Fibrous Clay Mineral-Polymer Nanocomposites

Chapter 13.3

Fibrous Clay Mineral–Polymer Nanocomposites E. Ruiz-Hitzky, P. Aranda, M. Darder and F.M. Fernandes Instituto de Ciencia de Materiales de Madrid, CSIC, Campus de Cantoblanco, E-28049, Madrid, Spain

Chapter Outline 13.3.1. Non-Intercalated and Non-Exfoliated Fibrous Clay Mineral–Polymer Nanocomposites 722 13.3.2. Fibrous Clay Mineral– Polymer Nanocomposites: Preparation and Interaction Mechanisms 723 13.3.3. Technological Aspects of Polymer Nanocomposites Based on Fibrous Clay Minerals 725 13.3.3.1. Thermoplastic Nanocomposites

Based on Fibrous Clay Minerals 727 13.3.3.2. Thermosetting Nanocomposites Based on Fibrous Clay Minerals 729 13.3.4. Bionanocomposites Based on Sepiolite and Palygorskite with Natural Polymers 731 13.3.5. Conclusions 736 References 737

The interaction of clay minerals with polymers is an old theme reported since the 1960s (Blumstein, 1961) and innovative new uses of clay mineral–polymer materials emerged in the 1990s as stressed in Chapter 13.1. A well-accepted definition of clay–polymer nanocomposites (CPN) (see Introduction of Chapter 13.0) is that the dispersed particles have at least one dimension in the nanometre range (nanofillers). Most references are addressed to smectite–polymer systems with hardly any mention of interactions of polymers with sepiolite and palygorskite (Ruiz-Hitzky, 2001; Ruiz-Hitzky et al., 2004; Ruiz-Hitzky and Van Developments in Clay Science, Vol. 5A. http://dx.doi.org/10.1016/B978-0-08-098258-8.00023-7 © 2013 Elsevier Ltd. All rights reserved.

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1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012

100 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012

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FIGURE 13.3.1 Evolution in the number of publications (A) and citations (B) on sepiolite and palygorskite nanocomposites in the past 20 years. Data from ISI Web of KnowledgeSM (February 9th, 2012) searching for (sepiolite* or palygorskite*) and (composite* or nanocomposite* or bionanocomposite* or nanobiocomposite*).

Meerbeek, 2006). Recently, the use of these fibrous clay minerals to prepare clay–polymer nanocomposites (CPN) of technological impact was reviewed (Ruiz-Hitzky et al., 2011a). This chapter critically reviews polymer and biopolymer nanocomposites based on sepiolite and palygorskite. This topic is drawing increasing interest (Fig. 13.3.1), in certain cases surpassing the interest in the more classic polymer nanocomposites based on smectites. The aim of this chapter is to call the attention of clay scientists and engineers to the numerous opportunities that this type of new materials offers. Another aim is to establish a base for further studies, to rationalize and comment on the processes used to prepare these compounds, and, finally, to illustrate some properties of these materials, such as thermal, mechanical, flammability and super-adsorbent properties. The use of biopolymers giving rise to the so-called bionanocomposites has opened the way to the design and preparation of new CPN that, specially in the case of sepiolite and palygorskite, offer relevant properties afforded by their counterparts of biological origin (Darder et al., 2007; Ruiz-Hitzky et al., 2008, 2011a).

13.3.1 NON-INTERCALATED AND NON-EXFOLIATED FIBROUS CLAY MINERAL–POLYMER NANOCOMPOSITES Contrary to the dispersion of smectite-based polymer nanocomposites, that of the fibrous clay minerals within the polymer matrix takes place by interactions with the external surface of the sepiolite and palygorskite particles, as these ones are non-swelling clay minerals. However, in some cases, polymers not only interact with the external surface of sepiolite, but also penetrate into the structural tunnels of the mineral (Inagaki et al., 1995; Sandi et al., 1999; Ruiz-Hitzky, 2001). Since these cavities are organized as nano-structured

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pores, the resulting materials are considered as CPN. There are other cavities of nanometric dimensions along the fibre axis, mainly attributed to defects formed during the particle growth, that are accessible to organic species (Kuang et al., 2003; Ruiz-Hitzky and Van Meerbeek, 2006).

