The dynamic viscoelasticity and water absorption characteristics of ...

Dental Materials Journal 2012; 31(1): 139–149
The dynamic viscoelasticity and water absorption characteristics of soft
acrylic resin materials containing adipates and a maleate plasticizer
Guang HONG 1 , Hiroki TSUKA 2 , Takeshi MAEDA 3 , Yasumasa AKAGAWA 2 and Keiichi SASAKI 4
1
Liaison Center for Innovative Dentistry, Graduate School of Dentistry, Tohoku University, 4-1 Seiryo-machi, Aoba-ku, Sendai 980-8575, Japan
2
Department of Advanced Prosthodontics, Graduate School of Biomedical Sciences, Hiroshima University, 1-2-3 Kasumi, Minami-ku, Hiroshima
734-8553, Japan
3
Department of Removable Prosthodontics, Osaka Dental University, 8-1 Kuzuhahanazono-cho, Hirakata, Osaka 573-1121, Japan
4
Division of Advanced Prosthetic Dentistry, Graduate School of Dentistry, Tohoku University, 4-1 Seiryo-machi, Aoba-ku, Sendai 980-8575, Japan
Corresponding author, Guang HONG; E-mail: hong@m.tohoku.ac.jp
The purpose of this study was to investigate the use of different plasticizers in soft acrylic resin materials to reduce leaching of the
plasticizer and thus increase the durability of tissue conditioners. Samples were prepared containing different combinations of three
types of polymer/copolymer powder and four types of plasticizer liquid (DEHM, DIBA, DAA and DINA). The dynamic viscoelasticity
of each sample was measured after water immersion using a dynamic mechanical analyzer. Water absorption, solubility and weight
change were also measured. A significant difference was found among the materials regarding dynamic viscoelasticity, water
absorption and solubility. The samples containing P-n-BMA had the most stable G’ and G’’ scores throughout the immersion. P-n-BMA
is the most suitable powder together with DEHM as the most suitable liquid component for a tissue conditioner. These results suggest
that it is possible to improve the durability of tissue conditioners by combining different polymers and plasticizers.
Keywords: Dynamic viscoelasticity, Soft acrylic resin, Maleate plasticizer, Adipates
INTRODUCTION
Soft acrylic resin materials are often used as resilient
denture liners and tissue conditioners in dentistry 1,2) .
Tissue conditioners are often used in edentulous patients
to treat lesions of the alveolar mucosa caused by ill-fitting
dentures, or to record dynamic functional impressions
utilizing their specific viscoelastic properties 3-5) . These
materials are supplied as powder and liquid. The powder
typically consists of poly(ethyl methacrylate) or a related
copolymer; and the conventional liquid contains a
mixture of an ester plasticizer and ethyl alcohol 6) .
Resilient denture liners only differ in the liquid
component, which consists of monomer, while the powder
constituent is almost the same 6,7) .
The main problem associated with this type of
material is a rapid loss of viscoelasticity in clinical use 8)
due to the leaching of ethyl alcohol and plasticizer into the
oral environment and water absorption 9-11) . Furthermore,
the leached plasticizer can plasticize the heat-polymerized
acrylic denture base resin and accelerate the deterioration
of the denture base material 12) . In addition, the estrogenic
effects of certain phthalates in vitro and in vivo have been
reported 13) . Hashimoto et al. 14) also reported that phthalate
esters, which are included in various dental materials as
plasticizers, showed estrogenic activity in vitro. Recently,
some European Union Directives have limited the use of
three of the most common phthalate esters (bis (2-ethyl
hexyl) phthalate, dibutyl phthalate, and benzyl butyl
phthalate) in toys and childcare articles, warning of their
risks 15) .
To minimize these risks, it is necessary to improve
the life span of soft acrylic resin materials by using an
alternative plasticizer. While the mechanical and
surface properties of soft acrylic resin materials have
been widely investigated 16-26) , the research to date has
still not found a solution to this problem 11,16-20) . When the
powder and liquid of soft acrylic resin are mixed, the
polymer dissolves in the plasticizer, polymer chain
entanglement takes place, and a gel is formed. The gel
has pseudo cross-links consisting of polymer chain
entanglements 16) . We surmise that when the best
conditions for formation of the pseudo cross-links have
been obtained by a suitable combination of soft acrylic
resin components, plasticizer leaching will be decreased,
and durability will be improved. Moreover, plasticizer
leaching will be decreased with the use of a high
molecular weight plasticizer in a liquid form. These
factors may be important for improving the longevity of
soft acrylic resin materials.
