Saturday, April 29, 2017

VELCRO coins

I bought a 15 pack of 5/8-inch VELCRO coins priced at $1.97 for a total of $2.14 with tax. The alternative to the VELCRO was 3M Command hanging strips, which are more expensive.

I'm using the VELCRO coins to adhere hooks to my window frame as anchor points for a curtain line. The hooks I'm using didn't come with the VELCRO coins. They're from a pack of peel-and-stick plastic hanging hooks I bought for $1.40 at a 99¢ store for my 1st attempt to mount the curtain line on the window frame. The combined cost of the VELCRO coins and the 99¢ store hooks ($2.14 + $1.40) is about the same as the price for a pack of 3M Command hooks and hanging strips. Oh well.

The 99¢ store hooks failed to hold the curtain line. Their adhesion seemed firm enough when I stuck them onto my window frame and waited more than a day to load them, but the hooks fell off the window frame and dropped the curtain after less than a day.

For the left and right-end anchor points, I removed the used stick-on pads from 2 of the failed hooks; I stuck 4 VELCRO coins in a diamond pattern onto the left-hand hook; I stuck 5 VELCRO coins in a square box+1 pattern on the right-hand hook. The curtain line is 1.5-millimeter or 1/16-inch kernmantle accessory cord, probably 100-pound strength. The left end of the curtain line is tied to its anchor point with a Siberian hitch. The right end is tied to its anchor point with a taut-line hitch, which means the curtain line is tensioned from the right side. The kernmantle accessory cord stretches under load, so I stuck 2 hooks between the anchor points to help distribute the load, guide the pull more across the line rather than down, and reduce the curtain sag to an acceptable amount. Unlike the load-bearing anchor hooks, the interior support hooks should be able to stay stuck on the window frame with just their original stick-on pads.

The product description lists a shear strength of 14.0 (I assume pounds) per square inch (avg.) and a peel strength of 1.2 per square inch (avg.). However, VELCRO's "Hook 88/Loop 1000, with 72 Adhesive System" technical data sheet lists a shear strength of 12.0 PSI.

I'm most interested in the shear strength, rather than the peel or tension strength, because almost all the load on the hooks should pull across the tensioned curtain line. I figure the shear strength should be about 21.875 PSI ((2*5/8)2 * 14.0) or 18.75 PSI ((2*5/8)2 * 12.0) for each hook. I believe there is also a peeling load from the weight of the curtain pulling down on the curtain line and the curtain line is a 1/4-inch off the VELCRO coins due to the structure of the hooks.

When the curtain line is tensioned with the taut-line hitch on the right side, the left-hand hook slides right 2/16 inch and the right-hand hook slides left 3/16 inch along the hook and pile of the VELCRO coins. I offset the right-hand hook 3/16 inch to the right to compensate for the 3/16-inch slide left when the taut-line hitch is tensioned. The result of offsetting the hook and pile coins seems to be a better aligned, tighter fit between the pile coins on the right-hand hook and hook coins on the window frame after the right-hand hook slides. I didn't offset the left-hand hook 2/16 inch to the left to compensate for the slide right. The next time I loosen the curtain line, I'll offset the left-hand and right-hand hooks before I re-tension the curtain line. I don't believe the adhesive is creeping.

The application instructions on the package say the VELCRO coins are ready for use 15 minutes after they're peeled and stuck on. However, product websites recommend, "for 0172 adhesive, allow the full curing time of 24 hours before loading or cycling" and the pressure-sensitive pre-coat "achieves full bond strength in 24 hours". I loaded and cycled the square box on the right-hand hook at about 23 hours after peeling and sticking on the VELCRO coins. I only waited about 45 minutes to load and cycle the +1 5th coin which I added as an adjustment to the right-hand hook after initially loading and cycling the square box. I wonder whether I lost bond strength, especially in the +1 coin, because I didn't wait for the 0172 adhesive to cure for 24 hours before loading and cycling the VELCRO coins on the right-hand hook. I'm waiting a full 24 hours before I load and cycle the diamond on the left-hand hook.

The loops - or pile - easily shed from the VELCRO coins. I hope that doesn't indicate a defective product with a weaker than expected shear, peel, and tension strength or cycle life. The cashier said I can return the purchase within 90 days if I'm not satisfied even if the product is opened and used, which I'm not sure about because her assurance isn't the same as the store's printed return policy. If the VELCRO coins are holding up the curtain fine at the end of the return period, I'll call it good.

The origin of the VELCRO brand name is "[Inventor George d]e Mestral decided to name his revolutionary invention an amalgam of the French words “Velour” (velvet) and “Crochet” (hook) hence the name of the brand."

13MAY17 update: Failure. The first observed sign of failure was the right middle support hook popping off the window frame at night. The next observed sign of failure was infrequent mysterious single or double-staccato ripping sounds. The following day, with the ripping sounds becoming more frequent, I examined the curtain line. The left middle support hook was hanging loose with its adhesive back partially torn from the top. The left anchor hook was crooked on a left-up, right-down diagonal with the left side of the hook raised off the coins. The left anchor hook was barely connected to the right half of the top middle coin of the diamond and maybe still the right lower border of the diamond. I expected to find the hook and pile had disconnected, but was more disappointed to find the left anchor hook had torn loose from the adhesive. The coins were still connected firmly to each other and the window frame. Meanwhile, the right anchor hook with the square+1 coins appears to be working fine. I thought there would be greater strain on the right anchor hook since I tensioned the curtain line from the right side, but it was the left anchor hook that failed. About 1/5-1/4 of the later-added +1 coin protrudes from the left edge of the right anchor hook; I wonder if that has helped level the hook against the tension while perhaps the left anchor hook tipped over on its right edge and thus placed greater strain on the left side of the hook. The coin of the right tip of the diamond on the left anchor hook was placed on the edge of the hook, but it did not protrude from the hook. Another difference is on the right anchor hook, I placed the pile coins on the window frame and the hook coins on the right anchor hook. On the left anchor hook, I placed the pile coins on the left anchor hook and the hook coins on the window frame. I'm disappointed. Back to the drawing board.



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