Several of the major uses of polyethylene terephthalate (PET) film, like film capacitors, barrier packaging and metal evaporated videotapes require its metallisation. The role of the interface in these applications is crucial: as capacitor films become thinner, the importance of the interface grows; it determines gas barrier and metal adhesion in packaging applications as well as epitaxy of magnetic alloys in videotapes. This project therefore investigates the chemical and physical phenomena at the interface in order to sustain the fast technological change in the above industries.
A model of the aluminium polyester interactions has been developed and values of volume and surface conductivity applicable to aluminium metallised ultrathin films have been determined. Variations in the insulation resistance of heat treated and compressed capacitors have been explained and eliminated and a model for carrier injection at the interface has been established. Appropriate adhesion measurement techniques have been developed and different surface treatments to improve adhesion have been compared. The influence of the polyester manufacturing and processing conditions on gas barrier and metal adhesion has been assessed and the surface topography needed for the epitaxial growth of magnetic alloys has been defined. A metal evaporated tape based on cobalt nickel oxide on PET has also been developed.
The trend to miniaturization means that film capacitors must use thinner and thinner films as dielectrics which increases the relative importance of the interface. One major question is the effect of a proportionally larger interface on dielectric behaviour or on leakage currents. The leakage behaviour of a material is characterized by its insulation resistance which is the ration of applied voltage to leakage current. A comprehensive analysis of conduction mechanisms in metallized polyester was carried out to improve the polyester's IR, and, specifically, to explain observed IR variations after heat and pressure treatments.
Standard ASTM methods for measuring these currents were not applicable in the case of ultrathin films and new measuring techniques have been developed. It was shown that, for standard capacitor geometries, the volume currents dominate the surface currents by roughly two orders of magnitude and thus determine the capacitor's insulation resistance.
The conduction at the interface has been shown to proceed via carrier injection at a Richardson-Schottky barrier. Contrary to expectations, however, the nature of the interface does not influence the volume charging currents. For temperatures below the polymer's glass transition, they are largely determined by the film's processing history, that is by bulk phenomena such as free volume relaxations in the amorphous phases of the polymer. This recognition has led to a modification in the processing conditions and thus to improvements in the insulation resistance of the final components, eliminating one of the problems which plagued the capacitor industry for a long time.
With the coming advent of high definition television, the current generation of video tapes will have to be replaced by higher performance tapes, capable of handling the increased data flow per time unit. This will require a bit size of 1 to 2 square micrometres, several times smaller than the 6 square micrometres achieved by a state of the art metal particle (MP) tape. We studied 2 metal alloys of possible use:
obliquely deposited cobalt nickel oxide, whose intrinsic magnetic direction is adapted to the field distribution of a conventional video head;
cobalt chromium with the magnetization perpendicular to the film plane, which provides the highest recording densities when used with a so called single pole head.
The polyester substrates were investigated and optimized, followed by the development of functional tapes based on the evaporation or the sputtering of cobalt nickel oxide and cobalt chromium layers. In addition, a comprehensive study of the magnetic properties of metallic thin films was carried out and available surface treatment have been analyzed and rated. Laboratory prototypes of the promised types of metal evaporated (ME) tapes exist now. Bit sizes below 2 square micrometres have been reached, with still frame times of several hours. For the cobalt chromium tape, a bit size of 1 square micrometre has been achieved with a sputtered layer of cobalt-79 chromium-21, using a substrate covered with a germanium underlayer at a temperature of 200 C.
The goals of the research were to increase metal polyester adhesion in food packaging materials, especially in a humid environment, and to increase the barrier to oxygen permeation of the metallized film.
The site of chemical bonding between aluminum atoms and polyester molecules was shown to be the carbonyl groups. The first monolayers of aluminum on a polyester surface are strongly oxidized, whereas the bulk of the aluminum is in the metallic state, growing in columns. Failure of the metal polyester bond occurs in most cases within the polyester, except in humid environments which lead to interfacial failure. Significant effort was put into the improvement and development of adhesion tests. The dependence of both adhesion and the oxygen barrier on the different parameters of the metallization process was assessed. The influence of thepolyester film manufacturing process was investigated and optimal parameters defined. The effect of the surface treatments on both properties was studied.
Improvements in adhesion and reduced oxygen permeation were achieved.
METALLIZED POLYESTER FILM IS GOING TO SEE STEADY GROWTH IN THE COMING YEARS IN NEW OR EXISTING APPLICATIONS, LIKE 8 MM VIDEO TAPES, CAPACITORS, FLEXIBLE PACKAGING, ELECTRONIC APPLICATIONS, FLOPPY DISCS.
LITTLE IS KNOWN AT THIS STAGE ON THE MECHANISM OF ADHESION OF EVAPORATED METAL LAYERS ON POLYESTER, OR ON THE ROLE OF THE INTERFACE IN ELECTRICAL APPLICATIONS. IN MAGNETIC APPLICATIONS AN EPITAXIAL COLUMNAR GROWTH OF THE METAL LAYER IS NECESSARY. WEAR RESISTANCE AND LUBRICATION OF THE METAL LAYER NEED TO BE OPTIMIZED.
FOR METALS WITH HIGHER MELTING POINTS THAN ALUMINIUM, MAGNETRON SPUTTERING AND ELECTRON BEAM EVAPORATION NEED TO BE OPTIMIZED.
NEW ADHESION PROMOTION TECHNIQUES LIKE IRRADIATION WITH FAR UV LIGHT NEED TO BE DEVELOPED.
Funding SchemeCSC - Cost-sharing contracts