Ó 2014 Eur J Oral Sci

Eur J Oral Sci 2015; 123: 53–60 DOI: 10.1111/eos.12167 Printed in Singapore. All rights reserved

European Journal of Oral Sciences

Oxygen inhibition layer of composite resins: effects of layer thickness and surface layer treatment on the interlayer bond strength

Jasmina Bijelic-Donova1, Sufyan Garoushi1,2, Lippo V. J. Lassila1, Pekka K. Vallittu1,3 1

Department of Biomaterials Science and Turku Clinical Biomaterials Centre-TCBC, Institute of Dentistry, University of Turku, Turku, Finland; 2Department of Restorative Dentistry and Periodontology, Institute of Dentistry, Libyan International Medical University, Benghazi, Libya; 3City of Turku Welfare Division, Oral Health Care, Turku, Finland

Bijelic-Donova J, Garoushi S, Lassila LVJ, Vallittu PK. Oxygen inhibition layer of composite resins: effects of layer thickness and surface layer treatment on the interlayer bond strength. Eur J Oral Sci 2015; 123: 53–60. © 2014 Eur J Oral Sci An oxygen inhibition layer develops on surfaces exposed to air during polymerization of particulate filling composite. This study assessed the thickness of the oxygen inhibition layer of short-fiber-reinforced composite in comparison with conventional particulate filling composites. The effect of an oxygen inhibition layer on the shear bond strength of incrementally placed particulate filling composite layers was also evaluated. Four different restorative composites were selected: everX Posterior (a short-fiber-reinforced composite), Z250, SupremeXT, and Silorane. All composites were evaluated regarding the thickness of the oxygen inhibition layer and for shear bond strength. An equal amount of each composite was polymerized in air between two glass plates and the thickness of the oxygen inhibition layer was measured using a stereomicroscope. Cylindrical-shaped specimens were prepared for measurement of shear bond strength by placing incrementally two layers of the same composite material. Before applying the second composite layer, the first increment’s bonding site was treated as follows: grinding with 1,000-grit silicon-carbide (SiC) abrasive paper, or treatment with ethanol or with water-spray. The inhibition depth was lowest (11.6 lm) for water-sprayed Silorane and greatest (22.9 lm) for the watersprayed short-fiber-reinforced composite. The shear bond strength ranged from 5.8 MPa (ground Silorane) to 36.4 MPa (water-sprayed SupremeXT). The presence of an oxygen inhibition layer enhanced the interlayer shear bond strength of all investigated materials, but its absence resulted in cohesive and mixed failures only with the short-fiber-reinforced composite. Thus, more durable adhesion with shortfiber-reinforced composite is expected.

Restorative filling composite resins have been recently improved with regard to the composition of the resin matrix and the shape of the filler particles. Enhancing the physical and mechanical properties of resin composites by incorporation of millimeter-scale short-fiber fillers and a resin matrix encompassing a semi-interpenetrating polymer network (semi-IPN) structure has proved promising (1, 2). Altering the chemical structure of monomers has also been investigated, and recently dimethacrylate-based monomers have been replaced with silorane-based ring-opening polymerization systems (3). This monomer system combines an oxirane ring (which opens during polymerization) and siloxane (that increases the hydrophobicity of the resin composite). The most important difference between the two monomer systems is that dimethacrylates are cured with a free radical, whereas silorane-based composites are cured via cationic polymerization.

Dr Jasmina Bijelic-Donova, TCBC, Institute of €inen Dentistry, University of Turku, Ita €katu 4 B, FI-20520 Turku, Finland Pitka E-mail: [email protected] Key words: fiber-reinforced composite; inhibition depth; shear bond strength; surface treatment; unpolymerized layer Accepted for publication December 2014

The curing reaction in siloranes is a photoinitiated cationic ring-opening polymerization reaction of epoxy monomers with an iodonium salt, an electron donor, and camphorquinone (CQ) as the photoinitiators. The presence of CQ enables the usage of conventional dental-curing units because of the matching light spectrum, whereas the other two initiators are needed to generate reactive cationic species that start the ring-opening polymerization. The irradiation of the photoinitiator induces the fragmentation of the iodonium salt and subsequently an acidic cation is released. This cation protonates the oxirane groups of the monomer and begins cationic polymerization by opening the oxirane ring (3). Although the cationic ring-opening polymerization is insensitive to oxygen (4), the formation of an oxygen inhibition layer on the surface of the freshly polymerized silorane-based composite is still possible (5).

