GYPSUM and it’s various products used in dentistry

Gypsum and it’s products
Dental material

1. GYPSUM PRODUCTS & ITS APPLICATIONS IN DENTISTRY PRESENTOR : DR. AKSHITA JAIN MDS (1ST YEAR)

2. OBJECTIVES Understand the role of gypsum products in dentistry. Learn about the properties of gypsum products. Understand the classification and production of gypsum products. Identify methods to control setting and expansion. Understand the infection control practices for gypsum products.

3. Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002 CONTENTS INTRODUCTION APPLICATIONS OF GYPSUM IN DENTISTRY NATURE OF PARTICLES & SETTING MECHANISMS SETTING REACTIONS THEORIES W/P RATIO TYPES OF TESTS FOR WORKING, SETTING, AND FINAL SETTING TIME CONTROL OF THE SETTING TIME : Retarders and Accelerators SETTING EXPANSION CONTROL OF SETTING EXPANSION HYGROSCOPIC SETTING EXPANSION STRENGTH TYPES OF GYPSUM PRODUCTS PROPORTIONING, MIXING, AND CARING FOR GYPSUM PRODUCTS INFECTION CONTROL

4. Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002 INTRODUCTION The Dihydrate form of calcium sulfate, called gypsum, usually appears white to milky yellowish and is found in a compact mass in nature. The mineral gypsum has commercial importance as a source of plaster of Paris. The term plaster of Paris was given because it was obtained by burning the gypsum from deposits near Paris, France. Gypsum products probably serve the dental profession more adequately than any other material .

5. APPLICATIONS OF GYPSUM IN DENTISTRY . For preparation of study model for oral and maxillofacial structures . A uxiliary materials for dental lab operations used in production of dental prosthesis . U sed to form molds and casts on which dental prostheses and restorations are constructed . Used as gypsum dental investment to form molds for the casting of dental restorations with molten metal. Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002

6. Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002 CLASSIFICATION OF GYPSUM PRODUCTS ADA/ANSI Specification No. 25 Type 1 — Impression plaster Type 2 — Dental plaster Type 3 — Dental stone (Hydrocal,Alpha hemihydrate) Type 4 — Dental stone (High strength, low expansion) [die stone, die, Modified alpha hemihydrate] Type 5 — Dental stone (High strength, high expansion)

7. CALCINATION B-HEMIHYDRATE . (Dental plaster) Modified ɑ- HEMIHYDRATE . (Die stone) ɑ- HEMIHYDRATE (Dental stone) Wet Calcination autoclave \ closed vessel steam (120 - 130 ”C ) Boiling with 30% calcium chloride \ magnesium chloride . Dry Calcination open kettle\ vessel ( 120- 180 ”C) PRODUCTION OF GYPSUM PRODUCTS

8. β - HEMIHYDRATE Large , irregularly shaped orthorhombic crystal particles with capillary pores and characteristic sponginess. Powder has low density and poor packing capacity Require large amount of water (as max is absorbed into the pores. i.e 50 ml water to 100 gm of powder ) Product obtained is weaker in nature as it has less particle/area. ɑ . - HEMIHYDRATE Smaller, regularly shaped crystalline particles in the form of rods or prisms. Powder has high density and pack well . ( non porous nature) Require less water to mix . i.e 30 ml water to 100 gm powder. Product obtained is stronger in nature as it has more particle/area and also during wet calcination sufficient water is present to allow complete solution conversion hence undergo recrystallization. The difference between a- and B-hemihydrates are a result of differences in crystal size, surface area and degree of lattice perfection:-

9. Powder particles of plaster of paris (b- hemihydrate) Powder particles of dental stone (a- hemihydrate) The B-hemihydrate particles absorb more water. - As the chief factors in gauging the amount of mixing water:- Particle size ( irregular ) total surface area ( larger ) particle distribution ( porous - far apart) Adhesion between the particles of hemihydrate ( less).

10. SETTING OF GYPSUM PRODUCTS The reverse of reaction describes the reaction of calcium sulfate hemihydrate powder with water to produce gypsum. The product of the reaction is gypsum and the heat evolved in the exothermic reactions equivalent to the heat used originally in calcination. Completely set material likely never attains 100% conversion to the dihydrate form unless exposed to high humidity for a long period of time. Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002

11. Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002 SETTING REACTIONS 3 T heories :- The Colloidal Theory Proposes that when mixed with water, plaster enters into the colloidal state through a 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. The Hydration Theory Suggests that rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set material.

