INDIAN DENTAL ACADEMY Leader in continuing dental education www.indiandentalacademy.com
INTRODUCTION GYPSUM CLASSIFICATION MANUFACTURE SETTING PROCESS PROPERTIES MANIPULATION COMPATIBILITY INFECTION CONTROL RECENT DEVELOPMENTS CONCLUSION www.indiandentalacademy.com
INTRODUCTION Auxiliary dental material. Glossary of Prosthodontic terms: Model: Facsimile used for display purposes, a miniature representation of something. Dental Cast: A positive life size reproduction of a part or parts of the oral cavity. Die: The positive reproduction of the form of a prepared tooth in any suitable substance. Direct link between the clinical phase of treatment and the technical laboratory procedures. Contributes significantlywww.indiandentalacademy.com to the ultimate success of the prosthesis.
GYPSUM: Originates from the Greek word â€˜Gyposâ€™ which means chalk. Dihydrate of calcium sulfate. Chemical formula CaSO4.2H2O. Sulfate mineral most commonly found. Usually white to yellowish white in color. Large beds of gypsum were formed when seawater evaporated, leaving dissolved Calcium and Sulfate ions to form deposits of gypsum. United States is the largest producer as well as the biggest consumer of gypsum. Others are Canada, France, Japan and Iran. www.indiandentalacademy.com
Gypsum mineral can be found in various forms:
1. ROCKâ€“GYPSUM: widely occurring massive dull coloured rocks.
2. ALABASTER: Large fine-grained white stones. Often used for carving into vases and ornaments. Also used in building of King Solomonâ€™s Temple.
KING SOLOMON’S TEMPLE
3. SELENITE: as transparent crystals.
4. SATIN SPAR: as fibrous crystals.
SYNTHETIC GYPSUM: • Also produced as a by-product of manufacture of phosphoric acid. • CHEMICAL GYPSUM.
3 H2SO4(l) + Ca3(PO4)2(s) + 6 H2O(l)
2 H3PO4(s) + 3 CaSO4·2H2O(s)
Found as prismatic, curved or twisted monoclinic crystals of vitreous luster.
Specific gravity: 2.3
It cleaves perfectly in one direction.
Moh hardness no: 2, which means that it can be scratched by a fingernail. www.indiandentalacademy.com
USES: 1. As a raw material for making Plaster of Paris. Plaster of Paris is called so, because the gypsum that was used to manufacture it came from a village called Montamarte, near Paris. It is used extensively in construction purposes. 2. Grounded gypsum (land plaster) is sometimes used as a fertilizer for soil that needs calcium. 3. Raw gypsum is also used to keep Portland cement from hardening too quickly. 4. It is also used to make paint (as a filler), filters, insulation and wall plaster. 5. Alabaster is used for carving ornaments and vases. 6. Selenite is sometimes used as an optical material. www.indiandentalacademy.com
Gypsum produced for dental application is nearly pure calcium sulfate dihydrate.
DENTAL USES OF GYPSUM PRODUCTS: • Impression plaster is used to make the impression of the edentulous mouth. • For preparation of study models of oral and maxillofacial structures. • To form cast and dies on which dental prosthesis are constructed. • As a mold material for processing complete dentures. • For mounting of casts on the articulator. • Also used as a binder for silica, gold alloy casting investment, soldering investment and investment for low melting point nickelchromium alloys. www.indiandentalacademy.com
GYPSUM PRODUCTS: Refers to the various forms of calcium sulfate, hydrous and anhydrous. Manufactured by the calcination of calcium sulfate dihydrate. Calcination can be controlled to produce partial or complete dehydration. ADA No: 25.
ISO No: 6873.
Gypsum products can be classified into five types:
GYPSUM PRODUCTS IMPRESSION PLASTER
( ISO TYPE I )
( ISO TYPE II )
HIGH STRENGTH PLASTER
STONE ( ISO TYPE III ) STONE, HIGH STRENGTH, LOW EXPANSION ( ISO TYPE IV ) STONE, HIGH STRENGTH, HIGH EXPANSION ( ISO TYPE V ) www.indiandentalacademy.com
CHEMISTRY OF GYPSUM: In the temperature range of 20o to 700oC that is important in dental manipulation of gypsum products, three phase changes occur in the CaSO4-H2O system. o o 40 – 50 C
CaSO4.1/2H2O + Water (Calcium Sulfate Hemihydrate) o o 90 – 100 C
γ-CaSO4 + Water (Hexagonal Form) (Soluble anhydrite)
o 300 – 400 C www.indiandentalacademy.com 4
CaSO (Orthorhombic CaSO4)
MONOCLINIC Calcium Sulfate Hemihydrate
HEXAGONAL Soluble anhydrite
Calcium Sulfate Dihydrate
ORTHORHOMBIC Insoluble anhydrite
MANUFACTURE: From the conversion temperature given before, it can be seen that calcium sulfate hemihydrate would be produced by heating gypsum to temperatures in the range of 50o to 90 oC. However, at these temperatures the reaction is slow; even at 90oC substantially complete conversion takes about 12 hours (Khalil et. al., 1971). Therefore, in commercial processes temperatures higher than this are used, for shorter times.
