Composites Part A: Applied Science and Manufacturing
Mode I interlaminar fracture behavior and mechanical properties of CFRPs with nanoclay-filled epoxy matrix
Introduction
Polymer nanocomposites reinforced with nanoclay particles have attracted significant attention because of unique mechanical, thermal and physical properties along with excellent transport characteristics that are offered by the layered structure of clay particles with extremely high aspect ratios. A significant improvement of modulus provided by the exfoliated nanocomposite structure was first reported on clay–polyamide-6 composites, with 90% increase due to 4 wt% of exfoliated clay [1]. Remarkable ten-fold increases in strength and modulus were also reported with exfoliated organoclays in rubbery epoxy [2]. These property gains were at the expense of ductility, which decreased with increasing clay content [3]. In contrast, the reinforcement efficiency offered by the similar organoclay in glassy epoxy matrices was not as remarkable as in the rubbery epoxy, and the tensile strength often showed lower values than that of the neat epoxy [4]. However, reported results of fracture toughness of clay nanocomposites showed no apparent consensus. For example, almost a ten-fold reduction in fracture energy was reported for a polyamide nanocomposite containing 4 wt% of exfoliated clay [5], which was attributed to the reduction of plastic deformation in the constrained polymer matrix. Studies on brittle thermoset matrices indicated mixed results: the silane treated clay–polyester system [6] exhibited improved quasi-static fracture toughness, whereas the fracture properties of clay–glassy epoxy nanocomposites were little influenced by the exfoliated silicate layers [7].
Our previous study revealed [8], [9] that both the moisture diffusivity and moisture permeability showed a systematic decrease with increasing clay content, which agreed with the prediction based on the simple tortuous path model. Increase in effective penetration path due to the very large aspect ratio of the silicate layers was responsible for the reduced permeability. The presence of organoclay in the epoxy matrix increased significantly the glass transition temperature, Tg, whether the nanocomposites were in a dry or wet condition. The organoclay was successfully treated with epoxy monomer to further improve the polar interactions with a poly(ethylene terephthalate–co-ethylene naphthalate) (PETN) matrix [10]. The morphological, thermal–mechanical, mechanical and gas barrier characteristics of the nanocomposites were evaluated using several characterization tools. It was found that the Cloisite 30B organoclay had better interactions with PETN and was more uniformly dispersed within PETN than the Cloisite 20A organoclay. Epoxy treatment of Cloisite 30B organoclay resulted in improvements in d-spacing between silicate layers, thermo-mechanical and tensile properties, as well as thermal stability, processing and gas barrier characteristics of the PETN/30B nanocomposites. These results suggest that the epoxy acted as the compatibilizer as well as the chain extender, improving the chemical interactions between PETN and organoclay, while discouraging the macromolecular mobility of polymer chains in the vicinity of clay particles.
While a significant progress has been made on the development of clay reinforced polymers, only a few studies have been conducted on the use of the modified polymers as the matrix material for fibre reinforced composites. An earlier work presented two approaches for fabricating continuous carbon fiber nanocomposite containing organoclay modified epoxy matrices [11]. The incorporation of 2 wt% of organically modified montmorillonite (Cloisite 25) resulted in carbon–epoxy composite laminates with microcrack densities 50% lower than those in the unmodified materials under cryogenic cycling [12]. The flexural modulus was relatively unaffected, whereas the corresponding flexural strength was marginally reduced due to the 2 wt% of organoclay reinforcement. There was a maximum of 50% improvement in initiation value of mode I interlaminar fracture toughness of unidirectional carbon–epoxy composites at an organoclay concentration of 7.5 wt% in the matrix phase [13]. However, there were negligible changes in interlaminar shear strength due to the incorporation of organoclays.
This paper is continuation of our previous work [8], [9], [10], which is part of a larger project on clay–epoxy nanocomposites for applications as adhesive and matrix for fibre reinforced composites in the construction industries. Main objectives of this study were to develop organoclay-filled epoxy nanocomposites as the matrix for carbon fibre reinforced composite laminates, and to characterize the mechanical properties and fracture resistance of the hybrid composites. Failure mechanisms associated with the incorporated organoclay were identified. Correlations were established between the fracture properties of the nanocomposite matrix and the interlaminar fracture toughness of the hybrid laminate composite.
