Play It Again, Paul | Magnetic Tape Deterioration 2: Coating Deterioration

Magnetic Tape Deterioration, Part 2: Coating Deterioration (November 2022)

I gratefully acknowledge the contributions to this article by Jeff Kaskey. Jeff is the owner of The LAST Factory, a business that provides audio media cleaning and preservative products, primarily to the audiophile community. Jeff provided his thoughts and perspectives on audio tape deterioration and remediation methods, and his insights helped shape and expand sections of what you’re about to read. He also provided product information beyond what was on his company’s web site, and I have now incorporated several LAST Factory products in my procedures for digitizing records and reel-to-reel tapes.

Part 1 of this series discussed deterioration of magnetic audio tape resulting from external magnetization and damage to the base. This article addresses problems associated with the binder (a coating that holds magnetizable and lubricating particles in place) and the back coating (a layer opposite the binder that reduces static electricity and provides additional lubrication). Such damage takes years to develop and will eventually ruin the tape.

Binder Deterioration

Binder Hydrolysis

Tapes can shed particles or slivers; they can become sticky; and they can emit a loud squealing noise when played or fast-wound. The predominant explanation attributes these symptoms to deterioration in the binder. The binder is composed primarily of polyurethane and essentially functions like glue holding magnetizable oxide or metal particles (domains) in a layer against the base (substrate). Binder deterioration goes by many names: Polyurethane Binder Deterioration, Soft Binder Syndrome, Archival Shed, and, most prevalently, Sticky-Shed Syndrome (SSS, often without the hyphen). There is a competing explanation, which I will address in the section on back coating deterioration.

According to the predominant explanation, the binder absorbs moisture from the air (hydrolysis), more so in high humidity. With the additional moisture, urethane particles that start out uniformly distributed in the substrate migrate to the surface, which then becomes viscous and potentially sticky. At its worst, the binder can stick to adjacent layers of tape and be peeled off, taking the domains with it and thereby permanently damaging that portion of the recording (see Figure 1). This is often more prominent in tape closer to the reel’s hub (which is where I typically encounter it in affected tapes) since the wind tends to be tighter there. The sticky residue can also gum up tape guides, lifters, rollers, and, most critically, heads. This sludge is a bear to clean up. (Cassette tapes don’t suffer from SSS caused by hydrolysis, although they can become sticky from fatty acids rising to the surface of the binder.)

Figure 1. Binder damage on a tape exhibiting Sticky-Shed Syndrome.

The stage was set for SSS in the early to mid-1970s when a new binder formulation became available, one using short-strand urethane particles. SSS appeared around a decade later, and it was determined that short-strand urethane absorbed significantly more moisture than did previous formulations. Formulations adopted by the mid-1980s changed to using urethane with mid-length strands, which absorb much less moisture. Although SSS can occur in the newer binder formulation, it appears to take longer to develop and, with tapes properly cared for, may not occur at all. (Only acetate tapes manufactured in their later years are subject to SSS since polyurethane wasn’t adopted as the binder layer until then.)

Several brands of tape were susceptible to SSS, though not uniformly across all of a brand’s lines, and SSS took longer to appear in some brands and lines than in others. There were variances even within lines attributable to different manufacturing facilities, region or country of origin, and binder formulation (manufacturers may not have been aware of whether they were receiving short-strand or mid-length-strand urethane). Cheaper tapes, notably “seconds” lines that used tape that failed first line quality control, were especially prone to SSS. The reputation of a brand isn’t always a reliable indicator. One brand I encountered began its commercial life producing high-quality tape but, over the years, degenerated into a seconds brand that became notorious for SSS.

Detection

The following are things you can do to check for SSS (I’ve performed most of these).

1. Check the tape’s box for mold or water damage (see Figure 2; there was also black mold inside the box, but I had cleaned it up before it occurred to me to take a picture). If either is present, it’s highly likely that SSS is, too.

Figure 2. Back of a cardboard tape box exhibiting moisture damage.

2. Check for dark rust-colored or brown particles in the tape’s box, indicating that the tape is shedding its binder.

3. Manually unwind the tape a few feet, allowing gravity to pull the end downward. If the tape sticks to the next layer on the reel, SSS is present. Note that this doesn’t detect where SSS is most likely to occur—near the hub.

4. Place a drop of water on the tape’s binder layer. If it remains approximately spherical, the tape is probably all right, at least where it’s being tested. If the drop flattens to where it’s noticeably shorter and wider, SSS is likely present. (Then gently dab off the water.)

