Elsevier

Vibrational Spectroscopy

Volume 40, Issue 1, 16 January 2006, Pages 133-141
Vibrational Spectroscopy

The use of near-infrared spectroscopy for the cure monitoring of an ethyl cyanoacrylate adhesive

https://doi.org/10.1016/j.vibspec.2005.07.009Get rights and content

Abstract

Near-IR reflectance spectroscopy has been used to study the curing of ethyl cyanoacrylate adhesive on polished dental glass and microscope slide substrates. The effects of changing the glue film thickness and the type of substrate on the curing rate have been investigated whilst maintaining a constant humidity. The FTIR spectral data has been used to calculate and plot the extents of cure versus time for various film thicknesses.

Introduction

Cyanoacrylates are one of many types of synthetic adhesives. They show exceptional adhesion to a wide range of materials such as metals, plastics, rubber, ceramics, woods, and fabrics. Besides their versatility, cyanoacrylates have several other assets. As they are one-part adhesives, no mixing or metering is required and only occasionally are substrate surface primers necessary. They contain no solvents and so, not only is there no need for solvent evaporation, they are also potentially 100% reactive. As only minimal amounts of the adhesive are required for optimal bond strengths to be achieved, cyanoacrylates are an economical adhesive [1]. Due to these unique properties, cyanoacrylates are the focus of this work.

Cyanoacrylates have been developed over the years to suit a continually increasing range of substrates and applications. Cyanoacrylates are ideal as industrial product assembly adhesives due to their ability to rapidly form bonds with many types of materials [1]. They have been used widely in the automotive, electronics, household appliance, home repair, furniture and hobby industries. Another more unusual application is in the detection of latent fingerprints in crime investigations [2], [3], [4]. Cyanoacrylates are also bacteriostatic [5] and have therefore found applications in medicine and dentistry. Some examples of such applications are plastic surgery [6], over-the-counter mouth ulcer dressings [7], ophthalmic surgery [8], nailbed repair [9], and post-extraction dressings in dentistry [5].

Recently, cyanoacrylates incorporated into glass-ionomer cement (GIC) formulations have been described [10]. These types of composites could be used for various general adhesive applications, but in particular, show potential as dental restorative cements. These types of cement would represent a new alternative to the already available resin-modified glass-ionomer cements (RMGICs: glass-ionomers modified by the addition of a resin component typically hydroxyethyl methacrylate) but with several advantages. For example, they would show enhanced adhesive properties due to the adhesive nature of the cyanoacrylate in comparison to the (non-adhesive) methacrylate resin. Due to the basic nature of the glass and the nature of the polymerisation reaction, these cements should also not suffer from incomplete polymerisation; a common problem for light-activated RMGICs [11], [12]. The need for incremental placement by the dentist would thus be avoided. In comparison to GICs, these cyanoacrylate-modified GICs would develop and strengthen more rapidly and again show improved adhesive properties. This study forms part of an ongoing, extensive program of investigation and development of these cement formulations. This work uses infrared spectroscopy to investigate the reaction between a cyanoacrylate adhesive and planar dental (GIC) glass, as this will be useful for understanding the more complex bonding that will be occurring between cyanoacrylates and powdered dental (GIC) glass. As will be described again later on, the curing of the cyanoacrylate will also be performed on a microscope glass slide substrate to compare the results for a different glass composition.

Cyanoacrylate esters are known to polymerise by both free radical and anionic mechanisms [13]. The latter mechanism has attracted more attention in the field of adhesives due to the ease of initiation and the rapid rates of polymerisation that can occur. Anionic polymerisation can be initiated by mild nucleophiles such as water or alcohols because of the electron withdrawing groups single bondCOOR and single bondCN groups on the α-carbon atom of the cyanoacrylate ester molecule. These groups not only reduce the electron density on the β-carbon thereby rendering this position susceptible to nucleophilic attack, they also significantly stabilise the anion formed at the α-carbon after such attack, by delocalising the negative charge. In most cases, ambient humidity in the air and moisture on the bonding surface are sufficient to initiate polymerisation within just a few seconds [1].

The mechanism for the anionic polymerisation of cyanoacrylates has been compiled from various literature sources [1], [4], [14], [15], [16], [17], [18] and is shown in Fig. 1. Despite the display of the chain transfer and termination steps in Fig. 1, these steps of the mechanism are much less clear [1] and are often not addressed in the literature [14], [15], [16]. However, it is believed a water molecule can react with a “living” polycyanoacrylate chain anion, thereby producing an inert polymer chain and a hydroxyl ion which can initiate further polymerisation of any remaining monomer molecules [1], [17]. It is thought that the inert or “dead” polymer chain may also act as chain transfer agent (not shown in Fig. 1) [19]. Termination occurs when the chain anion reacts with a species such as an acid [1], [4], [18].

