Elsevier

Polymer

Volume 45, Issue 25, November 2004, Pages 8501-8515
Polymer

Comparison of nanocomposites based on nylon 6 and nylon 66

https://doi.org/10.1016/j.polymer.2004.09.074Get rights and content

Abstract

Nylon 6 and nylon 6,6 organoclay nanocomposites were prepared by melt processing using a twin screw extruder. The effects of polyamide type and processing temperature on the mechanical properties and the morphology of the nanocomposites were examined. Mechanical properties, transmission electron microscopy (TEM), wide-angle X-ray diffraction (WAXD), percentage crystallinity and isothermal thermo-gravimetric analysis (TGA) data are reported. A particle analysis was performed to quantitatively characterize the morphology; these results are later employed in modeling the modulus of these materials using composite theory. No significant difference was observed in the mechanical properties and morphology of PA-6 nanocomposites processed at two different temperatures. PA-6 nanocomposites had superior mechanical properties than those made from PA-66. The tensile strength of PA-66 nanocomposites deviated from linearity at high levels of MMT. WAXD and TEM results show that the PA-6 nanocomposites are better exfoliated than the PA-66 nanocomposites, which exhibit a mixture of intercalated and exfoliated structures. Mechanical properties were consistent with the morphology. DSC reveals a higher percentage of crystallinity in the PA-66 samples. Isothermal TGA shows only a 5% difference in the degradation of the organic modifier on the organoclay processed at 240 °C versus 270 °C. Particle analysis shows a higher average particle length and thickness, and a lower average particle density and aspect ratio in nanocomposites based on PA-66 versus PA-6. The Halpin-Tsai and Mori-Tanaka composite theories predict satisfactorily the behavior of the PA-6 nanocomposites, while the PA-66 nanocomposites were predicted acceptably up to a certain volume fraction where the non-linear behavior takes effect. All the results indicate that there is a lower degree of exfoliation in the nanocomposites produced with a PA-66 matrix apparently stemming from the chemical differences between PA-6 and PA-66.

Introduction

Polymer layered silicate nanocomposites have attracted a great deal of interest over the last few years as a result of the potentially superior properties these materials can exhibit relative to conventional composites. Numerous studies have shown that a very low percentage of layered silicates can lead to a significant enhancement of many properties, such as stiffness and strength [1], [2], flame retardancy [3], [4], gas barrier properties [5], [6], ionic conductivity [7], [8], thermal stability [9] and tunable biodegradability [4]. All these properties make these materials interesting prospects for a wide variety of applications, such as in automotive, electronics, food packaging, biotechnology and many others.

Polyamides are widely used materials due to their tunable properties; polyamide 6 (PA-6) and polyamide 66 (PA-66) account for the majority of the commercial polyamide production and application. These polyamides physically differ in terms of melting point, glass-transition temperature, crystallinity, and tensile modulus, among other things. PA-66 has a melting point of 262 °C, which is higher than that of PA-6 at 219 °C; its glass-transition temperature is 65 °C, versus 52 °C for PA-6; the crystal structure of PA-66 is triclinic while PA-6 has a monoclinic structure; and its tensile modulus is around 2.9 GPa, while it is a little lower for PA-6. Some of these differences can be traced to the difference in symmetry of their repeat units and to the difference in configuration of functional units at the chains ends. PA-6 generally has one amine and one carboxylic acid group at the end of each chain; whereas, PA-66 contains a mixture of chains that have only amines, only acid groups, or a combination of the two at their ends. Reports in the literature have shown that the differences in end group configuration can lead to significant differences in the morphology and properties of blends with functionalized polymers made from these two materials [10], [11], [12].

Work from the Toyota research laboratories sparked a large interest in PA-6 based silicate nanocomposites. Their papers describe PA-6 nanocomposites made by an in situ polymerization process with superior strength, modulus, heat distortion temperature, and water and gas barrier properties with respect to pure PA-6 [13], [14], [15], [16], [17]. Their results indicate that this process leads to a large number of polyamide molecules, in their nylon 6-clay hybrids (NCH), that are ion-bonded to the silicate layers via the protonated amine chain ends, NH3+, and that the enhancement in mechanical properties can be due to the large surface area and to the ionic bonds between the organic polymer and the inorganic silicate [18], [19].

