Variables affecting stress development and resin conversion in light-cured dental composites

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L U L E Å I U N I V E R S I T Y , J k ^

O F T E C H N O L O G Y

2 0 0 4 : 0 8

DOCTORAL THESIS

Variables Affecting Stress Development and

Resin Conversion in Light-Cured Dental Composites

Nazanin Emarni

Department of Applied Physics and Mechanical Engineering

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Variables Affecting Stress Development and Resin Conversion in Light-Cured Dental Composites

Nazanin Emami

Avd för Polymerteknik

Institutionen för Tillämpad Fysik, Maskin och Materialtekink Luleå Tekniska Universitet

A k a d e m i s k a v h a n d l i n g

Som med vederbörligt tillstånd av Tekniska fakultetsnämnden vid Luleå tekniska universitet för avläggande av teknologie doktorsexamen, kommer att offentligt försvaras i universitets sal E246, fredag den 7 maj 2004, kl 13.00.

Fakultetsopponent: Professor David Watts, University of Manchester Dental School. Manchester, United Kingdom

Doctoral Thesis 2004:08 ISSN: 1402-1544

ISRN: L T U - DT- - 04/08 - - SE

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Variables Affecting Stress Development and Resin Conversion in Light-Cured Dental Composites

Nazanin Emami

O F T E C H N O L O G Y

D i v i s i o n o f P o l y m e r Engineering

Department o f A p p l i e d Physics and Mechanical Engineering L u l e å U n i v e r s i t y o f Technology, L u l e å , Sweden

L u l e å 2 0 0 4

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Cover illustration:

S E M image of spherical glass particle (Paper V ) B y J . G r a h n .

F T - R a m a n spectrograph, calibration curve for dental resin (Paper I I )

Printed in Sweden by the Printing office o f the Luleå University of Technology

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To my Family

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Variables Affecting Stress Development and Resin Conversion in Light-Cured Dental Composites N . Emami

A B S T R A C T

Objective: T h e a i m o f this study was to investigate and i d e n t i f y factors that can be used to m i n i m i z e stress development i n visible l i g h t - c u r e d dental composites ( V L C D C ) w i t h o u t c o m p r o m i s i n g the conversion level o f the p o l y m e r .

Materials and Methods: M o d u l u s o f elasticity, p o l y m e r i z a t i o n contraction strain, degree o f conversion and shrinkage o f light-cure dental composites were determined f o r t w o commercial V L C D composites after c u r i n g w i t h three d i f f e r e n t light p o w e r densities where total irradiated energy ( J / c m²) kept constant. F T - Raman spectroscopy was e m p l o y e d to determine the degree o f conversion. The cure kinetic o f the light-cured resins was studied b y use o f photocalorimetry (photo-DSC). D y n a m i c mechanical thermal analysis ( D M T A ) was used to investigate h o w d i f f e r e n t light c u r i n g methods affected glass transition and tangent delta o f l i g h t curable dental resins when the temperature changed f r o m 0 to 2 0 0 ° C . L i g h t attenuation through V L C D composites was studied. Three d i f f e r e n t f i l l e r types, t w o d i f f e r e n t surface treatments and eight d i f f e r e n t f i l l e r fractions per f i l l e r type and surface treatment were investigated. L i g h t transmission was measured f o r the d i f f e r e n t composite compositions at sample thicknesses o f 1 to 5 m m b y use o f a universal p o w e r meter.

Results: A s l o n g as the total l i g h t energy remained the same, the modulus o f elasticity remained constant f o r each composite, even though the power density d i f f e r e d . Composite thickness, irradiance time, c o m p o s i t i o n o f the V L C D composite and irradiation value had significant impact on degree o f conversion.

The irradiance value d i d not s i g n i f i c a n t l y affect the transition temperature.

Initiator, co-initiators and light irradiance had all s i g n i f i c a n t impact on cure behavior. D i f f e r e n t f i l l e r types and f i l l e r surface treatments had s i g n i f i c a n t effects on light absorption. I n general, l i g h t absorption increased l i n e a r l y w i t h increasing f i l l e r f r a c t i o n and sample thickness o f the cured composites.

Conclusion: L o w e r l i g h t irradiance value decrease stress levels i n V L C D composites and produce comparable conversion levels as l o n g as the total light energy remains the same f o r l o w versus high irradiance. B y increasing the composite thickness above 2 m m but not exceeding 6 m m , energy levels as h i g h as 30 J are needed to achieve acceptable conversion levels. D i f f e r e n t irradiance values do not a f f e c t the f i n a l T g o f tested composites as l o n g as the total light energy remains the same. B y using appropriate p h o t o initiator/co-initiator combination and soft-start c u r i n g , i t is possible to achieve slow c u r i n g and high

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degree o f conversion ( D C ) w i t h i n 40 s. A s expected, d i f f e r e n t f i l l e r particle properties have significant effects on l i g h t absorption d u r i n g c u r i n g , m a k i n g it important to consider these differences when one tries to develop a general light c u r i n g strategy f o r V L C D composites.

Key words: Dental composites; light-cure, irradiance; elastic modulus; shrinkage;

Raman spectroscopy; depth o f cure; glass transition; D M T A ; photo-calorimetry;

L E D ; l i g h t absorption; laser; optical properties, light attenuation.

Correspondence to: Nazanin E m a m i , L u l e å U n i v e r s i t y o f T e c h n o l o g y , SE-971 87 L u l e å , Sweden. Fax: +46-920-491084.

E - m a i l : nazanin.emami@sirius.luth.se 2004:08 • I S S N : 1402-1544

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Variables Aftecting Stress Development and Resin Conversion in Light-Cured Dental Composites N . Emami

C O N T E N T S

P R E F A C E 11

A B B R E V I A T I O N S 12

I N T R O D U C T I O N 13 - Power density (or irradiance) o f light source 14

- Variation among curing units and light activated

composites 15 - Degree of conversion 16

- The effect of filler and silane content on

polymerization of resin-based composites 18

- Polymerization shrinkage 19 - Methods for measuring polymerization shrinkage... .20

- Clinical significance of conversion level and

polymerization contraction stress induction 20

A I M S 23

M A T E R I A L S & M E T H O D S 24 - Paper 1 24 - Paper I I 25 - Paper III 26 - Paper IV 27 - Paper V. 27

R E S U L T S 30 - Paper 1 30

- Paper 11 31 - Paper III 32 - Paper IV 33 - Paper V 35

D I S C U S S I O N 37

C O N C L U S I O N R E M A R K S 42

A C K N O W L E D G M E N T S 44

R E F E R E N C E S 45

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P R E F A C E

This dissertation is based on the f o l l o w i n g papers, w h i c h w i l l be referred to by their Roman numerals:

I . E f f e c t o f light power density variations on b u l k curing properties o f dental composites*

I I . H o w light irradiance and c u r i n g time a f f e c t monomer conversion i n light- cured resin composites*

I I I . D y n a m i c mechanical thermal analysis o f t w o light-cured dental composites

I V . Influence o f light-curing procedures and p h o t o - i n i t i a t o r / c o - i n i t i a t o r composition on the degree o f conversion o f l i g h t - c u r i n g resins

V . H o w f i l l e r properties, f i l l e r f r a c t i o n , sample thickness and l i g h t source affect light attenuation in particulate f i l l e d resin composites

*Paper I and II are reproduced with kind permission of Journal of Dentistry and European Journal of Oral Sciences respectively.

