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Gypsum Products

Gypsum Products used in Dentistry

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Gypsum Products
Good Morning. . .
Presented By:
Dr.VAIBHAV BUDAKOTI
PG 1st yr
Dept. of Prosthodontics,
Crown , Bridge &
Implantology
GYPSUM
PRODUCTS
CONTENTS
• Introduction
• Production of Gypsum Products
• Types of Gypsum Products
• Setting of Gypsum Products
• Setting Expansion
• Strength of Set Gypsum Products
• Manipulation of Gypsum Products
• Infection Control
• Specialized Gypsum Products
INTRODUCTION
• Gypsum (CaSO4•2H2O; calcium sulfate
dihydrate) is a mineral mined in various parts
of the world
• ADA Specification no. 25
• ISO Specification no. 6873:1998
TYPES
 Albaster- pure white,
fine grained and
translucent
Satin spar - fibrous
needle like with silky
lustre.
Selenite - colourless,
crystalline and
transparent.

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Gypsum Products

  • 3. Presented By: Dr.VAIBHAV BUDAKOTI PG 1st yr Dept. of Prosthodontics, Crown , Bridge & Implantology GYPSUM PRODUCTS
  • 4. CONTENTS • Introduction • Production of Gypsum Products • Types of Gypsum Products • Setting of Gypsum Products • Setting Expansion • Strength of Set Gypsum Products • Manipulation of Gypsum Products • Infection Control • Specialized Gypsum Products
  • 5. INTRODUCTION • Gypsum (CaSO4•2H2O; calcium sulfate dihydrate) is a mineral mined in various parts of the world • ADA Specification no. 25 • ISO Specification no. 6873:1998
  • 6. TYPES  Albaster- pure white, fine grained and translucent Satin spar - fibrous needle like with silky lustre. Selenite - colourless, crystalline and transparent.
  • 7. USES • Building construction • Soil conditioning • Food additives • Pharmaceuticals • Medical devices
  • 9. APPLICATION IN DENTISTRY Phillips Pfaff introduced gypsum to dentistry. • For cast preparation. • Models and dies. • Impression Material • As Investment Material • Mounting of Casts • As a mould material for processing of complete dentures
  • 10. PRODUCTION OF GYPSUM PRODUCTS Calcination: The process of heating gypsum to manufacture plaster and stone products is known as calcination. i) Dry Calcination ii) wet Calcination iii) Dehydration by boiling with chemicals iv) Synthetic method
  • 11. Dry Calcination: • Commercially gypsum is finely ground and subjected to high temperatures of 110-130 °C in open kettle or kiln to eliminate the water of crystallization • Beta hemihydrates (Type I & II Gypsum products) are made with this method
  • 12. Wet Calcination: • Gypsum is ground and steam heated to a temperature of 110-130 °C in a closed kiln, kettle or autoclave • Alpha hemihydrates (Type III) are made through this method
  • 13. Dehydration by boiling chemicals: • Gypsum is calcined by boiling with 30% CaCl2 solution . The chlorides are then washed away in the presence of 0.5% Sodium Succinate. • α-modified hemihydrate (Type IV & V) Improved stone is manufactured by this method.
  • 14. Synthetic Gypsum Products: • alpha & beta hemihydrate can also be formed from the by-products or waste products of the manufacture of Phosphoric Acid. • More expensive than natural gypsum products • when product is properly made its properties are equal or exceed to natural gypsum products.
  • 17. Types of Gypsum Products According to ADA Specification no. 25 there are 5 types of Gypsum Products Type 1- Plaster impression Type 2- Plaster model Type 3- Dental stone Type 4- Dental stone high strength Type 5- Dental stone High strength high expansion
  • 19. Type I: Impression Plaster • Plaster of Paris is called so because it was obtained by burning gypsum from deposits near Paris. • Seldom used as replaced by less rigid materials like hydrocolloids and elastomers. Composition: • Dental plaster • K2SO4(Potassium sulphate) • Borax • Colouring and flavouring agent
  • 20. Type II: Model Plaster/ Laboratory Plaster • Used for Diagnostic cast Articulation of stone cast Art portion of working cast Flasking procedure for acrylic dentures Composition Βeta hemihydrate + Modifiers
  • 21. Type III: Dental Stone • Also known as Class I stone or Hydrocal. Used for • complete or partial denture models • Orthodontic models • Flasking procedure for acrylic dentures • It requires less water and is stronger than plaster • Composition: Alpha hemihydrate + 2 to 3% coloring agent+ K2SO4 (accelerator) + borax (retarder)
  • 22. Type IV: Dental Stone high strength Also called as Class II Stone or die stone or densite or improved stone. • About 2 times stronger than type II. Uses, Used when high strength and surface hardness is required. CAD /CAM dies Dies for fabricating inlay, crown and bridge wax patterns.
