Surface Properties and Adhesion PDF

CHAPTER 5 Surface Properties and Adhesion The surface phenomenon is increasingly playing a major role in the understanding of the behavior of biomaterials. Particularly important is the characterization of solid surfaces, the wetting of solids by liqui...

CHAPTER 5 Surface Properties and Adhesion The surface phenomenon is increasingly playing a major role in the understanding of the behavior of biomaterials. Particularly important is the characterization of solid surfaces, the wetting of solids by liquids and adhesion. Obtaining adhesion between dental restoratives and tooth is a highly important prerequisite. Definitions: a. Adhesion: It is bonding between dissimilar materials through chemical reaction of their atoms and molecules.  Examples of adhesion: Denture retention is accomplished by the adhesive action of a thin film of saliva between the soft tissue and the denture base. b. Cohesion: Is the bonding between similar materials.  Examples of cohesion: Bonding two pieces of pure gold together under pressure is an example of cohesion. The bonding in such case results from metallic bond and is called pressure welding c. Adhesive: Is the liquid material used to produce adhesion. d. Adherend: Is the solid substance to which the adhesive is applied. N.B: For adhesion to take place: - Materials being joined must be in close contact (intimate contact). - The adhesive between them must be applied in the liquid state to produce a thin layer. Types of adhesion: True adhesion Mechanical attachment Definition Bonding between dissimilar Bonding between dissimilar materials through bonding materials through "mechanical between their atoms and interlocking" i.e. no actual molecules "chemical reaction". bond is formed. Examples Glass ionomer and zinc Amalgam, composite, zinc polycarboxylate with the tooth. phosphate cement with the tooth. Mechanism Both glass ionomer and A liquid flows into pores or polycarboxylate contain CooH irregularities in the surface of a group that react chemically solid and set (harden) forming a with calcium of the tooth. strong mechanical bond. Coo Coo Ca Tooth structure Factors affecting the strength of adhesive junction: I-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. The degree of wetting is measured by the contact angle. The smaller the contact angle (the more acute), the better the wettability, (figure 57). - For an adhesive to produce good wetting with the adherent, the contact angle must be zero or less than 90° i.e. forces of adhesion  between them 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. Figure (57): Contact angle  Importance of wettability in dentistry: 1. Good wetting is important in soldering. 2. Good wetting is a factor in better denture retention. 3. A more natural appearance is achieved if restorative materials are wetted by a thin film of saliva. 4. To produce a smooth surface of casting, the wax pattern is coated by surface active agent (wetting agent or debubblizer) before investing. This will improve wax wettability thus, producing smoother surface.  Factors affecting wetting: Surface tension and surface energy: - Atoms and molecules at the surfaces of liquids and solids possess more energy than do those in the interior because the outermost atoms are not equally attracted in all directions. - The forces of attraction between the surface atoms of liquids are called "surface tension", where as the forces of attraction between the surface atoms of solids are called "surface energy", (figure 58). - In the case of liquids, as 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. This results in a surface tension, which causes the liquid to form drops. - Surface tension is decreased by increasing the temperature and impurities as they reduce the cohesive force between liquid atoms at the surface, and by minimizing the surface area. (a) (b) Figure (58): (a) surface tension of liquid, (b) surface energy of solids 1. The surface energy of the adherend (S.E.): (the reactivity of the solid surface): Increased S.E. of the solid, increases wettability. S.E. of the solid wetting Examples: 1. Metals usually have a higher surface energy and therefore they are relatively 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 (S.T.): Increasing S.T. of the adhesive, decreases tile wettability. S.T. of the adhesive wetting Therefore, using adhesive liquid of low surface tension will increase its wettability to a solid. N.B. 1. For good wetting, the surface tension of the liquid should be equal or less than the surface energy of the adherend (solid). 2. 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 mean spreading of liquid on the solid surface. 