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Ultra-high-molecular-weight polyethylene fiber reinforced dental composites: Effect of fiber surface treatment on mechanical properties of the composites Niloofar Bahramian a , Mohammad Atai b,∗ , Mohammad Reza Naimi-Jamal c a b c
Department of Biomaterials, Science and Research Branch, Islamic Azad University, Tehran, Iran Iran Polymer and Petrochemical Institute (IPPI), P. O. Box 14965/115, Tehran, Iran Department of Chemistry, Iran University of Science and Technology, 16846 Tehran, Iran
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Poor interfacial adhesion between the fibers and resin matrix in the ultra high
Received 19 December 2014
molecular weight polyethylene (UHMWPE) fiber reinforced composites (FRCs) is the main
Received in revised form
drawback of the composites. This study aims to evaluate the effect of corona and silane
28 April 2015
surface treatment of the fibers on the mechanical properties of the UHMWPE FRCs.
Accepted 25 May 2015
Methods. UHMWPE fibers were exposed to corona discharges for different periods of time
Available online xxx
(0 s, 5 s, 7 s). The surface characteristics of the UHMWPE fibers were investigated by attenuated total reflectance Fourier transformed infrared spectroscopy (ATR-FTIR), atomic force
Keywords:
microscopy (AFM), scanning electron microscopy (SEM) and nanoindentation technique.
Fiber reinforced composites (FRCs)
The flexural strength and flexural modulus of the FRCs made of the treated fibers were
UHMWPE fibers
determined on 2 mm × 2 mm × 25 mm specimens. The fracture toughness (the critical stress
Corona treatment
intensity factor, KIC ) of the composites was also evaluated using a three-point single edge
Mechanical properties
notch beam (SENB) bending technique. Statistical analysis of the data was performed with ANOVA and the Tukey’s post-hoc test. The fiber-resin interface and the fracture surface were investigated using SEM. Results. The change in the surface mechanical properties and chemistry of the corona treated UHMWPE fibers were monitored. The fibers exposed to corona for 5 s showed higher surface nanohardness. In the FRCs, the specimens reinforced with 5 s corona treated silanized fibers showed higher mechanical properties (flexural modulus, flexural strength, and fracture toughness), SEM images revealed a better adhesion between the resin and fibers after 5 s fiber corona treatment and silanization. Significance. Corona and silane treatment of UHMWPE fibers provide dental FRCs with improved mechanical properties. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author. Tel.: +98 2148662446; fax: +98 2144787023. E-mail address: [email protected] (M. Atai).
http://dx.doi.org/10.1016/j.dental.2015.05.011 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Bahramian N, et al. Ultra-high-molecular-weight polyethylene fiber reinforced dental composites: Effect of fiber surface treatment on mechanical properties of the composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.05.011
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1.
Introduction
Development of fiber-reinforced composites (FRCs) have offered dental researchers the possibility of making resinbonded, mechanically and esthetically good, and metalfree restorations for teeth replacement [1,2]. Variety of dental applications, such as bridges, splints, posts, spacemaintainers, orthodontic retainers, denture bases, clasps and connectors, and implant prostheses made of FRCs have successfully been used in dental practice providing acceptable mechanical properties [3,4]. In general, in a FRC, fibers are the main load-carrying members, while the surrounding matrix acts as a load transfer medium and protects the fibers from environmental harms [5]. The mechanical properties of FRCs are dependent upon the mechanical properties of each component, quality of impregnation of fiber with resin, adhesion between the fiber and matrix, quantity of fibers in the matrix resin, the fiber volume fraction, fiber architecture, and the fiber type [6,7]. UHMWPE fibers in comparison with other plastic materials are found to have high impact resistance, toughness, and tensile strength. The fibers are also light weight, corrosion and wear resistant [8,9]. Adhesion of UHMWPE fibers to the matrix resin is, however, difficult because the UHMWPE is non-polar in nature with low surface energy [10]. To provide a good adhesion between the UHMWPE fibers and the polymer matrix, the surface of the fibers has been modified using different methods such as chemical modification [11], chemical grafting [12], corona discharging [13,14], oxygen plasma [15–17], high energy laser [18], UV [19], and gamma irradiation [20]. Improving the adhesion performance of UHMWPE may results in improved dental FRCs. Corona discharge, is a process by which ionized gas species are formed around an active high potential electrode due to the electrical current [21]. In corona treatments of polymers the ionized species transfer their charge to the polymer surface resulting in increase in surface energy by introducing polar groups on the surface. The corona discharge may also change the surface roughness [22]. The process can impart hydrophilicity to polymer surfaces which may improve the surface affinity and sticking strength with some hydrophilic polymers [13]. It can also improve the bonding characteristics of materials by raising the surface energy, wettability, oxygen to carbon ratio, number of functional polar groups on the surface, and specific surface area leading to enhanced adhesive capabilities, increase in the delamination force [23,24] and improvement in the fracture toughness and flexural properties [25]. Silane treatment, on the other side, is a well-known method for improvement of surface wettability and interfacial adhesion between matrix resin and OH-covered substrates [26–28]. Therefore, the combination of these two surface modification methods may result in FRCs with improved properties. This study, consequently, designed to test the hypothesis that the double surface treatment of UHMWPE fibers improves the mechanical properties of an experimental dental FRC containing the fibers as reinforcing phase and Bis-GMA/TEGDMA as matrix.
2.
Materials and methods
2.1.
Materials
Materials used in this study are presented in Table 1.
2.2.
Methods
2.2.1.
Surface corona treatments of UHMWPE fibers
UHMWPE fibers were treated for 5 s and 7 s under atmospheric pressure air corona discharge using SpotTEC corona power Generator (Tantec, Lunderskov, Denmark, 6.5 kV/25 kHz and 550 W). The distance between electrode and UHMWPE fiber surface was 5 cm.
2.2.2.
