Epoxy resin Polysulfide

Epoxy Resins. Polysulfides may also be cured by reaction with epoxy resins (qv) according to the reaction in equation 2. Amines or other catalysts are used and often primary or secondary amine resins are cured together with the polysulfide. 

Composite Particles, Inc. reported the use of surface-modified rubber particles in formulations of thermoset systems, such as polyurethanes, polysulfides, and epoxies [95], The surface of the mbber was oxidized by a proprietary gas atmosphere, which leads to the formation of polar functional groups like —COOH and —OH, which in turn enhanced the dispersibility and bonding characteristics of mbber particles to other polar polymers. A composite containing 15% treated mbber particles per 85% polyurethane has physical properties similar to those of the pure polyurethane. Inclusion of surface-modified waste mbber in polyurethane matrix increases the coefficient of friction. This finds application in polyurethane tires and shoe soles. The treated mbber particles enhance the flexibility and impact resistance of polyester-based constmction materials [95]. Inclusion of treated waste mbber along with carboxyl terminated nitrile mbber (CTBN) in epoxy formulations increases the fracture toughness of the epoxy resins [96]. 

The liquid polymer is converted to the rubbery state by reagents that react with mercaptan (-SH) and side groups of the polymer segments by oxidation, addition or condensation to effect sulfide (-S-S-) bond formation. The oxidation reactions are exothermic and accelerated by an alkaline environment. The most commonly employed oxidizing agents which are suitable for curing liquid polymers are cobalt or manganese or lead octoate, p-quinonedioxime and di- or tri-nitrobenzene. Epoxy resin also reacts with liquid polysulfide polymers by addition in the presence of an aliphatic or aromatic amine and polyamide activator as shown in Equation 5.8 ... 

Epoxy-Liquid Polysulfide Blends Epoxy resins in combination with liquid PSs appear to possess many added advantages as the elastomeric PS segments in epoxy chains impart permanent flexibility [18]. Epoxy-liquid PS blends have been reported as binders for Army illuminating formulations [8]. Similarly, novolac epoxy-liquid PS blends have been reported for inhibition of composite propellants [19]. 

Early in the development of solid propellant, the asphalt composites were found to have poor physical properties, such as cracking under normal temperature cycling, poor tensile characteristics, etc. They were replaced with the elastomeric polymers which have become the present-day binders. The first of these was Thiokol rubber, a polysulfide rubber, whichgives the propellant with good physical properties. The presence of the sulfur atom in the Thiokol rubber decreases the performance compared to a CHO polymer thus the most frequently used binders are polyurethane, polybutadiene acrylic acid (PBAA), epoxy resin, etc. The choice of the latter binders is made with regard to physical properties rather than performance. 

Polysulfides can be cured by themselves with an oxidizing agent as a catalyst. They can also be used to cure epoxy resins (see Chap. 5) however, the rate of cure is very slow for a practical adhesive. Thus, polysulfide resins are generally added into epoxy formulations as a modifier to increase flexibility. In applications where maximum flexibility is required, the level of polysulfide may be greater than the epoxy resin present in the formulation. 

For adhesive systems, the liquid epoxy resins most widely used with LP-3 polymers are liquid unmodified and diluent-modified bisphenol A resins and liquid blends of bisphenol A and bisphenol F resins. Solid bisphenol A, multifunctional, and aliphatic diepoxy resins have also been used. Ratios of liquid polysulfide polymer to epoxy are in the range of 1 2 to 2 1. The effect of various degrees of polysulfide on cure properties of a DGEB A epoxy is shown in Table 7.7. An increase in elongation and impact strength is the result of increased amounts of the liquid polysulfide polymer. 

The addition of the polysulfide resin acts as both an elastomeric modifier and a diluent for the epoxy resin. The low viscosity of LP-3, for example, can drastically reduce the viscosity of the overall formulation. This provides greater ease of mixing and application and the ability to be applied and sprayed without a solvent. 

Suitable curatives for the polysulfide-epoxy reaction include liquid aliphatic amines, liquid aliphatic amine adducts, solid amine adducts, liquid cycloaliphatic amines, liquid amide-amines, liquid aromatic amines, polyamides, and tertiary amines. Primary and secondary amines are preferred for thermal stability and low-temperature performance. Not all amines are completely compatible with polysulfide resins. The incompatible amines may require a three-part adhesive system. The liquid polysulfides are generally added to the liquid epoxy resin component because of possible compatibility problems. Optimum elevated-temperature performance is obtained with either an elevated-temperature cure or a postcure. 

Polysulfide resins combine with epoxy resins to provide adhesives and sealants with excellent flexibility and chemical resistance. These adhesives bond well to many different substrates. Tensile shear strength and elevated-temperature properties are low. However, resistance to peel forces and low temperatures is very good. Epoxy polysulfides have good adhesive properties down to -100°C, and they stay flexible to -65°C. The maximum service temperature is about 50 to 85°C depending on the epoxy concentration in the formulation. Temperature resistance increases with the epoxy content of the system. Resistance to solvents, oil and grease, and exterior weathering and aging is superior to that of most thermoplastic elastomers. 

Another common method of flexibilizing epoxy adhesives is by blending the primary epoxy resin with other, more elastic polymers. Epoxy-nylon, epoxy-polysulfide, and to a certain extent epoxy-urethane hybrids use such a mechanism to provide flexibility. These flexibilizers are important additives for epoxy adhesives even though they may reduce certain... 

Data have been published dealing with successful applications of HAS in stabilization of other polymers than PO elastomers, styrenic polymers, polyamides, polycarbonates, polyacetals, polyurethanes, linear polyesters, thermoplastic polyester elastomers, polyacrylates, epoxy resins, poly(phenylene oxide) or polysulfide [12]. In spite of their basicity, HAS may also be used for stabilization of PVC. This application includes less basic derivatives of piperidine and 1,4-dihydropyridine [12,13,145,146]. 

ELP. [Morton IntT.] Epoxy terminated polysulfide polymers as concrete adhesives, for chemically resistant linings and coatings, bonding to metallic substrates, moi fier for other resins. 

Epoxy resin mortars have been used in repairing or conserving massive concrete structures. It has been found that the adhesive properties of epoxy resins modified with polysulfides is... 

The basic determinant of the final properties of a cured polysulfide polymer/epoxy resin compound is the weight ratio of liquid polymer to epoxy resin. By varying this ratio, the formulator is able to develop the optimum properties for a specific application. The effect of varying LP/EP ratios on the properties of a typical compound is shown in the following table. 

Polysulfides are used in synthetic rubber compositions [1], epoxy resin modifiers [2], coatings [3], adhesives [4], sealants [5], and many other products as described in recent Chemical Abstract issues. 

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