Interfacial interactions in carbon fiber reinforced polymer composites: surface characterization, chemistry and adhesion [antikvár]

Chapter 1 3 Chapter 1 Introduction Polymers, combined with other polymers, fillers or reinforcing fibers are used in increasing quantities in all fields of application. These materials contain the second component in quantities comparable to that of the matrix. Their growth rate exceeds those of commodity polymers and traditional structural materials like metals, wood or concrete. Arbitrarily they can be divided into three main categories: polymer blends, particulate filled polymers, and fiber-reinforced composites. All three offers some advantages compared to simple one-component polymers. Blends possess increased impact resistance, tailored properties, price advantage. Particulate filled polymers are stiffer, have good dimensional stability, improved processability, specific properties and price. Fiber reinforced polymers offer very high stiffness and strength, which are comparable to the same properties of steel at much lower weight, high heat deflection temperature and dimensional stability and chemical resistance (see Table 1.1). Blends, filled polymers and composites usually posses heterogeneous, two phase structure. The properties of such heterogeneous polymer systems depend on four factors: component properties, composition, structure and interfacial interactions. Component properties may vary in a wide range: the stiffness of the second component changes from a few pascals to hundred gigapascals as we proceed from elastomers trough liquid crystalline polymers to fillers and fibers. Composition may also cover a wide range from 5 wt% in nanocomposites to 80 vol% in advanced composites. The structure of these heterogeneous systems can be very complicated, e.g. two boundary structures can form in three-component PP/CaCOs/elastomer composites, which result in very different properties. The components maybe distributed separately in the PP matrix in a stiffness of 1.1 GPa, while the elastomer can embed the filler yielding a material of 0.5 GPa stiffness. Interaction of the components is also very important, it may determine the micromechanical deformation processes taking place during deformation and thus the macroscopic properties of the material. The effect of the last two factors is especially important in fiber reinforced polymers. Because of the very different properties of the fiber and the matrix (Table 1.2), their function and role is also dissimilar in the composite: the fibers carry the load, while the matrix distributes it among the fibers. However, this simple principle works only if some conditions are fulfilled: the fibers must be oriented in the direction of the load (structure) and the adhesion between the two components must be sufficient to make possible the transfer of stress (interfacial interaction). The first is achieved by the appropriate design of the composite; the compressive strength of composites is usually rather poor. The necessary adhesion strength is usually reached by the chemical coupling of the components, the creation of covalent bonds. However, the surface of the fiber must be modified and coupling agents must be used to reach this goal, since the surface of the fiber is chemically inactive. This thesis focuses attention mainly on the second question, on the surface properties of the fibers, on their characterization and on the modification of interactions.