irreversible hydrocolloids Flashcards
impression material definition
a substance or a combination of substances used to make an impression or negative reproduction of heard and soft oral tissues
purpose of impression
used in preparation of casts (positive replica) by filling the impression with dental stone or other model material
ideal requirements of an impression material
- Have pleasant taste, odor and esthetic color.
- Not contain any toxic or irritating ingredients.
- Be cheap.
- Have adequate shelf life.
- Be easy to use with minimum equipment.
- Exhibit dimensional stability.
- Have adequate strength (does not break or tear upon removal).
- Have elastic recovery properties with no permanent deformation.
- Have ability to be electroplated.
- Be compatible with die and cast materials.
- Have adequate wettability of oral tissues.
- Be readily disinfected without significant loss of accuracy.
colloids
Dissolution of solid (solute) in Liquids (solvent) could result in 1
of 3 states.
1. Solution: homogeneous mixture, consists of a single phase.
2. Suspension: mixture of two distinct phases.
3. Colloid: heterogeneous mixture, 2 non-readily differentiated
phases.
When the solvent of such colloid is water, the material is termed a
hydrocolloid.
hydrocolloids exist in two states
sol (viscous liquid)
gel (jelly-like)
sol can change into gel through:
physical (reversible) reaction
chemical (irreversible) reaction
2 types of hydrocolloid impression material
agar (reversible hydrocolloid)
alginate (irreversible hydrocolloid)
in order to make an impression using hydrocolloid materials:
The materials have to be introduced into patient’s mouth while
in the sol state (so that they have sufficient flow and register
details) and should then be left inside mouth to reach the gel
state (to achieve setting) before removal.
alginate
irreversible hydrocolloid
developed as a substitute for agar
algin - a mucous extract from some brown seaweeds
availability of alginate
in bulk or in tines or in sachets
in pre-weighed individual containers
a plastic scoop - dispensing the bulk powder
a plastic cylinder - measuring water
normal setting (type II)
1. mixing time
2. working time
3. setting time
- 1 minute
- 3 minutes
- 4 to 5 minutes
fast setting (type I)
1. mixing time
2. working time
3. setting time
- 45 seconds
- 1.25 to 2 minutes
- 1.5 to 3 minutes
over mixing results in
reduction in final strength
reduction in working time
under mixing results in
inadequate wetting, lack of homogeneity
poor recording of detail
setting time can be altered by water temperature
colder water - longer gelation time
warmer water - shorter gelation time
*control of setting time by changing w/p ratio is not recommended for alginate
composition
sodium or potassium triethanolamine alginate
calcium sulphate dehydrate
zinc oxide
potassium sulphate
potassium titanium fluoride
diatomaceous earth
sodium phosphate
colouring and flavouring agent
sodium or potassium triethanolamine alginate
dissolves in water and reacts with calcium ions 15%
potassium sulphate
accelerator - causes proper setting of gypsum that is poured against the alginate impression 3%
potassium titanium fluoride
gypsum hardener 3%
calcium sulphate dihydrate
reactor 16%
sodium phosphate
retarder 2%
zinc oxide
filler particles 4%
diatomaceous earth
filler particles 60%
colouring and flavouring agent
in chromatic alginate: changes color when setting is complete - traces
setting reaction
gelation
calcium sulphate + sodium phosphate =>
calcium phosphate + sodium sulfate
calcium sulfate + sodium alginate => insoluble calcium alginate
the speed of the reaction is affected by
l/p ratio
temperature
mixing rate
particle size of powder
additives
mixing and manipulation
Weigh powder- W/P ratio (16 g powder, 38 ml water)
Add powder to water
In rubber bowl or
In vacuum mixer
Mixed for 45 sec to 1 min
Place in tray
Remove after 2 to 3 minutes or after gelation (loss of
tackiness)
disinfection
Carried out by immersion in 1% Sodium Hypochlorite or 2%
Gluteraldehyde < 10 min immersion or preferably by spraying
with disinfectant.
the disinfectants in use
Sodium Hypochlorite/household bleach (1-10 dilution)
Iodophors
Synthetic phenols
dimensional stability
tends to be low
hydrocolloids have the ability to gain or lose water (imbibition, syneresis and dehydration => deformation
deformation will occur unless…..
there is immediate pouring of the impression in stone
storage of impression in 2% potassium sulfate or 100% humidity
100% humidity
achieved by wrapping the impression with a paper towel and storing both in sealed plastic bag
applications of alginate
elastic properties
In stock trays to make primary impressions.
Other applications:
Final impression for complete denture.
Impression for repairs to broken partial and complete dentures.
As wash material over primary compound impression.
Used for impression making when there are undercuts in mouth
with excessive flow of saliva.
For impressions to make study models, and working casts.
In fabrication of mouth guards, provisional restorations etc.
alginate hydrocolloid properties
- Alginate has pleasant taste and smell
- Good flexibility-ease of removal (jerking motion)
- Good elasticity and elastic recovery
- Fine reproduction of tissue detail
- Compressive strength – ranges from 5000-8000g/cm2
- Tear strength varies from 350-700 gm/cm2
- Dimensionally stable- but deformation likely
- Poorly adheres to the tray
- Biological properties - inhalation of silica particles harmful
(emergence of dust-free alginate).
dust free/low dust alginate
achieved by coating the material with glycerine or glycol. powder becomes denser than in uncoated state
chromatic alginate
contains acid/base indicator in their formulation or pH indicator. the alginate impression material with color indicators to avoid confusion with setting time
antiseptic alginate
contains a antiseptic such as glutaraldehyde and chlohexidine gluconate
advantages of alginate
low cost
minimum requirement of equipment
hygienic (fresh material used)
easy to mix and manipulate
comfort to patients
good surface detail even in saliva
accurate is properly handled
disadvantages of alginate
high viscosity
ability to displace tissues
poor tear strength
poor dimensional stability
distortion may occur if not handles properly
cannot be corrected
cannot to electroplated
grainy material
improper mixing, prolonged mixing, excessive gelation, w/p ratio too low
tearing
inadequate bulk, premature removal from the mouth, moisture contamination, prolonged mixing
irregularly shaped voids
moisture or debris on tissue
rough or chalky stone model
inadequate cleaning of impression, excess water left in impression, improper handling of stone, premature removal of impression, model left in impression for too long
external bubbles
undue gelation preventing flow, air incorporated during mixing
distortion
premature removal of impression
improper removal from mouth
movement of tray during gelation
impression not poured immediately