AFM and nanoindentation analysis
AFM imaging analysis was performed (SPM Dualscope/ Rasterscope C26, DME, Copenhagen/Herlev, Denmark) to study the surface topography and roughness of the fibers before and after corona treatment. Hardness (Hi ) of the fibers was determined using nanoindentaion technique (CSM, Peseux, Switzerland, indentor type: Berkovich, indentor material: diamond, max depth: 500 nm, loading and unloading rates: 1000 nm min−1 ). The elastic modulus of the specimen in depth-sensing indentation techniques, used in nanoindentation test, is determined from the slope of the unloading of the load-displacement response, Ei . Vickers hardness was also calculated from the Hi .
2.2.3.
Fourier-transform infrared spectrometer (FT-IR)
A FTIR spectrometer (EQUINOX55, Specac golden gate, ZnSe prism, Bruker, Germany) was used in order to identify functional groups formed on the fibers after corona and silane treatments. The spectra were collected in the wavenumber range of 4000–400 cm−1 , using attenuated total reflectance technique (ATR-FTIR). In this technique the infrared beam is directed into a crystal of relatively high refractive index (ZnSe) which is in contact with the fibers. Multiple reflections of the beam in the crystal collects the spectrum of the intimate fibers.
2.2.4.
Silanization of UHMWPE fibers
The chemical surface modification of UHMWPE fibers was carried out using a 2 (wt/vol)% ␥-MPS in ethanol/distilled-water (70/30 wt/wt, pH adjusted at 3.8 using acetic acid). Having the silane pre-hydrolyzed for 1 h, the fibers were immersed in the solution and left for 1 week at room temperature to dry.
2.2.5. Composite preparation and mechanical measurements The composites were classified in 5 groups (Table 2). Group 1, group 2 and group 3 were prepared to evaluate the effect of corona exposure time. All these three groups were made of 10 bundles for flexural test and 20 bundles for fracture toughness test. As the volume of the fracture toughness test mold was two times of the flexural test, twice fibers were inserted in the fracture toughness molds to keep the fiber content constant
Please cite this article in press as: Bahramian N, et al. Ultra-high-molecular-weight polyethylene fiber reinforced dental composites: Effect of fiber surface treatment on mechanical properties of the composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.05.011
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Table 1 – Materials used in the study. Country
City
Germany
Krefeld
Evonik
–
Resin monomer
Germany
Krefeld
Evonik
–
Germany
Munich
Sigma–Aldrich
–
Diluent monomer Activator
Germany China
Munich Shanghai
Sigma–Aldrich Surrey Hi-Tech
USA
Pittsburgh
Acros organics
– Roving continuous (SP1200/320) Diameter ≈20 m* 98% Purity
*
Company
Special properties
Role
Name 2,2-Bis-[4(methacryloxypropoxy)phenyl]-propane (Bis-GMA) Triethyleneglycol dimethacrylate (TEGDMA) N-N -dimethyl aminoethyl methacrylate (DMAEMA) Camphorquinone (CQ) Ultra-high-molecularweight polyethylene (UHMWPE)
Initiator Reinforcing fibers
Silane coupling agent
3-(Trimethoxysilyl)propyl methacrylate (␥-MPS)
Diameter is reported based on SEM observation.
in both tests. The fiber corona treatment time was 0 s, 5 s and 7 s for group 1, group 2 and group 3, respectively. The fibers in these three groups were silanized after corona treatment. Group 4 and group 5 were made of 8 fiber bundles for flexural test and 16 fiber bundles for fracture toughness test, corona treatment time was 5 s for both of them. The fibers in group 4 were silanized but the fibers in group 5 received no silane treatment to evaluate the effect of silane treatment.
strength (FS) was calculated in (MPa) using the following formula:
2.2.5.1. Flexural strength and flexural modulus. 0.5 wt% CQ
2.2.5.2. Fracture toughness. To determine the fracture tough-
and 0.5 wt% DMAEMA, as photo-initiator system, were mixed with the matrix resin (Bis-GMA/TEGDMA, 70/30 wt/wt). The fibers were then impregnated with resin and were cut with a sharp surgical blade and placed manually into the mold in unidirectional (UD) position. 5 bar shaped specimens were made of each composite group according to Table 2, using a metal mold with the dimensions of 25 mm × 2 mm × 2 mm. The mold was placed on a glass slide and manually filled with the preimpregnated fibers. A second glass slide was placed on and pressed against it. The composite was cured with a LED dental light source (L.E. Demetron 1, SDS/Kerr, Orange, CA, USA) with power density of 600 mW cm−2 for 40 s in three consecutive points, producing a partial overlapping. After curing, the specimens were polished with abrasive paper 600 and 1000 grits and stored in distilled water at 37 ◦ C (±2 ◦ C) for 24 h. Afterwards, they were submitted to a three point bending test with a universal testing machine (STM 20, Santam, Tehran, Iran) with a crosshead speed of 1 mm min−1 . The flexural
ness (KIC ), rectangular bar single-edge notch beam (SENB) specimens (n = 5) were fabricated in a 5 mm × 2 mm × 25 mm stainless steel mold. The mold was filled with impregnated fibers according to Table 2 and cured using the same procedure as the flexural specimens. A 2.5 mm notch was then cut at the center of each specimen using a 0.6 mm thick diamond wheel cutting machine (Mecatome T 201 A, Oxford, UK). A razor blade was then pressed on the notches to provide a sharp crack. The bending fracture test was performed at a cross-head speed of 1 mm min−1 [29,30] using the universal testing machine and the fracture toughness (critical stress intensity factor, KIC ) was calculated according to the following equation [31]:
FS = 3pL/2bd2 where p is the maximum load (N); L is the span length (mm); b is the width of the specimen (mm) and h, the height (mm). The flexural modulus was calculated from the slope of the initial linear region of stress–strain curve.
