Short glass fiber reinforced restorative composite resin with semi-inter penetrating polymer network matrix

d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1356–1362

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Short glass fiber reinforced restorative composite resin with semi-inter penetrating polymer network matrix Sufyan Garoushi ∗ , Pekka K. Vallittu, Lippo V.J. Lassila ¨ Department of Prosthetic Dentistry & Biomaterials Science, Institute of Dentistry, University of Turku, Lemminkaisenkatu 2, FI-20520 Turku, Finland

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objectives. The purpose of this study was to investigate the reinforcing effect of short E-glass

Received 4 July 2006

fiber fillers on some mechanical properties of dental composite resin with interpenetrating

Accepted 16 November 2006

polymer network (IPN)-polymer matrix. Materials and methods. Experimental composite resin was prepared by mixing short fibers (3 mm in length) with a fraction of 22.5 wt% and IPN-resin 22.5 wt% with silane treated sil-

Keywords:

ica filler 55 wt% using high speed mixing machine. Test specimens (2 mm × 2 mm × 25 mm)

Restorative composite

and (9.5 mm × 5.5 mm × 3 mm) were made from the experimental composite (FC) and con-

Water sorption

ventional particulate composite resin (control, Z250, 3M-ESPE). The test specimens (n = 6)

Fiber reinforce composite

were either dry stored or water stored (37 ◦ C for 30 days) before the mechanical tests. Three-

Mechanical properties

point bending test was carried out according to ISO 10477 and compression loading test

Degree of conversion

was carried out using a steel ball (Ø3.0 mm) with speed of 1.0 mm/min until fracture. Degree of monomer conversion (DC %) of both composites was determined by FTIR spectrometry. Water sorption and solubility of specimens were also measured. Scanning electron microscopy was used to evaluate the microstructure of the composite. Results. ANOVA revealed that experimental fiber composite had statistically significantly higher mechanical performance of flexural strength (210 MPa) and compressive load-bearing capacity (1881 N) (p < 0.05) than control composite (111 MPa, 1031 N). Degree of conversion of the FC (59%) and conventional composite (57%) was at the same range. Significance. The use of short fiber fillers with IPN-polymer matrix yielded improved mechanical performance compared to conventional restorative composite. © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Dental restorative filling composite resins have been introduced to dental community in 1960s [1]. Since then after many significant material improvements restorative composite still suffers lack of mechanical properties and problems related to polymerization shrinkage. Clinical studies have shown that direct fillings fail predominantly because of occlusal wear or secondary caries [2–5]. However, fracture of restora-

∗

tive composite is reported also as a common reason for replacement [3,5]. Due the failures of this kind, it is still controversial, whether restorative composites should be used in large high-stress bearing applications such as in direct posterior restorations [4,5]. The relatively high brittleness and low fracture toughness of current composites still hinder their use in these large stress-bearing restorations [12,17,18]. Studies have been undertaken to evaluate and improve restorative composite resin against wear and lower the poly-

Corresponding author. Tel.: +358 2 333 83 57; fax: +358 2 333 83 90. E-mail address: sufgar@utu.fi (S. Garoushi). 0109-5641/$ – see front matter © 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2006.11.017

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merization shrinkage. Attempts have been made to change type of filler or filler size and their silanization, by changing the polymerization kinetics of resins and to influence to degree of monomer conversion [6–9]. Reinforcing the resin with glass fibers [10–12], with fiber-reinforced composite (FRC) substructure [13], whiskers [14], particulate ceramic fillers (dense and porous) [15] and optimization of filler content [6] are among the methods that have been studied. Some other aspect relating to the indirect laboratory-made composites have been investigated by using post-curing to enhance composite strength and toughness [16]. Glass fibers have been investigated to reinforce dental polymers for over 30 years [19]. They have documented reinforcing efficiency and good esthetic qualities compared to carbon or aramid fibers [20–22]. The effectiveness of fiber reinforcement is dependent on many variables, including the resins used, the quantity of fibers in the resin matrix [23,24], length of fibers [24], form of fibers [25], orientation of fibers [26], adhesion of fibers to the polymer matrix [27], and impregnation of fibers with the resin [28]. Short random fibers provide an isotropic reinforcement effect in multidirections instead of one or two directions, as described by Krenchel [29]. Polymethyl methacrylate (PMMA)-based semi-interpenetrating polymer network (semi-IPN) matrix has been established as a polymer matrix in denture base materials [30]. Also some products of FRC use semi-IPN-polymer matrix [31]. However, dental restorative composite with semi-IPNpolymer matrix in combination with glass fibers has not been evaluated to the author’s knowledge. The aim of this study was to investigate the reinforcing effect of short E-glass fiber fillers on some mechanical properties of dental composite with semi-IPN-polymer matrix.

