Experimental hydrophilic vinyl polysiloxane (VPS) impression materials incorporating a novel surfactant compared with commercial VPS

Abstract

Objectives

To formulate experimental hydrophilic (Exp) VPS impression materials incorporating a novel surfactant (Rhodasurf CET-2), and to compare their contact angles (CAs) with commercial materials, before/after disinfection.

Methods

CAs were measured immediately after setting and after disinfection (1% NaOCl; 30 min and 24 h), together with their change whilst a droplet remained on the materials surface (over 10, 20, 30 60 and 120 s), on three commercial (Aquasil Ultra-Monophase [Aq M], Elite HD-Monophase [Elt M], Extrude Medium-bodied [Extr M]) and four experimental (Exp I–IV) materials, using the Drop Shape Analysis 100 technique. The results were compared statistically.

Results

CAs of all experimental materials were within the range of those obtained for the commercial materials, with the exception of Exp-IV, which presented with the lowest CAs at the three time points. The control Exp-I was hydrophobic at all three time points (CAs ∼100+), as was Elite. Immediately after setting, Aq M had low CAs but these increased significantly after 30 min of disinfection. After twenty four hours’ disinfection CAs of all Exp/commercial VPS increased significantly compared to immediately after setting. The CAs of droplets left on the material (120 s) decreased with time, even after disinfection, except for Exp-I.

Significance

The novel surfactant Rhodasurf CET-2 in Exp-III and IV, is an effective surfactant, retaining a low CA after disinfection, compared with Igepal CO-530 in Aq M. Disinfecting VPS impression materials for more than 30 min increases their surface CAs, and therefore prolonged disinfection periods should be avoided.

Introduction

The hydrophobicity/hydrophilicity of elastomeric impression materials is determined by the chemical structure and nature of these materials . To record fine details of the oral hydrated tissues with an impression material, and to transfer these details to die/cast materials by pouring with gypsum slurries, depend on the hydrophilicity and viscosity of the impression material . Vinyl polysiloxane (VPS) impression materials are inherently hydrophobic which makes them difficult to flow around the soft and hard tissues of the mouth, and they are not wetted by gypsum slurries. To overcome the problem of hydrophobicity, some manufacturers have incorporated non-ionic surfactants within them and have classed them as hydrophilic VPS (e.g. Aquasil) .

To prevent cross-contamination, impressions should be properly disinfected after removing from the mouth, since they are always contaminated with saliva, frequently with blood and bacterial plaque. Thus they serve as a potential source of infectious microorganisms to the dental health-care personnel (DHCP), who handle the impressions . The casts made from untreated impressions may also cause a spread of microorganisms to the DHCP . However, disinfecting solutions may adversely affect the dimensional stability of impression materials, particularly if they are hydrophilic. As an example, due to their hydrophilic nature, alginates, agar and polyethers are reported to be dimensionally unstable in disinfecting solutions . Conflicting results have been reported by various researchers who have investigated the wettability and dimensional stability of the so-called hydrophilic VPS, after disinfection.

The hydrophilicity (wettability) of impression materials can be examined by measuring the contact angle (CA) formed between the surface of the material and the curved surface of a drop of liquid on its surface . CAs can be measured by many methods for example, the sessile drop method (Drop Shape Analysis – DSA 100, Kruss GmbH, Hamburg, Germany) and the Wilhelmy method. There is not a single, accepted standard method to measure the CA of impression materials . Some researchers have used the Wilhelmy technique, for example, Lepe et al. who measured the CA of an aqueous solution of calcium sulphate on a solid sample placed in a fixed position, and following the liquid advancing on it (advancing CA), and then retreating the liquid to give the receding CA. However, most researchers have used the sessile drop method . This method measures the CA by capturing the profile of a liquid placed on a solid substrate surrounded by a gas, using high resolution cameras and software. According to the sessile drop method the following can be measured:

  • 1.

    Static CA

  • 2.

