Some parameters for testing deformation of elastomeric impression materials P. A. H. Blomberg* S. Mahmoodt R. J. Smalest 0. F. Makinsong
Key words: Elasticity, elastomers, impression materials.
Abstract Because of conflicting published data, the temperature rise in four elastomeric materials was measured with a thermistor during setting in the oral cavity and in metal and plastic moulds of varying shapes and volumes for 'elastic set' specimens. The clinical temperature rise was 2-3OC higher in the molar region than anteriorly, except for the polysiloxane. The temperature range attained in the set materials varied from 29OC to nearly 35OC for clinical and in v i m specimens. The average time taken by a group of operators to remove this type of impression from the mouth was five seconds. In a custom tray with light and heavy viscosity materials, the syringe material layer was only 0-0.15 mm thick and, essentially, the heavy viscosity material provided the elastic components for the impression. (Received for publication May 1990. Revised August 1990 and May 1991. Accepted August 1991.)
*Specialist practitioner, Townsville. ?Postgraduate student, Conservative Dentistry, The University of Adelaide. $Senior Lecturer, Conservative Dentistry, The University of Adelaide. §Reader, Conservative Dentistry, The University of Adelaide. Australian Dental Journal 1992;37(4):271-6.
Introduction In order to test elasticity of elastomeric impression materials in a subsequent study it was necessary that the temperature of the setting materials, the time of strain deformation during removal from the mouth, and the thickness of the syringe material be determined. There has been disagreement regarding the temperatures reached in polymerizing impression material. Elborn and Wilson' found that elastomers reached 31.2-32.1"C after four and one-half minutes intra-orally, while Sandrik and Same2reported the temperature after this time to be 35.3"C. The maximum temperature attained by Jamani et aL3 was in the range 30.1-34.5 "C, with a mean value of 33.1 "C. As far as the duration of strain during removal was concerned, findings varied from almost ~ e r ~ ~ . ~ to much longer times.6 It has been shown7-" that an increased duration of strain application resulted in an increased permanent deformation. Therefore, the average time taken by the operators to remove impressions from the mouth was included in the present study. As most brands of elastomeric impression materials are available in more than one viscosity, it was important to determine the viscosity type which forms the major component in an impression tray. Therefore, a number of clinical impressions were sectioned and the thickness of the syringed light viscosity materials was measured. The present study was designed to determine the above-mentioned three parameters - the temperature change within the setting material, the time 271
Table 1. Elastomers selected for the elasticity tests Chemical group
Proprietary name
Viscosity class
Batch code
Manufacturer
Polysulphide
Permlastic
High
Sybron/Kerr, USA
Condensation silicone
Delicron
Medium
Polyether
Impregum
Medium
Poly(viny1) siloxane
Reflect
Medium
Base 12057 Catalyst 11061 Base 6547B Catalyst 6672F Base H041 Catalyst H034 Base 0 1037 Catalyst 0 1036
Table 2. Type of test
Mould and data summary Specimen shape and size
Tensile Tensile Tensile Bend Bend Torsion
Cylinder 20.0 x 12.5 mm diam Rod 50.0 x 5.0 mm diam Bar 50.0 x 3.8 x 2.0 mm Bar 50.0 x 5.0 x 3.0 mm Plate 20.0 x 23.0 x 5.0 mm Rod 30.0 mm x 10.0 mm diam Rod 61.0 mm x 6.0 m diam
ESPE GmbH, Germany SybronlKerr, USA
31
Mould material ~~
Compression
Bayer, Germany
~~
3!
~~~
Stainless steel Acrylic Stainless steel
3r
Aluminium 3:
Aluminium 32
Acrylic Teflon
31
Acrylic
? -f 30 c
E
taken to remove impressions and thus to strain the elastomer and the thickness of the syringe material in impressions. The results were used in a subsequent elasticity study.
