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Photocured dental restorative materials: Effect of exposure time on curing, glass transition, modulus and water sorption
Clinical
Mqteria1.i 8 (1991) 145-153
Photocured Dental Restorative Materials : Exposure Time on Curing, Glass Transition, Modulus and Water Sorption Jan Kolarik,* Department
Claudio
Migliaresi,
Paola Capuana
& Luca Fambri
of Materials Engineering, University of Trento, 38050 Mesiano di Povo, Trento, Italy
Abstract: Two species of photocured dental restorative materials commercially available (Dimefill TL and Silux Plus) were employed to study the effect of exposure time and sample storage on the matrix polymerization, glass transition temperature, flexural modulus, water uptake and extractable fraction. The methods used for thermal analysis were differential scanning calorimetry and dynamic mechanical thermal analysis. The photoinitiator systems are decomposed within 40 or 20 s of illumination with visible violet light. The conversion is proportional to the logarithm of exposure time within the range 5-160 s. However, the conversion remains rather incomplete even after prolonged photocuring (160 s), as evidenced by low values of the glass transition temperature T, and flexural modulus E(37 “C). During the storage of photocured samples some progress of polymerization takes place, which is manifested by a perceptible additional polymerization increase in both T, and E(37 “C). An extensive attributed to living radicals can be observed at elevated temperatures (60--80 “C) if the post-curing is performed shortly after the illumination. Water uptake and extractable fraction diminish with the time of curing as the network density increases.
INTRODUCTION
GMA is very high, its conversion into polymer is rather limited ;2v7 to overcome this drawback, diluent methacrylate monomers of lower viscosity are added.‘,’ Incomplete conversion is sometimes regarded as beneficial because the shrinkage during polymerization is lower and the adhesion of the restorative material to the dental tissue may be better.2,10,11 On the other hanid, however, the unreacted vinyl groups may adversely affect color stability, aging, wear resistance, e:tc.2g6,7 Inorganic fillers represent an essential component of all dental restorative materials because they reduce the polymerization shrinkage and codetermine the resulting mechanical and other physical properties. To increase their maximum packing fraction,12 it is advisable to use fillers with a very wide particle size distribution or to combine suitably various fillers in one formulation. However, very small particles, e.g. below 0.1 pm in diameter, with a large specific surface area, may adsorb and immobilize a substantial fraction of the matrix.
Advanced dental restorative materialsl,’ consist of a polymer resin, inorganic fillers (up to 65 % wt), an initiator system effective under clinical conditions and otlher components, like pigments, stabilizers, inhibitors, etc. According to initiator systems, the formulations can be divided into two groups: (i) self-cured composites3 and (ii) photopolymerized composites.4 Most dental restorative composites are baised on dimethacrylates,5,6 mainly on 2,2bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]l-propane (frequently denoted as bis-GMA), on its analogues or on similar urethane dimethacrylates. I, 6 It is believed that densely crosslinked backbones consisting of bulky monomer units impart rigidity to the matrix.‘,2 However, as the viscosity of bis* On leave from the Institute of Macromolecular Czechoslovak Academy of Sciences, Heyrovskeho 06 Prague 6, Czechoslovakia.
Chemistry, nam. 2, 162 145
Clinical il4auterials 0267-6605/91
/SOS.50 @ 1991 Elsevier
Science Publishers
Ltd, England
146
Jan Kohik
Thus, fillers with a particle size in the range 0.5-10 pm are most convenient for dental composites.2 To ensure a good and durable adhesion to the matrix, fillers are routinely treated with silane coupling agents.’ Recently, composites of the bisGMA resin reinforced with short glass fibers have been tested as candidate materials for restorative dentistry.13 The self-cured formulations’ consist of two batches to separate two csmponents of a redox initiator system during the stsrage. After mixing the batches, the redox system produces radicals initiating the polymerization. ue to the presence of oxygen and added inhibitors, the polymerization proceeds very slowly during the induction period which secures a time interval 30 s> for t batches mixing. Afterwards, merization proceeds very fast, which is reflected in an abrupt increase in viscosity and, eventually, in the rapid hardening of the cement (time for processing is about 2 mm). The photopolymerized formulations employ photoinitiation reactions activated by ultraviolet 0r visible violet light. Current photocured cements usually contain diketones (e.g. camphoroquinone) as the photo-oxidant and tertiary amines as the These restorative materials are photo-reducer.l,* easy to process as no time limit is set. During the illumination the rate of reaction is bigher near the irradiated surface than in the gradients in mechanical and be expected. The formulations curable with visible violet light can be applied in layers up to 3 mm, otherwise, layering techniques are recommended.lv2 Due to the low mobility (rate of diffusion) and, consequently, long lifetime of growing radicals, the polymerization is believed to proceed for hours after the exposure.4*14 Routinely, the polymerization of 40-70 % of vinyl gro s is achieved.2v7 It should ant positive correlation be noted that a sign between improved mechanical properties and increased conversion was observed.” Sorption of water has a profound in the performance of dental composites.‘~“,8 Formulations with hydrophobic properties possess a better dimensional stability. Water uptake leads to groscopic expansion which was believed ts beneficial in closing margin gaps at t composite interface. On the other hand, it is generally known that sorbed water frequently destroys interfacial adhesion between constituents and deteriorates the ultimate mechanical properties of composites. Silane coupling agents suppress
et al.
water
take and enhance
extensively
the resistance
studied fr0
hs oral
Is of view. It is
ensity determine t glass tra~$~ti~~ Be Tg which is an importa characteristic of any polymer based material. latively little is known
on the physical
properties
‘of
Materials
compositions
given
~~~~~~ UL : matrix-Es-6 dimet~~c~ylate (S), (7) ;
a~~fac~rers
are the
Photocured dental restorative materials
surface. A pair of identical samples was always prepared: one was used in DSC measurements immediately after photocuring; the other was stored for a periiod of time before the measurements were implemented. Test pieces for dynamic mechanical thermal analysis (DMTA) were about 30 mm long, 10 mm wide and 1 mm thick. They were exposed to violet light from both sides to prevent a transversal gradient in mechanical properties. Pairs of identical samples were prepared for the same purpose as given abolve. Methods
DSC measurements were run on Mettler TA 3000 with the heating rate 20 “C/min in the interval - 50 to 200 “C. As soon as the first scan had been completed, the sample was cooled down (100 “C/ min) and the second scan was carried out to check completion of the detected polymerization reactions. DMTA, measurements were performed with the aid of the Dynamic Mechanical Thermal Analyzer” manufactured by Polymer Laboratories, Loughborough, UK. The apparatus worked in the single cantilever mode with an effective sample length 5 mm. The frequency of forced vibrations was 1 Hz; a small amplitude (20 pm) was selected to prevent fatigue fracture of the specimens in the course of measurements. The temperature interval in the first scan was - 50 to 170 “C (Dimefill TL) or - 50 to 120 “C (Silux Plus), in the second scan it was -50 to 200 “C. The heating rate used in all measurements was 3 “C/min. Water sorption was determined at 37 “C after 40 days of immersion by using samples identical with those for DSC. Complete desorption of water was achieved by drying under vacuum at 60 “C to constant weight. IRESURTS AND DISCUSSION Differential scanning calorimetry
Polymerization of Dimefill TL or Silus Plus dental composites is readily initiated at room temperature by visible violet light and the conversion is controlled by the exposure time. In order to employ DSC for an analysis of the progress of polymerization reactions, it is inevitable to run the experiments over the whole temperature interval in which the reactions can take place, though it may II
(a) I
I
I
I
I
0
1
100
I
'C
200
(b) Fig. 1. Effect of the exposure time on polymerization reactions in (a) Dimefill TL and (b) Silux Plus. Exposure time given in seconds. DSC measurements performed immediately after photocuring.
