Morphology and mechanical properties of glass fiber reinforced Nylon 6 nanocomposites
Graphical abstract
Introduction
Fiber reinforced polymer composites have been widely used for applications requiring high stiffness and strength, e.g., aerospace, automotive, marine, and sporting goods [1], [2], [3]. In most cases, rather high loadings of glass fibers are required to achieve the desired performance; this leads to an undesirable increase in specific gravity, decreased melt flow, and increased brittleness [4].
In recent years, polymer nanocomposites have attracted great interest due to the considerable enhancement in stiffness realized at very low filler loadings [5], [6], [7], [8], [9], [10], [11], [12]. Substantial improvements in mechanical [13], [14], barrier [15], [16], thermal [17], [18], and flammability [19], [20] properties have been reported while maintaining similar density and optical properties to those of the neat polymer matrix. Among these, nanocomposites based on polyamides have received attention due to their excellent compatibility with specific organoclays [13], [21], [22], [23].
Although polymer nanocomposites reinforced by montmorillonite (MMT) have exhibited improved thermal and mechanical properties at very low filler contents; loadings of more than 10 wt% MMT lead to poor dispersion and processing characteristics [9], [10], [12]. On the other hand, glass fiber loadings of 30 to 50 wt% are quite common [4]. It is of interest to explore whether combining these two fillers would give the desired performance at low to intermediate filler loadings. Such materials could be viewed as a polymer matrix containing two different types of fillers of rather different size scales. Alternatively, one might view these materials as a nanocomposite matrix filled with glass fibers because the clay platelets are so much smaller than the glass fibers. Although there have been extensive reports on polymer nanocomposites and micro-composites, only a few preliminary studies have been reported on the structure and properties of glass fiber reinforced polymer composites where the nanocomposite plays the role of the matrix as illustrated by the conceptual vision shown in Fig. 1.
Recently, several efforts have been made to investigate the combined effects of fillers at two different size scales, i.e., micro and nano. Akkapeddi [4] reported that the nature of the increase in properties of polyamide 6 composites reinforced by MMT and short glass fibers is not completely additive, but there is clearly an increase over single filler based composites. Similar enhanced mechanical properties are reported for the composites based on polyamide 6 [24], [25], [26], [27], [28], and some thermosetting polymers [29], [30], [31], [32]. The slight enhancement of tensile properties by two fillers were reported for glass fiber reinforced polypropylene nanocomposites using glass fiber mat [33], [34]. More recently, Isitman et al. [35] observed a synergistic flame retardancy effect of organoclays for glass fiber reinforced nylon 6 with conventional flame retardants.
Because of our experience with nylon 6 nanocomposites [13], [21], [22], [36], [37], [38], [39], [40], [41] and glass fiber composites [42], [43], [44], [45], [46], nylon 6 was chosen as the polymer for this study. The purpose of this work is to explore the morphology and mechanical property changes upon the incorporation of short glass fibers into nylon 6/MMT nanocomposites. In other words, this study investigates the reinforcement from two fillers of very different size scales: glass fibers with diameters on the order of 10 μm and MMT platelets with thickness of 1 nm.
Section snippets
Materials
Table 1 summarizes the materials used in this study. A commercial grade of nylon 6 was chosen that is commonly used for injection molding and extrusion applications. The organoclay was formed by cation exchange reaction between sodium montmorillonite and trimethyl hydrogenated-tallow ammonium chloride, designated here as M3(HT)1, and was obtained from Southern Clay Products. The data below are reported in terms of weight percent montmorillonite, wt% MMT, in the composite rather than the amount
Morphology
SEM micrographs of fracture surfaces for nylon 6 nanocomposites with 5, 10 and 15 wt% glass fiber are shown in Fig. 2. Over the entire range of glass fiber contents, the glass fibers are generally well dispersed in the nylon 6 matrix, and many fibers are pulled out from the matrix. In order to fabricate effective composites, it is necessary to minimize the attrition of fiber length and to have excellent interfacial strength. Fiber length has an important effect on the mechanical properties of
Mechanical properties
Fig. 10 shows representative stress-strain curves for glass fiber reinforced nanocomposites that fracture prior to yielding. The break stress increases while the break strain decreases as the glass fiber content increases. This trend is consistent with observations for adding fillers to a brittle matrix [50], [57].
The moduli of glass fiber reinforced nylon 6 nanocomposites are shown in Fig. 11 versus the glass fiber content for a fixed MMT contents of 0 and 5 wt% and versus the MMT content for a
Model predictions of modulus
There have been numerous attempts to model the properties of nanocomposites and to correlate the experimental data with composite models [21], [24], [37], [49], [55], [58], [59], [60], [61], [62], [63]. The assumptions typically made include the following: the polymer matrix is not affected by the presence of the filler, e.g., no change in crystallinity, the filler is perfectly aligned, there is good bonding between the matrix and the filler, the matrix and the filler are isotropic, and there
Conclusions
The structure and property relationships of glass fiber reinforced nylon 6 nanocomposites prepared by melt processing have been investigated to explore the reinforcing effects from two fillers of very different size scales (micro- and nano-). The micro- and nano-structures of the two fillers in the composites were assessed by SEM and TEM and quantified by detailed particle analyses. These analyses showed a reduction in glass fiber length as fiber loading increased, as expected, and a quite
Acknowledgements
This work was supported in part by a grant from General Motors Global Research and Development; the authors would like to thank William R. Rodgers for his continued interest. The authors express their appreciation to Honeywell and Southern Clay Products, Inc. for providing the polymers and the organoclays and to Don Sage of Owens Corning for helpful discussions and for providing the glass fibers that made this investigation possible.
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