Handbook for Sound Engineers

Magnetic Recording and Playback 1073

Second, avoiding elevated temperatures and stray

magnetic fields during use and storage will decrease the

excitation of the thermal idiots. Third, exercising the

tape by shuttling the tape from reel to reel several times

will partially erase the printed particles. The flexing and

rubbing of the tape produce enough activation energy to

neutralize some of the printing. For this reason, never

copy a stored master tape without exercise. The worst

possible print through level exists on the very first pass

of the tape. In some cases print through can drop as

much as 4–6 dB with five shuttle cycles.

28.3.12.5 Sticky Shed and Tape Baking

As mentioned earlier, the goal is to attach a maximum

number of perfectly stacked and oriented magnetic

particles onto the surface of the backing material.

Anything that interferes with this goal by displacing

some of the magnetic particles, such as additives for

lubrication, fungicides, and static charge reduction,

degrade the performance of the tape. Most important in

this category is the very binder that holds the particles

in place. Every bit of binder displaces some of the

useful magnetic particles.

The best choice is to use a very strong binder that

can do the job with the minimum amount of glue,

allowing space for more magnetic particles. The

winner is the highly crosslinked thermoset polymers in

use today. Starting around 1970, these binders with a

high urethane content gave a big boost to tape perfor-

mance. Unfortunately, long term experience with these

tapes now shows that the binder can break down

during storage. The symptoms are a buildup of residue

on the head and guides and a tendency for the tape to

stick to these residue buildups, in some cases actually

dragging the tape to a stop. The popular name for this

problem is sticky shed. The problem is usually

discussed in terms of binder breakdown, but there

appears to be a second major problem related to lubri-

cant oozing that is also present.

Urethane Binder Breakdown. The urethane binder
contains long polymer chains that provide the high
strength of the binder. Water in the surrounding air
enters the tape and breaks the long chains through a
process known as hydrolysis.
As a result of the chemical breakdown of the long
polymer chains, the binder is weakened enough for the
surface of the tape to begin to rub off onto the stationary
guides and heads. Depending on the design of the trans-
port, this residue can clog the heads in just a few
seconds. Machines with rotating guides and low tape

tensions take longer to clog, but the damage to the tape

is still intolerable.

Fortunately, the hydrolysis is somewhat reversible.

Tapes can be baked at a moderate temperature to reverse

the hydrolysis and restore strength to the binder.

Although this may seem like a bit of witchcraft, thou-

sands of baked rolls of archived tapes have proven the

technique.

The electric oven must provide a well-controlled

temperature of about 120–140°F (50–60°C). Large

dehydrators or fruit dryers are popular because of their

size and limited temperature range. Only an electric

oven should be used. The oven should be preheated and

checked for temperature stability with a high-resolution

thermometer such as a candy thermometer. The tape,

wound onto a metal reel, is placed into the oven hori-

zontally with generous space above, below, and around

the reel for air circulation. The tape is baked for 15–20

hours, and then allowed to cool to room temperature

undisturbed in the oven.

The baking process creates a low-humidity environ-

ment that draws some of the excess water from the tape

binder. The short polymer chains may recombine with

their neighbors to produce a better bond, but the break-

down process is not fully reversed.

Lubricant Oozing. The second failure mechanism also

involves the binder, but in this case the culprit is the

oxide. A change of oxide particles also changes the

chemistry needed to make a liquid binder that can:

1. Hold all the magnetic particles in suspension.


2. Be smoothly coated onto a polyester backing
   material.


3. Have the volatile byproducts evaporated in the
   drying ovens.

In the early 1970s Phizer introduced a new

high-output oxide with excellent signal characteristics,

but the particle required a reformulated binder with a

low pH in order to meet the above requirements for a

usable dispersion. This particle was utilized by 3M in

the 226 family of tapes (226, 227, 806, 807, 808, 809)

and by Ampex in the 456 family.

The new binder formula included a component that

served primarily as a lubricant. Unfortunately, however,

this lubricant would migrate to the surface of the tape

and concentrate into a sticky residue.

The baking operation described in the previous

section warms the concentrated lubricant enough to

allow the lubricant to flow and be reabsorbed into the

depths of the coating.