The structure and dynamics of networks formed by rod-shaped particles can be indirectly investigated by measuring the diffusion of spherical tracer particles. This method was used to characterize cellulose nanofibril (CNF) networks in both dispersed and arrested states, the results of which were compared with coarse-grained Brownian dynamics simulations. At a CNF concentration of 0.2 wt% a transition was observed where, below this concentration tracer diffusion is governed by the increasing macroscopic viscosity of the dispersion. Above 0.2 wt%, the diffusion of small particles (20-40 nm) remains viscosity controlled, while particles (100-500 nm) become trapped in the CNF network. Sedimentation of silica microparticles (1-5 mu m) in CNF dispersions was also determined, showing that sedimentation of larger particles is significantly affected by the presence of CNF. At concentrations of 0.2 wt%, the sedimentation velocity of 5 mu m particles was reduced by 99 % compared to pure water.

This paper presents an experimental characterization of the prebreakdown phenomena in liquid/solid interfaces. The characterization is devoted to the 2nd mode positive streamers initiated and propagated along interfaces of mineral-oil and solids with different chemical composition and physical properties. Polymers of low density polyethylene (LDPE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) polyvinylidene fluoride (PVDF) and papers made of kraft paper and a kraft fibril paper (made from cellulosic micro and nano fibrils), lignin-free paper and paper with high lignin content (referred to as k107 kraft paper) are used as the solid to study their influence on the streamer inception and propagation. The streamers are initiated at the interface by applying steps of voltage to a point-plane electrode arrangement with a solid (dielectric barrier) into the gap. The solid is placed diagonal to the oil gap and near to the point electrode. Shadowgraphs, charge and light intensity recordings are obtained during the inception and propagation of the streamers. Thus, estimations of the streamer length, velocity, current and average charge, are also presented. A time delay has been observed before the initiation of the streamer. This delay is probably correlated to the initiation process and formation of the gaseous phase of the streamer near to the interface. The threshold propagation voltage of the 2nd mode streamers at mineral-oil/solid interfaces is shown to be independent of the interface. However, the inception voltage is highly influenced by the interface. Additionally, the observed characteristics of streamers propagation (e.g. current, length, velocity, etc) along the tested interfaces cannot be fully explained by a capacitive coupling effect (permittivity mismatch). This open a discussion for the possibility that properties of the solid such as chemical composition, wettability and surface roughness can influence the streamer propagation.

Chemically modified cellulose micro- and nanofibrils were successfully used as paper strength additives. Three different kinds of cellulose nanofibrils (CNFs) were studied: carboxymethylated CNFs, periodate-oxidised carboxymethylated CNFs and dopamine-grafted carboxymethylated CNFs, all prepared from bleached chemical fibres of dissolving grade, and one microfibrillated cellulose from unbleached kraft fibres. In addition to mechanical characterization of the final paper sheets the fibril retention, sheet density and sheet morphology were also studied as a function of addition of the four different cellulose fibrils. In general, the cellulose fibrils, when used as additives, significantly increased the tensile strength, Young’s modulus and strain-at-break of the paper sheets. The effects of the different fibrils on these properties were compared and evaluated and used to analyse the underlying mechanisms behind the strengthening effect. The strength-enhancing effect was most pronounced for the periodate-oxidised CNFs when they were added together with polyvinyl amine (PVAm) or poly(dimethyldiallylammonium chloride) (pDADMAC). The addition of periodate-oxidised CNFs, with pDADMAC as retention aid, resulted in a 37% increase in tensile strength at a 2 wt% addition and an 89% increase at a 15 wt% addition (from 67 to 92 and 125 kNm/kg, respectively) compared to a reference with only pDADMAC. Wet-strong sheets with a wet tensile index of 30 kNm/kg were also obtained when periodate-oxidised CNFs and PVAm were combined. This significant increase in wet strength is suggested to be the result of a formation of cross-links between the aldehyde groups, introduced by the periodate oxidation, and hydroxyl groups on the lignocellulosic fibres and the primary amines of PVAm. Even though less significant, there was also an increase in wet tensile strength when pDADMAC was used together with periodate-oxidised fibrils which shows that the aldehyde groups are able to increase the wet strength without the presence of the primary amines of the PVAm. As an alternative method to strengthen the fibre network, carboxymethylated CNFs grafted with dopamine, by an ethyl dimethylaminopropyl carbodiimide coupling, were used as a strength additive. When used as an additive, these CNFs showed a strong propensity to form films on and around the fibres and significantly increased the mechanical properties of the sheets. Their addition resulted in an increase in the Young´s modulus by 41%, from 5.1 to 7.2 GPa, and an increase in the tensile strength index of 98% (from 53 to 105 kNm/kg) with 5 wt% retained dopamine-grafted CNFs.

