desorption cycles

d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 259–266

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journal homepage: www.intl.elsevierhealth.com/journals/dema

Hygroscopic dimensional changes of self-adhering and new resin-matrix composites during water sorption/desorption cycles Yong-jie Wei a,b , Nick Silikas b , Zhen-ting Zhang a,∗ , David C. Watts b,∗∗ a

Department of Prosthodontics, School of Stomatology, Capital Medical University, Tiantan Xili No. 4, Chongwen District, Beijing 100050, PR China b Biomaterials Science Research Group, School of Dentistry and Photon Science Institute, the University of Manchester, Higher Cambridge Street, Manchester M15 6FH, UK

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objectives. To study hygroscopic dimensional changes in new resin-matrix composites dur-

Received 11 August 2010

ing water sorption/desorption cycles.

Accepted 25 October 2010

Methods. Five materials were examined: a self-adhering flowable composite: Vertise® Flow (VF), a universal composite: GC Kalore (GCK), two micro-fine hybrid composites: GC Gradia Direct Anterior (GDA) and GC Gradia Direct Posterior (GDP), and a posterior restorative

Keywords:

composite: Filtek® Silorane (FS). Five disk-shaped specimens of each material were pre-

Dental resin-matrix composites

pared (15 mm diameter × 2 mm thickness) according to ISO 4049. The mean diameter of

Hygroscopic expansion

each specimen was measured by a custom-built laser micrometer (to a resolution of 200 nm)

Water sorption

periodically over 150 d water immersion and 40 d recondition periods at (37 ± 1) ◦ C. Perspex

Desorption

controls were used. Data analysis was performed by repeated measures ANOVA, one-way

Shrinkage

ANOVA and Tukey’s post hoc test (p < 0.05).

Self-adhering

Results. Differences in hygroscopic expansion were found for all test materials during sorp-

GPDM

tion, ranging from 0.74% (±0.05) for FS to 4.82% (±0.13) for VF. The differences were significant

Silorane

for all materials (p < 0.001), except between GCK and GDA. The mathematical relationship

DX-511

between diametral expansion and square root of time was non-linear. VF exhibited signifi-

UDMA

cant dehydration shrinkage. Significance. The silorane composite FS had the lowest hygroscopic expansion. The extent of compensation of polymerization shrinkage by hygroscopic expansion depends on materials, specimen dimensions and time-scale. So the clinical situation must be taken into consideration in the application of these findings. © 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

A dramatic growth in applications of esthetic resin-matrix composites has arisen in restorative dentistry. One principal

∗

limitation can be dimensional instability during polymerization and after restoration. The initial volumetric reduction, which is generally known as polymerization shrinkage, may develop detrimental stress on the cavity walls [1,2]. Consequently, de-bonding, micro-leakage through marginal gaps,

Corresponding author. Tel.: +86 0 10 6701 2783; fax: +86 0 10 6701 3995. Corresponding author. Tel.: +44 0 161 275 6749. E-mail addresses: [email protected], [email protected] (Z.-t. Zhang), [email protected] (D.C. Watts). 0109-5641/$ – see front matter © 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2010.10.015 ∗∗

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d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 259–266

O

O

O

OH O

P

Si

Si

O

O

OH

Si Si

O

O O

O O

O O

O

GPDM

Silorane O O

C

NH

O

O NH C

O NH C

O

O O

C

m' O

O

n

O

O

O O

O

NH

m O

DX-511

O

O

O O

NH

O

O

NH

O

O

O

UDMA Fig. 1 – Chemical structures of the main organic matrixes.

