Composite Manufacture

7/29/2019 Composite Manufacture

1/10

INTRODUCTION

The field of dental restorative resin composites is still

open for improvement and thus a much active area of

research1). In fact, despite the recent advancements

both in chemistry of the resins and in development

of novel nanoparticles and related technologies2,3),

many combinations thereof have to be explored and

the respective recipes have to be optimized yet. The

main issue with the dental resin composites is that

still their mechanical properties can only barely

approach those of dental amalgam4). Unfortunately, the

industrial manufacturers of dental composites, which

have the resources in both man-power and accessible

instrumental techniques, on the one hand are forced to

focus on the health issues of the novel materials, and

on the other hand are pushed to deliver new products

on the market as soon as possible to win the race with

their competitors. As a result, materials that are poorly

characterized in physical and mechanical properties

are sometimes released and promoted beyond their real

performances, as it has been the case in the last decade for

several flowable composites5,6). In this scenario, a deeper

understanding of the combined effects of the variousparameters of composite materials is to be attained, by

preliminary investigations carried out in the laboratory.

In this work we investigated simple composite systems

without filler-matrix bonding agent, and within this

limitation we studied the effect of different filler particle

materials on the respective elastic properties. Three

different types offillers were selected, namely ball-milled

glass, and commercial particles of titania and silica. The

ball-milled glass was chosen for the potential control

of the particle size in our laboratory in view of future

developments, and the commercial silica particles were

chosen as the standard counterpart for comparisons.

Titania has been chosen additionally as a novel filler

material, thanks to its mechanical stiffness due to

the crystalline particles nature, and to the interesting

possible photo-activity. In fact, it is known that titania is

a good absorber of UV light potentially driving chemical

reactions (photocatalytic effect7)), which could be used to

trigger e.g. periodic biofilm removal and/or changing its

surface wettability to superhydrophobic character8).

In most existing literature the mechanical properties

of dental composites are investigated by means of static

loading methods, such as nanoindentation to measure

elastic modulus and hardness, or universal tester to

measure flexural strength and fracture toughness.

However, indentation only reports about the response

to compressive stress, whereas flexural stress loading is

also important in restorations, which exhibit interfaces

bonded to the native tooth cavity. On the other hand,

strength and toughness tests are destructive, making

it impossible to repeat testing of the same specimensover the time. Furthermore, both methods are static and

can only partially account for the viscoelastic nature

intrinsic in the polymeric matrix phase of the dental

restorative composites. Therefore, we decided to use

dynamic mechanical testing as the main experimental

technique, to provide a more realistic characterization of

the materials considered.

Preparation and characterization of a BisGMA-resin dental restorativecomposites with glass, silica and titania fillers

Sanjay THORAT 1,2, Niranjan PATRA 1, Roberta RUFFILLI 3, Alberto DIASPRO 1 and Marco SALERNO 1

1 University of Genova, viale Causa 13, I-16145 Genova, Italy2Istituto Italiano di Tecnologia, Department of Nanophysics, via Morego 30, I-16163 Genova, Italy3 Istituto Italiano di Tecnologia, Department of Nanochemistry, via Morego 30, I-16163 Genova, Italy

Corresponding author, Marco SALERNO; E-mail: [email protected]

A photo-polymerizable Bisphenol-A diglycidylether methacrylate resin was characterized by Fourier transform infrared spectroscopy

after its irradiation under different conditions to identify the best curing. Bonding-agent free composites with particles of ball-milled

glass, silica and titania at loading of 10 and 50%wt were prepared, and their viscoelastic properties investigated by dynamic

mechanical analysis, in experimental conditions close to the working environment in the mouth. All composites showed good stability

at the considered conditions. The stiffest composite was the silica one, which was based on the smallest primary particles. The

storage moduli close to room temperature (25C) and mastication frequency (1 Hz) were extracted as reference bending moduli for

the materials, and compared to static compressive moduli measured by nanoindentation performed by atomic force microscopy.

Nanoindentation showed qualitative results in agreement with dynamic mechanical analysis as to the ranking of different materials,

while resulting in approximately two-fold elastic modulus.

Keywords: Dental restorative composites, Inorganic fillers, Dynamic mechanical analysis, Nanoindentation, Elastic modulus

Color figures can be viewed in the online issue, which is avail-

able at J-STAGE.

Received Dec 12, 2011: Accepted Apr 2, 2012

doi:10.4012/dmj.2011-251 JOI JST.JSTAGE/dmj/2011-251

Dental Materials Journal 2012; 31(4): 635644


2/10

MATERIALS AND METHODS

Resin matrix

2,2-bis[4-3-(methacryloxy-2-hydroxy-propoxy)-phenyl]-

propane (BisGMA) and triethylene glycol dimethacrylate

(TEGDMA) were mixed in 7:3 ratio by weight. The

former is the traditional resin monomer used in dental

restorative composites since more than half a century9).

