Fiber reinforcement of a resin modified glass ionomer cement
Carina B. Tanakaa, Frances Ershadb, Ayman Ellakwab, Jamie J. Kruzica*
a - School of Mechanical and Manufacturing Engineering, University of New South Wales (UNSW Sydney), Sydney NSW 2052, Australia b - School of Dentistry, The University of Sydney, Westmead NSW 2145, Australia
*Corresponding author. Jamie J. Kruzic Address: School of Mechanical and Manufacturing Engineering, University of New South Wales (UNSW Sydney), Sydney NSW 2052, Australia. Tel.: +61 2 9385 4017 E-mail address: [email protected]
Abstract Objectives. Understand how discontinuous short glass fibers and braided long fibers can be effectively used to reinforce a resin modified glass ionomer cement (RMGIC) for carious lesions restorations. Methods. Two control groups (powder/liquid kit and capsule) were prepared from a light cured RMGIC. Either discontinuous short glass fibers or braided polyethylene fiber ribbons were used as a reinforcement both with and without pre-impregnation with resin. For the former case, the matrix was the powder/liquid kit RMGIC, and for the latter case the matrix was the capsule form. Flexural strength was evaluated by three-point beam bending and fracture toughness was evaluated by the single-edge V-notch beam method. Compressive strength tests were performed on cylindrical samples. Results were compared by analysis of variances and Tukey’s post-hoc test. Flexural strength data were analyzed using Weibull statistical analysis. Results. The short fiber reinforced RMGIC both with and without pre-impregnation showed a significant increase of ~50% in the mean flexural strength and 160 – 220% higher fracture toughness compared with the powder/liquid RMGIC control. Reinforcement with continuous braded fibers gave more than a 250% increase in flexural strength, and pre- impregnation of the braided fibers with resin resulted in a significant improvement of nearly 400% relative to the capsule control. However, for the short fiber reinforced RMGIC there was no significant benefit of resin pre-impregnation of the fibers. The Weibull modulus for the flexural strength approximately doubled for the fiber reinforced groups compared to the control groups. Finally, compressive strength was similar for all the groups tested. Significance. By using a RMGIC as a matrix, higher flexural strength was achieved compared to reported values for short fiber reinforced GICs. Continuous braided polyethylene fibers can give much higher flexural strength than discontinuous glass fibers, and their effectiveness is enhanced by pre-impregnation of the fibers with resin. However, the short fibers were more effective to toughen the RMGIC matrix.
Keywords: Resin modified glass ionomer cement, fiber reinforcement, resin Composite, mechanical properties, fracture toughness
1. Introduction
The maintenance of oral health in geriatric patients is a significant issue in dental care. Age-related salivary changes such as xerostomia combined with other factors including poor oral hygiene or gingival recession exposing root surfaces tend to result in a higher prevalence of caries in elderly patients [1]. Currently, the minimal intervention approach known as Atraumatic Restorative Treatment (ART) has been the treatment of choice for these patients [2]. The ART technique has numerous advantages compared to conventional adhesive restorations such as it does not require local anesthesia, a rubber dam for isolation, or drilling and can be performed in a shorter treatment time, thus reducing patient discomfort. However, restorations placed using the ART technique only achieve high survival rates for relatively small cavities with sufficient support of tooth structure, such as in class I occlusal restorations [3]. In addition, clinical studies have shown that restorations placed in the root and cervical surfaces are one of the least durable types of restorations [4, 5]. Indeed, they commonly experience high loss of retention, failure at the restoration margins, and secondary caries [4, 6]. The high failure rates of complex cavities and Class V restorations are related to the poor mechanical properties of the materials used [7]. Currently, the preferred material for root and cervical lesion restorations are glass ionomer cements (GICs) and resin modified GICs (RMGICs) [8, 9] due to their excellent adhesion to the tooth and their natural capacity