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BDS 2: Dental Materials > Alginates > Flashcards

Alginates Flashcards

1
Q

What are impression materials used for?

A

Used to produce a detailed replica of the teeth and oral tissues

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2
Q

What are the 3 types of elastic impression materials?

A
  • Hydrocolloids
  • Elastomers
  • Non-elastic materials
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3
Q

Give an example of a reversible and irreversible hydrocolloid

A
  • Reversible = Agar

- Irreversible = Alginate

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4
Q

What are the 3 types of elastomers?

A
  • Silicones
  • Polysulphides
  • Polyethers
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5
Q

Name 3 non-elastic impression materials

A
  • ZOE
  • Impression plaster
  • Impression compound
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6
Q

What makes alginates irreversible hydrocolloids?

A

The reactions sets by a chemical cross-linking reaction

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7
Q

What are alginates extracted from?

A

Extracted from seaweed with a hydroxide

Ie with NaOH forms sodium alginate

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8
Q

What happens when alginate is mixed with water?

A

When alginate is mixed with water, it forms a colloidal suspension in it, and then it sets by a chemical cross-linking reaction

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9
Q

What is a typical alginate powder composition? (6)

Include percentage composition

A
  • Na/K alginate (12%)
  • Diatomaceous earth (70%)
  • CaSO4 (12%)
  • Na3PO4 (2%)
  • Sodium silicofluoride or Sodium fluorotitinate (4%)
  • Magnesium oxide (3%)
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10
Q

What is the role of Diatomaceous earth in alginate powder?

A

Acts as a filler to strengthen alginate

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11
Q

What is the role of CaSO4 in alginate powder?

A

Cross-linking agent

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12
Q

What is the role of Na3PO4 in alginate powder?

A

Retarding agent

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13
Q

What is the role of Sodium silicofluoride, Sodium fluorotitinate and Magnesium oxide in alginate powder?

A

pH controller

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14
Q

In sodium alginate polymer where does cross linking occur?

A

At COONa

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15
Q

Define cross link

A

Chemical bonds joining two polymer chains together

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16
Q

What joins 2 polymer chains of alginates?

What is displaced?

A
  • Ca

- Na in COONa is displaced

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17
Q

What is the function of the retarder in alginate powder?

A

Without retarder alginate gels immediately, so a retarder gives working time

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18
Q

How much cross-linking occurs when phosphate ions are present in the powder?

A

The cross-linking reaction is suppressed while the phosphate ions are being used up

So no cross-linking

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19
Q

Why are pH controllers added to alginate powders?

A
  • When setting pH of alginate initially decreases to below pH 3.5 and rises to pH 9 on setting
  • When set gel is too acidic or alkaline it will give a poor plaster/stone surface
  • So pH controllers are added to decrease the pH to near neutral (~8)
  • In order to make it compatible with casting materials
20
Q

What feature does Chromoclone have that makes the alginate easier to use?

A

Powder colour changes at different pH’s

21
Q

With Chromoclone what colour appears when water is added to alginate?

A

Purple

22
Q

What colour change of chromoclone needs to occur in order to indicate the alginate is ready to be loaded in a tray?

A

Purple to Pink

23
Q

Which colour change indicates that chromoclone in a tray can be inserted into the mouth?

A

Pink to light peach

24
Q

What are some advantages of alginates? (2)

A
  • Very good setting behaviour

- Cheap and reliable

25
Q

What are some disadvantages of alginates? (5)

A
  • Once alginates are set they lose water in air
  • Immersing the alginate in disinfectant causes initial swelling then shrinking causes poor dimensional stability
  • Poor tear strength
  • Does not adhere well to tray
  • Highly viscoelastic
26
Q

Alginates losing water air once set means what for general properties? (2)

How do you manage this?

A
  • Continual shrinkage in air
  • Poor dimensional stability
  • Impressions must be covered with a damp napkin and placed in a sealed plastic bag
27
Q

Explain step by step the disinfection protocol for alginates (5)

A
  1. Remove set alginate/tray from the mouth
  2. Rinse in tap water to remove blood/saliva
  3. Immerse in disinfecting solution for only 10 minutes
  4. Remove from disinfecting solution and rinse in tap water
  5. Wrap in a damp napkin and seal in a polythene bag (100% relative humidity)
28
Q

What is a disinfectant solution used in alginates?

A

Sodium hypochlorite

29
Q

Why do you need to shake the alginate tub before using?

A
  • Density of the various ingredients of an alginate powder differ
  • So there is a tendency for settling out in the tin/tub
30
Q

Why are dust free alginate powders used today?

What particle in particular is the cause of this?

A

Silica particles in the dust are a possible health hazard

31
Q

How do you produce dust free alginate?

A

Ball-mill (mix/grind) alginate powder with a small amount of ethylene glycol

32
Q

What are the different types of alginate bases? (4)

A
  • Sodium alginate
  • Potassium alginate
  • Sodium and Potassium alginate
  • Triethanolamine alginate
33
Q

What were polyvinyl alcohol sachets?

A
  • Pre-weighed alginate powders in sachets
  • Sachet made from polyvinyl alcohol, a water soluble plastic
  • Sachets would dissolve in water
  • No longer used as expensive
34
Q

By what process is agar set?

A

Set by a Physical Process

35
Q

What state is agar in at room temperature?

What state is agar in at 60°?

What occurs to agar when cooled form 60°?

A
  • A solid (gel) at room temperature
  • A viscous liquid at 60°C
  • On cooling, it reverts to the gel
36
Q

What is the composition of agar impression material? (4)

A
  • Agar (13-17%)
  • Borates (0.2-0.5%)
  • Potassium sulphates (1-2%)
  • Thixotropic materials (0.5-1%)
37
Q

What is the function of agar in agar impression materials?

A

Colloid

38
Q

What is the function of borates in agar impression materials?

A

Strengthens gel

39
Q

What is the function of potassium sulphates in agar impression materials?

A

Accelerates setting of stone

40
Q

What is the function of Thixotropic materials in agar impression materials?

A

Filler

41
Q

Why are agar dispensed in tubes?

A

Prevent water loss

42
Q

What is agar squeezed into?

A

Special metal tray

43
Q

What occurs before placing agar into its metal tray?

A
  • Tubes are put into a water bath at 60°C

- Transferred to a bath at 40°C before use as 60°C is too hot for patient

44
Q

How is agar cooled?

A

Cooled by water circulated via cooling tubes attached on outer surface of tray

45
Q

What are some advantages of agar? (3)

A
  • Once the equipments’ set up it is easy to use
  • The material is cheap
  • Impressions have good surface detail
46
Q

What are some disadvantages of agar? (5)

A
  • Impressions must be cast up immediately to prevent shrinkage
  • Agar absorbs water during disinfection
  • Poor compatibility with model materials (ie stone)
  • Poor tear strength
  • Highly viscoelastic

BDS 2: Dental Materials (13 decks)

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