Light Aircraft Association

I have a query regarding perspex/acrylic and equivalents for aircraft side windows.
For a given thickness of material, are some products more rigid than others?
The old side window I am to replace (2 mm thick) is much more rigid than the new acrylic window I want to install.
Thanks
Tony Nowak

Is it possible that the old Perspex has become more rigid due to aging as the plasticiser leaches out?
Plastic fuel pipes spring to mind.

Trevor,

That is a possibility as the old side window is probably 55 years old.

Thanks
Tony