Effect of the ladder-like aminopropyl silsesquioxane on the curing and properties of epoxy composition
Introduction
Epoxy materials – reinforced plastics, adhesives, casting, sealings and anticorrosion coatings – are used in almost all areas: from design to aerospace. Epoxy resins have the potential to produce chemically and heat resistant materials with high-strength, good adhesion and low shrinkage. However, the majority of cured epoxy resins have a number of weak points, such as insufficient toughness and brittleness. To eliminate these disadvantages, various methods are used, including modification.
The creation of epoxy/silica composites is a well-known method to improve properties of both types of polymers, aimed at eliminating their inherent disadvantages and imparting new properties. Silsesquioxanes (SSQO) are among the most promising modifiers to add in epoxy compositions [1,2]. They vary in structure, presence or absence of reactive substituents [3]. The introduction of silsesquioxane (SSQO) in epoxy resin may cause improvement in strength, heat and water resistance, plasticity, thermal stability, flame retardancy and other properties of epoxy composites [[4], [5], [6], [7], [8], [9]]. SSQO can also have a negative effect on some properties of epoxy [10], which may be due to the structural features, high molecular weight and low or extremely high reactivity of this type of modifiers.
There are different explanations of SSQO influence on the properties of epoxy resins. In some cases, interpenetrating or semi-interpenetrating epoxy-silica networks can be formed as a result of curing of both ingredients [11,12]. Another researchers view SSQOs as “inorganic nanofillers” of polymer systems. They believe that introduction of reactive SSQOs into epoxy compositions under proper conditions could bring to the formation of “inorganic” domains, well-dispersed in an organic (epoxy) matrix on the molecular or nanosize level [2,13]. It helps to avoid the problem of homogeneous dispersing of nanoparticles in a matrix, which is always a problem. From the other side Pistol et al. suppose that formation of SSQS-domains (aggregates) is preferred [14]. Molecules of domains are chemically bonded to the epoxy phase, which leads to an increase in the average size of the cooperative microstructure and provides a decrease in fragility and an increase in the glass transition temperature.
Aromatic silsesquioxoxanes with amine groups are used as hardeners or co-hardeners of epoxy resins [15]. Their curing ability depends on the amount of active hydrogens in a molecule and the type of amine groups: primary amines are more reactive than secondary amines.
Many aromatic amines are solids. The necessity to rise temperature of a mixture for introducing them in epoxy resins may cause rapid and irregular curing of composites, especially when a hardener contains primary amine groups.
Various methods to introduce amine SSQO into epoxy resins are used: heating [8], dissolving and forming prepolymer [10], mixing SSQO with aminoalkoxysilanes [5] or low viscosity amines, mostly ethyleneamines. For example, the mixtures of triethylenetetramine and amine-SSQO with secondary amine groups were used in Refs. [7,16,17] to cure Bisphenol-A epoxy resins. Amine-POSS containing secondary amine groups were used in all these studies. Ni et al. conducted curing by step process at 0–100 °C and found that in such systems two types of reactions are observed: at first triethylenetetramine reacts with epoxy resin, after – amine-SSQO. Obviously, this, as well as steric difficulties, caused significantly different rate of curing of epoxy resins by those hardeners, which lead to the formation of domains enriched by amine- SSQO [14].
There is another weak point: curing of epoxy resins with aromatic amine hardeners requires elevated temperatures. A curing process is often carried out at about 180–200 °C, which sometimes is not technologically convenient. Thus, it is necessary to find out the ways to reduce curing temperature and to maintain high level of properties of the cured composites. It is especially important when epoxy binder is used for production of large-size items.
It is possible to reduce the temperature of the curing in the presence of various catalysts, accelerators or low molecular weight amines [17]. Those mixtures are usually called curing systems. They allow to obtain products with low viscosity and to cure epoxy composites at moderate temperatures.
Thus, choice of curing system and curing conditions is an important problem that affects features of forming network and properties of cured epoxy composite. At the present study we used a new ladder-like (methyl-phenyl)silsequioxane with primary aminopropyl groups. The goals of the research were to find a compatibilizer to introduce NH2-SSQO in epoxy resin and to produce epoxy composites with improved strength and cracking resistance at moderate temperatures.
Section snippets
Materials
Bisphenol A-epichlorohydrin epoxy resin ED-20 (Epital, Russia) and ladder-like aminopropyl (methyl-phenyl)silsesquioxane (NH2-SSQO), which was obtained from GNIIChTEOS (Russia), were examined. The properties of ED-20 and NH2-SSQO are shown in Table 1, Table 2.
NH2-SSQO structure is illustrated in Fig. 1. Aramine (Stekloplastic, Russia) was used as a diluent and co-hardener. The properties of Aramine are presented in Table 3.
Methods
IR spectroscopy. The interaction between NH2-SSQO and ED-20 was studied
IR spectroscopy
Amines are well-known hardeners of epoxy resins. But, as it was mentioned above, NH2-SSQO has low curing activity. We used IR spectroscopy (ATR-IR) and Soxhlet extraction to prove chemical reaction between ED-20 and NH2-SSQO.
Fig. 3 shows the IR spectra of NH2-SSQO, ED-20 and their blends, cured at different temperatures (modes A and B). In binary blends non-stoichiometric amount ED-20:NH2-SSQO was used (1:1 instead of 1:1.97) because of the difficulties in combining the components in
Conclusions
The new aminopropyl (methylphenyl) silsesquioxane (NH2-SSQO) was used to cure epoxy resin, based on bisphenol A and epichlorohydrin. The introduction of the modifier into the epoxy resin was carried out using the liquid amine hardener Aramin, which served as a diluent and a co-hardener. Aramine cures epoxies at moderate temperatures. Using IR spectroscopy and Soxhlet extraction, it was confirmed that the modifier is able to cure the epoxy resin at 70 °C. Thus, a mixture of these hardeners
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.
Acknowledgements
The authors wish to acknowledge Anatoly G. Ivanov (GNIIChTEOS) for synthesis of NH2-SSQO and Maria Khairova for language help.
This research has not received any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Yulia V. Olikhova635103949500 PhD, Asstisant Professor, Department of Technology for Processing and Applying Plastic Mixtures of Dmitry Mendeleev University of Chemical Technology of Russia. Fields of research: epoxy and epoxy-silicon compositions for reinforced plastics and adhesives; polymer additives for road construction
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Cited by (0)
Yulia V. Olikhova635103949500 PhD, Asstisant Professor, Department of Technology for Processing and Applying Plastic Mixtures of Dmitry Mendeleev University of Chemical Technology of Russia. Fields of research: epoxy and epoxy-silicon compositions for reinforced plastics and adhesives; polymer additives for road construction
Vladimir S. Osipchik635317500 PhD, Professor, Department of Technology for Processing and Applying Plastic Mixtures of Dmitry Mendeleev University of Chemical Technology of Russia. Fields of research: development of new polymers with adjustable properties, as well as effective technologies for their production and processing
Natalia V. Kostromina635127000 PhD, Asstisant Professor, Department of Technology for Processing and Applying Plastic Mixtures of Dmitry Mendeleev University of Chemical Technology of Russia. Fields of research: anticorrosive polymer coatings, fire-resistant polymer materials, silicone rubbers, materials for road and civil engineering
Oleg I. Loban6356540500 Postgraduate, Department of Technology for Processing and Applying Plastic Mixtures of Dmitry Mendeleev University of Chemical Technology of Russia. Field of research: conductive adhesives