DENTAL PLASTER

ACKNOWLEDGMENT

We would like to express our special thanks of gratitude to our teacher Dr. Sanjeev Joshi and Dr. Parveen who gave us the golden opportunity to do this wonderful project on the topic dental plaster, which also helped us in doing a lot of research and we came to know about so many new things we am really thankful to them.

Secondly we would also like to thank our parents and friends who helped us a lot in finalizing this project within the limited time frame.

INDEX

1. INTRODUCTION: GYPSUM PRODUCTS2. ITS USES IN DENTISTRY3. TYPES OF GYPSUM PRODUCT4. INTRODUCTION: DENTAL PLASTER5. PRODUCTION6. SETTING REACTION7. GRAPH8. MANIPULATION9. WORKING TIME10.SETTING TIME AND MIXING TIME11.TESTS FOR SETTING TIME12.CONTROL OF SETTING TIME13.MODIFIERS FOR SETTING TIME14.STRENGTH15.COMMERCIALY AVAILABE DENTAL PLASTER16.REFRENCES

INTRODUCTION: GYPSUM PRODUCTS

• Gypsum (CaSO4.2H2O); (Calcium Sulfate Dihydrate) is a mineral mined in various parts of the world.

• Gypsum products are supplied as fine hemihydrate powders that are produced by heating ground gypsum particles.

• After mixing with water, the mixture reverts back to gypsum, this unique property of gypsum products has led to numerous applications through history.

• Gypsum and gypsum products are used today for many applications including building construction, soil conditioning, food additives, pharmaceuticals, medical devices and dental applications.

USES OF GYPSUM PRODUCTS IN DENTISTRY •Gypsum products are used in dentistry for the preparation of study models for oral and maxillofacial structures.

•Various types of dental plaster are produced, modified for specific property requirements and used to form molds and casts on which dental prostheses and restorations are constructed.

• Plaster mixed with fillers is known as gypsum dental investment. Such dental investment are used to form molds for the casting of dental restorations with molten metal.

•It is also used as an important auxiliary materials for dental laboratory operations that are involved in the production of dental prostheses.

• To mount cast on articulator.

• For bite registration.

• For dental investment

TYPES OF GYPSUM PRODUCTS1. Impression plaster (type I)

2.Dental plaster (type II)

3.Dental stone (type III)

4.Dental stone, high strength (type IV)

5.Dental stone, high strength, high expansion (type V)

IMPRESSION PLASTER•These impression materials are composed of plaster of Paris, to which modifiers have been added to regulate the setting time and the setting expansion.

• Impression plaster is rarely used anymore for dental impression.

• Its has been replaced by less rigid materials, such as the hydrocolloids and elastomers.

• Plaster is primarily restricted to use as a final impression in the construction of full dentures.

DENTAL PLASTER• Dental Plaster or laboratory Type II plaster is the beta form of calcium sulfate hemihydrate (CaSO4 .1/2H2O)

• A mixture of Plaster of Paris and water is placed in an impression tray and pressed against the tissues of, for example, a patient’s edentulous jaw. The plaster is allowed to harden or set and the impression is taken

• It is now used to fill as flask used in denture construction when setting expansion is not critical and the strength is adequate according to limits cited in the ADA specification or ISO standard.

• It is usually marketed in the natural white color, thus, contrasting with stones, which are generally colored.

DENTAL PLASTER

POWDER PARTICLES OF BETA- HEMI HYDRATE

DENTAL STONE

• Dental Stone or laboratory Type III plaster is the alpha form of calcium sulfate hemi hydrate (CaSO4. ½ H2O).

• It is also called high-strength stone. Here, the gypsum is calcined by boiling it in 30% calcium chloride solution.

• It is intended for the construction of casts in the fabrication of full dentures to fit soft tissues.

• Type III stones are preferred for casts used to process dentures because the stone has enough strength for this purpose

DENTAL STONE

DENTAL STONE

POWDER PARTICLES OF ALPHA-HEMIHYDRATE

DIFFERENCE BETWEEN DENTAL PLASTER AND

DENTAL STONE

DENTAL STONE, HIGH STRENGTH• The principal requisites for a die material are strength, hardness and minimal setting expansion. To obtain these properties, modified alpha hemi hydrate.

• The cube-shaped particles and the reduced surface area produce such properties without undue thickening of the mix.

• This material is also called die stone.

• A hard surface is necessary for a die stone because the tooth preparation is covered with wax and carved flush with the margins of the die.

