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Hydrolytic Degradation of Dental Compositesand
Effects of Silane-Treatment and Filler Fraction on Compressive
Strength and Thermal Expansion of Composites
From the Department of Dental Technology Faculty of
Odontology
University of Umeå S-901 87 Umeå, Sweden
Department of Dental Biomaterials College of Dentistry
University of Florida Gainesville, Florida 32610, USA
andScandinavian Institute of Dental Materials
Forskningsveien 1 Oslo 3, Norway
A K A D E M I S K A V H A N D L I N G
som med vederbörligt tillstånd av odontologiska fakulteten vid
Umeå universitet
för vinnande av odontologie doktorsexamen offentligen försvaras
i Sal B, 9 tr
Odontologiska kliniken måndagen den 28 maj 1984 kl. 09.00
av
KARL-JOHAN M. SÖDERHOLMLeg. tandläkare, f il .k a n d ., m . ph
il.
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ABSTRACT
Söderholm, Karl-Johan, M. 1984. H ydrolytic degradation of
dental composites and effects of silane-treatment and filler
fraction on compressive strength and thermal expansion of
composites. Umeå University Odontological Dissertations No 19, ISSN
0345-7532.
Some researchers have suggested that the weakest link of dental
composites is the filler-matrix bond. However, due to
incompleteness of information dealing with this bond and its
stability in a humid environment, it was considered desirable to
investigate the effect of water on this region, as well as the
influence of filler bonding and filler fraction on compressive
strength and thermal expansion.
Experimental composites containing different filler fractions of
either silane-treated or untreated fillers were made. Compressive
strength and coefficient of thermal expansion were determined using
routine methods, while the hydrolytic degradation was investigated
by measuring changes in concentrations of elements in the storage
water using atomic absorption spectrophotometry. Scanning electron
microscopic investigations were made on fractured samples.The
diffusion coefficient of a representative resin system was
determined gravi- metrically. Seven commercial composites were
investigated regarding hydrolytic degradation. The filler
compositions of these composite materials were determined by
emission spectroscopy or energy-dispersive x-ray analysis before
storage in distilled water. This water was replaced and analyzed m
onthly using plasma spectrophotometry or atomic absorption
spectrophotometry.After completed water storage the samples were
fractured and investigated by use of scanning electron
microscopy.
From the results of these studies the following conclusions were
drawn:1. The compressive strength of composites changes linearly
with increased filler
fraction. Contrary to bonded fillers, composites containing
unbonded fillers lost strength with increased filler fraction.
2. W ater diffuses through the polymer matrix and attacks the
filler particles. This degradation is most pronounced for untreated
fillers containing glass modifying elements such as sodium, barium
and strontium.
3. The resin, used as a matrix, influences the speed with which
the hydrolytic degradation of the filler proceeds.
4. The hydrolytic degradation of the filler seemed to be
associated with micro-crack formation occurring in the matrix. Of
the investigated composites, the micro-filled resin showed the
lowest frequency of such crack formations.
5. The coefficient of thermal expansion decreases linearly with
increased filler fraction. Silane treatm ent did not influence this
coefficient.
6. Using a simplified model to predict stresses in a particle
filled composite indicates that rather high stress levels are
induced in the polymer matrix due to polymerization shrinkage. This
shrinkage induces radial compressive and tangential tensile
stresses with respect to the filler surface. Increased filler
fraction increases the tangential tensile stresses but reduces the
compressive radial stresses.
Key words: Internal frictional theory, filler-matrix debonding,
stress-corrosion, stress- influenced diffusion, micro-crack
formation, crazing, opacity, wear, osmotic pressure, hoop
stresses.
K-J. Söderholm, Departm ent of Dental Biomaterials, College of
Dentistry, University of Florida, Gainesville, FL 32610, U. S.
A.
-
ABSTRACT
Söderholm, Karl-Johan, M. 1984. H ydrolytic degradation of
dental composites and effects of silane-treatment and filler
fraction on compressive strength and thermal expansion of
composites. Umeå University Odontological Dissertations No 19, ISSN
0345-7532.
Some researchers have suggested that the weakest link of dental
composites is the filler-matrix bond. However, due to
incompleteness of information dealing with this bond and its
stability in a humid environment, it was considered desirable to
investigate the effect of water on this region, as well as the
influence of filler bonding and filler fraction on compressive
strength and thermal expansion.
Experimental composites containing different filler fractions of
either silane-treated or untreated fillers were made. Compressive
strength and coefficient of thermal expansion were determined using
routine methods, while the hydrolytic degradation was investigated
by measuring changes in concentrations of elements in the storage
water using atomic absorption spectrophotometry. Scanning electron
microscopic investigations were made on fractured samples.The
diffusion coefficient of a representative resin system was
determined gravi- metrically. Seven commercial composites were
investigated regarding hydrolytic degradation. The filler
compositions of these composite materials were determined by
emission spectroscopy or energy-dispersive x-ray analysis before
storage in distilled water. This water was replaced and analyzed m
onthly using plasma spectrophotometry or atomic absorption
spectrophotometry.After completed water storage the samples were
fractured and investigated by use of scanning electron
microscopy.
From the results of these studies the following conclusions were
drawn:1. The compressive strength of composites changes linearly
with increased filler
fraction. Contrary to bonded fillers, composites containing
unbonded fillers lost strength with increased filler fraction.
2. Water diffuses through the polymer matrix and attacks the
filler particles. This degradation is most pronounced for untreated
fillers containing glass modifying elements such as sodium, barium
and strontium.
3. The resin, used as a matrix, influences the speed with which
the hydrolytic degradation of the filler proceeds.
4. The hydrolytic degradation of the filler seemed to be
associated with micro-crack formation occurring in the matrix. Of
the investigated composites, the micro-filled resin showed the
lowest frequency of such crack formations.
5. The coefficient of thermal expansion decreases linearly with
increased filler fraction. Silane treatm ent did not influence this
coefficient.
6. Using a simplified model to predict stresses in a particle
filled composite indicates that rather high stress levels are
induced in the polymer matrix due to polymerization shrinkage. This
shrinkage induces radial compressive and tangential tensile
stresses with respect to the filler surface. Increased filler
fraction increases the tangential tensile stresses but reduces the
compressive radial stresses.
Key words: Internal frictional theory, filler-matrix debonding,
stress-corrosion, stress- influenced diffusion, micro-crack
formation, crazing, opacity, wear, osmotic pressure, hoop
stresses.
K-J. Söderholm, Department of Dental Biomaterials, College of
Dentistry, University of Florida, Gainesvilie, FL 32610, U. S.
A.
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4
CONTENTS
1. PREFACE 6
2. INTRODUCTION 7
General background
The f i r s t commercial composites
The tra n s it io n from amalgam to composite resins
Development trends
Present shortcomings regarding composites
Summing-up
3. OBJECTIVES 19
4. EXPERIMENTAL PROCEDURES AND RESULTS 20
I . RELATIONSHIP BETWEEN COMPRESSIVE YIELD STRENGTH AND 20
FILLER FRACTIONS OF PMMA COMPOSITES
M ateria ls and methods
Results
I I . DEGRADATION OF GLASS FILLER IN EXPERIMENTAL COMPOSITES
22
M ateria ls and methods
Results
I I I . WATER SORPTION IN A BIS-GMA/TEGDMA RESIN 25
M ateria ls and methods
Results
IV. LEACHING OF FILLERS IN DENTAL COMPOSITES 27
M ateria ls and'methods
Results
V. HYDROLYTIC DEGRADATION OF DENTAL COMPOSITES 29
M ateria ls and methods
Results
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5
VI. INFLUENCE OF SILANE-TREATMENT AND FILLER FRACTION ON 32
THERMAL EXPANSION OF COMPOSITE RESINS
M ateria ls and methods
Results
5. DISCUSSION 34
6. SUMMARY & CONCLUSION 50
7. ACKNOWLEDGEMENTS 53
8. REFERENCES 55
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6
PREFACE
This t h e s i s i s based on the fo l lowing pa per s , which
wi l l be r e f e r r e d
t o by t h e i r Roman numerals:
I. Söderholm, K-J . : R e la t i o n s h ip between compressive
y i e l d s t r e n g t h
and f i l l e r f r a c t i o n s o f PMMA compos i tes. Acta
Odonto l . Scand.
1982, 40:145-150.
I I . Söderholm, K-J . : Degradat ion of g l a s s f i l l e r
in exper imenta l
composi tes . J . Dent. Res. 1981, 60:1867-1875.
I I I . Söderholm, K-J . : Water s o r p t i o n in a
BIS-GMA/TEGDMA r e s i n . J .
Biomed. Mater. Res. 1984, 18:271-279.
IV. Söderholm, K-J . : Leaking of f i l l e r s in denta l
compos i tes. J .
Dent. Res. 1983, 62:126-130.
V. Söderholm, K - J . , Zigan, M., Ragan, M., F i s c h l s c h
w e i g e r , W., and
Bergman, M.: Hydro ly t ic degr ada t io n of denta l compos i
tes . J .
Dent. Res. ( submi t ted f o r p u b l i c a t i o n ) .
VI . Söderholm, K-J . : I n f luence of s i l a n e t r e a t m
e n t and f i l l e r
f r a c t i o n on thermal expans ion of composite r e s i n . J
. Dent. Res.
( s ub m i t t ed f o r p u b l i c a t i o n ) .
★Copyright Apri l 10, 1984. Repr in ted by permission of John
Wiley &
Sons, Inc .
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7
INTRODUCTION
General Background
The idea to make a to o th co lo red r e s t o r a t i v e m a
te r ia l by mixing
i n o r g a n ic f i l l e r p a r t i c l e s with a c o ld - c
u r in g r e s i n da tes back to 1951
(Knock & Glenn, 1951). The j u s t i f i c a t i o n f o r
adding f i l l e r was t o
reduce po ly me r i za t io n s h r in k a g e , thermal expans
ion , and w a te r so r p t i o n
of those r e s i n based too th co lo red m a t e r i a l s a v
a i l a b l e a t t h a t t ime
( Pa f f enba rg e r e t a l . , 1953; Rose e t a l . , 1955).
However, t h i s p r e c u r
sor to our modern dental composi tes never became a su c c e ss
. One
p o s s i b l e ex p lan a t i o n f o r i t s poor success
might have been t h a t t h i s
m a te r ia l was in t roduced a t a time when d e n t i s t s s
t a r t e d to exp e r ie n c e
some c l i n i c a l f a i l u r e s with the u n f i l l e d r
e s i n s (Caul & Schoonover,
1953; Kramer, 1954; Hedegård, 1955). T h e re fo re , r a t h e
r than r i s k and
f a i l u r e s by u'sing the new r e s i n - b a s e d , to o t
h co lo red r e s t o r a t i v ei
m a t e r i a l , most d e n t i s t s r e tu r n e d to the use
of s i l i c a t e cements w i th
which they had a t l e a s t some prev ious exper ience ( P a f
f e n b a r g e r , 1972).
