Alginate Flashcards
What is alginate?
irreversible hydrocolloid
colloid?
substance which is distributed evenly through another material
Two phases of alginate?
dispersed
continuous
What can alginate exist as?
either viscous liquid (sol) or as a solid (gel)
changes from solid to gel in the case of alginate
2 components of alginate?
powder and liquid
mixed into paste
What phase is alginate when it is a paste?
sol phase
What phase is alginate when it sets?
gel phase
composition of alginate?
sodium and potassium salts + alginic acid
What constituent of alginate dissolves in water to form a hydrogel with calcium?
potassium alginate or sodium alginate
Role of potassium alginate or sodium alginate?
What constituent of alginate dissolves in water to form a hydrogel with calcium
What slows down the setting reaction of alginate?
sodium phosphate
acts as a retarder by preferentially reacting with calcium ions
Role of calcium sulphate dihydrate?
reacts with soluble alginate to form insoluble calcium alginate
setting reaction of alginate?
What is the general term for the alginate setting reaction?
double decomposition reaction
What acts as a retarder by preferentially reacting with calcium ions?
sodium phosphate
When is the material set?
once the phosphate ions have been used up
pH change of the material upon setting?
11 -> 7
mixig time of alginate?
45-60 secs
working time of alginate?
45s (fast set)
75s (regular set)
setting time of alginate?
1-4.5 mins
can be variable to altering water temp
What occurs hewn you remove alginate form the mouth?
impression material compressed when removed
distortion results which could be significant
remove quickly from mouth - snap out
How to minimise distortion of impression?
reducing the amount of compression
reducing the rime the impression is under compression
allowing time for recovery before poring - decompress
Alginate and syneresis?
gel contracts on standing and exudes liquid
lose moisture form impression - distortion
- need to control relative humidity and proper storage important
How to minimise syneresis?
lose moisture form impression - distortion
- need to control relative humidity and proper storage important
imbition of alginate?
takes up water and swells
uncontrolled in extent and direction
How to overcome imbition and syneresis?
wrap in damp tissue and store in sealed polythene bag until the impression can be cast
what effects the strength f alginate
variation in water volume - reduction in powder/liquid ratio will weaken gel
insufficient spatulation - ingredients not dissolved sufficiently , inconsistent chemical reaction
over-spatulation - disruption of forming calcium alginate gel and reduce strength
tear strength of alginate?
tear strength poor
especially when removing from mouth
advantages of alginate?
easy flow
reproducible of detail adequate
fast set
minimal tissue displacement
cheap
pt tolerance good
disadvantages of alginate?
poor dimensional stability
poor tear strength
distorts if unsupported
3mm minimum thickness requires - otherwise distortion occurs were thin sections are found, such as interproximal
easy to include air
minimum thickness of tray?
3mm thickness needed for alginate impression
indications for alginate?
used in situations where less accuracy/ detail is required:
study casts to pan case/design prosthesis
denture construction (primary and working impressions)
teeth opposing an indirect cast restoration
removable orthodontics appliances
other dental devised such as occlusal splints
displace quickly or slowly form mouth?
quickly
clinical tips for usage?
smooth impression surface
smear some material into fissures
pace some material into high palatal vault
evert lip/retract cheeks
careful not to pull tray towards dominant side
hold still in the mouth
ensure material fully set
displace quickly from mouth
reasons for a grainy material?
inadequate or prolnged mixing
liquid/powder tatio too low
reasons for tearing?
inadequate bulk
premature removal from mouth
Reason for external bubbles?
air incorporated during mix
reason for rough or chalky model?
inadequate cleaning of impression
excess water left in impression
premature removal of model
reason for distortion?
delayed pouring of impression
movement of tray during impression taking
premature removal from mouth of model
product name at IOD?
tropicalgin
Steps for impression decontaminatio?
Can you leave gypsum in contact with alginate for hours?
affect surface quality of cast
- remove when fully set
