Gypsum products

Seminar on the topic of Gypsum product

1. GYPSUM PRODUCTS Dileep kishor BDS IInd yr Maharana pratap Dental College Kanpur

2. Introduction Classification Types of gypsum products Manufacture of gypsum products Setting reaction Theories of setting reaction Manipulation Setting time Factor affecting setting time Modifiers Properties of gypsum products Setting expansion Care of gypsum Infection control

3. INTRODUCTION Gypsum is a mineral that is mined in various parts of the world. Chemically it is CaSO 4. 2H 2 O i.e. calcium sulphate dihydrate.

4. CLASSIFICATION ACCORDING TO ADA SPECIFICATION NO. 24 TYPE I – IMPRESSION PLASTER TYPE II – DENTAL PLASTER TYPE III – DENTAL STONE TYPE IV – DENTAL STONE (die stone , high strength , low expansion) TYPE V – DENTAL STONE (die stone , high strength, high expansion)

5. TYPE I OR DENTAL PLASTER , IMPRESSION Impression plaster was one of the earliest impression material in dentistry. Because of its rigidity (not elastic), it often had to be fractured to remove it from undercuts areas in the mouth Composition : Uses : 1. For making impression in complete denture and maxillofacial prosthetic (not used currently). 2. Bite registration material. Dental plaster + K 2 SO 4 + Borax + Coloring and flavoring agents

6. TYPE II OR DENTAL PLASTER , MODEL Composition : Contains Beta hemihydrate and modifiers Uses : 1. For making study casts and models. 2. To make models for curing dentures. 3. For making casts on articulator.

7. TYPE III OR DENTAL STONE , MODEL COMPOSITION : Alpha hemihydrate 2 to 3% coloring matter K 2 SO 4 – Accelerator Borax – Retarder USES : For preparing master casts and to make molds.

8. TYPE IV OR DENTAL STONE , HIGH STRENGTH , LOW EXPANSION Class II stone , die stone , densite , improved stone. USES : Die stone is the strongest and hardest variety of gypsum product. It is used when high strength surface hardness required , e.g., dies used for inlay , crown and bridge wax pattern.

9. TYPE V OR DENTAL STONE , DIE STONE , HIGH STRENGTH , HIGH EXPANSION It is the most recent gypsum product having a higher compressive strength than type IV this achieved by lower w/p ratio and increase the setting expansion to compensate for alloy solidification shrinkage. USES : To prepare dies with increased expansion.

10. MANUFACTURE OF GYPSUM PRODUCT The process of heating of gypsum for the manufacture of plaster is known as Calcination . CaSO 4. 2H 2 O CaSO 4 ⅟ 2 H 2 O CaSO 4 CaSO 4 110-130◦C (Calcium sulphate dihydrate) (Calcium sulphate hemihydrate) (Hexagonal anhydrite) 130-200◦C 200-1000◦C (Orthorhombic anhydrite)

11. ALPHA AND BETA HEMIHYDRATE Depending on the method of calcination there are two forms of hemihydrate. Beta hemihydrate(plaster) Alpha hemihydrate(stone) Alpha modified hemihydrate(die stone)

12. Plaster Manufacture Dry calcination Particle size/shape Large , irregular porous W/P ratio 0.5 P orosity Porous Properties Lower strength and hardness Application Used when strength is not primary importance (diagnostic casts) Stone Wet calcination Smaller, regular and dense 0.3 More porous Greater strength and hardness Used when greater strength and hardness is required (e.g., dies , master casts

13. MANUFACTURE OF DENTAL PLASTER Gypsum is ground and heated in an open kettle on kiln at a temperature of 110 to 130◦C. The process is called dry calcination . β type of crystals are formed. Microscopically : Fibrous aggregate of fine crystals with capillary pores. CaSO 4. 2H 2 O CaSO 4. ⅟ 2 H 2 O Heat 110-130◦C

14. MANUFACTURE OF DENTAL STONE Gypsum is calcined under steam pressure in an autoclave at 120 to 130◦C at 17Ibs/sq. inch for 5 to 7 hrs. Thus the product is obtained much stronger than β hemihydrate. Microscopically : Cleavage fragments and crystal in the form of rods and prism. CaSO 4. 2H 2 O CaSO 4. ⅟ 2 H 2 O 120 -130◦C 17Ibs/sq. inch pressure

15. SETTING REACTION When plaster is mixed with water it takes up one and half molecule of water. Hemihydrate + water Dihydrate + Unreacted hemihydrate + Heat The product of the reaction is gypsum. The heat evolved in the exothermic reaction is equivalent to the heat used originally in calcination.

16. THEORIES OF SETTING Three theories have been proposed : 1. Colloidal theory 2. Hydration theory 3. Dissolution – precipitation theory

17. Colloidal Theory : Plaster enter in to a colloidal state through sol gel mechanism In the sol state , hemihydrate combines with water(hydrates) to form dihydrate Water is consumed the mass turns to a ‘solid gel’. Plaster mixed with water

18. Hydration theory : The hydration theory suggested that rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set material

19. Dissolution precipitation theory : When hemihydrate is mixed in water it forms a fluid workable suspension Hemihydrate dissolves until it forms a saturated solution Some dihydrate is formed due to the reaction . Solubility of dihydrate is much less then hemihydrate, the saturated hemihydrate is supersaturated with respect to the dihydrate. All supersaturated solution are unstable. So the dihydrate crystals precipitation out. As the dihydrate precipitates out, the solution is no longer saturated with hemihydrate and so it continues to dissolve. The process continues no further dihydrate precipitates out of the solution

20. MANIPULATION Recommended W/P Ratio Impression plaster : 0.50 to 0.75 Dental plaster : 0.45 to 0.50 Dental stone : 0.28 to 0.30 Die stone, Type 4 : 0.22 to 0.24 Die stone, Type 5 : 0.18 to 0.22 INSTRUMENTS Flexible rubber/plastic bowl, stiff bladed spatula.

