I'm surprised I couldn't find a thread dedicated to this, as popular as it is in outside full scale PBRs. Resistant to most chemicals, polyethylene tubing is sold by many packaging firms, and can often cost as little as 20$ for 1000' http://www.poly-bags.com/pc-64346-3062486-3-mil-clear-poly-tubing.aspx . Although I have seen a few good threads mentioning this readily adaptable material, I have not seen any real discussion of it in particular. The following information could help alot of people out on here: where is the cheapest place to buy this stuff? what is the light transmission on polyethylene, and how does it vary between suppliers, and notably, is there a place that sells optical quality polyethylene tubing? Do algae readily stick to this material? What is a good thickness to get, is 3 or 4 mil acceptable or is 6 mil the standard? Also, what thickness in inches or millimeters corresponds to a gauge of 6 mil?
I mention this because, even though spools of poly ethylene tubing are common on packing websites, they are clearly not geared towards this application, and there are large differences in transparency between suppliers. Also, this poly tubing is white, indicating it at least partially acts as a neutral light filter. Is it slightly opaque on the website I listed because it reflects a large majority of light, or because it absorbs it? I believe this is an important quality, as reflected light is better than absorbed light, especially in keeping temperatures down in possibly a desert setting.
Contribute what you can, all is appreciated.
I checked out the polytubing. Most of it is not UV stabilized and 3Mil (.003") is way too thin for this application. I found 48" poly tubing 6 mils thick, but it was not UV stabilized. You can imagine what would happen if you built a big PBR out of the packing grade tubing. The whole thing would start falling apart on you at about the same time, likely midway through its second season of use. I'm not saying that one of the packing mfg's couldn't do the job for you, but it is going to be a custom run for them.
Even if it was stabilized, the max warranty on most UV stabilized polyethylene will be 4 years... So that means the majority of your infrastructure will need to be replaced every 4 years? I thought that was the point of vertigro's greenhouse, to filter out UV light, saving the main infrastructure inside. Doesn't most of the UV light get absorbed by even thin layers of transparent materials? Like a thin layer of glass, or UV stabilized plastic?
I know I probably need 6 mils =/ Do you know if 6 mils will hold a foot of water head? Two feet?
Do you know, does vertigro use two polyethylene sheets just heat welded together? Most PE i've seen is slightly white opaque, and his reactors look alot more light transmissive than that.
Judging from the photographs, which is all I can do, I'd say that he is using 6mil polyethylene. He might be using some other material, I cannot say for sure. The rest of his system is PVC pipe and poly tubing, those items are easily recognizable.
I don't know how much head the polyethyene will hold. It is quite a bit stronger than you might think at first blush. I have seen some fairly big tanks made with it, but the builders used something like hogwire to support the sides of the plastic liner.
I am reliably informed that 6mil will very often last as long as six or seven years on a greenhouse. Mind you, the film on a greenhouse probably does not see the load that a PBR bag would see. The bags in a PBR are not the majority of the infrastructure involved. A lot of it is piping, steel, or wood in my case, and pumps and tanks. Kertz et al also have a big chunk of money tied up in greenhouses. Those are necessary out in west Texas because you have some pretty sharp temperature swings every day. I don't think that they will be necessary in my area.
If you were to say, use this polyethylene within a greenhouse who's roofing reflected/absorbed all the incident UV light, would the polyethylene last for decades instead? Is UV the only thing that is wearing on it or does fatigue wear it also?
I have heard that vertigro's reactors are made by simply heat welding two sheets of plastic together in that pattern. I have only heard of polyethylene sheets being heat welded like that, and only heard of polyethylene as possibly being mostly transparent and cheap.
I have heard HDPE can compete with aramid. Pretty strong, but that doesn't discuss fatigue deformation over time.
Also, do you know of anything that will bond polyethylene to things like glass, or acrylic/polycarbonate, or styrene? I will be researching this.
I believe time is the factor that is least considered. If half of your apparatus wears out once every two years, you might as well count it as a running cost of the machine, along with the costs in labor of replacing it =/
Thanks man
davtuner:If you were to say, use this polyethylene within a greenhouse who's roofing reflected/absorbed all the incident UV light, would the polyethylene last for decades instead? Is UV the only thing that is wearing on it or does fatigue wear it also?
