Transfer interphase filler

The interphase provided by the adhesion promoter may be hard or soft and could affect the mechanical properties. A soft interphase, for example, can significantly improve fatigue and other properties. A soft interphase will reduce stress concentrations. A rigid interphase improves stress transfer of resin to the filler or adherend and improves interfacial shear strength. Adhesion promoters generally increase adhesion between the resin matrix and substrate, thus raising the fracture energy required to initiate a crack. 

A positive effect of fillers may be observed during irradiation crosslinking. It was found that the yield of radicals in polyethylene was increased 50% when a small amount (0.05 %) of aerosil was added [21]. It has been assumed that a higher production of radicals takes place at the interphase aerosil-polyethylene, where macromolecules can be in the nonequilibrium state cf uncompensated strains. With a higher content of a filler, a transfer of energy from the filler to the polymer j se may occur and thus contribute to a higher yidd of free radicals. Combination of irradiation with reactive admixtures may, moreover, affect a localization of crosslinks along the poly-mo chain. 

The interphase is a three-dimensional (3-D) layer in the immediate vicinity of filler surface, possessing physical properties different from the two main phases or components in a composite (i.e., matrix and filler). For the purpose of this chapter, the term interphase is limited to the layers introduced on the filler surface intentionally in a controlled maimer—engineered interphase layers (EIL). In these layers, a gradient of chemical composition can also exist as well as a gradient of physical properties. The pivotal problem is therefore a definition and an evaluation of an interphase thickness and its properties, namely, stiffness and fracture toughness. Interphase behavior plays a paramount role in the ability to transfer loads from the matrix to the reinforcements, hydrolytic stability of the material, and fracture behavior of a particulate composite. 

Particle/particle interactions induce aggregation, while matrix/filler interactions lead to the development of an interphase with properties different from those of both components. Both influence composite properties significantly. Secondary van der Waals forces play a crucial role in the development of these interactions and they are modified by surface treatment. Occasionally reactive treatment is also used its importance is smaller in thermoplastics than in thermoset matrices. However, reactive treatment must always be used in fiber reinforced polymers in order to achieve the necessary adhesion between the components. Strong adhesion is a primary condition of good stress transfer, assuring the proper performance of the composite. 

The coverage of the surface of a filler with a polymer layer which is capable of interdiffusion with the matrix proved to be very effective both in stress transfer and in forming a thick, diffuse interphase with acceptable deformability. In this treatment the filler is usually covered by a functionalized polymer, preferably by the same polymer as the matrix, which is attached to the surface by secondary, hydrogen, ionic or sometimes even by covalent bonds. The polymer layer inter-diffuses with the matrix, entanglements are formed and, thus strong adhesion is created. Because of its increased polarity, in some cases reactivity, usually maleic anhydride or acrylic acid modified polymers are used, which adsorb to the surface of most polar fillers even from the melt. This treatment is frequently used in polyolefin composites, since other treatments often fail in them, on the one hand, and functionalization of these polymers is relatively easy, on the other. Often a very small amount of modified polymer is sufficient to achieve significant improvement in stress transfer [126, 127]. 

In the case of fiber-reinforced composites, even where the filler consists of wood fibers and other ligno-cellulosic or proteic fibers, the main issue is the interfacial adhesion between matrix and fibers, as a sine qua non requirement for the load transfer, as the fibers carry the load, while the matrix distributes and transfers it from fiber to fiber. Hence, the relevance of the interphase that may be formed spontaneously, no matter the fibers are raw or modified by a surface treatment, and which strongly affects the properties of the composite through its thickness, structure and properties [3]. 

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