Post-irradiation hardness development, chemical softening, and thermal stability of bulk-fill and conventional resin-composites.

JJOD-2393; No. of Pages 10 journal of dentistry xxx (2014) xxx–xxx

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/jden

Post-irradiation hardness development, chemical softening, and thermal stability of bulk-fill and conventional resin-composites Ruwaida Z. Alshali a,b, Nesreen A. Salim c, Julian D. Satterthwaite a, Nick Silikas a,* a

School of Dentistry, The University of Manchester, Higher Cambridge Street, Manchester, UK Department of Oral and Maxillofacial Rehabilitation, King Abdulaziz University, Jeddah, Saudi Arabia c Prosthodontic Department, University of Jordan, Amman, Jordan b

article info

abstract

Article history:

Objectives: To measure bottom/top hardness ratio of bulk-fill and conventional resin-composite

Received 10 November 2014

materials, and to assess hardness changes after dry and ethanol storage. Filler content and

Received in revised form

kinetics of thermal decomposition were also tested using thermogravimetric analysis (TGA).

5 December 2014

Methods: Six bulk-fill (SureFil SDR, Venus bulk fill, X-tra base, Filtek bulk fill flowable, Sonic

Accepted 7 December 2014

fill, and Tetric EvoCeram bulk-fill) and eight conventional resin-composite materials (Gran-

Available online xxx

dioso flow, Venus Diamond flow, X-flow, Filtek Supreme Ultra Flowable, Grandioso, Venus Diamond, TPH Spectrum, and Filtek Z250) were tested (n = 5). Initial and 24 h (post-cure dry

Keywords:

storage) top and bottom microhardness values were measured. Microhardness was re-

Bulk-fill

measured after the samples were stored in 75% ethanol/water solution. Thermal decompo-

Resin-composite

sition and filler content were assessed by TGA. Results were analysed using one-way ANOVA

Microhardness

and paired sample t-test (a = 0.05).

Post-irradiation

Results: All materials showed significant increase of microhardness after 24 h of dry storage

Cross-link density

which ranged from 100.1% to 9.1%. Bottom/top microhardness ratio >0.9 was exhibited by

Thermogravimetric analysis

all materials. All materials showed significant decrease of microhardness after 24 h of storage in 75% ethanol/water which ranged from 14.5% to 74.2%. The extent of postirradiation hardness development was positively correlated to the extent of ethanol softening (R2 = 0.89, p < 0.001). Initial thermal decomposition temperature assessed by TGA was variable and was correlated to ethanol softening. Conclusions: Bulk-fill resin-composites exhibit comparable bottom/top hardness ratio to conventional materials at recommended manufacturer thickness. Hardness was affected to a variable extent by storage with variable inorganic filler content and initial thermal decomposition shown by TGA. Clinical significance: The manufacturer recommended depth of cure of bulk-fill resin-composites can be reached based on the microhardness method. Characterization of the primary polymer network of a resin-composite material should be considered when evaluating its stability in the aqueous oral environment. # 2014 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: +44 1612756747. E-mail address: [email protected] (N. Silikas). http://dx.doi.org/10.1016/j.jdent.2014.12.004 0300-5712/# 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Alshali RZ, et al. Post-irradiation hardness development, chemical softening, and thermal stability of bulk-fill and conventional resin-composites. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.12.004

JJOD-2393; No. of Pages 10

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journal of dentistry xxx (2014) xxx–xxx

1.