Though most d e n t i s t s r e tu r n e d to the use of s i l i
c a t e s , t h e r e were
some c l i n i c i a n s and r e s e a r c h e r s who cont
inued to t r y to f i n d new ways
to use the co ld -c u re d r e s i n s c l i n i c a l l y . A
new concept in t r o d u ced
d ur in g t h i s per iod was a method f o r bonding r e s i n s
to the enamel by
e tch in g the enamel with an ac id (Buonocore, 1955). Epoxy r e
s i n was
proposed a t about th e same t ime as an exper imenta l m a te r
i a l s i nce i t
was b e l i eved t h a t t h i s type of r e s i n had the p o t
e n t i a l to bond t o
t o o t h s t r u c t u r e (Bowen, 1956). An in o rg an ic f i
l l e r was added to the
epoxy r e s i n to reduce shr inkage and to improve bonding
along t h e
e n a m e l - r e s in i n t e r f a c e . In 1963, Bowen a l s
o in t r odu ced th e concept of
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8
a coupling agent to achieve a chemical bond between r e s i n
and f i l l e r s .
Although the l a b o r a t o r y r e s u l t s wi th t h e s e
exper imental m a t e r i a l s were
very promis ing , Bowen soon r e a l i z e d t h a t the epoxy r
e s i n he was
i n v e s t i g a t i n g could not be used c l i n i c a l l y
as i t d id not cure in a
m o is t environment (Bowen & Molineaux, 1969). To ach ieve
cur ing
i n t r a o r a l l y , a new r e s i n was formula ted by l i n
k in g two methyl
m e t h a c r y l a t e molecules to the two epoxy groups of the
epoxy r e s i n
molecule . This m o d i f i c a t i o n of the epoxy r e s i n
was done e i t h e r by t h e
r e a c t i o n of bisphenol A and g l yc idy l m e th a c ry l
a t e or by th e r e a c t i o n
of the d ig ly c id y l e t h e r of bisphenol A and m e th a c
ry l i c a c id . Each o f
t h e s e r e a c t i o n s r e s u l t e d in th e so c a l l e
d BIS-GMA monomer (Bowen 1963,
1965). Through t h i s approach, the epoxy r e s i n was conver
ted to a
d i m e t h a c r y l a t e which could be polymerized in the
same way as the co l d -
cured methyl m e th a c r y l a t e . Some of the advantages of
t h i s monomer
compared to p re v io u s ly -u sed methyl me th ac ry la te s
in c lu de : l e s s
polyme r iza t ion s h r in k a g e , l e s s v o l a t i l i t
y , and an inc rease d c r o s s l i n k
ing p o t e n t i a l dur ing p o l y m e r i z a t i o n .
However, t he BIS-GMA monomer
proved to be ex tremely v i scous and i t was d i f f i c u l t
to mix i t w i th
f i l l e r s wi thout i n c o r p o r a t i n g and r e t a i n
i n g a s i g n i f i c a n t
c o n c e n t r a t i o n of vo id s . To reduce the v i s c o s
i t y , the BIS-GMA monomer
was d i l u t e d wi th o th e r more f l u i d a c r y l a t e
s such as t e t r a e t h y l e n e
glycol d i m e th a c ry l a t e before the s i i arie t r e a t
e d f i l l e r s were added
(Bowen, 1965).
The F i r s t Commercial Composites
Although f a i l i n g to chemica l ly bond to to o th s t r u c
t u r e , Bowen's r e s i n
has se rved as the mat r ix f o r most o f th e c u r r e n t
composite m a t e r i a l s
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9
(Asmussen, 1975; Ruyter & S j 0 v i k , 1981). The c l i n i
c a l success o f t h i s
r e s i n for mu la t io n was r e a l i z e d soon a f t e r i
t s i n t r o d u c t i o n in 1964 by
th e 3M Corporat ion as a commercial p r o d u c t , Addent 35
(Pe te r son e t
a l . , 1966). This success was a t t r i b u t e d to the c l i
n i c a l d u r a b i l i t y o f
t h e composi tes compared to t h a t o f th e s i l i c a t e s
(Bowen e t a l . , 1968;
Brauer, 1977). However, th e composi tes e x h i b i t e d some
major c l i n i c a l
shor tcomings such as poor c o l o r s t a b i l i t y , a rough
s u r f a c e which
enhanced plaque adhes ion , and u n s a t i s f a c t o r y wear
r e s i s t a n c e
( M o s t e l l e r , 1972; E r i k sen , 1974; P h i l l i p s
e t a l . , 1973). These
f i n d i n g s , combined wi th th e prev ious poor c l i n i c
a l per formance o f
c o l d - c u r e d methyl m et hac ry la te m a t e r i a l s ,
would have been expec ted to
ad v e r se ly a f f e c t the marke ting and development of
composite m a t e r i a l s .
F o r t u n a t e l y , t h i s s i t u a t i o n d id not occur
because o f the i n t r o d u c t i o n
a t t h i s t ime of the ac id e tch techn ique f o r composite
r e s i n s (B jo rv a tn
& Tornes , 1972). The su c c e s s fu l combinat ion o f the
a c id e tch
technique and composite r e s i n s made i t p o s s i b l e to
r e s t o r e t e e t h i n
ways p r e v io u s ly cons ide red imposs ib le and helped
compos ites overcome
what could have become the f i r s t " c r i s i s " (Jordan e t
a l . , 1977).
The T r a n s i t i o n from Amalgams to Composite Resins
Since the i n t r o d u c t i o n of the ac id e t ch t e c h n
iq u e , re se a rc h a c t i v i t y
has i nc reased toward the development o f improved composi tes
as well as
bonding agents (Bowen, 1979; Smith, 1982). This t r end i s r e
l a t e d t o
th e numerous new c l i n i c a l t r e a t m e n t p o s s i b
i l i t i e s t h a t a ro se from such
breakthroughs in dent a l m a t e r i a l s sc ie n c e .
Improved wear r e s i s t a n c e
o f composi te r e s i n s could mean t h a t th e s e products
would r e p la c e
denta l amalgam as th e most commonly-used r e s t o r a t i v e
m a t e r i a l . S i nce
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10
amalgams have cap tur ed approximate ly 75% of th e r e s t o r
a t i v e ma ter ia l
market toda y , the economical as pe c ts o f such a t r a n s i
t i o n a r e obvious.
A d isa dvan tage of dental amalgams i s t h a t a shor t age o
f meta l s used in
amalgams ( t i n , s i l v e r , mercury and copper) could a r i
s e in the n e a r
f u t u r e (Kusy, 1982). In c o n t r a s t , composi tes
should have a b r i g h t e r
f u t u r e as the f i l l e r c o n s t i t u e n t used in
these m a t e r i a l s i s a v a i l a b l e
in almost un l im i te d q u a n t i t i e s .
In a d d i t io n to the se impor tan t economical a s p e c t s
, den tal compos
i t e s a l s o possess a c o l o r s i m i l a r to the t o o t
h . As d e n t i s t r y i s
co n t in uous ly c o n c e n t r a t i n g on more-demanding e
s t h e t i c re qu i r em en ts , we
can expec t an inc re as ed demand from the consumer f o r th e
se
r e s t o r a t i o n s in the f u t u r e . Another t r e n d
in fa vo r of composi tes i s
an i nc reased concern from the publ ic r eg ard in g mercury t
o x i c i t y from
amalgam r e s t o r a t i o n s (S c h o l l e , 1983). The s c
i e n t i f i c evidence i s
in c o n c l u s i v e to j u s t i f y such a t r a n s i t i o
n to composi tes a t t he pr e se n t
t ime (Dauer & F i r s t , 1982). I n s t e a d , the e x t
e n s iv e c l i n i c a l
e x p e r i e n c e with amalgam th e ra py gained dur ing th e
l a s t ce n tu ry would
j u s t i f y the cont inued use of amalgams u n t i l r e se a
rch has proven t h a t
composi tes a re a t l e a s t as good an a l t e r n a t i v e
as amalgams (Skjf l r land,
1973; L e i n f e l d e r e t a l . , 1983; S tan ley e t a l .
, 1979).
Development Trends
During th e l a s t t en y ea rs in c re a se d a t t e n t i o
n has been focused on t h e
development of improved composi tes . Although r e se a rc h e f
f o r t s have
markedly in c re a se d w i t h in th e p as t seve ra l y e a r
s , the wear r e s i s t a n c e
o f composi tes has not improved s u b s t a n t i a l l y .
These e f f o r t s have
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11
r e s u l t e d in the a v a i l a b i l i t y of more products
today than eve r b e f o r e ,
making i t q u i t e d i f f i c u l t to s e l e c t the most s
u i t a b l e m a t e r i a l .
Several p u b l i c a t i o n s have r e c e n t l y summarized
informa t ion r e g a rd in g
t h e orga nic and ino rga nic components in most commercial
composite
products (Asmussen, 1975; Ruyter & Sj 0 v ik , 1981; Zinck e
t a l . , 1982).
These papers show t h a t the most commonly-used r e s i n
component i s 2,
2 -b is ( p - ( 2 1-h ydrox y-3 ' -methacryloxypropoxy)
phenylene) pr o p an e ,
sh o r te n ed to BIS-GMA, which i s d i l u t e d wi th o t h e
r l e s s v i scous
monomers such as t r i e t h y l e n e g lycol d i m e th a c ry
la te (TEGDMA). To
minimize po l ym er iza t io n shr ink age and to ins ure good c
o l o r s t a b i l i t y
of th e compos i te , a minimum d i l u e n t co nt en t has
been recommended
( Cowperthwaite e t a l . , 1979; Asmussen, 1981). Agains t t h
e se
recommendations a r e da ta showing t h a t high BIS-GMA c on te
n ts r e s u l t i n
composi te r e s i n s with r a t h e r low po ly me r iz a t io
n degree (Ruyter &
Svendsen, 1978). There has a l s o been concern f o r some time
among
ma nu fa c tu re rs about c e r t a i n problems a s s o c i a t
e d wi th BIS-GMA. These
problems a re high water s o r p t i o n , high v i s c o s i t
y , and low p u r i t y
(Glenn, 1979). However, dur ing the l a s t t en y e a r s , t h
e r e has a l s o
been a t r e n d to r e p la c e BIS-GMA wi th o t h e r d i m e
t h a c r y l a t e s . This
t r e n d i s most pronounced wi th r e s p e c t to the produc
t io n o f the
s o - c a l l e d m i c r o f i l l e d compos ites in which ure
tha ne d im e t h a c r y l a t e
r e s i n s a r e commonly used (Ruyter & S j0 v ik , 1981;
Zinck e t a l . , 1982).
The f i l l e r system of most modern composi tes c o n s i s t
s o f e i t h e r
q u a r t z , barium and s t r on t iu m g l a s s e s , or
pyrogenic s i l i c a p a r t i c l e s
(Zinck e t a l . , 1982). The s i z e s of t h e s e p a r t i c
l e s range w i t h in and
between pr od uc ts . Most q u a r t z and g l a s s f i l l e r
s a r e 1-10 jum, whi le
the s i z e o f the pyrogenic s i l i c a , used in the m i c r
o f i l l e d com pos i te s ,
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12
O
a r e j u s t a few hundred A in s i z e (Ruyter & 0ysaed,
1982; Zinck e t a l . ,
1982). A d e f i c i e n c y of m i c r o f i l l e d m a t e r
i a l s i s t h e i r r e l a t i v e l y low
f i l l e r c o n t e n t , which render s th e se m a t e r i a
l s l e s s d im ens ion a l ly
s t a b l e in an environment with changes in tem pera ture and
humidi ty .
