Fibre dispersion

The properties of polymers reinforced with chopped glass fibres dispersed in the polymer matrix depend on ... 

Carbon fibres are used in various forms for polymer reinforcement short fibres dispersed in the matrix... 

ACh, cholinomimetics and acetylcholinesterase inhibitors have been shown to inhibit the movement of flatworms and to cause flaccid paralysis (for reviews, see Fbx et a/., 1996 Halton et a/., 1997 Halton and Maule, 2004 Ribeiro et a/., 2005). This is in contrast to the mainly myoexcitatory effects of ACh at the vertebrate neuromuscular junction and in many other invertebrates including nematodes. Although ACh has predominantly inhibitory actions on flatworm muscle it has been shown to have inconsistent effects on the muscle of the monogenean, Diclidophora merlangi (Maule et a/., 1989), to induce the contraction of muscle fibres dispersed from planarians (Blair and Anderson, 1994 Moneypenny eta/., 2001) and, to cause increased muscle activity... 

Nearly every polymeric system absorbs some moisture under normal atmospheric conditions from the air. This can be a difficult to detect, very small amount as for polyethylene or a few percent as measured for nylons. The sensitivity for moisture increases if a polymer is used in a composite system i.e. as a polymeric matrix with filler particles or fibres dispersed in it. Hater absorption can occur then into the interfacial regions of filler/fibre and matrix [19]. Certain polymeric systems, like coatings and cable insulation, are for longer or shorter periods immersed in water during application. After water absorption, the dielectric constant of polymers will increase due to the relative high dielectric constant of water (80). The dielectric losses will also increase while the volume resistivity decreases due to absorbed moisture. Thus, the water sensitivity of a polymer is an important product parameter in connection with the polymer s electrical properties. The mechanical properties of polymers are like the electrical properties influenced by absorption of moisture. The water sensitivity of a polymer is therefore in Chapter 7 indicated as one of the key-parameters of a polymeric system. 

Fibre dispersion can also be a pitfall when the IRF of a DOT system is to be recorded. If the angular distribution of the fibre illumination is not exactly the same as for the subsequent DOT measurement a wrong IRF is recorded. A reasonably accurate IRF can be recorded by putting a thin scattering medium between the source and the detection fibre bundle [443]. 

Within the field of materials science, the term composite is appUed to a material that consists of a combination of phases. Typically, there are two phases, a continuous one, known as the matrix, and a discontinuous one, dispersed within the matrix phase [1]. The majority of composites consist of an inorganic filler, either particles or fibres, dispersed in a matrix of organic polymer. However, other technically useful composites exist, including all-ceramic composites [2] and metal matrix composites [3]. 

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 major factors that govern the properties of short-fibre thermoplastic composites are fibre volume fraction, fibre dispersion, fibre aspect ratio and length distribution, fibre orientation and fibre-matrix adhesion. Each of these parameters is briefly discussed below. 

There are two major factors affecting the extent of fibre dispersion fibre-fibre interaction, such as hydrogen bonding between the fibres, and fibre length, because of the possibility of entanglements. As mentioned above, one of the specificity of ceUulose fibres as reinforcement is their poor dispersion characteristics in many thermoplastic melts, due to their hydrophUic nature. Several methods have been suggested and described in the Uterature to ovCTcome this problem and improve the dispersion. Among them are ... 

Fibre surface modification. The surface energy is closely related to the hydrophilicity of the UgnoceUulosic fibres. Use of dispersing agents, such as stearic acid or a mineral oil. The dispersion of lignocellulosic fibres can be improved by pretreatment with lubricants or thermoplastic polymers. An addition of 1-3 per cent stearic acid is sufficient to achieve a maximum reduction in size and number of aggregates in PP and polyethylene [7]. The use of stearic acid in HDPE/wood fibres was reported to improve the fibre dispersion and the wetting between the fibre and the matrix [9]. 

The production of the composites is optimized in relation to temperature, pressure and moulding time. High temperatures degrade the cellulose thus, negatively affecting the mechanical properties of the composites. Inefficient fibre dispersion in the matrix causes fibre agglomeration which decreases the tensile strength. 

Wet laid media are derived from paper manufacturing processes and are composed of short staple fibres dispersed in water to form a slurry that is fed to a moving wire screen on which it is dewatered. The randomly orientated fibres form a web which is dried by a sequence of heated rollers. An adhesive or binding agent may be dispersed in the original slurry or sprayed onto the web during or after dewatering. 

Akcay, B., Tasdemir, M.A., 2012. Mechanical behaviour and fibre dispersion of hybrid... 

The curves in Figure 8.22 a show Kj and as functions of angle 6. For comparison, the values obtained for short fibres dispersed at random in horizontal planes (2D) are also given. 

Different parameters are responsible for this cracking behaviour, the dominant ones being, as far as we can see today, the fibre spacing, the specific fibre surface [SFS], the degree of fibre dispersion in the material and the fibre matrix bond. 

Macroscopically heterogeneous materials, such as concrete, are even in the virgin state severely cracked. The size distribution of these cracks is directly governed by that of the particles in the material. Steel fibres dispersed in such a matrix have a length roughly twice the maximum grain size. A composite model of steel fibre concrete on mesolevel inevitably has to encompass particles, cracks and fibres. They mutually interact in a complicated way when the material body is stressed. Only somewhat simplified concepts are however accessible for design purposes. 

The average reduction of the reinforcement ratio in the boundary zone is obtained upon averaging over all possible values of t. A value of 0.58 is found for a 3-D UR fibre dispersion. Substitution in eq (13) for values of m between roughly 0.2 and 0.4 demonstrates the reinforcement ratio of a partially planar fibre composite to decrease in the boundary layer to 70 to 80% of its bulk value ... 

The use of coupling agents promotes a significant increase on the mechanical properties and in several studies it was showed that reduces the moisture content. Several of these studies revealed significant improvements on the modulus of elasticity, stiffness, tensile and/or flexural strength and fibre dispersion. 

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