The stable phase at these higher temperatures is hexagonal calcium sulfate, so the initial product of calcination is partly of very largely this anhydrous form. However, on cooling to temperatures below 85 oC and exposure to atmospheric moisture, the hexagonal calcium sulfate rehydrates to form the hemihydrate. www.indiandentalacademy.com
In the production of plaster, the gypsum is ground to a fine powder, impurities such as sulphur (S) and quartz (SiO2) are removed, and then it is subjected to calcination. Mineral Gypsum
dehydration by heat or other means
Plasters Synthetic Gypsum
High strength dental stone
Low High expansion expansion
PLASTER OF PARIS: ďƒź Traditional hemihydrate plaster. ďƒź Produced by the dry calcination of ground gypsum in open containers (pan, kettles or rotary kilns) at temperatures in the range of 120o to 180oC.
DIFFERENT FORMS OF HEMIHYDRATES: Depending on the method of calcination, different forms of hemihydrate can be obtained. α –hemihydrate α modified hemihydrate β – hemihydrate The α and β designations are retained because of tradition and convenience. They are without chemical significance and their use is solely to indicate the particular morphological appearance of the crystals. The differences between the α and β hemihydrates are a result of differences in the crystal size, surface area, and degree of lattice perfection. www.indiandentalacademy.com
Crystal size LARGER
LOOSELY PACKED AMPLE SPACE B/W CRYSTALS
CLOSELY PACKED LITTLE SPACE B/W CRYSTALS
Surface area/wt. Example
DENTAL PLASTER DENTAL STONE IMPRESSION PLASTER www.indiandentalacademy.com
The ﾎｱ竏知odified hemihydrate is made by boiling gypsum in a 30% aqueous solution of calcium chloride and magnesium chloride. This process yields the smoothest, most dense powder particles of the three types, and the powder is used primarily for dies.
POWDER PARTICLES OF β-HEMIHYDRATE ( X 400.)
POWDER PARTICLES OF DENTAL STONE ( x 400.)
Crystals of Îą-hemihydrate powder before grinding ( X 3000.)
PARTICLES OF β−HEMIHYDRATE ( X 400.) www.indiandentalacademy.com
PARTICLES OF α-MODIFIED HEMIHYDRATE ( x 400 )
THE SETTING PROCESS: THE SETTING REACTION:
Crystalline theory. Gel theory. Hydration theory
Crystalline theory: Proposed in 1887 by Henry Louis Le Chatelier. In 1907, it received the full support of Jacobus Hendricus vantâ€™s Hoff. Also called as Dissolution-precipitation theory. Based on the dissolution of hemihydrate powder and instant recrystallization of gypsum, followed by interlocking of the crystals to form the set solid. The differences in the solubilities of calcium sulfate dihydrate and hemihydrate causes the setting of these materials.
The setting mechanism may be viewed as follows:
Step 1. INDUCTION CaSO4.Â˝H2O
Step 2. CRYSTALLISATION CaSO4(aq)
STAGES: DISSOLUTION SUSPENSION SATURATION SUPERSATURATION NUCLEI FORMATION GROWTH OF NUCLEI SPHERULITE FORMATION
When the hemihydrate powder is mixed with water in the correct proportions it forms a thick slurry. The hemihydrate is sparingly soluble in water (6.5g/L at 20o C), so only a small amount can dissolve. Initially, therefore, the mix is a twophase suspension of hemihydrate particles in a saturated aqueous solution. The stable hydrate at temperatures below 40o C is the dihydrate which is even less soluble than the hemihydrate (2.4 g/L at 20o C). The aqueous solution is therefore supersaturated with respect to the dihydrate, which crystallizes out at suitable nucleation sites in the suspension. The gypsum crystals normally are acicular in habit, and often radiate out from the nucleation centers in the form of spherulitic aggregates. www.indiandentalacademy.com
As the dihydrate precipitates, the solution is no longer saturated with the hemihydrate, so it continues to dissolves. Dissolution of the hemihydrate and the precipitation of the dihydrate proceeds as either new crystals forms or further growth occurs on the crystals already present. The process is continuous and continues until no further dihydrate precipitates out of the solution.
