d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 1585–1592

avai lab le at www.sc iencedi rec t .com

journa l homepage: www. int l .e lsev ierhea l th .com/ journa ls /dema

Effect of filler particle size and morphology on force/workparameters for stickiness of unset resin-composites

Muhammad Kaleema,b,∗, Julian D. Satterthwaitea, David C. Wattsa

a School of Dentistry, University of Manchester, Manchester, UKb Army Medical College, National University of Science and Technology, Rawalpindi, Pakistan

a r t i c l e i n f o

Article history:

Received 3 July 2009

Accepted 4 August 2009

Keywords:

Resin-composite

Rheology

Stickiness

Filler morphology

Filler size

a b s t r a c t

Objectives. To investigate the effect of variation in filler particle size and morphology within

an unset model series of resin-composites on two stickiness parameters: (1) maximum

probe separation-force and (2) work-of-separation. This study was to complement pre-

viously reported measurements of composite stickiness in terms of a strain-parameter,

‘peak-height’.

Materials and methods. Eleven experimental light cured resin-composites were selected. All

had the same matrix (Bis-GMA, UDMA and TEGDMA, with 0.33% camphoroquinone) and

the same filler volume fraction—56.7%, however filler particles varied in size and shape

and were either unimodal or multimodal in size-distribution. Each material was placed in a

cylindrical mould (ϕ = 7 mm × 5 mm depth) held at 26 or 37 ◦C. The maximum force (Fmax, N)

and work of probe-separation (Ws, N mm) were measured. A flat-ended stainless-steel probe

(ϕ = 6 mm) was mechanically lowered onto and into the surface of the unset sample, until a

compressive force of 1 N was reached, which was held constant for 1 s. Then the probe was

moved vertically upward at a constant speed; either 2 or 8 mm/s. The tensile force produced

on the probe by the sticky composite was plotted against displacement and the maximum

value was identified (Fmax). Ws was obtained as the integrated area. Data was analyzed by

multivariate ANOVA and multiple pair-wise comparisons using a Tukey post hoc test to

establish homogenous subsets (at p = 0.05) for Fmax and a Games–Howell was used for Ws.

Results. As potential measures of stickiness, Fmax and Ws showed more coherent trends

with fillersize when measured at the lower of the two probe speeds, 2 mm/s. For unimodal

resin-composite Fmax ranged from 1.04 to 5.11 N and Ws from 0.48 to 11.12 N mm. For the

multimodal resin-composite they ranged from 1.64 to 4.13 N and from 2.32 to 8.34 N mm

respectively. Temperature increase tended to slightly reduce Fmax, although this trend was

not consistent. Ws generally increased with temperature.

Conclusion. Filler particle size and morphology influences Fmax and Ws of uncured resin-

composite which partly express the handling behaviors of resin-composites.

© 2009 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

∗ Corresponding author at: Biomaterials Research Group, School of Dentistry, University of Manchester, Higher Cambridge Street, M15 6FHManchester, UK. Tel.: +44 161 2254624.

E-mail address: dr kaleem78@hotmail.com (M. Kaleem).0109-5641/$ – see front matter © 2009 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.dental.2009.08.002

s 2 5 ( 2 0 0 9 ) 1585–1592

Table 1 – Formulation of unimodal resin-compositesused in the study.

Filler particles Resin-composite code

Shape Size (nm) wt%

Spherical 100 72.3 RZD102250 72.6 RZD107500 72.6 RZD106

1000 72.5 RZD105

Irregular 700 76.4 RZD1081000 76.4 RZD1091500 76.4 RZD110

The resin matrix consisted of Bis-GMA, UDMA, TEGDMA and thefiller volume fraction was constant at 56.7%, for all materials. Spher-ical (SiO2); irregular (Ba–Al–B-silicate glass). The consequence ofvarying the filler sizes, while maintaining filler volume fraction was

1586 d e n t a l m a t e r i a l

1. Introduction

Initially resin-composites were introduced as anterior restora-tive materials. Subsequently, developments in resin andfiller technology and demand for tooth coloured restorationsfacilitated their employment as an aesthetic alternative toamalgam for posterior restorations [1,2]. However, their use forlarge restorations is still controversial and fracture of restora-tions in the posterior region has been found to be a commoncause for restoration failure [3,4].

