by CRAIG STARK
TAPE BACKINGS
New, improved oxides and smoother, more uniform coatings seem to get the major share of tape publicity today, but a reader inquiry prompts me to consider what these marvelous materials are, literally, based upon.
For twenty years acetate tape film has held its own with the slightly newer polyester backings, but recently the latter have taken over almost completely. My correspondent has heard, however, that polyester stretches more than acetate, so that a taped voice and music commentary will gradually fall out of synchronization with his home movie and slide shows.
This concern is given apparent credibility by the fact that you can often snap an older cellulose acetate tape apart cleanly, put the broken ends together in a splicing block, apply a splice, and never hear the break. Try the same trick with polyester-backed tape, and you end up with an elongated length of useless tape that must be cut out at both ends, resulting in a permanent loss of the material in the stretched section. This is progress? Actually, it is, for my reader combined several partial truths to reach the false conclusion that acetate was the better tape base. First, it is true that, being plastic materials, acetate and polyester will stretch when sufficient force is ap plied to them. Up to a point, however about 5 percent elongation-they will return to their original length when the force is removed, and the important fact to note is that in this vital area polyester is 15 to 20 percent stronger than acetate. Moreover, the tape tensions on a properly adjusted recorder don't come close to causing a 5 percent elongation except, perhaps, on the very thinnest tapes (for example, a C-120 cassette or "triple-play" open-reel tape), where pre-stretched ("tensilized") polyester base is always used.
Second, and more important in keeping a tape at a constant length, is the fact that acetate is about eight times as susceptible to the humidity in the air as polyester is. A roll of polyester tape that might change length (and thus running time) by two seconds with a 60 percent change in humidity would change about seventeen seconds if acetate were used.
And that's not all. Suppose you make a recording in the summer when the relative humidity is a dripping 95 percent.
At this point the cellulose has soaked up moisture from the air and expanded about as much as it can. As you record, the tape is taken up at normal tension.
Then, however, you put it away and get it out on a winter's evening, when the humidity may have dropped to around 15 percent. During the storage interval the tape has lost its summer moisture and contracted, creating an enormous internal pressure--perhaps enough to exceed the elastic limit of the inner layers, thus permanently deforming them.
Conversely, if you record in the dry winter and then try to fast-forward the tape in the summer, it may have become so loose that it will "cinch," that is, form a series of accordion-like pleats in the middle of the reel. Polyester is not totally immune to humidity changes, but in addition to its greater tensile strength it's eight times more resistant to humidity effects than acetate.
Third, acetate tapes contain a plasticizing agent which, with age, tends to dry out. This makes them stiff and brittle, so they may break (cleanly, to be sure), when subjected to much less than the force needed to make a fresh tape reach the critical 5 percent elongation point. Polyester needs no plasticizer, and so resists the aging process better.
Which is probably one reason for the "clean break" reputation of acetate. Fresh acetate will stretch about 25 percent, polyester about 100 percent before breaking--both well beyond the elastic recovery limit. The other is that the slightest edge damage (such as a nick) will cause acetate to break very easily, but affects polyester far less. All in all, then, the shift to polyester is one more layer added to tape progress.
Also see:
THE SIMELS REPORT, STEVE SIMELS
NEW PRODUCTS: A roundup of the latest in high-fidelity equipment
Source: Stereo Review (USA magazine)