Polystyrene reprocessing

Some materials are able to withstand quite lengthy thermal histories , a term loosely used to describe both the intensity (temperature) and the duration of heating. Polyethylene and polystyrene may often be reprocessed a number of times with little more than a slight discoloration and in the case of polyethylene some deterioration in electrical insulation properties. 

Nova Chemicals HDPE, LDPE, LLDPE, MDPE, Melt Compounded Black Polystyrene HIPS, GPPS, Reprocessed, Specialty Polystyrene... 

Figure 2.18 Example of a reprocessable elastomer, with the cross-linking polystyrene domains shown by the circles. Reprinted with permission from J. E. Mark, Physical Chemistry of Polymers, ACS Audio Course C-89, American Chemical Society, Washington, DC, 1986. Copyright 1986, American Chemical Society.

Thermoplastics are plastics which undergo a softening when heated to a particular temperature. This thermoplastic behaviour is a consequence of the absence of covalent bonds between the polymeric chains, which remain as practically independent units linked only by weak electrostatic forces (Figure 1.4(a)). Therefore, waste thermoplastics can be easily reprocessed by heating and forming into a new shape. From a commercial point of view, the most important thermoplastics are high density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyethylene tereph-thalate (PET), polyamide (PA), polymethyl methacrylate (PMMA), acrylonitrile-butadiene-styrene copolymer (ABS), and styrene-acrylonitrile copolymer (SAN). 

The decomposition temperature has been reported to be 364°C for polystyrene and lower than 327°C for PMMA. Consequently, any reprocessing of these polymers would have to be done at temperatures considerably below 327 C at such temperatures there are only small regions of composition in which the polymers are compatible (that is, do not phase-separate) in the melt. 

The parameters so obtained can be tested by using them to predict values of the entry rate coefficient in a system which was not used in the data fitting. Such a test is shown in Figure 5.7, which is for a polystyrene seed of very different radius from that in Figure 5.6. The values in this figure were reprocessed firom steady-state n data given in [23], using the relation derived from the Limit 2a result (appropriate for this system) of Equation (5.22) ... 

Gas At a Polystyrene Foam reprocessing plant. Blowing Agent Gas was accumulated caused by the choke of closed type volume reduction apparatus in operation, and explosion broke out due to static electrical spark ignition... 

Six types of plastics are commonly used for packaging apphcations. In 1988, manufacturers adopted recycling code numbers developed by the Society of the Plastics Industry (Table 16.2). Because the plastics recycling industry stiU is not fully developed, only PET and HDPE are currently being recycled in large quantities. LDPE, which accounts for about 40% of plastic trash, has been slow in finding acceptance with recyclers. Facilities for the reprocessing of poly(vinyl chloride) (PVC), polypropylene (PP), and polystyrene (PS) exist but are rare. 

Apart from the reprocessors of post consumer plastics, other companies have continued to specialize in waste plastics from industrial and commercial operations. The recycling of plastic pallet wrap is expanding across the country. Used electrical wire and cable is being reprocessed primarily for the recovery of metals however a number of specialized operations recycle the cable insulators, polyethylene and polyvinyl chloride. Similarly a few facilities exist to recycle waste automotive trim and foam polystyrene packaging. A considerable effort is being expended to find applications for plastic auto shredder residue which is the by product of the automobile recycling industry which currently recovers metals from end of life automobiles. At present viable applications are still understudy and current residues continue to be landfilled. 

An example of a cross-Unking technique currently under development is the preparation of triblock copolymers such as those of styrene-butadiene-styrene. This system undergoes phase separation in such a way that relatively hard polystyrene domains act as temporary, physical cross-links, as is shown in Fig. 1.43 [228]. The resulting elastomer is thermoplastic, and it is possible to reprocess it by simply heating it to above the glass-transition temperature of polystyrene. It is thus a repm-cessible elastomer. There is a need to develop thermoplastic elastomers that are less expensive than the Kraton styrene-butadiene-styrene triblock copolymers. The leading candidates are stereochemical copolymers of polypropylene, and chemical copolymers of ethylene and comonomers such as hexene-1 [229-231]. 

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