Catalyzed moisture cures

Catalyzed Moisture Cures - This two component system is the same as the one package moisture cme system except a tertiary amine catalyst such as N,N-dimethylethanolamine is added just before application. This shortens cure time and increases the crosslink density of the film for better chemical resistance. As soon as the catalyst is added there is limited pot-life before gelation occms. 

Scheme 23.2 Synthesis of TES-MPEO in a two-step process of allylation and hydrosUylation. The functional PEO was combined with an additional crosslinker, TEOS, and cured using a tin-catalyzed, moisture cure process to produce a covalently linked block copolymer sflicone elastomer. Redrawn with permission from [38] Copyright 2004 Elsevier.

Catalysts serve a dual purpose in one-component moisture-curing urethanes. The first purpose is to accelerate the prepolymer synthesis. The second purpose is to catalyze the curing reaction of the adhesive with moisture. The most common catalysts used to promote both prepolymer formation (NCO/OH) and later the adhesive curing reaction (NCO/H2O) are dibutyltin dilaurate and DMDEE ((tertiary amine. A stabilizer such as 2,5-pentanedione is sometimes added when tin is used, but this specific stabilizer has fallen from favor in recent years, due to toxicity concerns. DMDEE is commonly used in many one-component moisture-curing urethanes. DMDEE is one of the few tertiary amines with a low alkalinity and a low vapor pressure. The latter... 

An emulsion polymer-isocyanate adhesive, a crosslinked polyvinyl acetate adhesive, a resorcinol-formaldehyde adhesive, a phenol-resorcinol-formalde-hyde adhesive, and an acid-catalyzed phenolic-formaldehyde adhesive developed bonds of high shear strength and wood failure at all levels of acetylation in the dry condition. A neoprene contact bond adhesive and a moisture-curing polyurethane hot-melt adhesive performed as well on acetylated wood as untreated wood in tests of dry strength. Only a cold-setting resorcinol-formal-... 

Two-part systems Single-part, cured via catalyst or hardener Moisture-curing adhesives Radiation (hght, ultraviolet, election heam, etc.)-curing adhesives Adhesives catalyzed hy the substrate... 

Three general elasses of urethane resins or vehicles are available today amine catalyzed, two-eontainer systems, moisture-cured urethane, and urethane oils and alkyds. The first and seeond types eontain unreaeted isoeyanate groups that are used to aehieve final eure in the eoating. In the first ease, an amine is used to catalyze a cross-linking reaction, which results in a hard, insoluble film in the second, moisture in the air acts as a cross-linking agent. 

Silicone They have excellent heat resistance up to 260°C (500°F), chemical resistance, good electricals, compatible with human body tissues, etc. and a high cost. There are the room temperature vulcanizing (RTV) types that cure and cross-link at ambient temperatures, catalyzed by moisture in the air. It is a good sealant and excellent for making flexible molds for casting. It is widely used for human implants. 

The formulation ofmoisture-ujriiig silicones includes a silicone polymer, filler, a moisture-reactive cross-linker, and sometimes a catalyst. A newer class of silicone sealants are known as the silicone latex sealants. These sealants are silicone-in-water emulsions that cure by evaporation of the emulsifying water. Addition-curing silicones in general are two-part systems that cure by the platinum-catalyzed reaction of a silicone hydride with typically a vinyl group attached to silicon. Because no by-products are generated by the cure, there are few volatiles and no shrink in thick sections. 

It was found that 5-nm-thick resist-mask polysilane films worked well in a direct lithography process on silicon substrates, resulting into a line width of 40 nm prepared by scanning probe microscope lithography, using a carbon nanotube tip.57 Thin PMPS films of 6—8 nm, with a molecular weight of 30,000 were prepared by spin casting and cured at 150°C to obtain a smooth surface. It has been interpreted that moisture was essential for the oxidation of the polysilane. The proposed mechanism involved dissociation of Si-Si bonds in polysilane by the electron injection from the carbon nanotube tip catalyzed by moisture. 

Where temperature of normal curing is not available they may be catalyzed with REZCAT 3 1%. When handled in this way a cure of no more than 250-275F. is all that is required when coating is moisture free. 

Siloxane-based sealants cure either by condensation or addition reactions (22). One-package systems are available that cure in the presence of atmospheric moisture. The cross-linking reaction results in the elimination of either acetic acid or methyl ethyl ketone depending on the resin system. The more conventional two-package systems use metallic soaps such as dibutyl tin dilaurate to catalyze the cross-linking with the subsequent elimination of ethyl alcohol. Addition cures can also be accomplished with vinyl-containing polysiloxanes and a transition metal catalyst. 

The base-catalyzed reaction of an epoxy resin with the phenolic resin produces a cross-linked polyether structure that is resistant to chemicals and heat and is a good moisture vapor barrier. Since the curing mechanism does not produce byproducts, thick sections may be obtained without voids and low shrinkage. Applications that employ the advantages of epoxy-phenolic formulations Include molding materials, laminates, coatings, and adhesives. 

The primary problem with epoxy resins appears to be the swellability of the resin in moisture (18). This is likely due to the preponderance of hydrophilic groups in the cured epoxy-adhesive. Urethanes have the additional problem that metal ion catalyzed hydrolysis of the urethane bond can take place. 

Unsubstituted and nonheat-reactive phenolic resins are used to a very limited extent due to their nonfilm-forming character. These resins are generally used as hardeners with epoxy resins to produce thermoset systems with high-quality engineering plastic properties. The base-catalyzed curing produces a crosslinked polyether structure, as shown in reaction (30), which is resistant to chemicals and heat and shows good barrier properties against moisture vapour. 

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