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The effect of internal fiber arrangement on the delamination failure in hybrid composite dental prostheses
S.R. Dyer12, L.V.J. Lassila2, and P.K. Vallittu2
1 Oregon Health and Sciences University, School of Dentistry, Department of Restorative Dentistry, Division of Biomaterials
and Biomechanics, Portland, Oregon, 97239, USA
2 University of Turku, Institute of Dentistry, Department of Prosthetic Dentistry and Biomaterials, Turku, FIN-20520, Finland
Objective: To test dental fiber-reinforced composite (FRC) substructure arrangements within a bi-layer beam on the load to initial (IF) and final failure (FF) in a modified mode II delamination test. Methods: Three groups of FRC and particulate reinforced composite (PRC) were tested: PE (Ultra high molecular weight polyethylene ribbon, Connect, and BelleGlass, Kerr), W (woven E-glass, Vectris Frame, and Targis, Ivoclar), and U (unidirectional R-glass, Vectris Pontic, and Targis, Ivoclar). Twelve specimens were fabricated by placing an FRC layer into a 25x6x0.75 mm3 mold. PRC (10x6x0.75 mm3) was added to the specimen center. The FRC was placed into unpolymerized PRC layer at 3 depths to the veneer surface: 0 % (750 ^m), 50 % (300 ^m), or 100 % (< 10 ^m). After light-polymeriza-tion, the specimens were standardized and then stored 1 week in 37 °C water. Four specimens were randomly selected for sectioning/pretest evaluation. The remaining 8 specimens (= n) were loaded in a static 4-point bend test (20 mm upper/ 10 mm lower spans) veneer side down until failure (recording IF and FF). A failure ratio (FR) was developed (IF/FFx100). Means and standard deviations (SD) were calculated (ANOVA, Tukey post hoc,p < 0.05). Results: No position of PE fiber increased the load to elicit IF. For the woven E-glass and unidirectional R-glass, the 50 % fiber position increased the load to IF. Conclusion: Glass fiber positions with 50 % penetration into the PRC significantly increased the load to IF in a modified mode II delamination.
1. Introduction
The use of fiber reinforcement has been attempted with dental polymers for over 40 years to create tooth replacing prostheses [1]. Wide success of fiber-reinforced (polymer matrix) composites (FRC) has not, however, been realized in dentistry like ceramic particulate-reinforced (polymermatrix) composites (PRC). Early FRC clinical studies resulted in less than adequate performance [2, 3]. In 1998, Dyer and Sorensen showed preliminary in vitro results of FRC fixed partial denture or dental bridge designs. The results emphasized the importance of correct fiber volume within the bridge to reduce delamination and fracture failure [4]. Freilich and colleagues reported that early clinical failures were minimized after such fiber volume modifications were made [5]. However, delamination or separation of the esthetic PRC veneer (providing tooth color and form) from the underlying FRC continues to be a primary cause of failure [4, 5] (Fig. 1).
FRC materials for dental prostheses are used in pre-preg and non-impregnated forms. The composite designer in most cases is a dental technician or dentist with minimal or no training in conventional composite fabrication techniques. Most dental prostheses are fabricated using a modified pre-preg lay-up technique, creating an underlying substructure
using the FRC pre-pregs. The FRC substructure is subsequently veneered with tooth colored PRC [6]. In practice, the fabricators of dental prostheses are typically limited regarding parent constituents, volume fraction (VF), and interface selection when using these dental composite systems. However, the designer is often able to determine a cross-sectional arrangement of fiber within a prosthesis, potentially affecting the mechanical properties and failure performance [7].
The experimental rationale was to maintain the parent constituents, fiber-matrix interface, and fiber VF within the specimens, but to change the internal fiber arrangement. Instead of creating a typical clear distinction between the FRC and PRC, portions of the FRC base were introduced into the veneer portion of the structure. Therefore, instead of creating two composite phases in the structure (100 % FRC and 100 % PRC), three phases were fabricated (100 % FRC, a hybrid zone of FRC/PRC, and 100 % PRC). While the overall fiber volume was not changed, the interfacial region received a relative increase in the local fiber VF. The fibers were attempted to be configured to allow for tension loading when shear forces approached the interfacial region. The overall objective of this study was to determine the ability to arrange fibrous components within
© S.R. Dyer, L.VJ. Lassila, and P.K. Vallittu, 2004
a composite in a manner that decreased delamination failure. It was hypothesized that these mesoscale fiber adjustments in arrangement, placing reinforcement constituents into the typical interfacial zone, would decrease the macroscopic delamination failure.