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The polymerization reaction with dimethacrylatebased composite materials induced by light-irradiation leads to the decomposition of the two-component initiating system (CQ and tertiary amine) and results in the generation of reactive free radicals, which are able to add to the double bonds of dimethacrylate groups, thereby creating new radicals. In the propagation reaction, these radicals will react until no additional monomer is able to react or the propagation reaction has terminated (6). Compared with a monomer molecule, atmospheric oxygen has a greater ability to react with the propagating free radicals (7), oxidizing them into stable spaces, known as peroxides, which have low reactivity towards the monomer. This leads to retardation or inhibition of the free-radical polymerization reaction. Consequently, an unpolymerized monomer layer will appear on the surface of the freshly cured resin, when resin is cured in air. This layer is known as the oxygen inhibition layer. The components of the oxygen inhibition layer are similar in composition to those of the uncured resin with consumed or reduced amounts of photoinitiator (7, 8). The oxygen inhibition layer is also known as an unpolymerized (uncured) layer of resin and its thickness as inhibition depth (9), which is affected by numerous factors (5, 10–16). The thickness of the oxygen inhibition layer could influence the interlayer bonding properties of the composite resins because the oxygen inhibition layer is known to: (i) impair the interfacial homogeneity (7); (ii) permit complete interdiffusion of the freshly overlaid composite through the oxygen inhibition zone, if thin (8); and (iii) compromise the mechanical strength, if thick (17). Hence, the thickness of the oxygen inhibition layer is crucial for the integrity of the layer itself and this for the quality of the interlayer connection. In addition, oxygen inhibition raises the problem of interlayer adhesion (18–23). The effect of oxygen on the bonding properties of the newly developed short-fiber-reinforced composite (everX Posterior) is unknown. This short-fiber-reinforced composite is a dimethacrylate-based composite, in which Bis-GMA and TEGDMA are used as monomers and millimeter-scale short-fiber fillers with a semiIPN structure (1, 2) are incorporated into the resin matrix. Fibers are known to restrict the polymerization shrinkage (24, 25), but as a result of the monomer composition, the polymerization reaction still proceeds by free radicals. Thus, formation of an oxygen inhibition layer on the surface of the short-fiber-reinforced composite is expected, although the viscosity of the resin matrix is high. It is also unknown how the short fibers within the composite are affected by oxygen diffusion. It was hypothesized that the oxygen inhibition layer will enhance the shear bond strength of the newly cured upper layer to the cured lower layer both of shortfiber-reinforced composite and that the thickness of the oxygen inhibition layer will give measurable values. Furthermore, Silorane was not expected to develop an inhibition layer and thus no effect was expected on the interlayer shear bond strength between incrementally placed layers of silorane-based composite.

The present investigation included dimethacrylatebased composites (short-fiber-reinforced composite, microhybrid, and nanofilled composite) and low-shrinkage silorane-based composite. The study aims were: (i) to assess variation in the thickness of the oxygen inhibition layer on the surface of the short-fiber-reinforced composite in comparison with freshly polymerized microhybrid-, nanofilled-, and silorane-based composite resins under the same conditions; and (ii) to evaluate the interlayer shear bond strength between successive depositions of same composite material with present, removed, water-spray and ethanol treated oxygen inhibition layer. Fracture mode evaluation of the interlayer surface following the shear bond test was also assessed.

