J. Dent. 1992; 20: 115-l
115
20
Light-activated glass polyalkenoate (ionomer) cements: the setting reaction* A. M. Bourke*, *Department UK
A. W. Walls*
and J. F. McCabet
of Operative Dentistry
and tDental
Materials Science Unit, The Dental School, Newcastle
upon Tyne,
ABSTRACT The setting reaction of two light-activated glass polyalkenoate cements has been investigated using differential thermal analysis and surface hardness measurements. One material was found to have two distinct phases to its setting reaction. Light activation resulted in a rapid initial set with a large exotherm. A slower setting reaction was detectable when the cement was allowed to set in the absence of light. The surface hardness of this cement increased for some time after the cessation of light activation, indicating continuation of the chemical setting reaction within the material. The ultimate hardness of the cement was significantly higher than its hardness at the termination of light activation. Light activation of the second material increased the rate of set of the cement but no evidence of a dual setting reaction was observed. This material remained very soft and flexible for over 1 h after light activation. A 30 s exposure with a visible light source produced some immediate hardening up to 1.5-2.0 mm below the surface for both materials. KEY WORDS: Glass polyalkenoate (ionomer) cements, Light curing, Physical properties, Setting reaction J. Dent.
1992;
20:
1 15-I
20
(Received 18 June
1991;
reviewed
1 August
1991;
accepted 2 October
1991) Correspondence should be addressed to: Dr A. W. Walls, Department of Operative Dentistry, The Dental School, Framlington Place, Newcastle upon Tyne NE2 48W. UK.
INTRODUCTION The glass polyalkenoate cements were originally developed as aesthetic filling materials (Wilson and Kent, 1972). Development of these cements has led to improvement in their physical properties and in their handling characteristics permitting wider clinical applications. One such application is the use of these cements to bond composite resins to dentine (McLean et al., 1985). This technique involves the use of a glass polyalkenoate to replace missing dentine. When the cement has set it is etched, washed and dried, a resin bonding agent is then applied and cured prior to placement of a composite resin to replace any missing enamel. One disadvantage of this technique is the time required for the cement to set prior to placement of the resin restoration. Bond strength tests have found that the glass polyalkenoate/composite bond fails cohesively within the cement (Hinoura et al., 1987; Welburyet al., 1988). This indicates that the bond strength *This research was carried out in partial fulfilment of the requirements for the Degree of Master of Science in Restorative Dentistry at the University
of Newcastle
@ 1992 Butterworth-Heinemann 0300-5712/92/020115-06
upon
Tyne. Ltd.
is limited by the physical properties of the cement and may explain the findings of Causton et al. (1987) that etching the cement at an early stage in its setting reaction resulted in a weak glass polyalkenoate/composite bond. Even when a fast setting lining cement is used, the bond strength is found to increase with time when the cement is left for up to 15 min before etching (Chin and Tyas, 1988). One approach to overcome the delay required to permit the cement to set prior to placement of the resin has been the development of a number of visible light-activated glass polyalkenoate lining cements. These cements are said to have a dual setting reaction. The primary setting reaction is initiated by exposure to visible light in the region of 470 nm. The method of activation/initiation of set is similar to that utilized in light-activated composite resins. It is accepted that the depth of cure of lightactivated glass polyalkenoate cements will be influenced by those factors which affect the quality of cure of light-activated composite resins. These will include the thickness and opacity of the material (Swartz ef al., 1983),
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J. Dent. 1992; 20: No. 2
the distance from the light source to the surface of the material (Yearn, 1985) and the intensity of the light source at 470 nm (Watts et al., 1984). The secondary setting reaction is an acid-base reaction which commences during mixing and continues after light activation. This leads to maturation of the cement. Failure to light cure these cements will result in a slow, chemical setting reaction. The advantage of this dual setting reaction would be to permit placement of a composite immediately after light activation of the lining. These cements also bond chemically to composite resins as well as to dentine and thus eliminate the need to etch the glass polyalkenoate prior to placement of a composite restoration. If the claims made for these light-activated cements are true then their use in the composite resin/glass polyalkenoate sandwich technique would be expected to decrease the clinical time. However, the effect of the lightactivated setting reaction on the secondary acid-base reaction is unknown. It is possible that the rapid hardening produced by light activation would inhibit the acid-base reaction. If this occurred the resultant material may not exhibit all of the usual properties of the glass polyalkenoate cements including adhesion to dentine. This study was carried out to examine the effect of light activation on these cements and to determine if the chemical setting reaction continues within the cement after light activation or whether the light-activated setting reaction prevents the acid-base reaction occurring.
MATERIALS
AND METHODS
Two light-activated cements were examined, Vitrebond (3M Co., St Paul, MN, USA) and XR Ionomer (Kerr UK Ltd, Peterborough, UK). The materials were mixed at room temperature according to the manufacturer’s instructions at all times. Vitrebond is described as a hydrous type of glass polyalkenoate cement. The liquid component is an aqueous solution of a modified poly(acrylic acid) with a methacrylate end group, a small amount of HEMA (2hydroxyethylmethylacrylate)-less than 10 per cent-and a photoinitiator. The powder is a radiopaque, ionleachable, fluoroaluminosilicate glass which is claimed to be photoactive. Mixture of the powder and liquid components initiates an acid-base type reaction as found in conventional glass polyalkenoate cements. The presence of the HEMA retards this reaction giving a prolonged working time. Subsequent exposure to light from a visible light curing unit results in rapid hardening of the material due to polymerization of the HEMA and the methacrylate end groups. The acid-base reaction continues after light activation and is responsible for the final physical properties of the cement. Failure to light activate Vitrebond will produce a slow setting material with inferior physical properties. The powder component of XR Ionomer is similar .to that of Vitrebond, i.e. a fluoroaluminosilicate glass with the addition of a crystalline photoinitiator. The liquid
component is an aqueous solution of a poly(acrylic acid) with a number of methacrylate groups added but without the addition of aqueous HEMA as in Vitrebond. Mixing the powder and liquid components of XR Ionomer also initiates an acid-base setting reaction while light activation results in polymerization of the methacrylate end groups to provide early strength and resistance to moisture contamination. The final physical properties of the material are due to the continued acid-base setting reaction.
