Polyethylene-Based Blends, Composites and Nanocomposities

By Visakh P. M. and María José Martínez Morlanes

The book focusses on the recent technical research accomplishments in the area of polyethylene-based blends, composites and nanocomposites by looking at the various aspects of processing, morphology, properties and applications. In particular, the book details the important developments in areas such as the structure-properties relationship of polyethylene; modification of polyethylene with radiation and ion implantation processes; stabilization of irradiated polyethylene by the introduction of antioxidants; reinforcement of polyethylene through carbon-based materials as additives; characterization of carbon-based polyethylenes composites, polyethylene-based blends with thermoplastic and thermoset; characterization of polyethylene-based thermoplastic and thermoset blends; polyethylene-based blends with natural rubber and synthetic rubber; characterization of polyethylene-based natural rubber and synthetic rubber blends; characterization of polyethylene-based composites.

• Language: English
• Publisher: Wiley
• Release date: Jul 6, 2015
• ISBN: 9781118831298

Book preview

Preface

This book on Polyethylene-Based Blends, Composites and Nanocomposites summarizes many of the recent research accomplishments across the current spectrum of polyethylene-based blends, composites and nanocomposites.

The first chapter gives an overview of the area of state-of-the-art, new challenges and opportunities of polyethylene-based studies and research.

The second chapter presents a structure of ultra high molecular weight polyethylene (UHMWPE) for orthopaedic devices: structure/property relationships. This chapter explains the many subtopics such as HDPE and UHMWPE, chemical structure, crystallinity and melting behaviour, molecular weight, mechanical properties, and sterilization by gamma rays.

The third chapter mainly concentrates on stabilization of irradiated polyethylene by the introduction of antioxidants (vitamin E). The discussion of this chapter has two parts. The first part delineates the main types of antioxidants, stabilization by Vitamin E, structure and biological function of vitamin E, mechanism of stabilization of vitamin E, methods of incorporation of vitamin E, vitamin E stabilized polyethylenes; the second part discusses the analysis of the content of vitamin E, using such instruments as FTIR, UV, HPLC and thermal methods.

The fourth chapter, which discusses polyethylene-based conducting polymer blends and composites, comprehensively reviews research work based on intrinsically conducting polymers in conjunction with PE. Various methods of fabricating the blends and composites are briefly outlined, and possible interactions among these polymers are discussed, with reference to a variety of characterization techniques, including spectroscopy, microscopy and thermal analysis. The effect of incorporating intrinsically conducting polymers into PE on mechanical strength, electrical conductivity, free-radical scavenging capacity and antimicrobial activities is also addressed very carefully in this chapter. A range of applications of these bends and composites is also discussed, and the opportunities for utilizing these materials in industrial and consumer products and the associated challenges are summarized.

The fifth chapter reviews polyethylene composites with lignocellulosic material, with the aim of describing in detail the advances in polyethylene reinforced with lignocellulosic material. Basic concepts and terminology adopted in the lignocellulosic composite materials are reviewed, and — in the context of polyethylene-lignocellulosic composites — ongoing research is then summarized. The main focus is on the principal methods used for the improvement of interfacial adhesion and the main adopted processing routes and the composite properties, concluding with a discussion of the applications, new challenges and opportunities of these polyethylene-lignocellulosic composites.

The sixth chapter reviews a study report on the use of layered double hydroxides (LDH) with the hydrotalcite-type structure as fillers of composites and nanocomposites in a polyethylene matrix. The properties of the LDH are described, as well as their preparation procedures; the most common methods to prepare the composites and nanocomposites are described and discussed; and, finally, the properties of these compounds, which define their applications in different fields (e. g., mechanical, thermal, electric, chemical, etc. properties) are discussed.

Chapter 7 illuminates the advantages and complexities of ultrahigh molecular weight polyethylene (UHMWPE). The importance of UHMWPE on arthroplasty, including the advantages, the limitations and the strategies devised to overcome the known drawbacks is discussed in the first section. Subsequent sections review and discuss the biocompatibility, the manufacturing processes, the tribological behaviour, the aging by oxidation and irradiation of UHMWPE and UHMWPE-CNT nanocomposites. The final section analyses the viscoelastic behavior of UHMWPE and its implications on the long-term survival of total joint arthroplasty.