Material and methods Specimen preparation A previously suggested microscopic technique (9, 10, 26, 27) for measuring the depth of oxygen inhibition was used in the present study. Table 1 shows the restorative composite materials investigated and their composition. Three groups were prepared for each material tested (n = 3). Each group differed regarding the treatment used to remove the oxygen inhibition layer from the surface of the polymerized specimen; in addition, there was a control group, in which the oxygen inhibition layer was left intact. An equal amount of each composite resin was applied onto the middle of a horizontally placed glass microscope slide. A constant sample thickness of 0.1 mm was ensured by placing two glass plates (20 mm 9 20 mm), as spacers, on each side of the composite resin, which was covered by another microscope slide as a cover slip. The specimens were then polymerized through the cover slip for 40 s using a light-emitting diode (LED) light-curing unit (Elipar S10; 3M ESPE, St Paul, MN, USA) with a tip diameter of 28 mm, producing an averaged irradiance of 1,200  2.5 mW cm 2 and a wavelength range of 430– 480 nm, with a maximal peak at 455 nm. This method secured formation of the oxygen inhibition layer only at the outer sides of the specimens because air–resin contact was possible only at the resin boundary between the polymerized composite material and the spacers. The groups prepared for this testing were as follows: group 1, the oxygen inhibition layer was not treated (i.e. was left intact); group 2, the oxygen inhibition layer was treated by wiping it from the surface of the cured specimen using alcohol sponges soaked in 99 wt% ethanol (Etax Aa; Altia, Rajam€ aki, Finland) for 20 s and then gently air dried for another 20 s; group 3, the oxygen inhibition layer was treated with water applied as a water spray for 20 s from a distance of ~5 mm perpendicular to the specimen surface and then gently air dried for another 20 s. The depth of the inhibition layer was measured using a stereomicroscope (Wild, Heerbrugg, Switzerland) in 10 locations around the periphery of each specimen at a magnification of 940, with a calibrated micrometer disk. The results were recorded in micrometers between the outer boundary of the specimen and the polymerized–unpolymerized resin interface. In addition, micrographs were taken of each specimen using the computer imaging program, Leica DC Twain (Leica, Cambridge, UK), in order to provide a visual record of the inhibition layer.

Oxygen inhibition of short-fiber composite

55

Table 1 Materials used in the study and their basic composition

Composite

Type of material

Manufacturer

Lot No.

Resin composition

Filler composition

Filler content wt/vol%

everX Posterior

Short-fibre composite

GC, Tokyo, Japan

1307292

Bis-GMA, TEGDMA, PMMA

E-glass fibre, barium borosilicate

74.2/53.6

Filtek Z250

Microhybrid

3M ESPE, St Paul, MN, USA

9BN (A3)

Zirconia/silica

78/60

Filtek Supreme XT

Nanofilled

3M ESPE

9AL (A3)

Bis-GMA, Bis-EMA, UDMA, TEGDMA Bis-GMA, Bis-EMA, UDMA, TEGDMA

Aggregated zirconia/ silica cluster and non-agglomerated/ non-aggregated silica filler

78.5/59.5

Filtek Silorane

Microhybrid silorane

3M ESPE

OFH (A3)

ECHCPMS, BECHEPMS, MBP, MEMSBP, MEFDOSBP, MASBP, BTPFPMEPMPI

Quartz, yttrium trifluoride

76/55

Filler size 1–2 mm of the individual glass fibre and 0.1–2.2 lm of the barium-borosilicate filler 0.01–3.5 lm

20 nm of the silica filler and 0.6–1.4 lm of the Zr/SiO2 cluster particle (5–20 nm of the primary particle) 0.1–2.0 lm

BECHEPMS, bis-3,4-epoxycyclohexylethyl-phenyl-methylsilane; Bis-EMA, bisphenol-A-dyethoxy dimethacrylate; Bis-GMA, bisphenolA-glycidyl dimethacrylate; BTPFPMEPMPI, borate(1-),tetrakis(pentafluorophenyl)-[4-(methylethyl)phenyl](4-methylphenyl)iodonium; DMAEMA, dimethylaminoethyl methacrylate; ECHCPMS, 3,4-epoxycyclohexylcyclo-polymethylsiloxane; MASBP, a mixture of alpha-substituted by-products; MBP, a mixture of other by-products; MEFDOSBP, a mixture of epoxyfunctional di- and oligo-siloxane by-products; MEMSBP, a mixture of epoxy-mono-silanole by-products; PMMA, polymethylmethacrylate; TEGDMA, triethylene-glycol dimethacrylate; UDMA, urethane dimethacrylate.

