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Bond strength of permanent soft denture liners bonded to the
denture base
Thomas J . Emmer , J r , DMD, a Thomas J . Emmer , Sr, DDS,
b
Jaya lakshmi Va idynathan , PhD, c and Tr i ta la K. Va
idynathan , PhD d
University of Medicine and Dentistry of New Jersey, New Jersey
Dental School, Newark, N. J.
The purpose of this study was to character ize denture and soft
l iner adhes ion and to determine the adhes ive and/or cohesive
strength of different soft t issue l iners bonded to the denture
base by use of a new technique. Two groups of f ive permanent soft
l iners (dry or exposed to water for 6 months) were tested by use
of a tensi le mode to character ize the fai lure character ist ics
of soft l iners bonded to denture base resin. The method dif fered
from previous test methods because of the specimen's abi l i ty to
al ign axia l ly dur ing the test. The results ind icated s igni f
icant di f ferences in the bond ing of l iners to the denture base,
and l ight-cure systems exhib i ted the greatest amount of stress
needed for fai lure. Low bond strength was observed when the adhes
ion was poor or when the cohes ive strength of the soft l iner was
low and lead to pure adhes ive or cohesive fai lure. When both
adhes ive and cohesive bonds were strong, fai lure occurred at h
igh stresses. Combinat ions of adhes ive and cohesive fa i lures
(mixed mode) were also observed in intermediate cases. (J PROSTHET
DENT 1995;74:595-601.)
Permanent soft denture liners have been a Valu- able asset for
dentists and, because of their viscoelastic properties, they act as
shock absorbers and reduce and distr ibute the stresses on the
denture-bearing tissues. 1-2 Their use for patient comfort and the
treatment of the atrophic ridge, bone undercuts, bruxism,
xerostomia, and dentures opposing natural teeth has been known to
be clinically beneficial. 3 Although these attr ibutes are posi-
tive, there are also disadvantages to the use of permanent soft
liners. One of the major drawbacks of the permanent soft l iners is
the lack of a durable bond to denture. 4-9 De- bonding of soft l
iners from the denture is a common clin- ical occurrence. Debonding
results in localized unhygienic conditions at the debonded regions
and often causes func- tional failure of the prosthesis.I~ Although
there are pub- lished reports on the bond strength of soft l iners
bonded to denture base resin, different methods such as peel 9, 11
or tensile 12 tests have been used to measure the bond strength.
Although the previously used tests have provided valuable
information, there are l imitations to some of these meth- ods. In
particular, direct gripping of the specimen in the tensile testing
machine may complicate or compromise the
aResearch Associate, Department of Prosthodontics and Bioma-
terials.
bAssociate Clinical Professor of Prosthodontics and
Biomaterials. CAssociate Professor of Prosthodontics and
Biomaterials. dprofessor of Prosthodontics and Biomaterials.
Copyright 9 1995 by The Editorial Council of THE JOURNAL OF
PROSTHETIC DENTISTRY.
0022-3913/95/$5.00 + 0. 16/1/68284
specimen al ignment 1~ and also damage the sample integ- r ity
at the gripped regions. There is therefore a need to de- velop a
tensile test method that permits axial self-align- ment of the
specimen.
This study was designed (1) to characterize the debond- ing
characteristics of soft denture liners bonded to denture resin
mater ia l with the following specific objectives, (2) to develop a
tensile method to characterize the failure modes and strengths of
soft liners bonded to denture base mate- rial, and (3) to use this
method to evaluate the bonding and/or the cohesive strength of
selected permanent soft reline materials bonded to a denture base
material.
MATERIAL AND METHODS
The reline materials included selected materials from light- and
heat-polymerized systems currently available. There are significant
differences in the chemical makeup of different materials (Table
I). Whereas Triad (Dentsply/ York Div., York, Pa.) and Astron
(Astron Dental, Wheeling, Ill.) reline materials use light
polymerized resins based on urethane dimethacrylate and Bis-GMA
dimethacrylate monomers, Molloplast-B reline material (Buffalo
Dental Mfg. Co., Syosset, N. Y.) is based on silicone. Other
systems such as PermaSoft (Nue Dent, Cambridge, Mass.) and Su- per
Soft (Coe Laboratories, Chicago, Ill..) are plasticized polymethyl
methacrylate (PMMA) that is mixed with polyethyl methacrylate
(PEMA). The denture base mate- rial used was Lucitone 199
(Dentsply/York Div.), a heat- processed PMMA based system.
Lucitone 199 denture mater ia l blocks (100 80 10 mm) were
flasked and processed for 6 hours at 164 ~ F and I hour at 212 ~ F.
