MECHANICAL TESTING OF A PHOTOCURED CHOPPED FIBER REINFORCED DENTAL COMPOSITE

Abstract

The objective was to validate strength improvements by incorporating chopped fiber into commercial particulate filled visible-light cured dental pastes. Photocurable resin preimpregnated 3-mm high purity quartz fibers were blended into two universal commercial composites, 3M Corp. Z100 and Kerr Corp. XRV at 35 wt% fiber. Four groups, consisting of the Z100 and XRV controls, along with Z100 and XRV both containing fibers, were prepared in a mold 2×2×25-mm then cured at equivalent irradiation intensities monitored by a Demetron Radiometer. Evaluation was accomplished on ten specimens from the four different categories according to ASTM Standard C 1161-94 configuration-A utilizing a fully-articulated four-point bend fixture and 20-mm span. Five samples out of each group were further assessed using a modified Izod Plastic Impact Tester per ASTM D 256-00. Both highly strengthened fiber reinforced composites attained a marginal error of uncertainty for statistical significance over the commercial controls (p=0.001 AVOVA posthoc Newman-Keuls) with maximum bending stress, flexural modulus, work of fracture and Izod impact toughness. From the results, it can be concluded that photocured chopped fiber reinforced composites will produce improvements over particulate filled compounds that can be considered breakthrough.

1. INTRODUCTION

In the development of alternative materials to replace silver-alloy dental fillings, the fundamental emphasis has been on polymer-based composites using particulate filler (1–3). Following the discovery of chemical initiator and accelerator chemistries for ambient temperature cure with acrylic and epoxy before World War II (4–6), several improvements have combined toward the progress of advanced composites in dentistry. Hybrid resins adding methacrylate functional groups onto a longer saturated epoxy resin, ethylene glycol of bisphenol A, (Bis-GMA) and use of dimethacrylate monomer diluent crosslinking agents (3,7,8) decreased cure shrinkage of acrylic (10) while permitting highly glass-filled systems at 75–90wt% (1–3, 9). Consequently, wear rates have been reduced for dental composites that now approach amalgams for smaller fillings (1,2). Although dimethacrylate crosslinking agents lower mixing viscosity and improve cured chemical resistance, such monomers also increase polymerization shrinkage (3). The early unfilled dental acrylic fillings suffered from shrinkage by approximately 6–8-volume percentage (11). But now with both the longer saturated backbone Bis-GMA polymer chain having less double bond reactivity and heighten glass-filler loading nearing 70 volume % that controls the amount of resin cured, shrinkage values are down to 1.0–4.0 volume % (10) with lower internal residual polymerization stresses (12). Photocuring in times of 6–12 seconds depending on light intensity (13) further eliminates air mixing porosity thereby minimizing oxygen inhibition of free radical curing (2,3) in addition to lessening void stress concentrators. Finally, UV cure was replaced by longer-wavelength visible light for faster and greater degree of cure through deeper composite sections (3,14).

Nevertheless, a major problem arises as cavities enlarge particularly when two connecting tooth surfaces are lost so that fracture becomes a crucial dental composite failure mechanism. The most obvious posterior composite requirement regarding strength with the American Dental Association Seal of Acceptance Program is that at least 75% of the cavities in clinical trial testing must be class II (15,16). A class II filling describes a posterior tooth where the cavity extends beyond the top occlusal class I region onto at least one lateral tooth wall. During a 20-year long Department of Defense study, loss of such an outer tooth exterior beyond the class I occlusal area resulted in a 468% composite fracture increase (17). Other principal dental composite failure patterns related to mechanical strengths or dimensional stability would include microcracking, microleakage and wear that contribute toward recurrent decay (17).