Section snippets
Fabrication of nanocomposites and composite laminates
The laminate composites were fabricated from carbon fibre plain woven fabric and organoclay-filled epoxy resin. The epoxy resin was a diglycidyl ether of bisphenol A (DGEBA) epoxy (Epon828, supplied by Shell Corp.). A eutectic mixture of 1,3-phenylenediamine (mPDA) and 4,4-methylenedianaline (MDA, both supplied by Aldrich) at a ratio of 40:60 by weight was used as curing agent. The addition of MDA was aimed to reduce the viscosity of the epoxy-hardener mixture without affecting the resultant
Morphologies and mechanical properties of clay–epoxy nanocomposites
Typical TEM photographs of nanocomposites containing 5 wt% clay are presented in Fig. 2, indicating that the I30P organoclays were well dispersed within the epoxy with a mixture of exfoliation and intercalation. The interlayer distance estimated from the TEM photograph gave a measurement larger than 10 nm, which is consistent with our previous morphological study based the X-ray diffraction (XRD) and TEM analyses [9]. The basal (0 0 1) reflection appeared at 3.7° for the I30P organoclay with an
Conclusions
The mechanical and fracture properties are studied of organoclay modified epoxy nanocomposites, as well as the CFRP composite made therefrom. The following conclusions can be highlighted from the study.
- (1)
The organoclay brought about a significant improvement in flexural modulus: an increase of 26% was registered with the addition of 3 wt% clay. The flexural strength gradually decreased with increase in clay content.
- (2)
There was a direct or inverse relationship between the fracture toughness and clay
Acknowledgements
This project was supported by the Research Grant Council of Hong Kong Special Administration Region (Project Number HKUST6184/03E). Part of the experiments was carried out when NS was a visiting scholar at Hong Kong University of Science and Technology (HKUST) from the National Engineering and Scientific Commission of Pakistan. Assistance with experiments by Mr. Alexander Wieser as an exchange student from the Technical University of Munich, Germany is much appreciated. Technical assistance
References (22)
- et al.
Synthesis of epoxy-clay nanocomposites: influence of the nature of the clay on structure
Polymer
(2001) - et al.
Moisture barrier characteristics of organoclay–epoxy nanocomposites
Compos Sci Technol
(2005) - et al.
Effects of epoxy treatment of organoclay on structure, thermo-mechanical and transport properties of poly(ethylene terephthalate–co-ethylene naphthalate)/organoclay nanocomposites
Polymer
(2005) - et al.
Organoclay-modified high performance epoxy composites
Compos Sci Technol
(2005) - et al.
Epoxy nanocomposites with high mechanical and tribological performance
Compos Sci Technol
(2003) - et al.
Fracture toughness and failure mechanisms in silica-filled epoxy resin composites: effect of temperature and loading rate
Polymer
(1993) - et al.
High strength, high fracture toughness fibre composites with interface control—a review
Compos Sci Technol
(1991) - et al.
Fracture toughness of CFRP with modified epoxy matrices
Compos Sci Technol
(1992) - et al.
Nylon 6-clay hybrid
Mat Res Soc Proc
(1990) - et al.
Clay reinforced epoxy nanocomposites
Chem Mater
(1994)
Characterization of epoxy-clay hybrid composite prepared by emulsion polymerization
J Appl Polym Sci
Cited by (296)
Enhancement in fatigue performance of FRP composites with various fillers: A review
2023, Composite StructuresMultiscale numerical methodology for assessing fracture toughness enhancement due to nanoclay inclusion in fiber-reinforced polymer composites
2022, International Journal of Solids and StructuresImproving the performance of advanced fiber-reinforced polymer (FRP) composites using nanoclay
2022, Advanced Fibre-Reinforced Polymer (FRP) Composites for Structural Applications