Composition

As previously noted, most polymethyl methacrylate resin systems include powder and liquid components (Figure 19-1). The powder consists of prepolymerized spheres of polymethyl methacrylate and a small amount of benzoyl peroxide, termed the initiator, which is responsible for starting the polymerization process. The liquid is predominantly nonpolymerized methyl methacrylate monomer with small amounts of hydroquinone. Hydroquinone is added as an inhibitor, which prevents undesirable polymerization or “setting” of the liquid during storage. Inhibitors also retard the curing process and thereby increase working time.

A cross-linking agent can also be added to the liquid. Glycol dimethacrylate is used commonly as a cross-linking agent in polymethyl methacrylate denture base resins. Glycol dimethacrylate is chemically and structurally similar to methyl methacrylate. Therefore, it can be incorporated into growing polymer chains (Figure 19-2). One should note that methyl methacrylate possesses one carbon-carbon double bond per molecule and glycol dimethacrylate possesses two double bonds per molecule. As a result, an individual molecule of glycol dimethacrylate can participate in the polymerization of two separate polymer chains that unites the two polymer chains. If sufficient glycol dimethacrylate is included in the mixture, several interconnections can be formed and solvent swelling may occur, such as that caused by exposure to ethanol in alcoholic beverages. These interconnections yield a netlike structure that provides increased resistance to deformation. Cross-linking agents are incorporated into the liquid component at a concentration of 1% to 2% by volume.

Storage

Manufacturers of heat-activated resin systems generally recommend specific temperature and time limits for the storage of powder and liquid components. Strict observance of such recommendations is essential. If recommendations are not followed, components can undergo changes that may affect working properties of these resins as well as the chemical and physical properties of processed denture bases.

Preparation of the Mold

Before mold preparation, prosthetic teeth must be selected and arranged in a manner that will fulfill both esthetic and functional requirements. In turn, the completed tooth arrangement is sealed to the master cast.

At this stage, the master cast and completed tooth arrangement are removed from the dental articulator (Figure 19-3, A). The master cast is coated with a thin layer of separator to prevent adherence of dental stone to the master cast during the flasking process. The lower portion of a denture flask is filled with freshly mixed dental stone, and the master cast is placed into this mixture. The dental stone is contoured to facilitate wax elimination, packing, and deflasking procedures (Figure 19-3, B). Upon reaching its initial set, the stone is coated with an appropriate separator.

The upper portion of the selected denture flask is then positioned atop the lower portion of the flask. A surface tension-reducing agent is applied to exposed wax surfaces and a second mix of dental stone is prepared. The dental stone is poured into the denture flask. Care is taken to ensure that the investing stone achieves intimate contact with all external surfaces of the mounted teeth. The investing stone is added until all surfaces of the tooth arrangement and denture base are completely covered. Incisal and occlusal surfaces are minimally exposed to facilitate subsequent deflasking procedures (Figure 19-3, C). The stone is permitted to set and is coated with separator.

At this point an additional increment of dental stone is mixed and the remainder of the flask is filled. The lid of the flask is gently seated and the stone is allowed to harden (Figure 19-3, D).

After the stone has hardened, the record base and wax must be removed from the mold. To accomplish this task, the denture flask is immersed in boiling water for 4 min. The flask is then removed from the water and the appropriate segments are separated. The record base and softened wax remain in the lower portion of the denture flask while the prosthetic teeth remain firmly embedded in the investing stone of the remaining segment (Figure 19-3, E). The record base and softened wax are carefully removed from the surface of the mold. Residual wax is removed from the mold cavity using wax solvent. The mold cavity is subsequently cleaned with a mild detergent solution and rinsed with boiling water.

Selection and Application of A Separating Medium

The next step in denture base fabrication involves the application of an appropriate separating medium onto the surfaces of the mold cavity. This medium must prevent direct contact between the denture base resin and the mold surface. Failure to place an appropriate separating medium can lead to two major difficulties: (1) If water is permitted to diffuse from the mold surface into the denture base resin, it can affect the polymerization rate as well as the optical and physical properties of the resultant denture base. (2) If dissolved polymer or free monomer is permitted to soak into the mold surface, portions of the investing medium can become fused to the denture base. These difficulties often produce compromises in the physical and esthetic properties of processed denture bases. Hence the importance of employing an appropriate separating medium should not be overlooked.

Currently, the most popular separating agents are water-soluble alginate solutions. When applied to dental stone surfaces, these solutions produce thin, relatively insoluble calcium alginate films. These films prevent direct contact of denture base resins and the surrounding dental stone, thereby eliminating undesirable interactions.

