BDS 2nd year notes nd books🦷

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Working time of cements * ZOE- 4 to 10 min *ZnPO4-5.5 min *GIC-7 min *resin-2 to 4 min

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Astringents These are the substance that precipitate proteins,but do not penetrate cells, thus affecting the superficial layer only. They r intended to toughen the surface making it mechanically stronger and decrease exudations. Tannic acid .... strengthening the gums Heavy metal salts...zinc chloride, zn sulfate, aluminium chl., Ferric chl., Strontium chl...mouth washes Alcohol is a potent astringent but not used in mouth as it causes burning sensation

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The viscosity of many dental materials decreases with increasing strain rate until it reaches a nearly constant value. Less viscous and more fluid. This is pseudoplastic viscosity. Liquids that show the opposite behaviour are dilatant and become more rigid as the rate of deformation increase.

Thixotropic; The viscosity of most of fluids decreases rapidly with increasing temperature. Viscosity may also depends on previous deformation of the liquid. Such fluids become less viscous and more flowable upon repeated application of pressure and are termed Thixotropic. Dental prophylaxis pastes, pop, resin cements, and some impression material. Importance; material does not flow out of mandibular impression tray until it's placed over dental tissue. Viscosity as a function of time can also be used to measure the working time of a material that undergoes a liquid-to-solid transformation.

Brinell hardness no. Or BHN; A hardened steel ball is pressed into the polished surface of a material under a specific load. BHN= load/ Area of indentation Rockwell hardness no.( RHN); a conical diamond point for measuring depth n also usefull for brittle n plastic materials Vickers hardness test(vht); a diamond in the shape of square pyramid is used. Used for dental casting gold, it is suitable for brittle material n for tooth structure. Knoop hardness no. (KHN); a diamond indentation tool is used. 1)The value for both exceedingly hard and soft materials can be obtained from this test. 2)the knoop and Vickers test are classified as micro hardness tests. 3) the Brinell and Rockwell- macro hardness test

Dentifrices The tooth cleaning substances such as toothpaste, gels and powders. Functions; 1) their abrasive and detergent action provide more efficient removal of debris, plaque n stained pellicle compared with use of toothbrush 2)they polish teeth to provide inc. light reflectance and superior esthetic appearance 3)they act as vehicle for the delivery of therapeutic agents with known benefits example fluorides, tartar control agents

Surface roughness is defined as relatively finely spaced surface imperfections whose height, width, finely spaced surface imperfections whose height, width, and direction establish the predominant surface pattern. Surface irregularities are isolated imperfections, such as nodules, that are not characteristics of the entire Surface area.

Defects in casting can be classified as; 1) distortion 2) surface roughness and irregularities 3)porosity 4) incomplete or missing details

Filler metal for soldering (Brazing) Requirements; 1)low flow temp. 2) ability to wet the substrate metal 3) sufficient fluidity at the flow temp Others; color, adequate hardness, strength, tarnish and corrosion resistance

Ductility and malleability Ductility represent the ability of a material to sustain a large permanent deformation under a tensile load up to the point of fracture.( Drawn into thin wire) Au> Ag>Pt Ductility is the relative ability of a material to be stretched plastically at room temperature without fracturing. Eg plastic strain indicated is a relative measure of the ductility of the solid material. Test used for ductility 1) the percent elongation after fracture 2) the reduction in area of tensile test specimen 3) the maximum no. Of bends performed in a cold bend test. Malleability is the ability of a material to sustain considerable permanent deformation without rupture under compression, as in hammering or rolling into a sheet. Au>Ag>Cu