Introduction

Over the past decades, nanoclays have been widely used as additives to improve the strength as well as the fire performance of polymers, as evidenced by applications and a large number of studies reported in the literature. The mechanism of action of nanoclays is now relatively well understood, despite some aspects remaining unclear, such as the phenomena controlling ignition time. During the burning of polymer nanocomposites, a surface layer is formed on top of the virgin polymer, which acts as a mass and heat shield slowing down mass transfer of pyrolyzed gas to the surface, because less heat is transferred to unpyrolyzed material. Furthermore, in the presence of nanoparticles, the temperature at the surface of the surface layer increases far beyond the so-called ignition temperature of the polymer, which results in increased surface reradiation losses and, hence, decreased heat transfer to the solid. The formation of this surface layer has been observed in a number of studies using the cone calorimeter, where a significant reduction of the peak heat release rate (PHRR) compared with the corresponding pure polymer was observed for relatively thin samples. Zhang, Delichatsios, and Bourbigot also studied the effect of the surface layer numerically, finding that the reduction in heat transfer at the interface of the surface layer and the virgin polymer is inversely proportional to the number of nanoparticles that remain on the surface after degradation of the polymer (if the concentration of nanoparticles is less than about 10%).

Introduction

The development of polymer/clay nanocomposites as commercial materials faces the problem of limited miscibility of inorganic hydrophilic layered silicates and organic hydrophobic polymers. Intensive studies have led to various strategies, including the use of surface-active organic compounds, chemical modification of the polymer matrix, and application of macromolecular compatibilizers that produce a desired improvement of miscibility and therefore facilitate the formation of nanostructure. The application of organically modified clays provides certain properties to nanocomposite materials superior to those of systems containing sodium montmorillonite. However, ammonium salts, which are most frequently applied, suffer from thermal degradation during the fabrication and further processing of nanocomposites. This leads to changes in the surface properties of clays resulting in alteration of nanocomposite structure and related properties and facilitates the occurrence of some unwanted side reactions and the contamination of polymeric material with the products of thermal degradation of an organic modifier, which may be responsible for enhanced thermal degradation of the polymer matrix, accelerated aging, color formation, plasticization effects, and so forth. The need to improve the thermal stability of organoclays applied in the preparation of polymeric nanocomposites has motivated the search for an organic modifier combining high thermal stability with high efficiency in facilitating dispersion of a nanofiller in a polymer matrix.

Introduction

The term “nanocomposite” is widely used to describe a very broad range of materials, where one of the phases has a submicrometer dimension . In the case of polymer-based nanocomposites, this typically involves the incorporation of “nano” fillers with one (platelets), two (fibers, tubes), or all three dimensions at the submicrometer scale. However, strictly speaking, simply using nanometer-scaled fillers is not sufficient for obtaining genuine/true nanocomposites: these fillers must also be well dispersed down to individual particles and give rise to intrinsically new properties, which are not present in the respective macroscopic composites or the pure components. In this chapter, we shall use a broader definition, encompassing also “nanofilled polymer composites”, where – even without complete dispersion or in the absence of any new/novel functionalities – there exist substantial concurrent enhancements of multiple properties (for example, mechanical, thermal, thermomechanical, barrier, and flammability). Further, we shall limit our discussion to one example, focusing on poly(ethylene terephthalate) (PET) with mica-type layered aluminosilicates.

Introduction

The use of plastic materials, from construction materials to consumer electronics, has been increasing substantially in the past few decades. Easy processing, low density, and possible recycling make plastic the first choice of materials for many applications, such as automobile parts and food packaging structures. Comparing with traditional materials such as metal and concrete, plastic materials are combustible in a fire. Enhancing the flame retardation of plastic materials has been a priority in material development for many researchers. Many fire retardation standards have been established for relevant industries. Several trade organizations have also created industrial standards for fire safety standards and testing procedures. Underwriters Laboratories (UL) is an independent product safety certification organization that has been testing products and writing standards for safety for more than a century. UL has extensive standards and testing protocols for building materials, energy, lighting, power, and control, as well as wire and cables. The International Electrotechnical Commission (IEC) also publishes extensive standards for materials used in the electrical and electronic industries.

