Epoxy clay nanocomposites – processing, properties and applications: A review
Section snippets
Epoxy resin
The term ‘epoxy resin’ refers to both the prepolymer and its cured resin/hardener system. The former is a low molecular weight oligomer that contains one or more epoxy groups per molecule (more than one unit per molecule is required if the resultant material is to be crosslinked). The characteristic group, a three-membered ring known as the epoxy, epoxide, oxirane, glycidyl or ethoxyline group as shown in Fig. 1 is highly strained and therefore very reactive. Epoxy resins can be cross-linked
Polymer nanocomposites
Polymer nanocomposites have attracted great interest, both in industry and in academics, because they exhibit remarkable improvement in material properties compared with virgin polymer or conventional micro and macro composites [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Conventional composites usually require a high content (>10%) of the inorganic fillers to impart the desired mechanical properties. Such high filler levels increase their density of the
Clays
Clays are hydrous silicates or alumino silicates and are fundamentally containing silicon, aluminum or magnesium, oxygen and hydroxyl with various associated cations. These ions and OH groups are organized into two dimensional structures as sheets. Clay minerals are also called layered silicates or phyllo silicates because their structural frame work is basically composed of 1 nm thick silicate layers comprising silica and alumina sheets joined together in various proportions and stacked on top
Montmorillonite
Among the different types of clay minerals, montmorillonite is the most commonly used for the preparation of polymer clay nanocomposites [6], [7], [8], [9], [10], [11], [12], [13]. Montmorillonite owes special attention among the smectite group due to its ability to show extensive inter layer expansion or swelling, because of its peculiar structure as shown in Fig. 2.
The crystal structure of montmorillonite consists of layers formed by sandwiching an edge shared octahedral sheet of alumina
Organic modification of clay
Generally clays are hydrophilic in nature. In order to make compatible with organic polymers, the surface of the clay minerals should be modified to organophilic prior to its use. Organic cations such as an ammonium ion or phosphonium ion are the commonly used organic modifiers for clay minerals [6], [7], [8], [9], [10], [11], [37], [38]. Modification involves the exchange of interlayer inorganic cations with organic onium salts. The organic modification causes the expansion of the interlayer
Structure of polymer clay nanocomposites
Depending on the nature of the components, processing condition and strength of the interfacial interactions between polymer and layered silicates (modified or unmodified), either conventional composites or nanocomposites can be formed as shown in Fig. 4 [6], [7], [8], [9], [10], [11], [24]. In a conventional composite, the polymer cannot diffuse between the clay layers and the clay particles exist in their original aggregated state. Properties of these composites are similar to the micro
Preparation of nanocomposites
There are several methods to prepare clay based polymer nanocomposites. These include in situ polymerization, melt intercalation and solution casting.
Epoxy clay nanocomposites
Among polymer layered silicate (clay) nanocomposites, epoxy based systems has been reported in detail, due to the ease of processing as well as its versatile applications in various fields. The structure and properties of epoxy clay nanocomposites are influenced by the curing agents, clay modifier and processing method.
Curing agents
In epoxy clay systems, there have been used amine and anhydride based curing agents, which rendered different properties and morphology to the epoxy systems. It was found that when anhydride was used as curing agent, an exfoliated morphology while diamino diphenyl methane (DDM) gave an intercalated morphology, since anhydride is a liquid and can easily diffuse into the clay gallery unlike DDM which is a solid [58]. Similar observations were made by Xu et al. [59] for diethylenetriamine and tung
Clay modifier
Generally based on the organic modifier used in the modification clay, the structure and properties of the nanocomposites varies considerably. Intercalated nanocomposite is generally produced with quaternary and tertiary alkyl ammonium surfactants due to low bronsted acidity of the surfactants. The fixed layer separation of clay layers is unable to provide optimum level of reinforcement. Exfoliated nanocomposite is produced with primary and secondary alkyl ammonium surfactants or quaternary
Processing method
Processing methods influence the clay morphology. The usual processing methods to disperse the clay layers in epoxy matrix are mechanical stirring, ultrasound sonication [79], [80], high shear mixing [81], [82], ball milling [83], etc. Lam et al. [79] reported that 10 min ultrasonication result an optimum micro-hardness at 4 wt.% clay containing epoxy nanocomposite. Zunjarro et al. [80] reported that high speed shear mixing yielded better mechanical properties compared to ultrasonication, even
Properties of epoxy clay nanocomposites
Epoxy clay nanocomposites show enhanced thermo mechanical properties even with a small amount of layered silicate (⩽5%). Improvements comprise higher modulus, increased strength, heat resistance, decreased gas permeability, reduced coefficient of thermal expansion and decreased flammability. The main reason for this improved property in nanocomposites is the large interfacial interaction between the matrix and layered silicate and also the high aspect ratio of the dispersed clay particles.
Mechanical properties
Mechanical properties of polymer–clay nanocomposites depend on the microstructure in which how the clay layers are dispersed in the polymer matrix. Generally the well dispersion of the clay particles in the polymer matrix yields enhanced tensile modulus, storage modulus and tensile strength. Even though, the tensile strength and modulus tend to increase with increasing clay content, the increasing trend is more noticeable for the tensile modulus. The reinforcing effect of clay layers on the
Thermal properties
Polymer clay nanocomposites are known for its high thermal stability and flame retardancy. The improved thermal stability is attributed to the action of clay layers as superior insulator and mass transport barrier to the volatile products generated during decomposition as well as assisting in the formation of char after thermal decomposition [6], [110], [111], [112]. The slowing down of the escape of the volatile products in nanocomposites is because of the labyrinth effect of the silicate
Barrier properties
Clay layers in the polymer matrix can act as an effective barrier to the penetrants. The enhanced barrier property of polymer nanocomposites is due to the labyrinth or tortuous path (Fig. 12) that retards the diffusion of gas molecules through the polymer matrix.
Neilson’s equation is proved to be a reliable estimate [124] of gas permeability of polymer-layered silicate nanocomposite systems. The Neilson’s equation is as follows:where Pn represents the permeability of the
Applications
Because of the large improvement observed in the mechanical, thermal and barrier properties, epoxy clay nanocomposites can be used for many specific applications in aerospace, defense and automobile industries. These composites are also used in high performance structural and functional applications such as laminates and composites, adhesives, sealants, tooling, molding, casting, electronics and construction.
Epoxy clay nanocomposites reinforced with a high strength carbon/glass fiber have the
Conclusion
Epoxy nanocomposites are highly versatile polymer systems for the new era of making lighter structural composites for various aerospace and automobile applications. The final morphology, physical, chemical and barrier properties of the nanocomposites were influenced by processing method, clay modifier and curing agents. Epoxy clay nanocomposites showed remarkable improvement in tensile, flexural and fracture toughness properties. Thermal stability and barrier properties were significantly
Acknowledgements
This work was supported by the Center for Science & Technology Research (CSTR) grant funded by the Korea government (MEST) (CSTR-002-100701-03) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0023106).
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