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SETTING CHARACTERISTICS OF THREE COMMERCIAL VINYL POLYSILOXANE IMPRESSION MATERIALS MEASURED BY AN OSCILLATING RHEOMETER.

Byline: SHAHAB UD DIN, MUHAMMAD HASSAN, SANDRA PARKER and MANGALA PATEL

Abstract

The aim of this study was to determine the setting characteristics of three commercial vinyl polysiloxane (VPS) impression materials at two different temperatures (23C and 37C). The objects were to investigate the effect of temperature and to compare the setting profile of these materials with each other using an oscillating rheometer. The three commercial VPS used were Aquasil Ultra Monophase (Aq M), Elite HD Monophase (Elt M) and Extrude Medium bodied (Extr M). For each material (n=5), 2 g VPS was extruded on to the lower plate of the rheometer and then the upper plate was positioned on top. The temperature was controlled using a thermostatically controlled water bath. Working time was calculated as 95% of the full trace recorded and setting time when the trace became parallel. Increasing the temperature significantly decreased the working and setting time for all materials tested.

At 23C there was no significant difference between the working time of Aq M and Extr M, which were significantly longer than Elt M. Extr M had the highest mean working time at 23C and Elt M had the lowest mean working time. At 37C the working time for Elt M was significantly shorter than Extr M and there was no significant difference with Aq M. The setting and working time for Elt M at 23C and 37C were significantly shorter compared with Aq M and Extr M. Aq M and Extr M at 23C showed no significant differences. The working and setting time of all the materials were temperature dependent.

Key Words: Impression material; elastomers; vinyl polysiloxane; working and setting time.

INTRODUCTION

Among all elastomeric impression materials, the Vinyl polysiloxane (VPS) impression materials are widely used because of their greater elastic recovery, superior accuracy and better dimensional stability.1-4

These materials are used to record impressions where reproduction of fine details is needed, such as in the construction of crowns and bridges.5,6 The VPS impression materials were first introduced in 1970's and since then these materials are most commonly used for recording impressions. 4,7-9

The VPS impression materials are supplied as a two pastes system. The base paste contains poly (methylhydrosiloxane) as well as poly (dimethylsiloxane) prepolymer with vinyl terminal groups. The poly (methylhydrosiloxane) is a moderately low molecular weight prepolymer in which some of the methyl groups are replaced by pendant or terminal hydrosilane groups. The catalyst paste contains vinyl-terminated poly (dimethylsiloxane) and a catalyst such as chloroplatinic acid.8,10-12 The fillers, such as amorphous silica or fluorocarbons, are added to both the pastes to improve mechanical properties.7,10 The VPS impression materials are presented in different viscosities; extra light-bodied, light-bodied, medium-bodied, heavy-bodied and putty.8,10,13 When the two pastes are mixed, an addition polymerisation reaction occurs forming a cross-linked molecules in the set state.7,12,14,15

Working time of elastomeric impression material is the period from the start of mixing of the two pastes to the commencement of elasticity and loss of plasticity, where further manipulations will introduce distortion.16

Ideally working time should exceed the time required for mixing, filling the tray, injecting the material on the prepared tooth/teeth, and seating the tray in the mouth.17 Setting time of elastomeric impression material is the time between the start of mixing until the polymerisation process has advanced sufficiently and developed elastic properties that permit the removal of the impression from the mouth over undercuts.8,18. Usually there is a correlation between the working and setting time; longer the working time, longer will be the setting time and vice versa.18 Ideally an impression material should have a rapid setting time provided the material has enough working time.17

Working and setting time of elastomeric impression materials increases with a decrease in temperature.15,19

To extend their working time two methods have been employed. Some manufacturers incorporate a retarder into their compositions (e.g. oleic or stearic acid) without compromising other properties. But the widely used method is to refrigerate impression pastes, which can extend the working time by approximately 1.5 minutes, without affecting the material's accuracy.7,20,21 Bonsor and Pearson15 reported the working and setting time of elastomeric impression materials at two different temperatures (23C and 37C), to simulate the ambient temperature of the dental surgery and patient's mouth respectively. Their data showed that the impression materials tested had faster working and setting time at the elevated temperature. They also demonstrated that polysulphide impression materials had much longer working time (4 to 6 minutes) and setting time (12 to 16 minutes) at 23C and 37C respectively, compared to vinyl polysiloxane (VPS), condensation silicone and polyether impression materials.

The amount of filler content also affects the working and setting time; with an increase in filler content there is a decrease in working and setting time.8 The manufacturers supply elastomeric impression materials with two different setting time (regular setting and fast setting) to save time for dental staff and for the comfort of the patient.22

The importance of impression materials and their optimum use cannot be under mined. Special emphasis should be paid to the setting characteristics of these materials in order to make the best use in clinical settings. The working and setting time vary according to the manufacturer. This study attempts to measure these timings and corresponding temperatures accurately. The purpose of this study was to evaluate the setting characteristics of three medium bodied VPS impression materials at two different temperatures (23C and 37C) to simulate the ambient temperature of the dental surgery and patient's mouth respectively.

METHODOLOGY

I. Materials

The commercial VPS impression materials used in this study were hydrophilic according to the literature provided by the manufacturers (Table 1). These materials were supplied through auto-mixed cartridge delivery system.

