I am sorry for such a long quote, but I could not find a URL for the
particular article and I thought it would be of great interest to
some on the list.
Nishimura is based at the Rochester Inst. of Technology Image Permananence
Institute.
========
INDUSTRY NEWS
Warning: Negative base deterioration
If you haven't been using polyester based film (such as Kodak Estar
base films), then I expect that most of you won't have any negatives
left within a few decades. Let me give you the sad story first before
I talk about the whys and hows. I got a call around 1992 or so from
Evelyn New York photographer known for her coffee table books in the
1950s and 60s of European cities. She called because she went into
her negative collection and found that they were all badly distorted
and the emulsions were lifting off. We had been researching this
problem since 1988 and were very aware of what the problem was. I had
to tell her that her life's work (other than what books and prints
were already out in the world) was gone and there was nothing that
could be done. A few could be saved by special methods, but it's so
labor intensive that of her thousands of negatives, it would only be
worth treating a couple.
Think about it.....this was only 30 years or so.........Any film that
you're shooting that isn't polyester (also called poly(ethylene
terephthalate), trade names include Estar and Mylar) then you're on
cellulose triacetate. All of the cellulose acetate film bases are
made by taking cellulose (the main constituent of paper and trees)
and modifying with acetic acid. (I'm oversimplifying here, but this
is the general idea.) This is why it's cellulose acetate. Kodachrome
film was on a variation of this base until about 1976 and then it
switched to triacetate. Ektachrome sheet film was also on another
variation, but all of these bases behave in the same way. The plastic
base reacts with water and the humidity in the air is sufficient. The
water in the presence of acids (and alkalines) pulls off the acetates
as acetic acid (or vinegar) and you can smell it. Water alone can
also do this, but the acid or alkaline acts as a special kind of
accelerator called a catalyst. Consider then that one acid and water
goes in and two acids (the original plus a vinegar) comes out. Those
two can go into a reaction to produce four acids and so on and so on.
This is what we call an autocatalytic reaction...The stuff is
generating it's own accelerator to destruction. If we measure the
deterioration over time, it starts out very slowly and slowly gets
faster. Eventually it generates enough acids that it reaches what we
call the "autocatalytic point" and the reaction really accelerates
and gets faster and faster.
This reaction is acid (or alkaline) catalysed, but is not
autocatalytic. Removal of the large acetate groups as acetic acid
vapors causes the film base to shrink. It will ultimately shrink
about 10% to 15%. At the same time, the gelatin emulsion is trying to
stay attached, but it's not shrinking so eventually the adhesion
between the base and the emulsion gives out and we see this as
"channelling." You literally find tunnels formed by the emulsion (on
top) and the separated base on the bottom often about 1/8 inch in
diameter. Sheet films with an anti-curl layer will also do that on
the anti-curl layer side as well. Breaking of the plastic molecules
into short pieces meanwhile causes the base to get brittle. I've
shocked my students by giving them a sheet of deteriorated film and
letting them flex it. It eventually has the flexibility of fresh
potato chip. I've crumbled a negative between my fingers like crisp
bacon. Meanwhile plasticizers in the base become incompatible and
they start coming out as perhaps feathery crystals or as oily liquid
filled blisters, or as solid plugs. The feathery crystals are about
the most common plasticizer, tri-phenyl phosphate. The oily liquid
plasticizer is a phthalate, often dimethoxyethylphthalate. How fast
can this happen? If you stored your negatives constantly at a
comfortable 70F (21C)/50% relative humidity, we would expect to see
fresh acetate film hit the autocatalytic point in about 40 years. In
the case that I mentioned above, every summer she would close-up her
Manhattan apartment and head to Europe so the negatives literally
baked in the humid New York City heat which greatly reduced the life
of her negatives. To give you an idea about how fast this thing can
go, let me give you an example. Suppose that we track the acidity in
the base over time. It starts out very very close to zero and at the
auto-catalytic point has x amount of acid. Now I mentioned that at
70F (21C)/50% RH it will take about 40 years to create x amount of
acid. How long do you think it would take, if we left it stored under
the same conditions, to double in acidity or to reach 2x acidity? Let
me tell you that it will be about another 5 years....and to increase
to 3x acidity would take around another 2 years. Most of you don't
have 70F/50% RH storage. Temperature and humidity play a big role. If
you were at 75F (24C)/50% RH rather than 40 years, it would take
roughly 25 years to reach the autocatalytic point and another 4 years
or so to double. Humidity also plays a big role (since the reactions
need water.) If you were at 70F/80% RH not only would I worry about
mold, but you'll reach the autocatalytic point in about 17 years.