Differential
thermal
analysis
The temperature changes produced by the setting reaction of the test materials, when light activated and when allowed to set chemically, were measured using a differential analysis (DTA) unit (Stanton Redcroft DTA 671, Stanton Redcroft Ltd, London, UK). This has been described previously (McCabe, 1985; Walls et al., 1988). The parameters measured were the time taken for the specimen to reach peak exotherm (T,,,), the time taken for the specimen to cool to a temperature corresponding to 5 per cent of the peak exotherm (5 % Tmax), and the magnitude of the exotherm. Ten samples of each material were mixed according to the manufacturer’s instructions and placed in an aluminium crucible (5.5 mm diameter X 2.15 mm depth). The samples were then covered with a Melinex matrix strip and placed upon the sample thermocouple platform of the DTA unit (Fig. I), operating in isothermal mode at 37”C, between 90 and 120 s from the commencement of mixing. A crucible containing a precured sample of the same material was placed on the reference platform prior to this. Half of the samples of each material (five) were then illuminated for 30 s using a Heliomat light source (Vivadent, Schaan, Liechtenstein), with a twin fibreoptic cable (Coltene, Altstatten, Switzerland) held 2.2 mm from the surface of the material. The light output at 470 nm from the two cables was 898 lux and 964 lux. To ensure that the test and reference samples received equal
Twin fibre-optic cable
Reference aluminium
Fig.
material crucible
in
1. The differential thermal analysis set-up.
Bourke et al.: Setting
reaction of glass polyalkenoate
cements
117
Table 1. The differential thermal analysis parameters and statistical analysis using the Student r-test
Setting reaction
Time to T max (min)
Virrebond Rise in Time to 5% T max temp. (min) (“C)
Weight lmgl
Time to T max (min)
XR lonomer Time to 5% Rise in T max temp. (min) (“C)
Light activated
0.76 (0.04)
4.25 (0.44)
20.33 (2.06)
90.07 (4.0)
1.82 (0.35)
6.49 (0.45)
Chemically cured
3.53 (0.25)
16.19 (6.19)
1.54 (0.46)
87.76 (3.88)
2.95 (0.48)
12.01 (1.29)
Statistical analysis
P < 0.001
P < 0.001
NS
P < 0.01
P < 0.001
P < 0.001
Weight fmg)
5.00 (1.58)
90.10 (8.37)
(E5)
87.44 (3.44)
NS
NS
Figures in parentheses are 1 standard deviation. irradiation, and hence heating, the tibre optic cables were aligned so that illumination of empty crucibles placed on the thermocouple platforms produced zero displacement of the differential recorder trace. A jig was constructed so that the fibre optic cable could be relocated in this position accurately. Following illumination of the test and reference samples the tibre optic cable was removed and a lightproof cover placed over the entrance to the DTA chamber. For the other live samples of each material the cover was placed over the DTA chamber immediately without any irradiation from the light source. All specimens were weighed at the end of the reaction. Five further tracings were obtained for each test material using cured samples on both the reference and sample thermocouple platforms which were irradiated for 30 s. The mean temperature rise produced by a 30 s irradiation was then calculated for both materials and used to differentiate the rise in temperature produced by the light source from that produced by the exothermic setting reaction of the material when light activated.
Hardness
tests
The hardness of the upper and lower surfaces of samples of test materials of varying thickness were measured with respect to time using a Vickers hardness tester (Leitz GmbH, Wetzlar, Germany). Samples of each test material were produced using polypropylene moulds of 0.5 mm, 1.0 mm, 1.5 mm and 2.0 mm in depth with a diameter of 6.0 mm. The moulds were placed on a Melinex matrix strip and filled with the test material which was mixed according to the manufacturer’s instructions. The material was covered with a Melinex matrix strip and a glass slide was placed firmly over this to express any excess cement. The top surface of the specimen was illuminated for 30 s with the same light source as used in the DTA experiments but with a single fibre optic cable which was placed against the glass slide. Samples were then placed in a light-proof container prior to hardness testing. Specimens that were to be stored for more than 30 min were placed in liquid paraffin 1.5 min after irradiation to prevent dehydration of the cement. For each thickness of material the hardness of the top and bottom surfaces was measured at 3 min, 10 min,
15 min, 30 min, 1 h, 4 h, 1 day and 7 days from the commencement of mixing. Five samples of each thickness were measured at every time interval and three readings were taken from each surface tested. The hardness of each surface of a sample was only measured at one time interval as repeated exposure to the light incorporated in the microscope of the hardness tester was found to affect the hardness of the cements.
RESULTS The DTA parameters for the test materials when light activated and when allowed to cure chemically are shown in Table I. Statistical analysis of the DTA parameters for the light-activated and chemically cured samples of the test materials was carried out using an unpaired Student r-test. The results of these tests (Table I) show that light activation of Vitrebond produced a faster setting reaction with a greater exotherm compared with that when it is allowed to cure chemically. Light activation of XR Ionomer on the other hand produced a faster setting reaction but there was no significant difference in the peak exotherm produced. The rise in temperature induced by a 30-s light activation of precured samples was 5 “C for both materials. The mean Vickers hardness number (VHN) for the top and bottom surfaces of the different thicknesses and postexposure times of the test materials are shown in Tables II and III. The results for XR Ionomer include only hardness values from 1 h onwards. The cement was sufficiently soft and flexible within 1 h of mixing so that although the hardness tester produced indentations which were visible to the naked eye, these could not be measured using the microscope as no sharp margin could be defined between the indentation and the surface of the material. However, it was noted that the bottom surfaces of the 2.0 mm samples were unset immediately after irradiation. Comparison of the top and bottom surfaces for each thickness of Vitrebond and postexposure time using an unpaired Student r-test (Table II,) suggested that the rate of set of the two surfaces was affected by the thickness of the specimen and the postirradiation time. A three-way
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J. Dent 1992; 20: No. 2
Table II. The mean hardness with a visible light source
(VHN)
of the top and bottom
JOP
thicknesses
l.Omm Bottom
3 min IOmin 15min 30 min Ih 4h 1 day 7 day
1.51(0.54) 1.48(0.34) 2.04(0.66) 5.1 l(1.68) 9.56(1.83) 1 1.36( 1.89) 14.57(2.83) 14.66(2.61)
l?mes
from the commencement
are taken
of various
of Vitrebond
with time, following
a 30-s
irradiation
Sample thickness
0.5 mm Time
surfaces
1.23(0.27)* 1.52(0.38) 2.06(0.58) 5.1 l(1.48) 8.90(1.20) 10.70( 1.68) 14.71(2.53) 13.82(1.24) of mixing.
0.74(0.1 l)** 1.45(0.33) 1.77(0.52) 3.58(0.90) 9.69( 1.55) 9.84(1 .Ol) 13.06(2.51) 13.13(2.14)
0.96(0.36) 1.44(0.32) 1.90(0.52) 3.77(0.74) 10.02(1.73) 10.36( 1.34) 14.25(2.59) 12.84(1.10) Figures
1.5mm
Bottom
Jop
in brackets
are 1 standard
Jop 0.93( 1.32) 1.48(0.40) 2.71(1.73) 4.33(0.76) 8.10(1.53) 1 1.52(1.06) 14.07(2.19) 13.24(1.42) deviation.
