Gary C. Stevens

Surface modification of polymer surfaces: atmospheric plasma versus vacuum plasma treatments

An atmospheric pressure non-equilibrium plasma (APNEP) has been developed in the UK by EA Technology Ltd and is currently being investigated in collaboration with the University of Surrey. The main focus is the use of atmospheric pressure plasmas to modify the surfaces of commercially important polymers including polyolefins, poly(ethylene terephthalate) and poly(methyl methacrylate). These surface modifications include surface cleaning and degreasing, oxidation, reduction, grafting, cross-linking (carbonization), etching and deposition. When trying to achieve targeted surface engineering, it is vital to gain an understanding of the mechanisms that cause these effects, for example, surface functionalization, adhesion promotion or multi-layer deposition. Hence comparisons between vacuum plasma treated surfaces have also been sought with a view to using the extensive vacuum plasma literature to gain further insight. In this paper, we will introduce the APNEP and compare the key characteristics of the plasma with those of traditional vacuum plasma systems before highlighting some of the surface modifications that can be achieved by using atmospheric plasma. Data from the analysis of treated polymers (by spectroscopy, microscopy and surface energy studies) and from direct measurements of the plasma and afterglow will be presented. Finally, our current understanding of the processes involved will be given, particularly those that are important in downstream surface treatments which take place remote from the plasma source.

Adhesion enhancement of polymer surfaces by atmospheric plasma treatment

An atmospheric pressure non-equilibrium plasma (APNEP) developed in the UK by EA Technology Ltd is currently being investigated in collaboration with the University of Surrey. Of the many applications of surface modification that can be induced using plasmas, adhesion enhancement is one of the most commercially important. In this paper, we illustrate the use of an atmospheric plasma to enhance the adhesion characteristics of low-density polyethylene (LDPE) and poly(ethylene terephthalate) (PET). The polymers were treated in the remote afterglow region of an atmospheric pressure plasma to avoid the thermal effects that can cause degradation for thermally sensitive materials when placed in direct contact with the plasma. Reactive (oxygen containing) and inert (oxygen free) atmospheric plasmas rapidly impart adhesion enhancement by a factor of two to ten as measured by 180° peel tests. However, extended exposure to the atmospheric plasma does not impart additional adhesion enhancement as the surface is ablated revealing the underlying polymer with poor adhesive characteristics. In contrast, vacuum plasma treated LDPE and PET show increased adhesion with extended plasma treatment. An adhesion enhancement in excess of two to three orders of magnitude was found to be achievable for vacuum plasma treatment times greater than 10 min.

The Dielectric Response of Polar and Non-Polar Nanodielectrics

In order to understand the relationship between the dielectric properties and the structure of polymer nanocomposites, especially the role of the interface on dielectric response, two polymers with different polarity were used to form nanocomposites with nanoalumina powder, a typical polar inorganic compound. Consequently, two kinds of nanodielectrics with very different interfaces were produced. In the epoxy nanodielectric, broad band dielectric spectroscopic measurements show that no difference is found between the unfilled resin and the nanocomposite under dry conditions. However, their dielectric responses are very different when they contain about 0.4 % b.w. absorbed water, a concentration that occurs under ambient exposure conditions. The findings suggest that the sites for absorbed water in the epoxy nanocomposites are very different to those in the unfilled thermoset and relate to the large interfacial internal surface in these materials. In this case all the relaxation processes can be well described by the Havriliak-Negami function, which give useful information on the dynamics and activation energies associated with the relaxation processes. In polyethylene nanocomposites, dielectric measurements show that the incorporation of nanoalumina produces a dielectric loss peak in the low frequency. This relaxation peak moves to high frequency with increasing nanoalumina content and the peak becomes broader. In the presence of 0.06 % b.w. absorbed water this peak also shows the same type of unusual dielectric loss behavior as that observed in the undried epoxy nanodielectrics. This behavior is thought to arise from the interplay between interfacial water mobility and bonding rather than the host polymer.