Introduction

Light cured resin-composites are the most commonly used direct dental restorative materials nowadays.1 One of the main limitations of the light-curing process is the limited depth of cure; generally, only increments up to 2 mm thick should be placed to ensure adequate light transmittance and full curing of a restoration. For filling a deep cavity, the incremental placement technique is time-consuming and may increase the risk of moisture contamination during the filling procedure. To overcome this problem, bulk-fill resincomposite materials that can be placed in either a single step or two-steps and cured in bulk have been introduced.2 Compromised depth of cure of a resin-composite material results in insufficient polymerization of deeper portions with subsequent degradation, poor physical properties and adverse biological reactions owing to leaching of the monomeric components of the uncured resin-composite.3 Depth of cure studies using microhardness depth profiles and bottom to top hardness ratio, show adequate cure at depths up to 5 mm for some bulk-fill materials, usually linked with increased light transmittance of these bulk-fill materials.4,5 On the other hand, some studies show significantly less depth of cure of some bulkfill materials than that claimed by manufacturers.6,7 Surface hardness of resin-composite is an important property that indirectly reflects the mechanical performance and extent of polymerization of the material.8 Researchers have extensively investigated composite’s surface hardness and the effect of material composition and post-cure ageing on surface hardness.9–14 Surface hardness is strongly influenced by the filler fraction of resin-composite5,14,15 and for conventional resin-composites will develop over time after discontinuation of the curing light which is mainly attributed to post-irradiation polymerization.16–18 An increase of hardness of 50–80% has been observed for some novel resin-composites including an ormocer, and a silorane based material.19 Factors related to composition and characteristics of the material that may affect the extent of post-irradiation polymerization and hardness development are not fully understood. Also, the nature of the post-irradiation developed polymer network and its effect on the final material properties has not yet been assessed. Cross-link density is a critical property in terms of hygroscopic and chemical stability of a polymeric material and its viscoelastic performance.20,21 Increasing cross-link density results in reduced free volume and porosity of the polymer network with close proximity of the polymer chains. This provides limited space and pathways available for solvent molecules and residual monomers to diffuse in and out of the structure.20 The highly functional dimethacrylate based polymer networks used in dental resin-composites are generally characterized by highly cross-linked structures; however, relative differences in the viscoelastic and hygroscopic behaviour attributed to slight variations in cross-link density are observed when chemically different dimethacrylate monomers and different light curing procedures are used.22,23 Methods to assess the degree of cross-link density include assessment of thermal stability in terms of the glass transition temperature measured by differential scanning calorimetry (DSC) and initial decomposition temperature measured by

thermogravimetric analysis (TGA).24 TGA is an extremely powerful thermal technique although it gives no direct chemical information. It has many applications in polymer science including assessment of composition, thermal stability and decomposition behaviour, and degree of moisture absorption.25 Thermal stability measured in terms of the onset degradation temperature is enhanced as the degree of chemical cross-linking increases.26–29 TGA analysis of dental resin-composites has been mainly used to assess the weight percent inorganic filler content.14,30–32 Another indirect method to assess cross-link density is assessing the degree of softening when a material is stored in a solvent solution: higher degrees of softening usually indicate less cross-link density.14,33 Ethanol solution is one of the most commonly used solvents and food simulating liquids and studying its effect on dental materials is highly relevant.33 In a study of the chemical softening effect of ethanol and other food simulating solvents on dental composites, it was found that the greatest softening effect is obtained by pre-conditioning BisGMA-based resin-composite samples in a 75% ethanol/water solution.34 Ethanol causes softening of the resin-composite surface by penetration into the polymer structure and replacement of the inter-chain secondary bonds and thereby pulling apart and dissolve linear polymer chains, oligomers, and residual monomers.35 Regardless of the degree of conversion, the softening effect of ethanol is expected to be more pronounced with a linear or less cross-linked polymer structure since it is difficult for the solvent molecules to overcome primary valence cross-links.24 In view of the limited research in this area, the aims of this study were: (i) to assess initial and post-irradiation surface hardness, and bottom/top hardness ratio of selected bulk-fill and conventional resin-composite materials; (ii) to explore any correlation between initial hardness and the extent of post-irradiation hardness development; (iii) to assess the degree of softening after storage in a solvent solution; (iv) to assess the filler content, onset decomposition temperature, and thermal decomposition kinetics of the different resincomposites using TGA; and (v) to explore the correlation between thermal stability measured by TGA and the degree of chemical softening as two different techniques for assessing the degree of polymer cross-link density. The first null hypothesis was that there would be no difference between bulk-fill and conventional resin-composite materials in terms of initial and post-irradiation surface hardness, bottom/top hardness ratio, the degree of chemical softening, and onset decomposition temperature. The second null hypotheses was that there would be no correlation between initial hardness and the extent of post-irradiation hardness development, and between thermal stability measured by TGA and the extent of chemical softening of the different materials.