2.0 mm
0.67(0.18)** 0.96(0.17)** 1.71(0.36)** 2.58(0.55)** 6.01(1.12)** 11.32(0.97) 12.89(2.15)* 13.14(0.92)
*P < 0.05,
**P
< 0.01
Bottom
Jop
Bottom
0.86(0.40) 1.85(0.55) 2.07(0.56) 2.30(0.39) 8.01(1.59) 10.52( 1.73) 1 1.37(2.03) 11.61(0.94) for top vs bottom
** 0 0.86(0.37)** 1.24(0.26)** 1.43(0.53)** 6.85(1 .16)** 8.54(2.42)** 1 1.85(1.45) 11.33(0.81)
using an unpaired
Student
t-test.
Jab/e 111.The mean hardness visible light source
(VHN)
of the top and bottom surfaces of various thicknesses
of XR lonomer with time, following
a 30-s irradiation with a
Samp/e thickness 0.5 mm Time lh 4h 1 day 7 day
JOP 1.5 l(O.44) 7.10(1.50) 12.15(1.88) 20.01(2.27)
Bottom 1.48(0.37) 6.54( 1.78) 13.25(1.74) 19.31(3.07)
Bottom
JOP 2.22( 1.30) 9.96(2.97) 12.34(2.47) 19.56( 1.54)
1.95( 1.05) 1 1.64(3.70) 13.17(2.93) 19.28(1 .12)
Times are taken from the commencement of mixing. Figures in brackets are
2.0 mm
1.5mm
l.Omm
1 standard
analysis of variance (ANOVA) found that the relationship between the hardness of the two surfaces differed significantly with changes in the thickness of the material (P < 0.001) and the postirradiation time (P = 0.002). Comparison of the top and bottom surfaces for each thickness and postexposure time of XR Ionomer (Table 111) showed little difference in the hardness of the two surfaces.
DISCUSSION The DTA parameters used in this study were those measured by Walls et al. (1988) and McCabe (1985). It has been suggested that the time taken for a material to reach T max is closely related to the clinical setting time of the material (McCabe and Wilson, 1980). The time taken for a specimen to cool to a temperature corresponding to 5 % T max is used as a measure of the actual setting time (Walls et al., 1988).
Vitrebond The setting characteristics of Vitrebond when light activated and chemically cured differ significantly (Table I). The times to T,,, and 5 % T,,, for the light-activated samples are considerably shorter than the same parameters for a conventional glass polyalkenoate lining cement measured at 23°C (Walls et al., 1988). The temperature rise produced is much closer to that of lightactivated composite resins (McCabe, 1985), than that of conventional glass polyalkenoate cements (Walls ef al., 1988). As irradiation of the precured samples of Vitrebond
JOP 1.82(0.89) 7.24( 1.29) 12.95(2.22) 18.98(1.93) deviation.
?? P < 0.05
Bottom 1.97(0.89) 7.33(0.92) 14.00(1.44)’ 20.1 1(1.87)* for top vs bottom
Bottom
Jop 3.24(0.90) 8.45(1.68) 16.59(5.21) 22.88(1.57) using an unpaired
3.33(0.78) 8.18(1.89) 14.69(4.00) 23.49(2.14) Student
t-test.
produced a temperature rise of 5”C, the large rise in temperature of the cement upon light activation is mainly due to the heat of reaction of the cement. The DTA parameters measured for the chemically cured samples of Vitrebond are similar to those of conventional glass polyalkenoate cements (Walls et al., 1988). The difference in the DTA parameters for the lightactivated and chemically cured samples of Vitrebond suggest that two setting reactions are occurring. This supports the manufacturer’s claim that light activation causes polymerization of the HEMA and modified poly acid molecules in the cement and that an acid-base chemical reaction occurs after mixing of the cement. However, it was not possible to discern from the DTA tests whether this chemical reaction continued after light activation, as has been claimed by the manufacturer. The large exothermic reaction produced by Vitrebond when light activated is a cause for concern. The manufacturer claims that there is no need for a sublining in deep cavities unless there is a pulpal exposure. However, as Zach and Cohen (1965) found that an increase in intrapulpal temperature of as little as 5.55 “C produced a marked pulpal response in monkeys, sometimes leading to pulpal necrosis, the use of Vitrebond in unlined deep cavities is questionable. The changes in hardness with time of the top (Fig 2) and bottom (Fig. 3) surfaces for the various thicknesses of Vitrebond indicate that the cement continues to undergo a setting reaction after irradiation. The hardness of the top surfaces increases rapidly for up to 1 h after light activation and then continues to increase gradually for up to 24 h, after which there is no significant increase, and
Bourke et al.: Setting
reaction
of glass polyalkenoate
cements
119
15
12
9
3
0 00 Time
(h)
Fig. 2. The hardness of the top surfaces of the various thicknesses of Vitrebond with time (log scale). Sample thickness: -, 0.5mm; ---, l.Omm; -----, 1.5 mm; - - - - - - -, 2.0 mm.
in some cases a small decrease in hardness. There are two possible explanations for this prolonged improvement in physical properties, either continuation of the methacrylate-based cross-linking reaction or of the ‘glass polyalkenoate’ type, acid-base chemical setting reaction. The main increase in hardness of light-activated composite resins has been found to occur within 10 min after irradiation (Hansen, 1983), although it may continue to increase slowly for up to 24 h (Leung et al., 1983). Glassionomer cements have a much slower setting reaction, with maturation of the cement occurring for up to 24 h after mixing leading to an increase in the physical properties of the cement including hardness (Crisp et al., 1976). The light-activated setting reaction would be expected to be responsible for a rapid increase in hardness of Vitrebond in the first lo-15 min, after which its effects would decrease significantly. As the hardness of the top surfaces continue to increase for 24 h after irradiation this implies that the chemical setting reaction continues within the cement after light activation. The bottom surfaces of the 2.0 mm samples were unset at the 3-min time interval (approximately 1 min after light activation allowing for mixing, placement and irradiation of the cement). This implies that the depth of cure of Vitrebond produced by a 30-s illumination is less than 2.0 mm. It also implies that hardness of the 2.0 mm samples, and the other sample thicknesses, at this time interval is due mainly to the light-activated setting reaction. Statistical analysis using ANOVA found that the relationship between the hardness of the top and bottom surfaces varied significantly with changes in the thickness and postirradiation time. Examination of Fig. 4, which plots the ratio of hardness (top/bottom) against time, gives an indication of how the thickness of the sample affects the light-activated setting reaction. The tracings for the 0.5 mm and 1.0 mm sample thicknesses indicate that they do not attenuate the passage of light significantly. The small delay in the hardening of the bottom surface of these
Time
[h)
Fig. 3. The hardness of the bottom surfaces of the various thicknesses of Vitrebond with time (log scale). Sample thickness: -, 0.5mm; ---, l.Omm; -----, 1.5 mm; - - - - - - -, 2.0 mm.