5. Mount the tape and thread it directly onto the take-up reel, bypassing tape guides and heads. Wind the tape by hand from one reel to the other, and feel if and where the tape fails to unwind evenly. This is admittedly tedious, but it’s a safe way to check the entire length of the tape. You can also repeat the water drop test wherever you think there might be a problem. (While I might do this with 3” and 5” reels, I fear my patience would wear thin with anything larger, and I would skip to the next step.)

6. Assuming you haven’t found any evidence of deterioration yet, transport the tape across some lint-free material (e.g., Pellon sew-in interfacing) without any contact between the tape and the guides and heads. My preferred method is to thread the tape from the supply reel directly to the take-up reel, wind several layers of a 1”-wide strip of material around a small cylinder (a cheap ballpoint pen works nicely), hold the pen so the tape passes around the material at an angle of at least 20⁰ as if it were a tape guide (see Figure 3), and “play” the tape as slowly as possible. (I’ll call this the “pseudo-guide technique.”) You might need to hold your finger against the supply reel’s flange edges to reduce the speed, especially as the tape approaches its end where SSS effects are likely at their worst (you may want to wear a glove to protect your fingers). Another method, which I’ll call the “threading technique,” is to attach one end of a long double strip of material to the top of the head stack cover, loop it snugly around the tape guides, rollers, and heads, attach the other end to the other side of the head stack cover’s top, and “thread” the tape around the material (see Figure 4). Whichever method you use, listen for tearing sounds or squealing from the supply reel, and note especially if the tape grabs adjacent layers on the supply reel. When finished, examine any residue or particles that have accumulated on the material; they’ll provide an indication of what deterioration is present. If necessary, repeat this test as indicated below.

Figure 3. Test material strip wrapped around a pen.

Figure 4. Test material strip wrapped around the threading path.

a. A very light brown streak on the material is to be expected, and the tape is safe to play. Figure 5 shows light streaking from a 3600’ reel of Maxell 35-180B using the “pseudo-guide technique.” Maxell tapes have a reputation of being SSS-resistant.

II-5 Swatch (pen) - Nominal residue swatch.jpg

Figure 5. Swatch of test material showing nominal residue.

b. The presence of dry binder particles indicates that deterioration has begun but may not have progressed enough to affect the quality of the audio. I suggest repeating a “play” test. If there is no shedding on the second pass, the tape may be played—once (preferably to duplicate or digitize it). If binder particles are present after the second pass, or if your intent is to keep the tape to play it again in the future, I recommend remediation before playing it at all.

c. A brown stain indicates that binder deterioration is more advanced; the darker the color, the worse the condition of the tape. Playing the tape risks accumulating residue on guides and heads, and remediation is warranted. Figure 6 shows thick, dry slivers that accumulated from a 600’ reel of Scotch 176 tape using the “threading technique." Scotch 176 is typically safe to play, but there have been reported instances of particle shedding and squealing, probably due to batch variations. The accumulation from this test indicates this particular tape is not safe to play without remediation.

II-6 Swatch (path) - Dry accumulation.jpg

Figure 6. Swatch of test material with accumulation of solid residue.

d. Sticky residue indicates advanced SSS. Figure 7 shows sticky accumulation from two tests of the same tape using the “pseudo-guide technique." The blotches are about ¼” square; there were no markings on the reel or box indicating the brand. The tape is not safe to play without remediation.

Figure 7. Swatch of test material with sticky residue indicating pronounced Sticky-Shed Syndrome.

Remediation

There are several methods for counteracting binder deterioration, and each involves dehydration. The intent is to remove the moisture that has been absorbed into the binder and to get the urethane particles reabsorbed deeper into the matrix.

The safest method of addressing SSS is to store an affected tape in a vacuum chamber for one to two days. This removes moisture from the binder and reduces viscosity to a level safe for playing.

A second way is to store affected tapes for several weeks in environmentally controlled conditions: 65⁰ F. or less and 24% or less relative humidity. The disadvantages of this approach are creating the environment to achieve and maintain those conditions and the lengthy delay before the tape becomes playable (which I doubt my customers would agree to).

A similar method is to store an affected tape in a sealed container with several desiccant packets. Silica gel packets will suffice, though clay is more effective at lower humidity. While much easier to set up, the tape would need to remain sealed for several weeks for the procedure be effective.

The two methods just described may cause acetate-based tapes to become more brittle because of the reduced humidity. I would recommend immediate application of a tape preservative followed by rehydrating as described in Part 1. At present, it is unknown if this admonition applies to vacuum chambers.

