|Year : 2016 | Volume
| Issue : 3 | Page : 204-209
Effect of storage time and temperature change on the dimensional stability of polyvinyl siloxane and polyether impression materials: An in vitro study
William Thomas1, Pradeep Kumar2, Susan Mathew3, Sudeep Sarathchandran4, P Jayanthi5
1 Department of Prosthodontics, Azzezia Dental College, Kollam, Kerala, India
2 Department of Prosthodontics, KMCT Dental College, Kozhikode, Kerala, India
3 Department of Prosthodontics, Mar Gregorius Dental College, Chelad, Kerala, India
4 Department of Prosthodontics, PMS Dental College, Trivandrum, Kerala, India
5 Department of Oral Pathology, Azzezia Dental College, Kollam, Kerala, India
|Date of Web Publication||10-Oct-2016|
Department of Oral Pathology, Azzezia Dental College, Kollam, Kerala
Source of Support: None, Conflict of Interest: None
Background: Synthetic elastomers such as polyvinyl siloxane and polyether are the most commonly used impression materials. The accuracy and dimensional stability of these materials are influenced by the temperature, humidity, and storage time.
Aim: The aim of this study was to assess the effect of storage time and temperature on the dimensional stability of polyvinyl siloxane and polyether impression materials.
Materials and Methods: A stainless steel die was used to prepare the molds from three brands of polyvinyl siloxane (Elite HD, Express STD, Aquasil) and one brand of polyether (Impregnum F) impression materials. The samples were stored at five different temperatures (10°C, 180°C, 280°C, 350°C, and 450°C) for 24, 48, and 72 h, respectively. The linear change in the dimension of the materials were recorded and analyzed.
Results: When stored at 280°C and 450°C, polyvinyl siloxane did not show any significant dimensional change in 24, 48, and 72 h when compared to control samples. At 10°C, significant dimensional change was noted in 24 h (P = 0.00) and at 180°C, significant change in the dimension was noted in 24 and 48 h (P = 0.01,P= 0.00, respectively). At 350°C, polyvinyl siloxane showed expansion in 24 and 48 h (P = 0.00,P= 0.04, respectively) whereas no further change occurred in 72 h. Significant changes in the dimensional stability were noted for polyether at all the five storage temperatures (10°C, 180°C, 280°C, 350°C, and 450°C) when compared to control samples.
Conclusion: All the three brands of polyvinyl siloxane and the one brand of polyether tested showed statistically significant dimensional changes in 10°C, 180°C, 350°C, and 450°C, whereas at 280°C, all the tested materials showed no significant changes. Most of these changes occurred within 24 h, with very little changes occurring in 48 and 72 h.
Keywords: Dimensional change, elastomers, polyether, polyvinyl siloxane
|How to cite this article:|
Thomas W, Kumar P, Mathew S, Sarathchandran S, Jayanthi P. Effect of storage time and temperature change on the dimensional stability of polyvinyl siloxane and polyether impression materials: An in vitro study. J NTR Univ Health Sci 2016;5:204-9
|How to cite this URL:|
Thomas W, Kumar P, Mathew S, Sarathchandran S, Jayanthi P. Effect of storage time and temperature change on the dimensional stability of polyvinyl siloxane and polyether impression materials: An in vitro study. J NTR Univ Health Sci [serial online] 2016 [cited 2019 Jul 22];5:204-9. Available from: http://www.jdrntruhs.org/text.asp?2016/5/3/204/191848
| Introduction|| |
Impression making is one of the most important stages in the production of dentures, crowns, and bridges. The accuracy and the functional efficiency of the prosthesis depend upon how well the cast replicates the natural oral tissues, which in turn depends on the accuracy and dimensional stability of the impression in which it was cast. The ideal requirements of an impression material include factors which affect the accuracy and dimensional stability of the impression, setting characteristics, ease of handling, and cost of the material. Of these, the dimensional stability and accuracy of the impression are considered to be the most important factors because they directly affect the fabrication of prosthesis.