Materials and methods The impression materials tested are listed in Table 1. All the materials were supplied as paste systems requiring mixing of equal lengths of base and catalyst. The manufacturers' instructions regarding proportioning and mixing were followed. T o examine the temperature of polymerization, the study was done in two parts, clinical measurements, and in vitro temperature rise during curing in moulds of different materials and different shapes. For the clinical study, custom-made acrylic trays (1 with occlusal stops were fabricated spaced to provide an elastomer thickness of approximately 2 mm. Temperatures were recorded with a thermistor probe! placed lingually to a maxillary central
1 Formatray. Sybron/Kerr, USA. (Model 425 C Telethermometer. Yellow Springs Instrument Co., USA. 272
E
29
l-
2a
27
26
25
24
I
23
0
I
2
4
6
a
lo
'
12
Time (min)
Fig. 1. -Temperature rises recorded intra-orally.
incisor and lingually to a maxillary first molar. The probe was secured within the mass of the impression material. Recordings for two impressions were made at each position. Recordings were made when Australian Dental Journal 1992;37:4.
33 32
Permlastic
1
.---.-.--
-
9 31Y
2 3
c
$
30-
-
0.
Compression - metal Compression - acrylic Tensile - vertical -- Tensile - horizontal Bend - flat Bend - round Torsion
E
---
l- 29-
----
28 -
--- - .
27’1 0
I
I
I
2
I
I
I
4
I
I
I
1
I
I
8
6
10
12
Time (rnin) Fig. 2. -Temperature rises recorded within moulds for Permalastic.
33 32
31
1
Delicron
-
-
9 Y
2
30-
3
c
! a) a 29-
i
//
l-
28
/’/
-
’
/’
-
--- Tensile - vertical
/
__ Tensile - horizontal -.-.- Bend - flat _- Bend - round
/
i /
/
27 -
26’1
......Torsion
I
0
I
2
Compression - metal
- Compression - acrylic
4
I
I
6
I
a
I
I
I
10
1
12
Time (min) Fig. 3. -Temperature rises recorded within moulds for Delicron.
the impression was first inserted in the mouth (one and one-half minutes after the start of mixing), and every thirty seconds thereafter, for ten minutes. For the in v i m study, eight moulds designed for Australian Dental Journal 1992:37:4.
the subsequent elasticity tests (compression, tensile, bend and torsion) were used. The specimen dimensions, the mould materials, mass and thermal characteristics are given in Table 2. The polymer273
33
Impregum
-...........
_./.--
...#G-.K..
32
p 31 Y
-
2 3 E
30
0)
P
E
I- 29
/
---
/'
Bend - flat
--Bend - round
28
...... Torsion
27
I
l
l
l
2
l
l
l
l
6
4
l
l
t
i
10
8
12
Time (min) Fig. 4. -Temperature rises recorded within moulds for Impregum.
Reflect
33
/*4
,'-
32
p 3' Y
2
3
5
30
-
P)
CL
Compression - metal Compression - acrylic Tensile - vertical -- Tensile - horizontal Bend - flat Bend - round Torsion
k I- 29
---
---
/
28
..-.-.
/ /'
/'
27
0
1
1
2
1
1
4
1
1
1
1
6 8 Time (min)
1
*
I
10
I
12
Fig. 5. -Temperature rises recorded within moulds for Reflect.
ization temperature was recorded by placing the probe sensor in the approximate centre of the specimen. One specimen of each material was tested following the time schedule for the intra-oral recordings. 274
In order to determine a clinically relevant duration of strain application, the times taken to remove full arch impressions were measured. Final-year students were asked to indicate the time of initial force application and of final impression removal. Australian Dental Journal 1992;37:4.
Fig. 6. -Cross-seaion of an impression illustrating the thin nature of the syringe material layer (up to 0.15 mm) except in the gingival crevice.
Seventeen recordings to the nearest half second were made. Clinical impressions were examined to determine the contribution of each viscosity type ,syringe light body and tray heavy body material, in a double-mix technique made in custom impression trays. Fifty impressions from student clinics were sectioned in the region of the preparation, and the syringe material thickness was measured with a vernier microscope.
