The effect of fiber orientation on the polymerization shrinkage strain of fiber-reinforced composites

Dental Materials (2006) 22, 610–616

www.intl.elsevierhealth.com/journals/dema

The effect of fiber orientation on the polymerization shrinkage strain of fiber-reinforced composites A. Tezvergil*, L.V.J. Lassila, P.K. Vallittu Department of Prosthetic Dentistry and Biomaterials Science, Institute of Dentistry, University of Turku, ¨isenkatu 2, FIN-20520 Turku, Finland Lemminka Received 31 March 2005; accepted 16 May 2005

KEYWORDS Fiber-reinforced composite; Polymerization Shrinkage; Strain gage; Resin composite

Summary Objective. The aim of this study was to characterize the linear polymerization shrinkage strain of glass fiber-reinforced composite (FRC) according to the fiber orientation. Methods. Test specimens (nZ5) (10.0!10.0!1.5 mm) were prepared from different brands of photopolymerizable resin-preimpregnated FRC; unidirectional continuous FRC, experimental random-oriented FRC, and bidirectional continuous FRC. As control materials, particulate filler composite resin and unfilled dimethacrylate monomer resin were used. Two uniaxial strain gages (gage length 2 mm) were used to measure shrinkage strains in two directions: longitudinally and transversally to the fiber direction. The uncured composite or resin was placed on top of the strain gages, covered with a separating sheet and a glass plate, and irradiated for 40 s with a light-curing unit. The shrinkage strain was monitored for 300 s. ANOVA and Tukey’s posthoc test were used at a significance level of 0.05. Results. ANOVA revealed that orientation of fiber and brand of material had a significant effect (P!0.05) on shrinkage strain. The unidirectional FRC revealed no shrinkage longitudinally to the fiber direction, whereas the shrinkage occurred transversally to the fiber direction. Particulate filler composite resin and unfilled resin revealed equal shrinkage strain in both of the measured directions. Significance. Anisotropic nature of FRC exists with regard to polymerization shrinkage strain. The variation of polymerization shrinkage strains of FRC compared to those of particulate filler composites and unfilled resin might be important for future clinical applications. Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Introduction * Corresponding author. Tel.: C358 2 333 83 75; fax: C358 2 333 83 90. E-mail address: [email protected] (A. Tezvergil).

In the last decade, the use of composite resin restorative materials has been widely accepted in

0109-5641/$ - see front matter Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2005.05.017

Polymerization shrinkage strain of FRC dental practice [1]. The clinical performance of composites has been improved markedly through modifications in formulation. However, they are still far from ideal, because of the limitations, including mechanical deficiencies, polymerization shrinkage and susceptibility to degradation in the oral environment [2,3]. Therefore, a search for high-strength materials with esthetic properties has been carried out by adding specifically oriented fillers such as fibers to the matrix [4–6]. Resinimpregnated fiber-reinforced composites (FRC) have been shown to possess adequate flexural modulus and flexural strength to function successfully in the oral cavity. The improvement in their handling properties and preimpregnation with light polymerizable resin have extended the use of FRC material in direct chairside applications [5,7] for minimal invasive restorations. There are two main types of resin systems that have been used in the impregnation of fibers. One is based on the use of photopolymerizable dimethacrylate monomers, and the other on a combination of dimethacrylate monomer resin and linear polymer, which forms a semi-interpenetrating polymer network (semi-IPN) after polymerization [8,9]. The dimensional stability of the composites is affected to a large extent by the polymerization reaction of the matrix phase. The polymerization of composites is usually based on free radical polymerization of bifunctional dimethacrylate monomers, which inevitably results in shrinkage of the composite [3,10]. This reaction produces a gelation, in which the material is transformed from the liquid form of the plastic phase into a solid polymer network that is formed throughout the material [3]. Shrinkage strain arises during situations where the material is either unconstrained or semi-constrained by bonding, whereas shrinkage stress occurs when the material is bonded and not free to shrink [11]. During shrinkage, the stresses are distributed to the adhesive and bonded tissues, which can result in damage to the host tissue or to the interface, or to the restoration itself, thereby leading to early failure of the system [12]. Many factors affect the development of shrinkage in dental composites, including matrix composition, filler content, degree of conversion and polymerization methods [3,13,14]. Particulate filler composites (PFC), such as restorative composites and veneering composites are isotropic, having no specific filler orientation in their structure. Due to isotropicity, the properties of PFC are the same in all directions. In contrast, the properties of FRC are related to the fiber direction. Continuous unidirectional FRC gives anisotropic properties, whereas bidirectional fibers (weaves) give