The latter is one among several monomers usually

mixed with BisGMA to make the paste more fluid

and handy during manipulation (viscosity decreasing

co-monomer)1). The system was carefully blended by

spatulation for 3 min. The co-monomer mixture was

further added with the photopolymerization system,

consisting in camphorquinone (CQ) as a photoinitiator

and dimethyl amino ethyl methacrylate (DMAEMA) as

an amine reducing agent. CQ and DMAEMA were added

in 1:1 weight ratio, such that their total amount was 0.5%

wt of the total co-monomer. All products were supplied

by Sigma-Aldrich (Milan, Italy). The overall system was

blended again by spatulation for additional 3 min. Incase offiller loading, the respective particles were added

in 10 or 50% wt proportion of the overall organic matrix

paste, and the system was spatulated again for up to

additional 15 min for the highest loading cases. The

paste was then poured into a clean antisticking mold

of Teflon, and placed in a bell rest chamber pumped to

low vacuum (~100 mbar) to remove air bubbles formed

during spatulation.

Filler materials

Two of the three types offillers used were commercial

materials, namely silica and titania particles

(Sigma-Aldrich products no. 718483 and 232033,

respectively). The silica particles are described as

nanopowder with average diameter of 12 nm, and

are amorphous in character. The titania particles are

described as powder without any size specification,

and are crystalline in character (anatase). The third

type offillers used was ball-milled glass. For the source,

optical microscopy specimen coverslips were used

(Menzel-Glser, Germany), of size 20200.3 mm3.

These coverslips are made of borosilicate glass (D 263 M

type, Schott AG, Germany). Despite the large planar

dimensions, these slides are so thin that it was possible

to load them directly into the ball-miller jar without

preliminary fragmentation. The ball-miller used was

a PM100 (Retsch, Germany), operated with a 50 mLzirconia jar, filled with 100 g of glass and zirconia balls.

We used 30 balls of 5 mm diameter together with 60 balls

of 3 mm diameter. The milling was carried out in 20 mL

isopropanol (IPA, Sigma-Aldrich) at a rotating speed of

450 RPM, with alternating cycles of 1 min clockwise and

1 min counterclockwise rotation without resting time,

for a total milling time of 5 h. After milling, the hot jar

(~80C) was let to cool down to room temperature (RT)

in place, before opening it. The suspension was then

poured into a previously weighted glass beaker, and

was let to dry in an oven at 90C for 4 h. After drying,

the glass appeared to be agglomerated in large plates

sticking to the jar bottom, but these could be easily

broken down into fine powder by simply touching them

with steel tweezers. It was thus possible to mix this

powder thoroughly with the resin paste, similarly to the

commercial particles.

In order to compare the commercial nanosilica and

titania powders with the glass ball-milled in IPA, the

particle size of each filler material was first measured by

dynamic light scattering (DLS) in IPA suspensions. To

this goal, polystyrene cuvettes were used in a Nano-ZS

setup (Malvern Instruments, UK). We started from

concentrated IPA suspensions (~5 g/L for the commercial

powders, and as collected material from the ball-milled

glass), and then moved on to a more diluted suspension

of the same material, decreasing the concentration to

50% at each step. We went on with dilution until the

measurement quality resulting from the instrument

report remained acceptable, as to the sufficient optical

density required for a good statistical analysis. Also, for

each suspension 3 series of 3 measurements each were

repeated and averaged.In addition to DLS measurements in IPA, after

drying the IPA suspension of ball-milled glass the filler

particles were measured again in DLS upon mixing them

into the resin. Since the mixtures had to be manipulated

in the light for several minutes, for these measurements

we used BisGMA and TEGDMA only without

photopolymerizing system (CQ-DMAEMA). The missing

CQ-DMAEMA part should not affect significantly the

rheology of the system, due to the low percentage (0.5

wt% of the co-monomer). Finally, microscopic imaging

was also used to further assess the size of bare primary

particles and particle aggregates.

Photo-curing conditions

The elongated beam shape of the specimens (see

Dyamic mechanical analysis for the size) required

three irradiation cycles for each specimen, which

were applied starting from the central region first and

moving to the two side regions later on. For selection of

the most appropriate photo-curing lamp, the intensity

of various light sources available in our laboratory was

preliminary measured with a power meter Nova II

(Ophir, USA). Since CQ has a peak of light absorbance

at 470 nm wavelength, which efficiently starts the

polymerization reaction after amplification by the

electrons extracted from DMAEMA, we measured both

the full spectrum power (white light) and the power ata selected blue region window only (blue light). To this

goal, a filter was taken from a fluorescence cube GFP

(Semrock, USA), with ~95% transmittance at a 455

490 nm wavelength pass band. As a result of this step,

we selected a X-Cite 120 lamp (EXFO, Canada), which

was placed at a distance of ~3 cm from the specimens to

be cured. This lamp is a high output power white light

source with a broad smooth spectrum, normally used

for optical microscopy imaging of specimens stained

with fluorescent dyes. The irradiance of this lamp was

evaluated in comparison with other light sources used in

both literature and recent clinical practice10-12), on which

636 Dent Mater J 2012; 31(4): 635644


3/10

basis we decided to set our irradiation time to 105 and

210 min for white and blue light, respectively, which

methods were both tested for curing the bare resin.