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A B B R E V I A T I O N S

V L C D C V i s i b l e light-cured dental composites G L M General L i n e a r M o d e l

Materials:

b i s G M A ( 2 , 2 - b i s [ 4 - ( 2 - h y d r o x y - 3 - m e t h a c r y l y l o x y p r o p o x y ) p h e n y l ] propane)

T E G D M A T r i e t h y l e n e g l y c o l dimethacrylate)

U E D M A ( N , N - b i s [ 4 - ( 3 - m e t h a c r o y l o x y e t h o x c a r b o n y l ] - l , 6 - d i a m i n o - 2 , 4 , 4 - trimethylhexane))

b i s E M A ö (bisphenol A polyethylene g l y c o l diether dimethacrylate) C Q Camphorquinone

P P D 1 -phenyl-1,2-propanedione

D M A E M A 2 - d i m e t h y l a m i n o e t h y l methacrylate C E M A N , N - c y a n o e t h y l m e t h y l a n i l i n e D A B E N , N - d i m e t h y l - p - a m i n o b e n z o i c acid ethylester

L i g h t socurce:

L E D L i g h t E m i t t i n g D i o d e

Units:

m W / c m ' Power density i n m i l l i w a t t s per square centimeter J/cm" Energy i n Joules per square centimetre

M P a Mega Pascals

Equipments and measurements:

D C Degree o f Conversion

F T I R Fourier T r a n s f o r m I n f r a r e d Spectroscopy FT-Raman Fourier T r a n s f o r m Raman Spectroscopy A T R A T R (Attenuated total r e f l e c t i o n ) D M T A D y n a m i c Mechanical T h e r m a l Analysis D S C D i f f e r e n t i a l Scanning C a l o r i m e t r y E ' Storage M o d u l u s

E " Loss M o d u l u s

T g Glass transition Temperature

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I N T R O D U C T I O N

V i s i b l e light-cured dental composites (particulate f i l l e d c e r a m i c - p o l y m e r composites) were introduced i n the 1970s. These materials and dentin adhesives have had a m a j o r impact on the acceptance o f composites as general purpose restorative materials [ 1 ] . H o w e v e r , compared to amalgam placement, visible light- cured composites are more technique sensitive. It also takes longer time to place a composite than a similar amalgam. The reason that composites are more time consuming relates to several factors. For example, to o p t i m i z e the success w i t h composite placement, the operation f i e l d should be isolated w i t h rubber dam. The dentin adhesive needs to be handled properly according to the protocols developed f o r the particular adhesive. Because o f l i m i t e d depth o f cure w i t h the light, composite increments need to be placed and each increment needs to be cured separately. A f t e r c o m p l e t i o n , most cured composites need to be adjusted to proper contour/occlusion, and then finished and polished, steps that add c o m p l e x i t y and time to the restorative procedure [ 2 ] .

R e d u c i n g the time needed to place a light-cured dental composite has been a m a j o r goal. Some l i g h t - c u r i n g technologies c l a i m that s i g n i f i c a n t l y shorter cure times can be used w i t h t h e m than w i t h traditional methods [ 2 ] . O f interest are plasma arc lamps w i t h p o w e r output levels up to f o u r times higher than that o f typical tungsten-halogen lamps [ 3 ] . A c c o r d i n g to other researchers w i t h these p o w e r f u l lights it is possible to cure a composite in 1/10 o f the time recommended by composite manufacturers [ 4 ] . Lasers and plasma arc l i g h t have also been utilized as l i g h t sources w h i c h are capable o f cutting the c u r i n g time [ 3 , 5 ] . D u r i n g the last f e w years, curing lamps based on l i g h t - e m i t t i n g diodes ( L E D ) [6,7] have been introduced. The p o l y m e r i z a t i o n e f f i c i e n c y o f both lasers and L E D : s is greater than that o f traditional tungsten-halogen lamps because o f the narrow wavelength range o f lasers and L E D : s i n the range o f the absorption peak o f the photo-initiator [ 8 ] . A n o t h e r strategy b e i n g suggested to decrease total c u r i n g t i m e has been to increase the thickness o f the increments that are placed and cured [9,10]. The advantage o f such a strategy remains to be f u l l y substantiated i n light o f potential negative consequences arising f o r m i n s u f f i c i e n t cure.

Contrary to the attempts to shorten cure t i m e , other studies have recommended that slower c u r i n g rates f a v o r a b l y moderate p o l y m e r i z a t i o n stresses [11-15] and may help i m p r o v i n g the marginal integrity o f a bonded composite [16- 2 2 ] . T o support o f these suggestions, stepwise and ramped c u r i n g lights have been

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developed. These lights are capable o f i m p r o v i n g the p o l y m e r i z a t i o n , either b y increasing the irradiance gradually during l i g h t - c u r i n g [ 2 2 ] , or b y delivering a short l o w irradiance pulse f o l l o w e d by a delay before the f i n a l h i g h irradiance stage i.e., pulse activation [ 2 3 ] . I n addition to l i g h t source, the through-the-tooth and three-sited c u r i n g techniques have been proposed to decrease p o l y m e r i z a t i o n rate [ 2 4 , 2 5 ] .

Researchers w h o intend to explore the relationship between rate o f p o l y m e r i z a t i o n reaction and stress development must also consider the impact o f c u r i n g rate and light penetration on c u r i n g o f the adhesive and the adhesive/composite interface. Thus, light penetration, or l i g h t attenuation, is an important factor to consider when light-curing o f dental composites is discussed.

P o w e r density of the light source

I m p r o v e d understanding o f the parameters a f f e c t i n g l i g h t - c u r i n g , w i l l help us to i d e n t i f y and use the potential benefits associated w i t h the above claims. A s a f i r s t step i n such a process t o w a r d better understanding, we need to study the e f f e c t o f the l i g h t source itself. The important parameters that should be considered include l i g h t p o w e r density, wavelength d i s t r i b u t i o n and light divergency o f the l i g h t r o d . A p r i m a r y parameter to consider is the amount o f l i g h t energy the l i g h t source generates via the light rod.

N u m e r o u s studies have tried to i d e n t i f y the best l i g h t sources, but remarkably little has been done to determine the amount o f l i g h t energy that participate i n the activation process o f both dental composites and bonding agents. D i f f e r e n t manufacturers recommend d i f f e r e n t amounts o f l i g h t energy needed f o r optimal c u r i n g . F o r example, the manufacturer o f the O p t i l u x c u r i n g unit ( D e m e t r o n / K e r r ) recommends that a power density greater than 300 m W / c m² should be s u f f i c i e n t to p o l y m e r i z e 3 m m t h i c k composite layers as l o n g as the c u r i n g time recommended by the manufacturer o f the composite is used [ 2 6 ] . The recommended irradiance time o f composites is usually 40 seconds, w h i c h w o u l d then mean that an energy amount o f about 12 J/cm² should be s u f f i c i e n t f o r c u r i n g the 3 m m t h i c k composite layer. Cunningham et a l , (1990) and Lee et al., (1994) have reported that compressive strengths values o f three composites were adversely a f f e c t e d when they received an energy density less than 12 J / c m [27,28]. Other

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investigators have f o u n d that an energy value o f 21 to 24 J / c m2 is needed to cure a 2 m m t h i c k composite layer [ 2 9 - 3 2 ] . A n important f a c t o r that also needs to be considered is that some studies have shown that many o f the curing units used i n dental c l i n i c s do not p r o v i d e adequate energy outputs [ 3 3 - 3 5 ] . This can be due to several factors such as; fluctuations i n line voltage, age o f the bulb, deterioration o f the r e f l e c t o r or filter, light guide wear or contamination, effect o f d i s i n f e c t i o n procedures on light transmission through the light guide, or m a l f u n c t i o n o f photoconductive fibers in the light guide [ 3 6 - 3 8 ] . Such f i n d i n g s may i m p l y that i n s u f f i c i e n t power density may have resulted i n poorly p o l y m e r i z e d light-cured composites. Poor l i g h t - c u r i n g w o u l d explain the poor results when compared to the c h e m i c a l l y activated composites [ 3 9 ] .