T h e i r low f i l l e r co n te n t i s r e l a t e d to
manufac tur ing problems with
in c o r p o r a t i n g high f r a c t i o n s of m i c r o f i
l l e d p a r t i c l e s in to a monomer,
s i n c e th e c o n c e n t r a t i o n of m i c r o f i l l e
d p a r t i c l e s d r a m a t i c a l l y changes
the v i s c o s i t y of the monomer ( J acobsen , 1981). To
solve t h i s problem,
t h e manufac turer d i l u t e s the monomer wi th so l v e n t
s such as ch loroform,
adds m i c r o f i l l e d p a r t i c l e s and evapora tes the
s o l v e n t . By doing s o ,
r e s i n s co n ta in in g a high f r a c t i o n o f m i c r o
f i l l e d p a r t i c l e s can be
produced. However, t h i s composite i s too v i scous to be
handled
c l i n i c a l l y and t h e r e f o r e the manufac turer
polymerizes t h i s f i l l e d
r e s i n and gr inds i t in to organic p a r t i c l e s co nt
a in in g the in o r g a n i c
p a r t i c l e s . These composite p a r t i c l e s a re then
mixed as a f i l l e r in to
the monomer and the product i s marketed as a m i c r o f i l l
e d composi te ,
a l th ou gh the organi c f i l l e r p a r t i c l e s could be
l a r g e r than
convent ional in or ga n ic f i l l e r p a r t i c l e s .
In a d d i t i o n to the developments of m i c r o f i l l e d
compos i tes,
manufac turers have r e c e n t l y focused t h e i r e f f o r
t s toward t h e
development of th e s o - c a l l e d hybr id or blend composi
tes (Dennison,
1982). These m a t e r i a l s a r e made by admixing m ic r o -
f in e and more
c o n v e n t i o n a l l y - s i z e d p a r t i c l e s i n to
one r e s i n b a se , thereby making i t
p o s s i b l e to in c r e a s e the f i l l e r f r a c t i o
n anothe r 10% by weight com
pa red to the more t r a d i t i o n a l composi tes (Dennison,
1982).
Even though the ma t r ix and the f i l l e r a re the main
components o f
a den tal compos i te , p robably the most c r i t i c a l reg
ion i s t h e i n t e r f a c e
-
13
between th e se two components (Bowen, 1972; Braden, 1978). To
ach ieve
a bonding between mat r i x and f i l l e r , th e ino rg ani c
f i l l e r p a r t i c l e s
a r e si l ane t r e a t e d , in most case s wi th / -m e th a
c ry lo x y p ro p y l t r im e th o x y
s i l a n e , in or de r to form a chemical bond between mat r
ix and f i l l e r s
(K o e l l i n g & Kolb, 1965; Plueddemann, 1970). During th
e l a s t ten
y e a r s , a t te mp ts have a l s o been made to process f i l
l e r s wi th such
topography t h a t they would be ab le to bond mechanic a l ly
to the r e s i n
(Bowen & Reed, 1976a,b; Ehrnford , 1976).
In a d d i t i o n to th e se e f f o r t s to improve the phys
ica l p r o p e r t i e s
of den ta l compos i tes , a t te m pt s to improve the handl
ing p r o p e r t i e s have
a l s o been made. For example, a t r a n s i t i o n from co ld
-c u re d r e s i n s
toward l i g h t - c u r e d r e s i n s has occur red dur ing
the l a s t seve ra l y e a r s .
During t h i s p e r i o d , composite r e s i n systems based
on a t e r t i a r y amine-
benzoyl peroxide a c t i v a t o r - i n i t i a t o r system
were rep la ced by
compos i tes a c t i v a t e d by an u l t r a v i o l e t (UV)
l i g h t source and i n i t i a t e d
by methyl e t h e r of benzoin. Due to concerns about the e f f
e c t o f UV
l i g h t on ora l t i s s u e s as well as problems r e l a t e
d to the
po lym er iza t ion depth of UV l i g h t - c u r e d r e s i n
systems , v i s i b l e
l i g h t - c u r e d systems were in t r o d u c e d . In th e
se v i s i b l e l i g h t - c u r e d
sys tems , camphoroquinone and N,N-dimethylaminoethyl m e th a c
ry la te a r e
a c t i v a t e d by l i g h t wi th a wavelength o f 468 nm
(Dar t & Nemcek, 1978).
Two advantages of t h i s system compared to the UV l i g h t
system are a
lower r i s k f o r s o f t t i s s u e damages and g r e a t e
r po ly me r i za t io n dept h ,
even though the po l ym er iza t ion depth i s not comple te ly
s a t i s f a c t o r y
(Swar tz e t a l . , 1983). However, r i s k s of o c u l a r
damage due to d i r e c t
and i n d i r e c t l i g h t exposure from the v i s i b l e l
i g h t source seem t o
e x i s t (Antonson & Benedet to , 1984).
-
14
P r e s e n t Shortcomings Regarding Composites
Although composite r e s i n s have provided many advantages in
r e s t o r a t i v e
t r e a t m e n t through the i n t r o d u c t i o n o f the ac
id e tch techn ique and
improved composite sys tems , composi tes s t i l l a re not s u
i t a b l e f o r
p o s t e r i o r r e s t o r a t i o n s except f o r Class I r
e s t o r a t i o n s where e s t h e t i c s
i s impor tan t (Schro tenboer e t a l . , 1983). The main shor
tcomings o f
p r e s e n t composite r e s i n a re inadequate wear r e s i s
t a n c e , d i s c o l o r a t i o n
p o t e n t i a l , and t h e i r p o s s i b l e a s s o c i a
t i o n with r e c u r r e n t c a r i e s
( L e i n f e l d e r e t a l . , 1980; Mjtfr, 1981). Of th e se
drawbacks, the poor
wear r e s i s t a n c e as well as th e p o s s i b l e r e l a
t i o n s h i p with r e c u r r e n t
c a r i e s have so f a r r e s t r i c t e d the usage o f
compos ites to a r ea s sub
je c t e d to r e l a t i v e l y low loads and to l o c a t i o
n s where i t i s easy t o
c l e a n the r e s t o r a t i o n and i t s sur rounding
margins . Regarding
r e c u r r e n t c a r i e s , the evidence i s not co nc lu s
iv e t h a t the composi tes
a v a i l a b l e today a re i n f e r i o r to amalgams (Mj0r,
1981). In a d d i t i o n to
th ese shor tcomings , t h e r e a re seve ra l qu es t io n s r
e g a rd in g
b i o c o m p a t i b i l i t y which must be answered befo re
one can s t a t e t h a t
composi tes a re as s a f e or s a f e r than denta l amalgams
(S ta n l ey e t a l . ,
1979).
An improvement in wear r e s i s t a n c e of composi te r e s i
n s has become
t h e most cha l l e n g in g re se a rc h problem in the e a r
l y 198 0 ' s . In the
denta l l i t e r a t u r e , the wear mechanism of denta l
composi tes has been
e x p la in e d as a r e s u l t o f exposure o f th e i no rg
ani c f i l l e r by lo s s of
r e s i n m a t r ix . Subsequent ly , f i l l e r p a r t i c l
e s a r e debonded and l o s t ,
and new r e s i n m a t r ix becomes exposed (Kusy & L e i n
f e l d e r , 1977).
However, t h i s d e s c r i p t i o n i s too general to e x p
la in the ba s i c f a i l u r e
mechanism. The wear process should be ana lyzed in g r e a t e r
depth to
-
15
dete rmine whether t h e r e i s any s p e c i f i c type of
wear mechanism which
i s dominant. The d i f f e r e n t types of wear processe s a r
e as fo l lo w s
( Burwel1 , 1957):
1. Abrasive wear
2. Adhesive wear
3. Corros ive or chemical wear
4. Sur face f a t i g u e
Abras ive wear can f u r t h e r be d iv id ed in to e i t h e r
two-body o r
th r ee -b o d y ty p e s . Each of th e s e types can occur in
the ora l c a v i t y .
During s l i d i n g of a cusp over and in d i r e c t c o n t a
c t with a r e s t o r a
t i o n , a two-body type of wear w i l l occur (Powers e t a l
. , 1974).
However, i f food or any d e b r i s i s d i s p e rs e d
between the cusp and t h e
r e s t o r a t i o n , a th r ee -bo dy type of a b r a s i v e
wear e x i s t s (Prasad &
C a l v e r t , 1980). Except f o r a b ra s iv e wear , adhes
ive wear can a l s o
o ccur under high p re s su re caus ing loca l c o n t a c t p o
in ts o f both occ lud
ing cusp and r e s t o r a t i o n to fu se t o g e t h e r fol
lowed by a t e a r i n g of t h e
s u r f a c e dur ing d is lo d g in g (Craig & Powers,
1976). The t h i r d ty pe ,
c a l l e d c o r r o s i v e or chemical wear , might a l s o
be a c o n t r i b u t i n g
f a c t o r , as chemica ls could r e a c t and weaken m a t r i
x , f i l l e r s , o r
f i l l e r - c o u p l i n g a g e n t , the reby f a c i l i t
a t i n g wear of the composi te
(Bowen, 1972; Söderholm, 1979; Wu & McKinney, 1982). The f o
u r t h type
of wear , induced by f a t i g u e , i s p robably a l s o an
impor tan t p rocess t o
c o n s i d e r in the breakdown o f composi tes s in ce wear i
s t ime-dependent
(Draughn, 1979; McCabe & Smith, 1981; McKinney & Wu,
1982).
Regarding t h e s e fo ur types o f wear , most r e s e a r c h
e r s dea l i n g wi th
the wear o f den ta l compos ites have focused on a b r a s i v
e wear (Craig &
Powers, 1976). The general f i n d i n g s from th e s e s t u d
i e s have so f a r
-
16
been r a t h e r d i s cou ra g i ng as th e r e s u l t s have
not c o r r e l a t e d well with
r e s u l t s from c l i n i c a l s t u d i e s (O'Neal &
Eames, 1973; Powers e t a l . ,
1974).