However, if you were in a very dry place at say 20% RH, it could take
about 90 years to reach the autocatalytic point. I should add that
the autocatalytic point isn't the end of life for your negative.
It'll stink, but will still be printable. Even at double the acidity,
you should still be able to use the film, but time left is very
short. Now before people start calling the manufacturers to complain,
it isn't their fault. In the early 20 th century when acetate bases
came out to replace nitrate, they ran accelerated studies and found
that the acetate hardly deteriorated at all while the nitrate was
falling apart like crazy so they thought that it was pretty darn
stable. Unfortunately they didn't realize that the reaction needed
moisture and they were running dry oven tests. (The RH was probably
only around 5% to 8%.) They started getting complaints in the 1950s
or 60s, but they were all from tropical countries (parts of India,
south-east Asia, South America,....) and they thought that it would
only happen in tropical countries. Much to their surprise it started
showing-up in north america starting in the early 1980s. People said
that the triphenyl phosphate also reacts with humidity to form phenol
(aka "carbolic acid") and phosphoric acid. This was true, but even
film that contained different plasticizers also deteriorated. Some
people said that it was the nitrate subbing layer. [Gelatin didn't
stick very well to cellulose acetate, but it did to cellulose nitrate
and cellulose nitrate also stuck very well to cellulose acetate so
they used a very thin layer of cellulose nitrate as a glue to stick
the emulsion to acetate bases up until at least the early 90s or so.)
Well, research showed that it wasn't the nitrate subbing either
because film with and without the subbing layer deteriorated at about
the same rate. (More recently companies have found how to make
gelatin stick and the nitrate subbing layer isn't used anymore.) Some
people worried that it was processing since people were using acid
fixing baths and acid stop baths. Well, it's not the processing
either. I have a large box of unprocessed sheet film that just
stinks.About the only thing that you can do is to store your negative
at a reasonably cool temperature. In fact the ISO and ANSI standards
recommend that collections store their acetate negatives below 45F
(and only then if they can be stored between 20% and 30% RH otherwise
they have to be even colder.) One of the lessons that we've learned
about this is that bad conditions count for more than good conditions
so even if your place is pretty good most of the year, but bad during
the summer, the summer will kill you. Consider my apartment. I keep
it pretty cool during the winter (I like the cold) and it sits at 55F
and a humidifier keeps it around 50% RH. If my apartment was like
that all the time, my negatives wouldn't reach the ACP for about 100
years. During the summer though its often 85 or worse since I don't
have a good flow of air through the place (due to window locations.)
The humidity also gets pretty high (although my bedroom is air
conditioned.) At 85F/70% RH I should reach the ACP in about 9 years.
What are the comparable effects of these two extremes? We'll ignore
the spring and fall and for simplicity say that my apartment is at
each condition roughly half the time (all other times are somewhere
in between.) The overall effect is that my negatives (if I stored
them there) would reach the ACP in 16 or 17 years. They would be
toast in less than 25 years. Now most people don't keep their places
as cold as I do in the winter. This is not to say that every negative
stored under these conditions will suffer this fate. We find it to be
like people. You find the odd 90-year old who still smokes a pack a
day. Who knows why they're still around? We also find the odd
negative that does better or worse that we would expect. However,
anecdotal evidence from collections suggest that on average, our
estimates are pretty good. We run a seminar every August and back in
the early 1990s I asked how many had acetate deterioration in their
collections and a few people put up their hands. Each year the number
of hands increased and now there's virtually no one who says no
(unless they don't have any acetate negatives.) The motion picture
industry spent millions on cold storage in the past 10 years just
because of this problem. They call it "vinegar syndrome" for good
reasons. People there ask how much vinegar can come out of film so I
ran a calculation based on typical triacetate film -- 1000 ft of 35
mm. This is roughly equivalent to 200 rolls of 36-exposure 35 mm film
or roughly 200 8 X 10 sheets. Ask yourself how much vinegar you might
expect if we took all of the acetate off of the film and turned it
into aqueous vinegar (as you buy it in the store.) The amount is just
a tiny bit less than one US gallon. You can smell acetic acid at very
low levels by the way-- roughly 1 ppm in the air. Polyester will more
or less do the same thing (producing ethylene glycol (anti-freeze)
and terephthalic acid.) However if it takes say 100 years for acetate
to reach some point of deterioration at 70F/50% RH, under the same
conditions, it would take polyester somewhere between 500 and 10,000
years to reach the same state. This is why we don't tend to worry
about it with polyester.
You may ask why the companies don't switch totally to polyester.
They're certainly trying to, but it's not necessarily all that easy.
