Сraig. Dental Materials

reached. Before packing, all stone surfaces of the mold are coated with an alginate separator and allowed to dry. The dough is then packed into the treated denture mold containing the artificial teeth and "trial packed" by repeated application of slow pressure with a flask press until no excess flash remains and the material has a glossy surface. Polymerization is accomplished by applying heat and pressure, which are maintained until polymerization is complete. The flask is then bench cooled to room temperature, and the denture is deflasked, finished, and polished.

The reaction involved in the heat-accelerated powder-liquid type of acrylic is outlined in a simplified equation above. The powder, consisting of the polymer plus the initiator, and the liquid monomer, containing the inhibitor, are proportioned in the ratio of approximately 3:l by volume.

Proportioning There must be sufficient liquid to completely wet the polymer powder. The powder and liquid are mixed with a stainless steel spatula and then kept in the sealed jar during the initial stages of reaction to avoid loss of monomer by evaporation. Incompletely wetted portions can result in a streaked or blanched

appearance in the denture because of incomplete polymerization of the plastic during processing. Care should be taken to avoid breathing the monomer vapor. Animal studies have shown that the monomer can affect respiration, cardiac function, and blood pressure.

The polymer-monomer mixture, on standing, goes through several distinct consistencies, which may be qualitatively described as

(1) sandy, (2) stringy or sticky, (3) doughy or puttylike, (4) rubbery or elastic, and (5) stiff. When the mixture is doughy it has desirable qualities for packing into a denture flask. Different products vary considerably in the time required to reach the doughy condition and the length of time they remain at the packing consistency. These stages or consistencies are represented by a model of the viscosity of the polymer/ monomer mix (Fig. 21-71. Nf represents the final viscosity and nf/2 is one half of this value. The powder-liquid mixture of a denture base plastic should be at the packing consistency when the mixture separates cleanly from the walls of the glass mixing jar and that this consistency be attained in less than 40 minutes from the start of mixing. The consistency, determined 5 minutes after the packing consistency is reached, should

be such that when 6 to 10 g of the material are placed over 0.75-mm diameter holes in a brass plate and loaded with a 5000 g weight, the dough intrudes into the holes to a depth of not less than 0.5 mm. If the material passes this test, adequate time should be available for trial packing of the denture mold and for final closure. During the various consistency stages, little polymerizationis taking place and the reaction occurring is physical in nature. This reaction includes some solution of the polymer in the monomer and some absorption of the monomer by the polymer, as well as wetting of the polymer particles. For conventional acrylic plastics, no substantial polymerization occurs until the denture flask is heated to above 70" C. If too much monomer is used in the mixture, polymerization shrinkage will be greater, additional time will be required to reach the packing consistency, and there will be a ten-

dency for porosity to occur in the denture. If too little monomer is used, the polymer will be insufficiently wetted, the dough will be difficult to manage, and the quality of the final denture will be compromised.

Packing The powder-liquid mixture should be packed into the flask at the doughy stage (Fig. 21-8, A) for several reasons. If it is packed at the sandy or stringy stages, too much monomer will be present between the polymer particles, the material will be of too low a viscosity to pack well, and it 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-stiff stage, the material will be too viscous to flow well under the pressure of the flask press, and metal-to-metalcontact of the flask halves will not be obtained. Delayed packing will result in

loss of detail in the denture, movement or fracture of the teeth, and an increase in the contact vertical dimension of the denture. Some plastics now have increased working time and remain in the doughy stage for periods approaching 1 hour. This permits the packing of several dentures at the same time. Other products, particularly the rubber-reinforced materials, have shorter working times, and in some instances only one or two dentures should be packed with one mix. The plastic dough should not be manipulated excessively with bare hands. The monomer is a good solvent for body oils and may pick up dirt from the hands, resulting in a nonesthetic denture. Monomer may also enter the bloodstream through the skin.

The acrylic dough is packed into the flask in slight excess by use of a hydraulic, pneumatic, or mechanical press (Fig. 21-8, B), and this excess is removed by trial packing procedures, with a damp cellophane or polyethylene film used as a separator for the upper half of the flask. The film separator allows easy separation of the flask halves during trial packing procedures. The closing force of the press is applied slowly during the trial packing to allow the excess or flash to flow out between the halves of the flask (Fig. 21-8, C). The flask is opened at intervals and the flash trimmed away (Fig. 21-8, D). Before final closure, the separating film is removed and discarded. During final closure of the flask, metal-to-metal contact of the flask halves is completed in the

press, the flasks are placed in a flask press that maintains pressure, and the denture is processed.

