GYPSUM PRODUCTS AND ITS CLINICAL IMPLICATIONS

Gypsum products in Dental Materials

1. GYPSUM PRODUCTS Guided by- Dr. Ajit Jankar Dr. Bhushan Bangar Dr. Shashi Patil Dr. Shirish Pawar Dr. Shital Wagh Dr. Yogesh Nagargoje Dr. Sneha Chole Dr. Vidya Vaybase Dr. Rutuja Tidke Presented by- Dr. Prashansa Vaikunthe

2. What we will learn after this seminar? Introduction Uses and applications in Dentistry Manufacturing of Gypsum products Manipulation of Gypsum Products Classification Setting reactions and tests for setting of Gypsum Setting Expansion Strength of Set Gypsum Products Infection control Modifications Conclusion References

3. Introduction Gypsum , the natural hydrated form of calcium sulfate dihydrate (CaSO 4 •2H 2 O ); gypsum dihydrate. Discovered by Egyptians, Gypsos - chalk or plaster M ineral mined in various parts of the world , particularly in Germany and Nova Scotia. First found in mines around the Montmartre city of Paris, so it is also called Plaster of Paris. (a misnomer)

4. Crystalline forms of Gypsum pure white, fine grained and translucent Satin spar Fibrous needle like with silky lustre Selenite colourless,crystalline and transparent Albaster

5. Cast, Die and Model Cast- A life-size likeness of some desired form. -GPT10 Die- The positive reproduction of the form of a prepared tooth in any suitable substance. -GPT10 Model- A positive likeness of an object.

6. Uses of Gypsum

7. Applications in Dentistry

8. Manufacturing of Gypsum products Calcination : The process of removing water of crystallization from dihydrate. The process of heating a solid material to drive off volatile chemically combined components, such as water and carbondioxide. -Philips Gypsum materials are produced by calcining Calcium sulphate dihydrate. Supplied as fine hemihydrate powders produced by heating ground gypsum particles, after mixing with H 2 O reverts back to gypsum

9. Commercially, the gypsum is ground and subjected to temperatures of 110 °C - 130 °C (230 °F to 266 °F) in open containers to drive off part of the water of crystallization. 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.

10. Dry Calcination Gypsum is ground heated in an open kettle or kiln at a temperature of 110-130 o C. Beta (β) type of crystals are formed. Microscopically, they are fibrous aggregate of fine crystals with capillary pores. Wet Calcination Gypsum is calcined under steam pressure in an autoclave at 120-130 degree celsius at 17 lbs/ sq. inch for 5-7 hours. The product obtained is α-hemihydrate. Microscopically, they are cleavage fragments and crystals in the form of rods and prisms.

11. Chemical Calcination If the calcination process occurs under pressure in 30% Calcium Chloride solution or in the presence of more than 1% of sodium succinate, the resulting hemihydrate crystals will be shorter and thicker than those produced in a closed container. Residual chemicals are removed by washing the powder with hot water. Gypsum product formed is called as modified α-hemihydrate or die stone. Microscopically, smooth, cuboidal and more dense powder particles.

12. Wet Calcination Gypsum is calcined under steam pressure in an autoclave at 120-130 degree celcius at 17 lbs/ sq. inch for 5-7 hours. the product obtained ( α-hemihydrate) is much stronger and harder than β-hemihydrate. Microscopically, they are cleavage fragments and crystals in the form of rods and prisms.

13. 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.

14. Proportioning: 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. 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.

15. Mixing: Manual: Rubber bowl and spatula. Powder is sifted into water to ensure good wetting and avoid clumps and air bubbles. 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

16. Air porosity may be reduced by vibrating the mix of plaster or stone in order to bring air bubbles to the surface. Mixing the material mechanically under vaccuum . Both

17. Pouring of the cast: The cast is composed of two parts which are prepared separately. The anatomical part (hard and soft tissue), here impression is 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: Single Step Method Both anatomical and art portions of the cast are prepared at the same time. This method requires skill and accurate timing.

18. b. Double Pour Method; Inversion Method In Inversion method, a mound of stone about 20mm high is made on a flat surface, such as a glass plate, with the remainder of the mixture or with a new mix using a lower W/P ratio. The filled impression, which should exhibit the initial set but not the final set, is then inverted over the mound of stone and the base is shaped with spatula before the stone sets. The dental stone model should be left undisturbed for 45-60 mins until the material has set completely. The dental stone model is now separated from the impression.

19. c. 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, 12 mm above the highest point of the impression.

