Polyamide-6/Clay Nanocomposites: A Critical Review

Transcription

1 Polyamide-6/Clay Nanocomposites: A Critical Review Polyamide-6/Clay Nanocomposites: A Critical Review Bhuvanesh Gupta, Marie-France Lacrampe and Patricia Krawczak* Ecole des Mines de Douai, Polymers and Composites Technology Department, 941 rue Charles Bourseul, BP 10838, F Douai Cedex, France Received: 2 December 2004 Accepted: 24 May 2005 SUMMARY The plastics business is gathering around nanocomposites because of the unusual performance of these materials in some technologically important applications, such as automotive parts, packaging, textiles and flame resistant materials. The use of clay as the reinforcing component of polymers has transformed the properties of the resultant materials. These nanocomposites are regarded as futuristic. However the science of claypolymer interaction is still a subject of intense investigation. The industrialization of nanocomposites is accelerating and efforts are being made to achieve still better performance by optimising of their compounding and processing conditions. Nanocomposites based on many different polymers have been, or are being developed. The current scenario in relation to polyamide-6/clay nanocomposites is presented in this article. Preparation, processing and application issues are addressed in particular. 1. INTRODUCTION Nanocomposites are gaining acceptance in the mainstream of the global plastics processing industry and there has been a great recognition of their enormous potential in relation to a wide range of product development. The term nanocomposite relates to the dispersion of nano-sized particles within the polymer matrix 1. In the last few years, a large number of nanomaterials consisting of particles measuring a billionth of a metre (one nanometre) or less have been commercialised. The new polymeric materials made by using these particles in an appropriate matrix may be classified as advanced materials because of their superior tensile strength, modulus and dimensional stability compared with conventional composite materials. The nanoparticles are so small and their aspect ratio is so high that the material properties improve even when the filler component is very low compared to conventional reinforcements like talc and glass. The nanocomposite materials also offer lower permeability to moisture, gases and hydrocarbons along with significant improvements in thermal behaviour, flame resistance, ionic conductivity, chemical resistance and bulk clarity 2-4. *Author to whom correspondence should be sent Rapra Technology Limited, 2006 Nanocomposites have been attracting attention since the 1990 s when Toyota, for the first time, produced polyamide-6/clay nanocomposites, used them to manufacture timing belt housings for the automotive industry, and demonstrated their unprecedented thermo-mechanical properties 5,6. This event proved to be the signal for a great deal of international academic and industrial research. Several other nanofillers, such as mica, graphite and carbon tubes are now also being exploited. Wu et al 7 have carried out a comparative evaluation of some of these nanoadditives. However, huge interest has arisen in layered silicate (clay) nanocomposites based on thermoplastics, elastomers or thermosets. In spite of initially slow progress, there has since been tremendous growth and success in product development, with significant achievements in the plastics processing industry. These new materials have already brought so many innovations in technologically important domains that they are expected to shake up the industrial world, for example in packaging industry (e.g. barrier films), automotive and aerospace industries (e.g. high strength polymers), textiles, and fire resistant or abrasion resistant materials. Figure 1 illustrates the worldwide market potential for nanocomposites. An annual growth rate of about 20% is forecast over the next five years, with a dominating position for thermoplastics 8. It is expected that the industry will be using tonnes of nanocomposites in 2010 compared to 1500 tonnes today 9. Polymers & Polymer Composites, Vol. 14, No. 1,

2 Bhuvanesh Gupta, Marie-France Lacrampe and Patricia Krawczak Figure 1. Worldwide futuristic scenario of nanocomposites (sales by value) 8 $ millions Thermoplastic Thermoset Year Nanocomposites have been or are being developed based on several different thermoplastics. Among them, considering its relatively low cost and high performance, polyamide-6 (PA-6) is an interesting polymer with several potential or actual applications, such as in films, textiles and engineering parts. The nanocomposites based on PA-6 are also attracting significant interest since they exhibit unexpected hybrid properties, synergistically derived from their two components 10,11. The PA-layered aluminosilicate nanocomposites have been quite promising because they offer exceptional reinforcement at a very low filler concentration (usually lower than 10 wt%). Typically, the type of silicate used in polymersilicate nanocomposites is montmorillonite (MMT), a 2:1 layered smectite clay, and extensive studies have been reported on the development of nanocomposites based on such PA/clay hybrids This type of clay is an inorganic mineral and has a natural platy structure with the individual platelets having a thickness of 1 nm and a width and length of several tens of nm, which explains their very high surface area ( m 2 /g) and aspect ratio (length/thickness > 100). Associating this type of nanofiller with a polymer matrix is a key challenge. Montmorillonite has a layered structure, where the silicate layers have sodium or calcium ions in the interlayer space. During nanocomposite fabrication, these spaces have to be filled with polymer, causing the clay to swell and increase the distance between the layers to an intercalated state once the electric charges and Van der Waals forces have been sufficiently lowered (Figure 2). Other types of clay such as saponite, laponite or boechmite may be used as well. However, the expected performance improvement can only be achieved when the clay platelets are completely dispersed, without aggregates in the matrix. The challenging issues of PA-6/clay nanocomposite preparation are therefore intercalation (introduction of polymer between the clay platelets), exfoliation (separation of individual platelets leading to an homogeneous dispersion within the matrix), and control of the polymer-filler interaction, which may be favoured by clay surface treatment with organic ions, so as to make it hydrophobic, and by adding compatibilisers. The resulting nanocomposite has a polymorphic structure, which may be modified by annealing and by changing the filler nature or amount. Processing conditions at high temperatures and high shearing rates are also likely to induce thermal degradation and preferential orientation of the platy structured fillers. These key issues, together with the possible applications and performance of polyamide-6/clay nanocomposites, are addressed in the following sections. 2. PREPARATION AND PROCESSING OF POLYAMIDE-6/CLAY NANOCOMPOSITES 2.1 Intercalation and Exfoliation Various efforts have been made to prepare nanocomposites by different routes 4. One promising strategy is the intercalation of monomers or polymers into the silicate hosts followed by exfoliation (Figure 3) 7. The term intercalation describes the case where a small amount of polymer moves into gallery spacing (interlayer) between the clay platelets to a level of 2-3 nm. The exfoliation or delamination 14 Polymers & Polymer Composites, Vol. 14, No. 1, 2006

