Sepiolite-based epoxy nanocomposites: Relation between processing, rheology, and morphology

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Abstract

Rheology of sepiolite-based epoxy suspensions as well as morphology and dynamic mechanical properties of the corresponding nanocomposites are discussed in this paper. The influence of the type of sepiolite used, i.e. non-modified, aminosilane and glycidylsilane surface modified, and of the process developed to prepare the epoxy suspensions were investigated. Except for low amount of filler, a shear thinning behavior was observed in the others sepiolite-based epoxy suspensions. The interactions developed between the sepiolite and the epoxy matrix are responsible for the magnitude of the shear thinning effect and are related to the morphology of the nanocomposites. The best dispersion of sepiolite was achieved using either an emulsion process or a glycidyl functionalized sepiolite.

Graphical abstract

Relation between the shear thinning behavior of sepiolite-based epoxy suspensions and the morphology of the nanocomposites. Influence of the process used to prepare the suspension.

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Introduction

Epoxy networks are among the most commercially successful thermosetting materials, especially as adhesives, coatings, encapsulation of electronic components and matrices of composite materials, etc. [1], [2]. The extensive applications of epoxy networks motivate intense studies having the objectives to prepare organic–inorganic nanocomposites with novel and improved performances. The properties of the nanocomposites are expected to arise from a synergistic combination of individual organic/inorganic component properties when these components are mixed intimately, i.e. at the nanometer scale. Three approaches to produce hybrid organic/inorganic thermosets are possible: (i) the first route involves the so-called sol–gel technique [3], [4], [5] consisting of hydrolysis and condensation of metal alkoxides (for example tetraethoxysilane), at a low temperature. The inorganic phase is formed in-situ in the organic medium; (ii) the second route is to use pre-formed nanoparticles, such as pyrogenic silica [6], [7], [8], [9], [10] or layered silicates [11], [12], [13]. In this case critical requirements for reaping the potential benefits of incorporating nanofillers are on one hand the dispersion of the particles at the nanoscale level, and on the other hand the rheological behavior of the suspension (i.e. increase of viscosity) which must remain compatible with the processing methods. These two points are closely linked; (iii) finally the third route which appears the past decade is to use well-defined inorganic clusters (also called nanobuilding blocks, for example polyhedral oligomeric silsesquioxanes) [14], [15], [16], which can assemble to form complex structures within the polymer matrix they are introduced in.

Our work focuses on the second approach and more precisely on the use of a non-conventional clay to prepare a new type of epoxy-based nanocomposites. When speaking about clay minerals as nanofillers in nanocomposite materials, scientists often think about montmorillonite (MMT), a well-known natural and low-priced layered silicate. If montmorillonite with its high aspect ratio, high surface area, and swelling ability, is an interesting candidate for the nanocomposite processing, sepiolite is also a valuable nanofiller. Geological deposits of sepiolite minerals are very scarce around the world and most of the world production comes from deposits of sedimentary origin located in Spain. With the ideal formula Mg8Si12O30(OH)4(H2O)4⋅8H2O, sepiolite structure is based on the typical structural units of phyllosilicates: (i) silica tetrahedra and (ii) Mg2+ or Al3+ octahedra [17]. Sepiolite clay is similar to montmorillonite in that both structures consist of two silica tetrahedral sheets enclosing one central octahedral sheet based on magnesia (sepiolite) or alumina (montmorillonite). Differences arise from the existence of discontinuities and inversions in the silica sheet that give rise to structural channels filled with water molecules and running along the axis of the particle [18], [19]. The presence of these inversions gives sepiolite characteristic needle-like morphology instead of plate-like structure like montmorillonite. The discontinuities in the external silica sheet also account for the high density of silanol groups located at the edges of each block at the external surface of the silicate.

Sepiolite is used as a technical and industrial additive for a wide variety of sectors and processes: its remarkable sorptive and rheological properties provide solutions for applications ranging from cat litters, carrier for chemicals, rheological additives for industrial paints, processing aids, binding additives, etc., but a very new application is also to be mentioned: the use as nanofillers in polymer systems. Sepiolite is potentially well suited for the design of hybrid nanocomposites because of its interesting needle-like morphology and the remarkable opportunities it offers. Owing to the great number of active centers on its surface (silanol groups and Mg2+-coordinated water), sepiolite induces a high potential interaction level between both nanofillers/nanofillers and nanofillers/matrix components. Moreover, although sepiolite is naturally hydrophilic, additional chemical treatments may be carried out to give organophilicity or reactivity to its surface. The chemical modification is generally done by similar treatments used for more conventional nanofillers (montmorillonite, silica), such as grafting of organosilanes and ion exchange. Contrary to the layered silicate-based nanocomposites that have been extensively studied, the sepiolite-containing epoxy systems have been scarcely reported [20], [21].

The objective of this paper is to report on the relation between the rheology of the sepiolite-based epoxy suspension and the morphology and thermo-mechanical properties obtained in the final crosslinked nanocomposites. Indeed, it is a difficult task to study the morphology of suspensions for two reasons: the suspensions are in an uncured state and the size of the sepiolite needles are in the nm scale; cryo-TEM would be helpful but is difficult to realize. In general, a good correlation exists between the rheological behavior of the suspensions before polymerization and the final dispersion state in the nanocomposites after the crosslinking reaction [6], [11]. Consequently, determining the rheological behavior of nanoparticle-filled epoxy resins prior to cure reaction may lead to better optimization of the process conditions, and therefore, to better dispersion state of the nanoparticles and better solid state properties. Therefore in this work, on one hand we varied the type of sepiolite used (non-modified and surface-functionalized) and on the other hand the type of process used to prepare the suspensions (“conventional” and “emulsion”) in order to study such relation between rheology, dispersion state obtained from electron microscopy and thermo-mechanical properties obtained by dynamic mechanical analysis. The influence of sepiolite on the kinetics of the reaction was also investigated through gel time measurements.

Section snippets

Materials

The thermosetting matrix considered in this work was obtained via the polycondensation of an epoxy–amine system. The diepoxy prepolymer used was a DGEBA (diglycidyl ether of bisphenol A, DER 330 from Dow Chemicals), it is liquid at room temperature. Its viscosity is equal to ca. 10 Pa s at 25 °C. The curing agent used, MDEA (4,4-methylene bis(2,6-diethylaniline), from Lonza), is a primary aromatic diamine. It presents a short and rigid chain so that the network obtained in combination with

General description

Fig. 1 shows the flow curve obtained for the DGEBA monomer and for epoxy suspensions containing different amounts of non-modified sepiolite. It can be seen that the epoxy monomer exhibits a Newtonian behavior. The addition of small amount of sepiolite (1.2 wt%) only induces a small increase in the viscosity and the behavior is still Newtonian. By contrast, the curve obtained for the epoxy suspension containing higher amounts of sepiolite (4.2 and 8.4 wt%) clearly indicates a shear-thinning

Conclusions

The general purpose of the present work was being able to control the dispersion state of nanoparticles and therefore, the morphologies generated in sepiolite-based epoxy networks. It was achieved through two key factors: (i) creation of appropriate interfacial interactions between nanoparticles and matrix, (ii) proper selection of the processing procedure and conditions. The first strategy involved the chemical modification of the nanoparticle surface. The sepiolite functionalization, glycidyl

Acknowledgments

The authors gratefully acknowledge the European Commission for the financial support (STRP Nanofire) and Tolsa SA (Spain) for providing the sepiolites. The authors thank Pierre Alcouffe who realized the TEM analysis at the Centre Technologique des Microstructures at the Université Claude Bernard (Lyon I).

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