Fourier transformed infrared spectroscopy

In order to verify the conversion of the co-monomer paste

into a polymerized resin, Fourier transformed infrared

spectroscopy (FTIR) was carried out for all the specimens

at RT in ambient air, both soon after pouring the paste

into the Teflon molds and after its irradiation. The FTIR

spectra were acquired by a Vertex 70 spectrometer

(Bruker, USA), in the range of 4004,000 cm1. The

samples were analyzed in attenuated total reflection

configuration, with an aperture diameter of 3 mm and a

spectral resolution of 4 cm1. For optimal signal-to-noise

ratio, 50 scans were averaged per sample spectrum,

and apodized by applying the Blackman-Harris 3-term

correction function for the Fourier transformation.

The interferograms were corrected using a zero-filling

factor of 2. All the spectra were baseline-corrected by

third order polynomial and were normalized thereafter.

Dynamic mechanical analysis

All the materials were shaped with the Teflon mold to

be rectangular beams of 13352 mm3 in size. On these

samples we performed dynamic mechanical analysis

(DMA) by means of a Q800 setup (TA Instruments,

USA), with instrument compliance of less than

0.2 m/N, as determined by a prior calibration in staticloading mode. We carried out DMA measurements

in single-cantilever mode, under strain control in the

materials linear regime. The maximum applied strain

was 35 m. Temperature sweeps at strain frequency of1 Hz were carried out, in a range of +2 to +62C (with

5C steps, 5C/min rate), since it should represent well

the limit values occurring in human mouth in normal

operating conditions, when ingesting from hot food to

icy drinks. For reaching the lowest temperatures in this

range, liquid nitrogen was used as a coolant. During

the temperature scans both the storage modulus E and

the loss modulus E were recorded, representing the

in-phase (real) and out-of-phase (imaginary) parts of a

complex modulus E*=E+iE, respectively, occurring due

to the stress lagging behind the applied strain with some

phase angle delay.

AFM nanoindentation

The same samples as prepared for the DMA were used,before carrying out the respective tests. We performed

nanoindentation by means of an AFM used in so-called

force spectroscopy mode, i.e. collecting force-distance

curves on given specimen sites. We used a MFP-3D AFM

(Asylum Research, USA) equipped with gold coated

silicon probes NSG20 (NT-MDT, Russia), made each of a

cantilever with nominal spring constant and resonance

frequency of ~60 N/m and ~450 kHz, respectively, and of

a terminal transverse pyramidal tip with apex diameter

and full aperture angle of ~20 nm and ~22, respectively.

The typical lateral scan size was 5 m, with spatial mapsampling of 302 pixels. The AFM probe was calibrated in

air for determination of the cantilever spring constant,

and again in de-ionized water for determination of

the appropriate optical lever sensitivity. In fact, the

force-distance curves were acquired in water to remove

the effect of aspecific tip-surface adhesion due to

ambient moisture. The curves had 1,024 datapoints, with

z actuation loops of 1 m range and 0.5 Hz frequency,low enough to minimize viscous drag effects, (data not

shown).

Preliminary to the AFM nanoindentation

experiments, occasionally AFM imaging was carried

out with the same probe (in Tapping mode, with 30 mscan size and 2562 pixels), which showed evidence of the

presence offillers in the composites, as compared to the

bare resin surface (data not shown). However, all the

composite samples were still flat and smooth enough at

the surface, such that they could be properly investigated

by nanoindentation at the considered low indentations

(up to limited maximum values ofmax~100 nm only).

After acquisition, the indentation was calculated from

the z movement of the actuator z and from the changein cantilever deflection D (partially compensating theformer) as =zD. The force-indentation data werefinally fit to the Hertz model of elastic contact, using the

unload part of the force loop to find the elastic modulus

values EAFM. Given the high number of parameters in the

Hertz model, (indenting tip size and shape, sample and

tip materials Poissons ratios, tip elastic modulus, actual

contact point (, F)=(0, 0)), both offsets of indentation and

force were let to fit automatically by the AFM software

within a broad range of forces, 25%75% of Fmax, such

as to minimize the deviations between fitting curve

and data-points (reduced 21,000). Additionally, theremaining parameters were adjusted on the bare resin

reference material, such that for this the compression

modulus EAFM resulting from nanoindentation was equal

to the respective flexural modulus E resulting from the

DMA experiments. After this initial setup, the same

values of the remaining working parameters were used

later also for the composites.

RESULTS

Resin curing evaluation

In Fig. 1a) the FTIR spectra of the uncured components

of the organic paste are presented, both individually

and blended in the resulting mixture (red line). Clearly,

BisGMA and TEGDMA dominate the spectrum of themixture, with some features, such as e.g. the peaks at

2,874 cm1 (symmetric stretching of CH3) and 1,126 cm1

(symmetric vibration of -C-O-C-) of TEGDMA increasing

the adjacent shoulders of BisGMA. The peaks of

DMAEMA at 2,771 cm1 (N(CH3)2 band) and of CQ at

1,747 cm1 (C=O vibration) do not appear in the mixture,

due to the low relative contribution of the respective

components. The 3,3503,550 cm1 band (OH stretching)

is slightly depressed.