3 Polyamide-6/Clay Nanocomposites: A Critical Review Figure 2. Polymer/clay nanocomposites: intercalated and delaminated or exfoliated structures Figure 3. Schematic route to the PA-6/clay nanocomposites preparation 7 Clay Na + Na + Na + Clay Intercalation CL/water Exfoliation Polymerisation Nylon-6 Intercalation/ exfoliation CL stands for caprolactum Nanocomposite occurs when the polymer further separates the clay platelets (e.g nm or more). The initial preparations were confined to the polymerisation of caprolactam in the presence of organophilic montmorillonite ions, exchanged with ammonium ions 10,17. This in-situ process was further improved by Liang et al 18, who dispersed clay into molten caprolactam at 90 C for 2-4 h and then carried out polymerisation at 260 C for 10 h under a nitrogen blanket in a reactor. However, although it was taken up by the Toyota Research group, the in-situ process has not been used in practice any more because of the difficulty of finding a suitable monomer and a compatible solvent system. So others elaboration strategies (such as melt compounding) have been designed and commonly used for commercial products, even if nanocomposites made by the insitu process have been found to out-perform other ones by a significant margin 19. The incorporation of clay into thermoplastics by melt compounding using a conventional extrusion process has become the favoured practice and will greatly influence commercial opportunities for this technology in the near future 20. Here, organophilic clay (organo-clay) is necessarily used to yield exfoliated compounds because the silicate layers do not exfoliate during compounding without organo-modification. The advantages of forming nanocomposites by melt processing are very interesting. The process does not require a solvent, so it is an eco-friendly approach. Moreover, it shifts nanocomposite production downstream, thereby giving end-use manufacturers freedom with respect to the final product specification. Efforts are currently directed to the optimisation of melt compounding conditions so as to achieve Polymers & Polymer Composites, Vol. 14, No. 1,

4 Bhuvanesh Gupta, Marie-France Lacrampe and Patricia Krawczak an homogeneous dispersion of the clay exfoliated platelets within the polymer matrix. For example, the influence of the extruder type and the screw design (single- or twin-screw, co-rotating or counterrotating, intermeshing or non-intermeshing, low/ medium/high shear intensity obtained according to screw geometry) has recently been investigated 19,21. It has been demonstrated that the degree of exfoliation (delamination) and dispersion of organomodified layered silicate nanocomposites (PA-6/ montmorillonite) is affected by the compounding conditions. For example, increasing the mean residence time in the twin-screw extruder generally increases the delamination and dispersion. However, there appears also to be an optimum shear intensity and an optimum extent of back mixing (a measure reflecting the broadness of the residence time distribution). Representative X-ray diffraction scans and transmission electron microscopy (TEM) micrographs are shown on Figures 4 and 5, while Table I summarises some results obtained over a wide range of process conditions. Further investigation are still required to differentiate the effects of shear intensity, mean residence time and mean residence time distribution, as well as feed rate, temperature profile and screw rpm, which are all critical to achieve the desired exfoliation 21. However, the compounding process is quite expensive. So natural (virgin) clay minerals without organo-modification have been studied with the aim of further developing the industrial process. Under such circumstances, a compatibiliser is needed to achieve the desired characteristics, and the clay minerals and polymers are compounded with alkylammonium salts and maleic anhydride 22,23. Recently, a novel compounding process has been reported using a sodium montmorillonite slurry in PA-6 where the silicate clay layers exfoliate and disperse homogeneously at the nanoscale level 24. The sodium montmorillonite is first blended with molten PA-6 in an extruder. Water is removed from the vent by applying a vacuum. The removal of water also helps to prevent the PA-6 degradation. 2.2 Polymorphism The evolution of polymorphism in polyamide nanocomposites has been observed by various authors PA-6 has two major crystalline phases (a monoclinic α-phase and a pseudoclinic hexagonal γ-phase) 29. The polymorphic structure of polyamides results from the different spatial arrangement in the hydrogen bonding between the oxygen in the carbonyl group of polyamide molecular chain and the hydrogen attached to the nitrogen in the neighbouring polyamide molecular chain 30. The principal difference between the two phases is the molecular packing. The α-phase consists of fully extended planar zigzag chains in which adjacent antiparallel chains are joined by the hydrogen bonds. Therefore, it is thermodynamically the most stable crystalline phase and can be obtained during slow cooling from the melt. The γ-phase consists of pleated sheets of parallel chains joined by hydrogen bonds. This is a less stable phase and is obtained during fibre spinning at high speed, or fast cooling 31. The γ-phase can be converted into the α-phase by melting and subsequent crystallization. If the polymer is crystallized isothermally, it shows an amorphous halo in the X-ray diffraction pattern, with a maximum at Evolution of its crystalline nature takes place with increasing annealing temperature, finally leading to the two-peak diffraction pattern 25. Figure 4. Wide angle X-ray diffraction scans for PA-6/3,1 wt% montmorillonite organoclay (non-optimised chemical treatment) prepared by melt compounding using different extruder/screw configurations Polymers & Polymer Composites, Vol. 14, No. 1, 2006