Results and discussion The temperature changes for the elastomers in the clinical tests are shown in Fig. 1. This details the average from the two impressions for each material over time for the temperatures in the incisal and molar regions for the four materials where the conditions apart from the thermistor lead were identical to the general clinical use of the materials. A difference was found between the clinical temperatures recorded incisally and in the molar region, as is evident from Fig. 1. This was expected because there is general agreement that the oral temperature becomes higher the more distal the measurement site ~ s e d . ~ As " ' ~elasticity is related to the degree of polymerization, it would seem advisable to use the lower temperature range of 30-31"C for preparing specimens for specification testing to simulate clinical conditions and even that is slightly higher than for the clinical values in the incisal region. This is reflected in those s t a n d a r d ~ ' ~which - ' ~ use a mould temperature of 32 "C as against the Australian standard," which specified 37°C. However, Jamani et d 3do not agree with this observation, as they suggest that a Australian Dental Journal 1992;37:4
temperature of 33 "C is a realistic value for assessing the properties of impression materials. The next group of results are shown in Fig. 2, 3, 4, 5. These detail the average temperatures within the elastomers for the two tests for each of the seven types of moulds. The temperatures of the materials within the moulds varied between 29 "C and 32 "C at the recommended setting times. Any change from storage of moulds from 32 "C would require separate experimental studies to test elasticity versus temperature values of the moulds. For the polysulphide, silicone and polysiloxane materials, the final temperatures of the mould specimens corresponded to the lower clinical temperatures in the incisal regions. For the polyether material, the results were midway between the clinical incisal and molar region temperatures. For the silicone and the polysiloxane elastomers, the temperature rises after the recommended setting time indicated considerable further polymerization. The time taken to remove an impression from the mouth measures the duration of strain on the impression and affects elastic recovery. For the duration of the strain, the removal time of impressions was determined to be a mean of 5.0 f 1.4 seconds for 17 impressions with a range of 2.5-8.5 seconds. Therefore, it was decided to select six seconds as the parameter for subsequent tests. This compares with a range from 1-2 seconds for the IS0 ~ t a n d a r d ' ~to 6 0 k 5 seconds for the former Australian standard." Sectioning the seventeen clinical impressions in the region of the preparations allowed the thickness contribution of the syringe light body material to be determined. A typical example is that shown in 275
Fig. 6. This indicates that the heavy bodied tray material displaces the light body material so that only a thin layer of the latter is left. On sectioning the impressions, it was found that for extra-coronal preparations, the thickness for syringe material ranged from 0.0 to 0.15 mm. for intra-coronal preparations a greater thickness of up to 1.0 mm was found trapped in internal point angles and extending past the gingival finishing margins (Fig. 6). It is concluded that higher viscosity tray materials contribute more to the elasticity of an impression than do the syringe materials. It is important then to note that the qualitative value of elasticity in an impressionis given by the heavier bodied material. The light body material might only contribute to the surface texture of the impression. It may be noted that there is a greater allowed set per cent deformation in standard specifications for tray materials than for syringe materials. The specification for standards testing should state the mould material and consider lower thermal conductivity materials for the more bulky cylindrical specimens for compression set tests. An area for further study is temperature rise as an indicator of the degree of polymerization of impression material and its relation to setting time.
Acknowledgements The authors would like to acknowledge the support and guidance provided by Associate Professor G. C. Townsend, Department of Dentistry, The University of Adelaide. References 1. Elborn A, Wilson HJ. Temperatures attained by impression materials in the mouth. Br Dent J 1965;118:80-2. 2. Sandrik JL, Sarna T. Temperatures of elastomeric impression materials while setting in the mouth. J Dent Res 1980;59:1985-6.
