Question about DECtape formulation

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,