Stiffness high-density polyethylene

Polypropylene polymers are typically modified with ethylene to obtain desirable properties for specific applications. Specifically, ethylene—propylene mbbers are introduced as a discrete phase in heterophasic copolymers to improve toughness and low temperature impact resistance (see Elastomers, ETHYLENE-PROPYLENE rubber). This is done by sequential polymerisation of homopolymer polypropylene and ethylene—propylene mbber in a multistage reactor process or by the extmsion compounding of ethylene—propylene mbber with a homopolymer. Addition of high density polyethylene, by polymerisation or compounding, is sometimes used to reduce stress whitening. In all cases, a superior balance of properties is obtained when the sise of the discrete mbber phase is approximately one micrometer. Examples of these polymers and their properties are shown in Table 2. Mineral fillers, such as talc or calcium carbonate, can be added to polypropylene to increase stiffness and high temperature properties, as shown in Table 3. 

High-density polyethylene (p = 0.94-0.96 g/cm ) has up to five times the stiffness of low-density polyethylene at ambient temperatures and can be used at much higher temperatures. Its chemical resistance is similar to that of the low-density grades, but the resistance to swelling by solvents is higher. 

Due to the low level of branching, there is little to hinder the crystallization of high density polyethylene. We routinely observe crystallinity levels in excess of 60%, which translate into densities ranging from approximately 0.94 to 0.97 g/cm3. High density polyethylene is the stiffest of all the polyethylene types. In some cases we incorporate small amounts of a comonomer, such as 1 -hexene, which reduce the crystallinity level. This improves toughness at the expense of stiffness. 

Chemical properties Low density (LDPE) and very low density polyethylenes (VLDPE) have branched chains which do not pack together well, imparting low density. High density polyethylene (HOPE) has longer, less branched chains, imparting stiffness and strength. No solvents at ambient temperature due to PE s high crystallinity, but chlorinated liquids can swell the plastic. 

Despite its origin form the nature, PLA s good stiffness and strength has enabled it to compete with other existing chemically based commodity plastics. Previous study on the mechanical properties of neat PLA by Jacobsen et al. [1] showed that PLA has great potential to be a substitute polymer for petroleum based plastics. The respective values of mechanical properties of PLA [2] with comparison of other petroleum based plastics e.g. polypropylene (PP) [3], polystyrene (PS) [4], high density polyethylene (HOPE) [5], polyamide (PA6) [6] shown in Fig. 11.2. 

Hyperfom HPN 210 M is a new nucleating agent for injection molded high-density polyethylene apphcations. It improves mechanical performance to the levels approaching those of polypropylene impact copolymers. It increases stiffness by up to 50%, heat distortion temperature by 10-40°C, and productivity. It has ability to create lamellar crystal orientation of PE in the polymer flow direction reducing shrinkage. ... 

However, the overall crystallinity and melting temperature decreased with increasing filler content. Tensile and dynamic mechanical analysis measurements indicated that expanded graphite was a better hller than untreated graphite. The overall improvement in mechanical properties was not great, but the mechanical strength and stiffness of high-density polyethylene were improved by the addition of the fillers. 

Thermoplastics used to blend with NR include PS, " polyamide 6, ethylene-vinyl acetate (EVA) copolymer, poly(methyl methacrylate) (PMMA), polypropylene (PP), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE) " and high-density polyethylene (HDPE). To improve the properties of TPNR, modified NR is also used. ENR is the most frequently used modified NR. TPNR blends are prepared by blending NR and thermoplastics in various proportions. The role of rubber is to improve the impact strength and ductility of the plastic. Depending on the ratio, materials with a wide range of properties are obtained. The stiffness of the rubber is increased with the incorporation of plastic into the rubber matrix. The mechanical properties of TPNR again depend on the proportions of the rubber and thermoplastic components. The elastic properties of TPNR are considerably... 

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