Week 8- Gypsum- Topic 16

Model (what we use) (def)

-replica used for observation
-ex. orthodontic study model

Cast (def)

-working model of more than one tooth ex. quadrant or arch
-fabricate an appliance or restoration on a cast
-must be strong and resistant to abrasion

Die (similar to cast, but single tooth) (def)

working replica of a single tooth or several teeth

Classification of Model Materials (3)

1-Gypsum Products (what we always use)
2-Metals
3-Resins

Gypsum Products (gen)

-Types I-IV
-Type I uses the most water and is the weakest
-Type IV uses the least amt of water and is the strongest

Gypsum Products- Type I

-(ADA Specification - Impression Plaster) Impression Plasters
-not used much any more, ie., edentulous impressions we discussed (ridgids)

Gypsum Products- Type II

-(ADA Specification - Model Plaster) Model or Lab Plaster
-Used for models or casts for demonstration(study models) or record keeping
-Usually white in color

Gypsum Products- Type III

-(ADA Specification - Dental Stone) Class I Stone or Hydrocal (good for bleaching trays)
-Naturally is white in color. Manufacturers color the product (ie., buff) to distinguish it
-Used for working models such as casts of full-arch impressions
-Casts of

Gypsum Products- Type IV

-(ADA Specification - Die Stone) Class II Stone, Densite or Improved Stone
-Hardest and strongest of all the products
-Used for stone dies to construct appliances and restorations such as gold inlays, crowns and bridges
-Often colored green or pink

Metal Products (gen)

-Less accurate than high-strength dental stone, but tougher and more abrasion resistant
-3 Types
---Electroplated Copper
---Electroplated Silver
---Low-fusing Alloys

Metal Products-Electroplated Copper

-makes outside of teeth very strong
-compound impressions or addition silicone rubber impressions
-surface of impression is electrically plated with copper
-acrylic resin, dental stone or a low-fusing metal is added to electroplated shell - then impressio

Metal Products-Electroplated Silver

-rubber-base impressions such as polysulfide, polyether, and some addition silicone rubber impressions
-surface of impression is electrically plated with silver
-acrylic resin, dental stone or a low-fusing metal is added to electroplated shell - then impr

Metal Products-Low-fusing Alloys

...

Resin (Epoxy Dies) (gen)

-Crowns, bridges, inlays epoxy dies - fabricated using polyether, polysulfide, silicone rubber (coating teeth with resin)
-Tougher and more abrasion resistant than high strength stone dies, but are not as accurate or as dimensionally stable
-Harden very s

Determination of Material Used

-Varies with impression material and purpose for which replica is to be used
-Often chosen because of requirements of a particular clinical situation
-Examples
---hydrocolloids - gypsum only
---polysulfide - gypsum, electroplated silver, epoxy
------ a) d

Chemical Composition of Gypsum (gen)

-All forms of gypsum materials are obtained from the mineral gypsum
-Gypsum is the dihydrate (2 waters) of calcium sulfate:
---Calcium Sulfate Dihydrate (2CaSO4 2H2O)
-Naturally occurs as one of two forms:
---Rock gypsum
---Alabaster

Chemical Composition of Gypsum- Rock gypsum

-a dull-colored rock
-common

Chemical Composition of Gypsum-alabaster

-compact, fine-grained, white & translucent mineral
-not as common as rock-gypsum

What is the most common type of gypsum used?

rock-gypsum

Chemical Composition of Plaster Type II

Plaster (AKA Plaster of Paris, Type II Gypsum)

Chemical Composition of Dental Stone - Types III and IV

-Calcium Sulfate Hemihydrate (CaSo4)2 H2O -All three have the same chemical formula, but the method of removing part of the water of crystallization is different for each form
-Basically, each is formed when gypsum is calcined (heated) to remove water (st

Manufacture of Plaster (AKA beta-hemihydrate,
B-hemihydrate)

-Gypsum is mined
-The gypsum is calcined in large, open vats (heated) 230-250 degrees F, 115 degrees Celsius
-reaction: (2 waters)
-2CaSo4x 2H2O + heat-> (CaSo4)2xH2O+3H2O
gypsum plaster. (plaster+steam)
-calcium sulfate dihydrate-->beta hemihydrate
-The

calcium sulfate dihydrate-->beta hemihydrate
(formula)

2CaSo4x2H2O + heat-->(CaSo4)2xH2O +3H2O
gypsum plaster (plaster+steam)

