Prevost XLII Skin Delamination Repair

[travelite]

(11-14-2014 17:19)CC_Guy Wrote:  The million dollar question would be, how can you determine if the adhesive will fail later? The answer is either there's no way of telling or they will all eventually fail. I have no idea which answer is correct.

Hi CC_Guy,

We'd like the bonded joint to last as long as the bulk of the adhesive (cohesive failure) as opposed to early failure due to substrate separation (adhesive failure) - adhesive failure seems to be the case in the photo I posted earlier.

Prevost has been using a single part polyurethane, either Sikaflex 221 or 255. Here's a list of the products used on my bus panel when I had a repair done to the converter modified panel under my bedroom slide:

Sika Cleaner-205
Sika Activator-205
Sika Primer-206 G+P
Sikaflex 255 FC
Bostik 70-03A

Arctic Bird posted about the heavy use of adhesive in the earlier pic. Single part polyurethane's require water and oxygen to cure. When the thickness of the bead goes beyond 1/2" the curing stops due to a lack of moisture. Manufacturers typically recommend small diameter beads of adhesive. This allows curing - also thinner beads elongate better than thicker beads. This may be why Prevost has moved to a two part Loctite epoxy, part number 5590. Two part epoxies continue to cure even if they've been covered or painted over and sealed from the atmosphere.

Something I noticed when Prevost was doing my panel is that they paid a lot of attention to panel and bus framework prep. Both sides were cleaned, activated, and primed with the recommended Sika products. Gloves were worn and panels were cleaned and untouched until mating time. Once the adhesive bead was applied the panel was put in place in a timely manner.

The skin on our buses reminds me of the "curtain walls" used in modern day skyscrapers. Exterior building panels are glued in place. They're not structural but they do need to withstand movement, expansion, vibration, UV, and temperature swings, and they do need to be weather tight. I'm not a building structure expert but I understand that at least one building code allows a degree of water infiltration as long as an exit path exists. I've always felt the same way about our skin. The adhesive is there primarily to hold the panels in place, and by that definition there is no failure. It's also true that the adhesive is serving double duty in that it's being asked to also act as a weather barrier. I'd like to see a design that separates the two functions with an easily replaced weather seal that can be serviced every so many years.

Even with proper material selection and perfect preparation and installation, bonded joints have a finite life span and will forever require regular inspection, repair, and replacement. It's not totally unlike rivets where rivet holes are areas of weakness where stress cracks can form and which have to be periodically checked for corrosion or loosening. It'd be great if Prevost could make these procedures as easy and painless as possible. A possible step in that direction with adhesives would be to separate the mechanical fastening system from the weather sealing system.

Back to your million dollar question. I think we have a pretty good idea of a typical cohesive failure interval, what we don't know is what the chances are of an early adhesive failure in a randomly selected XLII. I seem to recall at one point, circa 2013, Prevost Jacksonville saying there were something like 250 buses with early failure. If it hasn't suffered adhesive failure yet, then IMO it'll probably make it to cohesive failure.