post-operative sensitivity, secondary caries, enamel fracture and clinical failure of restorations could occur [3–5]. Clinically, stress might be relieved in various ways: by cavity design—taking configuration factor into consideration, applying liners, modulating curing initiation or by means of water sorption [6]. However, most of this information has been gathered in vitro or on the basis of well-educated assumptions. There is still the need for more direct clinical evidence to support these approaches. Resin-matrix composites are constantly exposed to an aqueous environment after intra-oral application. Water diffused into the matrix may contribute to the relaxation of the polymerization shrinkage stress to some extent [1]. In

addition, absorbed water may expand the polymer matrix and induce crazing [7] and cause hygroscopic expansion after occupying the microvoids and free volume between chains [8]. This might increase the bulk volume of resin-composites and possibly decrease marginal gaps generated by polymerization shrinkage [9]. On the other hand, water could elute some residual monomers and other components from certain resin composites, resulting in further shrinkage, reduced bulk [10], weakened mechanical properties in restorations and potential allergic reactions with some patients. Therefore, the dimensional change of a resin composite immersed in water is somewhat complicated and is material dependent [8].

Quarz (silane layer), radiopaque yttrium fluoride 76 7.9 Filtek® Silorane FS

20080514

77 6.6 GC Gradia® Direct Posterior GDP

0905201

73 5.2

GC Dental Products Corp., Tokyo, Japan GC Dental Products Corp., Tokyo, Japan 3M ESPE, St. Paul, MN, USA GC Gradia® Direct Anterior GDA

0901134

82 8.6 GC America Inc. USA GC Kalore GCK

0903171

Silica, prepolymerized filler, fluoro-alumino-silicate glass

UDMA and dimethacrylate co-monomers UDMA and dimethacrylate co-monomers Silorane

DX-511, UDMA and dimethacrylate co-monomers

GPDM and methacrylate co-monomers

Prepolymerized filler, barium glass, nano-sized colloidal silica, nano-sized ytterbium fluoride Prepolymerized filler (with lanthanoid fluoride), fluoro-alumino-silicate glass, strontium/barium glass, silicon dioxide, lanthanoid fluoride Silica, prepolymerized filler 70 5.2 Kerr Corporation, Orange, CA, USA Vertise® Flow VF

3172311

Fillers Filler loading (wt%) Elastic modulus (GPa) Batch no.

Five commercially available dental restorative resin composites were investigated (Table 1). 25 disk-shaped specimens (n = 5) were produced according to ISO FDIS 4049: 1999 [19] and their manufacturer’s instructions. Care was taken to minimize entrapped air while uncured material was placed into brass ring molds (15 mm diameter × 2 mm thickness) in a laboratory environment at 23 (±1) ◦ C and relative humidity of 50 (±2)%. The molds were sandwiched between two pieces of transparent polyester film with two glass microscope slides pressed on each side and then clamped. Five overlapping sections on each side of the specimen were irradiated respectively using a halogen curing light (Optilux 501, Kerr Corporation, USA; 11 mm exit window) under the standard curing mode (output wavelength range: 400–505 nm; output irradiance: 580–700 mW/cm2 ). Atmospheric oxygen was minimized by the polyester film during light curing. A radiometer was used to check the stability of the curing light periodically. Each specimen was carefully removed from its mold and irregularities were finished against 1000 grit silicon carbide abrasive paper. Any specimen with visual voids was discarded. Then specimens were stored separately in glass vials in a lightproof desiccator with anhydrous self-indicating silica gel at (37 ± 1) ◦ C, until the mass change of each specimen was less than 0.1 mg in a 24 h period, which indicated sufficiently completion of polymerization and dehydration. Each specimen was weighed using a calibrated electronic analytical balance with a precision of 0.01 mg (Ohaus Analytical Plus, Ohaus Corporation, USA). Two Perspex specimens (Polymethylmethacrylate, Imperial Chemical Industries, UK) were

Manufacturer

Specimen preparation

Material

2.1.

Code

Materials and methods

Table 1 – Test materials.

2.

