An Approach to the Production of Secure Durable Bank Notes (1974)

[More text appears: This film was produced for a meeting in 1974 attended by the RBA Governor and the CSIRO Chairman to demonstrate a proof of concept. The film has not been shown publicly since.]

[Old film rolls with title reading: An Approach to the Production of Secure Durable Bank Notes]

[Film slide: This film records the development of novel security tokens by the CSIRO Division of Applied Organic Chemistry under the joint sponsorship of CSIRO and the Reserve Bank of Australia.]

[Next film slide: We acknowledge the many useful meetings and discussions with the staff of the note issue department of the Reserve Bank of Australia.]

[Next film slide: Project Operating Committee: Dr D. H. Solomon (Chairman) – Chief, Division of Applied Organic Chemistry, CSIRO; Mr M. F. Brown – Manager, Printing Section, Reserve Bank of Australia; Mr R. A. S. Bywater – General Manager, Reserve Bank of Australia. (page scrolls down to reveal more Committee names) Dr S. D. Hamann – Chairman, Applied Chemistry Laboratories, CSIRO; Mr P. E. Morriss – Technical Officer, Reserve Bank of Australia; Mr P. A. Grant (Secretary) – Assistant Secretary (Industrial & Physical Sciences) CSIRO.]

[Next film slide: In addition, the following members of the CSIRO Division of Applied Organic Chemistry have been involved with the project: Mr K. Clark, Miss J. Kearns, Mr B. Loft, Dr C. McLean, Mr J. Ross, Mrs J. Swift]

[Next film slide: Photography and Direction: Roger Seccombe]

[Image shows a blank piece of paper on a table next to two Australian ten dollar notes, displaying the front and reverse design of the note]

Narrator: The perennial problem of note forgeries emphasises the need for more security features in notes. CSIRO’s initial involvement in the development of a more secure note began with a search for a paper with unique properties.

[Image shows a woman with three sheets of paper on the table in front of her and she tears the first piece in half]

Papers composed of polyvinyl alcohol fibrils and other synthetic fibres were considered but these had certain disadvantages and offered no obvious security features.

[Image shows the same woman holding up a piece of paper and then a close up of the torn edge]

Papers made from wool were also investigated but showed little promise and had poor tear resistance.

[Image changes to a man at a desk displaying the laminate note with two layers and the diffraction grating]

Interest in plastic film laminates began with the production by the Reserve Bank of a note composed of two melinex sheets containing a commercially available diffraction grating.

[Image shows a woman at a desk displaying the optically variable devices in turn as the narrator mentions them]

CSIRO then decided to insert devices which cannot be reproduced by photographic techniques, that is, optically variable devices. Those chosen were: liquid crystals, substances that changed colour with temperature variations; diffraction gratings, devices that change colour and pattern when the viewing angle changes; and Moire interference patterns, patterns that move when viewed from different angles.

[Image shows a man at a desk preparing the layers for the experimental note as the narrator describes]

The first experimental notes produced had identification printed on a specially prepared paper. Holes were punched in the paper to accommodate the diffraction grating, Moire interference pattern and section of liquid crystal sheet. Outer layers of plasticised polyvinyl chloride were then placed in position to protect the printing and encapsulate the optically variable devices. Heat and pressure were used to heat seal the laminates together.

[Image shows the same man placing the note into the machine press to perform this process and then removing the finished product]

This note satisfied the security requirements but suffered three unacceptable properties.

[Image shows the same man demonstrating these properties as the narrator describes them]

The cantilever strength was less than that of present day notes, printing was visible on the reverse side because of poor opacity, the tear strength was poor.

[Image shows a man adding white substances and then a red powder, to a machine hopper]

Work was carried out on the production of a more suitable plastic from which to construct the note. Low density polyethylene was pigmented with titanium dioxide to give the correct polymer sheet opacity and a coloured pigment was added to give a background colour. The ingredients were mixed together at elevated temperatures.

[Image shows the pink polymer coming out of the extruder and being placed between two sheets of white plastic]

Film was produced by subjecting the polymer to several tonnes pressure while maintaining the temperature above the softening point of the polymer, in this case, 150 degrees Celsius.