The easiest way to measure the output o f the c u r i n g l i g h t i n a dental clinic is to use a radiometer capable o f reading the irradiance value generated by the c u r i n g light [37,40,41]. Regular checks o f the l i g h t curing unit are needed to detect changes i n the irradiance values w h i c h should not be less than 400 m W / c m² [27,29,34]. U n f o r t u n a t e l y , even though such radiometers are used one should be aware that they are not as accurate as laboratory grade p o w e r meters [ 4 2 ] .

V a r i a t i o n among c u r i n g units a n d light activated composites

Photo-polymerization mechanism should be considered very c a r e f u l l y when the behaviors o f d i f f e r e n t non-traditional c u r i n g methods are studied. Such non- traditional c u r i n g methods include both high irradiance sources, e.g., plasma arc and laser, and more lenient c u r i n g techniques, e.g., soft c u r i n g and r a m p i n g . W i t h both c u r i n g techniques decreased p o l y m e r i z a t i o n stress is claimed. I n order to save t i m e d u r i n g the placement o f a restoration w i t h a high irradiance c u r i n g method, the use o f thicker composite layers is recommended. I n all o f these instances, the extent o f conversion is d e t e r m i n i n g factor because degree o f conversion affects the mechanical properties o f the cured composite [ 4 3 ] .

I n the literature factors that a f f e c t the amount o f l i g h t received by V L C D composites have been discussed. The design and size o f the l i g h t guide, distance o f the l i g h t guide f r o m the composite, p o w e r density, exposure duration, shade o f the composite, increment thickness, material composition and percentage o f f i l l e r loading, a l l affect the amount o f l i g h t energy received at the top and the bottom o f a composite restoration [36,37,44-50]. The energy delivered by the l i g h t - c u r i n g

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source can be approximated to be the product o f power density and l i g h t - c u r i n g time [ 3 8 ] . I f the restoration receives the same total energy, the fracture toughness and f l e x u r a l strength o f a resin composite may not d i f f e r despite differences i n curing time. H o w e v e r , the e f f e c t o f high power densities, e.g. l i g h t generated by plasma arc ( P A C ) light, is not k n o w n [ 5 1 ] . I f the composite restoration does not receive a s u f f i c i e n t amount o f energy, the degree o f conversion o f the m a t r i x w i l l decrease [29,45,49,52,53]. Such a decrease i n conversion level w i l l increase the c y t o t o x i c i t y o f the m a t r i x [ 5 4 ] , reduce its ultimate hardness [30,33,37,45,46,55], decrease its d y n a m i c elastic modulus [ 5 0 ] , and increase wear and marginal breakdown [ 5 6 ] . Incomplete c u r i n g may also result i n a weak b o n d at the tooth/adhesive/composite transition [28,38].

A m a j o r l i m i t a t i o n w i t h many studies where physical properties and polymerization shrinkage o f cured composites have been studied is that investigators often used d i f f e r e n t light sources. Each o f these l i g h t sources may produce d i f f e r e n t irradiance values [20,57]. However, i n most cases the same curing t i m e (40 s) has been used. A s a result, i d e n t i f i e d differences i n properties among tested materials may not o n l y be due to differences between materials, but also due to differences i n l i g h t energy values [ 3 8 ] .

Degree of conversion

Degree o f conversion i n dental composites can be measured w i t h several d i f f e r e n t methods. The percentage o f aliphatic carbon double bonds reacting d u r i n g p o l y m e r i z a t i o n o f the matrix resins can be used to express the degree o f conversion. Some o f the most important methods that have been used are; nuclear magnetic resonance spectroscopy [58,59], differential scanning c a l o r i m e t r y [ 6 0 - 6 9 ] , Raman spectroscopy [11,32], conventional i n f r a r e d spectroscopy [70-74] and f o u r i e r transform i n f r a r e d spectroscopy ( F T I R ) [75-80].

W h e n spectroscopic methods are used to investigate the cure o f methacrylate based resins, the most c o m m o n l y utilized absorption peak is the aliphatic carbon peak at 1638 cm" . D u r i n g p o l y m e r i z a t i o n , this absorption peak decreases as the aliphatic bonds react. C a l c u l a t i o n o f the conversion value can be done by comparing relative peak changes and h o w these changes occur over t i m e . C o m m o n l y used peaks f o r conversion determinations are the aliphatic/aromatic

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peaks present i n many dental composites. Because the aromatic peak does not react d u r i n g c u r i n g , it is used as an internal standard.

Photocalorimetry (photo-DSC) is based on the isothermal D S C m e t h o d and is used f o r d e t e r m i n i n g the amount o f heat released d u r i n g p o l y m e r i z a t i o n . Some o f the heat recorded d u r i n g photo-DSC measurement is heat released by the l i g h t source. The heat generated b y the light source is subtracted f r o m the total heat being released d u r i n g c u r i n g , and the resulting heat represents the amount o f heat released d u r i n g c u r i n g . B y using the energy value released by one m o l e aliphatic bonds d u r i n g c u r i n g , one can calculate how many percentages o f the aliphatic bonds have participated i n the polymerization reaction.

V L C D composites typically show l i m i t e d conversion values. The reason is that the cross-linked network p o l y m e r structure imposes severe restrictions on the m o b i l i t y o f reacting species; conversion values ranging between 4 0 to 80 percent are generally reported after i n f r a r e d spectroscopy and p h o t o - D S C determinations [32,73,60,74,78,81]. T o i m p r o v e conversion, diluents w i t h viscosities up to f i v e orders o f magnitude l o w e r than that o f a resin l i k e b i s G M A are used. These diluents are added to facilitate f i l l e r addition and m i x i n g . B o t h the i m p r o v e d conversion level and the increased f i l l e r f r a c t i o n contribute to i m p r o v i n g the mechanical properties o f the composite. The increased molecular m o b i l i t y caused by the incorporation o f diluents can increase the conversion level 2.5 times over that o f pure b i s G M A ( f r o m 2 6 % f o r pure b i s G M A to 6 6 % f o r 50/50 b i s G M A / T E G D M A ) [ 8 2 ] . Structural differences w i t h i n the diluents contribute to variations i n conversion because o f differences i n segmental m o t i o n o f the methacrylate groups [ 8 3 ] . However, differences i n properties o f d i f f e r e n t methacrylate based matrices complicate the interpretation o f correlations between conversion and mechanical properties o f V L C D composites. F o r example, b y increasing the concentration o f T E G D M A m i x e d w i t h b i s G M A , the measured f l e x u r a l strength decreases despite o f an expected positive e f f e c t on conversion [44,84].

The c u r i n g a b i l i t y o f V L C D composites depends not o n l y on m a t r i x composition but also on l i g h t exposure value, spectral d i s t r i b u t i o n generated by the light-source, presence o f oxygen, and the c o m b i n e d e f f e c t o f used photo initiator/co-initiator system. Therefore, proper combinations o f l i g h t sources, c u r i n g times and photo initiator/co-initiator systems are factors to consider when one tries to optimise l i g h t - c u r i n g o f V L C D composites.

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T h e effect of filler and silane content on polymerization of resin-based composite

F r o m the above we can conclude that composites w i t h higher conversion values w i l l not necessarily e x h i b i t superior mechanical properties. A c o m p l i c a t i n g factor in conversion determinations is the presence o f silane in a dental composite.

