An in vitro study of a custom-made device for thermoregulation of the mixing slab on the setting properties of zinc oxide eugenol impression paste

Divyansh Sinha; Laasya Shivanand Shettigar; Kishore Ginjupalli; Nagaraja P Upadhya; Abhishek Bhagat

doi:10.4103/jips.jips_337_22

RESEARCH

Year : 2023 | Volume : 23 | Issue : 1 | Page : 50-56

An in vitro study of a custom-made device for thermoregulation of the mixing slab on the setting properties of zinc oxide eugenol impression paste

Divyansh Sinha, Laasya Shivanand Shettigar, Kishore Ginjupalli, Nagaraja P Upadhya, Abhishek Bhagat
Department of Dental Materials, Manipal College of Dental Sciences, Manipal Academy of Higher Education, Manipal, Karnataka, India

Date of Submission	12-Jul-2022
Date of Decision	13-Oct-2022
Date of Acceptance	22-Oct-2022
Date of Web Publication	29-Dec-2022

Correspondence Address:
Nagaraja P Upadhya
Department of Dental Materials, Manipal College of Dental Sciences, Manipal Academy of Higher Education, Manipal - 576 104, Karnataka
India

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/jips.jips_337_22

Abstract

Aim: The present study was aimed to investigate the functional relationship between the mixing temperature and properties of a commercially available zinc oxide eugenol impression paste (ZnOE paste).
Settings and Design: In-vitro study.
Materials and Methods: A custom-made simulated mixing device was indigenously designed to maintain different mixing temperatures, simulating cold, ambient, and hot weather. A commercially available ZnOE paste was mixed according to the manufacturer's instructions in the simulated mixing device at the temperatures ranging from 10°C to 50°C. Initial setting time and consistency were measured according to A. D. A. Specification No. 16 (n = 8). A stainless-steel die having 25, 50, and 75 μm lines was used for surface detail reproduction. Detail reproduction of the stone casts of the impressions was evaluated with a stereomicroscope at 30 magnification (n = 8). The shear bond strength of ZnOE paste to self-cure acrylic tray resin was measured by using the UTM at a crosshead speed of 0.5 mm/min (n = 8).
Statistical Analysis Used: Data were analyzed by using one-way analysis of variance (ANOVA) and Tukey's post hoc tests at a confidence interval of 95% (alpha =0.05)
Results: Initial setting time, consistency, and detail reproduction of the ZnOE paste were affected by the mixing temperature (P < 0.001). Mixing ZnOE paste at a lower temperature of 10°C and higher temperatures of 40°C and 50°C resulted in shorter initial setting time, thicker consistency, and poor detail reproduction. However, no significant difference was obtained in the shear bond strength among the different mixing temperatures evaluated (P > 0.05).
Conclusion: Based on this in vitro study, it is advisable to perform the manipulation of ZnOE paste at a clinical/laboratory temperature of 30°C for optimum performance. The simulated mixing device used in this study can be an alternative for extreme climatic conditions.

Keywords: Zinc oxide eugenol impression paste, mixing temperature, initial setting time, consistency, detail reproduction, bond strength, mixing device

How to cite this article:
Sinha D, Shettigar LS, Ginjupalli K, Upadhya NP, Bhagat A. An in vitro study of a custom-made device for thermoregulation of the mixing slab on the setting properties of zinc oxide eugenol impression paste. J Indian Prosthodont Soc 2023;23:50-6

How to cite this URL:
Sinha D, Shettigar LS, Ginjupalli K, Upadhya NP, Bhagat A. An in vitro study of a custom-made device for thermoregulation of the mixing slab on the setting properties of zinc oxide eugenol impression paste. J Indian Prosthodont Soc [serial online] 2023 [cited 2023 Feb 6];23:50-6. Available from: https://www.j-ips.org/text.asp?2023/23/1/50/365948

