Will Epoxies Bond to Polyethylenes?
This question comes up an awful lot...and I'm always surprised. Much like the mythical white whale, there is plenty of heresay, plenty of speculation and plenty of anecdotal "evidence" on all sides of the issue. Will epoxies bond to polyethylenes in a fashion that will be sufficient for coating buried steel pipelines? Let me cut to the chase and possibly save you some time here: in my opinion: NO. Don't do it. Find another solution. Don't risk it. There are other better options out there, use one of them.
Was that clear enough? Now, there are all kinds of stories out there. Somebody was told that Manufacturer A says their epoxy gets a great bond with PE. Strangely, I don't see that listed on any data sheets anywhere and I don't see it in writing anywhere.
Other stories are "Yes, Manufacturer B has an epoxy developed specifically for bonding to PE" - again, I don't see any actual evidence of this.
I'm not going to name names, but there was a case of a manufacturer assuring an end user that their epoxy would bond well to a PE coated pipe. The job was somewhere in Europe - and for all I know, the outcome is still being determined by the courts. It was a big problem though....a big, big problem.
So let's just look at this from a practical point of view. When coating a field joint, you have a few critical, critical areas (I would argue that it is ALL important - but some areas are clearly more crucial).
- How the coating coats and bonds with the weld is critical.
- How the coating fills, flexes and interacts at the junction with the factory applied coating is critical.
- How the coating bonds to the adjacent factory applied coating is critical.
Of course this list could be almost infinite - how does it react to soil stress? CD? Peel (I happen to believe Peel is kind of a useless property...but I'll leave that for another article)? Electrical resistivity (believe it or not, electrical resistivity is actually a GOOD thing to have in a coating...don't believe the hype), Shear, Penetration resistance, Shore D Hardness, Hot water immersion, UV resistance, etc, etc, etc.
When considering this specific question (epoxies and PE), I'm really going to focus only on the middle one up there. How does the epoxy (generally talking about 2 part epoxies here) fill, flex and interact at the junction between the field joint coating (epoxy) and the factory applied coating (PE in this case).
In the shrink sleeve world, one of our goals when coating a field joint is to create a SEAL. To create this seal, we install a shrink sleeve wide enough to coat all bare steel and overlap onto the adjacent factory applied coating. This creates a seal on that factory applied coating...at least 3" away from the section of pipe that was not factory coated. This essentially 'moves' the critical 'seal' location a few inches (or more) away from the field joint.
In a case where a stand alone epoxy is installed on a field joint coating, that critical interface is right there at the edge of the field joint. That critical area is directly on top of bare steel (in many cases). That interface isn't really even protected. Why? Well you've got two different material types with dramatically different qualities and characteristics meeting right there over your steel. It would be like meeting a blind date at your house. No one does that! You meet somewhere else - you meet on neutral ground - you meet at a restaurant or a bar. That way if things are going well - if the two of you don't mesh...you can quietly sneak out the back and disappear back into anonymity (not that I've ever done that!).
In addition, you've got two different material types with different bending and flexing characteristics. That isn't going to work very well. When a hard material is up against a more flexible material...what happens? The hard material will tend to crack over time as any kind of movement occurs. That isn't good!