Surface Properties and Adhesion Bonding between dissimilar materials

Surface Properties and Adhesion: Bonding between dissimilar materials through chemical reaction of their atoms and molecules. Examples of adhesion: Denture retention by the adhesive action of a thin film of saliva between the soft tissue and the denture base. Cohesion: Bonding between similar materials. Examples of cohesion: Bonding two pieces of pure gold together under pressure, results from metallic bond

Adhesive: The material used to produce adhesion. Adherent: or substrate is substance to which the adhesive is applied. N. B. For adhesion to take place, ** Materials being joined must be in close (intimate) contact ** Adhesive must be applied in the liquid state to produce a thin layer.

Types of Adhesion Mechanical Adhesion (Attatchment) Restorative material Chemical (True) Adhesion - CH 2 - CH – CH 2 Restorative material Coo Ca Tooth structure

Mechanisms (Types) of Adhesion: Bonding by one of two mechanisms: I. Mechanical adhesion (attachment): By the flow and penetration of a liquid adhesive into microscopic or sub-microscopic irregularities in the surface of the adherend or substrate → Mechanical Interlocking i. e. No actual bond Examples: Amalgam, composite, zinc phosphate cement with the tooth.

II. True adhesion: Bonding between dissimilar materials through bonding between their atoms and molecules "chemical reaction". e. g. Glass ionomer and zinc polycarboxylate with the tooth. They contain COOH group that react chemically with calcium of the tooth. i. e. True adhesion = primary bonds (Adhesive dentistry) While: Attachment = mechanical interlocking.

Factors affecting the strength of adhesive junction 1) Wetting: Is the ability of an adhesive to wet the surface of the adherend. It is measured by the contact angle. ***Contact angle: It is the angle between the surface of the liquid and the surface of solid.

**Contact angle must be zero or less than 90° → Forces of adhesion between adherend & adhesive are more than the forces of cohesion between adhesive molecules together. ** Good wetting promotes adhesion and indicates strong attraction between the liquid and solid surface molecules.

Importance of wettability in dentistry: 1. Good wetting is important in soldering. 2. Good wetting for better denture retention. 3. A more natural appearance is achieved if restorative materials are wetted by a thin film of saliva. 4. Smooth surface of casting, the wax pattern is coated by surface acting agent (wetting agent or debubblizer) before investing This will improve wax wettability thus, producing smoother surface.

Surface Tension and Surface Energy Atoms and molecules at the surfaces of liquids and solids possess more energy than do those in the interior.

In case of solids, the **surface energy is greater than the internal energy, because the outermost atoms are not equally attracted in all directions. Surface energy is increased by: Increasing its surface area Solids tend to reduce this surface energy by adsorption of atoms or molecules. In case of liquids, the energy is called **surface tension the molecules at the surface are farther apart owing to loss of molecules by evaporation. This greater average separation leads to a net attraction between molecules and a higher energy of attraction. Surface tension is decreased by increasing the temperature and impurities. Liquids try to decrease its surface tension by minimizing its surface area.

Factors affecting wetting 1)The surface energy of the adherend (the reactivity of the solid surface): Increased Surface Energy of the solid, increases wettability. Examples: 1. Metals usually have a higher surface energy and are easy to wet by suitable adhesive. 2. Waxes are not easily wetted because they have low surface energy. 3. Teflon used in non stick cooking utensils has low surface energy.

2. The surface tension of the adhesive: Increasing Surface. Tension of the adhesive, decreases the wettability. Good wetting can be achieved if the molecules of adhesive are attracted to the molecules of the adherent more than they are attracted to each other which means → spreading of liquid on the solid surface. Liquids decrease their surface tension by minimizing its surface area (action of wetting agent or debubblizer)

3. Surface irregularities (roughness) of the adherend: Irregularities prevent an adhesive from completely wetting the adherend. Air pockets may be formed in small pit or crack and prevent the adhesive from penetrating into that area. 4. The viscosity of the liquid adhesive: Increasing the viscosity of the adhesive, decreases the wettability.