seem irrelevant from the clinical point of view. If violet light is not applied, the polymerization of both formulations occurs in the temperature interval 16&200 “C as evidenced by a large exothermal peak on the DSC diagrams (Fig. 1). Apparently, it is only at these temperatures that the initiator is thermally decomposed and polymlerization takes place. In the second scan over the same temperature level, no exothermal peak can be observed (Fig. 2(a)), which indicates that the polymerization was completed in the first run (as this holds for all samples measured, the second runs are not given in other figures). Figures 1 (a) and 1 (b) show that the ECM
8
Jan Kolarik et al.
r
0
L
I
1
0
100 (b)
Fig. 2. Effect of the storage of photocured samples on polymerization reactions in (a) Dimefill TL and (b) Silux Plus. (a) 1, Measured immediately after photocuring for 10 s; 2, second scan for the previous sample; 3, scanned 4 days after the photocuring for 10 s. (b) DSC measurements performed 4 days after photocuring. Exposure time given in seconds.
170 “C peak diminishes with the exposure time and disappears after 40 or 20 s of photocuring. At the same time, however, another exothermal peak at about 80 “C occurs due to photocuring. Its area reaches a maximum after 10 or 5 s of curing and afterwards decreases linearly with the logarithm of the exposure time (Fig. 3) in the interval 10-160 s ; simultaneously, the peak shifts towards higher temperatures from 70 to 105 “C (Fig. 1). A plausible interpretation of these data is as follows : the redox initiator system in Dimefill TL or
I-I
J
I__-_-
loo
If
Fig. 3. Effect of the photoc~ri~g time cm the area of exothermal peaks located at 80 “C (circles) and 170 “C (squares). (a) Dimefili TL: I’, 2’-measured immediately after photocuring (both curves involve data points for 2 series of samples); l”, 2”-measured after 4 days of storage at 37 ‘63 (curve 1” includes only 2 data points because in most cases , 3 80 “C peak was not detected); 1” a-measured after 4 days of storage at 22 ‘C (area of the 170 “6: peak was no (b) Silux Plus: I’, 2’-measured immediately after (the 170 ‘C peak was detectable only for exposures 0, 5, 10 s); 2”-measured after .5 days of storage at 37 “6: (the 80 “C peak was not observed).
decom lus is completely 20 s of the ~~otoc~ring by vi the experimental conditions initiate ~olyrner~~ati~~ at 1-05 produced rad ever, the conversion is far fr temperature. being c~rn~l~te within the time interval of composition of the photoinitiator; the no1 wit ization proceeds at room temperature 81 to 160 s or so, exposure time rops markedly wl merization rate conversion. If the light is
Photocured dental restorative materials
ditional polymerization (post-curing) starts to proceed fast at temperatures about 80 “C. To elucidate the role of living radicals in the postexposure curing, analogous sets of photocured samples were measured after 4-5 days of storage in the darkness. I-n this case, it is particularly instructive to analyze DSC diagrams of the samples in which a part of the initiating system was preserved, i.e. samplles with exposure times shorter than 40 or 20 s. Figure 2 clearly shows that due to the storage, the peak at 80 “C diminishes while the peak at 170 “C grows. Thus, the effect of storage is formally opposite to that of photocuring ; this reverse process proceeds faster at 37 “C than at 22 “C (Fig. 3 (a)), which reveals its dependence on the molecular mobility and,/or diffusion rate. Obviously, in a samples (cf. Figs 1 (a) and 1 (b)), which means that exposure, the thermally-induced additional polymerization occurred at about 80 “C, presumably due to the presence of living radicals produced during the preceding photocuring. However, this polymerization reaction was not observed for stored samples l(cf. Figs 1 (a) and 1 (b), which means that either the radicals were no longer present or the polymerization of unreacted double bonds was accomplished during the storage. The reappearance of the 170 “C peak (Fig. 3) proves that the conversion was not completed during the storage, though its progress due to a ‘dark reaction ’ has to be admitted because the present evaluation is not quantitative. Thus, we can conclude that the living radicals contribute to some extent to the conversion during the storage at 37 or 22 “C, but a substantial fraction of the remaining vinyl groups stays unreacted. Dynamic mechanical thermal analysis
The structure and elastic properties of composites can be effectively studied by means of dynamic mechanical measurements. The glass transition of the matrix gives rise to a pronounced peak of the loss modulus E” as well as of the loss factor tan 6 and, simultaneously, to a concomitant drop of the storage modulus 6. However, in the dynamic measurelments carried out at 1 Hz, the loss modulus peak is located about 15 “C above the glass transition temperature T, detected by volume expansion measurements. l6 As can be seen in Figs 4 and 5, th,e loss modulus peak is very wide and flat so that the sharper loss factor peak seems to be more convenient for the relative evaluation of the effects of photocuring and sample storage on the glass
149
logE’ Pa
logE"+
Pa
8
0
100
(a)
logE' Pa
9
0.2
logE"+I Pa
tan0
0.1
(b) Fig. 4. Effect of the photocuring time and of the thermal postcuring on the temperature dependence of the storage modulus I?‘, loss modulus _E”and loss factor tan S of Dimefill TL. Time of photocuring: (a) 20 s; (b) 160 s. ----, the first run -50 to 170 “C (measured immediately after illumination); -, the second run -50 to 200 “C.
transition temperature. However, Tables 1 and 2 indicate that the peak of the loss factor is located at temperatures about 50 “C higher than the peak of the loss modulus. Thus, it should be: borne in mind that the actual T, is, by 65 “C, lower on average than the temperature of the loss factor peak. Such differences also have to be taken into account for the data published earlier.g’15s1’‘,I3 Nevertheless, DMTA seems to be a convenient lmethod for the determination of T, because in the corresponding DSC diagrams (Figs 1 and 2), the: change in the specific heat is smeared out so that it is very difficult to assess the glass transition temperature location. The extended temperature interval1 of the glass transition brings evidence about an extremely wide spectrum of segmental mobility which can be ascribed to profound discrepancies in the local structure of the photocured matrix. The samples exposed to visible violet light for only 5 s from each side are so compact that they can 11.2
Jan Kolarik et al.
