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Viscoelastic behavior of dental restorative composites during setting
Dauvillier, B.S.
Publication date
2002
Link to publication
Citation for published version (APA):
Dauvillier, B. S. (2002). Viscoelastic behavior of dental restorative composites during setting.
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INFLUENCEE OF TEGDMA/BISGMA RATIO IN EXPERIMENTAL
RESINN COMPOSITES ON SELECTED MECHANICAL
PROPERTIES S
Abstract t
Thee shrinkage of resin composites, in combination with increasing stiffness, willl inevitably lead to stress development in the restoration and its surroundings. Onee way to relieve the shrinkage stress is by viscous flow of the composite from free,, unbonded surfaces. This study examines the possibility of stimulating viscouss flow by increasing the TEGDMA/bisGMA ratio in the resin of the experimentall composites. A series of experimental two-paste composites of varyingg TEGDMA-bisGMA composition in the resin were used to determine howw comonomer composition affects the shrinkage stress-strain, viscoelastic property,, and double-bond conversion during setting. In addition, the tensile strengthh of the composites after setting for one hour, and the wear process over aa period of one year were evaluated.
Varyingg the relative amounts of bisGMA and TEGDMA has a significant effect on thee mechanical properties of two-paste composites. It was found that the polymerizationn rate of bisGMA-TEGDMA composites is an indicative measure off viscoelastic behavior during setting: the higher the reactivity, the higher thee stiffness and viscosity. Composites with 50 wt% TEGDMA in the resin displayedd the highest maximum polymerization rate. Higher amounts of TEGDMAA in the resin resulted in an increased amount of converted double bonds,, increased wear, and reduced tensile strength. Higher amounts of TEGDMAA resulted in only a moderate increase in the pre-gel viscous flow of compositess in the early setting phase, and displayed high shrinkage stresses. Thee high post-gel shrinkage of this flowable composite is the decisive factor in thee high shrinkage stress development in the material.
I n t r o d u c t i o n n
Throughoutt its lifetime, the interface between a resin composite and thee tooth is subjected to the challenge of cyclical thermal and load stresses,, which can potentially cause debonding and lead to clinical failuree of the restorative treatment. Before the restored tooth is subjected too these stresses, the maturing restoration-tooth interface has first to survivee the build-up of shrinkage stress during the setting process of the resinn composite. Therefore, the success of a tight sealed restoration afterr preparation is rooted in minimizing shrinkage stress development inn the restoration.
Too prevent damage from shrinkage stress, dental literature has given considerablee attention to several factors, which reduce shrinkage stress inn the restorative material during setting [1]. Some factors are out of the clinician'ss control, i.e., resin, filler, and activation system formulation, whilee others are directly under the clinician's control, i.e., ratio two-paste system,, light-intensity, and preparation geometry (C-factor). These factorss not only affect the amount and direction of composite shrinkage, butt also the mechanical behavior of the restorative material when it transformss from a fluid substance to a solid [2]. Previous studies showed thatt the mechanical behavior of resin-based dental materials during settingg is viscoelastic by nature [3-5]. Up to the gel-point, the viscous flow11 property predominates, whereas after the gel-point, the elastic2 behaviorr predominates. We shall avoid the word solid for polymerized composites,, because the superficial appearance of the composite can be misleading.. A composite can look solid and yet achieve flow deformation overr a long period of time.
Shrinkagee stress development in the restoration can be low when shrinkagee is accompanied by predominant (pre-gel) viscous flow of the materiall [6]. In this way, volume change attributable to shrinkage can be compensatedd by material flow from the unbonded, outer surface. This chapterr is focused on the mechanical properties of experimental compositess with different TEGDMA/bisGMA ratio in the resin. Hence, dynamicc mechanical, volumetric shrinkage, and wear tests were performedd as a function of resin composition. For this purpose, conventionall resin formulation is briefly reviewed in the introduction. Forr a detailed description of the conventional monomer system, we referr to Peutzfeldt [7]
Thee viscous flow property in dental composites is provided by the resin phase,, as it changes from viscous to solid behavior after setting. Many
'viscouss flow: polymer chains or isolated cross-linked segments slipping past one another (Fig. 2.4).
2
today'ss commercially available dental resin composite materials utilize bisGMAA (Fig. 6.1) as major monomer in the resin. This bulky bifunctional monomerr has high reactivity, high molecular weight, undergoes low polymerizationn shrinkage, and produces a cross-linked, three-dimensionall resin network [7].
However,, due to presence of intermolecular hydrogen bonding (H-O—H-O)) in the monomer [8, 9], bisGMA possesses a very high viscosity.. For this reason, the resin phase of dental composites cannot consistt purely of bisGMA, because this would lead to poorly filled and toughh composite pastes, which handling property is not acceptable in generall dental practice. Deviatives of bisGMA have been developed byy selective replacement of the hydroxyl group by hydrogen or methyl groupss [10, 11]. As a consequence, the viscosity of the deviative was lowered,, but also the mechanical properties of the polymerized resin, as hydrogenn bonding restricts sliding of polymer segments relative to eachh other [10, 11].