Processing For heat-polymerized plastics the curing temperature must be maintained close to 74' C, because the polymerization reaction is strongly exothermic. The heat of reaction will be added to the heat used to raise the material to the polymerization temperature. The temperature rise at various positions in a denture flask during a curing cycle is illustrated in Fig. 21-9. The initial temperature increases are in the following

contact with the water bath. The temperatures in the different areas increase at approximately the same rate until the temperature of the plastic dough reaches about 70" C. At this point the

material becomes quite fluid, and the decomposition rate of the benzoyl peroxide initiator is rapid enough for a substantial amount of polymerization to take place. As the polymerization reaction proceeds, the exothermic heat of reaction increases the temperature of the plastic to values considerably above the surrounding materials and above the boiling point of the methyl methacrylate monomer. This occurs because both the plastic and stone are poor thermal conductors and the heat of reaction is dissipated slowly. Also the larger the mass or bulk of material,the higher the peak temperature attained in the plastic. In thick sections of a denture the temperature rise will be greater than in thin sections. It has been demonstrated that the strength of thin sections of a denture may be less than that of the thick sections because of a lower

656 Chapter 21 PROSTHETIC APPLICATIONS OF POLYMERS

degree of polymerization. Because of the excessive temperature rise, porosity will more likely occur in thick sections of the denture.

The porosity that developed in two plastics cured at 74", 8Z0, and 100" C is shown in Fig. 21-10. Product A showed no porosity at 74" C, a negligible amount at 82" C, and moderate porosity at 100" C. Product B, however, evidenced no porosity at 74" C, a moderate amount at 82" C, and severe porosity at 100" C. On the basis of a large number of studies, a satisfactory processing temperature for most products is between 71" and 77" C, although, as indicated in Fig. 21-10, some products can be processed at higher temperatures without serious difficulty. A satisfactory processing procedure is to cure the plastic in a constant temperature water bath at 74" C for 8 hours or longer. Longer curing times, such as overnight, will not result in any degradation of properties. Another satisfactory processing method, which permits curing in a shorter time, is to heat at 74" C for 1.5 hours and then increase the temperature of the water bath to boiling for an additional hour.

The porosity shown in the specimens in Fig. 21-10 is in the center. The absence of porosity in the periphery of the specimens results from lower temperatures in these areas because the heat can be dissipated to the surrounding dental stone. The center of the specimens experience higher temperatures because poor thermal con-

ductivity of the plastic prevents the dissipation of heat. Porosity in thick sections of dentures therefore may exist below the surface of the plastic, and in pigmented materials it may not be noticed until grinding or polishing exposes the deeper layers.

Porosity also results when insufficient pressure is maintained on the flask during processing, but the distribution of the porosity is different from that shown in Fig. 21-10. Porosity resulting from insufficient pressure is distributed uniformly throughout the material, rather than concentrated in the center.

Other problems associated with rapid initial heating of the acrylic dough above 74" C are production of internal stresses, warpage of the denture after deflasking, and checking or crazing around the necks of the artificial teeth. High internal stresses resulting from rapid heating combined with the heat of polymerization may be released later and cause distortion and misfit of the denture base.

A variety of other methods of supplying the necessary heat to accelerate the polymerization reaction have been used. They include steam, dry heat supplied by electric platens, dry-air oven, infrared heating, induction or dielectric heating, and microwave radiation. The results of various processing studies have shown that equally satisfactory clinical results may be obtained with any of these methods compared with

the water bath method if adequate temperature control and pressure are maintained.

