17.9 Gypsum Products, Waxes and Denture Base Polymers
Key Takeaways
- All dental gypsum products are calcium sulphate hemihydrate and differ only in particle shape and porosity, which determines the water/powder ratio.
- Die stone has a water/powder ratio of about 0.22 compared with about 0.5 for plaster of Paris, which is why it is stronger and more accurate.
- Inlay wax must burn out cleanly leaving under 0.1% residue, and wax patterns must be invested promptly because waxes have a very high coefficient of thermal expansion.
- Heat-cured acrylic must be packed at the dough stage and cured slowly, because exceeding the monomer boiling point of 100.8 degrees Celsius produces gaseous porosity.
- Gaseous porosity gives fine uniform bubbles in thick sections, contraction porosity gives irregular scattered voids, and granular porosity gives a chalky blanched surface.
Gypsum Products
All dental gypsum products are forms of calcium sulphate hemihydrate produced by driving water out of gypsum, and all set by the reverse reaction:
The differences between products arise entirely from the shape and porosity of the hemihydrate particles, which determine how much water is needed to mix them. Less water means a denser, stronger set material.
| Product | Type | Manufacture | Water/powder ratio | Use |
|---|---|---|---|---|
| Plaster of Paris | Type 2 | Open-kettle calcination; irregular porous particles | ~0.5 | Study models, mounting, flasking |
| Dental stone | Type 3 | Autoclave in steam under pressure; denser prismatic particles | ~0.3 | Working models for complete dentures |
| Die stone (improved stone) | Type 4 | Calcination in calcium chloride solution; dense cuboidal particles | ~0.22 | Dies for crown and bridge work; high strength, low expansion |
| Type 5 | High-expansion die stone | As type 4 with increased setting expansion | ~0.20 | Compensates for alloy casting shrinkage |
Practical points that generate examination items:
- Increasing the water/powder ratio weakens the model and lengthens setting time.
- Accelerators include potassium sulphate and increased spatulation; retarders include borax and sodium citrate.
- Setting expansion is normally 0.1% to 0.3% and can be increased deliberately by hygroscopic setting under water.
- Alginate impressions must be poured promptly; delay causes syneresis and imbibition and distorts the cast.
Dental Waxes
Waxes are used for pattern formation, registration and processing, and their defining property is a very high coefficient of thermal expansion, which is why a pattern must be invested promptly.
| Wax | Use | Key property |
|---|---|---|
| Inlay wax (types A, B, C) | Crown and inlay patterns | Low flow at mouth temperature, high flow when softened; burns out cleanly leaving under 0.1% residue |
| Casting wax | Cobalt-chromium partial denture frameworks | Supplied as sheets and preformed shapes |
| Modelling (baseplate) wax | Occlusal rims, denture trial bases | Softens at mouth temperature, so rims distort if left in a warm environment |
| Sticky wax | Temporary joining of components for repair | Brittle when set; adheres to dry surfaces |
| Boxing wax | Forming a base around an impression before pouring | — |
| Utility wax | Extending impression tray borders | Soft and adhesive at room temperature |
Wax distortion is the principal source of error in indirect restorations. It arises from released internal stresses and thermal contraction, and is minimised by manipulating the wax at a uniform temperature, avoiding repeated softening, and investing the pattern as soon as possible.
Denture Base Polymers
Heat-cured poly(methyl methacrylate) remains the standard denture base material. It is supplied as powder (pre-polymerised PMMA beads plus benzoyl peroxide initiator) and liquid (MMA monomer plus hydroquinone inhibitor and often a cross-linking agent such as ethylene glycol dimethacrylate).
Mixing passes through recognisable stages: sandy, stringy, dough (the correct packing stage), rubbery and stiff. Packing must occur at the dough stage.
Curing is by a slow heating cycle. The critical rule is that the temperature must not rise rapidly above the boiling point of the monomer, 100.8 degrees Celsius, or the monomer boils within the dough and produces gaseous porosity. Three distinct porosity faults are examined:
| Porosity | Cause | Appearance |
|---|---|---|
| Gaseous porosity | Curing too fast; monomer boils | Fine uniform bubbles in the thickest parts of the denture |
| Contraction (shrinkage) porosity | Insufficient packing pressure or inadequate dough | Irregular voids scattered through the section |
| Granular porosity | Loss of monomer from the mix, for example an uncovered mixing pot | Blanched, granular, chalky surface |
Other properties and faults:
- Polymerisation shrinkage of the packed dough is around 6% to 7% by volume but only about 0.5% linear in the finished denture, because most of the mass is already polymerised bead.
- Residual monomer causes mucosal irritation and, rarely, true allergy; a longer curing cycle reduces it.
- Water sorption and solubility — PMMA absorbs water slowly, causing slight expansion that partly offsets processing shrinkage.
- Self-cure (auto-polymerising) acrylic uses a tertiary amine activator instead of heat. It has higher residual monomer, lower strength and poorer colour stability, so it is used for repairs, additions and special trays rather than definitive bases.
- High-impact acrylic incorporates a rubber phase; cobalt-chromium bases give strength, thermal conductivity and thinner sections at the cost of weight and adjustability.
Exam link. A denture returning from the laboratory with fine uniform bubbles concentrated in the thickest part of the palate is gaseous porosity caused by too rapid a curing cycle allowing the monomer to boil, not by under-packing.
Processing Errors and Their Clinical Signs
The reason these laboratory materials appear in a written examination is that their handling errors present clinically, and candidates are expected to reason backwards from the fault. A denture that does not seat and rocks on the cast may reflect expansion or distortion of the model, an inaccurate impression, or processing strain released on deflasking. Increased vertical dimension of occlusion after processing is classically caused by incomplete closure of the flask, usually from excess dough or from flash trapped between the flask halves. Porosity in the denture base takes recognisable forms: gaseous porosity, appearing as fine spherical voids in the thickest part of the base, results from the resin reaching above the boiling point of the monomer during a too-rapid cure; contraction porosity, appearing as irregular voids, results from insufficient pressure or inadequate dough; and granular porosity results from monomer loss.
Crazing — fine surface cracks — results from stress relief, solvent contact or repeated thermal cycling. Residual monomer left by an inadequate curing cycle is both a plasticiser, weakening the base, and the usual cause of the mucosal irritation historically labelled "acrylic allergy", which is far more often a chemical irritation from residual monomer or an undiagnosed denture-induced stomatitis caused by Candida. Distinguishing true allergy from these commoner explanations is a favourite examination discrimination, and the correct first steps are to review denture hygiene, night-time removal and fit rather than to assume hypersensitivity.
Why Gypsum Class Matters
The four gypsum products differ in the water-to-powder ratio required, which determines strength and setting expansion. Plaster of Paris is weak and used for flasking and study models; dental stone is stronger and used for working casts; die stone and high-strength, low-expansion die stone are used where dimensional accuracy for a fixed restoration is critical. Adding more water than specified produces a weaker, more porous set; adding accelerators such as potassium sulphate speeds the set, while borax retards it.