To exp la in t he se d i s c r e p a n c i e s between l a b o r
a t o r y s t u d i e s and
c l i n i c a l f i n d i n g s , an in c r e a s i n g number
of r e s e a r c h e r s have sugges ted
t h a t i n t e r n a l s t r e s s e s as well as f a t i g u e
could be the major
c o n t r i b u t i n g f a c t o r s in th e wear process
(Draughn, 1979; McCabe &
Smith, 1981; McKinney & Wu, 1982). Another p o s s i b i l i
t y could a l s o be
t h a t chemica ls in the ora l environment might r e a c t with
any o f the
d i f f e r e n t components of a den ta l composi te (Bowen,
1972; Beech &
Brown, 1972; Söderholm & C a l v e r t , 1980; McKinney,
1984). However, no
u n i v e r s a l l y - a c c e p t e d theory has y e t been
developed to ex pl a in the poor
wear r e s i s t a n c e of denta l compos i tes . From a v a i
l a b l e c l i n i c a l d a t a ,
i t appears t h a t as the most w e a r - r e s i s t a n t
composi tes a re t h e
m i c r o f i l l e d m a t e r i a l s (Meier & Lu tz ,
1978; Jorgensen e t a l . , 1979;
Chr i s t ensen & C h r i s t e n s e n , 1982; Knudsen
& Jo rg en sen , 1983). In
a d d i t i o n to th e improved wear r e s i s t a n c e of t h
e s e m a t e r i a l s , the
m i c r o f i l l e d composi tes can be r e a d i l y po l i s
he d wi th smoother s u r f a c e s
than any o t h e r composite (Dennison e t a l . , 1979).
In a d d i t i o n to the m i c r o f i l l e d compo s i tes ,
some s t ron t iu m g l a s s
f i l l e r - c o n t a i n i n g composi tes have a l s o shown
some promising c l i n i c a l
r e s u l t s compared to q u a r t z - c o n t a i n i n g
composi tes (Moffa & J e n k i n s ,
197 8a ,b ) . However, some c o n f l i c t i n g da ta a re a v
a i l a b l e rega rd in g the
wear r e s i s t a n c e of the se s t r o n t i u m c o n ta in
in g composi tes ( L e i n f e l d e r
e t a l . , 1982).
Regarding the f i l l e r - r i c h hybr id composi tes and t h
e i r wear
r e s i s t a n c e , too few c l i n i c a l s t u d i e s a r
e a v a i l a b l e to g ive conc lus i ve
-
17
in f o r m a t i o n . However, a r e c e n t s tudy i n d i c a
t e s t h a t th e s e m a t e r i a l s
are not s u p e r i o r to o th e r composi tes (Ameye e t a l .
, 1984).
Summing-Up
Although severa l e f f o r t s have been made to improve the
wear r e s i s t a n c e
o f denta l compos i tes , no s u b s t a n t i a l b reak
through can be i d e n t i f i e d
s ince the f i r s t commercial composi tes were in t ro du ced
. As the coup
l i n g agent has remained r e l a t i v e l y unchanged, whi le
o t h e r r e s i n - f i l l e r
components have been t r i e d , the weak l i n k could very
well be t h e
f i l l e r - m a t r i x bond (Bowen, 1972; Braden, 1978). From
t h i s viewpoin t
i t i s s u r p r i s i n g t h a t only a small number of r e s
e a r c h e r s have s t u d i e d
t h i s v a r i a b l e . F o r t u n a t e l y , t h i s i n t
e r f a c e region has been
i n v e s t i g a t e d in aerospace and naval r e sea rch
programs, as
f i b e r - r e i n f o r c e d p l a s t i c s a r e very usefu
l m a t e r i a l s in th es e
d i s c i p l i n e s . From the se s t u d i e s , i t has been
shown t h a t water can
reduce the s t r e n g t h o f f i b e r r e i n f o r c e d r e
s i n s s u b s t a n t i a l l y . Water
can a l s o a t t a c k g l a s s f i b e r s in a r e s i n m a
t r i x , and f i b e r - m a t r i x
debonding as well as c rack formation might occur in t he se m a
t e r i a l s
with t ime (Er ickson & Plueddemann, 1974). Based on t h i s
i n fo rm a t io n ,
i t seemed rea son ab le to i n v e s t i g a t e th e e f f e c
t o f a f i l l e r - m a t r i x
bonding on compressive s t r e n g t h of denta l compos i tes.
S ince
p r o g r e s s i v e f i l l e r - m a t r i x debonding might
be induced by e i t h e r water
o r thermal f l u c t u a t i o n s , th e se two v a r i a b l
e s and t h e i r in f l u e n c e on
t h e f i l l e r - m a t r i x i n t e r f a c e were a l s o i
n v e s t i g a t e d . A p ro g re s s iv e
f i l l e r - m a t r i x debonding could expla in why denta l
composi tes f a i l due
t o wear a f t e r some t ime in s e r v i c e . Severa l l a b
o r a t o r y s t u d i e s have
shown t h a t composi tes con ta in in g bonded f i l l e r s a
re much more wear
-
18
r e s i s t a n t than th o se with unbonded f i l l e r s
(Powers e t a l . , 1974,
Prasad & C a l v e r t , 1980). In a d d i t i o n to the p
o s s i b l e e f f e c t s o f
debonding on th e wear p r o p e r t i e s , such a debonding
could a l s o
in f lu e n c e thermal expansion as a m a t r ix wi th unbonded
f i l l e r might
expand wi thout r e s t r a i n t in c o n t r a s t to a ma t r
ix wi th bonded f i l l e r s .
Debonding would a l s o be ab le to in f lu e n c e the c o l o
r of a compos i te , a s
s t a i n s could be accumulated in f i l l e r - m a t r i x
gaps . These gaps would
in tu r n in f lu e n c e the opa c i t y of the composite due
to a change i n
r e f r a c t i v e index a t th e f i l l e r - m a t r i x i n
t e r f a c e .
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19
OBJECTIVES
The p r i n c i p a l o b j e c t i v e s of the p r e s e n t s
tudy were:
1. To i n v e s t i g a t e the in f l u e n c e o f f i l l e r
f r a c t i o n as well as s i l a n e -
t r ea tm en t on the compressive s t r e n g t h of an exper
imenta l composi te
m a t e r i a l .
2. To determine the e f f e c t o f water on the f i l l e r - m
a t r i x i n t e r f a c e t o
e v a l u a t e whether f i l l e r - m a t r i x debonding
might occur wi th time
in a composite s to re d in a wet environment .
3. To s tud y the water s o r p t i o n c a p a c i ty and d i f
f u s i o n behav ior of
water in a r e s i n of s i m i l a r compos it ion as a
commercia l ly
a v a i l a b l e denta l composi te r e s i n .
4. To s tudy e f f e c t s o f water on some commercial p r od
uc ts .
5. To determine th e f i l l e r - l e a c h i n g behavior of
commercial composite
r e s i n s over a s i x month p e r io d .
6. To i n v e s t i g a t e t he e f f e c t s o f f i l l e r f
r a c t i o n s and f i l l e r t r e a t
ments on the thermal expansion c o e f f i c i e n t s o f den
ta l compos i tes .
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20
EXPERIMENTAL PROCEDURES AND RESULTS
I . RELATIONSHIP BETWEEN COMPRESSIVE YIELD STRENGTH AND
FILLER
FRACTIONS OF PMMA COMPOSITES.
The o b j e c t i v e of t h i s i n v e s t i g a t i o n was
to s tudy how volume f r a c t i o n
a s well as s u r f a c e t r e a tm e n t o f f i l l e r s i n
f lu e n c e th e compressive
s t r e n g t h of den tal compos i tes .
M a t e r i a l s and Methods
Soda lime g l a s s beads , 4-44 jum in s i z e were used as f i
l l e r s in t h e s e
exper ime nta l composi tes . In some compo s i tes , th e s e
beads were p r e
t r e a t e d with s i l a n e . This t r e a tm e n t was done
in a 250 ml s o l u t i o n o f
to lu ene co n ta in in g 2 v/o / - m e th ac ry lo xy pr opy l
t r im eth oxy s i l a n e , i n
which 50 g f i l l e r was s t i r r e d f o r 24 h. The si l a
n e - t r e a t e d f i l l e r was
then f i l t e r e d away from the s o l u t i o n and dr ie d a
t 60°C f o r ano the r 24
h.
The exper imenta l composi te m a t e r i a l s were made by
adding a
prede t e rmin ed volume f r a c t i o n f i l l e r (10 , 20,
30, 40, and 50 v /o) to a
20 ml j a r co n ta in in g some methyl m e th a c ry la te
monomer. Af t e r t h e
t o t a l volume of f i l l e r had been added, th e j a r was
comple te ly f i l l e d
with monomer and c l o s e d . In orde r to polymerize the
monomer, 1 w/o
benzoyl peroxide and 0 .3 v /o N ,N -d im e th y l - p - to lu
id in e were added to
the monomer befo re the f i l l e r was in c o r p o r a t e d .
Af te r the j a r had
been s e a l e d , the m a t e r i a l was mixed in a machine by
r o t a t i n g th e j a r 5
rpm f o r about 1 h. Following t h i s p r ocedure , the v
iscous m a te r ia l was
t r a n s f e r r e d to a sy r in g e and i n j e c t e d i n t
o g l a s s rods wi th an in ne r
-
21
d ia m e te r o f 4 .0 mm. A f te r s e t t i n g , th e m a t e
r i a l s were h e a t - t r e a t e d a t
90°C f o r 12 h, c u t and ground to le ng th s of 8 .0 mm, and
s t o r e d in a
d e s i c c a t o r a t room te m pe ra tu re . This technique
was used to make
u n f i l l e d PMMA samples.
The 0.2% compressive y i e l d s t r e n g t h was dete rmined
on t h r e e
samples from each group a t room tempera ture us ing an I n s t
r o n t e s t i n g
machine and a c r os s -h ead speed of 1.27 mm/min. The s t r e
n g t h v a lues
were analyzed by use o f l i n e a r r e g r e s s i o n a n a l
y s i s assuming t h a t a
l i n e a r r e l a t i o n s h i p between s t r e n g t h and
f i l l e r f r a c t i o n e x i s t e d .
A f t e r the compressive s t r e n g t h t e s t , the f r a c
t u r e s u r f a c e s were
coa ted with 50 A of gold and s tu d i e d in a scanning e l e c
t r o n microscope
a t lOkV. Spec ia l a t t e n t i o n was paid to i n v e s t i
g a t e the morphology of
the f i l l e r - m a t r i x i n t e r f a c e s .
Re sul t s
A l i n e a r r e l a t i o n s h i p was found between
compressive s t r e n g t h and
volume f r a c t i o n o f f i l l e r f o r 0
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22
I I . DEGRADATION OF GLASS FILLER IN EXPERIMENTAL
COMPOSITES.
The purpose of t h i s i n v e s t i g a t i o n was to e v a lu
a te th e e f f e c t s o f w a te r
on th e s t a b i l i t y o f the f i l l e r - m a t r i x i n
t e r f a c e in some exper imenta l
composi tes c on ta in in g soda- l ime g l a s s f i l l e r s
.
M a t e r i a l s and Methods
Four experimenta l den tal compo s i tes , a l l c o n ta in in
g about 40 v / o
f i l l e r , were prepared f o r s tu d y . Two o f th e se m a
t e r i a l s used co ld -
cured polymethyl m e th a c ry la te (PMMA) as a ma t r i x m at
e r i a l whi le t h e
o t h e r two m a t e r i a l s used a copolymer of b isphenol A
g ly c id y l methacry
l a t e / t r i - e th y l e n e g lycol d im e t h a c r y l a
t e (BIS-GMA/TEGDMA). The BIS -
GMA/TEGDMA composi tes conta ined 64 mol% BIS-GMA and 36 mol%
TEGDMA and
were made to s im u la te a commercial ly a v a i l a b l e
composi te . Th is
m a t e r i a l conta ined 1 w/o benzoyl peroxide and, in c o n
t r a s t to th e PMMA
m a t e r i a l s , i t was hea t - c u red a t 60°C f o r 1 h,
75°C f o r 1 h , and 90°C
f o r 4 h , and subseque nt l y i t was s lowly cooled to room
tempe ra ture .
