Plasticity semi-crystalline polymers

When designing plastic parts it is often recommended that the part have uniform thickness. This is especially true for semi-crystalline polymers where thickness variations lead to variable cooling times, and those in turn to variations in the degree of crystallinity in the final part. Variations in crystallinity result in shrinkage variations, which lead to warpage. However, it is often necessary to design parts in which a thickness variation is inevitable, i.e., extrusion profiles with thickness variations as shown in Fig. 6.14. 

It should be noted that, while this model was developed with the structure of a craze in a glassy polymer in mind, a generalized form, such as Eq. (22), should apply equally well for polymers which do not craze but form a cavitational plastic zone ahead of the crack tip, such as semi crystalline polymers near a hard interface. Such a situation will be examined in Sect. 6.2. 

Polyamide-6 (PA-6) and polypropylene (PP) are both semi-crystalline polymers and the combination of an engineering plastic (PA) and the best commodity product (PP) could lead to new blends with Interesting Intermediate properties. We tested systems containing 50 wt% of each product and the ones obtained by addition of 3% of the reactive PP-g-AM resulting from previous continuous grafting in the extruder. The blends were prepared by simple mixing in the ZSK 30 twin-screw extruder and the samples for mechanical testing were molded by injection in a BILLION equipment. 

Polyethylene is a major commodity plastic, with more than 33 billion pounds of the resin produced in the United States in 2000 [1]. Polyethylene encompasses a family of semi crystalline polymers with ethylene as the major building block [6]. The resins are loosely grouped into three classes low-density polyethylene (LDPE), high-density polyethylene (HD.PE), and linear-low-density polyethylene (LLDPE). LDPE is a homopoiymer of ethylene with side-chain branching at a frequency... 

ISO 3146, Plastics—Determination of melting behaviour (melting temperature or melting range) of semi-crystalline polymers, 1985. 

In Section 9.3 we present experimental studies of plastic deformation in two semi-crystalline polymers, HDPE and Nylon-6, both in tension and in plane-strain compression flow, from initial spherulitic morphologies to large plastic strains. In these studies, the evolving morphological alterations were monitored closely by a complementary array of techniques involving light microscopy and X-ray diffraction and scattering both in the crystalline and in the amorphous components. 

In the plastic response of semi-crystalline polymers the starting material has an initial spherulitic morphology and, in the process of simple extensional flow, either in tension or in plane-strain compression, ends up with a highly perfect... 

Whatever the morphology of a semi-crystalline polymer such as HDPE, the irreducible structural unit involved in the plastic flow is a pair of coupled crystalline lamellae and its complement of an amorphous layer in between, where the ratio of the thickness of the crystalline lamellae to that of the pair of crystalline and amorphous layers is, to a first approximation, equal to the crystallinity of the polymer. 

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