Chapter 8 explains the main aspects of montmorillonite polyethylene nanocomposites, focusing on the processing, properties and applications and the compounding and characterisation techniques to manufacture PE/Mt nanocomposites. Some of the topics addressed include the characterisation of Mt, morphology of PE/Mt nanocomposites and the influence in mechanical, thermal and other properties. Future work regarding PE/Mt nanocomposites is also included.

The ninth and final chapter discusses characterization methods for polyethylene-based composites and nanocomposites, explaining the most significant features of the processing, characterization, and properties of PE composites. In the section that discusses processing, different techniques used for preparation of PE composites are explained: extrusion, injection molding and compression molding. Another section of this chapter discussing characterization concentrates attention on these PE composites’ mechanical, thermal and morphology and rheological properties.

The editors express their sincere gratitude to all the contributors of this book who provided excellent support to the successful completion of this venture. We are grateful to them for the commitment and the sincerity they have shown towards their contribution in the book. Without their enthusiasm and support, the compilation of such a work would not have been possible. We thank all the reviewers who took valuable time setting forth critical comments on each chapter. We also thank the publisher John Wiley and Sons and Scrivener Publishing for recognizing the demand for such a book, and for realizing the increasing importance of the area of Polyethylene-based Blends, Composites and Nanocomposites.

Visakh. P. M.

María José Martínez Morlanes

May 2015

Chapter 1

Polyethylene-Based Blends, Composites and Nanocomposites: State-of-the-Art, New Challenges and Opportunities

Visakh. P. M.¹, and María José Martínez Morlanes²

¹Tomsk Polytechnic University, Russia

²School of Engineering and Architecture, Zaragoza, Spain

*Corresponding authors: [e-mail] and [e-mail]

Abstract

The chapter deals with a brief account of various topics in polyethylene-based blends, composites and nanocomposites. We discuss the different topics such as ultra high molecular weight polyethylene (UHMWPE) for orthopaedics devices, stabilization of irradiated polyethylene by the introduction of antioxidants, polyethylene-based conducting polymer blends and composites, polyethylene composites with lignocellulosic material, LDH as nanofillers of nanocomposite materials based on polyethylene, ultra high molecular weight polyethylene and its reinforcement/oxidative stability with carbon nanotubes in medical devices, montmorillonite polyethylene nanocomposites, and characterization methods for polyethylene based composites and nanocomposites.

Keywords: Polyethylene-based blends, polyethylene-based composites, polyethylene-based nanocomposites, carbon nanotubes, lignocellulose

1.1 Ultra High Molecular Weight Polyethylene (UHMWPE) for Orthopaedic Devices: Structure/Property Relationships

1.1.1 Introduction - HDPE and UHMWPE

HDPE is a linear polymer which can contain more than 1000 CH2 groups [CH2 – CH2]n. HDPE is also semi-crystalline. However, the amorphous regions are relatively small and it is said that HDPE is basically crystalline with uniformly-distributed flaws and imperfections. Karl Ziegler and Erhard Holzkamp together invented high-density polyethylene (HDPE) in 1953. As HDPE is a linear polymer, it can form a solid with very high percentage crystallinity values, between 60 and 80%. The melting behaviour of HDPE is very similar to that of UHMWPE. It too behaves like a glass solid below its glass transition temperature, and increasing the temperature above the Tg will see the material go from an elastic solid to a rubbery, tacky substance, known as the rubbery state.

Microstructurally, UHMWPE is a semi-crystalline polymer (45–55% crystallinity), so that it has two different regions: a crystalline and other amorphous [1, 2]. The crystalline region is formed by lamellar structures which result from the ability of the molecular chains to fold to adopt ordered configurations. In most UHMWPE, this lamellar morphology is observed since the high molecular weight prevents the occurrence of spherulites [3]. The crystal morphology of UHMWPE can be visualized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), which allow perceive the crystalline lamellae (light area) embedded within the amorphous region (gray area) (Figure 1.1).

Figure 1.1 TEM image (x60000) for UHMWPE.