12. 3. The Dissolution- precipitation theory This is based on dissolution of plaster and instant recrystallization of gypsum, followed by interlocking of the crystals to form the set solid. Thus the setting reactions is a simple hydrating reaction and is as follows: This dissolution of the hemihydrate and precipitation of the dihydrate proceeds as either new crystals form or further growth occurs on the crystals already present. The reaction is continuous and continues until no further dihydrate precipitates out of solution. As the dihydrate precipitates, the solution is no longer saturated with the hemihydrate,so it continues to dissolve Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002 This saturated hemihydrate solution is supersaturated. dihydrate precipitates out. The hemihydrate dissolves until it forms a saturated solution. When the hemihydrate is mixed with water, a suspension is formed that is fluid and workable.

13. Temperature rise during setting reaction :- The calcination was endothermic process, whereas this conversion is exothermic. The magnitude of temperature depends on bulk of material used (can be 25-30°C at the center) Maximum temperature rise is during the stage when final hardening occurs. Clinically:- Useful during flasking - softens the wax and enables easy removal from the mold. I- Initial set F-Final set

14. PROPERTIES :- Mixing time :- the time from the addition of the powder to the water until the mixing is completed. Mechanical mixing - 20 to 30 sec. Hand-spatulation- 1 min (at least) 2. Working time :- It is measured from the start of mixing to the point where the consistency is no longer acceptable for the product’s intended purpose - 3 mins is adequate A vibrator improves the flow and reduces voids, thereby improvin g 1 5 s trengt h and accuracy

15. 3. Setting Time :- The powder is mixed with water, and the time that elapses from the beginning of mixing until the material hardens is known as the setting time. 2 different setting times. INITIAL Setting time :- Clinically test- Loss of gloss test for initial set As the reaction proceeds , hemihydrate takes up excess water in forming the dihydrate so that the mix loses its gloss. This loss of gloss occurred at approx - 9 mins(ADA specification no 25) ( mass still had no measurable compressive strength ) —> not safely removed from the mold. Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002

16. II . Initial Gillmore Test for Initial Set The smaller one is most frequently used for testing the setting time of dental cements, but it is sometimes used on gypsum products. Initial gilmore test is marked by a definite increase in strength. Approx 8-16 mins Gillmore needles are of Two types—small and large. The small Gillmore needle has a 1/4 lb weight and a diameter of 1/12" (2.12 mm) while the large Gillmore has a 1 lb wt and diameter of 1/24" (1.06 mm ) Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002 III. Vicat Test for Setting Time Instrument, the Vicat penetrometer .

17. B . FINAL Setting Time Occurs when major crystallization i.e conversion of hemihydrate to dihydrate is essentially completed I. Clinically Test - roughly detected by dissipation of heat of reaction II. Gillmore needle test - use heavier Gilmore needle, [1.06 dia,453.6gm-wgt] . The elapsed times where the needle leaves barely a mark on the surface is called Final setting time. SETTING TIME of various materials - according to (ADA specification no. 25 ) Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002

18. Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002 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. It is not determined by any designated test; 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 hr . Most modern products reach the ready-for- use state in approximately 30 min.

19. CONTROL OF THE SETTING TIME Theoretically, at least three methods that can achieve such control. CLINICALLY :- A. Decrease the setting time B. Increase the Setting time Longer the mixing time Decreased w/p ratio- thicker mix Addition of chemical accelerators- eg . potassium sulphate , sodium chloride Shorter mixing time Increased w/p ratio - thinner mix Addition of chemical retarder - eg. borax (>0.08 gm/ lt ) , gums and gelatins. Increase the temperature Solubility of the hemihydrate Rat e of crystal growth No . of nuclei of crystallization Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002

20. Add graph we ratio n st Temp graph

21. ACCELERATORS AND RETARDERS RETARDERS:- ( ↑ Setting time ) ACCELERATORS :- (↓ Setting time) Organic materials ( glue, gelatin ) - f orms adsorbed Layer on the hemihydrate . (dec solubility) Borax (>0.08gm/ lt ) forms calcium borate to prevent further growth of crystals. Sodium C hloride (higher concentration) (> 2 %)/ P otassium chloride The gypsum (<20%) is typically added by including a small percentage of slurry water in the mixing water Potassium sulphate (>2%) - Sodium Chloride (2 %) Borax (<0.08gm/ lt ) . Probably the most effective and practical method for controlling the setting time.

22. Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002

23. SETTING EXPANSION This represents a LINEAR SHRINKAGE in the gypsum object of approximately 2.4%. According to these calculations, a volumetric contraction should occur during the setting reaction. However, a SETTING EXPANSION is observed instead; this phenomenon can be rationalized on the basis of the crystallization mechanism (0.06% - 0.5%) Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002

24. Since after the setting reaction --> gypsum is greater in external volume but less in crystalline volume --> the immediately set material must be porous --> composed of interlocking crystals, between which are micropores and pores containing the excess water required for mixing. On drying, the excess water is lost, and the void space is increased.

25. CONTROL OF SETTING EXPANSION 1. Water-to-Powder Ratio - Lower W/P ratio (thicker mix):  increases expansion   - Higher W/P ratio (thinner mix):  reduces expansion 2. Mixing Time & Spatulation - Long & vigorous mixing (especially hand) - inc expansio n 3. Chemical Modifiers - Accelerators   (e.g., K₂SO₄, NaCl up to ~2%) fast set- reduce expansion . - Retarders  (e.g., borax, citrates, acetates) slow set -  reduce expansion 4.Temperature & Humidity – Cold water (20C) — increased expansion Warm water (~37 °C) – reduced expansion Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002

26. STRENGTH The dry strength two or more times as high as the wet strength as excess water in cast reduces the cohesion of calcium sulfate dihydrate. WET STRENGTH DRY STRENGTH 2 TYPES :- - Strength measured when the excess of water within the cast is driven off . - Methods :- Heating cast at elevated temp (50 C for 1- 2 hrs. ) Cast at room temperature for approx 7 days. - Strength measured at 1 hour when the sample contains some or all the water in excess of the theoretical amount required for hydration (18.6 ml per 100 gm).

27. WHY ? Because when last traces of water leave, fine crystals of gypsum precipitate. These anchor the larger crystal .Therefore ,if water is added or if excess water is present, these small crystals are the first to dissolve, and thus the reinforcing anchors are lost . Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002

28. WATER/POWDER RATIO affecting the strength :- ↓ w/p ratio ( ↑ N o. of crystals ) - Advantage - ↑ setting expansion ↑ compressive strength Disadvantage - incomplete setting reaction ↑ w/p ratio ( ↓ N o of crystals )- Advantage - smooth and workable mix Disadvantage - ↓ strength ↓ setting expansion ↓ setting time

29. TYPES OF GYPSUM PRODUCTS The criteria for selection of any particular gypsum product depend on its use ,and the physical properties necessary for that particular use. Anusavice K.J, – “Phillips’ science of dental materials” 11th edition,2002

30. Type Main Uses Water-Powd er Ratio (W/P) 1- Hour Compressive Strength Setting Expansion Advantages Disadvantages I – Impression Plaster Preliminary impressions, bite registrations 0.50-0.75 4 MPa 0.00- 0.15% Good fluidity Poor strength; low accuracy II – Model Plaster Mounting, study models, flasking denture cases 0.45–0.50 ~ 9 MPa 0.00–0.30% Easy to use, economical Low strength; poor abrasion resistance III – Dental Stone Casts, study models, articulator mounting 0.28–0.30 ~20.7 MPa 0.00–0.20% Better strength & detail reproduction Slight expansion Lower than type 2. IV – Dental/ Die stone ( High-Strength, Low-Expansion ) Dies for crowns/bridges, detailed wax- ups 0.22–0.24 ~34.5 MPa 0.00–0.10% Excellent strength, low porosity minimal expansion , resistant to abrasion (ideal for precision dies) Higher cost; difficult to handle. ( Due to low W/P ratio. ) V – High-Strength, High-Expansion Dies for metal frameworks (base- metal alloys) 0.18–0.22 ~48.3 MPa Up to ~0.30% High strength (low porosity) & compensatory expansion for alloy shrinkage. Excessive expansion risk in inlays & onlays ( causes the metals to fit more tightly ) expensive .