5 Polyamide-6/Clay Nanocomposites: A Critical Review Figure 5. Transmission electron microscopy view for PA-6/3,1 wt% montmorillonite organoclay (non-optimised chemical treatment) prepared by melt compounding using different extruder/screw configurations 21 Single screw Co-rotating, low shear Co-rotating, medium shear Counter-intermeshing, medium shear Table 1. Influence of different extruder/screws configuration on PA-6/montmorillonite organoclay nanocomposites prepared by melt compounding 21 Extruder and screw type X-ray diffraction basal spacing (10-1 nm) X-ray diffraction area under curve TEM platelets or intercalates per 6.25 cm 3 (at X) Extruder mean residence time (s) Normalized variance (measure of back mixing) Single screw extruder Clay treatment A * Clay treatment B * Twin screw extruder Co-rotating intermeshing Low shear/clay treatment B Medium shear/clay treatment B Counter-rotating intermeshing Low shear/clay treatment B Medium shear/clay treatment B Medium shear/clay treatment A No Peak No Peak High shear/clay treatment B Counter-rotating non-intermeshing Low shear/clay treatment B Medium shear/clay treatment B No Peak No Peak High shear/clay treatment B * Montmorillonite clay chemical treatments A and B correspond to optimised and non-optimised organomodifications respectively. The organoclay A is designed to easily delaminate and disperse in PA-6 (better compatibility). The mineral content of the nanocomposites is 3,7 and 3,1 wt% for A and B respectively Polymers & Polymer Composites, Vol. 14, No. 1,

6 Bhuvanesh Gupta, Marie-France Lacrampe and Patricia Krawczak The scenario changes when PA is blended with nanoclay 15. The cooling pattern of PA and its nanocomposites is distinctly reflected in its X-ray diffraction behaviour. The X-ray diffraction pattern of virgin PA-6 and its nanocomposites shows that the α 1 and α 2 peaks are very prominent (Figure 6, plot a). The γ-phase shows a single peak at 21.5 (plot b). The slow cooling of the nanocomposite shows both α and γ-phases in the polymer (plot c). However, quench cooling of the composite shows a peak in the γ-phase, with virtually no evidence of the α-phase (plot d). These observations are well supported by subsequent studies by Wu et al 28 on PA-saponite nanoclay composites, where the clay addition changes the crystalline pattern of the nanocomposites. The addition of a small amount (2.5 wt%) of clay to the PA matrix leads to two crystalline peaks at 20.5 and 24, with no indication of a γ-phase. However, during fast cooling of the same film, a γ-phase peak also arises. It is important to mention that the high saponite content (5 wt%) in PA-6 favoured a sharp γ- crystalline peak and only traces of α-peaks during the fast and slow cooling of films. The authors proposed that the higher saponite contents promote heterophase nucleation of the γ-phase. An important finding of the study is that even the quenching of the nanocomposites leads to a matrix with the same crystallinity as pure PA. The polymorphic behaviour of PA-6/montmorillonite nanocomposites depends on the isothermal crystallization temperature 26. With increasing crystallization temperature, the α-phase in PA-6 converts steadily into the γ-phase and increases the perfection of the crystallites. Fourier Transform Infrared Spectroscopy (FT- IR) studies on monofilaments have confirmed the above observations that the cooling pattern has a strong influence on phase development in nanocomposites 15. Linkolm et al 32 suggested that the addition of clay layers forces the amide groups of PA-6 out of the plane formed by the chains. This results in conformational changes to the chains, which limits the formation of H-bonded sheets, so the γ-phase is favoured. The annealing process is thus an important factor controlling the phase transition in nanocomposites 26. Fast cooling such as quenching leads to a different X-ray diffraction behaviour for PA and PA nanocomposites. The PA-6 sample quenched in liquid nitrogen is amorphous (Figure 7a). Annealing at 100 C induces crystallinity in the polymer and a diffraction peak appears at 2θ=21.8 which is contributed by the γ-crystalline peak of PA-6. Subsequent annealing at 120 C introduces two additional peaks at 2θ=20.3 and 23.7, which are characteristic of α 1 and α 2, respectively. As the annealing temperature increases to 160 C, the α- peaks become more visible and the γ-peak begins to Figure 6. X-ray diffraction patterns of (a) α-phase PA-6; (b) γ-phase PA-6; (c) PA-6 nanocomposite is cooled down in oil bath from 250 to 20 C by natural convection; (d) PA-6 nanocomposite is removed from the 250 C oil bath and quenched in a water bath at 20 C; curves were vertically offset for clarity Polymers & Polymer Composites, Vol. 14, No. 1, 2006

