How much chlorine does it take to bleach a liner?

[TreeFiter]

I have been dealing with a fair amount of difficult pools this season, many of which require significant amounts of chlorine to reach shock levels due to high Cyanuric Acid levels. Just about every time I straighten out one of these pools, my boss questions the high FC levels out of fear that we might bleach a liner. So I have been wondering if anyone has seen any data showing just how much chlorine it would take to bleach a liner.

I put together a little mini experiment to get a rough sense of how risky it might be. I took a scrap piece of liner, and I cut it into small strips. Then I took four 1 liter bottles and filled two of them with water containing approximately 100ppm FC, and added a strip of liner to each. The other two were filled with water containing about 50ppm FC. Then I took one of each concentration and added a small amount of CYA to them. So I have 50 and 100 ppm FC with and without CYA. I put the bottles in a bucket on my back porch and covered it so the sun wouldn't affect the FC.

After a week, I opened the bottle with 100ppm FC and no CYA, the worst case scenario, and verified that the FC was still at 100ppm, and it was. I didn't see any noticeable fading of the liner. I didn't take the liner out of the bottle because it would be a bit of a pain to do, so it is possible that there might be a small amount of fading, but I intend to compare it side by side in a few days, which would be two weeks. I think that if I don't see any fading after two weeks at 100ppm FC and no CYA, the risk of bleaching a pool during a Shock or SLAM is non existent up to 100ppm FC, which is way higher than the necessary FC levels in the vast majority of situations.

Anyway, if anyone has seen real data from a reputable source, I'd love to see it. I can't help but think someone must have tested this out at some point before deciding to use Vinyl for swimming pool liners.

 

[woodyp]

I have no idea, but you just caused me to wonder what the "FC" level would be if you dunked a liner strip in a bottle of pure, undiluted bottle of 8.25% bleach. Oh chemgeek!!!!!!!

 

[chem geek]

See this link that notes the following:

Liner colors, other than medium shade blue, that have almost total resistance to chlorine bleaching and color fading are: white, turquoise, light blue, grey and dark royal blue. The print patters pool liners have excellent resistance to bleaching in most cases. The exceptions are medium blue prints, which can only be obtained with the inclusion of a bleach-able blue pigment.

Your tests were probably with colors other than medium blue so the inorganic pigments wouldn't be affected by chlorine. The vinyl itself (not just the colored pigments) is mostly affected by low pH, not by chlorine.

There is someone who was going to do a similar experiment but either never did it or they didn't report their results. See Liner fading from FC?.

Note that if you used 8.25% bleach for your 100 ppm FC experiment that the rise in pH from it (to around 8.5) would limit the active chlorine level to being roughly equivalent to having 17 ppm FC with no CYA at pH 7.5. If you were to use undiluted 8.25% bleach, it would actually result in a lower active chlorine level due to the even higher pH of 11.8 and would be equivalent to around 5 ppm FC. The peak active chlorine level comes from a 1:40 dilution where the pH is 9.55 and the active chlorine level is similar to 32 ppm FC.

You should also try doing similar experiments with just low pH to see how damaging that can be to liners making them brittle by destruction of the plasticizers that make vinyl softer and more flexible.

 

[TreeFiter]

If I were to adjust the pH down to 7.2, shouldn't I get higher active chlorine? I didn't even bother to check the pH in my experiment, but I can always test it later. I didn't use bleach, mainly because I typically don't have it on hand. I used Cal-Hypo, which left the water a bit cloudy.

I probably will play around with extreme pH conditions as well. This morning my boss made a comment about a liner that had a wrinkly bottom. He said it was the result of too much chlorine. I'm a bit doubtful based on what I'm seeing so far, but low pH could be the cause. It will be interesting to see what happens. I've also wondered what the effects of high pH are. Beyond the creation of scale, I haven't been able to find any real problems.

 

[chem geek]

Cal-Hypo will raise the pH as well, usually more than bleach but about the same as chlorinating liquid. You'd have to add acid if you wanted to lower the pH, but would need to be careful not to add too much or you'll produce chlorine gas. You'd add about 1/17th of a teaspoon (about half of 1/8th of a teaspoon) of full-strength Muriatic Acid per gallon to get the pH down to around 7.5. If you added 1/8th of a teaspoon per gallon the pH would get to around 6.7 so not a disaster. If you added 1/4th teaspoon or more per gallon, you'd exhaust the carbonate buffer and crash in pH.

High pH should not affect a vinyl liner except to be more likely to scale if CH and TA are present in sufficient quantities.

For the wrinkly bottom, if granular Trichlor were added (as is found in some "shock" products), then it is low in pH and slow to dissolve so would be bad sitting at the bottom of the pool.

 

[TreeFiter]

For the wrinkly bottom, if granular Trichlor were added (as is found in some "shock" products), then it is low in pH and slow to dissolve so would be bad sitting at the bottom of the pool.

Would it be the chlorine or the low pH that actually results in the wrinkling, or the combination of the two perhaps?

I also keep wondering if the bleaching I see in pools on a regular basis is a result of chlorine and sunlight combined. Some liners show fading below the water line in as little as a season. I keep thinking there should be some fading after a week at 100 ppm FC, but I haven't seen any yet. I can't help but think maybe sunlight speeds up the bleaching significantly.

 

[chem geek]

It's mostly the low pH that causes wrinkling as noted in this post and this post and it is vinyl liners that use more calcium carbonate fillers that are more sensitive to low pH conditions where they lose their flexibility (i.e. become more brittle) and can take up water and wrinkle. This is why low pH will affect some liners faster than others.

As for fading, I already explained that some colors won't fade at all, period, not in sunlight, not with high chlorine. The reason is that the colors come from inorganic pigments and they last a very long time -- think colored rock or other minerals. It is organic dyes that are sensitive to oxidation from chlorine AND from breakdown from the UV in sunlight. It is also true that when chlorine breaks down from the UV in sunlight that it produces hydroxyl radicals that are powerful though short-lived oxidizers so that is also a factor. So medium blue vinyl that is colored using organic dye will fade the fastest and some cheaper vinyl has less UV protection so would tend to fade the fastest. Such fading has different causes in different areas:

Above the water -- fading is largely from the UV in sunlight
Below the water on sunny side of pool -- fading is from chlorine and from the UV in sunlight including from hydroxyl radicals when chlorine breaks down
Below the water on darker side of pool -- fading is from chlorine

 

[TreeFiter]

I decided to take the sample out and compare it to a piece of liner that hasn't been exposed to chlorine. Very little difference. I think I can see very slight fading, but certainly not enough that I'd be afraid to raise FC to 100ppm as part of a SLAM. After 13 days at 100ppm, I expected something more dramatic.

20150817_220022.jpg - Google Drive

20150817_220152.jpg - Google Drive

 

[chem geek]

You could repeat your experiment putting the container out in sunlight. Of course, the chlorine will degrade quickly, but it will produce hydroxyl radicals in doing so and therefore would test the combination of UV and chlorine exposure, though vastly accelerated. The chlorine will be nearly all gone in less than one day of sunlight exposure -- probably only 1 ppm FC left after one full day assuming 50% loss per hour in equivalent direct noontime sun so figuring 8 such hours though it's actually over 14 hours but with sun at an angle much of the time.