Ceramics and composites - Orthopaedic Biomaterials in Research and Practice - page 217

Casting materials

Composition “Plaster bandage” and its modern replacements represent interesting

examples of nonimplantable composites. They are peculiar in design
also, since neither of their components is very strong. However, they are
combined to form a strong, tough composite.
The reinforcing or “fabric” component (cotton gauze in older prod-
ucts; fiberglass, polyester mesh, or a fiberglass-polyester interweave in
more modern products) is weak and flexible in compression but fairly
strong in tension. By comparison, the matrix (“plaster” [calcium sulfate],
a thermoset polymer, or a polymer-plaster mixture) is weak and flexible
in tension but quite strong in compression. Thus, the composite combines
these properties to produce a material that is fairly strong and tough. It
is highly “defective” in terms of the interfaces or voids, and although the
tensile strength of the fibers produces reasonable single-cycle strength,
these materials are fairly poor in resisting fatigue. However, the more
modern materials have high strength-to-weight ratios, permitting mul-
tiple layers to be built up to overcome this fatigue weakness.
The matrices “set” or harden by two different processes.

1. Plaster is usually partially anhydrous calcium sulfate, ground to a
fine powder. It sets owing to the recombination of water with the
CaSO 4 powder and the growth of an interlocking matrix of crys-
tals. The typical reaction is

(CaSO 4 ) 2 ∙H 2 O + 3H 2 O → 2[CaSO 4 ∙2H 2 O] + heat

Too little water may prevent stoichiometric rehydration or ade-
quate crystal growth; too much water or early motion may produce
many small, weak crystals.

2. Resins are a variety of materials including low-molecular-weight
polyesters and polyurethanes. They harden by either addition or
condensation polymerization, initiated by water, heat, or ultra-
violet illumination or some combination, forming interlocking
polymer networks. The setting of resins (and plasters) is modestly
accelerated by elevated temperature. However, both types of mate-
rial release heat during setting so that temperature elevations may
produce unacceptable temperatures at the patient's skin, especially
if applied over insufficient padding material.

Table 8.6 lists some of the many casting materials currently available.
They may be grouped into three main categories by matrix type: plaster,
plaster-resin, and resin. These groupings are shown in Table 8.7 with
some of the principal advantages and disadvantages of each system.
Despite the generic differences recited in Table 8.7, familiarity in use
may be the chief factor in selecting a casting material. This is the case,
even though there are significant differences in handling among appar-
ently similar synthetic (resin matrix) materials. After fabrication, materi-
als within each of the three generic categories are difficult to distinguish
except by appearance (color, etc.). However, isolated properties such as