for fluoride release. Furthermore, compared to resin-based composites, GICs are easier to handle and require less steps during the placement procedure. Although RMGICs represent a significant improvement in mechanical properties compared with conventional GICs, the strength and fracture resistance remains much less than typical resin composites [10, 11], and this is a significant concern when using them for permanent restorations. A few studies have shown that adding fiber reinforcements represents one potential way to improve the strength and fracture resistance for this class of dental materials [12-16]. However, one challenge is achieving good bonding between the matrix and the fibers [12], and no studies have examined fiber reinforcement of RMGICs. For fiber composite reinforcement, it is well known that control of the fiber/matrix interfacial properties is essential for achieving good mechanical properties [17]. In the field of resin based dental restorative composites, surface treatments such as cold gas plasma, silanization, or etching have been used to improve the interfacial bonding properties [18, 19]. In addition, studies show that fibers pre-impregnated with resin can have improved interfacial bonding, resulting in higher flexural strength [20], and this methodology has been successfully applied to porous continuous fibers for periodontal and post traumatic splints [21] and orthodontic retainers [22]. The purpose of this study was to evaluate the mechanical properties of fiber reinforced resin modified glass ionomer cements when using discontinuous/short glass fiber and braided long fibers. Additionally, the second purpose was to examine the effect of resin pre- impregnation of the fibers on the mechanical properties. The hypotheses of this study were that 1) by using a resin modified GIC matrix, enhanced properties would be achieved relative to fiber reinforced GICs that have been reported in the literature, 2) the RMGIC reinforced with long braided fibers would perform better than with short discontinuous fibers and 3) that fibers pre-impregnated with resin would give improved mechanical properties compared to non- impregnated fibers.
2. Materials and method
2.1. Specimen preparation
Two control groups (powder/liquid kit and capsule) were prepared from a light cured, resin modified glass ionomer cement (Riva Light Cure, SDI Limited, Australia) following the manufacturer dose and mixing recommendations. The hand mixed group was prepared with a powder-to-liquid mass ratio of 3.1:1. The capsule group was activated and mechanically mixed in an amalgamator for 10 s. Table 1 –Manufacturer and composition of the resin modified glass ionomer and composite materials used in this study.
Material Manufacturer Composition Resin-modified Riva Light-Cure Liquid: polyacrylic acid, tartaric acid, glass-ionomer Powder/Liquid and HEMA; cement Capsule powder: fluoroaluminosilicate glass (SDI Limited, Australia) *Filler (wt%): 30-60; *Filler composition: glass, oxide, fumed Construct™ Flowable resin silica; (Kerr, USA) resin composition: bis-EMA, TEGDMA, light-cure initiators, and stabilizers Abbreviations: HEMA: 2-hydroxyethyl methacrylate; bis-EMA: ethoxylated bis-phenol-A- dimethacrylate; TEGDMA: triethylene glycol dimethacrylate. *Further proprietary details not provided by manufacturer
Braided polyethylene woven fibers ribbons (Construct™, Kerr, USA) were used as a reinforcement with and without pre-impregnation with flowable resin (Construct™, Kerr, USA) in a laminate construction (Figure 1) that may be quickly applied in the clinical setting. For the impregnated group, the cut ribbons of fibers were placed on a glass slab and using a spatula the fiber was saturated with resin on both sides. The impregnated fiber was kept away from light until it was incorporated with the RMGIC and the whole composite structure was cured as described below. Additional details on the RMGIC and the flowable resin can be found in Table 1. For the flexural strength samples, the 1.0 mm wide fiber ribbons were cut into 25 mm lengths and placed at the base of a stainless steel split mold. The fiber ribbon was covered with the RMGIC capsule material to fill the mold. Compressive strength specimens were prepared with a 3 mm braided fiber ribbon. The fibers were stretched to 4 mm width and placed in the middle of the cylinder height between layers of RMGIC (Figure 1).