The size of the crystal lamellae is about 10–50 nm thick and 10–50 microns in length, and the average space between the lamellae is on the order of 50 nm [4]. The existence of tie molecules which act as a bridge connecting the crystalline domains through amorphous region that surrounds, provide better stress transfer to the polymer and greater mechanical strength. These microstructural characteristics, the high density of tie molecules and moderate crystallinity as well as its high molecular weight (3–6 × 10⁶ g/mol), are responsible for the high wear resistance and high elongation at break of polyethylene [5].

The melting behaviour of UHMWPE is dependent on the thickness and perfection of the crystals in the material, which is a function of the crystallisation temperature. In 1998, the nomenclature for UHMWPE was consolidated with availability of four grades for the worldwide orthopaedic market – GUR 4150, 1050, 1120 and 1020 resins. The first digit of the grade name was originally the loose bulk density of the resin. The second digit of the code is 1 if the polyethylene contains calcium stearate or 0 otherwise. Several studies showed that the presence of calcium stearate cause detrimental effects on both consolidation capacity and oxidative stability of UHMWPE so, in the late 90’s the use of this additive was abandoned [6–8].

1.1.2 Chemical Structure

The conformation of a polymer chain is the three dimensional spatial arrangement of the chain as determined by the rotation about backbone bonds. The conformation and configuration of the polymer molecules have a great influence on the properties of the polymer component. It is described by the polarity, flexibility and the regularity of the macromolecule. The helix is a typical ordered conformation type for polymers that contain regular chain microstructure. Typically carbon atoms are tetravalent, which means that in a saturated organic compound they are surrounded by four substituent in a symmetric tetrahedral geometry.

1.1.3 Crystallinity and Melting Behavior

In HDPE, chains fold to form the lamellae and propagate outwards three dimensionally, creating a sphere-like formation. This sphere-like formation is referred to as a spherulite. The spherulites are very small anisotropic spheres (1–5μm) only visible under very high magnification. Microstructural defects within the polymer chain generally cannot be incorporated into the crystal unit cell and have to be ejected during the crystallization process resulting in the formation of permanent amorphous phases. Below Tm (melting temperature), some polymers have high rates of crystallization, such as polyethylene, so it cannot be quenched quickly enough to prevent crystallization [9]. Since the crystals have a thickness of the order of 10 nm they have melting points that are thickness dependent. Additionally, the crystal thickness is a function of the crystallization temperature, resulting in materials having variable melting points, dependent on the crystallization conditions. Materials crystallized at lower temperatures show much smaller crystals and greater heterogeneity and therefore the melting temperature is lower and the spread is greater.

1.1.4 Molecular Weight

Ultra high molecular weight polyethylene (UHMWPE) is high-density polyethylene material. Chemically, high-density polyethylene (HDPE) and (UHMWPE) are identical; both are straight chain linear polymers. Molecular weight affects both UHMWPE and HDPE alike. This is because both materials are very similar. UHMWPE is in fact a high density polyethylene material, differing only in have a much higher molecular weight. Both UHMWPE and HDPE have long linear molecular chains, UHMWPE having longer chains than HDPE, this is why the molecular weight is greater [10]. The level of molecular weight also affects the degree of crystallinity which can be achieved within a material. Crystallinity has considerable effect on the mechanical properties of a material. Longer molecular chains, as within UHMWPE and HDPE are so entangled that they cannot completely reorganise into crystal structures. They are the weakest part of the material and they are the key element which reduces the mobility of the material, i.e. creep and wear resistance and impact strength [11].

1.2 Stabilization of Irradiated Polyethylene by Introduction of Antioxidants (Vitamin E)

1.2.1 Introduction

Polyethylene oxidative stability is actually relatively low: from a general point of view polyethylene undergoes degradation during processing, storage and use. C-H groups of PE react with oxygen to create unstable species leading to chain scissions and subsequent changes in the mechanical properties of polymer materials [12]. Since vitamin E is particularly adapted in the case of biomaterials, because of its low toxicity, its stabilizing mechanism and performances will be detailed in a later chapter, together with some experimental methods aimed at quantifying residual vitamin E after the complex elaboration process of hip. The existing literature will be reviewed so as to extract the kinetic parameters (rate constants for reactions with radicals, diffusion and solubility coefficients) necessary to perform kinetic modeling for a more complex description of the radio-thermal oxidation of PE + vitamin E.

