1. Impression materials

2. Market Share by Volume %

3. tooth
undercuts

4. stone
stone

5. Impression materials - types
Elastic
Nonelastic

6. Impression materials - types
Nonelastic
plastic
waxes (reversible)
impression compound (plastic, reversible)

7. Impression materials - types
Nonelastic
rigid
plaster (irreversible)
zinc oxide - eugenol impression paste (irreversible)

8. Impression materials - types
Elastic
hydrocolloids (aqueous)
agar (reversible)
alginate (irreversible)
elastomers (nonaqueous)
polysulfide
polyether
silicone (condensation, addition)

9. Aqueous Impression Materials
Hydrocolloids
Aqueous Impression Materials

10. Elastic impression materials Hydrocolloids
Colloid - a two phase mixture in which one of the phases (the dispersed phase) is very small ( nm).
1000 nm = 1 μm

11. liquid - liquid
milk
emulsion
gas – liquid
soap foam
foam
liquid – gas
fog
liquid aerosol
solid – gas
smoke
sol aerosol

12. Elastic impression materials Hydrocolloids
solution - solute and solvent are not physically separate.
sol - dispersed phase and dispersion medium are physically separate.

13. Elastic impression materials Hydrocolloids
sol - a colloid in which small solid particles are suspended in a liquid.
typical particles - 1 nm to 500 nm
1000 nm = 1 μm

14. Elastic impression materials Hydrocolloids
hydrocolloid - a sol in which the dispersion medium is water

15. Reversible Hydrocolloid
Agar Hydrocolloid
Reversible Hydrocolloid

16. sulfuric ester of a polymer a gallactose molecular wt. ~ 150,000
Agar - chemistry
a polysaccharide
sulfuric ester of a polymer a gallactose
molecular wt. ~ 150,000
insoluble in water

17. Agar – a polysaccharide (sulfuric ester of a polymer of gallactose)2 OH
OH . HSO4 HO HO H 8 N
~ 90

18. Agar – structure of the gel
agar fibrils arranged in interpenetrating tangles – “brush heaps”
water is held in the interstices of the brush heap

19. Agar Hydrocolloid - composition
Water
Potassium*
Borax**
Fillers %
80 – 85 %
1 – 2 %
0.2 %
* hardens gypsum
** strengthens the gel

20. Agar Hydrocolloid – reversible reaction
Forms sol 71 – 100oC
reversible
Forms gel 30 – 50oC

21. Agar Hydrocolloid – Conditioning Baths
Temp o C
Time
Boiling
100
Storage
66 – 69
Tempering
< 15

22. Agar Hydrocolloids – 3 bath system
storage
boiling
tempering

23. Agar Hydrocolloid – tempering bath
Removing tubes of agar hydrocolloid from the tempering bath.

24. Agar hydrocolloid: water cooled trays
A water-cooled tray is used.
The initial investment in agar
(baths and water-cooled trays)
is relatively high.

25. Irreversible Hydrocolloid
Alginate
Irreversible Hydrocolloid

26. Alginate hydrocolloid:

27. Alginate Hydrocolloid - composition
brown seaweed – algae
linear polymer with numerous carboxyl groups
anhydro - β - d – manuronic acid
called alginic acid

28. Alginate Hydrocolloid - reaction
H2O + Na-alginate + CaSO4
(paste)
soluble in water
Ca-alginate + Na2SO4
(gel)
insoluble in water
Reaction is irreversible

29. Alginate Hydrocolloid - mixing
gelation time dependent on water-powder ratio – faster with less water
do not change water-powder ratio – may affect strength
warm water decreases gelation time – each 10oC will halve the gelation time

30. Alginate\ Hydrocolloid – storage in air
Alginate impression of metal die
simulating three-unit FPD
preparation. Initial.
4 hours later.

31. Storage at 100% humidity minimizes wt. loss

32. Alginate Hydrocolloid – dimensional stability
set alginate exudes fluid – syneresis
rinse impression – exudate could inhibit setting of the surface of dental stone
dimensions will change at a rate that depends on the humidity at which the impression is stored
best advice – pour stone immediately

33. Both Hydrocolloids – elastic recovery
neither agar nor alginate is perfectly elastic
permanent deformation:
alginate: 1.5 %
agar: 1.0%

34. Both Hydrocolloids – reducing elastic recovery
To reduce permanent deformation:
use the hydrocolloid in a thicker layer – smaller percentage deformation
remove the impression faster – permanent deformation is reduced at higher strain rates

35. Combined agar – alginate technique
alginate is tray material
agar is syringe material
Basic idea: take advantage of the ability of agar to do a better job of replicating fine detail.

36. Combined agar-alginate technique:
A single water heater is used to
boil and store the agar cartridges.
No need to temper. The agar will
cool as it is being ejected from the
syringe.
agar is the syringe material

37. Combined agar-alginate technique:
Injecting the agar wash around
the prepared tooth. The
impression is completed using
the alginate in non-water-cooled
tray.
Note that the agar wash defined
the margins of the impression.

38. Combined agar – alginate technique - method
boil agar – 5 minutes
5 minutes at 70oC
Syringe agar over prepared teeth (syringe cools the agar)
Place tray filled with alginate material over the agar

39. Combined agar – alginate technique - advantages
can use custom tray
rapid set (3 minutes)
reduced equipment costs: syringe & one heater
low cost per impression

40. Hydrocolloids - disadvantages
low tear strength
large dimensional changes during storage
poor transfer of detail to stone
electroplated dies not possible

41. End