Organic chemicals, resistance

Swelling (e.g., neutral volatile organic chemicals), resistance increases. 

Chemical Resistance. Table 2 shows the chemical resistance of PVA fiber (40). The fiber exhibits markedly high resistance to organic solvents, oils, salts, and alkaU. In particular, the fiber has unique resistance to alkaU, and is hence widely used in the form of a paper principally comprising it and as reinforcing material for cement as a replacement of asbestos. 

In some cases, a pigment s thermal and chemical resistance can be improved by the encapsulation of the pigment particles by an iasoluble, colorless layer of metal oxide or oxide—hydroxide, eg, siUca, Si02. The function of such a shell is to prevent direct contact and reaction between the pigment surface and the organic matrix ia which the pigment is dispersed (11). 

Other Materials. Benzoguanamine and acetoguanamine may be used in place of melamine to achieve greater solubiHty in organic solvents and greater chemical resistance. Aniline and toluenesulfonamide react with formaldehyde to form thermoplastic resins. They are not used alone, but rather as plasticizers (qv) for other resins including melamine and urea—formaldehyde. The plasticizer may be made separately or formed in situ during preparation of the primary resin. 

Extractives and Ash. The amount of extractives in wood varies from 5 to 20% by weight and includes a wide variety of organic chemicals (11). Many of these function as intermediates in tree metaboUsm as energy reserves or participate in the tree s defense mechanism against microbiological attack. The extractives contribute to wood properties such as color, odor, and decay resistance. 

In the 1960s and 1970s, additional elastomers were developed by Du Pont under the Viton and Kalrez trademarks for improved low temperature and chemical resistance properties using perfluoro(methyl vinyl ether), CF2=CFOCF3, as a comonomer with vinyUdene fluoride and/or tetrafluoroethylene (12,13) (see Fluorine compounds, organic-tethafluoroethylene polypous and copolyp rs). 

Polypropylene Polypropylene (PP) pipe and fittings have excellent resistance to most common organic and mineral acids and their salts, strong and weak alkahes, and many organic chemicals. They are available in sizes V2 through 6 in, in Schedules 40 and 80, but are not covered as such by ASTM specifications. 

Carbon steel is easily the most commonly used material in process plants despite its somewhat limited corrosion resistance. It is routinely used for most organic chemicals and neutral or basic aqueous solutions at moderate temperatures. It is also used routinely for the storage of concentrated sulfuric acid and caustic soda [up to 50 percent and 55°C (I30°F)]. Because of its availability, low cost, and ease of fabrication steel is frequently used in services with corrosion rates of 0.13 to 0.5 mm/y (5 to 20 mils/y), with added thickness (corrosion allowance) to assure the achievement of desired service life. Product quahty requirements must be considered in such cases. 

Asphalt Asphalt is used as a flexible protective coating, as a bricklining membrane, and as a chemical-resisting floor covering and road surface. Resistant to acids and bases, alphalt is soluble in organic solvents such as ketones, most chlorinated hydrocarbons, and aromatic hydrocarbons. 

Asbestos may be used for improved heat and chemical resistance and silica, mica and china clay for low water absorption grades. Iron-free mica powder is particularly useful where the best possible electrical insulation characteristics are required but because of the poor adhesion of resin to the mica it is usually used in conjunction with a fibrous material such as asbestos. Organic fillers are commonly used in a weight ratio of 1 1 with the resin and mineral fillers in the ratio 1.5 1. 

As a very general rule it may be said that the amines are fast curing and give good chemical resistance but most are skin sensitive. The organic anhydrides are less toxic and in some cases give cured resins with very high heat distortion temperatures. They do not cross-link the resins at room temperature. 

Some corrosion-resistant materials for concentrated aqueous solutions and acids are given in Tables 4.10 and 4.11. The resistance of some common polymers to organic solvents is summarized in Table 4.12. The attack process is accelerated by an increase in temperature. The chemical resistance of a range of common plastics is summarized in Table 4.13. 

Red and blue acid-resistant bricks are resistant to all inorganic and organic chemicals, except for hydrofluoric acid and hot concentrated caustic alkalis. Acid-resistant fireclay bricks are used for conditions involving elevating temperatures and corrosive condensates. Highly vitrified materials such as chemical stoneware, porcelain and basalts are used for extremely severe duties or where contamination of the process liquors is undesirable. 

Epoxy resin paints, inferior to chlorinated rubber for resistance to strong acids, are excellent for dilute acids and strong alkalis. They produce a harder, more abrasion-resistant coating than does chlorinated rubber and are much better for resistance to fats, oils and many organic solvents. Table 3.50 gives data on the chemical resistance of epoxy resin coatings to different materials. 

A large range of man-made polymeric materials is available, from polyethylene, which is attacked by most organic chemicals, to fluorinated products such as polytetrafluoro-ethylene and polyethyletherketones, which have exceptional resistance to virtually all chemicals. All polymers have their own adhesive, welding and fabrication limitations which must be taken into account in the design of the coated item. These materials can also be used in solid form. 

These test procedures and standards are subject to change, so it is essential to keep up to date if one has to comply with them. It may be possible to obtain the latest issue on a specific test (such as a simple tensile test or a molecular weight test) by contacting the organization that issued it. For example, the ASTM issues new annual standards that include all changes. Their Annual Books of ASTM Standards contain more than seven thousand standards published in sixty-six volumes that include different materials and products. There are four volumes specifically on plastics 08.01-Plastics 1 08.02-Plastics 11 08.03-Plastics III, and 08.04-Plastic Pipe and Building Products. Other volumes include information on plastics and RPs. The complete ASTM index are listed under different categories for the different products, types of tests (by environment, chemical resistance, etc.), statistical analyses of different test data, and so on (56,128,129). 

The presence of these ionic groups gives the polymer greater mechanical strength and chemical resistance than it might otherwise have. Indeed the ionomer is resistant to dissolution in many solvents because of its unconventional chemical character, often being too ionic to dissolve in non-polar solvents and too organic to dissolve in polar solvents. 

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