> I have an orthotic patient who is planning to walk to the North Pole next
> year.
> I need to know the most suitable material to use for his Arctic boot
> orthoses.
> The likely air temperature will be -40C most of the time. Will
> Polypropylene
> become
> brittle at these low temperatures?
Marie,
Polypropylene becomes brittle as it approaches its glass transition point.
If you freeze a piece of polypropylene and strike it or drop it, it can
shatter in a somewhat glass like manner (but less fragmented). I don't
remember the exact glass transition temperature but it is slightly above the
freezing point of water. The warmer the polypro, the more flexible it
becomes and the cooler it is, the more stiff and brittle it becomes. For
example, if a patient is working in a factory in a warm climate and wearing
a boot that retains heat, their orthosis will be more flexible. The more
force and the more heat, the greater the tendency there is for material
creep or shape change, but the less tendency there is for fracture. I had
one case of an individual who worked inside a large oven in a carbon plant
and his polypro orthoses actually had melted on the lateral, plantar edge
due to the heat. Fortunately he had Spenco like covers on them.
Freezing temperatures will make this patient's orthoses less flexible and
potentially more susceptible to breakage if the temperature within the boot
becomes to cold. If his boots are off and his orthoses become exposed to
the cold, then they could fracture when he first starts to walk with them.
I would advise him to make sure that the devices get warmed up inside his
boots before he bears any weight on them. Any type of insulating top cover
might allow the orthoses to become colder within the boot than they would if
they had no top cover or just a thin, vinyl or leather cover. In other
words, there will be better heat conduction between the feet and orthoses if
the orthoses don't have any type of insulating top cover on them. You might
want to bottom fill the entire plantar surface of his orthoses with EVA.
This would act as an insulating layer between the ground (snow or ice) and
the orthosis and would help retain body heat within the polypro shell. It
would also act to support the shell against the boot and help resist
breakage by reducing orthotic deflection, which might prevent breakage if
the devices do become very cold.
Respectfully,
Jeff Root
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