Fillers general properties

The mechanical properties of pure polymeric materials are often inadequate for particular applications, and to overcome this problem these materials may be reinforced in some way. The most common method is to include a substantial amount of a rigid filler or fillers, generally as finely divided powder, or as rods or fibres. For certain materials, elastomeric particles may be used, and these have the effect of reducing brittleness. 

Fillers generally represent one of the major components by weight in an adhesive formulation. However, their concentration is quite often limited by viscosity constraints, cost, and negative effects on certain properties. The degree of improvement provided by a filler in an epoxy formulation will heavily depend on the type of filler and its properties (particle size, shape, size distribution, and concentration), surface chemistry, dispersion characteristics, dryness, and compatibility with the other components in the formulation. Table 9.3 summarizes the properties of selected fillers. 

Nonconductive fillers are employed with electrical-grade epoxy adhesive formulations to provide assembled components with specific electrical properties. Metallic fillers generally degrade electrical resistance values, although they could be used to provide a degree of conductivity as discussed above. 

The one exception where certain fillers can provide electrical property improvement is in arc resistance. Here hydrated aluminum oxide and hydrated calcium sulfates will improve arc resistance if cure is sufficiently low to prevent dehydration of the filler particles. Electrical-grade fillers generally improve the arc resistance of cured epoxy systems, as indicated in Table 9.10. 

Flexibilizers generally cannot be used to counteract internal stress in high temperature adhesive because of their relatively low glass transition temperature and thermal endurance properties. However, most high-temperature adhesive systems incorporate metallic fillers (generally aluminum powder) to reduce the coefficient of thermal expansion and degree of shrinkage. 

Mechanical properties of semi-crystalline thermoplastics polymers can be improved by incorporating various modifier particles with different physical properties [1]. Particulate mineral fillers generally enhance the stiffness but reduce the fi acture strength and toughness, while toughening rubbery inclusions reduce stiffiiess [2, 3]. However, it is possible to improve... 

Typical concentration range general range - 20-50 wt% with some fillers mechanical properties decrease even at low loadings (10%) calcium carbonate - 10-60 wt%, kaolin - 20-40 wt% talc - 20-40 wt% glass beads - 20-50 vol% " carbon black - 10-30 wt% glass fiber - 1-60 01% ° magnesium hydroxide - 60-65 wt% (for V-0 classification) antimony trioxide - 10 wt% (for V-0 classification)... 

GENERAL PROPERTIES OF LIGNOCELLULOSIC FIBER AS FILLERS Chemical Composition... 

GENERAL PROPERTIES OF LIGNOCELLULOSIC FIBER AS FILLERS TABLE 3.7 Mechanical properties of filled Nylon 6 composites [135]... 

In general, most fillers increase the heat distortion temperature (HDT) of plastics as a result of increasing modulus and reducing high-temperature creep. Thermoelastic properties such as coefficient of thermal expansion (CTE) are also affected by the presence of fillers and have been modeled through a variety of equations derived from the rule of mixtures [8]. For directional fillers, this property is strongly orientation-dependent, and because of the difference between the CTE of the filler and that of the matrix, internal stresses may lead to undesirable warpage. 

Partici Shape and Size. The most common morphology of conductive fillers used for ICAs is flake because flakes tend to have a large surface area, and more contact spots and thus more electrical paths than spherical fillers. The particle size of ICA fillers generally ranges from 1 to 20 /rm. Larger particles tend to provide the material with a higher electrical conductivity and lower viscosity (45). A new class of silver particles, porous nano-sized silver particles, has been introduced in ICA formulations (46,47). ICAs made with this type of particles exhibited improved mechanical properties, but the electrical conductivity is less than ICAs filled with silver flakes. In addition, short carbon fibers have been used as conductive fillers in conductive adhesive formulations (36,48). However, carbon-based conductive adhesives show much lower electrical conductivity than silver-filled ones. 

Fillers, generally inorganic materials such as calcium carbonate, barite, silicate, kaolin, or china clay, are used primarily to lower costs and also to impart special properties such as hardness, abrasion resistance, and no sticking. 

Nano-clays are also discussed, as they are starting to show commercial promise. For completeness, fillers like antimony oxide, which work in combination with other additives, such as halogens, are also briefly discussed. This chapter concentrates on fire retardant effects, the synthesis and general properties of the various fire retardant fillers having already been described in Chapter 2. 

The methods are generally very simple. The information gleaned from these tests gives a good indication of the filler s properties but is usually not sufficient to use as a set of data for screening fillers for potential applications. Substantially more information is required to assess quality of particular product. 

Fillers are generally inorganic particulates added to the adhesive to improve working properties, strength, permanence, or other qualities. Fillers are also used to reduce material cost. By selective use of fillers, the properties of an adhesive can be changed tremendously. Thermal expansion, electrical and thermal conduction, shrinkage, viscosity, and thermal resistance are only a few properties that can be modified by use of selective fillers. 

The theory, processes, and characterization of short fiber reinforced thermoplastics have been reviewed by De and White [31], Friedrich et al. [32], Summerscales [33], in an introductory text by Hull and Clyne [34], and in a handbook by Harper [35]. Natural fibers and composites have been reviewed by Wallenberger and Weston [36]. The introduction of new composite materials, called nanocomposites, has resulted in new materials that are being applied to various industrial applications. These materials have in common the use of very fine, submicrometer sized fillers, generally at a very low concentration, which form novel materials with interesting morphology and properties. Nanocomposites have been discussed in a range of texts including two focused on polymer-clay nanocomposites by Pinnavaia and Beall [37] and Utracki [38]. 

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