The aims of this study, firstly, to obtain high degree of clay exfoliation in the epoxy matrix by three dispersion methods such as normal mixing, shear mixing, and high-speed mixing and, secondly, to investigate corrosion behavior of epoxy/organoclay nanocomposite, immersion test, weight change, and penetration behavior were conducted. From the three mixing methods, the high-speed mixing method showed larger clay interlayer distance, smaller clay aggregate, and more homogeneity and expectedly resulted in high anticorrosive properties. Penetration depths of these nanocomposites showed a small difference; however, the most noticeable improvements in anticorrosion performance for epoxy/organoclay nanocomposites under high-speed mixing method were found to reduce penetration and weight uptake which are described via the model of nanoparticulate-filled structure and discussed in corrosion protection mechanism against environmental liquid penetration.
Polymer-layered silicate nanocomposites (PLSNs), materials consisting of a polymer matrix reinforced with inorganic nanoparticles, are an expanding field of study due to unexpected improvement on mechanical, thermal, and barrier properties over conventional unfilled polymer resins. Due to the property enhancement at small clay loading (1–10 wt%), PLSNs have attracted increasing interest from scientific [
Montmorillonite (MMT) is one of the most common inorganicfiller. MMT, which has an alumino-silicate structure, consists of two tetrahedral layers sandwiching an octahedral layer with Na+ or Ca2+ ions residing in the layer. Several PLSNs have successfully been prepared using MMT with polyamide as the most successful matrix [
In the past decades, mechanism of clay dispersion into the epoxy matrix has been one of the most interesting issues of polymer research. Sancaktar and Kuznicki [
However, PLSNs using MMT with epoxy resin as the matrix have not convincingly proved to be a complete exfoliation of the layered structure in the epoxy matrix. Epoxy resins are characterized by the presence of epoxide rings which can be reacted with curing agents or catalytically homopolymerize to form a crosslinked polymeric structure. Crosslinked epoxies exhibit outstanding properties such as good adhesion to a variety of surfaces, low shrinkage during curing, no emission of volatile products, resistant to thermal and chemical attack, high glass temperature, and high mechanical modulus.
These intercalated organoclay/epoxy nanocomposites show improved properties including enhanced mechanical properties at low nanofillers loading [
Focusing on corrosion studies, there are still few articles publish about it. Rana et al. [
The main goal of this study is to obtain the optimum clay dispersion method into epoxy matrix via three different mixing methods and to study corrosion behavior of epoxy/organoclay nanocomposites under immersion test, from weight change and penetration behavior points of view.
Epoxy composites used in this study were bisphenol A type (Epomik R140) from Mitsui Chemical Co., Ltd, diamine curing agent (Jeffamine D230) from Huntsman Corporation, and montmorillonite-based organoclay (Nanomer I.30E) from Nanocor Inc., USA, employed to produce the nanocomposites.
Epoxy/organoclay nanocomposites were prepared at various conditions. Initially, epoxy and 1 part per hundred resin (phr) of organoclay were mixed via three different mixing methods: (1) normal mixing at 1,000 rpm via propeller mixer for three hours, (2) shear mixing via three-roll calender (EXAKT 50, Exakt GmbH) for three times as shown in Figure
Illustration of mixing mechanism by three-roll calender.
Cross-section of high-speed homogenizer (a) and mechanism of rotor-stator (b).
Interlayer distance (
The organophilic nature of the clay allows the dispersion of organic solvent or monomer between the clay layers. XRD measurement can be used to characterize the interlayer distance. If diffraction peaks were observed in the low-angle region, such peaks would indicate the
The nanoclay composite structure and clay distribution were examined using transmission electron microscopy (TEM). TEM images were obtained by a JEOL 2010F equipped with a field emission gun operating at 200 kV. TEM samples were prepared by ultramicrotome to be thin sections of the polymer nanocomposite with a diamond knife. These thin sections were then captured on coated Cu grids. The samples were tested at different magnifications from 0.5
The immersion test specimens of 60 × 25 × 2 mm3 for measurement of initial weight were dried at 50°C in oven for more than 75 hours according to the previous study [
Typical apparatus of immersion test.
Specimens were removed at regular time intervals, washed by water, and then carefully wiped to remove excess of acid on the specimens. A microbalance was used for measuring the specimen weight. The weight change was determined by obtaining the change in mass of specimen in 10 different times from 0 h up to 625 h, relative to the initial weight, and was calculated by
To observe and measure penetration depth and penetration profile of sulfur (S) element of epoxy/organoclay nanocomposites, energy dispersive X-ray spectroscopy (EDS) is an analytical technique used for the elemental analysis of samples. Energy dispersive X-ray spectroscopy JEOL JEM-5310LV was employed by investigating cross section of the immersed specimens as shown in Figure
Schematic diagrams of sample (a) and typical micrograph (b) of EDS analysis.
