Basic Science for Dental Materials.pdf

Basic Science for Dental Materials Many of the materials that a dental student learns about will be obsolete when that student is a practicing dentist • Dentist’s responsibilities: o To appreciate features of materials o what makes them useful for their chosen application o Material limitations o Be...

Basic Science for Dental Materials Many of the materials that a dental student learns about will be obsolete when that student is a practicing dentist • Dentist’s responsibilities: o To appreciate features of materials o what makes them useful for their chosen application o Material limitations o Be able to select the most appropriate material on a case by case basis • Clinical performance of dental restorations depends on: o appropriate material selection, based on a knowledge of each material’s properties o the optimum design of the restoration o knowledge of how the material will interact with the biological environment Basic Science for dental materials • structure of materials • atomic bonding • metals, ceramics and polymers • the growing need to be aware of the safety aspects of dental materials • the care that has to be taken when sourcing materials from across the world • terminology used in the description of the physical, chemical and mechanical behavior of materials • the principles of adhesion The 3 solids: • ceramics o ionic bond (e.g crystalline and amorphous form, also glass) • metals o metallic bond • molecular solids o covalent, secondary bonds, polymers The 3 principle types of stress: • tensile • compressive • shear Stress • force per unit cross-sectional area that is acting on a material Strain • fractional change in the dimensions caused by force The 8 Mechanical Properties • Elastic Limit and Plastic Flow o Point where material exceed elastic limit, begin permanent deformation o Material does not return to normal shape after stress removed • Young’s Elastic Modulus: Modulus of elasticity • Fracture Strength: Ability of material to resist propagation of a preformed crack • Yield Stress and Proof Stress: Restoration will permanently deform if tensile stress exceeds yield stress (e.g. 3 unit metal bridge) • Fracture Toughness: Stress required to break material • Ultimate Tensile Strength: Max stress that specimen can withstand • Ductility: Amount of plastic strain produced in specimen at point of fracture • Resilience and Toughness: Amount of energy a material can absorb without any permanent deformation Mechanical Tests • Tensile Test o Material is stretched and recovery of normal dimensions noted • Compression test o Sample is compressed • Hardness Test o Resistance of material to an indenter or cutting tool • Impact Test o Resistance of material to application of load • Fatigue Test o Materials subjected to fluctuating stresses rather than static loads • Creep Test o Influence of constant stress that will eventually lead to material fracture Physical Properties • Rheological Properties (flow of materials) o Viscosity § As substance flows under force bonds, chemical bonds are broken and remade to form resistance flow o Thixotropy § Molecular rearrangement from agitation/mixing § Longer mixing at shear rare = lower shear stress, lower viscosity o Viscoelasticity § Solids showing capacity to rearrange molecules under applied load, when load released the material may/may not return to original shape • Thermal Properties (influence hot/cold sensation; cause mechanical failure from expansion/contraction) o Thermal conductivity § Ease of heat transference through material o Specific Heat § Heat energy required to raise temp of unit volume by one degree centigrade o Thermal Diffusivity § Rate of rise of temperature at one point due to a heat source at another point o Thermal Expansion § Ideal restorative material will have coefficient of expansion identical to that of tooth tissue bc thermal mismatch could give rise to marginal gap formation and breakdown of adhesive bonds • Optical Properties (the light source, the object, and the observer) o Color § Eye most sensitive to green-yellow, least sensitive to red/blue • Chroma = strength of hue (how vivid color is) • Value = brightness/darkness of object, black and white for diffusive/reflective objects and black to clear for translucent objects o Translucency § Allows passage of light in such a way that little distortion takes place o Surface Texture § As surface gets rougher, light scatters and gives matte appearance § restored tooth can be spoiled by the restoration having a matte surface finish, making it stand out from the rest of the tooth Chemical Properties • • • • • • oral environment very hostile materials may dissolve in the water present in saliva, or release soluble components corrosion may tarnish due to acid presence Materials may discolor or break down due to absorption of substances from saliva These possibilities can adversely affect the chemical stability of the materials and limit their durability The products released may have an adverse effect on the biological environment, both locally and systemically Degradation of Polymers • Water sorption and soluble fraction o Solvent molecule forces the polymer chains apart, causing swelling. o As strength of bond decreases, the polymer becomes softer, the glass transition temperature is reduced and strength is lowered (e.g. Nylon and Soft denture liners) • Bond rupture o Scission: degradation of covalent bonds in polymers o from radiation, heat, chemical attack, GI cements Degradation of Ceramics • Resistant to electrochemical corrosion • Susceptible to chemical corrosion Tarnish and Corrosion of Metals • Tarnish o Surface discoloration from formation of hard/soft deposits (e.g. sulfides, chlorides) o Tarnish DOES NOT cause deterioration of material itself o Ugly but easily removed from surface by polishing the metal • Corrosion o Chemical rxn between material and environment (more serious than tarnish) o Decrease in free energy as metal reacts with liquid or gas § In metals, corrosion process is electrochemical via oxidation reduction o Types of corrosion: § Dry Corrosion: All metals will form surface oxide coating when in contact w oxygen in air (NOT gold and other noble metals) § Wet Corrosion: Takes place in neutral, acid or alkaline environments. Metal ions and electrons released into water § Galvanic Corrosion: two dissimilar metals combined resulting in corrosion of one of the metals being increased § Crevice Corrosion — metal ions are released into space forming corrosion products — electrons are unable to react because of the lack of oxygen — electrons ravel thru metal to get to oxygen for oxidative reduction, pick up crevice anode and cathode — material lost from crevice based, metal weakness — damage done out of proportion to amount of material destroyed in corrosion process — localized worse than uniform