Interestingly, PA-6 nanocomposites prepared by melt compounding using a twin screw extruder show comparable properties to those prepared by the in situ technique [20]. In melt compounding, one would not expect the amine to be protonated because it is only a physical blending process. Nevertheless, a comparable enhancement in properties is obtained. This preparation process is of great interest because of its enormous advantages for the commercial production of these materials; as has been pointed out in the literature [20], [21]. These prior studies show that the melt viscosity and the residence time in the extruder are very important to obtain a well-exfoliated structure. The degree of exfoliation of the organoclay in the polymer matrix has a direct effect on the modulus and the strength of the nanocomposite [20]. Matrices with higher molecular weight produce a higher degree of exfoliation which improves composite properties such as stiffness and strength with a marginal loss of ductility [21]. This phenomenon is attributed to the higher melt viscosity which translates to a higher shear stress imposed on clay particles inside the extruder.

There is some literature concerning PA-66 nanocomposites, starting with the work by Goettler et al. [22], [23], that analyzes the effect of compounding method, molecular weight, amine/carboxyl end group ratio, and cation exchange capacity of the organoclay on the mechanical properties of nanocomposites made with PA-6, PA-66, blends, and copolymers of PA-6 and PA-66. Some other studies report on the hydrogen bonding, crystallization behavior, thermal stability and flammability, mechanical properties, morphology, and molecular modeling of PA-66 nanocomposites [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34].

To our knowledge, a direct comparison of the morphology and properties of nanocomposites made with PA-6 and PA-66 has not been made using the same melt processing conditions. The purpose of this paper is to analyze the effect of the polyamide type and processing temperature on the mechanical properties and the morphology of the nanocomposites formed from organically modified layered silicates by melt processing. Nanocomposites based on high molecular weight PA-6 and PA-66 were prepared using a twin screw extruder. Mechanical properties, transmission electron microscopy (TEM), wide-angle X-ray diffraction (WAXD), percentage crystallinity, and isothermal thermo-gravimetric analysis (TGA) data are reported. A particle analysis was performed to quantitatively characterize the morphology; these results are later employed in modeling the modulus of these materials using composite theory.

Section snippets

Materials

The materials used in this study are described in Table 1. Extrusion grades of PA-6 and PA-66 were chosen in order to have a high melt viscosity and promote the exfoliation of the organoclay. The organoclay was donated by Southern Clay Products; it was formed by a cation exchange reaction between sodium montmorillonite and octadecyltrimethyl ammonium chloride. The organoclay was selected based on a study, performed in our laboratories, on the effect of organoclay structure on the exfoliation

Results and discussion

In order to make a valid comparison between the extent of exfoliation of the clay platelets in PA-6 and PA-66, it is necessary to do so at comparable process conditions. For example, two key issues are the melt viscosity of the polymer matrix, as outlined by Fornes et al. [21], and the degradation of the organic modifier on the clay. These have a direct effect on the degree of exfoliation of the clay, which in turn affects the mechanical properties of the nanocomposites. PA-6 and PA-66 have

Conclusion

PA-6 and PA-66 nanocomposites were prepared using a co-rotating twin screw extruder. PA-6 nanocomposites had superior mechanical properties than those made from PA-66. The tensile strength of PA-66 nanocomposites deviated from linearity at high levels of MMT. WAXD and TEM results show that the PA-6 nanocomposites are better exfoliated than the PA-66 nanocomposites, which exhibit a mixture of intercalated and exfoliated structures. The mechanical properties are consistent with the morphology;

Acknowledgements

This work was funded by the Air Force Office of Scientific Research and by the Consejo Nacional de Ciencia y Technologia (CONACyT) of Mexico. The authors would like to thank Southern Clay Products Inc. for providing the clay materials and WAXD analyses and Dr Ji-Ping Zhou for his help on the TEM.

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