X-ray diffraction curves of organoclay and 1 phr epoxy/organoclay nanocomposite are illustrated in Figures
X-ray diffraction curve of organoclay.
X-ray diffraction curve of 1 phr epoxy/organoclay nanocomposite under high-speed mixing method.
All specimens: neat resin (EP), normal mixing (NMA), shear mixing (RHA), and high-speed mixing (DME) nanocomposites were immersed in 10% of sulfuric acid solution at fixed temperature, 60°C. The aim of this analysis is to determine whether the diffusion of sulfuric acid through epoxy/organoclay nanocomposites is following Fick’s law, and to investigate weight change of samples which are immersed in 10 mass % sulfuric acid solution at 60°C as a function of square root of immersion time in Figure
In this report, corrosion in the composite is described by its weight change behavior and penetration depth. The diffusion behavior of sulfuric acid in both neat resin and nanocomposites follows Fick’s law in weight change. As Figure
Weight change of neat epoxy (EP), normal mixing (NMA), shear mixing (RHA), and high-speed mixing (DME) nanocomposite in terms of square root of time in 10% of sulfuric acid solution at 60°C.
In order to discuss the behavior in details, all samples have almost similar penetration rates; however, high-speed mixing sample and neat epoxy resin reached equilibrium at similar time and earlier compared to normal mixing and shear mixing samples. It would be for the reason that layered clays themselves do not assist to improve anticorrosion performance as labyrinth effect in terms of penetration rate, moreover, the conventional and intercalated clays act as a moisture scavenger by absorbing the solution into themselves. As a result, both conventional and intercalated clay structures, which still remain layer-layer structure, in the epoxy matrix via normal and shear mixing occur the second absorption and postpone the equilibrium to longer time.
Therefore, high degree of intercalation/exfoliation of high-speed mixing specimen has the lowest weight saturation of the other two methods. This suggests that the mixing method of clay, especially via high-speed mixing method, significantly affects corrosion behavior in terms of solution equilibrium. The behavior can be explained that the random orientation of clay plates in the bulk material, which is reasonably effective in improving property, especially corrosion resistance. As a result, it could be effectively against penetration and weight uptake.
Penetration profiles of sulfur of neat epoxy and one of three mixing methods, high-speed mixing (DME) nanocomposite at different immersion time under 10% of sulfuric acid solution using EDS analysis, are shown in Figure
Penetration profiles of sulfur of neat epoxy (EP) and high-speed mixing (DME) nanocomposite at different immersion time under 10% of sulfuric acid solution.
Sulfur penetration depth of neat resin and epoxy/organoclay nanocomposite versus square root of time is shown in Figure
Penetration depth of neat epoxy and epoxy/organoclay nanocomposite under different mixing methods in terms of square root of time.
Figure
TEM images of epoxy/clay nanocomposites via normal mixing method at different magnifications.
Figure
TEM images of epoxy/clay nanocomposites via shear mixing method at different magnifications.
Figure
TEM images of epoxy/clay nanocomposites via high-speed mixing method at different magnifications.
Figure
Model of three kinds of nanocomposites made by normal mixing, shear mixing, and high-speed mixing. (Black dot represents crosslink between epoxy resin and curing agent, and gray dot represents onium ion.)
Normal mixing
Shear mixing
High speed mixing
Normal mixing includes randomly dispersed stacked fillers with small gaps as shown in Figure
Moreover, layered clays benefit in fixing the movement of polymer network. Onium ions which is contained on the surface of treated clay nanofillers would react with epoxy monomers (as the gray dots in illustration) and form crosslink. These phenomena could affect to decrease polymer mobility. Consequently, it would be expected to declaim weight uptake of the materials.
Solution equilibrium of epoxy/organoclay nanocomposite via high-speed mixing method is noticeably different compared to other samples as illustrated in Figure
Mechanism of sulfuric acid diffusion in epoxy network and intercalated layered filler.
Mixing method of epoxy nanocomposites plays an important role in clay dispersion in epoxy resin and is a main key parameter of epoxy/organoclay nanocomposites in the improvement of corrosion performance such as weight change, penetration depth, penetration profile and size of penetration depth. In order to obtain knowledge of anticorrosion performance of epoxy nanocomposites, three sample composites were prepared by different three mixing methods. Corrosion behavior was investigated under immersion test, and evaluated by penetration depth and penetration profile. All samples have almost similar penetration rates. On the other hand, high-speed mixing sample and neat epoxy resin reached equilibrium at similar time and earlier compared to normal mixing and shear mixing samples. Epoxy/organoclay nanocomposite preparation under high speed mixing method which creates exfoliated clay dispersion in the epoxy matrix performs better corrosion resistance compared to normal and shear mixing methods regarding to weight gain and penetration depth.
The authors would like to thank the Center for Advanced Materials Analysis, National University Corporation Tokyo Institute of Technology, for XRD and TEM analysis.