    Dynamic, advancing and receding CAs

For VPS impression materials, static CA measurements are preferred over dynamic due to the fact that the materials are rubbers after setting. However, the surfaces of the specimens should be clean, smooth and horizontal, since this method is very sensitive to contaminated and uneven surfaces . When water is the wetting liquid, materials with CAs higher than 90° are considered hydrophobic and indicate poor wetting, whereas materials with a CA lower than 90° are considered as hydrophilic. Materials with complete spreading of the liquid on their surface indicate CAs of 0° and possess perfect wetting properties .

Different brands of commercially available impression materials have different compositions and consequently these materials have different properties, such as wettability, viscosity and compatibility with gypsum slurries. Despite the inconsistencies in the properties of these materials, most of the researchers have investigated commercial products . Oh et al. however, developed their own compositions of hydrophobic, as well as hydrophilic, VPS impression materials containing a surfactant (nonylphenoxy poly[ethyleneoxy] ethanol; Fig. 1 ). They placed a sessile drop of deionized water (DW) on the surface of the material and after 2 min the CA was measured, using a computer aided Kruss G 10-System program (KRUSS Company, Hamburg, Germany). They found that the surfactant reduced the CA of their VPS impression materials compared to the control. Lee et al. also developed their own compositions of VPS impression materials following a modified version of Oh et al’s protocol, where they varied the concentration of surfactant within the formulations (0.5%, 1.5% and 2.5%). Their CA results were similar to Oh et al’s results and, they further explained, that there was a strong negative correlation between the concentration of the surfactant and CA.

Fig. 1
Structural formula of nonylphenoxy poly(ethyleneoxy) ethanol (non-ionic surfactant) showing hydrophilic (ethyleneoxy) and lipophilc (nonylphenoxy) sites.

Both, Oh et al. and Lee et al. studied CAs prior to disinfection of their formulations; however, it should be noted that disinfection may adversely affect the wettability of impression materials . It is believed that this occurs due to migration of the hydrophilic surfactant from the hydrophobic impression material and into the disinfection solution. Therefore, there appears to be a need to compare CA’s of both commercial and experimental, hydrophilic VPS impression materials, before and after disinfection, in order to identify the effect of the incorporated surfactant. Hence, the research presented in this paper investigated whether experimental VPS impression materials incorporating a novel non-ionic surfactant, Rhodasurf CET-2 (ethoxylated cetyl-oleyl alcohol), recommended by Sigma-Aldrich, could retain reduced contact angles after disinfection of the materials.

The corresponding aims of this study were to compare between three hydrophilic commercial VPS impression materials and four experimental materials containing a novel surfactant, Rhodasurf CET-2, in terms of the following characteristics:

  • 1)

    The change in contact angles (surface wettability) following disinfection in 1% sodium hypochlorite solution (NaOCl) after 30 min and 24 h.

Therefore, the null hypotheses can be summarised as the mean contact angle change for all materials was the same following twenty four hours’ disinfection, and the contact angle of a droplet placed on the surface of all materials was unchanged following a 120 s dwell time.

Materials and methods

Three commercial VPS impression materials were included in this study:

  • (i)

    Aquasil Ultra Monophase (Medium-Bodied), (Aq M) from Dentsply, USA

  • (ii)

    Elite HD Monophase (Medium-Bodied), (Elt M) from Zhermack, Italy

  • (iii)

    Extrude (Medium-Bodied), (Extr M) from Kerr, USA.

These were classed as hydrophilic according to the literature provided by their manufacturers, and were supplied as auto-mixed cartridge delivery systems.