611 orthotropic properties for the material in plane, and randomly oriented fibers (chopped fibers) give isotropic properties in plane or even three-dimensionally [15]. Previously, the fiber orientation was shown to have a pronounced effect on the mechanical properties [16], bonding properties [17] and thermal expansion of the FRC [18]. However, to the authors’ knowledge, no studies have been undertaken to determine the effect of fiber orientation on the polymerization shrinkage strain of dental FRCs. The purpose of this investigation was to characterize the linear polymerization shrinkage strain of FRCs according to the fiber direction, and compare it to that of commercially available particulate filler composites and unfilled resin.

Materials and methods The materials used in this investigation, including two PFCs, six different preimpregnated FRCs and an unfilled resin, are listed in Table 1. The FRC materials used were unidirectional continuous FRC: Stick (Group SC), Vectris Pontic (Group VP), everStick (Group ES), experimental randomoriented wool FRC (Group EW), and bidirectional FRCs: everStickNet (EN), StickNet (Group SN). Among the FRC materials used in this study, four were resin preimpregnated with photopolymerizable dimethacrylate monomer systems (Groups, ES, VP, EW, and EN) and two (Groups SN, SC) were preimpregnated with porous polymethylmethacrylate (PMMA), followed by further impregnation with a photopolymerizable dimethacrylate monomer resin system [8]. Further impregnation time of the PMMA-preimpregnated fibers with the dimethacrylate resin monomer system (SR) was 24 h, during which time the fibers were stored in a dark container. As control materials, the unfilled photopolymerizable lightcuring resin (Group SR), hybrid type particulate filler composite resin (Group ZO), and a flowable particulate filler composite resin (Group TF) were used. The polymerization shrinkage strain was monitored using the strain gage method. This method was previously described by Sakaguchi [19–21]. The uncured materials were placed in a silicon mould (10.0!10.0!1.5 mm) on top of two uniaxial strain gages as shown in the schematic figure (Fig. 1). The uniaxial strain gages (KFG-2N-120, Kyowa Ltd, gage length 2 mm) were used to measure shrinkage strains in two perpendicular directions: longitudinally and transversally to the fiber direction.

612 Table 1

A. Tezvergil et al. The materials used for this study.

Materials

Code

Manufacturer

Composition

Material type

Stick

SC

E-glass, PMMA

EverStick

ES

Vectris Pontic

VP

Polymer-preimpregnated continuous unidirectional FRC Resin-preimpregnated continuous unidirectional FRC Resin-prereimpregnated continuous unidirectional FRC

EverStickNet

EN

Wool FRC

EW

StickNet

SN

Stick Resin

SR

Tetric flow

TF

Filtek Z250

ZO

StickTech Ltd, Turku, Finland StickTech Ltd, Turku, Finland Ivoclar Vivadent AG, Schaan, Liechtenstein StickTech, Turku, Finland StickTech, Turku, Finland StickTech Ltd, Turku, Finland StickTech, Turku, Finland Ivoclar Vivadent, Schaan, Liechtenstein 3M, St Paul, MN, USA

E-glass, PMMA, bisGMA E-glass, bisGMA, TEGDMA PMMA, bisGMA, E-glass fibres PMMA, bisGMA, E-glass fibers E-glass, PMMA bisGMA, TEGDMA