Other important factors to consider when these
materials are used for tissue conditioning and temporary
relining in clinical situations include their dynamic
viscoelastic properties, dimensional stability, surface
properties and water absorption 4, 11, 16, 18, 27, 28) .
The aim of this study was to investigate the use of
alternative plasticizers to try and produce a more durable
tissue conditioner. We therefore examined the influence
of composition on dynamic viscoelasticity, water
absorption, solubility and weight change of soft acrylic
resin materials.
MATERIALS AND METHODS
Sample preparation
Table 1 lists the polymer powders and plasticizers used
Received Aug 22, 2011: Accepted Oct 18, 2011
doi:10.4012/dmj.2011-185 JOI JST.JSTAGE/dmj/2011-185

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Table 1
Polymer powders and plasticizers used
Polymer Code M w P s
Polyethyl methacrylate PEMA 3.39×10 5 20 µm
Poly(n-butyl methacrylate) P-n-BMA 3.68×10 5 200 µm
Copoly (n-butyl/i-butyl methacrylate)
P-n-BMA/i-BMA
(composition ratio: 4/6)
1.99×10 5 200 µm
Plasticizer Code M w CAS No.
Di(2-ethylhexyl) maleate DEHM 340.5 142-16-5
Diisobutyl adipate DIBA 258.4 141-04-8
Di-n-alkyl adipate (C=6,8,10) DAA 370.4 106-19-4
Diisononyl adipate DINA 398.6 33703-08-1
Ethyl alcohol EtOH 46.1 64-17-5
M w: average molecular weight; Ps: particle size
Table 2 Formulations components
Code Powders Liquids P/L by weight
PEMA+DEHM PEMA 100wt% DEHM 95wt%+EtOH 5wt% 1.35
PEMA+DIBA PEMA 100wt% DIBA 95wt%+EtOH 5wt% 1.35
PEMA+DAA PEMA 100wt% DAA 95wt%+EtOH 5wt% 1.35
PEMA+DINA PEMA 100wt% DINA 95wt%+EtOH 5wt% 1.35
P-n-BMA+DEHM P-n-BMA 100wt% DEHM 95wt%+EtOH 5wt% 1.35
P-n-BMA+DIBA P-n-BMA 100wt% DIBA 95wt%+EtOH 5wt% 1.35
P-n-BMA+DAA P-n-BMA 100wt% DAA 95wt%+EtOH 5wt% 1.35
P-n-BMA+DINA P-n-BMA 100wt% DINA 95wt%+EtOH 5wt% 1.35
P-n-BMA/i-BMA+DEHM P-n-BMA/i-BMA 100wt% DEHM 95wt%+EtOH 5wt% 1.35
P-n-BMA/i-BMA+DIBA P-n-BMA/i-BMA 100wt% DIBA 95wt%+EtOH 5wt% 1.35
P-n-BMA/i-BMA+DAA P-n-BMA/i-BMA 100wt% DAA 95wt%+EtOH 5wt% 1.35
P-n-BMA/i-BMA+DINA P-n-BMA/i-BMA 100wt% DINA 95wt%+EtOH 5wt% 1.35
in this study. Three polymer powders (Negami Chemical
Industrial Co., LTD., Ishikawa, Japan); one polyethyl
methacrylate (PEMA), one poly(n-butyl methacrylate)
(P-n-BMA) and one copoly(n-butyl/i-butyl methacrylate)
(P-n-BMA/i-BMA) were used in powder form. Four
plasticizers (Tokyo Chemical Industry Co., LTD., Tokyo,
Japan); one maleate plasticizer; di (2-ethylhexyl) maleate
(DEHM), three adipate plasticizers; diisobutyl adipate
(DIBA), di-n-alkyl adipate (DAA) and diisononyl adipate
(DINA); and ethyl alcohol (EtOH) (Wako Pure Chemical
Industries Ltd., Osaka, Japan) were used in liquid form.
