Hi, First of all, sory for my bad English. We have found internal and external blisters (bubbles)in composite Polypropylene / Aluminium / Polypropylene pipes for heating. The osmtoic blisters are full of condensed water and the aluminium layer is OK, so, the moisture should come from the water of the pipe or from the outside moisture... The blisters are so big that the water flow of the pipe was completely obstructed. This happened after more or less one year in an underfloor heating installation.
This composite pipeline is made by coextrusion. There are five multilayers:
1) Polypropylene
2) Adhesive (Malaeted Polypropylene)
3) Aluminium
4) Adhesive (Malaeted Polypropylene)
5) Polypropylene
The blisters with water grow between layers 2) and 3) and less frequently, between layers 3) and 4.
We could observe the same problem in an acelerated test of 10000 cycles of 15 minutes, at 20 degrees celcius and 95 degrees celcius, at 10 bar. After 500 or 600 cycles, we could already find the blisters (or bubbles)
We know that Polypropylene has a good (low) Water Vapor Transmission Rate, but maybe there is an "osmotic driven blister" problem... we don't know.
What about the chemical potential of water in the blister? I'll appreciate any help or advise you can give me.
Thanks in advance.
follow up posts
On 04/17/2008 Composite Analytica posts:
Dear Holger,
Thank you for sharing your experience and observations with us.
Blistering in a composite occurs usually as a result of osmosis. Osmosis is caused by a macroscopic solution in the composite. In this case, the macroscopic solution could consist of water and some other components, like for example monomers of the adhesive that is used. The macroscopic solution ought to be prevented in composites, especially in case of water (becuase of it's small size and capability for hydrolysis)
Macroscopic solutions in this kind of industrial applications are initiated by diffusion from the environment. This can be diffusion from the internal environment where it gives a constant load of hot water (95 degrees Celsius), but also and simultaneously, from the external environment (e.g. 20 degrees Celsius and 70% relative humidity). In this case the internal water is hot, which results in a relative high solubility in - and diffusivity through - the layer of polypropylene. Mind that temperature effects on the permeability behavior may be enormous with this type of polymeric materials: a mass transfer increase with a factor 1000(!) compared to 20 degrees Celsius is not uncommon. This reduces the time lag, which is the time the water requires to obtain a (critical) solubility at the polypropylene - aluminium interface, where the adhesive is.
With the adhesive many things can be wrong that would promote the occurrence of a macroscopic solution. I will mention a few:
(1) an adhesive that is susceptible to water hydrolysis,
(2) un reacted parts or cells in the adhesive due to reaction process conditions that are insufficient,
(3) a polluted surface of the metal (condensed particles / mixtures)
(4) etc....
Another or simultaneous phenomenon is internal stress by temperature. The materials have different degrees of thermal expansion, which can result in stresses with a higher energy than the interfacial stress.
IMPORTANT NOTICE:
A total collapse usually occurs as a result of the above described water diffusion with subsequent chemical attack of the interfacial region in combination with temperature initiated mechanical stress. Our chemical-physical simulation programmes can analyze this situation and provide a material/adhesive and processing suggestion.
Sincerely,
Composite Analytica
[responses: 2]
On 10 Jul 2008 at 22:56:45 Holger posts:
Little late, tough many thanks for your assistance with blister formation in polypropylene (PP), polyethylene (PE) and crosslinked polyethylene (PEX) on top of Aluminium and advice on chemical vapour deposition (CVD) of the Titanium layer!
[responses: 1]
On 12 Jul 2008 at 21:37:13 Alan posts:
Maybe a little off-topic but perhaps you can answer my question:
Is electricity from an osmosis membrane (seperating salt sea water from normal water) a reliable and efficient to produce energy? What sort of polymer membrane materials are used for this, and are the mass transfer equations to be solved with Fick's law, or do we need the Maxwell-Stefan equation since an electric and osmotic pressure potential is involved?
Any reputable reference on osmosis and related diffusion equations are welcome!
Alan
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