Mechanical Interlocking

This adhesion involves the penetration of the bonding agent into surface irregularities or porosity in the substrate surface. Gross examples of this mechanism include the retention of dental filling materials in mechanically prepared tooth cavities and of crowns by dental cements on teeth (Fig. 1) and the fixation of artificial joint components by acrylic bone cement (Fig. 2). Even apparently smooth surfaces are pitted and rough at the microscopic level, and strong bonding can arise with an adhesive that can penetrate at this level. The use of primers (chemical pretreatments) can create surface irregularities or porosities at the microscopic level, or can deposit porous layers that similarly provide effective micromechanical interlocking. Examples include the etching of dental enamel by 35-40% phosphoric acid (Fig. 3) and primer treatment of tooth dentin with acidic agents (Fig. 4). In each case the unpolymerized bonding agent penetrates 5—50 /im into the surface, creating numerous resin "tags" that provide a strong bond.

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