7 Polyamide-6/Clay Nanocomposites: A Critical Review Figure 7. X-ray diffraction patterns of (a) PA-6 and (b) PA-6/clay nanocomposites annealed at different temperatures 26 Annealing produces isothermal crystallization, which has recently been investigated by Zhao et al 25. These authors showed that the overall crystallinity increases as the crystallization temperature (annealing temperature) increases (Figure 8). This is essentially related to the segmental mobility, which increases at higher temperatures. The variation in crystallinity takes into consideration both the α and γ-phases. Above all it was observed that there is a gradual transition of the γ-phase to the α-phase in pure PA-6. However, the addition of montmorillonite to the PA-6 at various temperatures results in a strong tendency of the PA-6 to crystallize, and the α-peaks become dominant at higher annealing temperatures. 2.3 Polymer/Clay Interactions Clay Nature and Content The nature of the clay plays a crucial role in structural developments relating to PA-6 based materials. Wu et al 33 have carried out an interesting study on nanocomposites based on montmorillonite and saponite clays. In contrast to dioctahedral smectites with predominantly octahedral substitution in the montmorillonite, the saponite is composed of trioctahedral smectites with mainly isomorphous substitution of Si 4+ by Al 3+ in the tetrahedral sheets. For this reason, the montmorillonite structure is in the form of hexagonal lamellae, while the saponite structure is in the form of ribbons and laths 34. A comparison of these two types of clay in PA-6 is depicted in Figure 9. decrease. With a subsequent increase in temperature to 180 C, the α-peaks become more prominent. However, at 200 C, α-peaks still dominate and the γ-peak becomes smaller. In nanocomposites, the situation is a sharp contrast to that of PA-6. The quenched sample is still crystalline and characterized by a sharp γ-peak (Figure 7b). The annealing up to 140 C still maintains the γ character. The two diffraction peaks arise when the annealing temperature is further enhanced to 160 C and α- peaks become more prominent at the temperature of 180 C. However, the γ diffraction peak dominates and at 200 C an almost complete transformation of the α-form to the γ-form is evident. This is a very interesting observation because the polymer would undergo significant changes in crystalline structure, depending on the processing temperature and the residence time at that temperature, during moulding. PA-6 samples were prepared by hot pressing at 240 C and then cooled down to room temperature at 10 C/min. As already mentioned in the previous section, PA-6 contains multi-crystalline forms and usually exhibits a more stable α-crystalline state in the form of two α-crystalline peaks at 2θ=20.5 and 24 along with an additional distinct diffraction peak at 2θ=21.5 characteristic of the γ-phase (Figure 9, curve a). The amount of saponite clay plays a key role in the structural development of the nanocomposites. The addition of 2.5 wt% synthetic saponite to the PA-6 matrix leads to a diffraction pattern with two α-crystalline peaks, without any indication of the γ-phase diffraction peak (curve b). As the saponite content increases to 5 wt%, the X-ray diffraction pattern exhibits a sharp γ-crystalline peak and small traces of two α-crystalline peaks (curve c). However, these diffraction patterns are very different from the one obtained with montmorillonite clay. Here, the X-ray diffraction pattern for 2.5 wt% clay content Polymers & Polymer Composites, Vol. 14, No. 1,

8 Bhuvanesh Gupta, Marie-France Lacrampe and Patricia Krawczak Figure 8. Variation of crystallinity as a function of crystallisation temperature in PA-6/montmorillonite nanocomposites 25 Figure 9. X-ray diffraction scans for (a) PA-6, (b) 2.5 wt% saponite in PA-6/clay nanocomposites, (c) 5 wt% saponite in PA-6/clay nanocomposites, (d) 2.5 wt% montmorillonite in PA-6/ clay nanocomposites and (e) 5 wt% montmorillonite in PA-6/clay nanocomposites after being hot pressed into films and slowly cooled to room temperature 33 shows only one sharp α-crystalline peak at 2θ=24 and traces of another α-crystalline peak at 2θ=20.5. There is no γ-crystalline peak. However, as soon as the montmorillonite content increases to 5 wt%, an additional γ-crystalline peak also appears at 2θ=21.5 on the X-ray diffraction pattern Organo-Modification Montmorillonite is hydrophilic, which makes exfoliation very difficult. Modification of the clay with organic agents is therefore needed to enlarge the interlayer spacing of the silicate sheets 20 Polymers & Polymer Composites, Vol. 14, No. 1, 2006

9 Polyamide-6/Clay Nanocomposites: A Critical Review and to introduce hydrophobicity, giving better compatibility with the polyamide. PA-6 and may be reflected in the high mechanical strength of the nanocomposites. The very first step in nanocomposites fabrication is thus the hydrophobation of the clay. The sodium ions are replaced by alkyl ammonium salts by means of a cation exchange process. As a result, intercalation or delamination of the clay is achieved 35,36. During the processing of nanocomposites, these montmorillonite silicate layers are exfoliated and are dispersed into the matrix homogeneously at the nanometre scale. Some of the organic substances used to form organo-clays by ion exchange with sodium montmorillonite are presented in Figure The ammonium ion usually has one or more alkyl tails, formed from naturally occurring oils. These tails have a low level of unsaturation. The other groups on the nitrogen atom may be methyl, hydroxyethyl or benzyl, etc. Another amine, 12-aminodecanoic acid (ADA), which is suitable for direct food contact applications, has been used as the surface modifier in nanocomposite preparation by in-situ polymerisation 6. It is claimed that the additive offers additional advantages in terms of the tethering effect, which enables the nanoclay s organically modified surface to link up with the polyamide chain during polymerisation. This is an attractive feature of the ADA additive, because this enhances the compatibility of the montmorillonite with the Extensive X-ray diffraction studies have been carried out to investigate structural changes arising out of blending clay with PA-6 25,38. It has been found that montmorillonite shows a well-defined diffraction pattern, as presented in Figure On organic modification of montmorillonite, the peak shifts to lower value (4.6 ). Once montmorillonite (10 wt%) is added to PA-6, the intercalated matrix shows a diffraction peak shifting to a lower angle (2θ=1.85 ). With a subsequent decrease in montmorillonite content to 5%, the diffraction peak for montmorillonite disappeared completely, suggesting that the intercalated silicate layers are exfoliated into nanoscale layers and are randomly dispersed in the PA-6 matrix. So the key task is not only to design these nanomaterials for various applications, but to monitor structural changes within the matrix, which play a crucial role in the ultimate product characteristics. The modifying agent in montmorillonite can affect the ultimate nanocomposite structure. Ma et al 39 investigated the influence of organo-modification using n-dodecylamine, 12-aminolauric acid and 1,12-diaminodecane. Based on X-ray diffraction and differential scanning calorimetry (DSC), it was observed that these materials are intercalated into the nanocomposites and they influence the Figure 10. Structure of some organic amines used to form organo-clay by ion exchange with sodium montmorillonite (C = cocoa (C 12 ), T = tallow (C 18 ), R = rapeseed (C 22 )) 37 Polymers & Polymer Composites, Vol. 14, No. 1,