• The average dry surface hardness of the die stone is approximately 92.

DIE STONE

DENTAL STONE, HIGH STRENGTH, HIGH EXPANSION

• The gypsum product exhibits an even higher compressive strength than the type IV dental stone.

• Higher expansion is required in the stone die to aid in compensating for the alloy solidification shrinkage.

• One should avoid the use of type V stones for producing dies for inlays and on lays since the expansion may lead to an unacceptably tight fit.

POWDER PARTICLES OF TYPE IV AND TYPE V DENTAL STONE

PROPERTIES OF GYPSUM PRODUCTS

PRODUCTION• These materials are produced by calcining calcium sulfate dihydrate (gypsum).

• The gypsum is ground and subjected to temperatures of 110 C to 130 C ( 230 F ͦ ͦ ͦto 266 F) in open containers to drive off part of the water of crystallization. This ͦcorresponds to the first step the reaction.

• The principal constituent of gypsum- based products is calcium sulfate hemihydrate (CaSO4.1/2 H2O)

• The resulting particle is a fibrous aggregate of fine crystals with capillary pores known as plaster of paris or dental plaster in dentistry.

• As the temperature is further raised, it becomes an anhydrite. This process is known as calcination.

• Calcination Process:

CaSO4.2H2O 110 – 130 C ͦ CaSO4. ½ H2OGypsum (calcium sulfate

dihydrate)Dental Plaster (calcium sulfate

hemihydrate)

SETTING REACTION

• There are three theories of gypsum setting.

• The colloidal theory proposes that, when mixed with water, hemihydrate enters into the colloidal state through a sol-gel mechanism. In the sol state, hemihydrate particles are hydrated to form dihydrate, thereby entering into an active state. As the measured amount of water is consumed, the mass converts to a solid gel.

• The hydration theory suggests that rehydrated plaster particles unite through hydrogen bonding with sulfate groups to form the set material.

• The most widely accepted mechanism is the dissolution-precipitation theory, which is based on dissolution of the hemihydrate particles in water followed by instant recrystallization to the dihydrate. This reaction has become possible because the solubility of hemihydrate in water is four times greater than that of the dihydrate near room temperature.

CaSO4. ½H2O +3H2O 2 CaSO4.2H2O + unreacted (CaSO4)2.2 H2O + HEAT

Calcium sulfate hemihydrate Calcium sulfate dihydrate

REACTION :

MANIPULATION OF DENTAL PLASTER:

PROPORTIONING• The strength of a plaster is inversely proportional to the W/P ratio, it is important to keep the amount of water as low as possible.• However, it should not be so low that the mix will not flow into every detail of the impression.• Once optimum W/P is determined the same proportions should be used subsequently.• The water and powder should be measured by using an accurate graduated cylinder for the water volume and a weighing balance for the powder.• The powder should not be measured by volume, because powder varies from product to product and does not pack uniformly. Powder will pack harder as the container remains unused.• If the container is shaken, the volume increases as a result of entrapment.• Pre-weighed envelopes, have become popular, because they promote accuracy waste, and save time..

W/P RATIO

• The ratio of the water to the hemihydrate powder is usually expressed as W/P Ratio. The ratio is usually abbreviated as W/P .

• If 100 g of plaster is mixed with 60 ml of water, the W/P ratio is 0.6.

• The W/P ratio is important factor in determining the physical and chemical properties of the final gypsum product.

• As the W/P ratio increases, the setting time increases, the strength of the gypsum product decreases, and the setting expansion decreases.

• Although the W/P ratio varies for the particular brand of plaster the following are some typical recommended ranges. Type II plaster, 0.45 to 0.50.

• Using plaster or stone that is supplied in preweighed bags sometimes results in mixes that are too thick or too thin.

VARIOUS EQUIPMENT REQUIRED FOR DENTAL PLASTER PROPORTIONING

MIXING AND POURING

• If mixing is performed by hand, the bowl should be parabolic in shape, smooth and resistant to abrasion.• The spatula should have a stiff blade and a handle that is convenient to hold.• Entrapment of air in the mix must be avoided to prevent porosity, leading to weak spots and surface inaccuracies.• Place a measured amount of water in the bowl, and shift the weighed powder into the water as initial hand mixing is performed.• Then stir the mixture vigorously, periodically wiping the inside of bowl with the spatula to ensure the wetting of all of the powder and the breaking up of any agglomerates, or lumps.• Continue mixing until a smooth mix is obtained, usually within a minute. A• We must add the measured water first, followed by gradual addition of preweighed powder.• The powder is incorporated during approximately 15 sec of mixing with a hand spatula, followed by 20 to 30 sec of mechanical mixing under vacuum.

