Gypsum products and investment

Most gypsum products are optained from natural gypsum rock. It’s the dihydrate form of Calsium sulfate ( CaSo4.2H2O) On heating , it loses 1.5 g mol of its 2 g mol of water and is converted to calsium sulfate hemihydrate ( CaSo4. ½ H2O). When hemihydrate form mixed with water , the reverse reaction takes place. CaSo4. ½ H2O+ 11/2 H2O CaSo4.2H2O+ 3900 cal/g mol Hemihydrate form called β-calsium sulfate hemihydrate

Hydrocal: gypsum 125 °c + pressure Plasters : gypsum 110-120 ° c β-calsium sulphate hemihydrate İrreguler shape and porous Modelling and lab. Plasters Hydrocal: gypsum 125 °c + pressure Uniform in shape and denser than plasters α-calsium sulphate hemihydrate Densite: gypsum boiling in a %30 calsium chloride High strength dental stone

Types of gypsum Plasters Low strength dental stone High –strength / low expansion dental stone High –strength / high expansion Alternatively as type 2,3,4 and 5 in ADA spesification no,25

Ideal properties of die materials Dimensional accuracy. Ease of use. Accurate reproduction of fine detail. Inert. Good strength. Inexpensive. Abrasion resistant. Good color contrast

Chemical Reaction 100 g plaster + 18.6 g water = 118 g calsium sulfate dihydrate 100 g plaster + 45g water = 45- 18.6 = 26.4

Recommended Ratios of Water to Powder for Gypsum Products

Methods to Accelerate and Inhibit the set ACCELERATE_ speed Potassium sulfate Sodium chloride(salt) Slurry- from unclear bowl or spatula HEAT INHIBIT slow Borax Hydrocolloid gels Sodium citrate BLOOD COLD

Effect of Spatulation . When the chemical reaction starts, and some calcium sulfate dihydrate is formed. During spatulation the newly formed c.s.d. Breaks down to to smaller crystals and starts new centers of nucleation An increase in the amount of spatulation (either speed or time or both) causes more nuclei centers to be formed and that shortence the setting time.

Effect of Temperature Increasing the temperature from room temperature of 20° c to 37° c increases the rate of the reaction slightly and shortence the setting time . However , as the temperature is raised over 37°c the rate of reaction decreases.and the setting time is lengthened. At the 100°c the solubilityies of dihydrate and hemihydrate are equal . In which case tno reactiıon occures.and plaster does not set.

Effect of colloidal systems Colloidal systems such as agar and alginate retard the setting of gypsum products. If these materials are in contact with calsium sulfate hemihydrate during setting, a soft , easily abraded surface is optained. Acceleratores such as potassium sulfate are added to improve the surface quality of the set against agar or alginate.

Effect of Humidity When the relative hummidity increases to %70 and above , moisture in the air can cause some coversion of hemihydrate to dihydrate Because dihydrate crystals can accelerate the reaction by providing more nuclei for crystalization, the initial result is acceleration of setting.

Properties and behaviour Chemical properties: (exothermic reaction) Ca sulfate dihydrate heat water loss ground Ca sulfate hemihydrate The production of various types of gypsum is basically the same: If the heating process occurs in atmospheric pressure at 115°C, the powder particles are porous and irregular (b hemihydrate, plaster). if heating process is under pressure, powder particles are regular and less porous (a hemihydrate, or dental stone).

A further increase in pressure and refining of the powder by grinding results in denser stone, high-strength or die stone When stone is mixed with silica: dental investment, a material that can withstand high heat and stress produced when molten metal is forced into molds to form indirect restorations

Gypsum products Plaster of Paris Dental stone Die stone

Physical properties Type W/P Porosity Compressive strength Abrasion resistance Setting expansion plaster 0.45 High 8.8 MPa Low Stone 0.30 moderate 20.6 MPa Moderate High strength stone 0.23 34.3 MPa High strength High-expansion stone 0.20 48.0 MPa

Physical properties (cont.) Strength and hardness: Affected by Porosity Shape of particles (large irregular particles don’t condense well leading to decreased density). Amount of water mixed with the powder. Higher porosity requires more water to be mixed to produce a cast which will be weaker.

Dimensional accuracy: The higher the setting expansion, the lower the accuracy. Setting expansion results from the growth of crystals as they join. Solubility: Set gypsum is not highly soluble The greater the porosity the greater the solubility.

Reproduction of detail: Greater porosity decreases surface detail production The impression material should be compatible with the gypsum to improve detail reproduction. The best compatibility is between silicon and polyethers and gypsum

Classification of gypsum Impression plaster (Type I) *seldom used* Model plaster (Type II): used for Diagnostic cast Articulation of stone cast Art portion of working cast Flasking procedure for acrylic dentures (cast end) The mix produces a weak cast compared to dental stone. It’s available is fast and regular sets. Impression (negative replica), poured into gypsum to make a cast (positive replica

Plaster Plaster Flask

Dental stone (Type III): used for making Full or partial denture models Orthodontic models Flasking procedure for acrylic dentures (teeth end) It requires less water, stronger than plaster Dental stone casts

Dental stone, high strength (Type IV): AKA die stone used in fabricating wax patterns of cast restorations (crown &bridge) Implants Precision attachments work Often colored pink or green Almost 2 times stronger than type III stone Die stone used in the fabrication of cast crown restoration

