INTRODUCTION Gypsum is a mineral that is mined in various parts of the world. The word gypsum derived from Greek word meaning ¶to cook· referred to burnt or calcined mineral. Chemically it is CaSo4;2H2O i.e. calcium sulphate dihydrate. Three types of gypsum products are available in dentistry ² 1. Model plaster 2. Dental stone 3. High strength dental stone or die stone
All these types differ in their physical form , but have the same chemical structure. The term Plaster of Paris was given to this product as this was obtained by burning the gypsum from deposits near Paris , France . OTHER FORMS OF GYPSUM Gypsum has variety of names that are widely used in the mineral trade1} Selenite 2} Satin spar3} Alabaster 4}Plaster of Paris
CLASSIFICATIONACCORDING TO ADA SPECIFICATION NO 25-TYPE I -IMPRESSION PLASTERTYPE II -DENTAL PLASTERTYPE III - DENTAL STONE (Hydrocal, Alpha hemihydrate)TYPE IV - DENTAL STONE (HIGH STRENGTH) (Die stone, Densite,Modified alpha hemihydrate)TYPE V - DENTAL STONE (HIGH STRENGTH, HIGH EXPANSION)
Types of gypsum productsGypsum Manufacture UseProductsImpression Earlier plaster was Preliminary impressionPlaster modified for use as of edentulous ridge in(Type ² I) impression material by fabrication of dentures.Now rarely used. addition of chemicals and [MUCOSTAIC flavoring agents IMPRESSION]Dental Plaster Heating the gypsum Weakest and least(Type ² II) powder in open container. expensive of all gypsumPlaster of paris This direct and rapid products. heating results in powder Making preliminary castF-hemihydrate that consist of porous for CDModel plaster and irregular particles. To secure cast to articulator , to fill a flask in denture construction
Dental Stone Made for gypsum by(Type ² III) carefully controlledHydrocal calcinations under steam Stone is stronger pressure in a closedClass I stone container. The method and more expensiveE-hemihydrate releases the water of than plaster. crystallization slowly and Use for making cast resultant powder particles for diagnostic purpose. are more uniform in shape Cast for CD and RPD and less porous. constructionDental Stone, Made by calcining in calcium It is strongest and mosthigh strength chloride solution. This expensive. Densite is(Type IV) calcination result in a the densest of all theClass II Stone, powder particles that is material.Densite, very dense and cuboidal in Used mainly for cast andImproved shape, with a reduced dies for inlay and crownstones, Die surface area. fabrication.Stone
Dental Stone, Same as Type IV, For making veryhigh strength, Added with strong dies (hashigh expansion accelerators and highest(Type V) retarders and compressive surface tension strength) this is reducing agent like achieved by LIGNON SULFATE lowering the w/p ratio and increasing the setting expansion to compensate for alloy solidification shrinkage.
Based on crystal structure:1. Beta hemihydrate- Type I , II fibrous aggregate of fine crystals with capillary pores. Fluffy porous and least dense2. Alpha hemihydrate- Type III, IV, V cleavage fragments and crystals in form of rods or prisms Higher density and more crystalline
MANUFACTURE CaSO4.2H2O CaSO4.1/2 H2O CaSO4 Gypsum 110-130oC Plaster or Stone 130-200oC CaSO4 Hexagonal CaSO4 anhydrite Orthorhombic anhydrite 200-1000oC Commercially, the gypsum is ground and subjected to temperature 110-120 0C to drive off the water of crystallization and produce calcium sulphate hemihydrate .