  • 23. • Type V: Dental Stone high strength high Expansion • E.g: Hard rock, Jade rock, Resinrock XL5, Denflo HX • Has higher compressive strength than type IV which is attained by lower W/P ratio. • Setting expansion has been increased from a maximum of 0.10% to 0.30% to compensate for the solidification shrinkage of some alloys used as base metals used for dental casting. • Most costly of all gypsum materials. • Should be avoided in production of dies for inlays as higher expansion may lead to tight fits.
  • 24. SETTING OF GYPSUM PRODUCTS • The reaction between gypsum products and water produces solid gypsum, and the heat evolved in the exothermic reaction is equivalent to the heat used originally for calcination. • Set gypsum products probably never attain 100% conversion unless they are exposed to high humidity for a long time. Therefore, there are unreacted hemihydrates remaining in the set materials.
  • 25. SETTING REACTIONS There are three theories of gypsum setting. Colloidal Theory: Mahaelis in 1893 The gypsum product when mixed with water enters into a colloidal stage through a sol-gel mechanism. When in sol stage, the hemihydrate undergoes hydration to form active dihydrate, which then undergoes gelation to form a solid set mass.
  • 26. • Hydration Theory: • Suggest that the hemihydrate undergoes hydration. • These rehydrated crystals are joined together through hydrogen bond to the sulfate groups forming a set solid mass.
  • 27. • Dissolution-precipitation theory: • French chemist Henry Louis Le Chatelier, 1887 • Widely accepted theory, based on the principle of gradual dissolution of hemihydrate and instant crystallization of dihydrate with inter locking of crystals forming a set mass.
  • 28. Stages of Crystallization • Dissolution • Suspension • Saturation • Super saturation • Nuclear formation • Growth of nuclei • Crystallization • Spherulite formation
  • 29. Physical stages of gypsum • Fluid • Plastic • Firable • Carvable
  • 30. Water-Powder Ratio and effect of low and high water powder ratio on Gypsum Products • Proportion of water and powder to make a workable mix. Water required per 100 gram powder.
  • 31. Effect of Temperature on Setting Reaction • Change in temperature produces two effects • 1) Change in Temperature causes change in solubility of the hemihydrate and dihydrate, which alters the rate of reaction. • As temperature increases Solubility ratio of hemihydrate to dihydrate decreases and vice versa • ↑Solubility ratio ↓Setting reaction & ↑Setting time • 2) Change in ion mobility:
  • 32. Effect of Temperature on Solubility • As expected increase in water temperature increases rate of reaction but this is not the case with Gypsum products • Effect of temperature on setting reaction varies from stone to stone • Little change occurs between 0°C to 50°C • If temperature of water exceeds 50°C, there is gradual retardation of the reaction • If the temperature approaches 100°C, no reaction takes place • Between 50 to 100 °C, reaction will get reversed.
  • 33. Mixing Time: • The time from addition of powder to the water until the mixing is completed. • Mechanical mixing : 20 to 30 seconds • Hand-Spatulation: 1 minute Working Time: • The time available to use workable mix. It is measured from start of mixing to the point where the consistency is no longer acceptable for the product’s intended purpose. Generally 3 minutes is sufficient. Setting Time: • Time that elapses from the beginning of mixing until the material hardens
  • 34. Initial Setting Time: • Time required for Gypsum products to reach a certain arbitrary stage of firmness which is represented by semi-hard mass that is no more workable, but is not completely set. Final setting time: • Time required for the reaction to be completed. It is the time at which the material can be separated from impression without any distortion or fracture. • Setting time is measured by various penetration tests.
  • 35. Test for Initial set • Loss of gloss test for initial set • Some of the excess water is taken up in forming the dihydrate so that the mix loses its gloss. It takes 12 +/- 4 minutes approximately
  • 36. • Initial Gilmore test for initial set • The smaller one amongst the Gilmore needles is used. • Mixture is spread out, needle is lowered onto the surface. • Time at which it no longer leaves an • impression is called initial set.
  • 37. Vicat Test for setting time • Instrument used: Vicat Penetrometer • The needle is held just in contact with the mix. Soon after the gloss is lost, the plunger is released. • Time elapsed from start of mixing till the needle no longer penetrates to bottom of mix is k/a setting time.
  • 38. Gilmore test for final setting time • The heavier one amongst the Gilmore needles is used. • Time elapsed at which this needle leaves only a barely perceptible mark on the surface is k/a final setting time.