One problem that needs to be dealt with is know as core-set. (This is
different from a corset. :^)) If you roll plastic film tightly onto a
small reel....like a 35 mm spool...you cause stress. The outside of
each lap of film is larger than the inside of the lap. (We see this
at the Olympic field events. The runner in the inside lane has a
shorter distance to run so must start further back to compensate.)
Inside the plastic these stresses cause the polymer (plastic)
molecules to slide against each other to relieve the stress. This is
known as "cold flow." The plastic is literally changing shape at room
temperature. The result is curly film. I remember when I was in high
school standing in the dark with this roll of incredibly curly film
trying to get it onto a film real. Ack. Cellulose acetate takes on
core-set faster than polyester. However, what happens to the curly
roll after processing? It hangs nice and flat (pretty much) certainly
nothing like the horrible mess you were trying to get onto the reel.
This is because acetate film absorbs a fair amount of water during
processing (a few percent by weight) and this causes the core-set to
be released. Polyester, on the other hand, only absorbs a fraction of
a percent water by weight during processing so 35 mm polyester film
would come out of processing about a curly as it went in. Imagine how
hard it would be to print curly negatives. As far as I know, the only
35 mm polyester film on the market right now is Techical Pan. The
industry is working on the problem though and may even now have a
solution. (And they may be producing much more 35 mm polyester film.)
Polyester, as I've mentioned in regard to photo enclosures, is also
fairly expensive and tends to dull cutting blades pretty easily. By
the way, you may hear of people saying that this base deterioration
is just a "diacetate problem." It's not true. When diacetate film was
made starting in the late teens they didn't add as much plasticizer
and one of the consequences was a larger degree of solvent pooling.
Have any of you ever tried to dry tomatoes? If you don't cut them up,
the outside dries quickly forming a hard shell outside. Water from
the inside can't pentrate this hard shell so the tomato stays wet
inside and eventually rots. Acetate film base has the same problem.
It's cast by mixing the plastic with methylene chloride (and a few
other solvents) to form a viscous syrup called "dope." This is poured
onto a very large, continuously rotating wheel. As the wheel turns,
heaters and fans (effectively) evaporate off the solvents so that by
the time the wheel has made one rotation, the film base can be peeled
off for coating. The solvents evaporate off the surface very quickly
but this forms a solid outside while there's still solvents trapped
inside that can't get out. One of the things that the plasticizer
does is to reduce the amount of trapped solvent. (It's primarily
added as a fire retardant.) By the time you get the film, it contains
around 2olvent by weight and this slowly evaporates out over time.
(Causing the film to slowly shrink very slightly.) When they made
diacetate, they didn't include as much plasticizer so the there was a
greater amount of solvent loss shrinkage and since acetate
deterioration also manifests itself as a shrinkage, people assumed
that they were the same problem. (Diacetate will also get acetate
deterioration, but the initial rapid shrinkage found with diacetate
was solvent loss and not base deterioration.) On a historical note,
nitrate film was the first cellulose based film to be case on a
wheel. Prior to that they used a glass table at Kodak that was made
from a single piece of perfect glass 200 feet long and 40 inches
wide. The glass had to be absolutely flat with no scratches. In the
afternoon they poured the nitrate dope onto the table and then they
turned on big fans and opened all the windows. (Obviously there was
no EPA then.) In the morning, they closed everything up and poured on
the emulsion (in the dark.) When the emulsion had set, they peeled
the film and cut it up then poured the next batch of base. Now as bad
as this sounds, prior to this, they used to shave sheet film base off
a block of cellulose nitrate.
One last thing.... polyester doesn't suffer from solvent loss
shrinkage because it has to be melt cast rather than solvent cast.
The melt is extruded into a sheet and then undergoes several physical
modifications. One thing that they do is known as biaxial
orientation. They take the extruded film and they heat it up and pull
it in the length direction they repeat the process in the width
direction as well. I never can remember the order and it may be width
first. What this does is causes the plastic molecules to line-up in
the plane of the film. (If this was a bed full of kids, all of them
are lying down in different directions, but no one is standing up.)
The last thing they do is to add dimensional stability by partly
crystalizing the plastic. (For the chemists out there, of course that
can't be done, but parts of the molecule end-up in a closest packing
state.) If this wasn't done, if the film was heated (say in a
projector), the base would shrink.Anyway, I'm hoping that if I can
spread the word through the world of photographers I won't have to
tell anyone else that there's nothing left of their life's work.
Douglas Nishimura Research Scientist, Image Permanence Institute
- --
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Winsor Crosby
Long Beach, California
mailto:wincros@xxxxxxxxxxx
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