A novel zirconia fibre-reinforced resin composite for dental use

A novel zirconia fibre-reinforced resin composite for dental use

journal of the mechanical behavior of biomedical materials 53 (2016) 151–160 Available online at www.sciencedirect.com www.elsevier.com/locate/jmbbm...

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journal of the mechanical behavior of biomedical materials 53 (2016) 151–160

Available online at www.sciencedirect.com

www.elsevier.com/locate/jmbbm

Research Paper

A novel zirconia fibre-reinforced resin composite for dental use Ting Wang, James Kit-Hon Tsoin, Jukka Pekka Matinlinna Dental Materials Science, Faculty of Dentistry, The University of Hong Kong 4/F, Prince Philip Dental Hospital, 34 Hospital Road, Hong Kong SAR, PR China

art i cle i nfo

ab st rac t

Article history:

Aims: The purpose of this study was to evaluate and compare some biomechanical

Received 30 March 2015

properties such as fracture toughness, Vickers hardness and compressive strength of an

Received in revised form

experimental fibre-reinforced composite (FRC) filled with various percentages (0 wt%, 1 wt

3 August 2015

%, 3 wt%, and 5 wt%) of zirconia (ZrO2) fibres.

Accepted 9 August 2015

Materials and methods: A resin matrix (78.4 wt% bis-GMA, 19.6 wt% MMA, 1-wt% CEMA and 1 wt

Available online 17 August 2015

% CQ) with different percentages of silanized zirconia fibres (0%, 1%, 3%, and 5% by weight of

Keywords:

the resin matrix) was prepared. Silanization was carried out using an experimental silane

Fibre reinforced composite

blend (0.5 vol% bis-1,2-(triethoxysilyl)ethaneþ1.0 vol% 3-acryloxypropyltrimethoxysilane in

Zirconia fibres

ethanol, at pH 4.0). Each group of specimens was stored in two conditions – either at room

Silane coupling agent

temperature for one day or water storage at 37 1C for 7 days. They were randomly divided into

Fracture toughness

study groups according to the test method. For fracture toughness, a notchless triangular prism (NTP) test (n¼6) was undertaken. Hardness values (n¼6) were measured by using a Vickers hardness testing machine and compressive strength (n¼6) was tested. Scanning electron microscopy (SEM) images were taken at the fracture sites after fracture toughness test. The data were analysed by 1-way ANOVA (analysis of variance) and Bonferroni post-hoc tests (α¼0.05). Results: The ANOVA test revealed that the experimental FRCs with 1 wt% and 3 wt% zirconia fibres showed statistically significant differences in Vickers hardness at dry condition and NTP fracture toughness after 7-day water storage, respectively. However, compressive strength of experimental groups exhibited no significant difference (p40.05). Conclusion: Silanized zirconia fibres reinforcement in resin is a novel FRC which have shown promising biomechanical properties. & 2015 Elsevier Ltd. All rights reserved.

n

Corresponding author. Tel.: þ852 2859 0303; fax: þ852 2548 9464. E-mail address: [email protected] (J.K.-H. Tsoi).

http://dx.doi.org/10.1016/j.jmbbm.2015.08.018 1751-6161/& 2015 Elsevier Ltd. All rights reserved.

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1.

