SynopsisThe traditional melt index test (ASTM D 1238) is widely used in the plastics industry to characterize polymer processability. The data from this measurement must be interpreted cautiously, however, because polymers are usually processed under conditions that are far removed from those of the melt index test. In this study, melt index ( M I ) values from a series of polystyrene, polypropylene, linear low-density polyethylenes (butene and octene copolymers), and high-density polyethylenes were measured and related to molecular weight distributions of these materials. It was found that a simple relationship between 1/MI versus (where z = 3.4-3.7) was followed for the linear polymers with similar polydispersities. For branched polymers, the best correlation was that of -In ( MI) versus In ( Mu). A general relation for shear modifiable polymers like polyethylenes cannot be obtained unless the rheological state of the material can also be defined.

A major stream of post-consumer plastics is a commingled blend of polyethylenes and polypropylenes, which usually exhibits lower performance compared with that of the homopolymers due to the incompatible nature of polyethylenes and polypropylenes. Various studies have been carried out to characterize the structure, morphology, mechanical, thermal, and rheological behavior of this blend. The effect of processing, modification, and compatibilization have also been studied by various workers. This article is a review of the work done on polyethylene-polypropylene blends to help in the understanding of this important blend. ost-consumer plastics waste has become the P focus of legislation and environmental concerns.I3 The major polymeric components of these plastics waste streams are polyethylenes, polypropylenes, styrenics, polyvinyl chlorides, and polyethylene terephthalates. Separation of the plastics * To whom correspondence should be addressed.waste into an individual single polymeric type by various methods is costly and complete sorting is economically nonviable and sometimes impossible. Hence in the field of plastics recycling, we are usually faced with a recycled product of a commingled mixture. Due to the incompatible nature of most polymer^,^,^ the commingled polymeric mixtures are poor in their mechanical performance, aging behavior, and resistance toward thermal and chemical environment. In the case of recycling of polyolefins, a commingled blend of polyethylenes (consisting of linear low, low, and

Monodisperse, crosslinked polystyrene latexes were prepared by the dispersion technique. Some general observations regarding the effect of initial reagent concentrations on final particle size and size distribution are offered, in addition to a detailed discussion concerning the problems encountered with the use of the crosslinker divinylbenzene (DVB) in latex preparation. Particles synthesized in very polar media were found to reach their growth plateau sooner than those made in less polar surroundings. This trend was proposed to be the result of more effective nucleation in polar environments, which increases available surface area, thereby allowing the rapid replacement of monomer consumed within the particle phase during the polymerization. Attempts to favorably influence the growth rate and size distribution of particles during the reaction were unsuccessful, underlining the importance of the nucleation period in defining particle size characteristics. Up to 1% DVB was successfully incorporated in the synthesis of coagulum‐free, monodisperse, 5 μm beads, by controlling the entry of the crosslinker into the particle phase during the major particle growth period. Latex stability is proposed to be largely dependent on the mobility of the adsorbed steric stabilizer. © 1995 John Wiley & Sons, Inc.

SYNOPSISThe amount of divinylbenzene (DVB) that can be incorporated in a one-shot dispersion copolymerization with styrene to form a coagulum-free latex is low (<0.5%), a consequence of steric stabilizer immobilization and the inability of the system to provide sufficient quantities of new stabilization materials. Inclusion of a polystyrene solvent in the charge and/or addition of excess stabilizer shortly after the nucleation period enables 0.7% to be inserted, and up to 1% DVB can be copolymerized successfully by linear addition over the major particle growth period. Immobilization of some adsorbed stabilizer chains results in deviations from sphericity. Microparticles ranging from classic smooth-surfaced spheres a t low DVB concentrations to node covered cups and heavily dented spheres at higher levels, can be synthesized. Furthermore, batch addition of 1-6% DVB at least 7 h after nucleation produces monodisperse, stable latexes comprising particles bearing deformities due, not to graft immobilization, but to phase separation within the crosslinked particle skin that forms after DVB absorption. Lower amounts of divinyl monomer batch added before the 7-h mark permits synthesis of a variety of spheroidal and pod-shaped microparticles.

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