Many attempts have been made to overcome this problemby altering the composition of the monomer system, intro-ducing new filler technology and reinforcing the matrix withfibres [5,6]. However, silane treatment of inorganic fillers andchanges in their shape, size and ratio of the filler resin matrixphase has affected the viscosity of the resin-composite [7–10].Viscosity is directly related to the handling characteristicsof the resin-composite, including malleability, ease of place-ment, contouring of the restoration and stickiness to toothstructure or the instrument [11–16]. Thus viscosity influencesthe technique sensitivity of the restorative material, whichaffects the chair side operating time and quality of the restora-tion. An ideal material should flow around and into everycorner of the prepared cavity and stay there when the load isremoved. Ideally there should be good stickiness to the dentaltissues, and low stickiness to instruments [16].

Several studies have focused on the effect of filler size,morphology and variations in formulation on the mechanicalproperties of cured resin-composites [17–21]. Very few studieshave investigated the effect of filler particle morphology, sizeand resin filler compositional ratio on the rheological behaviorof uncured resin composite [22].

Regarding the stickiness of resin-composites, a study by Al-Sharaa and Watts described a new method for evaluation ofthe stickiness of light-cure resin-composites [16]. They useda flat-ended steel probe to measure stickiness of eight resin-composites. This was measured as the peak height, obtainedby curing the material shortly after it separated from theprobe. The present study is designed to complement the pre-viously reported stickiness measurement of ‘peak-height’, byfocusing on effect of the filler particle size and morphology onthe force required for probe separation from unset material.

The aim of this study was to investigate the effect of varia-tions in filler particle size and morphology on maximum probeseparation-force (Fmax) and work-of-separation (Ws) and toassess the effect of temperature and speed of probe separa-tion. The null hypotheses were that changes in filler particlesize, morphology, speed and temperature have no effect onthe stickiness of unset resin-composites.

2. Materials and methods

Ten experimental formulations [RZD] were used in this study(Tables 1 and 2), all were visible-light cured (Ivoclar Vivadent,

Schaan, Liechtenstein). Their matrix was a combination of Bis-GMA, UDMA and TEGDMA, with 0.33% camphoroquinone. Allof the RZD composites had a particulate dispersed phase ofthe same volume fraction—56.7%, which was treated with a

that the number of particles per unit volume varied systematically.

silane coupling agent (methacryloxypropyltrimethoxysilane).The filler particles were systematically graded in size, and fur-ther were either spherical or irregular. The spherical particleswere silica and made from solution, the irregular particleswere ground glass melts (Ba–Al–B-silicate glass).

Maximum force (Fmax, N) and work of probe-separation(Ws, N mm) were measured with a texture analyzer (Fig. 1)(TA.XT2i, Stable Micro Systems, Godalming, Surrey, UK).The analyzer comprised a stainless-steel cylindrical probe(ϕ = 6 mm) connected to a force transducer, which measuredthe force acting on the probe. Modifications were carried outto measure the handling characterizes. A thermostaticallycontrolled frame (ϕ = 70 mm) with a cylindrical cavity (ϕ = 7,depth = 5 mm) was constructed (Fig. 2). This frame was fixedto the stainless-steel stand. The temperature of the mouldcavity was regulated using an adjustable power supply unitwith a thermocouple in close proximity to the sample. Eachspecimen was placed in the mould cavity at 26 or 37 ◦C.

The flat-ended stainless-steel probe was mechanically low-ered onto and into the surface of the unset sample. Whena ‘trigger’ compressive force of 0.05 N was registered, data-acquisition commenced. The probe descended further, intothe sample surface layer, until a compressive force of 1 N wasrecorded, which was held constant for 1 s.

During the ‘debonding’ phase, the probe was moved verti-cally upward at a predetermined constant velocity of 2 and8 mm/s. As the specimen material adhered to the probe,it became elongated and exerted a tensile force on thetransducer. The magnitude of the force and the elongationdepended on the viscoelastic properties of the material.

