The normal spill I give for how fiberglass is finished goes like this. "When cloth is woven in the US the yarn has an oily starch like material known as Poly Vinyl Alcohol (PVA) that helps it slide and prevent breakage during the weaving. After the cloth is woven the starch oily stuff is burnt off and the silane or Volan is added which helps create a bond to the glass". For Silanes I always say something like "the silicone dioxide in the glass bonded with the methoxy groups on the the silane and one variable organic group was left available to bond with whatever resin was being used. Volan was done the same way except for Volan forming a chrome complex with the glass". This was a huge oversimplification of the process but at least gave some degree of an understanding of fiberglass finishing.
Why Fiberglass Cloth is Finished:
Fiberglass Cloth in the Greige is in the loom state or what it is just after weaving. At this stage it still have the PVA oily film on it. Cloth like this has a slippery feel to it and you can actually smell the distinct odor it has on the cloth. Silicone rubber coaters, that we have sold to anyway, have to have it like this. It may be that for the rubber/glass matrix to be flexible it needs this barrier to prevent the Silicone to Silicone bonding that would occur if the cloth was heat cleaned or post finished even further with Silanes or Volan. We sent roll samples to one company, by their request, having various finishes and none would work but the cloth in the greige.
Fiberglass cloth in the greige with the oily PVA on it will not work with resins. It will not even wet out. You can't even get the white out. I know because I tried just for curiosity sake back in the 80's when I was trying to come up with my own backyard finishing process. I got samples of silanes, baked the cloth in the oven, washed the cloth, made a makeshift bath with the silane - what a bunch of work - good experience and made me really appreciate the sophistication of the finishing process done by the weavers. This is much more involved than one would assume. There are some finish processes where some of the pva is left on and silanes added - all for specific end results - one of which being for ballistics where the end result is a glass/resin matrix that doesn't have a good bond that allow the glass to slide under impact like bullets or shrapnel. A process similar to this is used for melamine laminates. There is also cloth that comes from overseas that was woven with the yarn already in the finished state before weaving. This cloth is inferior because of the breakage of the yarn with the resin compatible finish on the yarn sliding less than the oily PVA. U.S. weavers may have tried this also. I'm not totally sure. But for the most part all US made fiberglass cloth has the pva on the yarn during weaving to prevent breakage then goes through very expensive processes to clean this slippery film off by heating and washing then pulling the cloth through a bath to add the Silane or Volan.
I have seen some Silanes that are normally soft be stiff on the same style of fiberglass fabric. When I was at one of the weaver's plants I asked and was told that if their technicians don't determine enough finish was added they run it back through a second time. What I think happens, and I'm sure there are many people out there reading this that know for sure, is that there is some molecular cross linking between the glass strands.
One group of Fiberglass Cloth Styles that seems to be effected more than any other are the satin weaves, particulary 4 harness satin weaves and still more specific are the 3 ounce crowfoot weaves. Style 120, for example, is greatly effected by the finish it has. 120 can range all the way from soft with a good Volan finish like the 504 to really stiff with something like the Z6040 finish or variants thereof. Some silane finishes on 120 that can be pretty soft are BGF's 497A and sometimes the 538 Amino silane "soft"version. 550 is a Volan/Silane hybrid that's pretty soft. To add to the confusion there is a 4 harness satin weave in the same weight range, thread count but with a thicker filament, known as style 220 by some weavers and 2220 by others. Having the thicker filament makes the cloth a little stiffer and the finish just adds it's effect also.
One way I'm using to indicate the softness or drape of any specific style with varying finishes is shaping the cloth over a baseball.