Figure 1. Schematic of the braided fiber specimens for flexural strength (A) and compressive strength (B) tests. Discontinuous short glass fibers (S-2 glass® fibers, AGY, USA) with a diameter of 5 m were cut by the manufacturer into a uniform length of 0.5 mm to create fibers with an aspect ratio of ~100. Adding fibers to the capsules was trialed but did not allow for good mixing. So, instead the short fibers were hand mixed with the bulk powder/liquid kit formulation of the same RMGIC (Riva Light Cure, SDI Limited, Australia). The short fibers were treated in two different ways: (1) wetted with the mixing liquid contained from the RMGIC kit or (2) pre-impregnated with flowable resin (Construct™, Kerr, USA). For the former group, the short fibers (5 vol% = 25 mg) were wetted with two drops of the mixing liquid. For the latter group, the short fibers were placed on a glass slab and the fibers were saturated with 75 mg of flowable resin. Two doses of powder were dispensed onto the mixing pad with two drops of liquid. Half of the powder was mixed with the liquid for 15 seconds. Then the wetted fibers were added with the other half of the powder followed by another 15 seconds of hand mixing. The mixture was then placed in a stainless-steel split mold for curing. For all groups, the mixed material was pressed flat in the mold with clear polymer strips and a glass slide. The beam specimens were light cured by three adjacent exposures of 20 s and the cylinder specimens were photoactivated for 20 s from each side of the split mold. All samples were light-cured in continuous mode with an irradiance of 1200 mW/cm2 using a commercial light curing unit (Radii-Cal, SDI Limited, Australia). Any excess material at the mold edges was removed by polishing and a thin coating of petroleum jelly was applied to each sample, which is the typical clinical procedure. The samples were stored in deionized water at 37 °C for 24 hours prior to mechanical testing. All subsequent mechanical tests were performed using a universal testing machine (Instron 4501, Instron Corp., USA) with a crosshead speed of 0.75 mm/min. 2.2. Flexural strength
Flexural strength experiments were performed in general accordance with ISO standard 4049 [23] using three-point bending with a support span distance of S = 20 mm. The unnotched flexural strength specimens (n = 15 for each group) were loaded to fracture. The flexural strength values, FS, for the control samples and for the short fiber reinforced RMGIC were calculated according to [23]:
3푃퐿 퐹푆 = Equation 1 2푏ℎ2 where P was the load at the fracture point, L was the support span distance, b was the specimen width, and h was the specimen height. For the braided long fiber samples, the fibers were oriented parallel to the length and on the tension side of the bend beams (Figure 1). Due to the laminate nature of the beam samples, the flexural strength was calculated using the transformed section method [24]. Using this method, a laminate of two materials with different elastic modulus can be transformed to an equivalent cross-sectional shape of homogeneous material, which in this case was chosen as the RMGIC. To obtain the transformed section, the effective beam width of the braided fiber layer was increased based on the modulus ratio according to:
′ 퐸RMGIC 푏braided fiber = 푏braid Equation 2 퐸braid where b' was the transformed width and b was the actual width.
The modulus of elasticity of the RMGIC, ERMGIC, was calculated from the slope, 훼, of the linear elastic region of the load-displacement curves during the control sample flexural tests according to:
퐿3 퐸 = 훼 Equation 3 푅푀퐺퐼퐶 4푏ℎ3
This procedure gave a value of ERMGIC = 7 GPa.
To obtain the modulus of elasticity for the braided fibers, Ebraid, five pre-impregnated fiber specimens of approximately 20 mm length were prepared for tension testing. The wetted fiber braids were pressed flat with a glass slide and light-cured by three adjacent exposures of 20 s. The braided fibers were tested in tension at a crosshead speed of 0.2 mm/min using a 150 N capacity load cell in a loading stage (MICROTEST 2kN, Deben UK Ltd, UK). The loading stage was placed under an optical microscope to measure the sample displacements. Ebraid was determined according to:
휎퐿 퐸 = 0 braid ∆퐿 Equation 4 where 휎 was the applied tensile stress, L0 was the initial fiber length, and ∆퐿 was the fiber length change. This procedure gave a value of Ebraid = 9 GPa.
