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

As described in the article " Moisture absorption in polymers ", the presence of humidity on the external surface and inside the polymeric masses creates different types of problems with the characteristics of the raw material to be used.

Both hygroscopic and non-hygroscopic recycled plastic polymers are subject to the damaging effect of humidity, which can be absorbed during processing, transportation or storage, through seeking balance with the environment in which they are found.

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

Humidity, whether found on the surface or inside the granule, negatively affects the aesthetic and mechanical appearance of the final product and, therefore, to produce a plastic raw material that does not encounter these problems, it is necessary to dehumidify it first to use it.

The percentage of residual humidity tolerated by the recycled raw material is usually indicated by producers through the quality control of outgoing goods and can vary depending on the type of polymer taken into consideration and the type of product intended to be created.

It must be taken into consideration that recycled plastic materials, after the packaging phase, spend more or less long times in transport and storage operations, times in which it is possible for the polymers to take on new humidity.

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

• ABS

 Air temperature: 80°

 Treatment time: 2-3 hours

 Residual humidity: 200 ppm

• PE

 Air temperature: 90°

 Treatment time: 1 hour

 Residual humidity: 100 ppm

• PP

 Air temperature: 90°

 Treatment time: 1 hour

 Residual humidity: 200 ppm

• PVC

 Air temperature: 70°

 Treatment time: 1 hour

 Residual humidity: 200 ppm

The most common system for dehumidifying recycled polymers consists in bringing the plastic material together with a stream 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 its saturation and this quantity varies depending on the increase in temperature. For example, 1 kg of air is made saturated at:

• 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 air from the environment, defined as simple drying, only in favorable temperature and humidity situations. While previously dried air, called dehumidified, can be used to achieve a significant drying action even in unfavorable conditions.

We keep in mind that a dry granule, if put in contact with the air, begins to absorb humidity , the percentage of which will vary according to the climatic conditions in which it is found, therefore the type of dehumidification intervention will vary depending on of these parameters.

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

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

• Type of polymer

• Starting humidity of the granule

• Process air dew point

• Residual humidity required

• Drying time

• Process air temperature

• Granule size

The entire drying process revolves around the accepted residual humidity , depending on the type of product to be produced and the production technique and, the shorter the drying process, the higher the temperature used, without prejudice to the technical limits of each polymer on the degree of softening and release 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 granule moved in the hopper, the air temperature and power consumption.

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

Category: news - technology - plastic - recycling - dehumidification

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