Gypsum products Flashcards
What are some alternative die materials?
Resins, Ceramics, Amalgam
Resins include acrylic and epoxy; ceramics provide maximum hardness; amalgam can be used in specific impressions.
What is the purpose of disinfecting models, casts, and dies?
To eliminate pathogens and ensure safety in dental practices
Disinfection methods include iodophor spray and sodium hypochlorite immersion.
List the advantages of gypsum products.
- Good dimensional stability
- Good accuracy
- Inexpensive
- Easy to use
List the disadvantages of gypsum products.
- Inferior mechanical properties
- Brittle
- Can easily fracture
- Retarding effects when used with alginate
What are anti-expansion agents?
Chemical modifiers that control setting expansion in gypsum products
Accelerators increase setting rate; retarders slow it down.
Fill in the blank: Gypsum sets with a measurable linear _______.
expansion
What factors affect the setting time of dental plaster?
- Manufacturer-controlled factors
- Operator-controlled factors
What happens if impurities are present in gypsum products?
They can act as nucleating agents and shorten the setting time
Impurities may arise from incomplete calcination during manufacturing.
Define hygroscopic expansion.
Expansion that occurs when the material is placed in water during initial setting
What is the difference between initial setting and final setting?
- Initial setting: material develops weak solid properties
- Final setting: material becomes hard and strong
What is the hydration theory in gypsum setting?
It suggests that rehydrated plaster particles unite through hydrogen bonding to form the set material.
What is the dissolution-precipitation theory?
The most widely accepted mechanism for setting, based on the dissolution of hemihydrate followed by recrystallization to dihydrate.
What is the recommended water-powder ratio for dental plaster?
18.6ml of water for every 100g of powder
Any excess water lengthens setting time and reduces strength.
What are the components of dental plaster?
- 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%
What is the effect of temperature on the setting reaction of gypsum products?
Higher temperatures decrease the setting time, while temperatures above 37.5°C increase it.
What are the three theories of gypsum setting?
- Colloidal theory
- Hydration theory
- Dissolution-precipitation theory
What are the requirements for model/die materials?
- Dimensional accuracy
- Dimensional stability
- High strength
- Compatibility with impression materials
- Good color contrast
How is calcium sulphate hemihydrate produced?
By heating calcium sulphate dihydrate to drive off part of its water
The commercial process involves grinding gypsum and heating at 110-120°C.
What types of dental gypsum products are identified according to ISO standards?
- 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
True or False: Gypsum products are primarily used for making positive replicas of hard and soft tissues.
True
What is the compressive strength of dental plaster?
Approximately 9 MPa
Die stone has a much higher compressive strength of about 34 MPa.
What is the role of hardening solutions in gypsum materials?
They improve abrasion resistance and increase compressive strength
Hardening solutions are mixed with gypsum instead of water.
What is the chemical composition of gypsum?
Calcium sulphate dihydrate (CaSO4·2H2O)
Gypsum is a naturally occurring mineral used in various applications, including dentistry.
What is Plaster of Paris derived from?
Calcium sulphate hemihydrate (CaSO4·½H2O)
It is a gypsum product commonly used in dentistry.
What are the uses of dental gypsum materials?
- 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.
What is the final setting time for gypsum?
45-60 minutes
This is when the conversion of hemihydrate to dihydrate is complete.
What is the initial setting time for gypsum?
5-10 minutes
This is the interval during which the mix can no longer be poured.
How is initial setting measured?
- Loss of gloss
- Needle penetration test using Gilmore needles
These methods help determine when the material can begin to be handled.
What are the two types of Gilmore needles used in testing?
- Lighter Gilmore needle (initial setting)
- Heavier Gilmore needle (final setting)
The different weights help ascertain different stages in the setting process.
What methods can be used to manufacture calcium sulphate hemihydrate?
- Autoclaving gypsum
- Boiling gypsum in a salt solution
These methods yield different types of dental stone with varying properties.
What is the theoretical water-to-powder ratio for gypsum?
18.6ml of water: 100g of powder
This is the ideal ratio for complete hydration, though excess water is often needed for workability.
What is the water-to-powder ratio for soft plaster?
50-60ml of water: 100g of powder
This higher ratio is due to its porous nature.
What is the water-to-powder ratio for hard plaster?
20-35ml of water: 100g of powder
Hard plaster requires less water due to its denser structure.
What is the final product of the hydration of hemihydrate?
Gypsum model or die
This process is essential in creating accurate dental reproductions.
What is the difference in compressive strength between soft and hard plaster?
- Soft plaster: Low compressive strength
- Hard plaster: High compressive strength
This difference affects their respective applications.
What is the boxing technique used for?
To create a mould for gypsum by wrapping boxing wax around the impression
This technique helps contain the mixed gypsum during pouring.
What is the effect of humidity on gypsum materials?
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.
True or False: Hard plaster is always white.
False
Hard plaster is pigmented, while soft plaster is typically white.
What is the purpose of the needle penetration test?
To measure the ability of plaster to resist penetration and determine setting times
This test provides a practical way to assess material strength.
What is the importance of minimizing air inclusion during mixing?
To prevent porosities within the set material, which can weaken it
Air bubbles can lead to inaccuracies in the final model.
