Сraig. Dental Materials

the rate of stress application. Despite this characteristic, the proportional limit determined under standard conditions is important. A plastic with a low proportional limit will begin to deform permanently at a low stress. If the percent elongation is relatively high, it may deform permanently to a considerable extent before rupture. If the proportional limit is high, considerable stress is required before permanent deformation will occur. A denture material should have a proportional limit sufficiently high that permanent deformation does not result from the stress applied during mastication. Permanent deformation may result in loss of retention or loosening of the teeth embedded in the denture base. The values reported in Table 21-3 for the proportional limit of poly(methy1 methacrylates) and polyvinyl acrylics are approximately the same and the dimensional stability of dentures made of these materials would be expected to be similar.

Impact Strength Impact strength is a measure of the energy absorbed by a material when it is broken by a sudden blow.

The impact strength for polyvinyl acrylics is about twice that of poly(methy1 methacrylates) (see Table 21-3), which indicates that polyvinyl acrylic absorbs more energy on impact and is more resistant to fracture.

Although the addition of plasticizers may increase the impact strength of plastics, the increases are accompanied by decreases in hardness, proportional limit, elastic modulus, and compressive strength. Ideally, a denture base plastic should have a sufficiently high impact strength to prevent breakage on accidental dropping, but not at the expense of the other properties.

The impact strength of several denture base materials is listed in Tables 21-4 and 21-5. There are differences in relative impact strength with all tests. However, the impact strength of rubberreinforced acrylic plastic is considerably higher in all instances. When surface defects are present, this improved impact resistance is significantly reduced.

Conventional heat-cured acrylic

Vinyl acrylic Rubber-reinforced

acrylic Hydrophilic acrylic

*Adapted from Soni PM, Powers JM, Ctaig RG:J Mich Dent

Assoc 59:418, 1977.

tAdapted from Stafford GD, Bates JF; Hugget R, Handley

RW:J Dent 8:292, 1980.

NA, Not applicable.

Transverse Strength and Deflection

In the evaluation of denture plastics, transverse strength measurements are used to a greater extent than either tensile or compressive strength, because this test more closely represents the type of loading in vivo. Transverse strength is determined by applying an increasing load until fracture at the center of a test specimen, as shown in Fig. 21-2. The deflection in millimeters at the middle of the plastic specimen is recorded at a load of 3500 and 5000 g. Transverse strength is therefore a combination of tensile and compressive strength and includes some of the elements of proportional limit and elastic modulus. The values listed in Table 21-4 show that the transverse strength varies from 78 to 92 MPa for various denture base materials.

A transverse deflection test has been used to evaluate denture base resins. The requirements are that the deflection in the center of the specimen shall be no more than 2.5 mm for a load of 1500 to 3500 g, and between 2.0 and 5.5 mm for a load of 1500 to 5000 g. All specimens are tested in water at 37' C after storage in water for 2 days at 37' C.

A comparison of the transverse deflection of several types of materials is seen in Table 21-2.

Dial gauge for deflection

1

A Weinhts

Specimen

Fig. 21-2 Simplified sketch of equipment for determining transverse deflection and strength.

Note that the pour-type resin fractured in this test and the rapid heat-cured acrylic was slightly less flexible than the other materials.

Fatigue Strength Dentures are subjected to a large number of smaller cyclic stresses during mastication. For this reason, the fatigue properties of denture plastics are important. Fatigue strength represents the number of cycles before failure at a certain stress. Values of the fatigue strength at a stress of 17.2 MPa for poly(methy1 methacrylates) and polyvinyl acrylic plastics are 1.5 x lo6 and 1 x lo6, respectively. Because the current denture base plastics hold up well in service, a value of 1 x lo6 cycles at 17.2 MPa is apparently an adequate fatigue strength value.

There are many flexural fatigue tests reported in the literature for denture base resins. Results from one of these tests are shown in Fig. 21-3. The samples were repeatedly flexed under a load of 3650 g at 342 flexures per minute in a fatiguetesting machine. The flexural fatigue strength of

PMM-Pour type

PMM-Thermosetting

Vinyl

PMM-Grafted butadiene

Materials

Fig. 21-3 Flexural fatigue of various types of denture base materials.

(Adapted from Johnson EP: J Prosthet Dent 46:478, 1981.)

the rubber-reinforced acrylic plastic was superior to the other materials, and the pour type of acrylic plastic had the lowest value.