To counteract inadequate strengths for larger fillings, small chopped fibers combining high-purity quartz are being utilized toward advanced composite development. Fiber addition builds viscosity rapidly over particle filler (18,19) allowing clinically relevant amalgam alloy-like condensation forces with applied pressures (20–22). Such heightened compaction loading enables routine reestablishment of the interproximal contact following replacement of a tooth surface between the posterior teeth (20–22). Furthermore, sticky particulate filled dental composites are placed with such unsubstantial pressures that voids occur (20, 23–25) whereas fiber reinforcement affords a compression process that eliminates packing defects (20). Chopped fiber accounts for 60% of the overall fiber-reinforced composite market because of simplified molding compound processing capability inside intricate die shapes (26). The merger of chopped 1–3 mm fibers with particulate filled compound will thus allow consolidation of an advanced strengthened composite into small-complicated cavity designs to meet the critical lengths for dental fillings (20). Chopped fiber reinforcement can then approach near continuous strengths through selective application of the appropriate filament lengths by supplying a mixed assortment of various sizes.

When comparing composite fiber reinforcement with particulate filler, several essential advantages are clearly apparent as the diameter-length aspect ratio extends to include: increased composite strength (27–32), fatigue strength (29) impact strength (27,31,32) modulus (27,29,31, 32), tear strength (31), fracture toughness (30,32) and wear resistance (33,34), depressed cure shrinkage (27,35,36) and improved dimensional stability (27,36) with lower residual stresses and less creep (36). The operator can even control fiber reinforcement directional shrinkage during random planar placement where contraction is minimized along the axes (27,35–37). Initial surface roughness for a dental composite is a function of either the surface contact (20) or polishing sequence (38,39). And, finishing normally produces Ra surface roughness values much lower than smooth contact enamel at approximately 64 nanometers (38). However, over time with particulate composite loss on the order of 2–10 microns/year across the masticatory surfaces (1), eventual occlusal surface roughness becomes dependent on the stability of material reinforcement encountering loads that degrade strengths responsible for wear resistance (33). The primary degeneration regarding wear is subsequently a function of reinforcing aspect ratio in relation to the average plowing groove (33) along with chemical degradation (40).

Although fillers are most frequently mentioned industrially to reduce overall composite cost, dental particulate provides several benefits that reduce wear (1,2), lower shrinkage (11, 32), increase modulus, strength and hardness (24,30,32), provide x-ray diagnostic contrast (25) and improve the resin viscosity for rheology control (41). Dentistry has investigated micro-fiber filler, but average dimensions are now only about 40–60 microns (9), so the critical length, which describes tensile failure of the fiber rather than shearing at the fiber ends (29,42), is not achieved. High atomic elements are added in dental silica particulate fillers for x-ray radiopacity which generally increase glass solubility that reduce composite hardness in moisture environments (40,43,44). Barium is the most common radiopaque element added in glass fillers for commercial dental composites where solubility has been a major concern (40). Another heavy element, zirconium, is an exception toward reducing glass properties due to the stable ion position in the silica crystal lattice by not being an alkali earth element (43,44,45). Although radiolucent, high purity quartz furnishes an ideal reinforcement to compliment current silane coupling technology with fibers now attaining higher purity levels than previously at 99.99% silica (46) to support the silanol hydrolysis reaction for ensuing matrix resin interaction. Quartz silica purity supplies chemical inertness, complete water insolubility, no water adsorption (40,46), and a low coefficient of thermal expansion, while also achieving the lowest mineral fiber dielectric and loss tangent (46). A related concern for microleakage, considered a source for secondary decay, is correlated with materials not having a low coefficient of thermal expansion due to poor dimensional stability during temperature changes (47). An extension of quartz fiber capability indicates performance above fiberglass over twice as long through high impact environmental aircraft radome studies (48). Quartz fiber with an individual filament tensile strength of 6 GPa, tests higher than E-glass and S-glass (46). Quartz filament ductility at 7.7 % (46) further improves chopped fiber reinforced ability to conform into complex spaces.