3. The viscosity of the liquid adhesive: Increasing the viscosity of the adhesive, decreases the wettability. Viscosity wetting Low viscosity of the adhesive is required to allow its easy flow on the surface of the adherend. This increases the strength of adhesion. II) Irregularities on the adherend: - If surface roughness is irregular and deep, so air pockets formation is more common, where air pockets will prevent the adhesive from penetrating into that area, therefore no intimate contact between adherend and adhesive will be formed weak adhesive bond, (figure 59). - But if surface roughness is regular and shallow, so there will be no chance for air pockets formation. This results in intimate contact between adherend and adhesive good adhesive bond. air pocket Figure (59): surface irregularities III) Cleanliness on the adherend: - Any debris or surface contaminations prevent the adhesive from coming into the 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. IV) Detrimenal stresses at the interface: i. Stresses due to setting contraction of adhesive: Liquid adhesives undergo contraction during setting. This contraction results in the creation of stresses at the interface that severely decreases the strength of adhesion. ii. Thermal stresses: The adhesive and adherend have always different thermal coefficients of expansion. Changes in temperature will produce stresses at the interface between adhesive and adherend. Close matching in coefficients of thermal expansion is required to minimize stresses and so increases the strength of adhesion. V) Thickness of the adhesive The thinner the adhesive film, the stronger is the adhesive junction, the less air voids are present. Thin adhesive film allows; - more intimate contact - Less thermal stresses - Less stresses due to setting contraction of adhesive. VI) The type of bond formed: No doubt that 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: If an adhesive bond is tested in tension, one of three things may happen, (figure 60): i. Adhesive failure (adhesive-adherend separation). ii. Cohesive failure of the adhesive. iii. Cohesive failure of the adherend. Figure (60): Failure of adhesive junction Dental considerations: The following conditions prevent ideal adhesion in the oral cavity: 1) The inhomogeneous composition of enamel and dentin: Enamel and dentin are inhomogeneous in their composition, being partly organic and partly inorganic. An adhesive which would adhere to the organic component would not be able to adhere to the inorganic portion. At the same time materials that would adhere to enamel would not be able to adhere to dentin. Thus adhesion would not be uniform over the entire surface. 2) Surface irregularities in the prepared cavity: The surface of the prepared cavity is full of pits and fissures. These morphologic roughness are further increased by the scratches which are produced by the dental burs used in preparing the cavity. It is difficult to design an adhesive that flows into these minute irregularities and wets the entire surface area of the cavity preparation. 3) Debris in the prepared cavity: Microscopically, the tooth surface is covered with debris that is formed when the dentist prepares the cavity (smear layer). Thorough cleaning of the cavity cannot remove this debris completely. This debris prevents adhesive from complete wetting of an adherent. 4) Presence of water in the prepared cavity: Of major importance is the presence of water. This is not water from saliva, but a microscopic single molecule layer of water which is always on the tooth surface. This film prevents the adhesive from coming into intimate contact with the tooth. Bonding to tooth structure - Surface treatments should be performed in order to help bonding of materials to enamel and dentin. Two mechanisms of adhesion (bonding) may be distinguished: chemical and mechanical. - The most widely used technique is acid etching of enamel and dentin to produce mechanical bonding between the tooth and composite filling materials. A- Bonding to enamel; (acid etching technique): - The most commonly used acid etch is 30-50 % phosphoric or citric acid. - Etch the surface of enamel by applying the acid for 15-30 seconds. The acid removes about 5 microns of the surface of enamel and produce microtags into which the adhesive will penetrate, then resin composite (filling material) bond to the adhesive, (figure 61).  Acid etching helps bonding to enamel by: 1- Removal of surface debris producing clean surface. 2- Producing pores in the surface into which resin penetrates to form tag-like extensions, giving mechanical interlocking. 3- Increasing the surface energy of the enamel, causing better wetting. 4- Exposure of greater surface area of the enamel to the resin.  (a) (b) Figure (61): (a) Unetched enamel, (b) Etched enamel B- 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 N.B; 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. 1. Etchant: - Dentin etching is done by applying the acid etchant for 10 to 15 seconds, (figure 62). This will lead to: 1. Partial or complete removal of the smear layer (debris layer). 2. Demineralization of dentin surface. Figure (62): Etched dentin N.B; On the other hand, etching of dentine by acid will lead to reduction of surface energy of dentin, as the demineralization of dentin will lead to exposure of more collagen that have low surface energy. 2. Primer: - It is used to elevate the surface energy of dentin to improve wetting.  - Because composite resins are hydrophobic, the primer should contain hydrophilic and hydrophobic materials. - The hydrophilic part should be designed to interact with the moist dentin surface, whereas the hydrophobic part bond to the restorative resin. 3. Dentin bonding agent: - Bond the primed dentin surface to resin composite restoration, and the bond results is micromechanical rather than true chemical adhesion. The final successful bond that is aimed to be produced, should have a continuous layer along the dentin surface called hybrid layer (resin infiltrated dentinal layer), which is a resin reinforced layer part is tooth and part is resin.