KIC = [3PL/2BW 3/2 ]Y
Y = (1.93(a/W)
1/2
− 25.11(a/W)
) − (3.07(a/W)
7/2
3/2
+ 25.8(a/W)
+ 14.53(a/W)
9/2
5/2
)
Table 2 – Characteristics of the studied groups. Groups
1 2 3 4 5
Aim of study
Effect of corona exposure time Effect of corona exposure time Effect of corona exposure time Effect of silane treatment Effect of silane treatment
Fibers corona treatment time (s) 0 5 7 5 5
Silanization of fibers with ␥-MPS Yes Yes Yes Yes No
Number of fiber bundles for (flexural test) 10 10 10 8 8
Number of fiber bundles for (fracture toughness test) 20 20 20 16 16
Please cite this article in press as: Bahramian N, et al. Ultra-high-molecular-weight polyethylene fiber reinforced dental composites: Effect of fiber surface treatment on mechanical properties of the composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.05.011
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Fig. 1 – SEM micrographs of UHMWPE fibers exposed to corona at (a) 0 s, (b) 5 s, and (c) 7 s. The images show the fiber surface details at 30,000× magnification.
Fig. 2 – AFM images show UHMWPE fibers (a) with no corona treatment, (b) treated for 5 s corona discharge, and (c) treated for 7 s corona discharge.
where P is load at fracture (N), L, W, B, and a are length, width, thickness, and notch length (in mm), respectively. The fractured specimens were then observed under scanning electron microscope.
2.2.6.
SEM analysis
silane treatments. The flexural strength, flexural modulus and fracture toughness of the FRCs (groups 1–3, Table 2), are shown in Fig. 4(a and b). The composite containing 5 s corona treated fibers have statistically significant higher flexural strength (225.7 ± 22.2 MPa), flexural modulus (45.2 ± 5.1 GPa) and fracture toughness (6.8 ± 1.2 MPa m1/2 ) among the other groups
The fiber surface after treatment and fracture surfaces of the specimens in the fracture test were observed by (FESEM, Mira 3-XMU, Brno-Kohoutovice, Czech Republic) and (TESCAN, VEGAII, XMU, Brno-Kohoutovice, Czech Republic). The samples were gold coated by a sputter coater before SEM observations.
Statistical analysis
The mechanical properties results were statistically analyzed (at least 5 repeat for each test) using one-way ANOVA and the Tukey’s post-hoc test at the significance level of 0.05.
3.
Results
b c
Transmittance (arb. units)
2.2.7.
a
d e ν(OH)
ν(C=O)
ν(C-O-C), ν(Si-O-Si), ν(Si-O-R)
SEM and AFM microscopic imaging of the UHMWPE fibers show different surface topographies after corona treatment (illustrated in Figs. 1 and 2). The hardness, modulus and roughness of corona treated and untreated fibers are tabulated in Table 3. An increasing and then decreasing trend is observed upon corona discharge duration. ATR-FTIR (Fig. 3) spectrums of the fibers before and after treatments show appearance of functional groups on the fiber surface after corona and
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 3 – ATR-FTIR spectra of UHMWPE fibers (a) untreated fibers, (b) 5 s corona treated, (c) 7 s corona treated, (d) 5 s corona treated and silanized, (e) 7 s corona treated and silanized.
Please cite this article in press as: Bahramian N, et al. Ultra-high-molecular-weight polyethylene fiber reinforced dental composites: Effect of fiber surface treatment on mechanical properties of the composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.05.011
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Table 3 – Surface properties of UHMWPE fibers before and after corona treatment. Corona time (s) 0 5 7
Hi (MPa)
HV
Ei (GPa)
110.4 (38.8)*a 314.1 (102.7)b** 140.0 (20.6)a
10.2(3.6)c 29.1 (9.5)d 13.0 (1.9)cd
7.0 (0.9)e 21.4 (8.0)f 9.1 (6.9)ef
Sq (nm) 90.5 171 43.6
Hi : Indentation Hardness. HV: Vickers Hardness. Ei : Indentation Modulus. Sq: root-mean-square (Rms) (average surface roughness). ∗ Numbers in the parenthesis represent the standard deviations. ∗∗ Means with the same letter are not significantly different at p ≤ 0.05.
Table 4 – Mechanical properties of the composites made of unsilanized and silanized fibers corona treated for 5 s. Silane concentration (wt%) 0 (Group 5) 2 (Group 4) ∗ ∗∗
Flexural strength (MPa)
Flexural modulus (GPa)
Fracture toughness, KIC (MPa m1/2 )
45.9 (9.4)* a 197.2 (26.4)b **
12.8 (3.0)c 39.3 (8.0)d
1.7 (0.7)e 5.9 (1.8)f
Numbers in the parenthesis represent the standard deviations. The letters show significant difference at p ≤ 0.05.
300
a
fracture (Figs. 5 and 6) improvements in the adhesion between fibers and matrix resin is observed after corona and silane treatments.
50 40
200
30
150
20
100
50
10
0
2
4
6
8
Flexural modulus (GPa)
Flexural strength (MPa)
250
0
Corona treatment time (sec)
Fracture toughness, KIC, (MPa.m1/2)
FS
FM
b
8
6
4
2
0
0
2
4
6
8
Corona treatment time (sec)
Fig. 4 – Mechanical properties of the FRCs corona exposure time, (a) flexural strength and flexural modulus, (b) fracture toughness.
(p < 0.05). The FRC with silanized fibers, in Table 4, shows higher flexural strength (197.2 ± 26.4 MPa), flexural modulus (39.3 ± 8.0 GPa) and fracture toughness (5.9 ± 1.8 MPa m1/2 ) compared to FRC made of unsilanized fibers (p < 0.05). According to the SEM images of composite specimens after
4.