Of th e two types of composi tes us ing the same mat r ix sys
tem, one
o f th e f i l l e r s had been s i l a n e - t r e a t e d and
th e o t h e r f i l l e r was
u n t r e a t e d . The f i l l e r s used in a l l th e s e
composi tes were 4-44 jum
so da - l im e g l a s s beads. S i l a n e t r e a t m e n t
and sample p r e p a r a t i o n were
performed as d e sc r ib ed in Paper I .
Groups of t h r e e samples of each m a te r i a l were s t o r
e d in 20 ml
d i s t i l l e d water in se a le d a c r y l i c b o t t l e s
a t 60°C. These samples were
f r a c t u r e d by a d iame t ra l compressive t e s t a f t e
r 16, 32, and 180 days
i n w a te r . A f t e r f r a c t u r e t h e samples were coa
ted wi th gold in
p r e p a r a t i o n f o r scanning e l e c t r o n
microscopy.
-
23
Another s e r i e s was des igned to determine changes in th e
pH and
the sodium con te n t o f the s t o r a g e water wi th t ime .
An 80-day s t o r a g e
p e r i o d was a r b i t r a r i l y s e l e c t e d and during
t h i s t ime th e pH was
r e g u l a r l y determined with a pH meter and a g l a s s e l
e c t r o d e . The
sodium c o n c e n t r a t i o n of the s t o r a g e w ate r
was de te rmined a t the end of
t h i s 80-day per iod by atomic abs o rp t io n sp ec t ro p h
o to m e t r y , us ing a
hol low cathode lamp, an a i r - a c e t y l e n e f lame, a 0
.7 nm wide s l i t , and a
wavelength of 589.0 nm.
Three dry samples of th e BIS-GMA/TEGDMA composi te wi th s i l
a n e -
t r e a t e d g l a s s beads were cyc led in compression with
4400 loads f o r one?
hour us ing a s t r e s s leve l ranging from 0 to 15 N/mm .
Three more
samples of the same m a te r i a l were cyc led f o r one hour
wi th the same2
f requency , but with a s t r e s s i n t e r v a l of 0-45 N/mm
. These specimens,
c l a s s i f i e d as low- and h i g h - s t r e s s e d
samples , r e s p e c t i v e l y , were
t h e r e a f t e r immersed in 20 ml d i s t i l l e d water
and kept a t 60°C f o r 80
day s . The s to ra g e water o f th e s e samples was a l s o
ana lyzed r ega rd ing
pH and sodium c o n c e n t r a t i o n .
R esul t s
Degradat ion of th e f i l l e r , e x h i b i t e d as a p e e
l in g of th e f i l l e r
s u r f a c e , could be seen on some o f the u n t r e a t e d
g l a s s beads i n s i d e the
BIS-GMA/TEGDMA mat r ix a f t e r 16 days in w a te r . This f i
l l e r d e g r a d a t i o n
was lo c a t e d c lo se to th e o u t e r s u r f a c e o f th
e samples.
Af te r 32 days in w a te r , s u r f a c e changes of the f i l
l e r s were seen
i n a l l composi tes wi th u n t r e a t e d f i l l e r s .
The d e f e c t s seen in the
BIS-GMA/TEGDMA compos ites were more pronounced than those in th
e PMMA
compos i tes .
-
24
The PMMA compos ites c o n ta in in g u n t r e a t e d f i l l
e r s s u f f e r e d from
severe f i l l e r - s u r f a c e deg ra da t io n a f t e r
the s to r a g e t ime was ex tended
t o 180 days. This degr ada t io n reached a depth o f about 0
.9 juni, and
the degraded g l a s s l a y e r had a swollen appearance . In t
h e
BIS-GMA/TEGDMA composi tes c o n ta in in g u n t r e a t e d f
i l l e r s , th e g l a s s
s u r f a c e had a swollen appea rance , too .
No d i r e c t s i g n s of f i l l e r - m a t r i x debonding
were seen in compos
i t e s c o n ta in in g s i l a n e - t r e a t e d f i l l e r
s . However, l e s s e l e c t r o n dense
a r e a s and p r e c i p i t a t e - l i k e format ions could
be d e te c te d on some of
the se beads .
The pH o f w ater in which the d i f f e r e n t composi tes had
been
s t o r e d inc re ase d wi th t ime . This i n c r e a s e was
most pronounced f o r
composi tes us ing PMMA and u n t r e a t e d f i l l e r s ,
reaching a pH of 9 .5
a f t e r 80 days in w a te r . I t was a l s o demons tra ted t
h a t th e pH
inc re ase d when the f i l l e r had been s i l a n e - t r e a
t e d , but to a l e s s e r
e x t e n t . The i n c r e a s e in pH was remarkably small f o
r the
BIS-GMA/TEGDMA compos ites con ta in in g s i l a n e - t r e a
t e d f i l l e r , but became
l a r g e r a f t e r a s e r i e s of c y c l i c s t r e s s e
s .
The sodium ion c o n c e n t r a t i o n of the d i s t i l l e
d water in which t h e
compos i tes were s t o r e d showed an in c r e a s e from 0 .
1 - 0 . 2 ppm to 5-9.5
ppm a f t e r 80 days. No d i r e c t r e l a t i o n s h i p s
could be seen between t h e
s i l a n e - t r e a t e d and u n t r e a t e d f i l l e r
compos i tes .
-
25
I I I . WATER SORPTION IN A BIS-GMA/TEGDMA RESIN.
The p r e s e n t s tudy was under taken to determine some water
s o r p t i o n
p r o p e r t i e s of a copolymer used as a m a t r i x in a
dent a l compos i te . The
p r o p e r t i e s i n v e s t i g a t e d were l i n e a r s w
e l l i n g , water s o r p t i o n c a p a c i t y ,
l e a c h i n g , and d i f f u s i o n c o e f f i c i e n t s
.
M a te r ia l s and Methods
A combinat ion of BIS-GMA and TEGDMA r e s i n s were mixed and
polymer ized
a s de sc r ib ed in Paper I I . Before p o ly m e r i z a t i o
n , th e monomer was c a s t
in to a g l a s s rod approximate ly 10 cm long and wi th an i n
n e r d ia m e te r
o f 21 mm whereupon the rod was sea led from both ends. A f t e
r po lymer i
z a t i o n , f i v e d i s c s , each 0 .8 mm t h i c k , were
c u t and smoothed by
p o l i s h i n g the c u t s u r f a c e s with sandpaper (250,
400, and 600 g r i t ) .
The samples were weighed and s to re d in a d e s i c c a t o r
u n t i l s t a b l e
we ights had been ga ined . The d iameters of t he d i s c s
were measured
with a measuring microscope and the t h ic k n e s s of the d i
s c s were
measured wi th a micrometer .
Af te r dehydra t ion the d i s c s were i n d i v i d u a l l y
s t o r e d in P e t r i
d i s h e s co nt a in in g 10 ml d i s t i l l e d water a t e
i t h e r 60°C or 37°C depend
ing on the scheduled t r e a t m e n t . A f te r 15 and 30 min
of s to r a g e i n
w a t e r , th e samples were weighed, whereupon th e weight
gain was mea
sured every h a l f - h o u r dur ing the next 1.5 hours , and
then every hour
d ur in g t h e fo l low ing 6 hours . Su bse quen t ly , th e
weight ga in was
recorded every 12th hour u n t i l a s t a b l e va lue was
reached . From
measurements of t h e d i ameters of th e samples a f t e r s a
t u r a t i o n and
cooling to room tempera ture in w a te r , th e l i n e a r
expansion caused by
-
26
w a t e r s o r p t i o n could be de te rmined. The d e so rp t
i o n process was
s tu d i ed by s t o r i n g the w a t e r - s a t u r a t e d d
i s c s in a d e s i c c a t o r a t e i t h e r
60°C or 37°C, depending upon the d es i gna te d t r e a t m e n
t , and th e weight
loss was determined accord ing to the same time schedule as used
du r ing
t h e s o r p t i o n s t u d i e s .
The d i f f u s i o n c o e f f i c i e n t s of water i n to
the polymer d i s c s were
de te rmined by p l o t t i n g the M^/M^ r a t i o as a fu n c
t i o n of t h e square
roo t o f t ime where was the weight gain a f t e r a t ime , t
, and M^was
the f i n a l weight ga in . During d e s o r p t i o n , a s i
m i l a r curve was p l o t
t e d , but in t h i s case was th e weight lo ss a f t e r a t
i m e , t , and
was the f i n a l weight los s a f t e r deh ydr a t i on .
By measuring the changes in weight wi th t ime and p l o t t i n
g t h e s e
r e s u l t s a t l e a s t f o r M̂ /M̂ 0 . 5 , a s t r a i g h
t l i n e could be drawn f o r
the i n v e s t i g a t e d m a t e r i a l s . The method of l
e a s t squares was used t o
de te rm ine the equa t io n f o r t h i s l i n e . This equa t
ion pe rm i t t ed an
e s t i m a t i o n of the time i t took to reach M./M = 0 .5 .
Knowing t h a tt oo
t i m e , t i9 the d i f f u s i o n c o e f f i c i e n t s
could be c a l c u l a t e d from the 2
r e l a t i o n s h i p ,
D = 0 .049/ ( t j_ /L 2 )2
2 -1where D = d i f f u s i o n c o e f f i c i e n t (cm s ) ,
and L = t h ic k n e s s of d i s c
(cm) (Crank, 1957). The d i f f u s i o n c o e f f i c i e n t
o f the polymer was
determined by tak in g the mean va lue of the s o r p t i o n
and d e s o r p t i o n
c o e f f i c i e n t s as t h e " t ru e" d i f f u s i o n c o
e f f i c i e n t (Crank 1957).
-
27
Results
The weight gain occur r in g dur ing the f i r s t s o r p t i o
n a t 60°C was 3 .3 9
w/o. The weight gain was 3 .68 w/o dur ing the second cyc l e a
t th e same
te m pera tu re . This in c re a s e in water s o r p t i o n c
a p a c i t y dur ing t h e
second c y c le a t 60°C was s t a t i s t i c a l l y s i g n i
f i c a n t acco rd ing to the
S t u d e n t ' s t - t e s t (p < 0 . 0 0 5 ) .
