Journal of Dentistry, 5, No. 1, 1977, pp. 39-41. Printed in Great Britain
A simple thermal cycling device for testing dental materials F. Morley and P. B. Stockwell Laboratory of the Government Chemist, Department of Industry, London
ABSTRACT Sudden temperature changes cause a deleterious effect on many bonded dental materials. An apparatus has been designed and built which rapidly cycles the materials through a temperature change between 4 and 60 °C. The apparatus has been used to compare adhesiveproperties of various cements and also to study marginal leakage.
INTRODUCTION Over the past few years the Laboratory of the Government Chemist has developed a reputation in the field of dental materials, first by the development of a novel dental material, the glass-ionomer cement, and also in the general area of material performance testing. As part of this general performance testing, the ability to simulate realistic oral conditions, particularly those of sudden temperature changes that occur as a result of the ingestion of hot and cold drinks and foods, was considered important, and this paper describes a simple apparatus designed and built to achieve this objective automatically. Temperature cycling (Peyton and Craig, 1971) has a deleterious effect on the adhesive properties of dental materials and can also cause leakage around non-adhesive Idling materials. The usual test for adhesion is a pull test to determine the breaking point. Cements mechanically bonded to the substrate weaken and part with temperature cycling. Also, marginal leakage between the tooth and the nonadhesive Idler, which increases the risk of additional caries around or below the filling material, can be accelerated by temperature cycling. The instrument described in this paper provides an accelerated method of testing the effects o f temperature cycling and a means of comparing the available cements and bonded materials. Currently, the instrument is being used to produce a comparison of the adhesive properties of glass-ionorner cements with zinc polycarboxylate cements, zinc phosphate cements and composite resins. Studies on marginal leakage for such materials as amalgams, dental cements and composite resins are being planned using radioactive tracer techniques to examine the extent of leakage. The apparatus consists of a sample container and two temperature-controlled baths at 60 and 4 °C respectively. The sample container is alternately Idled and emptied of water from each of the baths on a simple timed cycle. The apparatus requires only water and electricity and will operate completely automatically over long periods. C O N S T R U C T I O N OF T H E U N I T Fig. 1 shows a diagram of the unit, which consists of three basic parts: the liquid supply system, the three-position valve and the sample chamber and liquid control system. These three parts are discussed in more detail below.
40
Journal of Dentistry, Vol. 5/No. 1 WATER DRAIN
:
HOT
~
COLD
DRAIN
I
EE-POSITION VALVE
C
i i_o.o, E "QUICK DRAIN" j r i " VALVE f " "
i. ~
I I
CONTROL MODULE
Fig. 1. Diagram of the thermal cycling device.
The liquid supply system This consists of two stainless steel tanks (,4 and B) which are continuously supplied with l/quid, a Fixed drain providing a constant pressure head. Tank A has a volume of 2000 ml and is maintained at 60 + 1 °C by means of a heating element and stirrer mounted in the tank. The temperature is sensed by a thermistor also mounted in the tank, remote from the heater. The resistance change of this thermistor is monitored and used to control the power supplied to the heater by means of a solid state controller. Tank B has a volume of 4000 ml and is maintained at 4 + 1 °C by an external recirculating cooling unit (Techne 1000). Both temperatures are adjustable over limited ranges. Normally, water is used as the liquid supply, but it is possible to use a weak solution of lactic acid, although in this case an additional supply pump must be provided.
The three-position valve The three-position valve (c'~ was constructed from PTFE, which was chosen since it is both chemically resistant and self-lubricating. The valve was arranged to have two inlet ports, one outlet port of large bore (about 0.6 cm) and a midway shut-off position. The valve was constructed to be physically small (2.5 cm diameter X 5 cm) yet robust; it is operated from a reversible motor having a rotation speed of l tee/rain. The starting and direction of rotation of the motor are controlled from the sample chamber sensors and it is automatically stopped when the valve reaches any o f its three positions.