150
Table 2. Effect of illumination time on the tcrn~e~at~re location sf the glass transition peak and on the WexuraP modulus of Silux Plus
log E’ Pa
E
I.2
log E’+I Pa
tan6
1.1
Exposure time (SP 5”
40 120
-8 42
48
2
9.10
20
77 151
20 RI
922 9.33
40
70 149
30 110
80
75 353
160 320
EO 0
100
"C
200
b)
-7---__ --._
logE’
‘.
Pa
logE”+l
E’ __--
___--’
c‘-,
‘.;
.2
=:::::::.I\:-
Pa
tan 6
.I
100
0
"C
Fig. 5. Effect of the photocuring time and thermal post-curing on the temperature dependence of the storage modulus E’, loss modulus _!?’and loss factor tan 6 of Silux Plus. Time of photocuring: (a) 20 s; (b) 320 s. ----, the first run -50 to 120 “C; (a) measured immediately after photocuring; (b) the second run measured after 5 days of storage at 37 “C; -, -50 to 200 “C. Table 1. Effect of illumination time on the temperature location of the glass transition peak and on the flexural modulus of Dimefill TL
Exposure time (S! 5b
peak tan 6 (“c)
E”
After storage 4 days”
peak log E’(37”G) tan 6 (Pa) (“C)
52 82
-3 22
8.80 9.08
54 70
20
55 100
15 50
8.95 9.09
80
65 105
28 65
160
68 105
25 50
30 35 em L! 60
9‘17 9~22 9.16 9”17 9.25 925
9.12 9.20
52 63 32 64
50 205
9.16 9~32
77 8%
9.25 9.22
70 150
60 108
9.13 9.31
79 76
9.30 9”34
73 152
46 106
9.26 942
82 82
9 27 934
a Storage at 37 “C. * First run : - 50 to 120 “C; second run: t specimen, -50 to 200 “C (holds for all exposure
9.22 ‘9.36
times).
200
(W
After illumination
8~76 9.13
E”
log E’(37 “G) (Pa)
-10 3
8.67 8.82
72 90
30 42
8.95 9.12
9.12 9.17
74 89
20 38
8.90 9.17
9.27 9.35
77 89
30 40
9.18 9.20
a Storage at 22 C. b First run: - 50 to 170 “C; second run: the same test specimen, -50 to 200 “C (holds for all exposure times).
undergo dynamic mechanical testin Figs 4 and 5 and in Tables 1 and 2, t of the glass transition temperature, i 1 Hz) - 15 “G or T,,,(tan 6 ~olymethacrylat~ matrices in are rather low in comparis methyacrylates.12”s Dimefill by violet light (i.e. without treatment) have the glass ~~a~s~~i~~ t~rn~er~t~~e about l&15 “G or 35-40 “C, i.e. below or close to igh filler content, t e glass transition magnitude. For this reason, the about one order dental ~~rrn~~at~~n~ is rapidly modulus of test decreasing with temperature just in the interval around 37 “C (Figs 4 and 5); ~onseque~~tly~ E(J7 “C) of the studied dental formulat
and E(37 “C) increase Tables 1 and 2 sh with the time of ocuring, obviously due to the rising rnole~~~~~ mass and network density. Even at sential increase in excessive exposure times, no these quantities can be achiev It has to be noted that associated with the glass t at temperatures low
Photocured dental restorative materials
of the reactions observed in DSC diagrams. To identify the effect of thermal post-curing (taking place during the first run), the second DMTA runs were performed. The first runs were terminated at 170 “C (Dimefill) or at 125 “C (Silux), i.e. at temperatures beffore the polymerization associated with the thermal decomposition of the initiator could occur. In other words, only polymerization reactions initiated by living radicals (present after photocuring) were believed to take place. Tables 1 and 2 show that the thermal post-curing implemented during the first DMTA run accounts for a noticeable increase in T, and E’(37 “C). The T, rises with the time of the preceding photocuring in the region of exposures shorter than 40 or SOs ; beyond this interval, the glass transition temperature seems to approach a limit, i.e. T&&Y”, 1 Hz) = 5&65 “C for Dimefill TL and 105-l 10 “C for Silux Plus which is markedly higher than the values attained in the photocuring. Figures 4 and 5 also prove that the thermally post-cured resins create a true network because their modulus rises with temperature in the region of the rubber-like state (well above T,). (In intended applications, however, the thermal post-curing cannot be utilized.) Regardless of the large scatter, the DMTA data indicate--in conformity with DSC-that the conversion attainable in practice via photocuring is rather incomplete, for which there may be two reasons: (i) premature depletion of the initiator system; (ii) severe reduction of the diffusioncontrolled polymerization rate as the glass transition temperature approaches (with increasing molar mass and network density) the ambient temperature. The beneficial effect of prolonged photocuring, exceeding the period required for the photodecomposition of the initiator system, can then tentatively be ascribed to some enhancement of molecular mobility and diffusion rate in the matrix. This explanation is also in accord with the DSC diagrams, (Fig. l), showing that the corresponding exothermal peak shifts to higher temperatures (from 70 to 105 “C) with advancing conversion. Nevertheless, even a prolonged exposure to visible violet light makes it possible to approach the limit properties of dental composites observed1 after thermal post-curing. The values of T, and E(37 “C) cannot be used directly for an estimate of the conversion because any quantitative interrelations are difficult to establish; the dependences could ble calibrated by using data on unreacted vinyl groups obtained by a direct method, e.g. infrared spectrsscopy.6
151
Table 3. Effect of illumination time on water extractable fraction and equilibrium water uptake Dimefill
Exposure Extract (3) 5
(% wt)
TL
Silux Plus ___-Uptake Extract Uptake (% wt) (% wt) (% wt)
20 40
4.99 3.03 1.92 1.31
3.39 2.60 1.76 1.53
80 160 320
1.07 0.86 0.86
1.39 1.48 1.39
10
3.07 1.46 1.24
2.80 2.60 2.46
1.13 1.08 0.98 0.91
2.51 244 2.42 2.49
The effect of the storage of photocured samples on their properties can be estimated from Tables 1 and 2. In the first run, the stored salmples show a higher T,,, than the samples m.easured after illumination, which evidences som’e progress of polymerization during storage. The observed differences are larger for Silux Plus, jwhich may be ascribed to progress in the ‘dark’ polymerization due to the higher storage temperature. In conformity with these results, Silux Plus also displays a somewhat higher E’(37 “C) ; for Dilmefill TL, this trend is not observed. However, if the data obtained in the second runs are confronted, it is quite clear, despite the large scatter, that the samples measured (and thermally post-cured) immediately after the exposure have T,,, and E’(37 “C) hdgher than the stored samples. In conformity with DSC, DMTA also shows in a qualitative manner ,that the ability of living radicals to initiate additional polymerization diminishes with time, i.e. the radicals probably undergo terminating reactions. IFrom the practical point of view it is important to note that the sample storage at 37 “C has a small, yet positive effect on T, and E(37 “C). Water uptake
The data summarized in Table 3 show that the extractable fraction of both restorative materials decreases with increasing time of photocuring in the interval from 5 to 160 s, which corresponds to rising conversion and network build-up. For samples cured for more than 80 s it represents less than 1% by weight. In parallel, the equilibrium water uptake of Dimefill TL shows reduction ; Silux Plus absorbs an almost constant, but larger amount of water at exposures longer than 20 s. When confronted with literature data for other dental formulations,20~21 both tested materials show a cornparable water
152
Juan Kolarik et al.
sorption. Though the overall water uptake is certainly proportional to the hydrophilicity of all constituents, its decline with the exposure time reflects the build-up of a three-dimensional polymer network. Thus, the sorption and extraction data are in conformity with preceding findings that the photocuring of Silux Plus proceeds faster and is completed at shorter illumination times CONCLUSIONS
The authors are indebte for kindly s~pply~~~ the tested materials.
