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Title: Influence of plasma pre-treatment of polytetrafluoroethylene (PTFE) micropowders on the mechanical and tribological performance of polyethersulfone (PESU)-PTFE and impact modified polyamide (PA66)-PTFE compounds
Author: Hunke, Harald
ISNI:       0000 0004 7426 245X
Awarding Body: University of Bolton
Current Institution: University of Bolton
Date of Award: 2017
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Polytetrafluoroethylene (PTFE), (CF2-CF2)n exhibits a low coefficient of friction (CoF, 0.07 - 0.15) and is therefore widely used as an additive in polymer compounds to improve their friction and wear properties. However, its applications in polymer compounds are limited due to the PTFE’s inherent poor chemical reactivity, low wettability and poor adhesion with other polymers preventing the establishment of good interfacial bonding. To overcome this, the work introduces plasma treatment of PTFE micropowders in a low-pressure 2.45 GHz microwave plasma reactor, using NH3 and H2 as process gasses to chemically modify their surface by introducing surface nitrogen and oxygen polar groups leading to more hydrophilic surfaces. The presence of these functional groups enhances the surface energy of the plasma treated PTFE and enhances the compatibility with thermoplastic polymers. It must be emphasised that nearly all the literature existing on the surface functionalisation of PTFE focuses on the PTFE films only. The first publication from this research work, in the group of three, reports on the effects of the H2, NH3 plasma processing time (2.5, 10 h), resulting defluorination (1.13 for NH3/10h, 1.30 for H2/10h as compared to 1.86 for pristine PTFE) and measurement of changes in wettability and crystallinity of the PTFE micropowders. The resulting plasma modified PTFE micropowders were further used as dry lubricants to enhance the tribological and mechanical properties of amorphous polyethersulfone (PESU) and semi-crystalline α-olefin-copolymer impact-modified polyamide 66 (PA66i) polymer compounds resulting in the next two publications. At the same loading levels (10wt%) of PTFE, prepared using twin-screw compounding, the pinon- disc tribological measurements of the PESU-PTFE compounds revealed a lowCoF from 0.55 for pristine PESU to 0.20 along with corresponding reduction in the wear rates from 5.75 x 10-06 mm3/Nm (pristine PTFE) to 4.70 x 10-06 mm3/Nm (for H2 treated PTFE) to 3.42 x 10-06 mm3/Nm (for NH3 treated PTFE). In the PA66i matrix, the wear rates of the pristine and plasma treated PTFE were observed to be similar for the sliding speeds up to 2 m/s. However, at the higher sliding speeds, the benefits of plasma treatment became more apparent. At the sliding speed of 3 m/s, the wear rate of pristine PTFE-PA66i compound was 1.1 x 10-06 (± 0.2) mm3/Nm whereas the wear rate of H2 treated PTFE was 0.7 x 10-06 (± 0.1) mm3/Nm and the wear rate of NH3 treated PTFE was 0.6 x 10-06 (± 0.1) mm3/Nm. These improvements in the tribological and mechanical properties have been ascribed to the enhanced dispersion of PTFE in the host matrix with the plasma processing introduced functional polar groups providing enhanced intermolecular bonding (as confirmed using Fourier transform infrared spectroscopy, differential scanning calorimetry and dynamic mechanical-thermal analysis) between the components. Therefore, the incorporation of functional groups into PTFE micro-powders by plasma treatment is an effective and efficient route for enhancing the mechanical and tribological properties of engineering polymer compounds such as PESU-PTFE and PA66i- PTFE offering significant cost and environmental benefits over the existing e- beam and wet chemical technologies.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available