13.3.2 FIBROUS CLAY MINERAL–POLYMER NANOCOMPOSITES: PREPARATION AND INTERACTION MECHANISMS Conventional CPN based on smectites can be prepared by three methods (direct incorporation of polymers, in situ polymerisation of monomers and polymer-templated clay synthesis (see Chapter 13.1)). The preparation procedures for sepiolite and palygorskite polymer nanocomposites are limited to the first two methods because these fibrous clay minerals have not yet been prepared by chemical synthesis. These two procedures can be performed with different forms of sepiolite and palygorskite, such as powder, films and colloidal dispersions in water or in other polar liquids. Assembling of the fibrous clay minerals with polar monomers and polymers can be easily carried out by direct adsorption from pure liquids or in solution. Both sepiolite and palygorskite containing coordinated water molecules and covered by silanol (^SidOH) groups are strongly hydrophilic (RuizHitzky, 2001). This is especially relevant for low-polar polymers where the mineral surface must be modified by exchanging with long-chain alkylammonium ions or by grafting of organosilanes and other suitable reagents. An example of in situ polymerization is the polymerization of acrylonitrile (AN) previously inserted into the structural tunnels of sepiolite, replacing zeolitic water and interacting with coordinated water molecules by hydrogen bonds (Ferna´ndez-Saavedra et al., 2004). The polymerization of AN to PAN is initiated by 2,20 -azobisisobutyronitrile. Penetration of the monomers into the tunnels of sepiolite as well as polymerization with a second type of lowpolar monomers was reported for isoprene and styrene (Inagaki et al., 1995). On the basis of adsorption isotherms, the authors proposed that the monomers were adsorbed on both the external surface and in the tunnels of the mineral. The strong Brnsted acid sites of sepiolite are assumed to catalyze the process of in situ polymerization. However, the occurrence of acid sites in sepiolite is uncertain because the adsorbed basic species such as pyridine were not protonated (Ruiz-Hitzky, 2001; Kuang et al., 2003). Other molecules such as pyrrole and thiophene polymerize incompletely when adsorbed on sepiolite fibres (Inagaki et al., 1995). The formation of polymers from such monomers, however, is of potential interest for the development of conducting nanowires, whose size and shape are imposed by the geometry of the sepiolite. Under drastic conditions, unsaturated monomers such as ethylene can be polymerized in the tunnels of sepiolite (Sandi et al., 1999). If the sepiolite–PE

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nanocomposites are subsequently carbonized by pyrolysis, carbon nanofibres useful for solid-state lithium batteries can be produced. Similarly, carbonization of sepiolite–PAN nanocomposites can yield sepiolite–carbon materials that can be useful as electro-active components in electrochemical devices. The elimination of the sepiolite fibres by reaction with HCl and HF acids yielded carbon nanofibres of ca. 1-mm length and 20–30 nm diameter (Ferna´ndez-Saavedra et al., 2004). Penetration of polymers into the tunnels of sepiolite can be promoted by microwave (MW) irradiation. MW-assisted melt intercalation had previously been applied to the preparation of montmorillonite–PEO nanocomposites (Aranda et al., 1998, 2003). Similarly, PEO can be assembled with sepiolite fibres from the melt by MW irradiation (Ruiz-Hitzky, 2001; Aranda et al., 2006). The most salient feature is that the polymer chains can penetrate into the structural tunnels of sepiolite to replace, at least partially, zeolitic water in the cavities in an apparently irreversible process. Most polar monomers or polymers can be easily assembled with pristine fibrous clay minerals. Because these clay minerals are hydrophilic, polymers of low polarity are in general incompatible with the raw clay minerals. The less polar compounds can be assembled only if the clay minerals have previously been modified by exchange with alkylammonium ions or by grafting with suitable species. Sepiolite has a much lower cation exchange capacity than smectites (about 0.15 vs.  1 meq/g). Nevertheless, the reaction with quaternary ammonium salts yields organophilic solids (Alvarez et al., 1987). As in the case of smectites, additional surfactant ion pairs (cations plus the counterions) can be adsorbed (see Chapter 10.3). These types of materials show excellent compatibility with low-polar organic media and are used as paint thickeners and in many other industrial applications (Alvarez et al., 1984, 1985; Ruiz-Hitzky et al., 2011a). When the polymer is processed at high temperature (>250 C), a silane coupling agent is needed. These agents can also be used effectively to prepare CPN, since they make the clay mineral surfaces organophilic (Ruiz-Hitzky, 1974). Silane coupling agents have extensively been used to prepare micro-composites based on clay minerals, silica, fibreglass, etc. Organosilane coupling agents contain ^SidX groups (X ¼ OR, Cl) able to react with silanol groups on the clay mineral surface, giving stable siloxane bridges (Eq. 13.3.1). ½surface  Si  OH þ X  Si  ½R1 R2 R3  ! ½surface  Si  O  Si (13.3.1)  ½R1 R2 R3  þ HX Smectites and vermiculites have a low content of silanol groups because these groups occur only at the particle edges. In contrast, sepiolite and palygorskite are rich in such reactive hydroxyl groups because of the discontinuity of the silicate layers (Ahlrichs et al., 1975). The silanol groups located on the external surface, at the edges of the structural channels, are directly accessible to the reagents. After grafting with organosilanes (Ruiz-Hitzky and Fripiat,

