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Ogila et al. – eXPRESS Polymer Letters Vol.11, No.10 (2017) 778–798 Figure 6. Internal cooling set up roughness enhanced and pin molds produced cycle time reductions of 57, 65, and 70% respectively. 5. Range of materials Due to the duration and temperature characteristics of RM, the polymers suitable for this process are fewer in number as compared to those available for most other polymer processing techniques. This has secured the position of PE as the main RM material; a position it has reinforced with its low cost and re- sistance to chemical and UV degradation. Nonethe- less, materials other than PE are applied in RM, albeit with varying degrees of complexity. These include: PP, polycarbonate, polyamide, ethylene-vinyl ac- etate, ethylene butyl acrylate, polymethyl methacry- late, styrenics, cellulosics, polyurethanes (PU), sili- cones, unsaturated polyesters and epoxies. An in depth description of these RM materials and their properties is given in reference [96]. 5.1. Single site polymers The advent of SS polymers, alongside traditional ZN catalyzed polymers somewhat broadened the range of materials available for RM. However, taking into consideration that SS catalysts are suitable only for olefin polymerization; it is clear that these materials contribute minimally. Different grades of PE [24, 97–101] and PP [102] have, nonetheless, demon- strated their suitability to the RM process. For in- stance, experiments by Xin et al. [98] and later by Hay et al. [97] showed that under similar molding conditions SS PEs possessed; better mechanical properties, wider processing windows and higher bubble dissolution rates at lower temperatures than ZN catalyzed polymers. 5.2. Thermotropic liquid crystal polymers Thermotropic liquid crystal polymers (TLCPs) ex- hibit both the characteristics of an amorphous poly- mer and a solid crystal when they are heated above either their glass transition temperature (Tg) or Tm. They are especially attractive materials due to their; high tensile strength and moduli, excellent gas bar- rier properties, high resistance to a range of chemi- cals and good dimensional stability [103, 104]. Rangarajan et al. [105] studied the sinterability and rheology of a TLCP (Vectra B230), and determined that both were acceptable for RM. The TLCP mold- ings also possessed superior mechanical properties. However, the bulk density of the milled Vectra B230 powder was very low; which resulted in the lumping of its particles and led to higher porosity. The addi- tion of various quantities of HDPE powder resulted in better particle coalescence and improved bubble removal. Scribben and Baird [106] discovered that the low bulk density of TLCP powder resulted from a change in its morphology instigated by shearing forces dur- ing extrusion. These forces caused the liquid crys- talline regions of the polymer orient themselves into fibrillar structures; which broke down into the result- ing fibers on the TLCP particles upon milling. They improved the bulk density by melt extruding the TLCP with an immiscible polymer (PP); which they retrieved after quenching and powdering the mix- ture. The flow characteristics, particle coalescence and melt viscosity of the TLCP powder were accept- able for RM conditions. Melt densification, however, remained relatively poor and moldings made from the material possessed numerous surface pores. The authors attributed this to the aforementioned gas bar- rier properties of the polymer. 5.3. Bio-polymers Bio-polymers possess enormous potential as mate- rials not only for RM but for the entire plastics indus- try. The main advantage of these polymers is their 790PDF Image | Rotational molding: A review
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