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Effect of radiant heat on the surface hardness of glass polyalkenoate (ionomer) cement
J. Dent. 1994; 22: 360-363
Effect of radiant heat on the SW hardness of glass poly cement M. J. Woolford Department of Conservative Dentistry, The University, Dundee, UK
ABSTRACT The use of heat to improve mechanical properties of materials is a widely accepted phenomenon. It has been studied in dentistry with a view to improving the properties of resin composite. Dental cements may benefit by the application of heat, in particular with regard to their early surface properties. This study was carried out to examine the effect of the application of radiant heat to the surface hardness of one type of glass polyalkenoate cement. It was found that raising the temperature of the surface of the cement to a maximum of 60°C significantly improved the early surface hardness of the material. The application of a high level of heat also improved the surface hardness of the cement after 24 h compared to cement which had not been heat treated. The use of heat would appear to accelerate the matrix-forming reaction of the material and although further work is necessary this technique may have clinical application. KEY WORDS: Materials, Heat, Glass polyalkenoate, Surface J. Dent. 1994; 22: 360-363 January 1994)
(Received 16 August 1993; reviewed 24 September 1993; accepted 17
Correspondence should be addressed to: Dr M. J. Woolford, Department of Conservative Dentistry, University of Dundee Dental School, Dundee DDl 4HN, UK.
INTRODUCTION
MATERIALS AND METHODS
Heat has been shown to be beneficial to the properties of a number of dental materials, in particular resin composite lp5. Conversely some authors have shown a detrimental effect of the application of heat to this group of material@,‘. Little work has been carried out with regard to the application of heat to dental cements. Brune8 studied the effect of infrared radiation on a number of properties of a variety of dental cements. Dental silicate cement was studied in a similar manner by Brune and Evje9. No work has been published on the effect of the potential for the application of heat to a modern formulation of glass polyalkenoate cement. Heat is applied to the surface and so the effect is most likely to be seen, in the first instance, as a surface phenomenon. The following study was undertaken, therefore, to examine the effect of radiant heat on the surface hardness of one type of glass polyalkenoate cement.
The material used for this series of experiments was a commercially available glass polyalkenoate cement (Chemfil II, DeTrey Division, Dentsply, Weybridge, Surrey, UK) at a powder : liquid ratio of 6.8 : 1 w/w. The cement was mixed by hand using an agate spatula on a glass slab. The glass slab was kept at room temperature of 22S”C (k 2’C). Specimens were constructed using a polytetrafluoroethylene (PTFE) mould 5 mm in diameter and 2 mm in depth. The PTFE mould was held on a glass slab and had a cover of cellulose acetate sheet acting as a matrix. A glass microscope slide was placed on top of the cellulose acetate sheet to give the specimen a flat surface. The slide was held in position by a 1 kg weight. The mould was preheated to 37°C and the mould was returned to the incubator, once the cement had been inserted. for the period of set of the cement. This particular glass polyalkenoate cement has an initial set time of 4 min (manufacturer’s data) at which point the cement is no
o 1994 Butterworth-Heinemann Ltd. 0300-5712/94/060360-04
. Woolford:
longer capable of manipulation. The material reaches its clinical set point after 7 min (manufacturer’s data) and it is only after this time that it may be finished. At the end of the clinical set time the specimen was carefully removed from the mould and placed on the surface hardness machine. The surface hardness of a group of 20 glass polyalkenoate cement specimens was determined with a Vickers Hardness Testing Machine (Leitz GMBH, Wetzlar, Germany). A stopwatch was used to time all procedures from the start of mixing. The surface hardness of the glass polyalkenoate cement specimens will be increasing as the material sets. Glass polyalkenoate cements do not reach their fully set properties for approximately 24 h