Polystyrene—polyethylene blends, mechanical properties

Kung, E. Lesser, A. J. and McCarthy, T. J. (1997) Mechanical properties of polystyrene/polyethylene blends prepared by using supercritical CO2 processing, ACS Polymer Preprints, 38,462-463. 

We focus here on the polystyrene/high-density polyethylene (HDPE) system. We have studied this system in greater detail than any other and describe here the phase behavior of this system, the blend synthesis, and some mechanical properties of the composites. 

Barensten, W. M. Heikens, D. Mechanical Properties of Polystyrene/Low Density Polyethylene Blends. Polymer 1973, 14, 579. 

Fayt, R. Jerome, R. Teyssie, P. Molecular Design of Multicomponent Polymer Systems. XIV. Control of the Mechanical Properties of Polyethylene-Polystyrene Blends by Block Copolymers. J. Polym. Sci., Part B Polym. Phys. 1989, 27, 775. 

Fayt R, Jerome R, Teyssie P (1989) Molecular design of multicomponent polymer systems. XIV Control of the mechanical properties of polyethylene-polystyrene blends by block copolymers. J Poly Sci Part B Poly Phys 27(4) 775-793... 

Sanchez-Solis, A. Estrada, M.R. Cruz, J. Manero, O. On the properties and processing of polyethylene terephthalate/styrene-butadiene rubber blend. Polym. Eng. Sci. 2000,40 (5), 1216-1225. Luzinov, I. Xi, K. Pagnoulle, C. Huynh-Ba, G. Jerome, R. Composition effect on the core-shell morphology and mechanical properties of ternary polystyrene/styrene butadiene rubber polyethylene blends. Polymer 1999, 40 (10), 2511-2520. 

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,... 

As stated previously, styrene-diene triblock copolymers are the most important category of thermoplastic elastomers. Unlike most other TPEs, they can be blended with large quantities of additives without a drastic effect on properties. In almost all applications, the actual triblock copolymer content is less than 50%. Oils are used as a processing aid and do not result in a significant loss of properties if the polystyrene domains are not plasticized. For this reason, naphthalenic oils are preferred. The use of inert fillers such as clays or chalks reduces the cost of the final material. Unlike conventional rubbers, inert fillers do not have a substantial effect on the mechanical properties of TPEs. Thermoplastics such as polyethylene or polypropylene are also used to improve the solvent resistance and can increase the upper service temperature. Polystyrene homopolymer is used as a processing aid, which also increases the hard phase weight fraction and causes the material to stiffen. 

A series of studies of the emulsifying effect of hydrogenated butadiene/styrene block copolymer on the morphology and mechanical properties of blends of high density polyethylene with polystyrene has been presented by Fayt et al. (see also Ref. ). 

Exterior door handles are another application that has turned to plastics to balance chemical resistance and mechanical properties. Many filled thermoplastics such as blends of PC and polybutylene terephthalate (PBT), polyethylene terephthalate (PET), and nylon have been tried or used in this application, with nylon as the clear wirmer. Exterior mirror housings likewise use many thermoplastic solutions such as ABS, PC/ABS, blends of polyphenylene oxide (PPO) and polystyrene (PS), nylon, blends of PP and ethylene propylene diene monomer (EPDM), and weatherable ABS. Again, nylon clearly dominates this application in terms of volume. Many other exterior parts continue to adopt thermoplastic solutions. Figure 14 shows an impingement shield constructed from LGF PP. 

Weld lines and mechanical properties of injection molded polyethylene/polystyrene/copolymer blends. Polym. Eng. Sci.. 34,1202-1210. 

The examples given above are, however, exceptions. In general, polymer blends are immiscible. Immiscible polymer blends usually exhibit ultimate mechanical properties such as elongation at break inferior to either of the pure components. This is notably the case in polyolefin-polystyrene blends. Another example of this inferiority of ultimate properties is found in studies of the tensile test energy to break in blends of polyethylene and polyamides [89]. 

We have previously discussed recycled polymer blends in Section 5.11 and 6.7. We noted that binary (or indeed ternary) blends involving polyethylene, polystyrene, polyvinyl chloride, polyamides, etc., have poor mechanical properties because of a lack of adhesion between the components (Section 5.11). We indicated in Section 6.7 that introducing compatibiHzing agents such as styrene-hydrogenated butadiene-styrene triblock copolymer and maleated polyolefins can enhance the mechanical properties. However, this in itself is not enough. 

The compatibilizer improves the mechanical properties of PE/starch, and addition of a plasticizer is actually detrimental to the finished products. Although PE is used here to demonstrate the results of this invention, results are practically the same with other combinations of polymer and compatibilizer as disclosed therein. Incorporation of compatibiHzer is easily accomplished by mechanical blending of the polymer, starch, and compatibilizer prior to extrusion. Typically, the compatibilizer is composed of the same polymer as the primary polymer itself. The polymer component of the compatibilizer may be selected from the group consisting of polyethylene, polypropylene, polystyrene, polybutylene, poly(styrene-ethyl-ene-butylene-stryrene), poly(ethylene terephthalate), polyvinyl fluoride, polyvinyl chloride, or derivatives thereof [6]. 

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