Haitham.doc

J Bagh College Dentistry Vol. 27(2), June 2015 The effect of

Restorative Dentistry 6

The effect of glass flakes reinforcement on the surface
hardness and surface roughness of heat-cured poly

(methyl methacrylate) denture base material

Haitham T. Abdulrazzaq, B.D.S. (1)
Mohammed MM. Ali, B.D.S., M.Sc. (2)

ABSTRACT
Background: Heat-cured poly (methyl methacrylate) the principal material for the fabrication of denture base have
a relatively poor mechanical properties. The aim of this study was to investigate the effect of glass flakes used as
reinforcement on the surface hardness and surface roughness of the heat-processed acrylic resin material.
Material and method: Glass flakes (product code: GF002) pretreated with silane coupling agent were added to
Triplex® denture base powder using different concentrations. A total of 100 specimens of similar dimensions (65 x 10 x
2.5) mm were prepared, subdivided into 2 main groups of 50 specimens for each of the study tests. Ten specimens for
the control group and 40 specimens for each of the experimental groups (2%, 3%, 5%, and 7%) glass flakes content.
The surface hardness was evaluated using the Shore D hardness test, while the surface roughness was evaluated
using a profilometer device that detect the geometry of the specimen unpolished surface. Results were analyzed
using the Wilcoxon rank sum test and the 1-way analysis of variance, (P-value< 0.05).
Results: The surface hardness tended to increase significantly p 0.05 with the increasing flakes concentration, as an
increase of 5.12% was recorded in surface hardness for the highest loading level; while the roughness showed a
significant increase that remained within the tolerable range –less than 2µm– (significant bacterial colonization would
occur if the surface roughness is more than 2µm).
Conclusion: The addition of glass flakes to heat-cured poly(methyl methacrylate) enhanced the hardness of the
material, the improvement was statistically significant for the higher glass flakes concentrations (5% and7%), while for
the surface roughness there were a constant increase in roughness along with the increasing glass flakes content
Key words: Glass flakes, acrylic resin, hardness, roughness. (J Bagh Coll Dentistry 2015; 27(2):6-10).

INTRODUCTION
The material most commonly used for the

fabrication of dentures complete or partial is heat-
cured poly (methyl methacrylate) (PMMA). This
material is not ideal in every respect and it is the
combination of virtues rather than one single
desirable property that accounts for its popularity
and usage; but, it is still far from ideal in fulfilling
the mechanical requirements of prosthesis
(1).PMMA continues to be used because of its
favorable working characteristics, processing
ease, accurate fit, and stability in the oral
environment, superior esthetics and use with
inexpensive equipment. Despite these excellent
properties, there is a need for improvement in the
fracture resistance of PMMA (2). Methods to
improve the inherent material properties of
PMMA have included using alternate polymers
such as polycarbonate (3), nylon (4), chemical
modification by including butadiene-styrene
rubber co-polymers (5,6) and the addition of
reinforcing agents including particulates and
fibers (7).

Altering a material by adding functional fillers
to improve some of its properties may introduce
deleterious effects on other properties; hence the
incorporation of glass flakes into PMMA attempt-
(1) Master student. Department of Prosthodontics, College of
Dentistry, University of Baghdad.
(2) Assistant professor, Department of Prosthodontics, College
of Dentistry, University of Baghdad.

ting to improve its fracture resistance may
adversely affect other properties such as surface
hardness and surface roughness.

Hardness is broadly defined as the resistance
to permanent surface indentation or
penetration.Hardness is indicative of the ease of
finishing of a structure and its resistance to in-
service scratching (8). Based on this definition of
hardness, it is clear why this property is so
important in dentistry. It is important that the
surface roughness (Ra) of materials used for
dental prostheses is determined before their use in
the mouth. Rougher surfaces can cause
discoloration of the prostheses, be a source of
discomfort to the patient, it contributes to
microbial colonization, biofilm formation, and the
accumulation of plaque and the adherence of
Candida albicans (9-11).Increased presence of
Candida species is reported in denture-related
stomatitis (12). The intaglio surface of the denture
is not polished prior to insertion; the rough areas,
areas of imperfections and porosities serve as a
breeding ground for opportunistic oral fungi,
that's why roughness as a surface property is so
crucial for polymers used in the construction of
dentures (13).

Glass flakes, a high aspect ratio reinforcing
additive with many commercial applications.
Glass flakes has been used in many industrial
polymers, their manufacturers claim that its
addition to some thermoplastics has resulted in a

J Bagh College Dentistry Vol. 27(2), June 2015 The effect of

Restorative Dentistry 7

significant improvement in flexural properties and
planar reinforcement (14). The current study was
conducted to investigate the effect of glass flakes
added as a strengthener on the surface hardness
and surface roughness of conventional heat-
processed poly (methyl methacrylate) denture
base resin.