The silane is used as a c o u p l i n g agent between the inorganic f i l l e r surface and the resin. The silane also improves the h y d r o l y t i c stability o f the filler-silane interface

[ 8 5 ] . The o r g a n o f i l i c y - m e t h a c r y l o x y p r o p y l t r i m e t h o x y s i l a n e ( y - M P S ) , is extensively used i n c o m m e r c i a l l y available V L C D composites [ 8 6 ] . F o r m a t i o n o f a siloxane bond between filler and the c o u p l i n g agent contribute to i m p r o v e d stability o f the composite, w h i c h is achieved by treating the f i l l e r w i t h y - M P S or w i t h other silane c o u p l i n g agents [87-89]. The silanol end o f the silane molecule reacts w i t h the f i l l e r surface, w h i l e the methacrylate group o f the silane can react w i t h the m a t r i x resin. The f u n c t i o n a l i z e d f i l l e r particles w i t h their methacrylate groups can therefore also contribute to the absorbance peak seen at 1638 c m¹. That peak represents the vibrational mode o f that bond, and the peak is detectable by use o f i n f r a r e d spectroscopy [70,71,73,75,80]. Nevertheless, covalent bonding m i g h t occur between o r g a n o f u n c t i o n a l group o f the silane and reactive groups o f the resin m a t r i x [90,91]. The later reactivity depends not only on the chemical nature o f the reactants, but also on the spatial arrangement o f silanol on the f i l l e r .

There is no doubt that silane treatment o f the filler particles improves the interface strength between f i l l e r particles and resin m a t r i x . I f f i l l e r is not surface treated, stresses w i t h i n the flexible p o l y m e r m a t r i x cause debonding between f i l l e r and m a t r i x , and fractures may be initiated [92,93). Furthermore, the c o u p l i n g agent w i l l reduce the f i l l e r degradation due to hydrolysis [94,95,96], something that w i l l affect the stability o f the composite.

Under certain conditions, the silane methacrylate groups may not react to any greater extent and their contribution to the aliphatic absorbance peak may be great enough to a f f e c t the calculated m a t r i x conversion value [ 2 ] . The i n a b i l i t y o f the instrument to distinguish between o f silane methacrylate and matrix methacrylate makes it impossible to precisely determine the conversion o f the m a t r i x o n l y . A s the f i l l e r f r a c t i o n increases, the amount o f silane increases. However, the silane amount is not directly determined by v o l u m e f r a c t i o n f i l l e r , but rather by filler surface area. F i l l e r surface area is i n turn determined b y particle size, particle size distribution, particle shape and f i l l e r v o l u m e f r a c t i o n , and a l l these variables a f f e c t the mechanical properties o f the composite.

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Another, and probably more i m p o r t a n t e f f e c t that silane m i g h t have on the conversion level, is h o w silane affect l i g h t penetration and depth-of-cure o f V L C D composites. L i g h t penetration and l i g h t scattering are affected b y differences i n refractive indices between silane, f i l l e r particles and resin m a t r i x .

Polymerization shrinkage

V L C D composites shrink d u r i n g p o l y m e r i z a t i o n . The magnitude o f that shrinkage is related to the number o f aliphatic bonds per v o l u m e unit that reacts.

The more the number o f reacting bonds is, the more the material w i l l shrink.

Consequently, p o l y m e r i z a t i o n contraction depends on both m o n o m e r molecular size and the degree o f conversion. A n o t h e r important f a c t o r is the v o l u m e o f inert f i l l e r particles that has been added to the curable m a t r i x monomer. A monomer molecule such as b i s G M A is a relatively b i g , and its s t i f f structure and a b i l i t y to f o r m hydrogen bonds between adjacent molecules make the b i s G M A very viscous.

The h i g h viscosity o f b i s G M A makes i t d i f f i c u l t to add large f i l l e r fractions to that monomer. Because f i l l e r is added to decrease p o l y m e r i z a t i o n shrinkage o f a composite, T E G D M A w i t h its l o w e r m o l e c u l a r w e i g h t and l o w e r viscosity is added to facilitate f i l l e r incorporation. H o w e v e r , b y increasing the T E G D M A v o l u m e , conversion level and p o l y m e r i z a t i o n shrinkage increase. That shrinkage, though, decreases as the f i l l e r content increases and as the f i l l e r l o a d i n g increases, the mechanical properties improve. H o w e v e r , as the f i l l e r l o a d i n g increases, the viscosity o f the composite paste increases too, something that can make composite adaptation to cavity walls more d i f f i c u l t . Therefore, the characteristic properties f o r c o m m e r c i a l l y available V L C D composites are to a great extent determined by f i l l e r particle size, f i l l e r size d i s t r i b u t i o n and f i l l e r f r a c t i o n . Because the c o m p o s i t i o n o f the composite w i l l determine the physical properties o f the composite, the clinicians should consider composite c o m p o s i t i o n when they choose a product f o r a certain application.

F r o m the above discussion it can be concluded that there are several variables a f f e c t i n g p o l y m e r i z a t i o n contraction o f V L C D composite. I n a d d i t i o n to m o n o m e r c o m p o s i t i o n and f i l l e r fraction some other variables such as size o f restoration, cavity c o n f i g u r a t i o n , placement technique (incremental or b u l k ) and c u r i n g procedure must be considered [ 9 7 ] .

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The shrinkage o f a composite can be reduced by considering how d i f f e r e n t variables a f f e c t the shrinkage o f the dental composite. For example: a) U s i n g h i g h molecular weight (oligomers and prepolymers) resins results i n reducing the number o f reactive double bonds, which w i l l reduce shrinkage and heat generation caused by the p o l y m e r i z a t i o n process, b) Use a photo-curing protocol under o p t i m a l conditions. Such a p r o t o c o l can control initiation and propagation rates o f the p o l y m e r i z a t i o n process. C ) H i g h f i l l e r loading reduces the amount o f resin, w h i c h i n turn decreases the overall shrinkage.

Methods of measuring polymerization shrinkage

D i f f e r e n t methods have been used to measure p o l y m e r i z a t i o n shrinkage o f dental composites. D i l a t o m e t r y has been one o f the most popular methods [98-

100], H o w e v e r , i n some studies linear dimensional changes i n one direction have been measured [57,101-104]. Some investigators have used the bonded-disc methods f o r measuring shrinkage-strain kinetics [103,105]. Others have used a m o d i f i e d version o f the A m e r i c a n Society f o r Testing and Materials methods D 7 9 2 "'Specific G r a v i t y and Density o f Plastics by Displacement"' [ 1 0 6 ] .

C l i n i c a l significance of conversion level and polymerization contraction stress induction.

M a n y o f the cure-dependent properties that are expected to a f f e c t c l i n i c a l performance o f V L C D composites have been studied i n v i t r o . Such properties include hardness [79, 107,108], solubility [79,109], wear resistance [110], and fracture toughness [ 1 1 1 ] . Despite years o f studies, w e s t i l l d o n ' t k n o w to w h i c h extent we need to cure a dental composite i n order to make a l o n g lasting restoration. F r o m an ethical p o i n t o f view, i t is d i f f i c u l t to j u s t i f y such an i n v i v o study, w h i c h partly explains w h y such studies are l a c k i n g [ 2 ] .

Nevertheless, based on the prevalence o f u n d e r - p e r f o r m i n g c u r i n g units i n dental clinics [112-114], one should expect that incomplete p o l y m e r i z a t i o n is a c o n t r i b u t i n g factor causing restoration failures. T h i s possibility, at least i n part, may explain the higher f a i l u r e rate o f light-cured composite restorations evaluated in cross-sectional studies w h e n compared to c o n t r o l l e d , longitudinal i n v i v o

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investigations [125]. I t is important, though, to realize that such comparisons are questionable, because they rely on i n f o r m a t i o n f r o m d i f f e r e n t studies carried out under d i f f e r e n t conditions and that the impact o f operator on the f i n a l result is a significant f a c t o r to consider. I t has been shown that at l o w irradiance power i.e., less than 300 m W / c m², restoration performance is questionable [ 2 6 ] . A t intermediate levels o f light output, i.e., 300 to 600 m W / c m², the impact o f the irradiance on the restoration's performance is even more unclear. Variables that need to be considered include material properties and h o w l i g h t intensity affects p o l y m e r i z a t i o n contraction. For example, f o r both u n f i l l e d and f i l l e d resins, direct relationships exist between linear contraction and degree o f conversion. A reduction i n conversion is accomplished by a p r o p o r t i o n a l reduction i n contraction [11,116,115]. M a n y researchers have reported that stresses developing in bonded V L C D composite are closely related to the conversion o f the double bonds [115,117,118]. Therefore, i f the matrixes d o n ' t reach h i g h , i.e.. optimal, degree o f conversion because o f l o w l i g h t power, the residual stresses should decrease. I n a bonded restoration, reduced contraction w i l l decrease the stress level at the composite-tooth interface. I t is w o r t h m e n t i o n i n g that by using intermediate irradiance longer c u r i n g times are required to a v o i d l o w e r f i n a l conversion.