1.2.2 Vitamin E Stabilized Polyethylenes

First, vitamin E scavenges free radicals propagating oxidation. It is hence not surprising to observe that it significantly reduces the post irradiation effects observed in UHMWPE. Kinetic curves in post irradiation phase always display the same characteristic shape with a maximal increase rate at the beginning of post irradiation. In the case of unstabilized PE, concentration in peroxides (POOH) or carbonyl reach a pseudo-plateau after ca 1000 h at which [POOH] is on the order of 10–3–10–2mol l–1 [13]. This relatively high value can be explained. Stabilizing vitamin E UHMWPE prior to consolidation and irradiation is expected to have an adverse effect with the irradiation effect [14]. Vitamin E acts as a scavenger of radiation-induced free radicals and it will be consumed relatively fast during irradiationso that the thermal/oxidative stability in the post irradiation phase will be low (and even, in some cases, comparable than for unstabilized) [15]. The incorporation of this antioxidant before irradiation also produces another negative effect: loss of efficiency in crosslinking. However, Vitamin E offers the possibility to trap residual free-radicals and avoid some post irradiation effects. It could be an interesting strategy for add vitamin E by an impregnation method in the crosslinked UHMWPE. Although this method avoids the loss of efficiency in the aforementioned cross-linking process, it introduces a lack of homogeneity of vitamin E concentration through the polymer.

1.3 Polyethylene-Based Conducting Polymer Blends and Composites

1.3.1 Introduction

Intrinsically conducting polymers (ICP) are multi-functional polymers that can offer all of these valuable properties. Incorporating ICPs in the PE matrices thus offers the possibility of employing a single additive to achieve antistatic, antioxidant as well as antimicrobial properties. The electrical conductivity of ICPs, which is minimal in the neutral state, is elevated by several orders of magnitude through doping [16]. The mechanism behind the bulk electrical conductivity of ICPs is transportation of charge along and between polymer molecules via generated charge carriers. Other than electrical conductivity, the ICP-based blends and composites have been investigated as electrochromic element [17], catalytic surface [18], sensors [19], etc. Other conductive fillers, such as carbon black, carbon fibers and metallic powders [20, 21], can be used as a component of conductive polymer composites.

1.3.2 Preparation

The common methods of achieving PE/ICP blends and composites are through in situ polymerization of ICPs, melt processing and solution blending. The polymerization of a monomer in a polymer matrix is known to be a versatile and practical method for the in situ preparation of new materials [22]. This method yields a more intimate mixing of two components, which may not be achieved by mechanical blending due to the incompatibility of most polymers. The monomer can also be polymerized on the surface of a polymer matrix.

In situ oxidative polymerization has also been used to prepare/PE/polyaniline composites. In one approach, microporous PE films were immersed in a solution of aniline hydrochloride and polymerization was started by the introduction of ammonium peroxydisulfate [23]. Similarly, Wan and Yang (1993) obtained PE/polyaniline composites using iron (III) chloride as the oxidant [24]. Solution blending is largely utilized to prepare blends and composites where one or more of the components do not melt easily or are heat susceptible. ICPs like polyaniline are difficult to process due to their aromatic structure, interchain hydrogen bonds and effective charge delocalization in their structures.

Hosier et al. (2001) [25] prepared 2% (w/v) solutions of dodecylbenzene sulfonic acid doped polyaniline and PE separately in hot xylene. Appropriate volumes of the two solutions were then mixed and further refluxed to ensure complete dissolution of the polymers. Finally, the blend solution was poured into cold acetone and the PE/polyaniline blends were precipitated as a green solid. Pereira da Silva et al. (2001) [26, 27] obtained PE/polyaniline blends by casting a mixture of hot solutions of camphorsulfonic acid doped polyaniline in m-cresol and PE in decalin onto glass substrates. Melt blending usually involves dispersing the infusible ICPs into melt thermoplastic matrices [28]. Due to the insolubility of PE, as well as the ICPs, in common solvents, the melt blending route is commonly preferred over solution blending. Moreover, for industrial applications and large scale productions, melt blending is more desirable than solution blending as it is an easier process with low cost implications. ICPs degrade at higher temperatures and lose their functionality [29].