Investigationss have been carried out in the use of monomethacrylates as diluentt for bisGMA [12, 13]. Some concern can be addressed to the impactt this type of diluent may have on the degree of cross-linking in the resinn after setting. Monomethacrylates tend to give linear polymers whenn polymerized by themselves and may still do so in mixtures. Therefore,, conventional glycol dimethacrylates, which are cross-linking agents,, are used as diluent for bisGMA. The monomer most often used forr this purpose is TEGDMA (Fig. 6.1). To put the viscosities in
HH CH 3 OH W C H 3 W OH C H 3 H bisGMA A (2,2-bis[4-(2-hydroxy-3-methacrylyloxypropoxy)phenyl]-propane) ) OO CH3 u HH CH3 O TEGDMA A (triethyleneglycoll dimethacrylate)
q> > ^ ^
oo p e r s p e c t i v e , t h e viscosity of w a t e r is a p p r o x i m a t e l y 1 mPa.s (23 °C),
EE while the viscosity of bisGMA is approximately 1,000,000 mPa.s (23 °C), 88 a n d t h a t of TEGDMA a p p r o x i m a t e l y 10 mPa.s (23 °C) [14].
« «
V) )
«« However, TEGDMA has been s h o w n to adversely affect the properties of óó the resin by increasing the water sorption and polymerization shrinkage ££ [15, 16]. The increase in polymerization shrinkage is a consequence of the
SS fact that TEGDMA is a small molecular dimethacrylate molecule, which °° d e l i v e r s m o r e vinyl (C=C) g r o u p s p e r v o l u m e u n i t to the resin system. 22 B e c a u s e of t h e r e l a t i o n s h i p b e t w e e n d o u b l e - b o n d c o n v e r s i o n a n d SS p o l y m e r i z a t i o n s h r i n k a g e strain [1], increasing the a m o u n t of d o u b l e -QQ b o n d conversion increases volume shrinkage d u r i n g setting of the resin, 22 u n d e r a s s u m p t i o n of similar reactivity. H o w e v e r , the relative ease of \\ flow of t h e s e r e s i n c o m p o s i t e s m a y c a u s e a l o w e r p o l y m e r i z a t i o n 55 s h r i n k a g e s t r e s s e s .
55 To examine this potential advantage of material flow, we determined the 22 m e c h a n i c a l b e h a v i o r d u r i n g s e t t i n g of c o m p o s i t e s w i t h d i f f e r e n t q>> T E G D M A / b i s G M A r a t i o in t h e r e s i n . In a d d i t i o n , the d o u b l e - b o n d fcfc c o n v e r s i o n d u r i n g s e t t i n g and the w e a r p r o p e r t y after setting of the —— c o m p o s i t e s w e r e s t u d i e d to u n d e r s t a n d t h e effect of T E G D M A
c o n c e n t r a t i o n in the resin.
CO O
M a t e r i a l ss a n d m e t h o d s
P r e p a r a t i o nn of c h e m i c a l l y a c t i v a t e d r e s i n c o m p o s i t e s
Inn this study, three chemically activated resin composities, with different b i s G M A - T E G D M AA c o m p o s i t i o n in t h e r e s i n , w e r e e v a l u a t e d . The i n g r e d i e n t ss for the preparation of the composites were used as received. B i s G M AA a n d T E G D M A , p u r c h a s e d from R ö h m ( A p p e n d i x B), w e r e
i n h i b i t e dd w i t h 200 p p m Topanol O* a n d 100 p p m HQME3 respectively.
T h ee t r a d e d e s i g n a t i o n s , abbreviations, a n d sources for all materials are g i v e nn in Table 6.1.
TEGDMAA w a s a d d e d to bisGMA in p r o p o r t i o n s of 30, 50, a n d 70 % by weight.. The m o n o m e r mixes were prepared by weighing the components i n t oo glass v i a l s a n d r o t a t i n g h o r i z o n t a l l y at r o o m t e m p e r a t u r e u n t i l h o m o g e n e o u s .. T h e m i x t u r e s w e r e d e g a s s e d for 15 m i n u t e s u n d e r v a c u u mm p r i o r to d e t e r m i n e the d e n s i t y by p y c n o m e t r y . The v o l u m e of t h ee p y c n o m e t e r w a s d e t e r m i n e d with d e i o n i z e d w a t e r (23 °C).
Forr p r e p a r i n g the c o m p o s i t e , each m o n o m e r m i x t u r e w a s d i v i d e d into
3
Tablee 6.1 Materials used for preparation of two-paste resin composites. Compound d
Bisphenol-A-glycidyll dimethacrylate Triethylenee glycol dimethacrylate Benzoyll peroxide ( 2 5 % H20 )
NN ,N-bis(2-hydroxyethyl)-p-toluidine Butylatedd hydroxytoluene
Silanated44 glassfiller3 (LAB7373,
p=2.64g/cm33 (23 , dp'' =1.53 urn, BET=5.00 m2 /g) Abbreviation n bisGMA A TEGDMA A Initiator r Accelerator r Inhibitor r Filler r Supplierr (appendix B) Röhm m Röhm m Aidrich h Fluka a Fluka a SCHOTT T a500 wt% S i 0
2, 20 wt% SrO, 15 wt% B203, 15 wt% A l203, and 1 wt% BaO b
meann particle size
twoo portions. The activator was dissolved in one part (base resin) and the initiatorr was dissolved in the other part (catalyst resin). Finally, an inhibitorr was added to both resins. The amounts of activator, initiator andd inhibitor were chosen to mimic, as close as possible, commercially availablee conventional dimethacrylate composites (Table 6.2). Finally, all resinss were loaded with silanated glass filler (mean particle size about
Tablee 6.2 Composition of the resins for the 3 two-paste composites.
