The PMMA polymer powder is composed of small spheres called beads or pearls. It may also be in a fine granular form in some products. This enables the polymer to dissolve more readily in the monomer as its surface area to volume ratio is higher. The size of these particles is approximately 150 μm in those resins which are processed by heat, the most commonly used method. Other chemicals are added to the PMMA poly-mer to modify the final product (Table 23.1). For example, PMMA is inherently translucent and so pigments and opacifiers must be added to change its appearance unless a clear acrylic is desired.

Table 23.1 Typical chemical constituents of the powder component of a heat-cured resin

^* Phthalates are now regarded as hazardous materials, which, when used in excessive concentrations, are potential carcinogens. Manufacturers are looking at alternatives to these chemicals, such as citrates and benzoate esters.

Benzoyl peroxide coats the surface of the polymer beads. It is important that these beads are not contaminated as only a very small amount of polymer is required to start the reaction. Contamination can potentially initiate a premature polymerization reaction. If kept in good condition, the powder is very stable and as such has a very long shelf-life.

The liquid monomer is mainly MMA. This is a volatile liquid whose boiling point is 100.3°C. It is toxic if inhaled for a prolonged period and it is also highly flammable. It should therefore be handled in a well-ventilated room or preferably a fume cupboard. As with the powder, additional chemicals are added to the liquid to modify the final product (Table 23.2). In some products higher methacrylate monomers such as ethyl and butyl are substituted for the methylmethacrylate because they are less irritant.

The polymerization reaction may be initiated prematurely either by ultraviolet light or by free radicals forming spontaneously within the liquid. In order to prevent this occurring, the monomer is supplied in a dark bottle and hydroquinone is added to the monomer to preferentially react with any random free radicals which may be produced. The action of the hydroquinone produces stabilized free radicals which are not able to initiate the polymerization process. In this respect the hydroquinone acts rather like a sponge, mopping up free radicals until it is saturated. During the polymerization process all the hydroquinone must be used up before the polymerization reaction may take place.

It is advantageous that the mechanical properties of the acrylic are improved to increase the wear and fracture resistance of the material and also its resistance to the action of organic solvents which may cause surface cracking or crazing. This is achieved by adding cross-linking polymers such as diethylene glycol dimethacrylate. These different monomer units (co-polymers) can react with another growing chain at each end of the molecule when the polymer chains are growing so linking the chains. They are present in relatively small amounts and they have little effect on the transverse strength or hardness of the denture base material.

Plasticizers are often added to acrylics to vary their mechanical properties. Chemicals such as dibutyl phthalate do not take part in the polymerization reaction but are distributed throughout the polymerizing mass. They prevent the interaction between polymer molecules. Since they are not part of the structure, the plasticizer leaches out slowly as the denture becomes saturated with water. This presents two problems:

Concerns have been voiced with respect to the biological effects of the phthalates and manufacturers are looking at alternative plasticizers for use in denture base resins. Other chemicals such as the esters octyl or butyl methacrylate can also be used for materials which are made intentionally softer such as soft denture linings. Since these methacrylates will polymerize as part of the overall polymerization process, they are much less likely to leach out with time. Failure to initiate polymerization of these materials will, however, lead to leaching of the monomer and subsequent hardening of the denture base over time.

A few manufacturers supply the denture base material in a gel form. Here all the components are mixed together and formed in a thick sheet. The nature of the material precludes the use of chemical initiators and storage of the unprocessed material is important. It is generally recommended that the sheets are stored in the refrigerator in the dark before heat processing in the normal way.

PMMA has low tensile strength and a low elastic modulus. This means that it will flex in use. This flexure due to cyclical loading will, over time, result in fracture of the denture base. This is known as fatigue fracture. The process takes some time and is the least likely reason for fracture to occur. It also depends on the degree of flexure and the load applied. For example, an upper denture will flex about the midline of the palate, particularly if there is a bony ridge in this region. The degree of loading is determined by the chewing force applied during mastication.

