| Rubber-plastic blends have been commercialized as thermoplastic elastomers (TPEs). The mechanical and rheological properties of these blends depend not only on those of constituent polymers but also on the morphologies of blends. The outstanding properties of dynamically vulcanized rubber-plastic blends have been known and utilized for almost two decades. A thermoplastic vulcanizate (TPV) is a rubber-plastic blend, which contains particles of vulcanized rubber dispersed in a thermoplastic (generally crystalline) resin. This type of TPE is formed by the vulcanization of the rubber during its mixing with the thermoplastic resin in the molten state. In order to improve the properties of thermoplastic elastomers based on rubber-plastic blends, one should understand the relationships between morphology, processing, and properties of these materials.;This study has dealt mainly with the phase morphology and the mechanical properties of blends of natural rubber (NR) with crystalline hydrocarbon plastics, i.e., high density polyethylene (HDPE), syndiotactic-1,2 polybutadiene, and trans-1,4-polyisoprene, with and without dynamic vulcanization.;A specimen-preparation technique was established for the microscopic study of NR/HDPE blends under the scanning electron microscope (SEM). Osmium tetroxide vapor staining of a cryogenic microtomed surface of the specimen was used for NR/high density polyethylene (HDPE) and NR/syndiotactic -1,2-polybutadiene blends.;In the cases of unvulcanized NR/HDPE blends, an increase in the mixer rotor speed or in the volume percent of NR in a batch mixer gives a rise to a faster evolution of the phase morphology and finer particle sizes.;The effect of matching viscosities of polymeric components of NR/HDPE blends is to increase the rate of the evolution of the phase morphology. It is also found that the better is the dispersion, during mixing, before the curative addition, the better are the final properties of the thermoplastic vulcanizate (TPV) produced.;For the NR/syndiotactic-1,2-polybutadiene system, the phase morphology started as that of NR dispersed in the polybutadiene matrix. The two phases then fuse together and finally coalescence occurs; resulting in a single-phase. It was found that NR crosslinks more rapidly than does syndiotactic-1,2-polybutadiene or trans-1,4-polyisoprene in these miscible two phase systems.;Dynamic vulcanization significantly improves the properties of blends in the cases of NR/HDPE and NR/syndiotactic-1,2-polybutadiene. However, the blend properties also depend on the properties of constituent polymers. In the case of the miscible NR/syndiotactic-1,2-polybutadiene blends, dynamic vulcanization selectively prevents the NR from forming of a single continuous phase. It was also possible to dynamically vulcanize NR/ trans-1,4-polyisoprene blends. However, the selectively crosslinked, miscible blends are not as easily processable as are TPVs based on immiscible blends, i.e., NR/HDPE blends. |