High strength, high expansion dental stone (Type V) Colored blue or green Most costly of all gypsum materials Lowest W:P ratio, higher compressive strength The need for higher expansion (0.1%) was to compensate for the solidification shrinkage of some alloys used as base metals used for dental casting

Manipulation Selection: based on the desired properties and dental application. e.g.: For a diagnostic cast, plaster can be used. For a working cast, strength is required and accuracy, dental stone is the gypsum of choice Working models for cast restorations require die stone

Manipulation (cont.) Proportioning (W:P ratio) Golden rule: Manufacturer instructions should always be followed. Variations in W:P ratio affect the set materials’ properties such as strength and accuracy. Too much water? Too little water? Too much water, runny mix, weaker cast Too little water, mix is difficult to manipulate, does not produce a flowable mix.

Manipulation (cont.) Mixing: Manual: Rubber bowl and spatula. Powder is sifted into water to ensure good wetting and avoid clumps, and avoid air bubbles Strong vigorous mixing motion against sides of the bowl. Mix for 1-2 minutes until mix is smooth, homogenous and glossy.

Mechanical (also used with vacuum)

Manipulation (cont.) Initial setting time-working time Working time start after mixing for 1 minute Initial setting time: time elapsed from the start of mix until loss of gloss (8-10 minutes) 6-10 minutes of working time are available to pour the gypsum.

Final setting time: is reached when the materials can be safely handled, the gypsum is cool (exothermic reaction is over). Setting expansion: Results from crystal growth during setting. Can be decreased by the addition of potassium sulfate, sodium chloride, borax. Hygroscopic expansion. If gypsum soaked during setting, water fills pores and increases volume It’s recommended to separate the cast from impression after 1 hour. Strength increases 2-3 times after 24 hours

Tests for initial setting, and final setting times Loss of gloss test for initial setting time: loss of gloss occurs as water is taken up by gypsum to form the dihydrate. The materials does not have measurable compressive strength. Initial Gillmore test for initial set: needles are used to indent the material until no indentation can be seen = initial setting time.

A Vicat penetrometer (A) for setting time: used for the next stage of setting (refer to figure) after loss of gloss, the plunger rod is released onto the mix. Time elapsed until the rod no longer penetrates is the setting time. Gillmore test (B) for final setting time: a heavier Gillmore needle to determine final setting time. B B

Ready for use criteria The ability to judge readiness of gypsum to be handled improves with experience. Technically, the material is considered ready when compressive strength reaches 80% of the strength attained after 1 hour. Most products are ready to use in 30 minutes.

before separating cast from impression, ensure that no part of the tray is connected to the gypsum if alginate impression dried before cast separation, soak in water for 15 minutes.

How to control setting time Changing water : powder ratio Increasing water Decreasing water Retarded setting Shorter setting time Weaker model or cast Mix difficult to manipulate Inaccurate model Bubbles inclusion in mix --------

Spatulation: rapid and prolonged spatulation accelerates setting and also increases setting expansion. Temperature: increasing water temperature to a certain level will accelerate setting. Water temperature above 100°F will retard setting, above 212°F gypsum will not set.

Accelerators and retarders: Manufacturers add accelerators or retarders to gypsum. These chemicals increase or decrease gypsum solubility respectively which will alter setting time accordingly. Clinicians can add accelerators such as potassium sulfate or set gypsum (slurry water), they act as sites for crystallization. Setting reaction retarders: blood, saliva, alginate. If left on impression, can affect surface details of impression. Impression surface need to be properly rinsed before being poured.

Pouring of the cast: the cast is composed of two parts which are prepared separately The anatomical part (hard and soft tissue), impression poured using a vibrator Art portion or base, which is important to aid in handling and articulating the casts. Can be poured in different ways:

Double-pour method 1 2 3

Single step Both anatomical and art portions of the cast are prepared at the same time. This method requires skill and accurate timing. Difficulty encountered: If mix is too runny? If mix started to initially set?

Boxing method A strip of wax is fitted around the impression then gypsum is poured. The wax border should extend at least 0.5 inch above the highest point of the impression.

Storage and clean up Gypsum should be stored in airtight dry area. Prolonged exposure to moisture can retard setting due to decreased solubility of powder. Relevant equipment should be kept clean to avoid unwanted acceleration of setting by set gypsum.

Infection control Casts should have set for 24 hours before being disinfected if necessary. Spray rather than immerse Disinfectants commonly used: Sodium hypochlorite iodophors Chlorine dioxide

Trimming Plaster bases are recommended since trimming them is easier than dental stone. If base is made from stone, it should be soaked in water for 5-10 minutes to soften it before trimming. Important considerations when trimming?

Trimming considerations Proportion of base to anatomical part Parallelism Use of wax bite registration Outer border of cast Shaping of anterior part of upper and lower arches

Other types of Die stone Metal plated die stone: silver or copper plated to increase abrasion resistance Epoxy die stone: resin and a hardener are used to increase abrasion resistance. Resin reinforced die stone: resin is incorporated into the gypsum material to increase abrasion resistance.

Investment materials Used to form metal casting through the lost wax technique

References Dental materials, clinical applications for dental assistants and dental hygienists. Chapter 12 Phillips’ science of dental materials. Chapter10