Introduction

Zinc oxide eugenol impression paste (ZnOE paste) is an old, rigid as well as irreversible impression material used for recording secondary impressions of edentulous arches. It has been popular as a secondary impression paste due to its unique properties such as good surface accuracy, detail reproduction, dimensional stability, adequate working time, and good adherence to dried surfaces of resin custom trays without using any tray adhesive.^[1] It has been considered as the gold standard impression material for recording complete denture impressions.^[2] It is dispensed as a two-paste system with a base paste containing zinc oxide (ZnO) and a reactor paste containing eugenol/clove oil. Fillers, plasticizers, accelerators, etc., are invariably added to get the required characteristics for a certain application of the impression paste.^[2],[3] The material sets by a chemical reaction in which the phenolic – OH group present in eugenol provides hydrogen ions, thus, behaving as an acid. It triggers an acid-base neutralization reaction with ZnO. As a result of the reaction, zinc eugenolate salt is formed.^[4],[5] However, for the initial reaction of forming zinc hydroxide, water is a must. It is needed for the generation of reacting ionic species and to act as a reaction solvent. Upon the formation of zinc eugenolate, water is released as a byproduct and is available for further reaction.^[5],[6] The set impression consists of grains of unreacted ZnO getting interspersed in a matrix of long, sheath-like crystals of zinc eugenolate matrix, together with an excess of eugenol being absorbed by both ZnO and zinc eugenolate particles.^[5],[7]

Usually, ZnOE paste is available with a wide range of properties that have been carefully and skillfully compounded by the manufacturer. The factors such as type and particle size of ZnO, addition of chemical modifiers to one or both pastes are solely under the control of the manufacturer to control the rate of the reaction.^[4],[5],[8] Improper handling of the material like mixing ratio and mixing time alters the properties of the ZnOE paste. Previous studies have reported that a change in the ZnO to eugenol mixing ratio influenced the consistency and setting of the products.^[9],[10] Similarly, longer spatulation time, within practical limits, shortened the setting time of the product.^[11] Further, studies on commercially available ZnOE paste indicated variation in the properties due to disparities in setting characteristics, viscosity, wettability, and quality of the mix.^[9],[12]

It is important to note that the manufacturers formulate the ZnOE paste and optimize its performance under standard laboratory conditions with temperature and humidity control. However, during the clinical use of the material, these factors are likely to be significantly different than the standard conditions. Such deviations in manipulative and environmental conditions may affect the setting characteristics of the material and thus its clinical performance. The initial viscosity of the mixed paste governs its ease of handling and is also related to the initial setting time, detail reproduction, and adhesion to the custom tray. In this regard, the existing literature indicates that changes in humidity and temperature significantly influence the rate of setting of ZnOE paste.^{[11],[13],[14],[15]} However, it is not clear if these changes affect the clinical performance of the material. In this regard, it would be logical to investigate the effect of mixing temperature on the clinically relevant properties of ZnOE paste such as initial setting time, consistency, detail reproduction, and its bond strength to self-cure acrylic tray resins. The main aim of the present study was to establish a simple indigenous, reproducible, cost-effective simulated mixing device for the manipulation of ZnOE paste and to investigate the functional relationship between the variable mixing temperature and properties of the ZnOE paste such as initial setting time, consistency, detail reproduction, and shear bond strength. The hypothesis of the study was that the mixing temperature would influence the properties of ZnOE paste.

Materials and Methods

In this study, a commercially available ZnOE paste (Dental Products of India, Batch number 8173, India) was manipulated as per the manufacturer's instructions at various temperatures. The desired temperature during the manipulation was maintained by using a custom-made simulated mixing device. A schematic drawing of the simulated mixing device is shown in [Figure 1]a and [Figure 1]b and the original image is shown in [Figure 1]c.

Figure 1: (a) Simulated mixing device. (b) Simulated mixing device - lateral view. (c) Clinical image of simulated mixing device

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The simulated mixing device was constructed by using two thick Thermocol blocks for the upper and lower layers.^[16] A cuboid of the exact size of the glass slab was cut into the upper layer to fit the glass slab snugly into the cavity. A smaller and deeper cavity was then cut, that acted as the reservoir for water. The configured upper layer was then placed on the lower layer and then glued together. A total of five temperatures simulating cold (10°C and 20°C), ambient (30°C) and hot climatic conditions (40°C and 50°C) were selected. Water at the desired temperature was taken in a container in which a spatula made of rigid stainless steel and a glass slab were kept immersed for 5 min to attain the temperature same as that of water, before using them for mixing the ZnOE paste. The temperature of water was monitored regularly with a laboratory thermometer. Water from the container was then poured into the apparatus and the glass slab was snugly fit into the reservoir such that the lower surface of the glass slab was in contact with temperature-controlled water. Any debris or water condensation was gently dabbed away with cotton before dispensing the material onto the glass slab.

