Chapter 9: Gypsum Products

DHY 204

Chapter 9: Gypsum Products

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Gypsum

• Gypsum products: Fine powered that are mixed with water to form a fluid mass that can be poured and shaped and that subsequently hardens into a rigid, stable mass → Which in turn are used for making study models or casts, that are positive replicas of the oral cavity

• Cast: Positive reproduction (on which a restoration or appliance is fabricated)

  

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• Die: A working replica of a single tooth

  

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• Gypsum: Obtained from negative reproductions (alginate)

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Diagnostic Study Model Use

•  Patient education
• Treatment plan (especially for orthodontics)
• Tracking treatment progress
• Provides a legal record

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Working Cast Use

• Fabricating intraoral prosthesis directly and is more accurate than a study model

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Gypsum Formulation

• Gypsum is the dihydrate form of calcium sulfate

• CaSO4 . 2H2O

• 1 part calcium sulfate and 2 parts water

• Gypsum is commonly found in earth and has to be mined- in rock solid form, grounded up, add water, heated to high temperatures, the calicinated

• Heating process is called calcining

• Calcining: Heating gypsum to remove moisture

• Dihydrate → Hemihydrate

• Dihydrate (turns into power)

• Hemihydrate (after calcining)

• Hemi (half) part water

• Pure gypsum is white, but in most deposits, it is discolored by impurities

• Gypsum products are used in dentistry, medicine, homes, and industry. In homes, gypsum is used to make walls; in industry, it is used to make molds

• Three types of gypsum products are discussed in your chapter: plaster, stone, and high-strength or improved stone

• Chemically, all three are calcium sulfate hemihydrate, and they are produced as a result of heating gypsum and driving off part of the water of crystallization from the crystal

• Process is known as calcination

• Plaster, stone, and improved stone differ in the physical characteristics of their powder particles as a result of differing calcination methods → thus, making them suitable for various uses

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Plaster

• Beta-hemihydrate

• It is created through calcination using the open kennel technique

• Irregular shaped particles that need a lot of water to expand and grow

• First gypsum product created

• Weakest and least expensive out of the three types of gypsum products

• What is the chemical name of the gypsum products that is heated by the open kettle technique?

• Beta-hemihydrate

  

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Stone

• Alpha-hemihydrate

• In order to extract water out of stone, we use the steam pressure technique

• Stronger and more expensive than plaster & needs less water to mix  

• Water will attach very quickly because the stone crystals are smooth

  

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Chemical Reaction of Gypsum

• CaSO₄ . 2H₂O + heat → (CaSo₄)₂ . H₂O) Or… CaSO₄ . ½ H₂O) + 1 ½H₂O

• Gypsum rock is calcium sulfate dihydrate

• Gypsum end products: Plaster, stone, high-strength or improved stone

• As a result of calcination → calcium sulfate hemihydrate 

• What does the chemical name of calcium sulfate dihydrate stand for?

• Gypsum

• Calcium sulfate dihydrate can also be produced synthetically

• The chemical reaction of gypsum is not reversible

• Exothermic

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Types of Gypsum Products

• 1.) Dental or model plaster (white)
• 2.) Dental stone (typically yellow)
• 3.) High strength dental stone/die stone/improved stone
• Same chemically but physically different
• The differences between plaster, stone, and high-strength stone are the physical forms of the hemihydrate formed when the water is driven out, not the chemical make-up

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Plaster

• Dental plaster is the weakest

• Gypsum is heated in an open kettle:

• Porous

• Small, irregular particle shape

• Referred to as beta-hemihydrate (or Type II)

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Dental Stone

• Dental stone (usually yellow, can be obtained in other colors)

• Gypsum is heated under steam pressure in a closed container

• Larger, denser

• Less porous

• More uniform + more regular in particle shape

• Referred to as alpha-hemihydrate, (Type III), or Hydrocal

• High-strength dental stone → strongest (Type IV)

• Book mentions newly developed high-strength stone (Type V)

•  High-strength dental stone is heated under more pressure in calcium chloride solution + is further refined by grinding

• Very dense powder particle, cuboidal in shape with reduced surface area

• Least porous

• Most regular particle shape

• Most packing ability

• Often referred to Type IV stone, die stone, densite, or modified alpha-hemihydrate

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Physical Properties of Gypsum

• 1.) Strength + hardness (resistance to abrasion)
• 2.) Dimensional accuracy
• 3.) Reproduction of detail
• 4.) Solubility

