Lower Extremity Thermoplastics: An
Overview
William Clover Jr.
Thermoplastic Sheet
Thermoplastics over the past several years
have increased in popularity in both orthotics and prosthetics. There are many characteristics that make thermoplastic very attractive for use in the field. Clarity, flexibility,
rigidity, faster processing times, localized
adjustment by the use of heat, inert material
and surface quality are just a few of the benefits associated with thermoplastics.
Thermoplastics like any other type of
technology is subject to a learning curve
where there is a lack of understanding and
failures occur. This process quite often discourages the user and brands the material or
process as an unacceptable method of fabrication. Through education and close attention to detail, thermoplastics can become
one of the most valuable technologies available to prosthetists and orthotists.
Thermoplastic Sheet Handling
The handling of the sheet of plastic is critical in the success of the finished device. The
manufacturers of basic resins produce many
different types to meet the needs of many
industries. The manufacturing of the basic
resin requires the plastic to be heated to a
molten state twice, once to make the resin
and a second time to blend in the additives
that affect its final characteristics such as
melt index, heat deflection, ultraviolet light
stabilizers, surface modifiers, to name a few.
After the addition of stabilizers, the resin is
in pellet form. The pellets are then sent to an
extruder for forming into a sheet. The sheet
extrusion process requires that the resin be
heated again to form the plastic into a sheet
adding another heat history. During each
heating process, there is a degradation of the
physical characteristics of the plastic. Many
things can happen to the plastic as it is being
extruded that will affect the quality of the
sheet you receive. There is the possibility
that the extruder will add regrind to the resin
to use up his scrap material. This reground
plastic has an additional heat history and
may come from another batch of resin that
has different characteristics than the primary
resin. This may produce a sheet with unknown characteristics.
As the sheet is formed, it passes over heated rollers. The temperature and speed of the
rollers along with the tension and exit cooling rates all affect the internal stresses in the
plastic. As small volume users, it is important to have a close relationship with a plastic
vendor who is monitoring the sheet materials
that we will use in our facilities. It is wise to
develop a relationship with vendors who are
sensitive to the particular needs of the O&P
industry and are closely monitoring the quality and characteristics of the sheet materials
they are supplying. The additional cost associated with premium plastic and the services
that these vendors provide is an inexpensive
insurance policy to prevent failures that
could result in costly redos.
As the fabricator of the sheet, it is our
responsibility to continue to handle it with
great care so as not to abuse the material
which may lead to premature failures or unsatisfactory products. The heating process
used to fabricate a device will add a fourth
heat history to the material. It is important
to heat the material in compliance with the
manufacturer's specifications. The convection heating process transfers heat from the
surrounding air to the surface of the plastic;
the heat then migrates into the center or core
of the sheet. At elevated temperatures, the
surface temperature of the sheet may exceed
the maximum temperature of the plastic,
causing the physical characteristics to degrade while the core of the sheet is below the
minimum forming temperature. Referring to
Table 1
, it can be noted that the temperature
differential for polypropylene is minimum
forming temperature 290°F and maximum
forming 310°F-325°F, only 35°F. The maximum safe temperature before degradation
of the material is 33°F. This shows that
there is a very small temperature window to
work within. As the material falls below
290°F, the crystalline structure begins to
form and create links or bonds. When these
bonds are mechanically stretched below
290°F by the vacuum forming process, internal stress, spring back and accentuated
shrinkage can occur.
The optimum conditions for vacuum
forming parts are: (1) warm mold, (2) hot
plastic and (3) fast vacuum. The warm mold
(see Table 1
for mold temperature for each
type of plastic) reduces the thermal shock to
the hot plastic as it touches the mold surface.
Thermal shock and rapid cooling introduce
internal stress that can eventually lead to
failure. The molded part and the plastic
should be allowed to cool down slowly together over a long period of time. A word of
caution, be very careful in heating plaster
casts that have moisture inside. Steam will
form and cause casts to explode.
Attention to sheet selection, slower controlled heating and thermal shock will add a
small amount of time to the fabrication process. However, it will yield the best quality
plastic part for the end user. This small concession in time to guarantee patient satisfaction and prevent costly redos is worth the
time and the effort.
Thermoplastic Sheet Selection
Polypropylene homopolymer is a thermoplastic polymer with low specific gravity and
good resistance to chemicals and fatigue.
Entering commercial production in 1957, its
historical significance is understood by the
fact that it remains the fastest growing major
thermoplastic, having reached worldwide
production of 21 billion pounds in 1988.
The rigidity, strength and resistance to fatigue allow polypropylene homopolymers to
be typically used in lower extremity prosthetic application such as AK frames, AK
sockets, BK frames and BK sockets. Typical
shrinkage is 1 1/2 percent to 2 percent.
Copolymer - Polypropylene copolymer
has more resilience than homopolymer. This
is accomplished by the addition of polyethylene at levels of 5 to 25 percent. Copolymer
has less rigidity and processes at a slightly
lower temperature. These qualities allow copolymer to be spot modified more easily
than homopolymers. The white marking on
the surface of the copolymer in high stress
areas is not necessarily an indication of failure as associated with the same marks that
occur around failed areas on homopolymer
parts. The white marks will generally clear
and return to a natural color when spot heated. Copolymers are more commonly used in
orthotics, but are gaining acceptance for AK
and BK sockets. Typical shrinkage is 1 1/2
percent to 2 percent.