E~ERENCE 1. Antonucci, overvi GUShi
I, NH., igliaresi
sin based dental composites-An in Medicine 11, ed. E. Chiellini, F. & E. Wicolais. PPenum Press, New
XOdC,
The initiator system of photocured dental resins decomposes after about 40 s (Dimefill TL) or 20 s (Silux Plus) of exposure to visible violet light at room temperature (under conditions described above). The polymerization reactions proceed further with the exposure time up to 160 s; the resulting conversion is then approximately proportional to the logarithm of the exposure time. Despite the prolonged photocuring, the final conversion is rather incomplete, as evidenced by low values of the flexural modulus E’(37 “C) = 1.5-2 GPa and of the glass transition temperature T, which is about 10-l 5 “C for Dimefill or 35-40 “C for Silux Plus. The limited conversion may be brought about by two factors : (i) premature depletion of the initiator system; (ii) a profound decrease in the polymerization rate as the glass transition temperature approaches-with rising molecular mass and network density-the ambient temperature at which the photocuring is carried out. Progress in conversion during the prolonged photocuring can tentatively be ascribed to some enhancement of molecular mobility and monomer diffusion rate. If the illuminated samples are stored at 37 “C, the living radicals bring about some advancement in conversion, as evidenced by an increase in the glass transition temperature T, and modulus E(37 “C>. The conversion and network build-up can be completed by thermal post-curing at elevated temperatures (70-100 “C) carried out shortly after the photocuring as long as the produced radicals are living. The limiting values of T, and E(37 “C) achieved in this way (presumably corresponding to complete conversion) cannot be attained via photocuring under clinical conditions. It can be recommended, however, to use a prolonged time of photocuring (80-160 s) because it has a beneficial influence on conversion, glass transition temperature, modulus, water uptake and extracted fraction.
2. Cook, eech, D. R. & Tyas, M. .I., Structure and properties of methacrylate based dental restorative materials. 6 (19X5) 362-8. 3. Brawl-, 6;. Initiator-accelerator systems for acryllac resins and composites. In Biomedical and DenCui AppEications of Polymers ) ed. C. 6. Gebeiein & F. F. Kubhtz. ress, New York, B981, pp. 395409. 4. .I., The application of photochemistry to dental In Biomedicd and Dental ~~~~~~a~~~~s oj Polymeq ed. C. 6. Cebelein & F. F. Kublitz. Plenum Press, New York, 1981, pp. 41 l-17. 5. Antonncci, I. h/l., New monomers for use in dentistry. In Biomedical and Dental Applications qf Polymers, d. C. G Gebelein Bz F. F. Kublitz. Plenum Press, New York, i 982,
pp. 357-71. 6. Ruyter, I. E. onomer systems and polymerization. In Posterjor Co site Resin Dental Restorative Mate&is, ed. 6. Vanherle & D. C. Smith. Peter Xzuie Publishing, Netherlands, 198.5, pp. lQ9-35. 7. Cook, W. D., Polymerization defects rn composite resins. In Posterior Composite Resin Dental Restorative Materials, ed. G. Vanherle & C. Smith. Peter Sznle Publishing, Netherlands5 1985, pp: 273-97. 8. Cowperthwaite, 6. I?., Foy, .I. I., 8 Malloy, M. A , ‘li‘hc nature of the crosslinking matrix found in dental composite filling materials and sealants. In ~~~~~~~~a1 and Dental Applications of Polymers, e&s. C. G. GebeEein & F. F. Kubiits. Plenum Press, New York, 1981, pp. 379-S. 9. Ferracane, 4. I_,. 62 Greener, E. II., The effect of resin formulation on the degree of conversion and mechanical properties of dental restorative resins. J. .&omed Mater. Res., 26) (1986) 121-31. 10. Vankerckhoven, II., Lambrechts, P., Van Beylen, hr.? avidson, C. L. & ‘Vanherle, G., Unreacted methacrylate groups on surfaces of composite resins. Jo Dmr. Rcs.. 6B (1982) 791-6. 11. Schulein, ‘I. AM., AL Chalkiey, Y., Bond strength and bard activated composte resins. J. Biomed Mater. Res., I8 (1984) 789-96. 12. Nielsen, I. E., Mechanical Properties of Pollymeus aizd Composites. Marcel Dekker, New York, 1974. 13. Krause, W. EL, Park, S. II. & Straup, R. A., Mechanical properties of bis-GNIA resin short glass fiber composites” J. Biomed. Mater. &a., 23 (1989) 1195.-211. 14. Kandil, S. II., Kamar, A. A., Shaaban, S. A., Taymour, M. M. $i Morsi, S. E., Effwt of temperature and ageing on the mechanicai properti materials. BiomatPrials, IS. Clarke, R. I+ Dynamic mechanical thermal analysis of dental polymers. I. Bleat-cured poly(metby1 methacry!ate)based materials. Biomaberids, IQ (1989) W&t?. 16. Ilavsky, M. & Kolarik, .I., The viscoelastic beha.viour of butyl nnetPlacryla5c-ethyleneglycol mo~om~~~~acrylat~ copolymers in the main transition region. CoUectim Czech, Chem. Commun., 314(1969) 2473x.
Photocured
dental restorative
17. Clarke, R. L., Dynamic mechanical thermal analysis of dental polymers. II. Bis-phenol A related resins. Biomaterink, 10 (1989) 549-52. 18. Clarke, R. L., Dynamic mechanical thermal analysis of dental polymers. III. Heterocyclic methacrylates. Biomaterials, 10 (1989) 63&3. 19. Kolarik, J., Secondary relaxations in glassy polymersHydrophilic polymethacrylates and polyacrylates. Adv. Polym. Sci., 46 (1982) 119-68.
materials
153
20. Carfanna. C.. Guerra. G.. Nicolais. L. & Tartaro. S.. Effects of post-curing ‘and’water sorption on the mech: anical properties of composite dental restorative materials. Biomaterials, 4 (1983) 228-30. 21 Bastioli, C., Romano, G. & Migliaresi, C., Water sorption and mechanical properties of dental composites. Biomaterials, 11 (1990) 219-23.
