Elsevier

Polymer

Volume 42, Issue 3, February 2001, Pages 1083-1094
Polymer

Nylon 6 nanocomposites by melt compounding

https://doi.org/10.1016/S0032-3861(00)00380-3Get rights and content

Abstract

Nylon 6–organoclay nanocomposites were prepared via direct melt compounding using a conventional twin screw extruder. The mechanical properties and morphology of these nanocomposites were determined and compared to similar materials made by an in situ polymerization process. The organoclay was well exfoliated into the nylon 6 matrix when compounded with the twin screw extruder but use of a single screw extruder was far less effective. The mechanical properties of the organoclay nanocomposites were significantly increased with marginal decrease of ductility and showed much greater values than glass fiber composites.

Introduction

A variety of inorganic materials, such as glass fibers, talc, calcium carbonate, and clay minerals, have been successfully used as additives or reinforcement to improve the stiffness and strength of polymers. The extent of property enhancement depends on many factors including the aspect ratio of the filler, its degree of dispersion and orientation in the matrix, and the adhesion at the filler–matrix interface. Generally, inorganic materials neither have good interaction with organic polymers to achieve good dispersion nor adequate adhesion, and, as a result, surface treatments are common. Mica-type silicates like montmorillonite, hectorite, and saponite have received a great deal of attention recently [1], [2], [3] as reinforcing materials for polymers owing to their potentially high aspect ratio and unique intercalation/exfoliation characteristics. Such clay minerals have a layer structure (typically ∼1 nm in thickness) which if properly exfoliated can lead to platelets (approaching 1 μm in lateral dimensions) with very high stiffness and strength dispersed in the polymer matrix. To achieve a better interaction with organic polymers, the cations (typically sodium) present on the surface of montmorillonite to balance the net negative charge of the aluminum/magnesium silicate layer are exchanged with organic molecules with a cation group, e.g. alkyl ammonium ions to produce an organoclay.

The incorporation of organoclays into polymer matrices has been known for 50 years. In 1950, Carter et al. [4] developed organoclays with several organic onium bases to reinforce latex-based elastomers. In 1963, the incorporation of organoclay into a thermoplastic polyolefin matrix was disclosed by Nahin and Backlund of Union Oil Co. [5]. They obtained organoclay composites with strong solvent resistance and high tensile strength by irradiation induced crosslinking. However, they did not focus on the intercalation characteristics of the organoclay or the potential properties of the composites. In 1976, Fujiwara and Sakomoto [6] of the Unichika Co. described the first organoclay hybrid polyamide nanocomposite. One decade later, a research team from Toyota disclosed improved methods for producing nylon 6–clay nanocomposites using in situ polymerization similar to the Unichika process [7], [8], [9], [10]. They also reported on various other types of polymer–clay hybrid nanocomposites based on epoxy resin, polystyrene, acrylic polymer, rubber, and polyimides formed using a similar approach [11], [12], [13], [14], [15]. They reported that these polymer–clay nanocomposites exhibit superior strength, modulus, heat distortion temperature, water and gas barrier properties, and with comparable impact strength as neat nylon 6 [13], [15], [16], [17], [18], [19]. Numerous research groups have also described clay nanocomposites based on a variety of polymers including polystyrene [20], [21], [22], epoxy resin [23], [24], [25], poly(methyl methacrylate) [26], polycaprolactone [27], [28], polyolefins [29], [30], [31], [32], polyurethanes [33], polyimides [34], among others.

According to Vaia et al. [20], [21], [35], [36], nanocomposites can be obtained by direct polymer melt intercalation where polymer chains diffuse into the space between the clay layers or galleries. They suggest that this approach can be combined with conventional polymer processing techniques such as extrusion to decrease the time to form these hybrids by breaking up clay particles and increasing sample uniformity.

The incorporation of organoclays into thermoplastic matrices by conventional polymer melt compounding processes is a promising new approach for forming nanocomposites that would greatly expand the commercial opportunities for this technology. If technically possible, melt compounding would be significantly more economical and simple than in situ polymerization processes. This approach would allow nanocomposites to be formulated directly using ordinary compounding devices such as extruders or other mixers according to need without the necessary involvement of resin producers. However, there are very few studies on formation of nanocomposites by direct melt compounding [37], [38], [39], [40]; and, therefore, the corresponding knowledge about this process and what can and cannot be accomplished is still very incomplete. In this process, the rheological and thermodynamic character of the materials can be important parameters that affect the degree of exfoliation and properties of the final composites. The dispersion of filler agglomerates can be achieved when the cohesive forces of the agglomerates are exceeded by the hydrodynamic separating forces applied by the matrix fluid [41]. In the case of the intercalated organoclay, the amount of exfoliation appears to be strongly affected by the conditions of mixing. Generally, the degree of dispersion is governed by the matrix viscosity, average shear rate, and the mean residence time in the mixing process.

In a recent study, we describe the preparation of nylon 6 nanocomposites by melt compounding and investigate their degree of exfoliation and mechanical properties in terms of the type of organoclay, extruder and screw configuration [42]. We have found that the mechanical properties of nylon 6 nanocomposites are affected by the degree of exfoliation, which is dependent on both processing conditions and the clay chemical treatment.

In this paper, we explore nylon 6 nanocomposites formed by melt compounding and compare their properties with other nancomposites and glass fiber reinforced composites. We also investigate optimization of processing parameters such as processing temperature, residence time, and amount of shear.