after mixing. Therefore measurement of the surface hardness must have a time-dependent factor, if examined within the first 24 h from mixing. The times at which the readings for the surface hardness were taken were carefully recorded in this series of experiments to ensure that readings were taken for all the specimens in each group at the same time after mixing to ensure that the results would be comparable between each group. It was found during initial experimentation that each reading took 3 min to carry out. Readings were therefore taken at 7, 10 and 13 min after the completion of mixing. The surface hardness was measured for three different areas on the surface of 20 specimens and a mean value calculated from these. A second group of 20 specimens was allowed to reach the initial set point 4 min after completion of mixing, at which time they were placed under an infrared light source (Phillips Electrical, Holland) for 60 s. The particular infrared light source used was a large reflecting 150 W infrared bulb, mounted in a holding case to allow easy manipulation and the ability to set the direction of the infrared radiation. The intensity of the radiation could only be controlled by adjusting the distance of the specimen from the bulb. The temperature produced from the source was measured using a thermocouple attached to the surface of a specimen. A gap of 2 cm between the specimen and the infrared source was found to give a temperature at the surface of the cement of up to 80°C after 60 s. All specimens were placed 2 cm from the source and the length of time of exposure accurately measured to ensure that all specimens were exposed for a similar time and to a similar intensity of radiation. During the period of exposure the cement was covered with the acetate sheet to prevent excess drying and loss of surface water. Immediately after exposure the specimen was removed from the mould and the surface hardness determined, a careful note being taken of the time from completion of mix of the cement to the point when each reading was taken. The heat was applied at the initial set time of the cement (4 min) as it was felt that this would be the most appropriate time from the point of view of using the technique in the clinical situation. The size and shape of the infrared source used meant that it had no potential for intraoral use. A readily
Heat effect on glass polyalkenoate
cement
361
Table 1. Results for the surface hardness of glass polyalkenoate cement at the clinical set point when subjected to radiant heat from a quartz halogen fibreoptic light source for various times at the initial set point Time of application of heat Mean surface hardness (VHf’J) Standard deviation
0
20
40
60
24.1
43.3
41.6
48.9
5.0
2.3
2.1
5.6
available quartz halogen tibreoptic light source (Litema GSD, Baden-Baden, Germany) had the capacity to allow unfiltered high-intensity quartz halogen light through the tibreoptic handpiece. This allowed the use of a radiant heat source which had potential intraoral use. This radiant heat source was kept 1 mm from the surface of the cement by using a sleeve over the end of the fibreoptic tip which protruded by a distance of 1 mm. The effect on early surface hardness of varying the period of exposure of the specimens to radiant heat from a high-intensity quartz halogen light source was studied by constructing groups of 20 specimens, as previously described, and each group was exposed to the highintensity light source for 0,20,40 and 60 s. In order to study the long-term consequences of the application of radiant heat on the surface hardness of this particular glass polyalkenoate cement further groups were exposed to the high-intensity quartz halogen light source for 0, 10,30,45 and 60 s at the initial set point of the cement. Each specimen was coated with two layers of waterproof varnish (Special Varnish, De Trey). then placed in distilled water and stored in an incubator at a temperature of 37°C for 24 h. At the end of this time the varnish was gently removed using a tissue and the surface hardness determined. The 24 h time period was chosen as this marks an accepted fully set point for this group of materials and so the control group specimens which were not exposed to radiant heat will have reached an optimum surface hardness.