Non-destructive measurement of the degradation of transformer insulating paper

Knowledge of the condition of power transformer winding insulation paper is fundamental to making optimum asset replacement decisions in the power industry. The ability to assess the aged condition of Kraft paper quickly and non-destructively using portable instrumentation would significantly increase the opportunities for gaining this knowledge. Insulation paper degrades over time in-service and its degree of polymerization (DP) reduces, eventually affecting its mechanical strength. At low DP levels the insulation may start to disintegrate and the risk of electrical breakdown increases. Currently-used methods of estimating DP are either approximate or destructive. The use of spectroscopy together with multivariate statistical analysis (MVSA) provides a powerful non-destructive evaluation of the condition of paper. From initial feasibility studies, we have developed a simple, portable system (TRANSPEC) using fiber-optics and broad-band spectroscopy that can measure the degree of polymerization of various aged transformer papers to a precision of approximately 30 DP units with a spatial resolution of 14 mm. The system can also measure the chemical composition and condition of the insulating mineral oil. MVSA regression models were constructed from library spectral data, and these models are used to predict the DP of other papers with parameters that fall within the range spanned by the set of calibration samples. Separating oil and moisture information from wetted paper is possible and will be reported in a separate publication. With a single TRANSPEC system, non-destructive in-situ analysis of the DP of insulating paper is possible, providing a rapid cost-effective method for transformer insulation condition assessment and monitoring, which correlates well with current destructive methods.

Spherulitic banding in metallocene catalysed polyethylene spherulites

Abstract Atomic force microscopy (AFM) and transmission electron microscopy (TEM) have been used to examine the influence of molecular weight on the melt-crystallised morphology of a series of permanganically etched metallocene catalysed high-density polyethylenes (mHDPEs). The effect of increasing molecular weight is manifest by different morphologies, viz. banded spherulites and lamellar domains structures above a critical molecular weight, and sheaf-like structures below a critical molecular weight. In addition, the incipient spherulite development has been detected in a low molecular weight mHDPE. AFM height imaging of permanganically etched sections has revealed an apparent band period spacing decreasing from the centre of any given spherulite in non-diametral sections of isothermally crystallised samples. This observation was also confirmed by exhaustive TEM measurements on similar non-diametral sections. AFM height measurements show that band height amplitude varies across bands as a function of radial position and is a characteristic feature of all the banded spherulitc structures studied. The band amplitude and period are dependent on molecular weight and are sensitive to the degree of etching with fast etching rates for low molecular weight mHDPEs. Both the AFM and TEM measurements of the apparent band spacing converge with radial distance to the true band period spacing. Homopolymers and copolymers show a different relationship of true band spacing with molecular weight.

Thermoplastic cable insulation comprising a blend of isotactic polypropylene and a propylene-ethylene copolymer

There is much interest in the development of replacement materials for crosslinked polyethylene (XLPE) that are both recyclable (i.e. thermoplastic) and capable of high temperature operation. Thermally, polypropylene is the ideal choice, although its stiffness and low electrical breakdown strength make for a challenging materials design problem. We report here on the compositional optimization of a propylene homopolymer/propylene-ethylene copolymer blend in terms of its dynamic mechanical properties and thin film electrical breakdown strength. The extrusion of a trial minicable using the optimized blend is also discussed, which is shown to exhibit a significantly improved electrical performance, as gauged by its DC breakdown strength, than an XLPE-insulated reference.

