|NANOCOMPOSITES – TECHNOLOGY TRENDS & APPLICATION POTENTIAL|
Sangeeta Baksi, P R Basak & Soumitra Biswas
Nano-composites have gained much interest recently. Significant efforts are underway to control the nano-structures via innovative synthetic approaches. The properties of nano-composite materials depend not only on the properties of their individual parents but also on their morphology and interfacial characteristics.
By optimized fabrication process and controlled nano-sized second phase dispersion, thermal stability and mechanical properties such as adhesion resistance, flexural strength, toughness & hardness can be enhanced which can result into improved nano-dispersion.
The possibilities of producing materials with tailored physical & electronic properties at low cost could result in interesting applications ranging from drug delivery to corrosion prevention to electronic/automotive parts to industrial equipment and several others.
The nano-composite material is an innovative product having nano (one-billionth of a meter) fillers dispersed in the matrix. Typically, the structure is a matrix-filler combination where the fillers like particles, fibers, or fragments surrounds and binds together as discrete units in the matrix.
The term nano-composite encompasses a wide range of materials right from three dimensional metal matrix composites, two dimensional lamellar composites and nano-wires of single dimension to zero-dimensional core-shells all representing many variations of nano-mixed & layered materials.
Though various composite materials like fiberglass and reinforced plastics are now in wide use for numerous applications, there has been continued demand for novel composites with desirable properties for many other applications.
Nano-particles and nano-layers have very high surface-to-volume and aspect ratios and this makes them ideal for use in polymeric materials. Such structures combine the best properties of each component to possess enhanced mechanical & superconducting properties for advanced applications.
The properties of nano-composite materials depend not only on the properties of their individual parents but also on their morphology and interfacial characteristics. Some nanocomposite materials could be 1000 times tougher than the bulk component. The general class of nanocomposite organic/inorganic materials is a fast growing area of research.
The inorganic components can be three-dimensional framework systems such as zeolites, two-dimensional layered materials such as clays, metal oxides, metal phosphates, chalcogenides, and even one-dimensional and zero-dimensional materials such as (Mo3Se3-)n chains and clusters. Thus, nanocomposites promise new applications in many fields such as mechanically reinforced lightweight components, non-linear optics, battery cathodes, nano-wires, sensors and other systems.
By engineering the polymer-host interactions, nanocomposites could be produced with the broad range of properties. Lamellar nano-composites can be divided into two distinct classes viz. intercalated and exfoliated. In the former, the polymer chains are alternately present with the inorganic layers in a fixed compositional ratio and have a well-defined number of polymer layers in the intralamellar space.
In exfoliated nano-composites, the number of polymer chains between the layers is almost continuously variable and the layers stand >100 Å apart. The intercalated nano-composites are useful for electronic and charge transport properties. On the other hand, exfoliated nano-composites possess superior mechanical properties.
For example, the electronics industry utilizes materials that have high dielectric constants and that are also flexible, easy to process, and strong. Finding single component materials possessing all these properties is difficult.
The most commonly used ceramic materials with high dielectric constant are found to be brittle and are processed at high temperatures, while polymer materials, which are very easy to process have low dielectric constants.
Composite materials having micron-scale ferroelectric ceramic particles as the filler in liquid crystal polymer, fluoropolymer, or thermoplastic polymer matrices do not possess ideal processing characteristics and are difficult to form into the thin uniform films used for many microelectronics applications. Here comes the necessity of utilizing nanocomposite materials having a wide range of materials mixed at the nanometer scale.
Polymer nanocomposites consist of a polymeric material (e.g., thermoplastics, thermosets, or elastomers) with reinforcement of nano-particles. Polymer could be incorporated either as the polymeric species itself or via the monomer, which is polymerised in situ to give the corresponding polymer-clay nanocomposite. Most commonly used nano-particles include:
• Montmorillonite organoclays (MMT)
Thermosets and thermoplastics used as matrices for making nanocomposites include:
This in turn could result in significant weight reductions particularly in military and aerospace applications, greater strength and increased barrier performance for similar material thickness, whereas, the micro-dimensional particles/additives require much higher loading levels to achieve similar performance.
There are a few disadvantages associated with using nanoparticle viz. toughness and impact performance. Some researches have shown that nanoclay modification of polymers such as polyamides could even reduce impact performance.
