Page 24 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 2. Characterization of interfaces 1
2.2.1. Adsorption and wetting

Good wetting of fibers by matrix material during the impregnation stages of
fabrication is a prerequisite to proper consolidation of composites, particularly for
composites based on polymer resins and molten metals. It is well understood that
physical adsorption of gas molecules to solid surfaces is ascribed to the attraction
arising from the quantum mechanical effect due to the valence electrons present in
the constituents as a free gas. The physical attraction between electrically neutral
bodies is best described by the wetting of solid surfaces by liquids. Bonding due to
wetting involves very short-range interactions of electrons on an atomic scale which
develop only when the atoms of the constituents approach within a few atomic
diameters or are in contact with each other.
Wetting can be quantitatively expressed in terms of the thermodynamic work of
adhesion, WA, of a liquid to a solid using the Dupre equation
WA = YI + ?2 - 712 . (2.1)

W, represents a physical bond resulting from highly localized intermolecular
dispersion forces. It is equal to the sum of the surface free energies of the liquid, yl ,
and the solid, y2, less the interfacial free energy, y12. It follows that Eq. (2.1) can be
related to a model of a liquid drop on a solid shown in Fig. 2.2. Resolution of forces
in the horizontal direction at the point A where the three phases are in contact yields
Young’s equation

Ysv = YSL + YLV cos 3 (2.2)

where ysv, ysL and yLv are the surface free energies of the solid-vapor, solid-liquid
and liquid-vapor interfaces, respectively, and 8 is the contact angle. Liquids that
form contact angles greater and less than 90” are respectively called ‘non-wetting’
and ‘wetting’. If the liquid does not form a droplet, i.e. 8 = O”, it is termed
‘spreading’ and the relationship given by Fiq. (2.2) becomes invalid. In this case, the
equilibrium is expressed by an inequality
. (2.3)
Ysv - Yst > YLV

Vapor

‘A
Fig. 2.2. Contact angle, I), and surface energies, yLv, ysL and ysv. for a liquid drop on a solid surface.