Nevertheless, significant differences have been reported even at the occlusal surface o f restorations that have been cured d i f f e r e n t l y . A t the occlusal surface, m a x i m u m cure can be obtained w i t h relatively modest exposures [ 1 1 , 2 9 ] . H o w e v e r measurable differences i n c l i n i c a l wear have been observed f o r relatively small reductions i n the m a x i m a l conversion levels [ 1 1 1 ] .

Due to lack o f well-conducted c l i n i c a l studies targeting the effect o f l i g h t - curing, the clinical l i g h t - c u r i n g protocols that are used are based on laboratory studies. Hardness measurements have given us considerable i n f o r m a t i o n about the cure p r o f i l e throughout l i g h t cured composites [29,45,107,121,122], Investigators have also used hardness measurements to i d e n t i f y suitable exposure conditions f o r various l i g h t activated composites. These investigations have revealed that parameters a f f e c t i n g the hardness o f cured composites include the irradiance value o f the c u r i n g unit, and exposure time and l i g h t attenuating material properties e.g., f i l l e r type and material shade. M a n y studies have tried to i d e n t i f y a m i n i m a l l y acceptable output o f the c u r i n g l i g h t and a m i n i m a l c u r i n g t i m e . Acceptable cure values are achieved w i t h as l o w irradiance values as 185 to 400 m W / c m² when the composite thickness ranges f r o m 1 to 2 m m and c u r i n g times up to 60 s are used [29,32,119,120]. H o w e v e r , the broad ranges i n light transmittance among d i f f e r e n t

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composite brands make such values potentially c o n f u s i n g f o r c l i n i c i a n . The fact that commercial radiometers available on the market can d i f f e r as much as 19%

has not s i m p l i f i e d this issue [41 ] .

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A I M S

There is no doubt that more knowledge is needed to understand h o w d i f f e r e n t variables a f f e c t light-cured dental composites. Therefore, the intention w i t h this study was to focus on the effects o f light and c u r i n g times on dental composites, h o w d i f f e r e n t c o m b i n a t i o n o f photo and co-initiators and c u r i n g methods a f f e c t conversion, and h o w d i f f e r e n t optical variables affected l i g h t absorption i n dental composites. The f o l l o w i n g aims were investigated w i t h i n this study.

1. The first a i m was to study the e f f e c t o f d i f f e r e n t l i g h t power density (light irradiance) on degree o f conversion, modulus o f elasticity, p o l y m e r i z a t i o n shrinkage and linear contraction (Paper I )

2. The second a i m was to determine the e f f e c t o f variations i n resin m a t r i x composition, c u r i n g time and irradiance value on depth o f cure (Paper I I )

3. The third a i m was to compare the viscoelastic behavior and dynamic mechanical properties o f light cured composites under temperature scanning (Paper I I I )

4. The f o u r t h a i m was to assess the e f f e c t o f d i f f e r e n t c o m b i n a t i o n o f photo and co-initiators i n relation to d i f f e r e n t c u r i n g methods on rate o f p o l y m e r i z a t i o n and f i n a l degree o f conversion when the c u r i n g t i m e was kept constant (Paper I V )

5. The f i f t h a i m was to establish a mathematical model f o r optical properties o f l i g h t cured composites w i t h respect to the type o f f i l l e r particles, f i l l e r surface treatment, composite thickness and the type o f l i g h t (Paper V )

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M A T E R I A L S A N D M E T H O D S

P a p e r I : E f f e c t of light power density variations on b u l k c u r i n g properties of dental composites

Properties such as contraction strain, heat generation, v o l u m e t r i c shrinkage, degree o f p o l y m e r i z a t i o n and Y o u n g ' s modulus were measured and compared f o r t w o c o m m e r c i a l l y available V L C D composites. D i f f e r e n t l i g h t power densities and d i f f e r e n t curing times were used.

Materials and Methods

Dental composites (Z100 and Z 2 5 0 , 3 M ESPE) were investigated.

Specimens were cured w i t h l i g h t intensities o f 200, 450 and 800 m W / c m² f o r 140, 60 and 35 s f r o m a distance o f 7 m m . Strain-gauges were used f o r contraction strain measurements (Figure 1). FT-Raman (Fourier T r a n s f o r m Raman spectroscopy) was used to determine the degree o f conversion ( D C % ) at the top and the bottom o f 4 m m t h i c k specimens. V o l u m e t r i c p o l y m e r i z a t i o n shrinkage was determined using a water displacement method. E-modulus was determined i n tension on composite specimens.

Light source

Strain-gauge

Thermocouple Metallic mould

Figure 1: Strain-gauge attached to the side of the metallic mould in contraction strain measurement

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P a p e r I I : How light i r r a d i a n c e a n d c u r i n g time affect monomer conversion in light-cured resin composites

L i g h t energy value per unit area, calculated by m u l t i p l y i n g the irradiance value w i t h the c u r i n g time [ 3 8 ] , was used to determine how l i g h t c u r i n g should be conducted to optimize conversion. T o test whether that assumption was correct FT-Raman spectroscopy was used to determine the degree o f conversion o f t w o d i f f e r e n t composites. These t w o composites were cured w i t h d i f f e r e n t l i g h t irradiance levels f o r d i f f e r e n t time intervals.

Calibration curve: A calibration curve was developed to relate the aliphatic:aromatic peak ratio to the degree o f conversion. This was done before the degree o f conversion o f the t w o composites c o u l d be determined. The calibration curve was developed by m i x i n g 50 w t - % b i s G M A and 50 w t - % T E G D M A . That mixture was then hydrogenated according to a method described by Rueggeberg et al., [ 8 0 ] . Hydrogenation set up is shown i n Figure 2. The peak heights o f the standard solutions were measured f o r the aliphatic and aromatic C = C at 1639 cm"' at 1608 cm"¹ respectively. The aliphatic:aromatic peak ratios were then plotted as a f u n c t i o n o f k n o w n concentration o f hydrogenated carbon double bonds.

Figure 2: Hydrogenation process (2a) and Evaporation of the solvent (2b)

Degree of conversion: T w o c o m m e r c i a l dental composites, Z 1 0 0 and Z 2 5 0 ( 3 M ESPE, St Paul, M N , U S A ) and both o f shade A 3 , were selected. These

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composites were cured w i t h three d i f f e r e n t light irradiance values over d i f f e r e n t curing times. The composite specimens were light-cured w i t h d i f f e r e n t irradiation settings (200, 450 and 800 m W / c m²) during d i f f e r e n t time intervals (5, 10, 20, 40, 60 and 140 s). The tested specimens were 2, 4 or 6 m m thick, and the degree o f conversion values were measured after 1 m i n . w i t h Raman spectroscopy on the top and the bottom surfaces o f the specimens

P a p e r H I : D y n a m i c m e c h a n i c a l thermal analysis of two light-cured dental composites

Clinical observations suggest that some composite resins are more o f t e n linked to post-operative sensitivity than others. These differences may relate to differences i n modulus o f elasticity and polymerisation rates among materials.