Discussion
The characteristics of the interface between the UHMWPE fibers and the polymer matrix is an important factor determining the mechanical properties of UHMWPE fiber-reinforced composites [24]. With the aim of improving the mechanical properties of UHMWPE-based dental FRCs double surface treatment of the fibers were performed using corona discharge and silanization. The results verified the hypothesis that the treatments improved mechanical properties of the composites through improving the interfacial adhesion between the fibers and the matrix resin. The SEM images (Fig. 1a) shows the smooth surface of untreated UHMWPE fibers, only a few shallow longitudinal crevices is seen which are formed during the spinning of the fibers. After corona treatment, the crevices are almost removed but micro-pits are formed on the surface of 5 s corona treated fibers (Fig. 1b). Fig. 1c, however, shows deepening of the crevices after 7 s corona. Up to 5 s the fibers receive enough energy for melting the superficial polymer and disappearing the longitudinal grooves along with the formation of some micro-pits on the surface. At longer exposure times (here 7 s), high corona energy may degrade the polymer or melt the bulk of the polymer leading to the deformation and/or rupture of the fibers which are seen in Fig. 1c. This morphological change may be attributed to excavating the fibers surface [32] and partly broken UHMWPE polymer chains under the high energy corona [33]. The presence of micro-pits on the fiber surface can improve the interfacial adhesion of fiber–matrix through the mechanical interlocking between the micro-pits and the resin [17]. The changes in fiber surface roughness are also presented in Table 3. The AFM images (Fig. 2) clearly show the increasing (Fig. 2b) and decreasing (Fig. 2c) of the surface roughness in 5 s and 7 s corona exposure times, respectively.
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Fig. 5 – Fracture surface of composite made of (a) untreated fibers, (b) 5 s corona treated fibers, (c) 7 s corona treated fibers.
UHMWPE fibers are not adequately bonded to the polymer matrix because of their chemical inertness, low surface energy, and the absence of any polar groups on the surface of fibers. Corona treatment may introduce polar groups into the polymer surfaces and increase the surface energy, substrate wettability, hydrophilicity and adhesion characteristics [10,34]. The main chemical mechanism of corona treatment is oxidation which introduce oxygen containing groups on the fiber surface [34,35]. ATR-FTIR (Fig. 3) spectrums of the treated and untreated fibers illustrates the appearance of stretching vibration of hydroxyl groups ((OH) at 3200–3600 cm−1 ), stretching vibration of carbonyl groups ((C O) at ∼1710 cm−1 ), and stretching vibration of C O C groups (at ∼1100 cm−1 ). These groups change the surface chemistry and surface energy of the fibers [36] which, consequently, affect the adhesion characteristics of the fibers. Considering the absorption peak of in-plane bending (scissoring) vibration of CH2 groups (ıs at 1464 cm−1 ), which are not changed during corona treatment, as internal reference, the ratio of (C O)area /ıs (CH2 ) was calculated. The ratio is 0.07 for 5 s exposure and 0.09 for 7 s treatment, which indicates that more oxygen-containing groups are formed on the surface with increasing corona exposure time.
The surface elastic modulus and hardness (Table 3) shows a significant increasing trend up to 5 s (p < 0.05) which then plateaued out or decreased with further corona exposure. The increase in the hardness and modulus of the fibers may be attributed to the surface crosslinking of the polymer chains. Peroxy radicals formed on the surface of the fibers further to corona exposure may cause either crosslinking of the polymer chains or chain scission. Under normal treatment conditions the chain crosslinking reaction is dominant and under severe conditions which polymer receive higher corona energy the chain scission is prevalent [35]. UHMWPE is a semi-crystalline polymer [37,38]. Crosslinking create covalent bonds between neighboring chains primarily in the amorphous region [39] resulting in higher surface modulus and hardness. At higher exposure times, however, chain scission along with the melting of the surface polymer layer may decrease molecular weight and crystallinity of the UHMWPE which consequently decrease the mechanical properties of the polymer (Table 3). Therefore, it can be concluded that although micro-pits formed on fiber surfaces may strengthen fiber–matrix bond, higher exposure may cause great loss of fiber strength. Therefore, a trade-off between the fiber–matrix bond and fiber strength loss should be considered [40].
Fig. 6 – Fracture surface of composite made of (a) fibers with no silane treatment (group 5), (b) fibers treated with 2% silane (group 4). Please cite this article in press as: Bahramian N, et al. Ultra-high-molecular-weight polyethylene fiber reinforced dental composites: Effect of fiber surface treatment on mechanical properties of the composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.05.011
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Placing fibers in the resin toughens the composite through different mechanisms among them are blunting the crack front path or changing the crack direction [41]. The effect of incorporation of the fibrous reinforcements, however, strongly dependent upon interfacial adhesion between the resin and the fiber [42]. In this study composite containing 5 s corona treated fiber exhibited significantly higher flexural strength, flexural modulus, and fracture toughness than other groups (p < 0.05) (Fig. 4). It has been reported that corona treatment of carbon fibers significantly increased the flexural strength of composites made of the fibers [14]. The 5 s corona treatment of the UHMWPE fibers provides micro-pits on the surface of fibers and change the surface chemistry through introducing polar oxygen-containing groups. The former provides micromechanical interlocking between matrix resin and fibers and the latter enhances the wetting of the fibers by resin matrix. Both modifications increase the interfacial adhesion and enhance the ability of the FRC to absorb energy and exhibit higher mechanical properties. SEM fractographs of the FRCs (Fig. 5) also illustrate a good interfacial adhesion between resin matrix and fibers in the composites reinforced with 5 s corona treated fibers while a poor adhesion is observed in the FRC containing untreated fibers. The FRC reinforced with 7 s corona treated fibers not only shows lack of good interfacial adhesion but also reveals the deformation of fibers due to the exposure to high energy corona at longer times. Silane treatment of the fibers resulted in FRCs with higher mechanical properties (p
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Ultra-high-molecular-weight polyethylene fiber reinforced dental composites: Effect of fiber surface treatment on mechanical properties of the composites Niloofar Bahramian a , Mohammad Atai b,∗ , Mohammad Reza Naimi-Jamal c a b c
Department of Biomaterials, Science and Research Branch, Islamic Azad University, Tehran, Iran Iran Polymer and Petrochemical Institute (IPPI), P. O. Box 14965/115, Tehran, Iran Department of Chemistry, Iran University of Science and Technology, 16846 Tehran, Iran
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Poor interfacial adhesion between the fibers and resin matrix in the ultra high
Received 19 December 2014
molecular weight polyethylene (UHMWPE) fiber reinforced composites (FRCs) is the main
Received in revised form
drawback of the composites. This study aims to evaluate the effect of corona and silane
28 April 2015
surface treatment of the fibers on the mechanical properties of the UHMWPE FRCs.
