Mechanical Test of LDPE and HDPE Processed by Extrusion, Injection Molding, Compression Molding and Sheet Extrusion
LDPE and HDPE were processed by extrusion, injection molding, and sheet extrusion. Their mechanical properties such as tensile strength and percent elongation were measured by tensile test and analyzed statistically (Table 2). During the extrusion process, both polymers underwent die swelling. The water cooled polymer cords have a higher tensile strength but lower % elongation compared to the air cooled cords. HDPE has a much higher strength than LDPE due to its high crystallinity. LDPE and HDPE samples processed by injection molding and sheet extrusion show the same tendency in the extent of yield strength and elastic modulus. For sheet extrusion, the heat treated polymer sheet has a higher strength than the non-heat treated sheet because heating leads to an increase in both crystallinity and crystallite size. The specimen in rolling direction also has a higher strength than those in transverse direction due to the alignment of the polymer chains in rolling direction. UHMWPE (Ultra-high-molecular-weight polyethylene) saucer was processed by compression molding. The cross section of the saucer was examined by optical microscopy. Further, the melting temperature of PEO was determined to be 74.0°C ~ 78.9°C. Introduction
A polymer is a chemical compound or mixture of compounds consisting of repeating structural units created through a process of polymerization.1 The units composing polymers derive from molecules of low relative molecular mass. When all the repeating units along a chain are of the same type, the resulting polymer is called homopolymer. Chains composed of two or more different repeat units are termed copolymers. The physical characteristics of a polymer depend both on its molecular weight and shape, and the structure of the molecular chains. The chain structures include linear polymer, branched polymer, crosslinked polymer and network polymer. The polymer synthesized in this experiment, LDPE and HDPE, have different chain structures (i.e. LDPE is a branched polymer and HDPE is a linear polymer.). The polymer chain structure has a significant influence on polymer crystallinity, which is defined as the packing of molecular chains to produce an ordered atomic array.2 The mechanical properties that investigated in this paper, such as tensile strength, elastic modulus and percent elongation, greatly depend on the crystallinity of the polymer sample. Polymers play an essential and ubiquitous role in everyday life from those of familiar synthetic plastics and other materials of day-to-day work and home life, to the natural biopolymers that are fundamental to biological structure and function.1 Quite a variety of different techniques are employed in the forming of polymeric materials. Molding is the most common method for forming plastic polymers. The several molding techniques used include extrusion molding, compression molding, blow molding and injection molding.3 During molding, crystal regions in polymer melts upon heating. The resulted polymer melts are non-Newtonian fluids, and their viscosity depends on the shear rate. Melt index (MI) could be used to indicate the viscosity of the fluid. It is defined as the mass of polymer flowing in ten minutes through a capillary of a specific diameter and length by a pressure applied.4 Polymer melts are formed into a continuous charge of viscous fluid. The viscous fluid then solidifies into polymer product with specific shapes. During the solidification process, polymer melts recrystallizes and forms spherulite structure consisted of both amorphous region and lamellar. Experimental Procedure
Crystallization of PEO
Crystallization of PEO (Sigma-Aldrich Co., St. Louis, MD) was observed by optical microscopy. Sample of PEO powder was placed on glass slide and heated up using hot stage. Both the melting and the solidification processes were...
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