journal of the mechanical behavior of biomedical materials 53 (2016) 151 –160

Introduction

Fibre-reinforced composites (FRC) are a relatively new material group, consisting of a plastic matrix with reinforcement by various fibres. Structurally, they are usually composed of four components: the polymer matrix, the fibres, the initiator/activator system and a silane coupling agent (with a glassy fibre material) (Vallittu, 2014). Fibre plays a significant role to improve the mechanical properties of such a composite by transferring the stresses under an applied load from the weaker resin matrix to the stronger fine fibres. A typical initiator system for setting includes a photo-initiator, e.g., camphorquinone (CQ) which is a monomer to generate free radicals that could be activated and carry out a quick polymerization (or curing) by using blue light. An essential constituent, a silane coupling agent, is used on the fibre surface if the fibres are some type of SiO2glass fibres. The chemical hydrogen and covalent bonds are thus built to enhance strong bonding between the resin and fibres. Consequently, silanes have become an indispensable part of FRC as they profoundly influences the adhesion strength (Puska et al., 2014; So et al., 2012). Thermoset polymers are one of the popular polymers used in dental materials, since the molecules can join together by strong cross-links and polymer networks. It has been defined (Tsoi, 2007) that a cross-link as “a (chemical) bond, group, molecule or chain which connects two polymer chains at other than their ends.” Such a cross-link could form a part of the backbone in the polymer, and thus make it more rigid and add stability. Furthermore, with two (or more) cross-linked polymers through winding and locking, eventually an interpenetrating polymer network (IPN) system would be formed (Vallittu, 2009). The polymers in the IPNs are at least partially bolstered and interweaved one by one by interpenetrating networks. However, in some IPNs, one or more linear polymers (usually with a shorter chain length) are needed to extensively penetrate into by one or more cross-linked polymers. In brief, linear polymers, which are molecules without branches or cross-linked structures, are reticulated into the cross-linked polymers and structurally could store more mechanical energy with its spring elasticity than the cross-linked polymer. Therefore, a so-called “semi-interpenetrating polymer network (SIPN)” system was formed. Surprisingly, a semi-IPN could afford more force to resist stress from outside and more widely used in FRC than the IPN system (Vallittu, 2009). Fibres are the principal constituents in a FRC, where the fibres share and transfer the major portion of the load. Various studies (Abdulmajeed et al., 2011; Tsue et al., 2007) have provided laboratory evidence to confirm that the

proper selection of the fibre type, fibre length, diameter and orientation are important to the composite. This is because most of the mechanical characteristics, e.g., electrical and thermal conductivities, fatigue strength, compressive strength and modulus, of the composite are influenced (Zhang and Matinlinna, 2012). In dentistry, FRCs have become a fashionable and clinically interesting technology that could offer a new affordable option for both patients and clinicians. In fact, in addition to periodontology and prosthodontics, FRCs can be used as a root canal post in endodontics (Perea et al., 2014). In prosthetic dentistry, indirect restorations, such as a 3-unit bridge, are feasible and it is economical for patients and furthermore, usually one visit is enough. FRCs also can be applied in orthodontics for retention and as denture baseplate reinforcement (So et al., 2012). However, due to the different properties of various fibres, researchers have made many efforts in order to put diverse FRCs into practical and clinical use (Meiers and Freilich, 2006; Shinya et al., 2009; Vallittu and Sevelius, 2000). Gradually, different kinds of fibres, such as E-glass fibres, are used in FRCs today. Zirconia, a ceramic biomaterial, is used widely nowadays in dentistry. In particular, the properties such as good biocompatibility (Lung and Matinlinna, 2012; Mallineni et al., 2013), osseointegration (Zhang and Matinlinna, 2012) and its in general high strength (Shenoy and Shenoy, 2010) have allowed zirconia to be used in various applications, such as crowns and bridges (Perea et al., 2014), implant fixture (“screw”) and abutments (Tuusa et al., 2005), and orthodontic brackets (Keith et al., 1994). Nevertheless, due to its inertness and chemical stability zirconia cannot not be etched easily like porcelain (Lung and Matinlinna, 2012) using hydrofluoric acid, and some more harsh conditions such as heat (Liu et al., 2015) and much longer duration (Lee et al., 2015) might be necessary. Resin-to-zirconia or porcelain-to-zirconia adhesion represents different aspects (Liu et al., 2013). To tackle this, efforts has been put such as using laser (Liu et al., 2013), resin-infiltrated coatings (Liu et al., 2014) and zirconate primers (Cheng et al., 2014) to improve adhesion. Perhaps interestingly, not much attention has been paid to zirconia fibres. Considering the well-established chemistry (Lung et al., 2012), ease of use, biocompatibility and various wellknown applications in dentistry, silanes are still dominating and might be one of the best choices for zirconia adhesion improvement. One of the promising silane developed by Matinlinna et al. (2013a), Matinlinna et al. (2013b), which is so-called a novel silane system, has shown an improved performance in resin-zirconia adhesion under laboratory tests such as water storage and thermo-cycling. On the other hand, there is little information how such silanization might affect zirconia fibres. The use of FRC in the oral environment