Compound d bisGMA A TEGDMA A Initiator r Activator r Inhibitor r paa (g/cm3) Totall C=C (moi/cm3) ) 300 wt% TEGDMA Basee resin Catalyst resin
(wt%)) (wt%) 69.055 69.05 29.600 29.60 1.33 3 1.33 3 0.022 0.02 1.13410.002(23.66 ) 5.473-10"3 3 500 wt% TEGDMA
Basee resin Catalyst resin (wt%)) (wt%) 49.322 49.32 49.322 49.32 1.33 3 1.33 3 0.022 0.02 1.1199 2 (23.1 ) 6.088-10'3 3 700 wt% TEGDMA
Basee resin Catalyst resin (wt%)) (wt%) 29.600 29.60 69.055 69.05 1.33 3 1.33 3 0.022 0.02 1.09810.001(24.55 ) 6.660-10-3 3 a
Meann density value (n=3) of base resin is statistically similar to catalyst resin (p<0.05). .
"Thee glass filler was silanated by Heraeus Kulzer by means of a dry-blending process [17]. In this process,, the glass was grinded in a ball mill for about half an hour. The volume proportion between glasss and balls should be nearly 50/50. The silane coupling agent y-methacryloxypropyl-trimethoxysilanee (MPS) was added 50 % in excess to the theoretically necessary amount for covering thee glass surface fully with one layer of silane obtaining a monolayer and the milling continued for three hours.. This mixture was heated four hours at 100 to 104 C in an oven. Finally, the glass was sievedd by 200 urn.
1.55 ^m) to a content of 70 wt%. This filler content 4 % in volume, iss characteristic for commercially available flowable composites [18]. The resinn was mixed with filler using the following procedure. The resin was spreadd over a Teflon plate and a small fraction of the glass filler amount wass spread as a thin layer over the resin. The filler and resin were gentlyy mixed by shearing the mixture between a Teflon spatula and thee Teflon plate until homogeneous. Again, the mixture was spread overr the Teflon plate and the next small fraction of the glass filler amountt was spread as a thin layer, and mixed with the same shear movementt by the Teflon spatula. After mixing the mixture with the finall glass filler fraction, the composite was degassed for 15 minutes in ann excicator. The composite was stored for 2 days at room temperature. Finally,, the composite was mixed again on the Teflon plate, degassed for 155 minutes, and finally stored in a refrigerator at 6 °C.
Exceptt for the wear experiments, the amount of composites was kept constantt (0.114 cm3) and the measurements were performed under dry conditionn at room temperature 1 °C).
Dynamicc test: oscillatory sinusoidal cycles
Thee viscoelastic property of the resin composites during setting was measuredd using the oscillatory sinusoidal strain test on an automated universall testing machine (H10KM, Hounsfield). Details of the testing machinee are described extensively in chapter 3 of this thesis. The freshly mixedd resin composite (1:1 w / w ) was bonded between opposing steel diskss with diameter (d) of 5.4 mm and separated by a distance (h) of 5.0 mm,, creating a C-factor of 0.5 (=d/2h). During the measurement, the upperr disk performed an oscillating sinusoidal deformation with amplitudee of 1.0 jim. (=0.02 % strain) and frequency of 0.1 Hz around the originall height of the specimen (Fig. 6.2).
Onee hour after the start of the experiment, the resin composite was subjectedd to tensile loading (5 ^ m / m i n ) until fracture. The measurementss were repeated 3 times at room temperature 1 °C). Duringg the measurement, the data (time, load, and displacement signal) weree collected simultaneously at a sample rate of 18 points per second. All measurementss were started within 2 minutes after mixing the composites.
Volumetricc shrinkage
Crosss head Loadd cell Apply y K^K^ lL!l " 3 Response 200 200 3000 3700 Timee (s) 3800 0 TCP" "
TrBatëiT' '
re re 0. . 5 5 </> > la a (2) ) 200 200 3000 3700 Timee (s) 3800 0Figuree 6.2 Dynamic test method on an a u t o m a t e d universal testing m a c h i n e . A n
o s c i l l a t o r yy s t r a i n , m e a s u r e d by t w o ( 2 ) L V T t r a n s d u c e r s is a p p l i e d to t h e (1) s p e c i m e n ,, w h i c h is kept w i t h i n 1 urn at a c o n s t a n t h e i g h t . T h e s t r e s s r e s p o n s ee t o t h e o s c i l l a t o r y i n p u t is s u p e r i m p o s e d o n t h e p o l y m e r i z a t i o n s h r i n k a g ee s t r e s s .
thee specimen was not measured, because the oscillatory deformation was p e r f o r m e dd a r o u n d the original h e i g h t of the specimen. H o w e v e r , t h e d i s p l a c e m e n tt caused by axial s h r i n k a g e m u s t be t a k e n into a c c o u n t whenn modeling the stress data recorded by the dynamic test method. For thiss reason, volumetric shrinkage m e a s u r e m e n t s (n=3) w e r e performed
byy a mercury dilatometer at 1 °C, using the procedure described by
Dee Gee et al. [19]. All m e a s u r e m e n t s w e r e started within 2 m i n u t e s after mixingg the composites.
P a r a m e t e rr i d e n t i f i c a t i o n o n s t r e s s - s t r a i n d a t a
Forr this investigation, it w a s assumed that the mechanical properties off the composite are isotropic (the same in all directions), a n d that the viscoelasticc behavior of the composites in the early stage of setting can bee described by the Maxwell model [20]. The load and displacement data fromm the oscillatory s t r a i n m e a s u r e m e n t w e r e c o n v e r t e d to n o r m a l s t r e s ss (G) a n d s t r a i n (£) d a t a by u s i n g E q u a t i o n (6.1) a n d (6.2) respectively: :
(77 = (6.1) )
axial axial
AL AL (6.2) )
inn which A is the cross-sectional area of the cylindrical s p e c i m e n (m2),
FF the recorded load response of the specimen (N), AL is the displacement
beforee setting (m). The shrinkage and dynamic component in the stress dataa were isolated with the standard Fast Fourier Transform (FFT) smoothingg procedure in Origin (version 5.0, Microcal).
