::: Intelligence in Rheometry
Tips and Tricks from Joe Flow Rheology and humidity: Time to get wet!
Why is the ambient humidity important for the rheology of a substance?
In this latest issue, everything revolves around the subject "humidity".
Along with the temperature and the water content of a sample,
the relative humidity in the measuring environment is also an
important factor in rheological measurements.
Humidity can play a particularly decisive role in the measurement
of food. For example, the "crunchiness", consistency and mouthfeel of food are considerably influenced by its water content,
which is dependent on the ambient temperature and humidity.
For this reason, checks on these parameters are very important
for the production and storage of food.
of humidity on the stickiness of chewing gum. Fig. 2 shows that
the surface of the chewing gum is not sticky at a relative humidity
of 10 %. If the relative humidity is increased to 80 % it is possible
to detect adhesive forces between the chewing gum surface
and the upper measuring surface. With increasing temperature
the measurements show that the adhesive forces are effective
over a larger distance between the upper and lower surface.
This indicates that at higher temperatures a larger amount of the
water-soluble components of the chewing gum (e.g. sugar, sugar
substitutes or sweeteners) is dissolved out of the chewing gum
surface.
The relative humidity of the surroundings can also play a role
when using adhesives. The example of wallpaper paste shows
clearly how dependent the material properties are on the relative
humidity. Whereas a low relative humidity in the room to be wallpapered can lead to the thin film of paste drying out too quickly,
with high relative humidity you have to wait patiently for the wallpaper paste to dry and the wallpaper to adhere properly.
Is it possible to control the ambient
temperature and humidity during rheological
measurements?
To measure rheological properties under pre-defined temperature
and humidity, you can use a convection oven which is modified
so that an external humidity generator adjusts the relative
humidity along with the temperature.
Example: Chewing gum
As shown in Fig. 1, at a low relative humidity of 30 %, chewing
gum shows stable rheological behavior over 60 minutes with
constant storage modulus (G´) and loss modulus (G´´).
An increase in temperature from 25 °C to 37 °C only results in a
slight reduction of the moduli. Therefore, the mechanical
properties of chewing gum at a relative humidity of 30 % are
time-independent at both 25 °C and 37 °C.
Fig. 2: Adhesive force between chewing gum surface and measuring
surface as a function of the distance between chewing gum and surface,
measured at different relative humidity and ambient temperature values.
Example: Sealant
Particularly important in the sealants industry are onecomponent silicone sealants, which cure at room temperature
(RTV silicones). These silicone sealants cure via a reaction with
the water in the ambient air. The curing is therefore dependent
on the relative humidity. To seal the joints in sanitary areas, for
example, acetoxy silicone sealants are widely used which react
with water to cure and give off acetic acid. The complete curing
of these sealants therefore requires sufficient available water.
How dependent the curing of such a sealant is on the relative
humidity is shown in Fig. 3. With increasing relative humidity the
sealant's G´ and G´´ moduli increase more rapidly, and reaction
times become shorter until the elastic properties dominate (G´ >
G´´). Thus, the curing speed increases with relative humidity and
the time required for curing depends on the relative humidity.
Fig. 1: G´ and G´´ of chewing gum at different relative humidity and
ambient temperature values as a function of the time.
When the relative humidity increases to 80 %, a continuous
decrease in the moduli can be observed. Here the speed of the
decrease of G´ and G´´ is influenced by the temperature, which
indicates that the water absorption of the sample increases with
increasing temperature.
Using tack tests it is also possible to characterize the influence
Fig. 3: Curing of silicone sealants at different relative humidity values as a
function of time.
Summary
Rheological parameters can be used as a measure to evaluate
sensory attributes such as the mouth-feel, crunchiness and
consistency of food. All of these properties are considerably
influenced by the moisture content of the sample, which is
dependent on the ambient temperature and relative humidity.
In the processing of adhesives and sealants there are also many
examples which show the importance of humidity in these
applications.
The above-mentioned examples illustrate how significantly the
environmental conditions can affect the properties of different
samples. At the same time, it is clear that characterizing these
samples using rheological investigations is a way of evaluating
the quality, storage and processing properties of these materials
as a function of the environmental conditions.