The method of addressing SSS that has received the most attention involves incubating affected polyester tapes (acetate tapes should not be heated!), referred to in the vernacular as “baking” a tape. Ampex developed and patented the procedure, but the patent application did not offer specific guidance regarding temperature and time. While the recommended combination was 122⁰ for at least 8 hours, Ampex claimed success at 129⁰ for 3 hours, cited the best approach as 122⁰ for 12-16 hours, and recognized that some tapes required incubation for 24 hours (temperature not specified). Underlying some of this variance is that Ampex used an iterative approach in their testing. If a tape still tested poorly after initial incubation, it was reheated, sometimes more than once, until the tape became playable.

The Ampex patent was never enforced, and it has since expired. As a result, audio engineers, restoration professionals, and enthusiasts have experimented with incubation over the years and have devised their own “recipes” based on their experiences. Not unexpectedly, there are considerable differences among them. Recommended target temperatures vary from 122⁰ to 131⁰, and cited temperature ranges include 120-131⁰ and 130-140⁰. (I came across one recipe with a range that went up to 150⁰, but I would not recommend it.) It is important to note cautions derived from evidence: Temperatures below 122⁰ are ineffective; temperatures above 122⁰ may cause a 0.5 dB loss in frequencies above 10 KHz; and temperatures above 140⁰ may exacerbate other kinds of deterioration (“Loss of Lubricant,” described below, and “print-through,” discussed in Part 1, as well as damage to the base).

Similar divergence is evident in the recommended baking times, though some of the variance can be attributed to the size of the tape and the severity of SSS symptoms. Recommendations include 4-6 hours, 1-8 hours, 12 hours, 10-48 hours, and many others.

Several devices have been used for incubating tapes. The list includes dehydrators (such as those used to dry fruit or meat), convection ovens (cited in the Ampex patent application), hair dryers, and jury-rigged arrangements of light bulbs and fans. Whatever device is used, it should achieve and maintain the target temperature within one or two degrees. A laboratory-quality thermometer should be used to confirm temperature accuracy and how much the temperature drifts before the device triggers an adjustment. Tapes’ base and coatings expand and contract at different rates, and rapid heating or cooling can deform tape, which in turn leads to distortion of the audio—assuming the tape remains playable at all. Consequently, heating and cooling need to be done very gradually. The increase from ambient to the target temperature should take 24 hours; similarly, tapes should be cooled over a 48-hour period—likely very difficult to achieve with hair dryers or light bulbs.

Opinions on whether to bake a tape are as varied as recipes and devices. Some professionals always incubate tapes before doing anything else; others do so when they’re unsure whether SSS is present. Among the more conservative, some incubate tapes only when there is evidence of SSS while others bake tapes only as a last resort when other remediations have failed.

Regardless of the recipe and device, baking is only a temporary fix, as deteriorated conditions will reappear unless a tape preservative is applied. But incubation can make the tape playable for roughly a month, during which time it can be duplicated or digitized. Repeated baking results in distortion appearing in the audio, and that distortion increases the more times a tape is baked. It’s better to get it right the first time.

Looking to procure my own device and having ruled out hair dryers and light-and-fan contraptions, I was discouraged by the accumulation of negative dehydrator reviews describing both underheating and overheating, even on items classified as commercial grade. Wanting to avoid damaging customers’ tapes at all costs, I chose to purchase a laboratory-quality incubator, one with a digital target temperature setting, a feature that constantly adjusts temperature to keep it at the target value, and forced air to maintain a uniform temperature throughout the incubation chamber. I hadn’t researched vacuum chambers until after I ordered the incubator. When I did, I found that capacity was most frequently expressed in gallons with no indication of interior dimensions—critical for making sure a unit would accommodate a 10.5” reel or a large pancake tape. Given unfavorable reviews of “household” vacuum cannisters, I would recommend an industrial-grade device.

If you’re adventurous enough to attempt baking a tape, please keep these caveats in mind:

Splices are prone to failure after incubation. I suggest threading an incubated tape through tape guides and fast-forwarding and rewinding it before playing it, resplicing if necessary.
Do not use gas heat. Combustion of gas produces water vapor, which is what one is trying to eliminate.
Do not use a microwave oven; it will destroy the tape.
Make sure the oven will stop heating below 140⁰ (or lower, depending on what target temperature you decide to use). Recommended temperatures for tape incubation are lower than home ovens’ Low setting.
Use a laboratory-quality thermometer to ensure the target temperature is reached and maintained. Oven settings and oven thermometers are not sufficiently accurate. (Just for fun, I tested my home oven on its lowest setting using an oven thermometer and two meat thermometers. I got readings of 180⁰, 190⁰, and 193⁰—insufficiently reliable to use for tape incubation.)
Increase temperature over a 24-hour period, and cool tapes over a 48-hour period.
Do not bake acetate tapes; they will be destroyed. Hold the reel of tape up to a bright light; if you can see light through the pack, it’s acetate.
Do not use an oven or dehydrator to heat tapes and food at the same time. Compounds evaporating from food can become affixed to the tape. I’m not sure what tape vapors might be absorbed into food, and I’d rather not find out.

