Page 204 - Engineered Interfaces in Fiber Reinforced Composites

186 Engineered interfaces in fiber reinforced composites

At the end of the fiber manufacturing processes, a size is normally applied to the
carbon fibers for use as reinforcement of PMCs. Sizing of carbon fiber involves
application of an organic film to protect the fiber during fabrication into structural
parts and components. The amount of sizing varies between 0.5-1.5 wt% of the fiber
depending on the type and application of fibers. Sizes are intended:
(1) to protect the fiber surface from damage,
(2) to bind fibers together for ease of processing,
(3) to lubricate the fibers so that they can withstand abrasive tension during
subsequent processing operations,
(4) to impart anti-electrostatic properties, and
(5) to provide a chemical link between the fiber surface and the matrix and thus to
improve the bonding at the interface.
Sizing for different fabrication processes serves different purposes. Specifically, the
sizing for filament winding is designed to hold the tow of fibres as a relatively
cohesive bundle so that it can pass through the eyelets and guide without spreading.
At the same time, the size must also be sufficiently flexible to allow the tow to be
opened up and readily impregnated by the liquid resin. Similar requirements are
necessary for weaving. In contrast, the primary role of sizing in prepregging is to
hold down loose fiber ends and gather them into small bundles to avoid severe
misalignment in the final prepreg sheet. Apart from these purposes of sizing, there
are no appreciable effects on the mechanical properties of composites when
compared with those containing unsized fibers (Bascom and Drzal, 1987).

5.3.2. Surface treatments of carbon jibers

5.3.2.1. Types of surface treatment
The poor shear strength of carbon fiber reinforced polymers, those reinforced
with high modulus fibers in particular, is generally attributed to a lack of bonding at
the fiber-matrix interface. Extensive research has been directed toward the
development of surface treatment techniques for carbon fibers to improve the
fiber-matrix interface bonding. The mechanisms of bonding between carbon fibers
and polymer matrices are as complex as that of glass fibers, and there are more
complications associated with the carbon fiber surface because it is highly active and
readily absorbs gases, A range of active functional groups can be produced by
surface treatment. Reviews on this subject, such as important parameters controlling
the effectiveness of various surface treatment methods, can be found in numerous
references including Scolar (1974), Delmonte (198 l), Riggs et al. (1982), Donnet and
Bansal (1984), Ehrburger and Donnet (1985), Wright (1990) and Hughes (1991).
Surface treatments of carbon fibers can in general be classified into oxidative and
non-oxidative treatments. Oxidative treatments are further divided into dry
oxidation in the presence of gases, plasma etching and wet oxidation; the last of
which is carried out chemically or electrolytically. Deposition of more active forms
of carbon, such as the highly effective whiskerization, plasma polymerization and
grafting of polymers are among the non-oxidative treatments of carbon fiber
surfaces.