2.2
j :: B 4 z v)
r I
0
2.01.81.6-
z 6 : _c 'c 0 .0 u d
1.41.2l.O-
Time
(h)
Fig. 4. Ratio of hardness (top/bottom) of the various thicknesses of Vitrebond with time (log scale). Sample thickness: -, 0.5mm; ---, l.Omm; -----, 1.5 mm; - - - - - - -, 2.0 mm.
thicknesses compared with the top surface is similar to that found with light-activated composite resins (Leunget al., 1983). Examination of the tracing for the 2.0 mm samples provides a contrasting picture. The initial ratio is infinity and decreases to near 1.O by 1 day and remains at this level. The fact that the bottom surface was unset at 3 min does not necessarily imply that the increase in hardness of this surface was due entirely to the chemical setting reaction. Leung et al. (1983) found that the bottom surface of a 2.5 mm sample of composite resin showed no detectable polymerization-as measured by surface hardness-immediately after a 15-s light exposure but that the hardness of the bottom surface increased with postirradiation time for up to 1 day, indicating that lightactivated polymerization was occurring. These authors also noted that in cases where the bottom surface was significantly softer than the top surface immediately after irradiation, the hardness of the bottom surface remained
120
J. Dent 1992; 20: No. 2
significantly lower. However, it can be seen from Table II and Fig 4 that there is no significant difference in the hardness of the two surfaces of the 2.0-mm samples at 1 and 7 days. This provides further evidence that the chemical setting reaction is responsible for the ultimate hardness of the cement. The initial ratio for the 1.5 mm samples of about 1.4 implies that this thickness of Vitrebond produces some attenuation of the light resulting in less polymerization of the HEMA at the bottom surface. The ratio then increases to 1.6 at 30 min after which it decreases to near 1.0 by 4 h. Swartz et al. (1983) found that the hardness of the deeper layers of composite resins did not increase at the same rate as the top and this may explain the increase in the hardness ratio up to 30 min. However, as the effects of the chemical setting reaction become more marked the ratio decreases again. Therefore it can be seen that the sample thickness of Vitrebond affects the rate of the light-activated setting reaction. As the sample thickness increases from 0.5 mm to 2.0 mm the difference in the rate of set between the upper and lower surface increases considerably.
XR lonomer Light activation of XR Ionomer does not produce as rapid a setting reaction as seen with Vitrebond although the setting reaction is faster than that of Ketac Bond at 23°C (Walls et al., 1988). The rise in temperature of lightactivated XR Ionomer (5°C) is similar to that produced when a cured sample is illuminated for 30 s. However, this does not mean that there is no exothermic reaction occurring when XR Ionomer is light activated. This cement produced a much broader peak for the DTA tracing than Vitrebond which suggests that the illumination may be responsible for the early temperature rise and the exothermic reaction keeps the sample at this temperature for longer. Comparison of the DTA variables shows that failure to light activate XR Ionomer results in a slower setting cement. However, as there is no difference in the exotherm produced whether the cement is light activated or not, this suggests that the magnitude of the light-activated setting reaction is much less than that of Vitrebond. This is probably due to the polymerization of the HEMA component of Vitrebond but there is also the possibility that light activation of XR Ionomer does not produce a separate setting reaction and that the heating effect of the light source increases the rate of set of the acid-base reaction. The hardness tests on XR Ionomer suggest that light activation has little affect on the rate of set of this cement. However, these tests do reveal that the final properties of the cement are due to the acid-base reaction as the hardness continued to increase up to and beyond 24 h,
while earlier work has shown that the main setting reaction of light-activated materials is complete at 10 min (Hansen, 1983). The hardness tests also found that the immediate depth of cure produced by light activation is less than 2.0 mm. The two cements studied here are both marketed as lightactivated glass polyalkenoate cements for use as a lining material. While the final physical properties of both the cements have been found to be the result of the acid-base reaction, the effect of light activation on the rate of set of the two cements is markedly different. The effects of the different rates of set produced by light activation on the bond strength of the cements to dentine will be investigated in a further study.
References Causton B. E., Sefton J. and Williams A. (1987) Bonding class II composite to etched glass ionomer cement. Br. Dent. J. 63, 321-324. Chin Y. H. and Tyas M. J. (1988) Adhesion of composite resin to etched glass ionomer cement. Amt. Dent. J. 33, 87-90. Crisp S., Lewis B. G. and Wilson A. D. (1976) Characterisation of glass-ionomer cements. I. Long term hardness and compressive strength. J. Dent. 4, 162-166. Hansen E. K. (1983) After-polymerisation of visible light activated resins: surface hardness vs. light source. Stand. J. Dent. Res. 91,406-410. Hinoura K., Moore B. K. and Phillips R. W. (1987) Tensile bond strength between glass ionomer cements and composite resins. J. Am. Dent. Assoc. 114, 167-172. Leung R. L., Fan P. L. and Johnson W. M. (1983) Postirradiation polymerisation of visible light-activated composite resin. J. Dent. Res. 62, 363-365. McCabe J. F. (1985) Cure performance of light activated composites by differential thermal analysis (DTA). Dent. Mater. 1, 231-234. McCabe J. F. and Wilson H. J. (1980) The use of differential scanning calorimetry for the evaluation of dental materials. J. Oral Rehabil. 7, 103-110. McLean J. W., Prosser H. J. and Wilson A. D. (1985) The use of glass-ionomer cements in bonding composite resin to dentine. Br. Dent. J. 158,410-414. Swartz M. L., Phillips R. W. and Rhodes B. (1983) Visible light-activated resins-depth of cure. J. Am. Dent. Assoc. 106, 634-637. Walls A W. G., McCabe J. F. and Murray J. J. (1988) Factors influencing the setting reaction of glass polyalkenoate (ionomer) cements. J. Dent. 16, 32-35. Watts D. C., Amer 0. and Combe E. C. (1984) Characteristics of visible-light-activated composite systems. Br. Dent. .I 156, 209-215. Welbury R. R., McCabe J. F., Murray J. J. et al. (1988) Factors affecting the bond strength of composite resin to etched glass-ionomer cement. J. Dent. 16, 188-192. Wilson A. D. and Kent B. E. (1972) A new translucent cement for dentistry. Br. Dent. J. 132, 133-135. Yearn J. A (1985) Factors affecting cure of visible light activated composites. ht. Dent. J. 35, 218-225. Zach L. and Cohen G. (1965) Pulp response to externally applied heat. Oral Surg. Oral Med. Oral Pathol. 19, 515-530.