Interlayer shear bond strength The shear bond strength of each composite material listed in Table 1 was investigated. In addition to the groups described above (groups 1–3), one further group (group 4) was included in this investigation. In group 4, the oxygen inhibition layer was removed by grinding the surface with 1,000-grit Federation of European Procedures of Abrasives (FEPA) silicon carbide (SiC) abrasive grinding paper (Struers, Copenhagen, Denmark) at 250 r.p.m. under water cooling using an automatic grinding machine (LaboPol-25; Struers). After grinding, the surface was gently dried with air-spray for 20 s. In this experiment, the same restorative material was used as the substrate and the adherent material. The corresponding unpolymerized composite restorative material was inserted into the round-shape retentive cavity (of 5 mm diameter and 3 mm depth) prepared in an acrylic resin block, flattened, and then light-cured for 40 s with an LED polymerization light. Following polymerization, the adherent material was applied onto the substrate in an increment of 2 mm using a translucent polyethylene mold with an inner diameter of 3.6 mm and then polymerized for 40 s. The adherent material was polymerized on the composite surface that had been cured in air and for which the oxygen inhibition layer had been untreated (group 1), treated with ethanol (group 2) or water spray (group 3), or removed (group 4). Twelve specimens were prepared for each group. The specimens were either dry stored at 37°C for 7 days or

thermocycled (alternating immersion of the samples in distilled water of a temperature of 5° and 55°C) for 6,000 cycles, with a dwell time of 30 s and a transfer time of 5 s before testing. The specimens were stored in distilled water for 48 h at room temperature (23  1°C) before thermocycling and were tested immediately afterwards. The shear bond strength test was performed using a universal testing machine (Lloyd; Lloyd Instruments, Fareham, UK) at room temperature (23  1°C) and the results were recorded using PC software (Nexygen; Lloyd Instruments). The specimens were mounted in a mounting jig (Bencor Multi-T shear assembly; Danville Engineering, San Remon, CA, USA) with the shearing rod against and parallel to the flat prepared substrate sites. A circular edge blade created the shear type load positioned over the interface between the substrate and the adherent material at a crosshead speed of 1.0 mm min 1 until fracture. The shear load at failure was recorded in N and converted to MPa as a function of the area under test, automatically by the software. Shear bond strength values are presented in MPa. In order to analyse and to determine the fracture type, all fractured surfaces were visually examined under light microscopy at a magnification of 940. Statistical analysis The data were statistically analysed using ANOVA at a significance level of P < 0.05 with SPSS version 19 (Statistical

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Package for Social Sciences, SPSS, Chicago, IL, USA). Tukey’s post-hoc analysis was performed following ANOVA to determine differences among the groups.

Results

inhibition region (i), the diffusion (i.e. transition) zone (d), and the polymerized region (p) were observed for all specimens, regardless of the surface treatment. The oxygen inhibition layer was present on the surfaces of all dimethacrylate-based composites (Fig. 1A–C) and also on the surface of the silorane-based composite (Fig. 1D).

Thickness of the oxygen inhibition layer

Optically measurable thickness of the oxygen inhibition layer was evaluated for all materials tested. The results are presented in Table 2. The inhibition depth ranged from 11.6  2.4 lm for the water-spray-treated silorane-based composite to 22.9  4.7 lm for the waterspray-treated short-fiber-reinforced composite. The grinding procedure probably removed the oxygen inhibition layer completely. For this reason the oxygen inhibition layer thickness for all specimens of group 4, irrespective of the material, was assumed to be zero. Two-way ANOVA showed that both the type of material and the surface treatment had significant effects (P < 0.001) on the thickness of the oxygen inhibition layer. Although differences were observed among the materials, treatment with ethanol and water spray did not affect the inhibition depth when each composite material was observed individually. An exception was that the water spray cleaned surface of the microhybrid composite, which showed a statistically significant difference compared with its control and ethanoltreated counterparts. Figure 1 shows the oxygen inhibition layer detected on the surfaces of the investigated materials after ethanol treatment. Distinct lines between the oxygen

