Recent developments in fibre science have rendered commercially available new polymeric fibres characterised by high mechanical properties and low weight. Generally, these materials show an unsatisfactory adhesion with thermosetting matrices, limiting their use in composite fabrication. However, different methods, such as corona treatment and cold plasma exposure, are promising in increasing the bonding capacity of fibre surface. In this work a non traditional cold plasma, generated by means of a glow discharge working in air flow, was employed to obtain a localised surface treatment, in order to modify the surface properties of Vectran® polymeric fibres under form of fabric. The exposure time and gas flow rate were suitably varied, to study the effect of these process parameters on the fibre-matrix adhesion. Preliminary tests devoted to the measurement of Mode I delamination energy by double cantilever beam specimens were unsuccessful, owing to the invalid failure modes observed. Simple peel tests were then adopted in order to quantitatively assess the effect of surface treatment. Irrespective of the actual process parameters employed, the energy necessary for delamination increased steadily with increasing the treatment time, until a constant value was achieved. The maximum delamination energy was apparently independent of the particular gas flow rate. The only effect of the latter was in the time necessary to reach the best mechanical properties: the lower the gas flow rate, the longer was the critical time. Compared to the untreated fibres, the fully treated ones exhibited a delamination energy about 30% higher. However, also the scatter in the experimental data was higher, probably reflecting the features of the plasma reactor used, in-house made, not permitting a strict control of the process parameters. Different time intervals (delay times) were let to elapse between the treatment end and impregnation, to verify the stability of the treatment in air. A slight decrease in delamination energy with increasing delay time was observed. The loss in efficiency was about 10% within 30 min, and was not substantially influenced by the parameters set during the treatment. Scanning electron microscopy was applied to examine the fracture surfaces of treated and untreated fibres. As expected, the failure was adhesive at the fibre-matrix interface in the case of untreated fibres. On the contrary, the adhesive interface strength for treated fibres exceeded the transverse strength of the highly anisotropic Vectran® fibres: this gave rise to cohesive, intrafibre failures developing along the fibre axis, resulting in the formation of thin filaments clearly evidenced by the SEM images. It was concluded that the maximum delamination energy permitted by the plasma treatment finds a higher bound in the fibre properties, rather than depending on the specific process parameters used.
Improving fibre-matrix interface by cold plasma surface treatment
LEONE, Claudio;
2000
Abstract
Recent developments in fibre science have rendered commercially available new polymeric fibres characterised by high mechanical properties and low weight. Generally, these materials show an unsatisfactory adhesion with thermosetting matrices, limiting their use in composite fabrication. However, different methods, such as corona treatment and cold plasma exposure, are promising in increasing the bonding capacity of fibre surface. In this work a non traditional cold plasma, generated by means of a glow discharge working in air flow, was employed to obtain a localised surface treatment, in order to modify the surface properties of Vectran® polymeric fibres under form of fabric. The exposure time and gas flow rate were suitably varied, to study the effect of these process parameters on the fibre-matrix adhesion. Preliminary tests devoted to the measurement of Mode I delamination energy by double cantilever beam specimens were unsuccessful, owing to the invalid failure modes observed. Simple peel tests were then adopted in order to quantitatively assess the effect of surface treatment. Irrespective of the actual process parameters employed, the energy necessary for delamination increased steadily with increasing the treatment time, until a constant value was achieved. The maximum delamination energy was apparently independent of the particular gas flow rate. The only effect of the latter was in the time necessary to reach the best mechanical properties: the lower the gas flow rate, the longer was the critical time. Compared to the untreated fibres, the fully treated ones exhibited a delamination energy about 30% higher. However, also the scatter in the experimental data was higher, probably reflecting the features of the plasma reactor used, in-house made, not permitting a strict control of the process parameters. Different time intervals (delay times) were let to elapse between the treatment end and impregnation, to verify the stability of the treatment in air. A slight decrease in delamination energy with increasing delay time was observed. The loss in efficiency was about 10% within 30 min, and was not substantially influenced by the parameters set during the treatment. Scanning electron microscopy was applied to examine the fracture surfaces of treated and untreated fibres. As expected, the failure was adhesive at the fibre-matrix interface in the case of untreated fibres. On the contrary, the adhesive interface strength for treated fibres exceeded the transverse strength of the highly anisotropic Vectran® fibres: this gave rise to cohesive, intrafibre failures developing along the fibre axis, resulting in the formation of thin filaments clearly evidenced by the SEM images. It was concluded that the maximum delamination energy permitted by the plasma treatment finds a higher bound in the fibre properties, rather than depending on the specific process parameters used.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