31. SYNTHETIC GYPSUM It's also possible to make a - hemihydrates and B - hemihydrates from the byproducts or waste products of phosphoric acid production. The product is more expensive than that made from natural gypsum, and its properties are equal to, or exceed those of the latter. SPECIALIZED GYPSUM gypsum products that have been characterized for special purposes For example, the orthodontist prefers a white stone or plaster for study models and may even treat the surface with soap for an added sheen. These products generally have a longer working time for ease of trimming “Mounting stones” for articulation ( fast set , low expansion)

32. GYSUM as a INVESTMENT MATERIAL Investment material – A material that is suitable for forming a mood into which a metal or alloy is casted . Used – gold alloys ( onlays , inlays , crown ) Component :- GYPSUM ( Binder )- A- hemihydrate. 15% SILICA ( refractory material) – quartz , cristobalite and tridymite  α quartz  β quartz, (573 °C) , (linear expansion of 0.45%) α cristobalite  β cristobalite( 200 °C and 270 °C) α tridymite  β tridymite 117 °C and 163 °C, - to regulate thermal expansion 3. Modifiers * reducing agent – carbon / copper - non oxidizing atmosphere in mold * boric acid / NaCl – regulates ST / SE , - prevents shrinkage of gypsum

33. HYGROSCOPIC SETTING EXPANSION depends on :- Composition - particle size of the silica- Finer Amount of water added - proportional to the amount of water added during the setting period until maximal expansion occurs. No further expansion is then evident regardless of the amount of water added. VALUE – 1.2% - 2.2 % ( 5-6 timers higher) NORMAL SETTING EXPANSION Depends on thermal expansion of the wax pattern Amount of heat produced depends on - Increasing gypsum content Dec w/p ratio – increases setting exp Clinically influenced by :- softer and Thinner wax VALUE - 0.6% maximum  A, Normal setting expansion, B-Hygroscopic setting expansion. The investment was immersed in water 5 minutes after the beginning of mixing; the water/powder ratio was 0.30.

34. THERMAL EXPANSION Value – 1%-1.6% at 700C The thermal expansion investments are influenced by :- REFRACTORY MATERIAL Type - cristobalite – 1.6% (250C) - quartz –1.4% (600C) , atleast 75 % should be present - tridymite – 1% Particle size of the silica – Finer , uniform GYPSUM BINDER – a hemihydrate W/P RATIO – decrease CHEMICAL MODIFIERS – boric acid , NaCl , KCl -increases expansion and hardens the set material Cristobalite Quartz

35. CARING FOR GYPSUM PREVENTION - Therefore it is important that all types of gypsum products be stored in a dry atmosphere The best means of storage is to seal the product in a moisture- proof metal container. When gypsum products are stored in closed containers, the setting time is generally retarded only slightly, approximately 1 or 2 min per year.

36. INFECTION CONTROL Importance : Gypsum casts can carry pathogens from impressions or clinical procedures, risking cross-infection to staff and technicians. Hence, disinfection is mandatory. Methods to Avoid Infection of Casts/Models Disinfecting impressions Can cause dimensional changes, especially in hydrocolloid and polyether impressions if immersed too long. Disinfecting casts/models 1. Adding disinfectants to mixing time:- 5% phenol or 2% glutaraldehyde. Disadvantage : Both are tissue irritants 2 . Immersion method : - 1:10 dilution of 5% sodium hypochlorite for 30 mins. 3. Sterilization : - Gypsum models can be sterilized with ethylene oxide. 4. Surface spraying method : - Spray hardened gypsum with iodophor, glutaraldehyde, or phenol and then store in sealed plastic bag for 10 mins.

37. REFERENCES Anusavice, K.J.Phillips’ science of dental materials. 11th ed, WB Saunders, Philadelphia,2003 Anusavice KJ. Phillips’ science of dental materials. 12th ed. Anusavice KJ, Shen C, Rawls HR, editors. Saunders; 2014. Powers JM, Craig R. Restorative Dental Materials. 11th ed. London, England: Mosby; 2001. Hamdy TM. Effect of aluminum oxide addition on compressive strength, microhardness and setting expansion of dental plaster. Int J Adv Res. 2019 Aghbolaghi N, Maleki Dizaj S, Negahdari R, Jamei Khosroshahi AR, Rezaei Y, Bohlouli S, Ghavimi MA. Effect of adding silica nanoparticles on the physicochemical properties, antimicrobial action, and the hardness of dental stone type 4. Int J Dent. 2022 Yan M, Takahashi H. Effects of magnesia and potassium sulfate on gypsum-bonded alumina dental investment for high-fusing casting. Dent Mater J. 1998;17(4):301-13. Yan M, Takahashi H. gypsum-bonded alumina dental investment for high-fusing casting. Dent Mater J. 1998;17(3):174-185. Berg E, Nielsen Ø, Skaug N. High-level microwave disinfection of dental gypsum casts. J Prosthet Dent. 2006;96(2):123-127.

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