Thermoset-matrix materials

For thermoset-matrix materials, heat is usually added as a catalyst to speed the natural chemical reaction of polymerization. Two-part epoxies, such as found in your local hardware store, consist of a tube of epoxy and a tube of chemical hardener that react when mixed. Heat... 

Figure 13.7 Flow chart showing interrelationship of submodels for filament winding with thermosetting matrix materials...

Polymer composites are plastics within which fibres are embedded. The plastic is known as the matrix (resin) and the fibres dispersed witbin it are known as the reinforcement Thermosetting matrix materials include polyester, vinyl ester and epoxy resins. For higher temperature and extreme environments, bismaleimlde, polyimide and phenolic resins are used. Composites can be used to replace metal parts but care must be taken during design. Most engineering materials have similar properties in any direction (called isotropic) where composites have not This can however be offset by arranging the reinforcement layers in varying directions. 

The selection of a suitable thermoplastic matrix material mostly depends on the desired mechanical properties and the desired long-term service temperature. Depending on the application area, there are other decision criteria known from thermoset matrix materials, which can be of major interest, for example the chemical resistance or the water absorption properties. In contrast, thermoplastic composites are normally featured with an improved toughness compared to their thermoset competitors. Table 8.2 gives an overview on properties of different common polymers which are used for the pultrusion process. 

Polymer features that lead to miscibility with polysulfone should be further quantihed to be able to optimize the membrane separation characteristics of polymer mixtures. On the other hand, in the case of immiscible polysulfone blends, it is desirable to better define the features of the blend components that lead to a particular morphology. Some of those features are perhaps going to be different in the case of thermoplastic and thermoset matrix materials, but viscosity is certainly going to be relevant in both cases. However, in order to best utilize the polysulfone blends that have been discussed in this chapter, more work is required to better comprehend their structure-property-processing relationships. 

The generic thermosets are the epoxies and the polyesters (both widely used as matrix materials for fibre-reinforced polymers) and the formaldehyde-based plastics (widely used for moulding and hard surfacing). Other formaldehyde plastics, which now replace bakelite, are ureaformaldehyde (used for electrical fittings) and melamine-formaldehyde (used for tableware). 

The term cocuring means that two parts that must be fastened together are cured simultaneously and in contact to achieve permanent bonding between them. The process applies equally to thermoset-matrix composite materials and to thermoplastic-matrix composite materials (except the cocuring of two thermoplastic-matrix parts is not, of course, permanent). 

Describe and discuss thermoset-matrix and thermoplastic-matrix materials. Contrast their production times if you were to build a composite structure with both materials. 

Polymer-matrix materials include a wide range of specific materials. Perhaps the most commonly used polymer is epoxy. Other polymers include vinyl ester and polyester. Polymers can be either of the thermoset type, where cross-linking of polymer chains is irreversible, or of the thermoplastic type, where cross-linking does not take place but the matrix only hardens and can be softened and hardened repeatedly. For example, thermoplastics can be heated and reheated, as is essential to any injection-molding process. In contrast, thermosets do not melt upon reheating, so they cannot be injection molded. Polyimides have a higher temperature limit than epoxies (650°F versus 250°F or 350°F) (343°C versus 121°C or 177°C), but are much more brittle and considerably harder to process. 

StructurPly I and II by Multi-Axial are thermoplastic matrix resin prepregs used in the construction of primary-load-carrying composite structures. Unlike the few hours typically required by the thermoset resin materials to cure, StructurPly needs a few minutes at 196°C. 

In general, due to the simpler manufacturing process and the cheaper polymer matrix, the thermoset- or thermoplastic-based composite plates have lower cost than that of carbon/carbon composite plates. The thermoset-based composite and thermoplastic-based composite plates have their own advantages and limitations, which are largely determined by the properties of the matrix materials and the ratio between the filler and mafrix. 

Most structural PMCs consist of a relatively soft matrix, such as a thermosetting plastic of polyester, phenolic, or epoxy, sometimes referred to as resin-matrix composites. Some typical polymers used as matrices in PMCs are listed in Table 1.28. The list of metals used in MMCs is much shorter. Aluminum, magnesium, titanium, and iron- and nickel-based alloys are the most common (see Table 1.29). These metals are typically utilized due to their combination of low density and good mechanical properties. Matrix materials for CMCs generally fall into fonr categories glass ceramics like lithium aluminosilicate oxide ceramics like aluminnm oxide (alnmina) and mullite nitride ceramics such as silicon nitride and carbide ceramics such as silicon carbide. 

In most applications, polyester and vinyl ester resins are used as the matrix materials. Epoxies are also used, although they require longer cure times and do not release easily from the pultrusion dies. Hence, thermosetting resins are most commonly used with pultrusion, although some high-performance thermoplastics such as PEEK and polysulfone can also be accommodated. In addition to the resin, the resin bath may contain a curing agent (initiator, cf. Section 3.3.1.2), colorants, ultraviolet stabilizer, and fire retardant. 

The use of advanced composites has increased significantly in the last decade. The properties of high-specific strength and stiffness make composites ideal for many aerospace, automotive, and infrastructure applications. Fiber-reinforced composites, which commonly use thermosetting resins such as epoxies as the matrix material, have some inherent deficiencies. These include the need for multistep processing, limited shelf-life, low toughness, sensitivity to moisture, and the inability to reprocess or reform the material [1]. 

As pointed out in the beginning of the chapter—and perhaps also noticeable from the process description earlier—the pultrusion process at first appears rather simple and straightforward however, several intricacies arise on closer inspection. The key technology issues of pultrusion of thermoset-matrix composites are usually considered to be resin formulation, temperature control, material guidance, and die design (not in order of importance). It is typically the skill in these areas that distinguishes a successful pultruder from his hapless competitor. 

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