10 Bhuvanesh Gupta, Marie-France Lacrampe and Patricia Krawczak Figure 11. X-ray diffraction patterns for montmorillonite (a), organically modified montmorillonite (b), PA-6/ montmorillonite 90/10 (c) and 95/5 (d) 25 Figure 12. DSC second heating scans for (a) PA-6, (b) 2.5 wt% saponite in PA-6/clay nanocomposites, (c) 5 wt% saponite in PA-6/clay nanocomposites, (d) 2.5 wt% montmorillonite in PA-6/ clay nanocomposites and (e) 5 wt% montmorillonite in PA-6/clay nanocomposites after cooling from 300 C to room temperature at 10 C/ min 33 crystallization process. The increase in clay content led to a decrease in the nanocomposite crystallinity, perhaps because the clay inhibits chain mobility, as evident from the crystallinity decrease (from 35% to 26%) observed when organo-clay was added. The two phases of PA-6 are known to have different thermal histories, as the α-phase of PA-6 melts at a higher temperature (221 C) than the γ-form (215 C). Therefore, critical information regarding the microstructure of nanocomposites may also be achieved from DSC investigations 7,33,36, which can quite often be correlated to the X-ray diffraction patterns 33. Variation in the structure of PA and its nanocomposites has been reported by different authors, which may be due to the large differences in processing conditions DSC heating scans of PA-6 and montmorillonite- and saponite-based PA-6 nanocomposites are presented in Figure 12. The virgin PA-6 has two melting peaks; the highest temperature peak corresponds to the α-crystalline form and the lowest to the γ-crystalline form. The addition of montmorillonite or saponite clay (2.5 wt%) to PA-6 leads to the elimination of the low temperature peak and leaves behind only one high temperature peak corresponding to the α-form. As the amount of saponite increases to 5 wt%, the DSC thermograms are altered. Two melting peaks corresponding to the α and γ-crystalline forms appear. However, the presence of 5 wt% montmorillonite in the nanocomposites still leads to a single melting peak, corresponding to the α-crystalline form of PA- 6. These observations are in agreement with those from the X-ray diffraction patterns suggesting that the saponite induces heterogeneous nucleation of PA-6. Other research groups have, on the other hand, observed a single peak thermogram for virgin PA in the DSC scan, comprising of the α-form. However, the addition of montmorillonite clay to the PA-6 changes the melting pattern of the polymer 42. These authors observed that the single melting peak of the α-crystalline form at 221 C was transformed into a dual peak endotherm where the predominant first peak at 213 C and a smaller peak at 220 C correspond to the melting of the crystalline γ and α-crystalline forms, respectively 41, Compatibilisers The interfacial adhesion between the clay surface and PA-6 is an important aspect of nanocomposite fabrication. The PA-6/clay nanocomposites (PA-6CN) show some brittleness induced by the addition of filler to the polymer. Therefore, a compatibiliser that would be able to improve interfacial interaction between the two components may be thought to lead to a final 22 Polymers & Polymer Composites, Vol. 14, No. 1, 2006

11 Polyamide-6/Clay Nanocomposites: A Critical Review product having the desired properties. Liu et al 43 have investigated PA-6/clay nanocomposites containing polypropylene-g-maleic anhydride (PP-g-MAH) as the compatibiliser. Although those nanocomposites with a compatibiliser showed slightly lower tensile strength and tensile modulus, the impact strength was improved significantly and furthermore it increased with the compatibiliser content to as high as 30%. As an example, it increased from 23 J/m with no compatibiliser to 129 J/m for 30% compatibiliser in the nanocomposite (Table 2). 2.4 Thermal Degradation Thermal degradation during the processing of nanocomposites is a common problem. The polymer/clay nanocomposite preparation is carried out at high temperatures, irrespective of the fabrication route. If the processing temperature is higher than the thermal stability of the organic substances, decomposition of the constituent will take place. Thermogravimetric analysis (TGA) coupled with Fourier Transform Infrared Spectroscopy is a powerful tool for investigating thermal degradation patterns 44, which change completely after organomodification of the clay. The major difference between the two configurations lies in the temperature range of C, where the organoclay undergoes maximum degradation whereas virgin clay loses little weight. This suggests that the organic components in the modified clay are highly vulnerable to thermal degradation. This needs to be considered during the processing of polyamides at elevated temperatures. The evolution of the cyclic monomer (caprolactam) and other volatile gases like carbon dioxide or ammonia were observed in these materials. However, the onset temperature (for 5% weight loss) of the nanoclay composite was slightly enhanced to 404 C compared with virgin PA (400 C) but only for clay contents of 2.5 wt%. Interestingly, for the higher clay contents of 5 and 7.5 wt%, the onset temperature remained unchanged. It is supported by the studies of Liu et al 45 who observed distinct clay agglomeration in nanocomposites with high clay contents, whereas a well-exfoliated structure was observed with 2.5 wt% clay. It seems that morphological and interactive features play a crucial role in the degradation behaviour of the nanocomposite. At low clay contents of 2.5 wt%, the nanocomposite exhibits an interactive structure with an exfoliated matrix. At higher contents, the clay undergoes agglomeration and does not participate in homogeneous interactive bonding with PA-6. Studies of Kashiwagi et al 46 have also shown that the thermal stability of PA-6 nanocomposites does not differ much from that of virgin PA-6. Using clay contents of 2 and 5 wt%, hardly any appreciable change in the degradation behaviour was observed, except for a very little weight loss in the nanocomposite at a temperature of about 350 C. This could arise from the thermal degradation of the organic component in the clay. Forne et al 47 have carried out an interesting study on the degradation of the polymer matrix during melt processing. An excellent review of the degradation of alkyl ammonium montmorillonite organo-clay has been made by Xie et al 48. The thermogravimetric analysis (TGA) shows that the organic component of the organo-clay begins to break down at temperatures as low as 180 C in an inert atmosphere. This leads to chemical changes in the additive, and may affect the polymer melt intercalation and interfacial bonding between the two components. It has been found that the extent of degradation depends on both the type of PA-6 and the nature of the organoclay 49. High molecular weight PA-6 was found to experience more matrix degradation, as well as colour formation. The degradation stems from a chemical reaction between the organo-clay surface Table 2. Properties of the PA6CN/PP-g-MAH alloys 43 Properties of PA6CN/PP-g-MAH (wt:wt) = 100/0 90/10 80/20 70/30 0/100 Tensile strength (MPa) Tensile modulus (GPa) Flexural strength (MPa) Flexural modulus (GPa) Elongation at break (%) Notched Izod impact strength (J/m) Polymers & Polymer Composites, Vol. 14, No. 1,