Resin-preimpregnated continuous bidirectionally oriented FRC Resin-prereimpregnated randomly oriented FRC Polymer-preimpregnated continuous bidirectionally oriented FRC Unfilled resin

bisGMA, UDMA, bisEMA 39.7 vol% fillers

Flow type particulate filler composite resin

bisGMA, UDMA, bisEMA 60 vol% fillers

Hybrid type particulate filler composite resin

bisGMA, Bisphenol-A-glycidyl dimethacrylate; TEGDMA, triethylene glycol dimethacrylate; UDMA, urethane dimethacrylate; bisEMA, bisphenol-A-polyethylene glycol diether dimethacrylate; PMMA, polymethylmethacrylate; FRC, fiber-reinforced composite; Eglass, electrical glass fibres, silanated.

The materials were placed on the polyimide backing of the strain gages on the opposite side of the electrical resistance foil without any adhesive. The adhesion between the composite paste and the strain gage was previously shown to be sufficient to transfer all the contraction strain from the composite to the gage [21]. Polymerization shrinkage data were acquired from two strain gages using a strain measurement module (PCD-300A, Kyowa Ltd), which had been initially balanced at zero. The sampling rate of the module was 10 Hz. Data collection started 5 s before the start of polymerization and continued for 300 s. Both upper and lower surfaces of the specimens were covered with a separating sheet and a glass plate and irradiated for 40 s with a hand-held light-curing unit (Optilux 501, Kerr) at 740 mW/cm2 light intensity. The lightcuring tip (14 mm diameter) was maintained at a 2 mm distance above the glass slide with the use of a reference plate. Five test specimens were tested for each group. The initial temperature of the specimens and devices was about 23 8C. The shrinkage strain data at 300 s were evaluated using analysis of variance (ANOVA) and Tukey’s posthoc test at the 95% significance level (P!0.05) with SPSS (Statistical Package for Statistical Science 11.0, SPSS, Inc., Chicago, IL, USA) to establish the effect of brand and fiber orientation.

Results The ANOVA revealed that both orientation of the fibers and the brand of the material had

Figure 1

Illustration of experimental test set-up.

0.03 (0.03) 0.03 (0.01) K0.01 (0.02) 0.14(0.01) K0.01 (0.01)

VP

1.01 (0.07) K0.05 (0.04)

ES 1.90(0.08)

Isotropic Transverse Longitudinal

0.90(0.09)

0.55(0.05)

SC ZO TF SR

The average shrinkage (%) of the tested materials.

Mean shrinkage strain (SD) (%)

Table 2

613

Figure 2 Linear shrinkage strain (mstrain) of continuous unidirectional FRC materials. 0.60 (0.07) K0.02 (0.04)

SN EN

EW

Polymerization shrinkage strain of FRC

a significant effect (P!0.05) on the shrinkage strain. The mean shrinkage strain percentages (%) and standard deviations (SD) after 300 s are listed in Table 2. The mean strain vs. time curves calculated for unidirectional continuous FRC materials are shown in Fig. 2. At the beginning of polymerization, minor expansion of the material was observed in all groups (Figs. 2–4). At the time point of switching off the light-curing device, a small change in the graph could be observed (Figs. 2–4). Continuous unidirectional FRCs revealed slight expansion longitudinally to the fiber direction, whereas shrinkage of the FRC occurred at the transverse direction (Fig. 2). Even the different brands of three continuous unidirectional FRC materials revealed similar behavior; the absolute values varied among the groups. One of the FRC groups with bidirectionally oriented fibers (EN) showed an expansion followed by a slight shrinkage, whereas the other bidirectional material (SN) revealed higher shrinkage values following expansion (Fig. 3). FRC with randomly oriented fibers (EW) showed higher shrinkage compared to EN and SN (Fig. 3).

