By Head of Department Dr. Rashid Hassan Assistant Professor Science of Dental Materials Department RAWAL INSTITUTE OF HEALTH SCIENCES RAWAL COLLEGE OF DENTISTRY ISLAMABAD 1

 Atoms are bonded to each other by either primary or secondary bondS By attraction of positive and negative charges of ions.  Thus they form a regularly spaced configuration, known a s space lattice or crystal.  There are 14 possible lattice shapes and forms. 2

 Triclinic simple.  Monoclinic simple.  Monoclinic body centered.  Orthorhombic simple.  Orthorhombic base centered.  Orthorhombic body centered.  Orthorhombic face centered.  Hexagonal.  Rhombohedral.  Tetragonal simple.  Tetragonal body centered.  Cubic simple. 3

SIMPLE CUBIC BODY CENTERED CUBIC

5 FACE CENTERED CUBIC HEXAGONAL CLOSE PACKED ORTHORHOMBIC

 Structures other than crystalline forms can also occur in solid state.  Molecules are distributed in a random pattern.  Dental waxes are an example of amorphous structures. 6

 The increase in energy per unit area is called as surface energy or surface tension.  Interfacial tension that exists between the two surfaces due to unbalanced intermolecular forces.  For adhesion to exist the surfaces must be attracted to one another at their interface.  The energy at the surface is greater than that of its interior. 7

 At the surface the energy is greater because the outermost atoms are not equally attracted in all directions.  The surface atoms tend to form bond to other atoms in close proximity in order to reduce the surface energy.  When primary bonding is involved, the adhesion is called CHEMISORPTION 8

 Interfacial tension between a liquid and a solid resulting in a contact angle of less than 90°.  Adhesion is negligible when the surface molecules of the two materials are separated by a distance greater than 0.7nm.  In case of true wetting of the surface, adhesion failure should not occur. 9

 Cleanliness of the surface is very important to better adhesion.  Surface energy is very crucial.  Surface energy of waxes is very low that few if any liquid can wet their surface.  Sometimes a synthetic resin is used where adhesion is not desired.  e.g. Teflon (Polytetraflouroethylene) 10

 The extent to which an adhesive can wet the surface of adherend can be determined by measuring the contact angle between the adhesive and the adherend.  Contact angle is the angle formed at the interface of the adhesive and the adherend.  If the molecules of adhesive are attracted more to the molecules of the surface, the adhesive will spread completely over the surface of the solid. 11

 Complete wetting occurs at 0° and no wetting occurs at 180°.  Entrapment of air bubbles during adhesive application leads to voids and pores.  When a thermal energy is applied to such adhesive, stress concentrations develop.  These voids may lead to crack propagation. 12

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 Tooth is a composite material.  The composition of both enamel and dentin varies.  The materials that adhere to the organic component of the tooth may not adhere to the inorganic component.  Surface contamination (smear layer) reduces the adhesion in cavity preparation.  Inorganic component has strong affinity for water. 16

QUESTIONS??? 17 Questions ???