Mqteria1.i 8 (1991) 145-153
Photocured Dental Restorative Materials : Exposure Time on Curing, Glass Transition, Modulus and Water Sorption Jan Kolarik,* Department
Claudio
Migliaresi,
Paola Capuana
& Luca Fambri
of Materials Engineering, University of Trento, 38050 Mesiano di Povo, Trento, Italy
Abstract: Two species of photocured dental restorative materials commercially available (Dimefill TL and Silux Plus) were employed to study the effect of exposure time and sample storage on the matrix polymerization, glass transition temperature, flexural modulus, water uptake and extractable fraction. The methods used for thermal analysis were differential scanning calorimetry and dynamic mechanical thermal analysis. The photoinitiator systems are decomposed within 40 or 20 s of illumination with visible violet light. The conversion is proportional to the logarithm of exposure time within the range 5-160 s. However, the conversion remains rather incomplete even after prolonged photocuring (160 s), as evidenced by low values of the glass transition temperature T, and flexural modulus E(37 “C). During the storage of photocured samples some progress of polymerization takes place, which is manifested by a perceptible additional polymerization increase in both T, and E(37 “C). An extensive attributed to living radicals can be observed at elevated temperatures (60--80 “C) if the post-curing is performed shortly after the illumination. Water uptake and extractable fraction diminish with the time of curing as the network density increases.
INTRODUCTION
GMA is very high, its conversion into polymer is rather limited ;2v7 to overcome this drawback, diluent methacrylate monomers of lower viscosity are added.‘,’ Incomplete conversion is sometimes regarded as beneficial because the shrinkage during polymerization is lower and the adhesion of the restorative material to the dental tissue may be better.2,10,11 On the other hanid, however, the unreacted vinyl groups may adversely affect color stability, aging, wear resistance, e:tc.2g6,7 Inorganic fillers represent an essential component of all dental restorative materials because they reduce the polymerization shrinkage and codetermine the resulting mechanical and other physical properties. To increase their maximum packing fraction,12 it is advisable to use fillers with a very wide particle size distribution or to combine suitably various fillers in one formulation. However, very small particles, e.g. below 0.1 pm in diameter, with a large specific surface area, may adsorb and immobilize a substantial fraction of the matrix.
Advanced dental restorative materialsl,’ consist of a polymer resin, inorganic fillers (up to 65 % wt), an initiator system effective under clinical conditions and otlher components, like pigments, stabilizers, inhibitors, etc. According to initiator systems, the formulations can be divided into two groups: (i) self-cured composites3 and (ii) photopolymerized composites.4 Most dental restorative composites are baised on dimethacrylates,5,6 mainly on 2,2bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]l-propane (frequently denoted as bis-GMA), on its analogues or on similar urethane dimethacrylates. I, 6 It is believed that densely crosslinked backbones consisting of bulky monomer units impart rigidity to the matrix.‘,2 However, as the viscosity of bis* On leave from the Institute of Macromolecular Czechoslovak Academy of Sciences, Heyrovskeho 06 Prague 6, Czechoslovakia.
Chemistry, nam. 2, 162 145
Clinical il4auterials 0267-6605/91
/SOS.50 @ 1991 Elsevier
Science Publishers
Ltd, England
146
Jan Kohik
Thus, fillers with a particle size in the range 0.5-10 pm are most convenient for dental composites.2 To ensure a good and durable adhesion to the matrix, fillers are routinely treated with silane coupling agents.’ Recently, composites of the bisGMA resin reinforced with short glass fibers have been tested as candidate materials for restorative dentistry.13 The self-cured formulations’ consist of two batches to separate two csmponents of a redox initiator system during the stsrage. After mixing the batches, the redox system produces radicals initiating the polymerization. ue to the presence of oxygen and added inhibitors, the polymerization proceeds very slowly during the induction period which secures a time interval 30 s> for t batches mixing. Afterwards, merization proceeds very fast, which is reflected in an abrupt increase in viscosity and, eventually, in the rapid hardening of the cement (time for processing is about 2 mm). The photopolymerized formulations employ photoinitiation reactions activated by ultraviolet 0r visible violet light. Current photocured cements usually contain diketones (e.g. camphoroquinone) as the photo-oxidant and tertiary amines as the These restorative materials are photo-reducer.l,* easy to process as no time limit is set. During the illumination the rate of reaction is bigher near the irradiated surface than in the gradients in mechanical and be expected. The formulations curable with visible violet light can be applied in layers up to 3 mm, otherwise, layering techniques are recommended.lv2 Due to the low mobility (rate of diffusion) and, consequently, long lifetime of growing radicals, the polymerization is believed to proceed for hours after the exposure.4*14 Routinely, the polymerization of 40-70 % of vinyl gro s is achieved.2v7 It should ant positive correlation be noted that a sign between improved mechanical properties and increased conversion was observed.” Sorption of water has a profound in the performance of dental composites.‘~“,8 Formulations with hydrophobic properties possess a better dimensional stability. Water uptake leads to groscopic expansion which was believed ts beneficial in closing margin gaps at t composite interface. On the other hand, it is generally known that sorbed water frequently destroys interfacial adhesion between constituents and deteriorates the ultimate mechanical properties of composites. Silane coupling agents suppress
et al.
water
take and enhance
extensively
the resistance
studied fr0
hs oral
Is of view. It is
ensity determine t glass tra~$~ti~~ Be Tg which is an importa characteristic of any polymer based material. latively little is known
on the physical
properties
‘of
Materials
compositions
given
~~~~~~ UL : matrix-Es-6 dimet~~c~ylate (S), (7) ;
a~~fac~rers
are the
Photocured dental restorative materials
surface. A pair of identical samples was always prepared: one was used in DSC measurements immediately after photocuring; the other was stored for a periiod of time before the measurements were implemented. Test pieces for dynamic mechanical thermal analysis (DMTA) were about 30 mm long, 10 mm wide and 1 mm thick. They were exposed to violet light from both sides to prevent a transversal gradient in mechanical properties. Pairs of identical samples were prepared for the same purpose as given abolve. Methods
DSC measurements were run on Mettler TA 3000 with the heating rate 20 “C/min in the interval - 50 to 200 “C. As soon as the first scan had been completed, the sample was cooled down (100 “C/ min) and the second scan was carried out to check completion of the detected polymerization reactions. DMTA, measurements were performed with the aid of the Dynamic Mechanical Thermal Analyzer” manufactured by Polymer Laboratories, Loughborough, UK. The apparatus worked in the single cantilever mode with an effective sample length 5 mm. The frequency of forced vibrations was 1 Hz; a small amplitude (20 pm) was selected to prevent fatigue fracture of the specimens in the course of measurements. The temperature interval in the first scan was - 50 to 170 “C (Dimefill TL) or - 50 to 120 “C (Silux Plus), in the second scan it was -50 to 200 “C. The heating rate used in all measurements was 3 “C/min. Water sorption was determined at 37 “C after 40 days of immersion by using samples identical with those for DSC. Complete desorption of water was achieved by drying under vacuum at 60 “C to constant weight. IRESURTS AND DISCUSSION Differential scanning calorimetry
Polymerization of Dimefill TL or Silus Plus dental composites is readily initiated at room temperature by visible violet light and the conversion is controlled by the exposure time. In order to employ DSC for an analysis of the progress of polymerization reactions, it is inevitable to run the experiments over the whole temperature interval in which the reactions can take place, though it may II
(a) I
I
I
I
I
0
1
100
I
'C
200
(b) Fig. 1. Effect of the exposure time on polymerization reactions in (a) Dimefill TL and (b) Silux Plus. Exposure time given in seconds. DSC measurements performed immediately after photocuring.