Because o f the need f o r better understanding the molecular behaviour o f V L C D composites, we used D M T A to study t w o c o m m o n l y used posterior composites.

Materials and Method

T w o composites ( Z 1 0 0 and Z 2 5 0 by 3 M ESPE) were evaluated. Six specimens per composite and irradiance value (250, 500 and 850 m W / c m2) were made. The curing times were chosen to produce a f i x e d energy value o f 30 J / c m² independent o f irradiation value. D y n a m i c mechanical thermal analysis ( D M T A ) was performed. B e n d i n g loads were applied at frequencies o f 1, 3 and 10 H z i n single cantilever clamped mode w i t h i n the 0 ° C to 2 0 0 ° C temperature range at a dynamic scan rate o f 2 ° C / m i n .

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P a p e r I V : I n f l u e n c e of light-curing procedures a n d photo-initiator/co- initiator composition on the degree of conversion of light-curing resins

Proper c o m b i n a t i o n o f initiators/co-initiators cured w i t h suitable c u r i n g protocols can be used to i m p r o v e the degree o f conversion f o r light-curable resin composites w i t h favourable rate o f p o l y m e r i z a t i o n . D S C was used to determine the degree o f conversion f o r d i f f e r e n t c o m b i n a t i o n o f initiator/co-initiators. Three d i f f e r e n t c u r i n g methods were tested f o r each c o m b i n a t i o n . D u r i n g these evaluations, the o p t i m a l initiator/co-initiator system and c u r i n g protocols were i d e n t i f i e d . The reaction rates o f the d i f f e r e n t initiator/co-initiator combinations, light c u r i n g processes and their D C % values were determined.

E x p e r i m e n t a l resins [ b i s G M A : T E G D M A (50:50 by w e i g h t ) ] containing 0.02 m M photo initiator per gram resin [either camphorquinone ( C Q ) or 1-phenyl-1,2- propanedione (PPD)] and a co-initiator (0.04 m M / g resin) [either N , N - d i m e t h y l - p - aminobenzoic acid ethylester ( D A B E ) , N , N - c y a n o e t h y l m e t h y l a n i l i n e ( C E M A ) , or 2 - d i m e t h y l a m i n o e t h y l methacrylate ( D M A E M A ) ] were prepared. These six combinations were subjected to three c u r i n g conditions, standard c u r i n g , soft-start curing or L E D c u r i n g . The degree o f conversion were determined w i t h d i f f e r e n t i a l scanning calorimetry (Photo-DSC). The irradiation time f o r a l l d i f f e r e n t c u r i n g procedures was 4 0 s.

P a p e r V : How f i l l e r properties, filler fraction, sample thickness and light source affect light attenuation in particulate filled resin composites

One approach to enhance our understanding o f l i g h t absorption in dental composites is to e m p l o y Beer-Lambert's l a w . A c c o r d i n g to that l a w , successive absorptions occur i n the path o f a beam o f monochromatic radiation, and the transmitted optical p o w e r P measured by a detector vary as [123]

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P = P^a(l-R^F)exp{-ad) (1)

where P0 is the i n i t i a l optical power, RF is the total Fresnel reflectance c o e f f i c i e n t , a is the attenuation c o e f f i c i e n t and d the thickness o f the sample. T h r o u g h Beer- L a m b e r t ' s law we could develop the f o l l o w i n g equation,

In -{aa + a'a +as )d + (a'a- a')Vfd - a , (2)

where a = - l n ( l - R^F).

T o s i m p l i f y , the above equation can be rewritten as,

Z = A + B d + C d V (3) Based o n the above relationship, we hypothesized that by standardizing variables such as l i g h t sources, f i l l e r types and f i l l e r surface treatments, it should be possible to use Equation 3 to predict light absorption i n V L C D C o f d i f f e r e n t f i l l e r fractions and sample thickness values. A c c o r d i n g l y , the objective w i t h this study was to test whether Equation 3 is good mathematical model that can be used to predict l i g h t absorption i n V L C D C .

The objective w i t h this study was to determine h o w variables such as f i l l e r type, f i l l e r silanization, f i l l e r f r a c t i o n , specimen thickness and light source w o u l d a f f e c t l i g h t absorption in w e l l characterized experimental dental composites. B o t h l i g h t irradiance and attenuating power o f the material i n f l u e n c e depth o f cure [ 1 1 ] . D i f f e r e n t commercial visible light cure dental composites use d i f f e r e n t matrices and f i l l e r particles. Therefore it was important to c l a r i f y h o w the optical properties o f these V L C D C a f f e c t the depth o f cure to assess their c l i n i c a l usefulness.

Composition of the experimental materials: The resin system used i n this study consisted o f a m i x t u r e o f 50 w t % b i s G M A and 50 w t % T E G D M A to w h i c h a p h o t o - i n i t i a t o r (0.35 w t % champhorquinone [ C Q ] ) and a co-initiator (0.7 w t % o f dimethylaminoethylmethacrylate [ D M A E M A ] ) had been added. D i f f e r e n t amounts o f d i f f e r e n t f i l l e r types were added to that m i x t u r e . T w o o f the f i l l e r

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particle types ( S B B and H B B ) consisted o f spherical B a - A l - B - S i glass particles. A t h i r d f i l l e r type, called K U , consisting o f ground B a - A l - B - S i glass was also used.

T o characterize the e f f e c t o f aforementioned variables on light attenuation i n experimentally made V L C D composites the r e f r a c t i v e indices f o r monomers, silane and f i l l e r particles, and f i l l e r particle size distribution were measured.

Specimen preparation for the light absorbance determinations: The experimental materials were placed i n small w h i t e plastic c y l i n d r i c a l moulds. The thicknesses o f these moulds were 1, 2, 3, 4 or 5 m m thick. Samples 1, 2 and 3 m m thick were light-cured f r o m the top side only, w h i l e specimens 4 m m and thicker were cured f r o m both the top and the b o t t o m surfaces. The specimens were then kept dry i n a dark r o o m . For each material c o m b i n a t i o n ( f i l l e r type, f i l l e r surface treatment, f i l l e r f r a c t i o n and sample thickness), five samples were made ending up w i t h a total sample number o f 3*2*8*5*5= 1200 samples. L i g h t transmission was obtained f o r a halogen source and a c o l l i m a t e d laser, m a k i n g the total number o f observations 2400.

Experimental Determinations: The absorbance measurements were done w i t h t w o d i f f e r e n t light-sources; argon laser and a halogen c u r i n g unit. Figure 3 shows the set up f o r the l i g h t transmission measurement w i t h the laser. The set-up f o r the halogen source was made similar to the laser set-up.

Figure 3: Light transmittance measurement set up for laser source.

Curve fitting: D u r i n g the regression analysis o f the equation Z = AT, +K²V + K^d + K⁴Vd + K⁵V² + K⁶d² + K⁷V²d, i t was f o u n d that the parameters K², K⁵, K⁶ and K⁷ were undistinguishable f r o m noise o n the 95 % significance level, leaving equation Z = K, + K^}d + K⁴Vd as the best f i t model.

This equation is i n t e r m equivalent to Equation 3 i f K^t=A, K³=B and K⁴=C.

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R E S U L T S

Paper I

Polymerization strain level decreased significantly (p<0.05) w h e n composites were cured w i t h 200 m W / c m² rather than w i t h 800 m W / c m². Temperature rises were significantly d i f f e r e n t (p<0.05) f o r d i f f e r e n t composites and l i g h t intensity values. Reduction i n l i g h t intensity d i d not decrease the D C % values s i g n i f i c a n t l y at the top surfaces. The most dramatic differences existed between top and b o t t o m surfaces (p<0.05) rather than among curing groups. The D C % values o f Z 1 0 0 and Z250 at top and the b o t t o m surfaces are shown i n Figures 4 and 5.