During the f i r s t s o r p t i o n - d e s o r p t i o n cyc
le a t 60°C, a 0.31%
d e c r e a s e in weight oc cu r r ed . During th e fo l l
owing s o r p t i o n - d e s o r p t i o n
cyc le a t 37°C, the samples i n c reased in weight 0.07%, which
l a t e r was
l o s t dur ing the second cy c l e a t 60°C. The l i n e a r
expans ion caused by
water s o r p t i o n was 1.07-1.08% and did no t change s i g n
i f i c a n t l y between
t h e t h r e e c y c l e s . During th e f i r s t cyc le a t
60°C, th e de so rp t io n
d i f f u s i o n c o e f f i c i e n t was h ig he r than the s
o r p t i o n d i f f u s i o n c o e f f i
c i e n t , whi le the r e v e r s e o rde r p r e v a i l e d
dur ing th e second s o r p t i o n -
de so rp t io n cyc le a t 60°C ( p < 0 . 0 5 ) . At 37°C, no
s t a t i s t i c a l l y
s i g n i f i c a n t d i f f e r e n c e was found ( p > 0 .
0 5 ) between the s o r p t i o n and
the d e s o rp t i o n d i f f u s i o n c o e f f i c i e n t s
.
IV. LEACHING OF FILLERS IN DENTAL COMPOSITES
The aim of t h i s s tudy was to i n v e s t i g a t e the lea
ching r a t e of s i l i c o n
from f o u r commercial den ta l composi tes s t o r e d in d i
s t i l l e d w a te r , to
e v a lu a te whether h y d r o l y t i c degr ad a t i on of f
i l l e r s in commercial
p ro duc ts could be d e te c te d and r e l a t e d to t h e
leakage o f g l a s s -
modifying elements such as barium and s t r o n t i u m .
-
28
M ate r ia l s and Methods
Four com m erc i a l l y - av a i l a b l e dental compos ites
(Concise , P r o f i l e ,
F o t o f i l ® , and S i l a r ® ) which conta ined d i f f e r
e n t f i l l e r composi t ions
were s e l e c t e d f o r s tu dy .
The f i l l e r f r a c t i o n s of the d i f f e r e n t
composi tes were de termined
by weight d e te rm in a t io n of samples befo re and a f t e r
the m at r ix had
been burned away a t 575°C (ISO Standard 4049, 1978). The v a r
io u s
f i l l e r s were analyzed by o p t i c a l emiss ion spec t
rog raphy us ing photo
graphic reco rd ing fol lowed by i d e n t i f i c a t i o n of
elements based on
v i s u a l comparison with s ta n d a rd s .
X-ray d i f f r a c t i o n s p e c t r a were taken of one
sample of each
compos i te to e v a l u a t e whether th e f i l l e r was c r
y s t a l l i n e or amorphous.
For the leaching d e te rm in a t io n s i x c i r c u l a r d i
s c s , 15.0 mm i n
d ia m e te r and 0.85 mm t h i c k , were made of th e f o u r
composite produc ts .
A f te r p o ly m e r iz a t i o n , one s i d e of each sample
was s l i g h t l y roughened
by sandpaper to a s s u re exposure of f i l l e r p a r t i c l
e s on t h a t s i d e . All
s ix samples of each composite m a te r ia l were p laced in a p
l a s t i c b o t t l e
c o n t a i n i n g 10 ml d i s t i l l e d water and s t o r e
d in an oven a t 60°C. A f te r
30 days , the samples were t r a n s f e r r e d to ano ther b o
t t l e co n ta in in g 10
ml d i s t i l l e d w a te r whereupon ano the r 30 day per iod
o f s t o rag e
f o l l o w e d .
A f te r the t o t a l 60 day p e r i o d , the water samples f
o r the two 3 0
day i n t e r v a l s were ana lyzed by use of plasma spec t
rosc opy to dete rmine
the c o n c e n t r a t i o n s of Al , B, Ba, Mg, Sb, Sr , T i
, and S i .
-
29
Results
Except f o r the m i c r o f i l l e d r e s i n , each
composite conta ined about 0 .6
volume f r a c t i o n f i l l e r . The m i c r o f i l l e d
composi te con ta ine d only
about 0 .3 volume f r a c t i o n f i l l e r . The o p t i c a
l emission sp ec t r o g ra p h y
a n a l y s i s showed t h a t Concise® and Si 1 ar® mainly
conta ined S i , whi le(ß)P r o f i l e con ta ine d S i , Sr and
Al. The main c o n s t i t u e n t s o f the f i l l e r
o f Fotof i 1® were S i , Ba and Al.
X-ray d i f f r a c t i o n a n a l y s i s showed t h a t a l l
m a t e r i a l s , e x c e p t®
Concise , were amorphous in s t r u c t u r e . The c r y s t a
l l i n e s t r u c t u r e of
Concise was i d e n t i c a l to / 3 -q u a r tz .
The plasma s p e c t r o s c o p ic a n a l y s i s showed t h a
t P ro f i l e ® re l ea s e d Sr
and B, whi l e Fotofi 1 leaked Ba and B. All m a t e r i a l s l
eaked S i , wi th® ®
the leakage most pronounced f o r P r o f i l e and F o to f i l
dur ing the f i r s t
30 days.
Compared to the f i r s t 30-day p e r i o d , the leakage of Sr
, B, and Ba
dec rea se d dur ing th e second 30-day p e r i o d , as d id
the leakage o f S i .
An excep t ion was the m i c r o f i l l e d compos i te , which
showed a lmost tw ice
a s high leakage o f Si dur ing the second per iod .
V. HYDROLYTIC DEGRADATION OF DENTAL COMPOSITES
This s tud y was under taken to e v a l u a t e t he leaching
rpte o f s i l i c o n ,
s t ro n t ium and barium from four composite r e s i n s during
a s i x month
p e r i o d . Special a t t e n t i o n was paid to d e t e c t
any s ig ns of morphologi
cal changes which could i n d i c a t e ongoing h y d r o l y t
i c p rocesse s a t t h e
f i l l e r - m a t r i x i n t e r f a c e s .
-
30
M a t e r i a l s and Methods
® ® ®Three compos i tes , P - 10 , P r o f i l e , and Den-Mat
Class II , each
a d v e r t i s e d f o r use in p o s t e r i o r r e s t o r a
t i o n s , and one m i c r o f i l l e d®
r e s i n , I s o p a s t , were i n v e s t i g a t e d .
Five samples of each m a t e r i a l , 15.0 mm long and 4 .0 mm
in diame
t e r , were i n d i v i d u a l l y s t o r e d in 7 ml d i s t
i l l e d water a t 60°C. A f t e r
t h i r t y days , the samples were t r a n s f e r r e d to o t
h e r v i a l s cont a in in g 7
ml d i s t i l l e d w a te r whi le th e p re v io u s ly used
s t o r a g e w a te r was kept
f o r f u t u r e a n a l y s i s . This process was repea t ed
s i x t imes .
Five samples o f each composi te were ground f l a t and coa ted
with
go ld. Analysi s o f the f i l l e r compos i t ion was
performed us ing an
e n e r g y - d i s p e r s i v e x - ra y a n a ly z e r op era
t ed a t an a c c e l e r a t i n g v o l ta ge
of 15 kV. Af te r i d e n t i f i c a t i o n of the p r i n c i
p a l e leme nts , the compo
s i t i o n s were determined by use o f a computer program ab
le to perform a
q u a n t i t a t i v e a n a l y s i s .
Based on th e f i l l e r compos i t ions determined by en ergy-
d i s p e r s i v e
x - r ay a n a l y s i s , th e s t o r a g e water was ana
lyzed f o r c o n c e n t r a t i o n s of
s i l i c o n , s t r o n t i u m , and barium. These atomic a b
s o rp t io n s p e c t r o p h o to -
m e t r i c ana ly se s were performed by use of hollow cathode
lamps and a
n i t r o u s o x id e - a c e ty le n e f lame. S i l i c o n ,
s t r o n t i u m , and barium were
ana lyzed a t wavelengths of 251.6 nm, 460.7 nm, and 553.6 nm, r
e s p e c
t i v e l y , us ing a s l i t s e t t i n g o f 0 .2 nm. Analys
is o f va r ia nc e was p e r
formed on the atomic ab s o r p t io n da ta c o l l e c t e d
over th e six-month
p e r i o d .
Except f o r the a n a l y t i c a l p rocedu re s , l i g h t
microscopy and
scanning e l e c t r o n microscopy were used to i n v e s t i g
a t e the morphology
of the i n v e s t i g a t e d m a t e r i a l s both before and
a f t e r s t o ra g e in w a te r .
-
31
Results
®The en e rg y - d i s p e r s i ve x- r ay a n a l y s i s
showed t h a t the f i l l e r o f P-10
and I sopas t® mainly conta ined S i , whi le Pr o f i le ®
conta ined aluminum,®
s i l i c o n , and s t r o n t i u m . The f i l l e r composi
t ion of Den-Mat Class I I
was s i m i l a r to t h a t o f P r o f i l e except f o r a
small barium co n te n t and
a lower s t ro n t iu m c o n c e n t r a t i o n .
The atomic ab so rp t i o n s p e c t r opho to m e t r ic a n a
l y s i s r ev ea le d t h a t®
a l l m a t e r i a l s were leached of s i l i c o n . Except f
o r s i l i c o n , P r o f i l e®
a l so r e l e a s e d s t r on t ium whi le the Den-Mat Class
II composite l o s t
bo th s t r o n t i u m and barium. A c e r t a i n p r o p o r
t i o n a l i t y e x i s t e d between
s i l i c o n , s t r o n t i u m , and barium. Regarding the
lea ching p a t t e r n of t h e
d i f f e r e n t m a t e r i a l s , an i r r e g u l a r
change with t ime was found except®
f o r I s o p a s t . The g r e a t e s t and second g r e a t e
s t l each ing r a t e f o r
s i l i c o n were demonst rated by P ro f i l e ® and Isopast®
, r e s p e c t i v e l y ,
while P-10 showed the lowest l each ing r a t e ( p < 0 . 0 5
) .
Scanning e l e c t r o n microscopy showed t h a t c racks c lo
se to or a t® ® ®
the s u r f a c e s could be seen on P-10 , P r o f i l e and
Den-Mat Class I I
samples s to re d in w a te r . Of th e se t h r e e m a t e r i
a l s , a l a r g e number of® ®
cracks was observed in P r o f i l e and Den-Mat Class II
specimens b u t® ®
few cracks were found in th e P-10 specimens. I s o p a s t d id
not show
any d e t e c t a b l e c r a c k s . The d e te c te d c racks
were lo ca t ed a t t h e
m a t r i x - f i l 1er i n t e r f a c i a l zone and tended to
propagate away from th ese
regions a t sharp f i l l e r co rn er s to o t h e r m a t r i
x - f i l l e r i n t e r f a c i a l
a r e a s .
-
32
VI. INFLUENCE OF SILANE TREATMENT AND FILLER FRACTION ON
THERMAL
EXPANSION OF COMPOSITE RESINS.
The o b j e c t i v e s of t h i s i n v e s t i g a t i o n
were to s tudy the e f f e c t o f
f i l l e r f r a c t i o n on the c o e f f i c i e n t o f
thermal expans ion and to
e v a lu a te the i n f lu e n c e of t he f i l l e r - m a t r
i x bond q u a l i t y on t h e
c o e f f i c i e n t o f thermal expansion .
M a te r ia l s and Methods.