Surface discharge behavior of modified oil-impregnated paper (OIP) with nanoparticles (NPs), has been investigated under AC voltage with superimposed impulses. Surface Partial Discharges (PD) can develop at an oil-paper interface and lead to its degradation. Modified paper, made from fibers with adsorbed nanoparticles, can affect the partial discharge behavior of a paper in combination with oil at the interface between oil and fibers. Papers with two different concentrations (2 wt% and 6 wt%) of silica (SiO2), and paper with silanized zinc oxide (ZnO) nanoparticles (1 wt%) have been studied. Papers with SiO2 NPs showed lower impulse-induced surface PD activity. However, thorough purification during the production of SiO2 filled papers was necessary to achieve a good performance. With less purification, paper with 2 wt% of SiO2 did not show such significant improvements. Paper with 6 wt% of SiO2 NPs showed a large number of AC surface PDs, but low influence of impulse voltage on subsequent PD. Papers containing 1 wt% of silanized ZnO showed reduced relative permittivity, but no significant difference in surface PD behavior. The effect of high lignin content in Kraft paper has also been studied. Paper with higher lignin content showed better surface PD characteristics under the impulse. Paper with low concentrations of pure SiO2 NPs, and paper with high lignin content thus appear good candidates for further studies to improve the surface PD behavior of OIP.

This document presents an experimental study on the propagation of first mode negative streamers along mineral oil-solid interfaces. Samples made of an oil impregnated kraft paper and a low-porosity paper made from cellulosic micro and nano fibrils, as well as different polymeric films (low density polyethylene (LDPE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF)) are used as the solid. A comparison of the length, charge and velocity of streamers for all different mineral oil-solid interfaces is reported. It is shown that streamers propagate longer and faster along mineral oil-solid interfaces with low surface roughness, low porosity and higher electrical permittivity than mineral oil. Those streamers show a quasi-continuous injection of charge in the early stage of their propagation. This quasi-continuous charge injection consists of a sequence of small charge steps separated by few tens of nanoseconds in between. In comparison, the streamers that propagate along surfaces with similar permittivity to the mineral oil have lower injection of charge and higher stopping voltage conditions than streamers propagating free in the liquid without any solid barrier.

This work presents an experimental study ofsecond mode positive streamers propagating along mineral oilpaperinterfaces. A point-plane arrangement immersed inmineral oil with the paper inclined 60 degrees to the planeelectrode is used to create the liquid-solid interface. Kraft paperand a kraft fibril paper, made from cellulosic micro and nanofibrils, with higher density and lower surface roughness are usedas the solid materials. High speed shadowgraphy and chargerecordings are used to compare the propagation of second modepositive streamers along the mineral oil-kraft paper and mineraloil-kraft fibril paper. Streamers creeping along the mineral oilkraftpaper interface propagate mainly into the liquid, with oneor two main filaments. In comparison, the streamers propagatingalong the kraft fibril paper show a strong reduction of thebranching; these streamers consist of a single filament thatpropagates exactly on the solid surface. Streamers along the kraftfibril paper also have longer propagation time than for the casewith kraft paper. Mutual electrostatic shielding betweenfilaments is observed for the streamers creeping on the kraftpaper. An electrostatic analysis of the influence of permittivity,density and surface roughness of the solid in the electricalproperties of the streamer filaments is also performed.

Papers have been prepared from fibres that were modified by physical adsorption of silica nanoparticles. Cationic and anionic nanoparticles were adsorbed either directly onto wood fibres or using the layer-by-Iayer (LbL) technique where silica nanoparticles and a polyelectrolyte of opposite charge were adsorbed in consecutive layers. It was shown that it is possible to tailor the dielectric and mechanical properties of kraft paper by utilizing the pH-dependence of the charge density of both the nanoparticles and the polyelectrolyte during the build-up of layers onto wood fibres. With only one layer of cationic silica nanoparticles, 6.0 wt% of nanoparticles were adsorbed at pH 6, leading to almost complete coverage of the wood fibre surface, resulting in a paper with low dielectric losses and improved in-plane tensile properties.

Different wood-fibre based papers were characterized by dielectric spectroscopy, mechanical testing and microscopy. The data obtained were utilized to investigate the relationship between the chemistry, morphology and density of a paper and its permittivity and dielectric loss. The density strongly influences the dielectric response, but the response is not affected by the way the density has been achieved; by pressing the paper during drying or by mechanical treatment of the fibres before sheet preparation. The chemical composition of the pulp influences the polarization, dielectric loss and charge transport. It was found that paper-vacuum and paper-oil combinations can be represented by series-equivalent circuits. The permittivity of paper made from electrical grade kraft pulp, used in e.g. high voltage transformers, without any porosity is estimated to be 5.3 and tan delta to 0.01-0.02 at 50 Hz and 70 degrees C. The lignin and hemicellulose content of the kraft pulp do not affect the real part of the permittivity significantly, but the dielectric losses increase with increasing lignin and hemicellulose content in both oil and vacuum at 50 Hz and 70 degrees C.

This paper reports the probability distribution of negative streamers initiated in mineral oil with and without a solid interface. In addition, the charge injected by conduction currents prior to the streamer inception is presented. Impregnated paper and polymeric films made of PET, PTFE and PVDF are tested as solid materials. Comparison of the conduction charge for the different oil-solid interfaces is presented. It is found that the permittivity of the material used at the solid interface does not influence significantly the condition for streamer initiation in mineral oil. A nonlinear increasing of the charge readings for the impregnated paper, PET and PVDF cases are observed.