Organic matrix

Although hygroscopic expansion (strain) occurs slowly in resin-composites, eventually it is still expected to partially compensate polymerization shrinkage [6,11]. A greater expansion that exceeds the shrinkage is undesirable because the potential hygroscopic stress may cause micro-cracks or even cracked cusps in restored teeth [12], as well as adverse clinical consequences to any overlying restorations [13]. Dimensional changes of conventional dimethacrylatebased resin composites [14,15], core materials [16], a compomer [4], a calcium aluminate cement [17] and glass-ionomer cements [18] have been studied by several researchers. However, only prior work on resin-composites is of principal relevance to the present study. Hygroscopic expansion has not been previously reported on silorane, DX-511 monomer (with a long rigid core and high molecular mass, Fig. 1) or self-adhering resin-composites. Diametral and volumetric changes of newly introduced resin composites based on those monomers may significantly affect their clinical performance after either a short or relatively long-term restoration period. The aim of this study was therefore to evaluate the hygroscopic dimensional changes of five new resin-matrix composites when water stored at 37 ◦ C and subsequently reconditioned. The working null-hypothesis was that the hygroscopic expansion behavior of investigated resin composites does not vary between materials.

GPDM: glycerol phosphate dimethacrylate; UDMA: 1,6-bis(methacrylyloxy-2-ethoxycarbonylamino)-2,4,4-trimethylhexane/urethane dimethacrylate; silorane: 3,4-epoxycyclohexylethylcyclopolymethylsiloxane, bis-3,4-epoxycyclohexylethyl-phenylmethylsilane.

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d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 259–266

262

d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 259–266

used as control. One was stored dry in a lightproof desiccator and the other was kept in de-ionized water at (37 ± 1) ◦ C.

2.2.

Hygroscopic dimensional changes

Mean diametral measurements, and thus diametral changes, of the specimens were monitored by a custom-built noncontact laser micrometer. This was similar in some respects to a previously described instrument [11,13] though developed independently. The device incorporated a laser-scan micrometer (LSM) system (Measuring Unit LSM-503s and Display Unit LSM-6200, Mitutoyo Corporation, Japan), mounted on a heavy stainless steel base, 2.5 cm thick, with rubber feet. A disk specimen holder was rotated in a horizontal plane about a vertical axis by an electronic stepper-control unit. The LSM was interfaced, via the Display Unit, to a PC, with USB input, for further recording and data processing. The measuring LSM system (Fig. 2) obtained the specimen dimensional data rapidly and accurately using a highly directional parallel-scanning laser beam. The laser beam, generated by a laser oscillator, was directed at a polygon mirror rotating at high speed and synchronized by clock pulses. Then the reflected laser beam passed through a collimator lens and maintained its constant direction through the beam window toward the disk specimen. The measurement light rays traveled as ‘parallel beams’ toward a photo-electric detector unit, but they were partially obstructed by the disk specimen. The extent of the beam-obstruction was directly proportional to the disk diameter. The resulting electrical output signal changed according to the duration over which the beam was obstructed. This was processed by the Display Unit CPU and the disk dimension was displayed digitally. The laser beam system was able to measure each disk specimen diameter to a resolution of 200 nm. The LSM system was calibrated before each measurement using two reference gages, according to the manufacturer’s instructions. The stepper-control unit maintained stepwise rotation of the disk specimen (mounted on its holder) with a total of 800 steps per rotation. The speed of rotation was 28 steps per second. The scanning speed of the laser beam was 3200 scans per second and so 91,428 diametral measurements were taken per revolution and these were averaged, in sets of 1024, to give 89 recorded readings/revolution. At each sorption time-period, specimens were measured overfive complete rotations. Therefore, the diametral values presented for each specimen at each time point were obtained as overall averages of 445data values, which were transferred to an Excel file. The grand mean for the 5 specimens per group was then obtained for each sorption time-period. The initial mean diameter of each specimen was measured and denoted as d1 . Then specimens were immersed in 10 mL de-ionized water at (37 ± 1) ◦ C for periods of time. Diametral measurements were taken at different time intervals until equilibrium was reached. The mean diameters measured at time t and at equilibrium were respectively denoted as d2 (t) and d∞ . For measurement, each specimen was carefully taken out, dried on filter paper until there was no visible moisture and then mounted on the specimen holder. The diameter was measured after which the specimen was returned to

water storage. After 150 d water immersion, the specimens were reconditioned in the desiccator using the cycle described above until the values of the diameter were constant. The percentage diametral change from water-immersion was calculated by: d (%) =

d2 (t) − d1 × 100 d1

(1)

The percentage hygroscopic dimensional change (volumetric change) can be calculated by the following equation [11], which assumes isotropic expansion behavior:

 V (%) =

d(%) 1+ 100

3

 −1

× 100

(2)

If the diametral expansion at a given time is Dt = [d2 (t) − d1 ] and the corresponding value at equilibrium is D∞ = (d∞ − d1 ), then Dt /D∞ versus t0.5 can be plotted.

