Effect of μPlasma Modification on the Wettability and the Ageing Behaviour of Glass Fibre Reinforced Polyamide 6 (GFPA6)

Abstract

Glass fibre reinforced polyamide 6 (GFPA6) thermoplastic composites (TPCs) are promising materials with excellent properties, but due to their low surface free energy they are usually difficult to wet, and therefore, possesses poor adhesion properties. μPlasma modification offers potential solutions to this problem through functionalisation of the GFPA6 surface. In this study, the effect of μPlasma on the wetting behaviour of GFPA6 surfaces was investigated. Following single μPlasma treatment scans of GFPA6 samples, a substantial enhancement in wettability was observed. However, the effect of the μPlasma modification was subject to an ageing (hydrophobic recovery) phenomenon, although the enhancement was still partially maintained after 4 weeks. The ageing process was slower when the GFPA6 material was pre-dried and stored in low humidity conditions, thereby demonstrating the importance of the storage environment to the rate of ageing. Orientation of the fibres to the observed contact angle was found to be crucial for obtaining reproducible measurements with lower deviation. The influence of testing liquid, droplet volume and surface texture on the repeatability of the measured contact angle were also investigated.

The Ageing of μPlasma-Modified Polymers: The Role of Hydrophilicity

“…Although the total surface free energies of PA6 and PA12 were found to be similar, the ratio of the polar components to the total surface free energy of PA6 was significantly higher (11%) than that of both PA12 and HDPE (4.7%) and (2.2%). The wetting envelopes shown in Figure 4 were determined using these surface free-energy values (the associated method is described in detail elsewhere [ 20 ]. Complete wetting (where the contact angle = 0°) can be expected when the values of polar and dispersive components of the liquid surface energy lie within the enclosed area of the wetting envelope.…”

“…The μPlasma modification was carried out using a Roth & Rau Pixdro LP50 plasma inkjet printer (InnoPhysics, Eindhoven, The Netherlands) with an InnoPhysics POD24 print-head as shown in Figure 3 a,b. A printing rate of 20 mm/s, a working distance from the sample surface to the tips of the printing needles of 100 μm, and an accelerating voltage of 7 kV were chosen as the μPlasma modification parameters, which had previously been reported to induce the most significant enhancement in wettability [ 20 ]. Additionally, in this study, the polymer surfaces were modified using a single treatment scan.…”

“…The apparatus was adjusted to allow for samples to be placed in line with the camera and light source to ensure a clear and flat view of the droplet, which, therefore, enabled comparable measurements between polymers. An optimised measuring technique from our previous work was utilised [ 20 ]. Ionised water droplets with a volume of 6 μL were formed on sample surfaces using a calibrated pipette.…”

“… ( a ) Schematic of μPlasma modification setup, ( b ) top view of μPlasma discharge pattern generated by the 24 needle electrodes, and ( c ) μPlasma modification pattern. Reproduced with permission from [ 20 ]. …”

“…Degradative processes may involve chain scission, crosslinking, and oxidation [ 17 , 18 ], whereas non-degradative processes may involve hydrophobic recovery that involves chain reorientation and diffusion back towards the bulk of the material. Previous studies have shown that post-treatment ageing is affected by temperature [ 19 ], relative humidity [ 20 ], and polymer crystallinity [ 21 ].…”