Medical Design Briefs - November 2021 - 16

Biomedical Balloons
Dynamic mechanical analyzer (DMA) tests
were performed in tension mode.
To evaluate orientation and character5000
Nylon
Pebax
500
ize
phase formation, FTIR experiments
were carried out by recording infrared
spectra with a resolution of 4 cm-1 and
an accumulation of 16 scans. Thermal
properties were analyzed using a differential
scanning calorimeter (DSC).
The samples were heated from -30 to
200 °C at a heating rate of 10 °C/min,
then cooled at the same rate to -30 °C
and heated again to 200 °C. Tensile tests
were performed with a crosshead speed
of 127 mm/min. Puncture tests were performed
based on ASTM F1306 with a
probe size of 1.6 mm diameter.
50
5
50
Fig. 1 - Viscosity curve for nylon and Pebax at 230 °C.
parameters play important roles in producing
tubes with different physical and
mechanical properties. The tubes are
then stretched axially and radially in a
balloon forming machine using a spe -
cific recipe that includes combinations
of temperature and air pressure to make
the final balloon.
The room temperature aging of nylon
12 increases the glass transition temperature
(Tg), making the samples stiffer due
to the cold crystallization during the
amorphous phase. There are two crystalline
phases of α and γ for nylon 12 for
both oriented and nonoriented samples.
The crystalline structure of nylon, in addition
to the orientation of crystals, contributes
to the mechanical properties of
polyamide. The γ form is more ductile
than the α form; however, a transformation
of the γ form to α will occur for nylon
6 if extruded samples are stretched.6
For Pebax, the soft-to-hard segment
ratio plays an important role in controlling
the physical and mechanical properties
of the balloon. The hard segments
have been reported to be (50 ± 20) nm
wide by (300 ± 150) nm long. The extrusion
process is responsible for hard segment
orientation along the axial direction.
During the balloon forming pro -
cess, the crystalline structure is broken
and rearranged into smaller crystals and
becomes more oriented. The γ crystals
transform to α crystals during stretching.8
Materials
Grilamid L25 (nylon 12) from EMS and
Pebax® 6333 SA01 MED (a thermoplastic
elastomer from Arkema) tubes were produced
using a one-inch extruder.
Balloons with dimensions of 6 × 100 mm
were produced using a Confluent Medical
Technologies balloon forming machine.
500
Shear Rate (1/s)
5000
Results
The viscosity data was extracted from
the data provided by the suppliers, and
the viscosity curves (viscosity versus shear
rate at 230 °C) are plotted in Figure 1.
The polymer viscosity is an important criterion
for the extrusion process because it
correlates with pressure applied at the die.
The pressure at the die determines the
consistency and stability of the melt flow
and is important because it maintains the
dimensions for the extruded tube.
The extrusion process for tubes is usually
designed for a shear rate range of 100-
1000 s-1. For this study, the range is
between 400 and 800 s-1 at the die, where
shear thinning behavior is more pronounced
for nylon than for Pebax.9
In the extrusion process, materials are
melted and then enter a water bath where
they crystallize and form a morphology.
The morphology and orientation within
each tube determines the physical and
mechanical properties. The Pebax as is a
copolymer of nylon 12 (PA) and polyether
(PEO). The morphology of the Pebax
PA-segment
Non-crystallized
PA-segment
crystallized
PEO-segment
crystallized
PEO-segment
Non-crystallized
0.1 mm
Fig. 2 - Optical microscopy of the Pebax balloon surface at magnification of 40X.
16
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Medical Design Briefs, November 2021
Viscosity (Pa.s)
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