The 12 formulations used in this study and the
powder/liquid (P/L) ratio are listed in Table 2. The
powder and liquid were mixed by hand for 2 min (at
23±2°C and 70% humidity), and then poured into a
poly(propylene) container. Glass plates were placed on
top of the containers and the containers were placed in a
37°C incubator for 30 min. After the incubation, the
specimens were removed from the polypropylene
containers and immersed in 100 mL distilled water in
dark brown containers at 37°C.
Dynamic viscoelasticity measurement
Five pairs of specimens of each formulation were
prepared in layers measuring 10×10×2 mm. The
dynamic viscoelasticity was measured using an
automatic dynamic mechanical analyzer (DMA Q800,
TA Instruments Co., New Castle, USA) (Fig. 1) at 37°C
with the use of a shear sandwich sample clamp at 0 (30
min after specimen preparation), 1, 3, 7, and 14 days
after immersion in distilled water. This device is based
on the principle of non-resonance-forced vibration and
consists of a measurement operation block, furnace,
sample clamps, and main unit. The measurement
operation block consists of an optical encoder, air
bearings and driver motor. Viscoelasticity is studied
using dynamic mechanical analysis where an oscillatory
force (stress) is applied to a material and the resulting
displacement (strain) is measured. During testing a
suitable shear and dynamic displacement are applied to
the specimen, and the shear storage modulus (G’), shear
loss modulus (G’’) and loss tangent (tan ) are detected.
In this study, the G’, G’’ and tan were determined over
a frequency range of 0.05–100 Hz with a 0.08% strain.
Figure 2 shows the schemata of relationship between

Dent Mater J 2012; 31(1): 139–149 141
Fig. 1
Block diagram of the automatic dynamic mechanical analyzer (DMA Q800).
Fig. 2
Schematic of relationship between stress and strain.

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stress and strain. In purely elastic materials (a), the
stress and strain occur in phase, so that the response of
one occurs simultaneously with the other. In purely
viscous materials (b), there is a phase difference between
stress and strain, where strain lags stress by a 90° (/2
radian) phase lag. Viscoelastic materials (c) exhibit
behavior somewhere in between that of purely viscous
and purely elastic materials, exhibiting some phase lag
in strain. The shear storage modulus (G’) in viscoelastic
materials measures the stored energy, representing the
elastic portion; and the shear loss modulus (G’’) in
viscoelastic materials measures the energy dissipated as
heat, representing the viscous portion. The loss tangent
(tan ) is a useful parameter and a measure of the ratio
of energy dissipated to energy stored.
Water absorption, solubility, and weight change
measurement
Five specimens of each formulation were prepared in
layers measuring 30×20×2 mm. The samples were
weighed using an analytical balance (AW220, Shimadzu
Co., Kyoto, Japan) to an accuracy of 0.001 g at 30 min
after sample preparation, and then after 1, 3, 7, and 14
days of immersion. After 14 days of immersion, the
samples were removed from the water and stored in
desiccators containing freshly dried silica gel at 23°C.
The weight measurement was repeated every day until a
constant mass was obtained, i.e. until the loss in mass of
each specimen was no more than 0.0002 g between
successive 2 days weightings. The water absorption
(W sp), solubility (W sl) and weight change (W ch) of the
samples were expressed as a percentage using the
following formulae:
W sp (%)=(M 2−M 3)×100/M 1
W sl (%)=(M 1−M 3)×100/M 1
W ch (%)=(M−M 1)×100/M 1
where M 1 is the initial weight of the materials before
immersion in water (after sample preparation), M 2 is the
weight after absorption and desorption, M 3 is the final
weight after desiccation, and M is the weight of the
materials after 1, 3, 7, and 14 days of immersion.
Statistical analysis
All data, consisting of dynamic viscoelasticity (G’, G’’ and
tan ), water absorption, solubility and weight change
data, were analyzed independently by one-way analysis
of variance (ANOVA) combined with a Student-Newman-
Keuls (SNK) multiple comparison test at a 5% level of
significance. Two-way ANOVA was used to determine
the influence of the composition and immersion time on
the dynamic viscoelasticity, water absorption, solubility
and weight change of the materials. A frequency of 1 Hz
was selected for statistical analyses. The differences
among materials and among immersion times were
tested with an SNK test at a 5% level of significance. All
analyses were computed with SPSS for the Windows
operating system (SPSS 12, SPSS Japan Inc., Tokyo,
Japan).