Data were entered into statistical software (SPSS ver.16, SPSS Inc., Illinois, USA). For unimodal resin-composites,multivariate ANOVA was used with Probe-withdraw speed,temperature, and particle size as independent variables and,Fmax and Ws as dependent variables. The analyses were doneseparately for spherical and irregular resin-composite. Homo-geneity of variance was calculated by using Levene statistics,according to which multiple pair-wise comparisons using a

Tukey post hoc test was conducted to establish homogenoussubsets (at p = 0.05) for Fmax and Ws the Games–Howell posthoc test was used.

d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 1585–1592 1587

Table 2 – Formulation of multimodal resin-composites used in the study.

Filler particles Resin-composite code

Shape Size (nm) wt%

Spherical 100, 250 and 1000 (1:1:2) 72.0 RZD 114100 and 1000 (1:3) 72.0 RZD 113

Irregular 450 and 1000 (1:3) 76.4 RZD 111450, 700 and 1500 (1:1:3) 76.4 RZD 112

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difference between resin-composites with filler particle sizesof 1000 nm and the other three filler particle sizes of 100 nm(p = 0.009), 250 nm (p = 0.018) and 500 nm (p = 0.009).

The resin matrix consisted of Bis-GMA, UDMA, TEGDMA and the fillerirregular (Ba–Al–B-silicate glass). The consequence of varying the fillparticles per unit volume varied systematically.

. Results

he tensile force (N) data were plotted against the distance ofpward movement (mm). Two patterns of force/displacementehavior were observed, depending mainly on the materialnd probe withdrawal speed. Type I (Fig. 3) showed a primaryeak, followed by a secondary peak or shoulder. Fmax wasefined as the maximum force of this graph. Tensile force

ncreased essentially linearly with probe displacement, to aield point when the adhesive started to void and ultimatelyetach from the probe surface at Fmax. The work of probe-eparation Ws was obtained, as the area under the curve fromhe point when the probe started moving upwards until the

aterial was completely detached from the probe.Fmax and Ws were taken as potential measures of stickiness

nd are reproducible (Figs. 3 and 4). The mean and stan-ard deviations of Fmax and Ws for all the resin-compositesre summarized in Tables 3 and 4 respectively. For resin-omposite with unimodal spherical and irregular fillers meannd standard deviation values of Fmax at the withdraw speedf 2 and 8 mm/s are given in Figs. 5 and 6) showed a sin-le peak. Type II (Fig. 4 respectively. Similarly Figs. 7 and 8epresent the Ws mean and standard deviation values at with-raw speed of 2 and 8 mm/s respectively. For the multimodalller resin-composites, Fmax and Ws mean values and stan-ard deviations at 26 and 37 ◦C are graphically presented in

igs. 9–12.

For the unimodal resin-composites, irregular filler parti-le resin-composites showed overall low Fmax and Ws valuest all withdrawal speeds and temperatures, as compared to

Fig. 1 – Experimental setup used fo

me fraction was constant at 56.7%, for all materials. Spherical (SiO2);es, while maintaining filler volume fraction was that the number of

spherical filler particles (Figs. 5–8). An increase in particle sizeresulted in a decrease in Fmax and Ws at both temperaturesand both probe withdraw-speed for irregular fillers, whereas,for the spherical filler particle resin-composite, no specific pat-tern was followed.

With spherical filler resin-composites, multivariate ANOVAshowed a significant influence of the probe withdraw-speedand particle size on Fmax and Ws (p < 0.001) and a strong inter-action was also found between these factors for both Fmax andWs (p < 0.05). In the case of Fmax, a post hoc Tukey test identi-fied two subsets for the effect of filler particle size (Table 5), andfor Ws, the Games–Howell post hoc test showed a significant

Fig. 2 – Mould holding the materials with temperatureregulation.

r the stickiness measurement.

1588 d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 1585–1592

Table 3 – Mean values and standard deviation of maximum force Fmax (N) at 26 and 37 ◦C.

