1
Q
why is the gypsum water ratio important?
A
depending on the type of gypsum, if too much water is added, compressive strength can be extensively diminished, especially for type IV and V
should have 0.5 W/P for plaster, 0.3 for stone, and 0.22 for die stone
2
Q
define viscoelasticity
A
the ability of a material to strain instanaeously like an elastic solid during rapid stretching or to resist shear flow and to strain lineraly over time when a stress is appllied slowly
3
Q
how to reduce compression set?
A
- use recommended mixing time
- maximize tray-tooth distance
- material miust be completely set before removal
- snap removal
- provide recovery time (8-30 min) before pouring
4
Q
what is compression set?
A
permanent deformation of a material when a force that was applied to it was removed
5
Q
why is working time important?
A
- if you go past the working time, the material will be partially set –> therefore it will not compress under the object, the elastic recovery will be too much and the negative will be smaller than the actual object
6
Q
what are the desirable properties of an impression material?
A
- have a pleasant taste, odor, and esthetic colour
- not contain any toxic or irritating ingredients
- be economical
- have adequate shelf life for storage and distribution
- be easy to use with minimal equipment
- dimensionalstability
- have adequate strength so that it will not break or tear while removing from the mouth
- possess elastic properties and lesser plastic deformation
- be compatible with die and cast materials
- must be hydrophilic
7
Q
study page 6
A
ye
8
Q
how are impression materials categorized on viscosity?
A
- mucostatic –> impression plaster, agar, light body elastomer
- mucocompressive - impression compound
- pseudoplastic – addition silicones
9
Q
what are the advantages of impression compound
A
- non toxic and non irritant
- hardens in a good time
- compatible with gypsum
- no separating medium required
10
Q
what are the disdv of impression compound?
A
- cannot record fine detail
- distorts over undercuts
- shrinkage upon cooling
- dimensional change on storage
11
Q
what are the properties of kemco impression compound?
A
- glass transition temp at 39 degrees centigrade
- fusin temp = 43/5 degrees centigrade
- poor conductors of heat
- coeff of linear expansion is HIGH
- good dimensional stability
- good dimension stability????
- fine repro of detail??
12
Q
what are the advantages of kemco impression material?
A
- can be reused several times
- innaccurate portions cna be remade without having to make entire impression
- acuracy can be improved by flaming surface material
13
Q
what ar ethe disadvantages of kemco impression compound?
A
- distortion due to poor dimensional stability
- compress soft tissue while making impression
- difficult to remove if there are undercuts
- difficult to record details due to high viscosity
14
Q
when would one use ZOE?
A
- cementing and as an insulating medium
- temp filling
- root canal filling material
- bite reg paste
- impression material for edentulous pts
- temp relining material for dentures
15
Q
what factors control setting time of ZOE?
A
- particle size of the ZOE powder
- longer mixing time = decrease setting time
- high atmospheric temp and himidity accelerates setting
- setting can be delayed via a cooling mixing slab, spatula, or adding small amount of retarder or oils or waxes
16
Q
what are the properties of ZOE?
A
- good consistency and flow
- good detail repro
- rigid and having good strength
- good dimensional stability
17
Q
what are the advantages of ZOE?
A
- enough working time to complete border molding
- can be checked in mouth repeatedly without deforming
- registers accurate surface details
- dimensionally stable
- does not stick to cast material–> no separating material
- minor defects can be corrected
18
Q
what are the disadv of ZOE?
A
- requires special tray for impression making
- sticky in nature and adheres to tissues
- burning sensation of eugenol –> tissue irritation
- very inelastic
19
Q
what are non-eugonal pastes for?
A
- alternatives to ZOE for pts who are sensitive to the burning and tissue irritation cause by ZOE
20
Q
what are the uses of agar hydrocolloid?
A
- as a tissue conditioner
- full mouth impression without deep undercuts
- cast dupes
21
Q
what are syneresis and imbibition of agar hydrocolloids and why is it significant?