RESULTS Preliminary investigations demonstrated that there was no statistical difference to hardness values taken at the beginning and end of the 9 min (3 X 3 min) period (ANOVA P > 0.05). It was considered valid, therefore, to use the mean value of the three readings as the surface hardness value (VI-IN) for each specimen. When infrared radiation from the infrared light source was applied to the surface of the cement the mean surface hardness (VI-IN) was 50.0 (s.d. = 5.5). For the nonirradiated control group the mean surface hardness (VI-IN) was 24.1 (s.d. = 5.0). Statistical analysis (t test) showed a highly significant difference (P < 0.001) between the irradiated and the non-irradiated group. Table Z shows the results for varying the length of time of application of
362
J. Dent. 1994; 22: No. 6
Tab/e II. Results for the surface hardness of glass polyalkenoate cement after 24 h when subjected to radiant heat from a quartz halogen fibreoptic light source for various times at the initial set point Time of application of heat Mean surface hardness
0
15
30
45
60
62.4
61.4
60.3
62.2
71.8
9.1
6.1
4.8
3.9
10.6
W-W Standard deviation
radiant heat from the high intensity quartz halogen tibreoptic light source to the cement surface as an immediate effect on the surface hardness. Statistical analysis (ANOVA) showed a highly statistically significant difference between the groups (P < 0.01). The differences were localized using a Tukey comparison of means. All the irradiated groups had a statistically significantly higher surface hardness than the specimens that were not exposed to radiant heat (P < 0.01). There was a statistically significant difference between those specimens irradiated for 60 s and all other groups (P < 0.01). There was no statistically significant difference (P > 0.05) between the groups irradiated for 20 s and those irradiated for 40 s. Table II shows the long-term effect on the surface hardness when varying length of time of application of heat from the fibreoptic light source used at the initial set point and the surface hardness measured after 24 h. Statistical analysis (ANOVA, Tukey comparison of means) showed that the group irradiated for 60 s was significantly higher than all the other groups (P < 0.01).
DISCUSSION Glass polyalkenoate cements are prone to loss of surface water and desiccation during setting. No satisfactory method to prevent some drying taken place was discovered whilst the surface hardness measurement was being carried out. The time period for each specimen being in air was kept as short as possible and was constant for each specimen. The humidity of the environment around the testing machine was also kept as constant as possible. Although there was the possibility of some drying of the specimens, this could not be prevented and it was felt that as this was the same for all specimens the results would still be comparable. The results clearly showed an increase in surface hardness for the material after the application of heat from both infrared and high-intensity quartz halogen libreoptic light sources. This is to be expected as the application of heat will accelerate the setting reaction of the cement and so during the early time period after mixing the stage of set will be more advanced and thus the surface hardness will be greater. The increased hardness after 24 h compared to the unheated specimen was not expected. This was only found to be true for cement exposed to 60 s of radiant heat from the high-intensity
quartz halogen light source. This may be explained by the potential for the setting reaction to be considerably accelerated by the heat in the early stages and that this scale of acceleration is maintained even after 24 h. This is the case only for the specimen exposed for 60 s, which suggests that this level of energy is required to have a significant long-term effect. The use of the infrared lamp had a slightly greater effect upon the surface hardness than the high-intensity quartz halogen fibreoptic light source. Measurement of the surface temperature produced by each source gave temperatures of up to 60°C for the quartz halogen tibreoptic light source and up to 80°C after 60 s for the infrared light source. It is clear that the temperature produced by the infrared lamp is greater and so may have a greater effect on the surface hardness of the cement. The infrared lamp, however, was inappropriate to use clinically and so all further experimentation was carried out using the high-intensity quartz halogen fibreoptic light source. The effect of the heat on the setting reaction is likely to be to make the poly(alkenoic acid) more active. Therefore when the acid contacts the surface of the glass it is able to break down the surface of the glass more readily. This will increase the rate at which the ions are leached and released from the glass. A more reactive acid and greater rate of release of ions will lead to a more rapid cement formation by gelation of the insoluble products of the acid and basic glass. The heat would also increase the diffusion rates of the various ions leached from the glass. The total effect of all these accelerated reactions will be to produce a more rapid formation of the calcium polyalkenoate matrix, the components of the first formed matrix. As the surface hardness of the material increases, at least for the first period of setting, it will demonstrate the production of the components of the first stage of the setting reaction. Glass polyalkenoate cements take a significant time to reach a set point at which they may be subjected to stress. the accepted time period being up to 24 h. The application of heat has been shown to accelerate the setting reaction, at least on the surface of the cement. The technique of applying heat to the surface of the cement intraorally may be beneficial, therefore, in improving the early properties of the material at a time when it is most susceptible to damage. Further work is necessary to establish the potential effect of heat, applied in this way, to the dental pulp and to other properties of the glass polyalkenoate cement. Acknowledgements This project was supported by Scottish Endowment Research Trust, Grant 892.
