Resistance, chemicals/media

The first barrier is the form of the waste, which will immobilize the radioactive materials. The waste form should not be damaged by heat or radiation nor be attacked by groundwater. The waste is placed in a steel canister, which is resistant to leaching. The canister is surrounded by packing materials that prevent radioactivity from escaping, and the entire repository is backfilled with a material that absorbs or resists chemical intrusion. The final barrier is the host medium that separates the repository from the surrounding area. 

In industrial filtration a common filter medium is canvas cloth, either duck or twill weave. Many different weights and patterns of weave are available for different services. Corrosive liquids require the use of other filter media, such as woolen cloth, metal cloth of monel or stainless steel, glass cloth, or paper. Synthetic fabrics like nylon, polypropylene, and various polyesters are also highly resistant chemically. 

Nylon is a thermoplastic material which can operate under light to medium loads, is tough, abrasion-resistant, resists chemical attack, is non-toxic and can be used continuously at temperatures up to about 90°C. 

Uses Reduction of iron ore in blast furnaces as source of synthesis gas refractory furnace linings in electrorefining of aluminum and other high-temp, service electrodes in electrolytic reduction of AI2O3 to aluminum filter medium fuel electrothermal prod, of phosphorus, silicon carbide, and calcium carbide plastics additive providing wear reduction in PTFE, creep resist., chemical inertia and stability, thermal conductivity and stability... 

Poly(vinyl acetate) (PVAc) and vinyl acetate (VAc) copolymer coatings in latex form for buildings possess good light resistance, a medium stability to chemicals and low cost. VAc-methacrylate copolymers form good paints for interior and exterior application [54, 93]. 

The selection of a suitable mounting compound is determined by the properties of the plastic, such as viscosity, shrinkage, adhesion to the sample, abrasion resistance, chemical resistance, and thermal resistance, as exhibited in the processes used to prepare the section and render the microstructure more visible. Another determining property is hardness, which serves as an indicator of possible edge rounding. For the most part, the requirements for a mounting medium are fulfilled by the plastics available on the market. 

Table 5.102 Chemical resistance of polyether block amides with various degrees of hardness A = good resistance, B = medium resistance, C = non-resistant [989]...

Basis for the evaluation of the chemical resistance of polyamides summarized here are exposure tests with standard specimens that typically were in contact with the specific chemical medium for 12 months, as well as practical experience with testing and application of polyamide parts. The chemical resistance of PA 6 and PA 66 is generally identical. However, some chemicals cause varying degrees of weight increase. Therefore, PA 6 and PA 66 are listed separately in Table A.21. The values listed are reference values only, because the weight increase depends on the degree of polymerization of the test specimen. 

Logarithmic mass-time diagrams for chemical resistance permit one to study the increase in solution absorption until penetration of the chemical medium and corrosion of the substrate have occurred [73, 74). 

These properties are related to their particular structural and chemical features (1) high surface area with pore diameters in the range of mesopores (2) very low soluhiUty in strong polar solvents (3) hydrolysis resistance (4) medium-strength Lewis and Brpnsted acid sites (5) the possihiUty of easily tuning the surface acidity or acidity/basicity (6) nanoscopic particle dimension. 

A reverse-wipe appHcation is seen ia the clothes dryer fabric softener sheet whereia the spunbonded fabric is coated with a complex combination of compounds that are released iato the environment of a hot clothes dryer to soften and perfume the clothes, as weU as provide an antistatic quaHty. The spunbonded sheet, which must be made of polyester or nylon for temperature resistance, provides a simple and cost-effective medium to store the chemical compounds prior to release ia the dryer. 

Nitrocellulose based lacquers often contain short or medium oil alkyds to improve flexibiUty and adhesion. The most commonly used are short oil non drying alkyds. Amino resins or urethane resins with residual isocyanate functional groups may be added to cross-link the coating film for improved solvent and chemical resistance. The principal appHcations are furniture coatings, top lacquer for printed paper, and automotive refinishing primers. 

Chlorinated rubberis often used in combination with medium od drying type alkyds. The alkyd gives better toughness, flexibdity, adhesion, and durabdity, and the chlorinated mbber contributes to faster drying and better resistance to water and chemicals. The principal appHcations are highway traffic paint, concrete floor, and swimming pool paints. 

Fabrics of Woven Fibers For cake filtratiou these fabrics are the most common type of medium. A wide variety of materials are available some popular examples are listed in Table 18-10, with ratings for chemical and temperature resistance. In addition to the mate-ri of the fibers, a number of construction characteristics describe the filter cloth (1) weave, (2) style number, (3) weight, (4) count, (5) ply, and (6) yarn number. Of the many types of weaves available, only four are extensively used as filter media plain (square) weave, twill, chain weave, and satin. 

Filter aids should have low bulk density to minimize settling and aid good distribution on a filter-medium surface that may not be horizontal. They should also be porous and capable of forming a porous cake to minimize flow resistance, and they must be chemically inert to the filtrate. These characteristics are all found in the two most popular commercial filter aids diatomaceous silica (also called diatomite, or diatomaceous earth), which is an almost pure silica prepared from deposits of diatom skeletons and expanded perhte, particles of puffed lava that are principally aluminum alkali siheate. Cellulosic fibers (ground wood pulp) are sometimes used when siliceous materials cannot be used but are much more compressible. The use of other less effective aids (e.g., carbon and gypsum) may be justified in special cases. Sometimes a combination or carbon and diatomaceous silica permits adsorption in addition to filter-aid performance. Various other materials, such as salt, fine sand, starch, and precipitated calcium carbonate, are employed in specific industries where they represent either waste material or inexpensive alternatives to conventional filter aids. 

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