Effect of aluminoborate whiskers on mechanical properties of polycarboxylate cements.

Abstract

The mechanical properties of polycarboxylate cements containing 20 mass% of four kinds of aluminoborate whiskers with different fiber lengths and diameters were evaluated. Bending strength of the cements increased with increase in fiber length, whereby the strength of the cement with the longest fiber was about two times greater than that of whisker-free cement. The diametral tensile strengths of the four fiber-reinforced cements were also about two times greater than that of whisker-free cement, but which was not dependent on fiber length. Compressive strength was the same or slightly higher than that of whisker-free cement. SEM observation of fractured specimen after diametral compression test showed high affinity between the cement matrix and the whisker.

Full text

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Dental Materials Journal 2008; 27(4): 561－564
Original Paper

Effect
Polycarboxylate Cements

of Aluminoborate Whiskers on Mechanical Properties of
Shigeaki KURATA and Kozo UMEMOTO
Department of Biomaterials and Devices, Kanagawa Dental College, 82 Inaoka-cho, Yokosuka, Kanagawa 238, Japan
Corresponding author, Shigeaki KURATA; E-mail: kuratas@kdcnet.ac.jp

The mechanical properties of polycarboxylate cements containing 20 mass% of four kinds of aluminoborate whiskers with
different fiber lengths and diameters were evaluated. Bending strength of the cements increased with increase in fiber
length, whereby the strength of the cement with the longest fiber was about two times greater than that of whisker-free
cement. The diametral tensile strengths of the four fiber-reinforced cements were also about two times greater than that of
whisker-free cement, but which was not dependent on fiber length. Compressive strength was the same or slightly higher
than that of whisker-free cement. SEM observation of fractured specimen after diametral compression test showed high
affinity between the cement matrix and the whisker.
Key words: Polycarboxylate cement, Whisker, Mechanical property

Received Aug 3, 2007: Accepted Feb 1, 2008
INTRODUCTION
On the mechanical properties of dental and bone
cements, it is important to improve not only the
compressive strength but also the tensile and
bending strengths. When compared to resin cements,
polyalkenoate cements offer some upper-hand char-
acteristics such as high fluoride ion release, lower
residual monomer content, and reduced irritant
action on the pulp. Despite these advantageous
features, the failure modes of polyalkenoate cements
after tensile testing frequently show cohesive fracture
within the cement matrix as well as low adhesive
strength between the
Therefore, to improve
polyalkenoate cements, it must be achieved by
increasing the tensile strength.
　　The authors have previously reported that the
diametral tensile strength of polycarboxylate cements
was excellently improved by the addition of
aluminoborate whiskers of 20－50 μm fiber length
and 1－5 μm diameter1). Conversely, the strength of
the cements containing
spherical fillers ― with the same composition as the
whiskers ― was not improved1).
　　The objective of this study was to investigate the
effect of whiskers on the bending and tensile
strengths of polycarboxylate cement, whereby the
latter contained the same composition of whiskers of
different lengths and diameters.
cement
the
and
performance
adherend.
of
irregular-shaped and
MATERIALS AND METHODS
Aluminoborate whiskers
Table 1 shows the four kinds of Alborex G® whiskers
(Shikoku Chemicals, Kagawa, Japan) used in this
study. Chemical composition of the whiskers was
Al2(SO4)3/H3BO3/K2SO4 and the specific gravity of
each whisker was 2.93. For Alborex GA (hereinafter
referred to as GA), the shortest among the four kinds
of whiskers, fiber size was 5 μm in length and 0.2－
0.5μm in diameter. The abbreviations of other
whiskers, as shown in Table 1, are GB, GC, and GD.
The mean length and diameter of the whiskers in
increasing order is GA<GB<GC<GD. Length and
diameter of the largest whisker GD were 20－50 μm
and 1－5 μm respectively.
Polycarboxylate cements
A commercial polycarboxylate cement (Calron SmFP,
Dentsply Sankin, Tochigi, Japan) was used as a basic
cement to compare the supplementary effects of the
whiskers. An amount of 20 mass％ of each whisker
was mixed with the polycarboxylate cement powder
in a mortar with pestle to avoid the smashing of
whiskers. Cement powder (1.08 g) containing the
whisker and the cement liquid (0.60 g) were mixed
on a mixing pad for 30 seconds using a plastic
spatula. The mixed paste was filled into a Teflon
mold, whereby rectangular-shaped plates of 30×3×3
mm were prepared as specimens for the bending test.
　　Specimens for the compression and diametral
tensile tests were prepared using the same method.
Size of the specimen was 6 mm in diameter and 12
mm in length for the compression test and 6 mm in

