2. Experimental
2.1. Materials and surface treatment
The PAN (Polyacrylonitrile)-based AKSAca A-42 carbon fibre with
bulk density of 1.78 g/cm3 and yield of 800 g/km was used. Bakelite®
EPR-L20 epoxy resin was used as matrix material. The resin was
mixed with EPH-960 hardener at the weight ratio of 100:35 and the
mixture was then degassed for approximately 30 min before curing.
The acid surface treatment employed a 3:1 (v/v) mixture of concentrated
H2SO4 and HNO3, with sonication at 60 °C. During the treatment, A-
42 carbon fibre was added to 60 ml of the mixed acid solution in a beaker.
To avoid of extensive reduction of tensile strength, the treatment
was carried out at different controlled times from 15 min to 60 min at
an interval of 15 min. The treated fibres were then placed in another
beaker and washed several times in DI water until the pH level was
around 7. Washing removed all water-soluble N- and S-oxides, replacing
them with hydroxyl groups through ion exchange; though direct
N- and S-bonded were retained [4].