Figure 3 Linear shrinkage strain (mstrain) of FRCs with continuous bidirectionally and randomly oriented fibers.

614

A. Tezvergil et al. 1000

–1000 0

50

100

150

200

250

300

–3000

µ-strain

–5000 –7000 –9000 –11000 –13000

ZO TF SR

–15000 –17000 –19000 –21000

Time (s)

Figure 4 Linear shrinkage strain (mstrain) of particulate filler composite resins and unfilled resin.

Among the tested materials, the unfilled resin showed the highest shrinkage strain (2.0%) 5 min after the beginning of photopolymerization (Fig. 4). FRC showed significantly lower shrinkage in the longitudinal direction compared to the particulate filler composite resin (P!0.05). However, the transverse shrinkage of the FRC showed variation depending on the brand. Among the unidirectional fibers, the highest shrinkage in the transverse direction was found in SC groups and the lowest with ES fibers.

Discussion Low polymerization shrinkage is one of the key issues in achieving the dimensional accuracy, avoiding microleakage, debonding, and improving the marginal integrity and longevity of the restoration [22]. The direct chairside application of FRC requires the placement of FRC in close contact to tooth tissue and in situ polymerization of the material by chairside hand-held light-curing units. During polymerization, strains are produced by the forces that arise along from the polymerization shrinkage of resin matrix. Thus, shrinkage behavior characterization of FRC is essential in order to understand the dynamics at the adhesion interface. Various methods had been used to measure the shrinkage of resin-based materials including dilatometry, bonded disc, dynamic mechanical analysis, linear displacement, and strain gage methods [3, 11,19,23]. Previously, it was shown that different methods gave significantly different shrinkage strain magnitudes. These differences were due to the test configuration, the specimen constraint factor, and instrument compliance [3,23]. The strain gage method is reported to measure postgel shrinkage, as the strain gage can only be deformed by the composite, when it has obtained sufficient elastic rigidity to transfer shrinkage

stresses [19]. Parametric studies of prepolymerized and photoinitiator-free materials have proved that strain gages can be used effectively for the qualification and quantification of post-gel shrinkage [20,21]. In the current study, due to the anisotropic nature of the FRC material, the strain gage method was preferred. The other methods provide information assuming the volumetric shrinkage of the material to be the same in all directions; however, the strain gage measures the dimensional change along a single axis. By placing two strain gages perpendicular to each other, it was possible to see the changes separately and simultaneously in two directions. Moreover, the possibility to evaluate the expansion due to the exothermic polymerization reaction is considered an advantage in this study. The polymerization shrinkage curves revealed that all the materials investigated in this study showed an expansion during the first seconds after switching on the polymerization device to initiate polymerization. Previously, the expansion of the composites at the early stages of polymerization was reported to be related to the exothermicity of the polymerization and due to the heat produced by the local effect of the light-curing unit tip [24,25]. This effect might cause minor inaccuracy in the results due to the different thermal coefficients of composites and strain gages [21]. However, when the amount of shrinkage strain exceeds the amount of thermal expansion, rapid shrinkage behavior was reported for the composite materials. Therefore, this inaccuracy was not considered relevant compared to the effect of polymerization. Previously, it has been shown that thermal expansion is affected by the ratio of fillers in the matrix [26] and orientation of fibers [18]. The same trend was observed in the current study. Unfilled resin showed higher thermal expansion compared to the PFC. On the other hand, unidirectional FRC showed higher expansion transverse to fiber direction, and lower expansion along the fibers, in a longitudinal direction. The expansion was followed by rapid shrinkage in unfilled resin and PFC specimens. The magnitude of the shrinkage in composites has been shown to correlate directly to filler content [27]. A resin with a high level of filler fraction shrinks less than that with a lower filler fraction, as an increase in the filler fraction relatively decreases the fraction of monomer in the composite. This was also apparent in this study as the PFC showed less shrinkage strain compared to the unfilled resin. In FRC, the orientation of fibers was an important factor in the linear shrinkage strain. In the case of

