Thermoplastics semi-crystalline polymers

With an amorphous thermoplast the polymer softens over a rather short temperature interval from the glassy to the rubbery state. With a semi-crystalline polymer a certain amount of softening takes place at Tg with further T- increase the stiffness drops very gradually up to the melting point... 

PE, PP and PA are semi-crystalline polymers melting and solidification go accompanied by a (though gradual) volume jump. PS, PVC and PC are amorphous thermoplastics upon solidification they show no volume jump, but only a bend in the V-T relation. 

Furthermore, it is not surprising that the thermal conductivity of melts increases with hydrostatic pressure. This effect is clearly shown in Fig. 2.3 [19]. As long as thermosets are unfilled, their thermal conductivity is very similar to amorphous thermoplastics. Anisotropy in thermoplastic polymers also plays a significant role in the thermal conductivity. Highly drawn semi-crystalline polymer samples can have a much higher thermal conductivity as a result of the orientation of the polymer chains in the direction of the draw. 

Brittleness is found with semi-crystalline polymers below their glass-rubber transition Tg. An example is PP, which becomes brittle at about T -10 °C. PE retains its ductile nature down to very low temperatures. Other polymers have a Tg of some tens of °C above room temperature, such as polyamides and thermoplastic polyesters. Various mechanisms are responsible for a reasonable impact strength at room temperature for polyamides this is, for instance, the absorption of water also secondary transitions in the glassy region may play a role. 

Figure 3.13 shows the shift factors aT determined from time-temperature superposition as a function of temperature for melts of two semi-crystalline thermoplastics as well as the Arrhenius plot. For the two polyethylenes (HDPE, LDPE), the progression of log ax can be described with the Arrhenius equation. The activation energies can be determined from the slope as Ea(LDPE) 60 kj/mol and Ea(HDPE) 28 kj/mol. Along with polyethylenes (HDPE, LDPE, LLDPE), other significant semi-crystalline polymers are polypropylene (PP), polytetrafluoroethylene (PTFE) and polyamide (PA). 

The approximated lines at the right side of Fig. 3.14 correspond to an Arrhenius activation energy of approximately 120 kj/mol, which is significantly higher than the flow activation energy of the semi-crystalline polyolefin melts shown in Fig. 3.13. The temperature dependence of the melt viscosity for amorphous thermoplastics is substantially higher than that of semi-crystalline polymers and increases dramatically as the temperature approaches the glass transition temperature. 

Those which do crystallise invariably do not form perfectly crystalline materials but instead are semi-crystalline with both crystalline and amorphous regions. The crystalline phases of such polymers are characterised by their melting temperature (TJ. Many thermoplastics are, however, completely amorphous and incapable of crystallisation, even upon annealing. Amorphous polymers (and amorphous phases of semi-crystalline polymers) are characterised by their glass transition temperature (T), the temperature at which they transform abruptly from the glassy state (hard) to the rubbery state (soft). This transition corresponds to the onset of chain motion below T the polymer chains are unable to move and are frozen in position. Both T and T increase with increasing chain stiffness and increasing forces of intermolecular attraction. 

In terms of polymer matrices for composite materials, there will be a compromise between solvent and water resistance. Thus non-polar resins are likely to be less resistant to hydrocarbon solvents, which have low polarity, but more resistant to moisture absorption. Polar resins behave in the opposite way. Strongly polar solvents, such as dimethyl sulphoxide or similar, can interact with polar structures in the resin and are difficult to resist. Crystalline thermoplastic polymers are often better for such applications. For example, polyethene will only dissolve in hydrocarbon solvents (of similar solubility parameter) at temperatures above the crystalline melting point. Polar semi-crystalline polymers such as the polyamides or nylons can be dissolved in highly polar solvents, such as cresol, because of a stronger interaction than that between molecules within the crystallites. High performance thermoplastic polymers such as polyether ether ketone (PEEK) have been promoted for their resistance to organic solvents (see Table 3.5) [12], The chemical resistance of unsaturated polyester and vinyl ester and urethane resins is indicated in Table 3.6 [15]. 

Generally all thermoplastic polymers can be processed by thermoforming, but there are significant differences regarding process windows. Amorphous polymers show a wider softening temperature range than semi-crystalline polymers, which results in a wider process window and a more stable process [7]. For this reason only few semi-crystalline polymers are used in thermoforming, for example PP, PE, C-PET and polyester. The most commonly used amorphous polymers are PVC, PS, ABS, SAN, PMMA, PC and A-PET. 

It is not the intention of this chapter to be a review of the literature (if the reader is looking for this. Ref [1] is a recent example). Its purpose is to serve as an introduction to the technique of MTDSC starting with fairly basic and practical matters than progressing onto more advanced levels. It is also intended to serve as a guide to understanding the remaining chapters that deal with three principal classes of polymeric materials, thermosets, thermoplastic polymer blends and semi-crystalline polymers. 

Amorphous thermoplastic polymers have polymer chains in a random coil arrangement without any degree of local order, whereas a semi-crystalline polymer would have some degree of order of the polymer chains. The chains are entangled and because they are not fixed, can slip past one another, whereas a thermoset resin, when subjected to local stress breaks, in a brittle maimer. The ability of the thermoplast to dissipate energy by chain slippage confers the property of toughness to the composite. 

The polymers used in injection moulding can be divided into three main classes, depending on their structure and properties. These classes are amorphous thermoplastics semi-crystalline thermoplastics rubbers. 

All injection-moulded thermoplastics are prone to sink marks and voids in areas where sudden changes in section thickness occur, or over ribs and bosses. Semi-crystalline polymers such as PP are more prone to sinks and voids. Voids occur when the external skin of the moulding is rapidly cooled and becomes sufficiently rigid to support the contraction of the underlying melt. Sinking or surface depression occurs in localised thick sections where internal mass contains sufficient heat to keep the polymer in molten stage and erystallises slowly producing sink marks. 

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