Tailored glass fiber interphases via electrophoretic deposition of carbon nanotubes: Fiber and interphase characterization
Introduction
Advanced fiber composites have been widely used in structural applications due to their high specific strength, outstanding corrosion resistance, and good design flexibility as compared with conventional structural materials, such as aluminum and steel. Mechanical and physical properties of composites are influenced by the constituent materials, the interface between the fiber and the polymer matrix, and the fiber architecture. In terms of mechanical properties, the interfacial region plays a critical role in the stress-transfer mechanisms between the fibers and matrix and affects the overall structural properties [1]. Therefore, an understanding of the mechanisms influencing fiber/matrix adhesion is essential for the development of improved composite materials. Carbon nanotubes (CNTs), with their high stiffness and strength combined with the nanoscale size – three orders of magnitude smaller diameter than the fiber – offer unique opportunities to strengthen the fiber/matrix interface.
To explore ways of modifying the interphase properties of advanced composites, systematic changes to the fiber coating systems have been made to initially establish the influence on the intrinsic fiber mechanical properties. A recent review summarizes the scientific and technological achievements in interphase tailoring via fiber sizing, nanomaterials, fiber surface modification and matrix modifications [2]. Tanoglu and co-workers [3] experimentally investigated the sizing effects on the strength and energy absorption of the E-glass-fiber/epoxy-amine interface using a dynamic micro-debonding technique. They found that the compatible sizing on the glass fiber increased chemical bonding and adhesion between the inorganic glass fibers and the organic epoxy resin and strengthened the fiber/matrix interphase. However, the sizing reduced the fiber surface roughness and decreased the energy absorbing capability as a result of the reduced frictional stresses in the interface. Iglesias et al. [4] studied sizing effects on the mechanical performance of glass fiber composites by SEM fractographic analysis. A series of glass fibers were pretreated with different silane coupling agents (SCA), infused with epoxy resin and cured and finally tested to failure in tension. Adhesive failure modes were observed and they concluded that crosslink density in the coupling region increased as the accessibility of the functional groups of coupling layer increased. Feih et al. [5] investigated different SCA-based fiber sizings on the effect of the strength and fracture toughness of glass fiber composites. Results indicated that a strong interface provided higher transverse strength and crack initiation loads, while a weaker interface led to an enhanced degree of fiber bridging resulting in a higher interlaminar fracture toughness. Other studies examined the interfacial shear strength [6] and fiber strength [7] with the addition of different sizings. The tensile strength of the unsized fibers was 10 times lower than theoretical values due to surface defects. The study was able to determine the failure probability for unsized and sized fibers, concluding that sizing helped reduce the influence of the surface defects, leading to higher strength. More recently, a fiber pull-out study was used to determine how different particle size and distribution of sizing agents affected both carbon fiber strength and interfacial shear strength (IFSS) [8].
Additionally, Gao and co-workers [9] demonstrated how the use of nanoscale silica incorporated in the fiber sizing could be used to design interfaces that possessed high energy absorption and good structural properties. Their study detailed the role of sizing chemistry and surface roughness on glass fiber/epoxy composites performance using the microdroplet test. The chemical bond formed with compatible SCAs helped increase the IFSS while the texture created by the presence of nano-scale silica particle on the fiber surface increased energy absorption. In separate studies a series of fiber-sizing formulations were chosen to compare the effect of the SCA and film former on the properties of glass fiber/epoxy interphases [10]. It was concluded that, in addition to the SCA and sizing chemistry, that film formers played a critical role in glass fiber-matrix adhesion by controlling resin wetting of the fiber. The fiber/matrix interfacial properties could be adjusted by choosing different SCA/film former combinations which, in turn, controlled the homogeneity of the interphase region.
Based on advancements in nanotechnology over the past two decades, nanoscale reinforcing fillers such as CNTs may be synthesized and integrated into the traditional composites to modify the interphase region, leading to enhancements in the mechanical and electrical properties of composites [11,12]. Specifically, due to the exceptionally high surface area and aspect ratio of nanofillers, such as CNTs, the area of the newly formed interface in the nanocomposites is dramatically modified and typically an order of magnitude greater than in traditional composites. As a result, by introducing the nanomaterials into the composite, it is possible to tune the level of adhesion at the fiber/matrix interface through adjusting both the heterogeneity of surface morphology and chemical reactivity [9,13,14] and then potentially increase the overall load-carrying capacity of the composite. For example, Godara et al. [15] have experimentally investigated the interfacial strength of glass fiber/epoxy composites that were modified by dispersing CNTs in the fiber sizing formulation or the matrix. They observed over 90% increase in IFSS in the composite with the CNT-modified sizing. In addition, our previous studies have successfully applied the electrophoretic deposition (EPD) method to coat CNTs on both carbon and glass fibers using functionalized CNTs [16,17]. Both CNT-modified carbon and glass/epoxy composites showed between 70% and 80% increase in the in-plane shear strength.