Interlayer shear bond strength

The results of the shear bond strength investigation between incrementally placed composite layers are presented in Fig. 2. A three-way ANOVA (material type, surface treatment, and storage condition) revealed that only the composite material type and the surface treatment had significant effects on the interlayer shear bond strength (P < 0.001). The difference between the dry and the thermocycled specimens was not statistically significant (P > 0.001). Furthermore, no difference was observed between the ethanol and the water-spray treatment, compared with each other or with their control counterparts, for any of the tested material observed individually. All tested materials demonstrated lower interlayer shear bond strength when the cured surface of the underlayered composite was ground with SiC paper. For dry-stored specimens, the shear bond strength values ranged from 5.8  1.8 MPa for silorane-based composite with a ground interface to 36.4  7.6 MPa for microhybrid composite with a water-spray-treated interface. The thermocycled specimens exhibited greater shear bond strength values, ranging from 8.3  3.8 MPa for silorane-based composite with a mechanically ground interface to 42.6  8.2 MPa for the untreated interface

Table 2 Thickness of the oxygen inhibition layer of the tested materials

Material everX Posterior

Filtek Z250

Filtek Supreme XT

Filtek Silorane

Group (surface treatment) Group Group Group Group Group Group Group Group Group Group Group Group Group Group Group Group

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

(untreated surface) (ethanol-treated surface) (water-spray-treated surface) (ground surface) (untreated surface) (ethanol-treated surface) (water-spray-treated surface) (ground surface) (untreated surface) (ethanol-treated surface) (water-spray-treated surface) (ground surface) (untreated surface) (ethanol-treated surface) (water-spray-treated surface) (ground surface)

Group abbreviation Ex-1 Ex-2 Ex-3 Ex-4 FZ-1 FZ-2 FZ-3 FZ-4 FS-1 FS-2 FS-3 FS-4 FSi-1 FSi-2 FSi-3 FSi-4

Thickness of the oxygen inhibition layer (lm) 20.1 20.8 22.9 – 14.5 15.2 18.8 – 17.1 19.2 18.5 – 13.8 12.0 11.6 –

(5.5)* (5.7)* (4.7)* (2.6)a (4.2)a (3.0)b (3.0)+ (3.1)+ (2.6)+ (3.4)# (2.2)# (2.4)#

Values are given as mean (SD). The grinding procedure was assumed to remove the oxygen inhibition layer from the composites’ surfaces. Thus, the oxygen inhibition layer thickness for all specimens in group 4 was assumed to be zero. The same superscript letters or symbols within a value represent a homogenous subset (P > 0.05) among the groups for each material individually.

Oxygen inhibition of short-fiber composite A

57

portions of the substrate (base) and interfacial surface in the fracture surface].

B

Discussion

C

D

Fig. 1. Microscopy images (940 magnification) of the inhibited zone from: (A) nanofilled composite (Filtek Supreme XT); (B) microhybrid composite (Filtek Z250); (C) shortfiber-reinforced composite (everX Posterior); and (D) siloranebased composite (Filtek Silorane) (all after ethanol treatment). The lower-case letters denote the inhibited layer (i), the diffusion (i.e. the transition) zone (d), and the polymerized material (p).

of the microhybrid composite. The higher bond-strength values obtained following thermal aging could be ascribed to the post-curing allowed by the prestorage of specimens in distilled water for 48 h at room temperature (23  1°C) before subjecting them to the thermal cycling procedure. Failure mode analysis (Fig. 3) showed three fracture types: adhesive (including breaks at the interface); cohesive (including failures within the substrate or the adherent composite); and mixed [including small