Thee Maxwell's material parameters (E=Young's modulus and T|=viscosity) weree determined by applying the parameter identification procedure, ass described in chapter 4, on small time intervals (ten seconds) in the stress-strainn data. The strain for the small time intervals was calculated by addingg the oscillatory strain of the dynamic experiment to the shrinkage strain,, which was considered as a linear function:
£(t)£(t) = £(t0)+At + Bsm(eot) (6.3)
inn which £(t0) is the strain at begin time interval, A is the slope of the
shrinkagee strain ( l / s ) , B the amplitude, and co the angular frequency ( r a d / s )) of the applied oscillatory strain. The shrinkage strain was obtainedd from the free volumetric shrinkage data by the conversion factorr provided by Feilzer and co-workers (Table 3.1).
Inn an evaluation procedure, the Maxwell model was loaded with the calculatedd parameters and shrinkage strain of the composite, to determinee up to where in setting time the model simulates the real behaviorr of the composite. Details of the evaluation procedure are describedd in chapter 4 of this thesis.
Stresss relaxation time
AA number of constraints must be placed on the Maxwell's model parameters.. All parameters have to be positive, and must increase monotonicallyy over time. Stress relaxation is an important material property,, involving the time-dependent decrease in stress at constant strainn by viscous flow. A measure of stress relaxation under constrained conditionss is the so-called stress relaxation time (t). This material property,, defined as rj/E for this model, is the time required for the stresss to decrease to 1 / e (=37 %) of its initial value.
Infraredd s p e c t r o s c o p y
Thee infrared measurements were performed on a Biorad 165 FT-IR spectrometer,, equipped with a DTGS-detector, and controlled by Win-IRR software (version 2.04, Biorad) on a Pentium (200 MHz) desktop computerr (Windows® 98 platform).
Tablee 6.3 Infrared measurement protocol for two-paste resin composites.
Typee Number of scans Resolution (cm-1
) Time (s)
Backgroundd scan 16 2 40 Insertionn individual paste
Staticc scan 64 2 160 Insertionn mixed pastes
Kineticc scan 4 2 10
Remarks s
Background d
Noo conversion spectrum performedd 1 time
Conversionn spectrum performedd 385 times
Alll s p e c i m e n s w e r e m e a s u r e d in a Teflon m o l d (d=5.4 m m , h=5.0 m m ) p l a c e dd d i r e c t l y on a G o l d e n G a t e (Single Reflection D i a m o n d ATR, 105000 series, Graseby). The top of the cylindrical specimens was covered w i t hh a g r e a s e d glass slide. A b s o r b a n c e spectra w e r e t a k e n before a n d d u r i n gg the setting of the resin composites, according to the m e a s u r e m e n t protocoll as specified in Table 6.3. The kinetic scan w a s started within 60
Figuree 6.3 Peak of methacrylate double-bond stretch vibration (1636 cm1)
andd 1,4-disubstituted phenylene stretch vibration (1582 cm1) as internal referencee [21]. The baseline (dotted line) for the methacrylic peak is drawn from approximatelyy 1655 to 1590 c m ' .
oo seconds after mixing the composites. The double-bond conversion (^) for gg each spectrum was determined by the following equation:
o o
55 M Ó Ó
«« = 1 - ^ - (6-4)
wheree ct is the ratio of the peak height of the methacrylate double-bond
stretchh vibration at 1636 cm' to the peak height of the internal reference •22 (Fig- 6.3) at time t, and u is the same (mean) ratio for the individual 22 pastes. The static and kinetic scans were measured 3 and 5 times SS respectively.
Q Q
^^ Wear
i i
<£JJ The erosive wear performance of the experimental composites was 55 tested by three-body wear tests in the ACTA wear machine (ACTA,
88 The Netherlands), shown in Figure 6.4. This wear test mimics the clinical o»» condition in which the occlusal surface of the restoration is exposed to 5== stresses and relative motions associated with chewing and a third-body *•• medium. The third-body medium contains components resembling HBB those, which are present in natural food. A detailed description of the •*•• wear machine and its functioning has been given by Pallav et al. [22-24].
^^ Each compartment in the sample wheel was filled with freshly hand o__ mixed (1:1 wt%) composite material, covered by a matrix, and -cc polymerized at room temperature. The sample wheel accommodates
threee specimens of each composite material. The commercially available dentall resin composite Z100 was also included, to serve as reference. This light-activatedd composite, covered by a matrix, was polymerized for 180 secondss in a halogen light curing cabinet (Dentacolor XS, Kulzer) at roomm temperature. After one hour, the specimens were glued onto the samplee wheel with cyanoacrylate, and ground with a diamond wheel (gritt 600), to obtain a uniform cylindrical outer surface. The starting profilee on the specimen surfaces was obtained in the wear machine, by cyclingg the wheel 25,000 cycles in used (old) slurry. After being finished, thee specimens were maintained at room temperature and kept wet at all times. .