GROWTH OF NUCLEI
EARLY STAGES IN SETTING OF A DIE STONE (X 400.) www.indiandentalacademy.com
The setting reaction is the reverse of the first stage of dehydration and so is exothermic.
CaSO4. 1/2H2O + 11/2H2O
CaSO4.2H2O. + 3900cal/gmol.
This chemical reaction takes place regardless of whether the gypsum product is used as impression material or a die material.
GEL THEORY: Also known as the colloidal theory. Proposes that when mixed with water, plaster enters into the colloidal state through the 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.
HYDRATION THEORY: Suggests that rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set material.
STAGES IN SETTING: The setting process is continuous, from the beginning of mixing until the setting reaction is complete, by which time the material has reached its full wet strength. However, important physical changes can be recognized during this process. The stages in setting may be designated as: Fluid Plastic Friable Carvable
Some X-ray diffraction studies suggests that hemihydrate particles remain in the set product. Approx. less than 50% gypsum is present in Type IV and Type V stones. Approx. 60 % in type III stones. Approx. more than 90 % in Type II stones. These results demonstrate a higher concentration of dihydrate in the weaker set material or more conversion rate is seen in weaker set materials.
Initially, there is a continuous aqueous phase present, and the mix is a viscous liquid, exhibiting pseudoplasticity so that it flows readily under vibration; in this stage the mix has a glossy surface giving specular reflections. As the setting reaction proceeds, gypsum crystals continue to grow at the expense of the aqueous phase, and the viscosity of the mix increases. When clumps of the growing gypsum crystals interact, the mix becomes plastic; it will not flow under vibration but can be readily molded. At this time the glossy surface disappears as the aqueous phase is drawn into the pores formed when the growing gypsum crystals thrust apart. Continued crystal growth converts the plastic mass into a rigid solid, weak and friable at first but gaining strength as the relative amount of solid phase increases. www.indiandentalacademy.com
WATER REQUIREMENTS: CaSO4. 1/2H2O + 11/2H2O
CaSO4.2H2O. + Heat
1 g. mol
1.5 g. mol
1 g. mol.
This 18.6 gm of water is called as Gauging water. In practice, powder cannot be mixed with such small amount of water and still develop a mass suitable for manipulation. Some excess water is required for mixing to obtain the desired viscosity of the mix, which can be easily manipulated. At the completion of reaction, the excess unreacted water remains in www.indiandentalacademy.com the set mass. This residual water weakens the cast.
REQUIRED AND EXCESS WATER FOR GYPSUM MATERIALS GYPSUM
MIXING WATER ( ml/ 100g powder)
REQUIRED EXCESS WATER WATER ( ml/100 g ( ml/100 g powder ) powder)
37 – 50
18 – 31
28 – 32
9 – 13
HIGH STRENGTH STONE
19 – 24
Different water requirements.
Mainly due to the differences in the crystal shape and surface area. In case of Î˛-Hemihydrate particles the factors that promote the adhesiveness of the particles in the dry powder persist when they are suspended in water. For this reason also the Î˛-Hemihydrate powder particles powder produces a flocculated suspension and needs a relatively high proportion of mixing water to give a mix of workable viscosity.
RATE OF SETTING REACTION: Water/Powder ratio Spatulation Temperature Colloidal system and PH Additives Accelerators Retarders
W/P RATIO Within wide limits, the rate of hydration during setting is independent of the W/P ratio. However, the rate at which the associated physical changes described above occur is highly dependent on the W/P ratio of the mix, because these changes occur from the interaction of clumps of gypsum crystals growing from nucleation centers in the slurry. Physical changes associated with the setting of the mix take place more rapidly as the W/P ratio is decreased. Thick mixes (low W/P ratios) harden more quickly because available nucleation sites are concentrated in a smaller volume; interaction of the growing solid phase occurs earlier. Manipulation and setting times are thus directly proportional to W/P ratio. www.indiandentalacademy.com
EFFECT OF WATER/PODWER RATIO ON SETTING TIME MATERIAL
MODEL PLASTER DENTAL STONE HIGH STRENGTH DENTAL STONE
W/P RATIO ( ml/g )
0.45 100 0.50 0.55 0.27 100 0.30 0.33 0.22 100 0.24 0.26www.indiandentalacademy.com
INTIAL (VICAT) SETTING TIME (min)
8 11 14 4 7 8 5 7 9
SPATULATION The mixing process, called spatulation has a definite effect on the setting time and setting expansion of the material.