Accepted 25 May 2015
Methods. UHMWPE fibers were exposed to corona discharges for different periods of time
Available online xxx
(0 s, 5 s, 7 s). The surface characteristics of the UHMWPE fibers were investigated by attenuated total reflectance Fourier transformed infrared spectroscopy (ATR-FTIR), atomic force
Keywords:
microscopy (AFM), scanning electron microscopy (SEM) and nanoindentation technique.
Fiber reinforced composites (FRCs)
The flexural strength and flexural modulus of the FRCs made of the treated fibers were
UHMWPE fibers
determined on 2 mm × 2 mm × 25 mm specimens. The fracture toughness (the critical stress
Corona treatment
intensity factor, KIC ) of the composites was also evaluated using a three-point single edge
Mechanical properties
notch beam (SENB) bending technique. Statistical analysis of the data was performed with ANOVA and the Tukey’s post-hoc test. The fiber-resin interface and the fracture surface were investigated using SEM. Results. The change in the surface mechanical properties and chemistry of the corona treated UHMWPE fibers were monitored. The fibers exposed to corona for 5 s showed higher surface nanohardness. In the FRCs, the specimens reinforced with 5 s corona treated silanized fibers showed higher mechanical properties (flexural modulus, flexural strength, and fracture toughness), SEM images revealed a better adhesion between the resin and fibers after 5 s fiber corona treatment and silanization. Significance. Corona and silane treatment of UHMWPE fibers provide dental FRCs with improved mechanical properties. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author. Tel.: +98 2148662446; fax: +98 2144787023. E-mail address: [email protected] (M. Atai).
http://dx.doi.org/10.1016/j.dental.2015.05.011 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Bahramian N, et al. Ultra-high-molecular-weight polyethylene fiber reinforced dental composites: Effect of fiber surface treatment on mechanical properties of the composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.05.011
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1.
Introduction
Development of fiber-reinforced composites (FRCs) have offered dental researchers the possibility of making resinbonded, mechanically and esthetically good, and metalfree restorations for teeth replacement [1,2]. Variety of dental applications, such as bridges, splints, posts, spacemaintainers, orthodontic retainers, denture bases, clasps and connectors, and implant prostheses made of FRCs have successfully been used in dental practice providing acceptable mechanical properties [3,4]. In general, in a FRC, fibers are the main load-carrying members, while the surrounding matrix acts as a load transfer medium and protects the fibers from environmental harms [5]. The mechanical properties of FRCs are dependent upon the mechanical properties of each component, quality of impregnation of fiber with resin, adhesion between the fiber and matrix, quantity of fibers in the matrix resin, the fiber volume fraction, fiber architecture, and the fiber type [6,7]. UHMWPE fibers in comparison with other plastic materials are found to have high impact resistance, toughness, and tensile strength. The fibers are also light weight, corrosion and wear resistant [8,9]. Adhesion of UHMWPE fibers to the matrix resin is, however, difficult because the UHMWPE is non-polar in nature with low surface energy [10]. To provide a good adhesion between the UHMWPE fibers and the polymer matrix, the surface of the fibers has been modified using different methods such as chemical modification [11], chemical grafting [12], corona discharging [13,14], oxygen plasma [15–17], high energy laser [18], UV [19], and gamma irradiation [20]. Improving the adhesion performance of UHMWPE may results in improved dental FRCs. Corona discharge, is a process by which ionized gas species are formed around an active high potential electrode due to the electrical current [21]. In corona treatments of polymers the ionized species transfer their charge to the polymer surface resulting in increase in surface energy by introducing polar groups on the surface. The corona discharge may also change the surface roughness [22]. The process can impart hydrophilicity to polymer surfaces which may improve the surface affinity and sticking strength with some hydrophilic polymers [13]. It can also improve the bonding characteristics of materials by raising the surface energy, wettability, oxygen to carbon ratio, number of functional polar groups on the surface, and specific surface area leading to enhanced adhesive capabilities, increase in the delamination force [23,24] and improvement in the fracture toughness and flexural properties [25]. Silane treatment, on the other side, is a well-known method for improvement of surface wettability and interfacial adhesion between matrix resin and OH-covered substrates [26–28]. Therefore, the combination of these two surface modification methods may result in FRCs with improved properties. This study, consequently, designed to test the hypothesis that the double surface treatment of UHMWPE fibers improves the mechanical properties of an experimental dental FRC containing the fibers as reinforcing phase and Bis-GMA/TEGDMA as matrix.
2.
Materials and methods
2.1.
Materials
Materials used in this study are presented in Table 1.
2.2.
Methods
2.2.1.
Surface corona treatments of UHMWPE fibers
UHMWPE fibers were treated for 5 s and 7 s under atmospheric pressure air corona discharge using SpotTEC corona power Generator (Tantec, Lunderskov, Denmark, 6.5 kV/25 kHz and 550 W). The distance between electrode and UHMWPE fiber surface was 5 cm.
2.2.2.
