Epoxy-polysulfide

Poly sulfide compounds that are compatible with epoxy resins are liquid elastomers at room temperature. The most significant commercial resin of this type is LP-3 from Toray. The predominant product is a mercaptan-terminated liquid polymer (LP) that contains approximately 37% bound sulfur (see Fig. 7.2). It is the high concentration of sulfur linkages that provides these products with their unique chemical properties. A sulfur odor is noticeable during processing, making ventilation important. 

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. 

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K. Akimitsu et al, Experimental Studies of Composite Propellants , KKZ 63, 1902—06 (1960) CA 57, 1140 (1962) (Composite propint NH4CIO4 + epoxy, polysulfide resin was found to be superior to proplnt NH4CIO4 + polyester resin in both its phys burning props)... 

There are a number of inert binders such as polyester, epoxy, polysulfide, polyurethane which have been reported as binders for composite propellants and plastic bonded explosives (PBXs). At present, hydroxy-terminated polybutadiene (HTPB) is regarded as the state-of-the-art workhorse binder for such applications. However, the recent trend is to use energetic binders such as poly [3,3-bis(azidomethyl oxetane)] [poly(BAMO)], poly (3-azidomethyl-3-methyl oxetane) [poly(AMMO)], PNP, GAP diol and triol, nitrated HTPB(NHTPB), poly(NiMMO), poly(GlyN) and nitrated cyclodextrin polymers poly(CDN) for PBXs and composite propellants in order to get better performance. 

Epoxy-polysulfide systems do not generate significant amounts of exotherm, and their cure rate does not rely on the exothermic reaction. Thus, epoxy-polysulfide compositions have a cure rate that is relatively insensitive to temperature, and they can cure at very low temperatures (below room temperature). 

Epoxy-polysulfide Epoxy-polysulfide adhesives and sealants have a very high degree of elongation but poor tensile strength compared to unmodified epoxies. They are used primarily as sealants or coatings in die building and construction industries. 

Alloyed blends consisting of epoxy-phenolic, epoxy-nylon, and epoxy-polysulfide adhesives... 

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... 

A good example of a flexible epoxy hybrid adhesive is the epoxy polysulfides. Figure 8.2 shows the improved impact resistance that can be realized by using increasing amounts of... 

The polysulfide additive provides excellent adhesion to concrete because of its chemical nature and low viscosity, resulting in good penetration. Table 11.16 describes a formulation for a flexible epoxy-polysulfide adhesive. This adhesive is claimed to provide excellent adhesion between new and old concrete surfaces. Table 11.17 describes several epoxy-polysulfide formulations for general-purpose adhesives. 

Epoxy-polysulfide systems are used in the construction, electrical, and transportation industries because of their unique combination of cured flexibility, adhesion to many substrates, and chemical resistance. They are typically used as adhesives for... 

Because of their excellent flexibility, epoxy-polysulfide adhesives are commonly used for bonding plastic substrates where differences in thermal expansion are a concern. 

TABLE 11.16 Formulation for a Flexible Epoxy-Polysulfide Adhesive23... 

Typical properties of several epoxy-polysulfide adhesives are shown in Table 11.18. Room temperature tensile shear strength ranges from about 500 to 3000 psi, depending on the substrate and the adhesive formulation. Generally, tensile strength properties reach a maximum at about 10 to 30 percent polysulfide concentration. Peel strength can be as high as 20 lb/in. 

The heat distortion temperature is only slightly affected by the incorporation of 20 percent polysulfide polymer, but at a 1 1 ratio the drop becomes significant. This property prevents the use of epoxy-polysulfide adhesives at elevated temperatures. 

Epoxy-polysulfide systems can be formulated either as a liquid DGEBA epoxy mixed with liquid poly sulfide polymer or as an epoxy-terminated polysulfide polymer either may be cured with a tertiary amine such as DMP-30. Table 11.19 describes the formulation and shows the physical properties of these epoxy-polysulfide adhesives compared to an unmodified epoxy adhesive. 