1.4 Polyethylene Composites with Lignocellulosic Material: A Brief Overview

1.4.1 Introduction

Combination of properties of both lignocellulosic materials and thermoplastics opened a new range of applications. One of those applications is related to the employment of polyethylene-lignocellulosic composites in the decking and construction, which has been a growing market over the past decades. Lignocellulosic materials are basically constituted of cellulose, lignin and hemicellulose and some species suberin [30, 31]. In the form of natural fibres, they are distinct from synthetic fibres. For the case of polyolefin-lignocellulosic composites the main problem is that natural fibres are highly heterogeneous materials both physically and chemically [32, 33]. Lignocellulosic fibres, natural fibres or bio-fibres offer several advantages over the traditional ones. The most important are low density and low cost, good specific strength properties, nonabrasive during processing, CO2 neutral when burned and biodegradability [34, 35].

1.4.2 Coupling Agents and Fibre Chemical Treatments

Lignocellulosic composites using polyolefins (including polyethylene) have gained increasing interest over the past two decades, both in the scientific community and industry [36]. Since high density polyethylene (HDPE) is relatively inert it is difficult to achieve good interfacial adhesion in composites. Often, maleic anhydride grafted polyethylene is added to HDPE to improve interfacial adhesion to the reinforcing fibres [37]. Lignocellulosic fibres are highly polar owing to the presence of hydroxyl groups [38, 39]. The hydroxyl groups are readily available for chemical bonding (hydrogen bonding) with compatible polymer matrices and physical interlocking (wetting) with the non-polar matrices such as polyethylene.

In lignocellulosic fibres, several chemical pretreatments performed before compounding have been investigated by a number of researchers showing potential to remove waxes, oils from the surface and make it rough, active readily available hydroxyl groups or the introduction of new reactive site/groups; and to stop water uptake [40–47]. In polyolefin with lignocellulosic fibres, co-extrusion technology has been pointed out as advanced polymer processing technology due to the unique capacity in creating a multi-layer composite with different complementary layer characteristics, and in making the properties of the final products highly tunable. There are several studies where compression moulding was used as a second melting process to give the final shape to polyethylene-lignocellulosic composite products. The combination of thermoplastic materials, as polyethylene, with the unique properties of the lignocellulosic materials opens the prospective of new uses and applications. polyethylene-lignocellulosic materials are found in outdoor deck floors, railings, fences, landscaping timbers, cladding and siding, park benches and indoor furniture. There is a variety of composites products in the market using polyethylene matrix with lignocellulosic fibres. As pointed before, these commercial products could originate from recycled or virgin polyethylene grades.

1.5 LDH as Nanofillers of Nanocomposite Materials Based on Polyethylene

In addition to MMT and other cationic clays, such as layered double hydroxides (LDHs) with the hydrotalcite-type structure, are another family of layered materials [48]. Despite they have been used in a lesser extent than MMT (and structure-related materials) as nanocomposite polymer fillers, have shown enormous more potentially advantages than natural clays, specially concerning their purity and crystallinity and particle size control;. In addition, they can be also easily functionalized in different ways with different agents, thus permitting a chemical modification of the layers environment to optimize their compatibility with the polymer [49, 50].

Chemical composition, structure and properties under heating of LDHs anticipate an increase in the thermal stability and the degradation temperature of composites containing this inorganic fillers, they have been scarcely used to prepare polymer/layered inorganic nanocomposites (PLNs). This is probably due to the low chemical compatibility between these materials and the polymers, thus making rather difficult to reach a good compatibility between the inorganic filler and the polymer. When LDHs are added to a polymer (well in appreciable concentrations or without any sort of modification), the mechanical properties result negatively damaged. Many studies have been reported in the literature to prepare organically-modified LDHs.