Among the impression materials available, synthetic elastomers such as polyvinyl siloxane (addition silicone) and polyether are more popular because of their excellent detail reproduction and good dimensional stability. Polyether impression materials are supplied in two pastes. The base paste consists of a polyether polymer, colloidal silica as filler, and a plasticizer such as glycol ether or phthalate. The accelerator paste also contains an alkyl aromatic sulfonate. The set polyether materials are relatively hydrophilic and absorb water under conditions of high humidity resulting in the impressions to swell and distort. Polyether material should, therefore, be avoided where humidity is high and efficient air conditioning is not available.
Polyvinyl siloxane, also called as addition curing silicones is supplied in two pastes. Each paste contains a liquid silicone prepolymer (polydimethyl siloxane) and a filler, and one of the pastes contain a catalyst which is normally a platinum containing compound such as chloroplatinic acid. The production of little or no by-products in the cross-linking reaction of the addition curing material results in a very stable impression. They offer a significant advantage where models need not be cast soon after recording the impression because they are known to remain dimensionally stable over a considerable period of time.
Because of their exceptional dimensional stability, few practitioners send unpoured impressions to distant commercial dental laboratories for fabrication or pour the impressions quite lately. While the impressions are being sent to laboratories, they may be subjected to various temperature changes in transit. The factors that result in dimensional changes are temperature, humidity, and polymerization shrinkage. Various studies have been reported regarding the effect of humidity and temperature change on the dimensional stability of these two impression materials., All these published studies were conducted in countries other than India. The present study attempts to evaluate the effect of different temperatures, as reported in different regions of the Indian subcontinent, as well as the effect of different storage time on the dimensional stability of these impression materials.
| Materials and Methods|| |
This study was conducted at the Department of Prosthodontics K.V.G. Dental College, Sullia, Karnataka, and the Department of Industrial Biotechnology: TIFAC CORE Center, National Institute of Technology, Karnataka.
The elastomeric impression materials used for this study were polyvinyl siloxane and polyether. Products from three different manufacturers of polyvinyl siloxane and a product from a single manufacturer of polyether that are commercially available in India were selected. [Table 1] lists the trade names and manufacturers of impression materials used in this study.
|Table 1: Trade names and manufacturers of polyvinyl siloxane and polyether impression materials|
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Fabrication of study samples and test procedure
A stainless steel die was used to prepare samples for the study. The die was fabricated according to American Dental Association (ADA) specification number 19 for elastomeric impression materials. The die is a three piece device consisting of a cylindrical stainless block, steel mold and steel disc or a riser. The steel block has a height of 30 mm and a diameter of 35 mm. The top of the cylindrical block is machined into 30 mm diameter with 2.5 mm shoulders. On the top surface of the machined part, 3 cross lines having 25 mm length and 5 mm depth are engraved. A stainless steel mold or ring having 6 mm height and 35 mm diameter was fabricated and placed on the top of the machined part of the block, resting on the 2.5 mm shoulders. This mold or ring acts as a tray for the materials during impression making, which provides a thickness of 3 mm to the material [Figure 1].
A stainless steel disc or riser having a thickness of 3 mm and 30 mm diameter was made. This was used to push out the set impression material from the confines of the mold or ring.
Equal amount of base and catalyst of the light viscosity polyvinyl siloxane was dispensed on the glass slab and hand mixed with a spatula for 30 s, as specified by the manufacturer. After a uniform mix was attained without any streaks, the material was immediately taken with the spatula and smeared over the ruled surface of the metal block. Equal amounts of base and catalyst paste of putty material was then mixed with hands for 30 s to obtain a uniform colored consistency without any streaks.
The ring or mold was kept over the top portion of the block, resting on the shoulders, and the putty material was inserted into the space created between the mold and the ruled surface over the light viscosity material that was already applied. The flat square plate was then kept on top of the combination and clamped with a “C” clamp. After it was secured tightly, the assembly was placed inside a temperature controlled incubator where the temperature was kept constant at 35°C and the material was allowed to set for 6 min and kept for another 6 min.
