Common properties / failures of elastomeric impression materials Flashcards
How to describe the viscosity of the material?
most viscous
- putty
- heavy bodied
- medium (or regular/universal) bodied
- light-bodied
- extra light bodied
least viscous
thixotropy?
how the material flows and stays where it is without dripping
- important from a handling perspective
thixotropy aka?
structural viscosity
how to measure viscosity?
45 seconds after start of mixing
why measure viscosity at 45 seconds?
approx the time at which the material is being injected into the mouth after mixing by hand but before substantial cross-linking has occurred
does viscosity increase of impression material in the mouth?
yes
effect of viscosity on impression?
syringing of the light-bodied material has to be quick
material must flow all over the preparations
failure of impression material to flow will cause irregularities in the impression and cast
standard impression technique?
putty of heavy bodied ad light-bodied wash in a stock tray
putty/heavy body provides support for the light-bodied material which on its own would distort under load during pouring of the model
light-bodied material is sufficient fluid to reproduce the fine details where required - it doesn’t have high level filler in it
different viscosity material bind together when set
advantages of heavy-bodied presentation over putty?
not quite so viscous as a putty –> no displacement of the teeth
no deformation of a non-rigid tray
no displacement of the light bodied material from around the preparation
no folding of the putty leaving a defect like a seam in the impression
universal/regular viscosity material in a stok tray?
universal viscosity material is injected around the preparation and also fills the tray
loaded tray is then seated over the syringed material
detail reproduction of the universal paste must be sufficient to provide an adequate representation of the preparation
monophase impression
use a non-perforated tray
usually used with the polyethers
what can affect the working time of a material?
ambient temp and humidity
what material has the longest working and setting time?
polysulphide
different consistencies of the same material exhibit diff setting times
dimensional change on shrinkage?
degree of shrinkage primarily determined by their setting reaction
what causes the dimensional change of setting of the material?
any by-product of the setting reaction
what causes the extreme shrinkage of c-silicones?
due to production of ethanol
what material shows the second greatest shrinkage?
polysulphide
what material show the least shrinkage?
polyether and addition silicones sow minimal
when are polyethers dimensionally unstable?
if stored in water
when decontaminating - leave in bath for correct period of time
dry thoroughly and send to lab to be cast
dimensional change in setting depends on what?
depends on filler loading
more filler means less rubber to shrink
putty has low shrinkage
can distortion be reversed after removing from the mouth?
no
rate the elastomers and their distortion?
least
addition silicones
polyether
c-silicones
polysulphides
most
how to describe the ‘snap out’ of elastomers?
viscoeleastic
remove as quickly as possible to material is strained for shortest time
what happens if you remove elastomer slowly from the mouth?
flow and strain from which it may not recover
define strain?
flexibility of the material
what is the most flexible material?
polysulphides
stiffest to most flexible material?
most flexible
polysulphides
c-silicones
addition silicones
polyether
least flexible
define hardness?
measure of the depth of an indentation in the material created by a given force under standard load
do polysulphide and addition silicones change hardness with time?
no
what features of an impression material make it hard to remove from the mouth?
low flexibility and high hardness values
use a more loose fitting tray with more material which allows more flexure within the body
tear strength?
Ability to withstand tearing when removing
do elastomers produce detail accurately?
yes
light-bodied materials contain less filler
why does light-bodied materials make detailed impressions?
contain less filler
flow more readily over preparation and there is less likelihood of air being trapped
filler size also determines the ability of the impression material to record fine detail
smaller the filler particles the finer the detail the material can record
elastomers hydrophobic or hydrophilic?
hydrophobic
need good moisture control
effect of moisture contamination on impression?
moisture acts as a separating medium so preventing the impression material from binding to the tray
delamination may occur
some haemostatic agents used to stop gingival bleeding have an adverse effect on the setting of tall impression elastomers
ratio of addition silicone putty?
1:1
When would you use elastomers for impressions?
required when precision is of utmost importance, like crown and bridge preparation and implants
Types of elastomers?
addition silicone, also known as polyvinyl siloxanes (PVS), condensation silicone, polyether, and polysulfide.
How to mix elastomers?
Mechanical/machine
Mix the 2 pastes together
1:1
Problems with imp?
Thin lingual area
Not much light-body
Plastic tray - question rigidity, could have distorted imp
Size of tray, not fully captured distal aspect
Tear strength of material - between 2nd and 1st molar, canine and first premolar, 1st molar and 2nd premolar
What has occurred on the left image?
Drag
Insufficient I’m material there
Or
When tray has been inserted, not inserted axilly but wiggled into place
Also folding
What is wrong with imp on right
Drag on upper anterior (put in place and dragged dramatically)
Light body not on all teeth
Tray is ok - rigid
Not enough imp material in tray - sulcus on upper right canine region
What is wrong with the imp on left?
Void
When cast it will be distorted
Mixing machine can reduce voids
What is wrong with image on right?
Inaccurate representation of margins - traction not great
Drag
Poor tear strength - imp distorted
Light bodied material displaced to sulcus and palate - putty material pushed it out
What is wrong with image on left?
Imp not been looked after roper
Overly mixed
Reaction between I’m material and cast (can see chalkiness)
Need compatibility between imp and cast material
