Brainscape's Knowledge GenomeTM

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


See full index

BDS2 Dental materials science > Gypsum > Flashcards

Gypsum Flashcards

You may prefer our related Brainscape-certified flashcards:
Biology 101 flashcards
1
Q

What is a study model

A

a POSITIVE REPLICA of dentition, produced from the impression (negative representation) of a patient’s dentition

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

what are the purposes of study models

A
  • records the position, shape of teeth
  • aids visualisation/ assessment of dentition
  • enables manufacture of dental prostheses
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what material is often used to create study models

A

gypsum

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

what are the uses of gypsum

A
  • cast (plaster/stone)
  • die (stone/ improved stone)
  • mould material (stone)
  • investment binder (stone)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

what determines the type of gypsum you end up with

A

manufacturing conditions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is the manufacturing process of gypsum

A

Calcium sulphate dihydrate, heats it, produces calcium sulphate hemihydrate and water

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

What are the different types of gypsum

A
  • plaster (beta-hemihydrate)
  • dental stone (alpha-hemihydrate)
  • densite (improved stone)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

what determines the properties of the gypsum

A

crystalline structure

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

How do you manufacture gypsum to get plaster (beta-haemihydrate)

A
  • heated in open vessel
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

How do you manufacture gypsum to get dental stone (alpha hemihydrate)

A
  • heat in autoclave
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

How do you manufacture gypsum to get densite (improved stone)

A
  • heat in the presence of Ca and Mg chloride
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is the crystalline structure like

A

large porous, irregular crystals

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

What is the crystalline structure of dental stone like

A

non-porous, regular crystals, requires less water

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

What is the crystalline structure of densite like

A

compact smoother particles

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

what is the basic reaction gypsum undergoes during the manufacturing process

A

dihydrate –> hemihydrate

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

what is the basic setting reaction gypsum undergoes

A

hemihydrate –> dihydrate

Powder + water = gypsum (study cast)

(reverse of manufacture)

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

What is the mixing ratio for plaster

A 
water = 50-60ml
powder = 100g
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What is the mixing ratio for stone

A 
water = 20-35ml
powder = 100g
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

what is the theoretical ratio for the mixing ratio

A 
water = 18.6ml
powder = 100g
20
Q

are the mixing ratios of powder and water the same for all types of gypsum

A

no

21
Q

what is excess water needed for

A

a workable mass, affects properties

22
Q

what happens during the the setting process

A
  • hemihydrate dissolves
  • dihydrate forms
  • dihydrate solubility low-supersaturated solution
  • dihydrate crystals precipitate on impurities as crystals
  • more hemihydrate dissolved
  • continues until all hemihydrate dissolved

Initial set

  • dihydrate crystals come into contact i.e. push apart
  • expansion starts
  • properties of weak solid and will not flow
  • can be carved

Final set

  • strong and hard enough to be worked
  • strength continues to develop

During setting:
excess water is trapped in the powder mass

On completion of setting:

  • excess water evaporates
  • voids produced (porosity)
23
Q

How are initial and final setting times measured

A

using gilmore needles

24
Q

How is the strength of gypsum

A
  • compressive (~20-35MPa)

- low hardness

25
Q

How is the setting tie of gypsum

A

convenience

26
Q

What are the typical expansion values for plaster, stone and densite?

A 
plaster = 0.2-0.3%
stone = 0.08%-0.1%
densite = 0.05-0.07%
27
Q

What is the strongest out of plaster, stone and improved stone

A

improved stone (densite)

28
Q

when does strength develop

A

over 24 hr period

29
Q

why is stone stronger

A

it requires less water for workable mix (less porous)

30
Q

What does increase powder do to the setting time and expansion

A

decreases setting time

increases expansion

31
Q

What does increase spatulation do to the setting time and expansion

A

decreases setting time

increases expansion

32
Q

What does increase impurities do to the setting time and expansion

A

decreases setting time

increases expansion

33
Q

What does increase temp do to the setting time and expansion

A

depends on the brand of gypsum for setting time,

no effect on expansion

34
Q

What does increase chemicals do to the setting time and expansion

A

chemicals can increase or decrease the setting time, decreases expansion

35
Q

What does spatulation do

A
  • breaks down growing crystals
  • fragments act as nuclei of crystallisation
  • more growing crystals - come into contact sooner

HENCE:

  • decreases setting time
  • increases expansion
36
Q

What does increasing the powder in the powder/water ratio do

A
  • more nuclei of crystallisation per unit volume
  • crystals come into contact sooner
  • faster set and greater expansion
37
Q

why is it important we don’t get too much expansion on setting

A
  • so crowns, bridges and dentures won’t be too tight a fit when placed in mouth
38
Q

What are the factors which influence the effect of temperature on setting

A
  • rate of diffusion of ions increase with increasing temp
  • solubility of hemihydrate decreases with increasing temp

CONFLICTING mechanisms

39
Q

What chemical additives are there

A
  • potassium sulphate

- borax

40
Q

what is the effect of potassium sulphate

A
  • produces syngenite
  • crystallises rapidly (encourages growth of more crystals)
  • decreases setting time
41
Q

what is the effect of borax

A
  • forms calcium borate (deposits on dihydrate crystals)

- delays setting process (increases setting time)

42
Q

Whats important to know about compatibility with impression material

A
  • dental stone model surface detail depends on type of impression material
  • needs to be chemically compatible
  • must “wet” the impression material i.e. no resistance to flow over surface; avoid bubble formation
43
Q

What is gypsums role in the reproduction of surface detail

A
  • ideally gypsum reproduces the fine detail on the impression material
  • but, gypsum is inherently porous, resulting in a relatively rough surface (about 28-40µm)
44
Q

what is the impression material standard it has to meet

A

material must reproduce a 50µm wide groove

45
Q

Advantages of gypsum

A
  • dimensionally accurate and stable
  • low expansion (<0.1%) of stone/ densite)
  • good colour contrast
46
Q

Disadvantages of gypsum

A
  • low tensile strength
  • poor abrasion resistance
  • very brittle
  • surface detail less than elastomer
  • poor “wetting” of some impression materials

BDS2 Dental materials science (10 decks)

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