21. Procedure for Hand mixing Water is taken first to prevent adherence of dry powder to the sides of the bowl. Water and powder are dispensed according to the recommended W/P ratio. The powder is sifted in to water in the rubber bowl. Plaster/stone dispensers are also available It is allowed to settle for 30 sec to minimize air entrapment The mix is stirred vigorously. Continue till a smooth mix is obtained. Spatulation should be complete in 45 to 60 sec Vibrate the mix and pour it into the impression , taking care not to entrap air

22. SETTING TIME The time elapsing from the beginning of mixing until the material hardness is called setting time. Initial setting time : Hemihydrate crystals react to form dihydrate crystals. The viscosity of the mass is increased and it can no longer be poured. The material become rigid. It can be carved but not moulded. This is known as initial setting time. Final setting time : The time at which the material can be separated from the impression without distortion or fracture.

23. Measurement of setting time Loss of gloss method Exothermic reaction Penetration tests By using penetrometers Types of penetrometers Vicat needle Gillmore needles : 1.Initial Gillimore 2. Final Gillimore

24. Loss of gloss method : Occur when excess water in the mix is taken up in forming the dihydrate, so the mix loses its gloss. This occur approx. at 9mins and the mass has no measurable compressive strength. Vicat needle : It weight 300gm and the needle diameter is 1mm. The time elapsing from the start mixing till the needle does not penetrate to the bottom of the plaster is the setting time. The setting time obtained with the vicat needle is similar to the initial gillmore,

25. GILLMORE NEEDLES Two types of gillmore needles : 1.Small : ¼ Ib weight and diameter of 1/12” (2.12mm). 2.Large : 1 Ib wt and diameter of 1/24”(1.06mm)

26. Initial gillmore : The time elapsing from the start of mixing until the time when the point of the ¼ Ib Gillmore needle no longer penetrates the surface is the initial setting time. Final Gillmore : The time elapsing from the start of mixing until the point of the 1 Ib Gillmore needles leaves only a barely visible mark on the surface of the set plaster is known as the final setting

27. VICAT NEEDLE GILLMORE NEEDLE

28. Factors Affecting Setting time : 1. Manufacturing process 2. Mixing and spatulation 3. Water/Powder ratio 4. Temperature 5. Modifiers

29. Modifiers Modifiers are chemicals added in order to alter some of the properties and make it more acceptable to the dentist. If the chemical added decreases the setting time it is called an accelerator , whereas if it increases the setting time it is called a retarder .

30. Accelerators : The substances which decreases the setting time of gypsum products by increasing the rate of reaction are called accelerator. E.g. Tera alba(ground gypsum), 3.4% sodium and potassium sulphate (above 3.4% act as retarder), 2% sodium chloride(up to 2% low conc. act as retarder). Retarders : They are the substances which increases the setting time of gypsum products by decreasing the rate of reaction. E.g. Borax(1 to 2% most effective), sodium chloride(higher conc. 3.4 to 20%)

31. PROPERTIES OF GYPSUM PRODUCTS The important properties of gypsum products are : Setting expansion Strength Hardness and abrasion resistance Reproduction of detail

32. SETTING EXPANSION The dimensional increase that occurs concurrently with the hardening of various material. Setting expansion is of two types : 1. Normal setting expansion 2. Hygroscopic setting expansion

33. NORMAL SETTING EXPANSION All gypsum products show linear expansion during setting, due to the outward thrust of the growing crystal during setting. Crystal growing from the nuclei not only intermesh but also intercept each other during growth.

34. Control of setting expansion Mechanical mixing reduces setting expansion when compared to hand mixed stone. Increase in W/p ratio reduces the setting expansion. Modifiers generally reduces the setting expansion. Potassium sulphate 4% solution reduces setting expansion from 0.5 to0.06%. Sodium chloride and borax also decrease setting expansion.

35. HYGROSCOPIC SETTING EXPANSION

36. STRENGTH The strength increases rapidly as the material hardens after the initial setting. Factors affecting strength : W/P ratio : the more the water, the greater the porosity and less the strength. Spatulation : Within limits, strength increases with increased with increased spatulation Addition of accelerator and retarder lower strength

37. Wet strength : The strength when excess free water(more than is necessary for reaction) is present in the set gypsum. The wet strength( 1 hr compressive strength): Model plaster - 12.5Mpa Dental stone - 31Mpa Die stone - 45MPa Dry strength : The strength of gypsum when the excess free water is lost due to evaporation. It two or more times greater than wet strength.

38. HARDNESS AND ABRASION RESISTANCE Dies and casts are often used to construct restoration and prosthesis. A good surface hardness and abrasion resistance is therefore essential.

39. FLOW The greater the amount of water used(with in the clinical limit), the greater would be the flow. REPRODUCTION OF DETAIL Gypsum products reproduce detail accurately.

40. CARE OF GYPSUM The gypsum cast has to be soaked in water it must be placed in water bath. Storage of the powder As plaster is hygroscopic it should be kept in air tight container. It should be kept clean with no dirt or other foreign bodies.

41. INFECTION CONTROL Gypsum products may be disinfect by : Immersing cast in a disinfected solution (commonly used stone disinfectant are spray disinfectant, hypochlorites & iodophores.) Addition of disinfection solution. Overnight gas sterilization