Thee wear of the composites was tested once at a set time intervals after preparationn of the specimens: after 1 day, 3 days, 1 week, 1 month, 4 months,, and 1,4 years respectively. Each wear test run consists of 200,000 cycless of the sample wheel at a rotational speed of 1 Hz (chewing frequency).. The entire process lasts approximately 55 hours. After each
wearr test, the third-body medium, consisting of a slurry of white rice and millett seeds shells, was renewed. Before and after each wear test, ten separatee surface profiles of each specimen in the axial direction of the samplee wheel were recorded in an automated profilometer set up. As can bee seen from Figure 6.4, the surface profile includes of two unworn referencee planes flanking the worn area. The average depth of the worn surfacee relative to the reference planes represents the wear in microns (verticall loss of substance). By taking the reference areas outside the wearingg area, yet still on the material itself, any swelling by water sorptionn is canceled out in the measured wear.
bowll with rice and millet seed shells in water
Figuree 6.4 Schematic representation of the various steps in the wear test. Upperr left: sample wheel with some specimens polymerized and glued onto the wheel.. Middle: (1) sample wheel and (2) antagonist wheel rolling over each otherr in a slurry of rice and millet seed shells for three-body (erosive) wear. The antagonistt wheel is pressed against the sample wheel by a spring force of 15 N, andd the surface velocity has been adjusted to obtain slip rates of 15 % for occlusall contact-free wear. The rotational speed of the sample wheel was fixedd at 1 Hz (chewing frequency). Upper right: surface tracings from one unwornn reference to the other across the worn surface for wear measurements [221. .
Resultss a n d d i s c u s s i o n Shrinkagee stress-strain data
Thee material properties of dental resin composites depend on several factors,, related to the initiation system, the resin, the filler particles, and thee coupling between filler and resin. It is often difficult on the basis of thee published literature to know what has caused measured differences inn material properties, because the investigated materials were different inn several aspects. In this study, only one factor was varied, i.e., the weightt composition of bisGMA and TEGDMA in the resin.
Figuree 6.5 shows the shrinkage strain and shrinkage stress development duringg setting for the experimental composites with various bisGMA-TEGDMAA weights in the resin. The more bisGMA was replaced by TEGDMA,, the higher the composite shrinkage strain development. This observationn is in agreement with expectation, because the increase of TEGDMAA results in the increase of double-bond content in the resin (Tablee 6.2). From the shrinkage results presented in Table 6.4, we may noticee a doubling of shrinkage during the pre-gel phase of the composite att each increment step of TEGDMA. Although the presence of more TEGDMAA in the resin leads to more flowable composites, as noticable duringg handling, it does not provide the desired prolonged predominant viscouss state for the composite. At maximum flowability, only 20 % of thee total shrinkage had been completed before a stress response was registered. .
Thee stress-strain results show that bisGMA-TEGDMA composites reveal aa linear relation between the amount of post-gel shrinkage strain and the amountt of shrinkage stress after one hour setting. The effect of TEGDMA
Tablee 6.4 Volumetric shrinkage contribution of experimental resin composites afterr one hour setting.
Resinn composition 300 wt% TEGDMA 500 wt% TEGDMA 700 wt% TEGDMA Pre-gell shrinkage (vol%) ) 0.3 3 0.6 6 1.1 1 Post-gell shrinkage* (vol%) ) 3.2 2 4.0 0 4.6 6 Totall shrinkage (vol%) ) 3.5 5 4.6 6 6.7 7 ##
Post-gel shrinkage = part of the overall shrinkage wherein a shrinkage stress onn the composite is recorded.
onn the polymerization rate, stiffness, and viscosity development of the compositess will be presented and discussed in the next sections.
£ £
700 wt% TEGDMA 500 wt% TEGDMA 300 wt% TEGDMA *i 10000 2000 3000 Timee (s) 4000 0 700 wt% TEGDMA 500 wt% TEGDMA 300 wt% TEGDMA Maxwelll model 10000 2000 3000 4000 Timee (s)Figuree 6.5 (a) Volumetric shrinkage strain and (b) axial shrinkage stress developmentt (—measured, Maxwell model) of experimental resin composites (C-factor=0.5)) during setting. The onset in shrinkage stress (=gel-point) for the 30,, 50, and 70 wt% TEGDMA composite was 192, 180, and 190 s respectively.
Infraredd spectroscopy
Thee infrared results summarized in Table 6.5 provide valuable informationn about the effect of resin composition on the dynamics of the composite'ss polymerization reaction. The double-bond conversion rate wass determined from the time derivative of the mean conversion-time data. .
Tablee 6.5 Summary of the infrared results for two-paste composites in which the weightt composition of bisGMA-TEGDMA in the resin was varied.
Composition n (wtt % TEGDMA) 70 0 50 0 30 0 Maximum m conversion n ratee (%/s) 0.47 7 (0.03) ) 0.53 3 (0.02) ) 0.41 1 (0.03) ) Settingg time att maximum conversion n ratee (s) 143 3 163 3 186 6 Conversion n att maximum ratee (%) 27.00 (2.4) 25.55 (1.9) 15.11 (2.1) Conversion n afterr one hour {%) ) 57.33 (2.3)a 56.33 (3.8)a 44.22 (3.2) Converted d C=C C (mol/cm3 ) ) 1.937-10"3 3 1.723-10"3 3 1.208-10"3 3 Unreacted d C=C C (mol/cm3 ) ) 1.443-10"3 3 1.338-10"3 3 1.525-10"3 3 a
T h ee i n c r e a s e of TEGDMA in t h e resin p h a s e r e s u l t e d in h i g h e r d o u b l e -b o n dd c o n v e r s i o n of t h e c o m p o s i t e . T h i s is in a g r e e m e n t w i t h t h e l i t e r a t u r ee [12, 25]. The conversion after one h o u r setting w a s limited to v a l u e ss b e l o w 60 %. A h u n d r e d p e r c e n t c o n v e r s i o n is practically not feasible,, d u e to the formation of a h i g h cross-linked p o l y m e r n e t w o r k , w h i c hh limits the mobility of reacting species. Studies on light-activated d e n t a ll d i m e t h a c r y l a t e resins revealed final d o u b l e - b o n d conversions in t h ee r a n g e of 55-80 % [26, 27].