A
- syn = loss of water
- imbib = gain of water when immerse in water
important because can cause dimensional instabilities; to avoid, impressions should be poured immediately
22
Q
what is the use of alginate?
A
- orthodontic and study models
- prelim impressions
- cast: to fabricate mouth cards
- bleaching and fluroide trays
23
Q
what are the properites of alginate?
A
- alginate has pleasant taste and smell
- highly flexible
- good elasticity and elastic recoveru
- fine repro of detail
- compressive strength
- tear strength
- dimensional unstable
- poorly adheres to tray
- can be toxic bc of silica powder being inhaled
24
Q
what composes elastomeric impression materials?
A
- flexible matrix - (continuous phase)
- polymer
- x-linking agent
- curing agent
- modifiers
- accelerators
- retarders
- plasticizing
- flavoruing agents
- colourants
- fillers (dispersed phases)
25
polysulfides and condesnation silicones have the LARGEST DIMENSIONAL CHANGES during setting
ye
26
what are the types of non-aq elastomers?
- polysulphide rubber
- polyether
- silicone rubber
27
what are the adv of polysulphide rubber
- most hydrophilic of non-aq impression materials
| - best tear strength and elasticity
28
what are the disadv of of polysulphide rubber
- brown (non esthetic)
- stains clothes
- produces a strong odor
- non-well acepted by most pts
- long tooth prep
29
what are the advantages of polyether rubber?
- stable for long periods of time if stored dry
| - excellent impression accuracy and dimensional stability
30
what are the properties of polyether rubber?
- filled with silica for stability
- rxn affected by heat (is exothermic)
- stiff-- may break stone or teeth when separated from from impression
- allergenic hypersensitivity
- negatively affected by saliva, water, and or blood
- important to wait 20-30 minutes after stress relaxation occurs
31
polyether has low tear resistance
ye
32
what is silicone rubber made of?
polydimethyl siloxane in x-linked by alkyl orthosilicate molecules; silica is the PRIMARY filler, and you have ethanol as a by product
rxn: siloxane + ethyl silicate --> silicone rubber +ethyl alcohol
33
which impression materials require tray adhesives?
- condensation silicone rubber
34
what are the disadv of silicone rubber?
- polymerization shrinkage
- limited shelf life (silica fillers tend to settle out of material over time)
- poor wetting characteristics
- so syneresis or imbibition
35
what are the adv of silicone condensation reubber?
- no odor
- sets faster
- does not stain clothing
36
how is silicone rubber/VPS made?
polymerized by free rads from chloroplatinic acid --> catalyst is consumed to generate free rads, and H2 is created as a by-product
filled with silica
37
what is the polymer, x-linking agent, modifers, and fillwers for addition-cured silicone rubber?
polymer: double-bond-functional silicone polymer
x-linking agent = chlrotoplatinic acid
modifiers = colorants, flavorants, plasticizers
fillers = silica
- - during the first 3-4 hours small gas bubbles may edscape from the surface
- can collect at the bondaries and create bubbles
38
for ADDITION-CURED SILICONE, impressions do not need to be poured immediately
ye
39
study page 29
ye
40
What does compression set depend on?
Strain and time;
Strain depends on shape and tilt of tooth; thickness of mayerial between tooth and tray (thicker = reduced strain)
DM
flashcards
Decks in class (18)
# Cards
-
Dentin Bonding Agents II16
-
Dental Bonding Agents III22
-
Colour12
-
Dental Implants13
-
Impression Materials40
-
Waxes10
-
MEtals and Alloys49
-
MEtals 226
-
properties of dental materials11
-
biocomp22
-
Ceramics 115
-
Ceramics 228
-
Ceramics 319
-
Polymers for prosthetic applications20
-
Endo materials21
-
tissue engineering28
-
Wrought CS and SS33
-
Preventative materials4
