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Home  > Forum  > Testing & measuring  > Water resistance vs. water uptake

Sunday, 25 October 2020

Water resistance vs. water uptake

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Thursday 26 February 1998 1:00:00 am

When determining the water sensitivity of a paint film is it preferable to test water
sensitivity or water uptake. Water uptake is calculated as percentage water absorbed by
the dried paint film. Water sensitivity is tested by applying a spot of water to a dried paint
film on a glass substrate.


What is really being tested in these test methods? Is it accepted practise to want to test
water sensitivity on glass?

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Monday 16 March 1998 1:00:00 am

The determination of the percentage water uptake provides information about the diffusion
behaviour of the film examined under the influence of water. Permeability to water vapour is often
very important, particularly for architectural coatings, which should at the same time have low
permeability to water (e.g. rain). As a result, in porous substrates that undergo a change in volume
when they take up water, e.g. wood, concrete and plaster, a state of equilibrium may be reached
with the moisture. Under certain marginal conditions, a permeation coefficient can be determined by
such experiments, this being determined by way of the rate of diffusion and the solubility.

 

The determination of the sensitivity to water by applying a water droplet causes the dried film to
swell. When this happens, fundamental differences occur between solvent-based coatings and
aqueous systems. In the case of waterborne coatings, water entrapment occurs caused, for
example, by a residual glycol content in the system, or by the addition of water molecules to
hydrophilic groups contained in the binder. Due to the effect of the water, the polymer tangles of the
binder swell to some extent. The coating film becomes somewhat softer and this effect is often
associated with a milky cloudiness of the film caused by a slightly altered refractive index. This
phenomenon is reversible when the water taken up is subsequently released. The swelling of
solvent-containing systems is usually irreversible and may lead to impaired adhesion. A high swelling
capacity is harmful for the coating, in contrast to which a complete lack of swelling ability on the part
of the dried coating leads to blistering and hence to detachment of the film.

 

It is common practice to carry out the water sensitivity test on glass substrates. The effect of the
substrate is thereby minimised, without any swelling as might occur with wood, for example, or
corrosion in the case of metallic surfaces.

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