Deflasking and Finishing After polymerization, the flask is removed from the water bath and allowed to cool to room temperature. If the flask is opened prematurely while the plastic is still warm, warpage is likely to occur. Rapid cooling also tends to increase stresses in the denture, which may be released at a later time. During the cooling process, thermal shrinkage occurs. The thermal shrinkage takes place as a result of the relatively high thermal coefficient of expansion of the plastic. The magnitude of this thermal shrinkage will depend on the difference between the temperature at which the plastic hardens and room or mouth temperature, whichever is the final reference point. It is apparent that as long as the plastic is soft, it will shrink with the stone cast, which has a different thermal coefficient. When the plastic hardens, stresses are induced as cooling continues, because the plastic is forced to follow the shape of the cast despite the differences in thermal coefficients. Deflasking of the denture after cooling, however, allows some of the stresses in the denture to be released, and warpage occurs. Numerous studies dealing with the linear shrinkage of denture bases, measured across the posterior region, have shown that for normal processing conditions, the linear shrinkage is 0.3% to 0.5%. Usually more shrinkage is observed in mandibular than in maxillary dentures because of their shape.

A change in the contact vertical dimension of a denture during processing, as measured on an articulator, is also important. These changes are caused by variations in flask pressure, flask temperature, consistency of the dough, and strength of the stone mold. The pressure developed during closure of the flask is possibly the most important factor. If proper precautions are taken, the vertical opening may be held to 0.5 mm rather than reported variations of 2 to 5 mm.

After cooling, the denture is ejected from the flask and the stone is removed. Stone adhering to the plastic may be removed by shell blasting, a process where ground walnut shells are used to abrade the stone with little or no effect on the

plastic. After deflasking, the denture is trimmed with an arbor band and acrylic burs and polished. A wet polishing wheel and a slurry of pumice and water should be used to avoid heating the acrylic, which can cause measurable warpage. Tin oxide has been used to produce the final polish. However, tin oxide has been identified as a biological hazard and should be used with caution. After finishing, the denture should be stored in water.

Residual Monomer During the polymerization process the amount, of residual monomer decreases rapidly at first, then more slowly. The amount of residual monomer in a denture plastic processed at 70" C and at 100" C is shown as a function of the time and processing in Fig. 21-11. At zero time the monomer content was 26.2%.After 1 hour at 70" C it decreased to 6.6%, and at 100" C to 0.31%.After 4 hours the residual monomer was 4.0% and 0.29%, respectively. It required 168 hours at 70" C for the residual monomer to

Time (hours)

Fig. 21-11 Residual monomer concentration in the polymerization of methyl methaclylate in dental plastics at 70' C, A, and 100" C, 6, as a function of the time of processing.

(Adapted from Smith DC: Br Dent J 105:86, 1958.)

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1-hour terminal boil for processing dentures, as described earlier. However, the processing temperature should not be raised to boiling until most of the polymerization is completed, or porosity may result.

The highest residual monomer level is observed with chemically accelerated denture base plastics at 1% to 4% shortly after processing. Storing the denture for several days at elevated temperatures (up to 50" C) and excluding oxygen can significantly reduce the monomer level. However, this may be impractical. The rapid heat-cured denture base materials have significant residual monomer levels from 1% to 3% when they are processed in less than 1 hour in boiling water. If they are processed for 7 hours at 70" C and then boiled for 3 hours, the residual monomer content may be less than 0.4%.

If heat-processed materials are to be used for patients sensitive to residual monomer, processing for longer times in boiling water should reduce the monomer to an acceptable level. Because there is evidence that poly(methy1 methacrylate) monomer has poor biocompatibility, every effort should be made to eliminate residual monomer or reduce it to very low levels.

Dimensional Changes Dimensional changes take place when dentures are stored in water or are in contact with oral fluids. A considerable number of studies have reported on dimensional changes occurring during processing and storage of dentures. The area described with the most uniform results has been the posterior region. In general, heat-cured dentures stored in water show a linear expansion in this region of 0.1% to 0.2%, which partially but not completely compensates for the processing shrinkage of 0.3% to 0.5%. The net linear change can vary from a shrinkage of 0.1% to 0.4%. The major portion of the expansion in water takes place during the first month, and changes are insignificant after 2 months.

In a study comparing the fit of maxillary dentures made from five different commercial prod-

ucts, denture bases were placed on their respective casts after being stored in water at 37" C for 42 days to ensure equilibrium. The materials were ranked from best fit to worst fit by a panel of trained observers. The relative fit was determined by the space between the base and the cast in the palatal region. One rubber-modified acrylic had the worst fit, whereas another had a fit that was similar to the conventional acrylic. The best fit was observed with the vinyl-modified and the hydroxyethyl acrylics. There was no correlation with water sorption values, and therefore the fit of the denture bases may be related more to the glass transition temperature (glassy to brittle transition) than to water sorption for these materials.

