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BDS2 Dental Materials Sciences > elastomers > Flashcards

Flashcards in elastomers Deck (21)
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1
Q

2 types of elastomers

A

polyethers

addition silicones

2
Q

elastic behaviour is when

A

material Fits around tooth and on removal changes shape to overcome bulbous part of the tooth to allow removal

3
Q

why are clinical trials often not carried out for dental materials

A

expensive to conduct

4
Q

how should you evaluate a product information brochure

A

list information manufacturer thinks dentists are interested in and/or what they want the dentists to hear and focus on
- Tells you a positive story as want you to buy it

Need evidence

Pick out key points

Look at the source and evidence
- Where they independent or paid to assess
- Questions left unanswered
Unheard % were they unhappy with product
- How did they measure accuracy 
- Reliability
5
Q

name 8 common material properties that can be assessed

A

flow /viscosity

surface detail (reproduction)

contact angle / wettability

elastic recovery (%)

stiffness (flexibility)

tear strength only for IM

mixing time (min)

working time (min)

6
Q

2 different methods for preparing elastomers for use

A

Two cartridges – catalyst and base paste, mixed in the spiral tips

OR

Two tubs of PUTTY like materials, one the base, the other the catalyst putty
- Mix putties until homogeneous colour

7
Q

ISO standards

A

International Standard Organisation

Dental Standard – designed to assess relevant properties of a product to see whether it meets acceptable requirements for safe and effective use

industries use ISO for their produce as a badge, sign of quality
- But really just saying it is safe to use

8
Q

ISO standard for impression materials

A

is that grooves/indentations of either 20um or 50um (depending on material viscosity) are replicated

9
Q

how could good wettability of an IM be described

A

“they spread easily and adapt smoothly to dentine and moist oral tissue”

  • The IM will not be deturbed from foreign material on tooth surface
  • E.g. saliva
  • Will not leave a void between them
10
Q

ideal qualities of impression materials

A

Quality of surface interaction between material & tooth/soft tissue surfaces

  • Viscosity
  • Surface wetting (wettability)
  • Contact angle (BDS3)

Accuracy

  • surface reproduction (ISO)
  • visco-elasticity / elastic recovery

Dealing with removal & undercuts

  • Tear/tensile strength
  • Rigidity

Dimensional stability

  • Setting shrinkage
  • Thermal expansion/contraction
  • Storage
11
Q

contact angle

A

Determines how well the material envelops the hard/soft tissue surface (to record fine detail)

12
Q

viscosity

A

a measure of material’s ability to flow

determines a material’s potential for making close contact with hard/soft tissue surfaces
- so how well it records surface detail

Globules of impression materials spread out so minimal gaps between modules
- Big gaps = poor surface wetting, so not reliable

range: low, medium, high

13
Q

surface wetting (wettability)

A
surface reproduction (ISO4823)
few notches in substrate with standard notch dimensions
A = 20um
B = 50um 
C = 75um
Apply force after applying IM 
Push into notches
- Flows readily into gap C and B 
------B is the ISO norm
Doesn’t flow into A
14
Q

visco-elasticity/ elastic recovery

A

ideal: Material should relax back to normal once removed
- 100% elastic recover
Or recovery from deformation
- No permanent strain

IDEAL – low viscoelasticity (small deformation)

when impression material, having been stretched/compressed on removal from mouth, fails to return to its original dimensions/shape

  • there is permanent deformation (permanent strain)
  • not truly elastic
  • always some deformation or permanent strain

Elastomers are not ideal
- there is gradual increase in strain over time; gradual recovery which is incomplete

15
Q

impact on visco-elasticity is load time is less

A

Viscoelastic Behaviour
- influences tray removal method

If LOAD time is less -

  • impression removed with a sharp pull
  • less overall permanent strain (lower deformation)
16
Q

2 material properties which influence how material deals with removal and undercuts

A

tear/tensil strength

rigidity

17
Q

tear/tensile strength

A

Stress material will withstand before fracturing

Especially in undercut regions
– tear readily here

18
Q

rigidity

A

Stress/strain ratio

RIGID - large stress needed to cause material to change shape by 1%
- Composite resins

ideally impression material is FLEXIBLE
- has low rigidity, to ease its removal from undercut /interdental regions

19
Q

what is a material property which is critical for accurate impressions

A

elastic recovery

20
Q

if a dentition has many undercuts what standard of tear strength is it advisable to use

A

high tear strength

21
Q

5 factors to go through when deciding what material to use

A

Know the key properties of the material

Review product specification data

Know typical values expected for specific properties

Identify properties not mentioned

  • What have they missed out
  • Is it important/of note

REJECT claims not supported with scientific and/or clinical data