In Fig. 1b), the effect of photo-curing is reported.

In particular, a significant change in the bands

centered around 1,637 and 1,580 cm1 is observed, with

637Dent Mater J 2012; 31(4): 635644


4/10

Fig. 1 a) FTIR spectra of individual BisGMA, TEGDMA, CQ and DMAEMA, as well as of the (uncured) mixture

of the same compounds (red line). b) FTIR spectra of the resin mixture before (uncured, red) and after

irradiation, according to different conditions.

a relative increase of the second one, corresponding

to the stretching of the aliphatic and aromatic (i.e.

ring) C=C bonds, respectively. In fact, as a measure of

photo-polymerization, the degree of conversion (DC) of

the monomer mixture into the polymer (photo-cured

mixture) can be evaluated from the following equation,

according to a peak intensity method14):

(I1637 / I1580) polDC= 100 [ 1 - ] (1)

(I1637 / I1580) mon

where I1637 and I1580 are the peak intensities of thebands at the respective wavenumber positions, and the

subscripts outside the parentheses refer to the spectra

before (mon) and after (pol) photo-curing, respectively.

From eq. 1 applied to the spectra in Fig. 1b), it turns

out that the highest conversion is found for white light

curing (see Fig. 4). This condition (gray curve in Fig. 1b))

provided a DC of ~75% soon after cure, which also

showed the highest time-delayed increased one week

later (black curve in Fig. 1b), reaching ~94% (see Fig. 4).

Thus, spectral measurement repeated on white light

exposed resin after one week of storage in ambient light

at RT showed still significant ongoing conversion on that

time scale, (+25% in DC). However, when repeated again

after one month since irradiation, no further change was

observed, showing no effect of possible absorbance of

moisture or other ambient contaminants.

The blue exposed resin, on the contrary (blue curve

in Fig. 1b)), only reached ~57% soon after curing (Fig. 4),

and also showed lesser improvement at one week time

(~63%, i.e. +11%, curve not shown).

Filler size characterization

The apparent size of the filler particles was determined

by means of DLS, which was first applied on suspensionsof the respective powder in IPA. The adopted method of

progressively more diluted suspension should minimize

the effect of agglomeration, allowing for a determination

of particle size as close as possible to the primary particles.

In fact, after 3 to 5 dilution steps, for all materials the

apparent average particle size had decreased, reaching

either a more stable value or the limit of lowest

acceptable measurement quality mentioned above. The

final distributions of particles size in IPA are shown in

red in Fig. 2a)c). The apparent mean particle size in

IPA is ~650 nm for glass (Fig. 2a) red bars) and ~950 nm

for titania (Fig. 2b) red bars), respectively. For silica

638 Dent Mater J 2012; 31(4): 635644


5/10

Fig. 2 a)c): distributions of particle size (populations by particle number) obtained in IPA (red bars) and in

resin co-monomer (green bars), for the particles of: a) glass, b) titania, and c) silica. d)e): images offiller

particles drop-cast onto glass slides from IPA: d) AFM of glass particles, and e) SEM of titania particles.

(Fig. 2c) red bars) two populations showed up, centered

around ~180 and ~610 nm size, respectively.

In order to characterize a system closer to the final

samples, we also carried out DLS of the filler particles

dispersed in the resin. Since the 50 wt% filler loaded

mixtures were too viscous to be properly poured into

the DLS cuvettes, we restricted our measurements to

the 10 wt% samples. These results are also included in

Fig.2a)c), with size population distributions in green

color. The glass particles distribution in the resin (Fig.2a)

green bars) showed three peaks, with the intermediate

one (~700 nm) well overlapped with the distribution inIPA (red bars), and two more peaks on both the small

and large size sides, centered at ~240 and ~2000 nm,

respectively. The titania particles distribution in

the resin (Fig.2b) green bars) still showed a single

population peak same as in IPA, yet significantly shifted

to lower size, centered around ~200 nm. Finally, the

silica particles distribution (Fig.2c) green bars) is again

partly overlapped with the IPA one, but with the middle

shifted towards smaller size. In particular, two peaks at

~9 and ~24 nm appear, showing overall good consistency

with the nominal mean particle size of 12 nm.

In fact, for those fillers without any nominal

reference value for the primary particles size, i.e. titania

and glass, additional determination of particle size was

also carried out by microscopy. To this goal, the same

IPA solutions used for DLS were drop-cast onto glass

slides, and the surfaces were imaged. We first tried

AFM, which worked for the relatively larger and more

irregular glass particles (see Fig.2d)), whereas for the

titania particles it turned out into unstable images,

probably due to particle aggregates loosely bound to

the substrates and swept during the scan. Therefore

for titania we used SEM, after 5 nm platinum coating,

(see Fig.2e)). The images showed a general agreementwith DLS results in the resin. Indeed, for the glass the

presence of mostly submicrometer sized particles is also

accompanied by some larger ones (up to 23 m in atleast one direction). Similarly, the image of the titania

particles shows a primary particle size of 21060 nm

(meanstandard deviation), in agreement with the green

distribution in Fig.2b).

