Biomedical Engineering Reference
In-Depth Information
Ta b l e 3
.
29
(
Contd
)
Properties
Characterization
Thermal
properties
Polymers normally show a large variation in their properties
with temperature. At suficiently low temperatures, amorphous
polymers are hard and glass-like, compared to softer and more
lexible, when a critical temperature is reached usually the glass
transition temperature (
T
g
). The
T
g
of a plastic is one of the very
important set points in determining whether the polymer is
thermosetting or thermoplastic and hence our desired clinical
properties are affected.
Because of its esthetic properties, resin can be used for the
reproduction of lost tooth structure. There are two types of direct
illing resins. The older type is an unilled polymethyl methacrylate
(PMMA). Such a resin is often referred to as an acrylic resin. The other
type is the composite resin. Requirements for a denture polymers
are included in Table 3.29 [4, 80, 92, 106, 140].
3
.
4
.
1 Acrylic Resins
Acrylic resin is a general term for any one of the plastics (resin)
generated through chemical reaction by applying polymerization
initiator and heat to a monomer. Polymethyl methacrylate, one
of the more rigid variations of acrylic resin, was irst marketed in
Germany in 1927. A similar form of the resin known as Plexiglas
®
began to be manufactured in the United States in 1936. One of
the main characteristic features of PMMA is its high transparency.
With its high weather resistance, it has been known to last over
30 years; it does not easily turn yellow or crumble by sunlight.
Acrylic (unilled) resins are used as temporary crown material.
Temporary crowns are placed to protect the crown preparation and
provide patient comfort during the time the permanent crown is
being constructed. Acrylic resins are composed from powder and
liquid. The powder is composed of a polymethyl methacrylate, a
polymer, and contains some inert coloring pigments. The liquid is a
monomethyl methacrylate, a monomer.
Acrylic resins are used in individual impression trays and
orthodontic devices, in addition to dentures and artiicial crowns.
PMMA is well known for its property of being a bone cement (for
ixing hip implants) and of its use in making acrylic glass (as a base
for artiicial ingernails and varnish).
In respect to the diverse applications of polymers in denture
science, higher molecular weight methacrylate polymers such as