Its fracture toughness is also low and as it behaves as a brittle material on impact, a denture will frequently fracture if dropped onto a hard surface. To avoid this, patients should be advised to clean their denture over a bowl of water rather than directly over an empty hard ceramic bathroom basin (Figure 23.3). Dentures dropped on this type of surface will break very readily.

Fig. 23.3 An acrylic denture being cleaning over a sink filled with water to prevent its fracture should it be inadvertently dropped during the cleaning process.

During use, PMMA absorbs fluid. This water sorption causes the denture base to expand by about 2%, which is considered to be high. The polymer is sufficiently accurate for the proposed applications, in other words it achieves a close fit to the denture-bearing tissues despite the various inaccuracies encountered during the processing procedures.

The polymerization shrinkage which occurs during curing partly compensates for this expansion improving the fit of the denture. For this reason, it is essential to keep the denture wet as drying out of the PMMA denture base will cause shrinkage and crazing. Long-term water sorption often causes the polymer to discolour and stain. This is particularly the case if the patient drinks a lot of tea, coffee or red wine. However, generally its colour stability is good.

The dental technician must be careful to use the correct proportions of the monomer and polymer. It is generally accepted that for a heat-cured prosthesis, the powder:monomer ratio should be in the region of 3:1. Failure to follow these proportions can result in the formed prosthesis having inferior properties. If the ratio of polymer powder to monomer is too high, there will be insufficient monomer to wet the polymer and a granular resin structure will be produced. Excess monomer leads to increased shrinkage. The pure monomer shrinks by 21% but with correct proportioning, a 7% shrinkage may be achieved, which correlates to a 2% linear shrinkage. If a good technique is used during processing, this may be reduced to 0.5%, although stresses will be incorporated within the material, which will need to be relieved during the curing process. Manufacturers provide vials to ensure that the correct proportions should be used. The optimum ratio is usually 3 or 3.5:1 by volume or 2.5:1 by weight.

PMMA, even when it has been properly processed, retains some residual monomer in the order of 0.2–0.5%. The presence of free monomer leads to two problems:

The polymer may creep under load over the long term and deform the denture base. This phenomenon is more marked with cold-cure acrylics and can create greater problems. The addition of cross-linking agents is an attempt to minimize this effect.

Loss of material due to water solubility is low. However, the presence of organic solvents such as alcohols and chloroform has an adverse effect on methacrylates.

As mentioned earlier, crazing is the presence of fine cracks on the surface of the acrylic. They represent localized areas of plastic deformation of the polymer caused by stress relief of internal strain. Tensile stresses will also cause rupture of the polymer chains leading to a weakened denture. These areas may be filled with microscopic voids and a crack may result if the crazed area can no longer support stress.

A crack is formed as a result of brittle fracture, as the walls of voids in the region are thin in the region of the fracture. The crack grows under externally applied load such as a patient biting, eventually leading to a continuous crack. This can cause fracture of the denture base. Crazes may be caused by heat, the action of organic solvents (for example alcohol or if the monomer comes into contact with resin during a denture repair) or differences in coefficients of thermal expansion around (dissimilar) inclusions that form stress concentrations. The most obvious example of this is ceramic denture teeth on a PMMA denture base or the inclusion of stainless steel clasps. The adhesion of PMMA to metal components and ceramic teeth is primarily by mechanical retention.

Crazing may also be seen if the denture is not kept moist at all times during its life. This is due to mechanical stresses set up within the denture base as it contracts and expands during the drying and wetting cycle. The cross-linked resins are less likely to craze as they contain fewer lines of weakness (Figure 23.4).

Fig. 23.4A,B (A) Polymer with no cross-linking; and (B) a cross-linked polymer.

In order to prevent crazing, the polymer should not be allowed to dry out and so the finished denture should always be supplied to the dentist in water (Figure 23.5). The patient should then be advised to store the prosthesis in water when it is not in use. Wrapping in gauze or cotton wool is inadequate as the resin does not remain saturated.

Fig. 23.5 A complete upper acrylic denture being stored in a sealed polythene bag containing water to prevent the acrylic drying out.