Hospitals
References 1. Bausch JR, De Lange C and Davidson CL. The influence of temperature on some physical properties posites. J Oral Rehabil 1981; 8: 309-317.
of dental
com-
Woolford: Heat effect on glass polyalkenoatecement
2. Cook WD and Johansson M. The influence of postcuring on the fracture properties of photo-cured dimethacrylate based dental composite resin.JBiomedMaterRes 1987; 21: 979-989. 3. Wendt SL. The effect of heat used as a secondary cure upon the physical properties of three composite resins. I. Diametral tensile strength, compressive strength and marginal dimensional stability. Quintessence Int 1987; 18: 265-271. 4. Wendt SL. The effect of heat used as a secondary cure upon the physical properties of three composite resins. II. Wear, hardness and colour stability. Quintessence Int 1987; 18: 351-356. 5. Wendt SL. Time as a factor in the heat curing of composite resin. Quintessence Int 1989: 20: 259-263.
363
6. Draughn R Effects of temperature on mechanical properties of composite dental restorative materials. .I Biomed Mater Res 1981; 15: 489-495. 7. Greener EH, Greener CS and Moser JB. The hardness of composites as a function of temperature. J Oral Rehabil 1984; 11: 335-340. 8. Brune D. Heat treatment of glass ionomer, silicate, zinc phosphate and zinc polycarboxylate cements. Stand JDent Res 1982; 90: 409-412. 9. Brune D and Evje D. Improvement of erosion resistance of a silicate cement by infrared radiation. ScandJDent Res 1982; 90: 413-416..
Announcement
Relocation of Journal Publishing Office As from 1st October 1994, the publishing, editorial, production and reprint offices are moving to: Elsevier Science Ltd The Boulevard, Langford Lane, Kidlington, Oxford, OX5 IGB, UK Main switchboard: Tel: +44 (0)1865 843000 Fax: +44 (0) 1865 843010 All correspondence and queries relating to the journal should be sent to this address Newly submitted manuscripts should be mailed to the Editor
Effect of radiant heat on the SW hardness of glass poly cement M. J. Woolford Department of Conservative Dentistry, The University, Dundee, UK
ABSTRACT The use of heat to improve mechanical properties of materials is a widely accepted phenomenon. It has been studied in dentistry with a view to improving the properties of resin composite. Dental cements may benefit by the application of heat, in particular with regard to their early surface properties. This study was carried out to examine the effect of the application of radiant heat to the surface hardness of one type of glass polyalkenoate cement. It was found that raising the temperature of the surface of the cement to a maximum of 60°C significantly improved the early surface hardness of the material. The application of a high level of heat also improved the surface hardness of the cement after 24 h compared to cement which had not been heat treated. The use of heat would appear to accelerate the matrix-forming reaction of the material and although further work is necessary this technique may have clinical application. KEY WORDS: Materials, Heat, Glass polyalkenoate, Surface J. Dent. 1994; 22: 360-363 January 1994)
(Received 16 August 1993; reviewed 24 September 1993; accepted 17
Correspondence should be addressed to: Dr M. J. Woolford, Department of Conservative Dentistry, University of Dundee Dental School, Dundee DDl 4HN, UK.
INTRODUCTION
MATERIALS AND METHODS
Heat has been shown to be beneficial to the properties of a number of dental materials, in particular resin composite lp5. Conversely some authors have shown a detrimental effect of the application of heat to this group of material@,‘. Little work has been carried out with regard to the application of heat to dental cements. Brune8 studied the effect of infrared radiation on a number of properties of a variety of dental cements. Dental silicate cement was studied in a similar manner by Brune and Evje9. No work has been published on the effect of the potential for the application of heat to a modern formulation of glass polyalkenoate cement. Heat is applied to the surface and so the effect is most likely to be seen, in the first instance, as a surface phenomenon. The following study was undertaken, therefore, to examine the effect of radiant heat on the surface hardness of one type of glass polyalkenoate cement.