When composites are cured in air, as in clinical practice, an oxygen inhibition layer is formed on the surface of the freshly cured composite resin. In the present study, the inhibition of polymerization by oxygen was determined as the thickness of the low-polymerized outer layer on composite resin specimens, which were cured in the presence of air and at ambient temperature. The differences observed among the dimethacrylate-based composites could be a result of the filler content (13) and type (13, 15), which, in addition to the network density of the resin composite (10, 11), determined by the content of the diluent (TEGDMA) (5), also affect the oxygen inhibition depth. However, when a composite is reinforced with fibers, then the fibers and their orientation should also be considered as factors influencing the oxygen inhibition depth (27). The short-fiber composite used in the present investigation consisted of a cross-linked polymer network derived from dimethacrylate monomers (Bis-GMA and TEGDMA) and PMMA. During the polymerization process, this polymer matrix forms a semi-IPN structure (28). The differences between the short-fiber-reinforced composite and both dimethacrylate-based composites may be explained by the presence of fibers in shortfiber-reinforced composite, which are shown to affect the oxygen inhibition depth (27) and the internal voidspace formation (29). Orientation of the fibers in the previous study favoured the passage of oxygen (27), but it should be noted that the short-fiber-reinforced composite used in this study had randomly oriented short fibers, with length varying between 1 and 2 mm, and their influence on the oxygen inhibition depth could be less than in composites with oriented fibers.

Shear bond strength (MPa)

60

50 a

40 30

a b

20 c 10

0

Ex-1 Ex-2 Ex-3 Ex-4

FZ-1 FZ-2 FZ-3 FZ-4

FS-1 FS-2 FS-3 FS-4

FSi-1 FSi-2 FSi-3 FSi-4

Fig. 2. Shear bond strength of incrementally placed composite layers, with the oxygen inhibition layer present or removed, after dry storage (dark-grey bars) and thermal cycling (light-grey bars). The horizontal lines and the same superscript letters above the bars represent homogenous subsets (P > 0.05) among the groups. Ex, everX Posterior; FS, Filtek Supreme XT; FSi, Filtek Silorane; FZ, Filtek Z250. The numbers indicate the type of treatment applied to the oxygen inhibition layer on the polymerized specimen: 1, untreated; 2, ethanol treatment; 3, water-spray treatment; 4, ground.

58

Bijelic-Donova et al.

Failure type distribuon of specimens (%)

100 90 80 70 60 50 40 30 20 10 0 dry TC

Ex-1

dry TC

Ex-2

dry TC

Ex-3

dry TC

Ex-4

dry TC

FZ-1

dry TC

FZ-2

dry TC

FZ-3

dry TC

FZ-4

dry TC

FS-1

dry TC

FS-2

dry TC

FS-3

dry TC

FS-4

dry TC

FSi-1

dry TC

FSi-2

dry TC

dry TC

FSi-3 FSi-4

Fig. 3. Failure type distribution of specimens after the shear bond test. A significant finding was the high frequency of cohesive and mixed breaks (75%) with the everX Posterior applied on the surface of the ground substrate [i.e. that in which the oxygen inhibition layer has been removed (Ex-4)]. The corresponding treatment for the other materials (FZ-4, FS-4, and FSi-4) resulted in predominantly adhesive failures. Yellow bars indicate adhesive failures, grey bars indicate mixed (adhesive and cohesive) failures, and black bars indicate cohesive failures. Ex, everX Posterior; FS, Filtek Supreme XT; FSi, Filtek Silorane; FZ, Filtek Z250. The numbers indicate the type of treatment applied to the oxygen inhibition layer on the polymerized specimen: 1, untreated; 2, ethanol treatment; 3, water-spray treatment; 4, ground.