My opinion is that fatigue will eventually cause the bags to fail. Even without wind and vibration, the constant stress will eventually cause the polyethylene to split somewhere where it is contact with its support. Also, polyethylene is a hydrocarbon. As tightly bound as polyethylene is, I doubt that it would last in an atmosophere with oxygen for more than a decade.
Ultra High Molecular Weight Polyethylene can compete with aramids in some applications. How they compare in such service over prolonged periods of time I don't know. I know that they have led to some pretty amazing schtuff.
http://www.youtube.com/watch?v=v2Xs0mvJLK0
As for adhesives, the only thing I have found is a two part adhesive by 3M. It has a six month shelf life IF you store it in a freezer. As far as I know, 3M does not claim that it will make polyethylene to anything other than polyethylene. If you spend some time studying the Valcent video and image gallery, you will see evidence that Valcent has used an adhesive to attach PVC to polyethylene. That may be one of Kertz's patented or patentable secrets.
I think we can expect most of the PBR bags to last for four years. Greenhouse films with a four year rating last for seven years for the most part. I might well be wrong about that and will discover that some shorter time period is likely. We won't know for sure until I or someone else who will reveal what they learn, has conducted tests on such a system. I expect Valcent to keep right on playing their cards close to their vest. Were I a manager working for Valcent, that is what I would do.
Let's say that two years is the best we can expect from our bags. That becomes a deductable item from your income for tax purposes. This assumes, of course, that the bags are cheap enough that you can replace them and still turn a profit. We aren't going to get that answer by speculative calculation, either. Somebody will have to bell the cat.
I have found quite a few adhesives for polyethylene, I am guessing once the first one came out everyone jumped on the bandwagon. The field of adhesives isn't as abundant when working with polyethylene sure, but it's there. I don't think Kertz is big on materials engineering. Also, since generally plastic adhesives are made from some sort of plastic, if you can get an adhesive to stick to a plastic like polyethylene, but not to for example glass, you can find an adhesive that sticks to the adhesive bonded to PE and also to glass, like a double layer adhesive.
Actually, I believe the vertigro is made from polypropylene, since it is very transparent. Polypropylene seems to be more fatigue resistant, and stronger, although I am looking for good numbers on how much stronger it is. I also must find information on it's UV resistance. Does anyone know which photodegrades faster (is more sensitive to UVA/UVB light?) polyethylene or polypropylene? Sensitivity to UVB is much more advantageous, since UVB is much less common than UVA, and this partially explains why polycarbonate/PMMA takes 10 years to degrade compared to PE/4 years max. If someone can find me information on which photodegrades faster, and in which range of UV light it degrades, I'd really appreciate it
Polypropylene is more sensitive to UV, but is definately more clear. It is also naturally stronger and more impact resistantPolyethylene is only slightly cheaper.
Unstabilized PE's half life in regards to mechanical strength in the Florida sun was tested as 2.6 months. In New Mexico I'd say 2 months. Using a 99% UV blocking greenhouse plastic it should last 100 times longer, so around 12 years, correct? I guess that's why vertigro's costs are so exorbitant.
In regards to a business, do you get the full or a partial tax return on the loss? Thanks
davtuner:I have found quite a few adhesives for polyethylene,...
Holy smokes, davtuner! What are they? Who makes 'em? I looked for a long time and the 3M adhesive was the only one I found.
davtuner:In regards to a business, do you get the full or a partial tax return on the loss?
To get a solid opinion on this you need to talk to a CPA or a lawyer who specializes in the internal revenue code, but I would expect to be able to deduct most of my costs for bag replacement as a deductable expense. Also, that might not be the best way to handle it. There is more than one way to skin a revenuer. The bags are a part of the system that require scheduled replacement and do not constitute the whole of the system. With barb wire fence, and even this rule may have changed by now, you could deduct the cost of replacing all but one strand of a fence as maintainance. If you replaced all of the wires in one season, the IRS would call that new fence and you would have to carry that as a capital improvement.
davtuner: In regards to a business, do you get the full or a partial tax return on the loss?
