1.2.3 State of R&D
Polymer nanocomposites (PNCs) are expected to find applications in automotive and aeronautics industries. Polymer nanocomposites comprise polymer matrixes (resins), mainly thermosets, thermoplastics and elastomers, that are reinforced with nano-sized particles which have high aspect ratios.
Common processing technologies to produce polymer nanocomposites are incorporating nanoparticles into a polymer matrix and also through obtaining nanostructured polymers via modification of the polymer structure during the synthesis by nanopatterning. Automotive / aeronautics industry mainly use the former technology via dispersing inorganic or organic nanoparticles into either a thermoplastic or thermoset polymer. In this way it is not necessary to develop new resin systems and the main changes on the composite manufacturing processes are related to processing parameters.
Vacuum assisted processing (VAP) technologies are very favourable by the industry due to its cost, processing speed and environmentally friendliness. However, so far this method is not compatible for processing polymer nanocomposites.
For their widespread industrial application, PNCs should be net-shape mouldable and may be extruded, allowing consolidation of parts and reduction in assembly step. For example, nanofillers need to be easily incorporated into resin, which would be followed by forming pellets to be translated into automotive parts. Each step has great challenges to be met to produce competitive parts for specific uses. Each application may require variations in the process to match the part specifications and the processing methods used to make the part.
Hand lay-out and injection moulding methods are very common for the production of composite automotive parts. There are different structural injection moulding methods. Structural reaction injection moulding (SRIM), reinforced reaction injection moulding (RRIM) and sheet moulding compound (SMC) are the most common ones. Resin transfer moulding (RTM) is another common method used to manufacture both internal and external structural parts like floors, doors, side frames and bumpers and also under hood parts.
Nanoparticles used in PNCs can be three-dimensional spherical and polyhedral nanoparticles (e.g. colloidal silica), two-dimensional nanofibers (e.g. nanotube, whisker) or one-dimensional disc-like nanoparticles (e.g. clay platelet).
Dispersion of the nanofillers in the polymeric matrix is critical to make most out of their potential. The nanofiller dispersion can be obtained only by a fine optimization the processing. In fact the nanofillers show a tendency to remain agglomerated due to different kinds of physicals-chemicals interaction. The separation of nanofillers is in many cases difficult due to the dimensions of the polymer chains. Apart from dispersion, compatibility with the resin is very critical.
In the rest of this section different nanofillers and relevant processing technologies are explained in detail.
Nanoclays play an important role in composites due to their enhanced flame retardancy, barrier properties and improved balance of stiffness and toughness. Moreover, the fact that nanoclays are cheaper than other nanomaterials is a reason for their extended use in automotive and aeronautics industry. Nanoclays are produced in existing, full-scale production facilities which make them cheaper. Automotive and aeronautics industry uses nanoclays as fillers in polyolefin nanocomposites, thermoplastic olefin (TPO) clay nanoolefins, etc.
One general approach to prepare polymer nanocomposites is to employ intercalation chemistry of layered inorganic solids in which polymer is inserted into the interlayer space. Clay minerals are ones that are preferred as fillers for polymer nanocomposites due to their rich intercalation chemistry, high strength and stiffness, high aspect ratio of individual platelets, abundance in nature and low cost. One of the challenges for their use is the incompatibility between hydrophilic clay and hydrophobic polymer which causes agglomeration of clay particles in the polymer matrix. However, this could be overcome either with surface modification of clay through organic treatment or via other mechanical solutions as explained below.
Common methods for processing clay based polymer nanocomposites are in-situ polymerization, solution exfoliation and melt intercalation.
When layered clays are filled into a polymer matrix, either conventional composite or nanocomposite can be formed depending on the nature of the components and processing conditions. Schemes of conventional composite and nanocomposite are shown in Figure 1. Conventional composite is obtained if the polymer cannot intercalate into the galleries of clay minerals. The properties of such composite are similar to that of polymer composites reinforced by microparticles. There are two extreme nanostructures resulting from the mixing of clay minerals and a polymer providing a favour conditions. One is intercalated nanocomposite (I), in which monolayer of extended polymer chains is inserted into the gallery of clay minerals resulting in a well ordered multilayer morphology stacking alternately polymer layers and clay platelets and a repeating distance of a few nanometers. The other is exfoliated or delaminated nanocomposite (II), in which the clay platelets are completely and uniformly dispersed in a continuous polymer matrix. However, it should be noted that in most cases the cluster (so-called partially exfoliated) nanocomposite (III) is common in polymer nanocomposites.
