RHEOLOGICAL PROPERTIES OF DENTAL MATERIALS

SMIJAL

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 .

VISCOSITY

VISCOSITYViscosity 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.

VISCOSITY NOMENCLATURE

A liquid occupies space between 2 metal plates.

Lower plate fixed ; upper plate being moved at a velocity ‘V’

Force ‘F’ required to overcome frictional resistance to fluid flow.

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.

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

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…

CLASSIFIACATION OF FLUIDS BASED ON RHEOLOGY1.NEWTONIA

N

2.PSUDOPLASTIC

3.DILATANT

4.PLASTIC

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

NEWTONIAN FLUIDS

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.

PSEUDOPLASTIC FLUIDS

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

DILATANT FLUIDS

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..

PLASTIC FLUIDS

FACTORS AFFECTING VISCOSITYa. Temperature b. Pressure

a.Temperature:-inversely proportional . As temperature increases ,viscosity decreases.

Eg :-gheeB. Pressure :-

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

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.

Prophylaxis paste don’t flow out of a rubber cup until it is rotated against the teeth to be cleaned.

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.

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.

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.

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.

How to measure flow and creep???

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.

FLOW