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Questions and Answers
Which component is typically used as an accelerator in gypsum products?
What is the flexural strength of gypsum at 24 hours for Setting 2?
Which additive is classified as a retarder for gypsum setting?
For Setting 5, what is the compressive strength after 1 hour?
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What is the expansion percentage for Setting 3?
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In terms of detail reproduction, how does Setting 4 compare to Setting 3?
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What is the expected compressive strength at 24 hours for Setting 1?
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Which setting time corresponds to 0-0.15 minutes for Setting 1?
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What defines the working time of a gypsum product?
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Which reaction occurs when water is added to calcium sulphate hemihydrate?
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What characterizes the final set stage of a gypsum mix?
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Which type of dental gypsum product is classified as Type 5?
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What is the expected initial setting time for Type 1 dental impression plaster?
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In which stage of setting is the gypsum material still workable and can be shaped?
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What does the term 'setting reaction' refer to in the context of dental gypsum?
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Which factor affects the setting time of dental gypsum materials?
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Study Notes
Dental Gypsum
- Dental plaster is the B form of calcium hemihydrate.
- Dental stone is another form of calcium hemihydrate, it is stronger and more durable than plaster.
- W/P ratio refers to the ratio of water to calcium hemihydrate.
- Mixing time measures the time taken for powder to fully absorb water.
- Setting time measures the time from the start of mixing until the gypsum material fully hardens.
- Working time is the time allowed to work with the material before it sets.
- Setting reaction refers to the dimensional change that occurs over time as the gypsum sets.
Properties of a Good Impression Material
- Accuracy: it should reproduce all the details of the prepared teeth and surrounding tissues.
- Dimensional stability: it should not shrink or expand significantly during the setting process.
- Biocompatibility: it should not irritate the tissues of the mouth.
- Strength: it should be strong enough to resist fracture during manipulation and removal from the mouth.
- Flexibility: it should be flexible enough to be removed from the mouth without tearing or breaking.
- Taste and odor: it should not have an unpleasant taste or odor.
- Setting time: it should set quickly enough to be practical, but not so quickly as to interfere with the impression procedure.
Types of Dental Gypsum Products
- Impression plaster (Type 1): used for preliminary impressions for diagnostic models
- Dental plaster (Type 2): B form of calcium hemihydrate, used for study models and articulator models.
- Dental stone (Type 3) : stronger than plaster, used for making models for crowns, bridges, and other dental restorations.
- High strength, low expansion dental stone (Type 4): used for making dies for crowns and bridges.
- High strength, high expansion dental stone (Type 5) : used for making dies for crowns and bridges as they expand slightly when poured, which helps to compensate for the shrinkage that occurs during the setting of the resin.
Gypsum Reaction
- Calcium sulphate hemihydrate is dried in a kiln, water is given off as steam leaving calcium sulphate demihydrate.
- This is an exothermic reaction.
- The chemical equation for the drying process: CaSO
4·2H2O + ^HEAT^ → CaSO4·½H2O + 1½ H2O(steam) - The reverse reaction occurs when water is added to calcium sulphate demihydrate.
- The chemical equation for the setting reaction is: CaSO
4·½H2O + 1½ H2O → CaSO4·2H2O
Stages of Setting
- Fluid stage: the mixing stage, the gypsum is a fluid ready for pouring into a mold.
- Plastic stage: the gypsum is semi-glossy and still soft, it can be shaped and built up without folds or crumbling.
- Initial set: The gypsum is matte, it carves cleanly with a sharp blade and it is difficult to build up, it will crumble.
- Final set: the exothermic reaction ends and the gypsum reaches its maximum strength when dry.
Setting Time of Gypsum
- Setting time refers to the time from the beginning of spatulation to when the material fully hardens or the reaction is complete.
Factors Influencing Setting Time
- Water/Powder Ratio: Increasing the water/powder ratio slows down the setting time.
- Temperature: Increasing the temperature speeds up the setting time.
- Mixing Time: Longer mixing times can accelerate the setting time.
- Humidity: High humidity can slow down the setting time.
Classification of Dental Gypsum Products
- Type 1: Impression plaster - used for impression plaster
- Type 2: Dental plaster - B form of calcium hemihydrate
- Type 3: Dental stone - a form of calcium hemihydrate
- Type 4: Dental stone, high strength, low expansion
- Type 5: Dental stone, high strength, high expansion
Properties of Dental Gypsum Products
| PROPERT | TYPE | TYPE | TYPE | TYPE | TYPE | | Y | 1 | 2 | 3 | 4 | 5 | +===========+===========+===========+===========+===========+===========+ | Initial | 5-10 | 5-20 | 5-20 | 5-20 | | | setting | | | | | | | time (min) | | | | | | +-----------+-----------+-----------+-----------+-----------+-----------+ | Workin g | | | | | | | time (min) | | | | | | +-----------+-----------+-----------+-----------+-----------+-----------+ | Setting | 4 | 20 | 20 | 20 | 20 | | time (min) | | | | | | +-----------+-----------+-----------+-----------+-----------+-----------+ | Setting | 0-0.15 | 0-0.30 | 0-0.20 | 0-0.15 | 0.16-0.3 | | expansion | | | | | | | (%) | | | | | | +-----------+-----------+-----------+-----------+-----------+-----------+ | Compressi | 6 | 12 | 25 | 40 | 40 | | ve | | | | | | | strength | | | | | | | 1h (MPa) | | | | | | +-----------+-----------+-----------+-----------+-----------+-----------+ | Compressi | - | 24 | 70 | 75 | 75 | | ve | | | | | | | strength | | | | | | | 24h (MPa) | | | | | | +-----------+-----------+-----------+-----------+-----------+-----------+ | Flexural | 1 | 1 | 15 | 20 | 20 | | strength | | | | | | | 24h (MPa) | | | | | | +-----------+-----------+-----------+-----------+-----------+-----------+ | Detail | 75 | 75 | 50 | 50 | 50 | | reproduct | | | | | | | ion | | | | | | | (um) | | | | | | +-----------+-----------+-----------+-----------+-----------+-----------+
Accelerators and Retarders
- Accelerators are additives that increase the setting speed of gypsum.
- Examples of accelerators: sulfates and chlorides.
- Retarders are additives that slow down the setting process.
- Examples of retarders: acetate, borate, and tartrate.
Setting Expansion
Factors Affecting Setting Expansion
- Water/Powder Ratio: Increasing the water/powder ratio increases the setting expansion.
- Temperature: Increasing the temperature reduces the setting expansion.
- Mixing Time: Over-mixing can increase setting expansion.
- Type of Gypsum: Different types of gypsum have different levels of setting expansion.
Conclusion
- Different types of gypsum are used for different applications in dentistry, each with different properties and applications.
- The setting time and expansion of gypsum can be controlled by using accelerators and retarders.
- Understanding the properties and setting process of dental gypsum is crucial for creating accurate and durable dental restorations.
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Description
This quiz explores the properties and types of dental gypsum, including dental plaster and dental stone. It also covers the essential characteristics that make a good impression material in dentistry. Test your knowledge on the various aspects of gypsum setting times, working properties, and the evaluation criteria for impression materials.