The blocks were cut into 10 10 5 mm
DECEMBER 1995 THE JOURNAL OF PROSTHETIC DENTISTRY 595
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THE JOURNAL OF PROSTHETIC DENTISTRY EMMER ET AL
Table I. Denture reline materials
Triad Astron Molloplast B PermaSoft Super Soft
Polymerization Light Light Heat Heat Heat mode
Material chemical Urethane Composite ? Silicone Plasticized
Plasticized composition polyether (Information not polymethyl
polymethyl
dimethacrylate made available) methacrylate/ methacrylate/
polyethyl Polyethyl methacrylate methacrylate
Self Self Bonding agent Light cured chemical methyl- composition
methacrylate
How supplied Premixed paste Manufacturers Apply Triad
recommended bonding agent surface preparation
Self Saline
Powder liquid Premixed paste Powder liquid Powder liquid Apply
"wet" mix of Apply bonding Rinse denture Rinse denture
freshly mixed agent surface with surface with powder liquid
monomer monomer
Table II. Duncan multiple range tests of subsets
Sample group failure strength (MPa)
Triad Super Soft Astron Molloplast B PermaSoft
Stored 24 hours (72 ~ F), dry 7.43 2.94 2.60 1.21 1.50 Stored 6
months (72 ~ F), water 12.4 7.09 7.80 2.69 1.83
squares with a saw, and a water coolant was used. The squares
were attached to screws by use of autopolymeriz- ing acrylic resin
around the screw head. The opposite end that the screw was attached
to was roughened with a crosscut carbide bur (H 72E, Brasseler,
Savannah, Ga.) and randomly assigned to different groups.
For processing the light-polymerized materials, the in- dividual
squares were wrapped with clear Mylar film (Du Pont Co.,
Wilmington, Del.) and the surface of the squares was prepared
according to the manufacturer's recommen- dations (Table I). The
Mylar material that was selected had a high light transmission in
the wavelength necessary for polymerization. Soft l iner materials
were introduced to form a 5 mm thick layer between the two squares,
and placed in a Triad curing unit. The specimen was polymer- ized
for 10 minutes, inverted, and then polymerized for an addit ional
10 minutes. The Mylar wrap was then removed. For processing the
heat-polymerized materials, the squares were invested in type I I I
laboratory stone (SnapStone, WhipMix Corp., Louisville, Ky.) The
free end of the screw was part ial ly inserted into a prefabricated
plastic j ig to ensure their al ignment (Fig. 1). The opposing
flask was prepared in the same manner with an identical jig. The
height of the Lucitone 199 squares was adjusted by a nut attached
to the screw to ensure uniform sample height.
The surfaces of the squares were prepared to a thickness of 5 mm
according to the manufacturer's recommendations (Table I) before
receiving the l iner materials. The test ma- terial was packed
between the squares with two trial packings with cellophane as a
separator. The samples were deflasked with the walnut shell
blaster.
Ten samples of each mater ia l were tested at 72 ~ F within 24
hours of processing. Ten similarly prepared samples of each
material were also stored in water at 72 ~ F for 6 months and then
tested. The samples were placed in an MTS model 810 (MST System
Corp., Minneapolis, Minn.) connected to an X-Y recorder. The
samples were pulled apart at a crosshead speed of 1 mm/second. Fig.
2 illus- trates the specimen mounted in the machine and ready for
testing. The maximum tensile stress before failure, mode of
failure, and the total time elapsed preceding failure were re-
corded. The term'%ond strength" will not be used to describe the
maximum stress before fracture. A more accurate term, "failure
strength," is used because the samples did not always separate
because of interfacial debonding from the denture base (adhesive
failure). Tearing within the soft liner itself (cohesive failure)
or a mixed mode of failure that involved both cohesive and adhesive
failures were also observed.
Fai lure strength was recorded in megapascals (MPa). The mode of
failure was characterized as cohesive, adhe-
596 VOLUME 74 NtrMB~R e
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EMMER ET AL THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 1. Al ignment j ig for Lucitone 199 specimens in pro-
cessing flask.
sive, or mixed mode, dependent on whether the fracture surface
was in the soft l iner only, at the denture base-soft l iner
interface only, or in both. For evaluating mixed mode of failure, a
10 x 10 mm grid with a total area matching the substrate was placed
on the fracture surface, and the sur- face (with the grid) was
imaged on a monitor of the digitiz- ing system (LA-500, Pias Co.
