Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

DDS1-biomat lab > final exam questions > Flashcards

final exam questions Flashcards

1
Q

Two accelerators for gypsum?

A
  • Sodium sulfate (low conc)

- Potassium sulfate and sodium chloride (low conc)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

how does tri-sodium phosphate act as a retarder

A

tri-sodium phosphate reacts first with calcium sulphate. then sodium alginate reacts with calcium sulphate

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

should impression materials be plastic or elastic? why?

A

elastic; required to be stretched to a fairly large degree without suffering any permanent deformation (as in during removal of impression – put under compressive and tensile stress); plastic materials lack ability to record any undercut

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

why do u need adhesive on the tray

A

detached is grossly distorted and will lead to ill-fitting resto

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Two retarders for gypsum?

A
  • borax
  • sodium chloride (high conc)
  • sodium sulfate (high conc)
  • glue
  • agar
  • coagulated blood
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What provides the driving force for the setting reaction of gypsum?

A

water molecules in the dihydrate are found in alternating double layers
-dehydration process requires a lot of energy because the dihydrate is more stable at room temperature

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is the base, reactor, and retarder for alginate?

A

Base – Na alginate
Reactor – calcium sulphate
Retarder – tri-sodium phosphate

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

what is the activator for cold-cure PMMA

A

dimethyl-p-toluidine

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

criteria for good impression

A
  • uniform, homogenous mix
  • tray adhesive applied
  • rigid impression tray
  • no void or pulls on margins
  • detailed margin
  • no burn through of tray
  • strong bond between light and heavy
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

shrinkage of polymeric material is caused by

A
  1. VDW volume reduction (due to single bonds)

2. free volume reduction (less rotation and distance)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

4 reasons why we use custom trays

A
  1. Record most out of reach anatomical landmarks
  2. Minimize impression material distortion
  3. Prevent tissue distortion (can use low viscosity materials)
  4. Reduced cost
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

what can affect the mechanical/physical properties of a polymer

A

molecular weight and degree of polymerization

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

effect of oxygen on poly

A
  • decrease rnx velocity

- decrease deg of poly

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

syneresis

A

loss of water from alginate

-excess water in contact with gypsum

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

name all components of alginate

A
  1. na/k alginate
  2. calcium sulphate
  3. tri-sodium phosphate
  4. mineral/silicon fillers
  5. potassium fluoride
  6. pigment particles
  7. organic glycol
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

imbibition

A

alginate absorbs water from gypsum

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

why might you have lack of detail on impression

A
  • low temp (prolonged setting time; increase viscosity)

- too much pooled liquid (impression material doesnt flow)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What stage(s) does expansion occur during gypsum setting?

A

Plastic and friable stages

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

what type of impression material is alginate

A

irreversible aqueous hydrocolloid

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

Is alginate a hydrocolloid? what does it mean?

A

yes. hydrocolloids are uniform distribution of individual particles of one substance in water

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

list all properties of PMMA

A
  • colourless
  • any shade
  • radiolucent
  • chemically stable
  • knoop hardness 15 to 18
  • Tg=125 deg c
  • absorb water
  • low thermal conductivity
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

components of powder PMMA

A
  • pre-polymerized beads of PMMA
  • initiator: benzoyl peroxide
  • plasticizer: dibutyl phthalate
  • inorganic particles (hardness)
  • organic pigments and dyes
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

list all physical properties of MMA

A
  • transparent
  • dissolves PMMA
  • tg -45 deg C
  • excellent organic solvent
  • not chemically stable
24
Q

3 practical ways to decrease setting time of gypsum

A
  1. Add slurry water to water/powder mixture
  2. Use mixing bowl with some gypsum particles
  3. Mix water and powder faster and more vigorously
25
Q

What is gypsum?

A

Calcium sulphate dihydrate

26
Q

what can cause premature termination of poly and how

A

impurity in monomer; react with activated initiator OR activated growing chain

27
Q

What is the effect of low water/powder ratio?

A
  1. Quick setting
  2. High expansion
  3. More strength
  4. More surface hardness
  5. Less porosity
28
Q

properties of stone

A 
Uniform, dense, small particles
Rods and prism-like crystals
Pack tightly
Little water
Organized crystal growth
Strong structure (20% porosity)
29
Q

properties of plaster

A 
Irregular (size/shape), porous particles
Needle-like crystals with capillary pores
Loosely packed
Needs lots of water for mixing
Water absorbed into pores bw particles
Weak structure (35% porosity)
30
Q

Define the dissolution-precipitation theory.