Table 1 – Chemicals used for the resin matrix. Material

Manufacturer

bis-phenol A-glycidyl methacrylate (bis-GMA) Methyl methacrylate (MMA) Camphorquinone (CQ) N,N-cyanomethyl methylaniline (CEMA)

Accu-Chem Accu-Chem Accu-Chem Accu-Chem

industries industries industries industries

Inc. Inc. Inc. Inc.

Lot number

City and country

23823 1122 A0077555 T20100224

Melrose Melrose Melrose Melrose

Park, Park, Park, Park,

IL, IL, IL, IL,

Purity USA USA USA USA

AR AR Z 99.0% Z 99.5%

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journal of the mechanical behavior of biomedical materials 53 (2016) 151 –160

Table 2 – Chemicals for silane coupling agent. Material

Manufacturer

Purity/%

Lot. no.

(3-acryloxypropyl) trimethoxysilane bis-1,2-(triethoxysilyl)ethane Ethanol Deionized water

Gelest, Morrisville, PA, USA

95.0

Gelest, Morrisville, PA, USA Riedel-de Haën, Seelze, Germany Millipore, Bedford, MA, USA

Acetic acid (3.0 M)

From Centralized Research Laboratory (CRL), Faculty of Dentistry, HKU

N/A 99.8 Resistivity 18.2 MΩ cm N/A

5C6412 5L7926 03,550 N/A

is subject to saliva and acidic beverages or foods, all decreasing the mechanical properties. In order to reduce the effects from the changing oral environment and implement all the advantages of zirconia, zirconia fibres were chosen in the current study to reinforce the resin matrix. The aim of this study was to evaluate and compare the mechanical strength of an experimental FRC filled with various percentages of silanized zirconia (ZrO2) fibres. The hypothesis for the research was that zirconia fibres could provide a new FRC with higher Vickers hardness strength, superior fracture toughness, and a better compressive strength with the increase of fibre concentration.

2.

Materials and methods

2.1.

Materials

The chemicals for constituting the resin matrix and the silane coupling agent (a novel silane system) (Matinlinna et al., 2013a; Matinlinna et al., 2013b) are listed in Tables 1 and 2, respectively. Mesh type of zirconia (the average diameter: 3– 8 mm; length: 50–250 mm) was used throughout the study. All the chemicals were used as received without any purification.

2.2.

Preparation of FRC

2.2.1.

Preparation of resin matrix

The resin matrix was composed of 78.4 wt% of bis-phenol Aglycidyl methacrylate (bis-GMA), 19.6 wt% methyl methacrylate (MMA), 1.0 wt% of camphorquinone (CQ) and 1.0 wt% of N,Ncyanomethyl methylaniline (CEMA). These monomers and activators were weighed by using an analytical balance (AT 201, Mettler Toledo, Switzerland). The mixing of these chemicals was done in a beaker covered with an aluminium foil to protect the monomers from ambient light. Firstly, bis-GMA, CQ and CEMA were hand-mixed and carefully stirred together by a plastic stirring rod in the dark fume hood. Then, MMA was carefully added and mixed. Finally, the matrix was stored in sterilized plastic syringes which were wrapped with aluminium foil and kept in a refrigerator to stabilize for 48 h before use.

2.2.2.