Ltd., Osaka, Japan) by a video camera. The area percent of adhesive
failure was computed by counting the number of squares of the grid
in the denture base free of the liner (namely in the interfacial
area of failure). The area mean percentage determined for each
sample group was rounded to an interval scale with 20 intervals of
5% each. This interval method of evaluation was considered an
excellent way to characterize the mac- roscopic failure features of
the fracture surface. The time to failure was determined by a
single operator with a stop- watch to record (1) the time from the
start of the test (be- ginning arbitrar i ly at an approximate
force of 0.1 N) to the time corresponding to maximum stress and (2)
the time elapsed between the maximum stress and complete fail- ure.
To standardize the testing conditions for uniformity, the same
operator performed all of the tests. The time and stress data were
used to plot a qualitative deformation profile of each sample group
by l inear interpolation be- tween zero stress (at the start of the
test) to maximum stress and between maximum stress to zero stress
(corre- sponding to complete failure). This procedure was rela-
tively easy and accurate at the strain rate of 1 mm/second used for
the tensile test.
RESULTS
The mean and standard deviation (SD) values of failure strength
of both the dry and wet groups of samples are shown in Fig. 3.
One-way analysis of variance (ANOVA) revealed significant
differences of means (p < 0.001) be-
MTS jaws
Hook
Alignment arch attached to nut
Screw
Autopolymerized acrylic resin
Soft liner sample
Lucitone 199 blocks
Fig. 2. Overall test arrangement of specimen mounted in MTS
machine and ready for testing.
tween different brands in both the fresh and wet sample groups.
Duncan multiple range tests (a 0.05) showed dis- t inct homogenous
subsets (Table II).
Significant differences in failure modes were observed among the
sample groups. The percent of the denture base area that was free
of any liner was recorded as an area percent of adhesive failure.
The results are i l lustrated in Fig. 4. Scanning electron
microscopy (SEM) revealed typ- ical microstructures of failure
surfaces as presented in Fig. 5 (adhesive failure), Fig. 6
(cohesive failure), and Fig. 7 (mixed mode of failure). Fig. 8 i l
lustrates the deformation profiles obtained by l inear
interpolation between start of test at zero load and maximum stress
recorded and also between the maximum recorded stress and complete
fail- ure. Although the loading was performed under stroke control
in the actual test, the plot assumes a l inear load- ing and
unloading rate during the test period. Although this assumption may
not be accurate to describe the defor- mation profile, the method
is valid to characterize the duc- ti le/brittle failure behavior of
the reline material systems tested. The total t ime elapsed before
complete failure indi- cates the extent of plastic deformation
before failure under the constant strain rate conditions of the
test. Significant differences were observed in the failure
behavior.
Figs. 3 and 4 present trends result ing from water expo- sure
relative to fresh samples.
DECEMBER 1995 597
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THE JOURNAL OF PROSTHETIC DENTISTRY EMMER ET AL
MPa.
14-- 1 .4
12-
10-
8-
6- -
4-
2-
0-
Triad
78
Astron
2:9
Molloplast-B PermaSoft Super Soft
9 Dry @ 24 Hours.
Wet @ 6 Months
Fig. 3. Graph of mean and standard deviation (SD) of failure
strength (in MPa) of dry and wet sample groups of each soft liner
system tested.
100 100--
80-
60- 50
% 40 40 40 - 35
m 2O
20- ~ 0 0 0
o- ~ Triad Astron Molloplast-B PermaSoft Super Soft
9 Dry @ 24 Hours.
m Wet @ 6 Months
Fig. 4. Percent area of adhesive failure determined by fracture
surface area of denture base free of liner after completion of
test.
D ISCUSSION
Bonding material compatibility with denture base, liner
material, or both is an important factor to be considered in
studying failure strength. Plasticized PMMA (PermaSoft and Super
Soft) and PMMA denture base materials (Luci- tone 199) are similar
in chemical structure. Bonding agents are considered unnecessary
for these materials. Molloplast-B liner material is a silicone and
must be cou- pled with silane so that the liner bonds to the
silane, which in turn copolymerizes with the denture base resin.
Astron liner material uses a thin liquid-powder mix to prepare the
denture base surface, which results in bonding by co-
polymerization in addition to the potential mechanical adhesion
because of the roughened surface prepared before placement of the
full thickness of the liner material. The Triad system uses its own
universal bonding agent (unfilled resin) for copolymerization and
mechanical bond- ing.
The tensile strength, tear resistance, and deformation
characteristics of each material must also be considered. Triad and
Astron liner materials failed immediately after elastic deformation
with little stretching or plastic defoe- mation and recorded the
greatest failure strength values. Most of these failures were
internal (cohesive), which in-
598 VOLUME 74 NUMBER 6
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EMMER ET AL THE JOUKNAL OF PROSTHETIC DENTISTRY
Fig. 5. SEM shows microstructure of fracture surface of adhesive
failure. Absence of l iner material on fracture surface.
