|Year : 2014 | Volume
| Issue : 1 | Page : 6-12
The effect of temperature on linear dimensional stability of elastomers
Geerts Greta Aimée Virginie Maria, Muller Susanne Sanette
Department of Restorative Dentistry, University of the Western Cape, Durbanville 7551, South Africa
|Date of Web Publication||20-Mar-2014|
Geerts Greta Aimée Virginie Maria
Department of Restorative Dentistry, University of the Western Cape, P. O. Box 1027, Durbanville 7551
Source of Support: None, Conflict of Interest: None
Objectives: The objective of this study was to investigate the effect of temperature on dimensional stability of elastomeric impression materials. Materials and Methods: Polyether and polyvinyl siloxane impressions of an International Organization for Standardization-specified test-block were photographed at two temperatures and 3 time intervals. This resulted in 12 groups (n = 10). Images were digitally calibrated and measured. Mean percentage dimensional change (%DC) was analyzed using the Variance Estimation and Precision module of Statistica 10 (Statsoft®, Southern Africa) (95% confidence), with Fisher least significant difference correction for multiple pairwise comparisons. Results: For silicone, dimensional accuracy was highest when stored at 21°C for 14 days (%DC = 0.006). This was significantly better than all other silicone groups (P < 0.05). Dimensional accuracy was worst when stored at 21°C for 8 h (%DC = 0.237) and after cooling-off from 66°C (%DC = 0.300), with no difference between latter groups (P = 0.187). For polyether, dimensional accuracy was highest when stored at 21°C for 14 days (%DC = −0.272). This was significantly better than all other polyether groups (P < 0.05). Dimensional accuracy was worst when stored at 21°C for 8 h (%DC = 0.364) and after cooling-off from 66°C (%DC = 0.306), with no difference between latter groups (P = 0.199). Comparing polyether with silicone, %DC did not differ between the two materials when kept at 21°C for 14 days and when cooled-off after heat exposure for 8 h. At all other instances, the %DC between the two materials differed, with silicone always closest to zero %DC. Conclusions: Exposing silicone and polyether to high temperature doesn't lead to higher dimensional inaccuracy on the short-term compared to keeping them at 21°C. However, heat has an effect on shelf-life (14 days) of impressions, decreasing accuracy. This effect is worse for polyether.
Keywords: Dimensional stability, elastomers, temperature
|How to cite this article:|
Maria GA, Sanette MS. The effect of temperature on linear dimensional stability of elastomers. J Res Dent 2014;2:6-12
| Introduction|| |
Dimensional stability has been identified as an important property for dental impression materials. , Dimensional stability is defined as the ability of a material to retain its size and form over time.  When an impression material is dimensionally stable, the impression can be poured at a convenient time or allows transport to a dental laboratory without compromising the accuracy of the reproduction.
All elastomeric impression materials undergo some polymerization shrinkage.  The greatest dimensional accuracy occurs immediately after polymerization and declines with extended periods of time.  Polyethers exhibit a slight change, whereas additional silicones have the smallest change. ,,,, Addition-silicones acquire almost the ideal dimensional stability because there are no by-products formed during the chemical setting reaction. They can be poured immediately after the impression is removed from the mouth and they can still produce accurate casts even if poured within 1 to 2 weeks. 
The dimensional changes of the polyether impression materials are attributed to imbibition. ,, It is recommended that for optimal accuracy polyether impression materials should be poured within 60 min after removal from the mouth.  Based on a critical review of the literature, polyether impressions should not be stored at humidity higher than 50%. 
In 1981, Lacy et al. investigated delayed pour of elastomers and concluded that polyvinyl siloxanes are dimensionally stable.  Marcinak and Draughn investigated delayed pour for up to 1 week and found no significant change (−0.3%) for addition-cured silicones.  Williams et al. and Johnson and Craig agreed and reported that addition-cured silicones were the most accurate of all elastomeric impression materials after delayed pour. , Chen et al. investigated storage time and filler loading of elastomers and found the highest accuracy for polyvinyl siloxane and highly-filled materials. 
According to Purk et al. it is not unusual for parcels to stay in delivery vehicles for more than 8 h during the delivery process.  They reported that the temperature in a vehicle can reach up to 66°C when the outdoor temperature is 38°C. They investigated elastomers under extreme temperatures (from 10°C to 66°C) and found that all elastomers were unstable under these conditions.
