Substrate bonded plastics cure time

The cure reaction for many silicone sealants is initiated by acid added at low levels to the sealant formulation. Acids are chemically incompatible with concrete, marble, and limestone. When acid-containing silicone sealants are used in joints with these substrate materials, the acid reacts with the substrate bond surfaces, creating salts at the bond interface. These salts destroy the sealant/substrate adhesion and cause debonding and loss of the seal. In order to use a silicone sealant with these substrates, a silicone formulated without acid is required. Other known chemical incompatibilities are silicone and polychloroprene. Use of these two materials together in a sealant joint is to be avoided. Solvated sealant use in joints containing plastic or rubber materials should be undertaken only after chemical compatibility studies of the sealant with these materials is performed. Typical incompatibility will manifest itself over time by causing the sealant or substrate to soften, harden, crack, and/or craze. A standard test method for determining chemical compatibility is ASTM D-471. 

The solvent cement is generally applied to the substrate with a syringe or brush. In some cases, the surface can be immersed in the solvent. After the area to be bonded softens, the parts are mated and held under pressure until dry. Pressure should be low and uniform so that the finished joint will not be stressed. After the joint hardens, the pressure is released, and an elevated temperature cure may be necessary, depending on the plastic and desired joint strength. The bonded part should not be packaged or stressed until the solvent has adequate time to escape from the joint. 

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