RESULTS
Figures 3, 4 and 5 show the dependence of G’, G’’ and tan
on frequency for the each formulation 30 min after
specimen preparation. All samples exhibited higher
values of G’ and G’’ at higher frequencies excepted G’
values of samples of containing P-n-BMA. G’ of samples
of containing P-n-BMA increased as the frequency
increased from 0.05 to 10 Hz, then decreased again at
higher frequencies. The tan of all samples decreased as
the frequency increased from 0.05 to 10 Hz, then
increased again at higher frequencies.
Table 3 shows the mean and standard deviations
(SD) of the storage modulus (G’), loss modulus (G’’) and
loss tangent (tan ) of each formulation at 1 Hz before
immersion. Significant differences were found between
the different materials (p

Dent Mater J 2012; 31(1): 139–149 143
Fig. 3
Variation of storage modulus (G’) with frequency of materials 30 min after specimen preparation.
Fig. 4
Variation of loss modulus (G’’) with frequency of materials 30 min after specimen preparation.

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Fig. 5
Variation of loss tangent (tan ) with frequency of materials 30 min after specimen preparation.
Table 3 Initial storage modulus (G’), loss modulus (G’’) and loss tangent (tan ) of each formulation at 1 Hz
Code G’ (MPa) G’’ (MPa) tan
PEMA+DEHM 0.0714 (0.0085) a 0.0151 (0.0016) b 0.21 (0.004) g
PEMA+DIBA 0.0494 (0.0058) c 0.0105 (0.0012) cd 0.21 (0.002) g
PEMA+DAA 0.0610 (0.0103) b 0.0129 (0.0021) c 0.21 (0.003) g
PEMA+DINA 0.0626 (0.0085) b 0.0181 (0.0022) a 0.29 (0.013) f
P-n-BMA+DEHM 0.0106 (0.0007) de 0.0043 (0.0002) f 0.41 (0.004) e
P-n-BMA+DIBA 0.0115 (0.0009) de 0.0057 (0.0003) f 0.50 (0.012) c
P-n-BMA+DAA 0.0118 (0.0017) de 0.0055 (0.0007) f 0.47 (0.008) d
P-n-BMA+DINA 0.0118 (0.0011) de 0.0049 (0.0005) f 0.42 (0.009) e
P-n-BMA/i-BMA+DEHM 0.0182 (0.0020) de 0.0103 (0.0009) e 0.57 (0.019) b
P-n-BMA/i-BMA+DIBA 0.0085 (0.0032) e 0.0057 (0.0018) f 0.69 (0.049) a
P-n-BMA/i-BMA+DAA 0.0195 (0.0012) de 0.0109 (0.0005) cd 0.56 (0.027) b
P-n-BMA/i-BMA+DINA 0.0202 (0.0022) d 0.0111 (0.0009) cd 0.55 (0.015) b
Identical letters indicate no significant differences (p>0.05, SNK test)
recorded higher W sp and W sl values than samples
containing other plasticizers.
Weight changes with time for each formulation are
shown in Fig. 9. The ANOVA results indicate significant
differences among the materials and significant effects of
immersion time on weight change (p

Dent Mater J 2012; 31(1): 139–149 145
Fig. 6
Variations in storage modulus (G’) with immersion period at 1 Hz.
Fig. 7
Variations in loss modulus (G’’) with immersion period at 1 Hz.

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Fig. 8
Variations in loss tangent (tan ) with immersion period at 1 Hz.