Samples mechanical properties

We first carried out DMA on the bare resin samples,

to assess the mechanical properties resulting from

the different curing conditions. The respective

639Dent Mater J 2012; 31(4): 635644


6/10

Fig. 3 a) Storage modulus E and b) damping factor tan=E/E of bare resin samples cured in different conditions.

Fig. 4 Comparison between DC from the FTIR spectra

and storage modulus E at RT from the 1 Hz DMA

frequency scan in Fig. 3 (blue light curing curve),

stressing the possible correlation between the two

quantities.

measurements are presented in Fig. 3. In Fig. 3a)

the storage modulus E decreases monotonously in all

cases, as expected due to the increased fluidity of the

resin at higher temperatures. The white cured storage

modulus curve shows not only the highest values at all

temperatures, but also the slowest decrease slope than

the other curves. For an easier comparison with DC, the

E values at RT have been extracted from Fig. 3a) and

plotted again along with the respective DC values in

Fig. 4.

In Fig. 3b) we decided to plot the damping factor

tan=E/E rather than the loss modulus E or the total

complex modulus |E*|=(E 2+E2), as done sometimesin the literature15). In fact, only one out of the three

mentioned parameters is independent in addition to

E, such that each choice is allowed. However, whereas

|E*| is usually quite close to E (being E


7/10

Fig. 5 a) Storage modulus E and b) damping factor tan=E/E of resin composites with differentfiller materials

and loading, when available.

Fig. 6 Flexural (red bars) and compression modulus (green

bars) of cured specimens of all considered materials,

from left to right: bare resin, and composites with

glass, titania and silica fillers. The hardness from

the nanoindentation experiments (blue bars) is

also plotted.

the lowest decreasing slope with increasing temperature.

In fact, this curves starts at the lowest temperatures withE values only lower than for the glass composites, and

ends at the highest temperatures with higher E than

those samples. The cross-point is approximately placed

between the RT and the body temperature values.

For the damping factor plotted in Fig. 5b), this control

information shows again, similarly to the bare resin case

(Fig. 3b)), that no glass transition occurs in the considered

temperature range. Also in this parameter, describing

the amount of viscous character of the material, does the

silica composite perform better than the others, as its

curve is the most flat, showing high thermal stability

in the operating temperature range. Again, the glass

composites with different loading are very close to each

other (even more than for E in Fig. 5a)).

Finally, the results of the nanoindentation analysis

are summarized in Fig. 6, where the values of EAFM

modulus are compared with the E modulus from

DMA, for all the composites. The error bars of the

AFM quantities represent one standard deviations

of the populations of 302 data-points from the maps of

force-distance curves on the respective samples. These

error bars are approximately ~24% and ~33% of the

respective mean values, for the 50%wt and the 10%wt

composites, respectively. In fact, a larger deviation is

expected for the low (10%wt) loading composites, where

regions of dominating bare resin stiffness effect can be

found on the surface more likely than in the average

high loading (50%wt) composites. In Fig. 6 bars for the

measured hardness H have also been plot, which is an

additional information obtained from the Hertz fits. In

general, the relative error on HAFM is higher than that on

EAFM, due to the higher sensitivity of H to the uncertainty

in contact area with respect to E. Additionally, an even

higher error is found for both HAFM and EAFM for the low

loading composites, where the relative error with respect

to the mean is increased from ~32% to ~42%.Overall, the elastic modulus reported in Fig. 6

is in good qualitative agreement with DMA results.

Indeed, in both techniques the glass composites show

values significantly higher than the bare resin, and

not significantly different between them, for the two

different loadings considered. Also similarly to DMA,

the lowest moduli from nanoindentation come from the

titania composites. However, whereas in DMA at RT the

10%wt silica composite scores the same as the two glass

composites, in nanoindentation it is significantly higher,

and clearly the stiffest composite of all. Additionally,

whereas for the bad cured 50%wt titania composite DMA

641Dent Mater J 2012; 31(4): 635644


8/10

showed a modulus even lower than the bare resin, for

compressive nanoindentation test the presence offillers

makes the resulting modulus higher than the resin

even in that negative case. In fact, it is known that for

tensile measurement the presence of non-bonded fillers

can even act as detrimental defect sites, rather than as

reinforcing agents16).

DISCUSSION

Current commercial dentist lamps are based on LEDs

which present a typical emission light power of ~1,000 mW

in the spectral region of 460480 nm wavelength. In

common practice two 10 s irradiation cycles of these

lamps are used, for maximum 2 mm thick restorations.

The lamps are usually placed close to contact (~2 mm)

with the dental composite restoration, which has typical

diameter of 3 mm and is completely covered by a light

spot of approximately the same size, at the specified

distance. Therefore, a used irradiance of ~14,000 mW/

cm2

with a total delivered dose surface density of ~280 J/cm2 can be estimated.