12 Bhuvanesh Gupta, Marie-France Lacrampe and Patricia Krawczak and the polyamide. Therefore, any processing that offers greater dispersion of the organo-clay in the base polymer will lead to higher degradation. This is believed to arise from increased exposure of the organo-clay surface to the PA-6 as a result of increased platelet exfoliation. The chemical nature of the organo-clay is extremely important in introducing polymer degradation, depending on the level of unsaturation present in the organic component. The higher the number of double bonds, the greater the degradation and the deeper the colour formation. The molecular weight can decrease significantly, sometimes as high as 32%. A mechanism for clay degradation has been suggested and is presented in Figure The alkyl ammonium organo-clay decomposes to produce α-olefins, amines and other products where the elimination of the ammonium modifier takes place by substitution with a H-atom on the β-carbon. This mechanism has been supported by Davis et al as well 50. The organic part on the clay is more destructive when it is based on natural products such as coco, which have long unsaturated chains. This organo-clay degradation seems to be a regular feature, irrespective of the nature of the base polymer, and has been observed in other nanocomposites as well 51. However, the degradation can be minimised by using certain antioxidants. The crucial properties, such as the mechanical strength, are not much affected and colour formation in stabilised nanocomposites is almost the same as, or a little less than it is in unstabilized nanocomposites. Organo-clays contain a small amount of absorbed water that can also hydrolyse the polyamide at higher temperatures when processed. Davis et al 50 have observed that the melt processing of PA-6 at around 300 C leads to significant degradation of the nanocomposites, attributed mainly to bound water or to the dehydroxylation of the clay itself. Therefore, nanocomposite degradation during processing can be envisaged the cumulative effect of thermal, hydrolytic and induced decompositions. 3. PROPERTIES AND APPLICATION AREAS Some of the most promising applications of nanocomposites are currently found in the automotive industry and in the deployment of high thermo-mechanical behaviour polymers, barrier films for the packaging industry and fire resistant materials and textiles. There are also some interesting developments in medical products, abrasion resistant materials and UV protection systems (Figure 14). Although the addition levels are less than 10 wt%, for every 1 wt% addition, a property improvement of about 10% is usually induced. This filler content to performance ratio is known as the nano-effect Automotive Parts Nanocomposites made of polymers reinforced with exfoliated clay or carbon nanotubes are being considered for interior and exterior automotive parts. Although most of the current automotive market involves PP nanocomposites, PA-6 ones Figure 13. Proposed Hoffmann elimination reaction for organically modified montmorillonite (MMT) Polymers & Polymer Composites, Vol. 14, No. 1, 2006

13 Polyamide-6/Clay Nanocomposites: A Critical Review are slowly gaining acceptability in this sector. Toyota opened up the use of nanocomposites in the automotive industry, and the resulting new materials are capable of achieving 40% higher tensile strength, 68% higher tensile modulus, 60% higher flexural strength, 126% higher flexural modulus, than unfilled resins, while also having increased heat distortion temperatures (raised from 65 C to 152 C). The water permeability also decreases by 40% 13. These property improvements are attributed to the fact that a significant volume of polymer chains exfoliated clay lamellae, which have a high aspect ratio. The advantages of PA-6/clay nanocomposites (2.5 or 5 wt% clay) over virgin PA-6 are evident from Table 3. In general, montmorillonite-based nanocomposites have slightly better mechanical strength and modulus than saponite-based nanocomposites. The major difference lies in the elongation. PA- 6/saponite nanocomposites are characterised by elongations over 100%, whereas that of PA- 6/montmorillonite nanocomposites is less than 10%. The heat distortion temperature of the matrix is increased by 100 C in montmorillonite-based nanocomposites, whereas only a slight improvement is observed in the saponite-based nanocomposites. This confirms the essential role of the clay in the structural development of the two nanocomposites. Liu et al 52 have reported a significant and sharp improvement (from 3 GPa to ~7 GPa) in the tensile modulus with the increase in clay content from zero up to 17 wt% (Figure 15). The mechanical deformation of PA-6 is controlled by its orientation and by the slippage of the crystalline regions past each other along the plane 53. It is still an open question whether the crystallinity or crystalline form developing during the processing and fabrication of the nanocomposites has any influence over the mechanical properties of the material. The crystallinity of the polymer changes only marginally on the addition of clay to PA (It is only the polymorphism that changes in the nanocomposites). Earlier discussion in this paper reveals that the addition of clay leads to variations in the α- and γ-forms of the crystalline phase. However, the tensile modulus keeps on Figure 14. Application areas of nanocomposites Table 3. Mechanical and thermal properties of PA-6 and PA-6 nanocomposites 33 PA-6 PA-6/montmorillonite PA-6/saponite Clay content (wt%) Tensile strength (MPa) Tensile modulus (GPa) Flexural modulus (GPa) Notched Charpy impact strength (kj/m 2 ) Heat distortion temperature ( MPa) Elongation at break (%) >100 <10 <5 >100 >100 Polymers & Polymer Composites, Vol. 14, No. 1,