AFM and nanoindentation analysis
AFM imaging analysis was performed (SPM Dualscope/ Rasterscope C26, DME, Copenhagen/Herlev, Denmark) to study the surface topography and roughness of the fibers before and after corona treatment. Hardness (Hi ) of the fibers was determined using nanoindentaion technique (CSM, Peseux, Switzerland, indentor type: Berkovich, indentor material: diamond, max depth: 500 nm, loading and unloading rates: 1000 nm min−1 ). The elastic modulus of the specimen in depth-sensing indentation techniques, used in nanoindentation test, is determined from the slope of the unloading of the load-displacement response, Ei . Vickers hardness was also calculated from the Hi .
2.2.3.
Fourier-transform infrared spectrometer (FT-IR)
A FTIR spectrometer (EQUINOX55, Specac golden gate, ZnSe prism, Bruker, Germany) was used in order to identify functional groups formed on the fibers after corona and silane treatments. The spectra were collected in the wavenumber range of 4000–400 cm−1 , using attenuated total reflectance technique (ATR-FTIR). In this technique the infrared beam is directed into a crystal of relatively high refractive index (ZnSe) which is in contact with the fibers. Multiple reflections of the beam in the crystal collects the spectrum of the intimate fibers.
2.2.4.
Silanization of UHMWPE fibers
The chemical surface modification of UHMWPE fibers was carried out using a 2 (wt/vol)% ␥-MPS in ethanol/distilled-water (70/30 wt/wt, pH adjusted at 3.8 using acetic acid). Having the silane pre-hydrolyzed for 1 h, the fibers were immersed in the solution and left for 1 week at room temperature to dry.
2.2.5. Composite preparation and mechanical measurements The composites were classified in 5 groups (Table 2). Group 1, group 2 and group 3 were prepared to evaluate the effect of corona exposure time. All these three groups were made of 10 bundles for flexural test and 20 bundles for fracture toughness test. As the volume of the fracture toughness test mold was two times of the flexural test, twice fibers were inserted in the fracture toughness molds to keep the fiber content constant
Please cite this article in press as: Bahramian N, et al. Ultra-high-molecular-weight polyethylene fiber reinforced dental composites: Effect of fiber surface treatment on mechanical properties of the composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.05.011
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Table 1 – Materials used in the study. Country
City
Germany
Krefeld
Evonik
–
Resin monomer
Germany
Krefeld
Evonik
–
Germany
Munich
Sigma–Aldrich
–
Diluent monomer Activator
Germany China
Munich Shanghai
Sigma–Aldrich Surrey Hi-Tech
USA
Pittsburgh
Acros organics
– Roving continuous (SP1200/320) Diameter ≈20 m* 98% Purity
*
Company
Special properties
Role
Name 2,2-Bis-[4(methacryloxypropoxy)phenyl]-propane (Bis-GMA) Triethyleneglycol dimethacrylate (TEGDMA) N-N -dimethyl aminoethyl methacrylate (DMAEMA) Camphorquinone (CQ) Ultra-high-molecularweight polyethylene (UHMWPE)
Initiator Reinforcing fibers
Silane coupling agent
3-(Trimethoxysilyl)propyl methacrylate (␥-MPS)
Diameter is reported based on SEM observation.
in both tests. The fiber corona treatment time was 0 s, 5 s and 7 s for group 1, group 2 and group 3, respectively. The fibers in these three groups were silanized after corona treatment. Group 4 and group 5 were made of 8 fiber bundles for flexural test and 16 fiber bundles for fracture toughness test, corona treatment time was 5 s for both of them. The fibers in group 4 were silanized but the fibers in group 5 received no silane treatment to evaluate the effect of silane treatment.
strength (FS) was calculated in (MPa) using the following formula:
2.2.5.1. Flexural strength and flexural modulus. 0.5 wt% CQ
2.2.5.2. Fracture toughness. To determine the fracture tough-
and 0.5 wt% DMAEMA, as photo-initiator system, were mixed with the matrix resin (Bis-GMA/TEGDMA, 70/30 wt/wt). The fibers were then impregnated with resin and were cut with a sharp surgical blade and placed manually into the mold in unidirectional (UD) position. 5 bar shaped specimens were made of each composite group according to Table 2, using a metal mold with the dimensions of 25 mm × 2 mm × 2 mm. The mold was placed on a glass slide and manually filled with the preimpregnated fibers. A second glass slide was placed on and pressed against it. The composite was cured with a LED dental light source (L.E. Demetron 1, SDS/Kerr, Orange, CA, USA) with power density of 600 mW cm−2 for 40 s in three consecutive points, producing a partial overlapping. After curing, the specimens were polished with abrasive paper 600 and 1000 grits and stored in distilled water at 37 ◦ C (±2 ◦ C) for 24 h. Afterwards, they were submitted to a three point bending test with a universal testing machine (STM 20, Santam, Tehran, Iran) with a crosshead speed of 1 mm min−1 . The flexural
ness (KIC ), rectangular bar single-edge notch beam (SENB) specimens (n = 5) were fabricated in a 5 mm × 2 mm × 25 mm stainless steel mold. The mold was filled with impregnated fibers according to Table 2 and cured using the same procedure as the flexural specimens. A 2.5 mm notch was then cut at the center of each specimen using a 0.6 mm thick diamond wheel cutting machine (Mecatome T 201 A, Oxford, UK). A razor blade was then pressed on the notches to provide a sharp crack. The bending fracture test was performed at a cross-head speed of 1 mm min−1 [29,30] using the universal testing machine and the fracture toughness (critical stress intensity factor, KIC ) was calculated according to the following equation [31]:
FS = 3pL/2bd2 where p is the maximum load (N); L is the span length (mm); b is the width of the specimen (mm) and h, the height (mm). The flexural modulus was calculated from the slope of the initial linear region of stress–strain curve.
