Poly(methyl-methacrylate) nanocomposites with low silica addition Sebastian Balos, PhD, a Branka Pilic, PhD, b Dubravka Markovic, DDS, PhD, c Jelena Pavlicevic, PhD, d and Ognjan Luzanin, PhD e University of Novi Sad, Novi Sad, Serbia Statement of problem. Poly(methyl-methacrylate) (PMMA) represents the most popular current denture material. However, its major drawbacks are insufficient ductility and strength. Purpose. The purpose of this study was to improve the mechanical properties of PMMA in denture base application by adding small quantities of nanosilica. Material and method. Silica nanoparticles were added to the liquid component of the tested materials. The standard heat polymerizing procedure was followed to obtain 6 PMMA–silicon dioxide (/SiO 2 ) concentrations (0.023%, 0.046%, 0.091%, 0.23%, 0.46%, and 0.91% by volume). Microhardness and fracture toughness of each set of specimens was compared with the unmodified specimens. Furthermore, differential scanning calorimetry and scanning electron microscopy analyses were conducted, and the results obtained were correlated with the results of mechanical properties. Results. It was found that the maximum microhardness and fracture toughness values of the materials tested were obtained for the lowest nanosilica content. A nanosilica content of 0.023% resulted in an almost unchanged glass transition tem- perature (Tg), whereas the maximum amount of nanosilica induced a considerable increase in Tg. A higher Tg indicated the possible existence of a thicker interfacial layer caused by the chain immobility due to the presence of the particles. However, scanning electron microscopy results demonstrated extensive agglomeration at 0.91% nanosilica, which may have prevented the formation of a homogenous reinforced field. At a nanosilica content of 0.023%, no agglomeration was observed, which probably influenced a more homogenous distribution of nanoparticles as well as uniform reinforcing fields. Conclusions. Low nanoparticle content yields superior mechanical properties along with the lower cost of nanocomposite synthesis. (J Prosthet Dent 2014;111:327-334) Clinical Implications By adding low amounts of nanosilica with a hydrophobic surface layer, conventional poly(methyl-methacrylate) dentures can be made thinner, stronger, more resistant to cracking, and more durable. Thus, improvement of mechanical properties of denture materials improves the quality of life in patients. Poly(methyl-methacrylate) (PMMA) is the leading material for dentures today. Its advantages are related to its biocompatibility and esthetics, whereas its drawbacks are insufficient ductility and strength, which leaves opportunities for further improvement. 1,2 One method of improving the mechanical properties of PMMA may be nanoparticle addition, and one of the most common nano- particle fillers is nanosilica. Nanosilica has been successfully mixed with PMMA and other polymeric and dental composite materials, and several experimental studies have shown notable effects on mechanical and thermal properties. 3-8 However, nanoparticle loading also may result in decreased mechanical properties. 9 a Assistant Professor and Deputy Head of Department, Department of Production Engineering, Faculty of Technical Sciences. b Associate Professor and Vice Dean, Department of Material Engineering, Faculty of Technology. c Professor and Director, Clinic of Dentistry, Clinical Center of Voivodina, Department of Dentistry, Medical Faculty. d Assistant Professor, Department of Material Engineering, Faculty of Technology. e Assistant Professor, Department of Production Engineering, Faculty of Technical Sciences. Balos et al
Transcript
Polnan
Sebastian Balos,
aAssistant Professor and Deputy HeabAssociate Professor and Vice Dean,cProfessor and Director, Clinic of DedAssistant Professor, Department ofeAssistant Professor, Department of
Balos et al
y(methyl-methacrylate)ocomposites with low silica addition
PhD,a Branka Pilic, PhD,b
Dubravka Markovic, DDS, PhD,c Jelena Pavlicevic, PhD,d andOgnjan Luzanin, PhDe
University of Novi Sad, Novi Sad, Serbia
Statement of problem. Poly(methyl-methacrylate) (PMMA) represents the most popular current denture material. However,its major drawbacks are insufficient ductility and strength.
Purpose. The purpose of this study was to improve the mechanical properties of PMMA in denture base application byadding small quantities of nanosilica.