Many different adhesives have been tested and used in weldbonding. These include both high-strength structural adhesives, such as modified epoxies, and relatively low-strength adhesives, such as vinyl plastisols or epoxy-polysulfides that are commonly used for sealing and vibration damping. Table 14.10 shows that a variety of adhesives and substrates are compatible with the weldbonding process. 

Environment Epoxy + polyamine Epoxy + anhydride Epoxy + polyamide Epoxy- phenolic Epoxy- polysulfide Epoxy-nylon Phenoxy (thermoplastic)... 

Next to the epoxy-nylons and epoxy-urethanes, the epoxy-polysulfide adhesives show the greatest lap shear tensile strength at temperatures below 0°C. Bonded etched steel substrates show a tensile shear strength of 2900 psi at room temperature, and this increases to 3400 psi when the temperature is reduced to -156°C. Epoxy-nylon and epoxy-polysulfide are the only adhesives that show an increase in strength as the temperature is significantly reduced. 

The good low-temperature properties of epoxy-polysulfide adhesives are one reason why they have found significant use in the building and construction industry. Another reason is their excellent flexibility and ability to absorb stress and move with the thermal expansion and contraction of the substrates. However, in these applications the adhesive is generally not formulated for high tensile shear strength but rather for optimum elongation. 

Chlorinated Polyether. This thermoplastic resists most solvents and is attacked only by nitric acid and fuming sulfuric acids. Thus, it is not capable of being solvent-cemented. Chlorinated polyether parts can be bonded with epoxy, polyurethane, and polysulfide-epoxy adhesives after treatment with a hot chromic acid solution. Tensile shear strength of 1270 psi has been achieved with an epoxy-polysulfide adhesive. 

Adhesives commonly used on concrete must be formulated so that they cure well in the presence of moisture. The adhesive should have a viscosity that allows it to penetrate the concrete for mechanical bonding. Epoxy and epoxy-polysulfide adhesives are especially effective adhesives for concrete. The starting formulations given in Table 16.16 are for an ambient cure 100 percent solids epoxy and a waterborne epoxy adhesive. 

Starting formulation for a flexible epoxy adhesive containing polyamide curing agent and reactive diluent Starting formulation for a high-peel-strength adhesive Epoxy-polysulfide adhesive formulation Formulation for a flexible epoxy-polysulfide adhesive Epoxy-polysulfide adhesive formulations... 

Modified Polyphenylene Oxide (NORYL ) Conventional adhesives recommended include epoxies, polysulfide-epoxies, silicone, synthetic rubber, acrylics, cyanoacrylates, and hot melts (14). 

The epoxy polysulfide formulations are comparatively fast setting, with a short pot life. In filling a crack on a horizontal slab, the crack is widened by mechanical means to form a slight trough. The mixed compound may then be introduced into the crack by means of a caulking gun, or dispensed through a narrow spout. 

LP-3 and LP-33. Formulations with epoxy generally utilize ratios of liquid polymer to epoxy of 1/2 to 1/1. Some amine catalyst is required to speed the reaction. The ratio of 10 to 15 parts of amine per 100 parts of epoxy is normally used. The most common amine for this application is DMP-30 or EH-330. Typical properties of compounds of epoxy/polysulfide liquid polymers are listed in Table XII. 

Fettes and Gannon were the first to report reactions of liquid polysulfide polymers with epoxy resins [16]. In these adhesives, the epoxy resin is the major component. The polymer LP-3 in which n = 8 has been used most extensively. This polymer has a moleeular weight of about 1000 with viscosity in the range 7 to 12 P. The epoxy resins most widely used in adhesive formulations have viscosities the range 80 to 200 P with an epoxy equivalent of 175 to 210. Typical resins are Epon 820 and 828 (Shell), ERL-3794 (Union Carbide), and Araldite 6020 (CIBA). The epoxy-polysulfide reaction is prompted by organic amines (e.g. diethylenetriamine and benzyldimethylamine). The general reaction of a polysulfide polymer, epoxy resin, and amine hardner is... 

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