Reversing the Chemical Reaction
(formula)

(CaSo4)2xH2O+3H2O-->2CaSo4x2H2O + heat plaster water steam gypsum. exothermic e

Reversing the Chemical Reaction

-Note that heat is also a product of the reaction (exothermic heat of reaction)
-The amount of heat given off during the reaction is exactly the same amount of heat required to form the hemihydrate from the dihydrate
-Gypsum precipitates from the solution

Re: the chemical rxn, the crystals will continue to grow in size until all the plaster has what?

turned to gypsum

The centers of crystal growth are called ________ of crystallization

nuclei

Manufacture of Dental Stone (AKA alpha-hemihydrate -hemihydrate) (gen)

-The gypsum is heated under pressure and in the presence of water vapor at 125 degrees C
-If the gypsum is dehydrated in a solution of calcium chloride, a smoother crystal type is formed - it is also referred to as -hemihydrate
-Characteristics
---crystal

Manufacture of High-Strength Dental Stone (gen)

-Gypsum is boiled in a 30% solution of calcium chloride
-Characteristics
---crystals are the least porous of the three types of hemihydrate particles

Re:Manufacture of High-Strength Dental Stone, the crystals are the ________ (most/ least) porous of the three types of hemihydrate particles

least

Properties of Gypsum Products:
Water/Powder Ratio

-The amount of water used can affect the strength and hardness of the gypsum
-As water increases, the fewer crystals per volume and a resultant decrease in strength
-The minimum amount of water that can be used must be enough for a sufficient chemical rea

The amount of _______ used can affect the strength and hardness of the gypsum

water
-As water increases, the fewer crystals per volume and a resultant decrease in strength

The minimum amount of water that can be used must be enough for a sufficient _________ __________

chemical reaction

Additional water, beyond the minimum, is needed to insure _________ of all the particles. This water is referred to as "free water

wetting

It takes less water (a lower water/powder ratio) to mix ______ _______ (compared to plaster) because of the porous nature of the B-hemihydrate particles in plaster

dental stone

Properties of Gypsum Products: Setting Time (gen)

-The higher the number of gypsum particles left in the plaster the shorter the setting time (it takes less time to set)
-Setting time decreases with an increase in the number of gypsum particles (too much powder)
-The higher the amount of water (an increa

Re:Properties of Gypsum Products: Setting Time,-The higher the number of gypsum particles left in the plaster the _______ (shorter/ longer) the setting time

shorter

Re:Properties of Gypsum Products: Setting Time,Setting time ________(increases/ decreases) with an increase in the number of gypsum particles (too much powder)

decreases

Re:Properties of Gypsum Products: Setting Time,The higher the amount of water (an increased water/powder ratio)* the _______ (longer/shorter) the setting time

longer

Re:Properties of Gypsum Products: Setting Time, As amount of water increases, the setting time __________ (increases/ decreases)

increases

Re:Properties of Gypsum Products: Setting Time, The longer and more rapidly the product is mixed the ________ (shorter/ longer) the setting time

shorter

Re:Properties of Gypsum Products: Setting Time, An increase in water temperature causes a _________ (increase/ decrease) in setting time

decrease

Alter Setting Time- Accelerators

-Shortens setting time
---Potassium sulfate
---Sodium chloride (in small amounts)

Alter Setting Time- Retarders

-Lengthens setting time
---Borax
---Sodium Citrate

Properties of Gypsum Products: Setting Expansion (gen)

-All gypsum products expand during setting
-Expansion will lead to distortion of the cast
-Hygroscopic setting expansion refers to the increased expansion experienced by the hemihydrate allowed to set under water
-Chemical additions can reduce expansion a

Chemical additions can reduce expansion although not ________ it

eliminate

Properties of Gypsum Products: Strength of Gypsum Product (gen)

-The product must be able to withstand fracture or distortion during use (ie., fabrication of a dental appliance)
-Strength is determined under compressive stress
-The higher the water/powder ratio the weaker the product [Therefore, we know that dental pl

Re: Properties of Gypsum Products: Strength of Gypsum Product, product should not sit for more than ______ hour(s)

1

Properties of Gypsum Products: Hardness and Abrasion (gen)

-There is a direct relationship between the hardness and strength of gypsum products (the factors that increase or decrease strength generally do the same for the hardness)
-Abrasion resistance is an important property to consider in certain dental proced