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.

Introduction

With their ease of processing and high performance, polymeric materials have become a common and important part of modern life. However, because almost all polymers are composed predominately of hydrocarbons, these materials are flammable and thus greatly increase fire hazard to human life and property. As estimated for the United States, there are approximately 400,000 residential fires each year, 20% involving electrical distribution and appliances, and 10% concerning upholstered furniture and mattresses. These fires kill about 4,000 people, injure 20,000 people, and result in property losses amounting to about US$4.5 billion. Flame retardants are additives that can make flammable materials more difficult to ignite and significantly reduce the spread of fire. Use of flame retardants plays a major role in fire safety, saving lives, and preventing injuries and property damage. For example, in 1974, the number of recorded television set fires in the United Kingdom was more than 2,300, whereas this number had decreased to 470 in 1989, despite the number of television sets in use increasing many times. This is because effective flame retardants were developed for television sets.

Introduction

It has been reported that approximately 12 persons are killed and 120 are severely injured because of fire every day in Europe. Fire has considerable impact on the environment in terms of destruction of substructures and production of toxic and/or corrosive compounds such as CO, dioxins, HCN, and polycyclic aromatic compounds. Consequently, it is necessary to limit this kind of risk by designing new materials with improved flammability properties. Nowadays, many companies (building and civil engineering, transportation, cable-making and electrotechnical material, etc.) are directly concerned with this topic.

Buildings contain increasing calorific value in the form of highly combustible polymeric materials replacing more traditional materials (wood, alloys, metals, etc.) with the aim of improving the comfort of occupants (pieces of furniture, carpets, toys, household and leisure electric components, and data processing equipment, etc.). Potential sources of fire tend to growwith the multiplication of electric and electronic devices. The increasing sophistication and miniaturization of electronics (with increasingly powerful and fast microprocessors) have as a consequence a stronger concentration of energy, leading to an increased risk of localized overheating and thus of fire.

Layered silicates

The idea of flame retardant materials dates back to about 450 BC, when the Egyptians used alum to reduce the flammability of wood. The Romans (in about 200 BC) used a mixture of alum and vinegar to reduce the combustibility of wood. Today, there are more than 175 chemicals classified as flame retardants. The major groups are inorganic, halogenated, organic, organophosphorus, and nitrogen-based flame retardants, which account for 50%, 25%, 20%, and >5% of the annual production, respectively.

In many cases, existing flame retardant systems show considerable disadvantages. The application of aluminum trihydrate and magnesium hydroxide requires a very high portion of the filler to be deployed within the polymer matrix; filling levels of more than 60 wt% are necessary to achieve suitable flame retardancy, for example, in cables and wires. Clear disadvantages of these filling levels are the high density and the lack of flexibility of end products, the poor mechanical properties, and the problematic compounding and extrusion steps.

Background

Organic polymers are rapidly and increasingly taking the place of traditional inorganic and metallic materials in various fields owing to their excellent properties, such as low density, resistance to erosion, and ease of processing. However, organic polymers are inherently flammable; their use can cause the occurrence of large fires and, consequently, loss of lives and properties. Thus, enhancing the flame retardancy of these organic polymers is becoming more and more imperative with their wider application, especially in fields such as electronics where high flame retardancy is required.

For traditional flame retardants, on one hand, a very high loading is usually needed to meet flame retardancy demands, which can lead to the deterioration of mechanical properties; on the other hand, utilization of flame retardants can cause environmental problems.

Introduction

Thermodynamically, the introduction of a solid particle into a polymer matrix either decreases or increases the interfacial energy, depending on the degree of interaction between polymer chains and solid surfaces. If strong absorption of the polymer chains on the surfaces takes place, the system can be approached through minimization of the interfacial energy, reducing the energy factors. Furthermore, the minimization of interfacial energy can be optimized by increasing the interfacial area of solid particles. Therefore, in order to maximize reduction of the interfacial energy, the solid particles need a large aspect ratio, making both layered silicates and carbon nanotubes (CNTs) good candidates. In particular, layered silicates cation-exchanged with organophilic surfactants can be delaminated into a single silicate sheet in a polymer matrix and remain as nanosheets with aspect ratio 100–1000. Because of this unique delamination of organophilic silicates, polymer–organoclay nanocomposites are of great interest in industry and academia. Numerous research groups have characterized and predicted the microstructures of polymer/organoclay nanocomposites using advanced techniques.