Constitution of the blended silane solution

A published protocol for the experimental novel silane blend preparation (Lung et al., 2012) was used. In brief, a crosslinking silane, bis-1,2-(triethoxysilyl)ethane (BTSE) at a

N/A

concentration of 0.5 vol% in a solvent mixture of 90 vol% absolute ethanol and 10 vol% deionized water was prepared. The pH was adjusted to 4.0 with 3.0 M acetic acid. It was allowed to hydrolyse for 23 h. Then, 1.0 vol% of a functional silane coupling agent, 3-acryloxypropyltrimethoxysilane (ACPS) monomer was added. The final concentration of the blended silane coupling agent was 1.0 vol% ACPSþ0.5 vol% BTSE. The silane solution was then allowed to hydrolyse (activate) for an additional one hour before use and kept sealed in a refrigerator.

2.2.3.

Silane coating on zirconia fibres

The zirconia fibres were added into the blended silane solution (as prepared in Section 2.2.2) in the beaker with a ratio of 10.0 ml silane solution to 0.1 g zirconia fibres. They were allowed to agitate with a magnetic stirrer and allowed to react for 8 min. Then, the fibres were filtered out and placed into a desiccator for 48 h.

2.2.4.

Surface analysis

After the silane coating, some dry zirconia fibres were randomly selected and mounted on a specimen stub and examined by using a SEM/EDX instrument (SU1510, Hitachi, High-Technologies Corporation, Tokyo, Japan) to study the surface morphology and composition of the fibres. The elemental analysis, imaging and mapping was done by IXRF Software (Alvin DeVane Blvd Suite 130, Austin, TX, USA).

2.2.5.

Impregnation of zirconia fibres

After the surface analysis, four chosen weight % of silanecoated zirconia fibres, i.e. 0 wt% (control), 1 wt%, 3 wt%, and 5 wt%, were immersed and carefully into the resin matrix (as prepared in Section 2.2.1) for stabilization for 24 h in polypropylene Eppendorf vials. All the material groups were wrapped using an aluminium foil in order to prevent premature polymerization by light.

2.3.

Mechanical tests and specimen preparations

2.3.1.

Vickers hardness test

The experimental zirconia fibre FRC composite material was slowly and carefully inserted incrementally into brass split moulds with the dimensions of 2 mm  2 mm  25 mm (Zhang et al., 2014). During the process, the observable air bubbles were removed cautiously by carefully pressing with a plastic hand instrument. Then, the specimens were light-cured

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journal of the mechanical behavior of biomedical materials 53 (2016) 151 –160

Fig. 1 – Elemental mapping (left: zirconium, right: silicon) of zirconia fibres (a) before and (b) after silane treatment. by a light-curing unit (Elipar™ 2500, 3M ESPE, St. Paul, MN, USA) on both top and bottom sides of mould for four times of 40 s from left to the right, respectively. The average light intensity was 700 mW cm  2 and this was measured by using a Curerite™ Model 8000 radiometer, throughout the specimen preparation. After the light curing, 6 specimens for each study group, i.e., four groups (4 various concentrations with dryþwet storages) with a total of 48 specimens, were prepared and carefully detached from the moulds. A Leitz Micro-hardness tester (Leitz, New York, USA) and Leica QGo software program (Leica Microsystems Imaging Solutions, Wetzlar, Germany) were used to test the specimens. The applied load was 0.245 N and the loading (dwelling) time was 10 s for each specimen. Six measurements on two sides of the specimen were taken randomly for each specimen, and the mean value was calculated.

2.3.2.

Compressive strength test

The specimens for compressive test were produced similarly with the aforementioned method, but using smaller brass moulds with the dimensions of 2 mm  2 mm  10 mm. Six specimens for each study group, i.e., a total of 48 specimens were prepared and carefully detached from the moulds. The dimension of each cured specimens were measured by using a caliper. A universal testing machine (ElectroPuls™ E3000, Instron Industrial Products, Grove City, PA, USA) was employed for the compression test with cross-head speed of 2.0 mm/min at a load of 500 N. Then, the load and deflection were recorded with Console software (Instron Industrial Products, PA, USA) and the load-deflection curves were plotted. The maximum

load was applied to calculate the compressive strength (σ) of the upright rectangular specimen with the area of 2 mm  2 mm.