There is a need for better understanding of formulation/structure/property relationships to platelet exfoliation and dispersion etc. The improved properties vis-à-vis the disadvantages of the nano-particles & resultant composites are shown in Table 1:
Table 1: Important Characteristics of Nano-composites
Due to the higher surface area available with nanofillers, polymer nanocomposites offer the potential for enhanced mechanical properties, barrier properties, thermal properties and flame retardant properties when compared to conventionally filled materials.
Currently practiced processes for forming nanocomposites generally include individual steps for polymerizing each of the various
monomers followed by pelletization of each of the various polymers thus formed separately. After the individual polymers are pelletized, the formed pellets may be mixed with a nanofiller material in an extruder to form the nanocomposite material. While this process may be efficient for forming nanocomposites, at some instances they appear to be relatively expensive.
Three dimensional metal matrix composites, two dimensional lamellar composites and one dimensional nanowires and zero-dimensional core-shells all represent the various nano-mixed and layered materials. These method of construction combines the best properties of each of the components or give rise to new and unique properties for many advanced applications.
Nanocomposite could also be produced by dispersion of multi-layered silicate material into a thermoplastic polymer at a temperature greater than the melting or softening point of the thermoplastic polymer. The thermoplastic polymer is selected from the group consisting of a thermoplastic urethane, a thermoplastic epoxy, polyester, nylon, polycarbonate and their blends.
Published reports reveal that carbon nanotube-reinforced composites could be synthesized using a powder mixing process with a powder-powder blending between carbon nanotubes and ceramic powder or raw metal like aluminum or copper matrix followed by a conventional sintering process.
However, characterization of these carbon nanotube-reinforced composite materials has shown a decrease in mechanical properties. In particular, the relative density of the sintered composite materials becomes very low, ranging from 85% to 95%, which is important since low relative density means the existence of many fracture sources, such as pores and defects, which could be result in low mechanical properties.
The reasons behind these problems are due to severe agglomeration of carbon nanotubes on the metal powder surface and the use of conventional consolidation processes. However, carbon nanotubes agglomeration in a metal matrix could be prevented by homogeneous dispersion of carbon nanotubes in the metal matrix. For homogeneous dispersion, carbon nanotubes may be dispersed in a predetermindeddispersing solvent like water, ethanol, nitric acid solution, toluene, N,N-dimethylformamide,
dichlorocarbene and thionyl chloride to form a dispersed solution, which is further treated with ultrasonic wave. Water-soluble metal salts or metal hydrates are mixed with the ultrasonic wave treated dispersed solution, dried to remove water vapor, hydrogen, nitrogen and finally calcinated to produce a stable carbon nanotube / metal oxide nanocomposite powder.
Metal nanocomposite powders can be used as high-valued abrasive materials or wear-resistant coating materials. The metal nanocomposite powder could further be applied in industrial fields which utilize conventional metal composite materials, such as the aerospace, high-performance machine parts, and medical industry, because it has high sintering performance and easily becomes bulky.
3.2 Thermoplastic & Thermoset based Nanocomposites
Thermoplastic materials could be classified as metals, ceramics or polymers. However, the lower densities of polymeric materials offer an added advantage in applications where lighter weight is desired. The addition of thermally and/or electrically conducting fillers helps in developing conducting type nanocomposites.
Thermoplastic nanocomposites are used in a wide array of applications. These includes automotive sector for interior parts and under-the-hood applications, in packaging industry for carbonated beverage bottles, plastic wrap etc. The property comparison between thermoplastic & thermoset resins is given in Table 2.0.Table 2.0 Comparisons of Thermoplastic and Thermosetting Resin Characteristics
3.2.1 Nylon 6 Nanocomposites
Mica-type silicates are attractive nanoclays functioning as reinforcing fillers for polymers because of their high aspect ratio and unique intercalation and exfoliation characteristics. The incorporation of organoclays into polymer matrices has been known for many decades. In 1976, Fujiwara and Sakomoto of the Unitika patented the first organoclay hybrid nanocomposite.
Ten years later, a Toyota research team disclosed improved methods of producing nylon 6–clay nanocomposites using in-situ polymerization. The resulting nylon 6–clay nanocomposites exhibited increased solvent resistance, reduced permeability and increased flame retardant characteristics.
The nanocomposites could also be obtained by direct polymer intercalation, where polymer chains diffuse into the space between the clay galleries. This process could be combined with conventional polymer extrusion to reduce the time to form these hybrids, by shearing clay platelets leading to sample uniformity.
Dennis and Paul et al. demonstrated that both the chemistry of the clay surface and the type of extruder with its screw design affect the degree of exfoliation and dispersion of layered silicate nanocomposites formed from polyamide 6. Excessive shear intensity or back mixing also causes poor exfoliation and dispersion.