Finally, the flexural strength of the transformed section, FStrans, was calculated according to:
푃퐿푐 퐹푆푡rans = Equation 5 4퐼푥 where Ix was the moment of inertia of the transformed section, c was the distance from the centroid to the tensile surface, and all other variables were previously defined.
2.3. Fracture toughness
Single edge vee-notched bend (SEVNB) beam specimens (2 mm x 2.5 mm x 25mm) were fabricated for fracture toughness (n = 5 for each group) experiments. A sharp notch was made by inserting a razor blade into a groove in the split mold at the midpoint of each specimen’s length prior to curing. The notched specimens were tested according to ISO standard 23146 [25] using a three-point bending test arrangement (S = 20 mm; roller diameter = 2 mm).
KIC was calculated from the peak load at fracture according to the standard stress intensity factor equation for the SEVNB sample geometry [25]:
푎 1⁄2 푎 푎 푎 푎 2 푃푆 3 (푊) [1.99 − 푊 (1 − 푊) {2.15 − 3.93(푊) + 2.7 (푤) }] 퐾퐼퐶 = 푥 Equation 6 퐵푊3⁄2 푎 푎 3⁄2 2 (1 + 2 푊) (1 − 푊) where P is the load at the fracture point, S is the length of the support span, a is the notch crack length, and B and W are the specimen thickness and width, respectively.
2.4. Compressive strength Compressive strength was determined using cylindrical specimens with 4 mm diameter and 5 mm height (n = 10 for each group). The compressive strength, CS, was calculated according to:
푃 퐶푆 = Equation 7 퐴 where P is the load at the fracture point and A is the specimen cross-sectional area. For the braided fiber samples, the fibers were oriented perpendicular to the loading axis of the cylinder.
2.5. Statistical analysis One-way analyses of variance (ANOVA) with Tukey’s post hoc tests were performed for the mechanical properties data using Minitab 17 Statistical software (Minitab, LLC., USA). For all tests, the significance level was 95% (p= 0.05). The flexural strength data were fit to a Weibull statistical function (Equation 8) to obtain the Weibull modulus (m) and characteristic strength (휎0) parameters for the materials [26].
휎 푚 푃푓 = 1 − 푒푥푝 [− ( ) ] Equation 8 휎휃
In Equation 8, Pf is the probability for fracture for each value of fracture stress 휎. 3. Results Flexural and compressive strength, fracture toughness, and Weibull analysis results are shown in Table 2. The short fiber reinforced RMGIC both with and without pre-impregnation showed a significant increase in the mean flexural strength compared with the powder/liquid RMGIC (>50%), but this difference was less when compared to the capsule control RMGIC (~24-28%). Pre-impregnation of the braided fibers with the connect resin resulted in a significant improvement in flexural strength. This encouraging result motivated the pre- impregnation of the short fibers as well; however, pre-impregnation was not effective for the short fiber reinforced RMGIC and there was no significant difference in their flexural strengths. The compressive strength for all the groups tested, both with and without fiber reinforcement, was essentially identical with no significant differences. Fracture toughness analysis was carried out for the short fiber reinforced RMGIC and showed 160 – 220% higher
KIC compared with the unreinforced material. Pairwise comparison tests further demonstrated that there were no significant differences between short fibers with or without pre-impregnation with resin.
Table 2. Mean flexural strength, compressive strength, fracture toughness and standard deviation. Weibull modulus (m), characteristic strength (휎0) and standard error, 95% confidence interval. Identical letters in a column indicate no statistically significant difference (p > 0.05).
