Hydrocolloid Impression materials Flashcards
what kind of surface detail do high viscosity materials give?
less detail but can displace the tissues
useful for denture extensions but not flabby ridges
what kind of surface detail to low viscosity materials give?
more detail but tray wont contain
can viscosities change?
viscosity can change with time
what repels the impression material?
moist surfaces
hydrophobicity
what do flexible trays do?
distorts the material
must have effective bonding strength
what does an impression material need to be?
no changes when exposed to disinfectants no interaction with model/die materials non toxic easily manipulated palatable taste realistic working/setting time
types of impression materials?
non elastic - impression plaster/impression compound/zinc oxide and eugenol pastes
elastic - hydrocolloids/ synthetic elastomers
types of synthetic elastomers?
synthetic elastomers - polysulphides/silicone rubbers/polyethers
hydrocolloid - agar - reversible, alginate - irriversible
what do both types of hydrocolloids have in common?
both are colloidal suspensions of polysaccharides in water
what do both types of hydrocolloids exist in?
soluble - fluid/low viscosity/random chain arrangement
gel - increased viscosity/parallel chain arrangement, elastic properties
how to turn sol to gel?
gel to sol?
temperature/chemical
temperature
what is Agar?
supplied as a gel with?
complex polysaccharide made from seaweed
borax - to give more body
potassium sulphate - counteract retardant effect of borax on dental plaster
agar requires what?
what temperatures?
requires water bath conditioning 100degrees = gel to sol 65degrees = holding temp 45degrees = clinical usage temperature clinical use requires water cool trayed
advantages of agar?
take up moisture
re usable
disadvantages of agar?
special equipment
cost
cross infection concerns re use
low tear strength
why are agar materials dimensionally unstable?
water content
syneresis - loss of water from squeezed chains
imbibition - uptake of water - swelling due to chain seperation
clinical and laboratory uses of agar?
clinical - p/dentures, crowns, bridges
lab - duplicating material
how are alginates supplied?
powder to be mixed with water
composition of alginate?
- sodium of potassium salt of alginic acid - 11-16%, forms sol with water and in set becomes a cross linked gel
- calcium sulphate dihydride - 11-17%, calcium for cross linking
- sodium phosphate - 1-3% - controls working time
- inert filler 65-75% - body for manipulation
- reaction indicator in some to signify set
how does sodium phosphate control the working time?
sodium phosphate limits availability of calcium ions.
properties of alginate?
low viscosity - mucostatic
adequate working time - operator control with water temp
rapid set - at soft tissue first - dont disturb tray
elastic
must use no less than 3-5mm to prevent distortion on removal
susceptible to syneresis and imbibition - pour soon
what are modified alginates?
incorporation of silicone polymers
2 pastes
better tear resistance and better detail production
poor dimensional stability
what is impression disinfection?
immersion in freshly made solution
time immersion
rinse after
how much achtichlor to use?
10 tabs in 1000ml
alternatives to impressions?
CAD CAM
intraoral scanner
