Sikaflex 512 Failure

I found it very good when I fitted a solar panel on the roof, did the same as you and cleaned/degreased where fitting were going but I used emery paper to rough up the exact area marked with a pencil so the adhesive had something to grip on. it was still on when I sold the van.

Any adhesive needs something to key to. Very smooth surfaces, regardless of how clean, don't really provide this. Surface abrasion as described above by lornalou is perfect.

Thanks both you Guys, I also fitted a solar panel with it, 8 years ago, seems fine. As you said maybe I needed to roughen up the edge on that pp plastic, although I didn't on the van roof and I can't get that off except with a blade!

Our Thule wind out awning and locking door hand grip are both stuck on with Sikaflex nothing else,

This is not true, think for a moment how all the fixed windows in your car, the double glazing units in your home are bonded to the super smooth Float glass. Plus how most of the caravan's alloy sheet and alloy extrusions are bonded, all without abrasion.

Here, accepting the cleaning was adequate and there being no hint of bonding supports that belief, then the fact it breaks alone the Sikaflex to plastic moulding is indicating, either that was not clean or much more likely is made of a plastic not suitable for use with polyurethane based bonding products. Working exactly like release agents used to deliberately stop a bond.

Abrading here will make not the slightest difference if there is no "bond".

Thanks Ocsid, that was my initial feeling as the surfaces were spotless. I think the pp plastic simply does not bond, but Sikaflex works very well eg. with caravan paint, solar panel rear surface etc etc. Something to be aware of if you put anything on the van made of pp (I assume this is polypropylene?, Not that I know anything about chemistry), As it will fall off.

you cannot compare adhesives that are produced and tested to adhere car windows/glass with ones that are produced to adhere plastics or any other material, ie no more nails for timber. making the surface rough will help. I wonder why when you purchase a puncture repair kit for you bicycle it contains a piece of glass paper or grater to roughen the surface, even when you have a tyre repair on your car they roughen the inside of said tyre to help adhesion.  

You should use none setting mastic tape to put the roof light in. If it had stuck correctly you would never have got it out. Trouble is that roofs move and so do plastics when they get hot. Two dissimilar materials are going to expand and contract at different rates and break the joint.