Page 189 - ASOP ROT Study Guide

2.4 Resin Based Casting Materials
Many newer commercial casting bandages or tapes consist of glass fiber or other artificial fiber
bandages in association with polyurethane resins.
When exposed to heat and moisture, urethane monomer molecules polymerize (combine) to form
polyurethane. This chemical reaction also produces heat. There are very strong chemical links
between the polymerized urethane molecules in polyurethane. The strength of polyurethane resins is
considerably greater than that of plaster of Paris because of these chemical links.
The bandage consists of a woven net of glass fiber or another textile, such as terylene or
polypropylene. The bandage adds considerably to the structure of a cast because it is intimately
physically linked with the polyurethane resin.
The combination of polyurethane resin and bandage has properties of strength and flexibility which
are better than either the bandage or the resin alone. Materials, which in combination produce such
favorable properties, are known as composites.

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The disadvantage of polyurethane bandages is that the resin does not move around relative to the
bandage in the same way as plaster of Paris does. This means that the way the bandage is laid on
the limb to make the splint is very important and there is no scope for moving resin about to
compensate for poor siting of the bandage.
The advantage of composite polyurethane bandages are that they are light, flexible and very strong.
2.5 Sheet Materials
An alternative way of constructing a splint is to make it from a rigid sheet which may be cut to
shape and then made malleable by heating before being moulded to the limb and allowed to set.
Commercially available sheet materials either consist of polycaprolactones or isoprene rubbers.
These materials are chosen because they are strong enough to form a splint atthicknesses practical
to handle and they have low melting points so that they can be applied to the body for moulding
without risk of causing burns. The advantage of these materials are that they can be used to
make more complicated splints (e.g. for fingers and hands) or to construct functional braces.
The disadvantages of these materials are that they are difficult to work with and require the part of
the body to be stable during setting and construction of the splint. This makes them unsuitable for
fresh fractures which tend to be wobbly. These materials are also very expensive.
Materials with a higher melting point are available (polyethylene and polypropylene). These
materials are largely used to make longer lasting splints for those with chronic disability and have