Page 236 - Handbook of Adhesives and Sealants
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Surfaces and Surface Preparation  205


            interactions are considered possible, then they should be fully tested
            with prototype joints made in the production process.


            6.2.3  Other surfaces
            Many natural and man-made surfaces vary significantly in character-
            istics important for bonding or sealing. Wood and cellulose based prod-
            ucts, for example, will differ widely in surface roughness, pH, porosity
            and moisture content even within a single sample. The nature of these
            surfaces will also change with aging and oxidation. The presence of
            sap, pitch, resins, or preservatives will also affect bond strength. Gen-
            erally, freshly cut wood substrates are ideal for bonding because of
            their porosity. However, care must be taken to remove loose sawdust.
              Ceramic materials have smooth, glass-like surfaces, usually with
            very high surface energy. Since ceramics have high surface energies,
            they are usually easy to bond under normal conditions. However,
            many commercially important ceramics have glazed (glass-like) sur-
            faces. This glazed surface could provide another interface in the joint
            that must be addressed. The polar nature of the bonds between atoms
            in a ceramic material means that there will likely be an adsorbed layer
            of water and hydroxide ions. This layer is tightly held to the ceramic
            surface. Adhesives used with ceramics, as those used with metals,
            must be compatible with the surface moisture layer.
              Concrete is a substrate whose surface characteristics are likely to
            be affected by the environment in which it cures. Once cured, concrete
            has several surface characteristics that are hazardous for bonding or
            sealing. The concrete surface is extremely alkaline and will destroy
            any hydrolysis sensitive materials that are at the interface. It often
            has a weak, powdery surface layer that must be penetrated or re-
            moved. Thus, sealers are commonly used to moisture proof and
            strengthen the concrete surface prior to bonding.
              Bonding to painted or plated parts presents a problem not encoun-
            tered with other adherends. It is not recommended to bond to painted
            surfaces because the resulting bond is only as strong as the adhesion
            of the paint to the base material. Generally, the paint must be removed
            or abraded sufficiently so that any weakly attached areas are removed.
            Plated surfaces should also be tested before bonding to determine how
            strongly the plating is attached to the base substrate. Plated surfaces
            are often porous and usually exhibit poor resistance to moisture, es-
            pecially if the bonding agent does not seal the joint.
              Modern polymeric composites are being considered for light weight
            vehicles because of their high strength and low weight. Surfaces of
            these materials are usually liberally coated with mold-release agents
            such as silicone or fluorocarbon to aid release from the mold. Mold-
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