The measured E-modulus and volumetric shrinkage values were not significantly d i f f e r e n t (p>0.05) among the d i f f e r e n t l i g h t intensity groups.

The D C % values, the E-modulus values and the v o l u m e t r i c shrinkage values o f cured composites were not affected b y l o w l i g h t intensity c u r i n g . H o w e v e r , the contraction strain and p o l y m e r i z a t i o n ' s exotherms decreased.

80 -

70 -

60 - E S2 50

CD

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£=

O 40

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o o 30 - 0)

CT

CD

Q 20 - 10 -

0

• 800mW (40sec, B&T) ffl450mW (60 sec, B&T)

• 200mW (140 sec, B&T)

Z100 (Bottom and Top surfaces of 4mm thick specimen)

Figure 4: Degree of conversion (DC%) for Z100*at the top(2) and the bottom (1) surfaces of 4 mm thick specimens of composites cured with different curing methods (n=6). * Mean value ± Standard deviation

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80 - -p 0

70 - c o

60

conversi

50 40 - CD <D 30

Degi

20 10 -

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• 800 mW (40seo, B&T)

• 450mW (60sec, B&T)

• 200mW (140sec, B&T)

j , .

Z250 (Bottom and Top surface of 4mm thick specimen)

Figure 5: Degree of conversion (DC%)for Z250*at the top (2) and the bottom (1) surfaces of 4 mm thick specimens of composites cured with different curing methods (n = 6).* Mean value ± Standard deviation

Paper I I

The linear relationship proven by use o f the calibration curve suggested that the degree o f conversion can be determined f r o m a straight line relationship constructed by j o i n i n g the a l i p h a t i c a r o m a t i c peak ratio o f the uncured material w i t h o r i g i n . Based o n the obtained results f o r aiphatic:aromatic peak ratios o f uncured Z 1 0 0 (2.65 ± 0.12) and Z 2 5 0 (1.70 ± 0.04), the f o l l o w i n g relationships were f o u n d and used f o r determining the degree o f conversion o f the t w o composites.

DCzioo% = 100 - 37.757* R (4) and

D C^{Z 2 5}( ) % = 100 - 58.766*R (5)

where R represents the recorded aliphatic:aromatic peak ratio o f the i n d i v i d u a l samples after completed c u r i n g .

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G L M (general linear model) revealed that the material thickness, f o l l o w e d by irradiance time, is the most important variable to be considered, w h i l e material and irradiance level are less important (Table 1)

Table 1. Result f r o m the PROC G L M analysis

Source (df)Degree of

freedom

(Type I SS) sum of squares

Mean Square F Value P r > F

Material 1 10923.3778 10923.3778 171.53 <.0001

Irradiance 2 18075.1932 9037.5966 141.92 <.0001

Time 5 91980.0216 18396.0043 288.87 <.0001

Thickness 3 148033.6235 49344.5412 774.86 <0001

The highest conversion value o f one o f the Z 1 0 0 was j u s t below 60%, w h i l e the m a x i m a l conversion value o f the Z 2 5 0 was j u s t below 65%. That d i f f e r e n c e i n conversion values could be refered to differences i n monomer systems used i n the t w o composites.

B y considering light energy per square centimeter (J/cm²) rather than l i g h t irradiance ( m W / c m²) , it was f o u n d that equivalent energy values gave similar conversion values f o r a certain sample thickness.

P a p e r I I I

There were significant differences i n transition temperatures between the t w o materials (p<0.0001). The three frequencies ( 1 , 3 and 10 H z ) used f o r each material gave d i f f e r e n t glass transition temperature values (p<0.0001). A t the l o w e r "transitions", no significant differences existed. The glass transition o f Z 2 5 0 was l o w e r and narrower than that o f Z 1 0 0 . Z 2 5 0 exhibited l o w e r storage modulus values than Z100. The irradiance values d i d not affect any o f the transition temperatures significantly (p = 0.0985). Tables 2 and 3 comprise the results f r o m this study.

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Table 2: Glass transition temperatures f o r Materials A and B cured at d i f f e r e n t irradiance values ( w i t h standard deviation).

Light irradiance imW/cm')

Glass transition temperature for Material A at different frequencies

Glass transition temperature for Material B at different frequencies

Light irradiance

imW/cm') K i l l / 3 f t 1 Hz 10 Hz 3 Hz I Hz

850 180.8(1.9) 173.8(0.1) 167.8(1.3) 135.1(2.5) 130.3(2.2) 126.0(1.7) 500 181.6(1.7) 174.0(1.7) 165.9(1.5) 136.6(3.7) 131.8(3.3) 127.8(4.3) 250 180.1(1.8) 173.1(2.6) 167.2(2.0) 135.1(1.5) 129.8(1.5) 124.9(1.4)

Table 3: L o w e r " t r a n s i t i o n " temperatures o f Materials A and B cured at d i f f e r e n t irradiance values ( w i t h standard deviation).

Light irradiance (raW/cra*)

First peak at Tangent delta for Z100 at Afferent frequencies

Ftrst peak at Tangent delta for Z250 at different frequencies

Light irradiance

(raW/cra*) 10 Hz 3 Hz 1 Hz 10 H / 3 Hz 1 He

850 58.1(2.1) 58.9(1.7' 56.7(1.7) 54.6(5.1)" 53.3(5.6)" 53.1(6.1)"

500 55.3(4.6) 54.6(3.9) 52.5(4.4) 60.7(6.0)" 60.3(6.6)" 60.6(6.8)"

250 58.3(10.8) 57.7(10.7) 57.2(10.8) 55.0(4.7) 54.5(4.6) 5.3.4(3.7)

a. Samples cured at 850 m W / c m . One measurement d i d not show this peak b. Samples cured at 500 m W / c m². T w o measurements d i d not show this peak.

P a p e r I V

The G L M analysis conducted on the conversion levels generated after 3 s o f l i g h t curing is shown i n Table 4. A s seen f r o m Table 4, the l i g h t source was the most important variable closely f o l l o w e d by the initiator. O f the three light sources, standard c u r i n g yielded s i g n i f i c a n t l y higher conversion values and soft- start c u r i n g produced the lowest. The degree o f conversion f o r the L E D - c u r e was intermediate, s i g n i f i c a n t l y l o w e r than standard c u r i n g and s i g n i f i c a n t l y higher than

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soft-start c u r i n g . A s seen i n Table 5, the most important variable after 10 s o f curing was the initiator, f o l l o w e d by the l i g h t source.

The G L M analysis conducted on the conversion levels generated after 40 s o f light curing is shown in Table 6. A s seen f r o m that Table, initiator choice had no significant impact on conversion. A t this t i m e , light source was the most important variable, f o l l o w e d by co-initiator. O f the three light sources, standard curing gave the highest conversion value and L E D - c u r i n g the lowest.

Table 4: The G L M analysis conducted on the conversion levels generated after 3 s o f l i g h t c u r i n g .

Source DF Type I SS Mean Square F Value Pr>F Initiator 1 212.2403704 212.240307 128.92 <0.0001 Co-initiator 2 88.5664500 44.282250 26.90 <0.0001 Light 2 430.9679167 215.4839583 130.89 <0.0001

Table 5: The G L M analysis conducted on the conversion levels generated after 10 s o f l i g h t c u r i n g .

Source D F Type 1 SS Mean Square F Value Pr>F Initiator 1 8854.70341 8854.70341 410.49 <0.0001 Co-initiator 2 3371.31682 1685.65841 78.14 •cO.0001 Light 2 12882.03717 6441.01859 298.59 <0.0001

Table 6: The G L M analysis conducted on the conversion levels generated after 4 0 s o f l i g h t c u r i n g .

