Dental Material Assignment on Gypsum

DENTAL MATERIAL ASSIGNMENT ON GYPSUM

To Head of the Department: Dr Nosheen Khawar

Submitted by: Mansoor Ahmed Rahoojo Roll No: 75 1st year B.D.S Batch 20th

Chemistry of Gypsum Gypsum is a sedimentary rock, which settled through the evaporation of sea water trapped in lagoons. According to the nature of its impurities, gypsum can show various colors, ranging from white to brown, yellow, gray and pink. Gypsum selection and preparation (cleaning, classifying) are key factors to produce the best plasters. The chemical reaction is : (CaSO4, 2 H2O) + heat = (CaSO4, ½ H2O) + 1.5 H2O Several processes are available to calcinate gypsum into Plaster of Paris. We can distinguish two categories : 1st: Calcination under atmospheric pressure to produce Beta plaster ; 2nd: Calcination under elevated pressure to produce Alpha plaster. Controlling some critical calcination parameters is essential to master the growth of the plaster crystals. And the performance of the plaster depends a lot on its crystals' sizes and shapes. Plaster of Paris is a calcium sulfate hemi-hydrate : (CaSO4, ½ H2O) derived from gypsum, a calcium sulfate dihydrate (CaSO4 , 2 H2O), by firing this mineral at relatively low temperature and then reducing it to powder. Calcination of the gypsum at higher temperatures produces different types of anhydrites (CaSO4) Gypsum products used in dentistry are based chemically upon calcium sulfate hemihydrate.(1–3) Depending on the method of dehydration from natural occurring calcium sulfate dihydrate, different forms of hemihydrate can be obtained which are α– and β– forms. β–hemi-hydrate forms known as dental plaster is irregular, porous and spongy; while α– form known as dental stone is more regular and denser than β– form. Both products are subseq-uently modified by grinding. The differences in the particle’s shape, size and distribution affect the water/ powder ratio thus affect the physical properties of set material

Ref: ALI (Association Of Lifecasters Intl)

STRUCTURE The crystal structure of dental plaster is monoclinic II-β hemihydrates which are spongy and irregular due to its method of calcinations. Β hemihydrates produced from dry calcinations are slightly modified by some additions to obtain suitable properties so that they can be used as model plaster. Generally,they have low surface hardness and abrasive resistance.

Particles are irregular, fibrous and porous)

Particles of Dental stone,they are regular, dense and prism–like) as in fig: Ref: Al–Rafidain Dent J Vol. 3, No. 2, 2003

PROPERTIES The strength of gypsum depends,primarily,on the porousity of the set material and the time for which the material is allowed to dry out after setting.The porousity and hence the strength,is proportional to the W/P ratio.since stone is always mixed at a lower W/P ratio than plaster it is less [orous and consequently much stronger and harder. Gypsum is a very brittle material.The very low value of flexural strength of plaster is indicative of how fragile this material is.stone is less fragile but must be treated with care if fracture is to be avoided.It is relatively rigid but has a poor impact strength and is likely to fracture if dropped. The dimensional stability of gypsum is good.

Ref: Jofn F.McCabe Ch 3,p=37

PHYSICAL PROPERTIES Properties Ortho. Stone Quick stone w/p ratio 28:100 28:100 Working time 5-7 min 3-5 min Setting time 11 min 10 min Comp. strength (wet) 31MPa 27MPa Comp. strength (dry) 59MPa 55MPa

Manufacturing Gypsum is heated to a temperature of about 120 degree centigrade in order to drive off part of the water of crystallization.This produces irregular,porous particles which are sometimes reffered to as beta-paricles.Overheating the gypsum may cause further loss of water to form calcium sulphate anhydrite (CaSo4),Whilst underheating produces a significant concentration of residual dehydrate.The both components has a marked influence upon the setting characteristics of the resultant plaster.

Types Gypsum products used in dentistry are based on calcium sulphate hemihydrate (CaSO4.2H20) The current ISO Standard for Dental Gypsum Products identifies five types of material as follows:

I. II. III. IV. V.

Plaster, Impression Plaster, Model Dental Stone Dental Stone, High Strength Dental Stone, High Strength, High Expansion

Comparison between different products Dental plaster: (palster of paris) 1. Calcified ca-sulfate hemihydrates. 2. Hemihydrate particles are larger. 3. Particles are irregular,in size/ shape. 4. Plaster particles are porous to a considerable degree. 5. Are weaker in strength. 6. Setting time 3/5 min. 7. Used as a molding material and as impression.

Dental stone: 1.Autoclaved ca-sulfate hemihydrates. 2. Smaller particles, 3. More regular in size and shape. 4. They are virtually non-porous. 5. They are stronger in strength. 6. Used as model as a Die material.

Reference: 1. Philips science of dental materials – Anusavice 2. Dental materials a programmed review of selected topics – William. J. O’Brien 3. Restorative dental materials – Craig 4. Applied dental materials – J. F. Mc Cabe 5. Notes on dental materials – Combe 6. Applied dental materials – John. A. Anderson 7. Some properties of gypsum plaster- B.C.Combe and Dc Smith 8. Encyclopedia Americana – vol 13