rheological properties of dental materials
TRANSCRIPT
Rheological properties Dr Mumtaz ul Islam
Rheology
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
Viscosity
Viscosity
Resistance to flow Resistance to strain Low viscosity material needs less pressure to
flow High viscosity ….
Viscosity
Flow index
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
Pseudo plastic
Less shear stress produces more shear rate Flow index is less than 1 Viscosity is decreased due to shear rate Shear thinnig Pseuodoplastic
Dilatent
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
Bingham
Material shows no flow initially After yield stress, flows like newtonian or any
other type
Working time
Manipulation becomes impossible when viscosity has increased beyond a certain point, time taken to reach that point
Viscosity and time
Setting time
Time taken for the material to reach its final set state
Determination of setting
What is setting
Time required to produce a particular value of yield stress within the setting material
Thermal properties
Thermal changes Expansion and contraction Dimensional changes Tooth restoration interphase
Thermal conductivity
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
Thermal diffusivity
Thermal diffusivity
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
Measurement
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
Coefficient of thermal expansion
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)
Adhesion
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
Mechanical adhesion
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
Chemical adhesion
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
Wetting
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
Contact angel
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
Role of surface tension
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
Primers in dentistry
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
Role of Viscosity
Role of surface area
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
Surface area and viscosity
Miscellaneous physical properties
Dimensional changes during and after setting Density Appearance
Dimensional changes
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
Density
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
Appearance
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
Hue chroma and value
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
Translucent and opaque
Through which some of light passes Through which none of light can pass
Chemical properties
Chemical stability Chemical composition of material remains
unchanged Solubility how much a material is soluble Erosion dissolution of material with a mild
mechanical action
Role of pH
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 chalk-containing toothpastes extend this range from a lower end of pH 2 up to pH 12
Difference in performance
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
Standard tests of solubility
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
Leaching of constituents
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
Benefits of leaching
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