Reinforced plastics mechanical properties

The mechanical properties of plastics materials may often be considerably enhanced by embedding fibrous materials in the polymer matrix. Whilst such techniques have been applied to thermoplastics the greatest developents have taken place with the thermosetting plastics. The most common reinforcing materials are glass and cotton fibres but many other materials ranging from paper to carbon fibre are used. The fibres normally have moduli of elasticity substantially greater than shown by the resin so that under tensile stress much of the load is borne by the fibre. The modulus of the composite is intermediate to that of the fibre and that of the resin. 

Compounding to change and improve the physical and mechanical properties of plastics makes use of a wide variety of materials as reviewed throughout this book. The major and large market for these materials, such as additives, fillers, and reinforcements, continues to expand as the demand for reducing the cost of plastics, plastics to function in wider or extreme markets, and under stricter regulatory regimes continue to expand. 

The physical and mechanical properties of plastics, including reinforced plastics (RPs) have some significant difference from those of familiar metallic materials. Consequently in the past those not famihar with... 

During application, plastics are exposed to ambient conditions where temperature and humidity vary. This combined effect, known as the hygrothermal effect, may influence the mechanical properties of plastics. The properties of glass fiber-reinforced plastics are more affected by this combined influence than by one of the two factors alone. Moisture absorption in reinforced materials is mainly characterized by diffusion. The capillary effect and transport along microcracks is another way to absorb moisture. Moisture absorption by the latter two mechanisms is often a direct consequence of moisture absorption caused by hot and humid ambient conditions. 

The industrial value of furfuryl alcohol is a consequence of its low viscosity, high reactivity, and the outstanding chemical, mechanical, and thermal properties of its polymers, corrosion resistance, nonburning, low smoke emission, and exceUent char formation. The reactivity profile of furfuryl alcohol and resins is such that final curing can take place at ambient temperature with strong acids or at elevated temperature with latent acids. Major markets for furfuryl alcohol resins include the production of cores and molds for casting metals, corrosion-resistant fiber-reinforced plastics (FRPs), binders for refractories and corrosion-resistant cements and mortars. 

Table 13 is a representative Hst of nickel and cobalt-base eutectics for which mechanical properties data are available. In most eutectics the matrix phase is ductile and the reinforcement is britde or semibritde, but this is not invariably so. The strongest of the aHoys Hsted in Table 13 exhibit ultimate tensile strengths of 1300—1550 MPa. Appreciable ductiHty can be attained in many fibrous eutectics even when the fibers themselves are quite britde. However, some lamellar eutectics, notably y/y —5, reveal Htde plastic deformation prior to fracture. 

Nonoxide fibers, such as carbides, nitrides, and carbons, are produced by high temperature chemical processes that often result in fiber lengths shorter than those of oxide fibers. Mechanical properties such as high elastic modulus and tensile strength of these materials make them excellent as reinforcements for plastics, glass, metals, and ceramics. Because these products oxidize at high temperatures, they are primarily suited for use in vacuum or inert atmospheres, but may also be used for relatively short exposures in oxidizing atmospheres above 1000°C. 

Thermosetting unsaturated polyester resins constitute the most common fiber-reinforced composite matrix today. According to the Committee on Resin Statistics of the Society of Plastics Industry (SPl), 454,000 t of unsaturated polyester were used in fiber-reinforced plastics in 1990. These materials are popular because of thek low price, ease of use, and excellent mechanical and chemical resistance properties. Over 227 t of phenoHc resins were used in fiber-reinforced plastics in 1990 (1 3). PhenoHc resins (qv) are used when thek inherent flame retardance, high temperature resistance, or low cost overcome the problems of processing difficulties and lower mechanical properties. 

Polyester resins, reinforced with glass fibers, are used widely in the construction of process equipment. Some physical and mechanical properties are presented in Table 3.48. Table 3.49 lists various materials used as filler and the properties they impart to different plastics. 