Flexural Compressive Fracture Weibull Material strength strength toughness modulus 휎0 (MPa) 95% CI (휎0) (MPa) (MPa) (MPa√m) (m)
RMGIC Control d b — 6.6 40.7 (1.7) 37.5 –44.1 (powder/liquid) 37.9 (6.9) 0.5 (0.07)
RMGIC Control d a b 4.7 45.9 (2.7) 40.9 –51.5 (capsule) 42.1 (9.8) 109.7 (15.4) 0.5 (0.04)
RMGIC + 5% S- c a a 11.9 59.6 (1.4) 57.0 –62.4 glass Fibers 57.1 (5.9) 115.8 (11.2) 1.6 (0.48) RMGIC + 5% S- c a a glass Fibers 59.0 (5.9) 116.7 (9.3) 1.3 (0.26) 10.1 61.7 (1.7) 58.5 –65.0 + connect resin
RMGIC + b — — 5.9 115.7 (4.9) 98.0 –117.2 Braided Fibers 107.6 (18.9) RMGIC + a a Braided Fibers + 173.3 (14.0) 102.3 (13.4) — 14.8 179.4 (5.0) 165.3 –180.9 connect resin Plots of the cumulative probability of flexural failure are shown in Figure 2 along with
the Weibull fits. The characteristic strength (휎0) was higher for RMGICs reinforced with short fibers compared to the control groups, and again pre-impregnation with resin did not provide
any benefit. In contrast, the braided fiber reinforced RMGICs showed much higher 휎0 for the pre-impregnated group (173 MPa) compared to the bare fiber group (107 MPa). The Weibull modulus (m) approximately doubled for the fiber reinforced groups compared to the control groups, with the exception of the bare braided fibers (Table 2, Figure 2).
Figure 2. Weibull cumulative probability of failure curves for the flexural strength of the fiber reinforced and control groups. The dashed and dash-dot lines are the best fit of the data and the solid lined on each side represent the lower and upper limits of the 95% confidence interval.
All the braided fiber samples tested for flexural strength had the fracture process restricted to the GIC matrix and the braided fibers were not observed to fracture in the vicinity of the crack tip. The fractured GIC matrix was kept attached by the good adhesion to the fiber ribbon. Figure 3 shows the crack propagation process during fracture toughness testing of the short fiber reinforced RMGIC. The process starts with matrix fracture, and along the crack path the fibers act to bridge the crack faces and resist crack opening (Figure 3A). With further load application, the fibers pull out and their random orientations within the matrix are observed (Figure 3B). Both short fiber groups, with and without resin impregnation, showed similar fracture surface morphologies (Figure 3C & D).
Figure 3. (A) and (B) are in-situ optical micrographs of the fracture process for fracture toughness specimens. (A) During the early stages of the crack propagation fiber bridging occurs. (B) Later in the fracture process the fibers pull-out from the GIC matrix. In (A) and (B) the crack propagation direction was left to right. (C) and (D) are SEM images of flexural strength sample fracture surfaces showing similar fracture surface features for both the non- impregnated (C) and resin pre-impregnated (D) fibers. In (C) and (D) the crack propagation direction was bottom to top.
4. Discussion GIC materials are limited in their applications for restoring large cavities and are not recommended for class II restorations due to material loss on proximal surfaces [27]. One possibility to expand the use of GIC materials to more complex tooth cavities and longer-term restorations is to enhance the overall properties by short fiber reinforcement without compromising the excellent handling characteristics. Another possibility that the present authors have used in the clinic is to add continuous braided fibers in a sandwich technique to reinforce the proximal region and make the structure more resistant to occlusal forces. Either approach may benefit geriatric and other patients who have difficulties tolerating the rubber dam isolation that is strictly required for using composite restorations. The following subsections will examine the relative success of each approach in reinforcing a resin modified GIC.
4.1. Discontinuous short fibers
Discontinuous short glass fiber reinforcement contributed to an improvement in the mechanical properties of the RMGIC material regardless of the surface treatment. This can be observed in the higher flexural strength and fracture toughness relative to the relevant control group, and the higher Weibull modulus (Table 2). These results indicate that the fibers were efficient in toughening the matrix. Furthermore, the hypotheses that using an RMGIC matrix would give better mechanical properties than reported for fiber reinforced tradition GICs was accepted. This can be seen clearly by comparing the results in Table 2 with those in (Table 3), which shows the flexural strengths and fracture toughness achieved for various short fiber additions to traditional GICs taken from references [12-15].