[Image shows sheets being placed into the machine press and then the finished product, a flattened polymer sheet, being removed]

A variety of coloured films was prepared in this manner.

[Image changes to show several of these flattened sheets on a table in an array of colours]

In order to give the polymer film sufficient strength, woven terylene cloth was incorporated.

[Image shows a man adding the terylene cloth between the layers as the narrator describes and then attempting to tear the finished cloth]

The terylene was embedded in the polymer by using pressure and elevated temperatures. The resulting laminate possessed a tear strength greatly in excess of that of paper notes.

[Image shows a man performing the oxidisation process as the narrator describes and then displaying the finished product]

Without treatment, the surface of polyethylene is not receptive to printing inks and it was therefore necessary to oxidise the surface. This can be achieved on a small scale using hot chromic acid baths, followed by washing in a number of other solutions to neutralise the acid.

[Image shows the laminate moving through the printing press and he displays the finished product]

The oxidised centre laminate was printed by a letter press technique using ink specially formulated for adhesion to polyethylene surfaces.

[Image shows a man adding these features as the narrator describes]

An encapsulated liquid crystal mixture was prepared and applied to the required area by a spray technique. The Moire pattern and diffraction grating were added and the outer layers placed in position to protect the printing and optical devices. High density polyethylene was used in these layers to give the note the required cantilever strength. The laminate was then sealed together using pressure and heat.

[Image shows the same man placing the note into the machine press to perform this process and then removing the finished product]

This note had a typical plastic feel, which was unacceptable to the Reserve Bank. It was therefore decided to emboss the surface with a paper texture. A sheet of paper, the required size, was pressed into rigid polyvinyl chloride film. This process leaves an impression of the paper’s surface in the polymer film.

[Image shows a man adding paper to the layers, placing it in the machine press and the displaying the finished product]

The PVC film was mounted on a backing slab and a nickel and copper plate grown by electrodeposition techniques.

[Image shows the backing slab being placed in the electroplating bath]

Separation of the plastic from the metal left a metal replication of the original paper surface.

[Image shows a man displaying the metal replication, the complimentary plate and the note being placed between as the narrator describes]

A complimentary plate was produced by pressing the metal plate into a thermo-setting polymeric material. The note was placed between the embossing plates and subjected to elevated temperatures and pressure. This process transfers the surface texture of the embossing plates to the surface of the note.

[Image shows a man removing the embossed product from the machine press]

Extensive testing has been carried out to compare the new and conventional note materials. Tensile and tear strength tests show that the plastic laminate is considerably stronger.

[Image shows a laminate note and a regular note placed in a machine and the result of the regular note tearing first]

In the double fold test, the new material was 400 times better. In dirt retention tests, the new material retained less dirt than does the paper.

[Image shows the tests being performed as the narrator describes and the results of each test]

So far, we’ve shown the laborious methods by which the notes are made on a small scale. Large scale production is much simpler.

[Image shows close ups of bags of polymer beads with Union Carbide printed on the bags and then the beads being added to the hopper]

The polymer film would be produced on commercial film blowing equipment, such as that possessed by Union Carbide Australia Limited. Low density polyethylene granules are added to the hopper and forced along an extruder screw by a 40 horsepower motor.

[Image shows a white polymer bubble emerging from the extruder and being drawn up the tower and over the rollers as the narrator describes]

The molten polymer is forced out of a circular die in the form of a bubble. The bubble is drawn up a tall vertical tower. The size of this bubble, and hence the film thickness, is controlled by the air pressure within the bubble. The tube is collapsed over a series of rollers at the top of the tower and is produced as a continuous film.

[Image changes to show a red polymer bubble emerging from the extruder and then the flattened sheet on a roller]

The colour of the film is controlled by adding to the polymer feed batch, coloured pigments, in addition to titanium dioxide. To obtain flat sheet, one edge of collapsed tube is removed and a roll of centre fold sheet is obtained. On this machine, Union Carbide have cooperated in preparing a range of plastic films suitable for note production. High voltage equipment is used to render the surface of the film suitable for printing.

[Image shows machinery and a blue electric beam]

This process known as corona discharge, applies a high voltage between the surface of the film and an electrode. This results in a continuous electric discharge which oxidises the polymer surface and is a much simpler process than the chemical oxidation used on small scale runs.

