Melt mixing, disadvantage

The melt mixers are either batch or continuous type. The formers require lower investment cost, but are more labor-intensive, have low output and poor batch-to-batch reproducibility. Recent developments in process control and automation eliminated some of these disadvantages [Utracki, 1991]. The continuous melt mixers comprise extruders, continuous shaft mixers and specialty machines — these will be discussed in the following part of this chapter. A brief overview of the melt mixing devices is given in Table 9.8. 

Blends discussed in preceding sections were based on two or more preformed polymers. One disadvantage of such systems is that commercial melt mixing involves two viscous components. Also, intermolecular interactions, which might aid miscibihty, can further enhance the viscosity of the medium. An alternative procedure is to form at least one of the polymeric components in situ. There have been two studies to date which have involved oligomeric materials in generating PCL blends. 

Meta-phenylenediamine, a crystalline solid with a melting point of about 60°C, gives cured resins with a heat distortion temperature of 150°C and very good chemical resistance. It has a pot life of six hours for a 200 g batch at room temperature whilst complete cures require cure times of four to six hours at 150°C. About 14 pts phr are used with the liquid epoxies. The main disadvantages are the need to heat the components in order to mix them, the irritating nature of the amine and persistent yellow staining that can occur on skin and clothing. The hardener finds use in the manufacture of chemical-resistant laminates. 

The disadvantage of lasers with nanosecond-picosecond pulse duration for depth profiling is the predominantly thermal character of the ablation process [4.229]. For metals the irradiated spot is melted and much of the material is evaporated from the melt. The melting of the sample causes modification and mixing of different layers followed by changes of phase composition during material evaporation (preferential volatilization) and bulk re-solidification [4.230] this reduces the lateral and depth resolution of LA-based techniques. 

Metaphenylene diamine (MPDA) is one of the most common of the aromatic amines used to cure epoxies. This product is amber to very dark in color. It is a solid that melts at 65°C and is generally mixed with the epoxy resin at that temperature. The molten liquid or vapors from MPDA can stain the skin and nearby structures rather badly. The para-isomer is reported to be carcinogenic, but the meta-isomer is free from this disadvantage. 

The fuels for fast breeder reactors include alloys such as U-Pu-Zr and the ceramic materials UO2-PUO2, UC-PuC, and UN-PuN, but the mixed oxides, UO2-PUO2, are the choice for prototype fast breeder fuel elements because of their high melting temperature, compatibility with cladding and coolants, and relatively good irradiation stability and fission product retention. The disadvantages are the relatively low metal density, the... 

Phthalic anhydride is the cheapest anhydride curing agent, but it has the disadvantage of being rather difhcult to mix with the resin. Liquid anhydrides (e.g., dodecenylsuccinic anhydride and nadic methyl anhydride), low-melting anhydrides (e.g., hexahydrophthalic anhydride), and eutectic mixtures are more easily incorporated into the resin. Since maleic anhydride produces brittle products, it is seldom used by itself and is used as a secondary hardener in admixture with other anhydrides. Dodecenylsuccinic anhydride imparts flexibility into the casting, whereas chlorendic anhydride confers flame resistance. 

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