Material and method. Silica nanoparticles were added to the liquid component of the tested materials. The standard heatpolymerizing procedure was followed to obtain 6 PMMA–silicon dioxide (/SiO2) concentrations (0.023%, 0.046%, 0.091%,0.23%, 0.46%, and 0.91% by volume). Microhardness and fracture toughness of each set of specimens was compared with theunmodified specimens. Furthermore, differential scanning calorimetry and scanning electron microscopy analyses wereconducted, and the results obtained were correlated with the results of mechanical properties.
Results. It was found that the maximum microhardness and fracture toughness values of the materials tested were obtainedfor the lowest nanosilica content. A nanosilica content of 0.023% resulted in an almost unchanged glass transition tem-perature (Tg), whereas the maximum amount of nanosilica induced a considerable increase in Tg. A higher Tg indicated thepossible existence of a thicker interfacial layer caused by the chain immobility due to the presence of the particles. However,scanning electron microscopy results demonstrated extensive agglomeration at 0.91% nanosilica, which may have preventedthe formation of a homogenous reinforced field. At a nanosilica content of 0.023%, no agglomeration was observed, whichprobably influenced a more homogenous distribution of nanoparticles as well as uniform reinforcing fields.
Conclusions. Low nanoparticle content yields superior mechanical properties along with the lower cost of nanocompositesynthesis. (J Prosthet Dent 2014;111:327-334)
Clinical ImplicationsBy adding low amounts of nanosilica with a hydrophobic surface layer,conventional poly(methyl-methacrylate) dentures can be made thinner,stronger, more resistant to cracking, and more durable. Thus,improvement of mechanical properties of denture materials improves thequality of life in patients.
Poly(methyl-methacrylate) (PMMA)
is the leading material for denturestoday. Its advantages are related to itsbiocompatibility and esthetics, whereasits drawbacks are insufficient ductility andstrength, which leaves opportunities for
d of DepDepartmntistry, CMaterial EProductio
further improvement.1,2 One method ofimproving the mechanical properties ofPMMA may be nanoparticle addition,and one of the most common nano-particlefillers is nanosilica.Nanosilicahasbeen successfully mixed with PMMA and
artment, Department of Production Engineerinent of Material Engineering, Faculty of Technollinical Center of Voivodina, Department of Denngineering, Faculty of Technology.n Engineering, Faculty of Technical Sciences.
other polymeric and dental compositematerials, and several experimental studieshave shown notable effects on mechanicaland thermal properties.3-8 However,nanoparticle loading also may resultin decreased mechanical properties.9
g, Faculty of Technical Sciences.ogy.tistry, Medical Faculty.
Table I. Materials as specified by manufacturers
Materials andManufacturers
Powder/Liquid Ratio
Composition Lot Numbers
Powder Liquid Powder Liquid
Triplex Hot; IvoclarVivadent
23.4 g/10 mL PMMA, pigments, catalysts(0.5-1.5% benzoyl peroxide)
MMA, EDMA (2.5%-10%) N17392 N47618
Polyhot; Polident 22 g/10 g(30 mL/11 mL)
PMMA, pigments, catalysts MMA, other unspecifiedadditions
18 86
Biocryl-RN; Galenika 20 g/10 g PMMA, pigments and catalysts(0.31%)
In this study, the effect of nanosilicaaddition to 3 PMMA-based denture ma-terials was investigated. The approachused was aimed at determining gen-eral mechanical performance by testingfracture toughness and hardness. Frac-ture toughness offers answers related tothe behavior of materials in the presenceof a crack, whereas hardness providesinformation about the resistance of ma-terials to local plastic deformation.10-12
Nanosilica addition was kept at lowlevels (volume %, between 0.023% and0.91%) because low nanosilica contentand the posttreatment of the nano-particles may prevent or hinder agglom-eration.13-16 In this way, more uniformnanoparticle dispersion and distributionwas obtained, thus increasing the me-chanical properties.16,17 Another signifi-cant property of a nanocomposite maybe the thickness of a nanoparticle-basicmaterial interface.18-22 The null hypoth-esis was that no difference exists betweenthe mean fracture toughness or hardnessamong the 3 materials, with a total of6 silica levels.
