(105e) Thermoplastic Foaming Assisted By Microwave

Cell collapse can reduce the cell density and change the cell size of a foam structure. To avoid this phenomenon, different methods have been used such as crystallinity, branching or nanoparticles to increase the melt strength and elasticity [1-3].

Temperature-rise foaming is a common method to induce foaming in a vitrified thermoplastic polymer saturated with a blowing agent. In conventional heating systems (typically, by submerging the polymer/gas solution in an oil bath), the surfaces of the materials are in contact with the heating source and the heat is transferred inwards until the required temperature has been achieved. As plastics have low thermal conductivity, this process takes time and there is often a temperature gradient in the sample thickness, which of course reflects in the foam morphology and local density. In particular, in conventional heating an optimal foaming temperature is selected as a compromise between increasing the temperature to increase the polymer compliance and gas supersaturation, and decreasing the temperature to avoid weakening the polymer and limiting cell collapse. All of these processes must occur concurrently in the short time available for foaming. When attempting to foam large pieces, the described technique manifests its limits and, actually, the impossibility to achieve acceptable foam morphologies.

In this work, microwave heating is used in comparison with conventional heating. Microwave heating offers several advantages over conventional heating methods such as the use of a remote source, the relatively high speed of the process, and volume- and material-selectivity [4].

We used carbon nanotubes as the dispersed source of heating as well as for their well-known ability as bubble nucleators. In this way, foaming will occur near the hot nanotubes, where, the polymer is sufficiently compliant, while remaining strong elsewhere. As a model system, we tested polystyrene foamed with carbon dioxide and achieved thick foams characterized by a uniform morphology and low density.

References

1. Sharudin, R.W.B. and M. Ohshima, CO2�Induced Mechanical Reinforcement of Polyolefin�Based Nanocellular Foams. Macromolecular Materials and Engineering, 2011. 296(11): p. 1046-1054.

2. Garancher, J.-P. and A. Fernyhough, Crystallinity effects in polylactic acid-based foams. Journal of Cellular Plastics, 2012. 48(5): p. 387-397.

3. Moon, Y. and S.W. Cha, Study on viscosity changes with talc in microcellular foaming process. Fibers and Polymers, 2007. 8(4): p. 393-398.

4. Wu, T., Y. Pan, E. Liu, and L. Li, Carbon nanotube/polypropylene composite particles for microwave welding. Journal of Applied Polymer Science, 2012. 126(S2).