To test the hypothesis that high-purity chopped quartz fibers will improve the mechanical properties of photocured particulate filled dental resins, fully articulated four-point bend and Izod impact testing were completed. Two commercial dental composites acted as controls to compare the addition of preimpregnated chopped quartz fibers at 35wt%. The outlying error of uncertainty for statistical significance was set at p=0.001 Anova Newman-Keuls post hoc.

2. EXPERIMENTAL

2.1 Materials

Presilanated 3-mm quartz fibers, Quartz Products Co., Louisville, KY, were impregnated with Bis-GMA resin using a lower molecular weight diluent monomer, Triethylene glycol dimethacrylate (TEGDMA), Esschem, Inc, Essington, PA. The resins are photoactivated with initiators, camphorquinone, Aldrich, Milwaukee, WI, and Irgacure 819, Ciba, Tarrytown, NY, maximally degrading when exposed to light at 410 and 470 nm respectively which in turn excite a tertiary amine accelerator, 2-(dimethylamino)ethyl methacrylate, Aldrich, that increases electron free radical formation. Fumed silica, Cabot Corp. Tuscola, IL, is added to increase viscosity thereby preventing resin rich areas from developing under pressure. Two of the most popular particulate filled commercial dental composites 3M Corp. Z100, St. Paul, MN, and Kerr Corp. XRV, Orange, CA., were used as both controls and pastes to incorporate preimpregnated quartz fibers at 35wt%. Concerning quality control, in 1997 3M Corp. Dental Division became the second Health-care Company and first Dental Corporation to receive the Malcolm Baldridge National Quality Award from the Department of Commerce (49). 3M Z100 is reported with a 84.5 wt% zirconium silicate particle and Kerr XRV employs 78wt% mixed colloidal silica and barium silicate (50). Both Z100 and XRV use Bis-GMA and TEGDMA matrix resins (50).

2.2 Sample Preparation and Photocure

Composite samples were prepared from a 25×2×2 mm split-mold machined to specifications for American Dental Association (ADA) standard 27 on resin-based filling materials (15). Specific careful attention was made to utilize round-ended dental composite packing instruments when inserting the particulate filled commercial composites as the sticky paste is highly prone to air incorporation during placement (23,51). Fiber reinforced composites on the other hand consolidate under pressure thereby eliminating problems associated with sample defects (20–22) so that flat ended instruments were used. The split sample mold assembly was lightly coated with a True Vitality Release separating medium, DenMat Corp., Santa Maria, CA., and secured upon a 6.25 mm thick glass plate whereupon a small excess of uncured sample material could be inserted. The mold assembly containing uncured composite was then covered with a clear thin styrene panel and clamped under pressure provided by material resistance until the covering was flush with the mold. Visible light at 400 mW/cm2 with a peak intensity of 470 nanometers, monitored by a Demetron Radiometer, was used to irradiate all composite compounds beyond minimum manufacturer recommendations for equivalent times per unit area on both sides. The samples were separated from the split-mold after disassembly, the excess flash was removed, followed by a polishing sequence using 320, 600, and 600 wet grit silicon carbide. All cured composites were carefully inspected and discarded if any negligible voids could be visually detected by close examination. Before mechanical testing, specimens were measured to the nearest 0.0025-mm in depth and width.