The material used for this series of experiments was a commercially available glass polyalkenoate cement (Chemfil II, DeTrey Division, Dentsply, Weybridge, Surrey, UK) at a powder : liquid ratio of 6.8 : 1 w/w. The cement was mixed by hand using an agate spatula on a glass slab. The glass slab was kept at room temperature of 22S”C (k 2’C). Specimens were constructed using a polytetrafluoroethylene (PTFE) mould 5 mm in diameter and 2 mm in depth. The PTFE mould was held on a glass slab and had a cover of cellulose acetate sheet acting as a matrix. A glass microscope slide was placed on top of the cellulose acetate sheet to give the specimen a flat surface. The slide was held in position by a 1 kg weight. The mould was preheated to 37°C and the mould was returned to the incubator, once the cement had been inserted. for the period of set of the cement. This particular glass polyalkenoate cement has an initial set time of 4 min (manufacturer’s data) at which point the cement is no
o 1994 Butterworth-Heinemann Ltd. 0300-5712/94/060360-04
. Woolford:
longer capable of manipulation. The material reaches its clinical set point after 7 min (manufacturer’s data) and it is only after this time that it may be finished. At the end of the clinical set time the specimen was carefully removed from the mould and placed on the surface hardness machine. The surface hardness of a group of 20 glass polyalkenoate cement specimens was determined with a Vickers Hardness Testing Machine (Leitz GMBH, Wetzlar, Germany). A stopwatch was used to time all procedures from the start of mixing. The surface hardness of the glass polyalkenoate cement specimens will be increasing as the material sets. Glass polyalkenoate cements do not reach their fully set properties for approximately 24 h after mixing. Therefore measurement of the surface hardness must have a time-dependent factor, if examined within the first 24 h from mixing. The times at which the readings for the surface hardness were taken were carefully recorded in this series of experiments to ensure that readings were taken for all the specimens in each group at the same time after mixing to ensure that the results would be comparable between each group. It was found during initial experimentation that each reading took 3 min to carry out. Readings were therefore taken at 7, 10 and 13 min after the completion of mixing. The surface hardness was measured for three different areas on the surface of 20 specimens and a mean value calculated from these. A second group of 20 specimens was allowed to reach the initial set point 4 min after completion of mixing, at which time they were placed under an infrared light source (Phillips Electrical, Holland) for 60 s. The particular infrared light source used was a large reflecting 150 W infrared bulb, mounted in a holding case to allow easy manipulation and the ability to set the direction of the infrared radiation. The intensity of the radiation could only be controlled by adjusting the distance of the specimen from the bulb. The temperature produced from the source was measured using a thermocouple attached to the surface of a specimen. A gap of 2 cm between the specimen and the infrared source was found to give a temperature at the surface of the cement of up to 80°C after 60 s. All specimens were placed 2 cm from the source and the length of time of exposure accurately measured to ensure that all specimens were exposed for a similar time and to a similar intensity of radiation. During the period of exposure the cement was covered with the acetate sheet to prevent excess drying and loss of surface water. Immediately after exposure the specimen was removed from the mould and the surface hardness determined, a careful note being taken of the time from completion of mix of the cement to the point when each reading was taken. The heat was applied at the initial set time of the cement (4 min) as it was felt that this would be the most appropriate time from the point of view of using the technique in the clinical situation. The size and shape of the infrared source used meant that it had no potential for intraoral use. A readily
Heat effect on glass polyalkenoate
cement
361
Table 1. Results for the surface hardness of glass polyalkenoate cement at the clinical set point when subjected to radiant heat from a quartz halogen fibreoptic light source for various times at the initial set point Time of application of heat Mean surface hardness (VHf’J) Standard deviation
0
20
40
60
24.1
43.3
41.6
48.9
5.0
2.3
2.1
5.6
available quartz halogen tibreoptic light source (Litema GSD, Baden-Baden, Germany) had the capacity to allow unfiltered high-intensity quartz halogen light through the tibreoptic handpiece. This allowed the use of a radiant heat source which had potential intraoral use. This radiant heat source was kept 1 mm from the surface of the cement by using a sleeve over the end of the fibreoptic tip which protruded by a distance of 1 mm. The effect on early surface hardness of varying the period of exposure of the specimens to radiant heat from a high-intensity quartz halogen light source was studied by constructing groups of 20 specimens, as previously described, and each group was exposed to the highintensity light source for 0,20,40 and 60 s. In order to study the long-term consequences of the application of radiant heat on the surface hardness of this particular glass polyalkenoate cement further groups were exposed to the high-intensity quartz halogen light source for 0, 10,30,45 and 60 s at the initial set point of the cement. Each specimen was coated with two layers of waterproof varnish (Special Varnish, De Trey). then placed in distilled water and stored in an incubator at a temperature of 37°C for 24 h. At the end of this time the varnish was gently removed using a tissue and the surface hardness determined. The 24 h time period was chosen as this marks an accepted fully set point for this group of materials and so the control group specimens which were not exposed to radiant heat will have reached an optimum surface hardness.