Placement of an alginate-based separating medium is relatively uncomplicated. A small amount of separator is dispensed into a disposable container. Then a fine brush is used to spread the separating medium onto the exposed surfaces of a warm, clean stone mold (Figure 19-3, F). Separator should not be permitted to contact exposed portions of acrylic resin teeth, since its presence interferes with chemical bonding between acrylic resin teeth and denture base resins. Subsequently, the mold sections are oriented to prevent “pooling” of separator, and the solution is permitted to dry.

Polymer-to-Monomer Ratio

Proper polymer-to-monomer ratio is of considerable importance in the fabrication of well-fitting denture bases with desirable physical properties. Unfortunately, most discussions of polymer-to-monomer ratio provide little practical information for dental personnel. Furthermore, these discussions do not address relationships between molecular events and gross handling characteristics of denture base resins. The following paragraphs are intended to provide practical, unambiguous information.

The polymerization of denture base resins results in volumetric and linear shrinkage. This is understandable when one considers the molecular events occurring during the polymerization process.

Envision two methyl methacrylate molecules. Each molecule possesses an electrical field that repels nearby molecules. Consequently, the distance between molecules is significantly greater than the length of a representative carbon-to-carbon bond. When the methyl methacrylate molecules are chemically bonded, a new carbon-to-carbon linkage is formed. This produces a net decrease in the space occupied by the components.

Research indicates that the polymerization of methyl methacrylate to form polymethyl methacrylate yields a 21% decrease in the volume of material. As might be expected, a volumetric shrinkage of 21% would create significant difficulties in denture base fabrication. To minimize dimensional changes, resin manufacturers prepolymerize a significant fraction of the denture base material. This can be thought of as “preshrinking” the selected resin fraction.

In practice, the prepolymerized fraction is encountered as a powder, and is commonly referred to as polymer. The nonpolymerized fraction is supplied as a liquid, and is termed a monomer. When the powder and liquid components are mixed in the proper proportions, a doughlike mass results. Commercial products generally use a polymer-to-monomer ratio of approximately 3 : 1 by volume. This ratio provides sufficient monomer to thoroughly wet the polymer particles but does not contribute excess monomer that would produce increased polymerization shrinkage. Using a 3 : 1 ratio, the volumetric shrinkage can be limited to approximately 7%. The initial setting expansion of the gypsum mold and swelling of the polymerized acrylic resin after exposure to water in the denture flask and later, further compensate this shrinkage. The polymerization shrinkage of denture base resin is discussed in depth later in the chapter.

Polymer-Monomer Interaction

When monomer and polymer are mixed in the proper proportions, a workable mass is produced. Upon standing, the resultant mass passes through five distinct stages. These stages can be described as (1) sandy, (2) stringy, (3) doughlike, (4) rubbery or elastic, and (5) stiff.

During the sandy stage, little or no interaction occurs on a molecular level. Polymer beads remain unaltered, and the consistency of the mixture can be described as “coarse” or “grainy.” Later, the mixture enters a stringy stage. During this stage, the monomer attacks the surfaces of individual polymer beads and is absorbed into the beads. Some polymer chains are dispersed in the liquid monomer. These polymer chains uncoil, thereby increasing the viscosity of the mix. This stage is characterized by “stringiness” or “stickiness” when the material is touched or drawn apart.

Subsequently, the mass enters a doughlike stage. On a molecular level, an increased number of polymer chains enter the solution. Thus, monomer and dissolved polymer are formed. It is important to note that a large quantity of swollen, but undissolved polymer also remains. Clinically, the mass behaves as a pliable dough. It is no longer tacky and does not adhere to the surfaces of the mixing vessel or spatula. The physical and chemical characteristics exhibited during the later phases of this stage are ideal for compression molding. As a result, the material should be introduced into the mold cavity during the latter phases of the doughlike stage.

Following the doughlike stage, the mixture enters a rubbery or elastic stage. This is because monomer is dissipated by evaporation and by further penetration into remaining polymer beads. The mass rebounds when compressed or stretched. Because the mass no longer flows freely to assume the shape of its container, it cannot be molded by conventional compression techniques.

Upon standing for an extended period, the mixture becomes stiff. This can be attributed to continued evaporation of unreacted monomer. From a clinical standpoint, the mixture appears very dry and is resistant to mechanical deformation.

Dough-Forming Time

The time required for the resin mixture to reach a doughlike stage is termed the dough-forming time. American National Standards Institute/American Dental Association (ANSI/ADA) Specification No. 12 (ISO 20795-1:2008: Dentistry—Base polymers—Part 1: Denture base polymers) for denture base resins requires that this consistency be attained in less than 40 min from the start of the mixing process. In clinical use, the majority of denture base products reach a doughlike consistency in less than 10 min.

