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Dental materials > Gypsum products > Flashcards

Gypsum products Flashcards

1
Q

What are some alternative die materials?

A

Resins, Ceramics, Amalgam

Resins include acrylic and epoxy; ceramics provide maximum hardness; amalgam can be used in specific impressions.

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2
Q

What is the purpose of disinfecting models, casts, and dies?

A

To eliminate pathogens and ensure safety in dental practices

Disinfection methods include iodophor spray and sodium hypochlorite immersion.

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3
Q

List the advantages of gypsum products.

A
  • Good dimensional stability
  • Good accuracy
  • Inexpensive
  • Easy to use
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4
Q

List the disadvantages of gypsum products.

A
  • Inferior mechanical properties
  • Brittle
  • Can easily fracture
  • Retarding effects when used with alginate
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5
Q

What are anti-expansion agents?

A

Chemical modifiers that control setting expansion in gypsum products

Accelerators increase setting rate; retarders slow it down.

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6
Q

Fill in the blank: Gypsum sets with a measurable linear _______.

A

expansion

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7
Q

What factors affect the setting time of dental plaster?

A
  • Manufacturer-controlled factors
  • Operator-controlled factors
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8
Q

What happens if impurities are present in gypsum products?

A

They can act as nucleating agents and shorten the setting time

Impurities may arise from incomplete calcination during manufacturing.

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9
Q

Define hygroscopic expansion.

A

Expansion that occurs when the material is placed in water during initial setting

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10
Q

What is the difference between initial setting and final setting?

A
  • Initial setting: material develops weak solid properties
  • Final setting: material becomes hard and strong
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11
Q

What is the hydration theory in gypsum setting?

A

It suggests that rehydrated plaster particles unite through hydrogen bonding to form the set material.

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12
Q

What is the dissolution-precipitation theory?

A

The most widely accepted mechanism for setting, based on the dissolution of hemihydrate followed by recrystallization to dihydrate.

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13
Q

What is the recommended water-powder ratio for dental plaster?

A

18.6ml of water for every 100g of powder

Any excess water lengthens setting time and reduces strength.

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14
Q

What are the components of dental plaster?

A
  • Calcium sulphate hemihydrate: 75-85%
  • Calcium sulphate dihydrate: 5-8%
  • Soluble calcium sulphate anhydrite: 5-8%
  • Potassium sulphate (accelerator): 1-2%
  • Borax (retarder): 1-2%
  • Impurities: 4%
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15
Q

What is the effect of temperature on the setting reaction of gypsum products?

A

Higher temperatures decrease the setting time, while temperatures above 37.5°C increase it.

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16
Q

What are the three theories of gypsum setting?

A
  • Colloidal theory
  • Hydration theory
  • Dissolution-precipitation theory
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17
Q

What are the requirements for model/die materials?

A
  • Dimensional accuracy
  • Dimensional stability
  • High strength
  • Compatibility with impression materials
  • Good color contrast
18
Q

How is calcium sulphate hemihydrate produced?

A

By heating calcium sulphate dihydrate to drive off part of its water

The commercial process involves grinding gypsum and heating at 110-120°C.

19
Q

What types of dental gypsum products are identified according to ISO standards?

A
  • Type 1: Dental plaster: impression plaster
  • Type 2: Dental plaster: model plaster
  • Type 3: Dental stone: class I stone
  • Type 4: Dental stone: class II stone
  • Type 5: Dental stone: die, high strength and high expansion
20
Q

True or False: Gypsum products are primarily used for making positive replicas of hard and soft tissues.

A

True

21
Q

What is the compressive strength of dental plaster?

A

Approximately 9 MPa

Die stone has a much higher compressive strength of about 34 MPa.

22
Q

What is the role of hardening solutions in gypsum materials?

A

They improve abrasion resistance and increase compressive strength

Hardening solutions are mixed with gypsum instead of water.

23
Q

What is the chemical composition of gypsum?

A

Calcium sulphate dihydrate (CaSO4·2H2O)

Gypsum is a naturally occurring mineral used in various applications, including dentistry.

24
Q

What is Plaster of Paris derived from?

A

Calcium sulphate hemihydrate (CaSO4·½H2O)

It is a gypsum product commonly used in dentistry.

25
Q

What are the uses of dental gypsum materials?

A
  • Making positive replicas of hard and soft tissues
  • Making base of models
  • Impression material
  • Study models, cast and dies

These applications are crucial for creating accurate dental models.

26
Q

What is the final setting time for gypsum?

A

45-60 minutes

This is when the conversion of hemihydrate to dihydrate is complete.

27
Q

What is the initial setting time for gypsum?

A

5-10 minutes

This is the interval during which the mix can no longer be poured.

28
Q

How is initial setting measured?

A
  • Loss of gloss
  • Needle penetration test using Gilmore needles

These methods help determine when the material can begin to be handled.

29
Q

What are the two types of Gilmore needles used in testing?

A
  • Lighter Gilmore needle (initial setting)
  • Heavier Gilmore needle (final setting)

The different weights help ascertain different stages in the setting process.

30
Q

What methods can be used to manufacture calcium sulphate hemihydrate?

A
  • Autoclaving gypsum
  • Boiling gypsum in a salt solution

These methods yield different types of dental stone with varying properties.

31
Q

What is the theoretical water-to-powder ratio for gypsum?

A

18.6ml of water: 100g of powder

This is the ideal ratio for complete hydration, though excess water is often needed for workability.

32
Q

What is the water-to-powder ratio for soft plaster?

A

50-60ml of water: 100g of powder

This higher ratio is due to its porous nature.

33
Q

What is the water-to-powder ratio for hard plaster?

A

20-35ml of water: 100g of powder

Hard plaster requires less water due to its denser structure.

34
Q

What is the final product of the hydration of hemihydrate?

A

Gypsum model or die

This process is essential in creating accurate dental reproductions.

35
Q

What is the difference in compressive strength between soft and hard plaster?

A
  • Soft plaster: Low compressive strength
  • Hard plaster: High compressive strength

This difference affects their respective applications.

36
Q

What is the boxing technique used for?

A

To create a mould for gypsum by wrapping boxing wax around the impression

This technique helps contain the mixed gypsum during pouring.

37
Q

What is the effect of humidity on gypsum materials?

A

Gypsum materials are hygroscopic and can absorb water from the air, increasing setting time

This is why gypsum products should be stored in closed containers.

38
Q

True or False: Hard plaster is always white.

A

False

Hard plaster is pigmented, while soft plaster is typically white.

39
Q

What is the purpose of the needle penetration test?

A

To measure the ability of plaster to resist penetration and determine setting times

This test provides a practical way to assess material strength.

40
Q

What is the importance of minimizing air inclusion during mixing?

A

To prevent porosities within the set material, which can weaken it

Air bubbles can lead to inaccuracies in the final model.

Dental materials (1 decks)

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