seem irrelevant from the clinical point of view. If violet light is not applied, the polymerization of both formulations occurs in the temperature interval 16&200 “C as evidenced by a large exothermal peak on the DSC diagrams (Fig. 1). Apparently, it is only at these temperatures that the initiator is thermally decomposed and polymlerization takes place. In the second scan over the same temperature level, no exothermal peak can be observed (Fig. 2(a)), which indicates that the polymerization was completed in the first run (as this holds for all samples measured, the second runs are not given in other figures). Figures 1 (a) and 1 (b) show that the ECM
8
Jan Kolarik et al.
r
0
L
I
1
0
100 (b)
Fig. 2. Effect of the storage of photocured samples on polymerization reactions in (a) Dimefill TL and (b) Silux Plus. (a) 1, Measured immediately after photocuring for 10 s; 2, second scan for the previous sample; 3, scanned 4 days after the photocuring for 10 s. (b) DSC measurements performed 4 days after photocuring. Exposure time given in seconds.
170 “C peak diminishes with the exposure time and disappears after 40 or 20 s of photocuring. At the same time, however, another exothermal peak at about 80 “C occurs due to photocuring. Its area reaches a maximum after 10 or 5 s of curing and afterwards decreases linearly with the logarithm of the exposure time (Fig. 3) in the interval 10-160 s ; simultaneously, the peak shifts towards higher temperatures from 70 to 105 “C (Fig. 1). A plausible interpretation of these data is as follows : the redox initiator system in Dimefill TL or
I-I
J
I__-_-
loo
If
Fig. 3. Effect of the photoc~ri~g time cm the area of exothermal peaks located at 80 “C (circles) and 170 “C (squares). (a) Dimefili TL: I’, 2’-measured immediately after photocuring (both curves involve data points for 2 series of samples); l”, 2”-measured after 4 days of storage at 37 ‘63 (curve 1” includes only 2 data points because in most cases , 3 80 “C peak was not detected); 1” a-measured after 4 days of storage at 22 ‘C (area of the 170 “6: peak was no (b) Silux Plus: I’, 2’-measured immediately after (the 170 ‘C peak was detectable only for exposures 0, 5, 10 s); 2”-measured after .5 days of storage at 37 “6: (the 80 “C peak was not observed).
decom lus is completely 20 s of the ~~otoc~ring by vi the experimental conditions initiate ~olyrner~~ati~~ at 1-05 produced rad ever, the conversion is far fr temperature. being c~rn~l~te within the time interval of composition of the photoinitiator; the no1 wit ization proceeds at room temperature 81 to 160 s or so, exposure time rops markedly wl merization rate conversion. If the light is
Photocured dental restorative materials
ditional polymerization (post-curing) starts to proceed fast at temperatures about 80 “C. To elucidate the role of living radicals in the postexposure curing, analogous sets of photocured samples were measured after 4-5 days of storage in the darkness. I-n this case, it is particularly instructive to analyze DSC diagrams of the samples in which a part of the initiating system was preserved, i.e. samplles with exposure times shorter than 40 or 20 s. Figure 2 clearly shows that due to the storage, the peak at 80 “C diminishes while the peak at 170 “C grows. Thus, the effect of storage is formally opposite to that of photocuring ; this reverse process proceeds faster at 37 “C than at 22 “C (Fig. 3 (a)), which reveals its dependence on the molecular mobility and,/or diffusion rate. Obviously, in a samples (cf. Figs 1 (a) and 1 (b)), which means that exposure, the thermally-induced additional polymerization occurred at about 80 “C, presumably due to the presence of living radicals produced during the preceding photocuring. However, this polymerization reaction was not observed for stored samples l(cf. Figs 1 (a) and 1 (b), which means that either the radicals were no longer present or the polymerization of unreacted double bonds was accomplished during the storage. The reappearance of the 170 “C peak (Fig. 3) proves that the conversion was not completed during the storage, though its progress due to a ‘dark reaction ’ has to be admitted because the present evaluation is not quantitative. Thus, we can conclude that the living radicals contribute to some extent to the conversion during the storage at 37 or 22 “C, but a substantial fraction of the remaining vinyl groups stays unreacted. Dynamic mechanical thermal analysis
The structure and elastic properties of composites can be effectively studied by means of dynamic mechanical measurements. The glass transition of the matrix gives rise to a pronounced peak of the loss modulus E” as well as of the loss factor tan 6 and, simultaneously, to a concomitant drop of the storage modulus 6. However, in the dynamic measurelments carried out at 1 Hz, the loss modulus peak is located about 15 “C above the glass transition temperature T, detected by volume expansion measurements. l6 As can be seen in Figs 4 and 5, th,e loss modulus peak is very wide and flat so that the sharper loss factor peak seems to be more convenient for the relative evaluation of the effects of photocuring and sample storage on the glass
149
logE’ Pa
logE"+
Pa
8
0
100
(a)
logE' Pa
9
0.2
logE"+I Pa
tan0
0.1
(b) Fig. 4. Effect of the photocuring time and of the thermal postcuring on the temperature dependence of the storage modulus I?‘, loss modulus _E”and loss factor tan S of Dimefill TL. Time of photocuring: (a) 20 s; (b) 160 s. ----, the first run -50 to 170 “C (measured immediately after illumination); -, the second run -50 to 200 “C.
transition temperature. However, Tables 1 and 2 indicate that the peak of the loss factor is located at temperatures about 50 “C higher than the peak of the loss modulus. Thus, it should be: borne in mind that the actual T, is, by 65 “C, lower on average than the temperature of the loss factor peak. Such differences also have to be taken into account for the data published earlier.g’15s1’‘,I3 Nevertheless, DMTA seems to be a convenient lmethod for the determination of T, because in the corresponding DSC diagrams (Figs 1 and 2), the: change in the specific heat is smeared out so that it is very difficult to assess the glass transition temperature location. The extended temperature interval1 of the glass transition brings evidence about an extremely wide spectrum of segmental mobility which can be ascribed to profound discrepancies in the local structure of the photocured matrix. The samples exposed to visible violet light for only 5 s from each side are so compact that they can 11.2
Jan Kolarik et al.