CALCINATION CaSO4;2H2O boil, CaCl2 open kettle high pressure Type IV Type V steam Type II Type III
CaSO4.1/2H2O -hemihydrate -hemihydrate The difference between andhemihydrate are the result of differencesin crystal size, surface area and degree oflattice imperfection
-hemihydrate -hemihydratey When dihydrate is y When dihydrate is heated under steam heated in an open pressure kettle or kiln.y Dense prismatic y Spongy irregularly crystals shaped crystals.y Stronger harder y Less strong products products on mixing on mixing with water with water (Type I,II) (type III,IV,V)y Smaller w/p ratio y Larger w/ p ratio
SETTING OF GYPSUMPRODUCTSThe following reaction takes place in the hardeningprocessCaSO4.2 H2O+ 3H2O 2(CaSO4 .2H2O) + heat 3900Cal/g mol)The product of the reaction is gypsum.The heat evolved in the exothermic reaction isequivalent to the heat used originally in calcination.This chemical reaction takes place regardless ofwhether the gypsum material is used as impressionmaterial , Die material, or a binder in castinginvestment.
SETTING PROCESS There is distinct difference in the solubility of hemihydrate and dihydrate. Hemihydrate is 4 times more soluble in water than dihydrate near room temp.(200c)
T s tti g r cti c rst s f ll s: i y r t is ix it t r, s s si is f r t t is fl i r l . W t r c ss t r t it i y r t . T is s t r t s l ti is s rs t r t it i y r t ,s t l tt r r ci it t s t. l ti is l g rs t r t it i y r t . it c ti s t iss lv , till f rt r i y r t r ci it t s t f s l ti .
The process continues until most of thehemihydrate is converted to dihydrate.The crystals of dihydrate are spherulitic innature and grow from specific growth sitescalled nuclei of crystallization.
THEORIES OF SETTINGTheories of Settingof Gypsum products Colloidal Theory Hydration Theory Dissolution precipitation Theory or Crystalline Theory
Colloidal Theory: When mixed with water , plaster enters into the colloidal state through sol- gel mechanism . In the sol state ,hemihydrate particles are hydrated to form the dihydrate, thereby entering into an active state . As the measured amount of water isconsumed, the mass converts into a solid gel.
Hydration Theory This theory suggests that rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set material.Dissolution precipitation Theory: by Louis Chatelier in 1885 This theory is based on dissolution of plaster and instant recrystilization of gypsum, followed by interlocking of the crystals to form the set product
NUCLEI OFCRYSTALLIZATION SPHERULITIC GROWTHOUTWARD THRUST AS SPHERULITES MAKE CONTACT
PROPERTIES Water/Powder ratio-The proportion of water to powder used to make aworkable mix of a particular gypsum product is calledwater/powder ratioIt is the quotient obtained when the weight or volume ofthe water is divided by the weight of powder.It is usually abbreviated as W:PThe W:P ratio is an important factor in determining thephysical and chemical properties of the final gypsumproduct
Chief factors that determine the amount of gauging water required are:-1. Particle size2. Total surface area3. Particle size distribution
´For t l s , xc ss ount of sur ter is l ys necess ry ove the theoretic lly correct ount required for hydr tion hich is . 6%´The excess is needed to e or le ix that can e oured and shaped´The excess ater is distri uted as free ater in the set ass ithout taking part in chemical reaction and it contri utes to the subsequent porosities or voids in the set product
´The proper W:P ratio for each product depends on physical characteristics of powder particles´Therefore plaster requires more measured water (gauging water) to float the irregular porous particles than the dense particles of stone´A mixture of Calcium oxide (0.1 ) Gum arabic (1 ) reduces the amount of water necessary to mix gypsum products: resulting in Improved properties.
The excess water is distributed as free water in the set mass without taking part in the chemical reaction .When the set mass is dried the excess water evaporates leaves porosity in the structure weakening it.
Recommended ranges of w/p ratio:´Type I ³ 0.50-0.75´Type II ³ 0.45-0.50´Type III³ 0.28-0.30´Type IV ³0.22-0.24´Type V ³0.18-0.22
SETTING TIMEMixing time- Time from addition of powder to water until mixing is completed. Mechanical mixing:20-30 secs Hand spatulation:1 min.Working Time: The time available to use a workable mix, one that maintains an even consistency that may be manipulated to perform one or more tasks. General 3 min working time is adequate
Setting time When the powder is mixed with water, thetime that elapses from the beginning of mixinguntil the materials hardens is known as setting time.This is usually measured by some type ofpenetration test, using the following instruments- Vicat penetrometer Gilmore needle
1. Loss of gloss test for initial set. occurs hen excess ater in the mix is taken up in forming the dihydrate, so the mix loses its gloss. This occurs approx. at mins.and the mass has no measurable compressive strength. . Initial ilmore test for initial set This occurs approx at 13mins this time is marked by definite increase in strength.
