Defects in electrical cabling can range from being purely cosmetic to extremely serious. High voltage power transmission lines must be reliable. A defect causing failure of such a line will result in large scale disruption, as well as costly rerouting and repair. Power failures in public transport systems are also very disruptive, and failure of a fire defence system, or a power system in an industrial plant or oil rig could lead to loss of life.
Pirelli Cables Ltd makes insulated cables for energy distribution, cables for submarine power distribution, building and wiring cables, communications cables, and optical fibre. These products are installed in a variety of environments - underground, under sea, in industrial sites, in railway tunnels and in buildings, and are expected to give continuous service for long periods, often decades. Reliability is a key issue, and fire retardancy and low smoke emission are vital safety issues. Pirelli has to understand any process malfunction so that it can be eliminated, safeguarding product quality and eliminating scrap and rework.
Analytical Work at Pirelli Cables
The Pirelli central research and engineering laboratory is home to the analytical chemistry laboratory. Here, problems accessible by mid-range FTIR are investigated by infra-red microspectroscopy. These problems can be deposits from a plastic extruder, for example, where breakaway and subsequent incorporation into an extruded insulation results in rejection of the cable. Identifying the cause of such defects allows a reduction in their numbers and savings in costs.
Reflectance FTIR Microspectroscopy
Much of the work is done with cross-linked polymers and so simple pressing is unavailable as a sample preparation option, because the defects are typically less then 1mm across, irregular in shape and frequently relatively hard. ATR techniques are difficult to apply. Microspectroscopy allows interrogation of these and much smaller domains and can be applied in reflectance without sample preparation if necessary - if only one defect is available and there is a need to carry out other work on it then a non destructive technique is valuable. Reflectance spectroscopy has also been found valuable for the study of deposits on metal surfaces.
Transmission FTIR Microspectroscopy
Specimens of unfilled cross-linked material are also microtome cut for transmission work, and domains are interrogated down to the diffraction limit of the optics/radiation combination. Data has been obtained on samples up to lmm thick, containing areas of interest only 20 microns across.
FTIR Microspectroscopy Equipment
The microspectroscopy equipment used at the laboratory is a Perkin Elmer System 2000 FT IR spectrometer and i-Series microscope. The microscope has changed the way that analytical applications are approached. Spectra of polymer additives were taken normally by the time consuming method of preparing KBr disks. Now, crystals are examined in transmitted IR radiation directly, giving speedy and reliable identification. This application is also useful in identifying solvent extracted materials that are sometimes only available in small quantities.
Use of the microscope has also allowed some mapping of chemical changes along the length of cables subjected to destructive fire studies. This work was key to understanding material compatibility issues and the processes which take place in cables in fire. The instrument has been invaluable in a wide variety of investigations with benefits continuing as sampling techniques improve. Microanalysis of residues from separation techniques will also become increasingly important, and the microscope will be used because the amount of sample is so small.*