2. Methods
The experimental design was to test three fiber types from available prosthodontic FRC systems with three different amounts of penetration into the veneer portion of the specimen. The group designations of FRC and PRC were: PE (Ultra high molecular weight polyethylene or UHMWPE ribbon, Connect, and BelleGlass, Kerr), W (woven E-glass, Vectris Frame, and Targis, Ivoclar), and U (unidirectional R-glass, Vectris Pontic, and Targis, Ivoclar) (Table 1). The bi-layer specimens were fabricated by placing the FRC into a 6 mmx0.75 mmx25 mm restrictive silicone mold. The FRC was fabricated to 0.75 mm thickness. For the control groups “0 % depth”, the FRC bar was then photopolymerized for 120 seconds. All specimen photopolymerization occurred in a Targis Quick apparatus (Ivoclar Vivadent, Schaan, Liechtenstein). A clear thermoplastic matrix was used to apply a 10 mmx6 mmx0.75 mm thick layer of PRC veneer centered on the FRC bar (Fig. 2(a)). The veneer layer was then photopolymerized for 120 seconds.
For the first experimental group, “50 % depth”, the FRC bar was fabricated as described for the non-infiltrated group. However, before polymerization the most superficial layer of the FRC prepreg was reflected and lifted. For the groups, the outer layer of fiber was reflected. For the unidirectional
Fig. 1. Dental bridge with delamination failure (a); cross-section with dye penetration (b)
R-glass, 0.2 mm of unidirectional fiber was reflected. Four PRC ridges (0.3 mm highx2.0 mm wide) were then added to the unpolymerized FRC bar. These ridges were used to support the fiber extension into the veneering layer. The PRC ridge fabrication was standardized by using a clear thermoplastic matrix that created four ridges 0.30 mm in height, centered on the bar. The lifted superficial fiber composite layer was replaced. Next, the FRC bar with PRC supporting ridges was photopolymerized for 120 seconds. Last, the PRC veneer layer was added using the same clear matrix described earlier and photopolymerized for 120 seconds. The overall thickness of the veneer layer was 10 mmx6 mmx0.75 mm including the supporting ridges beneath the superficial fiber extension. For the “100 % depth” group, the protocol was the same as the 50 % penetrated group, except the four PRC support ridges were fabricated to a height of 0.70 mm.
All the specimens were finished on the veneer surface and polished using 240 and 600 grit carborundum finishing paper (Buehler, Lake Bluff, IL). Specimens were standardized to the dimensions of 25 mmx6 mmx 1.5 mm without exposing the FRC under the veneer surface. All specimens were inspected after fabrication with a light microscope (Nikon, Tokyo, Japan) for flaws.
Twelve specimens were fabricated per group and were stored in 37 °C water for one week prior to testing. Power analysis of pilot data indicated that for a 95 % confidence level, the sample size needed was 6. Consequently, four specimens were randomly selected per group for pretest evaluation and sectioning to verify the fiber arrangement.
The remaining 8 (= n) specimens were tested on a universal testing machine (Model LRX, Lloyd Instruments Ltd., Fareham, England), similar to a previous mode II delamination test [8]. The testing apparatus was configured to deliver force via two 2 mm diameter steel rods on the upright FRC side (crosshead speed: 1 mm/min) (Fig. 2 (b, c)). The
Table 1
Materials used (from manufacturers’ information)
Name General Composition
Belle-Glass HP Methacrylate ester monomer/BisGMA/ TEGDMA particulate composite (78 % filler: Ba-Silicate, SiO2)
Connect Gas plasma treated woven ultra high molecular weight polyethylene (reactive methacrylate ester monomers)
Connect Resin Methacrylate ester monomer/Bis-GMA/ TEGDMA particulate composite (74 % filler: Ba-Silicate, SiO2)
Targis Bis-GMA/UDMA/DDMA particulate composite (80 % filler: Ba-Silicate, S^)
Vectris Frame Woven E-glass (50% weight). Bis-GMA/UDMA/ DDMA/TEGDMA matrix (5 % filler: S^)
Vectris Pontic Unidirectional R-glass (65 % weight). Bis-GMA/ UDMA/DDMA/TEGDMA matrix (3.5 % filler: S^)
BISGMA = 2,2-bis[4-(2-hydroxy-3-methacrylyl-oxyporpoxy) phenyl]pro-pane; DDMA = Decandiol Dimethacrylate; TEGDMA = Triethylenegly-col-dimethacrylate; UDMA= UrethaneDimethacrylate
25 mm
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Fig. 2. Specimen dimensions and geometry (a); specimen and test set-up (b); loaded specimen (c)
underside PRC layer received tensile forces. The load span for the 4-point bend test was 10.0 mm. The base support span was 20.0 mm. The edges of the PRC veneer layer were positioned in the center of the beam in the testing apparatus. Each of the loading rods was coincident with the junction of the PRC veneering layer and FRC support base.
The beginning ofthe specimen crack formation was classified as the initial failure (IF) [9]. IF was denoted if at least two of the following conditions were present, 1) a sharp decline in the load/displacement curve or knee/corner, 2) visible signs of fracture, 3) audible emissions of crack formation and/or progression. The final failure (FF) of the specimen was characterized as the maximal load before the running load decreased by 50 % or catastrophic failure. A failure ratio was developed (IF/FFx 100) for this study to relate the failure process in terms of damage accumulation. The means and standard deviations (SD) were calculated.