2.3.3.

Fracture toughness test

For the fracture toughness test, notchless triangular prism (NTP) composite specimens with the size of 6 mm  6 mm  6 mm  12 mm were prepared (Ruse et al., 1996). In order to ensure a proper polymerization reaction of the monomers, all the specimens were light-cured on three sides of the triangular for 4  40 s, and further both bottom and top sides for 40 s, respectively. Twelve specimens were prepared for each zirconia fibre concentration, and six of them were subjected to dry storage whilst another six were subjected to wet storage. The NTP fracture toughness test used the same universal testing machine and the same recording software as for compressive strength test (Section 2.3.2). The NTP setting used was strictly according the literature (Ruse et al., 1996). In brief, a thin plastic sheet was used to create a 0.1 mm deep crack initiation point midway along one at the edges of the NTP mould, and test specimens were mounted in a custom-made jig. The assembly was then loaded in tension mode at a crosshead speed of 0.1 mm/min with the load of 500 N. Similarly, the load and displacement were monitored, recorded, and the loaddeflection curves were plotted. The fracture toughness (KIc) was calculated with the formula: KIc ¼

Pmax DW1=2

γn

where Pmax ¼ maximum load recorded during the test; D¼ 12 mm; W¼ 10.5 mm; γ*¼28, which is a dimensionless stress

journal of the mechanical behavior of biomedical materials 53 (2016) 151 –160

155

Fig. 2 – EDX analysis for zirconia fibres before silane treatment (EDX condition: 15.0 kV, take-off angle 35.01, elapsed live time 20.0).

Fig. 3 – EDX analysis for zirconia fibres after silane treatment (EDX condition: 15.0 kV, take-off angle 35.01, elapsed live time 20.0). intensity factor coefficient minimum as reported by Bubsey et al. (1982) and has been used to determine NTP fracture toughness in various dental composite research (Ruse et al., 1996; Yonaha et al., 2001; Zhang and Darvell, 2012).

2.4.

Storage conditions

To examine the effects of water storage on the FRCs and the control with four concentrations of reinforced zirconia fibres, the four study groups were randomly divided into two subgroups: water storage and dry storage. For water storage, the samples were immersed and kept in 50 ml of deionized water in a closed plastic container wrapped with aluminium foil. These plastic containers were stored in a wet incubator (Forsa Scientific, USA) at 37 1C for 7 days. For dry storage, samples were stored in the plastic container at room temperature (20 1C) and humidity (75%) in the dry condition for 24 h.

2.5.

Statistical analysis

The mechanical tests results were recorded, calculated and statistically analysed using Predictive Analytics SoftWare (PASW) Statistics 18.0 (Statisitical Package for Statistical Science, Chicago, IL, USA). The lever of statistical significance α was set as 0.05. One-way ANOVA and Bonferroni tests were performed.

2.6.

Scanning electron microscopy

Representative fractured specimens from each subgroup in the fracture toughness test with various storage conditions

were selected for investigation of fractured surfaces using scanning electron microscopy (SEM). Samples were mounted on a specimen holder and ion-sputtered with gold by using an IXRF System (Model: MSP-2S, IXRF System, Austin, TX, USA). Then, the samples were examined by using a Scanning Electron Microscopy (SU1510, Hitachi, High-Technologies Corporation, Tokyo, Japan) for the morphology of fracture surface and adhesion of fibres with the resin matrix.

3.

Results and discussion

3.1.