2) Irregularities on the adherend: ***Irregular and deep roughness, → air pockets → prevent adhesive from penetrating → no intimate contact between adherend adhesive → weak adhesive bond. ***Regular and shallow surface roughness, → no chance for air pockets formation This results in intimate contact between adherend adhesive → good adhesive bond.

3) Cleanliness on the adherend: Debris or surface contaminations prevent intimate contact which is necessary to produce adhesion. ** Adhesion to clean and dry surface of enamel and dentin is better than adhesion to wet contaminated one. 4) Thickness of the adhesive: Thinner adhesive film → stronger adhesive junction, ** less air voids are present. ** Less thermal stresses ** Less stresses due to setting contraction of adhesive.

5)Stresses due to setting contraction of adhesive: Contraction of liquid adhesives during setting → creation of stresses at the interface → decreases the strength of adhesion. 6) Thermal stresses: Different thermal coefficients of adhesive and + changes in temperature adherend → stresses in the bond. Close matching is required to minimize stresses and increases the strength of adhesion. 7) The type of bond formed: Primary bonds between adhesive and adherend produce stronger adhesion than if secondary bonds are formed (Soldered Joint is stronger than glued joint).

Failure of adhesive junction 1 - Adhesive failure (adhesive-adherend separation). 2 - Cohesive failure of the adhesive. 3 - Cohesive failure of the adherend. Adhesive Failure between adhesive and the adherend Cohesive Failure within the adhesive or within the adherend

Conditions that prevent ideal adhesion in the oral cavity 1) The inhomogeneous composition of enamel and dentin: Being partly organic and partly inorganic. Materials that would adhere to enamel would not be able to adhere to dentin. Thus adhesion would not be uniform 2) Surface irregularities in the prepared cavity: Scratches produced by the dental burs used in preparing the cavity result in morphologic roughnesses. 3) Debris in the prepared cavity: Tooth surface is covered with debris that is formed when the dentist prepares the cavity (smear layer). This debris prevents adhesive from complete wetting of an adherent. 4) Presence of water in the prepared cavity: Presence of water (not water from saliva) but a microscopic single molecule layer of water which is always on the tooth surface.

Bonding to tooth structure Bonding to enamel & acid etching technique Surface treatments should be performed → l l l help bonding of materials to enamel and dentin. Clean Etch: 30 -50% phosphoric acid for 15 - 30 sec Wash Dry Apply **Removal of surface debris “produce clean surface”. **Producing pores in the surface into which resin penetrates to form tag-like extensions, giving mechanical interlocking. ** Increasing the surface energy of the enamel, causing better wetting.

Bonding to Dentin - Dentin poses greater obstacles to adhesive bonding than does enamel due to; i) Presence of higher amount of water so it is strongly hydrophilic. ii) Presence of smear layer which will prevent proper adhesion. Smear layer is a 5 -10 microns thickness layer formed of a matrix of collagen containing tooth structure, blood, saliva and bacteria resulting from cavity preparation Dentin bonding involves three distinct processes: 1. Etching (conditioning). 2. Priming. 3. Bonding.

Bonding to Dentin Conditioning (Etching) Acid etchant for 10 to 15 seconds. This will lead to: *** Partial or complete removal of the smear layer (debris layer). *** Demineralization of dentin surface. Etching of dentine by acid Reduction of surface energy of dentin , as the demineralization of dentin will lead to exposure of more collagen that have low surface energy.

Primer l Bonding to dentin Elevate the surface energy of dentin to improve wetting. - Because composite resins are hydrophobic, Primer should contain both: hydrophilic and hydrophobic materials. Hydrophilic part should be designed to interact with the moist dentin surface, Hydrophobic part bond to the restorative resin. Hydrophobic Head Adhesive (Dentin bonding agent): Bond the primed dentin surface to resin composite micromechanical rather than true chemical adhesion. Successful bond should have a continuous layer along the dentin surface called hybrid layer (resin infiltrated dentinal layer), A resin reinforced layer part is tooth and part is resin.

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