Questions from the Floor
Ian Gilmour:
It appears that water is the main factor in degrading magnetic tape, in particular polyester-urethane, and that this is typically a reaction where the hydrogen ions and hydroxyl ions separate to form an alcohol and cartezolic acid which then accelerates the reaction. In order to confirm this, a number tests were conducted in which tapes were artificially aged in both inert gases such as nitrogen and in oxygen. The results suggest that, in order of severity, hydrolysis occurs first and then, at higher temperatures, oxidation and then, finally, pyrolysis at well above room temperatures. This has led us to want to store tapes in much lower humidities. I was wondering if you have any comments to make.
Michele Edge:
It really depends upon the specific structure of the urethane elastomer. There can be great differences between samples from different sources. In some urethane elastomers hydrolysis will dominate the reactions. In our studies, we have found that we can quite easily see the different peroxide levels in different samples and we have observed that the peroxides are decomposing so there must he oxidative degradation taking place as well. We found that the moisture that was present was serving as a plasticizer to actually enhance contact between the urethane elastomer and the pigment particles. This then facilitated the oxidation.
Jacques Lemaire:
We have been working a lot on the competition of the hydrolysis and the oxidation and in most cases oxidation precedes hydrolysis. For example, in polyester urethane the oxidation process leads to alpha carbonyl rafae which are very sensitive to water. So oxidation precedes hydrolysis.
Michele Edge:
That is precisely what we found in our results - the initial mechanism was the oxidation.
Gerry Gibson:
The chart that you showed from the Image Permanence Institute demonstrated, for example, that at 80%RH and at 10°C the tape had a life expectancy of 70 years. At 20%RH and at 24°C it had a life expectancy of 60 years. It would seem to indicate, therefore, that temperature has a major impact. That has not been addressed here. Could you comment?
Michele Edge:
The principal mechanism of the breakdown is that, to form the acid, you need moisture. All chemical reactions are subject to temperature; the higher the temperature, the faster the process takes place. There is, therefore, an interchange between the temperature and the humidity levels. If you go to very low humidity, there is no agent present to cause the reaction. If there is humidity present, then the lower the temperature the better.
Gerry Gibson:
Can we conclude, therefore, that cold and dry is best?
Michele Edge:
It depends upon how dry and how cold, because of other physical property changes. It ceases to be an issue of chemistry and becomes more a problem of physical properties.
Henning Schou:
One thing that I do not fully understand is that, if you do not know exactly how much acetic acid, which is a catalyst, is present in a film, how can you predict the lift expectancy of the film?
Michele Edge:
This is also a problem. This is why we are working on an indicator. We want to be able to give an indication of how much acid can accumulate before the film is no longer viable. For nitrate film this is easy. You can directly correlate the nitrate acids given off with image fading. With acetate film, the acids create problems with copying, the emulsion becomes sticky and there are physical property changes. A simple correlation is not possible.
David Stebbings:
I wondered if anyone had studied the possibility of a microbiological activity causing the decay of these polymers.
Michele Edge:
One major area of polymer research is into biodegradation. The types of polymers that we use for tape and film supports are, in fact, reasonably resistant to bacteria. When breaking these polymers down for recycling, other materials are frequently added to try to accelerate the breakdown. What usually happens, however, is that the bacteria attack the introduced material and leave the polymer alone. So the support polymers seem to be proof against bacterial attack.
Tulsi Ram:
I want to confirm one point about cellulose nitrate films. We have traditionally thought that it is degrading faster than the acetate materials. The latest data from our tests of films in storage is showing that the pH of the nitrate films is noticeably higher. (i.e. it is less acid) when kept in the ideal storage conditions than the acetate. Accelerated aging tests are also indicating that the behaviour of nitrate film is better than acetate.
Michele Edge:
It depends on the parameter that you are measuring. If you are measuring the acidity levels then I agree. If, however, you examine what is given off by nitrate film when its temperature is first raised, then you will find nitrogen dioxide which fades the image.
Friedrich Engel: May I point out that not all binders of magnetic tape include polyurethanes. Some tape binders, particularly in tapes of German manufacture, are PVC based and these are more stable.
