Page 228 - Handbook of Adhesives and Sealants
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Chapter
                                                                    6








                                                    Surfaces and

                                       Surface Preparation












            6.1  Introduction
            Since adhesives and sealants must function by surface attachment,
            the nature and condition of the substrate surface are critical to the
            success of any bonding or sealing operation. Four common criteria are
            generally recognized for an ideal bonding surface: cleanliness, conti-
            nuity, stability, and wetting of the surface by the adhesive or sealant.
              Cleanliness does not necessarily mean the absence of all surface
            films, since some surface films are very strongly attached to the bulk
            substrate and offer a suitable surface for adhesion. However, cleanli-
            ness does require the removal of unwanted or weak boundary layers
            such as oil, dirt, or corrosion. The purpose of cleaning the surface is
            to remove any weakly attached materials and to provide a surface that
            is relatively consistent from part to part.
              Discontinuities on the adherend surface, whether chemical or phys-
            ical, may adversely affect the apparent strength of the joint by cre-
            ating localized regions of poor bonding and stress concentration within
            the joint. Discontinuities may also make surface cleaning or treating
            processes non-homogeneous. These discontinuities could be due to in-
            consistent manufacturing processes or chemical inhomogeneity within
            the substrate.
              Stability of the substrate surface is important before bonding as well
            as after bonding. Unwanted boundary layers could form during the
            time between surface preparation and application of the adhesive, de-
            pending on the shop environment and the reactivity of the surface.
            Boundary layers could also form during the time period after the ad-
            hesive is applied and before it sets, depending on the reactivity of the
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