Two-part systems

Two-part room temperature condensation cure. Silicone can be formulated into two-part systems [3,12,14,33] that prevent the reactive groups from coming into contact before they are needed. The reactions in these systems are based on the condensation of a silanol group with an alkoxy silane group, catalyzed by organo-tin compounds (Scheme 9). 

PUs are also widely used as coating materials sold as finished polymers, two-part systems, and prepolymer systems. Water-based PU systems are now available allowing easy home use. Aromatic diisocyanate-derived coatings generally offer poor external light stability while aliphatic-derived systems offer good light stability. 

They are typically sold as two-part systems. By convention, the isocyanate prepolymer is designated Part A while the polyol, selected catalysts, and perhaps even water comprise Part B. 

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. 

Thicker sections can be cross-linked by hydrosilylation addition. This is the same chemistry used to produce fluorosilicone monomers with the vinyl functionality present on silicon. The catalyst reaction occurs between a vinyl group and silicone hydride.66 The advantage of this system is that it does not produce volatile by-products. On the other hand, the disadvantage is that it is available only as a two-part system.67 Recently, however, one-part, platinum-catalyzed products have been developed.66 The reaction is very rapid and at room temperature it is completed in 10 to 30 minutes. It is accelerated with increasing temperature and at 150°C (302°F) it is completed within a few seconds. This makes the compounds ideal for fast automated injection molding operations.68 One-part systems use the chemical complexing of the catalyst, which is activated at elevated temperatures, or its encapsulation into an impermeable shell, which is solid at room temperature and melts at elevated temperatures68... 

Catabolite repression is a two-part system. The first component is the small-molecule regulator, cyclic AMP. Glucose decreases cyclic AMP synthesis. The second component is cyclic AMP binding protein, CAP. CAP binds cAMP and thereby helps RNA polymerase bind to the promoter. When bound to cAMP, CAP binds to a sequence at the 5 end of the lac promoter. CAP binding bends the DNA, allowing protein-protein contact between CAP and polymerase. It therefore behaves in the opposite manner of repressor. Repressor (LacI) binds to operator DNA only in the absence of its small-molecule ligand, while CAP binds to promoter DNA in the presence of its small-molecule ligand. 

In two-part systems the adhesive is supplied as two materials in separate containers and to initiate setting the two must be mixed immediately before application. Mixed material either has to be used within the pot life of the system or must be discarded application equipment or tools used for the work either must be disposable or must be cleaned with a suitable solvent immediately after the cycle. 

This process is a take-off from compression molding that uses solid material male and female matching mold halves. This unique process uses a precision-made, solid shaped heated cavity and a flexible plunger that is usually made of hard rubber or TS polyurethane. This two-part system can be mounted in a press, either hydraulic or air-actuated. Rather excellent product qualities are possible at fairly low production rates. The reinforcement can be positioned in the cavity and the liquid TS resin is poured on it. Also used are prepregs, BMC, and SMC. 

ICG is available commercially fromAkom, Inc, as a two-part system the dry dye powder (25 mg) is dissolved into a volume of aqueous diluent and should be used within 10 hours. Amounts up to 40 mg of dye dissolved in 2 ml... 

Room temperature curing two-part systems require from as little as twenty-four hours to as long as seven days to reach ultimate properties. In most cases heat is used to accelerate cure and to speed up the manufacturing process. 

Elastomeric silicone sealants first appeared in the early 1950 s. The first of these, patented by J. F. Hyde of Dow Corning Corporation, was a two-part system consisting of an acid-ended polysiloxane and polysilicate.— With this system, materials which cured at room temperature were possible and the term "RTV" (Room Temperature Vulcanizing) was coined. These two-part systems required premixing in appropriate ratios before application and curing. 

Acrylics and urethanes are used as adhesives for laminations, also. These adhesives are known as two-part systems, and they require time to cure after the lamination is formed before they reach their final bond strengths. The chemical cross-linking reactions take place during the curing process. 

For bonding nylon to metal and other materials, various commercial adhesives, especially those based on phenol-formaldehyde and epoxy resins, are sometimes used. Epoxy adhesives (in two-part systems), for example, have been used to produce satisfactory joints between nylon and metal, wood, glass and leather. 

Selection of an adhesive whose adhesive or cohesive strength is initially marginal or poor. In the case of two-part systems, poor strength may be due to using non-stoichiometric ratios of resin to hardener. 

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