Medical Design Briefs - March 2023 - 19

is a rubber-Iike material that is soft at
room temperature.
The proportion of the styrenic phase
controls the hardness and stiffness properties
of this family. The greater the
proportion of the styrenic phase in this
block copolymer, the harder and stiffer
the product will be. The choice of soft
segment will control the chemical resistance
and thermal properties. The
polybutadiene SBS and polyisoprene SIS
have unsaturation present in their structures,
which leads to poorer chemical resistance
and reduced thermal oxidative
resistance. The poly(ethylene-butylene)
SEBS being saturated exhibit better
chemical resistance and are thermally
more stable.
SBC thermoplastic elastomers have
found a wide range of use in a number
of markets, including medical.
The TPV Family. TPV elastomeric
alloys are composed of mixtures of a
plastic and a rubber in which the rubber
phase is cured or crosslinked. The
plastic phase is commonly a polyolefin;
most notably polypropylene. However,
it is possible to make TPVs with a variety
of thermoplastics. Some that have been
used are nylon, SAN, ABS, acrylates,
polyesters, polycarbonates, and styrene.
The rubber phase is commonly an EPR
or EPDM rubber. Other rubbers that
have been used are nitrite, SBR, polybutadiene,
butyl, and CPE.
The end product is produced during
a dynamic vulcanization mixing process.
In this case the curing of the rubber
phase occurs during the mastication
with the thermoplastic resin. This
process gives a useful elastomeric alloy
with properties of a cured rubber but
has the processing characteristics of a
thermoplastic. It is important that the
mixing be continuous throughout the
masticating step, or a thermoset material
could result.
The TPU Family. Thermoplastic
polyurethane elastomers are made up
of two types: polyester and polyether.
The polyester-based TPUs are generally
characterized as having better physical
properties, oxidative stability, and oil
resistance. While the polyether-based
TPUs at a similar hardness exhibits
better low-temperature properties, hydrolytic
stability, and resistance to miMedical
Design Briefs, March 2023
Thermoplastic elastomers are unique synthetic compounds that combine some of the properties of rubber with
the processing advantages of thermoplastics. (Credit: Emilia/Adobe Stock)
crobial attack. TPUs are noted for their
inherent toughness, providing outstanding
abrasion resistance and tear
resistance.
Applications in medical are one of
the fastest growing market segments
for TPUs. Some new aliphatic specialty
grades have been developed for this
market as well. The applications here
include cardiovascular catheters and
vascular grafts. Other medical applications
include blood bags, IV sets, and
bioclusive dressings.
The COPE Family. COPEs are considered
engineering thermoplastic
elastomers because of their unusual
combination of strength, elasticity, and
dynamic properties. They show a high
degree of heat and chemical resistance
as well.
The COPA Family. The COPA family,
or polyether block amide elastomers,
are based on a block copolymer of nylon
12 and a polyether. Through the
proper combination of polyamide and
polyether blocks, a wide range of grades
that offer a variety of performance characteristics
is possible. The family is characterized
by durometer and flexibility.
The higher the durometer and stiffness
the higher the level of nylon 12 in the
block copolymer. COPAs have been limited
to niche markets due to their cost.
They are finding use in the medical market
for various types of catheters.
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Advantages of Irradiation of Polymers
In the early 1950s, interest began in the
potential use of high-energy radiation to
initiate polymerization and modify polymers
by crosslinking. The ability to cause
chemical reactions in the solid state by
well-controlled exposure of solid polymers
to high-energy radiation created a
whole new industry. This industry has resulted
in the creation of products such as
heat-shrinkable tubing and a number of
niche wire and cable products.
When considering the use of polymer
irradiation, it is essential to consider the
following:
* What are the advantages of irradiation?
* Which properties can be improved by
irradiation?
* What kind of radiation source is preferred?
*
What does irradiation cost?
There are a number of advantages of using
the irradiation process for polymerization
and crosslinking of polymers over the
conventional chemical process, such as:
* The crosslinking takes place at lower
temperatures than those involved with
chemical crosslinking.
* In the case of polymerization, the
monomers can be polymerized free of
catalyst contamination.
* Crosslinking and grafting can be performed
in situ on finished products.
* Coatings can be applied in monomeric
form eliminating solvents.
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Medical Design Briefs - March 2023

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