RESULTS Preliminary investigations demonstrated that there was no statistical difference to hardness values taken at the beginning and end of the 9 min (3 X 3 min) period (ANOVA P > 0.05). It was considered valid, therefore, to use the mean value of the three readings as the surface hardness value (VI-IN) for each specimen. When infrared radiation from the infrared light source was applied to the surface of the cement the mean surface hardness (VI-IN) was 50.0 (s.d. = 5.5). For the nonirradiated control group the mean surface hardness (VI-IN) was 24.1 (s.d. = 5.0). Statistical analysis (t test) showed a highly significant difference (P < 0.001) between the irradiated and the non-irradiated group. Table Z shows the results for varying the length of time of application of
362
J. Dent. 1994; 22: No. 6
Tab/e II. Results for the surface hardness of glass polyalkenoate cement after 24 h when subjected to radiant heat from a quartz halogen fibreoptic light source for various times at the initial set point Time of application of heat Mean surface hardness
0
15
30
45
60
62.4
61.4
60.3
62.2
71.8
9.1
6.1
4.8
3.9
10.6
W-W Standard deviation
radiant heat from the high intensity quartz halogen tibreoptic light source to the cement surface as an immediate effect on the surface hardness. Statistical analysis (ANOVA) showed a highly statistically significant difference between the groups (P < 0.01). The differences were localized using a Tukey comparison of means. All the irradiated groups had a statistically significantly higher surface hardness than the specimens that were not exposed to radiant heat (P < 0.01). There was a statistically significant difference between those specimens irradiated for 60 s and all other groups (P < 0.01). There was no statistically significant difference (P > 0.05) between the groups irradiated for 20 s and those irradiated for 40 s. Table II shows the long-term effect on the surface hardness when varying length of time of application of heat from the fibreoptic light source used at the initial set point and the surface hardness measured after 24 h. Statistical analysis (ANOVA, Tukey comparison of means) showed that the group irradiated for 60 s was significantly higher than all the other groups (P < 0.01).