There is always concern regarding what a net shrinkage of 0.1% to 0.4% represents clinically. A number of other steps in the preparation of a complete denture may cause dimensional inaccuracies, such as making the impression, pouring the stone cast, and preparing and investing the waxed denture. The oral tissues apparently accommodate small dimensional changes in dentures.

Injection Molding Denture Base Resins

Several manufacturers have re-introduced injection molding materials and equipment and the process is gaining in popularity. The flasking and boiling out of the waxed denture is similar to that used for compression molding. For injection molding, a hollow sprue connects the mold cavity created by wax boilout to an external opening on the flask and a high-pressure injection cylinder is connected to the opening. The denture base resin is mixed and placed in the cylinder. When the material reaches the proper consistency it is injected into the mold cavity under high pressure. The pressure is maintained during the polymerization cycle, and as polymerization shrinkage occurs additional material enters the flask. There are several studies noting increased dimensional accuracy with this technique. Injection molding is also used for microwavable and pour type resins.

CHEMICALLY ACCELERATED ACRYLIC DENTURE PLASTICS-COMPRESSION MOLDING

Chemically accelerated dental plastics, often called chemically curing resins, self-curing resins, cold-curing resins, or autopolymerizing resins, are similar to heat-accelerated dental plastics. The principal difference is that the polymerization reaction is accelerated by a chemical, such as N,N-dihydroxyethyl-para-toluidine, rather than by heat, as indicated by the simplified equation above (compare with the equation given in the earlier section on heat-accelerated acrylic denture plastics).

The amine accelerator reacts with the peroxide initiator at room temperature and sufficient free radicals are produced to initiate the polymerization reaction. Except for initiation, the remainder of the polymerization reaction is the same as for the heat-accelerated type. The reaction is exothermic and polymerization still results in volumetric shrinkage, but the plastic does not reach as high a peak temperature.

ManipulationandProcessing The general procedure for compression molding a chemically accelerated plastic is much the same as for the heat-accelerated type, except after final flask closure the dough is allowed to polymerize at room temperature or in a warm water bath in a pressure vessel.

The denture mold is packed when the polymer-monomer mixture reaches the doughy stage. Several trial closures are made and the flash removed. Care must be taken to make both the trial and final closure before the dough becomes so stiff that final closure is not possible. The chemically accelerated materials start to polymerize soon after the powder and liquid are mixed and proceed more rapidly through the various consistency stages than the heataccelerated types. The average time needed to

reach packing consistency is only 5 minutes for the chemically accelerated type, compared with 15 minutes for the heat-polymerized acrylic. It is more difficult to pack a number of denture flasks from one mix and still obtain complete flask closure. Additional working time may be obtained when the ingredients and the mixing jar are cooled in a refrigerator.

Properties After the flask is packed it should remain closed and under clamp pressure for a minimum of 2.5 hours to ensure polymerization. Compared with heat-cured plastics, chemically cured types do not reach the same degree of polymerization. The higher residual monomer acts as a plasticizer, which results in higher transverse deflection values and lower transverse strengths. After 15 days in water, however, chemically cured acrylics are nearly as hard as heat-cured. If, after 2.5 hours of curing at room temperature, the flask is boiled for 0.5 to 1 hour, properties comparable to the heat-cured type are obtained, and the residual monomer content is considerably reduced.