MATERIAL AND METHOD

Micronised glass flake, surface pre-treated
with silane coupling agent as it was ordered from
the manufacturer, product code: (GF002).
(Glassflake Ltd, Leeds, UK). This consists of
flake particles 1.3-2.3µm thick, and a range of
diameters of which 88% were below
50µm.Triplex® Hot (IvoclarVivadent AG, Schaan,
Liechtenstein) was selected as both the control
and the agent to be experimented.

The glass flake was mixed with the poly
(methyl methacrylate) denture base powder, using
Weight/Weight (W/W) ratio14, to be mixed with a
constant amount of liquid. The glass flake was
added in amounts of 2%, 3%, 5% and 7% by
weight of powder, an electronic balance (A&D®
HR-200, Japan) with accuracy of (0.0001g) was
used for this purpose.

Molds were prepared using dental stone in
standard denture flasks, by investing plastic
patterns (template) measuring (65x10x2.5) mm.
Powder and liquid were mixed in accordance to
the manufacturer instructions (23.4g/10ml), the
mixture was covered and left to mature until
dough was reached, dough was packed into the
prepared stone molds, trial closure of the flask
halves was carried out under 80 bar pressure in a
hydrolytic press (Bego®, Germany) using
transparent sheets then final closure of the flask
halves was performed and clamped before curing.
Curing (polymerization) was carried out by
immersing the clamped flasks in cold water in a
thermostatically controlled water bath (Kavo®
EWL 5501, Germany), heated until boiling
temperature (100 C˚), then boiling continued for
(45) minutes, this is the standard procedure which
is the curing method recommended by the
manufacturer. The flasks were left to cool to room
temperature before being opened. Specimens were
finished and polished (except the specimens for
surface roughness test)this latter group was
designated as "unpolished" and represents the
denture intaglio surface which does not undergo
any alteration prior to insertion intraorally;
Finishing and polishing was accomplished in a
way similar to that used in the fabrication of
complete dentures.

Testing procedure
The specimens used were with dimensions of

(65mm x 10mm x 2.5mm) (15) ±0.2mm;
Specimens were conditioned in distilled water at
37 ˚C for 48 hours before being tested (16).

I. Surface hardness
Ten specimens for the control and ten

specimens for each glass flakes concentration
were prepared to make a total of (50) specimen
for the surface hardness measurement.Each
specimen was indented using compact portable
indenter (Shore D hardness tester, HT-5610D,
China), the equipment generally consist of spring-
loaded metal indenter point (0.8mm diameter) and
a gauge from which the hardness was read
directly from the digital display. The device was
used along with its test block that controls both
the direction (leveling) and the amount of the
applied force as in (Figure 1). According to the
device manufacturer’s instructions, the test block
must be positioned above the specimen which was
supported on a flat surface and the indenter point
pressed firmly and in a steady motion through the
hole of the test block until metal to metal contact
obtained between the head of the device and the
test block to apply the same amount of load in the
same direction. The first indentation point (test
pattern) was carried out 10mm from the specimen
edge and it was repeated every ten millimeters
along a line that bisects the specimen surface as in
(Figure 2). Five measurements were performed
for each specimen, and the average of these
measurements was calculated and considered for
that single specimen.

Figure 1: Compact portable indenter (Shore
D hardness tester) with its test block.

10mm 10mm 10mm 10mm

Figure 2: Schematic diagram of the surface
hardness test specimen with five test sites.

J Bagh College Dentistry Vol. 27(2), June 2015 The effect of

Restorative Dentistry 8

II. Surface roughness

Ten specimens for the control and ten specimens
for each concentration were prepared to make a
total of (50) specimen for the surface roughness
measurement. Each specimen was tested for
surface roughness using a portable surface
roughness tester (TR220, Time High Technology
Ltd., China), which can measure small surface
variations by moving a diamond stylus (needle-
shaped) in contact with the surface, while moving
along the specimen surface, measurements were
done on the same selected area of each specimen
as in (figure 3). The vertical displacement of the
stylus was measured as the microgeometry of the
surface varies; these measurements were
processed, stored and displayed.
The tests were performed with a scan length range
of 11mm. The device was set at the zero level as
the baseline for measurement and for each
specimen before performing the surface roughness
measurement. Surface roughness was measured at
3 positions as in (figure 4) across each specimen
surface which was divided into areas (3 equal
thirds, 2 at each end and one in the middle), and a
final average was then calculated for that
specimen.

Figure 3: Profilometer (portable roughness
tester)

Figure 4: Schematic diagram of the surface

roughness test and the specimen.

RESULTS
Surface hardness

The control acrylic resin samples (0% glass
flakes) exhibited a mean surface hardness of
(18.15); when all the mean values of the test
groups are compared, there is an obvious trend of
surface hardness increase along with the increase

in the flakes addition percentage. Details are
presented in (table 1).

