Page 205 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 5. Surfaee treatments of:fibers and effects on composite properties 187
Dry oxidation: Dry oxidative treatments are normally carried out with air, oxygen
or oxygen containing gases such as ozone and C02 at low or elevated temperatures.
The dry oxidative treatment at a high temperature results in drastic changes in
surface properties, and often causes excessive pitting of the fiber surface, impairing
the fiber tensile strength (Novak, 1969). In this process, the surface layers simply
burn away unevenly to create pits in lines that coalesce into channels, resulting in a
high surface rugosity. These active sites could be related to the edge plane of the
fiber surface. Metallic impurities such as oxides of Cu, Pb, V and transition metals
are found to enhance the degradation rate even at a low temperature (McKee, 1970).
Plasma etching: Plasma treatment or electric discharge has become one of the
most popular methods for improving the fiber-matrix adhesion in recent years. Brief
reviews of this topic for surface treatments of carbon, aramid and polyethylene
fibers are given by Donnet et al. (1988), Yuan et al. (1991), Bascom and Chen (1991)
and Garbassi and Occhiello (1993), and a summary is presented below. Plasma is a
region of space in which high energy species, like electrons, ions, radicals, ionized
atoms and molecules, are present. Immersion of an object of any shape in a plasma
induces strong interactions of its surface with the energetic species present therein.
The fundamental principle of a plasma treatment technique is to induce the
formation of active species in a gas by a suitable energy transfer. Different types of
plasma can be generated depending on the experimental conditions. Among the
most frequently used are thermal (i.e. hot) plasma, glow discharge (i.e. cold plasma),
and corona discharge.
Thermal plasma of very high temperature is generated by coupling the energy into
a high pressure gas under equilibrium conditions. There are many different sources
of energy that include dc, ac, radio frequency and microwave. The result of
treatment is that many chemical bonds on the surfaces are broken, forming very
reactive species. Non-equilibrium corona discharges are generated at a high pressure
gas, such as air, by using highly charged wires or points. Cold plasma operates at a
low pressure under non-equilibrium conditions, and has been used extensively for
neon light tubes. The process is relatively easy to control and flexible compared to
other methods generating plasma, because any gas can be used. Schematic
presentation of a continuous cold plasma treatment system is shown in Fig. 5.12.
One of the major advantages of cold plasma treatment is that both etching and
deposition can be performed on the substrate surface. When a low pressure gas, such
as oxygen, chloride and fluoride, is introduced, active species are formed that can hit
and interact/functionalize the surface. This leads to abstraction of materials from
the surface (Le. etching). On the other hand, if hydrogen or fluorocarbon is excited,
radicals can be formed (i.e. deposition or grafting). One of the characteristic
differences between these processes is the treatment time: surface etching is very fast,
requiring only seconds, whereas the deposition of sizeable coating needs minutes.
Further details of deposition techniques are included in the discussion of the non-
oxidative treatment methods.
Wet oxidation: Several types of liquid-phase oxidizing agents, such as nitric acid,
acidic potassium permanganate, acidic potassium dichromate, dichromate perman-
ganate, hydrogen peroxide, ammonium bicarbonate and potassium persulfate, have