Source D F Type I SS Mean Square F Value Pr>F

Initiator 1 47.097615 47.097615 2.75 0.1004

Co-initiator 2 825.952919 412.976459 24.10 <0.0001 Light 2 3055.855735 1527.927868 89.17 <0.0001

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P a p e r V

The refractive indices f o r monomer resins, silane, and f i l l e r particles were respectively as the f o l l o w i n g s : b i s G M A / T E G D M A = 1.5020 ± 0.0002,

Y-methacryloxypropyl-trimethoxysilane = 1.4297 ± 0.0002, S B B =1.5509 ± 0.0002, H B B = 1.5481 ± 0.0001, and K U = 1.5454 ± 0.0002. These values show that all f i l l e r particles had s i g n i f i c a n t l y higher refractive indices than the matrix.

Filler density measurements showed that among the three f i l l e r types H B B f i l l e r had the highest density and the K U f i l l e r the lowest one. The H B B f i l l e r had a median diameter o f 1.81 ± 0.34 p m and S B B had a median diameter o f 1.44 ± 0.04 p m . The K U f i l l e r had the smallest median diameter o f 0.78 ± 0.07 p m .

Absorbance: The absorbance values o f the d i f f e r e n t materials were plotted against the differences i n f i l l e r fractions and sample thicknesses w i t h help o f Matlab. The 3 D representations o f the results o f HBB/nonsilanized/halogen and HBB/nonsilanized/laser combinations are show in Figures 6 and 7. I n these figures the absorption values are plotted i n relation to predicted values and the upper and lower 9 5 % significant level planes.

The analyses revealed that o f the t w o l i g h t sources, more l i g h t was absorbed by the composite when the laser light was used. A m o n g d i f f e r e n t f i l l e r types, the H B B f i l l e r absorbed most l i g h t and the K U f i l l e r the least. There were significant differences (p<0.05) i n l i g h t absorption between a l l three f i l l e r types.

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HBB-nonSilane-Halogen

CD Ü 0 8 C

Figure 6: The predicted absorption plane modelled for HBB/nonsilanized fdler particles where halogen was used as incident light in relation to 95% significant level boundaries.

HBB-nonSilane-Laser

1 2 -

Figure 7: The predicted absorption plane modelled for HBB/nonsilanized filler particles where laser was used as incident light in relation to 95% significant level boundaries.

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G E N E R A L D I S C U S S I O N

M a n y researchers have studied the e f f e c t o f irradiance values and d i f f e r e n t c u r i n g procedures on V L C D composites. U n f o r t u n a t e l y , too many opinions have clouded the issue, m a k i n g it d i f f i c u l t to draw solid general c o n c l u s i o n on this topic. Therefore, i n an attempt to c l a r i f y some o f these issues and i d e n t i f y h o w irradiance values and cure times affect stress development and degree o f conversion in V L C D composites d u r i n g p o l y m e r i s a t i o n , Papers I to I V focused on contraction strain, p o l y m e r i z a t i o n heat release, v o l u m e t r i c shrinkage, modulus o f elasticity, degree o f conversion, rate o f p o l y m e r i z a t i o n and d y n a m i c mechanical properties. These properties were measured on either c o m m e r c i a l light-cure dental composites or f o r experimentally made light-cure dental resins w h e n d i f f e r e n t c u r i n g methods were used.

Because o f the c o m p l e x i t y o f the optical behaviour o f V L C D composites and h o w the optical properties affect l i g h t attenuation when l i g h t passes through a composite disk, Paper V was designed to develop a better understanding o f l i g h t transmission and l i g h t absorption i n V L C D composites.

The m a i n results o f the studies, w h i c h are included i n this thesis, revealed that:

1. B y use o f l o w e r irradiance values, but under f i x e d energy per area conditions ( J / c m²) , l o w e r irradiance values are capable o f p r o d u c i n g polymers h a v i n g the same conversion levels as those cured w i t h higher irradiance values (Paper II). That requires though that the c u r i n g t i m e is s u f f i c i e n t l o n g . B y use o f the lower irradiance values, l o w e r stress levels are introduced (Paper I), something we relate to an extended f l o w t i m e o f the s l o w - c u r i n g p o l y m e r .

2. The amount o f heat generated d u r i n g p o l y m e r i s a t i o n does not necessary correspond to the amount o f polymerisation shrinkage (Paper I).

3. F r o m the obtained results i n Paper I, II & III i t was shown that Z 1 0 0 d i d not have as e f f i c i e n t conversion as Z 2 5 0 . Despite the higher conversion level o f Z 2 5 0 , it has lowest modulus o f elasticity, contraction strain and glass transition temperature.

4. B y c o m b i n i n g d i f f e r e n t photo-initiator/co-initiators/curing methods, l o w e r p o l y m e r i z a t i o n rates can be achieved w h i l e the f i n a l conversion w i l l be as high as those y i e l d e d by use o f faster c u r i n g procedures (Paper IV).

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5. Based on the Beer-Lambert's law it was possible to model the impact o f the composite's properties, i.e., f i l l e r particle size, refractive index, particle surface treatment and f i l l e r ' s v o l u m e f r a c t i o n , on light absorption i n d i f f e r e n t experimental light-cured dental composites (Paper V).

W h e n light-cured composites are used as restorative materials, and when there is no bond f o r m a t i o n between the composite and the cavity walls, the p o l y m e r i s a t i o n shrinkage w i l l lead to an almost homogeneous isotropic contraction. Such a contraction can result i n marginal gap f o r m a t i o n and leakage.

I f the composite is bonded, there w i l l be no free shrinkage. I n this case the shrinkage w i l l occur towards the cavity walls and contraction stresses w i l l f o r m . The magnitude o f these stresses w i l l be a f f e c t e d b y the amount o f shrinkage and the stiffness o f the material. Shrinkage and stress are in turn determined by molecular size and molecular m o b i l i t y w i t h i n the material d u r i n g curing. The number o f molecules that are i n v o l v e d i n the c u r i n g process can be determined f r o m the degree o f conversion value.

Comparison o f the t w o commercial light-cured composites i n paper I revealed that Z 2 5 0 caused s i g n i f i c a n t l y l o w e r strain i n the surrounding metal ring than ZIOO. F r o m c l i n i c a l point o f v i e w that f i n d i n g suggests that as l o n g as the geometrical conditions are the same, ZIOO should induce higher shrinkage stresses i n the surrounding cavity. The higher exotherm o f Z 2 5 0 can be explained by the higher number o f carbon double bonds converted to single bonds d u r i n g p o l y m e r i s a t i o n o f this material. However, the polymerisation contraction strain was s i g n i f i c a n t l y l o w e r f o r Z 2 5 0 . The higher degree o f conversion i n f u l l y cured Z 2 5 0 composite (Paper I I ) can be due to several factors. For example, it has been shown that U E D M A - b a s e d resins have higher conversion levels than b i s G M A - based resins [ 8 4 , 1 2 4 ] . Z 2 5 0 consists o f b i s E M A 6 , b i s G M A , U E D M A and T E G D M A and ZIOO o f b i s G M A and T E G D M A . Z 2 5 0 contains o f a monomer m i x t u r e h a v i n g a larger average molecular w e i g h t than ZIOO. The higher conversion level o f Z 2 5 0 should also be i n f l u e n c e d by the partially replacement o f the s t i f f and hydrogen bonded monomer b i s G M A w i t h the longer and m o r e flexible b i s E M A 6 molecules. The l o w e r aliphatic:aromatic ratio i n Z 2 5 0 is another reason. Since a f e w e r number o f aliphatic bonds react in Z 2 5 0 , therefore it has l o w e r aliphatic:aromatic ratio than i n cured ZIOO. However, probably the most important explanation is that Z 2 5 0 contains longer molecules, something that increases the distance between the polymerised molecules. Such an increase