Fracture Toughness Because the geometry of denture bases is complex and stresses can be concentrated in flaws on the surface or in frenum notches, cracks can occur in the denture base. Several tests are available for determining fracture toughness. One method for testing fracture toughness is to bend a notched specimen and record the force required for crack propagation (Fig. 21-4). Several denture base materials are compared in Table 21-6. The fracture toughness of the high-impact resin appears to be no better than that of vinyl or conventional acrylic resin. The rapid heat-cured resin had the highest fracture toughness, and the pour type of resin the lowest toughness. The fracture toughness of acrylic is greater when samples are saturated with water rather than dry.

Compressive Creep When denture base resins are placed under a load they will deform (creep) with time. The lowest compressive creep rates are found for heat-polymerized materials. Chemically accelerated acrylics, both dough and pour type, have higher values for compressive creep. At low stress levels, the type and quantity of cross-linking agents have no major affect on creep. However, at higher stress levels, creep values decrease with increasing quantities of cross-linking agents. For heat-polymerized mate-

Fig. 21-4 Sketch of specimens for measuring fracture toughness. p, Load; R, radius; thickness; 5,width. Dimensions in millimeters.

(Adapted from Stafford GD, Bates JF, Huggett R, Handley RW: J Dent 8:292, 1980.)

cyclic loading. Results demonstrate the viscoelastic nature of denture base polymers. A torsional pendulum also may be used to evaluate the viscoelastic properties of denture base resins. These tests are useful in evaluating the effect of free monomer, plasticizers, and degree of cross-linking.

M a t e r i a l M N / ~ - ~ "

rials, when the temperature is increased from 37" to 50" C the mode of failure changes from brittle to ductile. For autopolymerizing acrylics, the materials fail in a ductile manner at both temperatures.

Recovery After Indentation Indentation recovery of plastics in equilibrium with water is generally lower than when dry. The recovery from indentation of a 1.27-cm diameter steel ball loaded for 10 minutes at 30 kg is given in Table 21-3. The time allowed for recovery was 10 minutes. Recovery was 86% to 89% for dry specimens and 84% to 88% for wet specimens. The results for several new products are seen in Table 21-4.

A modified Wallace hardness tester has been used to measure the hardness, creep, and recovery of denture base polymers and the effects of

Hardness The low Knoop hardness number of the denture base plastics (see Tables 21-3 and 21-4) indicates these materials may be scratched easily and abraded. Cross-linked poly- (methyl methacrylate) is only slightly harder than regular poly(methy1 methacrylate) (about 1 kg/ mm2). The incorporation of fillers in plastics may alter the resistance to abrasion, but the hardness of the plastic matrix remains unchanged. Polishing, shell blasting, and cleaning denture bases by brushing should be carried out with this in mind.

Abrasion Resistance Abrasion resistance of denture base resins has been evaluated by abrading specimens against 600-grit silicon carbide paper for 1 hour under a stress of 0.26 MPa in water at 37" C and measuring loss of material (Table 21-7). All materials had similar wear characteristics. However, the vinyl acrylic had the best and the pour type of acrylic had the least wear resistance.

THERMAL CHARACTERISTICS

The thermal properties of plastics are important because plastic materials are usually processed at 74" C and in service are in contact with

hot and cold foods and beverages. If a chemical accelerator is used rather than heat, the material is still subjected to exothermic heat resulting from the polymerization reaction.

Thermal Conductivity Dental plastics are poor thermal and electrical conductors. Compared with gold, cobalt alloys, or even human dentin, which have thermal conductivities of 0.7, 0.16, and 1.3 x cal/sec/cm2 (O C/cm), respectively, the values for the various types of plastics listed in Table 21-8 are low. Low thermal conductivity allows plastic denture bases to serve as an insulator between the oral tissues and hot or cold materials placed in the mouth. It has been shown that the inclusion of sapphire whiskers in poly(methy1 methacrylate) increases heat conductivity.

Specifrc Heat Specific heat, or the heat required to raise the temperature of a gram of plastic loC, is a thermal property closely related to thermal conductivity. Although it is not obvious, it may be shown that the ratio of thermal conductivity to the product of specific heat and density is a constant for a particular material and represents the velocity of temperature disturbance in a plastic. The higher this ratio, termed diffusivity,the greater the velocity of heat transfer through a material. The specific heats for poly(methy1 methacrylates) and polyvinyl acrylics are similar, and the respective thermal conductivities are not greatly different; therefore the diff~~sivityof plastics will be roughly equivalent, 0.123 mm2/sec.