  • 39. Weight And Diameter Of Vicat And Gillmore Needle • Vicat needle : Weight - 300gm • Diameter – 1mm • Gillmore needle : I)Small : Weight - ¼ lb Diameter - 1/12’’(2.12mm) II)Large : Weight - 1lb Diameter – 1/24’’(1.06mm)
  • 40. Ready for Use Criterion • It is a subjective measure of the time at which the set material may be safely handled in the usual manner. • Technically material may be considered ready for use at the time when compressive strength is at least 80% of that which would be attained at 1 hr. • Most products reach ready-for-use state in 30 minutes. • Clinical tips:  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.
  • 41. How to Control Setting time? Theoretically there are 3 methods • Solubility of hemihydrates can be increased or decreased Solubility Setting reaction & Setting time • Number of nuclei of crystallization can be increased or decreased Nuclei of crystallization Gypsum crystal formation & Setting time • By increasing or decreasing the rate of crystal growth, setting time can be accelerated or retarded respectively.
  • 42. Fineness Finer the particle size of hemihydrate, faster the mix hardens. Fineness increases both dissolution as well as number of nuclei of crystallization. W/P Ratio More the water used for mixing  Fewer nuclei available per unit volume Consequently setting time is prolonged.
  • 43. Mixing • Longer and rapid mixing  Shorter the setting time • As mixing begins, formation of crystals increases. At the same time crystals are broken up by mixing spatula and are distributed throughout mixture resulting in formation of more nuclei of crystallization.
  • 44. Modifiers: Retarders and Accelerators RETARDER ACCELERATOR Chemical added to increase setting time Chemical added to decrease setting time Organic materials: Glue, Gelatin and gums. Act by forming adsorbed layer on hemihydrate to reduce its solubility and inhibit growth of gypsum crystals Materials like Potassium sulfate (>2%) or NaCl (2%), Sodium sulfate ( 3- 4%), slurry water (gypsum <20%), tera alba act as sites for crystallization Salts: Borax, Potassium Citrate, NaCl (20%). Form a layer of calcium salt that is less soluble than sulphate Balanced Stone: A stone with a setting time established by addition of proper quantities of both accelerators and retarders Blood, saliva, alginate act as retarders and If left on impression, can affect surface details of impression. Impression surface need to be properly rinsed before being poured.
  • 45. Tera-alba • It is finely powdered gypsum /white earth like substances used to control setting time of plaster of Paris. It acts by providing additional nuclei of crystallization.
  • 46. Setting Expansion • Occurs due to outward thrusting 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. • It may range from 0.06% to 0.5% • It is of two types Normal setting expansion: refers to the setting expansion when a gypsum product is allowed to expand in air (when placed on table or dry environment) Hygroscopic setting expansion: refers to the expansion of a gypsum product when it is allowed to set immersed in water.
  • 47. Difference and similarities between normal and hygroscopic setting expansion:
  • 49. • Increased expansion (more than twice) occurs in hygroscopic setting because of additional crystal growth permitted by allowing crystals to grow freely rather than being constrained by the surface tension when the crystals grow in air. • There are five stages in the process of expansion explained below • Stage I: Represented by 3 round particles of hemihydrate surrounded by water which constitutes the initial mix • Stage II: Crystals of dihydrate begin to form on the nuclei and water around the particles is reduced by hydration and particles are drawn more closely together because of surface tension. In the hygroscopic setting, water of hydration is replaced and distance between the particles remains the same.
  • 50. • Stage III: As crystals grow, they contact each other and setting expansion begins. Water around the particles decreases in normal expansion. The particles with their attached crystals are drawn together but contraction is opposed by the outward thrust of the growing crystals. However the crystals in hygroscopic setting are not inhibited because of replenishment of water. • Stage IV & V: More marked effect. Intermeshed and entangled crystals prevent expansion in normal setting. • Both types of expansion are physical in nature and do not involve chemical reaction.
  • 51. Properties of Gypsum Products %Setting expansion at hrs Comp str.(1hr) (MPa) Hardness (dry) (RHN) ( μm) Detail reproduction Type 1 0.15 max 4 min 8max 75±8 Type2 0.05 9 75±8 Type2 0.06min, 0.30 max 9 75±8 Type3 0.20 max 20 82 50±8 Type4 0.15 max 35 92 50±8 Type5 0.16min 0.30max 35 50±8
  • 52. Control of setting expansion • W/P Ratio: • At higher W/P ratios, fewer nuclei of crystallization per unit volume are present compared to low W/P ratio. • Inter nuclear spaces will be more in case of higher W/P ratio and there is lesser growth interaction of dihydrate crystals resulting into lesser outward thrust. • Most effective method of controlling setting expansion is by adding chemicals. • Increased spatulation increases setting expansion • Setting expansion can be reduced by adding Potassium Sulfate, Borax or Sodium Chloride
  • 53. Strength of Gypsum Products • Strength of set Gypsum Products is expressed in terms of compressive strength, although Diametral tensile strength can also be considered. • Wet Strength or Green Strength is determined when water in excess of that required for hydration of hemihydrate remains in the test specimen. When such excess water is removed by drying, the strength obtained is dry strength. a. Effect of water content: • Dry strength is 2 or more times the wet strength. Up to 8 hrs about 17.4% weight is lost. • Between 8-24 hours of drying only 0.6% weight is lost but strength almost doubles because fine gypsum crystals that precipitate act as anchor between large crystals
  • 54.  Effect of W/P Ratio: • As W/P ratio increases, compressive strength decreases  Effect of Manipulation and additives: • An increase in spatulation increases strength proportionally up to the limit of 1 minute equivalent to hand mixing. Beyond this overmixing results in break of crystals and decrease in strength. • Additives decrease the strength as they act as impurities reduce intercrystalline cohesion.