It would be a cost of doing bizness so it would be full. I would think plastic tubing that lasts 10 yrs would easily fall under a depreciating asset, even if it was just 3 years long. So instead of getting it all at once, you could piece it out under some yearly formula. This way you dont get the benefit 1 year and that's it, you can take the tax benefit all thruout the lifespan of that piece of equipment.
flectere si nequeo superos, Achaeronta movebo! -Virgil
http://www.reltekllc.com/adhesives-for-polyethylene.htm is just one, they all seem to be clones when I looked the first time, some amine compound.
"It would be a cost of doing bizness so it would be full. I would think plastic tubing that lasts 10 yrs would easily fall under a depreciating asset, even if it was just 3 years long. So instead of getting it all at once, you could piece it out under some yearly formula. This way you dont get the benefit 1 year and that's it, you can take the tax benefit all thruout the lifespan of that piece of equipment."
That's excellent, I can work with that... and yes, I figure eventually i'll talk to some tax guy about all this, I just don't want to get charged a lawyers rate for what should be easy access information.
Also, if one of you could validate my thought: lets say you have a UVI enhanced commercial packing grade polyethyene bag, 6 mil. Lets say in direct sunlight it takes 1 year to degrade. If you house all of this in a greenhouse (yes I know massive added cost, just as a thought game work with me), using a greenhouse plastic that blocks 99% of the incoming UV light, would the UVI polyethylene bag inside theoretically last for 100 years (math is 1 year/0.01 fraction of UV hitting)?
In so far as polyethylene, I figure using physical pressure might be cheaper than buckets of adhesive. Heat shrinkers would easily attach PE to a PVC tube, or even tie downs. Also, is there some easy non-adhesive way to join polyethylene to polyethylene, like through low heat welding?
Also, Froggy, Marc, or anyone else with experience, will 4 year poly film (like the stuff I saw at farmtek.com) block most if not all UV light? I can't seem to find this anywhere.... thank you
four year poly has 90% transmissivity, four year can last for ten years before they start to lose transmissivity..
marc
90% transmissivity of all spectra, or just visible light? Did the PE get brittle on you after a while, I heard it hardens badly, and possibly loses mechanical (tensile) strength =/
no it just turned amber in some places and the translucency diminished.
ecogenics3: no it just turned amber in some places and the translucency diminished. marc
Here is some actual research data from my part of the world.
I personally find that poly loses its clarity as the years go by. Not sure if its dust/dirt and/or chemical breakdown and/or...
Froggy,
thanks for posting that very informative power point presentation on greenhouse dynamics.
ive learned a lot from it already
davtuner:http://www.reltekllc.com/adhesives-for-polyethylene.htm is just one, they all seem to be clones when I looked the first time, some amine compound.
Thanks, Davtuner. The help is greatly appreciated. Polyethylene breaks down over time, especially in the presence of oxygen and ultraviolet light. This process can be retarded, but not stopped. Stop and think about what polyethylene is. It is all hydrogen and carbon. Basically what you see it do over time is to burn very slowly. I don't think fatique plays an important role in its breakdown until AFTER it has oxidized enough to become brittle.I think 100 years is completely out of the question insofar as the useful life of any polyethylene structure is concerned--save possibly in an oxygen and UV free environment.
Welding polyethylene to polyethylene is possible and is a standard practice. I posted a link to a tool to do just exactly that elsewhere on this forum. I thought about heat shrinkable tubing, The problem is getting a permanent seal between the PVC and the polyethylene. All the heat shrinkable tubing can do is to apply pressure to the joint. It might work if you take the time to cut a series of shallow grooves into the PVC. Such fittings are not sold anywhere that I know about and making them will take up both time and money. Mind you, I am not say that such a trick would be cost prohibitive if the demand for the fittings was there and that might eventually prove to be a good way to do it. Right now, though, I am struggling to find a cost effective method I can use right now, not ten years from now.
I have found a number of PVC bulkhead fittings that could conceivably be presssed into this service, but they cost over $4.00 each and that is more expensive than the plastic I would have to buy to make a 5' x 10' bag. They make hoses with special fittings to blow air between polyethylene sheets. These would likely work better than the bulkhead fittings that I have found, but the cheapest ones I have found so far are $32.00 each. I will likely resort to these for the time being until a better solution can be found.