Various separating media used are: • Tinfoil
• Cellulose lacquers
• Solution of alginate compounds • Calcium oleate
• Soft soaps • Sodium silicate • Starches.
Tin Foil
Tin foil was the material used earlier and was very effective. However, its manipulation is time consuming and difficult. It has been largely replaced by other separating media known as ‘tin foil substitutes’.
Sodium Alginate Solution
This is the most popular separating medium. It is water soluble. It reacts with the calcium of the plaster or stone to form a film of insoluble calcium alginate.
Composition
Two percent sodium alginate in water, glycerine, alcohol, sodium phosphate and preservatives.
Precautions to be Taken
• Waxes or oils remaining on the mould surface will interfere with the action of the separating medium.
• Mould should be warm, not hot. Continuity of the film will break if the mould is steaming hot.
• Coating of medium left on the teeth will prevent bonding of teeth with denture base.
Application
Separating media is applied using a brush, coating only the plaster surfaces, and not the acrylic teeth. One or two coats may be applied.
Mixing of Powder and Liquid
Polymer—monomer proportion = 3:1 by volume or 2:1 by weight.
The measured liquid is poured into a clean, dry mixing jar. Powder is slowly added allowing each powder particle to become wetted by monomer. The mixture is then stirred and allowed to stand in a closed container.
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If too much monomer is used (Lower polymer/monomer ratio) • There will be greater curing or polymerization shrinkage. • More time is needed to reach the packing consistency. • Porosity can occur in the denture.
If too little monomer is used (Higher polymer/monomer ratio)
• Not all the polymer beads will be wetted by monomer and the cured acrylic will be granular.
• Dough will be difficult to manage and it may not fuse into a continuous unit of plastic during processing.
Physical Stages
After mixing the material goes through various physical stages. No polymerization reaction takes place during these stages. A plastic dough is formed by a partial solution of the polymer in the monomer.
Stage I
Wet sand stage: The polymer gradually settles into the monomer forming a fluid, incoherent mass.
Stage II
Sticky stage: The monomer attacks the polymer by penetrating into the polymer. The mass is sticky and stringy (cobweb like) when touched or pulled apart.
Stage III
Dough or gel stage: As the monomer diffuses into the polymer, it becomes smooth and dough like. It does not adhere to the walls of the jar. It consists of undissolved polymer particles suspended in a plastic matrix of monomer and dissolved polymer. The mass is plastic and homogenous and can be packed into the mould at this stage.
Stage IV
Rubbery stage: The monomer disappears by further penetration into the polymer and/or evaporation. The mass is rubberlike, non-plastic and cannot be moulded.
Stage V
Stiff stage
Working Time
The working time is the time elapsing between stage II and the beginning of stage IV, i.e. the time the material remains in the dough stage (according to ADA Sp. No. 12, the dough should be mouldable for at least 5 minutes).
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The working time is affected by temperature. In warm weather when the working time is insufficient, the mixing jar is chilled to prolong the working time. Care is taken to avoid moisture.
Packing
The powder—liquid mixture should be packed into the flask at the dough consistency for several reasons:
• If it is packed at the sandy or stringy stages, too much monomer will be present between the polymer particles, and the material will be of too low a viscosity to pack well and will flow out of the flask too easily. Packing too early may also result in porosity in the final denture base.
• If packed at the rubbery to the stiff stage, the material will be too viscous to flow, and metal to metal contact of the flask halves will not be obtained. Delayed packing will result in movement or fracture of the teeth, loss of detail and increase in the vertical height of the denture.
Trial Closure
The acrylic dough is packed into the flask in slight excess. The excess is removed by trial packing, with a damp cellophane or polyethylene film used as a separator for the upper half of the flask. A hydraulic or mechanical press may be used to apply pressure (Fig. 8.6). The closing force is applied slowly during the trial packing to allow the excess dough, known as ‘flash’ to flow out between the halves of the flask. The flask is opened and the flash is trimmed away. Before final closure, the separating film is removed and discarded. The final closure of the flask or metal to metal contact of the flask halves, is then completed in the press.
FIGURE 8.6: Left—A hydraulic press (Kavo) is useful for trial and final closure of the flask. The pressure indicator dial indicates the pressure that is applied. Right—An automated curing bath (Kavo). The time and temperature of the curing cycle can be preset and regulated for optimum cure
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Curing (Polymerization)
After final closure, the flasks, are kept at room temperature for 30 to 60 minutes (sometimes called ‘bench curing’).
Purpose of Bench Curing
1. Permits an equalization of pressures throughout the mould.
2. It allows time for a more uniform dispersion of monomer throughout the mass of dough, since the last material added is usually drier than the first added to the flask.
3. If resin teeth are used, if provides a longer exposure of resin teeth to the monomer in the dough, producing a better bond of the teeth with the base material.
Curing Cycle
The curing or polymerization cycle is the technical name for the heating process used to control the initial propogation of polymerization in the denture mould. The curing cycle selected depends on the thickness of the resin.
Following are the recommended curing cycles.
• Heat the flask in water at 60-70° C for 9 hours (long cycle)
• Heat the flask at 65° C for 90 minutes, then boil for 1 hour for adequate polymerization of the thinner portions (Short cycle).
Electrically controlled curing baths are available which enable better control of the curing process (Fig. 8.6).
Cooling
The flask should be cooled slowly, i.e. bench cooled. Fast cooling can result in warpage of the denture due to differential thermal contraction of the resin and gypsum mould. Cooling overnight is ideal. However, bench cooling it for 30 minutes, and then placing it in cold tap water for 15 minutes is satisfactory.
Deflasking
The cured acrylic denture is retrieved from the flask. This is called deflasking . The flask is opened and the mould is retrieved. The mould separates quickly, because the surrounding plaster was poured in layers (3 pour technique). Plaster cutting forceps may be used to break up the plaster. Deflasking has to be done with great care to avoid flexing and breaking of the acrylic denture.