According to the manufacturer's guidelines, equal lengths of base and reactor pastes were dispensed on the glass slab positioned in the simulated mixing device and mixed for 45 s. The initial setting time and consistency were measured according to American Dental Association Specification No. 16.^[17]

Initial setting time

A metal ring of 25-mm diameter having a depth of 3 mm was taken and the freshly mixed paste was placed, and then was leveled off at the top. A steel rod of 2.4 mm in diameter with rounded ends weighing 10 g was applied to the surface of the paste at 20-s intervals. Initial setting time was measured as the time from the start of mixing until the material ceases to string out upon withdrawing the rod (n = 8).

Consistency

0.5ml of the mixed material was injected onto a glass plate. Exactly 1.5 min after the beginning of the mixing, another glass plate weighing 20 g and a 500 g weight were carefully placed. After 10 min, the weight was removed, and the average diameter of the resulting disc was noted in millimeters (mm) with a traveling microscope (n = 8).

Detail reproduction

A round stainless–steel die containing 3 parallel lines of 20, 50, and 75 μm width and 25 mm in length with 2.5 mm spacing was used for evaluating the detail reproduction of the impression. A specially designed tray that fits the die was used and was loaded with freshly mixed ZnOE paste. The loaded tray was then placed on the die and a load of 1 kg was applied. After setting, the impression was retrieved and subsequently cast with dental stone under vibration. The cast was retrieved from the impression after an hour. Subsequently, the cast was allowed to dry in air for 24 h and then evaluated for detail reproduction. Surface detail reproduction of the stone casts (n = 8) was evaluated by using a stereomicroscope at ×30 magnification.^[18] The details of the lines reproduced in the cast were assessed and graded as follows.

Grade 1 – Continuous well-defined lines with accurate details
Grade 2 – Lines are continuous, with little loss of accuracy
Grade 3 – Noncontinuous lines or marked loss of detail
Grade 4 – Absence of lines.

Shear bond strength

Rectangular custom trays of dimensions 4 cm × 2cm × 1 cm were made with self–cure acrylic tray resins. After 24 h of aging, freshly mixed ZnOE paste was extruded onto the custom tray to form a cylinder of radius 0.8cm and length 1.5cm and was allowed to set for 7 min (n = 8). After setting, the acrylic tray was attached to the lower platform of a universal testing machine (INSTRON 3366, UK), and loaded at the impression paste-acrylic resin interface at a crosshead speed of 0.5mm/min until debonding. The peak load observed during the test divided by the total bonded surface area of the specimen computed the shear bond strength in MPa.

Statistical analysis

The data were analyzed using the one-way ANOVA and Tukey post hoc analysis (GraphPad Prism) at a 95% confidence interval (α = 0.05).

Results

The effect of temperature on the initial setting time of ZnOE paste is shown in [Figure 2]. The results showed that a gradual decrease in initial setting time was observed with increasing slab temperature from 30°C to 50°C. The effect was reversed by reducing mixing slab temperature from 30°C to 20°C. One-way ANOVA showed a significant difference in the initial setting time at various temperatures studied (P < 0.001). However, the initial setting time at 10°C and 50°C was not significantly different (P = 0.0582) [Table 1].

Figure 2: Initial setting time of zinc oxide eugenol impression paste

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Table 1: Results for Tukey's post hoc test for initial setting time between the temperatures (10°C, 20°C, 30°C, 40°C and 50°C)

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[Figure 3] demonstrates the consistency of ZnOE paste manipulated at various temperatures. The ZnOE paste manipulated at ambient temperature 30°C showed maximum flow. Temperatures deviating on either side from 30°C were seen to reduce the amount of flow. The consistency values measured at different temperatures were significantly different from each other except between 20°C and 30°C (P = 0.4063) [Table 2].