MATERIAL AND METHODS
Three commercially available PMMA-based heat-polymerizing denture basebiopolymer materials and a nanoparticlemodifier were used in this study. Thematerials used in this study and theirproperties are given in Table I. Silica dis-persions (0.023%, 0.046%, 0.091%,
The Journal of Prosthetic Dentis
0.23%, 0.46%, and 0.91%) were preparedby mixing silica nanoparticles with themethyl methacrylate (MMA). Stable dis-persions of silica nanoparticles in MMAwere formed without phase separation orsedimentation for 24 hours. To betterdescribe various specimen groups, a des-ignation system was devised (Table II).Nanoparticles were weighed on a labo-ratory balance (accuracy, 0.0001 g) (Ad-venturer Pro; Ohaus). The mixing ofnanoparticles and the liquid component(MMA based) was performed by using amagnetic stirrer (MM-530; Tehtnica) witha speed of 500 rpm. The control groupdid not contain nanosilica particles.
After the introduction of silicananodispersions in MMA, the materialwas polymerized in the usual way. Theresulting 50 � 50 � 4-mm specimenswere cut with a standard metallographicabrasive cutting machine (Discotom;Struers) until exact specimen shapeswere obtained. Individual specimen di-mensions were achieved by using 1500
try
grit silicon carbide paper (P1500;Struers). The dimensions were checkedwith a micrometer (accuracy, 0.01 mm)(Altraco; Hyundai Measurement).
Fracture toughness was tested byusing a conventional tensile testingmachine (AT-L-118B; Toyoseiki) equip-ped with a 4-point bending device. Thecrosshead speed was kept constant at50 mm/min. The single edge V-notchbeam method was used, which com-prised a 4-point bending test and a20/40 mm distance between the sup-ports. Specimen dimensions were 45 �4 � 3 mm, with a notch cut at thelongitudinal center of the beam. Thepreliminary U notch was machinedmanually with a precision drill/grinder(FBS12; Proxxon) with a stroke of 3000to 15 000 min-1 fitted with a diameterof 20 � 0.8-mm silicon carbide disk. Tocreate the initial crack, a V notch wascut manually into the center of the Unotch with a commercial razor blade(Platinum; Gillette).14-16 The V-notch
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Table III. Fracture toughness, Vickers hardness, and their correspondingstandard deviations
Specimen
FractureToughness KIC,
MPam1/2Hardness HV0.3,
kg/mm2
T0 1.54 �0.11 19.6 �0.6
T002 2.09 �0.03 25.1 �0.5
T004 1.93 �0.10 23.1 �0.3
T009 1.78 �0.09 22.5 �0.8
T023 1.77 �0.08 21.6 �0.8
T046 1.74 �0.08 21.6 �1.1
T092 1.55 �0.06 24.8 �0.5
P0 1.38 �0.11 21.4 �1.0
P002 1.62 �0.04 25.2 �0.7
P004 1.40 �0.09 22.4 �0.3
P009 1.39 �0.07 23.5 �0.3
P023 1.36 �0.08 24.4 �0.6
P046 1.37 �0.08 25.0 �0.7
P092 1.40 �0.06 25.1 �0.6
B0 1.01 �0.11 22.1 �0.6
B002 1.59 �0.21 25.5 �0.6
B004 1.73 �0.17 25.2 �0.5
B009 1.58 �0.26 21.3 �0.4
B023 1.48 �0.11 22.2 �0.4
B046 1.51 �0.09 21.6 �0.7
B092 1.02 �0.02 23.4 �0.8
Biocryl Polyhot
2.2
2.0
1.8
1.6
1.4
1.2
1.00 0.023 0.046 0.460.092 0.23 0.92Fr
actu
re T
ough
ness
(M
Pa√
m)
Triplex
Silica Level (vol.%)
1 Fracture toughness versus nanosilica content: Triplex Hot,Polyhot, and Biocryl.
April 2014 329
depth was measured with a lightmicroscope (Orthoplan; Leitz/Leica).Fracture toughness was calculated ac-cording to the following equation:
where F is the maximum force (N), Sx isthe distance between the supports(mm), a is the geometric factorexpressed as a/W, a is the overall notchdepth (mm), B is the specimen width(mm), W is the specimen width (mm),and Y* is a dimensionless factor ofspecimen geometry. Fracture toughnesswas determined on the basis of 5specimens per tested group.