Table 4 Water absorption and solubility of each formulation
Code Water absorption (%) Solubility (%)
PEMA+DEHM 4.08 (0.17) d 1.52 (0.11) a
PEMA+DIBA 5.88 (0.67) e 6.40 (0.64) c
PEMA+DAA 4.09 (0.28) d 1.91 (0.28) a
PEMA+DINA 4.08 (0.33) d 2.26 (0.32) a
P-n-BMA+DEHM 1.26 (0.39) a 1.89 (0.26) a
P-n-BMA+DIBA 4.35 (0.62) d 7.52 (0.36) d
P-n-BMA+DAA 2.80 (1.03) c 2.09 (0.07) a
P-n-BMA+DINA 1.79 (0.97) abc 1.84 (0.19) a
P-n-BMA/i-BMA+DEHM 1.82 (0.27) abc 1.52 (0.13) a
P-n-BMA/i-BMA+DIBA 4.11 (0.65) d 5.60 (0.84) b
P-n-BMA/i-BMA+DAA 1.52 (0.03) ab 1.78 (0.19) a
P-n-BMA/i-BMA+DINA 2.58 (0.75) bc 1.65 (0.28) a
Identical letters indicate no significant differences. (p>0.05, SNK test)
DISCUSSION
The longevity of tissue conditioners is a significant
problem in prosthodontics 8) , and viscoelasticity is one of
the most important factors for determining their clinical
acceptance 13,14) . In clinical situations, tissue conditioners
are often replaced every three to four days 29) , and they
rarely last beyond two weeks. Therefore, in this study,
our samples were tested for two weeks. Dynamic
viscoelastic parameters at 1 Hz were selected for
evaluation in this study based on the research of Murata
et al. 11) who reported that these settings would simulate
behavior under typical masticatory conditions. The
shear storage modulus (G’) represents the elastic
component of material behavior, and the shear loss
modulus (G’’) represents the viscous component of

Dent Mater J 2012; 31(1): 139–149 147
Fig. 9
Weight change with time for each formulation.
material behavior. The lower G’ and higher G’’ means
that material has good flow characteristics, which would
allow the damaged denture-bearing mucosa to recover to
a healthy state and record the shape of the mucosa under
functional stress more effectively.
Differences in dynamic viscoelastic behavior were
found among the materials. The viscoelastic properties
of the all samples showed sensitivity to the changed in
frequency. The G’ describes elastic deformation under
stress whilst the G’’ describes viscous deformations. The
butyl system polymer based samples exhibited lower G’
and G’’, and higher tan than ethyl system polymer
based samples. This suggests that the butyl system
polymer based samples have good flow properties, which
would allow the damaged denture-bearing mucosa to
recover to a healthy state and more effectively record the
shape of the mucosa under functional stress. The glass
transition temperature (T g) of poly(ethyl methacrylate)
is higher than room temperature 16) , while the T g of
poly(n-butyl methacrylate) and related copolymers is
near or below room temperature 16) . For this reason, the
G’ and G’’ of samples containing P-n-BMA and P-n-BMA/
i-BMA are lower than those of the PEMA based samples.
The P-n-BMA samples exhibited the most stable dynamic
viscoelastic characteristics (G’ and G’’) throughout the
experimental period of all the materials except the
P-n-BMA+DIBA samples. Tissue conditioners are used
for three main clinical purposes: tissue conditioning;
dynamic impressions; and temporary relining. Murata
et al. 30) reported that materials with good early flow
properties, and a lower rate of change of flow with the
passage of time are suitable for tissue conditioning.
Therefore, soft acrylic resins containing P-n-BMA are
probably the most suitable for tissue conditioning.
The samples containing P-n-BMA/i-BMA, except for
the P-n-BMA/i-BMA+DIBA samples, exhibited rapid
changes in G’, G’’ and tan during the first day of water
immersion. Jones et al. 9) reported that the a total of
between 26–52% of the alcohol content of the gel
materials was lost during the various stages of
preparation prior to conducting the test. And he also
reported that the alcohol is completely lost within 24 h
from polymer gel materials stored in water. Supporting
these findings, Wilson 10) reported that most alcohol was
lost within the first 12 h. Based on these reports, we
suggest that the alcohol loss might be the reason for this
rapid changes in rheological parameters recorded during
the first day.
The samples containing P-n-BMA, except for the
P-n-BMA+DIBA samples, showed the most stability
coupled with a low change rate of G’ and G’’. Several
investigators have reported on the leachability of
plasticizers from soft dental polymers 6, 9, 32) . The loss of
viscoelasticity in tissue conditioners in clinical use may
be due to the leaching out of the plasticizer from the soft
polymer gel material, accompanied by water absorption.