The lamp used by us had a power measured in the

blue region of ~60 mW at 3 cm distance from the sensor

on a circular area of 13 mm diameter. This lamp was

the most powerful continuous wave source of blue light

available in our laboratory. Alternatively, we could use

pulsed lasers with high peak power, borrowed from

a spectroscopy laboratory close by, but we suspected

that the pulsed irradiation regime could modify the

photo-curing process dynamics or chemistry, after

heating effects. Therefore, we stuck to our continuous

light source and rather extended the irradiation times.

The selected times of 105 and 210 min for white and

blue light, respectively, were chosen to reach the same

dose surface density of the dentists lamp. We assumed

100% absorption of the delivered dose, and a simple

exposure reciprocity law between incident irradiance

at specimen pinc and exposure time t, pinca t=constant,

with a=1. Whereas this assumption can have limited

validity in some cases also depending on parameters

such as photoinitiator system, filler loading, and desired

depth of cure, it can still be considered valid in a first

approximation. In particular on the long time side of

this law, it has been shown that there is no minimum

irradiance under which no photopolymerization

starts13).

When discussing the FTIR spectra in Fig. 1 and therespective DC calculated and plotted in Fig. 4, it should

be kept in mind that it is not totally clear to what extent

thermal effects played a role in the curing, by eventually

modifying the reaction path and the resulting material

properties. In fact, when exposed to the white light

the resin was locally heated above 80C. Similarly,

thermal effects also applied in the FTIR measurement

repeated after DMA study (magenta curve in Fig. 1b)).

In this case, the heating cycle decreased the DC to ~26%

(Fig. 4). One possible reason for this effect could be the

partial vaporization of water molecules trapped inside

the resin, which hinders chain propagation during the

polymerization.

Independent on the highest DC obtained with the

white light exposure, for the curing of the composites

we decided to adopt the blue light exposure, such as to

follow more closely the conventional photo-curing adopted

in normal dental practice. In fact, one more problem

appears in the composites due to the fillers scattering

the light off its original straight path through the

organic phase. Actually, this can have two counteracting

effects. On one hand, some light exits the specimen and

is lost. On the other hand, most light incoming on the

fillers is not absorbed by them but rather redirected to

the locally surrounding matrix, which is also reduced in

quantity. Therefore, low loading composites should be

cured even faster than bare resin specimens. However,

in high loading composites the light could even not reach

at all the deepest matrix levels. Overall, we assumed a

general compensation of these effects on the considered

specimens (thickness ~2 mm), and applied to the

composites the same blue irradiation procedure used

for the bare resin specimens.Regarding the filler particle size from Fig. 2 it

clearly appears that all the average particles in IPA

are at maximum of micrometer size. In particular,

the ball-milled glass particles stay below that limit,

appearing even smaller than the titania particles. On

the other hand, for the only particles of known size, i.e.

the silica nanoparticles, the apparent size in IPA is much

larger than the nominal value. Clearly, some degree of

aggregation in suspension is present in IPA at least for

silica.

When repeating the DLS measurements of the

filler particles in the resin medium, obviously, due to

the higher viscosity as compared to solvent, the glass

particles were less free to move, and a lower degree of

both aggregation and sedimentation appears, resulting

in the presence of some primary and some very large

particles, respectively. For the titania particles, in turn,

clearly aggregation was less effective in the resin, and

the resulting size can be supposed to be the real primary

particle size.

Concerning the direct microscopic imaging of the

fillers, in Fig. 2d)e) we have shown images of bare

stand-alone particles only, and not of particles embedded

inside the cured composites. However, also given the

probably slow reaction rate following the low used

irradiance, there is no particular reason why the size of

either the primary particles or the particle aggregatesshould undergo major changes during photocuring with

respect to the size found in the DLS measurements in

the resin.

In Fig. 3a), the white cured sample was the one

with the best polymerized resin, which is in agreement

with the highest DC value observed (Fig. 4). In fact, the

two curves of the blue cured resin in Fig. 3a) present an

inversion of ranking when compared with the respective

DC (Fig. 4). Indeed, the decrease in DC resulting from

the DMA thermal scan did not correspond to a decreased

modulus but rather increased it, instead. Obviously,

polymer cross-linking is not the only mechanism

642 Dent Mater J 2012; 31(4): 635644


9/10

accounting for the increase in the mechanical properties

of the polymer, and thus only specimens processed in

exactly the same way should be compared in expected

mechanical properties with respect to their DC.

In Fig. 3b), the observed minor transition (curve

peak) for the blue cured resin (blue line) can partly

account for the inversion of E ranking with respect

to DC observed in Fig. 4. However, on repeating the

temperature scan, this weak maximum disappears

(magenta curve). Obviously, the respective transition, if

any, was related to orientation of some side groups of the

polymer or localized backbone motions (beta transition).

Finally, we conclude that no glass transition occurs for

the resin in the considered temperature range.

When discussing the results in Fig. 5, reporting

DMA measurements of the composites, it should be

mentioned that the respective DC values were similar

for all samples but for the 50%wt titania sample, which

showed much lower DC, as low as ~10%. In fact, the

50%wt titania sample also showed direct evidence of

incomplete curing as its surface was still gelly and stickyafter curing. On the contrary, the other composites were

properly hardened and had DC values between 60 and

75%, all higher than bare resin (~57% with blue light),

as expected due to the dominating effect of less resin

quantity to be cured in the composite for the same dose

delivered.

