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Dental Materials Second Half Sem > Impression > Flashcards

Impression Flashcards

1
Q

How do impression materials act?

A

Impression Materials behave as viscoelastic materials

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2
Q

what are some properties of impression materials?

A
  1. Strain Rate Dependence
  2. Elastic Recovery
  3. Tear Strength
  4. Viscosity - Working and Setting times
  5. Dimensional Stability
  6. Surface Resolution
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3
Q

What is strain rate dependance?

A

Mechanical response of dental materials (e.g. alginate, waxes,
polymers) depend on how fast they are loaded!

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4
Q

Higher strain rates =

A

~Higher strain rates ~ higher values of Modulus, UTS

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5
Q

what is elastic recovery?

A

–Ability to recover to the original shape after stretch

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6
Q

What is elastic recovery relevant for?

A

–Relevant for removal of IMs from the mouth

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7
Q

what is tear strength?

A

Ability to resist tearing in thin sections

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8
Q

what is dimensional stability?

A

Ability to retain its absolute dimensional size over time.

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9
Q

what is surface resolution?

A

–High degree of surface detail
Surface gets more “wet”
–Low degree of surface detail
Liquid don’t “wet” sufficiently
the surface

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10
Q

what are the classes of impression material?

A

hydrocolloid
elastomeric
bite registration
digital

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11
Q

What is a Hydrocolloid?

A

A dissolving solid (solute) is dispersed into liquids (solvent)
(example jello)

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12
Q

hydrocolloid particles are:

A

hydrophilic

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13
Q

what is sol state?

A

viscous liquid

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14
Q

what is gel state?

A

elastic solid

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15
Q

what is the chain of rxn for Alginate?

A
  1. Na-Alginate mix with water
    => sol of alginate chains
    Ca 2+ (released from
    dissolution of CaSO4 )
  2. Ca2+ ions replace Na- ions
    and cross-link the alginate
    chains
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16
Q

what is the working time?

A

The time allowed for mixing the alginate, loading the tray and positioning the tray into the patient’s mouth

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17
Q

what is the setting time?

A

The time required for the chemical reaction to be completed and the material
“solidify”

18
Q

How does temperature modify setting and working time?

A

Increase in temperature, shorten setting and work times
18 C < Temp < 24 C

19
Q

how does proportion of water to powder modify setting time?

A

Thinner mixes increase the setting time
**how ever this effects strength

20
Q

How does mixing effect setting/working time?

A

Under-mixing and over-mixing are detrimental to the strength

21
Q

ANSI-ADA requires at least _____ elastic recovery, no more than _____ permanent deformaion

A

95%
(no more than 5% permanent deformation) after compressed 20% for 5 seconds

22
Q

what factors increase permanent deformation? (there’s four)

A

-when time before testing decreases
-when amount of deformation increases during removal
-when time held under compression increases
-when thinner mix used

23
Q

what is the effect of consistency of the mix and time of testing on tear strength?

A

watery is less tear strength
thicker more

24
Q

what is the effect of rate of deformation on tear strength?

A

rate is slow in syringe
higher in regular (faster you load it the more mechanically sound it is)

25
Q

What is important to consider for storage of hydrocolloids?

A
  • Stored in air? Water evaporates, alginate shrinks
  • Stored in water? Water is absorbed, alginate expands
26
Q

what are some advantages of Alginate?

A
  1. Ease of mixing and manipulation
  2. Minimum equipment is necessary
  3. Flexibility of the set impression
  4. Accuracy if properly handled
  5. Low cost
27
Q

what are some disadvantages of Alginate?

A
  1. Low tear strength
  2. Limited elasticity
  3. Limited surface detail
  4. Storage dimensional stability
28
Q

what are two types of Elastomeric Impression Materials?

A

addition silicone
polyester

29
Q

elastomeric impression materials are described as

A

“Silicone rubbers”

30
Q

Compared to alginates, elastomeric have greater:

A
  • accuracy
  • dimensional stability with time (e.g. storage)
  • excellent record of details
  • tear strength
31
Q

Addition Silicone in are Supplied as 2-past systems incuding:

A
  • Paste 1: Low molecular weight silicone, vinyl, fillers,
    chloroplatinic acid catalyst
  • Paste 2: Low molecular silicone with hydrogen groups and
    fillers
32
Q

Addition Silicone has no generation of

A

volatile by-products

33
Q

what is the consistency of Addition Silicone

A
  • Low-Viscosity
  • Medium-Viscosity
  • High-Viscosity (may require dispensers)
    –mixing tips to mix pastes
34
Q

what are some properties of Addition Silicones

A
  • Dimensional change <-0.1%
  • Elastic recovery after mouth removal ~99.8% (permanent
    deformation of 0.2%)
  • Highest among all impression materials
  • Working time: short
  • Tear strength: mid-low
  • Available also as one paste - relies on shear-thinning behavior for dispensing
35
Q

Polyether Impression Materials are:

A

similar to Addition Silicones
good hydrophilicity

36
Q

what are some properties of Polyether impression materia?

A
  • Limited dimensional stability over time
  • Higher compared to other Elastomeric
  • Polyether absorbs water
  • No storage on water or sun contact
    –Shelf life >2 years
37
Q

what is the reaction behind polyester?

A
  • Base + Catalyst system
  • Base: moderately low-molecular- weight polyether containing ethyleneimine terminal groups
  • Catalyst aromatic sulfonic acid ester
38
Q

what is the consistency of polyester?

A

medium

39
Q

how is the Dimensional change compared to Addition
Silicones

A

greater

40
Q

two mixing systems for polyester:

A
  • Cartridge + mixing gun
  • Sausage + dynamic mechanical mixer
41
Q

what is the new way of impressions?

A

digital using intraoral scanners

42
Q

what is an example of digital impressions?

A

Cad/Cam
(CAD: Computer-Aided Design, CAM: Computer-Aided Manufacturing)

Dental Materials Second Half Sem (6 decks)

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