Tuesday, 1 January 2013

Stamp Phosphors


I have been asked by Dr. Frank Holland  if readers would be interested in his article entitled " How a little bit of a chemical helps the post to arrive"

I found it very informative, so without a to do here it is for your enjoyment


When I was consulting for a small company making fluorescents, they were asked by the Royal Mail (RM) to make both stamp phosphors (SP) and coding phosphors (CP). I developed suitable chemicals for both applications.

CPs were used to produce a machine readable set of dots representing the post code of a particular letter. The original CP we produced appeared as blue dots on the envelope and these were activated by long wave UV (whereas the SP was activated by short wave). However this system has been abandoned and replaced by a bar code system using a fluorescent ink.

SPs are used to produce a machine readable set of stripes on a stamp which enables the machine to orientate the letter the correct way up (stamp top right with address approximately central) so that the written post code can be read and the correct machine readable code stamped onto the envelope. An indication of this is given in the following

http://www.youtube.com/watch?v=1BVHPmGIxBQ&feature=related

There is only a small fraction of a milligram of phosphor on a stamp, and the letter bearing the stamp is passed at high speed past a bank of ultra violet (UV) lamps, which activate the phosphor. As the envelope leaves the UV lamps it is still glowing and this glow is detected by photoelectric detector which activates the next operation on that envelope (see video).

Thus the SP is a critical part of RM's sorting process, and from what I can work out RM handles about 19 billion items of post each year, about half of which use stamps, so about 10 billion pa. RM have been using the SP I developed for about 15 years, so that means 150 billion items using my SP.

There is an interesting essay on RM stamps and SPs in http://gbmachins.co.uk/html/phosphor_bands.html , which indicates the difference between first and second class stamps. The first class has two stripes of phosphor, the second one stripe. So the first class stamp will give two flashes of afterglow to the detector, which will send it to the first class channel, the second class will give one flash to the detector which will send it to the second class channel. Take this with the orientating use shown in the YouTube video and the SPs now have two machine readable functions in the sorting process
a) orientation
b) separation of first and second class mail

But there is another aspect, security. The common first and second class stamps could be produced, including perforations, gumming, and mounting into books and sheets, very easily and cheaply by a forger using modern scanning and printing techniques. But such forgeries would be easily detected by exposure to UV, and of course they would not be sorted by the sorting machines. With the prices of stamps recently increased to 60p and 50p there is obviously a big attraction to forgery (a possible overall market of £5 billion pa!).

Thus the SP is vital to both the operation of the sorting process and the financial aspects of the RM operation.

So how are these vital SPs made? The first attempts were made by the old General Post Office in the late 1950s, see GB 870504A, putting together what is now recognised as an antenna/ emitter system, whereby a polymer matrix system absorbs light and passes that energy onto a small component (usually described as the activator) which can later emit that energy as light. Thus the electrooptical properties of the activator can be influenced by embedding these luminescent systems in organic polymeric host matrices. If the energy levels of the polymer and the activator are appropriate, the organic matrix can act as an energy transferring system for the activator. After exciting the polymer, energy can be transferred via a cascade to the activator, which later emits at its characteristic wavelength.

As GB 870504A shows the polymer matrices can be made from nitrogen containing species such as urea, melamine, etc reacting with formaldehyde, and incorporating activators such as aromatic sulphonic acids and the like. But by the mid 1990s awareness of the antenna/emitter system helped me move on from GB 870504A and I produced different systems.

So the first thing is to produce a matrix which will act as an antenna, a good example is zinc sulphide. Pure ZnS will absorb UV, and under UV it appears very white, but there is very little or no afterglow when the UV is turned off. But add 60-90 ppm of copper and there is a quite long living afterglow. The ZnS has absorbed the UV and passed it onto the copper ion ZnS system and raised the energy level in that system. When the UV is turned off that energy is released into the the rest of the ZnS crystal which emits a green after glow.

For organic phosphors the same approach applies, make a matrix which will absorb UV, and then incorporate into such a matrix a compound which will react with the matrix and be able to receive the energy absorbed by the polymer and retransmit that energy later.

This is the work of Dr Frank Holland and must not be re produced without his permission.




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