Alginate Impression material is an Elastic, Irreversible
Impression material which is used to take the impression of both
Dentulous and Edentulous Impressions.
TYPES:Alginate Impression material الألجينلت Dental-impression

  • Type I fast setting : 1-2 min.
  • Type II normal setting : 2-4.5 min.
Available as :

  • Bulk powder
  • Preweighed packages
  • Color changing alginates:violet during mixing, pink when ready to be seated and white when set
Composition:

  • Ester salts of alginic acid(sodium or potassium or triethanolamine alginate) – 15%
  • Calcium sulphate (reactor) – 16%
  • Zinc oxide – 4%
  • Potassium titanium fluoride – 3%
  • Diatomaceous earth – 60%
  • Sodium phosphate – 2%
  • Coloring & flavoring agents traces
Setting reaction:

  • 2 Na3PO4 + 3 CaSO4 à Ca3(PO4)2 + 3Na2SO4
Setting Time:

  • Type I (fast set) : 1-2min
    Type II (normal) : 2-4.5 min.
Control Of gelation time:
Undercontrol by manufacturers:

  • By the amountt of retarder added during manufacturer
Under Control of clinicians

  • By altering the W:P ratio
  • By changing the mixing time.
  • By altering the temp.of water
  • Increase in temp. decreases gelation time & vice versa.
  • Premature gelation causes distortion in the impression & it is rendered useless.
  • Prolonged gelation time is tedious for both pt. & the dentist.
Manipulation :
The Powder is taken after inverting the can several time to produce
uniform distribution of the filler before mixing with water. The top of
the can should be taken off carefully to prevent the silica particles
from being inhaled. The proper W/P ratio as pacified by the manufacturer
should be used measured quantity of water and powder are added in the
rubber bowl & mixing is started, with a stirring motion to wet the
powder with water. Once its moistened rapid spatulation by swiping or
stropping against the side of the bowl is done. A vigorous figure eight
motion is used.

  • Mixing time:Working time
  • For fast set alginate45 sec.1 ¼ min.
  • For normal set alginate60 sec.2 min
Properties:

  • Taste & odor: ·Pleasant
  • Flexibility:·lower w/p ratio results in lower flexibility.
  • Elasticity & elastic recovery: highly elastic, permanent
    deformation less if the impression is removed from the mouth quickly.
  • Reproduction of the surface detail: ·lower than agar.
  • Strength: ·compressive strength : 5000-8000 gm/cm2
  • Tear strength : 350-700 gm/cm2
  • Dimensional stability: Poor
Biologic properties:

  • Silica present in the dust which rises from the can after fluffing alginate powder is a health hazard.
  • Dustless alginate is also present in the market which contain glycol.
Uses:

  • for making primary impression of edentulous patients with undercut ridges.
  • for preliminary impressions for complete dentures.
  • for making impressions for dentulous pts. For construction of study models & temporary removable prosthesis.
  • for making impression for orthodontic study models.
  • used for making impressions for the preparation of mouth protectors for athletes.
  • used as duplicating material.
Advantages:

  • ease of mixing & manipulation.
  • no elaborate equipment required.
  • material is elastic & comes out easily from undercuts.
  • economical
  • impression can be removed easily & has pleasant taste & odor, so comfortable for the patient.
Disadvantages:

  • can’t be electroplated.
  • poor tear strength especially in thin sections.
  • dimensional stability is poor.
  • distortion may occur if tray is shifted during setting.
  • cant be refined.
Types of failures:
1.Defects:

  • improper mixing
  • prolonged mixing
  • undue gelation
  • w/p ratio is too low.
2.Tearing:

  • inadequate bulk
  • moisture contamination
  • premature removal from mouth
  • prolonged mixing
3.External bubbles:

  • undue gelation preventing flow
  • air incorporated during mixing
4.Irregularly shaped voids:

  • moisture
5.Rough or chalky stone model:

  • inadequate cleaning of the impression
  • excess water left in impression
  • premature removal of model.
6.Distortion:

  • impression not poured immediately