Figure 3: Consistency of zinc oxide eugenol impression paste

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Table 2: Results for Tukey's post hoc test for consistency between the temperatures (10°C, 20°C, 30°C, 40°C and 50°C)

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The detail reproduction at various temperatures is shown in [Figure 4]. The ZnOE paste manipulated at 20°C and 30°C showed better reproduction of details compared to other temperatures. The casts obtained from the impressions manipulated at both lower and higher temperatures namely 10°C, 40°C, and 50°C showed grades of 3 or 4. There were no significant differences in the detail reproduction between 10°C and 40°C (P = 0.6133); 10°C and 50°C (P = 0.9963); 20°C and 30°C (P = 0.8194); 30°C and 40°C (P = 0.0519); 40°C and 50°C (P = 0.8194) [Table 3].

Figure 4: Detail reproduction grades of zinc oxide eugenol impression paste

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Table 3: Results for Tukey's post hoc test for detail reproduction between the temperatures (10°C, 20°C, 30°C, 40°C and 50°C)

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[Figure 5] depicts the shear bond strength of ZnOE paste manipulated at three different temperatures to acrylic resin tray. ZnOE paste manipulated at 30°C exhibited the highest shear bond strength. However, the shear bond strength of ZnOE paste mixed at different temperatures with acrylic resin was not found to be significantly different (P = 0.1328, 0.3407, 0.8077 between 20°C and 30°C; 20°C and 40°C; 30°C and 40°C) [Table 4]. Visual examination of the fracture mode indicated the adhesive type of fracture at the acrylic resin tray-impression paste interface.

Figure 5: Shear bond strength of zinc oxide eugenol impression paste

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Table 4: Results for Tukey's post hoc test for shear bond strength between the temperatures (20°C, 30°C, and 40°C)

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Discussion

This study was conducted to determine the impact of mixing temperature on the initial setting time, consistency, detail reproduction, and bond strength of commercially available ZnOE paste. The hypothesis that the mixing temperature affects the properties of ZnOE paste was accepted except for its bond strength when loaded onto self-cure acrylic resins.

The temperature of the mixing slab and spatula/environment plays an important part in the evaluation of the properties of ZnOE paste. The American Dental Association Specification No. 16 for impression paste stipulates the importance of temperature control (23.0 ± 2.0°C) during the manipulation.^[17] Clinically, best possible impressions can be obtained by manipulating ZnOE paste at ambient temperature. Any increase or decrease in the temperature of the mixing slab/spatula could alter the kinetics of the setting of the mixed ZnOE paste, thereby leading to compromised quality of the patient's impression. In the present study, a custom-made mixing device was prepared for thermoregulation of the mixing slab and spatula using two thick Thermocol blocks and water at the desired temperature. Thermocol was selected as the material of choice since it acts as a thermal insulator and would prevent any change of temperature of the water during the time taken for mixing the paste.^[16] The temperature of the mixing slab and spatula were controlled (10°C to 50°C) during the manipulation of the ZnOE Paste.

Initial setting time or working time is one of the important properties to be considered during the initial handling of the ZnOE paste. During the initial setting, both physical and chemical changes occur simultaneously. The initial viscosity of the mixed paste governs its ease of handling. Ideally, the impression material must have enough manipulation time for mixing, filling the tray, and seating the impression in the mouth.^[12] The improper handling during the manipulation phase could compromise their properties on subsequent placement into the mouth.

The initial setting time measurements obtained in a simulated mixing device indicated that the greatest decrease in initial setting time occurred at 10°C and 50°C. ZnOE paste mixed at these two temperatures did not comply with the minimum specification requirement of 180 s.^[17] There was an inverse relationship between the mixing temperature and initial setting time that can be attributed to increased/decreased ionic mobility due to increased/decreased kinetic energy of the ions. This corroborates the findings reported in the earlier investigations.^{[9],[11],[15]} The observation of this study has shown that a reduction in temperature below 20°C led to a shorter setting time. This anomalous behavior of the paste can be attributed to the possible incorporation of condensed water on the glass slab at a lower temperature. The addition of water increases the polar and ionic character of the material. An increase in the polar character favors the hydration of the ZnO to form Zn (OH)⁺ as the reacting species at a faster rate, whereas the ionic character favors the salt formation between zinc ions and eugenolate ions at a faster rate. The reaction occurs in two steps, i.e., generation of ions, followed by recombination of ions as shown below: ^{[5],[19],[20]}