T h ee i n c r e a s e of the d o u b l e - b o n d conversion rate to a m a x i m u m , even t h o u g hh t h e a m o u n t of m o n o m e r is d i m i n i s h i n g , is q u i t e c o m m o n in m u l t i f u n c t i o n a ll d i m e t h a c r y l a t e s y s t e m s , a n d is k n o w n as
auto-acceleration,acceleration, gel effect, or Trommsdorff-Norrish effect [28]. This effect is
a t t r i b u t e dd to the g r o w t h of p o l y m e r , w h i c h r e s u l t s in an increase of viscosity.. This, in t u r n , h a s an a d v e r s e effect on the diffusion process of r a d i c a l ss to t e r m i n a t e (Fig. 2.1), r e s u l t i n g in an i n c r e a s e in the c o n c e n t r a t i o nn of free radicals [29]. At a certain p o i n t in the reaction time,, the mobility of the monomer becomes even lower, and propagation (Fig.. 2.1) also b e c o m e s diffusion-controlled, along w i t h t e r m i n a t i o n . Thiss p h e n o m e n o n , k n o w n as autodeceleration, causes the rate of reaction too fall off s u b s t a n t i a l l y .
Inspectionn of the m a x i m u m conversion rates in Table 6.5 reveals that an increasee of TEGDMA in the resin causes the m a x i m u m conversion rate to shiftt significantly (p<0.05) to h i g h e r values, w i t h an o p t i m u m for the c o m p o s i t ee w i t h 50 w t % TEGDMA in the resin. This o b s e r v a t i o n is in agreementt with that of Lovell et al. [27], w h o found the highest m a x i m u m ratee for experimental light-activated resins in the range of 25 to 50 % in weightt TEGDMA. The researchers attribute the occurrence of a maximum ratee at this specific bisGMA-TEGDMA composition to the excellent diluent p r o p e r t yy of TEGDMA, a n d the high reactivity of bisGMA [30, 31]. In this specificc resin composition range, sufficient TEGDMA is present to reduce thee initial viscosity of the composite, and thereby increasing the mobility off the reactive bisGMA. When the composite contains too much TEGDMA inn the resin, t h e n t h e less reactive TEGDMA d o m i n a t e s the reaction, resultingg in a slower polymerization reaction.
Itt is s t r i k i n g t h a t the setting t i m e for m a x i m u m conversio n rate is not relatedd to the polymerization rate, b u t to the a m o u n t of TEGDMA in the resin.. The e x p l a n a t i o n for this o b s e r v a t i o n m a y be t h a t the composites d i dd not h a d the s a m e inhibition p e r i o d . The m o n o m e r s w e r e stabilized w i t hh different a m o u n t s of inhibitor. As the m o n o m e r s w e r e u s e d as receivedd (Table 6.1), the inhibitor concentration decreased w h e n bisGMA w a ss r e p l a c e d by TEGDMA, resulting in s h o r t e r inhibitor time p e r i o d s .
Examinationn of the setting time for maximum conversion rate (Table 6.5) withh the start of shrinkage stress development (Fig. 6.5) reveals that the transitionn of the auto-acceleration to autodeceleration occurs in the pre-gell setting phase of the composite. It was after 200 seconds, in whichh 17 % or more of the measured double-bond conversion had been completed,, that a stress response was registered. From statistical calculationss it is known that during the early stage of polymerization, monomerss are mainly converted into polymeric chains [32]. It may be expectedd that up to 17 % double-bond conversion, some cross-linking of thee polymer chains may have occurred, since both TEGDMA and bisGMAA are cross-linking agents. The fact that the composite structure iss still capable to flow predominantly up to 17 % double-bond conversion indicatee that the build up of the resin network proceeds with the formationn of isolated cross-linked polymer segments, which can slip alongg one another (Fig 2.4).
Maxwelll model
Thee evaluation results of the Maxwell model reveals that even in thee post-gel phase of setting, the composites can still flow permanently forr a considerable period in time. An explanation for viscous flow duringg post-gel setting, wherein double-bond conversion is mainly consumedd for cross-linking, may be that the cross-linking reaction proceedss faster in polymer segments than between polymer segments. Cross-linkingg between growing polymer segments proceeds slowly withh time, due to diffusion limitations. As a result, the polymer segments cann still slip along one another. As soon as all polymer segments are connectedd to each other, viscous flow is highly restricted to processes, suchh as local rearrangement in the polymer network, movement of the unreactedd vinyl groups, and to porosity in the composites, introduced by mixingg and formation of carbon dioxide (Fig. 2.1). From this point in settingg time, the Maxwell model is no longer valid, because it predicts permanentt viscous flow. In this study, this point in setting time is reachedd at approximately 15-20 minutes (Fig. 6.5b), wherein 40 % or moree double-bonds have been converted. On future studies, it might be usefulll to follow the setting process of dental composites with Nuclear Magneticc Resonance (NMR). This technique may be considered as a typee of chemical microscope, by which the bisGMA and TEGDMA units duringg the polymerization process can be monitored [33].