DISCUSSION Glass polyalkenoate cements are prone to loss of surface water and desiccation during setting. No satisfactory method to prevent some drying taken place was discovered whilst the surface hardness measurement was being carried out. The time period for each specimen being in air was kept as short as possible and was constant for each specimen. The humidity of the environment around the testing machine was also kept as constant as possible. Although there was the possibility of some drying of the specimens, this could not be prevented and it was felt that as this was the same for all specimens the results would still be comparable. The results clearly showed an increase in surface hardness for the material after the application of heat from both infrared and high-intensity quartz halogen libreoptic light sources. This is to be expected as the application of heat will accelerate the setting reaction of the cement and so during the early time period after mixing the stage of set will be more advanced and thus the surface hardness will be greater. The increased hardness after 24 h compared to the unheated specimen was not expected. This was only found to be true for cement exposed to 60 s of radiant heat from the high-intensity
quartz halogen light source. This may be explained by the potential for the setting reaction to be considerably accelerated by the heat in the early stages and that this scale of acceleration is maintained even after 24 h. This is the case only for the specimen exposed for 60 s, which suggests that this level of energy is required to have a significant long-term effect. The use of the infrared lamp had a slightly greater effect upon the surface hardness than the high-intensity quartz halogen fibreoptic light source. Measurement of the surface temperature produced by each source gave temperatures of up to 60°C for the quartz halogen tibreoptic light source and up to 80°C after 60 s for the infrared light source. It is clear that the temperature produced by the infrared lamp is greater and so may have a greater effect on the surface hardness of the cement. The infrared lamp, however, was inappropriate to use clinically and so all further experimentation was carried out using the high-intensity quartz halogen fibreoptic light source. The effect of the heat on the setting reaction is likely to be to make the poly(alkenoic acid) more active. Therefore when the acid contacts the surface of the glass it is able to break down the surface of the glass more readily. This will increase the rate at which the ions are leached and released from the glass. A more reactive acid and greater rate of release of ions will lead to a more rapid cement formation by gelation of the insoluble products of the acid and basic glass. The heat would also increase the diffusion rates of the various ions leached from the glass. The total effect of all these accelerated reactions will be to produce a more rapid formation of the calcium polyalkenoate matrix, the components of the first formed matrix. As the surface hardness of the material increases, at least for the first period of setting, it will demonstrate the production of the components of the first stage of the setting reaction. Glass polyalkenoate cements take a significant time to reach a set point at which they may be subjected to stress. the accepted time period being up to 24 h. The application of heat has been shown to accelerate the setting reaction, at least on the surface of the cement. The technique of applying heat to the surface of the cement intraorally may be beneficial, therefore, in improving the early properties of the material at a time when it is most susceptible to damage. Further work is necessary to establish the potential effect of heat, applied in this way, to the dental pulp and to other properties of the glass polyalkenoate cement. Acknowledgements This project was supported by Scottish Endowment Research Trust, Grant 892.
Hospitals
References 1. Bausch JR, De Lange C and Davidson CL. The influence of temperature on some physical properties posites. J Oral Rehabil 1981; 8: 309-317.
of dental
com-
Woolford: Heat effect on glass polyalkenoatecement
2. Cook WD and Johansson M. The influence of postcuring on the fracture properties of photo-cured dimethacrylate based dental composite resin.JBiomedMaterRes 1987; 21: 979-989. 3. Wendt SL. The effect of heat used as a secondary cure upon the physical properties of three composite resins. I. Diametral tensile strength, compressive strength and marginal dimensional stability. Quintessence Int 1987; 18: 265-271. 4. Wendt SL. The effect of heat used as a secondary cure upon the physical properties of three composite resins. II. Wear, hardness and colour stability. Quintessence Int 1987; 18: 351-356. 5. Wendt SL. Time as a factor in the heat curing of composite resin. Quintessence Int 1989: 20: 259-263.
363
6. Draughn R Effects of temperature on mechanical properties of composite dental restorative materials. .I Biomed Mater Res 1981; 15: 489-495. 7. Greener EH, Greener CS and Moser JB. The hardness of composites as a function of temperature. J Oral Rehabil 1984; 11: 335-340. 8. Brune D. Heat treatment of glass ionomer, silicate, zinc phosphate and zinc polycarboxylate cements. Stand JDent Res 1982; 90: 409-412. 9. Brune D and Evje D. Improvement of erosion resistance of a silicate cement by infrared radiation. ScandJDent Res 1982; 90: 413-416..
Announcement
Relocation of Journal Publishing Office As from 1st October 1994, the publishing, editorial, production and reprint offices are moving to: Elsevier Science Ltd The Boulevard, Langford Lane, Kidlington, Oxford, OX5 IGB, UK Main switchboard: Tel: +44 (0)1865 843000 Fax: +44 (0) 1865 843010 All correspondence and queries relating to the journal should be sent to this address Newly submitted manuscripts should be mailed to the Editor