2.3 Mechanical Testing

2.3.1 Fully Articulated Four-Point Bend

ASTM standard C 1161-94, analogous to MIL-STD 1942A, is an advanced ceramics flexural bend test developed primarily as a cost-effective means to more difficult error-prone tensile strength examinations of small samples (52–55). ASTM C 1161 specifications are included with small specimen configuration-A having a 2mm width and 1.5 mm depth (52) closely corresponding to the dental sample requirements in ADA standard #27 using both 2.0 mm width and depth during three-point flexural testing of resin-based filling materials (15). Dental particulate filled composites are not only prone to developing void defects (23) but are also extremely sensitive to air incorporation porosity (51). By contrast, fiber reinforced composites readily consolidate and pack easily (20–22). To reduce experimental error in highly flaw-sensitive advanced ceramic testing, four-point bend testing is “strongly preferred” over three point bend tests that are “intended only for material development, quality control screening, or to identify fracture origins in research studies.”(53–55) Important sources of error with four-point bend can further be minimized with freely moveable bearings for the loading noses and one of the support spans in addition to freely pivoting perpendicular rollers (52,56). Testing by fully articulated four-point bend is also reported as a progressive approach to lowering statistical variability (57). Therefore, taking into account the previous considerations while conforming to ADA specification #27 for a small dental sample size with identical 20 mm load span, ASTM standard C 1161 four-point A-configuration utilizing a fully articulated fixture was put to use, Figure 1. Both loading noses and supports at 2.0-mm diameter are the minimum sizes allowed (52) for a more critical test and duplicate requirements for ADA Specification #27. The loading noses were ¼- spaced at one half the length of the support distance. In order to secure the articulating fixture before mechanical loading, the upper four-point assembly was first inserted by precision fit into the overhead Instron test machine-arm attachment. The loading noses were then connected with elastics along both sides to rods extended from the test machine above for stabilization of the upper articulating group. The lower four-point articulating structure was also fastened with elastics from both sides on the roller span contacts to rods on the lower base coupled with the Instron, Figure 2. The mechanical test machine used for the evaluation was a Model 1123 Instron Series IX automated materials evaluation system with a constant crosshead speed of 0.5mm/min. The group sample size was set at 10.

Figure 1.

Schematic of a Fully Articulating Four-point Fixture. Copyright ASTM Reprinted with Permission

Figure 2.

Photograph of a Fully Articulating Fixture Showing Elastics Required to Secure the Upper and Lower Assemblies

2.3.2 Formulas

Flexural strength with a loading span of one half the support span or ¼ point flexure applying ASTM Standard C 1161 using calculation 9.1 equation 1 (52) from simple beam theory; also equivalent to ASTM D 6272 equation 6 (58). S=3PL/4BD²

Tangent Modulus of Elasticity corresponding to the formula from four-point bend standard ASTM D 6272 equation 12 (58) with loading nose one half the length of the support span using an identical loading configuration to ASTM C 1161 ¼ point span. E=0.17L³M/BD³

S-stress in the outer fiber throughout the load span, P-load, E-modulus of elasticity in bending, L-support span, B-width of beam, D-depth of beam, M-slope of the tangent to the initial straight line on the steepest portion within the elastic limit.

2.3.3 Work of Fracture

Application software integrated the areas under the load-deflection curves from four-point bend tests for Work of Fracture (WOF) at 5% displacement beyond initial tensile flexural failure measured at maximum bend strength.

2.3.4 Modified Izod Impact test

Performed by cantilever fixturing according to ASTM standard D 256-00 (59) without a machined notch necessary to concentrate impact stress for full sample energy values. A Tinius Olsen Plastic Impact Tester was operated for the Izod loading. Five samples were tested from each group. Note: Due to the miniature size of the test samples, the impact strength reported is a modified value and should not be used as a documented result per the ASTM specification. Neither the sample dimensions nor the Izod tester met the designation for a valid ASTM D 256 test. Significance should be adopted in a comparison manner only.

2.4 Surface Fracture and Imaging Characterization

Following mechanical testing

2.4.1 Visually gross macroscopic

irregularities were observed and noted for all samples.

2.4.2 Radiographs

All samples from each group were radiographed after being numbered and recorded. Fiber reinforced samples were alternated between particulate samples on Kodak Ektaspeed-Plus dental occlusal films to provide uniform beam energy distribution prior to exposure for 0.1 seconds at 70KeV with a General Electric Dental x-ray machine using a preset distance of 100 millimeters. An A/T2000Plus automatic processor developed the x-ray films.