An increase in the amount of spatulation (either speed of spatulation or time or both) shortens the setting time.
When the powder is place in water, the chemical reaction starts, and some calcium sulfate dihydrate is formed. During spatulation, the newly formed calcium sulfate dihydrate breaks down to smaller crystals and starts new centers of nucleation, around which calcium sulfate dihydrate can be precipitated. www.indiandentalacademy.com
W/P RATIO ( ml/g )
SPATULATION SETTING TURNS TIME ( min )
TEMPERATURE: Evidently the temperature has two main effects on the setting reaction of gypsum products:
Change in the relative solubilities of calcium sulfate hemihydrate and calcium sulfate dihydrate.
Change in the ion mobility
EFFECT ON SOLUBILITIES: The ratio of the solubilities of calcium sulfate dihydrate and calcium sulfate hemihydrate at 20 oC is about 4.5. As the temperature increases, the solubility ratio decreases, until 100 oC is reached and the ratio becomes one. As the ratio of the solubilities become lower, the reaction is slowed, and the setting time is increased.
SOLUBILITY OF CALCIUM SULFATE HEMIHYDRATE AND CALCIUM SULFATE DIHYDRATE AT DIFFERENT TEMPERATURES
Temperature CaSO4.1/2H2O CaSO4.2H2O ( g/100g ( g/100 g ( 0C )
ION MOBILITY: As the temperature increases, the mobility of the calcium and sulfate ions increases, which tends to increase the rate of reaction and shorten the setting time.
NET EFFECT: Practically, the effects of these two phenomena are superimposed, and the total effect is observed.
THE EFFECT OF TEMPERATURE ON SETTING TIME OF DENTAL STONE TEMPERATURE SETTING TIME AT (0C) W/P RATIO 0.3 ( min ) 5 25 30 35 40 45 50
65 75 60 50 45 40 55 www.indiandentalacademy.com
From E.C Combe: Notes on Dental Materials.
COLLOIDAL SYSTEMS AND PH: Colloidal systems such as agar and alginate retard the setting of gypsum products. Retard the reaction by nuclei poisoning. They get adsorbed on the CaSO4.2H2O nucleation sites or on the CaSO4.1/2H2O and thus interfering in the setting reaction. The adsorption of these materials on the nucleation sites retards the setting reaction more effectively than the adsorption on the calcium sulfate hemihydrate. Liquids with low Ph, such as saliva, retard the setting reaction. Liquids with high Ph accelerate setting.
ADDITIVES: Used in the formulation on dental plasters and stones for many years, mainly on an empirical basis, because at least in some cases their modes of action are not completely understood. ACCELERATORS
Citrates Tartrates Acetates
SODIUM CHLORIDE: Provides additional sites for crystal formation. The increased number of sites for nucleation also decreases the setting time of the material. Also increases the solubility of hemihydrate, so that it dissolves rapidly, thereby also decreasing the setting time. If it is present in high concentrations( >20%), the sodium chloride will deposit on the surface of crystals and prevents further growth. This decreases the reaction rate. www.indiandentalacademy.com
POTASSIUM SULFATE: Reacts with water and hemihydrate to form. Syngenite: K2 (CaSO4) 2.H2O. This compound crystallizes very rapidly and encourages the growth of more crystals, thus decreasing the setting time. When present as a 2 % solution in water, the setting time is decreased from 10 minutes to 4 minutes.
TERRA ALBA: ď‚§ Finely Powdered gypsum. ď‚§ In small amounts, it will provide additional sites for nucleation, decreasing the working time and setting times.
SODIUM SULFATE: Salts of relatively low solubility, such as sodium sulfate, acts as retarders in higher concentrations, by nuclei poisoning. As the setting proceeds, the amount of free water in the mix decreases and the concentration of the additive increases. When the limit of solubility is exceeded, the salt precipitates on the nuclei of crystallization, thus poisoning them.
BORAX: Sodium tetra borate decahydrate (Na2B4O5(OH)48H2O) When the plaster, or stone, powder is mixed with an appropriate aqueous solution of borax (0.9%w/w), the powder particles become coated with a thin layer of Ca2B6O115H2O (‘colemanite’) which is very insoluble and delays the dissolution of the powder. www.indiandentalacademy.com
CITRATES AND TARTRATES: Reaction of some additives with hemihydrate may occur; soluble tartrates and citrates precipitate calcium tartrate and citrate respectively. These act by nuclei poisoning. For a given anion, particular cation employed appears to affect the setting reaction markedly. Ca 2+ < K+ < H+
ACETATES: May act by: nuclei poisoning. www.indiandentalacademy.com
reducing the rate of solution of hemihydrate.
