main competitive polymers for foaming applications, namely polystyrene and polyethylene [1,2]. However, the foaming of conventional linear PPs in a continuous process is restricted by their rheological properties, especially their low melt strength [2]. Low viscosity and low melt strength lead to a rupture of the cell walls under the elongational forces occurring during cell growth.…”

Morphology development of polytetrafluoroethylene in a polypropylene melt (IUPAC Technical Report)

“…‡ Corresponding author: Tel. : +81-761-51-1621; Fax +81-761-51-1625; E-mail: m_yama@jaist.ac.jp studies proved that PTFE can be used as a processing modifier for isotactic polypropylene (PP), because it improves the melt elasticity [15][16][17][18], one of the most serious problems for conventional PP [19][20][21][22][23][24][25][26]. According to our previous paper [15], blending a small amount of PTFE enhances the storage modulus in the low-frequency region and normal stress difference.…”