Question about DECtape formulation

[Gary Oliver via cctalk]

I've always thought the physical tape wound on a DECtape spool was a 
fairly conventional 'sandwich' of mylar/oxide/mylar, but a recent 'test' 
makes me think there is something else involved.

I have a number of tapes I'm cleaning (removing dust, etc.)  to make 
ready to read on a restored (apparently) Astrotype dual DECtape drive 
and I was 'dressing' the leaders of the tape (removing ragged bits from 
old use.)  After trimming a wee bit from several tapes (.5 to 1 inch) I 
did a test.  Taking the bits of tape, I exposed them to various 
concentrations of isopropanol/water (from about 25% to 99% iso) and 
found than in all cases, some of the data side of the tape came off on 
the wipe.  The remaining tape fragment appears intact - the brown oxide 
was still there but both sides were now the same color, rather than the 
data side being darker (as were all my tapes before the test.)

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, other than the "...hydro- dynamic lubrication, relying on 
the viscosity of air to entrain it with the tape and provide the 
flotation medium." found in an "ELECTROMECHANICAL COMPONENTS & SYSTEMS 
DESIGN" from November,  1964.

All of my tapes, including DECtape brand, Scotch brand and even a couple 
of old "Microtape" brand from DEC (before 'dectape' name change) have 
this feature, so this doesn't appear to be something that appeared 
recently (as in late in DECtape production or due to old-age in the tapes.)

If someone has some detail information on the tape construction, I'd am 
curious to see it.

Thanks,