1.2.2 Short description
Automotive and aeronautics industry uses high amounts of advanced plastics. For example an average car has about 160 kilograms of plastic parts. For this reason, any further change in the plastic materials that would result in weight reduction of vehicles' components is very important and accordingly one of the main drives behind the research in polymer nanocomposites. Since PNCs have low amounts of additives which are lighter than traditional additives like glass fibers, they decrease the weight of the vehicles in considerable amounts.
Composite materials are a combination of two or more materials with different properties in the form of a matrix (metal/ceramic/polymer) material and a reinforcing material (glass/carbon/etc) or filler materials or additives to core materials. Composite materials exhibit different physical properties than their constituents even if the constituting materials retain their pristine structural properties. Reinforcing materials are surrounded by the matrix materials. The matrix holds the reinforcing material together while protecting it from external damage and stress. Reinforcements carry the load along the length of the fiber or to provide strength and stiffness in one direction.
In composites where the matrix is a polymeric material then matrix is named as resin. Resins are divided into two groups according to their behaviours as thermosets and thermoplastics.
Thermosets are usually liquids or low melting point solids in their pristine form. They are cured by the use of a catalyst, heat or a combination of the two when they are used to produce finished goods. Once cured, thermoset resins cannot be converted back to their original form. Accordingly, recyclability of pieces made out of thermosets is not possible. The most common thermosets used in the polymer industry are unsaturated polyesters (PE), epoxies, vinyl esters and phenolics. Applications of thermosets are very common due to their ease of processing.
Epoxies are usually used for fabricating high performance composites with superior mechanical properties, resistance to corrosive liquids and environments, superior electrical properties, good performance at elevated temperatures, good adhesion to substrate, or a combination of these benefits.
Vinyl Esters were developed to combine the advantages of epoxy resins with the better handling / quicker curing, which are typical for unsaturated polyester resins. Vinyl esters offer mechanical toughness and excellent corrosion resistance.
Phenolics are a class of resins commonly based on phenol (carbolic acid) and formaldehyde. Phenolic composites have many desirable performance qualities like high temperature resistance, creep resistance, excellent thermal insulation and sound damping properties, corrosion resistance.
A thermoplastic is a polymer that melts to a liquid when heated and freezes to a brittle, glassy state when cooled enough. Thermoplastic polymers can be remelted and remoulded; therefore, reshaped on the contrary to thermosets. Therefore, they can be recycled which makes their use more preferable. Among thermoplastics, polypropylene is commonly used in both plastic and composite auto parts. High performance engineering thermoplastics, used in more demanding applications include polyphenylene sulfide (PPS), polyether ketoneketone (PEEK) and several formulations of polyamide, commonly called Nylon.
A ‘polymer nanocomposite' (PNC) is a two-phase material where one of the phases has at least one dimension in the nanometre range. PNCs can have enhanced mechanical and electrical properties in terms of their strength, weight, flame retardancy and electrical conductivity due to the very high surface / volume ratio of its reinforcements. The interfacial properties and the interfacial area have a crucial role to obtain good final properties in composites. The interfacial area in PNCs is much higher compared to the traditional composites due to the ‘nano'size of the fillers which provide them those abovementioned advanced properties.
Today in automotive/aeronautics industry most commonly applied polymer nanocomposites are nanoclay incorporated polymers. They are preferred due to their enhanced physical properties eg. light weight, corrosion resistance, low thermal conductivity, low coefficient of thermal expansion, flame retardancy, noise dampening, high impact strength, high strength to weight ratio, dimensional stability, long term durability, directional strength, and etc. Another important reason for the interest in nanoclays is their cost and their availability in large scales.
In addition to main constituents of composites, resins and reinforcements, use of additional fillers has become very common both for composites and nanocomposites. Fillers not only improve the mechanical properties of the composite but also reduce the cost of it. Fillers can improve mechanical properties including fire and smoke performance by reducing organic content in composite laminates. Many other properties like water resistance, stiffness, dimensional stability and temperature resistance, etc. can be improved using fillers. One of the most common fillers used in nanocomposites is nanoclay. There have been also huge interest for using carbon nanofibers, carbon nanotubes, graphene and core shell as fillers for PNCs. Accordingly, many research groups from both industry and research institutes have been working on this field. However, so far there has been no industrial scale applications of nanofiller incorporated PNCs.
In addition to these filler materials, POSS (Polyhedral oligomeric silsequioxane) is researched as an alternative nanofiller to be used in polymer nanocomposites after its compatabilization.
Halloysite nanotubes (HNT) are also seen as promising filler materials that can be used in PNCs. HNTs, Al2O3Si2O4.4(H2O), are sort of clays which belong to Kaolin Group. They carry all the advantages of nanoclays. Additionally, they are more suitable for industrial processing due to their broad compatibility, low viscosity and better packaging features.
Most common clay mineral structures are montmorillonite, kaolinite and kanemite whose structures could be seen on the Image 1.2.1.
Montmorillonite is a natural clay which has a crystal structure consisting of nanometer thick layers (platelets) of aluminium octahedron sheet sandwiched in between two silicon tetrahedron sheets. Its formula is Mx(Al2-xMgx)Si4O10(OH)2.nH2O where M could be metal cations like Na, Ca, Mg, Fe and Li.
Kaolinite Al2Si2O5(OH)4 is another natural clay structure which consists of one aluminium octahedron sheet and one silicon tetrahedron sheet. Kanemite Na2Si4O9.5H2O is a layered silicic acid which is both naturally and synthetically available. The clays consist mainly of silicon tetrahedron sheets with different layer thickness.
Please check Chemistry-Materials sector for detailed information about carbon nanotubes' processing methods, properties and other application areas.
Carbon nanotubes are promising materials for use in PNCs due to their enhanced mechanical and physical properties (thermal stability, barrier properties, electrical properties i.e. for the dissipation of electrical charges. Since chemical vapour deposition (CVD) allows higher product output compared to other methods, CVD grown CNTs are preferred for polymer nanocomposite applications. Also CVD produced CNTs can be functionalized which is necessary for their incorporation into resin.
Carbon nanofibers are cylindric nanostructures with graphene layers arranged as stacked cones, cups or plates. Please see Chemistry-Materials sector draft report for details.
Resin transfer moulding
This method is also known as liquid moulding. RTM is a simple process which combines compression, moulding and transfer-moulding processes. It is comprised of a pairing, two part closed mould. Usually reinforcements fibers or a preform are placed in the mould in advance. Later on resin is pumped into the mould under low to moderate pressure. Resin should have low viscosity so as not to have problems during its injection into the mould. RTM is a practical method which is suitable for automated, serial manufacturing processes with a high efficiency.
Vacuum assisted resin transfer moulding (VARTM)
In VARTM resin is drawn into the mould under pressure through the special inlets designed on the mould. This is a very common method that is used in manufacturing composite automotive parts.
Prepregs are fibrous reinforcements like sheets, fabrics or mats which are already pre-impregnated with a pre-catalysed resin. Prepregs can be stored in cold places to be used later on. Prepregs are usually activated by heating. Prepregs are used by aeronautics and automotive industry in addition to wind energy, sporting goods and construction industries.
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