Materiall p a r a m e t e r s
Thee Maxwell parameters E (Young's modulus) and n (viscosity) were examinedd up to 15 minutes after mixing, because the Maxwell model had beenn proved valid during this setting phase (Fig. 6.5b). Table 6.6 summarizess the calculated values for several points in setting time of one measurement. .
Maximumm double-bond conversion is obtained when the amount of TEGDMAA in the resin is increased. The student's t test with pooled variancee (p<0.05) demonstrated that the maximum amount of double-bondd conversion after one hour setting, was similar for the 50 and 70 wt%% TEGDMA composite. As the viscosity and Young's modulus developmentt with setting time differs significantly (Table 6.6), one may concludee that the effect of TEGDMA on the viscoelastic property is not solelyy due to the degree of conversion. Probably, the amount and nature off the TEGDMA and bisGMA subunits in the polymer network also playss a role in the viscoelastic behavior of the composites. The aromatic bisGMAA molecule is relative stiffer than TEGDMA. The aromatic group inn the central part of the molecule (Fig. 6.1) causes much larger barriers too rotation about the bonds. TEGDMA is more flexible, because the etherr (C-O-C) linkages of the molecule give rise to only slight barriers to freee rotation about the bonds. Furthermore, bisGMA molecules are capablee of forming hydrogen bonding, which restricts sliding of polymer chains,, thereby increasing the viscosity of the system. The highest Young'ss modulus and viscosity development with setting time for the 50 wt%% TEGDMA composite can be explained by the fast polymerization reaction,, in combination with the build of an optimum bisGMA and TEGDMAA monomer ratio in the polymer network, and the low concentrationn of unreacted double-bond in the resin (Table 6.5). Too muchh TEGDMA in the resin results in a slower polymerization rate, a moree flexible polymer network, and more unreacted double-bonds in the resin.. As a result, the stiffness and viscosity development with setting timee is lower.
Clearly,, the polymerization rate of bisGMA-TEGDMA composites is ann indicative measure for the stiffness and viscosity development of the composite.. This is visualized in Figure 6.6. Instead of plotting the Maxwelll parameters individually, the ratio of viscosity to stiffness, the so-calledd stress relaxation time, is given for each composite. Stress-relaxationn is a measure of stress relief under constrained conditions. It representss the time required for the stress to decrease to 1 / e (=37 %) of itss initial value. The effect of TEGDMA on lowering the viscous state of thee composite, which is similar to the effect of increasing the
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D ee curves in F i g u r e 6.6 a n d Figure 6.5 s h o w t h a t for the m o s t reactive b i s G M A - T E G D M AA composites, the post-gel s h r i n k a g e is the decisive factorr in the s h r i n k a g e stress d e v e l o p m e n t of the c o m p o s i t e . Too m u c h TEGDMAA in t h e resin leads to a h i g h e r c o n c e n t r a t i o n of d o u b l e - b o n d conversion,, a n d t h u s higher post-gel shrinkage. As the relaxation curve iss only s l i g h t l y l o w e r t h a n for the 50 w t % TEGDMA c o m p o s i t e , the u l t i m a t ee s h r i n k a g e stress d e v e l o p m e n t is h i g h e r .
T h ee c o m p o s i t e w i t h t h e lowest c o n t e n t of TEGDMA u n d e r g o e s low post-gell shrinkage a n d develops low relaxation times. As a consequence, thee shrinkage stress is favorable low, which allow the b o n d to the cavity w a l ll a chance to form a n d remain intact. H o w e v e r , d u e to the restricted m o b i l i t yy of the b i s G M A m o n o m e r , less d o u b l e - b o n d s are c o n v e r t e d . Ass a result, relatively m o r e u n r e a c t e d m o n o m e r will be p r e s e n t in the c o m p o s i t e .. The p r e s e n c e of u n r e a c t e d m o n o m e r s is of c o n s i d e r a b l e concern,, because these monomers will slowly leach into the s u r r o u n d i n g
medium,, resulting in deleterious effects on the mechanical stability andd biocompatibility of the restoration [34, 35]. Further studies with bisGMA-TEGDMAA composites should be focused on two aspects, which cann reduce post-gel shrinkage of the composite. First, the most reactive bisGMA-TEGDMAA composition must be better specified by increasing thee TEGDMA in smaller increments. An optimum directed toward the usee of less TEGDMA should results in less post-gel shrinkage. Second, thee application of diatomites as dental filler must be tested [36]. This highh porosity material, with pore size in the range of 1.5-22 //m, allows thee formation of an integrated resin-filler system. In this case, shrinkage off the polymer system does not occur solely between filler particles, which,, ultimately, should lead to lower post-gel shrinkage.
Young'ss modulus and tensile strength after one h o u r setting Tablee 6.7 summarizes the elastic modulus and tensile strength of thee composites after one hour setting. All composites fractured cohesively,, whereby the fracture always started at one side at the peripheryy of the metal-composite interface, and propagated at a small anglee to the other side. The diameter of the fractured surface area did not derivatedd significantly from the diameter of the paper matrix.
Tablee 6.7 Material properties of experimental resin composites at a 60-minutee setting (mean - SD).
Materiall property 30 wt% TEGDMA 50 wt% TEGDMA 70 wt% TEGDMA Young'ss modulus (GPa) 6.97(0.1) 7.24(0.1) 7.08(0.1) Tensilee strength (MPa) 29.3(5.1) 20.0(4.0) 17.8(3.6)
Thee Young's modulus values are in agreement with the results of Asmussenn et al. for experimental light-activated bisGMA-TEGDMA compositess [37]. They found also a maximum in the Young's modulus. Duee to the smaller increments, the maximum was observed at a 64 wt% bisGMAA - 36 wt% TEGDMA composition in the resin. The existence of an optimumm bisGMA-TEGDMA composition toward less TEGDMA is favorable,, because this will ultimately cause less post-gel shrinkage strainn of the material.
