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26/01/2010

Article Published on Chemical Resistance of Polyethylene Pipelines

Chemical resistance is an important factor in planning strategic PE pipelines. As conditions of use vary from project to project, there are special cases that designers should be aware of, says David Hill, technical manager at GPS PE Pipe Systems.

Polyethylene is renowned for its good resistance to chemical attack. It does not rot, rust, pit, corrode or lose wall thickness through chemical or electrical reaction with the surrounding soil. It does not normally support the growth of, nor is affected by, algae, bacteria or fungi.



The degree of resistance to a specific chemical will depend on concentration, temperature and working pressure. The technical report ISO/TR 10358, containing information on the PE’s chemical resistance to a wide range of temperatures and concentrations, is a useful guidance when designing a piping system.

So PE has excellent chemical resistance – everyone seems to know that, but like everything, there are exceptions, and it is as well to know what they are. To explain this in some depth, it is necessary to differentiate between chemical resistance and permeation.

Permeation

Chemical resistance is an ability of solid materials to resist damage by chemical reactivity or solvent action. Permeation is a physical effect where a liquid or gas is able to diffuse through a solid without chemically altering it, though it may cause significant swelling and change in physical properties.

In the case of PE, those liquids most able to do this are low molecular weight hydrocarbons (e.g. petroleum fluids), and gases (hydrogen and carbon dioxide). That is not to say that other species cannot permeate, just that they do so more slowly, and in some cases so slowly that the effect can be ignored.

The main controllers for permeation are molecular size, and polarity. Big molecules are immobile and do not permeate easily. Polarity is caused by electric charge distribution within the molecular structure and this strongly influences intra-molecular forces. The general rule is that like permeates like more easily, so a non-polar solid is more permeable to a non-polar fluid and a polar solid to a polar fluid and so on. PE is non polar. For those non-chemically inclined this might all sound rather complicated, but there are a few basic rules of thumb that can help your way through.

Polar molecules frequently contain –OH groups, a case in point being H₂O (water). Liquids that completely dissolve in water are more likely to be polar, e.g. alcohols. For rapid permeation you need two things – small molecules and similar polarity (or in the case of PE none). A few examples:

Water (H₂O) is compact but polar, so permeation in PE (non-polar) is difficult.
Ethanol (C₂H₅OH) is compact but polar, so permeation in PE is difficult.
Carbon dioxide (CO₂) is compact but non polar, so permeation in PE is easy.
Benzene (C₆H₆) is fairly compact and non polar, so permeation in PE is easy.
Lubricating oil (C₃₀H₆₀₊) is very bulky and non polar, so permeation in PE is very difficult.
Benzopyrene (C₂₀H₁₂) is also bulky and non polar, so permeation in PE is difficult.

With mixed liquids things get more complicated if at least one of the species present is potentially permeable in the polymer, as this can swell the polymer making it easier for the rest to follow. If you are in doubt, it is recommended to ask rather than try to guess the answer. Nobody knows exactly what will happen in every possible circumstance, because there are infinite permutations of fluids that may be present. In an attempt to clarify and give as accurate view as possible, GPS PE Pipe Systems have extensively tested the compatibility of great many solvents with PE and would be happy to advise you on your particular case.

Returning to chemical resistance, we can say that with very few exceptions PE is immune to just about anything at room temperature (except for powerful oxidisers) and we can ignore anything from around pH=2 (highly acidic) to pH = 14 (highly alkaline). The difficult ones are things like fuming inorganic acids, halogens, peroxides and related chemicals.

Environmental Stress Cracking

But a note of caution: under certain specific circumstances, PE like many other thermoplastics is vulnerable to a phenomenon known as Environmental Stress Cracking (ESC), which causes unexpected brittle failure at low stress. This happens for reasons that appear to have little connection to straightforward chemical attack. Furthermore the required concentration of the offending chemical to cause failure can be very low indeed.

With PE the offending materials include detergents, ethylene glycol and related chemicals. Caution should always be applied when using these in contact with PE.

However, modern PE pipe grades have been developed to maximise resistance to ESC. This is particularly relevant to the UK, where the PE pipe materials used for many years, that is MDPE80 and PE100, have exceptionally good resistance compared to earlier PE pipe materials. Newly developed materials are available that have even more resistance.

The two circumstances in which any of these properties are most likely to come into play are:
a) when PE pipe is used for conveying fluids from chemical processes, chemical effluent, etc., and
b) when PE pipe is used for potable water or gas in contaminated land.

With a) it is a matter of making a judgement about the aggressiveness of the media at the expected temperature, and the affect on pipe properties. With b) main concern is usually the potential for permeation causing taste and odour issues with potable water,a barrier pipe such as Protecta-Line to be used or - with PE gas pipe - whether any change in physical properties will be significant (extremely unlikely).

Conclusion

Generally, PE pipes have excellent resistance to a large number of chemicals, including many commonly used chemicals and those naturally occurring chemicals found in the soil. However, there a few notable exceptions and special care is required in some industrial applications where effluents contain particular chemicals. On balance though, PE is resistant to corrosion, a genuine risk which limits the economic lifespan of many metallic pipe materials.

The polymer and pipe manufacturers have extensive test data regarding the chemical resistance of PE pipes. Advice should be sought if there is any doubt as to the suitability of PE for a particular application or environment. Here at GPS PE Pipe Systems, we offer over 100 years of expertise in the development of plastic piping and are happy to advise you.