Four experimental, hydrophilic VPS impression materials, of known compositions, were formulated ab initio so that the effect of the surfactant could be assessed on their CAs. The constituents used for preparing these (Exp-I, II, III and IV; Table 1 ) VPS impression materials were: vinyl-terminated poly(dimethylsiloxane) (pre-polymer; molecular weight-Mw 62700; Fluorochem, UK), Aerosil R812S (filler – from Lawrence Industries, UK), Rhodasurf CET-2 (ethoxylated cetyl-oleyl alcohol non-ionic surfactant, from Rhodia, UK), and the following were purchased from Sigma Aldrich, UK, poly(methylhydrosiloxane) (Mw 2270; conventional cross-linking agent), tetra-functional (dimethylsilyl) orthosilicate (TFDMSOS; Mw 328.73; novel cross-linking agent), platinum catalyst (0.05 M), palladium (˂1 μm; scavenger). The detailed compositions of the four experimental formulations (Exp-I–IV) are given in Table 1 .

Table 1
Formulations of novel VPS impression materials (Exp-I control) and Exp II–IV with 2%, 2.5% and 3% Rhodasurf CET-2 (surfactant) respectively.
Components Base paste (weight %) Catalyst paste (weight %)
Exp-I Exp-II Exp-III Exp-IV Exp-I Exp-II–IV
Vinyl-terminated poly(dimethylsiloxane), Mw 62700 39.90 37.95 37.46 36.98 40.72 39.51
Poly(methylhydrosiloxane), ∼Mw 2270 0.77 0.74 0.73 0.72
TFDMSOS, Mw 328.73 0.33 0.32 0.31 0.31
Platinum catalyst (0.05 M) 0.06 1.27
Rhodasurf CET-2 (ethoxylated cetyl-oleyl alcohol) (surfactant) 2.00 2.50 3.00
Palladium (<1 μm) 0.23 0.22
Aerosil R 812 S (filler) 9.00 9.00 9.00 9.00 9.00 9.00
Total 50% 50% 50% 50% 50% 50%

Exp-I was used as a control (with no surfactant) for Exp-II–IV, where the main difference was the incorporation of a novel non-ionic surfactant (Rhodasurf CET-2, in increasing amounts) to form hydrophilic formulations (Exp-II–IV). The catalyst paste was kept the same for all the hydrophilic formulations.

1% sodium hypochlorite (NaOCl) was used as a disinfecting solution. The disinfecting solution was supplied as 14% NaOCl, by Fisher Scientific UK Ltd, which was diluted to 1% NaOCl by mixing 100 ml of 14% NaOCl with 1300 ml of deionised water (DW).

Sample preparation for testing of CAs

Samples were prepared in rectangular stainless steel metal molds measuring 40 × 10 × 1 mm 3 , in a temperature controlled environment (23 °C ± 1 °C). An acetate sheet was placed on top of a metal plate, on to which the stainless-steel mold was positioned. The base and catalyst pastes (pre-packed in a double barrel cartridge), were mixed using an auto-mixing syringe and extruded directly into the mold cavity. Another acetate sheet was placed on top followed by another metal plate. Then the whole assembly was placed under a hand-operated hydraulic press (MESTRA MOD-030350, Talleres Mestraitua, S.L) and the pressure was slowly increased to 100 bars in order to distribute the material evenly in the mold cavity, flush out excess material and expel air bubbles . The materials were allowed to set at 23 °C (±1 °C). (i) for the time specified by manufactures for commercial materials and (ii) for 4–11 min depending on the overall composition of the experimental materials.

CA measurement using the Drop Shape Analysis (DSA)100 device

The Drop Shape Analysis (DSA)100 technique was used to measure the static (and dynamic) CA of commercial and experimental VPS impression materials.

The DSA 100 equipment was calibrated using the Young-Laplace method. As soon as a sample had set (n = 10 per material) it was carefully placed on the sample table and the video recorder with a charge-coupled device (CCD) camera was started. A droplet (∼1.5 μL) of DW was placed carefully onto the surface of the sample using a Gastight #1001 syringe (Hamilton Bonaduz AG, Switzerland – accuracy to 0.01 ml). The dynamic CAs (spreading of the droplets) were measured by capturing the profile of DW on the material, at 10, 30, 60 and 120 s (i.e. over two minutes) , using a mounted camera and processing using the software (DSA1).