II. Procedure

The working and setting time of commercial VPS (n=5) were determined at two different temperatures (23C and 37C). An oscillating rheometer (Fig 1), attached to a chart recorder17, was used to monitor the setting characteristics (Fig 1b). The equipment was calibrated each time before use. For each material (n=5), 2 g VPS was extruded directly on to the centre of the surface of the lower plate (diameter = 25 mm) of the rheometer and then the upper plate (diameter = 25 mm) was placed on top and fixed in position. The components of each VPS were weighed on a four figure balance, Mettler, Toledo Ltd, Model AG204, UK. The gap between the two plates was 2 mm. The oscillation was commenced when the material entered into the mixing nozzle of the auto-mixing syringe. The rheometer output produced a continuous trace over time on the chart recorder, which was set at a speed of 5 mm min-1.

The amplitude of the trace decreased with an increase in viscosity, as the setting proceeded. The recording was continued until the amplitude of the trace reached a constant width. The temperature (23C and 37C) was controlled using a thermostatically controlled water bath, which circulated water through the upper plate (Fig 1 c, d). Working time was calculated as 95% of the full trace recorded and setting time was calculated when the trace became parallel (Fig 1e).23,24

III. Statistical analysis

The results were analysed by one way ANOVA and Tukey's Honest Significant Difference (HSD) test using the SPSS PASW statistical22 software. One-way ANOVA was used to analyse the significant differences between the means of the groups.

TABLE 1: COMMERCIAL VPS IMPRESSION MATERIALS (REGULAR SET) USED IN THIS STUDY

Commercial VPS###Lot/batch Manufac-

###number###turers

Aquasil Ultra Monophase###090505###Dentsply,

(Medium-Bodied), (Aq M)###USA

Elite HD Monophase (Me-###95503###Zhermack,

dium-Bodied), (Elt M)###Italy

Extrude (Medium-Bod-###0-1068###Kerr, USA

ied), (Extr M)

TABLE 2: MEAN (N=5) WORKING AND SETTING TIME OF THREE COMMERCIAL VPS IMPRESSION MATERIALS

Tem-###Working time###Setting time

pera-###(seconds)###(seconds)

ture###Aq###Elt###Extr###Aq###Elt###Extr

###M###M###M###M###M###M

###164###124###170###460###320###482

###65###60###74###288###190###277

If the differences were significant between the groups, further analyses were carried out with post hoc test (HSD) to determine the significantly different mean value among groups. The differences were considered significant at the p[?]0.05 level.

RESULTS

The average working and setting time (seconds) of commercial VPS formulations at two different temperatures are shown in Fig 2 and 3 respectively. Increasing the temperature significantly decreased the working and setting time (p=0.03) of all the VPS impression materials tested. At 23C there was no significant difference (Tukey's HSD test) in the work- ing time of Aq M and Extr M (p=0.02), which were significantly longer (p=0.02) than Elt M (Fig 2). Extr M had the highest mean working time at 23C (170 sec 9 sec) and Elt M the lowest mean working time (124 sec 8 sec).

At 37C the working time for Elt M was significantly shorter (60 sec 6 sec) than Extr M, with the exception of Aq M where the difference was not significant. Aq M and Extr M did not show any significant differences between their working time. Extr M had the longest mean working time (74 sec 5 sec) at 37C. At 23C and 37C, the Elt M demonstrated significantly shorter setting time, while Aq M and Extr M showed no significant difference (Fig 3).

DISCUSSION

The working and setting time of commercial VPS were investigated at 23C and 37C to simulate the temperature of dental surgery (23C) and patients mouth (37C) respectively. The working and setting time of all the materials were temperature dependent; an increase in temperature (from 23C to 37C), without changing any other factors, resulted in shorter working and setting time, due to a faster rate of polymerisation (Table 2). Similar results were found by Berg, Johnson 19 after investigating the setting characteristics of two VPS and five polyether impression materials. On comparing the results of the current study with Bonsor and Pearson15 who investigated working and setting time of elastomeric impression materials (VPS, condensation silicone, polyether and polysulphide) at two different temperatures (23C and 37C), it was seen that all commercial materials of this study had lower working time compared to their VPS materials (Table 2).

Another study by Pae et al (2008) also showed similar results with this study. They investigated the effect of temperature on the rheological properties of five VPS and one polyether impression material. They found that the these materials had different viscoelastic properties and most of the materials showed different fluidity at 21C and 33C.25

The vinyl polysiloxane (VPS) impression material is inherently hydrophobic. However, some manufacturer have incorporated surfactants within these materials and have classed them as hydrophilic VPS impression materials. The commercial VPS impression materials used in this study were hydrophilic according to the literature provided by the manufacturer. Previous studies have reported that the addition of a surfactant increases the working and setting time of silicone impression materials.26 However, the materials evaluated in the current study showed their working and setting time were within the acceptable limits. It should be noted that the ISO482327, does not specify any working and setting time for elastomeric impression materials. They suggest one should refer to manufacture guidelines for working and setting time of these materials.

However, according to the ADA28, the maximum time for removal of the impression from patient's mouth should not be more than 10 minutes, but they do not specify whether the removal time includes working time.

Elt M's setting time was lower than Aq M and Extr M (Table 2). Therefore, it seems that the surfactant did not appear to affect setting time of Elt M. Elt M is sold as a hydrophilic impression material, but the latter property discussed implies that the surfactant incorporated is not as effective as those used in other materials (i.e. Aq M and Extr M). Hence, it is reasonable to infer that the addition of surfactant interfere with the setting of hydrophilic VPS materials.

CONCLUSIONS

* The working and setting time of VPS impression materials were temperature dependent; an increase in temperature resulted in shorter working and setting time without altering other factors.

* Elt M's working and setting time was lower than Aq M and Extr M.

* Addition of surfactant interferes with the setting of hydrophilic VPS impression material.

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Publication:Pakistan Oral and Dental Journal
Article Type:Report
Geographic Code:9PAKI
Date:Sep 30, 2016
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