Preparation and properties of advanced polypropylene materials from thermally expandable microspheres

Abstract

Thermally expandable microspheres with a shell composed of acrylonitrile, methyl methacrylate, and isooctyl acrylate encapsulating isooctane were synthesized and employed as foaming agents to fabricate advanced polypropylene foam. A systematic comparison was conducted between the nucleating effect of these microspheres and the foaming performance of the conventional chemical foaming agent azodicarbonamide. Characterization via infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy, and mechanical testing revealed that the thermally expandable microspheres exhibited well-controlled expansion behavior within the polypropylene matrix. Their maximum expansion temperature (185°C) aligned well with typical polypropylene processing temperatures. At an 8 wt% loading, the foam density decreased to 0.601 g/cm³, corresponding to a 32.2% reduction compared with polypropylene, while maintaining a moderate and uniformly distributed cell structure. Compared with the chemical foaming system, the polypropylene foamed with microspheres exhibited a more regular cellular morphology. The addition of a nucleating agent significantly enhanced the crystallinity of polypropylene, leading to improved impact and tensile strength. Moreover, the thermal conductivity of the foam was reduced to 0.090 W/m·K, achieving a balanced combination of lightweight characteristics and functional performance. This study confirms the efficacy of thermally expandable microspheres as foaming agents for polypropylene and demonstrates that their synergistic use with nucleating agents offers a promising strategy for producing low-density, high-performance foamed polypropylene. These materials show broad potential for applications in automotive lightweighting, building insulation, packaging cushioning, submarines, and aerospace.

Graphical Abstract