Initial gillmore test for initial set:- the mixture is spread out , the needle of ¼ lb weight and dia of 1/12µ is lowered onto the surface and the time at which it no longer leaves an impression is called the initial set. This time is marked by a definite increase in strength. Gillmore test for final setting time:- this uses a heavier gillmore needle(1lb, 1/24µ). The time elapsed when it leaves only a barely perceptible mark on the surface is called the final setting time. It is 45- 0 min.It is rarely used as an indication for the ready-for-use stage
3. icat test for setting time:it has an inbuilt gauge for judging the setting characteristics. The setting material is indented by a needle of 1mm dia. nder a load of 300gm.setting time is achieved hen needle can no longer penetrate to a depth of mm into the material. Vicat penetrometer
´ It consists of a rod eighing 300 g ith a needle of 1-mm diameter. A ring container is filled ith the mix, the setting time of hich is to be measured. The rod is lowered until it contacts the surface of the material, then the needle is released and allowed to penetrate the mix. When the needle fails to penetrate to the bottom of the container, the material has reached the icat or the initial setting time
Ready for use criterion´ It is a subjective measure of the time at which material may be safely handled´ Technically it is the time when compressive strength is atleast 80 of that attained at 1 hour.´ Most modern products reach the ready to use state in 30mins.
CONTROL OF SETTING TIME(S.T.) The .T. of gypsum products can be controlled by manufacturers particular formulation accordingly either a fast setting or a slow setting product can be purchased. Theoretically there are 3 methods1. The solubility of hemihydrate can be increased or decreased. . o. of nuclei of crystallization can be increased or decreased.3. ate of crystal growth is increased or decreased.
The operator can vary the S.T.within reasonBy altering W:P ratio mixing time.Decrease in setting time (fast setting) - o mixing q W:P ratio Addition of certain chemicals called accelerators. Increase in setting time (slow setting) ² q mixing o W:P ratio Addition of certain chemicals called retarders.
The operator can accelerate the setting timeby adding gypsum (.20 ),Potassiumsulfate or sodium chloride(.28 )Retarder act by forming an adsorbed layeron the hemihydrate to reduce its solubilityand on the gypsum crystals to present toinhibit growth. Organic materials like glue,gelatin and some gums behave in thismanner. Nacl when added in small quantities will act as accelerators , BUT in large quantities will act as retarder
IMPURITIES´ Presence of set gypsum particles, shortens the setting time because of the increase in the potential nuclei of crystallization.´ Orthorhombic anhydrite: Induction period is increased. Hence setting time increased.´ Hexagonal anhydrite: Induction period is decreased. hence setting time is decreased.
Fineness: Finer the particle size of the hemihydrate« «faster is the set.Temperature:´ Little change occurs between 0*C to 50*C.´ If temperature of the plaster exceeds 50*C then gradual retardation occurs.´ As the temperature approaches 100*C, no reaction takes place.´ At higher temperature there is a tendency for the gypsum crystals formed to be converted back to hemihydrate form
Setting Expansion´This thrust results in an external expansion with resulting internal porosity in set mass.
CONTROL OF SETTING EXPANSION(S.E)´Less the W:P ratio o S.E. o mixing time o S.E.´However the most effective method for control of S.E. is the addition of chemicals. (accelerators or retarders)´Accelerators or retarders have an effect of reducing setting expansion and are referred as Anti expansion agents.
