Perspex

Walter Castelijns via Robien E. van Gulik asked about Perspex and
distortion.

Perspex or Plexiglas is poly methyl methacrylate sheet and is
manufactured by casting the liquid monomer between glass sheets. On
polymerisation it shrinks considerably and freezes in the stresses
produced in manufacture. When the sheets are cut up to make objects,
the stresses are relieved somewhat but also concentrated in a
smaller area and the result is frequently distortion. Sculptors who
use cast acrylic sheet are always advised to anneal their work
because the built in stresses can show as surface cracks when the
object is exposed to UV or solvent. Annealing relieves the stresses
so that the object is less prone to environmental damage.

Most acrylic sheet is supported round the edges in use and this
normally helps the shape to hold. Also, most acrylic is made for
signs and therefore a long life expectancy is not required.

The suppliers of the acrylic sheet should be able to offer annealing
advice (ICI, manufacturers of Perspex have an excellent booklet),
but it is normally simpler to use physical fixings to prevent the
distortion.

Colin Williamson
Plastics Historical Society

                                  ***
                  Conservation DistList Instance 12:77
                 Distributed: Wednesday, March 31, 1999
                       Message Id: cdl-12-77-002
                                  ***