In formulating dental products, manufactures adjust the rate of setting or raw hemihydrates by adding accelerators and retarders, often as a balanced mixture. Many accelerators and retarders reduce the setting expansion, in some cases by changing the crystal habit of the growing gypsum crystals, thereby reducing the effect of growth pressure. This is accompanied by a reduction in the strength of the set material.
IMPURITES: If calcination is not complete and the gypsum particles remain, or if the manufacturer adds gypsum, the setting time is shortened because of the increase in the potential nuclei of crystallization. If orthorhombic anhydrite is present the setting reaction is delayed. If hexagonal anhydrite is present, the setting reaction is faster.
FINENESS: The finer the particle size, the faster the mix hardens as: Rate of hemihydrate dissolution is increased. Gypsum nuclei are more numerous. www.indiandentalacademy.com
THE MICROSTRUCTURE OF CAST GYPSUM: The set material consists of a tangled aggregate of monoclinic gypsum crystals, usually acicular in shape, with lengths in the range of 5 to 20µm. The aggregate exhibits inherent porosity, on a microscopic scale, which is of two distinct types: 1. Micro-porosity caused by the presence of residual unreacted water. Roughly spherical. Occur between clumps of gypsum crystals. 2. Micro-porosity resulting from the growth of gypsum crystals. Associated with the setting expansion. Smaller www.indiandentalacademy.com Appear as angular spaces between individual crystals in the
Micro-porostity due to presence of unreacted water
Micro-porosity caused due to growth of gypsum crystals
EFFECT OF W/P RATIO: The Relative amounts of both types of porosity are affected by the W/P ratio of the mix, but in two opposite ways:
1. A low W/P ratio leaves less residual water in the set mass and so decreases the amount of the first type of porosity.
2. A low W/P ratio increases the effect of the crystal growth during setting, because available nucleation sites are concentrated in a smaller total volume of mix; interaction of growing gypsum crystals occur earlier and is more effective so that the amount of the second type of porosity is increased.
ďƒ˜ In any W/P ratio, the total proportion of inherent porosity in the set mass is the sum of these two types. ďƒ˜ The effect of the first type predominates, so for any given plaster or stone there is always a decrease in the total inherent porosity of the set mass (i.e. an increase in apparent density) as the W/P ratio of the mix is reduced. ďƒ˜ Inherent porosity represents about 40% of the total cast volume at a W/P ratio of 0.5. and about 20% at a W/P ratio of 0.25. (Lautenschlager and Corbin, 1969). www.indiandentalacademy.com
VOIDS: Areas of air in the mix. Two types: â€˘Internal Voids: weaken the material. â€˘External Voids: do not record impression anatomy in that area. Ryerson NV(2000), investigated the effect of pressurized atmosphere on the size and number of voids in dental stones while setting. Increased atmospheric pressure reduced the size and number of voids. This method produces improved cast surfaces and fewer, smaller voids. Several pressure vessels are available for this purpose.
PICTURE SHOWING INTERNAL VOID www.indiandentalacademy.com
MIXING, WORKING AND SETTING TIMES SETTING EXPANSION STRENGTH SURFACE HARDNESS ABRASION RESISTANCE REPRODUCTION OF DETAIL SOLUBILITY
The important properties requirements of gypsum products according to ADA specification no. 25 are summarized in the table: TYPE
SETTING TIME (min )
SETTING EXPANSION RANGE ( %)
COMPRESSIVE REPRODUCTION STRENGTH OF DETAIL ( MPa ) ( Âľm ) Min. Max.
75 + 8
75 + 8
Type III +20 %
50 + 8
Type IV +20 %
50 + 8
Type V +20 % 0.16-0.30 35.0
50 + 8
MIXING, WORKING AND SETTING TIMES: MIXING TIME: Defined as the time from the addition of the powder to the water until the mixing is completed. Mechanical mixing of stones and plaster is usually completed in 20 to 30 seconds. Hand spatulation generally requires at least a minute to obtain a smooth mix. WORKING TIME: The time available to use a workable mix, one that maintains a uniform constituency 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 productâ€™s intended purpose. Generally, a 3-minute working time is adequate. www.indiandentalacademy.com
INDUCTION TIME: Time from the beginning of mix till the exothermic heat is felt.
