Corrosion-resistance Cross-linking

Phenylenediamines are used in a variety of other appHcations, such as corrosion inhibitors, cross-linking agents for epoxy resins, toners for electrostatic image development (35), and to improve wrinkle resistance of cellulose acetate fibers (36). 

One of the first attempts to produce polyurethane was from the reaction of an intermediate polyol of 1,3- and l,4-bis(hydroxyhexa uoroisopropyl)benzene m- and -12F-diols) by reaction with epichlorohydrin. This polyol was subsequentiy allowed to react with a commercial triisocyanate, resulting in a tough, cross-linked polyurethane (129,135,139). ASTM and military specification tests on these polyurethanes for weather resistance, corrosion prevention, bUster resistance, and ease of cleaning showed them to compare quite favorably with standard resin formulations. 

Because the heat distortion temperature of cured epoxy resins (qv) increases with the functionality of the curing agents, pyromellitic dianhydride is used to cross-link epoxy resins for elevated temperature service. The dianhydride may be added as a dispersion of micropulverized powder in liquid epoxy resin or as a glycol adduct (158). Such epoxies may be used as an insulating layer in printed circuit boards to improve heat resistance (159). Other uses include inhibition of corrosion (160,161), hot melt traffic paints (162), azo pigments (163), adhesives (164), and photoresist compounds (165). 

Corrosion. Aqueous solutions of citric acid are mildly corrosive toward carbon steels. At elevated temperatures, 304 stainless steel is corroded by citric acid, but 316 stainless steel is resistant to corrosion. Many aluminum, copper, and nickel alloys are mildly corroded by citric acid. In general, glass and plastics such as fiber glass reinforced polyester, polyethylene, polypropylene, poly(vinyl chloride), and cross-linked poly(vinyl chloride) are not corroded by citric acid. 

In the second, complementary, approach the polycondensation of silica polymer is followed by the formation of an organic network made by cross-linking reaction of monomers covalently bound to silicon compounds (Scheme 4.1) resulting in polymeric materials with outstanding protective abilities, including thermal, mechanical and corrosion resistance. 

Allylic plastics, which are produced by the polymerization of diallyl phthalate, have high heat deflection temperatures and high strengths. These cross-linked polyesters have solvent- and corrosion-resistant properties similar to those cited for alkyds. The properties of allylic plastics are shown in Table 15.3. 

The pnndpal unsaturated acids used are maleic and fumaric. Saturated acids, usually phthahe and adipic, may also be included. The function of these acids is to reduce the amount of unsaturation in the final resin, making it tougher and more flexible. The acid anhydrides are often used if available and applicable. The dihydroxy alcohols most generally used are ethylene, propylene, diethylene, and dipropylene glycols. Styrene and diallyl philialate are the most common cross-linking agents, Polyesters are resistant to corrosion, chemicals, solvents, etc. 

The reactive epoxies form a tight, cross-linked polymer network and are characterized by toughness, good adhesion, corrosive-chemical resistance, and good dielectric properties. 

Highly cross-linked epoxy resins combine high strength stiffness thermal, chemical, and environmental stability adhesion low weight processability excellent creep resistance and favorable economics. These resins are widely applied as coatings, casting resins, structural adhesives, and matrix resins of advanced composite materials. The broad spectrum of applications ranges from the automotive and aerospace industries to corrosion protection and microelectronics. 

Although both water and electrolyte can penetrate films, electrolyte tends to penetrate films only locally, as evidenced by studies of the electrolytic resistance of free films (41. 50. 51). The regions of direct electrolyte penetration have been shown to be small, localized, and randomly distributed. Furthermore, these regions have been demonstrated to correspond to initial sites of corrosion. The molecular origin of the direct electrolyte penetration regions is not entirely clear, but in some cases they have been associated with inhomogeneous cross-linking. 

Silver filler is more preferable due to excellent corrosion resistance and conductivity, but its high cost is serious disadvantage. Hence alternative conductive filler, notably nickel-carbon nanocomposite was chosen to further computational simulation. In developed adhesive/paste formulations metal containing phase distribution is determined by competitive coordination and cross-linking reactions. 

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