Anisotropy aramid

The properties are anisotropic to varying degrees (i.c.. mechanical, thermal, and electrical properties vary with direction in the material). The highest anisotropy is illustrated by the properties of a fully aligned 60% (by volume) carbon fiber epoxy laminate, where the properties parallel with the fiber direction can be thirty times greater than in the perpendicular direction, whereas in a molded short-fiber system the ratio of properties in perpendicular directions may only be a factor of two. The fibers themselves may have even higher anisotropy (e.g.. carbon and aramid fibers). 

Quantitative predictions of the effects of fillers on the properties of the final product are difficult to make, considering that they also depend on the method of manufacture, which controls the dispersion and orientation of the filler and its distribution in the final part. Short-fiber- and flake-filled thermoplastics are usually anisotropic products with variable aspect ratio distribution and orientation varying across the thickness of a molded part. The situation becomes more complex if one considers anisotropy, not only in the macroscopic composite but also in the matrix (as a result of molecular orientation) and in the filler itself (e.g., graphite and aramid fibers and mica fiakes have directional properties). Thus, thermoplastic composites are not always amenable to rigorous analytical treatments, in contrast to continuous thermoset composites, which usually have controlled macrostructures and reinforcement orientation [8, 17]. 

This chapter is concerned with the short-term mechanical properties — moduli and strengths — of glass, aramid and carbon fibres in a thermosetting resin matrix. A little information on reinforced thermoplastic matrix systems is also included. The data mainly refer to the room temperature properties of 55-65 v/o fibre, unidirectional, systems. The effects of the variation in fibre volume loading, method of test and instantaneous and long term exposure to temperature are briefly mentioned. Longitudinal properties tend to be fibre dominated, and so are compressive properties to some extent for glass and carbon fibres. The anisotropy of unidirectional materials is noticeable. 

These fibres are representative of the high performance aramid products currently available. The anisotropy of unidirectional composites is apparent as is the poor compression strength of aramid composites. The composite densities are extremely low. 

The anisotropy of the basic carbon and aramid fibres is clear as is the high longitudinal thermal conductivity of the very high modulus Amoco carbon fibres. 

The extreme thermal expansion anisotropy and very low longitudinal expansion of the carbon and aramid fibre systems are clear. All measurements are for room temperature or slightly above. Once the Tg of the polymer matrix has been exceeded the CTE in either direction will increase. 

The difference between insulators (e.g. aramid and glass composites) and conductors (carbon composites) is marked. In addition the anisotropy of resistivity is high for carbon composites. The results for Nicalon SiC were included to show the effect of systematically varying the resistivity of the fibre. The acid cure glass phenolic system has a relatively low resistivity and its tracking and arc resistance are poor compared with those of the phenolic prepreg system. 

Cryogenic properties of fibers and composites are given in Table 2. Most fiber-composite properties are governed by anisotropic features originating in the fiber arrangement and anisotropy and fiber-matrix interfacial bond. The fiber-matrix bond is discussed in refs. (36) and (39). In this article, the matrix materials discussed are epoxy resins, poljdmides, and polycarbonates and the fibers glass, carbon, and Kevlar, ie, aramid fibers. 

Due to the stiffness of the chain the thermo-mechanical stability of aramid fibre is appreciably better than that of the conventional synthetic fibres, as shown in Fig. 9. The coefficient of linear thermal expansion of P/ PTA fibre is negative, viz. — 4 x 10" °C S which can be explained from the anisotropy of the thermal motion of the atoms in the rigid chain. The longitudinal component of the thermal vibration will increase the length of the chain, while the transverse components shorten the chain. Because of the chain stiffness the longitudinal vibration is small compared with the transverse vibrations resulting in a negative linear thermal expansion coefficient. 

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