276
3. Jamani KD, Fayyad MA, Harrington E, Wilson HJ. Temperature changes of materials during impression taking. Br Dent J 1988;165:129-32. 4. Makinson OF. Elastic recovery from compression strains in some alginate impression materials. International Association of Dental Research Meeting (Australia and New Zealand Division) 1970:Abstr 14. 5. Harcoun JK. A review of modern impression materials. Aust Dent J 1978;23: 178-86. 6. McLean JW. Silicone impression materials. Dent Practitioner 1958;9:56-63. 7. Fairhurst CW, Furman TC, Schallhorn RJ, Kirkpatrick EL, Ryge G. Elastic properties of rubber base impression materials. J Prosthet Dent 1956;6:534-42. 8. Wilson HJ. Elastomeric impression materials: 1. The setting material. Br Dent J 1966;121:277-83. 9. Goldberg AJ. Visco-elastic properties of silicone, polysulphide, and polyether impression materials. J Dent Res 1974;S~ppl53: 1033-9. 10. Cook WD. Permanent set and stress relaxation in elastomeric impression materials. J Biomed Mater Res 1981; 15:449-63. 11. Macpherson GW, Craig RG, Peyton FA. Mechanical properties of hydrocolloid and rubber impression materials. J Dent Res 1967;46:714-21. 12. Bergstrom J, Varga G. Temperatures of the oral cavity in 50 healthy students. Scand Dent J 1971;64:157-64. 13. Maeda T, Stolze K, User A, Kroone H, Brill N. Oral temperature in young and old people. J Oral Rehabil 1979;6:159-66. 14. American Dental Association Council on Dental Materials and Devices. Revised American Dental Association Specification No. 19 for Non-aqueous Elastomeric Dental Impression Materials. J Am Dent Assoc 1977;94:733-41. 15. International Organization for Standardization. Dental materials - Elastomeric impression materials: 4823,1984. 16. British Standards Institution. Specification for Dental Elastic Impression Materials. British Standard 4269:1,1968. 17. Standards Australia. Dental Materials - Elastomeric impression materials. AS 1185,1984.
Address for correspondenceheprints: 0. F. Makinson, Department of Dentistry, The University of Adelaide, GPO Box 498, Adelaide, South Australia, 5001.
Australian Dental Journal 1992;37:4.
Key words: Elasticity, elastomers, impression materials.
Abstract Because of conflicting published data, the temperature rise in four elastomeric materials was measured with a thermistor during setting in the oral cavity and in metal and plastic moulds of varying shapes and volumes for 'elastic set' specimens. The clinical temperature rise was 2-3OC higher in the molar region than anteriorly, except for the polysiloxane. The temperature range attained in the set materials varied from 29OC to nearly 35OC for clinical and in v i m specimens. The average time taken by a group of operators to remove this type of impression from the mouth was five seconds. In a custom tray with light and heavy viscosity materials, the syringe material layer was only 0-0.15 mm thick and, essentially, the heavy viscosity material provided the elastic components for the impression. (Received for publication May 1990. Revised August 1990 and May 1991. Accepted August 1991.)
*Specialist practitioner, Townsville. ?Postgraduate student, Conservative Dentistry, The University of Adelaide. $Senior Lecturer, Conservative Dentistry, The University of Adelaide. §Reader, Conservative Dentistry, The University of Adelaide. Australian Dental Journal 1992;37(4):271-6.
