Nanocomposite
A nanocomposite is as a multiphase solid material where one of the phases has any one or all dimensions less than 100 nm, or structures having nanoscale repeat distances between the different phases that make up the material. This definition is more usually taken to mean the solid combination of a bulk matrix and nano-dimensional phase(s) differing in properties due to dissimilarities in structure and chemistry. By adding nanoparticles to a polymer matrix its performance can be enhanced greatly due to the nature and properties of the nanoscale filler. Polymer-based nanocomposites are becoming increasingly attractive in the field of industry. In order to achieve tailor-made properties, a large number of basic polymers are being applied together with minerals such as silicates, clays, carbon nanotubes, or metallic particles either as individual materials or as closely defined agglomerates.
Nano filler
Nano filler materials can be used to improve plastic composite materials. These materials require only less than 5 wt% of nano-filler as against conventional standard fillers to yield marked improvements in mechanical, thermal, electrical and other properties such as gas impermeability, dimensional stability and flame retardance. Furthermore, all of these benefits are available without sacrifices in composite density or light transmission of matrix resins. Nano fillers provide more than three times interfacial area per particle than conventional mineral fillers.
Nanoclays and nanotubes are two types of nano-fillers used in commercial products, such as carbon nanotubes and aluminum silicate nanotubes. But both types of nano-filler materials require surface modification prior to use.
Structure of clay material
Nanoclays for plastic composite refer to a category of clay minerals with a specialized structure, characterized by plate morphology such as modified montmorillonite clay with a 2-to-1 layered smectite clay mineral. Each layer has 2 tetrahedral sheets containing an octahedral sheet between them. Individual platelet thicknesses are just one nanometer, but surface dimensions are generally 300 to more than 600 nanometers, resulting in a very high aspect ratio. Hundreds or thousands of these layers are stacked together with van der Waals forces to form clay particles to have a thickness of only 1 nm.
Characteristics of clay material
For commercial applications, nanoclays are modified in a two-step process and incorporated into the resin matrix either during polymerization or by melt compounding. In the first step, the hydrophilic surface property of natural existing montmorillonite is changed to hydrophobic for compatibility with organophilic polymers that are generally used in composites. The most popular surface treatment method exchanges organic ammonium cations with existing inorganic cations on the clay’s surface and are known as organoclays.
Nanoclays normally exist as agglomerated bundles, consisting of thousands of platelets held together by van der Waals force. In the second step, individual clay platelets are separated from each other called exfoliation, but complete exfoliation is not always good. To get gas barrier function and promote good clarity of packaging materials, the platelets are fully exfoliated by combining surface compatibilizing agents and process shear in the plastic matrix. Complete exfoliation in polyolefin material decreases particle reinforcement, which may be a problem for automotive application. Commercially nanoclays are sold with more than 95% of exfoliation and even distribution within polyamide plastics.
Fabrication methods
Various fabrication methods such as in situ polymerization, Solution-induced intercalation and the melt process are used.
In situ polymerization
In this process polymer precursors are inserted between the clay layers to make them expand. The expanded clay layers are then dispersed into the matrix by polymerization to produce well-exfoliated nanocomposites applicable to a wide range of polymer systems, and are particularly useful for thermosetting polymers.
Solution-induced intercalation
This method can be used for commercial production. It uses solvent to swell and disperse clays into a polymer solution and mainly uses for water-soluble polymers because of the low cost of water as a solvent as well as its low health and safety risks. However, this method is not applicable to most engineering (i.e., non-water soluble) polymers because of high solvent cost, and health and safety concerns.
The melt process
This method can use traditional techniques such as extrusion and injection molding to produce nanocomposites and is easily adapted to commercial production. During the melt process, clays and polymers are intercalated with each other. The efficiency of intercalation is lower than in situ polymerization and often produces a partially exfoliated structure.
Applications
A combination of nylon and nanoclay is used for barrier layers in multilayer polyethylene terephthalate (PET) bottles and films for food packaging. Nanoclays in composite materials act as a barrier and arrest gas molecules permeation through the matrix resin. Also, because nanoclays do not degrade in the matrix, the gas barrier property is maintained over time. These clay nanocomposites are used for oxygen (O2) and carbon dioxide (CO2) sensitive products. Major applications of plastic nanoclay composite materials are in automotive parts, such as automotive body panels and under-hood components, and packaging. Nanocomposites provide high stiffness and impact resistance that are used for electrical parts, power-tool housing, appliance components, pallets, and dunnage.
An example of commercial nanocomposite materials on the market is timing belt cover and food packaging, based on nylon 6 with nanoclays marketed by Toyota. Many other companies produce plastic nanocomposite materials for packaging and automotive parts. Nanoclay can also be applied to reinforce fabrics by adding 1 wt% to improve mechanical properties of polypropylene fabrics. Nanoclay can potentially also be used in thermoplastic/natural fiber composites to improve their mechanical properties.