2.4.3 Philips Scanning Electron Microscope

(SEM) having digital storage capability was limited to only characterizing a fracture surface of one sample chosen closest to the mean for flexural bend strength. Samples were gold sputter coated under a vacuum.

2.4.4 Atomic Force Microscope

imaging was restricted to select specimens with minor surface flaws before polishing

2.5 Mechanical Test Results

Fiber reinforced composite samples produced extensive improvements over unadulterated particulate filled commercial controls ranging from approximately three fold increases during four-point flexural bending, to about ten fold improvements for Toughness measured through Work of Fracture or Izod Impact. All fiber reinforced samples increased strengths over particle filled composites for Maximum Bending Stress, Flexural Modulus, Work of Fracture and Izod Impact tests at statistically significant levels with p=0.001 ANOVA post hoc Student-Newman-Keuls, Figures 1–4.

Figure 4.

Flexural Modulus

Each test group passed the ADA Seal of Acceptance three-point flexural strength requirements, 50 MPa (15), for resin based composites using a more rigorous four-point advanced ceramics test. Toughness results between WOF at low strain rate and Izod impact high strain rate correspond well to one another in terms of representative values. Increases primarily for fiber reinforced composites from bend tests on initial tensile fiber damage calculated through integration for WOF are noted for comparison with unnotched full fracture Izod impact values.

2.6 Surface Fracture and Imaging Observations

All particulate filled composites failed catastrophically showing smooth brittle fracture through every sample tested without exception during four-point bending with the majority evidencing an initial thin horizontal fracture 0.5–1.0 mm on the tensile side followed by a vertical cleavage. X-rays demonstrated homogeneous radiopacity for all specimens presenting no observable voids. On the SEM particulate filled composite, material was packed uniformly with only a single void found on one specimen less than 10 microns, fracture characterization imaged in Figure 7. Izod impact samples were fractured through completely showing flush vertical surfaces.

Figure 7.

SEM glass particulate filled composite with void (scale 20 microns)

All four-point bend fiber reinforced samples were 100% intact while only two specimens displayed any fracture deviations of the tensile-compressive planes, at 16 and 30 degrees, in addition to a few minor delaminations on the tensile side. X-rays, however, showed that a large percent of the fiber reinforced composite samples proceeded from flexural tensile fracture onto faint interplanar failure horizontally out to the neutral axis. Radiopaque density was uniquely uniform for each group. Fiber reinforced Izod impact samples were fractured through completely showing surfaces with gross irregular ductile features combined with interplanar shearing. SEM imaging of fiber reinforced fractured surfaces was characteristic of random planar molding compound, demonstrating fiber fracture and fiber pullout, Figure 8.

Figure 8.

SEM random planar fiber reinforced composite (scale 100 microns)

Atomic Force Microscope imaging of minor defects from demolded unpolished samples indicates the potential for concentrating specialty additives to compliment fiber reinforcement. The extrusion of resins and particulate filler combines to form a layer on the surface or at a bonding interface that is approximately 1–6 microns thick.

3. DISCUSSION

3.1 Maximum Bend Strength Four Point Tests

This was an exciting discovery for dentistry regarding mechanical strength improvements of photocured fiber reinforced compounds over particulate filled counterparts. However, direct application should improve even more on chopped fiber properties as engineered principals predict higher relative strengths when comparing advanced composites. By controlling fiber lengths through manufacturing, near continuous strengths can be developed directly during clinical placement. Cavity mold design modifications that will conserve healthy tooth structure will also orient fiber direction inside thinner width fillings. The ASTM C 1161 four-point flexural test span used in the investigation was 20 mm with photocured chopped fibers at 3-mm lengths, whereas the average filling is only about 2–6 mm in length and 10–12 mm at the longest. So critical lengths will need to be determined for accurate clinical comparisons. However, generally discontinuous strengths are 50–60% of continuous reinforcement (26,60). If fibers exceed length requirements for a cavity space, reinforcing filaments with diameters of only 9 microns will bend, crush or break when forced into confined areas in a filling mold under condensing pressures that become extreme with light forces using small dental instruments. Quartz fiber silica purity further contributes exceptional ductility (46), which allows resin to conform around broken ends along the interfacial walls in a complex molding process.