FT Raman [4], [20], mid-infrared (mid-IR) [15], [21], [22], [23], [24], electron tunnelling [22] and nuclear magnetic resonance spectroscopy [15] have been used in the past to study the curing process of cyanoacrylates. The quantification of the extent of monomer conversion to polymers with time has been performed previously [4], [20], [23]. For example, Raman spectroscopic studies [20] have been used to quantify the extent of cure of a (unspecified) cyanoacrylate between an aluminium surface and a glass slide with time. The calculations were based on the changing intensity of the peak of the Csingle bondOsingle bondC bond at 840 cm−1 possibly corresponding to an oxirane ring present as an additive in the adhesive. However, as the role of the additive in the curing mechanism is not exactly known, the change in additive concentration may not be proportional to the change in monomer concentration. A more accurate assessment of monomer conversion may be obtained from the intensities of the peaks corresponding to the Cdouble bondC or neighbouring bonds (e.g. the Csingle bondH bond) because the Cdouble bondC functionality is disappearing during the polymerisation process.

Other Raman studies have investigated the polymerisation of an ethyl cyanoacrylate sealed with moisture in a glass tube (diameter 4 mm) [4]. It was estimated, based on the disappearance of the Cdouble bondC bond, that the polymerisation had proceeded to 85% completion after 92 days. The amount of added water in the system, which may affect overall conversion, was not indicated. The limited amount of water in the system possibly prevented complete monomer conversion even after 92 days.

Other mid-FTIR spectroscopic studies [23] of a curing (unspecified) cyanoacrylate on a borosilicate glass disc quantified monomer conversion with time based on the changing peak area corresponding to the Csingle bondH (in H2Cdouble bondC) bond, and determined the adhesive to be 100% cured in 4–5 min. The high extent of cure (100%) in comparison to that (85%) obtained from the Raman studies [4] described previously may be due to the fact that the cyanoacrylate was spread out on the substrate using a swab, in comparison to being contained sealed in glass tube [4]. Consequently, the cyanoacrylate samples in these experiments had different humidity conditions (or water access), which may explain the difference in results. In addition, the cyanoacrylate studied may have been a different type to that used in the Raman studies [4]. The rapid setting time (4–5 min) was possibly due to the basicity of the glass surface [25] and again the fact that the cyanoacrylate sample may have been spread as a thin film.

Studying the effects of film thickness on the curing of cyanoacrylates whilst maintaining a constant humidity, may provide a better understanding of the overall mechanism and in particular the role of water in the curing process. Ambient humidity in the air and surface absorbed water is usually sufficient to neutralise any acid stabiliser and then to initiate the curing reaction (anionic radical polymerisation). It has been claimed that in order to achieve a fast cure and to obtain a strong bond, a very thin film is required [1]. A thick film of cyanoacrylate between the adherends is known to produce a weak bond because the surface-initiated cure may not extend throughout the entire film thickness. The film thickness is therefore very important, as it determines the bond strength. The effect of changing the film thickness of a curing ethyl cyanoacrylate on an aluminium surface has been studied previously using spectroscopy [21], [22] but these studies focused on a cyanoacrylate/oxidised aluminium interface rather than the curing of the entire depth of the cyanoacrylate film. Spectral changes resulting from varying the film thickness were used to derive the vibrational spectrum of the adhesive molecule in the first monolayer and it was concluded that there might be hydrogen-bonding occurring between the surfaces. To date there appear to be no studies quantifying the effect of the cyanoacrylate film thickness on the extent of cure of the cyanoacrylate. This study therefore aimed to find a quantitative correlation between cyanoacrylate film thickness and extent of cure with time.