The absorbed water also acts as a plasticizer, reducing
the mechanical properties of the polymer network 33) .

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The samples containing P-n-BMA, except for the
P-n-BMA+DIBA samples, recorded lower values for
water absorption and solubility. This could be the reason
why these samples also showed the most stable
viscoelasticity. However, another reason for this
viscoelastic stability could be that these samples
constitute the most suitable combination of components,
providing the best conditions for the optimal formation
of the pseudo cross-links consisting of polymer chain
entanglements 24,34) .
When soft acrylic resin materials are immersed into
water, two processes take place: plasticizers and alcohol
are leached into the water and, at the same time, water
is absorbed by the material. In this study, the samples
containing PEMA recorded greater water absorption
than the samples containing other polymers. Samples
containing DIBA as the plasticizer also recorded greater
water absorption than other samples. The relative
hydrophilicity of the acrylic resin materials is the major
influence on their water absorption 31) . The hydrophilicity
increased as the number of hydrocarbon groups in the
polymers or plasticizers decreased. PEMA has fewer
hydrocarbon groups than P-n-BMA and P-n-BMA/i-BMA,
and DIBA has fewer hydrocarbon groups than the other
plasticizers used in this study. This could explain this
tendency for higher water absorption. And the different
molecular weight may be also influenced to these results.
The samples containing DIBA as a plasticizer
recorded higher solubility than the other samples. The
solubility of soft acrylic resin materials results from loss
of ethanol and plasticizer. Jones et al. 9) reported that
higher molecular weight plasticizers did not leach as
much from soft polymer gel materials as lower molecular
weight plasticizers. The molecular weights (M w) of
plasticizers used in this study are 340.5 (DEHM), 370.4
(DAA) and 398.6 (DINA), which are higher than DIBA
(M w: 258.4), and also higher than those of other
plasticizers usually used in commercial tissue
conditioners, such as dibutyl phthalate (M w: 278.4),
benzyl benzoate (M w: 212.3), benzyl salicylate (M w: 228.2)
and dibutyl sebacate (M w: 314.5). Based on this
information, we assume that the DIBA samples had a
higher level of leaching than the other plasticizers used
in this study, explaining why the DIBA samples exhibited
higher solubility.
All samples containing PEMA, except for the
PEMA+DIBA samples, displayed a significant weight
gain (maximum 2.56%). These samples recorded higher
water absorption (from 4.08% to 4.09%) than solubility
(from 1.52% to 2.26%). This may be because
P-n-BMA+DEHM samples, P-n-BMA+DINA samples
and all samples containing DIBA lost weight during
immersion in water, most likely due to the plasticizers
and alcohol leaching out at a greater rate than the water
being absorbed.
The results of this study indicate that the dynamic
viscoelasticity and water absorption characteristics of
soft acrylic resin materials are greatly influenced by the
combination of the components of these materials. The
flow of soft acrylic resin materials tended to increase in
resins containing butyl system polymers. The
P-n-BMA+DEHM sample showed the greatest stability
and dynamic viscoelasticity, and the lowest values for
water absorption and solubility. It should be noted,
however, that the present study did not completely
simulate clinical conditions because specimens were
tested for viscoelasticity in the dry state, and all
specimens were immersed in distilled water. To
overcome the limitations of the in vitro tests, artificial
saliva should be used as an immersion solution.
Furthermore, for a more complete understanding of how
different components affect durability, further research
is required into the components’ effects on other
properties such as working time, gelation time, surface
properties, dimensional change, and leachability of
plasticizer.
CONCLUSION
From the standpoint of dynamic viscoelasticity and
water absorption characteristics, poly(n-butyl
methacrylate) (P-n-BMA) is the most suitable powder
together with di (2-ethylhexyl) maleate (DEHM) as the
most suitable liquid component for a tissue conditioner.
Within the limitations of this study, our results show
that it may be possible to improve the durability of tissue
conditioners by judicious choice of components.
ACKNOWLEDGMENTS
This research was supported by Grant-in-Aid (No
21791899) for Scientific Research from the Ministry of
Education, Culture, Sports, Science and Technology,
Japan.
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