Thermosetting matrices

Figure 2.5 shows the share for polypropylene matrices versus thermoset matrices. 

For information,Table 6.23 shows some property examples of syntactic foams with thermoset matrices. 

Particle/particle interactions induce aggregation, while matrix/filler interaction leads to the development of an interphase with properties different from those of both components. Both influence composite properties significantly. Secondary, van der Waals forces play a crucial role in the development of these interactions. Their modiflcation is achieved by surface treatment. Occasionally reactive treatment is also used, although its importance is smaller in thermoplastics than in thermoset matrices. In the following sections of this chapter attention is focused on interfacial interactions, their modification and on their effect on composite properties. 

New possibilities appeared with the use of nonbreaking impact tests. The question is how a high-speed striker with a low energy can induce damages and what the residual energy is. These tests are very often performed on composites with thermosetting matrices. 

Organic matrices are divided into thermosets and thermoplastics. The main thermoset matrices are polyesters, epoxies, phenolics, and polyimides, polyesters being the most widely used in commercial applications (3,4). Epoxy and polyimide resins are applied in advanced composites for structural aerospace applications (1,5). Thermoplastics Uke polyolefins, nylons, and polyesters are reinforced with short fibers (3). They are known as traditional polymeric matrices. Advanced thermoplastic polymeric matrices like poly(ether ketones) and polysulfones have a higher service temperature than the traditional ones (1,6). They have service properties similar to those of thermoset matrices and are reinforced with continuous fibers. Of course, composites reinforced with discontinuous fibers have weaker mechanical properties than those with continuous fibers. Elastomers are generally reinforced by the addition of carbon black or silica. Although they are reinforced polymers, traditionally they are studied separately due to their singular properties (see Chap. 3). 

In recent years new thermoplastic matrices have been developed to improve the stiffness/toughness balance and the service temperature, in comparison to the epoxy thermoset matrices used in high performance composites. These materials, usually referred to as advanced thermoplastic matrices (8), include polymers that have great structural similarities, with aromatic moieties in the main chain spaced by groups of the type diagrammed below. 

In Table 15.3 are shown the chemical structures and Tg and of some representative thermoplastic polymers for use at high temperature (3,9). These matrices have high continuous service temperatures (120-200° C) even under wet environmental conditions. Advantages of thermoplastic over thermoset matrices are their shorter fabrication cycle (generally controlled environment storage is not required) and the possibility to be reprocessed and reconsolidated after manufacture. Poly(ether ether ketone) is a strong contender with epoxy resins for use as a matrix in composite prepregs with carbon fibers to be utilized in structural aircraft components. 

The surface energy of fibers is closely related to the hydrophilicity of the fiber [6]. Some investigations are concerned with methods to decrease the hydrophilicity. The modification of wood-cellulose fibers with stearic acid [10] hydrophobizes those fibers and improves their dispersion in polypropylene. As can be observed in jute reinforced polyester composites, treatment with polyvinylacetate increases the mechanical properties [10] and moisture repel-lence [60]. Several similar investigations on biobased composites based on thermoplastic and thermoset matrices have been reported by several authors [61-63]. Some of the major findings based on the authors own experimental investigations are discussed in the following sections. 

The matrix is the weakest component of the composite determining the allowed maximum stress and the maximum service temperature. Great efforts have been undertaken to develop thermosetting polymers with increased temperature resistance. Cyanate-esters (CEs), BMI, and polyimides are the actual thermosetting matrices with higher thermal resistance. 

KAD 06] Kaddami H., Dufresne A., Khelifi B., et ai, Short palm tree fibers Thermoset matrices composites , Composites Part A Applied Science and Manufacturing, vol. 37, no. 9, pp. 1413-1422, September 2006. 

Natural fibers can be classified as seed fibers (such as cotton), bast fibers (like flax, hemp, jute, kenaf, ramie), hard fibers (like sisal), fi-uit fibers (like coir), and wood fibers. The chemical composition and dimensions of some common agro-fibers are presented in Table 5.21. The origin of wood fibers can be sawmill chips, sawdust, wood flour or powder, cutter shavings, pulp or wood residues. As binders for these fibers, both thermosetting (like phenolic, epoxy, polyester) resins and thermosetting matrices [such as polyethylene (LDPE, HDPE), polypropylene (PP), poly(vinyl chloride) (PVC), polystyrene (PS)j can be used. Thermoplastic composites are, however, less expensive to process than thermosetting composites, in addition to their ability to be manufactured into complex shapes. 