Peak temperatures observed at various positions in a denture cured by chemical acceleration are not as high as for heat-polymerized types. The linear shrinkage across the posterior region of a chemically cured maxillary denture after deflasking is about 0.3% compared with 0.5% for a heat-cured maxillary denture. As in the case of heat-cured dentures, mandibular dentures prepared with chemically accelerated denture plastics show more dimensional change than corresponding maxillary dentures. The lower dimensional change of the chemically cured type results because less residual stress is produced in the denture during the processing cycle. This is substantiated by the fact that the chemically accelerated denture plastics produce less shrinkage when processed at 20" to 25" C rather than at 37" C. When chemically cured dentures are

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placed in water, an increase in the molar-to- molar distance is observed. After 1 month this expansion amounts to about 0.3%, which approximately compensates for the processing shrinkage of 0.3%. Additional storage up to 9 months results in a total expansion of 0.4%, and the net dimensional change at this time is +0.1%, compared with the master model. It should be noted that water sorption of the chemically cured acrylics is between 0.5 and 0.7 mg/cm2, or about the same as the heat-cured type. However, the solubility is 0.05 mg/cm2 for the chemically cured type compared with 0.02 mg/cmz for the heatcured type. This larger value for solubility is caused by the loss of residual monomer from the chemically cured acrylic.

On the basis of these observations, it can be concluded that chemically cured acrylic dentures are generally about 0.1% oversize after several months of service and heat-cured acrylic dentures are 0.3% to 0.4% undersize.

In addition to strength, water sorption, solubility, and dimensional changes, the color stability of chemically cured denture base should be considered. The presence of some amine accelerators may cause problems in color stability. These amines produce colored products on oxidation, and therefore the color stability of chemically cured acrylics may not be as good as that of heat-cured acrylics, although definite improvements have been made since their introduction. Several products are now available that comply with the ANSI/ADA Specification No. 12 color stability test. Activators such as organic sulfinic acids can be used to improve color stability, but these compounds have certain disadvantages, such as chemical instability.

FLUID RESIN ACRYLIC DENTURE PLASTICS

The fluid resin technique takes advantage of the flow properties of polymer-monomer mixtures in the early consistency stage and the smaller size of the polymer powder particles. A very fluid mix also results from a much higher monomerpolymer ratio of about 1:2.5.The polymer and

monomer are mixed and then poured into the mold in the denture flask, and no trial packing is required. This procedure is advantageous in the preparation of the saddles for partial dentures, in which trial packing is difficult and flow of plastic around the metal framework is required. It also requires less expensive equipment than the heataccelerated acrylic. Manufacturers claim that a denture can be produced in much less time using this procedure. This was not substantiated in a laboratory survey.

This technique involves the use of agar or alginate hydrocolloid or, less commonly, a soft stone or silicone mold. The presence of water in the hydrocolloid does not interfere with the polymerization of the slurry of polymer and monomer. The technique involves preparation of a hydrocolloid gel mold of the waxed-up denture, as shown in Fig. 21-12, A. After the gel is formed, the model, including the waxed denture, is removed. The wax is removed from the model and teeth, and the teeth are reinserted into the mold (Fig. 21-12, B). The model is repositioned in the mold after an alginate separator has been applied and allowed to dry. The denture flask is constructed so that two or three circular holes can be cut from the side of the flask through the gel to the posterior portion of the model. A slurry of chemically activated acrylic is poured through one of these holes into the mold space provided by the loss of the wax (Fig. 21-12, C). The remaining hole or holes provide vents for the excess acrylic slurry, thus ensuring adequate filling of the mold space. After pouring the acrylic the flask is placed in a pressure vessel containing warm water, and air pressure of 0.1 to 0.2 MPa is applied. Only 30 to 45 minutes are necessary for polymerization. After processing, the gel is easily broken away from the denture and the denture looks as clean as the one shown in Fig. 21-12, D. Because of higher polymerization shrinkage, dentures fabricated by this technique are slightly less accurate than heat-activated dentures processed in a stone mold. When compared with heat-cured resins, pour-type acrylics are characterized by lower impact and fatigue strengths,

higher creep values, lower transverse bend strength, lower water sorption values, and higher solubility. The technique is interesting, because the hydrocolloid mold is easy to prepare, processing time is shortened considerably, the agar hydrocolloid may be reused, problems of broken teeth are eliminated, and deflasking is simplified.

Commercial fluid resins may vary considerably in apparent viscosity. Fig. 21-13 demon-

strates the dramatic increase in apparent viscosity of six different fluid resins as a function of time and at a constant shear rate (rotational speed). It illustrates the importance of pouring the fluid resins into the mold soon after the powder and liquid phases are mixed if lower viscosities are to be obtained. The apparent viscosity of fluid resins appears to be an important factor when fluid resin denture materials are evaluated.