14 Bhuvanesh Gupta, Marie-France Lacrampe and Patricia Krawczak Figure 15. Effect of montmorillonite clay content on tensile modulus of PA-6 nanocomposites 52 showing an improvement as the amount of clay increases in the nanocomposites. It is therefore suggested that neither the overall crystallinity nor the polymorphism has any influence on the tensile strength improvements. It is the reinforcing effect of the clay fillers that plays a key role in improving the tensile strength of the nanocomposites. This viewpoint is also strengthened by the studies of Bureau et al 40. These authors prepared various samples by altering the processing conditions so that specimens with different crystalline forms and crystallinity levels were obtained. The samples were quenched PA-6, annealed PA-6, regular PA-6 and PA-6/clay nanocomposite (2 wt% organo-clay montmorillonite intercalated with ω- aminododecanoic acid). Surprisingly, neither the quenched nor the annealed samples showed any α-crystalline peak. The regular PA-6 had two peaks at 20 and 24, suggesting a high percentage of the α-crystalline form, and the nanocomposite had peaks for γ-phase as well. The tensile properties of these materials are presented in Figure 16 and Table 4. The quenched sample shows ductile behaviour and undergoes very high deformation (670%) along with significantly lower Young s modulus and yield stress. The annealing enhances the modulus and reduces the elongation to 120% besides being credited with a much higher level of crystallinity. The regular sample shows a still higher modulus but the yield stress is almost identical to that of the annealed one. This means that the higher content of the α-phase of the annealed sample does not alter the tensile strength. The higher modulus therefore is ascribed to the high crystallinity of the sample. Figure 16. Tensile stress vs. strain curves of the regular, quenched and annealed PA6 and the regular PA6/ montmorillonite clay with strain scales: (a) enlarged for low strains and (b) complete curves Polymers & Polymer Composites, Vol. 14, No. 1, 2006

15 Polyamide-6/Clay Nanocomposites: A Critical Review Table 4. Tensile test results for PA-6 and PA-6/ 2 wt% montmorillonite clay 40 Materials PA6 Moulding conditions Quenched Annealed Regular Young s Modulus (GPa) 2.4 ± ± ± 0.3 Yield Stress (MPa) 69 ± 2 91 ± 2 93 ± 1 Cold Drawing stress (MPa) 32 ± 2 51 ± 2 66 ± 2 Strain at Break (%) 670 ± ± ± 40 Crystallinity (%) PA6/clay 4.7 ± ± ± The nanoclay-based composite shows the highest rigidity and its elongation also goes down to 3% in spite of lower crystallinity, indicating the reinforcing effect of the nanofiller in the PA composite. The contribution of boechmite clay as reinforcing agent in PA nanocomposites has been explored by Ozdilek et al as well 54. Liu et al 55 have also reported that the mechanical properties are further improved if the montmorillonite clay is co-intercalated with a quaternary ammonium compound and an epoxy resin together. These authors explain that the exfoliation is much better in the epoxy containing organophilic clay, which is associated with the strong interaction between epoxy resin and amide end groups of PA-6. However, Shelley et al 56 have noticed a 200% increase in the tensile modulus and a 175% improvement in yield stress in stretched sheets of 5 wt% clay nanocomposites, and have attributed these variations to the reinforcement of PA-6 by clay particles, involving the complexation of midchain carbonyl groups with the exfoliated clay lamellae. Impact strength is an important characteristic of composites for laminated structures. The impact strength of nanocomposites remains similar to that of virgin PA-6 up to 5 wt% clay content (Table 3) and thereafter tends to decrease significantly. At 20 wt% clay the material becomes very brittle 19. The notched Izod impact strength of PA-6 at different organo-clay contents is reported on Figure 17 as a function of temperature. As the temperature is increased to reach the PA-6 glass transition temperature region, a brittle to ductile transition takes place. This brittle-ductile transition temperature is significantly enhanced by the increase in clay content. Above the glass transition temperature, the nanocomposites are super-tough, indicating high ductility. However, the advantages in thermo-mechanical properties are still frequently overshadowed by the deteriorating impact behaviour. Clearly, this is an issue that requires consideration wherever resistance to impact loading is required. Figure 17. Notched Izod impact strength of polyamide-6/organoclay nanocomposites as a function of temperature and clay content (0 to 20 wt%) 19 Polymers & Polymer Composites, Vol. 14, No. 1,