The data was subjected to one-way independent group analysis of variance and Tukey post hoc tests (Winks, TexaSoft, Cedar Hill, Texas). Statistical significance was set prior to analysis atp <0.05.
3. Results
The results are presented in Table 2. For each fiber type, the letters in Table 2 show the groups not statistically significant. No position of the PE fiber groups (50 % or 100 %) reinforced the specimen against damage initiation. For the woven E-glass, the 50 % fiber position significantly increased the load to IF. Regarding the unidirectional R-glass, the 50 % infiltrated fiber position also significantly increased the load to IF. The fracture ratio (FR) of the specimens varied between specimen groups and was affected by the internal fiber arrangement (Table 2).
4. Discussion
The results showed that both the type of fiber and the depth of the fiber penetration affected the IF. While the woven E-glass and unidirectional R-glass fibers showed an increase in the IF at the 50 % position, the UHMWPE showed no increase. The glass fibers did not show a significant increase in the initial failure at the 100 % infiltrated position. However, the UHMWPE showed significantly lower initial failure when the fiber was position just under the veneer surface (VD 100 %). The lack of reinforcement from the UHMWPE in these specimens may likely be attributed to the generally poor wetting and adhesion of polymers to the highly organized organic fiber. The lack of adhesion has been discussed in other areas of the dental and industrial literature [10-13]. For all the control groups, the primary fracture event occurred at the interface of the FRC and PRC.
The tests clearly demonstrated a difference between IF and FF. The fracture patterns also revealed that the various fiber positions predisposed the specimen to certain ratio of damage accumulation. For example, control specimens uniformly displayed simple delamination, while experimental groups showed signs ofveneer rupture. A damage mechanics approach to evaluating dental composite structures appears to be very useful. In terms of this study, it was evident that meaningful damage occurred after the initial failure. Monitoring lower limits in the oral environment is important because of fatigue loading and hygro-corrosive stresses. Consequently, the dental composite designer will want to design oral prostheses to resist damage initiation.
Table 2
Results
PE 0 % PE 50 % PE 100 % W 0 % W 50 % W 100 % U 0 % U 50 % U 100 %
IF (SD) in N 13.1 (1.5)a 12.3 (2.7)a 9.0 (3.1) 20.3 (2.5)d 29.3 (8.8)e 27.8 (8.0)d,e 47.6 (6.6)h 60.7 (13) 40.8 (8.8)h
FF (SD) in N 39.0 (10)y 37.6 (5.4)yz 29.7 (3.7)z 60.4 (8.4)t 62.0 (19)t 41.4 (14) 214 (40)w 188 (35)w 176 (35)w
FR (SD) 35.1 (8.3)l 33.1 (7.4)l 30 (8.9)l 34.4(7.1)m 52.2(26)m,n 71.1 (24)n 22.9 (5.5) p 33.5 (9.7)p 24.2 (8.0)p
A failure ratio was developed to compare the relationship between damage initiation and rupture. The failure ratio represents the ratio of initial failure to final failure. This omits information regarding resilience (work under Hoo-kean conditions) and the damage after final failure. In simple terms, the failure ratio describes the amount of damage accumulation compared with the fracture process. A determination of 100 % failure ratio indicates that the damage initiation and rupture were instantaneous. The failure ratio may be also interpreted as the relative rate of damage accumulation.
Failure ratios alone do not yield information suitable to predict the end use. These ratios need to be discussed in combination with other mechanical properties such as strength or fracture resistance (Fig. 3). A material with low initial failure and a high fracture ratio will rupture early and rapidly. A structure that has low initial failure and a low fracture ratio will fracture early, but will accumulate damage before its final rupture. Likely, the best combination for composite structures for intra-oral use will have high initial failure and a low fracture ratio. Generally, it is wise for critical structures have indicators signaling early damage [14]. Such features could have real benefits for medical or dental prostheses. Designers may consider fabricating dental prostheses with high initial failure loads with low fracture ratios.
The reinforcement mechanisms tested in this study may be exploited for dental and other structures. In practical terms, however, a dental technician may find such modifications time consuming or difficult. This manipulation of the FRC arrangement would likely need to be accomplished by a manufacturer or another intermediate. Further studies will need to include wear experiments of such experimental design modifications. Surface treatment of the fibers, especially of UHMWPE, need to be investigated. Shear/delamination modes I and III could also be tested.
5. Conclusion
Within the confines of these experiments, the results showed that the initial and final failures of the specimen were affected by the relative position of the fibrous composite within the structure. This effect occurred without an increase of FRC VF. The results also showed that both the type of fiber and the depth of the fiber penetration affected the initiation of damage. While the woven E-glass and unidirectional R-glass fibers showed an increase in the IF at the 50 % position within the PRC veneer, the UHMWPE showed no increase.
Fig. 3. Initial failure and failure ratio results. The same letter or symbol designates groups not significantly different
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