Fitness detection for polymer bank notes

 

 

By John Taylor, Note Issue Department, RBA

 

Paper presented at the Currency Conference; October 1996.

 

Introduction

 

All of Australia's banknotes are now produced on polymer (plastic) substrate instead of paper substrate. The change was introduced through the release of a new series of notes, the first of which was released in 1992. Other notes followed at roughly annual intervals culminating with the release of the $100 note in May 1996. Public reaction to the new series of notes is now very positive.

 

The move away from paper substrate was prompted by a desire to improve the security of Australia's banknotes. In addition to improved security, polymer notes are proving to be significantly more durable than paper notes.

The increased durability of polymer notes is a consequence of various factors, including:

  • the overcoating of finished notes with a clear varnish plus the non-porous and non- fibrous nature of the substrate, which mean that the notes do not absorb moisture (oils, sweat, beverages, etc) like paper notes. These same properties also mean that the notes do not stain or accumulate dirt as easily as paper notes do;

  • the non-fibrous nature of the polymer substrate, which means that the substrate does not physically breakdown with repeated folding, as occurs with paper notes which, in part, causes paper notes to go limp;

  • the toughness of the polymer substrate, which makes it much more difficult to initiate a tear in a polymer note compared to a paper note (although it is true that once a tear is initiated in a polymer note it propagates more easily than in a paper note, the initial toughness appears to be the overriding characteristic).

The different wear characteristics of polymer notes mean that the criteria that are used in manual or machine sorting of polymer notes are different in some ways to those that are used for paper notes. This paper discusses this issue and outlines our first hand experiences.

 

Initial experiences

 

In 1988, Australia conducted a field trial of polymer note technology to determine if the technology was viable. This trial involved putting into circulation, for a limited time, specially printed commemorative $10 notes. These notes replaced a significant number of the paper $10 notes that were in circulation. The $10 denomination was chosen because it was used heavily in day-to-day cash transactions.

 

At the end of the trial in 1989, the commemorative $10 notes had been in circulation for longer than the average life of a paper $10 note. The results of the trial were that:

  • polymer notes were much more durable than paper notes;

  • polymer notes did not soil like paper notes;

  • polymer notes did not go limp like paper notes;

  • polymer notes developed similar mechanical faults to paper notes. (Mechanical faults include tears (open, closed), tape, staples, corner folds, missing corners, holes, size variations.) Interestingly, the incidence of tears was even lower than anticipated, but, because of the tear propagation characteristics, a greater percentage of polymer notes with tears had sticky tape applied as compared to paper notes.

The field trial was judged a major success. The Bank was very encouraged by the results of a survey of public acceptance and performance conducted at the end of the trial. Some results of the survey were:

  • 88% of those surveyed perceived a that major advantage of polymer notes was their resistance to damage;

  • 87% appreciated the notes' cleanliness.

While it was clear that polymer notes were significantly more durable, it was difficult to determine precisely how much. The fact that the duration of the field trial was less than the average life of the commemorative $10 note did not make the job any easier. In the event, we estimated that the life of a polymer note (without an OVD like that used on the note) was twice that of its paper equivalent (the estimate for a note with an OVD like that used on the note was one and a half times). Of concern was that we did not know if some unknown factor or factors, in addition to mechanical faults, would emerge as important sorting criteria when polymer notes were left in circulation for extended periods. Laboratory simulations of wear, including those developed specifically for the much more robust polymer notes, gave no indication of what these other factors might be.

 

The Bank subsequently decided to introduce a new series of notes which would be produced using the new type of substrate. The Bank also decided that it would initially determine if a polymer note was fit for further use by solely checking to see if the note had mechanical faults. It was expected that the traditional soil reflectivity detector would be of little use. The laboratory based extended wear tests gave the Bank some confidence that it was not at great risk from relying on this one criterion. The plan was to monitor closely how polymer notes wore with extended use to identify as early as possible if an additional sorting criterion was required.

 

At the time we made the decision to introduce polymer notes, the Bank's CVCS systems were fitted with relatively crude detectors to check for mechanical faults (eg holes, tears, tape, missing corners). The Bank decided to get CSI to upgrade these to enable the Bank to more accurately discriminate between different levels of mechanical damage.

 

Extended experience from the new series of banknotes

 

Some of Australia's new series of polymer notes have now been in circulation for over 4 years. Our experience with how polymer notes wear in circulation has, therefore, increased significantly. This added experience has shown that the field trial involving the commemorative $10 note and the laboratory extended wear tests were not sufficient to fully understand how polymer notes wear with extended use in circulation. Additional knowledge about sorting criteria for polymer notes has been acquired by the Bank which is critical to the efficient sorting of such notes.

 

In many ways it is not surprising that additional factors were identified. Most mechanical faults in commemorative $10 notes were the result of mistreatment or impact (eg stapling a group of notes together and then tearing off individual notes from the group) rather than some fundamental breakdown in the substrate. The commemorative $10 note was not in circulation for a sufficiently long period to see the cumulative effect of prolonged heavy use.

 

Experience from early notes from the new note series has shown that our original estimate of life (twice that of the paper note equivalent) was far too conservative and badly underestimated the life of polymer notes. Experience has shown that polymer notes do not develop mechanical faults as quickly as we first anticipated. The basic polymer substrate has proved to be extremely robust.

 

The robustness of the substrate means that the printed surface of the note has to survive the rigours of circulation for much longer than initially anticipated. The polymer substrate is lasting so long that eventually the printed surface starts to wear. This ink wear has thus become an additional important sorting criteria for both manual and machine processing.