[Image shows the laminating machine and the processes as described by the narrator]

The films produced are used in a laminating machine built by CSIRO. A terylene web is fed between two filled low density polyethylene films and passed around a heated drum. Pressure is applied by means of a nip roller resulting in heat sealing of the three layers to give the required centre laminate. This equipment carries out the same operation as laboratory presses. The centre laminate is printed with the appropriate design and holes are punched for the optically variable devices.

[Image shows the holes being punched into the printed strip and other features added as described by the narrator]

The centre laminate is fed onto a film of high density polyethylene, the optical devices placed in position, and the covering layer of high density polyethylene film added. The three films are passed around a heated drum, through a nip roller to heat seal the components and over an embossing roller. The completed notes are then drawn off in the form of a continuous strip.

[Film slide: Optically Variable Devices]

[Next film slide: Diffraction Gratings]

Reflecting diffraction gratings consisting of segments of circles are available commercially.

[Image shows a board with squares of example diffraction gratings being moved back and forth to show the light bouncing off them]

Transmitting gratings are another type of diffraction grating available.

[Image shows a darkened room with a light beam passing through plastic to show the different colours reflected off it]

[Image changes to a diagram of diffraction grating as the narrator explains the process]

Gratings break up white light into various colours because of their three dimensional nature. The path length AA, travelled by light rays is different to path length BB. Hence in the plane XY, the two light rays interact, either reinforcing or cancelling one another according to the equations shown. This means that diffraction gratings cannot be reproduced by conventional photographic and printing techniques, because of the necessity to maintain the three dimensional surface.

[Image shows the superimposed Moire patterns being moved across one another as the narrator describes]

Dr Hamann has discovered that the patterns obtained from diffraction gratings may be predicted from the Moire patterns obtained from superimposing two plots, similar to the proposed diffraction grating. This enables us to design unique diffraction gratings, which will be obviously different from those available commercially.

[Image shows computer driven machinery and plotter in a dark room performing the process as the narrator describes]

One method of producing these gratings is by photographic reduction. The necessary artwork is produced on a computer driven plotting bed. The computer is programmed to draw the required pattern and calculates the X and Y coordinates for the plotter. The plotter draws a 650mm by 650mm diagram on a photographic film by means of a narrow light beam. In this case, 1,500 concentric circles. Safe lighting conditions are necessary as the photographic film is exposed on the plotting bed. The film is then developed.

[Image changes to show the finished product being placed into the reduction camera and the process being performed as the narrator describes]

The artwork is placed in a unique reduction camera, only available at the Weapons Research Establishment Salisbury, and reduced by a factor of 25 to give a 25mm by 25mm, three dimensional image on an optically flat glass slide. The glass is mounted on a steel back and pressed into rigid polyvinyl chloride film under carefully controlled conditions of pressure and temperature. This step represents a major simplification by us over conventional techniques which use a transfer to photo resist.

[Image shows the steel backed, glass slide being placed in the machine press and then the finished product]

The grating is replicated on the polymer surface which is then placed on a rigid backing plate. The gratings are placed in a vacuum deposition apparatus.

[Image shows the gratings being placed in the vacuum apparatus and then removed to be placed into the electroplating bath and then the finished product]

A thin layer of silver metal is evaporated onto the surface of the gratings to give a conducting layer suitable for electrodeposition. A chromium and copper backing is grown on the gratings in an electroplating bath. Separation of the metal from the plastic leaves a metal replication of the original diffraction gratings.

[Image shows a liquid being applied and spread over a glass plate and then the coated aluminium sheet]

To prevent a forger from replicating from the grating in a note, a unique polymer film has been developed. A thin film of this polymer is prepared on a backing sheet and coating of aluminium applied.

[Image shows items placed in the machine press and the finished product and spraying more polymer on as the narrator describes]

The grating is pressed into the polymer at elevated temperatures. This process embosses the surfaces with the diffraction grating and also partially cross-links the unique polymer. The grating is then filled with the unique polymer, which is also partially cross-linked to obstruct methods of exposing the grating surface.