In in-situ polymerization, monomers are intercalated into layered clays and polymerized (cured) via heat, radiation, pre-intercalated initiators or catalysts. In-situ polymerization is a conventional method for thermoset composites being suitable for low or non-soluble polymers e.g. nylon 6, epoxy, polyurethane, polystyrene, polyethylene oxide, unsaturated polyesters, polyethylene terephthalate. In this method, clay exfoliation depends on the extent of clay swelling and diffusion rate of monomers in the gallery. Olimer may be formed upon incompletely polymerization which leads to lack of quality.
In the solution exfoliation (also known as solution dispersion), layered clays are exfoliated into single platelets using a solvent in which the polymer is soluble. The polymer is then mixed with the clay suspension and adsorbed onto the platelets. Later on, the solvent evaporates leaving the clay-polymer complex. This method is preferable for water-soluble polymers as such epoxy, polyimide, PE, PMMA. However, this method has its disadvantages like (i) use of large quantities of solvent, (ii) unavailability of compatible polymer-clay solvent system and (iii) possibility of solvent and polymer co-intercalation.
In the case of melt processing (also known as melt compounding, melt intercalation, melt moulding), layered clays are mixed with the polymer matrix in the molten state. It is a preferred method applicable for thermoplastics due to the fact that no solvent is required and it allows processing of PNCs with conventional plastic extrusion and moulding technology. Nylon 6, PS and PET are some of the polymers that could be processed with this method. Melt intercalation is a convenient method for processing PNCs in industrial applications due to its speed.
All the previously mentioned methods allow use of very low amounts of nanoclay loading for polymer nanocomposites making the overall density similar to pure polymer, therefore does not increase the weight of the part. Also addition of nanoclays improve the processing capability for film or fibers, which is unlikely in conventional polymer composites.
The cost of nanoclay is reasonable for its use in automotive applications, 5 - 7 €/kg and considerably less amounts of nanoclay could replace for example glass fibres in a composite part. In a composite part which is comprised of 35% of glass fibre only a replacement of 7% nanoclay can be used if in-situ polymerization is preferred. In the case of melt processing incorporation of 5% nanoclay would be sufficient. In Europe aluminium trihydroxides (ATH) is usually used to improve flame retardance of composite parts in vehicles. They require loadings of 60 to 65% in composite parts. This loading ratio can be reduced to 15-20% just by adding 3 to 5 percent of the nanoclay.
Apart from decreasing the overall weight of the part, addition of nanoclays has a considerable impact on the processing conditions of the PNCs. For example, introduction of nanoclay, Cloisite 93A into nylon6 caused the melting temperature of the Nylon composite to drop about 3°C. The melting temperature of a polymer is dependent on the size of its crystal lamellae or region. After organoclay nano-sized platelets were introduced into the polymer melt, the platelets were dispersed between the free polymer chains through intercalation and exfoliation effects. As the polymer cools, the platelets may form obstacles restricting the formation of crystalline blocks, forming amorphous regions, and thus reducing the crystal size of the nylon6 in the PNC. As the size of the crystal lamellae decreases, the melting temperature of the polymer decreases.
The most suitable method for industrial applications of polymer nanocomposites is the melt-processing method. The main problem encountered in melt-intercalation is the thorough and uniform dispersion of fillers in the resin matrix to which different solutions have been developed.
In both melt processing and in-situ polymerization there is transition from liquid like behaviour to solid-like behaviour. For this reason, it is crucial to understand this behaviour to be able to better control the process.
Industrial case study for use of nanoclays in PNCs:
As mentioned before polypropylene is one of the most widely used thermoplastics in automotive parts. Accordingly, the polypropylene-clay nanocomposite (PPCN) is an attractive material due to its potential effect in weight reduction. Synthesis of PPCN via an in-situ polymerization process was reported. However, this method was not found to be fast enough for industrial production. Researchers from Toyota reported the industrial compatibility of the melt-compounding process for preparing PPCN. The drawback of this process in its existing form is that pre-treatment of the clay mineral is necessary to achieve a nanometer-scale dispersion of the silicate layers. This pre-treatment process is known as organo-modification and it has to be carried out separately from the melt compounding in an extruder. For organo-modification first, a granular clay mineral is dispersed in water. The organic cation is then added to the dispersion, followed by filtration, drying and milling. This makes the process tedious and costly.