The exfoliation and dispersion could be improved by increasing the mean residence time in the extruder. Increased residence time in a low-shearing or mildly shearing environment allows polymer to enter the clay galleries and peel the platelets apart. It is reported that the non-intermeshing, twin-screw extruder yields the best exfoliation and uniformity of dispersion.
The variables such as type of clay, the choice of clay pre-treatment method, the selection of polymer component and the way in which the polymer is incorporated has profound influence on the nature and properties of the final nanocomposite. The purity of the clay and homogeneity of dispersion of clay also affect the properties of nanocomposite.
Polymer and clay are intrinsically non-miscible due to difference in their polarity. Polymers constitute non-polar, organic material whereas the clay is more polar and inorganic material. Therefore for successful formation of polymer-clay nanocomposites, clay polarity needs to be altered to make the clay ‘organophilic’.
This could be carried out by using swelling agents like surfactants which increases the interlayer distance of clay structure before it is mixed with a monomeric material, which is then polymerized in the presence of the clay to form nanocomposites.
The clay-based nanocomposite could be produced in the form of an intercalated or exfoliated hybrid structure (Figure 1). In the case of an intercalate, the organic component could be inserted between the layers of the clay such that the inter-layer spacing is expanded, but the layers still bear a well-defined spatial relationship to each other. In an exfoliated structure, the layers of the clay are completely separated and the individual layers are distributed throughout the organic matrix. Complete clay particles could also be dispersed within the polymer matrix as conventional filler.
Figure 1: Formation of intercalated and exfoliated nanocomposites from layered silicates and polymersWhether particular organo-clay hybrid nanocomposites are synthesized as an intercalated or exfoliated structure depends on the exchange capacity of the clay, polarity of the reaction medium and the chemical nature of the interlayer cations.
3.2.3 Thermoset Based Nanocomposites
Thermoset nanocomposites are complex hybrid materials, which integrate nano-particles with polymers to produce a novel nano-structure, with extraordinary properties. Thermoset polymer nanocomposites have received less interest in their scientific development and engineering applications than their thermoplastic counterparts.
However, some of these materials could be relatively easy to bring into production. The understanding of characteristics of the inter-phase region and the estimation of technology-structure-property relationships are the current frontier researches in thermoset nanocomposites.
The experimental results of work on thermoset nanocomposites and analyses obtained from the collaboration of three research groups from Bulgaria, Greece and Italy reported that the engineering resin nanocomposites are restricted to the most commonly used thermosets, such as epoxy resins, unsaturated polyesters, acrylic resins, and so on.
Various nan-oparticles have been found to be useful for nanocomposite preparation with thermosetting polymers, along with smectite clay, diamond, graphite, alumina and ferroxides.
Thermoset nanocomposites results in improved dimensional/thermal stability, flame retardancy and chemical resistance and have potential applications in marine, industrial and construction markets. Such nanocomposite materials are particularly suitable to be used for a large variety of applications.
The materials are eminently processable and can be shaped in conventional shaping steps, such as injection molding and extrusion processes. Shaped articles of a variety of natures can be manufactured from such nanocomposite materials e.g. fibers, packaging materials and construction materials.
The number of commercial applications of nanocomposites have been growing at a rapid rate. It has been reported that by 2010, the worldwide production is estimated to exceed 600,000 tonnes and is set to cover the following key areas in the next five to ten years:
• Drug delivery systems
Improvements in mechanical property have resulted in major interest in nanocomposite materials in numerous automotive and general/industrial applications. These include potential for utilization as mirror housings on various vehicle types, door handles, engine covers and intake manifolds and timing belt covers.
More general applications currently being considered include usage as impellers and blades for vacuum cleaners, power tool housings, mower hoods and covers for portable electronic equipment such as mobile phones, pagers etc.
4.1 Food Packaging
The gaseous barrier property improvement that can result from incorporation of relatively small quantities of nanoclay materials has been shown to be substantial. Data provided from various sources indicate oxygen transmission rates for polyamide-organoclay composites, which are usually less than half of the unmodified polymer.
Further data reveals the extent to which both the amount of clay incorporated in the polymer, and the aspect ratio of the filler contributes to overall barrier performance. In particular, aspect ratio has been shown to have a major effect, with high ratios (and hence tendencies towards filler incorporation at the nano-level) quite dramatically enhancing gaseous barrier properties.
Development of a combined active/passive oxygen barrier system for polyamide-6 materials is underway at various laboratories across the world. Passive barrier characteristics are provided by nanoclay particles incorporated via melt processing techniques whilst the active contribution comes from an oxygen-scavenging ingredient.