2.

Materials and methods

2.1.

Study design

Bulk fill and conventional composite specimens were prepared using polytetrafluoroethylene (PTFE) moulds. All specimens



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were tested for Vickers microhardness (VH): initial and 24 h (post-cure dry storage) top and bottom Vickers microhardness values were measured. Specimens were then stored for 24 h in 75% ethanol/water (E/W) solution and microhardness was measured again. Thermogravimetric analysis (TGA) was used to study the thermal properties of the tested materials.

2.2.

Materials

Fourteen commercial resin-composite materials including six bulk-fill materials and eight conventional materials were tested. A list of composites studied, with details of filler percentage and organic matrix is given in Table 1.

2.3.

Microhardness assessment

Cylindrical samples were made using a 6 mm diameter 2 mm height PTFE mould for conventional composites while a 4 mm height mould was used for bulk-fill resincomposites (n = 5). The mould height complies with the maximum increment thickness specified by the manufacturer. Samples were fabricated by applying each material into the designated mould placed against a polyester matrix strip and a glass slab on a non-reflective background surface. The mould was slightly overfilled with the material and the excess was then extruded by applying another polyester matrix strip and a glass slab pressed firmly. Each sample was then cured from the top surface only for 20 s using an LED light curing unit (EliparTM, 3 M ESPE, USA) under standard curing mode. The light curing unit had a 10 mm tip with an output irradiance of 1200 mW/cm2 and wavelength range 430–480 nm. A calibrated radiometer system (MARC Blue-Light Analytics Inc., Halifax, NS, Canada) was used to verify the irradiance at each use of the light curing unit. Immediately after cure, the sample was gently pushed out from the mould, excess flash removed and the top surface of the sample was marked with a permanent marker. Initial top and bottom Vickers microhardness (VH) of the samples was measured using a microhardness instrument

(FM-700, Future Tech Corp., Japan). The test load was fixed at 50 g applied for 15 s and three sequential measurements were taken for each surface. Bottom/top surface microhardness ratios were calculated to assess the relative extent of cure of the bottom surface. For assessment of post-irradiation polymerization effect, the samples were stored dry for 24 h at 37 8C and the microhardness of the samples was then remeasured. The extent of post-irradiation polymerization was assessed by calculating the percentage increase of top surface microhardness after dry storage in relation to the initial microhardness value. For assessment of the degree of chemical softening (cross-link density), the same samples were stored in 75% E/W solution for 24 h at room temperature and the top surface microhardness was then measured. The percentage decrease of microhardness of top surface after storage in 75% E/W was calculated in relation to the microhardness value before storage.

2.4.

Thermogravimetric analysis (TGA)

Three specimens of each material were prepared using a PTFE mould (4 mm diameter, 1 mm thickness) and irradiated from one side for 20 s using an LED light curing unit (EliparTM, 3 M ESPE, USA) under standard curing mode with an output irradiance of 1200 mW/cm2. Samples were then stored at 37 8C for 24 h. Thermal decomposition analysis was performed using a calibrated thermal microbalance device (STA 449 C Jupiter, Netzsch-Gera¨tebau GmbH, Germany). Each sample was placed in the thermal micro-balance in an alumina crucible against an empty reference crucible and heated from 25 8C to 900 8C at a rate of 20 8C/min under a high purity Argon atmosphere (50 mL/min). Proteus analysis software was used to generate TGA thermo-analytical curves calculating the percentage weight loss (corresponding to the organic portion) as a function of increasing temperature. The extrapolation onset decomposition temperature as specified by ASTM and ISO was determined for each material.36 The weight percentage of the residual material after complete thermal decomposition corresponded to the filler loading.