Back Coating Deterioration

Back coating is a layer opposite the binder intended to reduce static electricity and to provide additional lubrication. It is typically composed of anti-static carbon black particles combined with polyester or polyurethane. While back coating serves its purposes, it is subject to degradation.

Back Coating Hydrolysis

Back coating degradation arises because carbon is a very versatile element capable of changing into other compounds under certain chemical and environmental conditions, among them carboxyl acid, alcohol, polymer fragments, and carbon particles. Additionally, carbon black is highly hygroscopic (i.e., it acts like a sponge), initiating and sustaining hydrolysis. The result is a breeding ground for mold and fungus that will change the back coating into a sticky material.

Back coating hydrolysis underlies the alternate explanation of SSS, espoused by audio engineer Charles Richardson: The sticky material resulting from back coating decomposition is in direct contact with and can transfer to the binder when a tape is on a reel. Further, that sticky material is, according to this explanation, the primary cause of hydrolytic deterioration of the binder rather than the binder’s absorption of moisture. (Proponents of this explanation concede that binders can also fail because of poor formulations, presumably the use of short-strand urethane particles.)

Reported findings supporting this explanation include the following:

Forensic analysis has found that back coating components make up most of the sticky material found on the binder of tapes exhibiting SSS.
Remediations focusing on the binder are generally unsuccessful in eliminating SSS.
Baking tapes that don’t have a back coating generally fail to remove SSS symptoms.
Tapes that don’t have a carbon black back coating rarely develop SSS.

I should immediately point out that there are many practitioners who would respectfully disagree with the last three points above. My own take on the two explanations is that binder hydrolysis is the more likely cause of SSS, and restoration professionals’ experiences and writings tend to support this. While the forensic analysis cited by Richardson documents carbon black back coating deterioration, the conclusion that it is solely responsible for SSS or triggers binder deterioration is likely an overgeneralization. And as shall be seen in the next section, the only remediation methods available to me target deterioration resulting from binder deterioration.

Remediation

The following have been offered as methods for addressing the symptoms of back coating deterioration.

Remove the back coating. Richardson created a device for performing this using isopropyl alcohol to dissolve the back coating (Figure 8). Although he applied for a patent of the device, it apparently never went into production, and no one to my knowledge has seriously pursued the process or its supporting findings. (Presumably, one would also need to remove the sticky material for this to be effective, but this was not addressed in the patent application.)
Remove the deteriorated carbon black back coating from both sides of the tape. (I have not discovered methods for performing this; tape cleaning solutions have escaped my Internet searches. Repeated wipes with link-free might cloth work, though I have no feeling for how successful this would be or what additional damage might result.)
Lubricate the tape with a fast-evaporating product designed specifically for this purpose. Note that applying lubricant can also address squealing irrespective of whether the cause is binder hydrolysis SSS or Loss of Lubricant, described below. Use only on polyester tapes, not acetate. (I am leery of this approach, fearing that tape lubricants can accumulate on tape deck components and interfere with contact between the tape and the heads; and that they can also capture loose binder particles and other dirt, transferring those contaminants to the tape deck.)
Incubate the tape only if none of the above is successful.

II-8 Back coating removal machine schematic.jpg

Figure 8. Richardson’s schematic of a machine for removing back coating.

Since Richardson’s device was never produced, tape cleaning solutions may not exist, and applying lubricant strikes me as counterintuitive, I’m left with dehydration as the sole method for addressing binder and back coating deterioration. For all practical purposes, this leads me to adopt the binder hydrolysis explanation of SSS.

Loss of Lubricant

I will mention in passing that some writers attribute squealing during playback to “Loss of Lubricant” (LoL) in the back coating. This attribution is suspect, however. One restoration professional has noted that there isn’t any supportive research for this claim, particularly any chemical analyses, and has recommended that “LoL” be stricken from discussion of tape deterioration. Moreover, the fact that squealing is not prevalent among tapes manufactured without a back coating and the additional lubrication it provides suggests that conditions other than LoL are likely causing the squealing.

Conclusion

I admit that this pair of articles on magnetic tape deterioration is loaded with technical terms, descriptions, explanations, and comments. Taking a step back, I believe the following are the important take-aways.

Magnetic audio media don’t last indefinitely.
Deterioration can often be treated, at least to allow for duplicating or digitizing.
Remediation procedures aren’t foolproof, and there are no guarantees that they’ll be successful.

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