Re: PEX
RJJ said:
Jasper: Interesting! is this a guess? where the info come from?
Yes, most of my conclusions are guesses, based on:
* The information in Lubrizol's letter
* Other websites I looked at when I wanted to improve wikipedia's article on PEX. I put some notes at:
http://en.wikipedia.org/wiki/Talk:Cross-linked_polyethylene#Comparison_of_types
* Basic "chemical kinetics". "Chemical kinetics" is the theory of how fast a chemical reaction uses up its reactant(s) to produce its product(s).
Each chemical reaction has a different rate. Most chemical reactions have the following properties:
* The rate depends on the chemicals involved, and what intermediate products are created along the way.
* The rate increases (roughly exponentially) with temperature. A 10 Celsius degree increase in temperature can double or triple the reaction rate.
* The rate increases (possibly non-linearly) with the concentration of each reactant.
* The rate depends on whether there is a catalyst.
* The rate depends on the solvent. In this case, the reaction happens in the tube material.
The degradation of PEX has the following reactants:
* Something that causes the damage, such as chlorine or ultra-violet light.
* The cross-linked polyethylene. (This has a fairly constant concentration, because it is most of the material.) Different kinds of PEX have different kinds of cross-links, so some kinds might be slower to be damaged.
* The anti-oxidants in the material. I assume the anti-oxidants are used up as they "try" to repair the damage. I assume that the tube will fail when it no longer has enough anti-oxidant in a spot to prevent a leak. Different kinds of PEX start out with very different amounts of anti-oxidants.
Basically, I am assuming that the amount of anti-oxidants used up is the sum of:
* The number of days exposure to sunlight * the amount of UV in the sunlight * a fudge factor, adjusted for the length of each day and the temperature of the PEX and whether the PEX contains any "sunblock". Many black plastic pipes use graphite as a "sunblock".
* The number of years exposure to chlorinated water * the concentration of the chlorine in the water * another fudge factor, adjusted for the temperature of the water and whether any linings prevent the chlorine from diffusing into the PEX.
I am also assuming that each PEX tube has a potential life, if it were never exposed to chlorine or UV. In effect, I assume that exposure to sunlight uses up some of the tube's life, and exposure to chlorinated water uses up more of the tube's life. Of course, you can only find out how long any particular tube will last when it fails. I hope that PEX tubes that are warranted against 180 days of exposure to sunlight have long potential lives (if they are kept out of sunlight).
A similar theory is used to estimate lung-cancer risk -- a smoker's risk of dying of lung cancer is roughly proportional to the number of cigarettes they have smoked, times the average time they kept the smoke inhaled. In the lung cancer analysis, body temperature is fairly constant, so it does not factor into the results. And yes, I knew a woman who never smoked who died of lung cancer.