Introduction In order to test elasticity of elastomeric impression materials in a subsequent study it was necessary that the temperature of the setting materials, the time of strain deformation during removal from the mouth, and the thickness of the syringe material be determined. There has been disagreement regarding the temperatures reached in polymerizing impression material. Elborn and Wilson' found that elastomers reached 31.2-32.1"C after four and one-half minutes intra-orally, while Sandrik and Same2reported the temperature after this time to be 35.3"C. The maximum temperature attained by Jamani et aL3 was in the range 30.1-34.5 "C, with a mean value of 33.1 "C. As far as the duration of strain during removal was concerned, findings varied from almost ~ e r ~ ~ . ~ to much longer times.6 It has been shown7-" that an increased duration of strain application resulted in an increased permanent deformation. Therefore, the average time taken by the operators to remove impressions from the mouth was included in the present study. As most brands of elastomeric impression materials are available in more than one viscosity, it was important to determine the viscosity type which forms the major component in an impression tray. Therefore, a number of clinical impressions were sectioned and the thickness of the syringed light viscosity materials was measured. The present study was designed to determine the above-mentioned three parameters - the temperature change within the setting material, the time 271
Table 1. Elastomers selected for the elasticity tests Chemical group
Proprietary name
Viscosity class
Batch code
Manufacturer
Polysulphide
Permlastic
High
Sybron/Kerr, USA
Condensation silicone
Delicron
Medium
Polyether
Impregum
Medium
Poly(viny1) siloxane
Reflect
Medium
Base 12057 Catalyst 11061 Base 6547B Catalyst 6672F Base H041 Catalyst H034 Base 0 1037 Catalyst 0 1036
Table 2. Type of test
Mould and data summary Specimen shape and size
Tensile Tensile Tensile Bend Bend Torsion
Cylinder 20.0 x 12.5 mm diam Rod 50.0 x 5.0 mm diam Bar 50.0 x 3.8 x 2.0 mm Bar 50.0 x 5.0 x 3.0 mm Plate 20.0 x 23.0 x 5.0 mm Rod 30.0 mm x 10.0 mm diam Rod 61.0 mm x 6.0 m diam
ESPE GmbH, Germany SybronlKerr, USA
31
Mould material ~~
Compression
Bayer, Germany
~~
3!
~~~
Stainless steel Acrylic Stainless steel
3r
Aluminium 3:
Aluminium 32
Acrylic Teflon
31
Acrylic
? -f 30 c
E
taken to remove impressions and thus to strain the elastomer and the thickness of the syringe material in impressions. The results were used in a subsequent elasticity study.
Materials and methods The impression materials tested are listed in Table 1. All the materials were supplied as paste systems requiring mixing of equal lengths of base and catalyst. The manufacturers' instructions regarding proportioning and mixing were followed. T o examine the temperature of polymerization, the study was done in two parts, clinical measurements, and in vitro temperature rise during curing in moulds of different materials and different shapes. For the clinical study, custom-made acrylic trays (1 with occlusal stops were fabricated spaced to provide an elastomer thickness of approximately 2 mm. Temperatures were recorded with a thermistor probe! placed lingually to a maxillary central
1 Formatray. Sybron/Kerr, USA. (Model 425 C Telethermometer. Yellow Springs Instrument Co., USA. 272
E
29
l-
2a
27
26
25
24
I
23
0
I
2
4
6
a
lo
'
12
Time (min)
Fig. 1. -Temperature rises recorded intra-orally.
incisor and lingually to a maxillary first molar. The probe was secured within the mass of the impression material. Recordings for two impressions were made at each position. Recordings were made when Australian Dental Journal 1992;37:4.
33 32
Permlastic
1
.---.-.--
-
9 31Y
2 3
c
$
30-
-
0.
Compression - metal Compression - acrylic Tensile - vertical -- Tensile - horizontal Bend - flat Bend - round Torsion
E
---
l- 29-
----
28 -
--- - .
27’1 0
I
I
I
2
I
I
I
4
I
I
I
1
I
I
8
6
10
12
Time (rnin) Fig. 2. -Temperature rises recorded within moulds for Permalastic.
33 32
31
1
Delicron
-
-
9 Y
2
30-
3
c
! a) a 29-
i
//
l-
28
/’/
-
’
/’
-
--- Tensile - vertical
/
__ Tensile - horizontal -.-.- Bend - flat _- Bend - round
/
i /
/
27 -
26’1
......Torsion
I
0
I
2
Compression - metal
- Compression - acrylic
4
I
I
6
I
a
I
I
I
10
1
12
Time (min) Fig. 3. -Temperature rises recorded within moulds for Delicron.
the impression was first inserted in the mouth (one and one-half minutes after the start of mixing), and every thirty seconds thereafter, for ten minutes. For the in v i m study, eight moulds designed for Australian Dental Journal 1992:37:4.
the subsequent elasticity tests (compression, tensile, bend and torsion) were used. The specimen dimensions, the mould materials, mass and thermal characteristics are given in Table 2. The polymer273
33
Impregum
-...........