Table 1 – Test materials and manufacturer information [bulk-fill (light grey) and conventional composite (dark grey)]. Material

Code

SureFil1 SDR1 flow Venus bulk fill X-tra base Filtek bulk fill flowable Tetric EvoCeram1 bulk fill SonicFilllTM Grandioso flow Venus1 Diamond flow X-flow FiltekTM Supreme XTE Grandioso Venus Diamond TPH13 Spectrum FiltekTM Z250

SDR VBF XB FBF TEC SF GRF VDF XF FS GR VD TPH Z250

Organic matrix Modified UDMA, Aliphatic UDMA, UDMA, BISGMA, BISGMA, UDMA, DEGDMA, BISGMA, BISGMA, UDMA, BISGMA, EBPADMA,

UDMA, EBPADMA, TEGDMA TEGDMA dimethacrylate, DEGDMA BISGMA, EBPADMA, Procrylat resin BISGMA TEGDMA, EBPADMA TEGDMA, HEDMA EBPADMA TEGDMA TEGDMA, Procrylat resin TEGDMA TCD-DI-HEA, EBPADMA EBPADMA, TEGDMA UDMA, TEGDMA, BISGMA

Filler %(weight) 68 65 75 64 77 83.5 81 65 60 65 89 64 75 82

UDMA: urethane dimethacrylate, EBPADMA: ethoxylated bisphenol A dimethacrylate (BisEMA), TEGDMA: triethylene glycol dimethacrylate, DEGDMA: diethylene glycol dimethacrylate, BisGMA: bisphenol A dimethacrylate, HEDMA: 1,6-hexanediol dimethacrylate, TCD-DI-HEA: bis(acryloyloxymethyl)tricyclo[5.2.1.02,6]decane. Increment thickness for bulk-fill is 4 mm and for conventional composite is 2 mm.



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Fig. 1 – A bar chart showing the mean Vickers surface microhardness (standard deviation) of top and bottom surfaces of six bulk-fill and eight conventional resin-composite materials immediately post-cure and 24 h aged dry at (37 8C).

2.5.

Statistical analysis

The data were entered into statistical software package (SPSS, V20, Chicago. USA). After the data were checked for normality, multiple paired-sample t-test were used to compare bottom and top surface microhardness of each material, initial microhardness and 24 h post-cure microhardness, and microhardness before and after chemical softening. One-way analysis of variance (ANOVA) was used to assess differences in decomposition temperature, filler loading, initial microhardness, 24 h post-cure hardness, percentage increase of microhardness after dry storage, and percentage decrease of microhardness after ethanol storage between all materials. This was followed by Tukey post hoc analysis for multiple comparisons (a = 0.05). Two-way ANOVA was used to assess

the interaction between material and storage factors on change of microhardness. A scatter plot and Pearson correlation analysis were performed to assess the relationship between 24 h post-cure microhardness and filler loading, percentage increase of microhardness after dry storage and percentage decrease of microhardness after 75% E/W storage, and extrapolation onset decomposition temperature and percentage decrease of microhardness after 75% E/W storage.

3.

Results

Mean microhardness for all tested materials are shown in Fig. 1 and Table 2. All materials showed statistically significant higher microhardness values after 24 h of dry storage

Table 2 – Mean (SD) Vickers microhardness for all tested materials under different storage conditions for both top and bottom surfaces [bulk-fill (light grey) and conventional composite (dark grey)]. Material

Immediately post-cure

Top surface VH SDR VBF XB FBF TEC SF GRF VDF XF FF GR VD TPH Z250

12.4 7.8 28.6 12.9 31.1 66.4 47.2 10.2 15.6 29.8 84.1 41.4 38.8 57.2

(1.1)a,b,c (0.8) a (2.4) d (0.5)b,c (2.7) d (2.6) h (1.7) f (0.6)a,b (2.1) c (2.1) d (2.7) i (2.7) e (0.7) e (4.1) g

Bottom surface VH 13.9 8.6 28.5 12.3 27.6 61.6 50.9 10.9 18.8 31.3 83.3 36.2 36.4 56.0

(2.8)b,c (0.7)a (2.5) d (1.2)a,b (1.7) d (2.1) i (3.1) g (0.5)a,b (1.7) c (4.2)d,e (2.6) j (0.7)e,f (2.2) f (1.9) h

24 h Post-cure dry storage at 37 8C

Top surface VH 24.7 14.8 36.3 22.0 43.0 75.3 51.5 20.2 23.5 41.2 97.5 56.6 50.1 71.9

(2.5) b (0.8) a (3.1) c (0.5) b (2.0) d (1.8) g (1.8)e,f (0.6)a,b (2.1) b (1.4)c,d (2.4) h (1.6) f (4.9) e (4.3) g