SETTING TIME: The time that elapses from the beginning of mixing until the material hardens is known as the setting time. An arbitrary setting time can be determined by using suitable penetrometers( e.g. Gillmore or Vicat needles).
LOSS OF GLOSS TEST FOR INITIAL SET: As the material sets, the mix loses its gloss. This occurs as some of the excess water is taken up in forming the dihydrate. This loss of the gloss is also sometimes considered as a indication of initial set of the material. www.indiandentalacademy.com
GILLMORE NEEDLES: Two types of Gillmore needles. The lighter Gillmore needle is constructed from a brass cylinder, of mass 0.25 lb (113.4 g), attached to a needle with a flat disk end of diameter 1/12" (2.12 mm). The larger needle consists of a 1lb mass (453.6 g) on a diameter of 1/24" (1.06 mm). The corresponding stresses are 0.3 MPa and 5 MPa, respectively. www.indiandentalacademy.com
GILLMORE NEEDLES USED TO DETERMINE THE SETTING TIME OF GYPSUM PRODUCTS
GILLMORE TEST FOR INITIAL SET: The mixture is spread out, and the smaller needle is lowered onto the surface. The time at which it no longer leaves an impression is called the initial set. This event is marked by a definite increase in strength. Acts as a guide to the time when the rigid material is strong enough to handle and, in particular, when it can be carved or trimmed to the final shape. The setting reaction continue for some time after this initial set.
GILLMORE TEST FOR FINAL SETTING TIME: 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. www.indiandentalacademy.com
VICAT TEST FOR SETTING TIME: Used to measure the initial setting time of gypsum products. Consists of a rod weighing 300 gm with a needle of 1 mm diameter. A ring container is filled with the mix. The needle with a weighted plunger rod is supported and held just in contact with the mix, then is the needle is released and allowed to penetrate the mix. The time elapsed until the needle no longer penetrates to the bottom of the mix is known as Vicat setting time. www.indiandentalacademy.com
VICAT PENETROMETER USED TO DETERMINE THE SETTING TIME OF GYPSUM PRODUCTS
In some cases, the Vicat and initial Gillmore measurements occur at the same time, whereas in other instances, there is small difference. If a dental manufacturer specifies a setting time, it will be a Gillmore or Vicat initial setting time.
READY-FOR â€“USE CRITERION: A subjective measure of the time at which the set material may be safely handled in the usual manner. Not determined by any designated test but by the ability to judge readiness improves with experience. Technically, the set material may be considered ready for use at the time when the compressive strength is at least 80% of that which would be attained at 1 hour. Most modern products reach the ready-for-use state in approx. 30 minutes.
MANIPULATION TIME: Recognition of the physical changes occurring in the mix during setting is important in the manipulation of plaster and stone. 1. When casting (e.g. pouring casts or dies), manipulation must be completed before the mix loses its fluidity. This change is marked by the disappearance of the glossy surface from the mix. 2. When molding (e.g. taking impressions or jaw registrations, articulating casts, flasking wax pattern dentures), manipulation must be completed before the mix loses plasticity and enters friable stage. There is no recognized objective method of measuring this time.
SETTING EXPANSION: VOLUME CHANGES DURING SETTING: Theoretically, calcium sulfate hemihydrate should contract volumetrically during the setting process. (CaSO4)2. H2O +
Eq. Volume 105.556 T. Volume
The volume of the calcium sulfate dihydrate formed is about 7% less www.indiandentalacademy.com than the sum of the volumes of calcium sulfate hemihydrate and water.
However, experiments have determined that all gypsum products expand linearly during setting. Instead of 7% contraction, about 0.2% to 0.4 % linear expansion is obtained. The setting reaction causes a decrease in the true volume of the reactants and under suitable conditions this contraction can be observed early in the setting process, when the mix is still fluid. However, once the mix begins to attain rigidity, marked by the loss of surface gloss, an isotropic expansion is observed, resulting from growth pressure of the gypsum crystals that are forming.