Filler morphology Filler particle size(nm)

26 ◦C 37 ◦C

2 mm/s 8 mm/s 2 mm/s 8 mm/s

Spherical 100 3.07 (0.30) 3.28 (0.37) 2.82 (0.28) 2.73 (0.58)250 2.77 (0.21) 1.91 (0.27) 2.60 (0.10) 4.33 (0.35)500 3.07 (0.30) 3.28 (0.37) 2.82 (0.28) 2.73 (0.58)1000 2.40 (0.26) 5.11 (0.42) 2.53 (0.47) 4.53 (0.12)

Irregular 700 2.22 (0.34) 2.88 (0.34) 1.85 (0.29) 3.87 (0.16)1000 1.14 (0.35) 2.61 (0.21) 1.41 (0.34) 1.85 (0.27)1500 1.30 (0.25) 1.29 (0.25) 1.04 (0.32) 1.22 (0.07)

Multimodal 100, 250 and 1000 (1:1:2) 2.18 (0.06) 3.85 (0.25) 2.14 (0.14) 3.77 (0.15)450 and 1000 (1:3) 1.64 (0.08) 4.03 (0.15) 2.16 (0.21) 4.13 (0.06)450, 700 and 1500 (1:1:3) 2.23 (0.02) 3.40 (0.32) 2.06 (0.47) 3.65 (0.08)

Table 4 – Mean values and standard deviation of work of probe-separation Ws (N mm) at 26 and 37 ◦C.

Filler morphology Filler particle size (nm) 26 ◦C 37 ◦C

2 mm/s 8 mm/s 2 mm/s 8 mm/s

Spherical 100 1.32 (0.10) 1.50 (0.02) 1.49 (0.43) 1.66 (0.56)250 1.55 (0.18) 0.88 (0.20) 2.19 (0.04) 3.18 (0.15)500 1.32 (0.10) 1.50 (0.02) 1.49 (0.43) 1.66 (0.56)1000 2.71 (0.41) 11.12 (1.52) 2.25 (0.48) 9.03 (0.16)

Irregular 700 1.15 (0.27) 2.48 (0.33) 3.39 (1.31) 3.26 (0.17)1000 0.63 (0.11) 1.54 (0.09) 1.01 (0.46) 1.58 (0.47)1500 0.67 (0.02) 0.79 (0.22) 0.48 (0.18) 0.68 (0.10)

Multimodal 100, 250 and 1000 (1:1:2) 2.32 (0.10) 5.98 (0.05) 2.67 (0.05) 8.34 (0.10)450 and 1000 (1:3) 3.07 (0.24) 4.60 (0.21) 2.46 (0.20) 6.79 (0.10)450, 700 and 1500 (1:1:3) 2.78 (0.24) 2.95 (0.42) 2.73 (0.31) 5.37 (1.10)

Table 5 – Filler particle size subsets identified by posthoc Tukey test for Fmax and were valid for bothtemperatures and speeds.

1 2 3

Fmax Irregular 700 nm 1000 nm 1500 nm

Spherical 100 nm 1000 nm250 nm500 nm

For the irregular filler resin-composites multivariateANOVA showed a significant influence of the particle size onF and W (p < 0.001) and probe withdraw-speed also had a

max s

significant influence on Fmax (p < 0.001). A strong interactionwas also found between probe withdraw-speed, temperatureand particle size for Fmax (p < 0.05), and between temperature

Fig. 3 – Typical Type I force/displacement curve.

Fig. 4 – Typical Type II force/displacement curve.

and particle size for Ws (p < 0.05). In the case of the Fmax, thepost hoc Tukey test showed three subsets for the effect of fillerparticle size (Table 5). For Ws, the Games–Howell post hoc testshowed a significant difference between all filler particle sizes(p < 0.05).

For the multimodal filler resin-composite, increase in with-draw speed from 2 to 8 mm/s resulted in an increase in Ws andFmax at both 26 and 37 ◦C but this effect was more prominentat 37 ◦C.