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20
Light-activated glass polyalkenoate (ionomer) cements: the setting reaction* A. M. Bourke*, *Department UK
A. W. Walls*
and J. F. McCabet
of Operative Dentistry
and tDental
Materials Science Unit, The Dental School, Newcastle
upon Tyne,
ABSTRACT The setting reaction of two light-activated glass polyalkenoate cements has been investigated using differential thermal analysis and surface hardness measurements. One material was found to have two distinct phases to its setting reaction. Light activation resulted in a rapid initial set with a large exotherm. A slower setting reaction was detectable when the cement was allowed to set in the absence of light. The surface hardness of this cement increased for some time after the cessation of light activation, indicating continuation of the chemical setting reaction within the material. The ultimate hardness of the cement was significantly higher than its hardness at the termination of light activation. Light activation of the second material increased the rate of set of the cement but no evidence of a dual setting reaction was observed. This material remained very soft and flexible for over 1 h after light activation. A 30 s exposure with a visible light source produced some immediate hardening up to 1.5-2.0 mm below the surface for both materials. KEY WORDS: Glass polyalkenoate (ionomer) cements, Light curing, Physical properties, Setting reaction J. Dent.
1992;
20:
1 15-I
20
(Received 18 June
1991;
reviewed
1 August
1991;
accepted 2 October
1991) Correspondence should be addressed to: Dr A. W. Walls, Department of Operative Dentistry, The Dental School, Framlington Place, Newcastle upon Tyne NE2 48W. UK.
INTRODUCTION The glass polyalkenoate cements were originally developed as aesthetic filling materials (Wilson and Kent, 1972). Development of these cements has led to improvement in their physical properties and in their handling characteristics permitting wider clinical applications. One such application is the use of these cements to bond composite resins to dentine (McLean et al., 1985). This technique involves the use of a glass polyalkenoate to replace missing dentine. When the cement has set it is etched, washed and dried, a resin bonding agent is then applied and cured prior to placement of a composite resin to replace any missing enamel. One disadvantage of this technique is the time required for the cement to set prior to placement of the resin restoration. Bond strength tests have found that the glass polyalkenoate/composite bond fails cohesively within the cement (Hinoura et al., 1987; Welburyet al., 1988). This indicates that the bond strength *This research was carried out in partial fulfilment of the requirements for the Degree of Master of Science in Restorative Dentistry at the University
of Newcastle
@ 1992 Butterworth-Heinemann 0300-5712/92/020115-06
upon
Tyne. Ltd.
is limited by the physical properties of the cement and may explain the findings of Causton et al. (1987) that etching the cement at an early stage in its setting reaction resulted in a weak glass polyalkenoate/composite bond. Even when a fast setting lining cement is used, the bond strength is found to increase with time when the cement is left for up to 15 min before etching (Chin and Tyas, 1988). One approach to overcome the delay required to permit the cement to set prior to placement of the resin has been the development of a number of visible light-activated glass polyalkenoate lining cements. These cements are said to have a dual setting reaction. The primary setting reaction is initiated by exposure to visible light in the region of 470 nm. The method of activation/initiation of set is similar to that utilized in light-activated composite resins. It is accepted that the depth of cure of lightactivated glass polyalkenoate cements will be influenced by those factors which affect the quality of cure of light-activated composite resins. These will include the thickness and opacity of the material (Swartz ef al., 1983),
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J. Dent. 1992; 20: No. 2
the distance from the light source to the surface of the material (Yearn, 1985) and the intensity of the light source at 470 nm (Watts et al., 1984). The secondary setting reaction is an acid-base reaction which commences during mixing and continues after light activation. This leads to maturation of the cement. Failure to light cure these cements will result in a slow, chemical setting reaction. The advantage of this dual setting reaction would be to permit placement of a composite immediately after light activation of the lining. These cements also bond chemically to composite resins as well as to dentine and thus eliminate the need to etch the glass polyalkenoate prior to placement of a composite restoration. If the claims made for these light-activated cements are true then their use in the composite resin/glass polyalkenoate sandwich technique would be expected to decrease the clinical time. However, the effect of the lightactivated setting reaction on the secondary acid-base reaction is unknown. It is possible that the rapid hardening produced by light activation would inhibit the acid-base reaction. If this occurred the resultant material may not exhibit all of the usual properties of the glass polyalkenoate cements including adhesion to dentine. This study was carried out to examine the effect of light activation on these cements and to determine if the chemical setting reaction continues within the cement after light activation or whether the light-activated setting reaction prevents the acid-base reaction occurring.
MATERIALS
AND METHODS
Two light-activated cements were examined, Vitrebond (3M Co., St Paul, MN, USA) and XR Ionomer (Kerr UK Ltd, Peterborough, UK). The materials were mixed at room temperature according to the manufacturer’s instructions at all times. Vitrebond is described as a hydrous type of glass polyalkenoate cement. The liquid component is an aqueous solution of a modified poly(acrylic acid) with a methacrylate end group, a small amount of HEMA (2hydroxyethylmethylacrylate)-less than 10 per cent-and a photoinitiator. The powder is a radiopaque, ionleachable, fluoroaluminosilicate glass which is claimed to be photoactive. Mixture of the powder and liquid components initiates an acid-base type reaction as found in conventional glass polyalkenoate cements. The presence of the HEMA retards this reaction giving a prolonged working time. Subsequent exposure to light from a visible light curing unit results in rapid hardening of the material due to polymerization of the HEMA and the methacrylate end groups. The acid-base reaction continues after light activation and is responsible for the final physical properties of the cement. Failure to light activate Vitrebond will produce a slow setting material with inferior physical properties. The powder component of XR Ionomer is similar .to that of Vitrebond, i.e. a fluoroaluminosilicate glass with the addition of a crystalline photoinitiator. The liquid
component is an aqueous solution of a poly(acrylic acid) with a number of methacrylate groups added but without the addition of aqueous HEMA as in Vitrebond. Mixing the powder and liquid components of XR Ionomer also initiates an acid-base setting reaction while light activation results in polymerization of the methacrylate end groups to provide early strength and resistance to moisture contamination. The final physical properties of the material are due to the continued acid-base setting reaction.