ZnO + H₂O ⇌ ZnOH⁺ + OH⁻

ZnOH⁺⇌ Zn²⁺ + OH⁻HE ⇌ H⁺ + E⁻

Recombination of ions

Zn²⁺ + 2E⁻⇌ ZnE₂ H⁺ + OH^-⇌ H₂O

ZnE₂ + H₂O ⇌ ZnE₂. H₂O

To validate the above findings, the entire simulated mixing device was transferred to a refrigerator where the temperature was maintained at 10°C. Refrigerators provide cooling with relatively less humidity than the environment by working on the principle of dry cooling. This resulted in little to no water condensation on the surface of the glass slab. The average initial setting time increased to around 215 (±15.4) s compared to the earlier reading of 131.5 (±21.9) s, as shown in [Figure 2]. This confirms that water condensation has a significant function to help the material set.

Consistency is the relative mobility of freshly mixed material to flow. It is related to the rheological changes occurring within the mixed paste during the mixing to setting time intervals. There is a gradual change in the consistency as the mixed material approaches a set condition. The consistency of the ZnOE paste is controlled by the rheology of the product, amount of pressure applied, the kinetics of the setting reaction, the nature of the environment i.e., temperature and humidity.^[21],[22] In this study, the diameter of the disc formed at various temperatures was in the range of 20–50 mm.^[17] However, a correlation was observed between consistency and setting time.^[9],[12] The ZnOE paste that showed less consistency or flow at 10°C, 40°C, 50°C temperatures also exhibited a shorter initial setting time. This could be attributed to the incorporation of water at 10°C and higher activation energy at 40°C and 50°C, leading to a rapid increase in viscosity, thus, limiting the spread of the material under load.^[23],[24] However, better consistency was observed at 40°C and 50°C compared to 10°C. Although the material showed a faster setting, an increase in the consistency can be attributed to the plasticizing component such as thermoplastic substances present in the material. The softening temperature of these substances is slightly higher than the oral temperature of 37°C, which facilitated the increased flow.^[11],[14]

Detail reproduction plays a vital role in the fabrication of any prosthesis. The factors such as viscosity, wettability, setting characteristics, and the presence of voids can influence the degree of surface detail obtained in the impression.^[25] In the present study, the casts obtained at 20°C and 30°C displayed good surface details, whereas poor detail reproduction was observed at 10°C and 50°C. It was observed that ZnOE paste appears to adhere to the stone cast at all the temperatures studied.^[9],[12] The ZnOE paste manipulated at 20°C and 30°C exhibited a gradual increase in the viscosity during the initial phase of the setting thus facilitating the flow of the material into the details. Detail reproduction deteriorated as the temperature decreased below 20°C and increased over 30°C due to accelerated setting, grainy nature of the mix, and incorporation of air voids.

The impression material must remain attached to the impression tray during placement and removal, hence the bond strength of the impression material to the tray becomes paramount.^[2],[16] Only three temperatures were selected as the ZnOE paste manipulated at 10°C and 50°C did not yield any promising result. The shear bond strength of ZnOE paste mixed at different temperatures with the acrylic resin did not show any significant differences. The ZnOE paste mixed at ambient temperature exhibited the highest shear bond strength. The low shear bond strength of the paste mixed at 20°C could be due to the slow rate of reaction, whereas at 40°C, it could be due to a faster rate of reaction that prevented the paste from adhering properly to the tray.

Conclusion

Conclusions drawn from the findings of the present in vitro study are listed below:

The properties of ZnOE paste were significantly affected by the temperature of the mixing slab
When it is not possible to achieve the ideal room or ambient temperature of 25°−30°C, it is advisable to follow the simulated custom-made mixing device to achieve consistent results
Extreme mixing temperatures of 10°C and 50°C did not yield clinically promising results and are not advised to be used.

Further clinical and laboratory experiments incorporating humidity are needed to illustrate the efficacy of this custom-made device.

Clinical Implications

For consistent and optimum clinical performance, it is advisable to manipulate ZnOE paste at ambient temperature.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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