It was found that by reheating addition-reaction silicone impression from room temperature to body temperature before pouring, the dimensional accuracy of the dies was improved.  Corso et al. tested the effect of temperature changes on the dimensional stability of polyvinyl siloxane and polyether impression materials.  They found that changes in storage temperature had a small but statistically significant effect on their dimensional stability.
There is no consistency in the literature in terms of methodology used for assessing dimensional stability of impression materials.  Some publications report the use of a standard, but most publications describe some kind of clinically related technique incorporating uncontrolled variables such as impression tray and casting materials.
Sometimes, measurements are made of the cast or of the impression itself.  It has been reported that alterations of acrylic trays and gypsum products may confound results.  This lack of standardization makes comparisons among different studies difficult. An accurate decision on the dimensional stability of impression materials can only be made if all confounding variables are excluded from the methodology. 
The International Organization for Standardization (ISO) published a standard for testing and minimum requirements for dental elastomeric impression materials (ISO 4823:2000).  This standard describes in detail the manufacturing of specimens using a custom-made test block with a special configuration of lines for determining detail reproduction and dimensional stability. This methodology was previously also described by the American Dental Association (ADA specification no. 19, Council on Dental Materials and Devices).  The ISO standard describes the use of a travelling microscope to measure distances. Again, there seems to be no consensus in the literature on the measuring device that should be used to evaluate the detail reproduction and dimensional stability of impression materials. Microscopes, travelling microscopes and calipers have been used. However, newer techniques are now available, like measuring software using images of specimens and measuring digitized impressions and casts using laser scanners. ,,
If relatively small temperature fluctuations, such as between room and body temperature, had an influence on the accuracy of elastomers,  one could speculate that a temporary exposure to higher temperatures, as in vehicles during transport, may cause reversible or even irreversible changes. Therefore, the purpose of this study was to investigate the effect of raised temperature on the linear dimensional stability of the two most popular types of elastomeric impression materials. The null-hypothesis was that temperature has no influence on the linear dimensional stability of elastomeric impression materials.
| Materials and Methods|| |
This in vitro controlled comparative study assessed the immediate (30 min) and intermediate (8 and 16 h) influence of exposure to temperature on the dimensional stability of two impression materials. It also looked at its effect on the shelf-life of the impressions (14 days).
A stainless steel test block and ring was manufactured according to the specifications of ISO 4823:2000 for testing dental elastomeric impression materials [Figure 1]. The surface of the test block was marked with three horizontal grooves (nos. 1, 2 and 3) intersected by two vertical grooves (nos. 4 and 5) [Figure 2].
|Figure 2: Schematic representation of test block surface. Red line shows the distance to be measured from groove no. 4 to groove no. 5 along groove no. 1, used for determination of percentage dimensional change (ISO 4823). Groove 1 is right-angled with width of 50 ± 8 μm at surface|
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Impressions of the test block's surface were made using two medium consistency impression materials: A polyvinyl siloxane material (S) (Affinis Precious Regular Body, Coltene/Whaledent, Switzerland) and a polyether material (P) (Impregum Penta, 3M ESPE, Germany). The impression materials were handled according to manufacturer's instructions.
A total of 12 groups of 10 specimens each were developed. The specimens subjected to temperature were placed in a pre-heated oven at 66°C and left in situ for 8 h. Their treatment in terms of time and temperature are shown in [Table 1]. A total of 40 specimens were made:
|Table 1: For each group (column 1) temperature exposure (column 2) and temperature at which readings were made (column 3)|
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- Specimens for groups S*21 30 min , S*21 8h and S*21 14 days were the same (n = 10)
- Specimens for groups S*66 8h , S*66 16h and S*66 14 days were the same (n = 10)
- Specimens for groups P*21 30 min , P*21 8h and P*21 14 days were the same (n = 10)
- Specimens for groups P*66 8h , P*66 16h and P*66 14 days were the same (n = 10).