In Fig. 5a), the more limited decrease in storage

modulus with increasing temperature for the silica

samples than for all the other samples may be due to the

smallest particle size, which provided better dispersion

and higher uniformity of the composite, resulting in less

regions of resin only domains. In this way, the decrease

in modulus, a peculiarity of the resin matrix, is less

pronounced, as the filler is maintaining the modulus

at relatively high levels during all the thermal cycle.

On the contrary, the glass fillers, similar to the titania

fillers (in the 10%wt sample) exhibit higher plasticizing

effect on the resin, probably due to the larger primary

particle size.

In Fig. 5b), the 10%wt titania sample, despite its

high DC (~75% vs 60% and 70% of the glass samples) also

shows high viscosity, probably due to some higher flow of

the titania particles in the resin with respect to the glass

ones, perhaps due to more difficult diffusion of the latter

after the higher size. The final maximum appearing for

the 50%wt titania sample is probably associated with

secondary effects such as the incomplete curing in caseof this sample, as mentioned above. The apparent glass

transition is probably due only to a higher fluidity of the

not polymerized material.

From the nanoindentation results, overall the

compressive modulus EAFM is approximately twice as

much as the flexural modulus E, with a highest value

of ~5.3 GPa for the nanosilica composite. Whereas this

value is still far from the minimum requirement for

dental restorative composite of posterior teeth, which can

set to ~10 GPa, it can already be considered a good result

for the relatively low loading achieved (10%wt, which is

less by volume, due to the filler being heavier than the

resin), in the absence of a bonding agent. In fact, the

latter effect can be the reason of a higher modulus found

in compressive mode of nanoindentation with respect to

flexural mode of DMA, where one side of the specimen

is under compression as well whereas the opposite side

is on the contrary under tension. Actually, it is known

that dynamic test mode should result in a higher value

of the measured modulus, due to the higher strain rate

applied17,18). Indeed, in our nanoindentation, the rate

of applied load is such that the maximum load Fmax is

reached in half the time period of the force-distance

loop. Being the frequency 0.5 Hz and the overall z sweep

1 m, this corresponds to 2 /s. In DMA, on the otherhand, the strain rate is such that a cycle with amplitude

of 35 m is performed with a frequency of 1 Hz, whichmeans 140 m/s, more than one order of magnitudefaster than during indentation. However, in the present

experiment, obviously the effect due to the different type

of loading scheme dominates, making the AFM moduli

always higher than the DMA ones.

Usually, fillers of glass or other milled materialhave relatively large size population, spanning at least

one order of magnitude (110 m and often 0.110 m),which makes the respective dental restorative materials

belong to the class of hybrids. These are normally

considered to allow for a better distribution of stress,

and increase strength and toughness and eventually

elastic modulus1). However, in our case the ball-milling

provided rather narrow particle size distribution (31%

of the mean, as from Fig. 2a)). This was also indirectly

confirmed by the AFM nanoindentation measurements,

which found monomodal distributions of material

stiffness (i.e. uniform composites) even at the low loading

of 10%wt.

Between the composites with silica and titania

fillers, one could expect a higher modulus for the titania

ones, due to the crystalline phase of the particles.

However, obviously, at the 10% loading the mostly

probed character of the composite is still assigned to the

resin matrix more than the filler particles. On the other

hand, at 50% the titania sample could not be properly

cured, due to the high reflectance of the titania, which is

well known for its use as a white pigment in paints. In

fact, previous investigations of titania filled composites

used thermal curing instead of photo-curing19). Indeed,

the crystalline titania composite with 50%wt loading as

measured by DMA showed elastic modulus even lower

than the bare resin.Finally, when comparing composites of differentfiller

materials as in the present case, it should be considered

that additional effects also arise from the different

effective values of loading by volume. In fact, the densities

of the various filler materials are rather different: 2.6 g/

cm3 for silica and 3.9 g/cm3 for titania, according to the

manufacturers data sheets, and 1.1 g/cm3 and 2.4 g/cm3

for bare resin and glass, as determined by us by weighting

and differential volumetric measurements of water in

graduated cylinders. Therefore, 10%wt corresponds to

volume loadings of 4.7, 4.4 and 2.7% for glass, silica

and titania, respectively, whereas 50%wt corresponds

643Dent Mater J 2012; 31(4): 635644


10/10

to 31, 29 and 20% for the same sequence of materials.

This calculation is made according to nominal densities

of bulk materials. However, the volume loading of our

nanosilica was obviously much higher, due to its fluffy

form, probably around 20% already for the 10%wt sample

and above 90% for the 50%wt sample, which could not

be made at all. One way to overcome this problem could

be using surfactant coated nanosilica, or suspend it in

a solvent also good for the resin phase, before mixing

everything together. However, this method will probably

raise other questions, regarding the solvent effects20).

