HDPE crystals

The extrapolated equilibrium melting point of orthorhombic HDPE crystals is 146 147 C. Actual measurements of slowly crystallized samples give the highest melting point. Tm. at 133- 138°C. 

Fig. 3.72 Contact mode AFM deflection images of thin films of HDPE crystallized on calcite surface with the film thicknesses of (a) 0.1, (b) 0.2, (c) 0.4 and (d) 0.6 pm. Reproduced with permission from [150], Copyright 1999. Elsevier...

More extensive investigations have been performed on HDPE/PP blends by Martuscelli et al. [1980] and Bartczak and Galeski [1986]. From the isothermal crystallization experiments, it was found that the rate of crystallization of the HDPE matrix was markedly reduced upon addition of small amounts of PP (10 wt%). The authors attributed this phenomenon to the increased melt-viscosity of the sample caused by the presence of solidified PP domains. Moreover, Plesek and Malac [1986] have calculated from the surface tensions of the homopolymers at T, that PP crystallization will not cause the nucleation of the HDPE phase, while in the reverse case HDPE crystals will induce the nucleation of PP. 

DSC results showed a melting temperature depression of HOPE caused by the dilution effect of the non-crystalline EVA and the probable co-crystallization of some EVA chains with HD PE chains [38, 40]. Changes in the crystallization and melting temperatures of EVA were determined mainly by the nucleation effect of HDPE crystals and the effect of partial miscibility between these polymers. Crystallization kinetics results showed that the addition of more HDPE into an EVA matrix caused more heterogeneous nucleation, while the addition of EVA would delay the nucleation of HDPE at the beginning of the cooling process. Intermolecular interaction in the melt facilitated the crystallization of both EVA and HDPE. 

HDPE crystallizes very rapidly. Articles such as films, filaments, pipes, and injection-molded articles exhibit some degree of molecular and crystal orientation. This orientation develops either spontaneously during melt flow and crystallization, or is introduced deliberately by stretching. The degree of orientation can be measnred by x-ray, polarization spectroscopy, acoustical methods, or birefringence. 

This might be due to a molecular chain scission of NR molecules caused by degradation during mastication taking place. According to the mastication degradation mechanism, long-chain molecular model and DSC results, it was revealed that partial NR chains were clamped by HDPE crystals and thus acted as tie molecules in the blend. 

Figure 13.1 Contact mode AFM deflection image of 0.2-itm thin films of HDPE crystallized on calcite surface. Reprinted from Bartczak, Z., et al. The morphology and orientation of polyethylene in films of sub-micron thickness crystallized in contact with calcite and rubber substrates. Polymer 1999, 40, 2367-2380, with kind permission from Elsevier, Copyright 1999.

Whitening. The coated conductor passes through water at about 80 °C and thus the HDPE crystallizes, precipitates out, and whitens the solution. 

Crystallization Kinetics. Crystallization of HDPE proceeds in two separate stages. During the first stage, HDPE crystaUizes rapidly. 

Orientation. Most articles made of HDPE, including film, fiber, pipes, and injection-molded articles, exhibit some degree of molecular and crystal orientation (21). In some cases, orientation develops spontaneously for example, during melt flow into a mold and its subsequent crystallisation. When blown HDPE film and fiber are manufactured, orientation can be introduced dehberately by stretching. 

Physical Properties. LLDPE is a sernicrystaUine plastic whose chains contain long blocks of ethylene units that crystallize in the same fashion as paraffin waxes or HDPE. The degree of LLDPE crystallinity depends primarily on the a-olefin content in the copolymer (the branching degree of a resin) and is usually below 40—45%. The principal crystalline form of LLDPE is orthorhombic (the same as in HDPE) the cell parameters of nonbranched PE are a = 0.740 nm, b = 0.493 nm, and c (the direction of polymer chains) = 0.2534 nm. Introduction of branching into PE molecules expands the cell slightly thus a increases to 0.77 nm and b to around 0.50 nm. 

The slow cooling of crystalline polymers, such as HDPE and PP, can allow large crystal formations to develop. Such crystals... 

Structural steels, tellurium in, 24 425 Structure(s), see also Chain structure Chemical structures Cocontinuous structures Controlled structure Crystal structure Molecular structure Morphology Phase structure of carbon fibers, 26 737-739 detersive systems for, 8 413t HDPE, 20 157-162 LLDPE, 20 182-184, 203-205 polyesterether elastomer, 20 72-73 polyester fiber, 20 21 polyether antibiotics, 20 137-139 polyimide, 20 276-278 polymer, 20 395-405 protein, 20 449 PTT, 20 68t... 

A number of organic pigments cause distortion in certain types of polyolefins, especially in HDPE. Pigments act as nucleating agents in such partially crystalline plastics i.e., they promote crystallization, which creates stress within the plastic product (Sec. 1.6.4.3). These pigments also enhance the shrinkage of polyolefins, particularly in the direction of the flow. 

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