Why a Surlyn® / Polyethylene moisture barrier is better than PolyKraft
Moisture barriers laminated to metal including Aluminium and stainless steel are widely used throughout the oil and gas industries to prevent galvanic corrosion of the cladding and also corrosion under insulation (CUI) from underneath. If water is present inside the system and makes contact with the metal jacketing or cladding corrosion can occur. The first barriers used were paint based, over time these evolved to using Kraft paper on polyethylene hence the name polykraft moisture barrier.
In order to protect the metal jacket or lagging the barrier must be resistant to water and scratches and not have any pin holes that would let water through. When comparing barriers only a Surlyn® / Polyethylene barrier such as DryMet is able to achieve all these aims.
Lets look at the properties in detail:
Pinholes – these are very small holes hard to see by naked eye but will let moisture through to the metal based on ASTM C1729 with a measurement of pin holes per 50ft2 paint would be 20 plus polykraft around 16 and DryMet zero.
Water ingress - WVTR(g/100 in2-day) paint not known, polyKraft poor 1.0 , Drymet <0.05
Scratch / wear resistance – Paint is easily scratched even by hand, polyKraft is paper so easily gets wet and retains moisture, DryMet using DuPont’s™ Surlyn® technology has excellent toughness and proven performance (Surlyn® Resins are used by all major golf ball makers).
Flammability – while paint is the best performer, Surlyn® is much better than polyKraft but with this being paper it’s to be expected. Paper has an auto ignition temperature of 232 deg C compared to Surlyn® above 315 deg C.
That’s why since the inception of moisture barrier they have evolved from paint to paper then Surlyn®. If you want a moisture barrier to perform and last you need a factory applied polyethylene and Surlyn® barrier like DryMet®. As a result most EPC and FEED engineers have moved away from polykraft its mainly old specifications that exist and are often replaced by Surlyn barriers due to ever decreasing supply and availability of polykraft.
A Surlyn® moisture barrier such as DryMet® can be combined with bare metal or exterior coatings such as paint (PVDF), Tedlar®. (PVF) to give additional benefits of corrosion resistance and high emissivity.
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