Metal Oxide Nanocomposites. Группа авторов
But they are seldom used in systems requiring high-pressure molding and high shear mixing due to their poor crush resistance than solid spheres. Both, solid or hollow microspheres demonstrated the properties which are directly related to their spherical shape and let them behave like minute ball bearing, resulting in improved flow properties. The microspheres are free of orientation and sharp edges and therefore produce smoother surfaces, whereby distributing the stress homogeneously throughout resin matrices.
2.6 Polymer Composites
In a polymer composite, either the constituent matrix material or the fiber is a polymer. The polymer matrix composites (PMCs) compose of a polymer resin as the matrix material and fibers as the reinforcement medium [24]. The maximum service temperature depends on the matrix because it normally softens, melts, or degrades at a much lower temperature than the fiber reinforcement. The polyesters and vinyl esters are the most widely utilized and least expensive polymer resins and so are used primarily for glass fiber-reinforced composites. Different resin formulations can induce wide range of properties in these polymers. The epoxies are utilized in commercial applications and PMCs for aerospace applications because of their superior mechanical properties and resistance to moisture as compared to polyesters and vinyl resins. High-temperature thermoplastic resins such as polyetherimide (PEI), poly(phenylene sulfide) (PPS) and polyetheretherketone (PEEK) can be used in future aerospace applications. PMCs are widely used in large number of applications due to their cost and facile fabrication. Polymer composites are classified according to reinforcement type (i.e., glass, carbon, and aramid) as follows:
2.6.1 Glass Fiber-Reinforced Polymer (GFRP) Composites
Fiberglass composites are produced in the largest quantities and they consists of glass fibers (continuous or discontinuous), contained within a polymer matrix. The glass which is most commonly drawn into fibers is referred to as E-glass and its diameter usually ranges between 3 and 20 m. Glass is widely used as a fiber reinforcement material due to its:
1 a) facile processing into high-strength fibers from the molten state.
2 b) wide availability and its easy processability into glass-reinforced plastic.
3 c) ability to produce high specific strength composite when used as reinforcement in a plastic matrix.
4 d) its chemical inertness, which makes it extremely useful in corrosive environments.
In glass fibers, the surface characteristics are important attribute because even a single or minute surface flaw can significantly affect the tensile properties. Surface flaws can be incorporated with much ease by rubbing or abrading the surface with another hard material. The surface of glass which is exposed to the normal atmosphere even for short time interval tends to have a weakened surface layer which interferes with bonding to the matrix. The fibers are coated during drawing with a thin layer of a material that protects its surface from damage and other environmental interactions [6]. There are various limitations to GFRP materials. Although they have high strengths, still they are not very stiff and also do not display the rigidity which is crucial for some applications (airplanes and bridges). The GFRP materials can work perfectly below 200 °C; where after, most polymers begin to flow or to degrade. However, this working temperature can be extended to 300 °C by utilizing high-purity fused silica for the fibers and high-temperature polymers such as the polyimide resins. The fiberglass is utilized in a variety of applications such as automotive and marine bodies, plastic pipes, storage containers, and industrial floorings. In transportation industries the glass fiber-reinforced plastics can decrease the vehicle weight and boost fuel efficiencies.
2.6.2 Carbon Fiber-Reinforced Polymer (CFRP) Composites
Carbon is a high-performance fiber material which is often used as reinforcement in advanced polymer-matrix composites. The reasons being that:
1 a) The carbon fibers hold maximum specific modulus and strength among all other reinforcing fiber materials.
2 b) They can retain their tensile modulus and strength at elevated temperatures.
3 c) The carbon fibers are not at all affected by moisture, acids, and bases at room temperature.
4 d) Due to their exciting physical and mechanical characteristics, the composite thus formed tends to have specific engineered properties.
5 e) Inexpensive and cost effective manufacturing processes for fiber and composite have been developed.
Carbon fibers are not purely crystalline, but they possess both graphitic and non-crystalline regions. These non-crystalline regions are composed of the 3-D ordered arrangement of hexagonal carbon networks which is also characteristic of graphite. The techniques to produce carbon fibers are relatively complex. Rayon, polyacrylonitrile (PAN), and pitch are used as organic precursor materials for producing carbon fibers [3]. The processing techniques are different for different precursors and also affect the resultant fiber characteristics. The carbon fibers can be classified on the basis of tensile modulus; which is further divided into four subclasses as standard, intermediate, high, and ultrahigh moduli. The diameters of both continuous and chopped fibers normally range between 4 and 10 μm. The carbon fibers are coated with a protective epoxy size which improves its adhesion with the polymer matrix. Carbon-reinforced polymer composites are utilized in sports and recreational equipment, pressure vessels, helicopters, aircraft (military and commercial) structural components and filament-wound rocket motor cases.
2.6.3 Aramid Fiber-Reinforced Polymer Composites
Aramid fibers materials possess high-strength, high-modulus and are known for their outstanding strength-to weight ratios, which is even superior to metals [9]. Chemically, this group of materials is known as poly (paraphenylene terephthalamide). Most common aramid materials are Kevlar (Kevlar 29, 49, and 149) and Nomex. The rigid molecules get aligned in the direction of the fiber axis, as liquid crystal domains during the synthesis process. These fibers have very high longitudinal tensile strengths and tensile moduli but are relatively weak in compression. They are also tough, resistant to creep, resistant to fire and quite stable at relatively high temperatures; in the range -200 and 200 °C [6]. These composites are prone to acidic degradation, yet are fairly inert in other solvents. Mostly, they mostly used in composites in which the matrix materials are either epoxies or polyesters. Owing to their great flexibility, these fibers can be easily processed using common textile operations. They are used in applications involving automotive brake and clutch linings, ropes, pressure vessels, sporting goods, bulletproof vests and armor, missile cases and sporting goods,
2.7 Composites Processing
Processing is the science which involves the transformation of shape of materials. Since the composites involve two or more materials, their processing techniques are different than those for metals [12]. There are a number of processing techniques available to develop different kind of resin and reinforcements. Therefore, the correct processing technique/conditions should be employed to meet the performance, production rate, and cost requirements of an application.
2.8 Composites Product Fabrication
Figure 2.6 shows an overview of various processes by which the composite products are generally synthesized by transforming the raw material into final shape. The individual products are fabricated, machined and further joined with other products as per the application requirement. The product fabrication process is a four-step process as follows:
1 a) Forming: In this step, as per the requirements, the feedstock is transformed into desired shape and size using the action of pressure and heat.
2 b) Machining: The extra/undesired material is removed by using machining operations such as cutting, grinding turning and drilling. Machining operations for composites require different operating conditions and tools than that by metals.
3 c)