Preparation of polymer-based nanocompounds with LDHs has to overcome the limitations due to the different chemical nature of the polymer and the inorganic filler. The preparation methods followed have been designed to make use of the advantages shown by the hydrotalcite structure when compared to other inorganic fillers, as the chemical composition can be fixed beforehand. Costa et al. [51] studied the effects of loadings of LDH up to 15% in polyethylene. This authors report an increase in the Young modulus when the concentration of LDH is increased, as well as a decrease in both, the elongation to break and the tensile strength. The chemical composition of the LDH also seems to have an effect on the mechanical response of the nanocomposite [52]. A PE/Zn, Al LDH nanocomposite showed an increase in the elongation at break when compared with the unloaded polymer. However, when Mg instead of Zn exists in the hydrotalcite layers of the PE/Mg, Al LDH, such a parameter largely decreases.

Ding and Qu [53] have reported an improvement of the thermal properties for LDH loadings not larger than 10% neat polyethylene. If a larger amount of LDH is added, not only layers of the inorganic hydroxides will exist, but also more organic substances which are intercalated between the layers of the LDH. One of the most significant parameters to evaluate the flammability properties measured through the cone calorimetric, is the reduction in the peak heat release rate (PHRR), which provides a measure of the intensity of the fire. As pointed out above, decomposition of the dispersed LDH layers in the nanomaterial favours charring of the polymer, acting as a fire retardant. Dispersion of the LDH layers in electrically active polymeric matrices increases the thermal stability of the nanocomposite [54]. Schönhals et al. [55] have studied the changes observed in the electrical properties of a LDPE when it is loaded with LDHs which had been organically modified with dodecyl benzenosulfonate (DBS), using PE-g-MA as a compatibilizer.

These results make LDHs excellent candidates as fillers for polymer matrices, to be used as degradant additives, which permits a precise control of the lifetime of the plastic product, decreasing its negative impact on the environment. The improvement in the thermal properties of PE/LDH nanocomposites is usually explained assuming that the LDH layers act as a sort of barriers. Yue et al. [56] claim formation of nanostructures from the dispersed LDH particles, which decrease heat transfer, thus stabilizing the polymer chains. Dispersion of the LDH particles thus improves the barrier properties of the polymer nanocompounds because of their easy exfoliation. Fluid diffusion, especially gases, is strongly affected by this barrier effect.

1.6 Ultra High Molecular Weight Polyethylene and its Reinforcement/Oxidative Stability with Carbon Nanotubes in Medical Devices

Most appropriate characteristics of the UHMWPE, when used as a bearing component in orthopedic implants, are its higher wear resistance. In spite of that the wear of UHMWPE in the form of microscopic particles led to osteolysis and later to the loosening of the implant which catalyze its failure on the long-term [57]. The most representative group of orthopedic devices using UHMWPE as a bearing material is the total joint arthroplasty (TJA). The most important groups are the total hip arthroplasty (THA) and total knee arthroplasty. According to the studies conducted on in vitro simulators, wear is dependent on kinematics of the articulating surfaces, the prosthesis design and the type of material [58]. An attempt to improve the performance of UHMWPE started almost immediately after its introduction as a bearing material in orthopedic devices. The motivation was the wear debris related osteolysis that was the main cause of failure in joint replacements based on UHMWPE inserts [59]. Clinical data regarding the performance of these formulations is still under way.

Although there are several compounds (Ascorbic acid (vitamin C) or alpha-tocopherol (vitamin E) which their antioxidant effect is proved to be effective in reducing the oxidation of UHMWPE [60], there are several studies that support the antioxidant capacity of carbon nanotubes (CNTs). For example, in the study of Zeynalov et al. [61] was conducted a simulation of thermo-oxidative processes which take place in the polymer chains, and the results showed an inhibition of oxidation of the polymer when carbon nanotubes are present. Also, P. Castell et al. [62] founded that the incorporation of low concentration of arc-discharged multiwall carbon nanotubes (MWCNTs) can act as inhibitors of the oxidative process on irradiated UHMWPE, proving the radical scavenger effect of this reinforcing material. Also, this study shows that the presence of MWCNTs enhances the chemical stability of the polymer.