4. Discussion

This experiment was designed to complement a previouslyreported ‘peak height’ measurement system for character-

d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 1585–1592 1589

Fig. 5 – Maximum force (Fmax) at the withdraw speed of2 mm/s by filler particle size. For the spherical fillers, atboth temperatures, analysis showed that 100, 250 and500 nm (but not 1000 nm) filler resin-composites werestatistically equivalent (p > 0.05). The irregular fillerresin-composites showed statically significant differences(p < 0.05) as size increased.

Fig. 6 – Maximum force (Fmax) at the withdraw speed of8 mm/s by filler particle size. For the spherical fillers, at bothtemperatures, analysis showed that 100, 250 and 500 nm(but not 1000 nm) filler resin-composites were statisticallyes

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Fig. 7 – Work of probe-separation (Ws) at the withdrawspeed of 2 mm/s by filler particle size.

Fig. 8 – Work of probe-separation (Ws) at the withdrawspeed of 8 mm/s by filler particle size.

quivalent (p > 0.05). The irregular filler resin-compositeshowed statically significant differences (p < 0.05) with size.

zing stickiness [16]. This may be compared with the workf Chuang et al. [23], on the stickiness of pressure sensitivedhesives. This new test method is very quick and highlyeproducible and gives a detailed representation of somespects of the stickiness behavior of the material, by measur-ng the maximum force (Fmax) and work of probe-separationWs). However, it remains to be seen, whether this will be morer less useful than the peak height method [16].

Two types (I and II) of force/displacement profile were

bserved overall (Figs. 3 and 4). The most common (Type I,ig. 3) exhibited a single peak. As the probe ascended from theesin-composite surface, any attached material was stretched,nd the force increased. When the composite paste started

Fig. 9 – Mean and standard deviation of maximum force(Fmax) for multimodal composites at 26 ◦C.

1590 d e n t a l m a t e r i a l s 2 5

Fig. 10 – Mean and standard deviation of maximum force(Fmax) for multimodal resin-composites at 37 ◦C.

Fig. 11 – Mean and standard deviation of work ofprobe-separation (Ws) for multimodal composites at 26 ◦C.

Fig. 12 – Mean and standard deviation of work ofprobe-separation (Ws) for multimodal composites at 37 ◦C.

( 2 0 0 9 ) 1585–1592

to void and then detach from the probe surface, the forcedecreased and ultimately fell to zero. In some cases (Type II,Fig. 4) the detachment process was more gradual and resultedin a secondary peak or shoulder.

The typical profiles obtained in this study (Figs. 3 and 4) arethe result of the combined response of the stress–strain behav-ior of resin-composites and the interfacial strength of thebond between probe and composites. In both cases Fmax is theheight of initial peak produced during the ascent of the probefrom the resin-composite surface and gives the tensile forceacting on the probe by the attached material, up to the pointwhere the resin-composite started to craze and detach fromthe probe surface. Fmax depends on the wetability of the resin-composite; level of resistance exerted against the debondingforce [23], roughness of both probe and material surfaceswhich will effect the true contact area [24], the presence ofair bubbles between the surfaces [24] and probe withdraw-speed [25]. It also must depend on the tensile strength andelasticity of the unset composite paste, at each measurementtemperature.

The second parameter Ws was the area under thestress–strain curve, which is the work done/energy requiredfor the probe separation from the resin-composites. This Ws isdirectly related to the shear performance of a material, whichin turn is related to level of cross-linking [23], which should bevery low or zero for the unset pastes. Again it is governed bythe extensibility of the paste.

The composition of resin matrix and filler size and shapeaffect the viscosity of the material. In particular, interlockingand interfacial interactions between filler particles and theresin monomers and matrix play an important role in con-trolling viscoelastic properties [12], which in turn affect thestickiness of the restorative material. In this study the effect offiller particle morphology on the force/work parameters wastested using ten experimental resin-composites of differentfiller sizes. Effects of temperature and withdraw-speed werealso investigated. The model resin-composites (RZD) used inthis study varied only in their filler size, shape and weight(%). All of them had the same organic resin matrix composi-tion (Bis-GMA, UDMA and TEGDMA) and same filler volumefraction (i.e. 56.7%) (Tables 1 and 2). This filler volume frac-tion plays a major role in relation to rheology because thehydrodynamic force which acts on the surface of particles oraggregates of particles, is generally unrelated to particle den-sity [26]. Thus any change in the stickiness behavior in thisstudy may be attributed principally to the influence of fillersize and shape.

