Asbestos particle shape

The majority of particles in the atmosphere are spherical in shape because they are formed by condensation or cooling processes or they contain core nuclei coated with liquid. Liquid surface tension draws the material in the particle into a spherical shape. Other important particle shapes exist in the atmosphere e.g., asbestos is present as long fibers and fly ash can be irregular in shape. 

Three particle systems were examined by image analysis and their aspect ratio and particle size distributions were measured [197,198]. The data were then used as a reference method for neural networking using a Malvern Mastersizer X and concentrations from 2 g f to 60 g I" . Particle shapes ranged from an ellipsoidal cracking catalyst, needle shaped asbestos and monoclinic sucrose crystals. 

It is well known that particle shape affects many secondary properties relevant to powder handling such as the bulk density, failure properties or particle-gas interaction. For non-spherical particles, the results obtained with different methods of particle size measurement are, in general, not comparable. From the point of view of powder handling, flaky or stringy particles like wood shavings, mica or asbestos fibres are known to be difficult because they interlock and form obstructions to flow. 

The mineral asbestos possesses some unique properties that cause lung injury and necessitate microscopic methods for evaluating its hazard. The characteristic fibrous shape of asbestos particles [see Fig. 1.4(b)] enables them to get past respiratory defense mechanisms and cause asbestosis (scarring of lung tissue), mesothelioma (cancer of the lining of the lung), and lung cancer. Microscopy is used to identify those asbestos fibers whose size and shape enable them to cause these diseases. 

Using in vivo techniques, natural and synthetic fibrous materials have been shown to induce fibrosis and carcinogenic responses that were directly related to dose, if the materials were placed on the target tissues. Chrysotile appeared to be more biologically active than the other UICC asbestos samples or fibrous glass, with particle size and shape having some influence on the response. In vitro experiments indicate that fibers can be cytotoxic and possibly mutagenic, increase the secretory activity of fibroblasts, and possibly initiate an immune cascade. 

As a consequence of the complications associated with the theoretical derivation of the relationship between electrophoretic mobility and the shape of the particle, attempts have been made to solve the problem experimentally. Unfortunately the results in this connection appear to be equally inconclusive. Abramson studied the movement of spherical particles of various oils and of needles of asbestos and of m-aminobenzoic acid coated with the same protein, e.g., gelatin or egg albumin, in each case the results showed that, provided the surface of the particle was completely covered with the protein, the electrophoretic mobility is independent of the shape of the moving particle. ... 

Oncologic makes predictions based on factors outside the scope normally covered by other expert systems—for example, it can predict the toxicity of asbestos-like materials on the basis of particle size and shape and surface... 

CaCOs compounds. Wollastonite s fiber-like shape provides similar properties as those of the glass fibers discussed below, and also like glass, its abrasiveness can damage processing equipment. However, unlike other mineral fiber, asbestos, its particles are nonhazardous [1-1, 3-4, 7-6, 7-13, 7-15]. 

The yield of retained fraction in the case of irregularly shaped particles was found to be 1.3-1.5 times that for particles with a rounded shape. In crushing operations, for example, particles of barite, quartz, feldspar, and magnetite form rounded particles, decreasing their adhesion. The wastes from low grades of asbestos contain irregularly shaped mineral dust particles, which adhere to the drum surface and are separated as the retained fraction [137]. 

Many authors used government and company websites as sources for updated Material Safety Data Sheets (MSDS) and information on the threshold limit values (TLV) for the airborne concentration of filler dusts in the workplace. Reliable information on possible risks to human health or the environment is extremely important to current and potential users of existing fillers, or new fillers of different origin and different particle size/shape characteristics. It should be recognized that health issues have been responsible in the past for the withdrawal from certain plastics markets of natural and synthetic fibrous fillers with unique properties such a as chrysotile asbestos, microfibers, whiskers and the recently mandated very low content of crystalline silica in mineral fillers. 

In the case of particle inhalation, their size and shape may determine whether they are retained in the lungs or are rejected. Long thin needle like fibres, such as asbestos, may be more readily retained than rounder particles, and may cause more physical irritation. 

In addition to the shapes already mentioned, there are also particles of fibrous or acicular form ft)risms, needles, fibers, etc.) having one dimension greatly exceeding the others. These include particles of zinc oxide (0.4-1 m), asbestos [(0.3-3) 100 m or (0.5-1.5) 1 m], tobacco virus [(1.0-2.0) (10-30) /x], etc. 

Microcrystalline Silicate n A derivative of chrysotile asbestos, consisting of tiny rod-shaped particles of / hydrated magnesium silicate. The particles have hydroxyl groups on their surfaces that bond with hydrogen-bonding sites on the molecules of a fluid in which they are incorporated. The material has also been used as a viscosity-building agent in unsaturated polyester and other resins. 

Here we consider colloidal sols, where discrete sohd particles are dispersed in a liquid. The sol particles can have three-dimensional (sphere-Uke), two-dimensional (rod-like) or one-dimensional (plate-like) forms, as exemplified by Fig. 3.3. Examples of these structures include dispersions of highly monodisperse spherical particles that can be obtained by emulsion polymerization of latex particles, dispersions of needle-shaped colloidal particles in cement and asbestos and plate-like particles in aqueous solutions that are the structural basis of clays. 

Figure 3.3 Typical shapes of colloid particles (a) spherical particles of polystyrene latex, (h) fibres of chrysotile asbestos, (c) thin plates of kaolinite. The scale bars indicate 1 pm. [Adapted with permission from D. H. Everett, Basic Principles of Colloid Science, Royal Society of Chemistry, 1988]...

Talc is white and soft, so it is a valuable filler. The flat shape of talc particles adds reinforcing value in addition to simply acting as a filler. Asbestos does occur in some talc deposits, so the location of the talc mine from where your talc supply comes could be important in some demanding applications. 

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