Polyether impression materials were supplied as a single phase, two paste system, a base paste, and an accelerator paste. Equal length of two pastes were expressed over the glass plate and hand mixed with the spatula for 45 s, as per the manufacturer's instructions. The material was placed over the metal block and the same procedure was repeated as done with polyvinyl siloxane.
At the end of 6 min, the assembly was taken from the incubator and kept at room temperature for 10 min. The assembly was then dismantled and the set impression material was removed from the mold with the help of the riser. The impression material was then viewed under the travelling microscope, and the length of the 3 lines that were recorded in the impression material was measured from one end to another. This reading was taken as control or baseline length.
The sample was transferred to the environmental incubator where the temperature can be adjusted from 10°C to 500°C. The samples were placed inside the incubator at 10°C, 180°C, 280°C, 350°C, and 450°C for 24, 48, and 72 h, respectively. At the end of each storage period, the sample were taken out of the incubator and kept at room temperature for 10 min. The samples were again viewed under the travelling microscope and the lengths of the 3 lines in the surface of the samples were measured. This reading was taken as the final length. The difference between the control length and the final length was taken as the measure of dimensional stability.
Ten such samples of three manufacturers of polyvinyl siloxane for three different storage periods, namely, 24 h, 48 h, and 72 h for five different temperatures (1°C, 18°C, 28°C, 35°C, and 45°C) were made. A total of 450 samples of polyvinyl siloxane impression materials were prepared. Ten samples of polyether impression material for each storage period at 5 temperatures were fabricated; a total of 150 samples of polyether impression materials were prepared and tested.
Statistical analysis was done using the Statistical Package for the Social Sciences (SPSS software (IBM Inc.)) version 11.4. The differences in the dimensional stability of the materials were compared using Student's t-test and analysis of variance. P value less than 0.05 was considered to be statistically significant.
| Results|| |
[Table 2] shows the change in dimension of various polysiloxane and polyether impression materials stored at different temperatures and storage times. When stored at 280°C (room temperature), polyvinyl siloxane did not show any significant dimensional change in 24, 48, and 72 h when compared to control samples. At 10°C, significant dimensional change was noted in 24 h (P = 0.00), but no further change was seen in 48 h and 72 h. At 180°C, significant change in the dimension was noted in 24 and 48 h (P = 0.01, P = 0.00), but no change was seen in 72 h. At 350°C, polyvinyl siloxane showed expansion in 24 and 48 h (P = 0.00, P = 0.04), while no further change occurred in 72 h. When the temperature was increased to 450°C, polyvinyl siloxane did not show any significant change in the dimensional stability up to 72 h.
|Table 2: Dimensional change of poylysiloxane and polyether materials at various temperatures and storage times|
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Brand-wise evaluation of polyvinyl siloxane revealed that Express STD did not show significant variations at any of the tested temperatures and storage periods whereas Aquasil showed variations only at very low (10°C) and high temperatures (450°C). The storage time period did not significantly affect the dimensional stability of Aquasil. Elite HD exhibited dimensional changes at 10°C, whereas at 280°C, 350°C, and 450°C, no significant changes were noted.
Significant changes in the dimensional stability were noted for polyether at all the five storage temperatures (10°C, 180°C, 280°C, 350°C, 450°C) when compared to control samples. The microscopic changes in the dimensional stability were found at 24, 48, and 72 h.
When polysiloxane and polyether were compared, dimensional changes were higher in polyether at all the temperatures, although statistically significant changes were noted only at 10°C, 280°C, and 450°C (P = 0.02, P = 0.03, P = 0.00, respectively).
| Discussion|| |
The primary factors affecting dimensional change in impressions are thermal contractions, polymerization shrinkage, and changes occurring due to loss of volatile by-products. The dimensional change is predominantly due to thermal shrinkage as the material cools from oral temperature to room temperature. Polyvinyl siloxane and polyether are the most dimensionally stable materials of all the existing impression materials because no volatile reaction by-products are released that may cause the materials to shrink. Hydrogen gas that is formed during the polymerization reaction of polyvinyl siloxane is not a reaction by-product but occurs as the result of abnormal proportion of pastes or due to the presence of impurities. The clinically set material is close to being completely cured, and therefore, there is little residual polymerization to contribute to the dimensional change. Polyethers do not have reaction by-products similar to polyvinyl siloxanes. Although the residual polymerization continues beyond the clinical set time for polyethers, it is for a much shorter period of time.
