UHMWPE seems like it might be an interesting material for clothes, and indeed, there is a startup (“SheerlyGenius”? I forget) making sheer UHMWPE pantyhose — the idea is that they will be considerably sturdier than nylon pantyhose, which is somewhat surprising to me.
UHMWPE fiber has 2.4 GPa of tensile strength, apparently. You could imagine a cloth sort of like rip-stop nylon made from it, with somewhat thicker fibers every millimeter and very thick fibers every centimeter. Say, 3× and 10× thicker than the regular fibers.
I don’t have a super great reference handy for the tensile strength of cellulose, but Heckballs: a laser-cuttable MDF set of building blocks says MDF’s UTS is 18 MPa, which is probably low but in the ballpark. ASTM A36 steel has a yield stress of 290 MPa, HIPS has 32 MPa UTS, and annealed aluminum’s yield stress is around 15–20 MPa.
So let’s say cotton’s tensile strength is 40 MPa. I have some sturdy cotton serge shorts here whose cloth is about 700 μm thick. Achieving similar strength with UHMWPE would require 40/2400 of the average thickness, or 12 μm; you could have 160 or so 6-micron-thick threads per millimeter, with an 18-micron thread each millimeter, and then a 60-micron thread each centimeter. This gives us an average thickness of roughly (60·60 + 18·10·18 + (10000-(60+18·10))·12)/10000 = 12.396 microns. Actually the extra 0.396 microns from the thicker and very thick threads should probably be doubled, making it 12.8 microns.
However, each 60-micron thread would break under a force of some 7 newtons, the weight of 700 grams. If you really want rip-stop strength, you likely need another approach. Something like knitting, for example, so that the force at the tip of a rip is distributed over more cloth, but ideally with knots frequently enough to prevent runs from spreading. Alternatively, you could scale the 60-micron threads up to, say, 300 microns — still half the thickness of the cotton, but now with a breaking force of 170 newtons, which would make the cloth unlikely to tear by accident.
Let’s suppose that we knit some shorts from 20-micron-thick UHMWPE threads, then, and that the knitted cloth is about 60 microns in thickness and about 60 microns per row of knits or purls. This might require 1100 mm × 550 mm of cloth, totaling 36 cubic centimeters and about 25 grams. The 550 mm of length is a bit over 9000 rows of knitting. The tensile strength of a leg of the shorts as a whole would be something like 2.4 GPa · 20 microns · 550 mm · 0.5, which works out to 13 kN, a bit over a tonne. You could still cut the cloth with scissors, but ripping it might not be feasible.
To make this garment opaque, you’d probably need to mix in a substantial amount of some very opaque pigment, such as finely divided titanium dioxide, carbon black, or gold. If this is within the body of the fibers, which is advisable for durability, it will weaken them, requiring a compensating increase in thickness.
Individual Dyneema fibers are supposedly 12–20 μm in diameter, as currently manufactured.