PLASTICS MOLDED AT ROOM TEMP
Mixing technique could save energy in plastics processing and aid recycling
Plastics usually must be heated to temperatures exceeding 200 °C to make them fluid enough to be extruded or molded into useful shapes. But a new component-mixing technique makes it possible to mold plastics at room temperature [Nature, 426, 424 (2003)]. The work, by professor Anne M. Mayes and coworkers at MIT, could lead to energy savings in industrial plastics processing and promote plastics recycling.
Five years ago, professor Tom Russell's group at the University of Massachusetts, Amherst, collaborated with Mayes's team to develop "baroplastics" that become disordered more easily under pressure. But in those studies, the pressure-induced transitions were observed at relatively high temperatures.
||COLD MOLD Mayes and coworkers molded these boxes at relatively low temperature (40 °C) from core-shell particles of polystyrene/poly(n-butyl acrylate) baroplastic (powder in central box).
"Designing materials with the right glass transitions that could exhibit pressure-induced mixing and possibly be processed at room temperature was a four-year road from there," Mayes says. Glass transition temperature (Tg) is the temperature at which polymers change from solidlike to meltlike.
In the new low-temperature baroplastics, a glassy high Tg component such as polystyrene is mixed with a rubbery lower-Tg component such as poly(n-butyl acrylate). Under pressure, the rubbery component tends to solvate the glassy one, causing the mixture to melt and flow at much reduced temperatures. The pressure exerted by some conventional molding and extrusion equipment is sufficient to make the low-temperature baroplastics workable.
The researchers demonstrated low-temperature processing of block copolymers and polymers made from two-component core-shell nanoparticles. They concede that block copolymers "have potential drawbacks as substitutes for today's commodity plastics in that their synthesis is generally more complex and expensive." However, core-shell nanoparticles are polymerized by free-radical synthesis--the most common type of industrial polymerization--and thus provide better prospects for industrial use.
"The ability to process and mold polymeric materials at room temperatures represents a tremendous advance in the fabrication and recycling of plastics," comments Craig J. Hawker of the Center on Polymer Interfaces & Macromolecular Assemblies at IBM Almaden Research Center. "The research represents a novel and general concept for eliminating the numerous problems associated with thermal degradation and may find general applicability for a wide range of materials."
"Honestly, I think it is too early to say how this technology will impact the plastics field, since we still have much to learn about its advantages and limitations," Mayes tells C&EN. "But it seems like there is a lot of opportunity. Pressure-based processing could save significant energy because of the reduced temperatures involved; allow for multiple recycling of plastics giving properties comparable to the virgin materials (since there is no thermo-oxidative degradation involved); reduce the need for stabilizers and other processing additives; and allow for incorporation of new components, such as biologically derived or other thermally sensitive materials."