While many of these experimental studies have proven the unique enhancements nanomaterials offer, these studies often neglect to study the interfacial region [11,18]. Some of the studies are theoretically based, creating high fidelity models of the interfacial region. However, due to the complex nature of the interfacial and interphase regions modified with nanomaterials, it is often difficult to design experiments that can confirm the specific interactions examined in the models. For similar reasons, most practical engineering models of composite structures do not include the influence of nanoscale interactions on macro-scale mechanical properties. Based on the successful interface research of conducting micro-droplet tests on glass fiber/epoxy composite materials by Gao et al. [9], in this study we have used similar test setup for CNT-modified glass fibers and focused on investigating the microscale load-transfer mechanisms at the fiber/resin interface.
Two main techniques are commonly utilized to integrate CNTs onto raw fibers; including both CVD [[19], [20], [21], [22], [23]] and EPD [16,17,[24], [25]]. The high temperature CVD process offers a direct method for perpendicular CNT growth on the fibers [[22], [23]] and complete coating coverage with high concentrations of CNTs [19,20,26]. However, the CVD approach can reduce the fiber strength and modulus at the elevated temperature [20]. EPD offers a commonly applied industrial two-step coating process in which charged particles migrate under an applied field to form a consolidated film on the electrode surface [27]. EPD is energy efficient and capable of homogenously coating a variety of practical materials with compact films [[28], [29], [30], [31], [32]]. Successful EPD relies on the functionalization of the dispersed material, which enables a surface charge to develop. The surface charge, or zeta-potential, is dependent on the solution pH and helps to aid dispersion and mobility under applied electric fields [[33], [34], [35]]. The EPD process has the benefit that the deposition may be carried-out under ambient conditions and allows for manipulation of the nanomaterial chemistry for the specific application. Our recent research has applied this efficient EPD process for depositing functionalized CNTs onto conductive carbon fiber [17] and non-conductive glass fiber substrates [16].
In our previous composite and model interphase study, significant improvements in strength and toughness have been achieved for the EPD-CNT reinforced fiber/epoxy composites. CNT modification the interphase contributed to these significant improvements by forcing the fracture path away from the matrix/fiber interface an into the CNT-rich interphase region. The aim of the current study is to characterize the micro-mechanical properties of E-glass fibers coated with CNTs via EPD. The initial measurements examined the intrinsic effect of the EPD treatment on glass fiber strength and micro-droplet tests provided insight into the influence of the CNT-modified interface on the micro-mechanical shear failure mechanisms occurring in glass/epoxy composites.
Section snippets
Materials and processing
To study the influence of CNTs on the fiber/matrix interface, single E-glass fibers were extracted from a tow in a unidirectional E-glass fabric (style 7721, 203 g/m2, aminopropylsilane, APS, sizing, Thayercraft Inc., USA). The measured average diameters of the fibers were 10 μm. Multi-walled CNTs (Hanwha, Nanotech, Korea) were functionalized using a novel ultrasonicated-ozonlysis (USO) process [16,17] to produce a stable 1 g/L aqueous dispersion. Moisture-free oxygen gas with a concentration
Fiber surface morphology
Fig. 3 shows the surface morphology of as-received E-glass fiber and CNT-PEI EPD coated E-glass fiber. It can be seen from Fig. 3a that the surface of the as-received E-glass fiber was not smooth with some small heterogeneous features from the sizing agent, indicating the SCA provided some level of surface texture, which could improve the IFSS by altering interfacial sliding mechanisms. Fig. 3b confirms that the EPD process was very effective, with evidence of a uniform PEI-CNT coating present
Conclusions
EPD was successfully applied to coat single E-glass fibers with ozone and PEI functionalized CNTs. Single-fiber tensile testing indicated that the CNT coating did not degrade the tensile strength or stiffness of the fibers. Statistical analysis of the fiber fracture strength suggested that the EPD-CNT coating reduced the influence of surface defects on the commercially sized fiber and led to slightly improved mechanical properties. Microdroplet testing showed increased IFSS with CNT-coated
Acknowledgement
Q.A. and E.T.T gratefully acknowledge the funding support from University of Delaware Graduate Student Office Fellowship Award and National Science Foundation (Grant CMMI-1254540: Dr. Mary Toney, Program Director). S.T. and J.W.G. would like to acknowledge a research sponsorship by the Army Research Laboratory, which was accomplished under Cooperative Agreement Number W911NF-12-2-0022. The views and conclusions contained in this document are those of the authors and should not be interpreted as
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