_./.--
...#G-.K..
32
p 31 Y
-
2 3 E
30
0)
P
E
I- 29
/
---
/'
Bend - flat
--Bend - round
28
...... Torsion
27
I
l
l
l
2
l
l
l
l
6
4
l
l
t
i
10
8
12
Time (min) Fig. 4. -Temperature rises recorded within moulds for Impregum.
Reflect
33
/*4
,'-
32
p 3' Y
2
3
5
30
-
P)
CL
Compression - metal Compression - acrylic Tensile - vertical -- Tensile - horizontal Bend - flat Bend - round Torsion
k I- 29
---
---
/
28
..-.-.
/ /'
/'
27
0
1
1
2
1
1
4
1
1
1
1
6 8 Time (min)
1
*
I
10
I
12
Fig. 5. -Temperature rises recorded within moulds for Reflect.
ization temperature was recorded by placing the probe sensor in the approximate centre of the specimen. One specimen of each material was tested following the time schedule for the intra-oral recordings. 274
In order to determine a clinically relevant duration of strain application, the times taken to remove full arch impressions were measured. Final-year students were asked to indicate the time of initial force application and of final impression removal. Australian Dental Journal 1992;37:4.
Fig. 6. -Cross-seaion of an impression illustrating the thin nature of the syringe material layer (up to 0.15 mm) except in the gingival crevice.
Seventeen recordings to the nearest half second were made. Clinical impressions were examined to determine the contribution of each viscosity type ,syringe light body and tray heavy body material, in a double-mix technique made in custom impression trays. Fifty impressions from student clinics were sectioned in the region of the preparation, and the syringe material thickness was measured with a vernier microscope.
Results and discussion The temperature changes for the elastomers in the clinical tests are shown in Fig. 1. This details the average from the two impressions for each material over time for the temperatures in the incisal and molar regions for the four materials where the conditions apart from the thermistor lead were identical to the general clinical use of the materials. A difference was found between the clinical temperatures recorded incisally and in the molar region, as is evident from Fig. 1. This was expected because there is general agreement that the oral temperature becomes higher the more distal the measurement site ~ s e d . ~ As " ' ~elasticity is related to the degree of polymerization, it would seem advisable to use the lower temperature range of 30-31"C for preparing specimens for specification testing to simulate clinical conditions and even that is slightly higher than for the clinical values in the incisal region. This is reflected in those s t a n d a r d ~ ' ~which - ' ~ use a mould temperature of 32 "C as against the Australian standard," which specified 37°C. However, Jamani et d 3do not agree with this observation, as they suggest that a Australian Dental Journal 1992;37:4
temperature of 33 "C is a realistic value for assessing the properties of impression materials. The next group of results are shown in Fig. 2, 3, 4, 5. These detail the average temperatures within the elastomers for the two tests for each of the seven types of moulds. The temperatures of the materials within the moulds varied between 29 "C and 32 "C at the recommended setting times. Any change from storage of moulds from 32 "C would require separate experimental studies to test elasticity versus temperature values of the moulds. For the polysulphide, silicone and polysiloxane materials, the final temperatures of the mould specimens corresponded to the lower clinical temperatures in the incisal regions. For the polyether material, the results were midway between the clinical incisal and molar region temperatures. For the silicone and the polysiloxane elastomers, the temperature rises after the recommended setting time indicated considerable further polymerization. The time taken to remove an impression from the mouth measures the duration of strain on the impression and affects elastic recovery. For the duration of the strain, the removal time of impressions was determined to be a mean of 5.0 f 1.4 seconds for 17 impressions with a range of 2.5-8.5 seconds. Therefore, it was decided to select six seconds as the parameter for subsequent tests. This compares with a range from 1-2 seconds for the IS0 ~ t a n d a r d ' ~to 6 0 k 5 seconds for the former Australian standard." Sectioning the seventeen clinical impressions in the region of the preparations allowed the thickness contribution of the syringe light body material to be determined. A typical example is that shown in 275
