Polymer Composites

MIRTA I. ARANGUREN

Encyclopedia of Surface and Colloid Science, Second Edition

Taylor & Francis/Marcel Dekker

Lugar: New York; Año: 2006; p. 4796 - 4810

INTRODUCTION Polymer composites are two phase materials usually made by combining a low density polymeric matrix with a high-modulus filler/reinforcement. A filler is a second component added mainly to reduce the overall costs of the material and sometimes to control the volume shrinkage of some commercial resins or to improve their thermal properties. The term reinforcement is reserved for those components whose addition results in an improvement of the final mechanical properties of the material. In general, fillers are particulated solids of different sizes and geometries, spheres, cubic, platelets or irregular particles, with similar dimensions in all the directions. The reinforcements are almost exclusively fibrous materials (continuous or short fibers). Although each of the main phases maintains its identity in the composite, the final properties achieved by this combination cannot be obtained by the components acting alone. This improved performance depends, to a large extent, on the interactions generated between the two main components; that is, of the adhesion between the polymer matrix and the reinforcement, therefore on the nature of the interface/interphase developed (Figure 1). Interface is defined as the surface boundary between matrix and reinforcement, that maintains the bond between the two components. The term interphase includes the interface concept, but it is also the region (volume) altered during the consolidation process, with properties that can vary progressively or stepwise from those of the fiber to those corresponding to the bulk matrix [1]. The modern trend to manufacture composites with increasingly more reduced size domains of the phases, such as microcomposites or nanocomposites enhances the contribution of the interfacial region to the overall composite behavior and makes the study and understanding of interfacial phenomena of utmost importance. The nature of the interphase can limit the overall performance of the bulk composite, since through this region the coupling between matrix and reinforcement/filler is achieved. Since the characteristics of this region can affect the fracture toughness and strength of composites under different loading configurations [2], it is evident that the knowledge of the relationship structure–properties at the interphase is essential to the rational design of composite materials and the correct tailoring of that interfacial region [1]. The overall behavior of the composites depends partly on the filler/reinforcement characteristics, such as geometry, size, orientation, volume fraction, and on the matrix characteristics, eg: degree of polymerization, crystallinity or crosslinking density. However, the mechanical and fracture behaviors are also strongly affected by the strength of the bond between the matrix and filler, which is responsible of the stress transfer across the interface. The study itself is difficult since interfacial surfaces can be rough; the contact with the matrix can be imperfect or the contact area be larger than calculated for simplistic geometric considerations that do not account for the actual roughness. The polymerization of a liquid matrix or the crystallinity of a thermoplastic and in general the mobility of the molecules near the filler/reinforcement is frequently reduced, so that the modulus in this region becomes higher than that of the bulk matrix. Concentration of stresses around the inclusions (unloaded samples) also occurs because of the different thermal expansion coefficients of the two main phases of the material. This initial state of stresses affects the capability of load transfer through the interface. This complex situation generated by the presence of fillers/reinforcements can generate voids or microcracks or other singularities in the material that will act as points where the failure of the material can initiate [3]. Unfortunately, there is not yet a single accepted model that can quantitatively relate the properties or characteristics of a given interphase to the bulk properties of the composites. The use of models that take into account the chemical–physical, thermodynamic and mechanical principles of the fiber–matrix adhesion, as well as the experimental information are the sources used to design an interphase that fit a desired composite property. The design is complex and many times conflicting requirements are to be satisfied in the design of polymer composites. High bond strength is necessary if the material is subjected to shear or compression, in order to reduce fiber debonding or buckling (compression). However, it will be a disadvantage in fracture resistance.