150
Table 2. Effect of illumination time on the tcrn~e~at~re location sf the glass transition peak and on the WexuraP modulus of Silux Plus
log E’ Pa
E
I.2
log E’+I Pa
tan6
1.1
Exposure time (SP 5”
40 120
-8 42
48
2
9.10
20
77 151
20 RI
922 9.33
40
70 149
30 110
80
75 353
160 320
EO 0
100
"C
200
b)
-7---__ --._
logE’
‘.
Pa
logE”+l
E’ __--
___--’
c‘-,
‘.;
.2
=:::::::.I\:-
Pa
tan 6
.I
100
0
"C
Fig. 5. Effect of the photocuring time and thermal post-curing on the temperature dependence of the storage modulus E’, loss modulus _!?’and loss factor tan 6 of Silux Plus. Time of photocuring: (a) 20 s; (b) 320 s. ----, the first run -50 to 120 “C; (a) measured immediately after photocuring; (b) the second run measured after 5 days of storage at 37 “C; -, -50 to 200 “C. Table 1. Effect of illumination time on the temperature location of the glass transition peak and on the flexural modulus of Dimefill TL
Exposure time (S! 5b
peak tan 6 (“c)
E”
After storage 4 days”
peak log E’(37”G) tan 6 (Pa) (“C)
52 82
-3 22
8.80 9.08
54 70
20
55 100
15 50
8.95 9.09
80
65 105
28 65
160
68 105
25 50
30 35 em L! 60
9‘17 9~22 9.16 9”17 9.25 925
9.12 9.20
52 63 32 64
50 205
9.16 9~32
77 8%
9.25 9.22
70 150
60 108
9.13 9.31
79 76
9.30 9”34
73 152
46 106
9.26 942
82 82
9 27 934
a Storage at 37 “C. * First run : - 50 to 120 “C; second run: t specimen, -50 to 200 “C (holds for all exposure
9.22 ‘9.36
times).
200
(W
After illumination
8~76 9.13
E”
log E’(37 “G) (Pa)
-10 3
8.67 8.82
72 90
30 42
8.95 9.12
9.12 9.17
74 89
20 38
8.90 9.17
9.27 9.35
77 89
30 40
9.18 9.20
a Storage at 22 C. b First run: - 50 to 170 “C; second run: the same test specimen, -50 to 200 “C (holds for all exposure times).
undergo dynamic mechanical testin Figs 4 and 5 and in Tables 1 and 2, t of the glass transition temperature, i 1 Hz) - 15 “G or T,,,(tan 6 ~olymethacrylat~ matrices in are rather low in comparis methyacrylates.12”s Dimefill by violet light (i.e. without treatment) have the glass ~~a~s~~i~~ t~rn~er~t~~e about l&15 “G or 35-40 “C, i.e. below or close to igh filler content, t e glass transition magnitude. For this reason, the about one order dental ~~rrn~~at~~n~ is rapidly modulus of test decreasing with temperature just in the interval around 37 “C (Figs 4 and 5); ~onseque~~tly~ E(J7 “C) of the studied dental formulat
and E(37 “C) increase Tables 1 and 2 sh with the time of ocuring, obviously due to the rising rnole~~~~~ mass and network density. Even at sential increase in excessive exposure times, no these quantities can be achiev It has to be noted that associated with the glass t at temperatures low
Photocured dental restorative materials
of the reactions observed in DSC diagrams. To identify the effect of thermal post-curing (taking place during the first run), the second DMTA runs were performed. The first runs were terminated at 170 “C (Dimefill) or at 125 “C (Silux), i.e. at temperatures beffore the polymerization associated with the thermal decomposition of the initiator could occur. In other words, only polymerization reactions initiated by living radicals (present after photocuring) were believed to take place. Tables 1 and 2 show that the thermal post-curing implemented during the first DMTA run accounts for a noticeable increase in T, and E’(37 “C). The T, rises with the time of the preceding photocuring in the region of exposures shorter than 40 or SOs ; beyond this interval, the glass transition temperature seems to approach a limit, i.e. T&&Y”, 1 Hz) = 5&65 “C for Dimefill TL and 105-l 10 “C for Silux Plus which is markedly higher than the values attained in the photocuring. Figures 4 and 5 also prove that the thermally post-cured resins create a true network because their modulus rises with temperature in the region of the rubber-like state (well above T,). (In intended applications, however, the thermal post-curing cannot be utilized.) Regardless of the large scatter, the DMTA data indicate--in conformity with DSC-that the conversion attainable in practice via photocuring is rather incomplete, for which there may be two reasons: (i) premature depletion of the initiator system; (ii) severe reduction of the diffusioncontrolled polymerization rate as the glass transition temperature approaches (with increasing molar mass and network density) the ambient temperature. The beneficial effect of prolonged photocuring, exceeding the period required for the photodecomposition of the initiator system, can then tentatively be ascribed to some enhancement of molecular mobility and diffusion rate in the matrix. This explanation is also in accord with the DSC diagrams, (Fig. l), showing that the corresponding exothermal peak shifts to higher temperatures (from 70 to 105 “C) with advancing conversion. Nevertheless, even a prolonged exposure to visible violet light makes it possible to approach the limit properties of dental composites observed1 after thermal post-curing. The values of T, and E(37 “C) cannot be used directly for an estimate of the conversion because any quantitative interrelations are difficult to establish; the dependences could ble calibrated by using data on unreacted vinyl groups obtained by a direct method, e.g. infrared spectrsscopy.6
151
Table 3. Effect of illumination time on water extractable fraction and equilibrium water uptake Dimefill
Exposure Extract (3) 5
(% wt)
TL
Silux Plus ___-Uptake Extract Uptake (% wt) (% wt) (% wt)
20 40
4.99 3.03 1.92 1.31
3.39 2.60 1.76 1.53
80 160 320
1.07 0.86 0.86
1.39 1.48 1.39
10
3.07 1.46 1.24
2.80 2.60 2.46
1.13 1.08 0.98 0.91
2.51 244 2.42 2.49
The effect of the storage of photocured samples on their properties can be estimated from Tables 1 and 2. In the first run, the stored salmples show a higher T,,, than the samples m.easured after illumination, which evidences som’e progress of polymerization during storage. The observed differences are larger for Silux Plus, jwhich may be ascribed to progress in the ‘dark’ polymerization due to the higher storage temperature. In conformity with these results, Silux Plus also displays a somewhat higher E’(37 “C) ; for Dilmefill TL, this trend is not observed. However, if the data obtained in the second runs are confronted, it is quite clear, despite the large scatter, that the samples measured (and thermally post-cured) immediately after the exposure have T,,, and E’(37 “C) hdgher than the stored samples. In conformity with DSC, DMTA also shows in a qualitative manner ,that the ability of living radicals to initiate additional polymerization diminishes with time, i.e. the radicals probably undergo terminating reactions. IFrom the practical point of view it is important to note that the sample storage at 37 “C has a small, yet positive effect on T, and E(37 “C). Water uptake
The data summarized in Table 3 show that the extractable fraction of both restorative materials decreases with increasing time of photocuring in the interval from 5 to 160 s, which corresponds to rising conversion and network build-up. For samples cured for more than 80 s it represents less than 1% by weight. In parallel, the equilibrium water uptake of Dimefill TL shows reduction ; Silux Plus absorbs an almost constant, but larger amount of water at exposures longer than 20 s. When confronted with literature data for other dental formulations,20~21 both tested materials show a cornparable water
152
Juan Kolarik et al.
sorption. Though the overall water uptake is certainly proportional to the hydrophilicity of all constituents, its decline with the exposure time reflects the build-up of a three-dimensional polymer network. Thus, the sorption and extraction data are in conformity with preceding findings that the photocuring of Silux Plus proceeds faster and is completed at shorter illumination times CONCLUSIONS
The authors are indebte for kindly s~pply~~~ the tested materials.