Life assessment of cable paper using slow thermal ramp methods

Abstract Rapid thermal ramp techniques are widely used to assess the properties of polymers (e.g. DTA, TGA, etc.). For certain types of processes, slow thermal ramp methods can assist the process of assessing the longer term ageing and life prediction of a polymeric material in complex environments. We will illustrate this in relation to recent work on the ageing of Kraft electrical insulation paper in insulating oil in relation to the longer term ageing of oil-filled power cables. In this case the key indicators of degradation were the degree of polymerisation (DP) and the production of gases capable of dissolving in the oil and measured by dissolved gas in oil analysis (DGA). Kinetic modelling of the change in the DP value was used to generate Arrhenius parameters and cable life prediction calculations were made for a range of potential cable operating temperatures. The DP decreases with age and increasing temperature in a complex chain scission reaction. In parallel, above 150°C there is a rapid exponential increase in the evolution of CO, CO 2 , CH 4 , C 2 H 4 and C 2 H 6 with increasing temperature. We found that the rate of degradation of the paper/oil system is dependent on the degree of containment of the ageing experiments. These results indicate that containment effects and resulting pressure effects exist in this type of accelerated non-isothermal ageing experiment and should be accounted for in practical ageing environments. The estimated life of the paper can be calculated if we assume that the Arrhenius parameters are representative of the entire degradation process and if suitable end-of-life criteria can be defined. It is also possible to calculate the average operating temperature of an electrical cable over a fixed time period, knowing the starting and finishing DP. Some of the attractions, potential weaknesses and use of the slow thermal ramp method in complex environments are also discussed.

Spectroscopic measurement and analysis of water and oil in transformer insulating paper

A portable fiber-optic spectroscopic system (TRANSPEC) has been developed for nondestructive measurement of aged transformer insulating paper. Following successful measurement of degree of polymerization (DP) of a range of transformer-aged paper samples, the system has now been shown to separate the oil and paper information for measurement of DP in oil-wetted paper samples. In addition, the system has been shown to be capable of the prediction of both oil and water content of paper to a high accuracy, and is also capable of identifying and quantifying different water species. Spectroscopic measurements have been used together with gravimetric water adsorption measurements to investigate the kinetics of uptake of water vapour into paper from air in the case of nominally dry and oil-impregnated samples. Relationships between water adsorption parameters and properties of the paper insulation have also been investigated using measurements under controlled conditions.

Thermal Properties of Composites Filled with Different Fillers

Electrical insulating polymers usually incorporate inorganic fillers to achieve specific electrical, mechanical, thermal properties and reduce cost. In this exploratory work, we have studied the thermal properties, at 40degC, of epoxy resin composites with different filler such as: boron nitride, alumina, diamond, silicon carbide, and silicon nitride, with particle size ranges from micro to nano. The results show that BN is the best material in enhancing the thermal conductivity of the epoxy composites despite the various fillers having comparable or higher thermal conductivities than BN. Our current model shows that large disparities in mechanical modulus - a metric for interfacial compatibility - can cause thermal contact resistance due to increased phonon scattering resulting from the large mismatch between filler particle and resin matrix.

A mathematical model and decision-support framework for material recovery, recycling and cascaded use

Abstract To achieve more than incremental reductions in resource consumption and waste, it will be necessary to develop new approaches to the systematic use and re-use of materials in the kind of system termed an “industrial ecology”. This paper presents a new methodology—CHAin Management of Materials and Products (CHAMP)—developed for modelling the flow of materials through a succession of uses with different performance requirements. Although developed specifically for polymers, the CHAMP approach is also applicable to other materials and products. Materials are characterised by a set of technical performance parameters, termed utilities. Geographical location is also treated as a utility to enable logistics—both distribution of products and collection of used products or waste—to be incorporated within the same modelling framework. Processing, transport and use are treated as activities through which a material can pass. The costs and environmental impacts of activities are included in the modelling framework, and are assessed on a life cycle basis by considering the complete supply chain of materials and energy used by each activity. The methodology includes acceptance criteria which determine whether a material is suitable for specific uses or activities. These criteria are applied within the model to guide selection of materials for specific applications and of successive uses for specific materials. A simple example of the CHAMP approach is given, to illustrate the kinds of problem to which it has been applied.