Supercritical CO2 foaming of pressure-induced-flow processed linear polypropylene

• A systematic study of pressure-induced-flow processed PP foaming was conducted.
• An optimization of pressure-induced-flow process was conducted.
• The effect of pressure-induced-flow on foaming of linear PP with different MFIs was investigated.
• Pressure-induced-flow is necessary for a critical foaming pressure and shorter saturation time.
• Low density PP foams with higher strength and thermal stability were obtained.

Linear polypropylene is very difficult to foam due to its low melt strength and high crystallinity. Pressure-induced flow (PIF) has been demonstrated to be able to improve the foamability of linear PP under a very high pressure. In this study, we report a systematic evaluation to optimize PP foaming process using supercritical CO2. PP was firstly processed by means of PIF at optimized conditions. For CO2 foaming of PP, the effect of foaming pressure and saturation time was investigated in detail. A threshold foaming pressure of 13.8 MPa was found to be needed to achieve low-density foams. Comparing to commercially available EPP and high melt strength PP foams, the PIF PP foams possess much higher compressive strength and thermal stability at lower densities.

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