[Film slide: Liquid Crystals]

[Image shows several sheets of dark laminate spread out on a table and then a close up of a woman placing her hand onto a sheet to show the colour change from her body heat]

Liquid crystals are substances that are capable of changing colour with variations in temperature. They are complex mixtures of cholesteryl esters and ester carbonates.

[Image pans along a row of chemical bottles as the narrator describes and then changes to an ester molecule diagram and model]

The molecules of the individual components are shaped in such a way as to align themselves in the same direction over a specific temperature range.

[Image shows a diagram demonstrating changing wavelength and crystal alignment]

Over this temperature range, the liquid crystals reflect light of a specific wavelength, depending on the temperature. Hence liquid crystals change colour as the temperature alters. Liquid crystals suitable for incorporation into plastic laminates require encapsulation to protect them during the laminating procedure.

[Image shows a man adding the ingredients to a metal vial as the narrator describes and then pouring the emulsified liquid into a conical flask]

The ingredients for the capsule walls are dissolved in water and the liquid crystal mixture is added. EMULSIFICATION takes place under carefully controlled conditions of pH and the encapsulating material coats the droplets of liquid crystal. The encapsulated liquid crystal is then applied to the printed background.

[Film slide: Moire Patterns]

Everyone has seen Moire patterns of one kind or another. They appear in the folds of a lace curtain and in the shadow of a picket fence.

[Image shows overlapping laminates and the interference bands as the narrator describes]

They’re most usually seen when a set of equally spaced parallel lines is viewed through another set. In this case the interference pattern is also a set of parallel, equidistant bands. In general, the shape and spacing of the lines determine what we see.

[Image shows both the circle patterns of lines as described by the narrator]

Here a circle, who’s spacings decrease in exact proportion to their areas. And here are some circles who’s spacings vary according to a sine wave function. These patterns are striking, and we might think of using them as security devices. But they are mathematically simple and a clever forger could work out how to make them.

[Image shows a page of uneven bands as the narrator describes]

For that reason, we’ve devised new and much more complex patterns based on randomly spaced lines. The lines you see here are not randomly spaced, but they’re slightly irregularly spaced and the Moire patterns are correspondingly irregular.

[Image shows a computer controlled plotter drawing lines and a close up of the computer display]

Let’s see what happens if the spacings become completely random. The best way to draw a set of randomly spaced lines is to use a computer controlled plotter, with a program like this one, for generating a sequence of pseudo-random numbers. The last digits in the second line are the random numbers which decide the position of each line. The computer can reproduce the same set of spacings or it can impose a modulating function, which varies the spacing slightly from one plot to another in a smooth manner, or it can alter the shapes of the lines.

[Image shows sets of two overlapping sheets of lines to form patterns and symbols as described by the narrator]

If two non-identical sets of randomly spaced lines are superimposed, a random interference pattern results. By contrast, two identical sets of lines, give a single sharp interference band. If we now modulate the spacings of one of those grids, with respect to the other, we produce an interference band with the shape of the modulating function. With more complicated modulating functions we can produce fairly elaborate symbols.

[Image shows the reproduced patterns as the narrator describes, including the red and green pattern]

To use these grids as security devices in notes, we reproduce them on opposite sides of a thin transparent film. When the film is tilted, the pattern moves from side to side. The most subtle of our devices so far, is a stereoscopic pattern formed by reducing the scale of one of the grids and altering the shapes of it’s lines. No forger could hope to produce this kind of pattern. For variety, we can make one of the grids coloured and then we can form intertwining red and green patterns.

[Image shows the pattern from the overlapping pages of dots as the narrator describes]

Finally, we’re not restricted to random lines. Here’s a set of randomly spaced dots, superimposed on an identical set. When both sets are positive prints, we see a white blob. But if one set is a positive print and the other is a negative, we see a black blob. In this case, it’s a stereoscopic one because we’ve shrunk one grid with respect to the other.

[Image shows overlapping pages of dots slowly moving to display the Reserve Bank of Australia’s logo]

In this case, we rotate the positive and negative films with respect to each other.

[Film slide: Produced for the Division of Applied Organic Chemistry by the CSIRO Film and Video Centre. 1974.]