Working Time

Working time is defined as the time a denture base material remains in the doughlike stage. This period is critical to the compression molding process. ANSI/ADA Specification No. 12 requires the dough to remain moldable for at least 5 min.

The ambient temperature affects the working time. Hence the working time of a denture resin can be extended via refrigeration. A significant drawback associated with this technique is that moisture may condense on the resin when it is removed from the refrigerator, which may degrade the physical and esthetic properties of a processed resin. Moisture contamination can be avoided by storing the resin in an airtight container. Following removal from the refrigerator, the container should not be opened until it reaches room temperature.

Packing

The placement and adaptation of denture base resin within the mold cavity are termed packing. This process represents one of the most critical steps in denture base fabrication. It is essential that the mold cavity be properly filled at the time of polymerization. The placement of too much material yields a denture base that exhibits excessive thickness and resultant malpositioning of prosthetic teeth. Conversely, the use of too little material leads to noticeable denture base voids or porosity. To minimize these difficulties, the mold cavity is packed in a well-defined sequence.

While in a doughlike state, the resin is removed from its mixing container and rolled into a ropelike form. Monomer is painted over the necks of the denture teeth to promote bonding to the denture base. Subsequently, the resin form is bent into a horseshoe shape and placed into the portion of the flask that houses the prosthetic teeth (Figure 19-4, A). A thin polyethylene separator sheet is placed over the master cast, and the flask is reassembled.

The flask assembly is placed into a specially designed press and pressure is applied incrementally (Figure 19-4, B). Slow application of pressure permits the resin dough to flow uniformly throughout the mold space. Excess material is displaced eccentrically. The application of pressure is continued until the denture flask is fully closed. Next the flask is opened and the polyethylene packing sheet is removed from the surface of the resin with a rapid, continuous tug.

Excess resin will be found on the relatively flat areas surrounding the mold cavity. This excess resin is called flash. Using a gently rounded instrument, the flash is carefully teased away from the body of resin that occupies the mold cavity (Figure 19-4, C). Care is taken not to chip the gypsum surfaces of the mold. Pieces of gypsum that have become dislodged must be removed so that they are not incorporated into the processed denture base.

A fresh polyethylene sheet is placed between the major portions of the flask, and the flask assembly is once again placed in the press. Trial closures are repeated until no flash is observed.

When flash is no longer apparent, the mold is closed for the last time with no polyethylene sheet interposed. The mold sections are properly aligned and placed in the flask press. Again, pressure is incrementally applied. Following definitive closure, the flask is transferred to a flask carrier (Figure 19-4, D). The flask carrier maintains pressure on the flask assembly during denture base processing. A cross-sectional representation of the denture flask assembly is presented in Figure 19-5.

Injection Molding Technique

In addition to compression molding techniques, denture bases can be fabricated via injection molding using specially designed flasks. One half of the flask is filled with freshly mixed dental stone, and the master cast is settled into this mixture. The dental stone is appropriately contoured and permitted to set. Subsequently, sprues or ingates are attached to the wax denture base, which lead to an inlet or pressure port (Figure 19-6, A). The remaining half of the flask is positioned, and the investment process is completed (Figure 19-6, B). Wax elimination is performed as previously described (Figure 19-6, C), and the flask is reassembled. Subsequently, the flask is placed into a carrier that maintains pressure on the assembly during resin introduction and processing.

Upon completion of the foregoing steps, resin is mixed and injected into the mold cavity (Figure 19-6, D). The flask is then placed into a water bath for polymerization of the denture base resin if a heat-curing resin is used. Upon completion, the denture is recovered, adjusted, finished, and polished.

Currently there is some debate regarding the comparative accuracy of denture bases fabricated by compression molding and those fabricated by injection molding. Available data and clinical information indicate denture bases fabricated by injection molding can provide slightly improved clinical accuracy.

However, the advantages of injection molding are not straightforward. For example, although tooth movement is minimized, resin viscosity must be considerably lower than that used in compression molding in order to facilitate injection.

This requires a substantially smaller polymer-to-monomer ratio, and consequently results in increased curing shrinkage. Hence, the palatal fit is compromised. This is illustrated in Figure 19-12D, below, in which a greater opening at the posterior palatal seal area is shown for injection molding than for any of the other molding processes.

From a clinical perspective, this requires greater (i.e., deeper) preparation of the post-palatal seal area (on the dental cast) to ensure contact of the polymerized denture base with the patient’s palatal tissues.