ACCELERATORS ANDRETARDERSAccelerators : It increases the solubility of hemihydrates without increasing the solubility of dihydrate. Thus accelerates the process.Eg. NaCl upto about 2 NaSO4 upto 3.4 K2SO4 2 Terra Alba/Set calcium sulfate dihydrate. Liquids of High pH accelerate the setting reaction while the liquids of low pH like saliva retard the setting reaction
Retarders : Certain chemicals form a coating on the hemihydrates particles and thus prevent the hemihydrates from going into solution in the normal manner.Eg. Citrates, Acetates, BoratesColloidal system such as agar alginate retard the setting reaction by being adsorbed on the hemihydrate or the dehydrate nucleation sites thus interfering in hydration reaction
HYGROSCOPIC S.E.´So far we have assumed that the plaster or stone is allowed to set in air.´If the setting process is allowed to occur under water, the setting expansion is more than double in magnitude.
Stage I : Initial mix is representedby three round particles of hemihydratesurrounded by water As the crystals of dihydrate grow, they contact each other, and the S.E. begins
Stage II : Reaction started and crystal of dihydrate start forming´In left, water around particles is reduced by hydration and particles are drawn more closely together by surface tension action of water.´In right, because the setting is taking place under water, the water of hydration is replaced and distance between the particles remains the same.
Stage III :´The water around particles is again decreased in the eg. on left.´The particles with their attached crystals tend to be drawn together as before but the contraction is opposed by the outward thrust of growing crystals.´On the other hand the crystals in right diagram are not so inhibited, as water is again replenished.
Stage IV Stage V: The effect become more marked. The crystals being inhibited on the left become intermeshed and entangled much sooner than those on right, which grow much more freely during early stages before the intermeshing finally prevents the further expansion.
´The basic mechanism of crystal growth is same in both instances, and both phenomenon are true setting expansion.´To distinguish between them, S.E. without water immersion is often termed as Normal setting expansion (NSE) where as the expansion that occurs under water is known as Hygroscopic setting expansion (HSE) .
FACTORS INFLUENCING HSE ´q W:P ratio o HSE ´o mixing time o HSE
STRENGTH´The strength of gypsum products is usually measured in terms of compressive strength.´As seen in setting reaction the strength develops rapidly during the first 30 to 45 min as the hydration is completed. The wet strength of dental plaster is 9MPa and Improved stone is 35MPa 1 hour tensile strength of model plaster is 2.3 MPa
´The strength depends on porosity of set material, which is related to W:P ratio necessary to make a workable mix.´Eg ² Plaster (which require the most gauging water to make a fluid mix) is the weakest, while improved stone is strongest.
´The presence or absence of excess of free water affects strength and two types of strength are recognized. Wet strength Dry strength
Wet trength : It is the strength measured when the sample containssome or all of the water is excess of the theoretical amount required for hydration. ry trength : It is the strengthmeasured when the excess of water is not present. ry strength is two or more times more than the wet strength.
FACTORS INFLUENCING THESTRENGTH´o W:P ratio q strength.´o mixing time o strength.´o accelerator or retarder q strength.
SURFACE HARDNESS AND ABRASIVERESISTANCE´ DEFINITION - IN MINEROLOGY SURFACE HARDNESS OF SUBSTANCE IS ITS ABILITY TO RESISTS SCRATCHING SURFACE HARDNESS ~ COMPRESSIVE STRENGTH
Detail Reproduction:ANSI/ADA Specification No.25 requires´ Type I Type II to reproduce a groove of 75 m´ Type III , IV V to reproduce a groove of 50 m.
PROPORTIONING, MIXING AND CARINGFOR PRODUCTSThe technical use of gypsum products is relatively simple requiring only´Mixing bowl´Mixing spatula´Room-temperature water´Appropriate gypsum product
PROPOTIONING Because the strength of a stone is indirectly proportional to the W:P ratio, it is most important to keep the amount of water as low as possible. However it should not be so low that the mix will not flow in every detail of the impression. Thus, water and powder must be proportioned accurately for optimum properties.
a. Measuring the Water The water is usually dispensed by volume in a graduated cylinder, as 1 gm of water has a volume of very close to 1 ml.b. Measuring the Powder The power can be weighed in grams with a simple balance or scale. Volume dispensers may be used, but volume dispensing of the power is not accurate because of varying packing effect on the powder. Weighing with a scale is a simple and convenient method to ensure accurate proportions.
