The determination of the degree of cure in epoxy paints by infrared spectroscopy

doi:10.1016/j.polymertesting.2005.02.013

Polymer Testing

Volume 24, Issue 5, August 2005, Pages 572-575

https://doi.org/10.1016/j.polymertesting.2005.02.013 Get rights and content

Abstract

The goal of this paper is to present a method to determine the time evolution of epoxy conversion for epoxy/amine and epoxy/amide formulations in heterogeneous systems such as paints by means of near infrared (NIR) spectroscopy. The cure reaction involves a simple addition mechanism between the oxirane ring and the amine/amide hydrogen functional groups. The extent of the reaction was calculated from the NIR absorption band at around 4530 cm⁻¹ which depends on the oxirane rings concentration. The NIR spectroscopic method, which has the advantage of real time analysis over other techniques such as chemical titration, can be employed with most of the usual, modern FTIR spectrometers.

Introduction

Since the first synthesis of this kind of polymer in 1947 [1], epoxy systems have steadily gained importance to become one of the most prominent families of polymeric materials. Epoxy systems have been used in adhesives, matrixes for composites and protective coatings.

The most commonly used epoxy resin is based on diglycidil ether of biphenol A (DGEBA), shown in Fig. 1. The oxirane rings at the end of the chains act as reactive sites for the cross-linking reaction. The most common curing agents are amines, which usually have more than three reactive site per molecule, so that a three-dimensional net can be formed when they react with the epoxy resin. Although the reaction between the oxirane rings and the amine groups can take place at room temperature, a proper curing agent must be chosen to ensure a complete reaction. The kinetics of the cure reaction is greatly ruled by the curing agent, which additionally determines the final performance of epoxy systems.

The time required for the complete cure of an epoxy system is of great importance in industrial applications. An eventual anomalous performance of the coating could be the consequence of incomplete cure. Traditionally, the degree of cure in epoxy resins and paints has been determined by chemical titration [2]. However, this is a time-consuming method and involves the use of toxic and pollutant chemicals.

As an alternative, infrared spectroscopy in the middle range (4000–400 cm⁻¹) has been suggested to determine the degree of cure [3]. The bands appearing in the middle IR are due to fundamental vibration modes of different organic groups. In particular, the oxirane ring presents a band at 915 cm⁻¹ the intensity of which should diminish as the cure reaction proceeds. However, this band overlaps with other bands due to different organic groups present in the resin [4]. The overlap of bands cannot be easily solved and the determination of epoxy conversion by means of FTIRS has never been widely accepted by the coatings industry.

In recent years, the near infrared (NIR, wave numbers higher than 4000 cm⁻¹) spectroscopy has been successfully employed to determine the degree of cure of epoxy resins in homogeneous systems [5]. In this infrared range, combination bands and overtones of C–H, O–H and N–H can be observed.

However, this method cannot be successfully applied in heterogeneous systems, such as paints, due to radiation scattering by pigments and other phases which prevent the acquisition of useful transmission spectra.

In the present paper, a method for the determination of the degree of cure of epoxy paints is presented by means of diffuse reflectance near infrared spectroscopy (DRFT-NIR) and the results are critically compared to those obtained by chemical titration.

Section snippets

Materials

All chemicals used in this work are commercially available and were used as received. The DGEBA based epoxy paints used were Kelcot E-401 and Kelcot E-402, which were cured with polyamines and polyamides, respectively. The cure reaction was carried out at room temperature and humidity (23 °C, 65% R.H.). The main physical properties of the paints are shown in Table 1.

Infrared spectroscopy

A Nicolet Magna 550 Series II spectrometer was employed, equipped with an Ever-Glo lamp, a KBr beam splitter and an MCT detector.

Epoxy/amine resins

The DRFT-NIR spectra obtained for the epoxy/amine Kelcot E-401 paint are shown in Fig. 2. When the two components of the paint are mixed, a heterogeneous, multiphase system is obtained and its physical properties change as the cure reaction proceeds. The baseline presents a non-zero slope due to radiation scattering caused by inorganic components which are included in paints, but not in pure epoxy/amine formulations such as inorganic pigments. However, it has been previously reported that

Conclusions

In the present work, we present a method for the spectrometric determination of the degree of cure in heterogeneous epoxy/amine and epoxy/amide paints, employing the NIR bands areas ratio of oxirane rings to phenyl groups.

The radiation scattering effects in heterogeneous systems, caused by pigments and other components in paints, can be circumvented with the use of the diffuse reflectance technique in a way that curing curves can be obtained with a quality comparable to those obtained using the

Acknowledgements

The authors thank Kelcot for gently providing the chemicals used in this work.

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Analysis MethodThe determination of the degree of cure in epoxy paints by infrared spectroscopy