The sample chamber and liquid control system The sample chamber (D), support gauze (F) and the 'quick-drain' valve (E) are all constructed from stainless steel; the t w o sensors (G and fir) are platinum wires and the solenoid (1)
Morley and Stockwell: Device for testing dental materials
41
is a Kuhnke 24-volt unit. The control module (J) contains all the necessary power supplies and relays for the valves and solenoids, and also a delay timer which controls the length of time that the sample is immersed in the liquid. The thermal cycle is initiated by switching a 'start/stop' switch to 'start' and can be halted at any time by switching to 'stop', whereupon the system returns to the off position. The first cycle can be made either a 'hot' or a 'cold' cycle by means of two push buttons on the control module. The switching of valve C is arranged so that a 'hot' cycle must always follow a 'cold' cycle and vice-versa. The opening of drain valve E is controlled from level sensor G which, when activated, initiates the delay timer and, after the elapsed time which is preset to between 0 and 4 minutes, the drain valve opens. The interlinking between sensors, valves and the timer can be best explained by listing the sequence of events for a complete cycle. After switching on, 20 minutes is allowed for the system to stabilize.
PERFORMANCE The sequence of events in the operation of the thermal cycling device is as follows: 1. Three-position valve C in 'cold' position and drain valve E shut. 2. Liquid fills chamber to activate first sensor H and then sensor G. 3. When sensor G is energized valve C is switched to the 'off' position and the delay timer is energized. 4. When delay time elapses drain valve E opens. 5. Liquid empties from chamber and first sensor G and then sensor H is deactivated. 6. When sensor His deactivated drain valve E shuts and valve C switches to 'hot' position. 7. Liquid fills chamber to activate first sensor H and then sensor G. 8. When sensor G is energized valve C is switched to 'off" position and the delay timer is energized. 9. When delay time elapses drain valve E opens. 10. Liquid empties from chamber and first sensor G and then sensor H is deactivated. 11. When sensor H is deactivated drain valve E shuts and valve C switches to 'cold' position. The cycle continues from 2. The unit is simple to operate and robust. It is able to test dental materials for extensive periods with little, if any, operator intervention. It has been operated successfully over a period o f 18 months and has recently been modified to incorporate twin cells to hold two samples which can be tested simultaneously. Future plans involve the construction of a system encompassing five cells.
Acknowledgements The authors would like to thank the Dental Materials subdivision for pointing out the problem and Dr S. Crisp for many helpful discussions. REFERENCE Peyton F. A. and Craig R. G. (¢d.) (1971) Restorative Dental Materials. St Louis, Mosby, pp. 53-54.
A simple thermal cycling device for testing dental materials F. Morley and P. B. Stockwell Laboratory of the Government Chemist, Department of Industry, London
ABSTRACT Sudden temperature changes cause a deleterious effect on many bonded dental materials. An apparatus has been designed and built which rapidly cycles the materials through a temperature change between 4 and 60 °C. The apparatus has been used to compare adhesiveproperties of various cements and also to study marginal leakage.
INTRODUCTION Over the past few years the Laboratory of the Government Chemist has developed a reputation in the field of dental materials, first by the development of a novel dental material, the glass-ionomer cement, and also in the general area of material performance testing. As part of this general performance testing, the ability to simulate realistic oral conditions, particularly those of sudden temperature changes that occur as a result of the ingestion of hot and cold drinks and foods, was considered important, and this paper describes a simple apparatus designed and built to achieve this objective automatically. Temperature cycling (Peyton and Craig, 1971) has a deleterious effect on the adhesive properties of dental materials and can also cause leakage around non-adhesive Idling materials. The usual test for adhesion is a pull test to determine the breaking point. Cements mechanically bonded to the substrate weaken and part with temperature cycling. Also, marginal leakage between the tooth and the nonadhesive Idler, which increases the risk of additional caries around or below the filling material, can be accelerated by temperature cycling. The instrument described in this paper provides an accelerated method of testing the effects o f temperature cycling and a means of comparing the available cements and bonded materials. Currently, the instrument is being used to produce a comparison of the adhesive properties of glass-ionorner cements with zinc polycarboxylate cements, zinc phosphate cements and composite resins. Studies on marginal leakage for such materials as amalgams, dental cements and composite resins are being planned using radioactive tracer techniques to examine the extent of leakage. The apparatus consists of a sample container and two temperature-controlled baths at 60 and 4 °C respectively. The sample container is alternately Idled and emptied of water from each of the baths on a simple timed cycle. The apparatus requires only water and electricity and will operate completely automatically over long periods. C O N S T R U C T I O N OF T H E U N I T Fig. 1 shows a diagram of the unit, which consists of three basic parts: the liquid supply system, the three-position valve and the sample chamber and liquid control system. These three parts are discussed in more detail below.
