Study on the reactivity and kinetics of primary and secondary amines during epoxy curing by NIR spectroscopy combined with multivariate analysis

https://doi.org/10.1016/j.vibspec.2019.102993Get rights and content

Highlights

  • Reactivity of primary and secondary amines in epoxy curing process was determined by NIR spectroscopy combined with multivariate analysis.

  • The wavelet-based second derivative and angle vector transform were combined to effectively eliminate the effect on MCR by scattering light.

  • Reactivity of primary is different from that of secondary amines during epoxy curing process.

  • There is a competitive reaction with epoxy group between PA and SA, their competitiveness can be quantitatively expressed by k2(SA)/k2(PA).

Abstract

The temperature of epoxy curing reaction and the change of amino reactivity for the amine curing agent have an effect on the three-dimensional network structure of the product, which in turn affect the performance of the epoxy resin. The study on the reactivity and kinetics of primary and secondary amines in epoxy curing process is of great significance to deeply understand the curing mechanism and realize the control of product quality. In this paper, an effective method for studying the reactivity and kinetics of primary and secondary amines in epoxy curing process by near-infrared (NIR) spectroscopy combined with multivariate analysis was proposed. The curing system of bisphenol A epoxy resin (E51) and M-xylylenediamine (MXDA) was studied, and the resins were isothermally prepared at 50, 60, 70, 80, 90 °C, respectively. The NIR spectra of the system for each curing reaction process were continuously collected online. The characteristic absorption bands of the primary and secondary amines overlap heavily in the wavenumber range of 6700–6330 cm−1. The wavelet-based second derivative together with multivariate curve resolution(MCR) were employed to resolve their respective absorption peaks. Then, the curves of conversion and conversion rate for primary and secondary amines were obtained. The study shows that the kinetics of the curing reaction follows the self-catalytic Kamal model in the early stage, and it follows the revised model of incorporating the diffusion factor after vitrification, in which the hydroxyl group plays a self-catalytic role in the reaction process; the reactivity of primary and secondary amines is different during epoxy curing process. There is a competitive reaction between them, and its competitiveness can be expressed by k2(SA)/k2(PA); the reactivity of primary and secondary amines varies with curing temperature, and k2(SA)/k2(PA) increases significantly when curing temperature exceeds 60 °C, which has a significant influence on the formed resin network structure.

Introduction

Epoxy resin is a thermosetting resin with good mechanical properties, thermal properties and solvent resistance. It has been widely used in coatings, adhesives, electronic insulating materials, composite materials and other fields [[1], [2], [3]]. Its performance is dependent on the three-dimensional network structure formed by the cross-linking of the epoxy monomer with the curing agent [4,5]. In the resin preparation process, an amine curing agent is added to the epoxy monomer, and the curing cross-linking reaction is carried out at a certain temperature. In the initial stage of the reaction, the secondary amines are formed when the primary amines of the curing agent react with the epoxy groups. Therefore, primary amines and secondary amines coexist during the reaction. Obviously, the primary and the secondary amines react with the epoxy groups to form a linear structure and a branched cross-linked structure, respectively. In the initial cross-linking stage, the reactivity of primary and secondary amines has an effect on the formation ratio of linear and branched molecular segments, which in turn affect the formed molecular network structure.

Mijovic et al. [6] used High Performance Liquid Chromatography and argentimetry to determine the concentration of raw materials, intermediates and products in the epoxy-amine reaction process, and a curing reaction mechanism was proposed. The kinetic parameters (reaction rate constant kSA/kPA) were calculated according to the concentration of each component in the reaction process, and the reaction kinetics model was studied. However, using HPLC and argentimetry as analytical methods not only takes a long time to determine but also HPLC cannot be used to determine the concentration of highly cross-linked products and the components in the whole reaction process, so that a precise mechanism model cannot be obtained. Therefore, it is of great theoretical and practical significance to study the effective determination methods of reactivity and reaction kinetics of primary and secondary amines in epoxy curing process for understanding the curing reaction mechanism and controlling the product quality.