A new production method for PPCN using non-pretreated clay was developed at Toyota. In this method, the nature of the clay material when it was exfoliated in the water was taken into consideration. The basic concept of this method was that clay slurry was achieved in the twin-extruder by adding the clay mineral and the water separately. For this purpose, a new twin-screw extruder was designed (for details of the design please see Kato et al, polymer Eng. and Science). This method facilitated the dispersion of the clay particles even for more than 5wt% clay.
As it is the case for nanoclays, also for CNTs, filler dispersion as well as interfacial interactions have been shown to be crucial for both incorporating CNTs into the resin and reaching enhanced mechanical properties in PNCs. One of the biggest challenges is to obtain a homogeneous dispersion of CNTs in a polymer matrix because van der Waals interactions between individual tubes often lead to significant aggregation or agglomeration, thus reducing the expected property improvements of the composite.
Different methods are used to optimize the CNT dispersion within the resin. Some of them are solution mixing, ultrasonification, coagulation, melt compounding, in-situ polymerization and chemical functionalization of the tube surface. The purpose of all of these methods are to separate the individual nanotubes so as to get a homogeneous dispersion throughout the matrix while chemical functionalization intends to bring some adhesion between the nanotubes and the polymer thus enabling effective stress transfer at the polymer interface. It needs to be taken into account that carbon nanotubes have to be functionalized for their incorporation into the resin independent of the method used for dispersion. All of these methods provide dispersion of CNTs within the resin. However, all of them lead to degradation of mechanical properties of CNTs which also effect the final properties and performance of the composite part.
CNT enhanced PNC parts can be manufactured via injection moulding, extrusion, melt compounding, resin transfer moulding, sheet moulding compound, etc. Processing of PNC parts using CNT enhanced prepregs is promising.
In the injection moulding process the polymer melt is forced to flow through the gate, runner and mould cavity system. The geometry of the flow channel can significantly affect the end property of the injection moulded products, especially when the materials being processed show anisotropic properties when oriented differently. For example, polymer nanocomposites reinforced with CNTs may show different properties at different directions as the CNTs with high aspect ratios may orient differently at different direction which also effect the final electrical properties of CNT enhanced PNC. In the flow orientation direction, drastic improvement in conductivity can be achieved, while in the transverse directions, no obvious improvement is observed. For this reason, controlled alignment of CNTs is important to get most out of PNCs when CNTs are used as fillers.
PNC processing methods that are used for nanoclays can also be used for composites where CNTs are used as fillers. The most common problems encountered are entangling and uncontrolled alignment of CNTs in the resin. And also increased viscosity which decreases the processing speed in all melt processing methods RTM, RIM, Vacuum assisted methods.
Another way to process CNT incorporated polymer composites parts is to prepare prepregs with carbon nanotubes. Vehicle parts can be processed using CNT enhanced prepreg. CNT enhanced prepregs are handled like traditional prepregs. Therefore, use of CNT does not lead to a change in the processing method of parts. Zyvex Performance Materials is the pioneer in use of CNT enhanced prepregs. [Retrieved on 14th of May 2009 from http://www.azonano.com/news.asp?newsID=11449]
Cost and production of large amounts of good quality CNTs is an important problem for their widespread use in polymer nanocomposites. Besides, there are problems related to their dispersion and also handling and manipulation of them for their incorporation into resins.
One method, which has been heavily investigated to overcome the problems related to integration of CNTs into resins is to grow them directly on carbon fibres. Some experts believe that this would be the real breakthrough. While others point out to the problems occur as a result of this alternative method: The adherence of grown carbon nanotubes onto the substrate, in this case it is carbon fibre, is not really under control. Consequently, it cannot be assured that whole structure is firm. Moreover, after the growth of carbon nanotubes, carbon fibres are partially destroyed which lead to degradation of the mechanical properties.
