What chemicals react with greenhouse plastic and cause it to turn white?

Question

I have a small PVC hoop house that I built last winter and plan on using again this winter. I found out first hand, then read about it later on, that the plastic used for greenhouses will react with PVC and turn white. Since I've already invested in the PVC, and it's a much cheaper alternative to metal pipe, I'm trying to come up with a way to isolate the plastic from touching the PVC.

The options I've come up with so far are to use some type of tape (felt, electrical, clear duct tape etc.) and apply it to the outside of the hoops where they come in contact with the plastic. My purlins are attached on the inside of the hoops so I don't need to worry about them. The felt shouldn't react since I've seen this sold on greenhouse supply websites for just this reason, but what about the tape? Are there any other low-cost options I'm missing?

The design is changing a bit the second time around, but this is what it looks like from the inside. As you can see very little PVC actually comes into contact with the plastic covering. The plastic I used was from Green-Tek which I think is really Dura-Film Super 4 from AT Films and it's what I plan on using again this year.

Answer 1

The UV stability of PVC aside I chose to use a roll of clear weather sealing tape from Frost King. This is used to seal windows which means it's not going to react with the greenhouse plastic I'm using. I've applied it to the top of all the hoops and will see how it fairs over the winter compared to last year without it. This is the roll I picked up

Update on 8/29/14: It's now been almost two years since deciding to use this product. My PVC structure has remained setup in the garden the entire time with this tape on it. I've had no issues with it coming undone, cracking, or turning my plastic white (aside from the general wear marks from wind which are much less than that of the chemical reaction). The structure has been up for a total of three years and the stability of the PVC looks to be no different than the day it was setup.

Answer 2

I know this is an old post but I want to thank you Brian for bringing this very important and potentially costly topic to our attention.

I am going through this same experience setting up 2 low tunnels for the first time as nighttime freezing temps creep closer. This pitfall was brought to my attention when purchasing my poly film through the Johnny’s Seeds website warning me of this chemical reaction b/t PVC & poly film.

Here is an article posted by the UD Cooperative Extensión on the topic: https://sites.udel.edu/weeklycropupdate/?p=4816

Their recommendation is to paint the PVC with latex-based paint. I struggled to find a clearly stated adhesive tape that would provide the same barrier and avoid all the setup, drying time , potentially missed spots, etc. in having to paint the PVC.

So your personal experience & testimony to the durability of the Frost King WeatherSeal tape after several years in place is huge boon for cold weather gardeners on a budget. Thank you again for providing us with this invaluable information.

I’m wondering if DA’s UV-resistant outdoor PVC pipe recommendation would offer the same “off the shelf” protection w/o needing pre-treat conventional PVC.

I also purchased clamps from Johnny’s Seeds to secure the poly film to PVC. The clamps are made of Acrylonitrile Butadiene Styrene (ABS). So far I can’t find any information whether this material will negatively react with the poly film. I have asked the Johnny’s Seeds customer care folks and they are helping me research it as well.

I will repost if I can find an answer. Either way I’m covering the clamps as well just to be safe!