Before use, the test block and ring were ultrasonically cleaned and placed in an oven at 35°C ± 1°C for at least 15 min. All the impressions were made using pre-packaged cartridges and mixed using a micro plastic dispenser MK II (Coltene/Whaledent) for the silicone and the Pentamix 2 electric mixing unit (3M ESPE) for the polyether. The ring was positioned over the block and the cavity that is formed in this way was filled with impression material. A polyethylene-covered glass plate was placed over the ring and pressed down until the glass plate touched the ring and all excess material was expelled. At 60 s after completion of the mix, this specimen-forming assembly was placed in a water bath at 37°C for the minimum time recommended by the manufacturer's instructions for leaving the impression in the mouth. Consequently, the specimen was separated from the test block. The specimen surface was rinsed with distilled water and dried using a gentle stream of clean air. The specimens were treated according to the regimens indicated for the 12 groups [Table 1]. After this, the specimens together with a ruler (0.5 mm scale) were photographed with a single-lens reflex digital camera (Nikon D80, macro lens) [Figure 3]. A standardized technique was used with lens, specimen and ruler all parallel. The distance between the two vertical grooves along groove no. 1 on the image of the impressions was measured 3 times up to 0.00001 mm accuracy. The mean was used for statistical analysis. The average of the three measurements was used to calculate the percentage dimensional change (%DC) for each specimen to the nearest 0.000001 using the equation: %DC = 100 (L1 - L2/L1). L1 = The distance measured on the test block and L2 = the distance measured on the specimen. According to ISO requirements, a specimen passes the dimensional stability test if the %DC is not more than 1.5. The ADA specifications are more stringent and set the limit at 0.5%DC. The software used for measuring the images was analyzing digital images, version 11, August 2008.  It allows scaling, calibration and analysis of digital images with at least nine times magnification.
|Figure 3: Image of specimen with 0.5 mm scale. Dimension of all images was 3872 × 2592 pixels|
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For the descriptive statistics, Excel 2010 (Microsoft Office 2010, Microsoft® ) was used. For the statistical analysis the Variance Estimation and Precision module of Statistica 10 (Statsoft® ) was used. Three factors were considered: (a) Material type (P or S); (b) temperature (21 or 66); and (c) time point for measurement (1, 2 or 3). It was this factor that caused the repeated measurement of the same sample, resulting in repeated measures analysis of variance. Interactions between all variables were also analyzed. To analyze exactly where the differences lay, a Fisher least significant difference correction was applied to correct for multiple pairwise comparisons (confidence interval 95%).
| Results|| |
The descriptive statistics for all the groups of both impression materials are represented in [Table 2] and [Table 3]. A negative %DC implies that the linear dimension of the specimen is larger than that of the test block and vice versa. For determination of the minimum %DC, the value closest to zero %DC is chosen and for the maximum %DC the value furthest away from 0%DC. For polyether, the highest mean %DC was for P*21 8h (0.364424) and the lowest mean %DC was for group P*21 14 days (−0.038932) [Table 2]. For silicone, the highest mean %DC was for S*66 16h (0.297070) and the lowest mean %DC was for group S*21 14 days (0.005887) [Table 3].
|Table 2: Descriptive statistics (%DC) for all P impression material groups|
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|Table 3: Descriptive statistics (%DC) for all S impression material groups|
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[Table 4], [Table 5], [Table 6] summarize the statistical differences between pairs of groups.
|Table 4: Pairs of groups and P values to demonstrate the effect of different materials on %DC|
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|Table 5: Pairs of groups and P values to demonstrate the effect of time on %DC|
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|Table 6: Pairs of groups and P values to demonstrate the effect of exposure to temperature on %DC|
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To establish the effect of material on dimensional stability, pairs of groups subjected to the same time/temperature regimen for each material were compared with each other [Table 4].
There was a significant difference in mean %DC for all the pairs, except between the groups left for 2 weeks at room temperature (S*21 14 days and P*21 14 days ) and between the groups exposed to 66°C, cooled off and measured after 8 h (S*66 16h and P*66 16h ).
To establish the effect of time on the dimensional stability of each material, pairs of groups of the same material and temperature scenario were compared [Table 5]. All the pairs showed a significant difference in %DC.
To establish the effect of exposure to temperature on the dimensional stability, same material groups with the same time intervals but exposed to different temperatures were compared [Table 6]. All pairs differed significantly.
[Figure 4] plots the effects of all the variables (temperature, material and time) on the mean %DC.
|Figure 4: Plots of the mean percentage dimensional change for the control (21 temperature scenario) and the effect of raised temperature (66 temperature scenario). Vertical bars denote 0.95 confidence intervals. Same letters indicate no statistical difference among groups; different letters indicate statistical difference among groups|
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| Discussion|| |
This study investigated the influence of temperature on the dimensional stability of two types of elastomeric impression materials. The null-hypothesis stating that temperature has no influence on the linear dimensional stability of elastomeric impression material is rejected.