In terms of comparable material results, four-point bend testing is more rigorous than three-point resulting in lower strength values with advanced ceramics (56,61). For example, sintered Beta Silicon Carbide A-size ceramic specimens increase maximum bend strength from an average of 312MPa to 394MPa when alternating ASTM C 1161 four-point to three point loading (61). Differences between four and three point testing are related to shearing effects that become pronounced as the loading noses are moved toward the supports. When comparing resin-based composites, the diameters for the four-point loading noses and supports were minimized at 2.0 mm using advanced ceramic specifications C 1161, identical to ADA Spec. #27, for a more critical test. Increased crucial stress was thus applied indicated from ASTM D 6272 that specifies polymer-based samples minimally use 3.2-mm diameter loading noses and supports otherwise pressure concentrating indentations can occur creating lower modulus and strength values (58).

In another regard toward additional strength, chopped fibers align in a random planar orientation in response to pressure against a mold wall. As fiber orientation angles that offset from the long axis decrease below 45-degrees, strengths increase rapidly near 20-degrees and improve dramatically for perfectly aligned 0-degree fibers (26,29). Photocure fiber-reinforced compound can thus be aligned down the major direction of the cavity where it is most required during flexural stress or tensile forces resulting through Poisson’s ratio dimensional change under material compression. Fiber orientation develops toward the long axis of a cavity when fibers attain lengths greater than the filling width by planar orientation through confinement by the mold walls, Figures 10a–c.

Figure 10.

a: 3-mm fiber 3mm wide mold random planar

b: 3-mm fiber 1.5 mm wide mold

c: 6-mm fiber 1.5mm wide mold

Dental cavity designs are already modified for composites to be smaller than amalgam alloy fillings due to the adhesive nature of resin-based composites (62). Fiber reinforced composites will thus promote such conservative filling design philosophy regarding tooth preservation using thinner widths in order to align fibers for increased strengths down the primary axis of the preparation. In fact, filling designs that are more narrow between the tooth cusps provide a stronger tooth with less chance of fracture (63). So, the operator should be able to design preparations for fiber orientation with premanufactured fiber lengths that can be placed for consolidation to approach both continuous and aligned strengths (20).

3.2 Modulus Values

Coupled orientation and continuous strength designing will also increase modulus values as photocured fiber reinforced composites proceed from the random planar state toward the more continuous orientated states (26,60) depicted in Figures 10a–c. Heightened fiber reinforced composite modulus values at 19–20 MPa already approximate human tooth dentin measured by Atomic Force Microscope at 20.6 MPa (64). A higher modulus will then provide improved stress transfer between the two separate mediums for the filling material and tooth at the marginal interface. Modulating forces can thereby equalize the loads more evenly so that minor weak areas of the composite are not intensified especially at stress concentration points or line angles during long term cyclic fatigue.

Modulus determinations calculated for initial failure on the tensile side by four-point bend can further be revised upwards when comparing other advanced composite materials. ASTM C 1161 was established to best compensate for flaws in advanced ceramics that are isotropic, as anisotropic materials will produce shear deflections at a much greater rate when the loading noses are moved from midspan toward the supports (52,58). Therefore, standards concerning advanced isotropic ceramics change when testing polymer-based materials that suggest increasing the span to depth ratios from 13 up to a minimum of sixteen and even up to as great as 60 for highly anisotropic materials (52,58). The fixture with ASTM C 1161 size A combining ADA specification #27 sample dimensions provided a span/depth ratio of only 10 contributing additional stress using ¼-point loading noses so that shearing effects cannot be underestimated. Also loading nose diameters should be at least 3.2 mm for polymer based materials to reduce stress concentrations and indentations that can reduce both modulus and strength values (58).