Polymerization shrinkage strain of FRC

615

unidirectional FRC, the shrinkage of composite was previously described by the formula [28] sl Z

sm Em Vm Ef Vf C Em Vm

(1)

ðshrinkage of FRC longitudinally to fiberÞ

st Z ð1 C nm Þsm Vm Ksl ðnf Vf C nm Vm Þ (2) ðshrinkage of FRC transversally to fiberÞ where sm refers to the shrinkage of unfilled resin matrix, E refers to the modulus, and V to volume percentage. Subscripts f or m refers to fiber or matrix. n refers to Poisson’s ratio. If it is estimated that E fV f [E mV m, then longitudinal shrinkage of FRC approaches zero, and transversal shrinkage of FRC could be simplified as follows st Z ð1C nm Þsm Vm . According to the formula, it could be expected that transversal shrinkage of the matrix could be even slightly higher than the unfilled resin alone. The results of the current study were in good agreement with the literature. In the case of continuous unidirectional FRC materials, the shrinkage strain along the fibers was low, whereas the main shrinkage occurred in the transverse direction to the fiber direction. After the minor expansion during the initial phase of polymerization, the shrinkage was internally constrained by the fibers in the direction parallel to the fiber alignment. This might explain why the specimen remained in an expansion state in the longitudinal direction. When shrinkage is constrained by the fibers, the polymer matrix settles in the compression state, which might be beneficial in improving the load transfer capacity from the polymer matrix to glass fiber [29]. On the other hand, a high shrinkage strain was observed transversal to the fiber direction. The fibers were not effective in constraining the shrinkage in a transverse direction. The third unidirectional FRC, VP, investigated in the present study, had lower fiber content compared to the other two unidirectional FRC materials. However, it showed a lower shrinkage strain compared to SC. This difference might be due to the differences in matrix structure. VP contains additional inorganic filler particles in the polymer matrix, which can lower the shrinkage and may balance the difference between two different directions. The differences between different

brands of unidirectional fibers might also be explained by the differences in matrix formulations. The monomer formulation, the amount of diluent monomer in the mixture, the degree of conversion, and rate of polymerization are known to be important factors in the development of shrinkage strain [3,30]. Two different semi-IPNbased polymer matrices were used in this study containing different amounts of PMMA in the matrix structure [17], which might result in the differences in shrinkage behavior [31]. Similar to the continuous unidirectional FRCs, the bidirectional FRC showed very little shrinkage strain in either direction. After initial expansion, minor shrinkage was observed in both directions. This might be due to the constraints applied by the fibers in both directions. During polymerization exotherm, the resin matrix expanded due to thermal expansion, but after this stage, stiff fibers might restrict the increase in compressive strains, which will than result in shrinkage. However, among the bidirectional materials, SN showed a higher shrinkage strain compared to EN. Although they have the same weave pattern, the difference in shrinkage strain might be due to their different matrix compositions. SN containing bisGMA-TEGDMA resin for further impregnation might be expected to show higher shrinkage compared to EN, which has bisGMA in the matrix structure. Obviously, after initial expansion, the shrinkage strain of SN matrix was able to overcome the restriction of the fibers and this resulted in shrinkage. The FRC with randomly oriented fibers (EW) showed low polymerization shrinkage, but was still slightly higher than the bidirectional FRC. Despite the fact that the matrix contained a low volume of fibers compared to the other materials, the short fibers were also effective in restricting the shrinkage. The present study suggests that the anisotropic nature of FRC exists with regard to polymerization shrinkage. The low polymerization shrinkage of FRC compared to unfilled resin and PFC might be important for future clinical applications. The difference in the shrinkage behavior between the transverse and longitudinal directions of unidirectional FRC and its effect on the longevity a restoration needs further investigation.

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