Thermosets processing fundamentals

There are a variety of ways to classify polymers according to their molecular structure and these will be covered in more detail later in Chapter 4. However, there are two general types that should be mentioned here. Most polymers can be broadly classified as either thermoplastics or thermosets. The fundamental physical difference between the two has to do with the bonding between molecular chains - thermoplastics have only secondary bonds between chains, while thermosets also have primary bonds between chains. The names are not only associated with the chemical structure of each but their general thermal and processing characteristics as well since... 

Both thermoplastics and thermosets can reap the benefit of fibre reinforcement although they have developed in separate market sectors. This situation has arisen due to fundamental differences in the nature of the two classes of materials, both in terms of properties and processing characteristics. 

The fundamental benefit of UV powder coatings is that they can be processed at considerably lower temperatures than traditional thermoset powders. While ordinary thermoset powders require temperatures in the range of 350 to 400°F (177 to 204°C), commercial UV curable powders require temperatures that are not higher than 220 to 250°F (104 to 121°C). A comparison of the two types is in Table 7.7. 

The Polymer Data Handbook offers, in a standardized and readily accessible tabular format, concise information on the syntheses, structures, properties, and applications of the most important polymeric materials. Those included are currently in industrial use or they are under study for potential new applications in industry and in academic laboratories. Considerable thought was given to the criteria for selecting the polymers included in this volume. The first criterion was current commercial importance—the use of the polymer in conunercial materials—for example, as a thermoplastic, a thermoset, or an elastomer. The second criterion was novel applications—a polymer that is promising for one or more purposes but not yet of conunercial importance—for example, because of its electrical conductivities, its nonlinear optical properties, or its suitability as a preceramic polymer. The hope is that some readers wiU become interested enough in these newer materials to contribute to their further development and characterization. Finally, the handbook includes some polymers simply because they are unusually interesting—for example, those utilized in fundamental studies of the effects of chain stiffness, self-assembly, or biochemical processes. 

These conclusions imply that fundamental studies of the phase separation process in polysulfone/thermoset nuxtures are needed to better understand the kinetics of the situation. General conclusions about the range of variables that allow for the development of a desirable morphology compared to others are needed to be able to take full advantage of the fracture toughness enhancements... 

Essentially, polymers can be defined as either thermoplastic or thermosetting. Although they often display similar properties, and indeed in the automotive industry frequently compete for the same applications, there are fundamental differences in structure and processing methods. These differences have implications for production costs and feasibility. 

We started this chapter with a description of the fundamental differences between thermoplastic and thermoset materials we will end it with an example of a subtle blend of their respective properties. A disadvantage of the three-dimensional network of EPDM, but actually of all thermoset materials, is the lack of recyclability. Crosslinked EPDM, both the waste from production and after use, cannot be processed in the melt again like thermoplastics. Reclaiming technologies have been developed for vulcanized mbber, degrading part of the network via high-temperature and shear treatment, but these technologies are less effective for EPDM vulcanizates, probably because the EPDM chains are so stable. A break-... 

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