STAGES: INITIAL MIX INITIAL CRYSTAL GROWTH SOLID PHASE CONTACT EXPANSION TERMINATION
The initial contraction is unlikely to affect the important dimensions of a gypsum cast, because in the still fluid mix it will occur mainly in the vertical direction. Gravity will keep the mix adapted to the anatomical portion of an impression. The expansion that is observed after the mix attains rigidity takes place in all directions and will affect the dimensions of the cast. The point at which the initial contraction ceases is used as zero in the laboratory measurements of effective setting expansion. The observed expansion that occurs when plaster or stone sets is a volumetric one. In dental testing a value is determined for linear setting expansion and the assumption is made that the expansion is isotropic. This assumption is not always justified; if restraint is imposed in some directions and not in others( e.g. by a rigid impression) setting expansion can be far from uniform. www.indiandentalacademy.com
LINEAR SETTING EXPANSION OF GYPSUM PRODUCTS SETTING IN AIR TYPE
SETTING EXPANSION (%)
From William J.O Brien: Dental materials- Properties and selection
EFFECT OF IMMERSION: Gypsum products exposed to additional water while setting ( e.g. by immersion) show a greater expansion than when setting in air, a phenomenon commonly called Hygroscopic expansion.
The most well accepted reason for the increased expansion when the hemihydrate reacts under water is the additional crystal growth permitted by allowing the crystals to grow freely, rather than being constrained by the surface tension when crystals form in air, also known as the Crystal interlocking theory.
It follows, therefore, that the basic mechanism of crystal growth is the same in both instances, and both phenomena are true setting expansions. The hygroscopic setting expansion is physical and is not caused by a chemical reaction any more than is the normal setting expansion. The reduction in W/P ratio increases the hygroscopic setting expansion and the normal setting expansion in the same manner. Increased spatulation results in increased hygroscopic setting expansion as well. The Hygroscopic setting expansion obtained during the setting of dental plaster or stone is generally small in magnitude. A dental stone may exhibit a normal setting expansion of 0.15%, with a maximum hygroscopic setting expansion of not more than 0.30%. Nevertheless, this difference may be sufficient to cause the misfit of www.indiandentalacademy.com the denture or similar device on the cast.
FACTORS AFFECTING SETTING EXPANSION:
ADDITIVES: Manufactures can reduce the setting expansion and at the same time control setting time by the addition of a balanced blend of accelerators and retarders to the raw hemihydrate base plaster. Typical combinations are: •
Potassium sulfate- borax
Potassium sodium tartrate- sodium citrate.
Combinations of accelerators and retarders, in solution, to control the setting expansion, are known as ANTI-EXPANSION SOLUTIONS. www.indiandentalacademy.com
The additives also reduce the strength of the set material. This is not a disadvantage in impression plaster, but in stones and die stones strength as well as dimension accuracy is important; formulation of the latter materials therefore involves striking a compromise between a desirable reduction in setting expansion and an undesirable reduction in strength
SODIUM CHLORIDE: Provides additional sites for crystal formation. The higher density of crystals limits the growth of crystals and hence reduces their ability to push each other apart. This results in decreased setting expansion.
POTASSIUM SULFATE: Reacts with water and hemihydrate to form ‘ Syngenite’ This compound crystallizes very rapidly and encourages the growth of more crystals. Results in decreased setting expansion.. www.indiandentalacademy.com
W/P RATIO: Setting expansion is inversely proportional to the W/P ratio. Reducing the relative amount of aqueous phase in the mix allows more effective interaction of growing gypsum crystals during setting and so increases the setting expansion. Because of their lower water requirement, the raw hemihydrates used to produce dental stones and die stones have a higher inherent setting expansion in normal mixes than does plaster. This effect is masked, however by the additives in their formulation.
STRENGTH: Brittle material. Weaker in tension than in compression. For set plaster, the tensile strength is about 20% of the compressive strength; for set die stone about 10%. In practice, fracture of set gypsum typically occurs in tension, tensile strength is a better guide to fracture resistance. Compressive strength gives a better indication of surface hardness.
FACTORS AFFECTING STRENGTH: •
WATER/POWDER RATIO: Strength properties are inversely related to the W/P ratio and to the total amount of inherent porosity. When maximum strength is required, a given material should be mixed with as low a W/P ratio as practicable. The limiting factor is the viscosity of the mix because it increases with decreasing W/p ratio and can become so high that the ability to pour casts is prejudiced. With any plaster or stone, using a low W/P ratio to obtain maximum strength properties also gives an increased setting expansion, which must be accepted. But in applications where dimensional accuracy is more important than strength (e.g. impressions), higher W/P ratio can be used.
MODEL PLASTER DENTAL STONE HIGH STRENGTH DENTAL STONE
W/P RATIO ( ml/g ) 0.45 0.50 0.55 0.27 0.30 0.50 0.24 0.30 0.50
COMPRESSIVE STRENGTH ( Mpa )
12.5 11.0 9.0 31.0 20.5 10.5 38.0 21.5 10.5
EFFECT OF DRYING: The free water content of the set product definitely affects its strength. For this reason, two strength properties of gypsum are reported: • The wet strength
also known as the green strength.