2.

Materials and methods

The materials used in the study are listed in Table 1. Experimental fiber composite (FC) was prepared by mixing 22.5 wt% of short E-glass fibers (3 mm in length) to the 22.5 wt% of dimethacrylate-PMMA resin matrix and then 55 wt% of filler particles of SiO2 (3 ± 2 ␮m in size) were added gradually. The mixing was carried by using high speed mixing machine for 5 min (SpeedMixer, DAC, Germany, 3500 rpm). Before the silica filler particles were incorporated into the resin matrix, they were silane treated, according to the technique defined in the previous studies [32]. In polymerization, the resin matrix of dimethacrylate-PMMA formed semi-IPN-polymer matrix for the composite FC. Three-point bending test specimens (2 mm × 2 mm × 25 mm) and compressive load bearing test specimens

Fig. 1 – Schematic drawing of the test specimen and the compression load-bearing capacity test.

(9.5 mm × 5.5 mm × 3 mm) were made from experimental fiber composite FC and conventional particulate filler dental composite (control, Z250, 3M-ESPE) (Fig. 1). Bar-shaped specimens were made in a half-split stainless steel mold between transparent Mylar sheets and cubic specimens in open silicon mold covered by Mylar. Cubic specimens were fabricated by incrementally placing the materials in a silicon mold. In order to simulate the clinical condition, one additional test group were made by placing a bottom layer of FC (2 mm) as substructure and then conventional composite (1 mm) was applied subsequently after light initiated polymerization of the FC. Polymerization of the composite was made using a hand light-curing unit (Optilux-501, Kerr, CT, USA) for 40 s from both sides of the metal mold and incrementally from the top of the silicon mold. The wavelength of the light was between 380 and 520 nm with maximal intensity at 470 nm and light irradiance was 800 mW/cm2 . The specimens from each group (n = 6) were either stored dry or water stored (37 ◦ C for 30 days). The dry-stored (room temperature) specimens were tested 24 h after their preparation. Three-point bending test was conducted according to the ISO 10477 (test span: 20 mm, cross-head speed: 1 mm/min, indenter: 2 mm diameter). All specimens were loaded in material testing machine (model LRX, Lloyd Instrument Ltd., Fareham, England) and the load-deflection curves were recorded with PC-computer software (Nexygen 4.0, Lloyd Instruments Ltd., Fareham, England). Static compressive fracture test was carried to determine the load-bearing capacity of each group using a universal testing machine. Specimens were loaded using a steel ball (Ø3 mm) until fracture.

Table 1 – Materials used in the study Brand Z250 everStick Stick resin

Manufacturer

Lot number

3M-ESPE, St. Paul, MN, USA StickTeck Ltd., Turku, Finland StickTeck Ltd., Turku, Finland

20040420 2050426-ES-125 540 1042

Composition Bis-GMA, UDMA, Bis-EMA PMMA, Bis-GMA, E-glass fillers 60% Bis-GMA–40% TEGDMA

PMMA, polymethyl methacrylate, Mw 220.000; Bis-GMA, bisphenol A-glycidyl dimethacrylate; TEGDMA, triethylenglycol dimethacrylate; UDMA, urethane dimethacrylate; Bis-EMA, bisphenol A-polyethylene glycol diether.

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Flexural strength ( f ) and flexural modulus (Ef ) were calculated from the following formula [33]: f =

3Fm I , 2bh2

Ef =

SI3 4bh3

where Fm is the applied load (N) at the highest point of loaddeflection curve, I the span length (20 mm), b the width of test specimens and h is the thickness of test specimens. S is the stiffness (N/m) S = F/d and d is the deflection corresponding to load F at a point in the straight-line portion of the trace. Toughness was calculated as the integral of the area under the stress/strain curve and reported in units of MPa. The degree of monomer conversion (DC %) of both composite FC and Z250 during and after photoiniated polymerization was monitored by Fourier transform infrared spectroscopy (FTIR) (Spectrum One, Perkin-Elmer, Beaconsfield Bucks, UK) with an attenuated total reflectance (ATR) sampling accessory. Materials were placed in 1.8 mm-thick ring molds with a diameter of 6.5 mm on the ATR-sensor (ZnSe-crystal). The upper surface of the specimen was covered with a Mylar sheet and a glass slide of 1 mm thickness and slightly pressed against the ATR to ensure the good contact of the specimen. The light source was placed in contact with glass surface. The substrate was light-polymerized with a hand-held light-curing unit (Freelight 2, 3M-ESPE, Elipar, Germany) for 40 s. The spectra during the polymerization process was recorded every 6 s

until 5 min. The DC % was calculated from the aliphatic C C peak at 1638 cm−1 and normalized against the aromatic C C peak at 1608 cm−1 according to the following formula:

