02 Preliminary Impressions

Question 1

What are the two driving ingredients in alginate and what are their respective weight percentages?

Answer 1

1. Potassium alginate (18%)

2. Calcium sulfate dehydrate (14%)

Question 2

What is the irreversible hydrocolloid reaction?

Answer 2

Calcium sulfate dehydrate + Potassium alginate + water => Cross-linked insoluble calcium alginate gel

Question 3

Syneresis

Answer 3

Distortion due to loss of water (not evaporation)

Question 4

Imbibition

Answer 4

Distortion due to uptake of water

Question 5

What are eight ideal properties for preliminary impressions?

Answer 5

1. Accuracy
2. Good tear strength
3. Elasticity (lack of deformation after strain)
4. Dimensional stability
5. Compatible with gypsum products, disinfectants
6. Pleasant odor, texture, consistency
7. Easy to use
8. Economical

Question 6

What are four issues with non-elastic materials?

Answer 6

1. Lack of fine detail
2. Displaces soft tissue
3. Rigid and brittle
4. Cannot use in undercuts

Question 7

What are three non-elastic impression materials?

Answer 7

1. Impression plaster
2. Compound
3. Zinc oxide eugenol

Question 8

When calcium sulfate dehydrate is heated, what happens?

Answer 8

It loses water and becomes calcium sulfate hemi-hydrate

Question 9

Difference in the calcium sulfate hemihydrate between plaster, dental stone, and high strength stone?

Answer 9

beta CaSO4 * 1/2 h2/o
alpha CaSO4 * 1/2 h2/o
CaSO4 * 1/2 h2/o

Question 10

Increasing water:powder ratio does what to setting reaction and expansion?

Answer 10

1. Slower reaction

2. Less expansion

Question 11

Increasing spatulation rate does what to setting reaction and expansion?

Answer 11

1. Faster reaction

2. Increases expansion

Question 12

Adding low concentration NaCl does what to setting reaction and expansion?

Answer 12

1. Faster reaction

2. Increases expansion

Question 13

Adding 1% K2SO4 and increasing water temp does what to setting reaction and expansion?

Answer 13

1. Faster reaction

2. No effect on setting

Question 14

What are four examples of gypsum retarders?

Answer 14

1. Borax
2. Acetates
3. Citrates
4. Tartrates

Question 15

What are three examples of gypsum accelerators?

Answer 15

1. Potassium sulfate
2. Sodium chloride
3. Terra alba

Question 16

Increasing water does what to gypsum compressive strength?

Answer 16

Decreases compressive strength

Question 17

Vacuum mixing does what to gypsum compressive strength?

Answer 17

Increases compressive strength

Question 18

Drying a cast does what to compressive strength?

Answer 18

Increases compressive strength

Question 19

What does trisodium phosphate do in alginate?

Answer 19

Retards reaction

Question 20

What does diatomaceous earth do in alginate?

Answer 20

Provides consistency and flexibility