40
Journal of Dentistry, Vol. 5/No. 1 WATER DRAIN
:
HOT
~
COLD
DRAIN
I
EE-POSITION VALVE
C
i i_o.o, E "QUICK DRAIN" j r i " VALVE f " "
i. ~
I I
CONTROL MODULE
Fig. 1. Diagram of the thermal cycling device.
The liquid supply system This consists of two stainless steel tanks (,4 and B) which are continuously supplied with l/quid, a Fixed drain providing a constant pressure head. Tank A has a volume of 2000 ml and is maintained at 60 + 1 °C by means of a heating element and stirrer mounted in the tank. The temperature is sensed by a thermistor also mounted in the tank, remote from the heater. The resistance change of this thermistor is monitored and used to control the power supplied to the heater by means of a solid state controller. Tank B has a volume of 4000 ml and is maintained at 4 + 1 °C by an external recirculating cooling unit (Techne 1000). Both temperatures are adjustable over limited ranges. Normally, water is used as the liquid supply, but it is possible to use a weak solution of lactic acid, although in this case an additional supply pump must be provided.
The three-position valve The three-position valve (c'~ was constructed from PTFE, which was chosen since it is both chemically resistant and self-lubricating. The valve was arranged to have two inlet ports, one outlet port of large bore (about 0.6 cm) and a midway shut-off position. The valve was constructed to be physically small (2.5 cm diameter X 5 cm) yet robust; it is operated from a reversible motor having a rotation speed of l tee/rain. The starting and direction of rotation of the motor are controlled from the sample chamber sensors and it is automatically stopped when the valve reaches any o f its three positions.
The sample chamber and liquid control system The sample chamber (D), support gauze (F) and the 'quick-drain' valve (E) are all constructed from stainless steel; the t w o sensors (G and fir) are platinum wires and the solenoid (1)
Morley and Stockwell: Device for testing dental materials
41
is a Kuhnke 24-volt unit. The control module (J) contains all the necessary power supplies and relays for the valves and solenoids, and also a delay timer which controls the length of time that the sample is immersed in the liquid. The thermal cycle is initiated by switching a 'start/stop' switch to 'start' and can be halted at any time by switching to 'stop', whereupon the system returns to the off position. The first cycle can be made either a 'hot' or a 'cold' cycle by means of two push buttons on the control module. The switching of valve C is arranged so that a 'hot' cycle must always follow a 'cold' cycle and vice-versa. The opening of drain valve E is controlled from level sensor G which, when activated, initiates the delay timer and, after the elapsed time which is preset to between 0 and 4 minutes, the drain valve opens. The interlinking between sensors, valves and the timer can be best explained by listing the sequence of events for a complete cycle. After switching on, 20 minutes is allowed for the system to stabilize.
PERFORMANCE The sequence of events in the operation of the thermal cycling device is as follows: 1. Three-position valve C in 'cold' position and drain valve E shut. 2. Liquid fills chamber to activate first sensor H and then sensor G. 3. When sensor G is energized valve C is switched to the 'off' position and the delay timer is energized. 4. When delay time elapses drain valve E opens. 5. Liquid empties from chamber and first sensor G and then sensor H is deactivated. 6. When sensor His deactivated drain valve E shuts and valve C switches to 'hot' position. 7. Liquid fills chamber to activate first sensor H and then sensor G. 8. When sensor G is energized valve C is switched to 'off" position and the delay timer is energized. 9. When delay time elapses drain valve E opens. 10. Liquid empties from chamber and first sensor G and then sensor H is deactivated. 11. When sensor H is deactivated drain valve E shuts and valve C switches to 'cold' position. The cycle continues from 2. The unit is simple to operate and robust. It is able to test dental materials for extensive periods with little, if any, operator intervention. It has been operated successfully over a period o f 18 months and has recently been modified to incorporate twin cells to hold two samples which can be tested simultaneously. Future plans involve the construction of a system encompassing five cells.
Acknowledgements The authors would like to thank the Dental Materials subdivision for pointing out the problem and Dr S. Crisp for many helpful discussions. REFERENCE Peyton F. A. and Craig R. G. (¢d.) (1971) Restorative Dental Materials. St Louis, Mosby, pp. 53-54.
