Question about DECtape formulation

[Paul Koning]

> On Jan 23, 2022, at 8:40 PM, Noel Chiappa via cctalk<cctalk at classiccmp.org>  wrote:
> 
>> From: Gary Oliver
>> I've always thought the physical tape wound on a DECtape spool was a
>> fairly conventional 'sandwich' of mylar/oxide/mylar ...
>> Was there some kind of 'lubricating' coat on the data side? It makes
>> sense, but none of my DEC documents or Googling has any mention of
>> lubrication ...
>> If someone has some detail information on the tape construction, I'd am
>> curious to see it.
> Dunno if you know of this:
> 
>  http://www.bitsavers.org/pdf/dec/dectape/3M_DECtape_Spec_Nov66.pdf
> 
> but it doesn't mention any lubrication, just a "Protective Overlay" layer,
> over the "Coating" (which I assume is the oxide). I'm a bit surprised that
> "some of the data side of the tape came off on the wipe", though, unless the
> "various concentrations of isopropanol/water" dissolved the Protective
> Overlay.


Looking at that spec some items pop out.

The coating is quite thin, much thinner than the backing.  Not too surprising actually, if it were thick it would reduce the max possible bit density.

There is a coating wear spec (as a ratio of wear resistance relative to an ordinary tape) but no other specs on the coating, such as solvent resistance.

There also is no description of what the coating is.  I also used to think of DECtape "sandwich" tape as mylar/oxide/mylar, but the documentation doesn't say that.  And at a coating thickness of 0.04 mils, it clearly isn't another mylar ribbon layer.  It sounds more like some sort of sprayed-on coating of some sort of abrasion-resistent material.  It might well be porous, which would be a possible explanation for the oxide coming off when rubbed with solvent.

	paul