VH after 24 h storage in 75% E/W solution

% Increase of hardness after 24 h dry storage at 37 8C

% Decrease of hardness after 24 h storage in 75% E/W

Bottom surface VH 25.5 16.9 38.2 24.0 40.5 70.6 55.7 21.2 26.0 40.1 96.1 52.1 48.2 72.5

(2.8) b (0.8) a (2.9) c (0.8) b (0.8) c (2.0) f (2.4) e (1.4)a,b (4.8) b (2.8) c (4.9) g (1.9)d,e (2.6) d (3.0) f

6.3 6.4 22.8 12.1 26.7 58.8 43.9 7.7 11.6 24.3 74.9 34.0 33.9 56.6

(0.4) a (0.3) a (2.1) d (0.7) c (1.3) d (3.8) g (1.7) f (0.8)a,b (0.8)b,c (1.0) d (3.0) h (2.0) e (0.9) e (2.3) g

100.1 90.3 27.4 69.9 38.7 13.3 9.1 98.0 52.7 38.8 16.1 37.2 29.0 25.9

(19.8) a (19.7)a,b (15.7)d,e,f (7.2)b,c (10.3)d,e (3.6)e,f (3.9) f (10.9)a,b (27.7)c,d (7.0)d,e (6.2)e,f (9.1)d,e,f (11.9)d,e,f (8.0)d,e,f

74.2 56.6 37.0 44.7 37.6 21.8 14.5 61.8 50.2 40.9 23.1 39.8 31.7 21.0

(1.9) h (3.9)f,g (2.7)c,d (2.8),d,e (4.7)c,d (5.9) a (3.9) a (4.0) g (6.2)e,f (3.1)c,d,e (3.7)a,b (3.5)c,d (6.2)b,c (4.4) a

Depth of bottom surface is 2 mm for conventional composites and 4 mm for bulk-fill composites as recommended by the manufacturers. Values with same superscript letters per column indicates statistically homogenous groups (Tukey test, a = 0.05).



Fig. 2 – The percentage increase of microhardness after 24 h of dry storage was negatively correlated to the initial microhardness values with R2 = 0.67.

compared to initial microhardness values. The percentage increase of microhardness ranged from 9.1% (SD 3.9) for GRF to 100.1% (SD 19.9) for SDR. The percentage increase of microhardness after 24 h of dry storage was negatively correlated to the initial microhardness values with Pearson correlation coefficient of 0.82 ( p = 0.0003) and linear regression R2 value of 0.67 ( p < 0.001) (Fig. 2). Top and bottom microhardness values were not significantly different with the bottom/top microhardness ratio higher than 0.9 for all materials and a tendency towards a lower ratio with higher filler content ( 0.57 Pearson correlation coefficient, p = 0.04). Top surface microhardness values at 24 h post-cure of all materials were variable and ranged from 14.8 (SD 0.8) for VBF to 97.6 (SD 2.5) for GR. Top surface hardness was positively correlated to filler content measured by TGA with Pearson correlation coefficient of 0.89 and linear regression analysis R2 value of 0.76 ( p < 0.001). After 24 h storage in 75% E/W solution, all materials showed a statistically significant decrease in microhardness

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compared to pre-storage microhardness values. The percentage decrease of hardness after 75% E/W storage ranged from 17.6% (SD 3.9) for GRF to 74.2% (SD 1.9) for SDR (Fig. 3). The extent of softening after 75% E/W storage was positively correlated to the extent of post-cure hardness development upon dry storage with Pearson correlation coefficient of 0.94 ( p < 0.001) and linear regression R2 value of 0.89 ( p < 0.001) (Fig. 4). Filler loading measured by TGA ranged from 58.3% weight (SD 0.9) for FF to 85.3% weight (SD 0.1) for GR. The extrapolation onset decomposition temperature ranged from 280.9 8C (SD 2.6) for SDR to 390.4 8C (SD 0.1) for SF (Table 3). SDR and VD both showed characteristic two-stage decomposition with a second stage decomposition temperature of 432.4 (SD 0.2) 8C and 415.3 (SD 0.1) 8C, respectively. XF showed an early weight loss prior to decomposition at a temperature of 140.1 8C. The rest of the materials showed single-stage decomposition behaviour on TGA (Fig. 5). Pearson correlation analysis between the percentage decrease of microhardness after 75% E/W storage and the extrapolated onset decomposition temperature of all the materials showed a strong negative correlation between the two variables. Pearson correlation coefficient was 0.70 ( p = 0.005) and linear regression R2 value = 0.50 ( p < 0.001) (Fig. 6).