Section snippets

Experimental

The nylon 6 used in this study is a commercially available material from AlliedSignal with Mn=29,300. The clay minerals were supplied by Southern Clay Products. Sodium montmorillonite (MMT) was received as a fine powder with an average particle size of 7 μm with a cation exchange capacity (CEC) of 95 mequiv./100 g. The organoclay (OCL) was formed by ion exchange of sodium montmorillonite with bis(hydroxyethyl) (methyl) rapeseed alkyl ammonium chloride. It contains 63 wt% of clay mineral. This

Melt exfoliation

Fig. 2shows X-ray powder diffraction patterns for the organoclay powder and composites with nylon 6 formed in the single and twin screw extruders. Fig. 3shows an SEM photomicrograph of organoclay particles (part a) and TEM photomicrographs of nanocomposites prepared in two different extruders (parts b–e). The particle shape of the organoclay powder (Fig. 3a) indicates a plate-like structure with an average particle diameter of about 6 μm. As indicated in Fig. 2, the interlayer platelet spacing

Rheology

For polymer composite systems, the size, shape and concentration of the filler can have a significant effect on the rheological properties in the melt state. Generally, the viscosity of molten reinforced composites exhibit shear thinning behavior at high shear rates; however, at low shear rates the viscosity usually increases with filler concentration and may even show an yield value. However, there have been relatively few detailed studies of the rheological properties of nanocomposites formed

Thermal properties

The thermal properties of nylon 6 and the various composites were determined by differential scanning calorimetry, DSC, and thermogravimetric analysis, TGA. Table 3provides a summary of the results. The DSC measurements indicate that the presence of filler does not affect the Tg of the nylon 6 matrix which occurs at approximately 53°C. The DSC melting peak (Tm) of the composites occurs at a slightly lower temperature than that of the neat nylon 6. This may be related to a slight reduction in

Effect of filler type

Typical stress–strain diagrams for nylon 6 and composites containing 5 wt% of fillers are shown in Fig. 5(at 5.08 cm/min) and Fig. 6(at 0.5 cm/min). A summary of the mechanical properties of these materials is shown in Table 4. The actual amount of mineral in each composite was measured by weighing the residue after burning them. As can be seen from Table 4, regardless of the type of filler, the strength and modulus were substantially increased relative to neat nylon 6 without significant

Conclusions

Nylon 6–organoclay nanocomposites were prepared by melt compounding using a typical co-rotating twin screw extruder and compared with composites containing glass fibers and untreated clay. Transmission electron microscopy and X-ray diffraction indicate that the organoclay was well dispersed into the nylon 6 matrix and the mechanical properties of these materials compare well with those from the literature for nanocomposites formed by in situ polymerization and by melt processing. Melt

Acknowledgements

This material is based in part upon work supported by the Texas Advanced Technology program under Grant number 003658-0017 and 0067. The authors would like to thank Southern Clay Products Inc., for providing clay materials, XRD, TEM analyses, and for many helpful discussions. We also would like to thank the University of Akron for use of the Instron Capillary Rheometer.

References (53)

  • A Okada et al.

    Mater Sci Engng C

    (1995)
  • L Biasci et al.

    Polymer

    (1994)
  • N Ogata et al.

    Polymer

    (1997)
  • T.J Pinnavaia

    Science

    (1983)
  • V Mehrotra et al.

    Mater Res Soc Symp Proc

    (1990)
  • E.P Giannelis

    J Minerals, Metals Mater Soc

    (1992)
  • Carter LW, Hendricks JG, Bolley DS. United States Patent No. 2531396 (1950) (assigned to National Lead...
  • Nahin PG, Backlund PS. United States Patent No. 3084117 (1963) (assigned to Union Oil...
  • Fujiwara S, Sakamoto T. Japanese Kokai Patent Application No. 109998 (1976) (assigned to Unichika K.K.,...
  • Y Fukushima et al.

    J Inclusion Phenomena

    (1987)
  • Okada A, Fukushima Y, Kawasumi M, Inagaki S, Usuki A, Sugiyama S, Kurauch T, Kamigaito O. United States Patent No....
  • Kawasumi M, Kohzaki M, Kojima Y, Okada A, Kamigaito O. United States Patent No. 4810734 (1989) (assigned to Toyota...
  • A Usuki et al.

    J Mater Res

    (1993)
  • Usuki A, Mizutani T, Fukushima Y, Fujimoto M, Fukumori K, Kojima Y, Sato N, Kurauch T, Kamigaito O. United States...
  • A Okada et al.

    ACS Polym Preprints

    (1991)
  • Yano K, Usuki A, Okada A, Kurauch T. United States Patent No. 5164460 (1992) (assigned to Toyota Motor Co.,...
  • K Yano et al.

    J Polym Sci Part A; Polym Chem

    (1997)
  • Y Kojima et al.

    J Mater Res

    (1993)
  • Y Kojima et al.

    J Mater Sci Lett

    (1993)
  • Y Kojima et al.

    J Appl Polym Sci

    (1993)
  • A Usuki et al.

    J Appl Polym Sci

    (1995)
  • R.A Vaia et al.

    Chem Mater

    (1993)
  • R.A Vaia et al.

    Macromolecules

    (1995)
  • A Akelah et al.

    J Mater Sci

    (1996)
  • M.S Wang et al.

    Chem Mater

    (1994)
  • T Lan et al.

    Chem Mater

    (1994)
  • Cited by (1080)

    View all citing articles on Scopus
    View full text