2.3.

Bonded-disk shrinkage-strain

The shrinkage of the materials was analyzed in the usual bonded-disk configuration [40]: 1 mm × 8 mm. The paste disks (n = 5) were irradiated in situ at 23 ◦ C for 60 s with 500 mW/cm2 blue light. The axial strain was measured continuously from start of irradiation for 1 h. The mean final (maximum) values are reported. These correspond essentially to volumetric values of strain, due to the strongly anisotropic shrinkage of the paste disks bonded to the glass surfaces.

2.4.

Statistical analysis

Data were analyzed using SPSS for Windows (Version 16.0, SPSS Inc., Chicago, USA) and SigmaPlot Version 11 (Systat Software Inc., California, USA). The percentage diametral changes, hygroscopic dimensional changes, means and standard deviations were calculated. Repeated measures ANOVA was performed for the percentage hygroscopic expansions (p < 0.05) and one-way ANOVA was performed for the percentage hygroscopic expansions at 150 d (p < 0.05), followed by Tukey’s post hoc test at p < 0.05.

3.

Results

Differences in hygroscopic expansion were found for all test materials. Repeated measures ANOVA showed significant differences in hygroscopic expansion of most test materials during 150 d water immersion. VF, however, reached maximum hygroscopic expansion at 90 d (Fig. 3). The hygroscopic dimensional changes over time during the water sorption/desorption cycle are presented in Fig. 4a–e. VF demonstrated significant dehydration shrinkage to a negative value. The control specimen of Perspex showed no difference in dimension. The correlation between diametral expansion and square root of time was non-linear at the early stage of the watersorption process (Fig. 5).

d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 259–266

263

% hygroscopic dimensional change

Fig. 2 – Configuration of the laser-scan micrometer (LSM) system.

5 VF GCK GDA GDP FS Perspex

4 3 2 1 0 0

20

40

60

80

100

120

140

160

time (d) Fig. 3 – Percentage hygroscopic dimensional change of test and control materials immersed in de-ionized water for 150 d.

The mean hygroscopic dimensional expansion after 150 d water immersion and the values of bonded-disk shrinkagestrain are shown in Table 2. The differences of hygroscopic expansion were significant for all groups (p < 0.001), except for group GCK and group GDA.

4.

Discussion

The current study found that hygroscopic dimensional changes varied between materials. Therefore the null hypothesis was rejected. In this study, our method determined the free expansion of the materials. In the situation of a constraining dental cavity it is possible that the expansion magnitudes could be different – probably lower – than in the free-expansion situation. From Table 2, it seems that the volumetric shrinkage of GCK and VF could be compensated – or even over compensated – by hygroscopic expansion after a long period of water immersion. However, this conclusion could not be drawn for the other three test materials. This coincides with the result of prior studies that the volumetric shrinkage could not be totally compensated by hygroscopic expansion in resin composites [4,12]. Each test material achieved over 70% of its final hygroscopic expansion within 28 d, showing a marked rise followed by a level-off. Such an approach to equilibrium is a normal