3.3 Impact and Work of Fracture toughness

WOF scales up well to Izod impact with both denoting energy adsorbed through different rates and distances during strain failure. Particulate WOF values closely resemble Izod impact demonstrating very little change for the brittle particulate filled composites whereas large increases are observed for the fiber reinforcement. The additional energy adsorption seen with Izod impact beyond early fracture integrated for WOF can describe the failure mechanisms for fiber reinforced materials. Fiber reinforced WOF data represents intact samples showing initial resin matrix cracking, tensile failure of the outer fibers and progressive damage accumulation with crack deflection and parallel debonding of the fiber resin interface through interplanar shear out toward the neutral axis. While on the other hand, Izod impact with a complete break moreover includes fiber fracture, fiber matrix debonding, fiber pull-out, fiber crack deflection and resin microcracking parallel to the fibers (65). Particulate filler composites conversely all broke during both flexural and Izod testing with complete brittle fracture exhibiting no more than about 1.0-micron average displacement or only 0.002% calculated tensile strain (58) after initial failure response during WOF bending. In terms of clinical relevance regarding toughnesss, impact was modeled to simulate macroscopic third-body hyperloading masticatory fracture while WOF is considered to have some relationship to subcritical long-term cyclic fatigue. Although these assumptions require direct correlation between laboratory and clinical findings, at least data is presented toward this end.

3.4 Fiber reinforced consolidation and void defects

Much of the problem associated with resin-based dental composites in posterior teeth is due to the original particulate filler development for placement in small front fillings, where esthetics is the major issue. So consequently beyond low strengths, insertion of a sticky paste into larger posterior teeth presents with difficult interfacial problems of air porosity incorporation. Macroscopic voids can even develop when small instruments withdraw causing a vacuum pullback defect. Although concerns for critical flaw size were removed during mechanical testing with this investigation using extreme meticulous care in sample preparation, clinical placement is not only more difficult due to considerably less visual access, but reasonable time constraints are placed upon the dentist to complete procedures in short acceptable periods. As an illustration, voids encountered with particulate filler dental composites are displayed using a plastic typodont model depicted in a x-ray image, Figure 11, where accidental operator error occurs. Voids then can create an artery for secondary decay in cavity areas where such flaws appear to be a natural source for bacterial colonization. By contrast, fiber reinforcement consolidates the entire bulk molding compound paste for routine uniform void-free samples that essentially eliminates large defect concerns (20).

Figure 11.

X-ray close-up of voids in a commercial dental composite with low viscosity using a resin system where hydroxyl groups have been removed from the Bis-GMA resin to reduce secondary bonding (51) thus accentuating defects during placement.

3.5 Future considerations

1. Continuous strengths with controlled manufacturing process providing pastes in tubes for extrusion, roll form, pellets or small tablets in different fiber lengths from 1–10mm.

2. Conservation of tooth structure to minimize filling dimensions for improved fiber directional strength from random planar orientation.

3. As material strengths advance, several other properties are generally included such as lower wear, reduced microcracking, less shrinkage and improved dimensional stability.

4. Strength increases and moduli are closely proportional to fiber volume (60). Although 35wt% fiber reinforcement is considered a common level for bulk molding compounds, higher levels up to 68wt% are considered with sheet molding (60). Advanced unidirectional tapes are another source for photocure fiber reinforced technology with combined continuous strengths, orientation, and higher fiber volume percent along with void-free fiber resin impregnation (60).

5. Strength properties should all increase when vacuum mixing equipment is used to also improve fiber wetting and lower porosity with reduced air incorporation associated with oxygen inhibition of free radical curing.

6. Fiber reinforced pastes have an advantage where additives can be concentrated in the compound at about double the fiber reinforced composite level on the surface or an interface for specific requirements.