Polyethylene - Low-density polyethylene
(LDPE) was first commercialized in the early 1940s as a wire coating. LDPL is the oldest
member of the polyethylene family of resins.
The flexibility, chemical inertness and lower
temperature processing make this material a
good choice for lower extremity socket liners. Typical shrinkage is 1 1/2 percent to 3
percent.
Linear low-density polyethylene differs
from LDPE in the formation of the molecular chains. In making branch polyethylene,
the crucial polymer parameter of density
which in a sense describes the closeness and
regularity (or crystallinity) of the packing of
the longer polymer backbones, is varied by
means of changes in reactor pressure and
heat; linear low-density polyethylene density, on the other hand, varies with the quantity of comonomer used with ethylene. The
comonomer forms short chain branches
along the ethylene backbone. The greater
the quantity of comonomer, the lower the
density of the polymer.
By contrast, branched low-density polyethylene has both short- and long-chain
branches (Figure 1a
and Figure 1b
). This is one reason properties of linear low-density polyethylene differ somewhat from those of its branched analog, low-density polyethylene. As the name
suggests, linear low-density polyethylene is
more linear, more crystalline, and thus processes differently and exhibits different end-use performance.
Linear low-density polyethylene finds applications in all areas of traditional polyethylene usages. It has improved tensile, puncture resistance, impact and tear properties
making linear low-density polyethylene a
good choice for AK and BK socket liners. It
should be noted that the lack of long-chain
branching in linear low-density polyethylene
allows the polymer chains to slide by one
another upon elongation without becoming
entangled. This phenomenon manifests itself
as an apparent increase in the shrink factor.
Typical shrinkage is 1 1/2 percent to 5 percent.
Copolyester - Commonly known in the
industry as Durr Plex, this material remains
amorphous, clear and virtually colorless
even in very heavy sections. It has high stiffness and hardness and good toughness.
These qualities make this material a good
choice for check sockets. Durr Plex can be
easily spot heated with a heat gun and modified easily during the fitting process. Typical
shrinkage is 0.5 percent to 0.75 percent.
Ionomer - Surlyn is the trade name for
Du Pont Company's Ionomer resins. Du
Pont Company is the sole U.S. producer and
supplier of ionomer resins. The long-chain
semi-crystalline polymer structure imparts
characteristics normal to a polyolefin. The
upper usable temperature range of unreinforced Surlyn is 120 degrees F to 170 degrees
F. The reduction of crystalline structure produces good optical clarity. Surlyn is processed using the same techniques as low- density polyethylene. Surlyn can be used for fitting modules and socket liners. Typical
shrinkage is 1 percent to 2 percent.
Polycarbonate - Polycarbonate has been
used for many years in the 0 & P industry as
a clear check socket material for both AKs
and BKs. The major drawback is that it is
hydrophilic and must be pre-dried to form
acceptable check sockets. A 3/8-inch thick
sheet must be dried for 48 hours at 275 degrees F before it can be used. For best results
and to reduce brittleness, work over a warm
mold (140 degrees F). Typical shrinkage is
0.3 percent to 0.7 percent.
Stress Relieving
The stress relieving process occurs after
the sheet is extruded. It is designed to relieve
stress in the material that was induced during
the extrusion process. There are several
methods of accomplishing stress relieving.
Some are inline processes that address the
cooling of the sheet as it exits the extruder.
Other methods use heat and pressure on cut
sheets. These processes were developed to
reduce warp and shrink in fabricated sheet
applications where the sheet is subjected to
elevated temperature, but are below the
minimum forming temperatures. An example would be where polypropylene sheet is
used to line the inside of chemical tanks that
hold hot liquids. If the material was not
stress relieved, at elevated temperatures, the
sheet might warp and pull away from the
sides of the tank.
It is the author's opinion that stress relieving has no effect on the quality of the part
produced in the O&P industry for several
reasons:
First, the stress relieving process occurs
under the minimum forming temperature. In
the O&P industry, the plastic is heated
above this temperature, which allows the
plastic to flow freely and relieve its internal
stresses. Stress may be reintroduced into the
plastic during the forming process.
Second, it is not apparent that there are
any chemical additives added to the plastic to
change the molecular structure so as to relieve stress. It is quite possible that some of
the commercial methods for stress relieving
could be adapted to relieve stress in formed
parts which would allow for a more reliable
finished part. There are some techniques
currently being explored for post-curing/annealing of the formed parts while still on the
cast.
During the past 25 years, William Clover Jr. has been
involved with the design and development of plastic products. The past seven years he has been
with Orthomedics - 4 1/2 years as director of engineering, 1 1/2 years as vice president of operations, managing both the manufacturing and custom fabrication areas and at the beginning of
1990, assumed the role of president of the products division. The information for this article is an
accumulation of many years of experimentation,
refinement, practical application and study of the
vacuum forming process as it relates to the O&P
industry. It is the intent of this article to share
information that is unique to this industry and will
serve to improve the quality and durability of the
devices delivered to the patients.
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