Adhesives, polysulfide

Besides water vapor and gas diffusion, other requirements for good edge sealants are water resistance, uv resistance, heat/cold resistance (—40 to 80°C), adhesion to glass and metal, and good characteristics for appHcation. Polysulfide sealants have maintained an exceUent record in use since the 1950s. Development of new polysulfides and sealant formulations continues in order to meet market needs of the 1990s. 

Aircraft Sealants. Polysulfides have been used for sealing fuel tanks and aircraft stmctural components since the 1940s. There are stringent requirements for these sealants. They must have outstanding resistance to fuels and exceUent adhesion to many different materials such as various metal aUoys and protective coatings used in aircraft constmction. The sealants must also perform in extremely variable weather conditions (10). The polysulfides have an exceUent performance record. 

Polyurethane sealant formulations use TDI or MDI prepolymers made from polyether polyols. The sealants contain 30—50% of the prepolymer the remainder consists of pigments, fiUers, plastici2ers, adhesion promoters, and other additives. The curing of the sealant is conducted with atmospheric moisture. One-component windshield sealants utili2e diethyl malonate-blocked MDI prepolymers (46). Several polyurethane hybrid systems, containing epoxies, siUcones, or polysulfide, are also used. 

Adhesives and Sealants. Various grades of chloriaated paraffins are used as nonvolatile iaert fire-retardant plasticizers and modifying resias ia adhesives and sealants (3). They find wide appHcation ia polysulfide, polyurethane, acryUc, and butyl sealants for use ia building and constmction. The low volatihty high chlorine types are also employed ia sealants for double- and triple-glazed wiadows. 

Polysulfides are more commonly used as sealants but are occasionally employed as adhesives as well. Polysulfides adhere well to a wide range of materials and are useful where some flexibility is needed in the joint. 

Liquid copolymers of butadiene and acrylonitrile with carboxyl end groups are used as contact adhesives in the automotive industry, Polysulfide elastomers, produced by the reaction of ethylene dichloride and sodium po-... 

Fig. 33 Effect of nanoclay content on adhesive strength of polysulfide elastomer/clay nanocomposites samples containing 0, 2, 4, and 8 phr of nanoclay are designated as PSCO, PSC2, PSC4, and PSC8, respectively...

Fig. 34 Peel force versus distance curves for polysulfide/nanoclay adhesive systems...

Fig. 35 a Failure pattern in the neat polysulfide adhesive sample PSCO. b Failure pattern in clay-loaded sample PSC8... 

Formulation. Polysulfide-based sealants are formulated with appropriate ingredients to obtain the desired properties for a particular application. A typical formulation contains liquid polysulfide polymer, curing agent, cure accelerators (bases) or retarders (acids), fillers, plasticizers, thixotropes, and adhesion promoters. 

Titanates have been instrumental in the bonding of fluorinated resins to packaging films, poly(hydantoin)—polyester to polyester wire enamel, polysulfide sealant to polyurethane (a phosphated titanate is recommended), polyethylene to cellophane using a titanated polyethylenimine, and silicone mbber sealant to metal or plastic support using polysilane (Si—H) plus polysiloxane (Si—OR) and titanate as the adhesive ingredients (450—454). Polyester film coated first with a titanium alkoxide, then with a poly(vinyl alcohol)—polyethylenimine blend, becomes impermeable to gases (455). 

One problem with early epoxy formulations is that they cured to a relatively brittle material. By using reactive flexibilizers, such as polysulfides, epoxy adhesive formulators have obtained the flexibility required for many applications in this industry. Polyamides and even coal tars have also been used to provide flexibility to epoxy base resins. 

Polysulfides and mercaptan Moisture resistance Quick set time Flexible Odor Poor elevated-temperature performance Poor tensile strength Adhesives and sealants Civil engineering Casting and encapsulation Coatings... 

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... 

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. 

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. 

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