Ultra-low-density neat HDPE foams via steam-assisted scCO2 foaming

Abstract

High-density polyethylene (HDPE) is difficult to foam because of its high crystallinity and low melt strength, and most strategies require crosslinking, blends or fillers. Here, we investigate an additive-free route based on one-step batch foaming with supercritical CO 2 (scCO 2 ) assisted by subcritical water in a custom rapid-ejection device. The effects of saturation temperature ( T sat = 110 – 160 ° C ), pressure ( P sat = 10 –40 MPa) and water load (0, 10 and 45 g, approximately 0, 15 and 70% of the 65 mL autoclave volume) were evaluated. Without water, low-density foams were obtained only within a narrow range, mainly at 20 MPa and 120 °C–130 °C. Adding water extended low-density foaming up to 150 °C at 20 MPa and, at 140 °C, reduced the minimum required pressure from 30 MPa to 20 MPa. Under optimal conditions ( T sat = 140 ° C , P sat = 20 MPa , 45 g H 2 O), the relative density reached ρ r ≈ 0.015 ( X ≈ 69 ), the lowest value reported for HDPE foams to the best of the author’s knowledge. Water also enabled open-cell contents from predominantly closed-cell structures to values approaching 100 %. These results suggest that rapid cooling of the carbon dioxide (CO 2 )–water fluid during depressurization, followed by heat transfer from the polymer to the fluid and possible additional water evaporation, increases the effective melt strength and enables ultra-low-density high-density polyethylene (HDPE) foams using only scCO 2 and water.