Fig. 6. This indicates that the heavy bodied tray material displaces the light body material so that only a thin layer of the latter is left. On sectioning the impressions, it was found that for extra-coronal preparations, the thickness for syringe material ranged from 0.0 to 0.15 mm. for intra-coronal preparations a greater thickness of up to 1.0 mm was found trapped in internal point angles and extending past the gingival finishing margins (Fig. 6). It is concluded that higher viscosity tray materials contribute more to the elasticity of an impression than do the syringe materials. It is important then to note that the qualitative value of elasticity in an impressionis given by the heavier bodied material. The light body material might only contribute to the surface texture of the impression. It may be noted that there is a greater allowed set per cent deformation in standard specifications for tray materials than for syringe materials. The specification for standards testing should state the mould material and consider lower thermal conductivity materials for the more bulky cylindrical specimens for compression set tests. An area for further study is temperature rise as an indicator of the degree of polymerization of impression material and its relation to setting time.
Acknowledgements The authors would like to acknowledge the support and guidance provided by Associate Professor G. C. Townsend, Department of Dentistry, The University of Adelaide. References 1. Elborn A, Wilson HJ. Temperatures attained by impression materials in the mouth. Br Dent J 1965;118:80-2. 2. Sandrik JL, Sarna T. Temperatures of elastomeric impression materials while setting in the mouth. J Dent Res 1980;59:1985-6.
276
3. Jamani KD, Fayyad MA, Harrington E, Wilson HJ. Temperature changes of materials during impression taking. Br Dent J 1988;165:129-32. 4. Makinson OF. Elastic recovery from compression strains in some alginate impression materials. International Association of Dental Research Meeting (Australia and New Zealand Division) 1970:Abstr 14. 5. Harcoun JK. A review of modern impression materials. Aust Dent J 1978;23: 178-86. 6. McLean JW. Silicone impression materials. Dent Practitioner 1958;9:56-63. 7. Fairhurst CW, Furman TC, Schallhorn RJ, Kirkpatrick EL, Ryge G. Elastic properties of rubber base impression materials. J Prosthet Dent 1956;6:534-42. 8. Wilson HJ. Elastomeric impression materials: 1. The setting material. Br Dent J 1966;121:277-83. 9. Goldberg AJ. Visco-elastic properties of silicone, polysulphide, and polyether impression materials. J Dent Res 1974;S~ppl53: 1033-9. 10. Cook WD. Permanent set and stress relaxation in elastomeric impression materials. J Biomed Mater Res 1981; 15:449-63. 11. Macpherson GW, Craig RG, Peyton FA. Mechanical properties of hydrocolloid and rubber impression materials. J Dent Res 1967;46:714-21. 12. Bergstrom J, Varga G. Temperatures of the oral cavity in 50 healthy students. Scand Dent J 1971;64:157-64. 13. Maeda T, Stolze K, User A, Kroone H, Brill N. Oral temperature in young and old people. J Oral Rehabil 1979;6:159-66. 14. American Dental Association Council on Dental Materials and Devices. Revised American Dental Association Specification No. 19 for Non-aqueous Elastomeric Dental Impression Materials. J Am Dent Assoc 1977;94:733-41. 15. International Organization for Standardization. Dental materials - Elastomeric impression materials: 4823,1984. 16. British Standards Institution. Specification for Dental Elastic Impression Materials. British Standard 4269:1,1968. 17. Standards Australia. Dental Materials - Elastomeric impression materials. AS 1185,1984.
Address for correspondenceheprints: 0. F. Makinson, Department of Dentistry, The University of Adelaide, GPO Box 498, Adelaide, South Australia, 5001.
Australian Dental Journal 1992;37:4.