Differential
thermal
analysis
The temperature changes produced by the setting reaction of the test materials, when light activated and when allowed to set chemically, were measured using a differential analysis (DTA) unit (Stanton Redcroft DTA 671, Stanton Redcroft Ltd, London, UK). This has been described previously (McCabe, 1985; Walls et al., 1988). The parameters measured were the time taken for the specimen to reach peak exotherm (T,,,), the time taken for the specimen to cool to a temperature corresponding to 5 per cent of the peak exotherm (5 % Tmax), and the magnitude of the exotherm. Ten samples of each material were mixed according to the manufacturer’s instructions and placed in an aluminium crucible (5.5 mm diameter X 2.15 mm depth). The samples were then covered with a Melinex matrix strip and placed upon the sample thermocouple platform of the DTA unit (Fig. I), operating in isothermal mode at 37”C, between 90 and 120 s from the commencement of mixing. A crucible containing a precured sample of the same material was placed on the reference platform prior to this. Half of the samples of each material (five) were then illuminated for 30 s using a Heliomat light source (Vivadent, Schaan, Liechtenstein), with a twin fibreoptic cable (Coltene, Altstatten, Switzerland) held 2.2 mm from the surface of the material. The light output at 470 nm from the two cables was 898 lux and 964 lux. To ensure that the test and reference samples received equal
Twin fibre-optic cable
Reference aluminium
Fig.
material crucible
in
1. The differential thermal analysis set-up.
Bourke et al.: Setting
reaction of glass polyalkenoate
cements
117
Table 1. The differential thermal analysis parameters and statistical analysis using the Student r-test
Setting reaction
Time to T max (min)
Virrebond Rise in Time to 5% T max temp. (min) (“C)
Weight lmgl
Time to T max (min)
XR lonomer Time to 5% Rise in T max temp. (min) (“C)
Light activated
0.76 (0.04)
4.25 (0.44)
20.33 (2.06)
90.07 (4.0)
1.82 (0.35)
6.49 (0.45)
Chemically cured
3.53 (0.25)
16.19 (6.19)
1.54 (0.46)
87.76 (3.88)
2.95 (0.48)
12.01 (1.29)
Statistical analysis
P < 0.001
P < 0.001
NS
P < 0.01
P < 0.001
P < 0.001
Weight fmg)
5.00 (1.58)
90.10 (8.37)
(E5)
87.44 (3.44)
NS
NS
Figures in parentheses are 1 standard deviation. irradiation, and hence heating, the tibre optic cables were aligned so that illumination of empty crucibles placed on the thermocouple platforms produced zero displacement of the differential recorder trace. A jig was constructed so that the fibre optic cable could be relocated in this position accurately. Following illumination of the test and reference samples the tibre optic cable was removed and a lightproof cover placed over the entrance to the DTA chamber. For the other live samples of each material the cover was placed over the DTA chamber immediately without any irradiation from the light source. All specimens were weighed at the end of the reaction. Five further tracings were obtained for each test material using cured samples on both the reference and sample thermocouple platforms which were irradiated for 30 s. The mean temperature rise produced by a 30 s irradiation was then calculated for both materials and used to differentiate the rise in temperature produced by the light source from that produced by the exothermic setting reaction of the material when light activated.
Hardness
tests
The hardness of the upper and lower surfaces of samples of test materials of varying thickness were measured with respect to time using a Vickers hardness tester (Leitz GmbH, Wetzlar, Germany). Samples of each test material were produced using polypropylene moulds of 0.5 mm, 1.0 mm, 1.5 mm and 2.0 mm in depth with a diameter of 6.0 mm. The moulds were placed on a Melinex matrix strip and filled with the test material which was mixed according to the manufacturer’s instructions. The material was covered with a Melinex matrix strip and a glass slide was placed firmly over this to express any excess cement. The top surface of the specimen was illuminated for 30 s with the same light source as used in the DTA experiments but with a single fibre optic cable which was placed against the glass slide. Samples were then placed in a light-proof container prior to hardness testing. Specimens that were to be stored for more than 30 min were placed in liquid paraffin 1.5 min after irradiation to prevent dehydration of the cement. For each thickness of material the hardness of the top and bottom surfaces was measured at 3 min, 10 min,
15 min, 30 min, 1 h, 4 h, 1 day and 7 days from the commencement of mixing. Five samples of each thickness were measured at every time interval and three readings were taken from each surface tested. The hardness of each surface of a sample was only measured at one time interval as repeated exposure to the light incorporated in the microscope of the hardness tester was found to affect the hardness of the cements.
RESULTS The DTA parameters for the test materials when light activated and when allowed to cure chemically are shown in Table I. Statistical analysis of the DTA parameters for the light-activated and chemically cured samples of the test materials was carried out using an unpaired Student r-test. The results of these tests (Table I) show that light activation of Vitrebond produced a faster setting reaction with a greater exotherm compared with that when it is allowed to cure chemically. Light activation of XR Ionomer on the other hand produced a faster setting reaction but there was no significant difference in the peak exotherm produced. The rise in temperature induced by a 30-s light activation of precured samples was 5 “C for both materials. The mean Vickers hardness number (VHN) for the top and bottom surfaces of the different thicknesses and postexposure times of the test materials are shown in Tables II and III. The results for XR Ionomer include only hardness values from 1 h onwards. The cement was sufficiently soft and flexible within 1 h of mixing so that although the hardness tester produced indentations which were visible to the naked eye, these could not be measured using the microscope as no sharp margin could be defined between the indentation and the surface of the material. However, it was noted that the bottom surfaces of the 2.0 mm samples were unset immediately after irradiation. Comparison of the top and bottom surfaces for each thickness of Vitrebond and postexposure time using an unpaired Student r-test (Table II,) suggested that the rate of set of the two surfaces was affected by the thickness of the specimen and the postirradiation time. A three-way
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J. Dent 1992; 20: No. 2
Table II. The mean hardness with a visible light source
(VHN)
of the top and bottom
JOP
thicknesses
l.Omm Bottom
3 min IOmin 15min 30 min Ih 4h 1 day 7 day
1.51(0.54) 1.48(0.34) 2.04(0.66) 5.1 l(1.68) 9.56(1.83) 1 1.36( 1.89) 14.57(2.83) 14.66(2.61)
l?mes
from the commencement
are taken
of various
of Vitrebond
with time, following
a 30-s
irradiation
Sample thickness
0.5 mm Time
surfaces
1.23(0.27)* 1.52(0.38) 2.06(0.58) 5.1 l(1.48) 8.90(1.20) 10.70( 1.68) 14.71(2.53) 13.82(1.24) of mixing.
0.74(0.1 l)** 1.45(0.33) 1.77(0.52) 3.58(0.90) 9.69( 1.55) 9.84(1 .Ol) 13.06(2.51) 13.13(2.14)
0.96(0.36) 1.44(0.32) 1.90(0.52) 3.77(0.74) 10.02(1.73) 10.36( 1.34) 14.25(2.59) 12.84(1.10) Figures
1.5mm
Bottom
Jop
in brackets
are 1 standard
Jop 0.93( 1.32) 1.48(0.40) 2.71(1.73) 4.33(0.76) 8.10(1.53) 1 1.52(1.06) 14.07(2.19) 13.24(1.42) deviation.