 

During their long life, polymer notes experience folding and crumpling many times. Eventually, the cumulative effect of abrasion along fold and crumple lines causes ink to wear. Initially, this is very minor and difficult to see as it usually involves ink being worn away in very thin lines. Eventually, after extended use in circulation, these areas of wear widen and deepen and become more noticeable. Because major fold lines tend to be concentrated around the centre of the note, ink wear also tends to be concentrated in this area as well.

 

The new series note which has been in circulation for the longest time is, on average, lasting around four times that of its paper equivalent before ink wear suggests that it could be taken out of circulation. This is around double our initial estimate.

 

As a result of this additional experience, the Bank has started to upgrade its CVCS systems for machine sorting to include an ink wear detector to complement the mechanical fitness detectors. For manual sorting, the major criteria used by cash handlers are the presence of tears, sticky tape, missing corners or ink wear.

 

New ink wear detector

 

Detecting the ink wear discussed above is not necessarily easy on a high speed note sorter. The ink wear is most obvious in transmitted light: when the note is held up to the light, the ink wear is seen as highlights against a dark background due to the decreased opacity of the note in the areas of ink wear. This, in fact, has become the basis for the approach taken by TechComm Group, an Australian company engaged by the Bank, to build an ink wear detector. As the note travels along the machine transport path it is backlit. A camera detects the resultant image which is analysed at high speed to determine the amount of abnormal light transmission which is a measure of the extent of ink wear. The detector can be set at different threshold levels to allow different quality standards to be implemented if desired.

 

Detection of soiling on polymer notes

 

As mentioned above, polymer notes do not soil like paper notes. This is not to say that polymer notes do not soil to some extent. One of the main causes of soiling on paper notes is what is usually called "self soiling". That is, as the raised intaglio ink (used to print the main design elements on notes) is worn away through abrasion, it is spread and deposited on the rest of the note. This and other soiling is most obvious on the unprinted or lightest areas of the note. Standard soil reflectivity detectors for high speed note sorters are designed to detect the gradual darkening of these areas over the life of the note and exploit the strong correlation between this darkening and the general decrease in the quality of the note. Some infra-red detectors are capable of detecting the spreading of the intaglio ink across the note and can be used as a form of soil detector.

 

The intaglio ink on polymer notes is also gradually abraded and spread across the notes in much the same way it is with paper notes. The extent of soiling and hence darkening is, however, significantly less than with paper notes due to the overcoating, non-porous and non-fibrous characteristics of polymer notes outlined in the introduction above.

 

At first glance, this might suggest that, despite the reduced extent of soiling on polymer notes, traditional soil detectors may have some applicability. However, it is our experience that the correlation between the darkening of a note that comes with soiling and the decrease in the note's general condition does not hold for the full life of a polymer note. After a period of time, the correlation reverses. That is, polymer notes that have been used extensively start to reflect greater amounts of light than new notes.

 

This difference in behaviour between polymer and paper notes is a result of two factors. First, the soiling on paper notes is cumulative as it gets deeper and more extensively embedded into the paper substrate's fibrous structure over time. This does not happen with polymer notes. Second, as ink is abraded off polymer notes the notes' colour lightens. This is because soiling is not cumulative with polymer notes and it is possible to abrade away more ink from polymer notes than from paper notes. With paper notes, some of the ink gets protected by being embedded amongst the fibres. Consequently, the relationship between the physical condition of a polymer note and light reflectivity is a non-linear one which is not well suited to form the basis of a simple detector.

 

At this stage we do not have any direct experience with infra-red soil detectors to get a feel for the likely applicability of these detectors with polymer notes. However, some indirect information to hand suggests that they will work.

 

Detection of changes in stiffness of polymer notes

 

For paper notes there appears to be a strong correlation between the increase in soiling and the decrease in stiffness of the note. Consequently, it can be reasonable to check for either one or the other. Do limpness detectors have any applicability with polymer notes? While we have not done extensive testing, it is our experience that limpness detectors (based around the decrease in the audible crackle a note makes as it is bent) may be applicable to polymer notes. Paper notes go limp because of moisture and a fundamental breakdown of the substrate. This reduces the crackle a note makes. With polymer notes, the reduced crackle that occurs as the note ages is not the result of the substrate breaking down, but of it becoming more flexible. If further study confirms the usefulness of limpness detectors, they may be an alternative for ink wear detectors or they may complement such detectors as currently happens with soil and limpness detectors for paper notes.

 

Summary

 

Polymer notes are much more durable and wear differently than paper notes. Extended real life experiences have been critical in learning how polymer notes perform in circulation. Australia is the only country with that experience.

 

Standard approaches for sorting fit from unfit paper notes involve detecting soiling and/or limpness, and mechanical faults. For polymer notes, the relevant criteria are ink wear, and mechanical faults. If potentially delicate features, such as certain optically variable devices, are used on polymer notes (or paper notes), then special fitness detectors for them may be required. The Bank has developed an ink wear detector for use with machine sorting. The detector is being fitted to the Bank's CVCS systems but can be fitted to other types of machines.

 

The detectors on machines used to detect mechanical faults in paper notes are equally applicable to polymer notes. From our experience, traditional soil reflectivity detectors are of little use with polymer notes. However, further study will be needed to determine if traditional limpness or infra-red soil detectors are applicable to polymer notes as well as paper notes. If they are, they may be substitutes for an ink wear detector or complement such a detector.

 

 

 

 

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