The study of the curing mechanism, kinetic and mechanical performance of polyurethane/epoxy composites using aliphatic and aromatic amines as curing agents

https://doi.org/10.1016/j.tca.2020.178598Get rights and content

Highlights

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    Cure kinetics of thermosetting polyurethane/epoxy composites.

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    It was shown that curing process comprises three stages.

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    Slow curing with separated stage of composite formation leads to composite with more regular structure and improved mechanical performance.

Abstract

The cure kinetics of thermosetting polyurethane/epoxy composites based on oligomeric diglycidyl ether of bisphenol A and branched oligomeric diphenylmetane diisocyanate and polymeric aliphatic amine or mixture of low molecular weight aromatic amines as hardener was studied. It was shown that curing process comprises three stages: at first, a polyurethane network is formed, after that amine hardener react with epoxy oxirane rings with formation linear oligomers, and at the final stage of curing, the network is formed by reaction of secondary amines obtained on the second stage with epoxy oxirane rings. The isoconversional methods of Flynn–Wall–Ozawa and Kissinger-Akahira-Sunose were employed to investigate activation energy as a function of curing degree. The correlation between Ea and α determined by these methods indicated that there were autocatalytic effects in curing systems under study. Interestingly, the mixture of low molecular weight aromatic amines curing compositions exhibited the highest tensile strength and elongation among all materials studied. It means that slow curing with separated stage of composite formation leads to composite with more regular structure which improved their mechanical performance.

Introduction

Epoxy resins are urgent thermosets as well as matrices in composites [1]. They find applications in widespread fields such as the aviation and marine industry, in electronics, and as coatings, or as glues. The key factors in the aforementioned applications are good thermal, mechanical and dielectric properties, their resistance to corrosion and low shrinkage upon cure. However, the enhancement in toughness still remains a problem because they are identified as rigid and brittle materials. The addition of a second phase can be proposed as a solution to this problem by improving the fracture toughness of epoxies [2]. A perusal of literature revealed the use of different types of polymers as a second phase [3]. In the quest of developing toughening epoxy, we found polyurethane as an attractive option because of its excellent material properties viz. good thermostability, high mechanical strength, excellent adhesive strength, high corrosion, weather and chemical resistance, dimensional stability, good processability etc [4]. Successful use of linear polyurethane for curing of epoxy is also cited in the literature [[5], [6], [7]].

Understanding the cure kinetics of modified of thermosets is essential for process development and quality control. Thus, the authors decided to carry out a study on bulk cross-linking of a polyurethane based on isocyanate terminated pre-polymer and a different amine compounds by using an epoxy as the cross-linker, in order to compare the efficiency of low molecular amines and branched polyamines on kinetic and mechanical performance. The main objective of this study is to corroborate the relationship among the reaction parameters such as temperature, time and degree of conversion, as well as to establish the function which discriebes the rate of the reaction. Thus, the usage of epoxy and polyurethane is the main highlight of the present study. Differential scanning calorimetry (DSC) was chosen as the tool to demonstrate the efficiency of the crosslinker. Compounds with different nucleophility (aromatic and aliphatic) and different molecular weight and branched architecture (low molecular weight mono-functional and polymeric multi-functional) were chosen as amine hardeners. As the cross-linking kinetics of polyurethane is very complex due to the involvement of multiple competing reactions we compared a number of kinetics models. DSC is well suited to carry out such kind of kinetics study [8,9]. Such optimization of reaction conversion based on time and temperature would be immensely helpful to design the toughness epoxy.

Section snippets

Materials

The epoxy resin, a diglycidyl bisphenol-A based epoxy with epoxy equivalent of 195−216 g/equivalent (ZAO Chimex Ltd, Russia), PEG-400 (hydroxyl number 260−290 mg KOH/g, PAO Nignekamsknephtechim, Russia), oligomeric MDI (PIC-200) with NCO content 32 % (OOO Isomer,Russia), aromatic hardener Etal-45 M (mixture of aromatic amines with sebacic acid, AO ANPC Epital, Russia), polyethylene polyamine PEPA with MM = 230–250 and amine number 1250 mg KOH/g (ZAO Chimex Ltd, Russia) were used as supplied.

DSC analysis

The

The cure kinetics

Fig. 1 shows a dynamic DSC plots of the compositions under study at different heating rates. The dynamic DSC measurements reveal exothermic on-set, peak and end-set temperatures of the cross-linking reactions. These parameters are given in the Table 2. Generally, cross-linking mechanism of polyurethane/epoxy mixture is multifarious in nature and involves many competing reactions. However, only one common exothermic peak is observed on DSC curves, this does not mean that the reaction follows

Mechanical properties of cured epoxy resins

The mechanical properties of curing compositions measured using the tensile tests are summarized in Table 5. The change of the mechanical properties is closely related to the structure of the composites. PU forms its own microphase regions in composites, and the microphase regions have the elasticity of PU, thus reducing their tensile strength [22]. The decrease in tensile strength is proportional to the fraction of PU in composition as well as to the increase in elongation. Overall decrease in

Conclusion

The effect of PU content on the non-isothermal curing kinetics and mechanical properties of DGEBA based epoxy curing with aliphatic polyamine polymer or mixture low molecular aromatic amines was investigated. It was shown that curing process have three stages: at first a polyurethane network is formed, then the amine hardener react with epoxy oxirane rings with formation linear oligomers, and in the end of curing the network is formed by reaction of secondary amines obtained on the second stage

Author statement

The authors have equal yield in this paper.

Declaration of Competing Interest

We support that we have not any conflict of interest.

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      This can be attributed to aggregation of particles or to flushing of small particles during synthesis. Epoxy-PU composition for the study was prepared as described in our previous paper [24]. A three-neck round bottom flask (250 mL) equipped with a nitrogen inlet tube, a thermometer, an oil bath and a mechanical stirrer was used for synthesis.

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      The different trends of activation energies for four systems could be attributed to their diverse structure of epoxy resin. Overall, the activation energy of curing reaction was determined by several factors [36-38]. At the early stage before gelation, the reaction was chemically controlled.

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      After curing, the characteristic peak of epoxy group disappears completely, the characteristic peak of ether group appears, and a wide and strong peak appears in the range of 3200 ~ 3650 cm−1, which is the characteristic peak of alcoholic hydroxyl group. According to the literature [46,47], it can be inferred that the epoxy group of polyurethane/epoxy resin reacts with the amine group of polyamine curing agent to form an IPN structure, and the reaction mechanism is shown as Scheme 1: (1) The active hydrogen in primary amine reacts with epoxy group to generate secondary amine and hydroxyl group; (2) The secondary amine reacts with epoxy group to generate tertiary amine and hydroxyl group; (3) The hydroxyl groups in the reaction products continue to undergo etherification reaction with epoxy groups until the epoxy groups are completely consumed, and the thermosetting resin with an IPN structure is generated.

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      Lastly, the as-prepared final product was dried at 60 °C for 20 h under a vacuum to collected Gr/PANI@EPOSS hybrids. The detailed experimental scheme is shown in Fig. 2 [28,29]. And the dispersion of Gr, Gr/PANI and Gr/PANI@EPOSS in ethanol is displayed in Fig.S1, it can be clearly seen that all of Gr nanosheets precipitated to the bottom of the bottle after only 1 h storage, indicating its poor dispersion in ethanol.

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