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# Rheological properties of dental materials

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### Rheological properties of dental materials

1. 1. Dr Mumtaz ul Islam
2. 2.     Study of flow or deformation of materials Applied for both solids and liquids In case of solids and elastomers it can be explained by viscosity and viscoelasiticity models Rheometers are used for studying changes in viscosity
3. 3.     Resistance to flow Resistance to strain Low viscosity material needs less pressure to flow High viscosity ….
4. 4.      Shear stress = K (Shear rate)n K and n are constants and n is the flow index n = 1 shear stress is directly proportional to shear rate Viscosity is constant and independent of shear rate Newtonian fluid
5. 5.      Less shear stress produces more shear rate Flow index is less than 1 Viscosity is decreased due to shear rate Shear thinnig Pseuodoplastic
6. 6.      More shear stress produces less flow Flow index is more than 1 Increase in shear stress produces more increase in shear rate Increase in viscosity Shear thickening
7. 7.    Material shows no flow initially After yield stress, flows like newtonian or any other type
8. 8.  Manipulation becomes impossible when viscosity has increased beyond a certain point, time taken to reach that point
9. 9.  Time taken for the material to reach its final set state
10. 10.  Time required to produce a particular value of yield stress within the setting material
11. 11.     Thermal changes Expansion and contraction Dimensional changes Tooth restoration interphase
12. 12.      Conduction Convection and Radiation Rate of heat flow per unit temperature gradient High values means good conductors Amalgam should be used with an insulator base Thermal conductivity is an equilibrium property
13. 13.   where K is the thermal conductivity, Cp is the heat capacity and r the density when transient thermal stimuli are applied a certain amount of heat will be absorbed in raising the temperature of the material itself. This will effectively reduce the quantity of heat available to be transported through the material
14. 14.    Thermocouple If the temperature recorded by the thermocouple rapidly reaches that of the liquid, this indicates a high value of diffusivity A denture base material, ideally, should have a high value of thermal diffusivity in order that the patient retains a satisfactory response to hot and cold stimuli in the mouth
15. 15.   The fractional increase in length of a body for each degree centigrade increase in temperature The values of α are often very small numbers (typically 0.000025ºC−1 for amalgam) they are often quoted as parts per million (ppm)
16. 16.    Interaction between two materials at an interface where they are in contact to prevent their separation Materials which are capable of bonding two surfaces together are called adhesives Material (surfaces) to which the adhesive is applied is termed the adherend
17. 17.    Adhesive simply engages in undercuts in the adherend surface If attachement involves only few micrometer area it is micromechanical attachement (adhesion) Undercut cavities if made for adhesion then it is macromechanical attachment
18. 18.    Adhesive has a chemical affinity for the adherend surface If the attraction is caused by Van der Waals forces or hydrogen bonds, the resultant bond may be relatively weak Ionic or covalent links may result in a stronger bond
19. 19.    Self explanatory Whichever mechanism of bonding is utilized the adhesive must be capable of wetting the adherend surface In mechanical or chemical adhesion flow of adhesive on all undercuts and surface area of adherend is desired
20. 20.    The ability of an adhesive to wet an adherend surface is evaluated by measuring the contact angle which is formed when a drop of adhesive is applied to the adherend Low contact angle good wetting High contact angle usually results in globule formation
21. 21.   The surface tension of the adhesive is the property which maintains it in the form of a droplet and acts to prevent wetting There must be sufficient energy liberation through the forces of attraction between the adhesive and adherend in order to break down the surface tension of the adhesive
22. 22.    Majority of resins used in dental fillings are relatively hydrophobic whilst dentine and enamel are relatively moist Adhesion is difficult to achieve in this situation Use of primers solved this problem they alters the nature of adherend surface
23. 23.    Surface area determines the adhesion More surface area covered and in contact by an adhesive more will be the adhesion More viscous an adherent wetting will be less
24. 24.    Dimensional changes during and after setting Density Appearance
25. 25.      The success of restorative procedures depends on dimensional changes which occur during and after impression recording Casting of alloys Setting of direct restorative materials Shrinkage and expansion of material An expansion at one stage can be used to partly counteract a contraction which occurs at another stage
26. 26.     Mass per unit volume Alters the design and technique Bulky design in heavy metal for upper denture always fails Even bulky design in acrylic impede stabilization of upper denture and difficulty in speech in case of lower denture
27. 27.  Colour of an object or material is not an inherent property of that material but results from a number of factors including the composition of the material and its thickness and surface roughness as well as the nature of illuminating light
28. 28.     Hue colour itself like blue red green Chroma intensity or magnitude of colour Value brightness or darkness The hue and chroma are inherent properties of materials whereas the brightness may be affected by factors such as surface finish
29. 29.   Through which some of light passes Through which none of light can pass
30. 30.     Chemical stability Chemical composition of material remains unchanged Solubility how much a material is soluble Erosion dissolution of material with a mild mechanical action
31. 31.    When assessing the solubility or erosion rate of materials it is important to consider the vast range of conditions which may exist in the mouth The pH of oral fluids may vary from pH 4 to pH 8.5 representing a range from mildly acidic to mildly alkaline Highly acidic soft drinks and the use of chalkcontaining toothpastes extend this range from a lower end of pH 2 up to pH 12
32. 32.   It is possible for a material to be stable at near neutral pH values but to erode rapidly at extremes of either acidity or alkalinity This partially explains why certain materials perform adequately with some patients but not with others
33. 33.  Often involve the storage of disc specimens of materials in water for a period of time the result being quoted as the percentage weight loss of the disc
34. 34.    Many materials, when placed in an aqueous environment absorb water by a diffusion process. Constituents of the material may be lost into the oral fluids by a diffusion process commonly referred to as leaching This may have serious consequences if it results in a change of material properties or if the leached material is toxic or irritant
35. 35.    Occasionally leaching is used to the benefit of the patient. For example, in some cements containing calcium hydroxide slow leaching causes an alkaline environment in the base of deep cavities. This has the dual benefit of being antibacterial and of encouraging secondary dentine formation