WATER FOR REACTIONMIXING WATER:-TOTAL AMOUNT OF WATER REQUIRED FOR CHEMICAL REACTION TO OCCURREQUIRED WATER:- THAT AMOUNT OF WATER THAT REACTS WITH HEMIHYDRATE IS KNOWN AS REQUIRED WATER.STANDARD VALUE 18.6 ML/100GM OF POWDER
MANUAL MIXING´ The preferred method of mixing is to add the measured water first, followed by the gradual sifting of the pre weighed powder into it.´ Guess work of repeatedly adding water and powder to achieve proper consistency is to be avoided, as it may cause an uneven set in the cast, resulting in low strength and distortion.´Hand mixing involves using spatula at a rate of 2 revolutions /sec.
MIXING´Done in a flexible plastic or rubber bowl with a stiff bladed spatula to combine the powder and water.´The mix should be smooth, homogenous, workable and free of air bubbles.´A minimum of air inclusion in the mixed product is desirable to prevent surface bubbles and internal defects.
VACCUM MIXING´Often, mixing is done mechanically with a vaccum mixing and investing machine.´This provides a gypsum mix that is free of air bubbles and homogenous in consistency.´Many other devices are available which will mix gypsum products mechanically and they are used where the application of product is critical
CARING FOR CAST Once the setting reaction in the cast have been completed, its dimension will be relatively constant thereafter under ordinary conditions of room temperature and humidity. However it is sometimes necessary to soak gypsum cast in water. If the stone cast is immersed in running water, its linear dimension may decrease approximately 0.1 for every 20 min of such immersion. The safest method is to soak the cast in saturated solution of calcium sulfate.
Storage of set plaster or stone at roomtemperature produces no significant dimensionalchange.However if the storage temperature is raised tobetween 90o and 100oc, a shrinkage occurs asthe water of crystallization is removed and thedihydrate reverts to hemihydrate. Therefore, it is not safe to store or heat a stonecast in air at temperature higher than 55oC
SYNTHETIC GYPSUM´This is done with waste products or by- products of phosphoric acid production´The synthetic product is usually much more expensive than that made from Natural gypsum, but when properly made its properties are equal to or exceed those of the latter.
SPECIAL GYPSUM PRODUCTS In addition to the standardized gypsummaterials there are some that have beencharacterized for special purposesFor Eg. Orthodontist prefer a white stone or plaster for study models and may even treat the surface with soap solution for added sheen. These products have a longer working time for ease of trimming.
´The mounting stones or plasters are used to mount the cast on articulator, are fast setting and have low S.E. The mounting plaster has low strength to permit easy trimming and to separate the cast readily from articulator´Since 1991, a plethora of new dental stones have appeared mostly as time savers.´One type is ready to use in 5 min. but it has little working time.
´Another product changes color to help denote when it is ready for use.´Most recently, the trend is to add a small amount of plastic or resin that reduces brittleness and improves the resistance to scratching during the carving of wax pattern.
INFECTION CONTROL´There is always a risk of cross contamination to a dental office personnel by micro-organism, including hepatitis B and HIV , via dental impressions.´Thus the impressions should be disinfected using spray and immersion disinfecting techniques
If the impression is not disinfected it is prudent todisinfect stone castUsual disinfectants for stone cast include spraydisinfectants, hypochlorites and iodophoresDisinfecting solutions can be used that do notadversely affect the quality of gypsum cast.Alternatively, dental stone containing disinfectantmay be employed.When patients with known cases of infection arebeing treated, overnight ethylene oxide gassterilization is an option.
REFERENCES´ Phillips Science of DENTAL MATERIALS- 10th 11th Edition´ CRAIG·S Dental Materials- Properties and manipulation. 8th edition´ Clinical aspects of dental materials .Theory, practice and cases. Marcia Gladwin.´ Applied dental Materials. McCabe