4.

Discussion

Microhardness of several resin-composite materials was tested in this study. Of the tested materials, VBF and VDF showed significantly lower VH after 24 h of dry storage (14.8 SD 0.8, 20.2 SD 0.6), which indicates inferior mechanical properties of both VBF and VDF compared to other materials. SF showed the highest VH value among other bulk-fill composites (75.4 SD 1.8), while GR showed significantly higher VH value compared to all materials (97.5 2.4). Lower VH values were obtained in this study compared to other studies4,5,14 which can be attributed to variations in test parameters especially the indentation load.37 The current study showed that 24 h post-cure top surface Vickers’s microhardness values of the

Fig. 3 – Percentage increase of micro-hardness after 24 h dry storage at (37 8C) (left axis) and percentage decrease of microhardness after 24 h storage in 75% ethanol/water (right axis) of six bulk-fill and eight conventional resin-composite materials. Please cite this article in press as: Alshali RZ, et al. Post-irradiation hardness development, chemical softening, and thermal stability of bulk-fill and conventional resin-composites. Journal of Dentistry (2014), http://dx.doi.org/10.1016/j.jdent.2014.12.004


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Fig. 4 – A strong positive correlation between % increase of top surface microhardness after 24 h dry storage and % decrease of microhardness after 24 h storage in 75% ethanol/water solution (R2 = 0.94).

different materials were directly related to the filler weight percent of the materials. These findings are in line with previous data that showed increasing the filler amount in a resin-composite improves its mechanical properties in terms of hardness, compressive strength, and wear resistance.38 All materials showed a significant increase of microhardness (%) after 24 h of dry storage at 37 8C. These in vitro results are consistent with a previous study that showed an increase of microhardness over a period of a month of dry storage at 37 8C with microhardness tending to increase rapidly within the first hour and then slowly reaching the maximum at about one week.16 Another study showed significantly higher hardness and degree of conversion values after one week

Table 3 – The mean (SD) values of extrapolated onset decomposition temperature and filler content from TGA of six bulk-fill (light grey) and eight conventional resin composite materials (dark grey). Material

SDR VBF XB FBF TEC SF GRF VDF XF FF GR VD TPH Z250

Extrapolated onset decomposition temperature (8C) 280.9 299.7 382.5 376.7 366.7 390.4 350.8 301.4 389.2 368.8 381.2 311.8 371.7 366.2

(2.6) a (0.7) b (1.9)h,i (1.2)g,h (1.2) f (0.1) i (5.8) e (1.9) b (2.6) (2.6) f (2.6) h (2.9) d (2.1)f,g (2.9) f

Filler content measured by TGA (wt%) 66.2 60.1 73.6 60.6 72.6 82.8 78.3 60.2 59.7 58.3 85.3 78.1 75.1 79.0

(0.2) c (0.1) b (0.0) d (0.2) b (0.1) d (0.2) g (0.9) f (0.1) b (0.5) b (0.9) a (0.1) h (0.2) f (0.1) e (0.2) f

Values with same superscript letters per column indicate statistically homogenous groups (Tukey test, a = 0.05).