Microhardness was measured withthe Vickers microhardness tester (HVS-1000; Huayin). A 2.94 N load wasapplied by using 15 seconds dwell time,and 5 indentations were made. Micro-hardness was determined as the averagevalue and was calculated with thefollowing equation:
HV ¼ 1:8544 F
d2(2),
where F is the load (kg) and d is theaverage between 2 measured indenta-tion diagonals (mm).
Mechanical properties were statisti-cally analyzed by using 2-way ANOVAand 2 predictive factors, material andsilica level. The Dunnett multiple com-parison procedure was performed todetermine the statistically significantdifference between the control groupmean and the fracture toughness andmicrohardness means of the modifiedgroups. Fracture surfaces were exam-ined by a scanning electron microscope(SEM) (JSM-6460LV; JEOL) operatingat 25 kV. The specimens were previouslycoated with gold (SCD-005; Bal-tec/Leica). Differential scanning calorimetry(DSC) analysis was performed by using aDSC device (Q20;TA Instruments). Theanalysis was conducted within a 40�C to160�C temperature range.
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RESULTS
Fracture toughness, Vickers micro-hardness results, and the correspondingstandard deviations for the control
group and modified groups are shownin Table III. Trends regarding the me-chanical properties and nanosilicacontent are represented in Figures 1and 2, which show fracture toughness
Biocryl Polyhot
19
21
23
25
27
0 0.023 0.046 0.460.092 0.23 0.92
Vic
kers
Mic
roha
rdne
ss (
kg/m
m2 )
Triplex
Silica Level (vol.%)
2 Microhardness versus nanosilica content: Triplex Hot,Polyhot, and Biocryl.
Table V. Analysis of variance for microhardness
Source Sum of Squares df Mean Square F Ratio P
Model 297.93 20 14.896 14.37 <.001
Residual 87.08 84 1.037
Corrected total 385.01 104
Table IV. Analysis of variance for fracture toughness
Source Sum of Squares df Mean Square F Ratio P
Model 6.72 20 0.336 21.74 <.001
Residual 1.30 84 0.015
Corrected total 8.02 104
Table VI. Multiple comparisons for fracture toughness by silica level
Contrast Significance Difference ± Limits
0.0203-0 * 0.455 0.123
0.045-0 * 0.374 0.123
0.091-0 * 0.272 0.123
0.227-0 * 0.224 0.123
0.456-0 * 0.229 0.123
0.917-0 0.010 0.123
P<.05 indicates significant difference.*Results of multiple comparisons suggest all pairs represent fracture toughness, except the lastone (0.91-0) show statistically significant differences at the 95.0% confidence interval.
330 Volume 111 Issue 4
versus nanosilica content and micro-hardness versus nanosilica content. Itcan be seen that all modified TriplexHot specimens have an increased
The Journal of Prosthetic Dentis
fracture toughness compared withthe unmodified specimens. The highestvalue was obtained for 0.023% ofadded nanosilica (T002). For the higher
try
contents of nanosilica, the fracturetoughness values gradually diminish.Similar trends were noted for the other2 tested materials, Polyhot and Biocryl.
The microhardness results for alltested materials are shown in Table IIIand Figure 2. The highest microhard-ness results were obtained at 0.023%nanosilica content. Microhardness ofspecimens with higher nanosilica con-tent was lower; however, a second-ary maximum can be observed atthe maximum nanosilica content of0.91%. The majority of modifiedspecimens had increased microhard-ness compared to the control groupspecimens.
The results of 2-way ANOVA statis-tical analysis are shown in Tables IVand V. The results of fitting a generallinear statistical model, which relatesfracture toughness to 2 predictive fac-tors material and silica level are shownin Table IV. The relation of microhard-ness to material and silica level issummarized in Table V. As indicated bythe P value, a statistically significantrelationship exists between both frac-ture toughness and microhardness andthe predictor variables at the 95.0%confidence interval.
The results obtained with theDunnett test are presented in Tables VIand VII. Results of multiple compari-sons suggest all pairs represent frac-ture toughness, except the last one(0.91-0) show statistically significantdifferences at the 95.0% confidence in-terval. However, multiple comparisonsof microhardness results indicate thata statistically significant differenceexists in all pairs, as shown in Tables VIand VII.