The oxygen inhibition layer was optically observed in this study (Fig. 1) for both dimethacrylate- and silorane-based composites. The generation of radicals with silorane composite is possible because of the presence of CQ. The absorption of light by CQ leads to an excited CQ formation and initiates an electron-transfer photosensitization reaction, which reduces the iodonium salt. Consequently, an iodonium salt free radical and a photosensitizer (CQ) cation-radical are yielded. The latter usually generates a strong acid, whereas the former decomposes (30). The acid generated in the further reaction donates a proton (hydrogen cation) to the epoxy ring, which is unstable and prone to combine with available hydrogen, therefore opening the ring (31). By protonating the oxirane groups, the acidic cation initiates the cationic polymerization. In other words, the polymerization of silorane-based composite involves two stages: an initial free-radical phase and a cationic phase. The radical concentration is significantly reduced (but not completely eliminated), which consequently leads to formation of a remarkably thinner oxygen inhibition layer on the surface of the siloranebased composite than observed with dimethacrylatebased composites. The finding of the present study is in agreement with the finding of SHAWKAT et al. (5). The differences observed between the two types of composites (dimethacrylate vs. silorane) could be attributed to distinct differences in their composition. Besides the filler content, which is lower for the silorane-based composite, other differences include: (i) the resin phase [i.e. organic matrix (dimethacrylate-based vs. epoxy based)]; (ii) type of initiator system [i.e. type and concentration of photoinitiator (CQ and tertiary amine vs. CQ, iodonium salt and electron donor)]; and (iii) the type of polymerization reaction (radical vs. cationic ring-opening). The specific characteristics of the silorane-based organic matrix and the epoxy-functional

silane agent restrict the use of particular fillers, resulting in a formulation with decreased filler content and distinct filler morphology (32). The irregularly shaped filler particles and the lower filler load might limit the wettability, which alters the thickness of the oxygen inhibition layer (33, 34). This, in addition to the silorane chemistry (expoxy-based ring opening system), could explain the variations of the effect of the oxygen inhibition layer on the interlayer shear bond strength within the groups for the silorane-based composite and its differences from the dimethacrylate-based composites. Oxygen inhibition layer-free surfaces have been produced for research purposes by curing the specimens at elevated temperatures (13), or in air-free argon (17, 22), nitrogen (5, 8, 21, 23), or carbon dioxide (15, 16) atmospheres. Water spray and ethanol treatments were used in the present study as momentary treatments for cleaning the surface of the oxygen inhibition layer, because they are clinically more practical methods. All dimethacrylate-based composites studied showed an insignificantly thicker oxygen inhibition layer after both treatments, which means that those treatments did not remove the oxygen inhibition layer from the exposed surface. The silorane-based composite exhibited a thinner oxygen inhibition layer after both types of surface treatment. This could be a result of the aforementioned explanation regarding the silorane chemistry, its filler content and morphology, and also the more hydrophobic nature of silorane’s resin matrix than the dimethacrylates’ resin matrix. Both the composite material and the surface layer treatment affected the interlayer shear bond strength. The outcome for both composite types was improved interlayer shear bond strength when the oxygen inhibition layer was present. This finding supports the influence of the physical surface properties of the oxygen inhibition layer on the bond strength between incrementally placed