 DC% = 1 −

Caliphatic /Caromatic Ualiphatic /Uaromatic

 × 100%

(1)

where is the Caliphatic is the absorption peak at 1638 cm−1 of the cured specimen, Caromtic the absorption peak at 1608 cm−1 of the cured specimen, Ualiphatic the absorption peak at 1638 cm−1 of the uncured specimen and Uaromatic is the absorption peak at 1608 cm−1 of the uncured specimen. The fraction of remaining double bonds for each spectrum was determined by standard baseline techniques using the comparison of maximum heights of aliphatic and reference peaks for calculations. Water sorption and solubility were measured, using eight specimens, each of them stored in 120 ml water for 30 days at 37 ◦ C. The dry weight (md ) of the specimens was measured with a balance (Mettler A30, Mettler Instrument Co., Highstone, NJ, USA), with an accuracy of 0.1 mg. During the water immersion, weight (mw ) of the specimens were measured at 1, 2, 3, 7, 14, 21, and 30 days. Solubility weight (mh ) was measured after the dehydration was stabilized at 60 ◦ C in air.

Fig. 2 – (a) Flexural strength of experimental FC composite and conventional restorative composite Z250. Vertical lines represent standard deviations (dry, after polymerization and conditioning; water, after water saturation for 30 days at 37 ◦ C; dehydrated, dehydration at 60 ◦ C). (b) Flexural modulus of experimental FC composite and conventional restorative composite Z250. Vertical lines represent standard deviations. (c) Flexural toughness of experimental FC composite and conventional restorative composite Z250. Vertical lines represent standard deviations.

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Water sorption and solubility were calculated as follows: Water sorption % =

mw − mh × 100%, md

Water solubility % =

md − mh × 100% md

Scanning electron microscopy (SEM, model 5500, Jeol Ltd., Tokyo, Japan) was used to evaluate the structure of polymer matrix, orientation of fibers and fracture surface (Fig. 6). Cross-sections of test specimens were wet ground using silicon carbide grinding paper (FEPA #4000) by a grinding machine LaboPol-21 (Struers A/S, Rodovre, Denmark). Mean values of flexural properties, load-bearing capacity, degree of monomer conversion and water sorption were statistically analyzed with analysis of variance (ANOVA) at the p < 0.05 significance level with SPSS version 13 (Statistical Package for Social Science, SPSS Inc., Chicago, IL, USA) to determine the differences between the groups.

3.

Fig. 4 – The degree of monomer conversion (DC %) of composite FC and composite Z250 light-polymerized with light-curing unit for 40 s.

Results

The mean flexural strength, flexural modulus and toughness together with load-bearing capacity and degree of conversion of tested groups with standard deviations (S.D.) are summarized at Figs. 2–4. ANOVA revealed that experimental FC composite had statistically significantly higher flexural strength of (210 MPa) and compressive load-bearing capacity of (1881 N) compared to control Z250 composite (111 MPa, 1031 N) (p < 0.001) in dry conditions. Water storage decreased the flexural strength and the load-bearing capacity in both materials and with both tests (p < 0.001) by an average of 20%. No significant difference was found in flexural strength between dry and dehydrated specimens (Fig. 2). Degree of monomer conversion after 5 min of photopolymerization of FC composite was 58% (1.8) and Z250 composite 55% (1.2). Water sorption after 30 days of FC composite was 1.54 wt%, which was slightly higher than sorption of Z250 composite (1.28 wt%). Solubility of FC composite was 0.34

Fig. 5 – Water sorption (%wt gain) of the tested materials during 30 days of storage in water at 37 ◦ C.

(0.05) wt%, whereas Z250 composite revealed lower solubility 0.27 (0.12) wt% (Fig. 5). SEM-micrographs of surface of FC composite revealed microstructure of combination of fibers and particulate fillers. Fibers acted as a crack stopper and provided increase in fracture resistance (Fig. 6)

4.