NIR spectroscopy can reflect the composition and structure information of a substance at the molecular level, and can also realize online and non-destructive measurement, which is suitable for the study of the polymerization process. Dannenberg [7] first began to analyze the functional groups in the epoxy curing process by NIR spectroscopy and established a standard curve of absorbance vs group content by multiple measurements. The concentration changes of various groups during the curing process were studied. However, this method is complicated in preparation and cannot directly obtain the content of each group. In addition, Schiering et al. [8] compared MIR with NIR in quantitative analysis of composition changes in mixed epoxy system. The bands of OH and secondary amine overlap in the MIR spectrum, however, they are separated in the NIR spectrum. Therefore, NIR is more suitable for studying the content of primary and secondary amines as a function of curing time and temperature. Xu et al. [9] obtained the spectral data of OH and secondary amine by NIR, and fitted the mechanism model of non-catalytic and catalytic reaction path after quantitative analysis. Recently, Sahagun et al. [10] used real-time NIR spectroscopy to study the effect of curing temperature on the molecular structure of epoxy resin products. The change of concentration for primary amines was obtained by non-convolution calculation at the absorption band in the range of 5100−5040 cm−1, and the change of concentration for secondary amines was calculated by convolution calculation at absorption band of 6720−6520 cm−1. The results were used to determine the ratios of linear structure and branched structure of polymer at gel point and vitrification point, and the law of temperature having an effect on the network structure of epoxy-amine curing system was studied. However, due to many influencing factors, such as the NIR characteristic bands of primary and secondary amines are severely overlapped; there is a phase transition (gel point begins to transform) in the curing reaction to produce a significant light scattering effect, resulting in severe spectral drift; the composition of the reaction process is complicated, which interferes with the determination of the number of components in deconvolution calculation, etc., so that the concentration of the amines calculated by deconvolution would have a large deviation.

The purpose of this paper is to extract the spectral characteristic information of primary and secondary amines during curing reaction by collecting the on-line NIR spectra. Multivariate analysis methods were adopted to calculate the curves of concentration as a function of time for primary and secondary amines, respectively. Furthermore, the reactivity and kinetics of the amines were studied in detail to provide a theoretical and technical basis for the in-depth understanding of the reaction process mechanism and the precise control of the reaction process.

Section snippets

Materials

The bisphenol A type epoxy resin (E51) used with molecular mass of 381.75 in the study was purchased from Baling Petrochemical Co. Ltd. (China), and the curing agent M-xylylenediamine (MXDA) from Shanghai Macklin Biochemical Co. Ltd. (China). The chemical structures of the E51 is shown in Fig. S1.

Sample preparation

14.78 g of E51 was weighed by electronic balance and put into standard laboratory beaker. It was placed into a vacuum oven for 48 h to remove the inside air bubbles. Then 2.52 g of MXDA was added to

Theory and algorithm

Since primary and secondary amines (hydrogen-containing groups) have strong characteristic bands in the NIR spectral range, this paper proposes to continuously collect the NIR spectral data during the entire epoxy curing process. According to Beer–Lambert law [11], the process spectral data is used to obtain the curves of concentration as a function of time for primary and secondary amines. And the curves of conversion and conversion rate are calculated according to their concentration curves.

Results and discussion

In this paper, we attempt to calculate the change curves of concentration for various types of amines according to their changes of spectral intensity during curing reaction, and then their reactivity was calculated. The curing reaction of epoxy is usually carried out according to the reaction Eqs. (4) and (5) [17,18],

Obviously, the concentration of the mixture of reactant and product varies with the curing reaction, leading to the spectral intensity varies with the curing reaction. According

Conclusion

In this paper, an effective method for determining the reactivity of primary and secondary amines in the epoxy curing process is proposed using the continuously collected NIR spectra of the whole reaction process, the wavelet-based second derivative, angular vector spectral conversion, and MCR-ALS. Furthermore, the concentration as a function of time for primary and secondary amines in the epoxy curing process is analyzed, and the reaction kinetics models are established to determine the

Data availability

The processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors acknowledge the support from the National Key Research and Development Program of China [No. 2016YFF0102504].

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