Table 3. Flexural strengths and fracture toughness values from the literature, obtained for traditional GICs reinforced with short fiber. Fracture REF Flexural Material toughness strength (MPa) (MPa√m) 5.0 wt.% E-glass fibers 13.2 – 11.6 - [12] silanized 37 - [13] 15 mass% E-glass fibers non-silanized 36 - [13] 20 vol% reactive fiber 8.9 – 15.6 (1.9) - [14] 5 wt% hollow glass fiber 29.8 (6) 1.3 (0.2) [15] 5 wt% solid glass fiber 28.0 (7) 0.7 (0.2) [15]
This effect of the fibers can be observed directly in Figure 3, which shows the fracture process of the short fiber reinforced RMGIC samples. The sequence of events observed in the fracture process started with the matrix cracking, and upon further load application fiber bridging and pull out was observed with no observations of fiber fracture.
The impregnation of fibers with resin had no significant effect on the flexural strength or fracture resistance of the material (Table 2), and no difference was observed in the fracture process or the fiber pullout behavior (Figure 3). Accordingly, the third hypothesis of this study was partially rejected. Overall, the KIC values of the short fiber reinforced RMGIC reached values similar to typical particle reinforced resin based dental composites [28-32], and higher than has reported for a fiber reinforced traditional GIC [15], which could potentially contribute to longer lasting restorations.
Fiber volume fraction is an important factor known to affect the mechanical properties of fiber reinforced GICs; however, properties do not always monotonically increase with volume fraction [12-15]. In this work, we initially explored using 5, 7.5 and 10 vol.% and found that 5 vol.% was the optimal fiber concentration that allowed the recommended powder-to- liquid mass ratio (3.1:1) to be maintained along with easy handling of the material. This is considered important because the mechanical properties of GICs are also strongly dependent on the powder/liquid ratio [33].
With 10 vol.% fibers, it was too difficult to handle the material and produce samples for mechanical testing. With 7.5% fibers, good handling could be achieved with the addition of extra liquid, but preliminary tests showed this compromised the strength. Another study on short fiber reinforced GICs which added the short fibers to the glass powder also reported that an extra amount of liquid was required to obtain the correct consistency of the material [14]. Thus, it is important to consider that the benefits of fiber reinforcement can be negatively counterbalanced by a weak matrix if extra liquid is required. In the current study, we aimed to maintain the 3.1:1 ratio recommended by the manufacturer, which gave both good handling and mechanical properties.
Achieving optimal interfacial bonding between the fibers and the glass ionomer matrix is thought to be critical for achieving superior brittle composite mechanical properties. Too strong of bonding will cause brittle fiber fracture and limit the toughening effect, while too weak of bonding will enable easy pullout without significant stress transfer to the fibers. In the present work, brittle fiber fracture was never observed, suggesting there may be opportunity to further enhance the mechanical properties by improving the fiber-matrix interface bonding. Several strategies for glass fiber modification have been used on the past for fiber reinforced GICs.
In one study, silane treatment of the glass fiber surfaces was shown to be ineffective for improving the mechanical properties of a fiber reinforced GIC [13] (Table 3). Another approach was to design an oxide fiber chemistry that was reactive with the GIC matrix [14]. While that study demonstrated a 75% improvement in flexural strength compared to the matrix when using 20 vol.% fibers, there was no control group with nonreactive fibers for comparison. In the present work, 5 vol.% bare fibers were able to improve the flexural strength by 50% relative to the relevant control group. Furthermore, by starting with a stronger RMGIC matrix, the overall flexural strength with was much higher than in [14] (~57 versus 16 MPa) with a much smaller volume fraction of fibers (5 versus 20 vol.%). In the present work, resin impregnation was attempted due to the encouraging results for the braided fiber samples (Table 2), and also because the matrix was a resin modified GIC designed for compatibility with resin. However, similar to with silane treatment [13], no statistically significant improvement was detected with the resin treatment.
