Description
of Application
Most of the plastic glazing materials used in boat building
are either clear acrylic sheet (PMMA), widely marketed
under trade names such as “Perspex” and “Plexiglas”
(the
latter manufactured by Rñhm und Haas), or polycarbonate
(PC), marketed by Rñhm und Haas as “Makrolon”
and by General Electric as “Lexan”.
All plastic glazing products possess certain characteristics
that must be clearly understood before these products
are installed or bonded with adhesives. In general,
incorrectly installed plastic glazing panels are prone
to stress cracking, which may be aggravated by the use
of certain polyurethane-based adhesives.
Plastic glazing products have a higher coefficient
of thermal expansion than conventional glass. Therefore,
when designing glazing installations, an expansion gap
of at least 5 mm all round must be incorporated between
the window rebate and the plastic glazing panel to accommodate
thermal movement. Similarly, any clearance holes for
fixing screws must be drilled oversize, i.e. larger
than the actual diameter of the screw shank.
To minimise the risk of stress cracking, flat sheets
of plastic glazing material should be installed completely
flat; they should not be forced to take up a curvature
by the use of mechanical fastenings. When the design
calls for curved glazing panels, these should be prefabricated
to order and properly tempered by a specialist supplier
to ensure a stress-free installation.
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