The DSC curves are presented inFigure 3. DSC curves for Triplex Hot,Polyhot, and Biocryl are shown inFigure 3. Low nanosilica addition(0.023 [0.046%]) contributes to amoderate rise in glass transition tem-perature (Tg) for Triplex Hot and Bio-cryl materials. However, in the case ofPolyhot, nanosilica causes a marginaldecrease of Tg.
SEM macrographs and micrographsof the fracture surface obtained for
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Table VII. Multiple comparisons for hardness HV0.3 by silica level
Contrast Significance Difference ± Limits
0.0203-0 * 4.265 1.010
0.045-0 * 2.557 1.010
0.091-0 * 1.427 1.010
0.227-0 * 1.717 1.010
0.456-0 * 1.697 1.0010
0.917-0 * 3.43 1.010
P<.05 indicates significant difference.*Results of multiple comparisons suggest all pairs represent fracture toughness, except lastone (0.91-0) show statistically significant differences at 95.0% confidence interval.
40 60 80 100 120 140 160
Hea
t Fl
ow (
W/g
)
Temperature (˚C)
97.1˚C0.92 vol.% SiO2
0 vol.% SiO2
0.023 vol.% SiO2
93.4˚C
92.5˚C
Universal v4.3A TA InstrumentsExo Up
A
40 60 80 100 120 140 160
Hea
t Fl
ow (
W/g
)
Temperature (˚C)
96.8˚C0.92 vol.% SiO2
0 vol.% SiO2
0.023 vol.% SiO292.7˚C
92.9˚C
Universal v4.3A TA InstrumentsExo Up
B
40 60 80 100 120 140 160
Hea
t Fl
ow (
W/g
)
Temperature (˚C)
108.8˚C0.92 vol.% SiO2
0 vol.% SiO2
0.023 vol.% SiO2
0.046 vol.% SiO2103.4˚C
103.0˚C
101.6˚C
Universal v4.3A TA InstrumentsExo Up
C
0.05 W/g
0.05 W/g
0.05 W/g
3 Differential scanning calorimetry thermograms of A, TriplexHot. B, Polyhot. C, Biocryl.
April 2014 331
Balos et al
the tested materials in an unmodifiedcondition and with nanosilica addit-ion are shown in Figures 4 to 6. SEMimages of the specimens with addednanosilica were obtained for 0.023%and 0.91% nanosilica, that is, for min-imum and maximum nanoparticle ad-dition, respectively. Shown in Figure 4are SEM macro images at a magnifi-cation of �30 of Triplex in unmodifiedcondition (Fig. 4A), with 0.023%nanosilica content (Fig. 4B) and 0.91%nanosilica content (Fig. 4C). A similarfracture mode for all the specimens isshown in Figure 4A-C. The intense riverpatterns near the initial crack indicate abrittle fracture mode.
Shown in Figure 5 is the fracturesurface of Polyhot at a magnificationof �5000, of the unmodified (Fig. 5A)specimen and the specimen modifiedwith 0.023%nanosilica addition (Fig. 5B).The crack propagated from the ini-tiation site, which created a radialstriped pattern, the occurrence of sta-ble crack propagation up to the nextradial striped pattern. Thus, smoothmorphology is noticed at lower mag-nifications, which indicates indiscrimi-nate crack propagation through thespecimen.
Agglomerates found in the speci-mens made of Biocryl modified with0.91% nanosilica are shown in Figure 6.A comparison of backscattering andthe secondary electron SEM mode ofoperation is shown in Figure 6A, B. Athigh magnifications (�50 000), a largeagglomerate of approximately 250 nmcan be seen to differ chemically fromthe PMMA matrix.
DISCUSSION
The results obtained in this investi-gation allow the rejection of the nullhypothesis. As the nanoparticle contentwas increased, fracture toughness de-creased for all tested materials. Suchresults may be explained by the degreeof filler agglomeration, particle dis-tribution, and the interfacial layer be-tween the nanoparticle and the PMMAmatrix.