Surface treatment of zirconia fibres

Elemental mapping was applied to examine whether the silane coupling agent (the novel silane system) successfully coated on the surface of zirconia fibres before and after the silane treatment. In particular, the silicon (Si) and zirconium (Zr) elements were of interest (Fig. 1). The magnified images well detected zirconium as an element but not silicon on the zirconia fibres before the silane treatment (Fig. 1a) which was expected. After the silane treatment, silicon was detected on the zirconia fibres (Fig. 1b), and EDX was also used to analyse and assess the quantities of elements on the zirconia fibres before (Fig. 2) and after (Fig. 3) the silane treatment. According to the EDX analysis, the zirconium to silicon weight percentage ratio on the fibre was 23.829/0.729¼ 32.69 before the silane treatment. However, after the silane treatment, the ratio decreased to 40.991/2.570¼ 15.95. Therefore, the silicon content was increased about 2 times after the silane treatment.

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Despite the silicon percentage on zirconia fibre surface has increased, the silane coupling agent treatment may be limited, i.e., there still existed some places where silicon cannot covered totally the fibre surface. Thus, the zirconia surfaces may not be perfectly wetted, and the interfacial bonding between the fibres and the resin matrix might not be optimal. Therefore, there may possibly be weak adhesion as a result. Furthermore, it is noteworthy that the mechanical properties of FRCs are determined by the fibre orientation, fibre shape, fibre distribution and proportion in the resin matrix (Abdulmajeed et al., 2011; Vallittu, 2014). Owing to the mechanical and interfacial properties of fibres, FRC depends on the uniformity of fibres. In addition, apart from this reason, the impregnation process of fibres with the resin matrix may lead to a reduced transparency and reinforcement effect if the fibres are aggregated or folded together. Therefore, the effectiveness of silanation and the dispersion and wetting of the fibres still need to be improved by further experiments in the near future.

3.2.

Fracture toughness

Using the NTP fracture toughness test, the fracture toughness was tabulated in Fig. 4 and Table 3. In particular, the 3 wt% zirconia fibre group indicated an increase in the fracture toughness after water storage, whilst other groups showed a decrease. For the water storage group, again the 3 wt% group showed a statistically significant higher fracture toughness than other groups (po0.05). However, no significant difference (p40.05) in fracture toughness was found in the dry condition after 24 h between groups. Two factors might contribute to this result. Firstly, as some previous studies (Mallick, 2007) have suggested that the fibres play a valuable role in FRC, the zirconia fibres are the integral part in transforming the stress force and sharing the major portion of the load. However, with the increase fibre percentages (say, up to 5 wt%), the fracture toughness has a tendency to reduce due to the viscosity of the FRC was increased. Thus, the higher viscosity makes the uniform dispersion of the fibres in the resin matrix more difficult. More voids and air bubbles were formed in the experimental composites which in fact became a barrier for transferring the stress. In addition, due to the white colour of zirconia fibres, the light for curing probably could not be transmitted. As a result, low degree of conversion occurred and this possibly reduced the mechanical strength (Zhang and Darvell, 2012). A study had already suggested that the suitable content of zirconia-silica nanofibre in a dental composite should be in the range of 2.5–5 wt% (Guo et al., 2012). Secondly, since the fibres were probably not entirely silanized by the silane coupling agent, the interfacial adhesion and wetting capability could only partially occur between the silanized zirconia fibres and the resin matrix. This said, a higher fibre content would need some extra resin matrix to wet properly the entire fibre surface. Consequently, reduced fracture toughness was observed. Moreover, the “crack bridging” mechanism (Xu, 2000), which refers to the micro-cracks existing inside the composite might also affect the fracture toughness. A deep understanding of the fracture toughness of FRCs is very important to their success and further development in dentistry. Fracture, alongside with the recurrent biting force and