compared to immediately post-cure values.18 A continuous increase of VH up to one week has been observed with about 92% of the maximum hardness achieved at 24 h and slightly higher VH hardness values at 37 8C compared to 23 8C storage temperature.19 Additionally, a significantly higher degree of conversion values 24 h post-cure compared to immediately post-cure for bulk-fill and conventional resin-composites using FTIR spectroscopy was confirmed recently.39 The increase in percentage microhardness after 24 h of dry storage in the current study is mainly attributed to progressive crosslinking reaction and post-irradiation polymerization, hence; it can be used as an indirect measure to assess changes in the degree of conversion of the resin matrix.8,16,18,40,41 The results also showed a significant negative correlation between the percentage increase of hardness after 24 h of dry storage and the initial microhardness values. This finding can be explained by the effect of mobility of reactive molecules in the initial polymer network.42 Mobility in the initially developed network can be restricted either by the initial high degree of monomer conversion or possibly by the initial high hardness values of the resin-composite associated with high filler content. The first assumption is clearly demonstrated by GRF which in a previous study showed considerably high degree of conversion value of 77.1 (SD 3.5) immediately postcure while SDR showed a low initial degree of conversion value of 58.4 (SD 0.5).39 The composites with initially higher degree of conversion are considered to have a rigid network that restricts the migration of reactive species and residual monomers and thus restrict further post-irradiation polymerization reaction. All tested materials showed no significant differences between top and bottom hardness with bottom/top hardness ratio higher than 0.9 for a thickness of 2 mm for conventional composites and 4 mm for bulk-fill composites. Assessment of bottom/top surface hardness ratio is a valuable indicator of the thoroughness of cure of a sample with ratio of 0.8 being suggested as a minimally acceptable value for light cured resin-composites.16,43 Accordingly, the results of the current study indicate sufficient curing of the bottom surface at the recommended manufacturer thickness, and efficient performance of the bulk-fill materials, which also indicates adequate polymerization throughout the whole increment depth since a linear relationship exists (R2 = 0.96) between bottom/top VH ratio and bottom/top degree of conversion ratio measured by FTIR.40 This finding is in a good agreement with a recent study which used Vickers hardness profile to determine the depth of cure of bulk-fill composites (VBF, XB, FBF, TEC, SF) and showed that SF attains sufficient bottom/top hardness ratio up to a depth of 5 mm (5). The same findings were also shown by a recent study in which the role of material translucency on depth of cure was investigated (4): it was found that the depth of cure is enhanced by increased translucency of all bulk-fill composites compared to conventional materials except SF (4). Moreover, the results of the current study showed a tendency towards decreased bottom/top hardness ratio with increased filler loading and viscosity of the materials which is explained by the decreased curing light transmittance associated with increased filler loading.44 A significant decrease in microhardness after 24 h storage in 75% E/W solution for all tested materials was detected



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Fig. 5 – Representative thermogravimetric curves for four different composites (SDR, VD, SF and XF). Note the two-stage decomposition of both SDR and VD. SF showed a typical one-stage decomposition thermogram. XF shows an early weight loss due to vaporization at 140.1 8C (arrow) prior to decomposition.

compared to pre-storage microhardness values (24 h postcure). GRF and SF showed the lowest percentage decrease of hardness value after storage in 75% E/W (14.5% SD 3.9 and 21.8 SD 5.9, respectively), while the highest decrease was shown by SDR (74.2% SD 1.9) (Fig. 4). These results are in line with recently published data which involved measurement of the ratio of VHN after 24 h ethanol storage to VHN after 24 h dry storage for bulk-fill and conventional resin-composites: SF showed a high ratio of 0.90 indicating minimal softening effect while a ratio of as low as 0.19 was shown by SDR.14 Interestingly, the results of the current study showed that the degree of softening by 75% E/W is significantly positively correlated to the degree of post-irradiation hardness development. There are no previous reports exploring this relation. We would hypothesize that the results of the current study may hint to the existence of two networks: the primary which is formed immediately after curing and has higher cross-link density, and the secondary, which is formed after curing and appears to be of more linear and poorly cross-linked structure

with compromised resistance to the chemical effect of solvents compared to the primary one.21 The degree of cross-link density has a considerable effect on mechanical behaviour of a polymer network with poorly cross-linked structures showing less rigidity and amore rubbery behaviour.45 The lower cross-link density of the secondary compared to the primary network could be due to slower movement of polymer chains during the secondary polymerization. This can also be due to primary cyclization reaction phenomenon that promotes secondary network inhomogeneity and reduces the mechanical strength compared to the initial microgel reaction that occurs in the primary network and results in highly cross-linked regions.46,47 The current results may also indicate that the long-term hydrolytic and mechanical resistance is more influenced by the quality of the primary network with only temporary and limited value of the secondary network. A similar effect of storage on mechanical properties that conform with our findings was observed in a previous study when post-cure heat-treated composites and