7. Pressure application with fiber reinforced pastes produces enhanced bonding over sticky particulate filler compounds that inherently include porosity along a bonding interface.

8. Outside applications for larger part repairs or thin veneering operating bigger curing units. Visible light can now be safely used instead of harmful UV energy for curing deeper sections.

4. CONCLUSIONS and SUMMARY

4.1 Conclusions

1. Photocurable resin preimpregnated chopped fibers will provide enormous improvements in mechanical strengths for particulate filled dental composites demonstrated through maximum bending stress, flexural modulus, work of fracture and Izod impact toughness.

2. Large amounts of energy adsorption tested through work of fracture and Izod impact are characterized by tough ductile fiber fracture surfaces, fiber bridging and resin debonding compared to simple particulate filled composite brittle cleavage failure seen visually and with SEM imaging.

3. Consolidation of fiber reinforced dental pastes simplifies sample preparation and routinely eliminates problems associated with porosity or voids recognized with particulate filler.

4.2 Summary

This study investigated the incorporation of photocurable resin preimpregnated chopped quartz fibers at 35wt% into two universal dental particulate filled pastes. The two particulate filled compounds were 3M Corp. Z100 and KERR Corp. XRV. Four sample groups included the two commercial pastes and two experimental groups with 35wt% fiber reinforcement added. A fully articulated four-point bend fixture designed for advanced ceramics conforming to ASTM specification C 1161-94 size-A was used to minimize flaw-sensitive material testing error. A short 20mm span with loading noses and span rollers set at 2mm diameter consistent with both ASTM C 1161 and ADA specification #27 were fabricated to accommodate samples prepared according to ADA specification #27 at 2×2×25-millimeters. Modified Izod impact tests were also performed on the same four sample groups per ASTM D 256-00. The marginal error of uncertainty is negligible (ANOVA post hoc p=0.001) when predicting strength improvements accompanying the addition of quartz chopped fibers to particulate filled composites during testing for maximum bending stress, flexural modulus, work of fracture and Izod impact toughness. Although particulate filled commercial pastes are clearly prone to incorporating porosity in sample preparation, close visual inspection and imaging indicates microvoidage was not a problem in this investigation due to intensive quality control. Chopped fibers on the other hand consolidate when combined in a particulate filler compound to such an extent that voids are routinely eliminated.

Figure 3.

Maximum Bending Stress

Figure 5.

Work of Fracture

Figure 6.

Izod Impact Toughnesss

Figure 9.

Atomic Force Microscope image of defect exposing a quartz fiber bundle and particulate filled paste extruded above to the surface interface.

Biographies

Richard C. Petersen: CEO for R&D Faculty, Inc.; Stanford University (Ebell Scholar); UCLA, D.D.S. 1975 (B.S. incorporated with Doctoral Degree through early acceptance, California State and Regents Scholar, highest score on the Science portion of the U.S. Dental National Boards 1973, Mosby Publishing Award 1975); Northwestern University, M.S. 1998 (advanced postdoctoral degree in Biomaterials, sponsored through the National Institutes of Health); Board Certified Healthcare Executive, CHE; Member American Dental Association; Certificate of Appreciation for NASA Outreach Space Exploration Initiative; Collaboration with Los Alamos National Laboratory through the Joint Association for the Advancement of Supercritical Technology, Department of Energy Ex-officio

Edward G. Wenski: Mechanical and Thermal Analysis Analytical Laboratory, United States Department of Energy Federal Manufacturing and Technologies, Operated by Honeywell International; Bachelors of Science in Mechanical Engineering, University of Missouri-Columbia 1983; Master of Science in Mechanical and Aerospace Engineering, University of Missouri-Columbia 1984; Professional Engineering License, State of Missouri 1991; Member American Society of Mechanical Engineers, American Society for Metals and past Chairman, Vice Chairman, Treasurer and Secretary of the Western Regional Strain Gage Committee, a sub-organization to the Society for Experimental Mechanics.