Applied surface phenomena by Dr Reham Mohammed Abdallah

Applied surface phenomena by Dr. Reham Mohammed Abdallah

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 The 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.

• Atoms or molecules at the surfaces of solids or liquids differ greatly from those in the bulk of the solid or liquid (Fig. 1). • The solid surfaces have atoms of higher energy than bulk atoms because of the absence of some neighboring atoms and thus readily adsorb ambient atoms or molecules.

Fig. 1: Top surface atoms of higher energy than interior atoms.

Theories of adhesion 1. Adsorption adhesion • Chemical bonding between the adhesive and the adherent. The force involved may be primary or secondary. (intermolecular forces are responsible for adhesion). Example of primary bond: 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

2. Mechanical adhesion • Interlocking of the adhesive with irregularities in the surface of the adherent (Attachment = mechanical interlocking). • Example: zinc phosphate cement with the tooth. • Mechanical adhesion could be classified into: Fig. 2: Mechanical interlocking

A. Macro-mechanical retention • Can be seen by naked eye and interlocking between two substances occurs (undercuts Fig. 3, screws). Fig. 3: Undercut preparation (dark area)

B. Micro-mechanical retention • Cannot be seen by naked eye where penetration and solidification of liquid substance into the surface micro-pores of solid surface occurs as in acid etching (Fig. 4). Fig. 4: Enamel surface before (left) and after (right) acid

3. Diffusion adhesion • A bond between two surfaces may be formed by interdiffusion of atoms across the interface. • The adhesive blends into surface of adherend. (Example; adhesion between porcelain and metal in PFM restorations and during repair of acrylic denture base). 4. Electrostatic adhesion • A difference in electrostatic charges between constituents at the interface creates attraction force.

Factors affecting adhesion 1. Factors related to adherent �Surface energy Atoms and molecules at the surfaces of liquids and solids possess more energy than do those in the interior. Fig. 5 : Surface and interior atoms

• 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 attraction force exists between the surface molecules of a liquid. • 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. • The higher the surface energy of the adherent, the stronger the strength of the adhesive junction. Fig. 6: Surface tension of liquids

Examples on surface energy: 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. Fig. 7: Teflon pan with low surface energy

�Irregularities and air pockets • Rough surface provide mechanical means of retention and increase surface area of adhesion but air pockets may be created (Fig. 8) → prevent complete wetting of the entire surface→ ↓ strength of adhesion. • Regular and shallow roughness of the surface decrease the chance of air pockets formation than irregular and deep surface roughness. Fig. 8: Airc pockets decreasing degree of wetting

�Cleanliness on the adherent • Any debris on the surface will prevent direct contact between adhesive and adherent→ ↓ strength of adhesion.

2. Factors related to adhesive �Surface tension • The higher the surface tension of the adhesive →↓ the wettability→↓ strength of adhesion. �Viscosity • Low viscosity of the adhesive → ↑flow on the surface of the adherent →↑ strength of adhesion.

�Thickness Thin film thickness of the adhesive →↑ strength of adhesion because less air voids are present, less thermal stresses and less stresses due to setting contraction of adhesive. �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. Fig. 9: Different contact angles with different degrees of wetting

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 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. Fig. 10: Effect of wetting on strength of adhesion • The more wetting of the adherent surface by the adhesive →↑ strength of adhesion.

3. Factors related to both �Thermal stresses • If the adhesive and adherent have different coefficient of thermal expansion →↓ strength of adhesion. • Close matching is required to minimize stresses and increases the strength of adhesion.

�Dimensional changes during setting • Liquid adhesives undergo contraction during setting and also the adherent may show contraction during setting → creation of stresses at the interface →↓ strength of adhesion. �Type of bond formed • Primary bonds → stronger adhesion than if secondary bonds are formed.

Mode of failure of an adhesive junction Adhesive failure: This occurs at the interface between adherent and adhesive Fig. 11: Adhesive failure Cohesive failure: fracture occurs within the adhesive or adherent materials. Mixed failure: where adhesive and cohesive failure occur (Fig. 13). Fig. 12: Cohesive failure

Fig. 13: Mixed type of failure in the adhesive joint

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 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, Fig. 14: Shape of dentin with and without smear layer