Marit-Helen Glomm Ese

Oxidation of paper insulation in transformers

Oxidation ageing experiments on paper insulation and transformer oil have been performed using two different techniques. One is a traditional ageing study wherein degree of polymerization (DP) is measured after conditioning in sealed bottles containing paper submerged in oil, where the partial pressure of oxygen over the oil is varied. Paper is sampled at certain intervals during 8 months and DP-values are measured. The other study was carried out by means of a microcalorimeter; continuously measuring the heat flow from the paper-oil sample. We find that the degradation reaction rate is not linear with oxygen concentration and that the activation energy of oxidation is lower than what has earlier been found for hydrolysis. The microcalorimeter appears to mimic ageing characteristics for oxidation and can potentially be a useful tool for quick ageing investigations.

Interactions between bovine serum albumin and Langmuir films composed of charged and uncharged poly(N-isopropylacrylamide) block copolymers

The thermoresponsive poly(N-isopropylacrylamide) (PNIPAAM) and NIPAAM block copolymer derivatives are attractive for drug delivery applications as they contract reversibly at lower critical solution temperatures (LCST) close to physiological conditions. In order to investigate biomaterial-protein compatibility, we have studied the interaction between PNIPAAM copolymer films spread at the air-water surface and bovine serum albumin (BSA) injected below the precompressed polymer films, using the Langmuir technique coupled with Brewster angle microscopy (BAM). A PNIPAAM homopolymer was applied together with a number of PNIPAAM-based di- and triblock copolymers, to assess effects of e.g., charge and hydrophobicity on protein-polymer interactions. The nature and strength of protein-polymer interaction was found to be tunable, ranging from complex formation (PNIPAAM homopolymer) to mixed monolayers and electrostatic cross-linking, according to the nature of the co-monomer. Results show that intercalation versus adsorption can be controlled through polymer composition.

Study of oxidation and hydrolysis of oil impregnated paper insulation for transformers using a microcalorimeter

An isothermal microcalorimeter has been used to study ageing of oil impregnated kraft paper. Under the assumption that the heat flow is proportional to the ageing rate of paper it is found that activation energy seems to be lower for oxidation than for hydrolysis. This observation corresponds well with results obtained by traditional ageing methods. The results have also been confirmed by measuring the changes in DP of the samples used in the calorimetric measurements. Comparing degradation of aged and unaged oil impregnated paper in air gave a higher heat flow for the unaged paper, but the activation energy for the processes remained the same. The process was also studied with another cellulosic material and in addition varying size of the sample holder of the calorimeter, giving similar results. Hence, since the microcalorimeter appears to give the same qualitative ageing characteristics for oxidation and hydrolysis as more time consuming methods, calorimeter could be a useful tool for quick ageing investigations.

Ageing and corrosion of paper insulated copper windings: the effect of irgamet® 39 in aged insulated oil

Copper sulphide can be formed on copper windings in high voltage transformers and cause dielectric breakdown in the transformers. To avoid transformer failures caused by copper sulphide, metal passivators are added to the insulating oil. Here, the commonly used metal passivator Irgamet® 39 was used in combination with copper surfaces, sulphides and insulating oil in order to investigate the effect of the passivator. Traditional methods such as ageing experiments supplemented by Quartz Crystal Microbalance experiments were used to investigate the effect of Irgamet® 39. The results showed no effect of Irgamet® 39 on the corrosion of copper in systems where the formation of copper sulphide had started, while the effect was found to be very good on fresh copper surfaces.

Esterification of low molecular weight acids in cellulose

The interaction between low molecular weight acids dissolved in mineral oil with cellulose has been studied. The formation of esters as a function of temperature has been demonstrated using attenuated total reflection infrared (IR) spectroscopy. Higher temperatures lead to a shift towards ester formation compared to lower temperatures. This reaction might be reversed by exposing the cellulose to an excess of water at elevated temperatures. However, this hydrolysis of the esters will not be complete, leaving some of the initially free acids trapped in the paper as esters. Regarding degradation of the cellulose, only free acids with a carbonyl group available will enhance the break-up of the glycosidic bonds in the cellulose polymer. Therefore it seems more appropriate to use cold water extraction in order to quantify the amount of free acids in the paper/pressboard.