A
  • dissolution of hemihydrate molecules

- recrystallization of dihydrate particles, resulting in interlocking of crystals, to form the set solid gypsum

31
Q

how can we control the speed of phase transformation from sol to gel

A

add tri-sodium phosphate as a retarder

32
Q

What are two properties that can control setting time of gypsum?

A
  1. Solubility of hemihydrate (higher solubility = shorter time)
  2. Number of nuclei of crystallization (more nuclei = shorter time)
33
Q

purpose of preliminary impression vs final impression

A

prelim: record dimensions of edentulous arch
final: record fine details of edentulous arch

34
Q

auto-mix or spatula mixed and why?

A

auto-mixed:

  • faster
  • more convenient
  • less technique sensitive
  • no voids
  • no risk of cross cont
  • ideal phys/mech properties
35
Q

How is beta-calcium sulphate hemihydrates formed?

A

For plaster: dry calcination, without pressure, heated to 120 deg C in an open vessel

36
Q

clinical acceptable range of alginate

A

1-3%

37
Q

Explain the 4 steps/mechanisms to dissolution precip theory

A
  1. hemihydrate powder mixed in water to form a workable suspension. Particles are only moderately dissolved in water
  2. continuous dissolution forms a super-saturated hemihydrate solution
  3. super-saturated solution forms dihydrate crystals by gradual precipitation
  4. solution is “not-saturated” so more hemihydrate particles are dissolved (cycle continues)
38
Q

6 criteria for custom trays.

A
  1. Close adaptation
  2. No impingement
  3. Adequate space
  4. Rigid, thick, and fracture resistant
  5. Handles don’t interfere with movements of oral structures
  6. Smooth surfaces and flanges, no sharp corners/edges
39
Q

Name a retarder for gypsum and how it works?

A

Agar and alginate can retard crystallization by being adsorbed on calcium sulfate nucleating sites and interfere with crystallization on the surface of gypsum model. Gives it a chalky view and powder texture

40
Q

How is alpha-calcium sulphate hemihydrates formed?

A

For dental stone: wet calcination, under pressure (117 Kpa for 5-7 hrs), heated to 125 deg C; For die stone: without pressure, boiled to 100 deg C in 30% CaCl2 solution

41
Q

what if you compress more than 1/3 original thickness of VPS or polyether

A
  • elastic recovery is not 100%
  • plastic deformation
  • distance to wall is twice the depth of undercut
42
Q

why do we use both light body and heavy body

A
  • light: very accurate; low dimensional stability

- heavy: support light body; maintain dimensional stability

43
Q

2 ways of keeping alginate fresh

A
  • store in 100% relative humidity inside a plastic bag

- wrap in damp (not dripping) paper towel

44
Q

properties of die stone

A 
Large, uniform, dense particles
Short and thick crystals
Tightly packed
Little water
Organized and extensive crystal growth
Strong structure (10% porosity)
45
Q

components of liquid PMMA

A
  • monomer: MMA
  • inhibitor: hydroquinone
  • cross linking agent: glycol dimethacrylate
  • activator (cold-cure): dimethyl-p-tonuidine
46
Q

clinical properties of alginate

A

acceptable surface detail; faster reaction with warm water; enough elasticity for moderate undercuts; time-dependent dimensional stability; non-toxic; can be manipulated

47
Q

what is the reaction transformation for alginate soln to gel

A

Na alginate +CaSO4 =>(water) Ca alginate + Na2SO4

48
Q

practical way of alternating setting time of alginate

A

higher temp of water (shorter setting time)

49
Q

What is the starting spot for dihydrate crystals?

A

Nucleation centers in the suspension

50
Q

What can act as nucleation centers for dihydrate crystals.

A
  1. Impurities, such as residual gypsum particles
  2. particles of gypsum
  3. strained areas on undissolved hemihydrate particles
51
Q

PMMA reaction for heat cure liquid

A

PMMA (activator: heat)+BP=active BP

active BP+MMA=active MMA=PMMA

52
Q

how long can you leave alginate on bench

A

30 min

53
Q

PMMA reaction for cold cure liquid

A

PMMA (activator: dimethyl-p-toluidine) + benzoyl peroxide = activated benzoyl peroxide
activated BP +MMA = activated MMA = PMMA

54
Q

function of inhibitors in MMA

A
  • inhibit polymerization to increase storage stability

- retard poly to adjust working time

55
Q

what does adding barium salts do to PMMA

A
  • reduce strength of denture base
  • reduce translucency and vital appearance of denture
  • more expensive

DDS1-biomat lab (8 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions