An interesting question, Mick
I'm inclined to agree with John
… When [pre-Henkel] I dealt with Loctite in Hemel Hempstead; their technical chaps were always very helpful.
That said: You might find this interesting **LINK**
http://www.plasticsintl.com/plastics_chemical_resistence_chart.html
It shows 'no data available' for propane and acrylic [which is roughly what cured 638 becomes]
You may be on the frontiers of research ![surprise surprise](data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==)
MichaelG.