Interfaces and voids

When a polymer and a reinforcement are mixed, interactions between these two distinct chemical components require the existence of contact areas interfaces) between the two components. The larger the area, the greater the probability of physical, chemical or physical-chemical interactions occurring between the two components (Yosomiya, 1990). In addition to the dependence on the extension of the contact area, interactions between the dispersed phase and the matrix phase at the interface also depend on the affinity between the components. A load can only be carried by a reinforcement if it is transferred to the reinforcement through the interface. The mechanical properties of composites thus depend on the chemical and physical properties of their constituents as well as their configuration (Frollini et al., 2004). 

Despite the compatibility among phenolic resins and some reinforcements, it is sometimes necessary to introduce modifications to improve adhesion at the interface. 

The adhesion between aramid fibers and resins is usually poor, mainly due to their high crystallinity. Treatment with cold plasma improved the interfacial adhesion between Kevlar fabric and phenolic resin (Guo et al., 2009 Frollini and Castellan, 2012). 

In addition to the search for conditions that produce a good interface for phenolic composites, other issues must be addressed to ensure that the end material has the desired properties. One critical factor is the identification of conditions that minimize or, preferably, eliminate the presence of voids. 

Phenolic resins such as resol-type resins generate water and formaldehyde during curing as by-products of condensation reactions. Hexamethylenetetramine (HMTA) can be used as a source of formaldehyde for curing novolac-type resins, producing formaldehyde and ammonia as by-products. During curing. 

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The moisture content in air-dry wood fibers ranges from 6% to 7%, but the processes for plastics manufacturing tolerate little or no water. Even 1% or 2% moisture is considered too high [1, 6]. Removal of water is critical because any moisture remaining in the wood-plastic blend turns to steam and manifests itself in the form of foam, disrupting processes, resulting in poor surface quality, weak wood-plastic interface, and voids that are unacceptable for final sale [3, 25]. As a result, particles must be predried for blending. 

Composites are complex, heterogeneous, and often anisotropic material systems. Their properties are affected by many variables, including in situ constituent properties reinforcement form, volume fraction, and geometry properties of the interphase, the region where the reinforcement and matrix are joined (also called the interface), and void content. The process hy which the composite is made affects many of these variables. Composites containing the same matrix material and reinforcements, when combined by different processes, may have very different properties. 

At elevated temperatures where titanium alloys could be the adherend of choice, a different failure mechanism becomes important. The solubility of oxygen is very high in titanium at high temperatures (up to 25 at.%), so the oxygen in a CAA or other surface oxide can and does dissolve into the metal (Fig. 12). This diffusion leaves voids or microcracks at the metal-oxide interface and embrittles the surface region of the metal (Fig. 13). Consequently, bondline stresses are concentrated at small areas at the interface and the joint fails at low stress levels [51,52]. Such phenomena have been observed for adherends exposed to 600°C for as little as 1 h or 300°C for 710 h prior to bonding [52] and for bonds using... 

The deposition temperature is above 1200°C and the deposit usually consists of an outer layer of MoSi2 and an intermediate layer of MoSi.PlP l Such reactions are difficult to control and often result in mechanical stresses and voids at the interface, which may cause adhesion failure. The direct deposition of the silicide is often preferred. This is accomplished by reacting a gaseous silicon compound with a gaseous metal compound, as shown in the following sections. 

Sometimes interdiffusion between two metals is uneven and may lead to the creation of vacancies or voids. This type of imbalance is the result of possible unequal mobilities between a metal couple. These voids occur individually near the common interface. The voids, like bubbles, coalesce, resulting in porosity and loss of strength. Many thin-fihn couples exhibit this phenomenon, which is referred to as Kirkendall void creation. Al-Au, Cu-Pt, and Cu-Au are just a few examples. To be specific, it has been found (7), for instance, that in the case of Au-Ni, about five times more Ni atoms diffuse into Au than Au atoms diffuse into Ni. 

Other than in polymer matrix composites, the chemical reaction between elements of constituents takes place in different ways. Reaction occurs to form a new compound(s) at the interface region in MMCs, particularly those manufactured by a molten metal infiltration process. Reaction involves transfer of atoms from one or both of the constituents to the reaction site near the interface and these transfer processes are diffusion controlled. Depending on the composite constituents, the atoms of the fiber surface diffuse through the reaction site, (for example, in the boron fiber-titanium matrix system, this causes a significant volume contraction due to void formation in the center of the fiber or at the fiber-compound interface (Blackburn et al., 1966)), or the matrix atoms diffuse through the reaction product. Continued reaction to form a new compound at the interface region is generally harmful to the mechanical properties of composites. 

Void, R. D., and Void, M. J., Colloid and Interface Chemistry, Addison-Wesley, Reading, MA, 1983. (Graduate and undergraduate levels. A textbook at a more advanced level than this book.)... 

Polysaccharides may exercise a protective action in an emulsion and foam as a thin film at liquid-liquid (emulsion) and liquid-air (foam) interfaces. The hydrophile-lipophile balance in the macromolecules as well as <(>, determines whether or not the emulsion is an oil-in-water or water-in-oil dispersion (Void and Void, 1983 Dickinson, 1992). 

The regions of coalescence for GaN stripes oriented along [llOO] contained occasional voids which originated from the LE0-GaN/Si02 interfaces and terminated within the LEO-GaN film thickness. A continuous, epitaxial, nearly defect free GaN layer always formed above these voids. Continuous voids or cracks throughout the LEO-GaN films have not been observed, as is commonly reported for selective growth of GaN on sapphire substrates [15],... 

The result of such shrinkage is internal stresses at the adhesive-substrate interface and the possible formation of cracks and voids within the bond line itself. Depending on the primary base resin, the adhesive formulator may need to reduce the amount of shrinkage when the adhesive hardens. This can be accomplished in several ways. 

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