Thermal Coefficient of Expansion

The temperature change from processing temperature to room temperature or mouth temperature indicates the importance of the thermal coefficient of expansion. Plastics have relatively high thermal coefficients of expansion (71 to 81 x 10-"/" C) compared with other dental materials. Gold, amalgam, and tooth structure have values

a linear function of the amount of the filler present. Thermal expansion is important in the fit of denture bases. It is apparent that a denture that fits a cast accurately at room temperature will not fit exactly the same at mouth temperature.

Heat Distortion Temperature Heat distortion temperature is a measure of the ability of a plastic to resist dimensional distortion by heat. This is the temperature at which a specimen loaded in a transverse manner at 1.8 MPa stress deflects a distance of 0.25 mm. These temperatures are sufficiently high that they are of little concern except in the repair of dentures. The heat distortion temperature for polyvinyl acrylic is 54' to 77" C and for poly(methy1 methacrylates) 71" to 91" C. These values suggest the need for keeping repair temperatures low by using chemically or light-polymerized materials for this purpose.

Plastics or polymers, when heated, are transformed from a glassy or brittle condition to a

Adhesion to acrylics Colorability

Color stability

Taste or odor

Tissue compatibility Shelf life

1

1

Good

Good

Yellows very slightly

None

Powder and liquid, good; gel, fair

1

1

1

Good

Good

Yellows slightly None

Ezldfair

rubbery condition. The temperature of this transition is called the glass transition, Tg. Molecular motions are allowed above this temperature that are not permitted at lower temperatures, thus plastics are easier to deform above Tg.

OTHER PROPERTIES OF DENTURE PLASTICS

Density The density, or the weight in grams of a cubic centimeter (g/cm3) of material, varies in plastics because of variations in molecular weight. Denture base plastics have densities ranging from 1.16 to 1.36 g/cm3 (Table 21-9), or slightly greater than the density of water.

1.18 g/cm3 for poly(methy1 methacrylate). This increase in density is mainly accounted for by an approximate 21% decrease in volume of monomer during polymerization. Because the ratio of polymer to monomer used in the preparation of dental poly(methy1 methacrylates) and polyvinyl acrylics is usually 3:1,the free volumetric shrinkage amounts to approximately 6%. The lightactivated denture base material has low polymerization shrinkage of 3% because higher molecular weight oligomers are used.

Adapted from Stafford GD, Bates JE Huggett R, Handley

RW:J Dent 8:292, 1980.

It should be pointed out that the linear shrinkage values reported in the literature are generally much less than would be expected (Table 21-10) on the basis of the free volumetric shrinkage, because a portion of the polymerization takes place after the plastic has attained a solid condition, resulting in residual stresses in the plastic rather than additional shrinkage. An ideal plastic would be one that had no polymerization shrinkage, but thermal dimensional change would still result from cooling the plastic from molding temperature to room temperature.

Dimensional Stability and Accuracy

The dimensional stability of the denture during processing and in service is important in the fit of the denture and the satisfaction of the patient. In general, if the denture is properly processed, the original fit and the dimensional stability of the various denture base plastics are good. However, excess heat generated during finishing of the denture can easily distort a denture base by releasing residual stresses.

It has been shown that chemically activated denture bases processed by dough molding with a dimensional accuracy of -0.1% were more accurate than heat-activated denture bases at -0.4%. The most accurate dentures were produced using either a chemically activated pour resin processed under pressure at 45" C or a microwave-activated resin. A visible lightactivated resin was more accurate than a conventional heat-activated resin. In the past, injection-

molded dentures were less accurate than compression-molded materials. Recent studies demonstrate that dentures processed by new injection molding methods are more accurate than standard compression molding. The increase in vertical dimension of occlusion was very small for the injection-molded acrylic when compared with the conventional compressionmolded acrylic.

In another report, six different denture base materials were processed by heat, light, or microwave energy. The denture bases were removed from the casts, finished, and polished. After storage in distilled water for 42 days to allow for water sorption, the dentures were placed back on the stone casts on which they were made and ranked for accuracy of fit by five evaluators. Denture bases processed by microwave energy, low heat at 45" C (autopolymerizing resins), or visible light fit better than resins processed at either 74' C (conventional resins) or 100" C (rapid heat-cured resins).

When evaluated in two dimensions, the dimensional stability of denture bases is usually reported at <I%. In one study the accuracy of maxillary dentures was measured at six locations from anterior to posterior. For all materials studied, the accuracy was better from the anterior of the denture to the middle of the palate (generally <I00 pm) and became worse toward the posterior of the denture. In a recent study of three-dimensional stability of several products, changes ranged from 0.2% to 8.1%in the frontal dimension, and 0.2% to 9% in the lateral dimension. Changes were greatest in the cross-arch dimension.