  • 55. Manipulation of Gypsum Products Selection: based on the desired properties and dental application. e.g.: • For a diagnostic cast plaster can be used. • For a working cast, strength and accuracy is required, dental stone is the gypsum product of choice • Working models for cast restorations require die stone. • For casting procedures dental investment is required.
  • 56. Proportioning • Golden rule: Manufacturer instructions should always be followed. Variations in W:P • ratio affect the set materials’ properties such as strength and accuracy. • The strength of a stone is inversely proportional to the w/p ratio. • It is better to keep the amount of water as low as possible. • Once the optimum proportion is noted , the same must be used subsequently • An accurate graduated cylinder has to be used for water and a proper weighing balance for powder has to be used • Powder should NOT BE MEASURED USING SCOOP as it doesn’t pack uniformly.
  • 57. MIXING • Manual: Rubber bowl and spatula. • Powder is sifted into water to ensure good wetting and avoid clumps and air bubbles • If the mixing has to be done by hand , the bowl must be parabolic , smooth and abrasion resistant • Spatula should have a stiff blade and a convenient handle • Stirring of the water powder during mixing has to be done more vigorously in order to avoid air entrapment • Wetting of all powder particles with water must be ensured Improper Mix Proper Mix
  • 58. 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:
  • 59. • Double Pour Method • Single Step Method 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?
  • 60. 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
  • 61. Care of the cast • If the cast surface is not hard and smooth when removed from the mould , then its accuracy is questionable • Cast once set will maintain the dimensions relatively stable with changes in humidity and room temp. • But in few cases the cast has to be immersed in water , negligible expansion may occur if the water is saturated with calcium sulphate. If not saturated, gypsum will dissolve. • The safest method of soaking the cast in water is to immerse the cast in water bath that contain plaster debris at the bottom of the container.
  • 62. Infection Control • There are chances of cross infection of HBV, HIV etc on clinicians due to impression which has lead to the idea of disinfection sprays and immersion techniques to disinfect the impressions and its effect on the surface of the cast. • If the impression is not disinfected then the cast should be disinfected using disinfectant solutions . Alternatively dental stone containing disinfectant can be used. • Commonly used stone disinfectants are spray disinfectants, hypochlorites and iodophores and glutaraldehyde. • Microwave irradiation has been tested both for drying and disinfection.
  • 63. Specialized Gypsum Products Dental Casting Investments: • Silica + dental plaster/stone. • Capable of withstanding high temperatures. • Used to prepare refractory molds for casting. Divestment: • Die-stone + Gypsum bonded investment mixed with colloidal silica • Used to make refractory dies
  • 64. Orthodontic Stone • Orthodontists prefer to use white stone or plaster • Have longer working time for pouring multiple models • Treated with “model glow”- model soap to produce glossy surface Resin modified stones • They are resin fortified die-stones. Synthetic resin + alpha Gypsum • Less brittle, improved surface smoothness, increased resistance to abrasion • Compressive strength as high as 79MPa
  • 65. Mounting Plasters • Used for attaching casts to the articulators • Low setting expansion of 0 to .05(important for accuracy of mounting), • low strength of 12 MPa (allows easy separation from the cast) and fast setting time (3 minutes)  Fast setting stone • Exceptionally fast setting stones (2 minutes) • Early high compressive strength (41 MPa at 1 Hr.), which allows separation of cast from impression in 5 minutes
  • 66. Soluble plaster • When impression plaster contain potato starch to make the impression soluble it is termed as ‘soluble plaster’. • MODE OF ACTION:
  • 67. Conclusion • A wide choice of Gypsum Products is available so we should choose the one or a combination which is most suitable for our desired requirements.
  • 68. References • Philips’ Science of Dental Materials, 11th ed. • S. Mahalaxmi Materials Used in Dentistry, 1st ed. • John J Manappallil Basic Dental Materials, 4th ed.