Interactions of α-Lactalbumin and Cytochrome c with Langmuir Monolayers of Glycerophospholipids

Interaction of cytochrome c (Cyt c), α-lactalbumin III (α-La III) with Langmuir monolayers of pure and mixed glycerophospholipids was investigated using surface pressure-area (Π-A) isotherms. The general trend was that maximum interaction between protein and phospholipid is observed for mixed (1:1 molar ratio) phospholipid monolayers. Interaction between the protein and the charged phospholipid films was found to be independent of global protein charge. Our data indicate that the proteins interact with the phospholipid films by inserting themselves into the monolayer rather than anchoring to the phospholipid head groups.

Ageing of oil impregnated thermally upgraded papers

Investigation of the difference in the ageing process for several thermally upgraded and non-upgraded papers in transformer oil was performed. We observed that the level of nitrogen in the papers is crucial for the resistance towards hydrolysis. One of the papers in the experiment did not fulfil the upgrade specification with respect to the nitrogen level (data sheet). For hydrolysis of wet oil impregnated samples in argon atmosphere the upgraded papers seem to degrade less than non-upgraded papers. The paper with highest level of nitrogen degrades less than those with lower levels. The Insuldur process seems to be the best way of thermally upgrading the paper (Upgrade 1) and this also gives the highest level of nitrogen. For one of the upgraded samples (Upgrade 2) the nitrogen disappears completely after startup and this paper behaves as non-upgraded for the highest temperature. Dry oil impregnated paper (0.2% water content) under argon atmosphere does not seem to be hydrolysed and ages at a very slow rate. In the case of oxidation of dry oil impregnated samples (0.2% water content) in air there are initially no significant difference between non-upgraded and upgraded papers. However, the upgraded papers seem to have an improved performance after the water production from ageing becomes significant. The oxidation activation energy for non-upgraded paper is lower compared to hydrolysis and for upgraded paper this difference is smaller.

Charge- and temperature-dependent interactions between anionic poly(N-isopropylacrylamide) polymers in solution and a cationic surfactant at the water/air interface

Due to their expanded coil to collapsed globule transition close to physiological temperatures, N-isopropylacrylamide (NIPAAM) polymers and derivatives thereof represent promising drug delivery vectors. Here, we report on the interaction between anionic PNIPAAM derivatives containing potassium 3-sulfopropylmethacrylate (SPMA) blocks of various sizes and a cationic lipid membrane mimic as a function of temperature, polymer charge density and molecular weight, by injection under a pre-compressed surfactant monolayer. The resulting film properties (including equilibria) were studied using surface pressure–area isotherms, Brewster angle microscopy, and interfacial 2D oscillatory rheology. Whereas mixed films of dioctadecyldimethylammonium bromide (DODAB) and an uncharged PNIPAAM homopolymer resulted in distinct polymer and surfactant domains, the anionic polyelectrolytes were found to ionically cross-link and contract the DODAB film. Film penetration, charge screening and surfactant film contraction were dependent on the molecular weight and size of the SPMA blocks on the polymers.

A study of the effect of copper and additives in hydrocarbon and ester based insulating liquids with isothermal microcalorimetry

An isothermal microcalorimeter has been used to study the effect of solid copper in hexadecane, vegetable oil (ester) and mineral oil. The experiments have shown a catalytic effect of copper in oxidation of hydrocarbons and also the anti-oxidative effect of dibenzyl disulphide when combined with copper addition. The catalytic effect of copper is found in hydrocarbon as base oil and hexadecane, but not in experiments using inhibited oil and ester as the dielectric liquid.

Isothermal microcalorimetry as a tool for studying oxidation stability of insulating liquids

The application of an isothermal microcalorimeter for the investigation of the oxidation stability of insulating liquids has been studied. We propose that the total integral heat signal (area below the calorimeter curves) is a good indicator of the degree of oxidation taking place in the sample during the experiment. Known and unknown insulating liquids have been used in this study, and the amount of available oxygen in the samples has been varied. The results show that isothermal microcalorimetry is well suited for characterizing the oxidation stability of insulating liquids.