There are numerous articles in the literature that report conflicting results for accuracy of denture base resins. However, it is encouraging that emphasis is being placed on dimensional accuracy and some of the newer materials appear superior to older products.

Water Sorption and Solubility The sorption of water also alters the dimensions of acrylic dentures. This change in dimension is, for the most part, reversible and the plastic may go through numerous expansions and contractions

when alternately soaked in water and dried. However, repeated wetting and drying of dentures should be avoided by the patient, because irreversible warpage of the denture bases may result. Denture plastics of the same type may vary considerably in water sorption because of the presence of additives. Poly(methy1 methacrylates) have relatively high water sorption values of 0.69 mg/cm2. Polyvinyl acrylics have lower values of 0.26 mg/cm2. The thickness of the plastic specimen and the type of polymer influence whether equilibrium water sorption will be attained in 24 hours.

Temperature also affects the rate at which water is absorbed, because the diffusion coefficient is increased by a factor of two between room and oral temperature and the equilibrium absorption value does not change.

Denture plastics can be tested for water sorption where a dried plastic disk 50 mm in diameter and 0.5 mm thick is stored in distilled water at 37" C for 7 days, after which the increase in water is determined and the sorption recorded in milligrams per square centimeter. In addition, the solubility of the plastic is measured on the same specimen by re-drying to constant weight in a desiccator and re-weighing to determine the loss in weight in milligrams per square centimeter. A denture plastic should have a water sorption value of not more than 0.8 mg/cm2 and a solubility not greater than 0.04 mg/cm2 (see Table 21-21,

Resistance to Acids, Bases, and Organic Solvents The resistance of denture plastics to water solutions containing weak acids or bases is good to excellent. Denture plastics are quite resistant to organic solvents, with poly(methy1 methacrylate) being more resistant than polyvinyl acrylic. Both are soluble in aromatic hydrocarbons, ketones, and esters. Alcohol will cause crazing in certain denture plastics. Ethanol also functions as a plasticizer and can reduce the glass transition temperature. Therefore solutions containing alcohol should not be used for cleaning or storing dentures. Incorporating ethylene glycol dimethacrylate as a cross-linking agent in denture base resins has little effect on water

sorption, but significantly improves solvent resistance.

ProcessingEase A comparison of the processing ease of the various types of denture plastics is difficult. In general, with the proper equipment and facilities all of the denture plastics have satisfactory processing properties.

Adhesion Properties The adhesion of denture plastics to untreated porcelain or metals is generally poor. As a result, porcelain teeth or combined metal and plastic bases should be designed so that the porcelain or metal is held by mechanical retention. It has been shown that the lack of adhesion between a plastic base and porcelain teeth provides an area in which microorganisms present in the oral fluids may incubate, and this makes maintenance of a clean denture more difficult. This problem can be avoided by the use of plastic teeth, which form a bond with denture base materials or by organosilane treatment of porcelain teeth (although this treatment is not routinely used). The silane coupling agent, y-methacryloxypropyl- trimethoxysilane, provides a bond between the porcelain and the plastic surface. Many studies have demonstrated the effectiveness of bonding denture base resins to partial denture alloys or titanium using the following ingredients alone or in combination: silica blasting, metal etching, silane coupling agents, 4-META adhesive resin, other organic primers such as 10- methacryloxydecyl dihydrogen phosphate.

Esthetics The esthetic qualities of denture base plastics include such properties as colorability, color stability, taste, and odor. The ability of the plastics to be colored and their compatibility with dyed synthetic fibers for characterization are both good. Color-measuring systems have been used to compare the actual color of denture base resins with the color of gingival tissues. Results indicate that few commercial products actually match the color of the tissue they are replacing. A color stability test requires that a specimen exposed for 24 hours to an ultraviolet light source shall not show more than a slight change in color

when compared with an original specimen. One study questioned the stain resistance of the lightactivated denture acrylic. Current denture products have no taste or odor when properly processed.