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Dent Mater J 2008; 27(4): 561－564562
diameter and 6 mm in length for the diametral
tensile test. Codes of the hardened cements prepared
from the four whiskers were GA, GB, GC, and GD.
Whisker-free cement specimens were prepared as a
control. Seventy-five specimens were stored in 37 °C
water for two days prior to testing.
Bending, compression, and diametral tensile tests
Bending strength was measured with a universal
testing machine (AGS-500, Shimadzu Co. Ltd., Kyoto
Japan) using a three-point bending fixture with a
span of 20 mm. Crosshead speed of the test was
adopted as 0.5 mm/min to facilitate the calculation
of the bending modulus. Bending modulus of
elasticity was calculated from the straight portion up
to the proportional limit of the load-time curve
obtained from the bending test.
　　In this study, reasons for obtaining the elasticity
modulus from the bending test were as follows: (1)
whiskers were added to the cement with a view to
improving the latter’s bending and tensile strengths;
(2) a stress-strain curve from a compression test is
unsuitable for obtaining the moduli of brittle
materials like this cement.
　　The compression and diametral tensile tests
were carried out using the same testing machine at a
crosshead speed of 2.0 mm/min. All data were
statistically analyzed by Student’s t-test.
SEM observation
To evaluate the affinity between the cement matrix
and the whisker, the fractured surface of the
specimen after the diametral compression test was
observed using a scanning electron microscope (SEM)
(JSM-35C, JEOL Ltd., Tokyo, Japan).
RESULTS
Mechanical properties
Table 2 shows the values of the compressive, bending,
and diametral tensile strengths, as well as the
bending moduli of elasticity. Bending strength of
whisker-free cement was 15.2±2.6 MPa, while that
of GA was 20.1±2.1 MPa. Therefore, there was a
statistically significant difference between GA and
the whisker-free cement (p<0.05). Bending strengths
of GB, GC, and GD were 26.0±2.4, 30.2±2.4, and
32.3±3.5 MPa respectively. The values of GC and
GD were about two times greater than that of
whisker-free cement and were significantly different
from those of GA and GB.
　　The elastic modulus of whisker-free cement was
3.99±0.35 GPa, and the values of the four cements
containing the whiskers ― namely GA, GB, GC, and
GD ― were higher than that of whisker-free cement
(p<0.05).
　　The diametral tensile strength of whisker-free
Alborex G
whisker
Mole ratio of composition
Al2(SO4)3/H3BO3/K2SO4 #
Length$
(μm)
Diameter$
(μm)
GA
GB
GC
GD
8
8
8
8
2
2
2
2
10
20
25
30
5 0.2－0.5
0.5－1
1－2
1－5
10－30
20－30
20－50
#: Mole ratio of metal
$: Measured by Shikoku Chemicals
Table 1 Compositions, lengths, diameters, and codes of the various Alborex G® whiskers
Cement
Compressive
strength
(Mpa)
Diametral
tensile strength
(Mpa)
Bending
strength
(Mpa)
Elastic
modulus&
(Gpa)
Control
GA
GB
GC
GD
63.8 (7.4)a
74.1 (1.5)b
72.0 (5.8)ab
69.2 (5.9)ab
64.3 (5.3)a
5.28 (0.8)a
9.88 (1.3)b
10.3 (1.2)b
11.0 (1.1)b
10.8 (1.3)b
15.2 (2.6)a
20.1 (2.1)b
26.0 (2.4)c
30.2 (2.4)d
32.3 (3.5)d
3.99 (0.35)a
4.86 (0.27)b
5.41 (0.40)c
4.91 (0.26)b
5.39 (0.60)bc
&: Bending modulus of elasticity
Mean value (SD)
Same superscript letters denote no significant difference test between groups (p>0.05)
Table 2 Mechanical properties of polycarboxylate cements containing 20 mass% of Alborex G® whiskers immersed in
water at 37°C for 2 days