STEPS OF MANIPULATION

MIXING OF DENTAL PLASTER

CUT SECTION OF A IMPROPERLY MIXED DENTAL PLASTER CAST

CUT SECTION OF PROPERLY MIXED DENTAL PLASTER CAST

CARING OF THE CAST• Dimensions of cast will be relatively constant under ordinary conditions of room and temperature and humidity.• However, it is sometimes necessary to soak the gypsum cast in preparation for other procedures.• When a dry cast is immersed in water, negligible expansion may occur if the water is saturated with calcium sulfate.• The safest method for soaking the cast is to place it in a water bath with gypsum debris remaining on the bottom of the container to provide a saturated solution of calcium sulfate.• It is not safe to store or heat or heat a stone cast in air at a temperature higher than 55 C ͦ(133 F) ͦ

INFECTION CONTROL• The possible cross-contamination of dental office personnel by microorganisms, including Hepatitis B virus, HIV via dental impressions has prompted study of the effect of spray and impression disinfecting techniques on impression materials.

• Disinfection solution can be used that do not adversely affect the quality of gypsum cast. Alternatively disinfectants can be incorporated either in the powder or dissolved water.

• The same microwave used in drying gypsum casts has also been evaluated in infection control. One study that a 5-min irradiation at 900 W reduced microorganisms counts.

• The widespread availability of a spectrum of disinfected dental plaster casts with proven efficacy and unimpaired physical properties will undoubtedly strengthen the barrier system of infection control in the dental laboratory.

MIXING TIME (MT)

Mixing time is defined as the time from the addition of the powder to the water until the mixing is completed. Mechanical mixing of plaster is usually completed in 20 to 30 seconds, hand spatulation generally requires at least a minute to obtain a smooth mix.

WORKING TIME (WT)

Working time is the time available to use a workable mix, one that maintains a uniform consistency to perform one or more tasks. It is measured from the start of mixing to the point where the consistency is no longer acceptable for the product’s intended purpose. For example, sufficient working time might be needed to pour an impression, pour a spare impression, and clean the equipment before the gypsum fully sets. Generally, a 3 min working time is adequate.

SETTING TIME (ST)

The powder is mixed with water and the time that elapses from the beginning of mixing until the material hardens is known as setting time. This is usually measured by some type of penetration test using the instruments. A number of stages occur in the setting of gypsum product. By use of an actual strength test on an dental plaster. In this figure 1 min is indicated for the mixing time (MT), with an additional 3 min for the working time (WT), that is, pouring into an impression

TESTS FOR SETTING TIME (ST)LOSS OF GLOSS TEST FOR INITIAL SET:

As the reaction proceeds, some of the excess water is taken up in forming the dihydrate so that the mix loses its gloss. This loss of gloss at approximately 9 min, at which the mass still had no measurable compressive strength. Therefore it could not be safely removed from the mold.

GILLMORE NEEDLE

GILLMORE TEST FOR INITIAL SET

•There are two types of Gillmore needles are shown. •The smaller one is more frequently used for the setting time of dental cements, but it is some times used on gypsum products. • The mixture is spread out, and the needle is lowered onto the surface. The time at which it no longer leaves an impression is called initial set.• This event is marked by a definite increase in strength.

VICAT TEST FOR SETTING TIME

• The next stage in the reaction is determined by use of another instrument, the Vicat penetrometer.• The needle with a weighted plunger rod is supported and held just in contact with the mix.• Soon after the gloss is lost the plunger is released. The time elapsed until the needle no longer penetrates to the bottom of the mix is known as the setting time.

VICAT NEEDLE PENETROMETER

GILLMORE TEST FOR FINAL SETTING TIME

The next stage in the setting process is measured by the use of the heavier Gillmore needle. The elapsed time at which this needle leaves only a barely perceptible mark on the surface is called the final setting time.

COMPRESSIVE STRENGTH OF A TYPE II MODEL PLASTER DURING SETTING

CONTROL OF THE SETTING TIME

Setting time must be controlled for different applications.Theoretically at least three methods can achieve such control.