Tissue Compatibility The tissue compatibility or allergic sensitization of the skin to the components of denture plastics or to processed plastics has been a subject of considerable contention. It may be concluded that completely polymerized poly(methy1 methacrylate) or polyvinyl acrylics rarely cause allergic reactions but that methyl methacrylate monomer or other trace components in the monomer may produce an allergic reaction. Dentures prepared by polymerization of methyl methacrylate with a chemical accelerator may have sufficient residual methyl methacrylate monomer present in the finished denture to cause an allergic reaction in patients who are sensitive to methyl methacrylate monomer. This reaction diminishes as residual monomer leaches out of the plastic. Allergic reactions to heat-processed denture base plastics also occur but less often than with chemically accelerated plastics. Again, residual monomer is considered the allergen, and strict adherence to processing instructions recommended by the manufacturer can keep the residual monomer to a minimum. When patients are known to have suffered from an allergic reaction, processing the denture for extended periods (such as 24' versus 8 hours) may be helpful. Residual monomer levels can also be reduced dramatically by processing heatpolymerized poly(methy1 methacrylate) in a water bath for 7 hours at 70" C, followed by boiling for 1 hour. Boiling has only a slight effect on the dimensional accuracy of the processed dentures. Vinyl acrylic or light-activated denture base materials are an alternative for those patients who are sensitive to methyl methacrylate monomer.

In one study, 53 patients wearing dentures and suffering from "burning-mouth syndrome" were evaluated for allergies related to various

components of denture base plastics. Epicutaneous patch tests were used. Approximately 15 patients demonstrated a positive skin test to one or more of the following: N,N-dimethyl-para- toluidine, hydroquinone, formaldehyde, methyl methacrylate, and p-phenylenediamine, as well as several metallic compounds. Pigments also may be toxic. There is one case reported of a patient who had an extensive systemic reaction to dentures. Patch testing was used and the patient had a positive reaction to the pure dye supplied by the manufacturer. She had no reaction to the unpigmented acrylic.

Plasticizers are used to lower the glass transition temperature of poly(methy1 methacrylate), which in turn produces a tougher, less brittle material. In addition, the use of plasticizers decreases water sorption because they are hydrophobic. There is evidence that plasticizers fill microvoids within acrylic. This "microvoid concept" is also related to water sorption where water may replace the plasticizer in the microvoids over time. Plasticizers are commonly used in fairly high concentrations in soft denture liners. Unfortunately, some plasticizers, particularly phthalates, are known toxins; products that contain these materials should be evaluated for biocompatibility. With the demand for safety from health-related products, greater attention will be focused on the tissue compatibility of denture base materials.

The growth of Candida albicans on the surface of dentures is a concern for many denture patients. This organism is associated with denture stomatitis. An in vitro study has demonstrated the effectiveness of chlorhexidine gluconate in eliminating this organism. The chlorhexidine apparently can bind to acrylic surfaces for at least 2 weeks. Treating acrylic with Nystatin, followed by drying, produces similar results. The use of phenolic disinfectants can cause surface damage to denture base resins and is not recommended.

In addition to Candida albicans, many other microorganisms can adhere to denture base acrylics, such as Streptococcus oralis, Bacteroides

gingivalis, B. intermedius, and S, sanguis. It is not surprising that many organisms adhere more to rougher surfaces than to those that are highly polished.

Shelf Life The shelf life, or useful storage time at room temperature, for denture base plastics varies considerably. Acrylic plastics packaged in the powder-liquid form have excellent shelf life, because the powder is almost indefinitely stable and the liquid usually is adequately protected from polymerization during storage by a hydroquinone inhibitor. Vinyl acrylic plastics, packaged as a gel in which the monomer is in contact with the polymer, must be stored at refrigerator temperature (about 2" C) to have a reasonable storage life of 1 to 2 years. The shelf life of light-activated denture base materials has not been reported.

Summary It is fortunate that many types of denture base materials are available that will produce satisfactory dentures. The requirements may vary for different patients, and the processing facilities can dictate which material to use. There are also significant differences between products in each category, and it is wise to choose materials that have passed ANSI/ADA Specification No. 12.

MANIPULATION AND PRWW OF DENTURE BASE PLASTICS " -,, ,[

As mentioned previously, denture base plastics are supplied in several forms. The techniques used to process these materials into a finished complete or partial denture are briefly described subsequently, as are the justifications for the various procedures are presented.

Texts on prosthetic dentistry describe impression techniques, pouring master stone casts, setting denture teeth, preparing the waxed denture, investing in a denture flask, removing the wax, and coating the stone mold to prevent adhesion of the plastic. A set of waxed dentures on stone casts with the teeth in position is shown in Fig. 21-5, and a flask that contains maxillary porcelain teeth ready for packing the plastic dough is shown in Fig. 21-6.

HEAT-ACCELERATED ACRYLIC DENTURE PLASTICS

The general method for processing a heataccelerated acrylic denture base material consists of proportioning and mixing the polymer powder and the liquid monomer and allowing the monomer to react physically with the polymer in a sealed jar until a doughy consistency is