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