KIC = [3PL/2BW 3/2 ]Y
Y = (1.93(a/W)
1/2
− 25.11(a/W)
) − (3.07(a/W)
7/2
3/2
+ 25.8(a/W)
+ 14.53(a/W)
9/2
5/2
)
Table 2 – Characteristics of the studied groups. Groups
1 2 3 4 5
Aim of study
Effect of corona exposure time Effect of corona exposure time Effect of corona exposure time Effect of silane treatment Effect of silane treatment
Fibers corona treatment time (s) 0 5 7 5 5
Silanization of fibers with ␥-MPS Yes Yes Yes Yes No
Number of fiber bundles for (flexural test) 10 10 10 8 8
Number of fiber bundles for (fracture toughness test) 20 20 20 16 16
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Fig. 1 – SEM micrographs of UHMWPE fibers exposed to corona at (a) 0 s, (b) 5 s, and (c) 7 s. The images show the fiber surface details at 30,000× magnification.
Fig. 2 – AFM images show UHMWPE fibers (a) with no corona treatment, (b) treated for 5 s corona discharge, and (c) treated for 7 s corona discharge.
where P is load at fracture (N), L, W, B, and a are length, width, thickness, and notch length (in mm), respectively. The fractured specimens were then observed under scanning electron microscope.
2.2.6.
SEM analysis
silane treatments. The flexural strength, flexural modulus and fracture toughness of the FRCs (groups 1–3, Table 2), are shown in Fig. 4(a and b). The composite containing 5 s corona treated fibers have statistically significant higher flexural strength (225.7 ± 22.2 MPa), flexural modulus (45.2 ± 5.1 GPa) and fracture toughness (6.8 ± 1.2 MPa m1/2 ) among the other groups
The fiber surface after treatment and fracture surfaces of the specimens in the fracture test were observed by (FESEM, Mira 3-XMU, Brno-Kohoutovice, Czech Republic) and (TESCAN, VEGAII, XMU, Brno-Kohoutovice, Czech Republic). The samples were gold coated by a sputter coater before SEM observations.
Statistical analysis
The mechanical properties results were statistically analyzed (at least 5 repeat for each test) using one-way ANOVA and the Tukey’s post-hoc test at the significance level of 0.05.
3.
Results
b c
Transmittance (arb. units)
2.2.7.
a
d e ν(OH)
ν(C=O)
ν(C-O-C), ν(Si-O-Si), ν(Si-O-R)
SEM and AFM microscopic imaging of the UHMWPE fibers show different surface topographies after corona treatment (illustrated in Figs. 1 and 2). The hardness, modulus and roughness of corona treated and untreated fibers are tabulated in Table 3. An increasing and then decreasing trend is observed upon corona discharge duration. ATR-FTIR (Fig. 3) spectrums of the fibers before and after treatments show appearance of functional groups on the fiber surface after corona and
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 3 – ATR-FTIR spectra of UHMWPE fibers (a) untreated fibers, (b) 5 s corona treated, (c) 7 s corona treated, (d) 5 s corona treated and silanized, (e) 7 s corona treated and silanized.
Please cite this article in press as: Bahramian N, et al. Ultra-high-molecular-weight polyethylene fiber reinforced dental composites: Effect of fiber surface treatment on mechanical properties of the composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.05.011
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Table 3 – Surface properties of UHMWPE fibers before and after corona treatment. Corona time (s) 0 5 7
Hi (MPa)
HV
Ei (GPa)
110.4 (38.8)*a 314.1 (102.7)b** 140.0 (20.6)a
10.2(3.6)c 29.1 (9.5)d 13.0 (1.9)cd
7.0 (0.9)e 21.4 (8.0)f 9.1 (6.9)ef
Sq (nm) 90.5 171 43.6
Hi : Indentation Hardness. HV: Vickers Hardness. Ei : Indentation Modulus. Sq: root-mean-square (Rms) (average surface roughness). ∗ Numbers in the parenthesis represent the standard deviations. ∗∗ Means with the same letter are not significantly different at p ≤ 0.05.
Table 4 – Mechanical properties of the composites made of unsilanized and silanized fibers corona treated for 5 s. Silane concentration (wt%) 0 (Group 5) 2 (Group 4) ∗ ∗∗
Flexural strength (MPa)
Flexural modulus (GPa)
Fracture toughness, KIC (MPa m1/2 )
45.9 (9.4)* a 197.2 (26.4)b **
12.8 (3.0)c 39.3 (8.0)d
1.7 (0.7)e 5.9 (1.8)f
Numbers in the parenthesis represent the standard deviations. The letters show significant difference at p ≤ 0.05.
300
a
fracture (Figs. 5 and 6) improvements in the adhesion between fibers and matrix resin is observed after corona and silane treatments.
50 40
200
30
150
20
100
50
10
0
2
4
6
8
Flexural modulus (GPa)
Flexural strength (MPa)
250
0
Corona treatment time (sec)
Fracture toughness, KIC, (MPa.m1/2)
FS
FM
b
8
6
4
2
0
0
2
4
6
8
Corona treatment time (sec)
Fig. 4 – Mechanical properties of the FRCs corona exposure time, (a) flexural strength and flexural modulus, (b) fracture toughness.
(p < 0.05). The FRC with silanized fibers, in Table 4, shows higher flexural strength (197.2 ± 26.4 MPa), flexural modulus (39.3 ± 8.0 GPa) and fracture toughness (5.9 ± 1.8 MPa m1/2 ) compared to FRC made of unsilanized fibers (p < 0.05). According to the SEM images of composite specimens after
4.