In this book no prior knowledge of plastics is assumed. Chapter 1 provides a brief introduction to the structure of plastics and it provides an insight to the way in which their unique structure affects their performance. There is a resume of the main types of plastics which are available. Chapter 2 deals with the mechanical properties of unreinforced and reinforced plastics under the general heading of deformation. The time dependent behaviour of the materials is introduced and simple design procedures are illustrated. Chapter 3 continues the discussion on properties but concentrates on fracture as caused by creep, fatigue and impact. The concepts of fracture mechanics are also introduced for reinforced and unreinforced plastics. 

The mechanical properties of composites reinforced with wood fibers and PVC or PS as resin can be improved by an isocyanate treatment of those cellulose fibers [41,50] or the polymer matrix [50]. Polymethylene-polyphenyl-isocianate (PMPPIC) in pure state or solution in plasticizer can be used. PMPPIC is chemically linked to the cellulose matrix through strong covalent bonds (Fig. 8). 

As is known of glass fiber-reinforced plastics, the mechanical and physical properties of composites, next to the fiber properties, and the quality of the fiber matrix interface, as well as the textile form of the reinforcement primarily depend on the volume content of fibers in the composite. 

Generally, the mechanical and physical properties of natural fiber-reinforced plastics only conditionally reach the characteristic values of glass fiber-reinforced systems. By using hybrid composites made of natural fibers and carbon fibers or natural fibers and glass fibers, the... 

Natural fibers compete with technical fibers, such as glass fibers or carbon fibers, as reinforcements for plastics. The advantages of technical fibers are their good mechanical properties, which vary only a little, but their recycling is difficult. 

The mechanical properties of composites are mainly influenced by the adhesion between matrix and fibers of the composite. As it is known from glass fibers, the adhesion properties could be changed by pretreatments of fibers. So special process, chemical and physical modification methods were developed. Moisture repel-lency, resistance to environmental effects, and, not at least, the mechanical properties are improved by these treatments. Various applications for natural fibers as reinforcement in plastics are encouraged. 

Glass-reinforced aluminum foil with either a bright polished or white lacquer surface is utilized with most types of insulant. Primarily it is used as a vapor control layer or as a means of upgrading the fire properties of plastic foams, but it does give a semi-decorative finish to the insulation. It is therefore often use where the insulation is open to view but located away from direct risk of mechanical damage. 

Table 3-1 gives typical mechanical property data for four materials, the exact values of which are unimportant for this discussion. Aluminum and mild steel have been used as representative metals and polypropylene (PP) and glass fiber-TS polyester reinforced plastics (GRP) as representative plastics. Higher-performance types could have been selected for both the metals and plastics, but those in this table offer a fair comparison for the explanation being presented. 

The term s plastic, polymer, resin, elastomer, and reinforced plastic (RP) are some-what synonymous. However, polymer and resin usually denote the basic material. Whereas plastic pertains to polymers or resins containing additives, fillers, and/or reinforcements. Recognize that practically all materials worldwide contain some type of additive or ingredient. An elastomer is a rubberlike material (natural or synthetic). Reinforced plastics (also called composites although to be more accurate called plastic composites) are plastics with reinforcing additives, such as fibers and whiskers, added principally to increase the product s mechanical properties. 

In general adding reinforcing fibers significantly increases mechanical properties. Particulate fillers of various types usually increase the modulus, plasticizers generally decrease the modulus but enhance flexibility, and so on. These RPs can also be called composites. However the name composites Utterly identifies thousands of different combinations with very few that include the use of plastics (Table 6-18). In using the term com-... 

A designer can produce RP products whose mechanical properties in any direction will be both predictable and controllable. This is done by carefully selecting the plastic and the reinforcement in terms of both their composition and their orientation, and... 

The frictional properties of TPs, specifically the reinforced and filled types, vary in a way that is unique from metals. In contrast to metals, even the highly reinforced plastics have low modulus values and thus do not behave according to the classic laws of friction. Metal-to-thermoplastic friction is characterized by adhesion and deformation resulting in frictional forces that are not proportional to load, because friction decreases as load increases, but are proportional to speed. The wear rate is generally defined as the volumetric loss of material over a given unit of time. Several mechanisms operate simultaneously to remove material from the wear interface. However, the primary mechanism is adhesive wear, which is characterized by having fine particles of plastic removed from the surface. 

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