Gypsum - 2.1 PDF

**[Gypsum - 2.1]** - - Dental plaster = B form of calcium hemihydrate Dental stone = a form of calcium hemihydrate W/P ratio = ratio of water to calcium hemihydrate Mixing time = time from addition of powder to water until fully absorbed Setting time = time from start of mixing until full...

**[Gypsum - 2.1]** - - Dental plaster = B form of calcium hemihydrate Dental stone = a form of calcium hemihydrate W/P ratio = ratio of water to calcium hemihydrate Mixing time = time from addition of powder to water until fully absorbed Setting time = time from start of mixing until fully hardened or reaction complete Working time = time from beginning to mix until material is no longer workable Setting reaction = dimensional change over time **Properties of a good impression material** - - - - - - **Types** - - - - - - - - - - - - - - - - **Reaction** Calcium sulphate hemihydrate is dried in kiln, water is given off as steam leaving calcium sulphate demihydrate Exothermic reaction CaSO~4~·2H~2~O + ^HEAT^ → CaSO~4~·½H~2~O + 1½ H~2~O(steam) When water is added to calcium sulphate demihydrate the reverse reaction occurs CaSO~4~·½H~2~O + 1½ H~2~O → CaSO~4~·2H~2~O **Stages of setting** Fluid stage = mixing ready for pouring into mould Plastic stage = semi-gloss, still soft, able to shape and build up without folds or crumbles Initial set = matt, carves cleanly with sharp blade, difficult to build up, gypsum will crumble Final set = exothermic reaction ends, max strength when model is dry **Setting time** Time from beginning of spatulation to when material fully hardened or reaction complete Factors influencing setting time: - - - - **Classification of dental gypsum products** Type 1 = dental impression plaster - used for impression plaster Type 2 = dental plaster - B form of calcium hemihydrate Type 3 - dental stone = a form of calcium hemihydrate Type 4 = dental stone, high strength, low expansion - Kaffir-D, Crystacal Type 5 = dental stone, high strength, high expansion - Die stone **Properties** +-----------+-----------+-----------+-----------+-----------+-----------+ | **PROPERT | **TYPE | **TYPE | **TYPE | **TYPE | **TYPE | | Y** | 1** | 2** | 3** | 4** | 5** | +===========+===========+===========+===========+===========+===========+ | Initial | 5-10 | 5-20 | 5-20 | 5-20 | | | setting | | | | | | | time | | | | | | | (min) | | | | | | | | | | | | | | Working | | | | | | | time | | | | | | +-----------+-----------+-----------+-----------+-----------+-----------+ | Setting | 4 | 20 | 20 | 20 | 20 | | time | | | | | | | (min) | | | | | | +-----------+-----------+-----------+-----------+-----------+-----------+ | Setting | 0-0.15 | 0-0.30 | 0-0.20 | 0-0.15 | 0.16-0.3 | | expansion | | | | | | | (%) | | | | | | +-----------+-----------+-----------+-----------+-----------+-----------+ | Compressi | 6 | 12 | 25 | 40 | 40 | | ve | | | | | | | strength | | | | | | | 1h (MPa) | | | | | | +-----------+-----------+-----------+-----------+-----------+-----------+ | Compressi | \- | 24 | 70 | 75 | 75 | | ve | | | | | | | strength | | | | | | | 24h (MPa) | | | | | | +-----------+-----------+-----------+-----------+-----------+-----------+ | Flexural | 1 | 1 | 15 | 20 | 20 | | strength | | | | | | | 24h (MPa) | | | | | | +-----------+-----------+-----------+-----------+-----------+-----------+ | Detail | 75 | 75 | 50 | 50 | 50 | | reproduct | | | | | | | ion | | | | | | | (um) | | | | | | +-----------+-----------+-----------+-----------+-----------+-----------+ **Accelerators and retarders** Accelerators = additive which makes gypsum set quicker - sulphates or chlorides Retarder = additive which slows gypsum setting - acetate, borate, tartrate **Setting expansion** Factors - - - - **Classification of dental gypsum products** ISO (international standards organisation) Type I impression plaster = dental impressions Type II dental plaster = investing dentures in dental flasks and articulating Type III stone = dental models, base pour, has an inherent setting expansion that makes it hard to reproduce tooth position 1. 2. 3. 4. ​​ **Water (ml)** **Powder (g)** **W/P ratio (ml/g)** ------------------- ---------------- ---------------- ---------------------- Plaster 50-60 100 0.55 Stone 20-35 100 0.30 Theoretical Ratio 18.6\* 100 0.186