Regarding to mechanical properties, it was reported the fabrication of UHMWPE/CNTs composites with properties close to the commercially available fibers like Kevlar [63]. Bakshi et al. [64] used a quite different technique to prepare films of UHMWPE/CNTs. Mechanical ball-milling to mix the raw UHMWPE with CNTs, and after that the powder composite was processed by compression molding in a hot press. The results have shown a homogeneous distribution of the CNTs into the UHMWPE. Xue et al. [65] reported the wear and creep resistance of a composite material consisting of UHMWPE and high density polyethylene (HDPE) polymer blend reinforced by MWCNTs. Campo et al. [66] reinforced UHMWPE with 0.5 wt.% MWCNTs and performed wear studies on a pin-on-disc wear tester for 1 million cycles. It was reported that the wear loss of pure UHMWPE was 130 mg, which was increased to 178 mg (36% higher) by reinforcing MWCNTs. Premnath et al. [67] reported that the crosslinking of UHMWPE dominated when irradiation was done in absence of air, whereas chain scission of polymer was observed to dominate in air environment.

1.7 Montmorillonite Polyethylene Nanocomposites

Several investigations have proven that polymer/montmorillonite (MMt) composites and nanocomposites present better properties than traditional polymer composites, like mechanical strength, barrier properties, and flame retardant properties. In the particular case of polyethylene (PE), it is difficult to get a well dispersion of Mt, as PE is a non-polar polymer. Sanchez Valdes et al. [68] compared the behaviour of CPN with two different compatibilizing agents MAH and a commercial zinc-neutralized carboxilateionomer, obtaining a slightly better mechanical performance with the first one. Zhai et al. [69] employed maleic anhydride grafted polyethylene (PE-g-MAH) to obtain OMt PE-g-MAH CPN with 1, 3 and 5 phr of OMt and compared the results with OMt PE CPN showing by XRD and TEM a much higher exfoliation degree of the OMt PE-g-MAH nanocomposites

Stoeffler et al. [70] also tried imidazolium and phosphonium salts together to pyridinium salts as surfactants for Mt to obtain PE nanocomposites. Most of authors modified the Mt with the amount of salt required to intercalate 100% of the CEC of the Mt. Mandalia and Bergaya [71] studied the effect of surfactant/CEC ratio, obtaining that the amount of surfactant had a direct effect on the interlayer separation of the clay, and with clay mineral having a high surfactant load (150 to 200% of CEC) the polymer intercalation was more homogeneous.

Sánchez-Valdés et al. [68] prepared the OMt LLDPE-g-MA CPN by melt blending in a twin screw extruder using two steps mixing and one step mixing. Most of authors describe no effect of the Mt on the thermal stability of PE composites and nanocomposites, but there are others who describe an increase or decrease in the stability. Anyhow, the described shifts in the thermal decomposition temperature are small and may depend on different factors, especially on the intrinsic stability of the modifying agent of the nanoclay. On the other hand, Zhai et al. [72] described a small increase in the decomposition temperature at the earlier stages of the decomposition process for OMt PE CPN containing 3 and 5 phr of OMt modified with an ammonium salt (with a C-18 tallow), but the higher increase in the thermal stability was found for the CPN containing 1 phr of OMt, which was justified by a higher exfoliation degree and which kept monolayer in the composite containing 1phr of OMt. The moduli is not significantly increased when OMt are used without compatibilizing agent. Despite the relatively soft compatibilizer molecules causes a decreases of the modulus (Morawiec et al. [73]) by themselves, the systems containing both, compatibilizers and nanoclay present higher modulus than the net PE. Sanchez-Valdes et al. [68] reported that the modulus was not significantly increased when the OMt were used without compatibilizing agent, but in those systems containing PE-g-MA the increase was around 70% (with 6% of OMt).

Sanchez-Valdes et al. [69] also found lower values of tensile strength and elongation at break for the nanocomposites than for pure PE, but those systems with compatibilizing agent had a less drastic reduction in deformation. The reduction in elongation at break was attributed to the fact that the inorganic mineral clay particles are rigid and could not be deformed by external stress, but acted only as stress concentrators during the deformation process. The oxygen permeability of the CPN obtained by these authors decreased as much as 40% when compared to the pure HDPE. According to Shahabadi and Garmabi [74], the interfaces between different phases of nanocomposites, especially the polymers and the inorganic particles, could deteriorate the barrier properties. OMt could also act creating voids (increasing the free volume), decreasing the polymer crystallinity, and also the