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Physical Properties of Gypsum: Strength + Hardness

• 2 factors contribute to the strength + abrasion resistance of the final product:

• Shape of the particle

• Porosity

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Physical Properties of Gypsum: Dimensional Accuracy

• Setting expansion occurs during the hardening of all gypsum products (plaster expands most)

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Physical Properties of Gypsum: Reproduction of Detail

• Compatibility of the impression material will influence that quality of the surface reproduction (high strength has most detail → less bubbles)
• The greater the porosity of the final gypsum product, the less surface detail is produced

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Physical Properties of Gypsum: Solubility

• Solubility: The weight of the material dissolved into the water is the solubility of that material
• Solubility is directly related to the porosity of the material
• Plaster is highly soluble
• High-strength stone is low solubility

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Product Selection

• High-strength stone → for precision

• The desired physical properties + behavior necessary for a particular use determine the criteria for selection of a gypsum product

• For educational purposes primarily → plaster is the choice

• Fabricate container or bleaching tray → stone is the choice

• Precision for crown + bridge → hi-strength is the choice

• Plaster is frequently used for diagnostic casts + articulation of study casts

• Stone is ideal for making full or partial denture models, orthodontic models, and casts requiring high strengths + abrasion resistance: working casts

• High-strength Stone is used for fabricating wax patterns of cast restorations (different die colors)

• The colors of Gypsum:

• Plaster- White

• Stone- White/yellow

• High-strength stone: Light green/mauve

• High-strength high-expansion stone: Green/blue/yellow

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Water/Powder Ratio

• The properties of gypsum products are directly related to their W/P ratio

• The amount of water in millimeters added to 100 grams of powder

• 19 ml water/100 g powder (typical for improved stone)

• Plaster needs excess water for lubrication (evaporates off during drying)

• All three products require different required amounts

• Every gypsum product has a recommended W/P ratio

• Water is measured in a graduated cylinder

• Measure powder on a scale

• Add powder to water slowly

• A change in the W/P ratio affects:

• 1.) Setting time (rate-working time)

• 2.) Strength (hardness)

• 3.) Setting expansion (viscosity)

• Increasing W/P Ratio:

• 1.) Thinner mix

• 2.) Increase in setting time (more H₂O has to evaporate)

• 3.) Reduce the strength + hardness (leaves more air bubbles or voids)

• 4.) Reduce the setting expansion (a thinner less viscous mix)

• Decrease W/P Ratio:

• 1.) Thicker mix

• 2.) Decrease the setting time

• 3.) Increase strength + hardness

• 4.) Increase setting expansion (a thicker more viscous mix)

• Control of setting times:

• Alternating the W/P ratio

• Spatulation

• Temperature of water

• Accelerator and retarders

• Why would you want to put powder in water?

• Because there is a chemical reaction when the powder hits the water, you need to make sure you are mixing at all time

• What temperature should the water be?

• If you use cold water, it will delay the setting expansions. Hot water accelerates it. Water should be room temperature

  

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Mixing-Spatulation

• For hand spatulation, use flexible rubber bowl and stiff spatula

• Recommended: 2 revolutions per second for 1 minute

• Some use vibrator to help mixing + get air bubbles to surface → looking for sour cream consistency

• For mechanical spatulation, use a vacuum device

• An increase in the time & rate of spatulation has a definite effect on setting time & expansion

• An increase in spatulation time + rate:        

• Shorter (decreased) setting time

• Increase in setting expansion

• Strength (hardness) is thicker

• A decrease in spatulation time + rate:

• Longer (increased) setting time

• Decrease in setting expansion

• Strength (hardness) (thinner)

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Avoiding Bubbles in the Model

• To avoid bubbles in the model:

• 1.) Sift powder into water

• 2.) Vibrate the mix

• 3.) Controlled rate of spatulation

• 4.) Use a flexible bowl and spatula

• 5.) Vacuum mix

• 6.) Add small increments initially

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Setting Times

• Initial setting time (working time) → starts the moment W/P is being mixed until the setting mass reaches a semi-hard state

• The semi-fluid mixture is poured into the impression

• The “loss of gloss” can be used as a determination of the initial setting or working time; it is typically 5-7 minutes

• Final setting time → occurs when a chemical reaction is complete and the model is cool to touch

• The model can be removed from the impression

• A time of 30-45 minutes

  