8


Fig. 6 – A negative correlation between the initial decomposition temperature (8C) for each tested composite material and % decrease of microhardness after 24 h storage in 75% ethanol/water solution (R2 = 0.50).

normally cured composites were aged in water.48 Although post-cure heat treated composites exhibited higher fracture toughness, flexural strength, and degree of conversion compared to normally cured ones, both composites exhibited parallel reduction in properties after they were aged in water.48 The filler weight content has a direct and positive correlation with mechanical behaviour as was shown by the microhardness values in this study and other mechanical properties assessed in various studies.15,49 Inorganic filler loading ranged from 58.3% weight (SD 0.9) for FF to 85.3% weight (SD 0.1) for GR as determined using TGA. Values of filler loading assessed by TGA were slightly lower than those reported by the manufactures owing to thermal degradation of the organic silane coating and evaporation of some inorganic component of the fillers.30 In case of TEC, however, the discrepancy was high which is attributed to the thermal decomposition of the pre-polymerized ‘organic filler’ fraction incorporated into this resin-composite material. Assessment of the TGA curves of the different materials showed variable extrapolated onset decomposition temperatures that ranged from 280.9 (SD 2.6) for SDR to 390.4 (SD 0.1) for SF. This reflects variations in thermal stability and degree of cross-link density of the polymer network of the different materials. All materials showed one-stage thermal decomposition except SDR and VD which showed clear two-stage decomposition that characterizes UDMA-based materials.46 The first stage may involve the decomposition of the side chains and lightly cross-linked primary cyclization sites of the organic matrix followed by decomposition of the highly crosslinked microgel regions. XF showed one-stage decomposition, however, a characteristic decrease in weight of about 1.5% was apparent at the very initial part of the TGA thermogram at a temperature of 140 8C. This may be due to vaporization of water absorbed into the polymer structure composed of the hydrophilic DEGDMA molecules. Also, it may involve vaporization of the low molecular weight residual DEGDMA monomer prior to thermal decomposition of the polymerized

matrix. Mechanisms involved in thermal decomposition of polymers include depolymerization, random chain scission, side group elimination, and oxidation of the polymer.50 In the current study, extrapolated onset decomposition temperature values were found to be strongly negatively correlated to the degree of chemical softening by 75% E/W solution. This may reflect that the mechanisms involved in initial thermal decomposition could be similar to those involved during solvent softening. According to the results of this study, bulk-fill resincomposites did not show a different behaviour as a group of materials than conventional resin-composites. Initial and postirradiation hardness of all tested materials were highly dependent on the inorganic filler content. Bottom/top hardness ratio of bulk-fill and conventional composites were comparable at the recommended manufacture increment thickness. Also, the degree of chemical softening and onset decomposition temperature were variable and mainly affected by the polymer network characteristics of each material independently. Thus, the first null hypothesis was accepted. The second null hypothesis was rejected since a relationship between initial hardness and the extent of post-irradiation hardness development and between thermal stability measured by TGA and the extent of chemical softening was detected.

5.

Conclusions

Within the limitations of this study the following can be concluded:

1. Bulk-fill materials showed comparable bottom/top hardness ratios to those of conventional composites despite their increased increment thickness. 2. The extent of post-irradiation hardness development and polymerization seems to be influenced by the initial hardness value (the lower the initial value the higher the increase of post-irradiation hardness). 3. There seems to be two networks in the polymerized resin matrix. The secondary (post-irradiation) seems to be of poor cross-link density with low resistance to degradative effects of solvents and the primary seems to be more stable. 4. Inorganic filler content, initial thermal stability, and thermal decomposition kinetics measured by TGA were variable for different resin-composites. 5. A strong relationship exists between Initial thermal decomposition and degree of chemical softening of the resin-matrix.

Conflict of interest statement None.

Acknowledgements The authors gratefully acknowledge Andrew Forrest from the School of Material Science (The University of Manchester) for



his assistance with the thermo gravimetric analysis measurements of the resin-composite samples.

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