Even though statistical differences were found, it is important to take note that all the groups for both materials showed a mean %DC considerably lower than the ISO recommended DC of not more than 1,5%. The mean %DC for all the groups were also within the limits set by the ADA; However, one specimen of the P*66 8h had a %DC of 0.607, being higher than the ADA limits [Table 2].
For the explanation of the statistical results, it is helpful to refer [Figure 4].
Storing the silicone impression material at room temperature for 14 days (S*21 14 days ) resulted in the highest dimensional accuracy for this material. This is shown by a %DC closest to the zero-value [Table 2]. In fact, the 14-day accuracy was significantly better than at 30 min after impression making (S*21 30 min ) [Table 5]. Given enough time to recover (14 days at room temperature) after an 8 h period of exposure to an elevated temperature, the silicone impression material returned to the dimension it had 30 min after impression making. This is evidenced by no statistical difference in %DC between groups S*66 14 days and S*21 30 min [Figure 4]. However, it did not reach the same level of dimensional accuracy as the group that was kept at room temperature for 14 days. This is shown as a significant difference between S*21 14 days and S*66 14 days [Table 6].
Keeping the polyether impression material at room temperature for 14 days (P*21 14 days ) resulted in the highest dimensional accuracy for this material. This is shown by a %DC closest to the zero-value [Table 1]. In fact, the 14-day accuracy was significantly better than at 30 min (P*21 30 min ) after impression making [Table 5]. Thongthammachat et al. reported that polyether needs to be cast within 24 h after impression making.  The present study does not support this, with the 14 days %DC closest to zero %DC. At 14 days, the polyether did not quite reach the same level of dimensional accuracy as the silicone material after 14 days, even though this difference was not significant (P*21 14 days and S*21 14 days ) [Table 4]. After exposure to a high temperature for 8 h, polyether impression material exhibit a significant expansion after 14-day recovery at room temperature (P*66 8h and P*66 14 days ) [Table 5].
For polyether, there was a significant difference between the group that was left for 14 days at room temperature (P*21 14 days ) and the group that was exposed to 66°C for 8 h and then left at room temperature for 14 days (P*66 14 days ) [Table 6]. It was for this time and temperature combination that the largest difference in %DC happened. Rubel reported that polymerization shrinkage causes smaller dimensions.  Indeed, the results of this study demonstrate an initial contraction (at 30 min and 8 h). Lacy et al. and Williams et al. reported that polyether material seem to be affected by imbibition resulting in expansion. , Indeed, the results of this study show an expansion of the material at 14 days. At 14 days, the group exposed to 66°C showed more expansion than the 21°C group. Modern materials still seem to be affected by dimensional fluctuations attributed to polymerization shrinkage and imbibition expansion.
When silicone specimens were kept at room temperature, the dimensional accuracy was lowest at 8 h after impression making. There was a significant difference between the %DC at 8 h and 30 min after impression making (S*21 8h and S*21 30 min ) and also between 8 h and 14 days (S*21 8h and S*21 14 days ). There was also a significant difference between the 30 min and 14 days readings (S*21 30 min and S*21 14 days ). This late expansion compensated for the shrinkage of the material during the first 8 h and resulted in the most dimensionally accurate group of the complete study (S*21 14 days ). Thongthammachat et al. also reported that silicone material has dimensional stability of up to 30 days.  The manufacturers of the silicone used for this study report that stone models can be poured after 30 min at the earliest and that the impression remains dimensionally stable for a practically "unlimited" period of time. The results of this study support these recommendations if the impression is stored at room temperature. However, exposing the material to a high temperature, does influence the accuracy at 14 days. Under these conditions, the shelf life of the material needs to be investigated further.
For both materials and temperatures, the %DC was the highest at 8 h after impression making for the 21°C groups and 16 h (8 h at 66°C plus 8 h of cooling-off) for the 66°C groups (i.e., S*21 8h ; S * 66 16h ; P*21 8h and P*66 16h ). This confirms the reported initial polymerization shrinkage of elastomeric impression materials.  Silicon impressions stored at room temperature for 8 h significantly differ from the impressions exposed to 66°C, with the "warm" impressions being more accurate (S21 8h and S66 8h ). However, cooling-off caused a significant contraction (S66 8h and S66 16h ). The same is true for the polyether material.