Fig. 3 – Compressive load bearing capacity of the test groups. Z250 + FC refers to specimen that was combined with a bottom layer (2.0 mm) of FC and covered with a 1.0 mm layer of Z250. Vertical lines represent standard deviations.

Discussion

The results of the mechanical test revealed substantial improvements in the load bearing capacity and the flexural strength of dental composite resin reinforced with short Eglass fiber fillers in comparison with conventional restorative composite. Clinical study reported by Dijken have shown that restorative composite with microfibers suffer extensive wear and fracture [34], which can be partly explained because of the used fiber length was well below of critical fiber length. In order for a fiber to act as an effective reinforcement for the polymers, stress transfer from the polymer matrix to the fibers is essential [11,35,36]. This is achieved, if the fibers have a length equal or greater than the critical fiber length [11,35]. It has been measured using fiber fragmentation test that the critical fiber lengths of E-glass with Bis-GMA polymer matrix vary between 0.5 and 1.6 mm [37]. Deteriorated or initially poor

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Fig. 6 – SEM photographs of polished surface of FC composite with a propagating crack (A). Fracture surface with different magnifications showing fractured glass fiber (B–D).

adhesion between the fibers and the polymer matrix increase the critical fiber length. In this case, the mechanical friction of the fibers to the polymer matrix at the interface can compensate the poor adhesion [38]. Based on this, the present study used fibers as filler in 3 mm length. The flexural test has been widely used to characterize the mechanical properties of dental restorative materials [36,39]. In the present study, the specimens of experimental FC composite had a flexural strength of 211 MPa. This in accordance with previous results of Petersen [11], who tested also with short fiber reinforced composite. However, Petersen carried out a four-point bending test, which causes more shear stress to the specimen compared to the three-point bending test, which is defined as an ISO standard test to measure the mechanical properties of dental composite (ISO 4049). According to Krenchel, short random orientated in 3D fiber provides strengthening factor of 0.2, whereas in 2D orientation gives 0.38, and unidirectional fibers gives factor of 1. Strength values of unidirectional FRC in the three-point bending are reported to be between 800 and 1200 MPa [40]. In addition, a two times higher load-bearing capacity of specimens made of composite FC was obtained compared to that of conventional particulate fillers restorative composite. The reinforcing effect of the fiber fillers is based on stress transfer from the polymer matrix to fibers but also the behavior of individual fiber as a crack stopper (Fig. 6). Random fiber orientation and lowered cross-linking density of the polymer matrix by the semi-IPN structure likely had a significant role in mechanical properties.

The FTIR has proved to be a useful technique for the analysis of the degree of monomer conversion in dental composites [41]. The setup used in this study was designed to simulate the conditions during the fabrication of direct restorations. The upper surface of the test material was exposed to the light source and the lower surface was in contact with the ATR crystal. Therefore, the experimental design in this work can provide information about how the polymerization propagates on the bottom of the test material. Experimental composite FC showed slightly higher degree of conversion, which could be due to lower filler content in comparison with conventional composite Z250. However, some of the difference could be also explained by differences between polymer matrices of pure thermoset and semi-IPN. Increase of the filler content can limit the mobility of free radicals in the polymer matrix [42]. Water storage decreased the flexural strength and the load-bearing capacity in all of the specimens. In the polymer matrix, water acts as a plasticizer increasing free volume and decreasing glass transition temperature of the polymer matrix [43–45]. Also, previously it has been reported that there is a potential deteriorative effect of water to the interfacial adhesion between the polymer matrix to the glass fibers through rehydrolysis of silane coupling agent [43–45]. Composite FC absorbed more water than conventional composite, which can be explained due to lower filler content of FC. The amount of the absorbed water is also affected by hydrophilicty of the polymer matrix and by the chemical stability of the filler particle against water [44].

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Polymerization shrinkage in direct filling composite restorations causes marginal leakage between the filling and cavity walls of teeth [46]. Efforts for reducing the polymerization shrinkage have included modification of resin compositions, using higher filler contents to reduce the amount of polymerizable resin [9]. The authors hypothesize that the use of fiber filler could reduce the polymerization shrinkage of the composite and the question will be evaluated in furthers studies.

5.

Conclusion

Short glass fiber reinforced semi-IPN composite resin revealed improvements in mechanical properties compared with the conventional particulate filler restorative composite. This could suggest better performance of glass fiber reinforced composite in high stress-bearing application areas.

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