Crazing of acrylic sheeting

Thomas Dixon in Cons DistList 10:53 asked about acrylic sheet
crazing. This is very common. When acrylic sheet is made it is
normally cast and the thickened monomer polymerises between two
sheets of glass. The shrinkage on polymerisation is significant
(>10%) and as a result internal stresses are built in to the sheet.
When the sheet is cut to manufacture a box or frame, etc., then the
internal stress is relieved at the point of the cut but still
remains in the rest of the sheet. Any slight imperfection on the
surface will cause a localised concentration of those stresses and
the sheet could then start to crack at that point. In practical
terms this is not normally a problem unless the sheet is exposed to
an aggressive environment. UV from outdoor exposure over a long time
(normally years rather than days) will cause surface cracks to
appear as the stress is relieved. These appear as crazing on the
surface. The most common occurrence is when the sheet is exposed to
non polar solvents as vapours.

I have seen the acrylic front of a notice board craze in less than a
week because the inside of the board had been faced with a textile
which was glued into place using a solvent based adhesive. The same
problem occurs when acrylic is used for outdoor sculptures.
Incidentally the polystyrene jars used to package whole cloves in
spice stores normally exhibit the same crazing after a few months
due to the clove oil.

This crazing results from the internal stress developed on
manufacture and can by minimised or even prevented by annealing the
final shape before it gets exposed to the environment. Annealing
involves heating in an oven and then slowly cooling over several
days. Needless to say, very accurate temperature control is
essential.

Most acrylic sheet manufacturers will have literature on annealing.
Here in the UK ICI have an excellent one for their Perspex sheet.

I think that extruded acrylic sheet is less prone to crazing than
the cast version, it is less optically pure, but also less
expensive, please note 'less' prone not 'not' prone.

Colin Williamson

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                  Conservation DistList Instance 10:55
                 Distributed: Monday, December 9, 1996
                       Message Id: cdl-10-55-001
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