Most of the studies related to dimensional stability of impression materials have been conducted in countries other than India where temperature and humidity are not very extreme and varied., Our study investigated the effect of five randomly selected temperatures from different parts of India, (10°C, 180°C, 250°C, 350°C, and 450°C) on the dimensional stability of polyvinyl siloxane and polyether when kept in three different storage periods (24 h, 48 h, and 72 h). This study tested three brands of polyvinyl siloxane available in India (Elite HD+: Zhermach, Aquasil: Dentsply, Detray, Express, 3M ESPE) and one brand of polyether (Impregum F: 3M ESPE).
In majority of the studies, the samples were prepared in customized plastic trays or in stock trays. Due to extreme temperatures and different storage times, a custom plastic tray can produce changes by itself that can influence the property of the material. It has also been reported that a material can shrink toward the tray and can result in larger casts. We have employed the test procedure of ADA specification no. 19, where a metallic ring which is a part of the die assembly acts as a tray that carries the material; after setting the material was removed.
In our study, all the three brands of polyvinyl siloxane showed statistically significant dimensional changes in 24 h and 48 h when stored at 10°C and 180°C, however, no further changes were noticed in 72 h. At 280°C, all the brands in all three storage periods showed no statistically significant dimensional change. At 350°C, all the brands showed statistically significant changes at 24 h and 48 h. At 450°C, dimensional changes were noted, however, the values were not statistically significant. Our results are in contrast to the studies done by Corso et al. who reported that, when polyvinyl siloxane was kept at 40°C and 400°C for 26 h, the materials showed slight expansion. None of the three brands of polyvinyl siloxane materials showed clinically evident superiority over the other. However, Elite HD exhibited significantly lower stability at 10°C and 180°C in all the storage periods, whereas Aquasil showed slightly lower stability over others at 450°C.
In our study, significant changes in the dimensional stability were noted for polyether at all the five storage temperatures at 24 h, 48 h, and 72 h. When mean readings of polyvinyl siloxane and polyether were compared, statistically significant dimensional change was seen at 10°C in 24 h, 180°C in 48 h, and 280°C in 24 h and 48 h. However, at 350°C and 450°C, no significant difference was seen. Again, this is in contrast with the study conducted by Boulton et al. who found that polyether showed excellent dimensional properties and reported polyether to be more dimensionally stable than polyvinyl siloxane. They also emphasized the importance of pouring the impressions as soon as possible after removal from the mouth to prevent dimensional change. The authors also suggested that, if significant delay in pouring were unavoidable, then the polyether material would be expected to exhibit less dimensional changes than other materials.
In our study, few polyether samples were disintegrated and distorted, the reasons for which were not investigated in this study. Corso et al. reported that ambient humidity affects the dimensional stability of polyvinyl siloxane and polyether impression materials. Endo et al. in his investigation found that polyether swells when in contact with moisture. Fusayama et al. also reported that polyether material expands with increase in humidity. The distortion of few polyether samples in our study may be due to moisture or humidity. This study evaluated only the linear dimensional changes of the impression materials and the effect of humidity was not evaluated.
| Conclusion|| |
All the three brands of polyvinyl siloxane and the one brand of polyether tested showed statistically significant dimensional changes in 10°C, 180°C, 350°C, and 450°C, whereas at 280°C all the tested materials showed no significant changes. Most of these changes occurred within 24 h with very little changes occurring in 48 h and 72 h. These results indicate that extreme changes in temperature may produce distortion of the impression materials, and therefore the casts should be poured within 24 h to prevent distortion. Furthermore, when polyvinyl siloxane and polyether are compared, polyvinyl siloxane seemed to be more stable than polyether in these test conditions.
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Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2]