Oxygen transmission results reveal substantial benefits provided by nanoclay incorporation in comparison to the base polymer (rates approximately 15-20% of the bulk polymer value, with further benefits provided by the combined active/passive system).
Increased tortuosity provided by the nanoclay particles essentially slows transmission of oxygen through the composite and drives molecules to the active scavenging species resulting in near zero oxygen transmission for a considerable period of time.
Such excellent barrier characteristics have resulted in considerable interest in nanoclay composites in food packaging applications, both flexible and rigid.
Specific examples include packaging for processed meats, cheese, confectionery, cereals and boil-in-the-bag foods, also extrusion-coating applications in association with paperboard for fruit juice and dairy products, together with co-extrusion processes for the manufacture of beer and carbonated drinks bottles. The use of nanocomposite packaging would be expected to enhance considerably the shelf life of many types of food.
4.2 Fuel Tanks
The ability of nanoclay incorporation to reduce solvent transmission through polymers such as polyamides has been demonstrated. Available data reveals significant reductions in fuel transmission through polyamide–6/66 polymers by incorporation of nanoclay filler.
As a result, considerable interest is now being seen in these materials as both fuel tank and fuel line components for cars. Of further interest for this type of application, the reduced fuel transmission characteristics are accompanied by significant material cost reductions.
The presence of filler incorporation at nano-levels has also been shown to have significant effects on the transparency and haze characteristics of films. In comparison to conventionally filled polymers, nanoclay incorporation has been shown to significantly enhance transparency and reduce haze.
With polyamide based composites, this effect has been shown to be due to modifications in the crystallization behaviour brought about by the nanoclay particles.
Similarly, nano-modified polymers have been shown, when employed to coat polymeric transparency materials, to enhance both toughness and hardness of these materials without interfering with light transmission characteristics. The ability to resist high velocity impact combined with substantially improved abrasion resistance was also demonstrated.
4.4 Environmental Protection
Water laden atmospheres have long been regarded as one of the most damaging environments, which polymeric materials can encounter. Thus an ability to minimize the extent to which water is absorbed can be a major advantage.
Available data indicate that significant reduction of water absorption in a polymer could be achieved by nanoclay incorporation. Similar effects could also be achieved with polyamide-based nanocomposites. Specifically, increasing aspect ratio diminishes substantially the amount of water absorbed, thus indicating the beneficial effects likely from nanoparticle incorporation compared to microparticle loading.
Hydrophobicity enhancement would clearly promote both improved nanocomposite properties and diminish the extent to which water would be transmitted through to an underlying substrate. Thus applications in which contact with water or moist environments is likely could clearly benefit from materials incorporating nanoclay particles.
4.5 Flammability Reduction
National Institute of Standards and Technology in the US has demonstrated the extent to which flammability behaviour could be restricted in polymers such as polypropylene with as little as 2% nanoclay loading. In particular heat release rates, as obtained from cone calorimetry experiments, were found to diminish substantially by nanoclay incorporation.
Although conventional microparticle filler incorporation, together with the use of flame retardant agents would also minimize flammability behaviour, this is usually accompanied by reductions in various other important properties. With the nanoclay approach, this is usually achieved whilst maintaining or enhancing other properties and characteristics.
Nanotechnology is revolutionizing the world of materials. It has very high impact in developing a new generation of composites with enhanced functionality and a wide range of applications. The data on processing, characterization and applications helps researchers in understanding and utilizing the special chemical and material principles underlying these cutting-edge polymer nanocomposites.
Although Nanocomposites are realizing many key applications in numerous industrial fields, a number of key technical and economic barriers exist to widespread commercialization. These include impact performance, the complex formulation relationships and routes to achieving and measuring nanofiller dispersion and exfoliation in the polymer matrix. Investment in state-of-the-art equipment and the enlargement of core research team’s is another bottleneck to bring out innovative technologies on nanocomposites.
Future trends include the extension of this nanotechnology to additional types of polymer system, where the development of new compatibility strategies would likely to be a prerequisite. Production of PVC-based systems is still some way off and challenges remain to be solved in PET nanocomposites. Additional reinforcement of clay nanocomposites by glass fibre is currently being investigated. There is also interest in the development of electrically conducting clay nanocomposites.
While considerable basic research activities are currently underway at Indian academic institutions & national research labs, immediate exercises on product development-cum-demonstration should be taken up in active collaboration with the industries in the country.