Rheological properties of dental materials

rheological properties of dental materials and their measurement

1. Rheological
properties
By
EnasElshenawy
Dental biomaterials department
Tanta university

2. What’sRheology???????????
Viscosity
Classification of fluids based on Rheology
Thixotropy
Viscoelasticity
Stressrelaxation&creepcompliance
Viscosityandelasticitymeasurements
Rheometry
Measurement of Rheological properties
Applications in dentistry
Outline

3. What’s Rheology????

4. Rheo-
flow
-ology
study
Rheology

5. •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 .

6. Flow characteristics

7. Viscosity

8. Viscosity
•Viscosity (η) is the resistance of a fluid to flow
and is equal to the shear stress divided by the
shear strain rate, or:
η= t / [de/dt]
•Itismeasuredinpoise(p)orcentipoise(cp).

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12. •Rearranging the equation for viscosity, we see that fluid behavior
can be described in terms of stress and strain, just like elastic solids.
t = η[de/dt]

13. Classification
of fluids based
on rheology

14. 1.Newtonian
•TheviscosityofaNewtonian
fluidisconstantand
independentofshearrate.
•e.g.somedentalcements

15. 2.Pseudoplastic
•Theviscosityofa
pseudoplastic fluid
decreaseswithincreasing
shearrate.
•e.g.monophaseelastomeric
impressionmaterial

17. 3. Dilatant:
•Theviscosityofadilatantfluid
increaseswithincreasing
shearrate.
E.g.fluidsindentistryincludethe
fluiddenturebaseresins.

18. Temperature
Factors
affecting
viscosity
Pressure

19. Thixotropy

21. Examples
Prophylactic paste Impression material

23. Viscoelasticity

24. •Awiderangeofmaterialsshowbehaviourthatisintermediate
betweenthatofaviscousliquidandthatofanelasticsolid.
•Viscoelasticmaterialsarestrainratesensitivematerial.

25. For elastic solid:
Forthespring,the
applicationofaloadresults
inanimmediatestrainthatis
maintainedforaslongasthe
loadisapplied.
Oncetheloadisremoved,
the spring returns
instantaneouslytoitsoriginal
state.

26. For viscous fluid
Ontheapplicationofaloadto
thedashpot,thereisagradual
increaseinthestrain,which
continuestoincreaseforaslong
astheloadisapplied.
Onremovaloftheload,the
strainisnotrelieved,andthe
dashpotremainsinitsnew
position.

27. For anelasticmaterial
(delayed elasticity)
Whenstressisappliedbelowthe
proportionallimitat(t0time)non-
linearincreaseofstrainwithtime.
Onloadremovalthestrainwill
nonlineardecreasetozerowith
time.
Gradualbut complete
recovery.

28. For viscoelastic materials
UponloadapplicationImmediatestrain
duetoelasticportionfollowedbygradual
non-linearincreaseinstraindueto
viscousandanelasticparts.
Uponremovalofload:theelasticstrainis
immediatelyrecovered.
Theanelasticstrainisgraduallyrecovered.
Theviscousstrainisnotrecovered,which
resultinsomepermanentdeformation.

29. •Themechanicalpropertiesofmanydental
materials,suchasalginate,elastomericimpression
materials,waxes,polymers,dependonhowfast
theyareloaded
•Forthesematerials,increasingtheloading(strain)
rateproducesadifferentstress-straincurvewith
higherratesgivinghighervaluesfortheelastic
modulus,proportionallimit,andultimatestrength

30. Two properties of importance to
viscoelastic materials
Stress relaxation
Creep

31. Stress
relaxation
& creep
compliance

32. Stress relaxation
evaluation of orthodontic elastic bands

33. Creep

34. Creep recovery curve, showing, A,elastic, B,
anelastic, and, C,viscous strain.

35. Creep compliance:
•Creep compliance( J
t ): is
defined as strain divided by stress
at a given time.
•Once a creep curve is obtained, a
corresponding creep compliance
curve can be calculated.

36. •if a single creep compliance curve is calculated from a family of
creep curves determined at different loads, the material is said to
be linearly viscoelastic.
•In this case, the viscoelastic qualities can be described concisely by
a single curve.

37. Creep compliance curves for elastomeric impression materials
Condensation silicon
polysulfide
polyether
Addition silicon

38. Viscosity and
elasticity
measurements

40. •Oscillation Testing

41. Complex modulus -G*

42. Phase Angle
•Different types of materials have a different lag / phase angle between the
deformation and response.

44. Storage & Loss Modulus
•Rheology language tends to use a combined form of complex modulus and
phase angle
› Storage (elastic) modulus G’
› Loss (viscous) modulus G’’
•If…
› G’ > G’’, phase angle less than 45°-SOLID LIKE
› G’’ > G’, phase angle greater than 45°-LIQUID LIKE

47. Rheometry

49. Measurement
of rheological
properties

51. Viscometers

52. 1. Ostwald viscometer
GravimetricCapillaryPrinciple
based on the relation between viscosity and
time.

53. 2.Falling Sphere Viscometer:

54. 3.Rotational Viscometer:
•Forhighlyviscousfluidsameasurementbasedon
gravitywouldtakefartoolong.Therefore,
rotationalviscometersuseamotordrive.
•Unlikecapillaryviscometers,rotational
viscometersprovidedynamicorshearviscosity
results.

55. Therearetwomainprinciplesinuse:

57. Rheometers

58. 1. Melt Flow Testing:
•ameasureoftheflow
ofapolymerwhen
melted.
•Theresultofamelt
flowtest,calledthe
meltmass-flowrate
(MFR)

59. 2. Rotational Rheometers
Controlled strain Controlled stress
•two types

61. Various sample geometries are available from the rheometer
manufacturers
Concentric cylinders cone& plate parallel plates

63. 3. Capillary Rheometers:
•mostlyusedtoexamine
processing behavior,
ratherthanjustdetermining
therheologicalparameters

64. Applications
in dentistry

65. Working and setting times
•Cementsanddirectfilling
composites
•measuredwiththeincrease
inviscositymonitored
duringsetting.

66. Gel point in polymers
•difference between the state before and after crosslinking
is obvious.

67. Gel point in polymers
Notthe true gel point