Polyoxymethylene foam: From an investigation of key factors related to porous morphologies and microstructure to the optimization of foam properties

Polyoxymethylene (POM) foam based on using of a blowing agent, i.e. azodicarbonamide (ADCA) is proposed. The viscosity represented by melt flow index (MFI), which is related to the dioxolane content in POM, serves as the main factor to control the spherical cell size and its uniform distribution, including cell density. The POM foam with 13 wt% dioxolane content (V20HE) produced at 200 °C with ADCA content at 1 part per hundred parts of resin (phr) under 0.20 MPa is confirmed to be the optimal condition. At this condition, ADCA initiates cyanic acid with CO2, and allows the monodispersed bubbles without significant degradation of POM. An increase in impact strength of V20HE foam confirms the role of the cell in absorbing the impact. The microstructure analysis based on the evaluation of folded chain crystal (FCC) and extended chain crystal (ECC) by FTIR reveals that the compression pressure applied primarily initiates the ECC. By simply varying the compression pressure, it is possible to fine tune the microstructure and control the impact strength, toughness, and tensile modulus of POM foam.