Thee relationship between tensile strength and monomer composition contrastss with the findings of Asmussen and co-workers. They observed ann increase of tensile strength with the amount of TEGDMA, while in this
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</> > • • > • • •C C <v v u u c c <D D 3 3 700 wt% TEGDMA -500 wt% TEGDMA' 300 wt% TEGDMA 0.2 2 0.44 0.6 Strainn (%) 0.8 8 1.0 0Figuree 6.7 Tensile stress-strain behavior of the experimental resin composites att one hour setting. Specimens were prepared during dynamic testing, and, therefore,, differs in shrinkage stress history (notice stress value at strain=0 %).
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study,, the opposite relation is observed. Many factors (methodology, specimenn preparation, initiation system, etc.) may contribute for the differencee in observation. The most decisive factor, probably, will be the specimenn preparation, as this affects the structural integrity, and, therefore,, the tensile strength of composites severely. In the study of Asmussenn et ah, specimens of light-activated composites were prepared inn a mold, under (shrinkage) stress free conditions. In this study, specimenss of chemically activated composites were prepared during dynamicc testing. In this test, the composite was loaded with shrinkage stresss (Fig 6.7). It is known that internal stresses stimulate microvoid formationn in the composite structure [36]. For this reason, it may be expectedd that the composites in this study contain microvoids. The higherr the shrinkage stress, the more microvoids may be expected. In addition,, hand mixing of composites introduce porosity, another source off microvoids formation. High amount of microvoids weakens the composite;; i.e., the difference between the real (bulk) value and measuredd value becomes greater. By stretching the composite until fracture,, the microvoids may grow substantially until they reached the criticall size, and initiated the fracture process. The higher the amount of microvoids,, the more the measured value deviates from the bulk material.. This might be the case in our study, while Asmussen and co-workerss measured more likely the bulk property of the composites.
Thiss observation of the tensile strength dependence on the shrinkage stresss is of clinical relevance in dentistry.
Thee reason for the good agreement between the Young's modulus in this studyy with Asmussen and co-workers is that the values for the Young's moduluss were determined from the initial (straight) stress-strain portion off the tension curve. In this region, the elongation of the composites, and, therefore,, the growth of the microvoids was relatively small. As a result, thee measured moduli are representative for the bulk material.
Wear r
Ann increase of TEGDMA in the resin phase results in a higher concentrationn of converted double-bonds in the composite (Table 6.5). Thiss benefits the adhesion between the resin and the filler, because moree converted double-bonds could contribute to bind the resin to the filler,, leading to increased wear resistance. The results of the erosive wearr test (Fig. 6.8) confirms this statement; the higher the TEGDMA concentration,, the lower the wear; i.e., higher wear resistance. The
150 0 125 5 100 0 GG 1 day OO 3 days DD 7 days 1 month 4 months 1.4 years Z100 0 300 wt% TEGDMA composite e 500 wt% TEGDMA composite e 700 wt% TEGDMA composite e
Figuree 6.8 The effect of maturation on occlusal contact-free wear for experimentall two-paste resin composites in comparison with reference (Z100) att 15 % slip and 15 N force (in m/200,000 cycles). Each sample wheel, which accommodatess three specimens of each composite material, was tested once. Alll means (n=3) were significantly different (p<0.05).
experimentall composites show a significant decrease in wear over time. Similarr behavior was observed for the composite Z100, and is commonly seenn in composites [38]. This phenomenon, which last for four months, hass been attributed to internal stress relief [39]; however, a continued conversionn of double-bonds also contributes to the increased wear resistancee [40].
Thee sudden increase of wear after one year is a phenomenon that can be observedd only when wear is measured with a single measurement. Usually,, subsequent measurements should reveal several microns less wearr for the same material. This effect is attributed to surface softening causedd by the prolonged exposure to water, and seems to be related to hydrolysiss and subsequent resin-filler debonding [41].
Thee thickness of the softened layer, however, is much less than the amountt of wear, which occurs clinically in the same time period. This meanss that the rate at which the surface softens is much less than the rate att which it wears off, and, therefore, it is unlikely that this softening has anyy clinical significance.
C o n c l u s i o n s s
Thiss research shows that the relative amounts of bisGMA and TEGDMAA in the resin affect the maximum polymerization rate and mechanicall properties of experimental composites. It was found that, in general,, bisGMA controls the reactivity of the polymerization reaction, whereass TEGDMA controls the mobility of the reacting dimethacrylate systemm and the composite shrinkage. Composites with 50 wt% TEGDMA inn resin displayed the highest polymerization rate and the highest viscoelasticc parameter development during setting. Although large amountss of TEGDMA in the resin resulted in a higher number of convertedd double bonds, it also increased wear and reduced tensile strength.. Substituting bisGMA for TEGDMA prolonged the predominant -- pre-gel - viscous flow property of composites to only a moderate degree.. At a high TEGDMA/bisGMA ratio, the relatively high post-gell shrinkage is the decisive factor in shrinkage stress development. The Maxwelll model is able to predict the viscoelastic behavior of two-paste bisGMA-TEGDMAA composites up to 15-20 minutes into the setting process. .
References s
1.. See chapter 2 of this thesis.
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