1326 M C. Paira et al. Carbon 38(2000)1323-1337 and water, can be considered identical ( w=y), so that containing a single fibre aligned in the centre, with a the non-dispersive interactions may be estimated by a dog-bone shape, according to the Din 53504(S3A) simplified form of Eq. (6). Using the two-liquid phase standard tensiometric method, cos osw is calculated as the ratio Fragmentation tests, consisting of the stretching of the between the forces FHy and Fuw [ll], that represent the single filament composites, were made on an Instron 4505 force exerted on the fibre as it enters the first liquid at a speed of 0.5 mm/ min, until the matrix started yielding (hydrocarbon), and the second liquid (water), respectively: At this point, it was considered that the fragmentation of the matri W--YCos esw/H (9) was much higher than the tensile strain of the fibre in air (more than 10 The non-d contribution to the surface energy of measured using optical transmission microscopy. For the fibre P75S and P120J (untreated) fibres, a few tests were performed with a Minimat testing device(Polymer Labora- tory)coupled to a polarised light microscope. In this way, it was possible to monitor the fragmentation process by optical microscopy and simultaneous video recording The surface free energy values for the liquids used in the calculations are presented in Table 2. 4. Results and discussion 33. Micromechanical tests 4. 1. Surface characterisation 3.3. 1. Single filament tensile test 4. 1. Scanning electron microscopy The single filament tensile test is a technique widely Scanning electron microscopy (SEM) of the carbon sed to obtain tensile strength data on fibre form materials fibres was performed with a LEICA-$360 apparatus using The test method used here is described in the literature [8 a microanalysis system LINKexlll. Observation of the and was adapted from the AsTM standard surface revealed some morphology alterations induced by The diameter of the circular fibres was determined by a plasma with the type of laser diffraction technique, with a 10 mw He-Ne laser beam [14. The cross-sectional area of the non-circular SEM micrographs of the P120J fibres are presented in fibres was obtained after embedding their extremity in a Fig. 1. Although the P120J fibres were untreated and block of resin and polishing [8]. The measurements were unsized, a considerable amount of thin flakes was observed done using optical microscopy and image analysis. on the surface that may result from contamination during The tensile tests were performed in an Instron 1122 the production process. The flakes were not significantly equipped with a load beam of 5 N, at a crosshead speed of affected by plasma treatment at 75 w, but their con- centration decreased when the power increased. Only traces were present after the 100-w treatment and they 3.3. 2. Fragmentation tes were almost absent at 150 W. For the 75 W/3 min plasma The samples were processed by compression moulding treatment no significant alteration of the surface morpholo- the polymer in a hot press [9]. Two plates were prepared in gy, relative to the untreated fibres, was observed with the controlled conditions, and kept in a steel frame used as a amplification used Higher treatment levels produced etch- mould. Five to six carbon fibres were positioned straight ing lines along the fibre axis direction( deeper etching for across one of the polymer windows and glued to the metal stronger treatments). a decrease in fibre diameter was also frame. in both extremities. The two frames were assem- observed bled, and the set was compression moulded and cooled In The ribbon fibres behave differently this way, five to six carbon fibres aligned and positioned ment, at 75 W/3 and 10 min. The y ser aces ma ng mother, as can be observed in Fig. Is no net btained. Composite bars were cut from the plaque decrease in the fibre's dimension Surface free energy characteristics of the liquids Xw(mJ m a-Bromonaphthalene 44.6 Octane 21.61326 M.C. Paiva et al. / Carbon 38 (2000) 1323 –1337 d and water, can be considered identical (g W H ¯g ), so that containing a single fibre aligned in the centre, with a the non-dispersive interactions may be estimated by a ‘dog-bone’ shape, according to the DIN 53504 (S3A) simplified form of Eq. (6). Using the two-liquid phase standard. tensiometric method, cos u is calculated as the ratio Fragmentation tests, consisting of the stretching of the SW/H between the forces FHV HW and F [11], that represent the single filament composites, were made on an Instron 4505 force exerted on the fibre as it enters the first liquid at a speed of 0.5 mm/min, until the matrix started yielding. (hydrocarbon), and the second liquid (water), respectively: At this point, it was considered that the fragmentation process reached saturation, as the yield strain of the matrix FHW HW g ] was much higher than the tensile strain of the fibre in air ] 5 ]cos u . (9) SW/H FHV HV g (more than 10 times). The fragment lengths obtained were measured using optical transmission microscopy. For the The non-dispersive contribution to the surface energy of nd P75S and P120J (untreated) fibres, a few tests were the fibres, g S , was estimated using the geometric mean performed with a Minimat testing device (Polymer Labora- type relation: tory) coupled to a polarised light microscope. In this way, nd nd nd ]] it was possible to monitor the fragmentation process by W 5 2 g g . (10) SW œ S W optical microscopy and simultaneous video recording. The surface free energy values for the liquids used in the calculations are presented in Table 2. 4. Results and discussion 3.3. Micromechanical tests 4.1. Surface characterisation 3.3.1. Single filament tensile test 4.1.1. Scanning electron microscopy The single filament tensile test is a technique widely Scanning electron microscopy (SEM) of the carbon used to obtain tensile strength data on fibre form materials. fibres was performed with a LEICA-S360 apparatus using The test method used here is described in the literature [8] a microanalysis system LINKexLII. Observation of the and was adapted from the ASTM standard. surface revealed some morphology alterations induced by The diameter of the circular fibres was determined by a plasma treatment. These changes vary with the type of laser diffraction technique, with a 10 mW He–Ne laser fibre analysed. beam [14]. The cross-sectional area of the non-circular SEM micrographs of the P120J fibres are presented in fibres was obtained after embedding their extremity in a Fig. 1. Although the P120J fibres were untreated and block of resin and polishing [8]. The measurements were unsized, a considerable amount of thin flakes was observed done using optical microscopy and image analysis. on the surface that may result from contamination during The tensile tests were performed in an Instron 1122 the production process. The flakes were not significantly equipped with a load beam of 5 N, at a crosshead speed of affected by plasma treatment at 75 W, but their con- 0.5 mm/min. centration decreased when the power increased. Only traces were present after the 100-W treatment and they 3.3.2. Fragmentation test were almost absent at 150 W. For the 75 W/3 min plasma The samples were processed by compression moulding treatment no significant alteration of the surface morpholo￾the polymer in a hot press [9]. Two plates were prepared in gy, relative to the untreated fibres, was observed with the controlled conditions, and kept in a steel frame used as a amplification used. Higher treatment levels produced etch￾mould. Five to six carbon fibres were positioned straight ing lines along the fibre axis direction (deeper etching for across one of the polymer windows and glued to the metal stronger treatments). A decrease in fibre diameter was also frame, in both extremities. The two frames were assem- observed. bled, and the set was compression moulded and cooled. In The ribbon fibres behave differently after plasma treat￾this way, five to six carbon fibres aligned and positioned ment, at 75 W/3 and 10 min. The surface becomes along the plaque length at middle thickness can be smoother, as can be observed in Fig. 2. There is no net obtained. Composite bars were cut from the plaques, decrease in the fibre’s dimensions. Table 2 Surface free energy characteristics of the liquids 22 d 22 22 Liquid g (mJ m ) g (mJ m ) g (mJ m ) L L LW a-Bromonaphthalene 44.6 44.6 – Octane 21.3 21.3 51.0 Water 72.6 21.6 –