Oxygen inhibition of short-fiber composite

composite layers and also its influence on the failure mode. Therefore, it can be suggested that the surface wettability provided by the oxygen inhibition layer is crucial for adhesion of the adherent surface. Factors influencing wettability, such as the surface free-energy of the solid and the surface tension of the liquid, have been discussed in detail elsewhere (33, 34). The finding of the present investigation is in accordance with studies (33, 34) which showed that both the physical and chemical surface properties of the oxygen inhibition layer depend on its thickness; if relatively thin, as in this study, the oxygen inhibition layer allows diffusion of the photoinitiator into the overlaying composite, thus improving the bond strength. In addition, owing to the presence of a proton donor, the oxygen inhibition layer mediates higher bond strengths to both enamel (34) and dentin (33). The ethanol and the water-spray treatments were applied for a controlled time of 20 s, and adverse bonding effects were not observed following either surface treatment. Cleaning the surface of the oxygen inhibition layer with ethanol and water-spray may have extracted some unpolymerized monomers. This might have influenced the bond strength, which was also evident in the more frequent occurrence of cohesive breaks observed with these groups. Predominantly cohesive fractures were observed for the ground short-fiber-reinforced composite surface (i.e. the oxygen inhibition layer was completely removed). This could be a result of the micro-mechanical interlock between the monomer from the overlaying composite and the fibers of the underlaying composite, which were exposed during the grinding procedure. In other words, the resin–fiber interaction was enabled at the interface of the successively placed short-fiber-reinforced composite layers, although the oxygen inhibition layer was removed by grinding. The ground surface resulted in the lowest interlayer bond strength in all groups, and adhesive failures were typical for the rest of the particulate filler composites with ground surfaces, irrespective of the storage condition. The grinding procedure has probably eliminated the oxygen inhibition layer and compromised the bond strength, owing to an inadequate wetting of resin to the fillers (18, 35). In general, our results are in agreement with the majority of other studies (14, 18, 20), in which the presence of an oxygen inhibition layer with adequate thickness at the surface between the adjacent composite layers improved the interfacial bonding. However, ELIADES & CAPUTO (7) reported inefficient polymerization within the oxygen inhibition layer that reduced the interfacial strength, which contradicts our results. The authors suggested that in order to achieve a strong bond with a new composite layer, this catalyst-free monomer layer should be removed. The silorane-based composite exhibited the lowest bond strength, which could be a result of the lower filler load as well as of the formation of a thin oxygen inhibition layer that led to defective interlayer connection. The present investigation supports acceptance of the first hypothesis regarding the influence of the oxygen

59

inhibition layer on the bonding properties of the shortfiber-reinforced composite and rejection of the second hypothesis regarding the influence of the oxygen inhibition layer on the bonding properties of the siloranebased composite. The presence of an oxygen inhibition layer on the surface of the cured short-fiber-reinforced composite underlayer improved the bond strength to the adjacent short-fiber-reinforced composite layer and the inhibition depth gave measurable results. In contrast to the second hypothesis, the oxygen inhibition layer was also required for improving the interlayer bond strength with the silorane-based composite and its absence caused significant reduction of the bond strength values, from 28.8 (7.8) MPa to 5.8 (1.8) MPa. In conclusion, the oxygen inhibition layer, acting as an intermediate layer, is retained on the surface of the composite after treatment with either ethanol or waterspray. The water-spray treatment would probably have no effect on removal of the oxygen inhibition layer, because of the oxygen dissolved in water, whereas treatment with ethanol could remove the oxygen inhibition layer, but probably a longer application time is needed. The presence of an oxygen inhibition layer improved the interlayer shear bond strength of adjacent composite layers and led to more durable adhesion, whereas the absence of an oxygen inhibition layer adversely affected the bond strength and led to adhesive interfacial failures. Hence, the oxygen inhibition layer should be provided and left intact after polymerization. Similarly to conventional composites, the oxygen inhibition layer also remained on the surface of the short-fiber-reinforced composite and did not adversely affect the bond strength to a successive layer of shortfiber-reinforced composite. The interlayer bond strength of short-fiber-reinforced composite was within the same range as that observed for the conventional dimethacrylate-based filling composites. However, the shortfiber-reinforced composite showed a tendency for cohesive breaks, regardless of whether the oxygen inhibition layer was present or removed. This finding supports the clinical reliability of adhesion of the short-fiber-reinforced composite and more durable adhesion should be expected with this composite material. Acknowledgements – The authors thank Adjunct Professor Niko Moritz for technical assistance with the figures and Robert M. Badeau, PhD, for the English language editing of this manuscript. We are also grateful to the 3M ESPE and GC for providing the materials used in this investigation. This study is part of the BioCity Turku Biomaterials Research Program (www.biomaterials.utu.fi). Conflicts of interest – All authors have agreed with the concept of the manuscript and they confirm that there is no economical benefit or any financial interest to report. Authors do not have conflicts of interests. Author Vallittu is a consultant to the Stick Tech Ltd., a member of the GC Group.

References 1. GAROUSHI S, VALLITTU PK, LASSILA LVJ. Short glass fiber reinforced restorative composite resin with semi-inter penetrating polymer network matrix. Dent Mater 2007; 23: 1356–1362.

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