2.0 mm
0.67(0.18)** 0.96(0.17)** 1.71(0.36)** 2.58(0.55)** 6.01(1.12)** 11.32(0.97) 12.89(2.15)* 13.14(0.92)
*P < 0.05,
**P
< 0.01
Bottom
Jop
Bottom
0.86(0.40) 1.85(0.55) 2.07(0.56) 2.30(0.39) 8.01(1.59) 10.52( 1.73) 1 1.37(2.03) 11.61(0.94) for top vs bottom
** 0 0.86(0.37)** 1.24(0.26)** 1.43(0.53)** 6.85(1 .16)** 8.54(2.42)** 1 1.85(1.45) 11.33(0.81)
using an unpaired
Student
t-test.
Jab/e 111.The mean hardness visible light source
(VHN)
of the top and bottom surfaces of various thicknesses
of XR lonomer with time, following
a 30-s irradiation with a
Samp/e thickness 0.5 mm Time lh 4h 1 day 7 day
JOP 1.5 l(O.44) 7.10(1.50) 12.15(1.88) 20.01(2.27)
Bottom 1.48(0.37) 6.54( 1.78) 13.25(1.74) 19.31(3.07)
Bottom
JOP 2.22( 1.30) 9.96(2.97) 12.34(2.47) 19.56( 1.54)
1.95( 1.05) 1 1.64(3.70) 13.17(2.93) 19.28(1 .12)
Times are taken from the commencement of mixing. Figures in brackets are
2.0 mm
1.5mm
l.Omm
1 standard
analysis of variance (ANOVA) found that the relationship between the hardness of the two surfaces differed significantly with changes in the thickness of the material (P < 0.001) and the postirradiation time (P = 0.002). Comparison of the top and bottom surfaces for each thickness and postexposure time of XR Ionomer (Table 111) showed little difference in the hardness of the two surfaces.
DISCUSSION The DTA parameters used in this study were those measured by Walls et al. (1988) and McCabe (1985). It has been suggested that the time taken for a material to reach T max is closely related to the clinical setting time of the material (McCabe and Wilson, 1980). The time taken for a specimen to cool to a temperature corresponding to 5 % T max is used as a measure of the actual setting time (Walls et al., 1988).
Vitrebond The setting characteristics of Vitrebond when light activated and chemically cured differ significantly (Table I). The times to T,,, and 5 % T,,, for the light-activated samples are considerably shorter than the same parameters for a conventional glass polyalkenoate lining cement measured at 23°C (Walls et al., 1988). The temperature rise produced is much closer to that of lightactivated composite resins (McCabe, 1985), than that of conventional glass polyalkenoate cements (Walls ef al., 1988). As irradiation of the precured samples of Vitrebond
JOP 1.82(0.89) 7.24( 1.29) 12.95(2.22) 18.98(1.93) deviation.
?? P < 0.05
Bottom 1.97(0.89) 7.33(0.92) 14.00(1.44)’ 20.1 1(1.87)* for top vs bottom
Bottom
Jop 3.24(0.90) 8.45(1.68) 16.59(5.21) 22.88(1.57) using an unpaired
3.33(0.78) 8.18(1.89) 14.69(4.00) 23.49(2.14) Student
t-test.
produced a temperature rise of 5”C, the large rise in temperature of the cement upon light activation is mainly due to the heat of reaction of the cement. The DTA parameters measured for the chemically cured samples of Vitrebond are similar to those of conventional glass polyalkenoate cements (Walls et al., 1988). The difference in the DTA parameters for the lightactivated and chemically cured samples of Vitrebond suggest that two setting reactions are occurring. This supports the manufacturer’s claim that light activation causes polymerization of the HEMA and modified poly acid molecules in the cement and that an acid-base chemical reaction occurs after mixing of the cement. However, it was not possible to discern from the DTA tests whether this chemical reaction continued after light activation, as has been claimed by the manufacturer. The large exothermic reaction produced by Vitrebond when light activated is a cause for concern. The manufacturer claims that there is no need for a sublining in deep cavities unless there is a pulpal exposure. However, as Zach and Cohen (1965) found that an increase in intrapulpal temperature of as little as 5.55 “C produced a marked pulpal response in monkeys, sometimes leading to pulpal necrosis, the use of Vitrebond in unlined deep cavities is questionable. The changes in hardness with time of the top (Fig 2) and bottom (Fig. 3) surfaces for the various thicknesses of Vitrebond indicate that the cement continues to undergo a setting reaction after irradiation. The hardness of the top surfaces increases rapidly for up to 1 h after light activation and then continues to increase gradually for up to 24 h, after which there is no significant increase, and
Bourke et al.: Setting
reaction
of glass polyalkenoate
cements
119
15
12
9
3
0 00 Time
(h)
Fig. 2. The hardness of the top surfaces of the various thicknesses of Vitrebond with time (log scale). Sample thickness: -, 0.5mm; ---, l.Omm; -----, 1.5 mm; - - - - - - -, 2.0 mm.
in some cases a small decrease in hardness. There are two possible explanations for this prolonged improvement in physical properties, either continuation of the methacrylate-based cross-linking reaction or of the ‘glass polyalkenoate’ type, acid-base chemical setting reaction. The main increase in hardness of light-activated composite resins has been found to occur within 10 min after irradiation (Hansen, 1983), although it may continue to increase slowly for up to 24 h (Leung et al., 1983). Glassionomer cements have a much slower setting reaction, with maturation of the cement occurring for up to 24 h after mixing leading to an increase in the physical properties of the cement including hardness (Crisp et al., 1976). The light-activated setting reaction would be expected to be responsible for a rapid increase in hardness of Vitrebond in the first lo-15 min, after which its effects would decrease significantly. As the hardness of the top surfaces continue to increase for 24 h after irradiation this implies that the chemical setting reaction continues within the cement after light activation. The bottom surfaces of the 2.0 mm samples were unset at the 3-min time interval (approximately 1 min after light activation allowing for mixing, placement and irradiation of the cement). This implies that the depth of cure of Vitrebond produced by a 30-s illumination is less than 2.0 mm. It also implies that hardness of the 2.0 mm samples, and the other sample thicknesses, at this time interval is due mainly to the light-activated setting reaction. Statistical analysis using ANOVA found that the relationship between the hardness of the top and bottom surfaces varied significantly with changes in the thickness and postirradiation time. Examination of Fig. 4, which plots the ratio of hardness (top/bottom) against time, gives an indication of how the thickness of the sample affects the light-activated setting reaction. The tracings for the 0.5 mm and 1.0 mm sample thicknesses indicate that they do not attenuate the passage of light significantly. The small delay in the hardening of the bottom surface of these
Time
[h)
Fig. 3. The hardness of the bottom surfaces of the various thicknesses of Vitrebond with time (log scale). Sample thickness: -, 0.5mm; ---, l.Omm; -----, 1.5 mm; - - - - - - -, 2.0 mm.