F ig. 6. SEM shows microstructure of typical cohesive failure.
Entire fracture surface is covered with liner.
dicated that these materials are brittle, strong, and bonded
strongly to the denture base. The adhesive strength was higher than
the cohesive strength for this material.
Molloplast-B l iner material stretched over t ime and showed a
low failure strength. The time elapsed before failure was high. It
also failed internal ly with many small fractures toward the end of
the elongation. This mater ia l is ductile and weak, and the
bonding at the interface is stronger than the cohesive strength of
the liner.
PermaSoft and Super Soft liner systems began to fail adhesively
prematurely. As a result, the remaining inter- facial area
decreased and resulted in an increase in the stress of the cross
section. Because of the configuration of the l iner-denture resin
interface to the direction of stress, this stress was now closer to
a shear type of stress than tensile (Fig. 9). Subsequent failure
resulted from shear stress within the liner. This type of failure
left a sharp cleft of the mater ia l over a large area. This mater
ia l is britt le and weak, and the bond strength to the denture
base is close to the cohesive shear strength of the material, caus-
ing either adhesive or mixed mode of failure in these sys-
tems.
The changes in the material properties after 6 months in water
warrant discussion. The failure strengths invariably increased on
water exposure and this may be an indication that the materials
became more brittle and probably less
Fig. 7. SEM shows microstructure of mixed mode of fail- ure.
Area A represents portion of fracture surface free of l iner and
area B shows liner material retained on surface.
DECEMBER 1995 599
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THE JOURNAL OF PROSTHETIC DENTISTRY EMMER ET AL
7.43 Triad
b
a Super Soft 2.94,
2.60 1.21
1.05
MPav ' I - I ~ ~ I_ I' Time Sec. 10 20 30 40
Fig. 8. Deformation profile of time elapsed before failure.
Profile is drawn by l inear in- terpolation of stress between start
of test and maximum stress (a) and between maximum stress and
complete failure (b). Total time to failure is t ime from start of
test to complete failure.
F ig . 9. Transformation of tensile stress to shear stress
through initial adhesive failure caused liner to reorient in stress
direction.
viscoelastic. This may also account for the nearly complete
adhesive debonding of some of the materials (for example,
PermaSoft), because they were able to resist deformation caused by
increased brittleness. The effect of water im- mersion on the
bonding agent may also be a factor in the adhesive failure of wet
samples.
There is a need to evaluate other effects such as temper- ature,
strain rate, and liner thickness on the adhesive properties, and
these were not included in this study. Nev- ertheless, the
differences in failure strength and modes are valuable in
understanding the adhesion characteristics of the soft l iners
studied. Moreover, the new methods used in this study to
characterize soft l iner-denture adhesion ap- pears to be a
valuable approach for future research.
CL IN ICAL S IGNIF ICANCE
Clinically, the abil ity of denture reline materials to re- sist
debonding from the denture and also internal fracture under
masticatory stresses are extremely important. In addition, the
liner material must remain stable in the sal- ivary oral
environment. In this study, the adhesive and cohesive strength
properties of selected soft liners were determined in a tensile
test method that ensured axial self-alignment of the specimen
during the test. The changes in the properties l isted caused by
water exposure for 6 months were also determined.
Typically, Triad and Astron l iner materials showed a britt le
type of failure that occurred cohesively within the l iner
material. Molloplast-B liner mater ia l failed in a duc- ti le
manner, but cohesively within the liner material. In contrast,
Permasoft and Super Soft l iner materials failed either adhesively
or in a mixed mode. All of the materials tended to become more
brittle on water exposure for 6 months. These differences in
failure characteristics of dif- ferent materials should be
considered in evaluating their clinical performance.
CONCLUSIONS
The tensile method developed in this study appears to be a
valuable procedure to characterize the stress magnitudes and modes
of failure of soft l iner bonded to denture base. There is a
significant difference in the bond strength
600 VOLUME 74 NUMBER 6
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EMMER ET AL THE JOURNAL OF PROSTHETIC DENTISTRY
between soft liners as function of brands (material types) and
curing modes. The failure is characterized by the in-
terrelationships between the properties, chemical charac- teristics
and/or compatibility of the liner, denture base, and bonding
materials. Prolonged exposure to water sig- nificantly increased
the failure strength, introduced brit- tle behavior to the liner,
and changed the mode of failure more toward adhesive failure.
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DECEMBER 1~5 601