It is the strength obtained when the water in excess of that required for hydration of hemihydrate is left in the test specimen. • The dry strength: When the excess water has been driven off by drying, the strength obtained is dry strength. The dry strength may be two or more times as high as wet strength. www.indiandentalacademy.com
The effect of drying on the compressive strength of set plaster is shown in the graph:
A somewhat similar change in the surface hardness also takes place www.indiandentalacademy.com during the drying process.
STRENGTH PROPERTIES OF GYPSUM PRODUCTS TYPE
W/P TENSILE COMPRESSIVE RATIO STRENGTH STRENGTH ( Mpa ) ( Mpa ) Wet Dry Wet Dry
From William J.O Brien: Dental Materials- Properties and Selection 5 th ed.
EFFECT OF SPATULATION: With an increase in the mixing time, the strength is increased. But this increase is only seen upto a hand mixing of 10 minutes. If the mixture is overmixed, the gypsum crystals formed are broken up, and less crystalline interlocking results in the final product.
EFFECT OF ADDITIVES: The addition of an accelerator or retarder lowers both the wet and the dry strengths of the gypsum product. Such a decrease in strength can be partially attributed to the salt added as an adulterant and to the reduction in intercrystalline cohesion. Prombonas A, Vlissidis D(1994) measured the Compressive strength and setting temperatures of mixes with various proportions of plaster to stone. The mix made with 1:1 ratio of stone to plaster, may be the www.indiandentalacademy.com material of choice for filling the upper half of the flask during complete
TENSILE STRENGTH: Important in structures in which bending tends to occur because of lateral force applications, such as removal of cast from impressions. Because of the brittle nature of gypsum materials, the teeth on the cast may fracture rather than bend. For brittle materials like gypsum products, diametral compressive strength test is used to determine the tensile strength of such products.
SURFACE HARDNESS AND ABRASION RESISTANCE: In general, hardness is defined as the resistance to penetration. For dental purposes, the surface hardness of a material is generally measured in terms of its resistance to indentation. The surface hardness of unmodified gypsum products is related in a general way to their compressive strength. High compressive strength of the hardened mass corresponds to high surface hardness. After the final setting occurs, the surface hardness remains practically constant until most excess water is evaporated from the surface, after which its increase is similar to increase in compressive strength. The surface hardness increases at a faster rate than the compressive strength, because the surface of the hardened mass reaches a dry state earlier than the inner portion of the mass. www.indiandentalacademy.com
Attempts have been made to increase the hardness of gypsum products by: â€˘
Impregnating the set gypsum with epoxy or methyl methacrylate monomer that is allowed to polymerize. According to Craig, increase in hardness was obtained for model plaster but not for dental stone or high-strength dental stone. Generally, impregnating set gypsum with resin increases its abrasion resistance, but decreases compressive strength and surface hardness. â€˘
Soaking the gypsum dies or casts in glycerine or different oils does not improve the surface hardness but rather makes the surface smoother, so that a wax carver or other instrument will not cut the stone as it slides over the surface.
Mixing high strength dental stone with a commercial hardening solution containing colloidal silica (about 30%) improves the surface hardness of set gypsum. The Knoop hardness numbers of two commercial high-strength dental stone were 54 and 77 kg/mm2. When hardening solution was used, these values increased to 62 and 79 kg/mm2 respectively.
The abrasion resistance of gypsum product is an important property in certain dental procedures. For example, if a wax pattern is to be carved and finished on a stone die, the metal instrument used to carve the wax may abrade off and destroy adjacent areas of the stone. Abrasion is a major concern when gypsum products are used for dies, leading to the frequent recommendation that surface hardeners should be used before waxing or scanning. www.indiandentalacademy.com
HARDENING SOLUTION: Composed of: Colloidal Silica: 30 % Water Modifiers May be used in place of water to mix gypsum products. Amount of solution used is less than if water were used alone because surface-active modifiers allow the particles to be more easily wetted. Affects the hardness and setting expansion of gypsum dies.
REPRODUCTION OF DETAIL: ADA specification no. 25 requires that:
Type I and II reproduce a groove 75µm in width.
Type III, IV and V reproduce a groove 50µm in width.
Gypsum dies do not reproduce surface details very well because the surface of the set gypsum is porous at microscopic level.
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Published on May 9, 2014
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