Electrospun nanofibres as a tool for controlling the gas bubble size distribution in fibre/thermoset-matrix composites R. Polanský, P. Prosr, M. Zemanová, J. Pihera, T. Džugan, et al. Composites Science and Technology, 2018 The influence of electrospun nanofibres made from polyamide 6 (PA6) on the gas bubble size distribution by volume in fibre/thermoset-matrix systems cured without a vacuum was comprehensively studied via various methods. An analysis of the microstructural, thermal and dielectric properties proved that the unique structure of PA6 electrospun nanofibres can withstand the curing conditions required for fibre/thermoset-matrix systems and that the overall dielectric behaviour of manufactured composites is not negatively influenced by the addition of nanofibres. Visual and numerical analyses of the tomograms obtained via X-ray computed microtomography (micro-CT) suggested that a nanofibrous structure can act as a solid grid limiting the growth of large gas bubbles or their random agglomeration during curing. The initial observations were independently supported by partial discharge analysis (PDA), which confirmed that the addition of nanofibrous layers leads to the presence of bubbles with generally smaller dimensions and that the nanofibrous layers significantly suppress the partial discharge (PD) activity inside a composite structure.
Properties of polyamide nanofibers treated by UV-A radiation Zdeňka Kolská, Radek Polanský, Pavel Prosr, Monika Zemanová, Petr Ryšánek, et al. Materials Letters, 2018 Nonwoven polyamide nanofibers have extensive applications in a wide range of areas. However, a general problem of polyamide nanofiber is a poor adhesion to other materials or to microorganisms. In this work, we exposed nonwoven polyamide nanofibers to ultraviolet, long-wave (UV-A) radiation for different times and studied the changes in surface and bulk properties by available techniques. Obtained results confirmed the changes in surface chemistry, polarity, charge, roughness and morphology. Also changes in crystallinity portion and alpha and gamma phases were observed. We confirmed that the UV-A radiation can be used for surface activation and adhesion improvement. These changes are more visible and significant after short radiation time (till 10 min).
Partial discharges of nonwoven nanofibers composite J. Pihera, R. Polansky, M. Zemanova, P. Prosr, J. Chvojka Annual Report Conference on Electrical Insulation and Dielectric Phenomena CEIDP, 2016 A promising technology of nanofibrous composites is studied nowadays as an alternative method to the well-known dielectric nanocomposites filled by various nanofillers like a metallic oxides, alumina, silica, carbon nanofibers or nanotubes. All of these nanofillers are known, more or less. The use is to improve some of the electrical, mechanical and thermal properties of nanocomposites. Unfortunately, its expensiveness and tendency to agglomeration remain as their main disadvantage. In the contrary, the nanofibers can be applied as a nonwoven fabric over the surface of the composite with no tendency to create the cluster agglomeration as the nanocomposites with nanoparticles. It was prepared experimental specimens of nonwoven nanofibers composites based on the modification of commonly used three-component mica-based electrical insulating material (epoxy, glass fibers and mica). The modification of these, well known, mica composites was done by incorporation of the nonwoven nanofibers layers (1, 2 and 3) to its structure always with different area density (1, 3 and 5 g/m2) of the nanofibers. The tested material was delivered in the form of resin-rich sheets on which the layers of the nanofibers made from Polyamide 6 were applied and specimens were subsequently cured using typical resin rich curing process. The influence of prepared modifications on the partial discharge characteristics and magnitudes of the resulting nanocomposites was studied. Partial discharges results show differences between the specimens' variations depending on nanofibers presence, the number of layers and surface density of nanofibers. The decreasing of the partial discharge activity is recognisable when the nanofabrics is incorporated into the composite. The obtained results proved that the nonwoven nanofibers based on Polyamide 6 seem to be a perspective material with certain resistance to partial discharge activity.
The usage of nonwoven nanofibers for improving properties of electrical insulation R. Polansky, P. Prosr, J. Pihera, T. Dzugan, M. Zemanova, et al. Annual Report Conference on Electrical Insulation and Dielectric Phenomena CEIDP, 2016 The nonwovens and nanofibers as a new material for possible improving properties of electrical insulation are introduced in this paper. Nanofibers were used to modify a commonly used two-component like mica-based high voltage electrical insulating material. The tested material in the form of resin-rich sheets was modified using a various number of layers (1, 2 and 3) of the nanofibers made from Polyamide 6 (PA6) in the various surface densities (1, 2 and 3 g·m-2). Specimens were manufactured at increased temperature and pressure. The volume resistivity, voltage dependence of dissipation factor and mechanical properties were tested. The structure of the nanofibers was examined by scanning electron microscopy (SEM), the manufactured composites were analyzed under optical microscopy. Obtained results showed that composites with layers of nonwoven nanofibers are characterized by lower values of dissipation factor at electric field intensity greater than 5 kV/mm when the partial discharge activity starts to occur. Mechanical tests showed that while the Young's modulus of the modified composites notably increased (of about 27 - 43.5 %), flexural strength stayed on the same values as unmodified composites.
Development of a measuring system for on-line in situ monitoring of composite materials manufacturing R. Polanský, J. Pihera, J. Komárek, R. Pavlica, P. Prosr, et al. Composites Part A Applied Science and Manufacturing, 2016 A unique portable measuring system using an impedance spectroscopy method with a self-adapting frequency of measurement is introduced. The system is intended for the on-line in situ monitoring of composite materials curing under industrial conditions. The capabilities of the developed system are demonstrated through the results obtained from on-line in situ measurements of unreinforced thermosetting resin, as well as of composites under real manufacturing conditions. Observations are supported by the results of other established methods for determining the degree of curing: temperature-modulated differential scanning calorimetry (MDSC), Fourier transform infrared spectroscopy (FT-IR) and broadband dielectric spectroscopy (BDS). Compressive and bending tests were also carried out on manufactured composites removed at different stages of the post-curing phase. Due to the self-adapting frequency, the system has enhanced sensitivity in the post-cure phase when the diffusion-controlled reactions proceed and, therefore, is suitable also for the analysis of hard post-cure samples.