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Dent Mater J 2008; 27(4): 561－564563
cement was 5.28±0.80 MPa. The values of the four
cements containing the whiskers were about two
times greater than that of whisker-free cement. The
compressive strength of GA was 74.1±1.5 MPa ― the
highest value among the four cements ― was signifi-
cantly different from that of whisker-free cement
(p<0.05). On the other hand, there were no
significant differences between the whisker-free
cement and the other three cements, GB, GC, and
GD.
SEM observation
Figure 1 shows a SEM photograph of the fractured
surface of GA after diametral compression test. It
was observed that the matrix of the cement adhered
to the rod-like whisker, indicating high affinity
between the whisker and the matrix (white arrows in
Fig. 1).
DISCUSSION
Previously, researches with a view to improving the
mechanical strength of polyalkenoate cements have
been conducted using powders composed of different
types of particles. Kerby and Bleiholder2) reported
that the mechanical properties of glass ionomer
cements were reinforced by the addition of stainless
steel or silver powder. In the same vein, Tsuchihashi
et al.3) reported that the strength of glass polyalkeno-
ate cements was improved by about 30％ with the
addition of a powder composed of irregular-shaped
particles. However, Walls et al.4) and Irie and Nakai5)
reported that the addition of silver powder hardly
improved the bending strength of glass ionomer
cements.
　　On adding fibers to reinforce glass ionomer
cements, Kobayashi et al.6,7) reported that the tensile
and bending strengths of glass ionomer cements with
short glass fibers of CaO-P2O5-SiO2-Al2O3 were
increased by about two times as compared to the
cement free of the short fibers. In this study, the
strength of the cements with Alborex G® whiskers
was obviously improved. On the contrary, cements
containing particulate fillers of the same composition
as Alborex G® whisker were not improved in
diametral tensile and bending strengths1). It seemed
that with particular filler as a reinforcement agent,
improvement in the mechanical properties of glass
ionomer cements was largely dependent on the shape
of the added powder particles. On the other hand, a
general improvement in the strength of glass ionomer
cements could be discerned with the addition of
whiskers.
　　The bending strength of the cements with
whiskers increased in the order of GA＜GB＜GC＜
GD. In particular, the bending strengths of the
cements with whiskers GC and GD were two times
greater than that of
Compressive strength was slightly improved by the
addition of whisker GA, but not so with the addition
of whiskers GB, GC, and GD. The diametral tensile
strengths of the four cements with whiskers GA to
GD were about two times greater than that of
whisker-free cement, and that there were no
significant differences among the four cements with
whiskers.
　　Compressive strength was not remarkably
improved in this study with the addition of whiskers.
It was thus thought that the matrix of the cement,
comprising the metal salt of polyacrylic acid, was the
weak layer in the cement. On the other hand, the
increase in the bending and diametral tensile
strengths of the cements could be ascribed to the
dynamic friction in the drawing process of the
whiskers on bending or tensile stress. From the
result of each test, it seemed that the addition of
whisker GB reinforced the cement with optimum
toughness. This could be ascribed to the aspect ratio
of the whisker, where the aspect ratio values of GA,
GB, GC, and GD calculated from the mean lengths
and diameters of the whiskers were 14, 26, 13, and
12 respectively.
　　On the SEM photograph of the fractured surface
of GB after the diametral compression test, it was
observed that the matrix of the cement adhered to
the rod-like whisker (white arrows in Fig. 1). Hirata
et al.8) reported that polyacrylic acid showed a good
affinity for alumina powder surface. As Alborex G®
whiskers contained alumina in the composition, it
was thought that the good mechanical properties of
whisker-free cement.
Fig. 1 SEM photograph of the fractured surface of cement
specimen containing the GB whisker after
diametral compression test. *: White arrows
indicate the cement matrix adhering to the
whisker.

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Dent Mater J 2008; 27(4): 561－564564
whisker-reinforced cements might be due to the high
affinity between the whisker and the matrix.
ACKNOWLEDGEMENTS
The authors thank the General Manager, Mr. Gen
Hata, and Mr. Toshimasa
Development Department, Marugame Plant, Shikoku
Corporation, Kagawa, Japan for supplying the
Alborex G® whiskers for this study.
Arai, Commercial
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