Long-term (14 days), for both materials, the room temperature groups stabilize closer to a zero-%DC than the 66°C groups. Therefore, it is recommended to avoid exposure to high temperatures in the first place. This is in line with Goncalves et al. who concluded after a critical review of the literature that impressions should be stored at 21 ± 2°C.  Corso et al. also found that polyethers expanded more after an initial contraction and after being exposed to 40°C. 
To investigate the influence of temperature, the specimens were subjected to a temperature of 66°C for the duration of 8 h. This temperature was chosen because it was recorded as a temperature that occurred in delivery vehicles during American summers when the outside temperature was 38°C. During South African summers, these temperatures are easily reached. The time interval of 8 h was an arbitrary time period for the 21°C group of specimens and it was thought that 8 h might be a realistic time lapse between impression making in the surgery and fabrication of the cast in the laboratory.
The manufacturers of the polyether material used for this study recommend that the impressions be poured anytime within 14 days after it has been made if stored under recommended conditions (dry place, 23°C). Therefore, 14 days was chosen as the "shelf-life" for both impressions.
Since no undercuts were incorporated into the ISO specified mold, no elastic recovery could have played a role in the fluctuations of the %DC. The influence of elastic recovery using different temperature scenarios could be investigated further.
Piwowarczyk et al. reported that any conclusions on dimensional stability of impression materials can only be made when any other variable is excluded from the methodology.  Following the ISO methods in this study, the specimens themselves were measured and not reproductions by means of casts. Furthermore, no support by means of trays was necessary. This eliminated any variable introduced by casting procedures and additional materials used in the process such as dental stones, tray and adhesive materials. Therefore, dimensional changes in this study are exclusively attributed to the impression material. Unfortunately, most scientific studies investigating dimensional stability of elastomeric impression materials have included these variables, obscuring the real property of the material and making comparison among studies difficult. The accuracy of elastomeric impression materials is higher than that of gypsum casts.  Therefore, measuring casts in order to determine the accuracy of impression materials is not indicated.
ISO requires a %DC for elastomeric impressions of not more than 1.5. A 1.5%DC over ten mm means dimensional inaccuracy of 0.15 mm; over longer distances, such as in cross-arch full fixed implant-supported prostheses, this error will be multiplied. This level of inaccuracy would result in micro-leakage and non-passive fit for multi-unit implant supported prostheses.
ISO recommends the use of a travelling microscope for measuring the distance on the specimens. However, locating a travelling microscope next to an incubator for temperature controlling the specimens was not possible. Therefore, it was decided to use a technique of measuring dimensions from digital images from the specimens and not from the specimens themselves. This had the benefit of minimizing temperature loss from the moment of removal from the incubator until the image was made: It was assumed that making a photograph of a specimen was faster than mounting the specimen on a travelling microscope and manipulating the apparatus to do three readings. By the time the third reading is done using a travelling microscope, the specimen must have cooled off. A second benefit was that the specimens are digitally stored and can be measured and re-measured at any given time.
The results of in vitro studies do not necessarily predict clinical performance. However, impression materials fall in the category of dental materials where laboratory tests have some level of correlation with the clinical performance since impression materials function for a short period of time and predominantly extra-orally. 
This study is limited because only one elevated temperature, 66°C, was chosen. However, it is improbable that impression materials will ever be exposed to higher temperatures. Even when exposed to such a high temperature, it is still safe to cast impressions 2 weeks after impression making.
Another limitation is the use of one brand for each type of elastomer. Extrapolation of the results to other brands of related materials must be done with care.
| Conclusions|| |
Within the limitations of this in vitro study, the following conclusions can be made:
- For both impression materials, storing at room temperature for 2 weeks resulted in the highest dimensional accuracy. For both materials, this accuracy was better than 30 min after removal from the mouth
- For both impression materials, dimensional accuracy was poorest at 8 h when stored at room temperature and at 16 h after being exposed to heat
- After being exposed to heat, the long-term (14 days) dimensional accuracy of both materials was negatively influenced, more so for the polyether impression material than for the silicone material
- Despite statistical differences, both materials were within ISO-specifications for dimensional stability, even when exposed to a high temperature for a prolonged period
- It is recommended that minimum ISO requirements are revised, especially in the era of implantology where a higher level of accuracy is required for passively fitting prostheses.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]