Critical Effects of Polyethylene Addition on the Autoclave Foaming Behavior of Polypropylene and the Melting Behavior of Polypropylene Foams Blown withn-Pentane and CO₂

Abstract

Polypropylene (PP) bead foams were prepared by autoclave foaming using n-pentane and CO2 as the blowing agents. PP foams blown with n-pentane had foaming temperatures of 92–96 °C, expansion ratios of 10–50 times, and a signal T m at 150.1 °C, while PP foams blown with CO2 had foaming temperatures of 151–153 °C, expansion ratios of 8–20 times, and dual melting peaks at 164.0 and 140.9 °C. Polyethylene (PE) addition was used to improve the foaming behavior of PP and to induce the formation of dual and multiple T m in PP/PE foams. A differential scanning calorimetry procedure was carried out to simulate the steam-chest molding of bead foams. Interbead bonding was found to be determined by the heat of fusion of T mc (crystal melting of the newly formed crystals during fast cooling), ΔH mc. Recrystallization of the PE component contributed to the increase of ΔH mc, which potentially improved interbead bonding of the molded PP/PE bead foams.

Improved cell morphology and thermal properties of expanded polypropylene beads by the addition of PP with a high melting point

“…The former two were investigated for all foams. The bead foaming processes, both nucleation and growth, were determined by the rheological and thermal properties, such as shear thickening, stress‐hardening, and crystallization . Due to the low melt strength and narrow foam temperature window, control of cell morphology remains challenging for EPP; cell merging occurs often and the cell density and cell size are difficult to regulate.…”

“…Due to the low melt strength and narrow foam temperature window, control of cell morphology remains challenging for EPP; cell merging occurs often and the cell density and cell size are difficult to regulate. In recent years, many effective methods have been adopted to modify PP, including crosslinking, long chain branching, blending, and composition modification . Long chain branching (LCB) and blending are the most commonly used techniques.…”

“…utilized PP ionomers to produce higher melt viscosity and better foamability compared to pure PP. The economical blending method is attractive due to the short preparation period and low cost required . Park et al .…”

“…The enthalpy and temperature of the first melting peak directly relate to the processing conditions for interbead bonding and the characteristics of the second melting peak relate to the mechanical properties of the final material. The dual peaks often occur in the autoclave foaming process, during which a new peak is created . The present main focus is to adjust the ratio of high and low melting peaks and to increase the difference of the two melting peaks which would improve the properties of EPP and widen the steam‐chest molding temperature .…”

“…found dual and multiple melting peaks for the LLDPE/HDPE (20 wt %), LLDPE/LDPE (20 wt %), and LLDPE/HDPE (10 wt %)/LDPE (10 wt %) blends blend foams. Zheng et al . intended to increase the heat of fusion (Δ H m low ) for sintering PP bead foams at lower melting temperatures by the addition of PE with a lower melt point.…”