20. According to ADA Specification no. 25 , there are 5 types of Gypsum Products ; Type 1- I mpression Plaster Type 2- Dental Plaster Type 3- Dental stone (Hydrocal) Type 4- Dental stone high strength ( Die-stone, Densite) Type 5- Dental stone High strength high expansion (Identical chemical formulae- CaSo 4 . 1/2H 2 O) Types of Gypsum Products

21. Types of Gypsum products According to ADA Specification no. 25 , there are 5 types of Gypsum Products ;

22. Type I: Impression Plaster Type II: Dental Plaster Used for - f lasking -mounting casts on articulators -bases for casts Disadvantages - low strength - porosity One of the earliest impression materials in dentistry. Its fluidity makes it suitable for making impressions of soft tissues in an uncompressed state. Composed of – BETA HEMIHYDRATE + MODIFIERS (to regulate setting time and setting expansion)

23. Type III: Dental stone Type IV: Dental Stone high strength Also called as Class II Stone or densite or improved stone. Used when high strength, surface hardness and minimal setting expansion is required. Dies for fabricating inlay, crown and bridge wax patterns Also known as Class I stone or Hydrocal . Used for Full or partial denture models Flasking procedure for acrylic dentures It requires less water and is stronger than plaster

24. 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 , as base metals used for dental casting. Should be avoided in production of dies for inlays as higher expansion may lead to tight fits . Type V: Dental Stone high strength high expansion

25. Specialized Gypsum Products

26. Setting Reaction When plaster is mixed with water it takes up 1 ½ molecule of water i.e. it regains water of crystallization and becomes calcium sulphate dihydrate. The reaction is exothermic . The reaction is continuous and repeated until hemihydrate is converted to dihydrate . 2CaSO 4 . ½ H 2 O + 3H 2 O 2(CaSO 4 . 2H 2 O) + Heat (3900 Cal)

27. Theories of Setting Reaction of Gypsum Products

28. Put forward by Mahaelis in 1893. When gypsum is mixed with water, it enters into a colloidal sol state through a sol-gel mechanism. As the amount of water is consumed, the mass converts into a solid-gel. Colloidal Theory

29. When hemihydrate undergoes hydration, the rehydrated crystals are joined together to hydrogen bond to the sulfate groups forming a set solid mass. Hydration Theory

30. Most widely accepted mechanism, propounded by French chemist Henry Louis Le Chatelier in 1887 . Based on dissolution of plaster and instant crystallization of gypsum followed by interlocking of the crystals to form set solid. This reaction has become possible because the solubility of hemihydrate in water is four times greater than that of the dihydrate near room temperature. Dissolution PrecipitationTheory

31. 2CaSO 4 . ½ H 2 O + 3H 2 O 2(CaSO 4 . 2H 2 O) + Heat (3900 Cal) The reaction is exothermic . The reaction is continuous and repeated until hemihydrate is converted to dihydrate . As the amount of gypsum formed increases, the mass thickens and hardens into needle like clusters called Spherulites . Finally intermeshing of crystals of gypsum leads to a strong solid structure .

32. Further crystal growth occurs as the hemihydrate dissolves and dihydrate precipitates forming new crystals. The precipitation of dihydrate occus due to the difference in solubility of dihydrate and hemihydrate leading to crystallization of dihydrate. Hence the name “Crystalline Theory” . The crystals of dihydrate are called spherulites due to their needle-like appearance. Spherulite -Greek word, sphaira - ball and lithos -stone.

33. Setting Time: Working Time: Mixing Time: Time that elapses from the beginning of mixing until the material hardens. The time from addition of powder to the water until the homogenous workable state is facilitated. Mechanical mixing : 20 to 30 seconds Hand-Spatulation: 1 minute The time 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.

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. TESTS FOR SETTING OF GYPSUM PRODUCTS Loss of gloss test: As the reaction proceeds, the excess water on the surface is taken up in forming the dihydrate, so that the mix loses the surface gloss ( approximately 9 minutes). Exothermic Reaction : Temperature rise of the mass. Loss of gloss Initial Setting Time Final Setting Time

36. Gillmore Test Initial The smaller one amongst the Gilmore needles is used. Time at which it no longer leaves an impression is called initial set. Final 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 called final setting time. 2.13mm diameter point under 0.25lb or 113.4gm weight Time: 13 minutes 1.06mm diameter point under 1 lb or 453.6gm weight Time: 20 minutes

37. Vicat Test 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 denotes final setting time . 300gms load 1mm diameter Setting time ranges from 20-30 minutes

38. Ready to Use Criteria 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.

39. Control of Setting time Theoretically there are 3 methods: Solubility of hemihydrates can be increased or decreased Solubility Setting reaction & Setting time 2. Number of nuclei of crystallization can be increased or decreased Nuclei of crystallization Gypsum crystal formation & Setting time 3. By increasing or decreasing the rate of crystal growth, setting time can be accelerated or retarded respectively.