Experimental compos i tes , c o n ta in in g 0 . 2 , 0 .4 or 0
.6 volume f r a c t i o n o f
e i t h e r s i l a n e - t r e a t e d or u n t r e a t e d
soda- l ime g l a s s beads were prepared
f o r s t udy . The mat r ix was t r i e t h y l e n e g lycol d
im e th a c ry la te (TE6DMA),
t o which 0.15 w/o benzoyl peroxide and 0 .2 v/o
N,N-dimethyl-p-
t o l u i d e n e were added to a c t i v a t e p o ly m er iz a
t i on . In a d d i t i o n to t h e s e
c o m pos i t e s , pure r e s i n samples were a l s o made.
These m a t e r i a l s were
placed i n t o g l a s s tubes 4 .0 mm in d i a m e te r , co l
d -cu red and cu t i n t o
5 0 .8 mm long samples . A f t e r c u r i n g , t h e samples
were s t o r e d a t 65°C
f o r 12 h.
The c o e f f i c i e n t o f thermal expansion of each sample
was d e t e r
mined twice in a d i l a t o m e t e r . The tempera ture range
and he a t i n g r a t e
f o r th e se measurements were 25°C to 90°C and 4°C/min, r e s
p e c t i v e l y .
A l i n e a r r e g r e s s i o n a n a l y s i s was performed
on c o e f f i c i e n t of
thermal expansion versus f i l l e r f r a c t i o n d a t a
.
To e s t i m a te p o s s i b l e s t r e s s l e v e l s in a
composi te a t room temper
a t u r e , a s i m p l i f i e d model o f a composi te co n ta
in in g s p h e r ic a l f i l l e r s
was in t r od uced . Based on s t r e s s e s in a t h i c k - w
a l l e d s p h e r ic a l sh e l l
caused by an i n t e r n a l p r e s s u r e , the h i g h e s t
and lowest s t r e s s l e v e l s
-
33
which might occur in the i n v e s t i g a t e d composite could
be es t imated
with t h i s model. The e f f e c t o f f i l l e r f r a c t i
o n on th e se s t r e s s e s
could a l s o be v i s u a l i z e d from t h i s model by o rd
in a ry c a l c u l a t i o n s .
R esu l t s
The l i n e a r r e g r e s s i o n a n a l y s i s showed t h a
t the c o e f f i c i e n t o f thermal
expans ion of the i n v e s t i g a t e d m a t e r i a l s fol
lows a l i n e a r r e l a t i o n s h i p
with f i l l e r f r a c t i o n ( p < 0 . 0 5 ) . Regarding
th e d i f f e r e n t f i l l e r
t r e a t m e n t s , no d i f f e r e n c e s could be d e t e
c t e d in thermal expansion f o r
a s p e c i f i c f i l l e r f r a c t i o n . However, thermal
c y c l in g seemed to cause
a dec rease in the c o e f f i c i e n t o f thermal expansion d
e t e c t e d during the
second c y c l e .
Combining the exper imental r e s u l t s with the s i m p l i f
i e d model of a
composi te p re sen t ed in t h i s s t u d y , i t was shown t
h a t f i l l e r p a r t i c l e s
a re surrounded by hoop s t r e s s e s induced dur ing po ly m
er i za t i o n . These
s t r e s s e s a r e compressive in a r a d i a l d i r e c t i
o n a t the f i 11e r - m a t r i x
i n t e r f a c e , while they a r e in te n s io n in a t a n g
e n t i a l d i r e c t i o n .
These s t r e s s e s decrea se wi th d i s t a n c e from the f
i 11e r - m a t r i x i n t e r f a
c ia l zone and become zero in a ra d ia l d i r e c t i o n a t
the s u r f a c e of t h e
proposed composite model. Residual t e n s i l e s t r e s s e s
remain in a
t a n g e n t i a l to the f i l l e r s u r f a c e . The
magnitude of th ese s t r e s s e s a r e
dependent on f i l l e r f r a c t i o n . Low f i l l e r f r a
c t i o n s g ive high compres
s ive s t r e s s e s and r e l a t i v e l y low t a n g e n t
i a l t e n s i l e s t r e s s e s . Higher
f i l l e r f r a c t i o n s reduce the compressive r a d i a l
s t r e s s e s but in c re as e
the t a n g e n t i a l t e n s i l e s t r e s s e s .
-
34
DISCUSSION
Paper I shows t h a t a good f i l l e r - m a t r i x bond i s
re q u i r e d when a d e n ta l
compos i te i s made. Lack o f such a bond w i l l weaken r a t
h e r than
s t re n g th e n a composite m a te r i a l i f more f i l l e
r i s added, a t l e a s t when
a s p h e r i c a l f i l l e r i s used. I f th e f i l l e r i
s well bonded to the
mat r ix me chanica l ly or c he m ic a l l y , an in c re a se
in compressive s t r e n g t h
can be expected wi th an i n c r e a s e in f i l l e r f r a c
t i o n ( I sh a i & Cohen,
1968; Pra sad , 1977).
Through the y e a r s the genera l t r e n d has been to use as
high a
c o u p l e d - f i l l e r f r a c t i o n as p o s s i b l e
(Dogon e t a l . , 1982). Attempts
have a l s o been made to improve the mechanical r e t e n t i o
n of f i l l e r
w i t h i n the mat r ix (Bowen & Reed, 1976a,b; Ehrnford,
1976, 1981, 1983).
The b e n e f i t s o f t h i s in c r e a s e in both f i l l e
r f r a c t i o n and f i l l e r
bonding have r e s u l t e d in h i ghe r compressive s t r e n
g t h and modulus of
e l a s t i c i t y , and lower po lym er iza t ion s h r in k a
g e , thermal expans ion , and
w a t e r - s o r p t i o n / w a t e r - s w e l 1ing o f the
denta l composite (Bowen & Rodr i
guez, 1962; Bowen, 1964).
Even though l a r g e r f i l l e r f r a c t i o n s and b e t
t e r f i l l e r - m a t r i x
bonding improve th e p r o p e r t i e s of compos i tes , i t
should be noted t h a t
the in d iv id ua l composite components r e t a i n t h e i r o
r i g i n a l p r o p e r t i e s
(H o l l i d a y & Mann, 1966). Accord ingly , the i no rg
ani c f i l l e r does not
improve the p r o p e r t i e s of t he r e s i n per s e , but
merely a c t s to reduce
t h e u n d e s i r a b le p r o p e r t i e s of the polymer.
Due to g r e a t d i f f e r e n c e s
in some phys ica l p r o p e r t i e s of the m at r i x and th
e f i l l e r , one can
s u s p e c t t h a t hygrothermal changes as well as po lym er
iza t ion shr inkage
could induce r e l a t i v e l y l a r g e s t r e s s e s a t
the f i l l e r - m a t r i x i n t e r f a c e s
-
35
(Hahn & Kim, 1978; H a r r i s , 1978). These s t r e s s e
s , as well as chemi
c a ls a s s o c i a t e d wi th the ora l envi ronment , might
cause f i l l e r debond
ing from the m at r ix with t ime (Söderholm & C a l v e r t
, 1980). Thus, the
d e t e r i o r a t i o n of the f i l l e r - m a t r i x bond
with t ime wi l l determine t h e
long- te r m p r o p e r t i e s of a composite r e s t o r a t
i o n . The e f f e c t of
f i l l e r - m a t r i x debonding was a l s o i n d i r e c t
l y demonst rated in Paper I .
In t h i s s tudy i t i s shown t h a t the compressive s t r e
n g t h of composites
con ta in in g 0.5 volume f r a c t i o n of unbonded f i l l e
r was only o n e - h a l f
t h e s t r e n g t h o f composi tes wi th bonded f i l l e r .
In a d d i t i o n to the
s i g n i f i c a n c e of good m a t r i x - f i 11er bonding
on compressive s t r e n g t h ,
i t s e f f e c t on th e wear p r o p e r t i e s of composi
tes has a l s o been shown
(Powers e t a l . , 1974; Prasad & C a l v e r t , 1980). In
one s tudy on wear ,
i t was shown t h a t composi tes c o n ta in in g unbonded f i
l l e r wore as f a s t
o r f a s t e r than the pure mat r ix m a t e r i a l , while
composi tes c o n t a i n i n g
bonded f i l l e r s were much more w e a r - r e s i s t a n t
(Pr asad , 1977).
Accord ingly, the poor c l i n i c a l long- te rm wear r e s i
s t a n c e of d e n ta l
compos i tes could be a r e s u l t o f m a t r i x - f i l l e
r debonding o ccur r in g in
the ora l c a v i t y wi th time (Bowen, 1972; Braden,
1978).
To i n v e s t i g a t e the proposed f a i l u r e mechanism, s
ev e ra l f a c t o r s
such as po l ym er iza t io n s h r in k a g e , hygrothermal i
n c o m p a t i b i l i t y , and
chemical p r ocesses must be con s i de red . Before any th e o
ry can be
proposed regard in g such a debonding mechanism, i t i s impor
tan t t o
c h a r a c t e r i z e th e f i l l e r - m a t r i x i n t e r
f a c e . The s u r f a c e between the
r e s i n and the f i l l e r can be i d e n t i f i e d as the
t r u e f i l l e r - m a t r i x
i n t e r f a c e . However, t h i s su r f a c e i s d i f f i
c u l t to s tudy due to i t s
i n f i n i t e s i m a l l y small volume. As most adhes ive
bond f a i l u r e s
d e s c r i b e d as i n t e r f a c e debonding occur in a reg
ion very c l o s e to but
-
36
n o t e x a c t l y a t the t r u e i n t e r f a c e , the term
f i l l e r - m a t r i x i n t e r f a c e is
in r e a l i t y synonymous with the i n t e r f a c e reg i on
. This reg ion in c lu d e s
a t h i n l a y e r of th e f i l l e r a t the f i l l e r s u
r f a c e , the coupl ing a gen t ,
and a t h i n l a y e r of the m at r ix a t the coupl ing age n
t -m a t r ix i n t e r f a c e .
Each o f th e se components have t h e i r own s p e c i f i c
phys ica l and chemical
p r o p e r t i e s (Kwei, 1965).
Regarding the s i l a n e l a y e r and i t s a b i l i t y to
bond to the f i l l e r
s u r f a c e , most r e s e a r c h e r s c u r r e n t l y
agree t h a t i t s f i l l e r bonding
a b i l i t y i s e i t h e r o f a co v a l en t or p o l a r
na tu re (Koel l ing & Kolb,
1965; Plueddemann, 1970). Whether th e s e two types of bonds, e
s p e c i a l
ly the hydrogen bond, a re s t a b l e in an aqueous environment
has been an
open ques t io n f o r severa l y e a r s in l i t e r a t u r e
de a l i n g wi th r e i n f o r c e d
p l a s t i c s (Er ickson & Plueddemann, 1974). The r e s u
l t s p r esen t ed i n
Paper I I i n d i c a t e t h a t the s i l a n e coupl ing
agent used in t h i s s tudy
was s t a b l e a g a i n s t water a t t a c k over the d u r a
t i o n of the s t u d y .