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1976), the hydrophilic sepiolite particles can be easily dispersed in lowpolarity compounds including polymers (Ruiz-Hitzky, 1974). The stability of these organic derivatives is excellent; the attached groups are eliminated only after heating at elevated temperatures by combustion or pyrolysis, that is, in the presence or absence of oxygen. Organosilanes containing unsaturated groups (vinyl and methacryloxy) or thiol functions (3-propylmercapto) give organic derivatives of sepiolite capable of further co-polymerisation reactions. CPN were prepared in which the polymer was covalently bound to the modified clay mineral (Ruiz-Hitzky, 1974). CPN were also prepared by hydrolysing mixtures of clay minerals and organosilanes with strong acids. The clay minerals involved were, for example, chrysotile, sepiolite and vermiculite (Fripiat and Mendelovici, 1968; Zapata et al., 1972; Ruiz-Hitzky and Van Meerbeek, 1978; Van Meerbeek and Ruiz-Hitzky, 1979). The extraction of octahedral cations such as Mg2þ by acid attack yields new silanol groups (Eq. 13.3.2),  Si  O  Mg  O  Si  þ½Hþ =H2 O ! ½surface  Si  OH þ Mg2þ (13.3.2) which react with the silanol groups produced by the hydrolysis of the chloro or alkoxy organosilanes as reported in Eq. (13.3.1). When the surface is made compatible, CPN can be prepared with many polymers. After reaction of sepiolite with vinylsilanes, the unsaturated groups are homogenously distributed. This was visualised by TEM by reacting the vinyl groups on the sepiolite with OsO4 (Barrios-Neira et al., 1974), confirming the arrangement of the grafted species to silanol sites along the edge of the structural channels. Sepiolite can also be grafted using compounds of various functionalities such as isocyanates and epoxides (Table 13.3.1). Examples of sepiolite and palygorskite modifications by grafting reactions for making them compatible with diverse polymers are discussed below.

13.3.3 TECHNOLOGICAL ASPECTS OF POLYMER NANOCOMPOSITES BASED ON FIBROUS CLAY MINERALS At present, CPN are still under development with very little commercialization. Nevertheless, their wide-scale introduction in the market is highly awaited as they fulfil several market needs, the most relevant of which are reinforcing the elastic properties of polymeric matrices, fire retardancy and enhanced resistance to thermal oxidation. These materials are expected to be marketed at a reasonable cost (Ruiz-Hitzky and Van Meerbeek, 2006). Natural or modified sepiolite and palygorskite were recently applied as inorganic nanofillers for different types of polymeric matrices taking into account their fibrous habit with dimensions at the nanometric scale (Nohales et al., 2006; Sangerano et al., 2009; Fernandes et al., 2011; Ruiz-Hitzky et al., 2011a).

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TABLE 13.3.1 Organic Derivatives of Silicates by Grafting of Isocyanates and Epoxides Function Isocyanates RdC]N^O

Grafted groups ^SidOdCOdNHdR

Epoxides

CH2-CH-R

^SidOdCH(CH2OH)dR

Reagent

Substrate

Butyl isocyanate Phenyl isocyanate Hexamethylene di-isocyanate 2,4-Toluene di-isocyanate

Sepiolite1,2 Sepiolite1,2 Sepiolite2 Vermiculite3 Sepiolite2 Vermiculite3

1,2-Epoxyethyl benzene (epoxystyrene) 1,2-Epoxybutane

Sepiolite4,5,6

1-Allyloxy-2,3epoxypropane (allylglycidyl ether) 2,3-Epoxypropyl methacrylate 3-Vinyl-7-oxabycycle (4,1,0) heptane

Sepiolite4,5,6

Sepiolite4,5,6

O

Sepiolite6 Sepiolite6

References: 1Ferna´ndez-Herna´ndez and Ruiz-Hitzky (1979), 2Ruiz-Hitzky et al. (1979), 3Siffert and Biava (1976), 4Casal and Ruiz-Hitzky (1977), 5Casal and Ruiz-Hitzky (1984), 6Casal et al. (1980).