CAs were measured 10 s after placing the drop on each sample i) immediately after setting ; ii) after 30 min in disinfecting solution ; iii) after 24 h disinfection , in order to compare the effect of the hydrophilic agent in the materials.

Statistical analysis

Statistical analysis was performed using SPSS Statistics for Windows (Version 22.0. Armonk, NY: IBM Corp). The null hypothesis that the mean contact angle change for all materials was the same following twenty four hours’ disinfection was tested using one-way Analysis of Variance (ANOVA). Tukey’s Honest Significant Difference (HSD) test was used for post-hoc analysis to perform multi-way comparisons of the change in mean contact angle amongst all materials.

Furthermore, one-way ANOVA was also used to test the null hypothesis that the contact angle of a droplet placed on the surface of all materials was unchanged following a 120 s dwell time. Tukey’s HSD test was used for post-hoc analysis of inter-material mean differences. A Bonferroni factor of two was used to account for multiple hypotheses and thereby the significance level for this study was set at α = 0.025.

Materials and methods

Three commercial VPS impression materials were included in this study:

  • (i)

    Aquasil Ultra Monophase (Medium-Bodied), (Aq M) from Dentsply, USA

  • (ii)

    Elite HD Monophase (Medium-Bodied), (Elt M) from Zhermack, Italy

  • (iii)

    Extrude (Medium-Bodied), (Extr M) from Kerr, USA.

These were classed as hydrophilic according to the literature provided by their manufacturers, and were supplied as auto-mixed cartridge delivery systems.

Four experimental, hydrophilic VPS impression materials, of known compositions, were formulated ab initio so that the effect of the surfactant could be assessed on their CAs. The constituents used for preparing these (Exp-I, II, III and IV; Table 1 ) VPS impression materials were: vinyl-terminated poly(dimethylsiloxane) (pre-polymer; molecular weight-Mw 62700; Fluorochem, UK), Aerosil R812S (filler – from Lawrence Industries, UK), Rhodasurf CET-2 (ethoxylated cetyl-oleyl alcohol non-ionic surfactant, from Rhodia, UK), and the following were purchased from Sigma Aldrich, UK, poly(methylhydrosiloxane) (Mw 2270; conventional cross-linking agent), tetra-functional (dimethylsilyl) orthosilicate (TFDMSOS; Mw 328.73; novel cross-linking agent), platinum catalyst (0.05 M), palladium (˂1 μm; scavenger). The detailed compositions of the four experimental formulations (Exp-I–IV) are given in Table 1 .

Table 1
Formulations of novel VPS impression materials (Exp-I control) and Exp II–IV with 2%, 2.5% and 3% Rhodasurf CET-2 (surfactant) respectively.
Components Base paste (weight %) Catalyst paste (weight %)
Exp-I Exp-II Exp-III Exp-IV Exp-I Exp-II–IV
Vinyl-terminated poly(dimethylsiloxane), Mw 62700 39.90 37.95 37.46 36.98 40.72 39.51
Poly(methylhydrosiloxane), ∼Mw 2270 0.77 0.74 0.73 0.72
TFDMSOS, Mw 328.73 0.33 0.32 0.31 0.31
Platinum catalyst (0.05 M) 0.06 1.27
Rhodasurf CET-2 (ethoxylated cetyl-oleyl alcohol) (surfactant) 2.00 2.50 3.00
Palladium (<1 μm) 0.23 0.22
Aerosil R 812 S (filler) 9.00 9.00 9.00 9.00 9.00 9.00
Total 50% 50% 50% 50% 50% 50%
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Nov 22, 2017 | Posted by in Dental Materials | Comments Off on Experimental hydrophilic vinyl polysiloxane (VPS) impression materials incorporating a novel surfactant compared with commercial VPS
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