E~ERENCE 1. Antonucci, overvi GUShi
I, NH., igliaresi
sin based dental composites-An in Medicine 11, ed. E. Chiellini, F. & E. Wicolais. PPenum Press, New
XOdC,
The initiator system of photocured dental resins decomposes after about 40 s (Dimefill TL) or 20 s (Silux Plus) of exposure to visible violet light at room temperature (under conditions described above). The polymerization reactions proceed further with the exposure time up to 160 s; the resulting conversion is then approximately proportional to the logarithm of the exposure time. Despite the prolonged photocuring, the final conversion is rather incomplete, as evidenced by low values of the flexural modulus E’(37 “C) = 1.5-2 GPa and of the glass transition temperature T, which is about 10-l 5 “C for Dimefill or 35-40 “C for Silux Plus. The limited conversion may be brought about by two factors : (i) premature depletion of the initiator system; (ii) a profound decrease in the polymerization rate as the glass transition temperature approaches-with rising molecular mass and network density-the ambient temperature at which the photocuring is carried out. Progress in conversion during the prolonged photocuring can tentatively be ascribed to some enhancement of molecular mobility and monomer diffusion rate. If the illuminated samples are stored at 37 “C, the living radicals bring about some advancement in conversion, as evidenced by an increase in the glass transition temperature T, and modulus E(37 “C>. The conversion and network build-up can be completed by thermal post-curing at elevated temperatures (70-100 “C) carried out shortly after the photocuring as long as the produced radicals are living. The limiting values of T, and E(37 “C) achieved in this way (presumably corresponding to complete conversion) cannot be attained via photocuring under clinical conditions. It can be recommended, however, to use a prolonged time of photocuring (80-160 s) because it has a beneficial influence on conversion, glass transition temperature, modulus, water uptake and extracted fraction.
2. Cook, eech, D. R. & Tyas, M. .I., Structure and properties of methacrylate based dental restorative materials. 6 (19X5) 362-8. 3. Brawl-, 6;. Initiator-accelerator systems for acryllac resins and composites. In Biomedical and DenCui AppEications of Polymers ) ed. C. 6. Gebeiein & F. F. Kubhtz. ress, New York, B981, pp. 395409. 4. .I., The application of photochemistry to dental In Biomedicd and Dental ~~~~~~a~~~~s oj Polymeq ed. C. 6. Cebelein & F. F. Kublitz. Plenum Press, New York, 1981, pp. 41 l-17. 5. Antonncci, I. h/l., New monomers for use in dentistry. In Biomedical and Dental Applications qf Polymers, d. C. G Gebelein Bz F. F. Kublitz. Plenum Press, New York, i 982,
pp. 357-71. 6. Ruyter, I. E. onomer systems and polymerization. In Posterjor Co site Resin Dental Restorative Mate&is, ed. 6. Vanherle & D. C. Smith. Peter Xzuie Publishing, Netherlands, 198.5, pp. lQ9-35. 7. Cook, W. D., Polymerization defects rn composite resins. In Posterior Composite Resin Dental Restorative Materials, ed. G. Vanherle & C. Smith. Peter Sznle Publishing, Netherlands5 1985, pp: 273-97. 8. Cowperthwaite, 6. I?., Foy, .I. I., 8 Malloy, M. A , ‘li‘hc nature of the crosslinking matrix found in dental composite filling materials and sealants. In ~~~~~~~~a1 and Dental Applications of Polymers, e&s. C. G. GebeEein & F. F. Kubiits. Plenum Press, New York, 1981, pp. 379-S. 9. Ferracane, 4. I_,. 62 Greener, E. II., The effect of resin formulation on the degree of conversion and mechanical properties of dental restorative resins. J. .&omed Mater. Res., 26) (1986) 121-31. 10. Vankerckhoven, II., Lambrechts, P., Van Beylen, hr.? avidson, C. L. & ‘Vanherle, G., Unreacted methacrylate groups on surfaces of composite resins. Jo Dmr. Rcs.. 6B (1982) 791-6. 11. Schulein, ‘I. AM., AL Chalkiey, Y., Bond strength and bard activated composte resins. J. Biomed Mater. Res., I8 (1984) 789-96. 12. Nielsen, I. E., Mechanical Properties of Pollymeus aizd Composites. Marcel Dekker, New York, 1974. 13. Krause, W. EL, Park, S. II. & Straup, R. A., Mechanical properties of bis-GNIA resin short glass fiber composites” J. Biomed. Mater. &a., 23 (1989) 1195.-211. 14. Kandil, S. II., Kamar, A. A., Shaaban, S. A., Taymour, M. M. $i Morsi, S. E., Effwt of temperature and ageing on the mechanicai properti materials. BiomatPrials, IS. Clarke, R. I+ Dynamic mechanical thermal analysis of dental polymers. I. Bleat-cured poly(metby1 methacry!ate)based materials. Biomaberids, IQ (1989) W&t?. 16. Ilavsky, M. & Kolarik, .I., The viscoelastic beha.viour of butyl nnetPlacryla5c-ethyleneglycol mo~om~~~~acrylat~ copolymers in the main transition region. CoUectim Czech, Chem. Commun., 314(1969) 2473x.
Photocured
dental restorative
17. Clarke, R. L., Dynamic mechanical thermal analysis of dental polymers. II. Bis-phenol A related resins. Biomaterink, 10 (1989) 549-52. 18. Clarke, R. L., Dynamic mechanical thermal analysis of dental polymers. III. Heterocyclic methacrylates. Biomaterials, 10 (1989) 63&3. 19. Kolarik, J., Secondary relaxations in glassy polymersHydrophilic polymethacrylates and polyacrylates. Adv. Polym. Sci., 46 (1982) 119-68.
materials
153
20. Carfanna. C.. Guerra. G.. Nicolais. L. & Tartaro. S.. Effects of post-curing ‘and’water sorption on the mech: anical properties of composite dental restorative materials. Biomaterials, 4 (1983) 228-30. 21 Bastioli, C., Romano, G. & Migliaresi, C., Water sorption and mechanical properties of dental composites. Biomaterials, 11 (1990) 219-23.