As with smectite–polymer nanocomposites, fibrous clay mineral–polymer nanocomposites are based on the nanoscale interaction of the disperse phase, that is, the clay mineral particles, with the continuous phase, that is, the polymer. Because of the large specific surface area (SSA) of these particles, even a considerably low content of the clay minerals induces a considerable effect on the properties of the composite material as a consequence of the large interface area between both phases. Because the interface area is critical for the reinforcing effect, a high SSA of the disperse phase implies that an extended interface is attained at low clay mineral contents. On the other hand, the nano-dimension of these fillers often induces aggregation due to van der Waals interactions, making reinforcement ineffective. As with all high aspect ratio nanoparticles used for mechanical reinforcement purposes, the diminutive dimensions of the fibrous clay mineral particles offer as much potential as they pose challenges. In this context, sepiolite and palygorskite present a key advantage because of their simple surface chemistry. As stated above, the silanol groups present along the edges of the nanoparticles can be used to tailor the chemical affinity between the clay mineral and the matrix either

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by grafting reactions or by modification with surfactants. Such strategies, which envisage tuning the chemical environment of the fibrous particles, serve, in two different ways, the same purpose: maximizing the dispersion of the clay mineral particles in the polymeric matrix. This is achieved by maximizing the affinity between the modifiers and the polymer and, in some cases, by the formation of a covalent bond between the modifying groups and the polymer matrix. The modification of the clay mineral surface, namely, by the introduction of voluminous end groups, end-tethered polymers or alkyl chains, acts as a steric stabilizers (see Chapter 8), which prevents the clay mineral particles from aggregation. Because of their versatile surface chemistry, clay minerals are not restricted to a specific type of polymer but rather can be tailored to achieve good dispersibility in most polymeric matrices. However, it should be noted that for non-modified clay mineral particles, polar polymers such as biopolymers are especially suited. However, most polymers used in day-to-day applications, and consequently those that represent the biggest share of the polymer nanocomposites market, do not correspond to biopolymers. Rather, they can be divided into two main categories, namely, thermoplastic and thermoset polymers.

13.3.3.1 Thermoplastic Nanocomposites Based on Fibrous Clay Minerals The use of fibrous clay minerals to reinforce thermoplastics is closely related to the properties of the fibrous clay minerals, especially the high aspect ratio. Also important is the cost reduction associated with the enhancement of the mechanical properties permitting to reduce the global amount of the material needed for a given application. The most notorious examples are polymers such as PE, which represent a technological challenge due to their non-polar character as well as an industrial goal due to their wide applicability and, hence, a substantial market to explore. Finally, these materials can also be considered as a green alternative to the preparation of non-reinforced materials since their optimised structural properties allow a substantial reduction of the need for oil-derived materials. Shafiq et al. (2012) recently studied the effects of raw and modified sepiolite on the properties of PE nanocomposites, with rather surprising results. After modifying sepiolite both in situ and ex situ with vinyl triethoxysilane, which was considered to enhance the chemical coupling between the fibres and the PE, the authors found that raw sepiolite presented the most favourable behaviour regarding both the thermal stability and mechanical properties of the CPN. Although this behaviour had already been reported for sepiolite–gelatin nanocomposites (Fernandes et al., 2011), it was highly unexpected for PE since there is no reasonable affinity between the two phases. The elastic modulus obtained at 2 phr loading of raw sepiolite in LDPE increased by 40% with respect to the non-reinforced

