Moisture content of powders

1. Moisture content definition

Most powders have the capacity to pick-up water and therefore are containing a certain amount of water. This water can come from the manufacturing process, for example the amount of water remaining after a drying step, or can migrate to the material later, for example by submitting the powder to a humid air. The mass of water contained in a mass of material is called the moisture content or water content.

The moisture content should not be confused with the water activity.

2. Calculation formula of moisture content

How to calculate the moisture content ? What is the water content equation ?

The ratio in between the water contained in the material and the material weight gives the moisture content :

u = m_w/m_s

With :

u = moisture content (kg of water / kg of material)
m_w = mass of water content in the solid weight considered (kg of water)
m_s = mass of dry solid (kg of material)

It should however be noted that the convention can be changed in certain applications : instead of the mass of dry solid, the mass of wet solid can be used as denominator.

3. How to measure the water content

The most widespread method to measure the water content of a powder sample is to use a oven that will heat up and then dry the sample. The sample is precisely measured before and after drying (in defined conditions) so that the difference gives the amount of water evaporated, thus the quantity of water that was in the sample.

m_w = m_{s_wet} - m_{s_dry}

Equation 2 : mass of water in a sample

With

m_w = mass of water content in the solid weight considered (kg of water)
m_{s_wet} = mass of wet solid (kg of material)
m_{s_dry} = mass of dry solid (kg of material)

One drawback of this method is that it will not be very accurate if the material is also containing some volatile compounds that will also be removed during heating.

In this case, the Karl-Fischer titration can be more adapted as it allows to analyze only water.

4. Interpretation and relation to flowability of moisture content

The "humidity of a powder", or rather its moisture content, is often mentioned as a usual root cause of bad flowability of powder. Indeed, if the moisture content is too high, a solid will be prone to caking, clumping, generally bad flowability. The water content is then high enough to create small water bridges in between particles and modify their flow behavior.

It is possible with a powder rheometer to measure the relation in between the "flowability" and the water activity. Powders of different water content are measured, if the energy to make them flow changes radically at a certain moisture content, then the powder is sensitive.

One important effect of water on powders is that it can play the role of a plasticizer, thus modifying the glass transition temperature Tg if the powder is made of an amorphous material (polymer, in plastic industries, but also in food industries). The glass transition temperature varies with the moisture content which means that, when submitted to a temperature T, the water content can bring the Tg below T and thus makes the material sticky changing radically its flow behavior.