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# Rheological properties of dental materials.ppt real

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### Rheological properties of dental materials.ppt real

1. 1. RHEOLOGICAL PROPERTIES OF DENTALMATERIALSSMIJAL
2. 2. What is rheology??? Rheology is the branch of physics that deals with deformation and flow of matter. (wikipedia) It is the study of deformation and flow characteristics of matter. (phillip’s) Dentists are subjected to manipulate materials which flow or deform when subjected to stress .The study of flow characteristics of materials is the basis for the science of rheology .
3. 3. VISCOSITY
4. 4. VISCOSITY Viscosity is the resistance of liquid to flow. This resistance to fluid flow is controlled by internal frictional forces within the liquid. Thus ,viscosity is a measure of the consistency of a fluid and its inability to flow. A highly viscous fluid flows slowly.
5. 5. VISCOSITY NOMENCLATUREA liquid occupiesspace between 2metal plates.Lower plate fixed ;upper plate beingmoved at a velocity ‘V’Force ‘F’ required toovercome frictionalresistance to fluidflow.
6. 6.  If the plates have an area (A) in cntact with the liquid ,a shear stress (S) can be defined as S=F/A The shear strain rate,is E=V/d Where ‘d’ is the shear distance of the top plate relative to the fixed lower plate. ‘V’ is the velocity f the top plate.
7. 7.  Viscosity is defined as the shear stress divided by strain rate. Viscosity=shear stress =S/E strain rate SI unit of viscosity- Cp(Centipoise) or Mpa per second. Pure water at 20 degree celsius has a viscosity of 1.00 c P
8. 8. Application Measure viscosity of dental materials . Materials manipulated in fluid state in oral cavity i.e, materials like cements and impression materials undergo a liquid to solid transformation in mouth. Gypsum products used in fabrication of dies are transformed from slurries into solid structures. Cements used as luting agents and bases…
9. 9. CLASSIFIACATION OF FLUIDS BASEDON RHEOLOGY1. NEWTONIAN2. PSUDOPLAST IC3. DILATANT4. PLASTIC
10. 10. NEWTONIAN FLUIDS Ideal fluid –shear stress proportional to strain rate. The plot is a straight line. Newtonian fluid has a constant viscosity regardless of the shear strain. About 10 % of liquids r newtonian Eg:-Water and newly mixed zinc phosphate cement
11. 11. NEWTONIAN FLUIDS
12. 12. PSUDOPLASTIC FLUIDS They are shear thinning fluids. Their viscosity decreases with increasing strain rate until it reaches a nearly constant value.Eg :-poly carboxylate cements, non water mixed glass ionomer cements elastomeric impression materials.
13. 13. PSEUDOPLASTIC FLUIDS
14. 14. DILATANT FLUIDS Shear thickening fluids Shows increase in viscosity with an increase in shear rate. Liquid becomes more rigid as rate of deformation increases. Eg:-Traditional resin composites
15. 15. DILATANT FLUIDS
16. 16. PLASTIC FLUIDS Plastic fluids are rigid until a certain yield stress is applied to them and they attain constant viscosity. The application of this yield stress to cause flow in a plastic material is called Bingham characteristic. Eg:- ketch up..
17. 17. PLASTIC FLUIDS
18. 18. FACTORS AFFECTING VISCOSITYa. Temperature b. Pressurea. Temperature:-inversely proportional . As temperature increases ,viscosity decreases.Eg :-gheeB. Pressure :-
19. 19. THIXOTROPICITY It’s a property exhibited by certain liquids that become less viscous and more fluid under repeated applications of pressure . If u shear them ,their viscosity decreases ;if it is left undisturbed ,then its viscosity will increase to its pre stressed level. Eg :-POP , resin cement, dental prophylaxis paste. Impression material starts to flow when placed in tissues and pressure applied .They don’t flow out of d impression tray until placed over dental tissues
20. 20. THIXOTROPICITY It’s a property exhibited by certain liquids that become less viscous and more fluid under repeated applications of pressure . If u shear them ,their viscosity decreases ;if it is left undisturbed ,then its viscosity will increase to its pre stressed level. Eg :-POP , resin cement, dental prophylaxis paste.
21. 21.  Impression material starts to flow when placed in tissues and pressure applied .They don’t flow out of d impression tray until placed over dental tissues. Prophylaxis paste don’t flow out of a rubber cup until it is rotated against the teeth to be cleaned.
22. 22. CREEP CREEP:- The time dependant plastic deformation of a material under static load or constant stress near its melting point. 2 types: static dynamic Static:- creep that takes place under constant load Dynamic creep:-under changing load.
23. 23. SAG SAG:- It is a form of creep that occurs in a metal at high temperature under its own weight. Clinical importance:-because of the low melting point of amalgam ,dental amalgam can slowly creep from a restored tooth site under periodic sustained stress , which can be destructive to the dental prosthesis. Creep test is requires in ANSI/ADA No:1 and Addendum 1a for dental amalgam products.
24. 24. CREEP Creep can also lead to an unacceptable fit of FPD frameworks when a cast alloy with poor creep resistance is veneered with porcelain at relatively high temperature.
25. 25. FLOW The term flow is used in dentistry to describe the rheology of amorphous materials such as waxes. The flow of wax is its potential to deform under a small static load ,even that associated with its own mass.
26. 26. How to measure flow andcreep??? A cylinder of prescribes dimension is subjected to a given compressive stress for a specified time and temperature. Creep or flow is measured as the percentage decrease in length that occur under these testing conditions.
27. 27. FLOW