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Putting the squeeze on plastics

Figure: Core-shell baroplastics processed from powders into rigid box tops at 40°C.

The production of plastics is an energy intensive process, consuming roughly 40,000 Btu for each pound of plastic generated. About half of this energy is used in producing the raw polymer materials, while the remainder is spent in manufacturing plastic goods from such raw materials. During manufacturing, much of the energy is used to heat the polymers to temperatures high enough to allow them to melt and flow, typically above 200° C (392° F).

Researchers in MIT's NSF-funded Center for Materials Science and Engineering have developed a new type of plastic that could substantially lower energy consumption related to plastics manufacture. As described in a recent Nature article,1 the new plastics, called "baroplastics," use similar manufacturing equipment as current commercial plastics but need little or no heating to be molded into desired shapes. Instead, these materials flow when large pressures are applied, due to their specially designed nanophase structure. At microscopic length scales, the raw materials, made by core-shell emulsion polymerization, are arranged like a Tootsie PopTM , with a soft polymer center and a hard polymer shell. When placed under pressure, the hard component partially mixes with the soft component, allowing the entire system to flow. Once the pressure is relieved, the plastic rehardens.  

Besides saving energy in plastics manufacture, baroplastics can potentially save energy consumed in raw material production, since the new plastics are also highly recyclable. Unlike traditional heat-based processing, which can thermally degrade polymer molecules and incorporated additives, pressure-based processing causes minimal degradation, allowing the new plastics to be molded over and over without changing their molecular structure. The substantial discoloration and loss of mechanical performance that is problematic in current plastics recycling can thus be avoided. MIT researchers have successfully shredded and remolded their materials as many as 10 times.

1 Gonzalez-Leon, J. A., M. H. Acar, S.-W. Ryu, A.-V. G. Ruzette and A. M. Mayes , "Low-temperature processing of baroplastics by pressure-induced flow," Nature 426 , 424-428 (2003).

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