16 Bhuvanesh Gupta, Marie-France Lacrampe and Patricia Krawczak The above-mentioned results clearly show that recent developments in PA-6/clay nanocomposites have led to improvements in short-term thermo-mechanical behaviour (tension, bending) with a relatively limited influence on impact behaviour. However, plastic parts are often subjected to long-term loading, especially in the automotive industry. It is thus of prime importance to pay attention to the static and dynamic fatigue behaviour of the nanocomposites as well. Bellemare et al 57 have characterized the effect of clay nanoparticles on the dynamic fatigue crack initiation and propagation mechanisms and on the fatigue properties of PA-6/montmorillonite nanocomposites (fatigue life S-N curve measurement and crack growth monitoring, based on fracture mechanics). Dispersing sheet-like inorganic nanoparticles in a PA-6 matrix increases the dynamic fatigue life of axial fatigue specimens subjected to a given tension-tension stress amplitude, but decreases the fatigue life at a given strain amplitude. Therefore, the effect of nanoparticles on the fatigue life appears to be similar to the one reported for inorganic fillers having dimensions in the microscale. This increase in fatigue life at given stress amplitude is thought to be a consequence of an increase in intrinsic resistance to crack initiation, as suggested by the tendency of nanoparticles to increase Young s modulus and decrease the displacement amplitude during cycling. On the other hand, compared with unfilled PA-6, resistance of nanoclay filled PA-6 to fatigue crack propagation decreases, which is attributed to an indirect effect of the nanoparticles tendency to increase the yield stress, favouring the formation of microvoids ahead the crack tip. These authors conclude that most of the effect of nanoparticles on the fatigue behaviour probably results from the mechanical reinforcement of the microstructure and its effects on the yield stress and Young s modulus, rather than from the fact that the inorganic filler acts as a stress concentrator. Finally, as far as under-the-hood automotive parts are considered, it is important to investigate the influence of prolonged contact with automotive fluids (such as oil, grease, and ethylene glycol) and pollutants (such as NO x ) on the mechanical properties and environmental stress cracking of the nanocomposites. Meanwhile, some concrete applications of PA-6/clay nanocomposites can already be found in automobiles such as the Toyota timing-gear housing (raw material supplied by Ube) or the Mitsubishi Motors engine hood (raw material from Unitika) Barrier Layers in Packaging The major application focus of PA-6 nanocomposites today is in high barrier packaging. The plastic food packaging sector actually requires specific film characteristics. A great deal of attention is therefore directed to nanocomposites that demonstrate improved oxygen and carbon dioxide barrier properties. The gas permeability of a film is the product of the diffusion coefficient and of the solubility of gas molecules within the polymer material. Adding any kind of impermeable fillers improves the barrier properties of the materials. This effect is all the more pronounced when the fillers have a high aspect ratio that increases the effective diffusion path length encountered by the gas molecules (increasing tortuosity) and thus reduces the diffusion coefficient (this effect is named passive barrier effect ). The solubility decreases when the filler content increases, following a relation that depends on orientation. The quality of the exfoliation and the dispersion in the matrix are also key issues (the existence of aggregates tends to overwhelm the positive effect on permeability reduction). That is why, with their very high aspect ratio, nanocomposites based on platy nano-fillers such as clay show barrier performances much higher than those of micro-composites based on brittle inorganic fillers such as mica. This means that it is possible to reach the same level of impermeability with much lower filler contents. It is thus possible to divide by a factor of two the gas permeability for 2 wt% montmorillonite and by a factor of 10 for 8 wt% montmorillonite. For example (Figure 18) Liang et al 18 have used 12-aminododecanoic acid modified montmorillonite in PA-6 composites and they observed that the oxygen transmission rate (OTR) at 65% RH falls by about 40% or 80% when the clay content reaches 2 wt% or 8 wt%. The orientation of the clay particles within PA-6 matrix is important, and may influence certain properties of the nanocomposites. In particular, highly oriented particles may be more effective in improving the barrier properties. Using a combination of FT-IR and Transmission Electron Microscopy (TEM), it was observed that the NH bonds in nanocomposites are less preferentially oriented along the plane of the film surface than in the PA-6 films Polymers & Polymer Composites, Vol. 14, No. 1, 2006

17 Polyamide-6/Clay Nanocomposites: A Critical Review Figure 18. Oxygen transmission rate at 65% RH of PA-6/montmorillonite prepared by in situ polymerisation for different clay contents 18 OTR (cc-mil/100in 2.day) S1 Clay content (Nanomer 1.24TL) (%) As far as commercially available materials are concerned, a PA-6 based compound that incorporates organo-clay hybrids (2 to 8 wt%) by a compounding process has been developed for film and sheet extrusion (supplied by RTP). It is said to exhibit properties as good as or better than those of typical mineral-filled composites with 20-30%- wt filler content. The significance of this product lies in the four-fold improvement in the oxygen transmission rate over unmodified PA-6 3. Moreover, good transparency is achieved in the thermoformed structures, which is an important criterion for the packaging industry when it is desirable to be able to view the packaged contents. further improvements in the barrier properties have been reported in the form of an "active-passive barrier" system, with clay as the passive barrier and a proprietary PA-6-specific oxygen scavenger as the active agent (supplied by Honeywell) 58. Such a material is claimed to achieve a 100-fold reduction in the oxygen transmission rate compared with PA-6 (Figure 19). Following such characteristics, this active-passive barrier PA-6 nanocomposite has been projected as the barrier system for beer storage 58. One further step would be the development of "active barriers", based on nanofillers modified so as to interact with the various gas molecules considered. Other PA-6 nanocomposites containing surface modified montmorillonite minerals (supplied by Nanocor, a leading clay manufacturing company) are commercially available from different suppliers (Bayer and Honeywell) 3. It is claimed that PA-6 with 2 wt% nanoclay has barrier properties three times better than those of PA-6 and 4 wt% nanoclay confers a six-fold improvement in the barrier properties. It is also worth mentioning the results obtained with another polyamide challenging PA-6 (Figure 20) 59. This polyamide is produced by the polymerisation of Figure 19. Oxygen transmission rate at 80% RH of PA-6 nanocomposites 58 Unfortunately, it is not possible at present to produce barrier nanocomposites based on nano-platelets using the only tortuosity effect, that is to say with matrix materials which do not already exhibit barrier properties on their own. This means that nanocomposites cannot fully replace traditional packaging solutions (such as multi-layer films) but they can enlarge the application areas of some polymers. While most of the past developments and current commercial materials are based on the above mentioned passive barrier effect (tortuosity effect), OTR (cc-mil/100in 3.day) Unmodified Nylon Nylon-6 nanocomposite 80% RH Active-passive Nylon-6 nanocomposite Polymers & Polymer Composites, Vol. 14, No. 1,