Water Hydrocolloids

Question 1

What is hydrocolloid?

Answer 1

-hydro: water is the dispersion medium
- colloid: solution in which the dispersed particles are larger than ordinary atoms/molecules but still invisible grossly

Question 2

How do hydrocolloids exist?

Answer 2

• gel: semisolid mass formed by decrease in temp or a chemical rxn
• sol(solution): aggregates of molecules dispersed in water behaving like a high viscosity fluid
  **molecules join to form a network of chains/fibrils which enclose the dispersion medium (medium)

Question 3

How do hydrocolloids behave?

Answer 3

• drying: evaporation of water with high ambient temp and low humidity -> shrinkage
• imbibition: water absorbed causing swelling and will not restore the original dimensions to gel
• syneresis: appearance of exudate on surface of impression independent of humidity

Question 4

Is alginate reversible or irreversible

Answer 4

irreversible- sets by chemical reaction

Question 5

What are the uses of alginate?

Answer 5

• preliminary impressions for CD and partial dentures/ study model impression
• not for the use of very accurate casts

Question 6

What is the benefit of running alginate across the teeth after filling the tray?

Answer 6

• reduce chance of air bubbles on surface of teeth

Question 7

In the composition of alginate, what does sodium alginate do?

Answer 7

reacts with calcium ions

Question 8

In the composition of alginate, what does calcium sulfate do?

Answer 8

reacts with sodium alginate

Question 9

In the composition of alginate, what does potassium sulfate, silicates, and borates do?

Answer 9

improve surface of stone, counteracts gypsum inhibition

Question 10

In the composition of alginate, what does sodium phosphate do?

Answer 10

retarder (slows down the chemical reaction, increasing the working time by ties up the Ca ions)

Question 11

In the composition of alginate, what does fillers do?

Answer 11

controls consistency

Question 12

Why does calcium alginate cause gelation?

Answer 12

because it is insoluble in water

Question 13

What is adjusted to provide regular and fast set materials?

Answer 13

• sodium phosphate (more retarder, slower. less retarder, quicker)

Question 14

What is the mixing time for alginate?

Answer 14

• regular set: 1 minute
• fast set: 45 sec

Question 15

What is the working time for alginate (including mixing time)?

Answer 15

• regular set: 3-4.5 min
• fast set: 75 sec-2 min

Question 16

How is setting time altered?

Answer 16

• changing water temp: warmer water-> faster set

Question 16

What is the setting time for alginate?

Answer 16

• regular set: no less than 2 or greater than 4.5 minutes
• fast set: no less than 1 or greater than 2 minutes

Question 17

How to increase tear strength and resistance to permanent deformation:

Answer 17

keep material in patients mouth as long as possible (2-3 min for alginate)

Question 18

What will increase deformation while removing tray?

Answer 18

• slow removal=increase deformation

Question 19

How long should you wait to pour up alginate impression (recovery)?

Answer 19

8 minutes

Question 20

What is one of the most flexible dental materials?

Answer 20

alginate

Question 21

tear strength removal increases with _____ rate of removal

Answer 21

increases

Question 22

Loss of water via evaporation or syneresis causes

Answer 22

shrinkage

Question 23

Gain of water via imbibition causes

Answer 23

expansion
- pour up quicker to reduce

Question 24

Best storage of alginate to maintain dimensional stability

Answer 24

store in 100% humidity for up to 2 hours

Question 25

shrinkage of impression material will cause the cast to be…

Answer 25

too big

Question 26

expansion of impression material will cause the cast to be…

Answer 26

too small

Question 27

Which part of alginate is brittle and which part allows it to be elastic?

Answer 27

• brittle part: pure calcium alginate
• elastic part: sodium alginate

Question 28

Initial particle composition at the beginning of setting:

Answer 28

• only outer surface of particle changes to calcium alginate
• center remains soft
• eventually, the impression will change to calcium alginate

Question 29

Unique feature of agar/gel hydrocolloids:

Answer 29

• reversible
• gel upon cooling, liquefy upon heating
• heating units/special trays are required to not overturn or overcool patients oral tissue (thermal shock)

Question 30

downsides of agar hydrocolloids:

Answer 30

• dimensional unstable
• difficult to register sub gingival areas
• thermal shock

Question 31

In the composition of agar hydrocolloids, what does agar do?

Answer 31

fibril structure of gel

Question 32

In the composition of agar hydrocolloids, what does potassium sulfate do?

Answer 32

improves surface of stone (same as alginate)

Question 33

In the composition of agar hydrocolloids, what does borax do?

Answer 33

improves strength of gel

Question 34

In the composition of agar hydrocolloids, what does alkyl benzoate do?

Answer 34

preservative

Question 35

In the composition of agar hydrocolloids, what is the highest content?

Answer 35

water (85%)

Question 36

What type of reaction do agar hydrocolloids undergo?

Answer 36

conversion reaction from gel to solution and vice versa
(heat to 212 and cool to 109)

Question 37

What is hysteresis?

Answer 37

difference between the gel to solution and solution to gel transformation temperatures (212 and 109 in agar hydrocolloids)

Question 38

At what temp do you store and temper agar hydrocolloids?

Answer 38

• store: 145-150F
• temper: 115F for 2 min