In summary, the addition of randomly orientated glass fibers gave significant strength and toughness improvements in comparison to the unreinforced RMGIC. By starting with a resin modified GIC matrix, the flexural strength values were much higher than has been reported for the fiber reinforcement of traditional GICs [12-16], and the fracture toughness became comparable to particle reinforced resin composites [28-32]. The Weibull modulus was approximately 70% higher compared with nanofill composites and similar to microhybrid resin (m=13.1) [34]. This study demonstrates that is possible to produce reinforced glass ionomers with mechanical properties as good as some dental composites. In addition to the attractive mechanical properties, another advantage of using short fibers is that the material can be accommodated to any type of cavity shape. The improved mechanical properties in addition with the benefits of bonding to the tooth structure make fiber reinforced RMGICs a potential material for long term restorations. Further optimization of the fiber and matrix interface is an area of future research that may enable further improvement of the mechanical properties.
4.2. Braided fibers
The results of this study show that braided continuous fibers are much more effective at enhancing the flexural strength of a RMGIC than discontinuous short fibers. The benefit of using continuous braided fibers over short fibers is that the continuous interlocked architecture promotes well distributed load transfer to the fibers [35]. Furthermore, pre-impregnation of the fibers with resin before applying the RMGIC matrix greatly enhanced the flexural strength and the Weibull modulus. The characteristic strength of the pre-impregnated group (173 MPa) was nearly twice than that for non-impregnated fibers (107 MPa). These results suggested that the pre-impregnation creates a stronger reinforcement by better bonding the fibers with the GIC matrix. The fracture toughness of these groups was not measured because the crack did not propagate through the braided reinforcement. The fracture was restricted to the GIC matrix and the fractured pieces were kept together by the unbroken braided fibers. However, the randomly distributed short fibers showed a better toughening mechanism for the RMGIC compared to the braided fibers whereby the multidirectional oriented fibers effectively reinforce the matrix and prevent cracking in the RMGIC. Thus, the second hypothesis that the RMGIC reinforced with long braided fibers would perform better than with short discontinuous fibers was partially rejected. While our results showed that long braided fibers gave the best overall flexural strength, the short fibers were more effective to toughen the RMGIC matrix and the compressive strengths were identical.
Compared the short glass fibers used in the study, it is important to note that the braided ribbon has a different composition and surface treatment. Our motivation for selecting these different materials is that they are commercially available materials that could be immediately and easily incorporated into commercial RMGIC materials. The braided fibers are made of polyethylene and received a cold plasma treatment on the fiber surface to improve the fiber wettability and fiber–matrix adhesion. Furthermore, the braided polyethylene fibers have relatively low modulus (Ebraid = 9 GPa) which better matches the resin modified glass ionomer cement (ERMGIC = 7 GPa) compared to the S-glass fibers (~94 GPa). According to fiber-matrix debonding theory [17], the fiber debonding toughness, Rd, is inversely proportional to the fiber elastic modulus. Since fiber debonding and pullout are clearly involved in the toughening process for the present materials (Figure 3), the higher modulus for the short glass fibers may also have played a role in the lower mechanical properties. However, the dominant difference in the present study is thought to be the continuous, long fiber geometry of the braided fibers.
5. Conclusion The main goal of the present study was to improve the mechanical properties of a resin modified glass ionomer cement (RMGIC) while maintaining the desirable handling characteristics of the RMGIC, which was largely achieved. Furthermore, the hypotheses that using an RMGIC matrix would give better mechanical properties than reported for fiber reinforced tradition GICs was accepted. The second and third hypotheses that the RMGIC reinforced with long braided fibers would perform better than with short discontinuous fibers and that pre-impregnation of the fibers with resin would improve the mechanical properties were partially rejected. While our results showed that long braided fibers gave the best overall flexural strength, the compressive strengths were identical. Furthermore, the short fibers provided an effective toughening mechanism for the RMGIC matrix giving KIC values comparable to many resin composites. Finally, resin pre-impregnation was found to be effective depending on the fiber type, with a strong flexural strength benefit for the braided continuous polyethylene fibers, but no benefit for the short discontinuous glass fibers.
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