physical phenomenon, where the expansion of the polymer network leads to an increasing restoring force opposing further sorption and expansion. During the process, water molecules occupied the previous space of any extracted leachable species. The relationship between dimensional changes and water sorption in resin-matrix composites was demonstrated by Hirasawa et al. in 1983 [9]. Hygroscopic expansion can be affected by various factors: the monomer, the polymerization rates, the cross-linking and pore size of the polymer network, the bond strength, the interaction between polymer and water, the filler and the resin–filler interface [8,13,20–23]. It is apparent that FS presented the lowest hygroscopic dimensional changes (Table 2 and Fig. 3). FS is based on a new monomer silorane synthesized from hydrophobic siloxane and low shrinkage ring-opening oxirane [24]. The photo-activated cationic polymerization in FS decreases the polymerization shrinkage and increases the degree of conversion [25–27]. Therefore, its relatively low hygroscopic expansion which might relieve some stresses between the fillers and the matrix in a constrained geometry could maintain molecular bonds for a longer term [24,25]. The GC composites incorporate UDMA, which was first introduced as a dental resin matrix in 1974 [28]. Mohsen et al. [29] suggested a mechanism of water sorption in UDMA polymer and composite. At an early stage, strongly bound water will form intramolecular hydrogen bonds with the UDMA polymer. Progressively, loosely bound water will interact with intermolecular hydrogen bonds of adjacent polymer chains, inducing chain slippage and polymer plasticization. In the meantime, water will accumulate at microvoids and resin–filler interfaces, leading to expansion. This could be an explanation for the significant difference of hygroscopic expansion between UDMA based composites and silorane based composites. Although GDA and GDP have the same co-monomer, the hygroscopic expansion of the anterior formulation was slightly greater than that for the posterior formulation. The main factor may be due to reduced monomer in GDP (Table 1). Table 2 shows that the hygroscopic expansions of GCK and VF were slightly higher than their measured bondeddisk shrinkages. GCK has the DX-511 monomer showing numerous hydrophilic groups in the structure. Hence a

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d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 259–266

a

% hygroscopic dimensional change

% hygroscopic dimensional change

5 4 3 2 1 0

b 2.0

1.5

1.0

0.5

0.0 0

50

100

150

0

200

50

100

2.0

150

200

150

200

time (d)

c

% hygroscopic dimensional change

% hygroscopic dimensional change

time (d)

1.5

1.0

0.5

0.0

1.6

d

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

-0.5 0

50

100

150

0

200

50

100

% hygroscopic dimensional change

time (d)

time (d) 0.8

e

0.6

0.4

0.2

0.0 0

50

100

150

200

time (d) Fig. 4 – (a) Percentage hygroscopic dimensional change of Vertise® Flow during the water sorption/desorption cycle. (b) Percentage hygroscopic dimensional change of GC Kalore during the water sorption/desorption cycle. (c) Percentage hygroscopic dimensional change of GC Gradia Direct Anterior during the water sorption/desorption cycle. (d) Percentage hygroscopic dimensional change of GC Gradia Direct Posterior during the water sorption/desorption cycle. (e) Percentage hygroscopic dimensional change of Filtek® Silorane during the water sorption/desorption cycle.

Table 2 – Percentage hygroscopic dimensional expansion (standard deviation) of test materials after 150 d immersion in de-ionized water at 37 ◦ C and bonded-disk shrinkage-strain. Difference between groups having the same superscript letter was not significant (p < 0.05). Materials ®

Vertise Flow GC Kalore GC Gradia Direct Anterior GC Gradia Direct Posterior Filtek® Silorane ∗

% hygroscopic dimensional expansion

% shrinkage

*a

4.40 1.70 2.40 2.18 0.99

4.79 (0.18) 2.03 (0.07) b 1.90 (0.05) c 1.51 (0.07) d 0.74 (0.05) b

The highest hygroscopic dimensional expansion [4.82% (±0.13)] of VF was reached at 90 d water immersion.

d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 259–266

1.0

Dt/D∞

0.8

0.6 VF GCK GDA GDP FS

0.4

0.2

0.0 0

1000

2000

3000

4000

√time (√sec) √ Fig. 5 – The diametral expansion over time of test materials. The early stage of the non-linear process indicates occupancy of water molecules into microvoids and free volume of the matrix.

