Found this interesting. Looked over at Challenge Sailcloth’s site at the specs for their various Ultra fabrics. Ultra 400 is listed at a weight of 4.65 oz/yd2 and with a breaking strength of 114.5 lbs in the warp and 117.6 lbs in the fill. Meanwhile, “4 oz” S grade fiberglass, which actually weighs 3.7 oz/yd2 has a breaking strength of 125 lbs in the warp and 125 lbs in the fill. I found it interesting and a bit surprising that it weighs almost an ounce per square yard less and yet has higher tensile strength (probably because of the polyester content in Ultra fabrics).
The downside of fiberglass is that it has less flexural durability/longevity than nylon, polyester, etc, and it would need to be encapsulated in some kind of stabilizing matrix/adhesive. As to the first part, I think this is less important in packs than in clothes, tarps, tents, etc, especially if you design the pack to be more stiff in the first place (which imo carries weight better)*. As to the second part, it is easy enough to apply a silicone coating. The other main issue is that it wouldn’t be sufficiently waterproof on its own, even with a silicone coating, but it could easily be silicone bonded and sewed to a silpoly fabric and just have the silpoly on the inside–the combo of fabrics plus extra silicone would provide a very high HH.
While it is not a perfect/ideal material because of the above issues, it has other highly attractive qualities besides the very high tensile strength. It is not affected by UV whatsoever. The silicone matrix is significantly more UV resistant than nylon and even more so than polyester, and so a pack fabric made out of a combo of fiberglass cloth and silicone would last a very long time even with a lot of intense sun exposure.
It also has practically no moisture regain (like the UHMWPE in Ultra) but is far more easily and strongly bonded to adhesives. It is inherently more stiff of a fabric (especially the S grade form of fiberglass), which again, imo is actually better for a pack. It is extremely heat and spark resistant. You could have your pack near a fire and get sparks on it with no issue whatsoever. In fact, siliconized fiberglass cloth is made/sold for welding protection, stove pipe exit cloth material, etc.
* If you had some kind of lid/closure flap (which would be much better than a roll top for a fiberglass cloth bag), you would want that fabric to be a different one than the fiberglass cloth, because that part of the bag will see a lot of folding flex. Obviously the shoulder straps and hip belt would also need to be a different fabric and probably need to be reinforced with a more regular fabric at the seams.