Fig. 4 – Fracture toughness test results. Dot denote dry storage for 24 h, inverted triangle denote 7 days water storage. Error bars are 71 standard deviation. marginal deterioration of materials, are one of the failure forms of restored teeth (White et al., 1996), as well as one essential element of the self-life of a resin composite compared with amalgams when used as restorative materials (Deligeorgi et al., 2011). Thus, the present study characterized the experimental resin composites reinforced by various percentages of zirconia fibres by testing and calculating their fracture toughness. The normal fracture toughness tests, including the singleedge notched beam fracture toughness test (SENB), indentation fracture toughness test (IF) and the notchless triangular prism fracture toughness test (NTP), are generally applicable in the evaluation of dental materials. However, it was reported (Shigenori et al., 2008) that some resin composites exhibited different fracture toughness values mainly due to the curing conditions and the testing methods. Since there was a high correlation (R2 ¼0.997) between the NTP and SENB (Yonaha et al., 2001), the NTP fracture toughness test method for resin composites was suggested. In particular, NTP has the advantage in maintaining the integrity of fibres in a FRC sample. Therefore, it was chosen in the current study.

3.3.

Hardness test

Fig. 5 and Table 3 summarize the results of Vickers hardness test after two storage conditions. In general, the groups with zirconia fibres exhibited a higher Vickers hardness than the control group whilst 1 wt% group and 5 wt% showed a higher hardness than the other study groups (po0.05), especially in dry condition after 24 h. Nevertheless, according to ANOVA, only 1 wt% group showed a significantly higher Vickers hardness than the other groups. This said, it seems that there was a slight trend of decrease in such way that the specimens immersed in water had a marginally lower hardness compared with those without water storage, except for 3 wt% specimens. However, different storage conditions did not make any statistically significant difference (p40.05). Some reasons might be vital for this. Some previous studies seem to claim that the hardness of a composite was indirectly reflected by a degree of conversion on the surface of material during the polymerization process (Anfe et al.,

journal of the mechanical behavior of biomedical materials 53 (2016) 151 –160

157

Table 3 – Fracture toughness, hardness and compressive strength (mean7SD) of the experimental zirconia-fibre reinforced composites at dry storage and 7 days water storage conditions. Superscript values marked with identical letters did not differ significantly (p40.05). Mechanical test

Fibre loading

0 wt%

1 wt%

3 wt%

5 wt%

Fracture toughness (MPa m1/2)

Dry storage 24 h 7 days water storage Dry storage 24 h 7 days water storage Dry storage 24 h 7 days water storage

3.02A,a71.18 2.37B,a70.52 22.56F,f70.31 22.55H,f72.23 64.21K711.14 47.19K79.97

1.41A,b70.75 1.20B,b70.51 27.22G,g72.84 22.27H,h72.11 61.36K79.96 54.03K79.25

1.77A,c70.55 3.69C,d70.97 23.33F,i70.69 24.55H,i72.50 58.46K79.99 54.62K713.86

2.31A,e70.79 1.74B,e70.59 26.59G,j73.93 24.04H,j70.91 64.83K77.47 56.50K79.47

Vickers hardness (kg f mm  2) Compressive strength (MPa)

Fig. 5 – Hardness test results. Dot denote dry storage for 24 h, inverted triangle denote 7 days water storage. Error bars are 71 standard deviation.

Fig. 6 – compressive strength test results. Dot denote dry storage for 24 h, inverted triangle denote 7 days water storage. Error bars are 71 standard deviation.

2008). Since hardness is a surface property for materials, it might only slightly affect the degree of the conversion at the resin at surface. This said, a higher degree of conversion of the composite's surface should yield a higher hardness. Thus, it could be explained that the resin matrix groups without a fibre addition showed a stable result of hardness, due to the isotropic nature of the material, i.e., it had similar degree of conversion throughout the material. The addition of 1 wt% fibre could be uniformly dispersed into the resin matrix. Given this, in such a concentration, the fibres not only showed their potential to enhance the mechanical strength (e.g., hardness) of the experimental FRC, but also the resin might get a higher degree of conversion as the most superficial part of FRC was still resin matrix not the fibres. Thus, the light-curing could sufficiently provide a reasonable polymerization and therefore increased the FRCs' hardness. However, when the percentage of zirconia fibres was increased, the hardness was declined. This was most probably due to the surface of the FRC which contained a lot of fibres and sufficient curing could not be done, which might have affected its hardness.