relatively high hygroscopic expansion is unsurprising. Furthermore, strontium and barium glass fillers used as glass modifiers contain leachable ions. These ions can be leached into water and then it has been suggested that water molecules could permeate into spaces previously occupied by those ions [30]. Consequently, the strength of resin–filler interface may be reduced and any unbound water absorbed at the interfaces may contribute to expansion in GCK. In the case of VF, this had the highest hygroscopic expansion after 150 d water immersion (Fig. 4a). In adhesive monomers, polymerizable groups and additional functional groups are linked by a custom-made spacer group, the molecular design of which will influence the hygroscopic expansion of the resulting polymer [31]. Therefore, the considerable expansion of VF, exceeding the measured bonded-disk shrinkage, is probably due to the hydrophilic acidic phosphate group and the short spacer group in the adhesive monomer GPDM. Also, as a flowable composite the monomer content was the largest in this group of materials. There are further structural factors relating to resincomposites that could influence dimensional stability, particularly hygroscopic expansion. For instance, higher degrees of conversion of C C double bonds can induce higher shrinkage in monomers which could give rise to porosity if the material is highly constrained by bonding within a clinical cavity [32]. Hence, the rate and degree of hygroscopic expansion may be limited in a clinical situation because of restraining forces from the cavity and increased free volume from the porosity that can be re-occupied by water ingress without undue bulk expansion [33,34]. Furthermore, both the polymerization shrinkage and the hygroscopic expansion of a restoration will vary over time [1]. This complexity means, as always, that in vitro data can only be extrapolated to a clinical situation with caution [35]. The data in Table 2 are a preliminary comparison between the unconstrained hygroscopic expansion and the bonded-disk shrinkage. Due to the arguable clinical consequence of hygroscopic expansion in a cavity [36], hygroscopic expansion of GCK and VF might fully counteract the

265

effect of polymerization shrinkage or even induce expansion stress over a period of approximately 90 d. The current study highlights the need for further research into the correlation of hygroscopic expansion and volumetric shrinkage in simulated clinical situations. During 40 d water desorption (Fig. 4a–e), the hygroscopic expansion of all test materials decreased toward zero while the specimens were reconditioned. VF showed significant dehydration shrinkage to a negative value, indicating that its final volume was smaller than the initial volume. This is probably due to changes in the GPDM monomer and degradation of the resin–filler interface [8]. While a number of components were eluted out from VF [37], their prior space could be occupied by absorbed water molecules. Then when the unbound water was desorbed during recondition process, the hydrolyzed resin network with reduced rigidity would collapse, causing significant volumetric reduction in VF. The relationship between the diametral expansion process and the square root of time is a non-linear sigmoid curve in Fig. 5, indicating that this was not a straightforward Fickian diffusion process. This behavior is consistent with rapid uptake of water. Several studies measured hygroscopic dimensional changes of resin composites by Archimedes’ principle [12,14,16,18]. It is necessary that the immersion liquid does not react chemically with the cured specimen apart from sorption. Nevertheless, the density of the immersion liquid, which may affect the absorption process, can be influenced by the solubility of the specimen and the environmental temperature. The impact of this cannot be neglected when precise density data are needed using a 0.01 mg balance [16]. Small hygroscopic dimensional changes are very difficult to measure using a manual caliper. Therefore, a custom-built laser micrometer was used in the current study. This non-contact system enabled very small hygroscopic dimensional changes to be measured accurately without damaging or otherwise changing the specimen [38]. Similar methods were used in a few previous studies [11,13,39]. The diverse hygroscopic behavior of the resin-composites investigated here shows the value of further studies of this important category of materials.

5.

Conclusions

Laser-scanning micrometry was found to be a highly accurate means of measuring dimensional changes of material specimens over time. The resin-composite specimens investigated were found to undergo progressive hygroscopic expansion over a period of several months. A silorane composite, however, showed the greatest dimensional stability in an aqueous environment. By contrast, a self-adhesive composite was the least dimensionally stable, due to the incorporation of hydrophilic monomers. The volumetric expansion compensated at least partially for the polymerization shrinkage on setting, but over a much slower time-scale.

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Acknowledgment This investigation was supported in part by the joint postgraduate program of foreign universities and Chinese universities based in Beijing from Capital Medical University, as well as the scientific and technological innovation platform project from Beijing Municipal Commission of Education, Beijing, China.

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