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Variation in Setting Times

• Increased setting time (slower-setting product)

• Retarders

• Borax

• Blood

• Saliva

• Decreased setting time (faster-setting product)

• Accelerators

• Potassium sulfate

• Model trimmer water

• Sodium chloride

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Setting Expansion

• Plaster

• 0.2 - 0.3%

• Stone

• 0.08 - 0.1%

• Improved stone

• 0.05 - 0.07%

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Strength

• Wet strength

• Dry strength

• Factors affecting strength

  

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Other Properties of Gypsum Materials

• Surface hardness
• Dimensional stability

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Technique of Use

• Measuring the water

• Measuring the powder

• Adding powder and water

• Mixing

• Hand mixing

• Vacuum mixing

• Filling the impression

   

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Temperature

• Gypsum is ideally mixed with room temperature water (70 degree Fahrenheit)

• An increase in temperature will accelerate the setting time up to 100 degrees Fahrenheit

• Remember that this decreases setting time

• Temperature above 100 degrees Fahrenheit will retard setting time

• Above 212 degrees Fahrenheit (boiling point) → no reaction

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Accelerators & Retarders Review

• Accelerators

• 2% potassium sulfate

• Terra alba gypsum particles

• Model trimmer “slurry” water

• Sodium chloride (table salt)

• Retarders

• Borax

• Blood

• Saliva

• Alginate

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Hardening Solution

• Increase hardness + abrasion resistance

• Increase setting expansion

• Composed of:

• 1.) Water

• 2.) Colloidal silica

• 3.) Chemical modifiers

** \n Storage & Infection Control**

• Storage

• Gypsum should be stored in airtight, moisture proof containers because it can absorb moisture from the air, which can adversely affect the powder

• Infection control

• Models may be sprayed with iodophor, 1:10 sodium hypochlorite, or chlorine dioxide

• Preferable to disinfect impression because it is easier, more effective + limits the introduction of cross-contaminated items in the lab

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Chapter 9 Review Questions

1.) The desirable strength of gypsum is related to the amount of water used.

1. Directly
2. Indirectly
3. Not
4. Partially

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2.) For gypsum products, a suitable accelerator and retarder, respectively, would be:

1. Ethyl alcohol and oleic acid
2. Oleic acid and glycerin
3. Borax and potassium sulfate
4. Potassium sulfate and borax

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3.) The gypsum material known as “high-strength stone” may also be referred to as:

1. Plaster
2. Dental stone
3. Type lll stone
4. Improved stone

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4.) The final setting time in minutes for gypsum products typically is:

1. 15 to 30
2. 30 to 45
3. 45 to 90
4. 90 to 120

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5.) To make a correct mix for dental stone when using 50 g of powder, the amount of water would be approximately:

1. 10 to 12 mL
2. 14 to15 mL
3. 28 to 30mL
4. 45 to 50mL

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6.) Cathy and her parents will meet with an orthodontist to discuss an orthodontic treatment plan. At a previous appointment, the orthodontist took impressions of Cathy’s maxillary and mandibular arches. The replicas made from each impression to discuss the treatment plan are termed:

1. Casts
2. Dies
3. Study models
4. Molds

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7.) Decreasing the setting time of a dental material results in a product that:

1. Will set faster (takes less time)
2. Will set slower (takes more time)
3. Does not affect the setting time
4. Enhances the properties

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8.) The weakest gypsum product is:

1. Improved stone
2. Dental stone
3. Plaster
4. Die stone

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9.) When excess water is not present in a gypsum product, it is known as the:

1. Wet strength
2. Dry strength
3. Initial setting time
4. Surface strength

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10.) The best way to mix gypsum products is to: (according to manufacturer’s directions)

1. “Eyeball” the amount of powder added to water
2. Add water to the powder
3. Add powder to the water
4. Add powder and water to bowl simultaneously

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11.) Initial and final setting times of gypsum can be determined with the use of the Gillmore needles. A practical, easy method to determine final setting time in a dental office would be to:

1. Set a timer for 20 minutes
2. Observe the change from wet to dry strengths
3. Watch for the loss of gloss
4. Attempt to penetrate the material with the metal spatula

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12.) When the setting time of a dental material is increased, which of the following is true?

1. The material sets slower
2. The material sets faster
3. The setting reaction does not change
4. The setting reaction is increased

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*Answers are in the back of textbook*

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