Thermosetting matrices, such as epoxies and thermosetting hot-melt adhesives, are used where increased reliability is required. Repair of anisotropically conductive interconnections assembled with thermoset adhesives is problematic, however, as the adhesive matrix must be removed completely from the substrate and device prior to reassembly. 

Table 12.1 gives typical values of the diffusion constant for a series of thermosetting matrices where the effect of polarity of the resin on the equilibrium on moisture concentration is illustrated. Epoxy resins are the network product of the reaction of a multifunctional epoxide monomer with a hardener. One of the important aspects of composite materials which needs to be recognised is that the material is synthesised at the same time as the component is manufactured. The precise chemistry of the final network can be uncertain but is a function of the chemical stmctures of the epoxide and hardener and/or catalyst, which determine the mechanism of cure. In this way, the cured resins can have differing polarities. 

The demand for better fuel efficiency based on the strict governmental regulations on safety and emission has led to the wide application of composites and plastics in the automotive industry in the place of the traditionally used steels [32]. Thermoplastic materials reinforced with natural fibers have reported to have excellent mechanical properties, recycling properties, etc. [33-36]. Several natural and biorenewable fibers such as wheat, isora, soybean, kenaf, straw, jute, and sisal are used in the fiber/plastic composite industry, and the use of namral fibers as reinforcements for composite has attracted many industries [37, 38]. Compared to polymer resin, polymer biocomposites that are reinforced with natural fibers have many applications due to its ease of processing, comparatively lower cost, and excellent mechanical properties [39]. For more than a decade, European car manufacturers and suppliers have been using natural fiber-based composites with thermoplastic and thermoset matrices. These biocomposites and bionanocomposites... 

Textiles, polymers and composites for buildings Table 4.2 Typical properties of thermosetting matrices... 

Marieta, C. Remiro, G. Garmendia, G. Harismendy, I. Mondragon, I. (2003). AFM approach toward understanding morphologies in toughened thermosetting matrices. European Polymer Journal, Vol.39, No.lO, (October 2003), pp. 1965-1973,... 

Thermoplastics added to thermoset matrices can also improve the degree of toughness and it is believed that PES and PEEK are used for this application in many of the new generation epoxy systems. 

MANUFACTURING PROCESSES FOR CARBON FIBERS IN THERMOSET MATRICES [61]... 

Other end uses in thermoset matrices 23.6.20.1 Model airplanes... 

Unidirectional carbon fibre reinforced epoxy resin as well as polycarbonate composites were manufactured with a fibre volume fraction of around 60 %. The main steps in manufacturing a composite are the impregnation, the moulding and the curing by thermoset matrices. Composites should be non- porous, crack-free and stress-free, with homogeneously distributed fibres embedded in the matrix. 

Depending on the adhesion between the filler and the polymer the particles can act as reMorcing agent or simply as stress concentrating. Usually the bond between the filler and the matrix is created by the pressure increase subsequent to the shriidcage during polymoization for thermosets matrices, or to tiie molding process for thermoplastics. 

Another serious drawback of using natural fibers with polymeric matrices is poor fiber-matrix interfacial adhesion, thus impairing composite mechanical and other properties. Optimization of interfacial adhesion between natural fibers and thermoplastic and thermoset matrices has been the focus of a large amount of research conducted during the past two decades (John and Anandjiwala 2008). Various... 

Most of the previous research on natural fibre composites has focused on reinforcements such as flax, hemp, sisal and jute, and thermoplastic and thermoset matrices. Some of these composites have been produced using matrices made of derivatives from cellulose, starch and lactic acid to develop fully biodegradable composites or biocomposites [52]. The emerging diversity of applications of natural fibre composites has seen the production of sandwich structures based on natural fibre composite skins (see Fig. 23.10). 

In this chapter, some results will be presented, focusing attention on the properties in terms of physical and chemical structure of the coconut and sugarcane bagasse fibers, processing behavior and final thermal properties of these fibers with thermoplastics or thermosetting matrices, paying particular attention to the use of physical and chemical treatments for the improvement of fiber-matrix interaction. 

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