Medical Design Briefs - March 2023 - 20

TECHNOLOGY LEADERS Tubing/Extrusion
Tensile strength @
break (Mpa)
% elongation
% hot creep elongation @
29 psi, 200 °C
0 Mrd 5 Mrd 10 Mrd 15 Mrd
59.5
65.7
400
Melted 55
53.4
350 200
61
42.9
125
58
20 Mrd
Broke
Broke
63
Table II. Performance characteristics of ASTM type-4 dog bones of 72 Shore D COPA compound with 2 percent
crosslinker following crosslinking at various dosages from 0 to 20 Mrd.
Property
Tensile strength @
break (Mpa)
% elongation
% hot creep elongation
@ 29 psi, 200°C
0 Mrd 2 Mrd 4 Mrd 6 Mrd 8 Mrd 10 Mrd
59.1
60.6
700 225
Melted 46.8
61.4
200
17.1
60.8
150
10.9
53.6
125
3.0
51.2
75
2.0
Table III. Performance characteristics of ASTM type-4 dog bones of 72 Shore D COPA compound with 2.5 percent
crosslinker following crosslinking at various dosages from 0 to 10 Mrd.
A number of properties are also improved
that yield additional advantages:
* Improved thermomechanical properties.
* Increased service use temperature.
* Improved dimensional stability.
* Heat memory induced in crystalline
polymers.
* Lower permeability and improved chemical
resistance.
* Reduced stress cracking.
* Generally improved physical toughness.
The irradiation source considered for
the process is dependent on the depth
of treatment required. If considerable
depth of treatment is necessary, radioactive
isotopes such as cobalt 60, sometimes
referred to as gamma radiation, is
preferred. Electron beam accelerators
are used for thinner products and are
dependent on the voltage ratting of the
equipment. For thin coatings, electron
beam of low voltage or ultraviolet light
can be used.
The cost of the irradiation process
is dependent on the type of polymer,
formulation shape, and thickness of
the fabricated product. It is also dependent
on the required irradiation dosage
needed to achieve the desired properties.
The cobalt 60 process is more expensive
and much slower. It is used for
thick products and materials that are
dose rate sensitive. The electron beam
process is for thinner products and can
be run at very high rates. It is also less
20
costly. The use of specifically designed
formulations can reduce cost, speed
up the process, and create even greater
overall advantages using the irradiation
process.
Experimental
Trial 1. For the first trial, a 72 shore
D durometer polyamide thermoplastic
elastomer was selected. This polyamide
thermoplastic elastomer was compounded
with a crosslink promoter on a corotating
twin screw extruder. The crosslink
promoter chosen was capable of being
processed at the temperatures needed
to mix and mold this polyamide thermoplastic
elastomer. The 72 D durometer
was chosen because it is the most widely
used and also has the highest temperature
rating.
After compounding tensile bars
were molded for testing. The molded
samples were then exposed to cobalt
60 as the irradiation source. The irradiation
dosage was 5, I0, 15, and 20
megarads. Following the irradiation,
the samples were conditioned at room
temperature for 24 hours. Samples
were tested for tensile strength and
elongation. Samples were also tested
to determine whether they crosslinked.
The ICEA Publication T-28-562
test used to determine whether the
samples crosslinked. This publication
provides a procedure that is suited
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for determining the relative degree
of crosslinking of polymeric electrical
cable insulation. The test method was
modified to accommodate this material.
The melting point of this material
is approximately 175 °C; therefore, it
was tested at 200 °C to be above the
melting point of the material. It
is
important to use a temperature that
is between 20° and 30 °C above the
melt point so that the true measure of
crosslinking can be made. The results
of this first trial appear in Table II.
Trial 2. The second trial also used the
72 D durometer polyamide thermoplastic
elastomer. The crosslink promoter
was the same, but the level of crosslink
promoter was increased by 1/2 percent.
This was done because the first trial
showed a good response at lower dosage
and a rather large loss of elongation at
the higher dosage. The goal was to obtain
maximum crosslinking with a low
irradiation dose. The reason for this
was because this material is used in the
manufacture of catheters, and a low dose
would simulate the type of dose used to
sterilize the catheter. The material was
compounded and molded the same way
as done in the first trial. The irradiation
dosage used was refined and done at 2,
4, 6, 8 and I0 megarads. The results are
shown in Table III.
Trial 3. The third trial consisted of
using the 72 D durometer polyamide
thermoplastic elastomer with the same
crosslinker and level used in the second
trial. In this trial, samples were molded
for tensile and elongation, flex modulus,
and heat deformation. The samples
were irradiated at a dose of 5 megarads.
The heat deformation test used a
Randal & Stickney gauge with a 2,000
gram weight. The temperature of the
test was 175 °C. This type of test is commonly
used to measure deformation
resistance of wire and cable insulations
and jackets. The results are shown in
Table IV.
Trial 4. The fourth trial evaluated the
crosslinking of some lower durometer
grades of polyamide thermoplastic elastomer.
The durometers chosen were 35
D, 55 D, and 70 D. The samples were
irradiated at I0 megarads. The samples
were tested for crosslinking only. The
results are shown in Table V.
Medical Design Briefs, March 2023

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Medical Design Briefs - March 2023

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