Plastics semi-crystalline

An important subdivision within the thermoplastic group of materials is related to whether they have a crystalline (ordered) or an amorphous (random) structure. In practice, of course, it is not possible for a moulded plastic to have a completely crystalline structure due to the complex physical nature of the molecular chains (see Appendix A). Some plastics, such as polyethylene and nylon, can achieve a high degree of crystallinity but they are probably more accurately described as partially crystalline or semi-crystalline. Other plastics such as acrylic and polystyrene are always amorphous. The presence of crystallinity in those plastics capable of crystallising is very dependent on their thermal history and hence on the processing conditions used to produce the moulded article. In turn, the mechanical properties of the moulding are very sensitive to whether or not the plastic possesses crystallinity. 

Typical Characteristics of Some Important Plastics (a) Semi-crystalline plastics... 

HDPE is a semi crystalline plastic, whose crystallinity varies from 40 to 80%. depending on the degree of branching and molecular weight. Polymer chains in crystalline HDPE have a flat zigzag configuration. The principal crystalline form of HDPE is orthorhombic, with a density of 1.00 g/cm3 and the cell parameters a =0.740, b = 0.493. and c ... 

Escaig B, G Sell C, Plastic Deformation of Amorphous and Semi-Crystalline Materials, Les Editions de Physique, Les Ulis (France), 1982. 

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. 

At room temperature, PE is a semi-crystalline plastomer (a plastic which on stretching shows elongation like an elastomer), but on heating crystallites melt and the polymer passes through an elastomeric phase. Similarly, by hindering the crystallisation of PE (that is, by incorporating new chain elements), amorphous curable rubbery materials like ethylene propylene copolymer (EPM), ethylene propylene diene terpolymer (EPDM), ethylene-vinyl acetate copolymer (EVA), chlorinated polyethylene (CM), and chlorosulphonated polyethylene (CSM) can be prepared. 

Polymers Elastomers (rubbers) Soft plastics (e.g. low density PE) Isotropic hard plastics ( Amorphous [ Semi-crystalline Conventional fibres (nylon, PETP) "Extended chain" fibres (Extended zig-zag chains (Extended helical chains ( Glassy. Cross-linked (Tg < 275 K lTg> 325 K 0.001 0.2 2.5- 3 2.5- 5 1-3 3 5-15 250fl ca 50fl... 

Plastic deformation of the original semi-crystalline structure... 

Copolymers are ubiquitous and important because they allow monomers to be combined in such a way so as to provide useful and sometimes unique properties. For example, linear polyethylene (PE) and isotactic polypropylene (i-PP) are both semi-crystalline plastics, but copolymers of ethylene and propylene (EPR) (usually with other comonomers) are rubbers at room temperature (depending on composition). The homopolymers are shown in the top two figures in Figure 6-2, and if you don t know which one s which by now you should collapse in deepest humiliation. A section of an EPR copolymer chain is shown at the bottom. 

For pharmaceutical materials moisture is known to affect a wide range of properties such as powder flow compactibility and stability (physical chemical and microbiological) (8 46-53). The interaction between moisture and a solid is complex and can occur in a variety of ways. For example water can be stoichiometrically incorporated into a solid s crystal structure in the form of a hydrate (pseudo-hydrate) as discussed previously in this section. In addition moisture can have non-stroichiometrical i.e., nonspecific interactions with a solid by adsorbing on the surface or being absorbed into the material and acting as a plasticizer. These non-specific interactions are more common in amorphous or semi crystalline materials and are the subject of this section. 

There have been many efforts for combining the atomistic and continuum levels, as mentioned in Sect. 1. Recently, Santos et al. [11] proposed an atomistic-continuum model. In this model, the three-dimensional system is composed of a matrix, described as a continuum and an inclusion, embedded in the continuum, where the inclusion is described by an atomistic model. The model is validated for homogeneous materials (an fee argon crystal and an amorphous polymer). Yang et al. [96] have applied the atomistic-continuum model to the plastic deformation of Bisphenol-A polycarbonate where an inclusion deforms plastically in an elastic medium under uniaxial extension and pure shear. Here the atomistic-continuum model is validated for a heterogeneous material and elastic constant of semi crystalline poly( trimethylene terephthalate) (PTT) is predicted. 

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. 

The ionomer which was isolated from the neutralization of sample SBD-2 was a brown-colored elastic network of moderate strength. Ionomer samples SBD-1 and SBD-2, neutralized to the stoichiometric end point using KOH, were compression molded at 140°C and examined for tensile properties. The results, as shown in Figure 16, illustrate the profound influence of crystallinity on the elastomeric inner block. The semi-crystalline material (SBD-1) behaves much like a rigid plastic, while the amorphous sample (SBD-2) is an elastomer of moderate strength. 

Aromatic polyesters having an amorphous molecular structure. Compared with other amorphous engineering plastics in terms of heat resistance, polyarylates are generally positioned between polycarbonate on the low side and sulfone and polyether polymers on the high side. Compared with crystalline and semi-crystalline engineering plastics, polyarylate resins offer better resistance to warping, and generally comparable mechanical properties. 

---