Wednesday, 20th january 2021 | Technical information
Marco Arezio - Consulente materie plastiche - The Importance of Dehumidification of Recycled Plastic Polymers

How and why it is necessary to reduce the moisture of recycled polymers before their use.

As described in the article the" Absorption of 'humidity in polymers ” the presence of humidity on the external surface and inside the polymeric masses creates different types of problems for the characteristics of the raw material to be used.

Both hygroscopic and non-hygroscopic recycled plastic polymers are subject to the damaging effect of moisture, which can be absorbed in the phases processing, transport or storage, through the search for a balance with the environment in which they are found.

As we have seen, in non-hygroscopic recycled polymers, moisture is retained on the surface, while in hygroscopic ones it will also be found inside the plastic granule.

Humidity, whether on the surface or inside the granule, negatively affects the aesthetic and mechanical aspect of the final product and, therefore, to produce a plastic raw material that does not encounter these problems, it must be dehumidified before using it.

The percentage of residual moisture tolerated by the recycled raw material is usually indicated by the producers through the quality control of the outgoing goods and can vary according to the type of polymer taken into consideration and the type of product to be made.

It must be borne in mind that recycled plastics, after the packaging phase, spend more or less long times in transport and storage operations, times in which it is possible that the polymers take on new moisture.

For this reason it is always recommended, before using the granule, to carry out the dehumidification operation which, according to the polymer , may require variable times, reaching a final residual humidity expressed as follows for a polymer sampling:


 Air temperature: 80 °

 Treatment time: 2-3 hours

 Residual moisture: 200 ppm


 Air temperature: 90 °

 Treatment time: 1 hour

 Residual moisture: 100 ppm


 Air temperature: 90 °

 Treatment time: 1 hour

 Residual moisture: 200 ppm


 Air temperature: 70 °

 Treatment time: 1 hour

 Residual moisture: 200 ppm

The most common way to dehumidify recycled polymers is to bring the plastic material together with a current of hot air , as this has the ability to retain humidity and the higher the temperature, the greater the volume of humidity removed.

The air has the ability to retain water until it is saturated and this quantity varies according to the temperature increase. For example 1 Kg. Of air is saturated with:

• 20 ° - 14.7 grams of water

• 35 ° - 36.6 grams of water

• 50 ° - 82.6 grams of water

In polymer dehumidification cycles it is possible to use the air coming from the environment, defined as simple drying, only in situations of favorable temperature and humidity. While previously dried air, called dehumidified, can be used to achieve an important drying action even in unfavorable conditions.

We take into consideration that a dry granule, when placed in contact with the air, begins to absorb moisture , whose percentage will vary as the climatic conditions in which it is found vary, therefore the type of dehumidification intervention will vary according to these parameters.

In a drying process the hot air will hit the granule, which will be placed in a hopper, which will release moisture from its surface and its interior. which will migrate to the created airflow.

The main variables, therefore, during a drying process are:

• Type of polymer

• Starting moisture of the granule

• Process air dew point

• Required residual moisture

• Drying time

• Process air temperature

• Size of the grain

The whole drying process rotates around the humidity residual accepted , depending on the type of product to be produced and the production technique and, the lower the drying process, the higher the temperature used, without prejudice to the technical limits of each polymer on the degree of softening and emanation of harmful substances.

The air requirement for the dehumidification processes will be expressed in cubic meters of air for each Kg. of polymer to be dried, taking into account the quantity of granules handled in the hopper, the air temperature and the current consumption.

The size and shape of the granule also have an influence in the dehumidification process, as, as its size and surface area per unit of measurement increase ( cube, cylinder or sphere) increases the drying time.

Automatic translation. We apologize for any inaccuracies. Original article in Italian.

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