HPL ?
AndrewG is a former chemist and tends to understand such things far better than I , but I thought I'd add this :
https://plasticsheetsshop.co.uk/what-is-hpl/
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Silly Moustache,
Just an old Limey acoustic guitarist, Dobrolist, mandolier and singer.
I'm here to try to help and advise and I offer one to one lessons/meetings/mentoring via Zoom!
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