Processing, thermoplastics crystallization

Phase Materials. Phase holograms can be recorded in a large variety of materials, the most popular of which are dichromated gelatin, photopolymers, thermoplastic materials, and photorefractive crystals. Dichromated gelatin and some photopolymers require wet processing, and thermoplastic materials require heat processing. Photorefractive crystals are unique in that they are considered to be real-time materials and require no after-exposure processing. 

In the last two decades, numerous experimental and theoretical studies dealing with reaction-induced phase separation in multiphase polymer systems (mostly porous matrices, toughened plastics, melt processable thermoplastics [143], molecular composites, polymer dispersed liquid crystals, etc.) have been reported. A newcomer in this field should get acquainted with hundreds (possibly thousands) of papers and patents. The intention of this review was to provide a qualitative basis (quantitative occasionally) to rationalize the various factors that must be taken into account to obtain desired morphologies. 

When processing thermoplastic materials, material properties not only dictate drying and processing conditions, viscosity, orientation, and shrinkage, but also the processing techniques and equipment that can be used. The next section reviews material properties with an emphasis on processing temperatures, particle properties, melt viscosity and elasticity, and orientation, relaxation, crystallization, and shrinkage. 

Some of the common types of plastics that ate used ate thermoplastics, such as poly(phenylene sulfide) (PPS) (see Polymers containing sulfur), nylons, Hquid crystal polymer (LCP), the polyesters (qv) such as polyesters that ate 30% glass-fiber reinforced, and poly(ethylene terephthalate) (PET), and polyetherimide (PEI) and thermosets such as diaHyl phthalate and phenoHc resins (qv). Because of the wide variety of manufacturing processes and usage requirements, these materials ate available in several variations which have a range of physical properties. 

Highly aromatic thermoplastic polyesters first beeame available in the 1960s but the original materials were somewhat difficult to process. These were followed in the 1970s by somewhat more processable materials, commonly referred to as polyarylates. More recently there has been considerable activity in liquid crystal polyesters, which are in interest as self-reinforeing heat-resisting engineering thermoplastics. 

Crystalline polyesters are highly important as adhesive raw materials. They are normally crystalline waxes and are highly symmetrical in nature, which can aid the crystallization process [26]. Poly(hexamethylene adipate) and poly(caprolactone), shown in Table 2, are only two of the many crystallizable backbones. Poly(ethylene adipate) and poly(letramethylene adipate) are also commonly used in urethane adhesives. The crystalline polyesters are used in curing hot melts, waterborne polyurethanes, thermoplastic polyurethanes, and solvent-borne urethane adhesives. The adipates are available mostly as diols. The poly(caprolactones) are available as diols and triols. 

The thermoplastic polyurethane (TPU) adhesives must, of necessity, contain low gel content because they must be processable in an extruder. Most adhesives are relatively linear, with a functionality of 2.0, although small amounts of branching may be introduced, usually at the expense of a lower melt flow. Good physical properties of TPU s are obtained when the thermoplastic urethanes have molecular weights of 100,000 or higher (see p. 56 in [63]). Most TPU adhesives are based on symmetrical polyesters with a fast crystallizing backbone or a backbone slightly modified to increase the open time. 

Thermoplastic urethane adhesives may be processed into an adhesive film. I,amination of two substrates can, in theory, be done immediately, but the film is often extruded onto one substrate, covered by a release liner, and allowed to cool. Crystallization follows to create a non-tacky film that may be cut into specific shapes. The release liner is then removed, and the shaped adhesive can be heat-activated on one substrate, using infrared lamps. The second substrate is then nipped under pressure, followed by a cooling press to speed crystallization. Once the backbone has crystallized, the bond should be strong. 

Liquid crystal polymers (LCP) are a recent arrival on the plastics materials scene. They have outstanding dimensional stability, high strength, stiffness, toughness and chemical resistance all combined with ease of processing. LCPs are based on thermoplastic aromatic polyesters and they have a highly ordered structure even in the molten state. When these materials are subjected to stress the molecular chains slide over one another but the ordered structure is retained. It is the retention of the highly crystalline structure which imparts the exceptional properties to LCPs. 

There has been considerable interest recently in an alternative type of ABA triblock structure, where the end blocks could form crystalline domains, by crystallization, rather than amorphous domains by phase separation. It was felt that, since such a crystallization process need not depend on the incompatibility between the blocks, it should be possible to have a homogeneous melt, which should exhibit a much lower viscosity, and hence much easier processing, than the heterogeneous media of the conventional triblock copolymers. Furthermore, thermoplastic... 

In addition to inherently faster crystallization kinetics in a water-heated mold, PBT offers several other benefits compared to PET. First of all, the lower processing temperatures required for PBT make it less susceptible to hydrolytic degradation and thus drying is not as critical as in the case of PET. Thermoplastic composites made from PBT also tend to have a higher % elongation to break (Table 15.2) [14, 15]. Although this attribute does not necessarily show... 

Poly(butylene terephthalate) (PBT) is a semicrystalline, thermoplastic polyester which is completely analogous to PET except that it has a longer, more flexible butylene chain linkage which imparts a rapid crystallization rate, thus making PBT well suited to injection moulding processes. This polyester is used widely for electrical and electronic components due to its high temperature resistance and good electrical properties (Chapter 8). 

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