CONCLUSION

After identifying the most appropriate conditions for

light curing our resin matrix, we carried out DMA

characterization of bonding-agent free composites

with different filler materials under flexural stress,

and compared it to static nanoindentation carried out

by AFM. The elastic modulus of the composites was

always improved or at least remained the same as thebare resin in all compression tests. In flexural tests the

elastic modulus was always lower than the compression

test values, and even decreased with respect to bare

resin in the 50%wt titania sample. Additionally, in glass

composite no increase in elastic modulus was observed

on increase of loading from 10 to 50%wt. In compression,

which is in many cases the most important loading mode

of dental restoration materials, the highest modulus was

observed for the nanosilicafiller, even if in 10%wt loading

only, corresponding in turn to ~4.4%vol. In this case,

an elastic modulus as high as 5.31.6 GPa was found.

Additionally, no thermal transitions were observed at

the investigated temperatures for any material, showing

good stability at operating conditions. Titania fillers

could be of interest in future applications where their

photocatalytic effect could be useful to promote local

antibacterial or remineralization reactions.

REFERENCES

1) Chen M-H. Update on Dental Nanocomposites. J Dent Res

2010; 89; 549-560.

2) Uskokovi V, Bertassoni LE. Nanotechnology in dental

sciences: moving towards a finer way of doing dentistry.

Materials 2010; 3: 1674-1691.

3) Sharma S, Cross SE, Hsueh C, Wali RP, Stieg AZ, Gimzewski

JK. Nanocharacterization in Dentistry. Int J Mol Sci 2010;

11: 2523-2545.4) Wahl MJ. AmalgamResurrection and redemption. Part 1:

the clinical and legal mythology of anti-amalgam.

Quintessence Int 2001; 32: 525-535.

5) Han L, Ishizaki H, Fukushima M, Okiji T. Morphological

analysis offlowable resins after long-term storage or surface

polishing with a mini-brush. Dent Mater J 2009; 28: 277-

284.

6) Salerno M, Derchi G, Thorat S, Ceseracciu L, Ruffilli R,

Barone A. Surface morphology and mechanical properties

of new-generation flowable resin composites for dental

restoration. Dent Mater 2011; 27: 1221-1228.

7) Chen X, and Mao SS. Titanium dioxide nanomaterials:synthesis, properties, modifications, and applications. Chem

Rev 2007; 107: 2891-2959.

8) Pignatelli F, Carzino R, Salerno M, Scotto M, Canale C,

Distaso M, Rizzi F, Caputo G, Cozzoli PD, Cingolani R,

Athanassiou A. Directional enhancement of refractive

index and tunable wettability of polymeric coatings due to

preferential dispersion of colloidal TiO2 nanorods towards

their surface. Thin Solid Films 2010; 518: 4425-4431.

9) Bowen RL. Use of epoxy resins in restorative materials. J

Dent Res 1956; 35: 360-369.

10) Kurachi C, Tuboy AM, Magalhaes DV, Bagnato VS.

Hardness evaluation of a dental composite polymerized with

experimental LED-based devices. Dental Mater 2001; 17:

309-315.

11) Silva Soares LE, Martin AA, Barbosa Pinheiro AL, PachecoMTT. Vickers hardness and Raman spectroscopy evaluation

of a dental composite cured by an argon laser and a halogen

lamp. J Biomed Opt 2004; 9: 601-608.

12) Hosseinalipour M, Javadpour J, Rezaie H, Dadras T, Hayati

AN. Investigation of mechanical properties of experimental

Bis-GMA/TEGDMA dental composite resins containing

various mass fractions of silica nanoparticles. J Prosthod

2010; 19:112-117.

13) Musanje L, Darvell BW. Polymerization of resin composite

restorative materials: exposure reciprocity. Dent Mater 2003;

19: 531-541.

14) Rueggeberg FA, Hashinger DT, Fairhurst CW. Calibration

of FTIR conversion analysis of contemporary dental resin

composites. Dent Mater 1990; 6: 241-249.

15) Ryou H, Niu L-N, Dai L, Pucci CR, Arola AA, Pashley DH,

Tay FR. Effect of biomimetic remineralization on the dynamicnanomechanical properties of dentin hybrid layers. J Dent

Res 2011; 90: 1122-1128.

16) Darvell BW. In: Darvell BW, editor. Materials science for

dentistry. 9th ed. Sawston, Cambridge: Woodhead Publishing

Limited; 2011.

17) Braem M, Davidson CL, Vanherle G, Van Doren V, Lambrechts

P. The relationship between test methodology and elastic

behavior of composites. J Dent Res 1986; 66: 1036-1039.

18) Jacobsen PH, Darr AH. Static and dynamic moduli of

composite restorative materials. J Oral Rehabil 1997; 24:

265-273.

19) Yoshida K, Taira Y, Atsuta M. Properties of opaque resin

composite containing coated and silanized Titanium dioxide.

J Dent Res 2001; 80: 864-868.

20) Patra N, Salerno M, Diaspro A, Athanassiou A. Effect of

solvents on the dynamic viscoelastic behavior of poly(methyl

methacrylate) film prepared by solvent casting. J Mater Sci

2011; 46: 5044-5049.

644 Dent Mater J 2012; 31(4): 635644

Documents

Composite Manufacture