4 Triplex fracture surfaces obtained after fracture toughness test. A, unmodified. B, 0.023% nanosilicaaddition. C, 0.91% nanosilica addition.
5 A, Polyhot in unmodified condition. B, Modified with 0.023% nanosilica.
332 Volume 111 Issue 4
Investigation of the Tg in nano-composites can lead to different results,depending on the materials involvedand the method of mixing the nano-particles with the polymer matrix. Theinfluence of the nanoparticles on Tg inpolymer composites is controversialbecause, in nanocomposites, Tg de-pends on a variety of factors.17-19 The
The Journal of Prosthetic Dentis
behavior of the silica-reinforced systemis influenced by the existence of theinterfacial layer caused by the chainimmobility within a few nanometers ofthe filler surface.20,21
The change of the composite Tgvalue is mainly associated with thepolymer immobility in the interfaciallayer. If the interfacial layer is thin and
try
a small amount of polymer is immo-bilized, then no change in Tg is ex-pected. Furthermore, if nanoparticlesare well dispersed at sufficiently smalldistances apart, their modifiedPMMA interfacial layers contact eachother, which forms a homogenousreinforced field.18 On the contrary, inthe specimens with larger amounts of
Balos et al
6 Biocryl modified with 0.91% nanosilica, scanning electron microscope image of agglomerates.A, Backscattering mode. B, Secondary electron mode. C, Groups of approximately 50 nm agglomerates.
April 2014 333
nanoparticles, agglomeration may beobserved and a larger portion of thepolymer chain may be immobilized,which results in higher Tg. Interfaciallayer thickness can be calculated withthe following equation:
where d is the interfacial layer thickness,dp is the nanoparticle diameter, ad isthe fraction of monomer (polymer) thatforms the interfacial layer, and yp isthe nanosilica content in the nano-composite.19 From Equation 3, it canbe seen that the interfacial layer thick-ness is directly proportional to thenanoparticle diameter. However, thecreation of agglomerates may cause atheoretical threefold decrease in thenumber of particles, which may lead tothe occurrence of gaps between thereinforcing fields. As a result, during
Balos et al
fracture toughness testing, the crackmay propagate between the reinforcedfields.21
As the SEM imagery shows, ag-glomeration was observed in speci-mens with 0.91% silica. However, silicananofiller particles appear to beembedded and semibonded to thePMMA matrix, which may indicate arelatively strong nanoparticle-matrixinteraction. Backscattering SEM imag-ery indicates that large agglomeratesmay fracture under load, while theirsilica layer may remain firmly bonded tothe PMMA matrix. After the nanofilleragglomerate fractures, the stress istransmitted to the PMMA matrix, whilethe crack propagates to the adjacentfiller particles. The lack of crack for-mation and propagation duringmicrohardness testing indicates thatagglomerates may not fracture duringindentation. The high microhardnessvalue of specimens that, for example,contain 0.91% of nanoparticles may beexplained by the high nanoparticle
content as well as the thick immobilizedPMMA layer around agglomerates.
A limitation of this study is thetesting of just 3 PMMA-based denturematerials. However, when bearing inmind that most denture materials arebased on PMMA, the 3 selected mate-rials may well represent the entiregroup. Future research should addressthe testing of a larger number of me-chanical properties in PMMA-baseddenture materials.
CONCLUSIONS
Within the limitations of this study,the follow conclusions can be drawn:
1. Low nanosilica content yieldsbetter mechanical properties than highnanosilica content in terms of micro-hardness and fracture toughness. Asnanosilica content increases, fracturetoughness decreases, whereas micro-hardness has a secondary maximumat 0.91% nanosilica content. These
334 Volume 111 Issue 4
trends can be explained by the crackdevelopment and its propagation aswell as the agglomeration, which occurswith increased nanosilica content.
2. High nanosilica content results inintensive agglomeration, where largerparticles are formed. This results indistribution and the formation ofvarious modified immobilized layersaround nanoparticles.
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Corresponding author:Dr Sebastian BalosUniversity of Novi SadTrg Dositeja Obradovica 6, 21000Novi SadSERBIAE-mail: [email protected]
Copyright ª 2014 by the Editorial Council forThe Journal of Prosthetic Dentistry.