The nature of the substrate the cyanoacrylate is bonding to may significantly affect the cyanoacrylate curing process. As clear from the above, only a limited number of different cyanoacrylate curing substrates have been investigated and these were isolated studies, which cannot be reliably compared due to different experimental conditions. This study therefore aims to investigate the curing of ethyl cyanoacrylate on two types of substrate: a planar dental glass (KG 23) disc and a standard glass microscope slide. As mentioned previously, a knowledge of how cyanoacrylates cure on planar dental glass will be useful for understanding the more complex bonding that will be occurring between cyanoacrylates and powdered glass. In addition, cyanoacrylates are known to show poor durability when bonding to planar glass. This is believed to be due to the basic nature of the glass causing rapid cyanoacrylate curing which leads to high stress in the bond line, and therefore renders the cyanoacrylate at the bond line particularly susceptible to chemical or physical degradation [25]. The comparison of cyanoacrylates curing on these two different compositions of glass in this study may further clarify this theory.

The vast majority of the reported cyanoacrylate curing studies [15], [21], [22], [23] have been carried out in the mid-IR range, showing fundamental absorptions. To date there have been no infrared spectroscopic studies of curing cyanoacrylates performed using near-infrared (near-IR) reflectance spectroscopy. Near-IR spectroscopy has recently become a popular technique and it has various advantages over mid-IR spectroscopy [26], [27], [28]. The presence of the fundamental bands in mid-IR sometimes hampers the identification of the absorption bands of interest. In contrast, the near-IR region is dominated by overtones and combination bands, which can be isolated more easily. In addition, the lower intensity of the near-IR bands may be used as an advantage because it is often difficult to obtain on-scale mid-IR spectra [29], [30].

Near-IR spectroscopy has recently been used to monitor the curing of unfilled [28], [31], [32], [33], [34], [35] and filled [34], [36], [37], [38], [39] dental resins. The majority of the dental resins studied in this way have been standard methacrylate mixtures, Bis-GMA/TEGDMA [28], [31], [34], [36], [37], [38], [39]. The majority of previous studies obtained spectra using standard spectrometers with enclosed sample compartments. Only limited studies [34], [37], [38] to date have involved near-IR spectroscopy using fibre optics and these were using different arrangements to that used in this study. As opposed to the studies referenced above, which used transmission spectroscopy, this current cyanoacrylate study uses an optical fibre probe to obtain transflectance [40] measurements. The general set-up for transflectance measurements is such that the incident light passes through the sample of interest, reflects off for example an aluminium plate, and then travels back through the sample before reaching the detector. Recording NIR transmission spectra for liquids or gels would be awkward, especially for cyanoacrylates due to the need for filling and emptying the cuvettes. By using transflectance, the need for a mould or cuvette to contain the liquid can be avoided.

Near-IR spectroscopy has been the chosen technique for this study because of the above-mentioned advantages and due to the following other practical advantages: working in the mid-IR region requires long, tedious sample preparation in comparison with near-IR; and the near-IR optical fibre probe (connected to a spectrometer) is flexible and relatively small in size, thus making the near-IR apparatus readily portable, and more convenient to use than a standard mid-IR spectrometer. Its portability is particularly convenient for the humidity effect studies. To help identify and confirm peaks in the near-IR spectra, mid-IR studies were also undertaken.

Section snippets

Materials

The ethyl cyanoacrylate used in this investigation was commercially obtained and is known under the trade name of Loctite Super Glue Control Liquid (product code: 0158589) [41]. The material was used as received. A fresh sample was used each time for each test. Attempts to establish the detailed composition of the adhesive using 1H NMR spectroscopy and mass spectrometry have so far been unsuccessful due to the very complex spectra obtained in both cases. Besides the major peaks representing the

Mid-infrared spectra

For clarity, only the mid-IR spectra recorded every minute between 0 and 5 min and finally at 100 min are displayed in the graphs (Fig. 3, Fig. 4, Fig. 6). These particular spectra were chosen because they gave the clearest indication of how the peaks were changing during the curing process. The ambient temperature was 23 °C, the relative humidity was 57% and the cyanoacrylate film thickness was 0.325 mm.

Mid-IR peak assignments of the cyanoacrylate system are presented in Table 1.

The three

Conclusions

This study demonstrates the feasibility of monitoring the curing of cyanoacrylates using near-IR reflectance spectroscopy. Near-IR spectroscopy has proved to be a versatile, simple tool for monitoring the curing of cyanoacrylates. The flexibility of the optical fibre probe makes this technique particularly convenient for use with a controlled humidity chamber. Both the type of substrate and thickness of the cyanoacrylate film have been found to have a strong effect on the cure curve profile.

Acknowledgements

The authors would like to thank Dr. Steve Ritchie and Mr. Gary Foster of Exeter Advanced Technologies for providing and assisting with the use of the humidity chamber, and Mr. Colin Lovell for the EDX microanalysis of the microscope slide glass.

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