PROCESSING ERRORS Porosity

When porosity is present on the surface, it

1. Makes the appearance of denture base unsightly.

2. Proper cleaning of the denture is not possible, so the denture hygiene and thus, the oral hygiene suffers.

3. It weakens the denture base and the pores are areas of stress concentration, thus, the denture warps as the stresses relax.

Porosity may be: A. Internal porosity B. External porosity

Internal Porosity

Is in the form of voids or bubbles within the mass of the polymerized acrylic. It is usually not present on the surface of the denture. It is confined to the thick portions of the denture base and it may not occur uniformly.

Cause Internal porosity is due to the vaporization of monomer when the temperature of the resin increases above the boiling point of monomer (100.8°C) or very low molecular weight polymers. Exothermic heat of the surface resin dissipates easily into the investing plaster. However, in the centre of the thick portion, the heat cannot be conducted away therefore the temperature in the thick portions may rise above the boiling point of monomer causing porosity.

Avoided by Dentures with excessive thickness should be cured using long, low temperature curing cycle.

External Porosity

It can occur due to two reasons:

1. Lack of homogenity If the dough is not homogenous at the time of polymeri- zation the portions containing more monomer will shrink more than the adjacent area. This localized shrinkage results in voids. The resin appears white.

Avoided by Using proper powder-liquid ratio and mixing it well. The mix is more homogenous in the dough stage, so packing should be done in the dough stage.

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2. Lack of adequate pressure Lack of pressure during polymerization or inadequate amount of dough in the mould during final closure causes bubbles which are not spherical. The resin is lighter in color.

Avoided by Using the required amount of dough. Check for excess or flash during trail closure. Flash indicates adequate material.

Crazing

Crazing is formation of surface cracks on the denture base resin (Fig. 8.11). These cracks may be microscopic or macroscopic in size. In some cases it has a hazy or foggy appearance rather than cracks.

FIGURE 8.11: Crazing

Crazing has a weakening effect on the resin and reduces the esthetic qualities. The cracks formed can cause fracture.

Causes Crazing is due to: • Mechanical stresses or • Attack by a solvent • Incorporation of water.

In poly (methyl methacrylate) crazing occurs when tensile stresses are present. The cracks are at right angles to the direction of tensile stress. Crazing is a mechanical separation of the polymer chains or groups under tensile stress.

Crazing is visible around the porcelain teeth in the denture, and is due to the contraction of the resin around the porcelain teeth during cooling after processing. Due to the action of solvents the cracks are randomly placed, e.g. weak solvents, i.e. alcohol.

Water incorporation during processing will form stresses due to evaporation of water after processing, causing crazing.

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Avoided by

• Using cross linked acrylics. • Tin foil separating medium. • Metal moulds.

Denture Warpage

Denture warpage is the deformity or change of shape of the denture which can affect the fit of the denture (Fig. 8.12). Warpage can occur during processing as well as at other times.

FIGURE 8.12: Denture warpage has resulted in a space between the palatal surface and the cast. Obviously this would affect the fit

It is caused by a release of stresses incorporated during processing. Some of the stresses are incorporated as a result of the curing shrinkage while other stresses may be a result of the uneven or rapid cooling. Packing of the resin during the rubbery stage can also induce stresses. Some stresses may be incorporated during improper deflasking. These stresses are released subsequently:

• During polishing, a rise in temperature can cause warpage • Immersion of the denture in hot water can cause warpage • Re-curing of the denture after addition of relining material, etc.

RECENT ADVANCES

Several modified poly (methyl methacrylate) materials have been used for denture resins, hydrophilic polyacrylates, high impact strength resins, rapid heat polymerized acrylic, light-activated denture base material (described earlier) and pour type acrylic resins (described earlier).

High Impact Strength Materials

These materials are butadiene-styrene rubber-reinforced poly (methyl-metha- crylate). The rubber particles are grafted to methylmethacrylate so that they will

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bond well to the heat polymerized acrylic matrix. These materials are supplied in a powder-liquid form and are processed in the same way as other heat-accelerated methyl methacrylate materials. Indicated for patients who drop their dentures repeatedly, e.g. senility, parkinsonism.

Rapid Heat—Polymerized Resins

These are hybrid acrylics that are polymerized in boiling water immediately after being packed into a denture flask. After being placed into the boiling water, the water is brought back to a full boil for 20 minutes (reverse cure). After the usual bench cooling to room temperature, the denture is deflasked, trimmed, and polished in the usual manner. The initiator is formulated to allow for rapid polymerization without the porosity that one might expect.