40. Impurities : Fine gypsum particle residues from incomplete calcination or addition by the manufacturer will shorten the setting time because of the increase in the number of nuclei. Fineness: Finer the particle size of hemihydrate, faster the mix hardens. Fineness increases both dissolution as well as number of nuclei of crystallization.

41. W/P Ratio- Weight/volume of water Weight of hemihydrate powder More the water used for mixing Fewer nuclei available per unit volume ratio  nuclei of crystallization , strength, setting expansion & setting tim e Longer & rapid mixing- nuclei of crystallization, setting time. water temp.- accelerate the setting time. little change- 0-50ºC gradual retardation->50ºC no reaction-100ºC   Effect of W/P ratio on setting reaction

42. Change in temperature produces two effects Change in Temperature causes change in solubility of the hemihydrate and dihydrate, which alters the rate of reaction. As temperature increases , s olubility ratio of hemihydrate to dihydrate decreases and vice versa . Solubility ratio Setting reaction & Setting time. Effect of Temperature on setting reaction

43. 2. Change in ion mobility: Superimposition phenomenon: I nc rease in temperature from 20 degrees to 37 degrees  shortens setting time. However, temperature above 37 degrees , rate of reaction decreases lengthening of setting time At 100 degrees no reaction(solubility ratio becomes 1)

44. 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. Temperature : The difference in solubility between hemihydrate and gypsum becomes smaller with increasing temperature, and this condition lowers the driving force for forming the dihydrate; it also results in a slower setting reaction.

45. 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 o C to 50 o C If temperature of water exceeds 50 o C , there is gradual retardation of the reaction. If the temperature approaches 100 o C , no reaction takes place Between 50 to 100 o C , reaction will get reversed.

46. Use of Modifiers

47. Setting Expansion Occurs due to the 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.

48. Schematic sketch of setting expansion under Normal and Hygroscopic setting conditions

49. Strength of Set Gypsum Products Strength of set Gypsum Products is expressed in terms of compressive strength. 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 .

50. 1. 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. 2. Effect of W/P Ratio: As W/P ratio increases, compressive strength decreases.

51. 3. 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 that reduce intercrystalline cohesion.

52. D etails : Type I & II- can reproduce a groove of 75 m width , whereas TYPE III, IV and V ; of 50 m width. A ir bubbles: Formed at the interface of impression and gypsum casts because freshly mixed gypsum does not wet silicone impression materials.  use surfactants. C ontamination : by blood or saliva (also retards the reaction) rinsing of impression  

53. Care of Cast: Cast once set will maintain the dimensions relatively stable under ordinary conditions of humidity and room temperature. 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. Safest- soak the cast in water containing gypsum debris. Store at temp. <55 If >55 - shrinkage with loss of strength occurs  

54. Infection Control There are chances of cross infection of HBV, HIV etc. on clinicians. If the impression is not disinfected then the cast should be disinfected Alternatively dental stone containing disinfectant can be used. Disinfectants can be incorporated in powder or dissolved in water 0.5% sodium hypochlorite 10% povidone -iodine solution 0.12% chlorhexidine gluconate Microwave irradiation has been tested both for drying and disinfection. ( 5min at 900W)

55. Modifications of gypsum 4. Balanced stone- contains equal amount of accelerators and retarders

56. Conclusion A unique dental material which is easy to manipulate and sets to a hard and strong mass. 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. Gypsum products are immensely useful in dentistry in general, and Prosthodontics in particular.

57. References Anusavice , Kenneth. J. Gypsum Products In John Dayson , Editor: Phillips’ Science of Dental materials eleventh edition. Elsevier/ Saunder’s ; 2003. p.255-280. Anusavice , Kenneth.J . Gypsum Products In Chiayi Shen, Editor: Phillips’ Science of Dental materials twelfth edition. Elsevier/ Saunder’s ; 2013. p. 1 Craig’s Restorative Dental Materials twelfth edition. Elsevier/ Mosby;2006. p. 313-24.