For unimodal spherical filler resin-composites, multivari-ate ANOVA showed a strong influence of probe-withdrawspeed and particle size on both Fmax and Ws (p < 0.001). At lowwithdraw speed of 2 mm/s, the increase in size of the fillerparticles resulted in decrease in the Fmax and Ws, that mightbe due to decrease in the active surface area of the particles,resulting in less resistance to upward movement of the probe.At the higher speed of 8 mm/s, resin-composite with a fillerparticle size of 1000 nm showed a high increase in the both

Fmax and Ws (Figs. 6 and 8). This was also identified by Tukeypost hoc test for Fmax (Table 5), whereas, in the case of Ws,the post hoc Games–Howell test again showed a significantdifference between 1000 nm filler resin-composite and all oth-

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rs (p < 0.05), that might be because at this size there was anncrease in the resin part of the resin-composite due to morepaces between the particles, which resulted in an increasen resistance to the upward movement of the probe due to areat shear resistance between the polymer chains [23].

Most commercial resin-composites have different fillerizes in order to increase the filler fraction. Their mean val-es are usually given by the manufacturer. For the multimodalesin-composite used in this study, it was difficult to calculatehe mean filler size, and they were analyzed separately fromhe unimodal resin-composites.

Multimodal filler resin-composites with spherical particleshowed higher Fmax and Ws compared to irregular ones. It isifficult to give a completer explanation as so many complexactors may be involved.

For the irregular resin-composite multivariate ANOVAhowed a very strong influence of the particle size on both

max and Ws (p < 0.001), as an increase in the filler particleize resulted in decrease in Fmax and Ws (Figs. 5–8). Especiallyn the case of Ws, the post hoc Games–Howell test showed aighly significant difference (p < 0.05) between all filler parti-le sizes. Similarly for Fmax the post hoc Tukey test identifiedhree subsets for the three resin-composite and showed a sig-ificant difference between them. The reason for this trendight be that irregular filler particles at higher volume frac-

ion can be more interlocked with each other so they exertore resistance to a deforming force, which in this case is

pward movement of the probe.There was no significant influence of temperature on both

max and Ws (p > 0.001) at both speeds. That might be dueo the fact that temperature mainly affects the resin part ofhe resin-composite as compared to the filler part. And allhe experimental materials used in this study have the same

onomer composition and are present in very low fraction.Temperature change had rather visible effects on the

arameters Fmax and Ws as seen in the graphical plots, fornimodal resin-composites (Figs. 5–8).

As temperature increases, it is to be expected that rela-ive slippage of the internal composite components may occur

ore readily. Hence the pastes are likely to be more extensi-le. Also the extended material is likely to break with a lowerorce. Hence the Fmax value could be expected to be lower at aigher temperature. If so, this means that Fmax is less useful asmeasure of stickiness than is ‘peak height’ [16]. Nevertheless,

he parameter Ws, as an integrated area of force and distances sensitive to changes in extensibility. So this may be moreuitable stickiness parameter than Fmax. It does give a greaterrend of increasing with temperature, although some of theata suggests no significant differences with temperature.

. Conclusions

Filler shape strongly influence the Fmax and Ws of the resin-composites.Probe-withdraw speed strongly effect the Fmax and Ws in

the case of the spherical filler particle resin-composites.Temperature has no strong significant effect on the Fmax

and Ws of resin-composites with this experimental resincomposition.

0 0 9 ) 1585–1592 1591

• Fmax and Ws may be used as potential measures of the stick-iness, with certain qualifications. These parameters are lesssensitive to temperature than probe height [16], that morefully expresses the clinically perceived increase of stickinesswith temperature.

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