Polyimide/clay nanocomposites

The aim of this book is to provide comprehensive information about the two most important facets of polymer nanocomposites technology, thermal stability and flame retardancy. These two effects ensure a large number of potential applications of polymer nanocomposites. This book provides information regarding their mechanisms of action, as well as practical examples of recent advances in the generation of polymer nanocomposites that are thermally stable and flame retardant.

Introduction

Polymer/clay nanocomposites have received considerable attention during the past decade, both in industry and in academia, because of their attractive improvement of material properties relative to pure polymers and conventional polymer composites. The improvements include mechanical, thermal, flame retardant, and gas barrier performance. It is believed that the improvements are mainly attributable to the nanometric size dispersion of the clay and the specific interfacial interaction between the polymer matrix and clay layers.

The structure and properties of clays

The clays commonly used in polymer nanocomposites belong to the family of 2:1 layered silicates or phyllosilicates. The crystal structure of the clay layers is made up of two tetrahedrally coordinated silicon atoms, which are fused to an edge-shared octahedral sheet of either aluminum or magnesium hydroxide. The layer thickness is about 1 nm and the lateral dimension of the layers may vary from 30 nm to several micrometers or even larger, depending on the particular silicate. There is a van der Waals gap between the layers, usually called a gallery or interlayer. Isomorphic substitution within the crystal structure of the layer (for example, Al3+ replaced by Mg2+ or by Fe2+, or Mg2+ replaced by Li+) generates negative charges that are counterbalanced by alkali and alkaline earth cations situated inside the interlayer.

Introduction

A variety of discontinuous (short) functional fillers may be combined with thermoplastic or thermoset matrices to produce composites. The fillers may differ in shape (fibers, platelets, flakes, spheres, or irregulars), aspect ratio, and size. When the fully dispersed (exfoliated or deagglomerated) fillers are of nanoscale dimensions, the materials are known as nanocomposites. They differ from conventional microcomposites in that they contain a significant number of interfaces available for interactions between the intermixed phases. As a result of their unique properties, nanocomposites have great potential for applications involving polymer property modification utilizing low filler concentrations for minimum weight increase; examples include mechanical, electrical, optical, and barrier properties improvement and enhanced flame retardancy.

Introduction

Polymer/clay nanocomposites exhibit remarkable improvement in material properties relative to unfilled polymers or conventional composites. These improvements can include increased tensile modulus, mechanical strength, and heat resistance and reduced gas permeability and flammability. There are various methods of preparing polymer/clay nanocomposites: (i) in situ polymerization, (ii) solution intercalation, (iii) melt intercalation, and (iv) in situ template synthesis.

Nanoclays are difficult to disperse in polymer matrices, because of the strong attractive forces among the clay platelets and the commonly hydrophobic nature of polymers. Thus, it is necessary to modify pristine nanoclays in order to (i) render them compatible with most polymers and (ii) enlarge the basal spacing of clay to favor polymer intercalation. Several approaches are used to modify clays and clay minerals.

Introduction

Inorganic fillers have conventionally been added to polymer matrices to enhance their mechanical strength and other properties, as well as to reduce the cost of the overall composites. Layered aluminosilicates, also popularly described as clays, are one such type of filler, which are responsible for a revolutionary change in polymer composite synthesis as well as for transforming polymer composites into polymer nanocomposites. Aluminosilicate particles consist of stacks of 1 nm–thick aluminosilicate layers (or platelets) in which a central octahedral aluminum sheet is fused between two tetrahedral silicon sheets. Owing to isomorphic substitutions, there is a net negative charge on the surface of the platelets that is compensated for by the adsorption of alkali or alkaline earth metal cations. Because of the presence of alkali or alkaline earth metal cations on their surfaces, the platelets are electrostatically bound to each other, causing an interlayer to form in between. The majority of the cations are present in the interlayers bound to the surfaces of the platelets, but a small number of cations are bound to the edges of the platelets. Though the use of layered aluminosilicates has been documented in some older studies, indicating their potential for substantially improving polymer properties, reports from Toyota researchers in the early nineties attracted serious attention. In these studies, polyamide nanocomposites were synthesized by in situ polymerization in the presence of clay with organic modifiers.