It is important to overcome the challenges related to processing of CNT incorporated PNCs because of the advances CNTs bring to composites. A recent discovery by Prof. Catalin Picu at RensselaerPolytechnic Institute shows that addition of CNTs into epoxies lead to tougher, stiffer, harder and more durable composites. Epoxy composites are preferred due to their lightweight. However, their low toughness and fatigue resistance has been a disadvantage until now. Incorporation of treated CNTs in epoxies lead to a 20 fold reduction in the crack growth rate under repetitive stress as compared to an epoxy composite frame made without nanotubes. [Rensselaer Polytechnic Institute (2009, April 3). Fitter Frames: Nanotubes Boost Structural Integrity Of Composites. ScienceDaily]
Vapour grown carbon nanofibers (CNFs), due to their high tensile strength, modulus, and relatively low cost, are drawing significant attention for their potential applications in nano-scale polymer reinforcement. They are synthesized from pyrolysis of hydrocarbons or carbon monoxide in the gaseous state, in the presence of a catalyst. Vapour grown CNFs distinguish themselves from other types of nanofibers, such as polyacyrlonitrile or mesophase pitch-based carbonfiber, in their method of production, physical properties, and structure. Thermoplastics such as polypropylene, polycarbonate, nylon, and thermosets such as epoxy, as well as thermoplastic elastomers such as butadiene-styrene diblock copolymer, have been reinforced with CNFs.
Carbon nanofibres contribute to improve the materials used in automotive industrial applications with the mechanical, electrostatical and tribological properties.
Graphene is a two-dimensional form of carbon which has attracted huge interest since its entrance into the game since 2006 due to its extraordinary mechanical and electronic properties. Graphene has been researched primarily for its potential applications in electronics. But its foreseen applications in composites especially for automotive and aeronautics industry are also promising. It is the toughest material ever known for now. Since they are two dimensional and have very low aspect ratio (height to length ratio) entanglement problems faced with its other counterparts, CNTs, CNFs and nanoclays do not arise in their use in PNCs. Besides, since two dimensional platelets of graphene can slide over each other, it does not increase the viscosity of the resin in the molten state. Apart from their easier incorporation and processing advantages, graphene is much cheaper than CNTs which makes its industrial uptake / application more realistic. According to a US company XG Sciences which provides graphene it costs 10$ / lb while SWNT costs 500$/lb. Graphene is stated to provide more or less the same thermal and barrier properties to a PNC while bringing drastic improvements in electrical and mechanical properties compared to nanoclays. [xgsciences.com/applications] The challenge for use of graphene in PNCs was to be able to have right kind of graphene sheets which was realized by Dr. Brenson et al. at Northwestern University. XG Sciences a spin-off company from Michigan State University, established a pilot plant to produce already graphene incorporated polymers. In this method, instead of individual platelets stacks of platelets are used which is claimed to be stiffer than its individual counterparts. [http://www.technologyreview.com/computing/20821/page2/]
When added into resin, graphene changes the glass transition temperature of the resin and makes lower temperature processing possible. Graphene could be added to thermosets, thermoplastics and elastomers. Melt processing methods, RTM, injection moulding and extrusion can be used for processing graphene platelets incorporated PNCs. [http://www.mfrtech.com/articles/1948.html]
Although graphene is cheap and its incorporation into polymer resin seems to be easier in its lab-scale applications, the research in this field has not been mature enough to state that graphene based polymer nanocomposites would be the most promising ones for automotive and aeronautics industry. Since it is possible to use graphene as a thin coating on different surfaces it would find diverse applications at parts where electromagnetic interference shielding is critical.
Core shell rubber:
Core shell structure is produced with a tangle of polymerized polyolefin rubber forming a ball with functionalized groups hanging out like bristles. When embedded in plastics like polypropylene, nylon, polycarbonate, epoxy resins, etc. these functional groups can combine with the plastic and improve the adhesion of the rubber with the plastic. As a result core shell rubber improved plastics become less brittle.
The researchers manufacture these tiny rubber balls in a one-pot procedure that causes the rubber components to cross-link into the shape of a tiny rubber ball with their functional groups intact. Addition of a surfactant causes the polymers to entangle into a ball with some of the functional groups sticking out from the surface. This way the process can be controlled better and the size of the particles from micron-sized to nanoparticles can be produced in a controlled manner.
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