Discussion
The characteristics of the interface between the UHMWPE fibers and the polymer matrix is an important factor determining the mechanical properties of UHMWPE fiber-reinforced composites [24]. With the aim of improving the mechanical properties of UHMWPE-based dental FRCs double surface treatment of the fibers were performed using corona discharge and silanization. The results verified the hypothesis that the treatments improved mechanical properties of the composites through improving the interfacial adhesion between the fibers and the matrix resin. The SEM images (Fig. 1a) shows the smooth surface of untreated UHMWPE fibers, only a few shallow longitudinal crevices is seen which are formed during the spinning of the fibers. After corona treatment, the crevices are almost removed but micro-pits are formed on the surface of 5 s corona treated fibers (Fig. 1b). Fig. 1c, however, shows deepening of the crevices after 7 s corona. Up to 5 s the fibers receive enough energy for melting the superficial polymer and disappearing the longitudinal grooves along with the formation of some micro-pits on the surface. At longer exposure times (here 7 s), high corona energy may degrade the polymer or melt the bulk of the polymer leading to the deformation and/or rupture of the fibers which are seen in Fig. 1c. This morphological change may be attributed to excavating the fibers surface [32] and partly broken UHMWPE polymer chains under the high energy corona [33]. The presence of micro-pits on the fiber surface can improve the interfacial adhesion of fiber–matrix through the mechanical interlocking between the micro-pits and the resin [17]. The changes in fiber surface roughness are also presented in Table 3. The AFM images (Fig. 2) clearly show the increasing (Fig. 2b) and decreasing (Fig. 2c) of the surface roughness in 5 s and 7 s corona exposure times, respectively.
Please cite this article in press as: Bahramian N, et al. Ultra-high-molecular-weight polyethylene fiber reinforced dental composites: Effect of fiber surface treatment on mechanical properties of the composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.05.011
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Fig. 5 – Fracture surface of composite made of (a) untreated fibers, (b) 5 s corona treated fibers, (c) 7 s corona treated fibers.
UHMWPE fibers are not adequately bonded to the polymer matrix because of their chemical inertness, low surface energy, and the absence of any polar groups on the surface of fibers. Corona treatment may introduce polar groups into the polymer surfaces and increase the surface energy, substrate wettability, hydrophilicity and adhesion characteristics [10,34]. The main chemical mechanism of corona treatment is oxidation which introduce oxygen containing groups on the fiber surface [34,35]. ATR-FTIR (Fig. 3) spectrums of the treated and untreated fibers illustrates the appearance of stretching vibration of hydroxyl groups ((OH) at 3200–3600 cm−1 ), stretching vibration of carbonyl groups ((C O) at ∼1710 cm−1 ), and stretching vibration of C O C groups (at ∼1100 cm−1 ). These groups change the surface chemistry and surface energy of the fibers [36] which, consequently, affect the adhesion characteristics of the fibers. Considering the absorption peak of in-plane bending (scissoring) vibration of CH2 groups (ıs at 1464 cm−1 ), which are not changed during corona treatment, as internal reference, the ratio of (C O)area /ıs (CH2 ) was calculated. The ratio is 0.07 for 5 s exposure and 0.09 for 7 s treatment, which indicates that more oxygen-containing groups are formed on the surface with increasing corona exposure time.
The surface elastic modulus and hardness (Table 3) shows a significant increasing trend up to 5 s (p < 0.05) which then plateaued out or decreased with further corona exposure. The increase in the hardness and modulus of the fibers may be attributed to the surface crosslinking of the polymer chains. Peroxy radicals formed on the surface of the fibers further to corona exposure may cause either crosslinking of the polymer chains or chain scission. Under normal treatment conditions the chain crosslinking reaction is dominant and under severe conditions which polymer receive higher corona energy the chain scission is prevalent [35]. UHMWPE is a semi-crystalline polymer [37,38]. Crosslinking create covalent bonds between neighboring chains primarily in the amorphous region [39] resulting in higher surface modulus and hardness. At higher exposure times, however, chain scission along with the melting of the surface polymer layer may decrease molecular weight and crystallinity of the UHMWPE which consequently decrease the mechanical properties of the polymer (Table 3). Therefore, it can be concluded that although micro-pits formed on fiber surfaces may strengthen fiber–matrix bond, higher exposure may cause great loss of fiber strength. Therefore, a trade-off between the fiber–matrix bond and fiber strength loss should be considered [40].
Fig. 6 – Fracture surface of composite made of (a) fibers with no silane treatment (group 5), (b) fibers treated with 2% silane (group 4). Please cite this article in press as: Bahramian N, et al. Ultra-high-molecular-weight polyethylene fiber reinforced dental composites: Effect of fiber surface treatment on mechanical properties of the composites. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.05.011
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Placing fibers in the resin toughens the composite through different mechanisms among them are blunting the crack front path or changing the crack direction [41]. The effect of incorporation of the fibrous reinforcements, however, strongly dependent upon interfacial adhesion between the resin and the fiber [42]. In this study composite containing 5 s corona treated fiber exhibited significantly higher flexural strength, flexural modulus, and fracture toughness than other groups (p < 0.05) (Fig. 4). It has been reported that corona treatment of carbon fibers significantly increased the flexural strength of composites made of the fibers [14]. The 5 s corona treatment of the UHMWPE fibers provides micro-pits on the surface of fibers and change the surface chemistry through introducing polar oxygen-containing groups. The former provides micromechanical interlocking between matrix resin and fibers and the latter enhances the wetting of the fibers by resin matrix. Both modifications increase the interfacial adhesion and enhance the ability of the FRC to absorb energy and exhibit higher mechanical properties. SEM fractographs of the FRCs (Fig. 5) also illustrate a good interfacial adhesion between resin matrix and fibers in the composites reinforced with 5 s corona treated fibers while a poor adhesion is observed in the FRC containing untreated fibers. The FRC reinforced with 7 s corona treated fibers not only shows lack of good interfacial adhesion but also reveals the deformation of fibers due to the exposure to high energy corona at longer times. Silane treatment of the fibers resulted in FRCs with higher mechanical properties (p
