All raw materials for producing rigid polyurethane
foams should be protected from moisture during
storage. Polyols are generally hygroscopic and
absorb water from the air, so the water content can
increase by a few percent in a short period of time
and can lead to shifts in characteristic values (see
Section 3.1), increased CO2-generation and therefore
lower foam density and reduced foam strength.
Protection against moisture is particularly important
in the storage of polyisocyanates. The polyurea
which forms during reaction with atmospheric moisture
is separated off as a solid so encrustations
occur in the storage container and particles are
deposited which can block the lines and nozzles.
For this reason, air entering the storage container
when raw materials are removed should be dry.
Polyurethane raw materials can only be stored for a
limited time. While most of the basic components
can be stored unmixed in sealed containers for
months and years, mixtures (formulations) are likely
to undergo a chemical change in a matter of weeks.
This is shown by discoloration, increased viscosity,
and a reduction in reactivity and foam stability.
High temperatures can sharply reduce the storage
stability of the raw materials. Care must be taken in
this respect particularly during long periods of
transportation and heating caused by solar radiation.
As a rule, the raw materials available on the
market are adequately stable over fairly short periods
of time (days to weeks) at temperatures up to
50 °C. This has to be the case as the temperature
of the raw material can often be 50 °C, for example
with tanker deliveries. A processing temperature of
20 to 25 °C is recommended in most cases, which
means that appropriate cooling, for example by
heat exchangers, should be arranged on receipt of
the raw materials. Temperature control of the raw
materials during prolonged storage is also appropriate.
High temperatures can impair the quality,