Polymerization Procedure

As previously noted, denture base resins generally contain benzoyl peroxide. When heated above 60 °C, molecules of benzoyl peroxide decompose to yield electrically neutral species containing unpaired electrons. These species are termed free radicals. Each free radical rapidly reacts with an available monomer molecule to initiate chain-growth polymerization. Since the reaction product also possesses an unpaired electron, it remains chemically active. Consequently additional monomer molecules become attached to individual polymer chains. This process occurs very rapidly and terminates by either (1) the direct connection of two growing chains (i.e., combination) or (2) the transfer of a single hydrogen ion from one chain to another. (See also Figures 6-7, 6-8 and 6-9 in Chapter 6).

In the system under discussion, heat is required to cause decomposition of benzoyl peroxide molecules. Therefore, heat is termed the activator. Decomposition of a benzoyl peroxide molecule yields two free radicals that are responsible for the initiation of chain growth. Hence, benzoyl peroxide is termed the initiator.

During denture base fabrication, heat is applied to the resin by immersing a denture flask and flask carrier in a water bath. The water is heated to a prescribed temperature and maintained at that temperature for a period suggested by the manufacturer.

Temperature Rise

The polymerization of denture base resins is exothermic, and the amount of heat evolved can affect the properties of the processed denture bases. Representative temperature changes occurring in water, investing stone, and resin are presented in Figure 19-7. The temperature profile of the investing stone (identified as “plaster”) closely parallels the heating curve for the water. The temperature of the denture base resin lags somewhat during the initial stages of the heating process. This can be attributed to the fact that the resin occupies a position in the center of the mold and therefore heat penetration takes a longer time.

As the denture base resin attains a temperature slightly above 70 °C, the temperature of the resin begins to increase rapidly. In turn, the decomposition rate of benzoyl peroxide is significantly increased. This sequence of events leads to an increased rate of polymerization and an accompanying increase in the exothermic heat of reaction. Because resin and dental stone are relatively poor thermal conductors, the heat of reaction cannot be dissipated. Consequently, the temperature of the resin rises well above the temperatures of the investing stone and surrounding water. This can cause the monomer to boil and produce porosity within the processed resin.

Internal Porosity

As indicated earlier, the polymerization process is exothermic. Because resin is an extremely poor thermal conductor, the heat generated in a thick segment of resin cannot be dissipated. When heating is poorly controlled, the peak temperature of this resin can rise well above the boiling point of monomer (100.8 °C). This causes boiling of unreacted monomer and produces porosity within the processed denture base.

Polymerization Cycle

The heating process used to control polymerization is termed the polymerization cycle or curing cycle. This process should be carefully regulated to avoid the effects of uncontrolled temperature rise.

The curing cycle presented in Figure 19-7 is unsatisfactory because of the marked temperature increase during the early stages of polymerization. Rapid heating of the resin mass may cause the monomer to boil, producing porosity within the polymerized denture base. Fortunately this process can be controlled by heating the resin more slowly during the polymerization cycle.

The relationship between the rate of heating and temperature rise within the denture base resin is illustrated in Figure 19-8. The polymerization cycle represented by curve C probably would yield porosity in thick portions of the denture, since the temperature of the resin exceeds the boiling point of the monomer (100.8 °C). On the other hand, the polymerization cycle represented by curve A probably would result in the presence of unreacted monomer, since the resin temperature fails to reach the boiling temperature of the monomer (100.8 °C). Thus, it is logical to assume that an optimum polymerization cycle lies somewhere between curves A and C.

Research has led to the development of guidelines for polymerization of denture base resins. The resultant polymerization cycles have been quite successful for denture bases of various sizes, shapes, and thicknesses.

One technique involves processing the denture base resin in a constant-temperature water bath at 74 °C (165 °F) for 8 h or longer, with no terminal boiling treatment. A second technique consists of processing in a 74 °C water bath for 8 h and then increasing the temperature to 100 °C for 1 h. A third technique involves processing the resin at 74 °C for approximately 2 h and increasing the temperature of the water bath to 100 °C and processing for 1 h.

Following completion of the chosen polymerization cycle, the denture flask should be cooled slowly to room temperature. Rapid cooling can result in distortion of the denture base because of differences in thermal contraction of resin and investing stone. To minimize potential difficulties, the flask should be removed from the water and bench cooled for 30 min. Subsequently, the flask should be immersed in cool tap water for 15 min. The denture base can then be deflasked and prepared for delivery. To decrease the probability of unfavorable dimensional changes, the denture should be stored in water until it is placed in the oral cavity.

Polymerization via Microwave Energy

Polymethyl methacrylate resin can be polymerized using microwave energy because the methyl methacrylate molecule is asymmetric. This technique employs a specially formulated resin and a nonmetallic flask (Figure 19-9). A conventional microwave oven is used to supply the thermal energy required for polymerization.