2.2
j :: B 4 z v)
r I
0
2.01.81.6-
z 6 : _c 'c 0 .0 u d
1.41.2l.O-
Time
(h)
Fig. 4. Ratio of hardness (top/bottom) of the various thicknesses of Vitrebond with time (log scale). Sample thickness: -, 0.5mm; ---, l.Omm; -----, 1.5 mm; - - - - - - -, 2.0 mm.
thicknesses compared with the top surface is similar to that found with light-activated composite resins (Leunget al., 1983). Examination of the tracing for the 2.0 mm samples provides a contrasting picture. The initial ratio is infinity and decreases to near 1.O by 1 day and remains at this level. The fact that the bottom surface was unset at 3 min does not necessarily imply that the increase in hardness of this surface was due entirely to the chemical setting reaction. Leung et al. (1983) found that the bottom surface of a 2.5 mm sample of composite resin showed no detectable polymerization-as measured by surface hardness-immediately after a 15-s light exposure but that the hardness of the bottom surface increased with postirradiation time for up to 1 day, indicating that lightactivated polymerization was occurring. These authors also noted that in cases where the bottom surface was significantly softer than the top surface immediately after irradiation, the hardness of the bottom surface remained
120
J. Dent 1992; 20: No. 2
significantly lower. However, it can be seen from Table II and Fig 4 that there is no significant difference in the hardness of the two surfaces of the 2.0-mm samples at 1 and 7 days. This provides further evidence that the chemical setting reaction is responsible for the ultimate hardness of the cement. The initial ratio for the 1.5 mm samples of about 1.4 implies that this thickness of Vitrebond produces some attenuation of the light resulting in less polymerization of the HEMA at the bottom surface. The ratio then increases to 1.6 at 30 min after which it decreases to near 1.0 by 4 h. Swartz et al. (1983) found that the hardness of the deeper layers of composite resins did not increase at the same rate as the top and this may explain the increase in the hardness ratio up to 30 min. However, as the effects of the chemical setting reaction become more marked the ratio decreases again. Therefore it can be seen that the sample thickness of Vitrebond affects the rate of the light-activated setting reaction. As the sample thickness increases from 0.5 mm to 2.0 mm the difference in the rate of set between the upper and lower surface increases considerably.
XR lonomer Light activation of XR Ionomer does not produce as rapid a setting reaction as seen with Vitrebond although the setting reaction is faster than that of Ketac Bond at 23°C (Walls et al., 1988). The rise in temperature of lightactivated XR Ionomer (5°C) is similar to that produced when a cured sample is illuminated for 30 s. However, this does not mean that there is no exothermic reaction occurring when XR Ionomer is light activated. This cement produced a much broader peak for the DTA tracing than Vitrebond which suggests that the illumination may be responsible for the early temperature rise and the exothermic reaction keeps the sample at this temperature for longer. Comparison of the DTA variables shows that failure to light activate XR Ionomer results in a slower setting cement. However, as there is no difference in the exotherm produced whether the cement is light activated or not, this suggests that the magnitude of the light-activated setting reaction is much less than that of Vitrebond. This is probably due to the polymerization of the HEMA component of Vitrebond but there is also the possibility that light activation of XR Ionomer does not produce a separate setting reaction and that the heating effect of the light source increases the rate of set of the acid-base reaction. The hardness tests on XR Ionomer suggest that light activation has little affect on the rate of set of this cement. However, these tests do reveal that the final properties of the cement are due to the acid-base reaction as the hardness continued to increase up to and beyond 24 h,
while earlier work has shown that the main setting reaction of light-activated materials is complete at 10 min (Hansen, 1983). The hardness tests also found that the immediate depth of cure produced by light activation is less than 2.0 mm. The two cements studied here are both marketed as lightactivated glass polyalkenoate cements for use as a lining material. While the final physical properties of both the cements have been found to be the result of the acid-base reaction, the effect of light activation on the rate of set of the two cements is markedly different. The effects of the different rates of set produced by light activation on the bond strength of the cements to dentine will be investigated in a further study.
References Causton B. E., Sefton J. and Williams A. (1987) Bonding class II composite to etched glass ionomer cement. Br. Dent. J. 63, 321-324. Chin Y. H. and Tyas M. J. (1988) Adhesion of composite resin to etched glass ionomer cement. Amt. Dent. J. 33, 87-90. Crisp S., Lewis B. G. and Wilson A. D. (1976) Characterisation of glass-ionomer cements. I. Long term hardness and compressive strength. J. Dent. 4, 162-166. Hansen E. K. (1983) After-polymerisation of visible light activated resins: surface hardness vs. light source. Stand. J. Dent. Res. 91,406-410. Hinoura K., Moore B. K. and Phillips R. W. (1987) Tensile bond strength between glass ionomer cements and composite resins. J. Am. Dent. Assoc. 114, 167-172. Leung R. L., Fan P. L. and Johnson W. M. (1983) Postirradiation polymerisation of visible light-activated composite resin. J. Dent. Res. 62, 363-365. McCabe J. F. (1985) Cure performance of light activated composites by differential thermal analysis (DTA). Dent. Mater. 1, 231-234. McCabe J. F. and Wilson H. J. (1980) The use of differential scanning calorimetry for the evaluation of dental materials. J. Oral Rehabil. 7, 103-110. McLean J. W., Prosser H. J. and Wilson A. D. (1985) The use of glass-ionomer cements in bonding composite resin to dentine. Br. Dent. J. 158,410-414. Swartz M. L., Phillips R. W. and Rhodes B. (1983) Visible light-activated resins-depth of cure. J. Am. Dent. Assoc. 106, 634-637. Walls A W. G., McCabe J. F. and Murray J. J. (1988) Factors influencing the setting reaction of glass polyalkenoate (ionomer) cements. J. Dent. 16, 32-35. Watts D. C., Amer 0. and Combe E. C. (1984) Characteristics of visible-light-activated composite systems. Br. Dent. .I 156, 209-215. Welbury R. R., McCabe J. F., Murray J. J. et al. (1988) Factors affecting the bond strength of composite resin to etched glass-ionomer cement. J. Dent. 16, 188-192. Wilson A. D. and Kent B. E. (1972) A new translucent cement for dentistry. Br. Dent. J. 132, 133-135. Yearn J. A (1985) Factors affecting cure of visible light activated composites. ht. Dent. J. 35, 218-225. Zach L. and Cohen G. (1965) Pulp response to externally applied heat. Oral Surg. Oral Med. Oral Pathol. 19, 515-530.