1. The solubility of the hemihydrate can be increased or decreased. For example if the solubility of the hemihydrate increased, super saturation of the calcium sulfate increases, and the rate of crystalline deposition also increased.

2. The number of nuclei of crystallization can be increased or decreased. The greater the number nuclei of crystallization, the faster the gypsum crystals form and the sooner the hardening of the mass occurs because of crystalline intermeshing.

3. The setting time can be accelerated or retarded by increasing or decreasing the rate of crystal growth, respectively

Thus the operator can vary the setting time within reason by changing the W/P ratio and mixing time

IMPURITIESIf the calcination is not complete and gypsum particles remain, or if

the manufacturer adds gypsum, the setting time is shortened because of the increase in potential nuclei of crystallization.

FINENESSThe finer the particle size of the hemihydrate, the faster the mix

hardens. Not only is the rate of the hemihydrate dissolution increased, but the gypsum nuclei are also more numerous. Therefore a more rapid of crystallization occurs

W/P RATIOThe more water used for mixing, the fewer nuclei there are per unit volume. Consequently, the setting time is prolonged.

MIXINGWithin practical's limit, the longer and more rapidly the plaster is mixed the shorter is the setting time

TEMPERATURELittle temperature change occurs between 0 C (32 F) and 50 C (120 ͦ ͦ ͦ

F). If the temperature of the plaster-water mixture exceeds 50 C ͦ ͦ(120 F) ͦa gradual retardation occurs. As the temperature approaches 100 C ͦ(212 F), no reaction takes place. ͦ

EFFECT OF W/P RATIO AND MIXING TIME ON SETTING TIME

MODIFIERS FOR SETTING TIME

RETARDERS• Retarders generally act by forming an adsorbed layer on the hemihydrate to

reduce its solubility and on the gypsum crystals present to inhibit growth.• Organic materials, such as glue, gelatin, and some gums, borax, potassium citrate and sodium chloride behave in this manner.•If the chemical added modifiers increases the setting time, its is known as retarder.

ACCELERATORS• The most effective and practical method for controlling the setting time is the

addition of certain chemical modifiers to the mixture of plaster.• If the chemical added modifiers decreases the setting time, its is known as accelerator.• The us of cooler water will not have a significant effect on the setting time. Thus, to ensure the working time , one should purchase a regular setting product and/or use minimum mixing times and slower rates of mixing.

COMPRESSIVE STRENGTH ON ADDITION OF

ACCELERATORS AND RETARDERS

STRENGTH

• The strength of gypsum products is generally expressed in terms of compressive strength, although tensile strength should also be considered in order to secure a satisfactory guide to the total strength characteristics.• Wet strength: It is the strength obtained when the water in excess of that required for hydration of the hemihydrate is left in the specimen.• Dry strength: When the excess water in the specimen has been driven off by drying, the strength obtained is dry strength.

COMPRESSIVE STRENGTH AS A FUNCTION OF W/P RATIO FOR THE FIVE TYPES OF GYPSUM PRODUCT

SETTING EXPANSION• Setting expansion is due to the outward thrust of growing crystals. It is observed based on the mechanism of crystallization i.e., The process of outgrowth of crystals from the nuclei of crystallization .

• Setting expansion may range from 0.06%-0.5%• It is of two types,

• Normal setting expansion• Hygroscopic setting expansion

• Normal setting expansion refers to the setting expansion when a gypsum product is allowed to expand in air.• Hygroscopic setting expansion refers to the expansion of a gypsum product when it is allowed to set, immersed in water.

DIFFERENCE BETWEEN NORMAL & HYGROSCOPIC SETTING EXPANSION

DIFFERENCE BETWEEN NORMAL & HYGROSCOPIC SETTING EXPANSION

LATEST ADVANCEMENT IN DENTAL PLASTER:

• Orthodontist prefer white stone for study model and may even treat surface with soap for an added sheen.• Nowadays mounting plaster have been developed which are fast setting and have low setting expansion.• A type of plaster changes color to denote when it is ready for use.• Another trend is the addition of a small amount of plastic or resin, which reduces brittleness and improves resistance

SOME COMMERCIALY AVAILABLE DENTAL PLASTER:

NEELKANTH DENTAL PLASTER

VICTOR DENTAL PLASTER

ANSWORTH DENTAL PLASTER

DENTEX DENTAL PLASTER

REFERENCES• Phillips Science of dental materials 11th edition.• Phillips science of dental materials 12th edition.• Google images.

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