Thermoset processing discussion

The field of research on the chemorheology of thermosetting polymers is new and exciting. As our knowledge of the curing processes expands, new materials will come into focus. These too, must be understood. The reader should also look beyond the application of these methodologies to the thermoset materials discussed herein, and use the information to develop further... 

These reactions are quite numerous and have been utilized for a long time. They also include all thermosetting processes of polymers. Many are discussed in previous chapters. 

Thermoplastic elastomers (TPEs) are polymeric materials that are processed into fabricated articles in the same manner as a conventional thermoplastic, yet these articles have the properties and functional performance of a thermoset rubber. They have been gaining a significantly larger market share over the past three decades with nontire applications, compared to the conventional thermoset elastomers discussed earlier. 

Some of the most common stabilization—soHdification processes are those using cement, lime, and pozzolanic materials. These materials are popular because they are very effective, plentiful, and relatively inexpensive. Other stabilization—soHdification technologies include thermoplastics, thermosetting reactive polymers, polymerization, and vitrification. Vitrification is discussed in the thermal treatment section of this article and the other stabdization—soHdification processes are discussed below. 

Details are given of the development of energy and material recycling processes for thermosetting polymer composites. Applications in the cement industry and in coal fired fluidised bed combustion plants are discussed. 3 refs. 

The characteristics of the three most common thermoset resin systems used in pultrusion are compiled in Table 11.2 [3]. It is noteworthy that unreinforced polyesters and vinylesters shrink 7-9% upon crosslinking, whereas epoxies shrink much less and tend to adhere to the die. These epoxy characteristics translate into processing difficulties, reduced processing speed, and inferior component surface finish. It is normal practice to use resin additives to improve processability, mechanical properties, electrical properties, shrinkage, environmental resistance, temperature tolerance, fire tolerance, color, cost, and volatile evaporation. It is normally the resin, or rather its reactivity, that determines the pulling speed. Typical pulling speeds for polyesters tend to be on the order of 10-20 mm/s, whereas speeds may exceed lOOmm/s under certain circumstances. Apart from the resins characterized in Table 11.2, several other thermosets, such as phenolics, acrylics, and polyurethanes, have been tried, as have several thermoplastics (as will be discussed in Sec. 11.2.6). 

It is surprising to realize how often the undercure produced by vitrification is completely ignored when performing the thermosetting polymerization by irradiation (UV, EB, X-ray) at room temperature. As there is no external heat source, once vitrification sets in conversion may only increase through the continuation of reaction in the glassy state. However, as we have discussed in Chapter 5, polymerization in the glassy state is a self-retarded and very slow process. 

Hydrolysis must appear in principle as a pseudo-zero-order process, except if the end-life conversion nf/E0 exceeds largely 10%, which seems unlikely. In linear polymers, the number of chain scissions n can be determined from molar mass measurements. In thermosets, it is considerably more difficult to determine. Some possible ways are discussed in the following section. 

In the field of high thermomechanical performance polymers, linear and thermosetting systems offer complementary properties. Among the thermosetting materials, BMIs and BNIs have been extensively studied and are now commercially available. In this chapter, firstly the main preparation and characterization methods are reviewed, and then the chemistry of the polymerization processes is discussed for both families. For the BMIs, due to the electrophilic character of their double bond, different polymerization pathways have been published, which is not the case for BNIs. Special attention has been paid to thermal polymerization which has already been used in industrial achievements however, on the other hand, the structure of these materials has been considered for the purpose of establishing relationships between processability, stability and thermomechanical properties. 

Baekeland succeeded where others had failed, not only because he carried out his process under pressure but also because he recognized the importance of controlling the relative proportions of phenol and formaldehyde. Too little formaldehyde, for example, and the product was weak and brittle. Baekeland also worked out conditions for reacting phenol with a small amount of formaldehyde under acidic conditions, to produce a thermoplastic material he called novolac. Novolac can be converted into a thermoset similar to Bakelite by heating with additional formaldehyde. Novolac became useful as a positiveworking photoresist for the manufacture of integrated circuits some 60 or 70 years later. We will discuss photoresists later in this chapter. 

Adhesives commonly used on thermosetting materials include epoxies, urethanes, cyanoacrylates, thermosetting acrylics, and a variety of nonstructural adhesive systems. The following discussion includes a very brief description of various thermosetting substrate materials, the properties that are critical relative to epoxy adhesion, and any special processes that should be noted for the particular substrate. 

The observation that an increase in temperature or a decrease in rate both result in the same fracture response points toward a viscoelastic influence on thermoset fracture behavior, especially crack initiation. This characteristic behavior of epoxies has been explained qualitatively by consideration of the temperature and strain rate effects on the plasticity of the material at the crack tip . In effect, test conditions which promote the formation of a so-called crack tip plastic zone, or blunt the crack by a ductile process, promote unstable crack propagation. This aspect of unstable fracture is subsequently discussed in more detail. 

The work described herein relates primarily to lamination and bonding processes. However, the techniques are generic to most forms of thermoset resin processing. In the discussion which follows many of the resin systems contain glycidyl amines. The bulk of the epoxy formulations used in the aerospace industry today are based on tetraglycidylmethylenedianiline, I (TGMDA) and with diaminodi phenylsulfone, II (DOS). Systems based on... 

It is beyond the scope of this review to discuss in detail viscoelastic properties at and after gelation. The gel point is one of the important characteristics in the epoxy resin curing process from both kinetic and rheological aspects. The gelation transition has been widely studied for epoxy resin systems [39,44,133-138] and already discussed in some of this series, such as by Malkin and Kulichikhin [28], Williams et al. [139], and Winter and Mours [140]. Rheological techniques for the determination of the gel point have been summarized for thermosetting resins by Halley et al. [29,141]. 

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