Table 1: Descriptive statistics of the surface
hardness test.

Test Group Mean S.D.

Surface
hardness

Control 18.15 0.7230
2% 18.62 0.5245
3% 18.88 0.4442
5% 18.94 0.3438
7% 19.08 0.1686

The one way analysis of variance (ANOVA)

was conducted between the test groups to
examine sources of variation, as shown in (table
2).

Table 2: One way ANOVA between the tested

groups regarding Surface hardness test

Surface

hardness
Sum of
squares df

Mean
square F-test Sig.

Between
groups 5.375 4 1.344

5.884 0.001 * Within groups 10.277 45 0.228

Total 15.652 49
* Indicate the presence of statistically significant

differences at a level less than 0.05

The Wilcoxon test (Wilcoxon rank sum test)
was conducted to investigate the difference
between each two test groups. A statistically
significant difference (p<0.05) was found between
the control group and the case groups of (3%, 5%,
and 7%) and also between the 2% and 7% groups;
comparison between all the other groups revealed
a statistically non-significant difference (p >
0.05). Further details are presented in (table 3).

Table 3: Wilcoxon test between tested groups

regarding Surface hardness
Comparison P-value % difference

Control & 2% 0.221 +2.58%
Control & 3% 0.038* +4.02%
Control & 5% 0.036* +4.35%
Control & 7% 0.012* +5.12%

* Indicate the presence of statistically significant
differences at a level less than 0.05

Surface roughness

The control acrylic resin samples (0% glass
flakes) exhibited a mean surface roughness of
(1.3394); when all the mean values of the test
groups are compared, there is an obvious trend of
surface roughness increase along with the increase
in the flakes addition percentage. Details are
presented in (table 4).

J Bagh College Dentistry Vol. 27(2), June 2015 The effect of

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Table 4: Descriptive statistics of the surface
roughness test

Test Group Mean S.D.

Surface
roughness

(µm)

Control 1.33 0.1793
2% 1.47 0.1314
3% 1.55 0.1303
5% 1.58 0.1050
7% 1.65 0.2020

The one way analysis of variance (ANOVA)

was conducted between the test groups to
examine sources of variation, as shown in (table
5).

Table 5: One way ANOVA between the tested

groups regarding Surface roughness test

Surface

roughness
Sum of
squares df

Mean
square

F-
test Sig.

Between
groups 0.592 4 0.148

6.259 0.000* Within
groups 1.064 45 0.024

Total 1.657 49
* Indicate the presence of statistically significant

differences at a level less than 0.05

Table 6: Wilcoxon test between tested groups

regarding Surface roughness

Comparison P-value % difference
Control & 2% 0.074 +10.34%
Control & 3% 0.028* +16.4%
Control & 5% 0.009* +18.6%
Control & 7% 0.007* +23.65%

* Indicate the presence of statistically significant
differences at a level less than 0.05

DISCUSSION

In an attempt to explain these results, one must
imitate the micro-structure of the reinforced
PMMA resin specimens; these specimens are
planar structures (having thickness much lower
than their other two dimensions i.e. length and
width), and during the fabrication of these
specimens using the compression-molding
technique, a considerable amount of these flakes

might align parallel to the specimen's principal
plane especially as the flakes approaches the
surface, and as these flakes are high aspect ratio
fillers, they offer greater opportunity of
overlapped surfaces; and since they possess an
elastic modulus greater than that of the denture
base resin so they are stiffer and deform less than
the acrylic matrix. In the result, much more
resistance was provided against the penetrating
indenter as more glass flakes were incorporated
into the acrylic resin samples. This finding agrees
with that of Al Momen (18) who indicated a
significant increase in surface hardness when
6.6% of glass fibers were added to PMMA matrix.
Chen et al. (19) stated that the knoop hardness was
decreased as compared to the control when glass
fibers was added in 1%, 2% and 3%
concentrations; they also stated that the most
prominent decrease in surface hardness was in the
1% concentration, the issue that may disagree
with the results of the current study.

the variance in surface roughness might be
attributed to the protrusion of flakes from the
surface of PMMA specimens, since these fillers
are micron-sized and as the samples were
prepared using the compression-molding; it was
assumed that the flakes were spread or forced
randomly within the thickness and across the
surface of the samples, acquiring different
orientations in a more free random manner as they
approach the core of the sample; while, as these
flakes reach the surface of the specimen they tend
to align parallel to each other’s and to the
specimen's principal plane; this assumption don’t
exclude that a considerable amount of these flakes
might take other different random orientations
having their edges protruding out of the polymer
matrix rendering the surface of the reinforced
specimens rougher. It’s worthy to say that, this
assumption might occur at higher scale as the
concentration of flakes was increased, to explain
why the roughness was increased with the
increasing glass flakes concentration.

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