Injectable polymers mechanical properties improvement

The reinforcement of thermoplastics, notably polypropylene (PP) and polyamide (PA), with short glass fibers is a classical method to improve polymer mechanical properties. Fibers are dispersed more or less homogeneously in the polymer by melt compounding techniques such as twin screw extrusion to produce compounds ready for injection molding. Molded parts benefit from the better mechanical properties of the reinforcing fibers as the fibers bear a considerable part of the load applied to the composite structure. 

Injection moldings are made with the melt at or above 300 C and molds at or above 135 C for crystalline parts, or much colder for amorphous parts. Amorphous parts will begin to crystallize when heated above 90 C. Very little PPS is used as the neat polymer in molded parts because it is too brittle. As with other crystalline polymers, mechanical properties are improved by using fiber glass in the molding compounds. Minerals are used as additional fillers in some compounds. As with other highly crystalline... 

Orientational order plays an important role in solid polymers. It is often induced by industrial processing, for example in fibers and injection- or compression-modulated parts. In polymers with liquid-crystalline properties of the melt or solution, the anisotropies generated by the flow pattern are particularly pronounced. In order to improve the mechanical properties of polymer fibers or films, the degree of orientation is intentionally enhanced by drawing. At the same time, anisotropy of mechanical properties can result in low tolerance to unfavourably directed loads. In many liquid-crystalline polymers, in the mesophase near the transition to the isotropic phase, electric or magnetic fields can induce macroscopic orientational order [1]. Natural polymers such as silk protein fibers, which are biosynthesized and spun under biological condition, also have good mechanical properties because of their ordered structure [2]. 

PSF/PET blends show a dispersed morphology. The combination of crystalline PET and amorphous polysulfone provides chemical resistance and warp-free properties. The amount of crystalline polymer is varied to meet requirements in thermal properties and the level of reinforcement is varied to tailor the modulus. The blends have electrical and mechanical properties similar to PET but only a third of its shrinkage and warpage. Also, stress crack resistance to common solvents is improved. Similarly, the service T is upgraded compared to PET. The blends are formed by injection molding and extrusion. 

Specifically, PVC blends with polyethylene, polypropylene, or polystyrene could offer significant potential. PVC offers rigidity combined with flammability resistance. In essence, PVC offers the promise to be the lowest cost method to flame retard these polymers. The processing temperatures for the polyolefins and polystyrene are within the critical range for PVC. In fact, addition of the polyolefins to PVC should enhance its ability to be extruded and injected molded. PVC has been utilized in blends with functional styrenics (ABS and styrene-maleic anhydride co-and terpolymers) as well as PMMA offering the key advantage of improved flame resistance. Reactive extrusion concepts applied to PVC blends with polyolefins and polystyrene appear to be a facile method for compatibilization should the proper chemical modifications be found. He et al. [1997] noted the use of solid-state chlorinated polyethylene as a compatibilizer for PVC/LLDPE blends with a significant improvement in mechanical properties. A recent treatise [Datta and Lohse,... 

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