Gypsum

Question 1

What is a study model

Answer 1

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

Question 2

what are the purposes of study models

Answer 2

• records the position, shape of teeth
• aids visualisation/ assessment of dentition
• enables manufacture of dental prostheses

Question 3

what material is often used to create study models

Answer 3

gypsum

Question 4

what are the uses of gypsum

Answer 4

• cast (plaster/stone)
• die (stone/ improved stone)
• mould material (stone)
• investment binder (stone)

Question 5

what determines the type of gypsum you end up with

Answer 5

manufacturing conditions

Question 6

What is the manufacturing process of gypsum

Answer 6

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

Question 7

What are the different types of gypsum

Answer 7

• plaster (beta-hemihydrate)
• dental stone (alpha-hemihydrate)
• densite (improved stone)

Question 8

what determines the properties of the gypsum

Answer 8

crystalline structure

Question 9

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

Answer 9

• heated in open vessel

Question 10

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

Answer 10

• heat in autoclave

Question 11

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

Answer 11

• heat in the presence of Ca and Mg chloride

Question 12

What is the crystalline structure like

Answer 12

large porous, irregular crystals

Question 13

What is the crystalline structure of dental stone like

Answer 13

non-porous, regular crystals, requires less water

Question 14

What is the crystalline structure of densite like

Answer 14

compact smoother particles

Question 15

what is the basic reaction gypsum undergoes during the manufacturing process

Answer 15

dihydrate –> hemihydrate

Question 16

what is the basic setting reaction gypsum undergoes

Answer 16

hemihydrate –> dihydrate

Powder + water = gypsum (study cast)

(reverse of manufacture)

Question 17

What is the mixing ratio for plaster

Answer 17

water = 50-60ml
powder = 100g

Question 18

What is the mixing ratio for stone

Answer 18

water = 20-35ml
powder = 100g

Question 19

what is the theoretical ratio for the mixing ratio

Answer 19

water = 18.6ml
powder = 100g

Question 20

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

Answer 20

no

Question 21

what is excess water needed for

Answer 21

a workable mass, affects properties

Question 22

what happens during the the setting process

Answer 22

• 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)

Question 23

How are initial and final setting times measured

Answer 23

using gilmore needles

Question 24

How is the strength of gypsum

Answer 24

• compressive (~20-35MPa)

- low hardness

Question 25

How is the setting tie of gypsum

Answer 25

convenience

Question 26

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

Answer 26

``` plaster = 0.2-0.3% stone = 0.08%-0.1% densite = 0.05-0.07% ```

Question 27

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

Answer 27

improved stone (densite)

Question 28

when does strength develop

Answer 28

over 24 hr period

Question 29

why is stone stronger

Answer 29

it requires less water for workable mix (less porous)

Question 30

What does increase powder do to the setting time and expansion

Answer 30

decreases setting time | increases expansion

Question 31

What does increase spatulation do to the setting time and expansion

Answer 31

decreases setting time | increases expansion

Question 32

What does increase impurities do to the setting time and expansion

Answer 32

decreases setting time | increases expansion

Question 33

What does increase temp do to the setting time and expansion

Answer 33

depends on the brand of gypsum for setting time, | no effect on expansion

Question 34

What does increase chemicals do to the setting time and expansion

Answer 34

chemicals can increase or decrease the setting time, decreases expansion

Question 35

What does spatulation do

Answer 35

- breaks down growing crystals - fragments act as nuclei of crystallisation - more growing crystals - come into contact sooner HENCE: - decreases setting time - increases expansion

Question 36

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

Answer 36

- more nuclei of crystallisation per unit volume - crystals come into contact sooner - faster set and greater expansion

Question 37

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

Answer 37

- so crowns, bridges and dentures won't be too tight a fit when placed in mouth

Question 38

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

Answer 38

- rate of diffusion of ions increase with increasing temp - solubility of hemihydrate decreases with increasing temp CONFLICTING mechanisms

Question 39

What chemical additives are there

Answer 39

- potassium sulphate | - borax

Question 40

what is the effect of potassium sulphate

Answer 40

- produces syngenite - crystallises rapidly (encourages growth of more crystals) - decreases setting time

Question 41

what is the effect of borax

Answer 41

- forms calcium borate (deposits on dihydrate crystals) | - delays setting process (increases setting time)

Question 42

Whats important to know about compatibility with impression material

Answer 42

- 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

Question 43

What is gypsums role in the reproduction of surface detail

Answer 43

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

Question 44

what is the impression material standard it has to meet

Answer 44

material must reproduce a 50µm wide groove

Question 45

Advantages of gypsum

Answer 45

- dimensionally accurate and stable - low expansion (<0.1%) of stone/ densite) - good colour contrast

Question 46

Disadvantages of gypsum

Answer 46

- low tensile strength - poor abrasion resistance - very brittle - surface detail less than elastomer - poor "wetting" of some impression materials