Does roughness of fibre influences the surface energy and contact angle? Chemical treatment can improve the surface energy but causes the increase in the hydrophilic characteristic of fibre?
In theory, the surface roughness should not influence the surface energy unless the roughness is on the order of the molecular size. However, quite often, the surface energy is determined using the contact angle measurement. In such a case, the surface tension of the liquid used enters into the equation. Therefore, the tens of nanometer level surface roughness influence of the wetting of the surface by the probe liquid and influence the calculated surface energy. This is the reason why the same material can be hydrophobic or superhydrophobic depending on the surface topology. The surface roughness and fiber-matrix interfacial bonding is not straightforward relationship. The surface energy matching will be favorable for wetting of the fiber my the matrix resin. Thus, it is a requirement for good adhesion. If, then, the surface of the fiber has roughness (or void) where the resin can form mechanical interlocking, then good adhesion can be realizes. However, if the pore (or void) shape is in such a way that interlocking cannot be formed, despite the surface may be rough, then there will not be improvement in adhesion. Thus, wetting alone is not sufficient condition for good adhesion. It is only the requirement. You need further conditions, such as covalent bond formation, or specific interaction formation such as hydrogen bonding or ionic bonding to have good adhesion. If the composite is very dry, van der Waals; forces is sufficient to obtain reasonably good adhesion. However, such adhesion can be weakened significantly when small molecules, such as water, is present. Thus, if you wish to achieve permanence in bonding (or hydrothermal stability) , you need covalent bond formation.
Thank you Prof Hatsuo,
surface energy is just a requirement. So is it possible to increase the surface energy of the fibre since high fibre surface energy results in improving adhesion level.
The fibre surface energy need to be higher than or equal to the matrix surface energy for the adhesion requirement?
There are surface treatments which can enhance surface energy like High Frequency Glow Discharge Treatment.
The higher the surface energy, the lower the contact angle and the higher the wettability, and vice-visa.
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