However, the weak p o in t o f th e i n t e r f a c e region was
the su r f a c e o f the
f i l l e r p a r t i c l e s , which showed s ig ns of h y d r
o l y t i c d e g r a d a t i o n ,
e s p e c i a l l y when th e f i l l e r was not s i l a n e t
r e a t e d . This degrada t ion
i s exp la ine d by assuming t h a t water reaching the f i l l
e r - m a t r i x
i n t e r f a c e s leach sodium ions from th e g l a s s s u r
f a c e (C h a r le s , 1958).
As a r e s u l t o f t h i s l each ing p r o c e s s , the s i
l i c a network becomes
n e g a t i v e l y charged due to lo s s of p o s i t i v e
sodium io ns . To compensate
fo r t h i s charge imbalance, hydrogen ions of the wa ter sur
rounding t h e
g l a s s s u r f a c e a r e a t t r a c t e d in to the s i l
i c a network. Through t h i s
p ro c e s s , hydroxyl ions of th e water accumulate o u t s i
d e the g l a s s
s u r f a c e , the reby i n c r e a s i n g the pH in t h a t
reg io n . At a c e r t a i n pH
l e v e l , th e se hydroxyl ions a t t a c k th e s i lo x an e
bonds inducing a
-
37
h y d r o l y t i c d e gr ad a t io n of th e f i l l e r s u r
f a c e . These r e a c t i o n s can be
de scr ibe d as fo l lows (C h a r le s , 1958):
- S i - 0 ' Na+ + H20 — -Si-OH + Na+0H" (1)
- S i - 0 - S i -OH + OH' — - S i - 0 ' + HO-Si-OH (2)
Thus, the r e a c t i o n s between the hydroxyl ions and the s
i l i c a
network r e s u l t in a breakdown of the s i l i c a s t r u c
t u r e . As one of
t h e s e r e a c t i o n produc ts i s - S i - 0 " , water w i
l l cont inue to r e a c t wi th
the s i l i c a network accord ing to the r e a c t i o n
- S i - 0 ’ + H20 — -Si-OH + OH’ (3)
Due to proton t r a n s f e r , new hydroxyl ions a r e formed
which
c o n t i n u e to r e a c t wi th the s i l i c a s t r u c t u
r e , and h y d r o l y t i c degrada
t i o n the re by becomes a u t o c a t a l y t i c (C h a r l e
s , 1958).
The r e s u l t s p r e se n t ed in Paper II i n d i c a t e t
h a t t h i s type of
r e a c t i o n occurs in a l l composi tes s t u d i e d , as
both pH and sodium ion
c o n c e n t r a t i o n i n c reased in t h e s to r a g e
water o f a l l the composites
with t im e .
According to the SEN i n v e s t i g a t i o n ( I I ) , the u n
t r e a t e d f i l l e r s in
t h e BIS-GMA/TEGDMA composi tes showed th e f i r s t s ig ns
of h y d r o l y t i c
d e g r a d a t io n , whi le the u n t r e a t e d f i l l e r
s in the PMMA composi tes showed
t h e most d ramatic changes a f t e r a s ix-month exposure to
w a te r . This
d i f f e r e n c e might be expla ined as a r e s u l t o f d i
f f e r e n c e s in t h e
d i f f u s i o n r a t e s o f w a te r , sodium, and hydroxyl
ions in the two r e s i n s .
-
38
According to Paper I I I and a previous i n v e s t i g a t i o
n by Braden (1964) ,
the exchange r a t e of water through the PMMA matr ix i s f a s
t e r than t h a t
th rough th e BIS-GMA/TEGDMA r e s i n . The leaching of sodium
from the
PMMA composite should then be i n i t i a t e d f a s t e r than
in the BIS-
GMA/TEGDMA compos i te . Even though the g ene ra t io n o f
hydroxyl ions as
a r e s u l t o f sodium ion-hydrogen ion exchange might be f a
s t e r in t h e
PMMA m a t r i x , the hydroxyl ions might a l s o d i f f u s e
away f a s t e r from the
f i l l e r s u r f a c e in t h i s m a te r i a l than from
corresponding reg ions of t h e
BIS-GMA/TEGDMA composi te . Such a d i f f e r e n c e in
hydroxyl ion d i f f u s i o n
could mean t h a t even though the hydroxyl ion format ion i n s
i d e a
BIS-GMA/TEGDMA composite i s s lower , th e c r i t i c a l pH f
o r i n i t i a t i n g
h y d r o l y t i c de gr ada t ion could be reached sooner i n
s i d e t h i s composi te .
When the c r i t i c a l pH va lue has been reached i n s i d e
th e PMMA
composi te , the a u t o c a t a l y t i c p rocess and f i l l
e r de gra da t io n could very
well proceed more r a p i d l y in th e PMMA composi te than in
the
BIS-GMA/TEGDMA composi te due to the f a s t e r d i f f u s i o
n of water in t h e
PMMA r e s i n . This may be demons tra ted through r e a c t i
o n s (2) and (3)
which show t h a t water i s n ecessary f o r a r e g e n e r a
t i o n of hydroxyl io ns
( C h a r l e s , 1958). Ac co rd ingly , the f a s t e r the
water molecules reach
the f i l l e r s u r f a c e , the f a s t e r the a u t o c a
t a l y t i c p rocess w i l l p roceed
when i t has been i n i t i a t e d .
The r e s u l t s p r esen ted in Paper I I I showed t h a t the
d i f f u s i o n
c o e f f i c i e n t o f wat e r in BIS-GMA/TEGDMA was
approximate ly o n e - te n t h
t h a t o f PMMA. Regarding th e changes in s o r p t i o n and
d e s o r p t i o n
c o e f f i c i e n t s between the f i r s t and second cyc le
a t 60°C, i t has been
hypothes ized t h a t s t r e s s e s induced dur ing sw el l in
g and dehy d ra t i o n
induced s u r fa c e c racks (Rosen, 1961). However, no a t te m
pt s were made
-
39
t o exp lo re t h i s th e o ry through SEM a n a l y s i s . F
ur th e r s t u d i e s o f t h i s
type should be performed to de te rmine whether or not such c
rack forma
t i o n o c c u r s , as such c racks would f a c i l i t a t e
wear o f th e mat r ix
m a t e r i a l .
In Paper I I I , i t was f u r t h e r shown t h a t the l i n e
a r swe l l i ng due
t o water s a t u r a t i o n was as high as 1.08%. The sw e l l
i ng process may
have e i t h e r a p o s i t i v e or ne ga t ive impact on th e
performance of a
composi te m a te r i a l (Asmussen & Jo rge nse n , 1972;
Hembree, 1983). Water
sw el l ing may compensate f o r th e s e t t i n g c o n t r a
c t i o n (Asmussen &
J 0 rg en se n , 1972). However, ex c e s s iv e sw e l l i ng
could induce s t r e s s e s
a t the r e s t o r a t i on-enamel i n t e r f a c e , c o n t
r i b u t i n g to debonding and
margina l leakage (Hembree, 1983).
Based on the d i f f u s i o n c o e f f i c i e n t s a t 37°C
and 60°C, and assum
ing t h a t the Arrhenius r e l a t i o n s h i p i s v a l i d
in t h i s i n t e r v a l , i t i s
p o s s i b l e to e s t i m a t e the a c t i v a t i o n
energy f o r wa ter d i f f u s i o n in t h e
BIS-GMA/TEGDMA r e s i n . The c a l c u l a t e d a c t i v a t
i o n energy va lue i s
approximate ly 10 kca l /mole . This va lue i s a lmost i d e n
t i c a l to t h e
v a p o r i z a t i o n energy of water a t th e se tempe ra tur
es and may i n d i c a t e
t h a t the energy needed to produce wa ter d i f f u s i o n
through a BIS-
GMA/TEGDMA r e s i n i s consumed p r i m a r i l y by water v a
p o r i z a t i o n (Moore,
1968). These r e s u l t s imply t h a t the d i f f u s i o n
of water in the i n v e s
t i g a t e d BISGMA/TEGDMA r e s i n i s a p rocess where water
molecules pene
t r a t e as water vapor in to th e network where they a r e a t
t r a c t e d by
s t r o n g p o l a r groups . When an o th e r a c t i v a t e
d water molecule reaches
such a hydrogen-bonded water molecule , i t can d i s p l a c e
the p r e v i o u s l y -
a t t a c h e d water molecule . In t h i s p r o c e s s , i t
s k i n e t i c energy i s l o s t ,
and th e molecule becomes bonded by the p o l a r group p re v
io u s ly occupied
-
40
by the d i s p la c e d molecule . This model i n d i c a t e s
t h a t th e d i f f u s i o n
of water through t h i s r e s i n could be q u i t e pa ss iv e
regard in g cha in
r o t a t i o n s o f the r e s i n (Thomas, 1951).
In Paper IV, the leaching behavior of four commercial comp os i
te s ,
Conc ise ®, P r o f i l e 0 , F o t o f i l 0 , and S i l a r 0
, was i n v e s t i g a t e d . This
s tudy was designed to de te rmine whether the h y d r o l y t i
c d e g r a d a t i o n ,
which was r e p o r t e d in Paper I I , a l s o r e s u l t e d
wi th f i l l e r s used in
commercial p rod uc ts . To be ab le to t e s t the hypo thes i
s t h a t a g l a s s
modi fy ing element such as sodium f a c i l i t a t e s the h y
d r o l y t i c
degr ad a t io n of f i l l e r s in compos i tes , two composi
tes c o n t a i n i n g
s t r o n t i u m or barium were s e l e c t e d . These m a t e
r i a l s , P r o f i l e and
F o t o f i l 0 , were compared to Concise0 and S i l a r ®
which c o n ta in e d
r e l a t i v e l y pure S i 0^ f i l l e r . The hypo the s i s
being t e s t e d was t h a t
P r o f i l e and F o t o f i l would e x h i b i t more
pronounced leaching than t h e
o t h e r two m a t e r i a l s . A f te r 30 days in w a t e r
, the r e s u l t s confirmed
the v a l i d i t y of th e proposed h y p o th e s i s .
However, during the second
p e r io d a decrease in leach ing occurred f o r a l l m a t e
r i a l s excep t f o r
S i l a r 0 . The S i l a r ® composi tes e x h i b i t e d
twice as high a s i l i c o n
le a c h in g r a t e dur ing the second p e r i o d . This f i
n d i n g , combined with
the f a c t t h a t the s i l i c o n was leached from the quar
tz f i l l e r i n
Concise® , c r e a t e d some doubt on whether only one
mechanism was
r e s p o n s i b l e f o r the leaching of g l a ss m od i f ie
r s and whether breakdown
o f the s i l i c a network was the s o l e ba s i s f o r the h
y d r o l y t i c
d egrada t ion of f i l l e r s in composi tes (C h a r le s ,
1958; Wiederhorn &
Bolz , 1970). However, the t o t a l f i l l e r su r f a c e a
rea o f th e d i f f e r e n t®
composi tes might e x pl a in why S i l a r r e l e a s e d more
s i l i c o n than any®
o t h e r m a t e r i a l . Based on th e assumpt ion t h a t S
i l a r has s p h e r ic a l
-
41
f i l l e r s G.04 /um in d i am e te r , while the d iameters
of the o t h e r f i l l e r s