Michele Edge:
As Prof Lemaire will, I’m sure, confirm, PVCs are notoriously unstable, particularly with regard to oxidation and breakdown. It really depends upon the exact formulation. I have been talking about the work done on urethane elastomers and cannot, therefore, comment greatly about the PVC binders because we have not examined these in detail. The research has involved examining old tape samples which contain urethane elastomers.
Dietrich Schüller:
A comment about the Vinegar Syndrome and sound tapes. As a result of/he Second JTS in Berlin in 1987, we examined the tapes held in the Phonogrammarchiv of the Austrian Academy of Sciences and found a tape that smelt of vinegar. It was removed from the store and placed on my desk in an open box. I have now lost an interesting demonstration item because it does not smell of vinegar any more and it is perfectly playable. I guess that there is a trade off between ventilation and enclosures; between the vinegar syndrome and dust and atmospheric pollutants.
Michele Edge:
You are right; it is the practicality. On one hand it is enclosure and the vinegar syndrome and on the other good ventilation and dust. If you merely punch holes in the boxes and cans, you run the risk of infecting adjacent films or tapes.
Ian Gilmour:
The point raised by Dietrich Schüller is timely. The indicators that have been discussed measure the concentration of acetic vapours in the air. There has been a lot of debate recently about whether it is better to aerate cans or whether it is better to keep them .sealed so that a decaying film does not infect its neighbours. Could you advise on this please.
Michele Edge:
I cannot really comment on this dilemma. It is really for you to decide which course of action will he best for your collection. My work is aimed at finding out what is happening to enable you to make informed decisions.
Morten Jacobsen:
Dr Edge said that she would group indicators into two groups: long-term and short-term. The indicator that my company make, the M-Check, is placed by her in the long-term group. If I put one of my indicators in a can with a film that is only marginally smelling of vinegar, even to the nose at the start of a day when it is fresh, my indicator will show a change within a few days. If a similar test is carried out with a strongly acidic film, the indicator will change within six hours. This I would call a short-term indicator.
We must also look at how long the indicator will stay in the can. Will it last far a generation or two or three? The indicator must survive as long as it is necessary for it to turn yellow.
The question of ventilation of cans is interesting. There are libraries in the USA where they hold the lid of the can above the base by means of small pegs. Others punch holes in the base and sides. Both increase ventilation considerably. There are countries, however, where the fire regulations do not permit this.
Michele Edge:
The grouping of indicators into long-term and short-term is merely a guide as to how long it will take to change colour. There are several good indicators available, including yours and the one Joan was speaking about, that change colour in a day or in a week or so. What is needed is an indicator that will change colour very quickly, ideally with a graded colour change that permits an estimate of the degree of acid to be made, so that films can be tested as they come into the archive.
Henning Schou:
May I ask Joan a question. You said that you use non-ionic resins for these tests. As the chemists amongst the audience will know, when you measure the pH it is, in fact, the negative logarithm of the ionised hydrogen. How confident are you that a non-ionic resin will give a realistic result?
Joan Whitehead:
I am confident because what I have done is to change the pH range of the indicator that I am using by buffering it. Also I am not measuring the pH of the film but the acetic vapour in the can. The results of the tests that I am conducting cannot be expressed in terms of a pH anyway.
Henning Schou:
An acid is only as strong as its medium and when we measure pH we talk about water as neutral at 10.7 of hydrogen etc. If the water is not present and we use non-ionic resins, presumably without water, how reliable are the results of your work?
Joan Whitehead:
I will be able to answer that point in three or four months time. The work is still in progress but the signs are that the test is going to be very reliable.
Henning Schou:
Would it he true to say that, at the moment, these indicators are at an experimental stage?
Joan Whitehead:
Yes.
Terry Watson:
The lithium red test that you described showed a colour change after one or two hours. Does this tell us where on the graph of acetate film that particular film is? Has it already passed the point of no return?
Joan Whitehead:
What I found was that a one millimetre linear colour change on the filter paper corresponded with a pH of about 4.0 or 4.5. I cannot he more specific because the lest is carried out at room temperature. Variations in the ambient temperature cause variations in the results. Tests carried out in the higher temperatures of the summer months reacted much faster than tests carried out in the winter. I have carried out tests on over 100 films and I can confidently say that if the filter paper shows a one millimetre colour change then its pH is between 4.0 and 4.5.