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PE. Also, the thermal stability indicators such as the Vicat softening temperature were consistent with the mechanical properties, as the values for non-modified sepiolite were systematically higher than for the modified sepiolite. These results directly question the assumption that the interactions between the matrix and the filler have a decisive influence on the behaviour of the CPN. Garcia et al. (2009) studied the thermal stability of PE reinforced by three types of nanometric particles of different morphology but closely related chemical properties: octyl trimethoxysilane-modified silica and sepiolite and the commercially available organo-montmorillonite, Cloisite 15A. Sepiolite protected the thermal degradation of PE as efficiently as Cloisite 15A and better than the silica nanoparticles. The adsorption capacity of the clay minerals might be the reason behind the suitability of clay minerals’ thermal-oxidative degradation of CPN. Sepiolite reinforcement was also studied for polyesters. These polymers also represent an important market share due to their wide application as synthetic fibres, for example, for fabric manufacture and plastic bottle production (PET). Duquesne et al. (2007) showed the influence of modified sepiolite particles in a polycaprolactone (PCL). To maximise the compatibility between the fibrous filler and the matrix, sepiolite was modified by a two-step procedure. Firstly, it was modified with aminopropyl triethoxysilane, yielding an amino-modified sepiolite. This amino function was used to promote ringopening of the e-caprolactone in the presence of tin(II) bis(2-ethylhexanoate), yielding a sepiolite hybrid with small polymer chains identical in nature to the bulk polymer. The CPN after melt-compounding and hot-plate pressing showed no significant changes in the melting temperature or crystalline fraction. Also, the degradation temperature was not enhanced compared to that of raw PCL. The enhancement of the elastic modulus by 30% at 3 mass% filler, however, revealed the suitability of caprolactone as a compatibilizing agent between the silicate and the polymer matrix. Once again, the reinforcing efficiency of the amino-modified sepiolite was lower than that of the raw sepiolite. Other polymers such as PP and PBT also reinforced sepiolite (Tartaglione et al., 2008). To evaluate the effect of the modifying agent on the properties of the sepiolite–PP nanocomposites, several reactions were performed on the sepiolite surface. Three surfactants with different chains were adsorbed on the sepiolite outer surface. In addition, a thiol-terminated alkoxysilane was grafted to the structural silanol groups on the edges of the sepiolite particles. The incorporation of sepiolite revealed a sensible effect on the thermooxidative degradation temperatures. The grafted sepiolite particles shifted the degradation process to distinctly higher temperatures. However, the degradation was not affected by pristine or modified sepiolite. Ma et al. (2007) also studied the reinforcement of PP by sepiolite. They prepared sepiolite–PP nanocomposites by supercritical CO2-assisted mixing

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and traditional melt compounding. The Young’s modulus and tensile strength were both enhanced with increasing sepiolite content but with sensible differences in the mechanical behaviour of the composites according to the processing technique used. Regarding the elastic properties, the traditional melt compounding performed in a twin-screw extruder yielded better results. However, the yield stress was superior when the CPN were processed using supercritical CO2-assisted mixing. Another interesting feature was the inefficiency of maleic anhydride to promote the adhesion between the clay mineral fibres and the polymeric matrix. Palygorskite has also found extensive application for the reinforcement of thermoplastic matrices (Gala´n, 1996) recently. Yuan et al. (2008) studied the influence of palygorskite on the thermal stability of PET. After in situ polymerization of terephthalate monomers in the presence of organically modified palygorskite, the samples were subjected to thermogravimetric analysis. The degradation temperatures of the CPN were highly dependent on the heating rate applied. Surprisingly, low heating rates (25 mass% (Matos et al., 2009; Darder et al., 2010; Ruiz-Hitzky et al., 2010a). All these properties may guarantee the utilisation of these foams as building materials. In another field of application, the biocompatibility of both components, the macroporous structure and good mechanical properties allow the use of these foams as scaffolds for the growth of living species, as for instance, microalgae (Chlorella vulgaris and Anabaena), enhancing the viability of the sepiolite– chitosan-based foams (Ruiz-Hitzky et al., 2010c). The use of these foams could be extended to biomedical applications with special emphasis on scaffolds for regenerative medicine.

1 cm

200 mm

5 mm

FIGURE 13.3.3 Alginate–sepiolite bionanocomposite processed as a rigid foam by freeze-drying (left image), SEM images at different magnifications showing the macroporous structure of this material (centre image) and a cross-section of the cell walls showing the arrangement of the material as well as the dispersion of the sepiolite fibres within the alginate matrix (right image).

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As biopolymers represent an important source of carbon, bionanocomposites can serve as intermediates in the development of carbonaceous materials. In previous studies, the silicate in CPN was employed as a template that could be easily removed to obtain nano-structured carbon (Kyotani et al., 1988; Aranda, 2007; Ferna´ndez-Saavedra et al., 2008). However, recent examples of bionanocomposites based on sepiolite and caramel, obtained by thermal transformation of sucrose under MW irradiation and submitted to carbonization at ca. 800 C, have demonstrated the benefits of retaining the silicate moiety in the clay mineral/graphene-like material, so as to incorporate new functionalities by grafting of suitable organo-alkoxysilanes (Go´mez-Avile´s et al., 2007), or due to its improved performance as electrodes in lithium batteries (Go´mez-Avile´s et al., 2010). These materials displayed electronic character, with a low activation energy (ca. 0.15 eV) and electrical conductivity values ranging from 102 to 1 S cm1 at room temperature (Aranda et al., 2010). Similarly, bionanocomposites comprising the structural protein gelatin and sepiolite were also transformed into conducting supported N-doped carbon materials at relatively low temperatures (