was observed with the addition of zirconia fibres into the resin matrix. According to ANOVA analysis, no statistically significant differences were demonstrated in compressive strength compare with various groups in both dry storage 1 day and water storage for 7 days. Two possible explanations might explain these results. Firstly, to improve the mechanical properties of the resin composite, both the mechanical properties of zirconia fibres and the interfacial adhesion between the fibre surface and the resin matrix are important. Even though the impregnation of the resin matrix and fibres improved the interfacial bonding strength, the strength may not strong enough to effectively reinforce the composites. The reason for this might be that the partially successful silanization of the zirconia fibres might not be strong enough, i.e., it is not well known how strongly silanes can bond onto zirconia. Secondly, the loads from the longitudinal compressive strength must be subjected to the fibre volume fraction. In fact, for the composite with a small fibre volume fraction, the fibres are supposed to be able to crack first under the load, and then the matrix will carry the load. On the contrary, for the large fibre volume fraction, once the fibre have failed, the matrix cannot take up the extra force and thus they would fail together (Lee and Suh, 2006). In general, when the fibre volume fraction is less than the critical volume fraction, the composite's longitudinal compressive strength would be less than the matrix compressive strength. In the current study, 1.0–5.0 wt%

3.4.

Compressive strength test

Fig. 6 and Table 3 reveal the outcomes of the compressive strength test of various test groups – dry and 7-day water storage groups. No uniform change of compressive strength

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Fig. 7 – SEM micrographs of various percentages of zirconia fibre-reinforced composite and stored in dry condition for 1 day. Note how the random-type fibres are evenly distributed in the resin matrix in red arrows. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 8 – SEM micrographs of various percentages of zirconia fibre-reinforced composite and stored in water condition in 7 day. Note the fibres were pulled out from the crack surface and the pulled-holes (red arrows), which suggests that the fibres were weakly bonded to the resin matrix. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

journal of the mechanical behavior of biomedical materials 53 (2016) 151 –160

weight percentages of fibres were used, which did not account for the critical volume in the composite, and thus the compressive strength might almost is the matrix compressive strength. This might explain why the result did not show any significant difference. The zirconia fibres in the current study were randomly distributed in the resin matrix, such that the fibre direction and orientation cannot be justified. This gives rise to the load transfer mechanism of the FRC matrix unpredictable. When fibre-reinforced composites are loaded in compression, the influence of the associated stress, which are also determined by the details of fibre packing (Matthews and Rawlings, 1999), could be greatly affected by the fibre orientation. Therefore, for the further studies, placing the reinforcing zirconia fibres with same direction along the axis of the specimen might have a positive effect.

3.5.

SEM

SEM observations revealed that the zirconia fibres were evenly distributed in the resin matrix, for various weight percentages of zirconia fibres at dry (Fig. 7) and water storage conditions (Fig. 8). Note that fibres were pulled out from the crack surface and the pulled-holes (as shown in red arrows in Fig. 8), which suggests that the fibres were weakly bonded to the resin matrix. Similar debonded surface was also observed in short glass-fibre reinforced PMMA surface (So et al., 2012).

4.

Conclusions

With the limitations of this laboratory study, the hypothesis was partially met: we may summarize that silanized zirconia fibres have the potential to reinforce an experimental resin composite. The reinforcement resulted in a significant increase in Vickers hardness and fracture toughness. The most effective fibre percentage to obtain a significant increase in fracture toughness was 3 wt% after water storage, and in Vickers hardness it was 1 wt% in dry condition. Compressive strength was not affected by silanization of zirconia fibres.

Acknowledgements This work was done in partial fulfillment of the requirements of the degree of MSc(DMS) for the first author at the Faculty of Dentistry, The University of Hong Kong. A part of the results has been presented in IADR/AADR/CADR General Session in Boston, USA, 2015. We would like to thank Mr. Tony Yuen, Mr. Paul Lee and Mr. Bentley Yeung for their technical assistance.

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