Water absorption is thought of in two ways. Total absorption and rate.

The total absorption is in wt% and measures the change in weight after
immersion in DI water for 24 hours. The sample is conditioned at
105-110C for multiples 1 hour until the "dry" weight stabilizes prior to
immersion.

% absorption =  (dry weight - weight after immersion) / dry weight X
100%

The absorption rate is a measure of the change in % absorbed over time.
While this could be measured by a series of timed weightings (as above)
it's more typically measured as permeability through a thin layer of
material.

Permeability = grams / square meter / mil thickness /day

Be mindful of the subtle distinction. Absorption rate follows an
exponential decay as the total absorption limit for a material is
approached while the permeability is a measure of the materials inherent
flow through rate at steady state.

The permeability of polyimide film (Kapton) is 54 g/sq m/mil/day and the
total absorption is 2.8 wt%. The absorption rate changes rapidly over
time with a large percentage occurring in the first 5 min. and nearly
reaching completion in 30 min to an hour depending on the film type and
thickness. Another polyimide film (Upilex) has only 1.7 g/sq m/mil/day
permeability and 1.2 wt% absorption so you must know what specific
materials you're considering.

Andy Magee
Flex Guru
[log in to unmask]

Yuan Li wrote,

What is the meaning of Water absorption rate (wt%)? If it is 2.4%, how
do I
interpret it? Thanks,

Yuan Li

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