Recycled Plastic Polymers: Drying or Dehumidification?

All plastic materials, virgin or recycled, in the form of granules or ground or densified, have a tendency to retain moisture, until they reach a balance with the external environment. This absorption capacity depends, as previously mentioned in another article, on the type of polymer, the air temperature and its humidity.

Based on the above considerations, the polymers can be divided into hygroscopic and non-hygroscopic.

In fact, in hygroscopic materials, water is absorbed inside the structure by chemically binding with it, while in non-hygroscopic polymers the moisture remains outside the mass, subsequently interfering in the manufacturing process.

The plastic polymers, expressed in the forms of granules, ground, densified or powders, are sent for their transformation according to the product to be made and the type of process established.

Whether the materials are hygroscopic or non-hygroscopic, the presence of humidity during the melting phase of the polymer mass creates considerable problems as water can become vapor, creating streaks, surface bubbles, irregular thermal shrinkage, structural tensions, deformations or breaks. .

Humidity is one of the main causes of imperfections or defects on plastic products made but, at the same time, it is a problem largely neglected or underestimated by operators who mainly use recycled plastics.

If we want to list some obvious defects caused by the presence of moisture in polymers, we can mention:

• Opaque appearance of the product

• Brown streaks

• Silver streaks

• Weak welding lines

• Incomplete pieces

• Burrs

• Bubbles

• Blowholes

• Reduction of mechanical properties

• Deformations of the element

• Degradation of the polymer

• Irregular aging

• Irregular withdrawals

To overcome these drawbacks, it is a good idea to dry the material before using it with air jets.

In this case we can list two intervention systems, similar to each other, but with different results, which are represented by drying and dehumidification.

For drying we can consider a process of insufflation of air sucked into the environment and introduced into a hopper in which the plastic material to be treated is located, for a certain time at a set temperature.

This system depends greatly on the weather conditions and the degree of humidity in the air and is recommended only for non-hygroscopic materials.

For hygroscopic materials, such as polyolefins, (PP, HDPE, LDPE, PP / PE just to name a few), the forced air drying system seen above is not sufficient, as the intrinsic moisture content in the polymer, makes the process of little effectiveness.

In this case it is advisable to dry the polymers through dehumidification, which involves the insufflation inside the hopper, no longer of air in variable environmental conditions, but of air dehumidified through a dryer at a set temperature.

The hopper must be insulated to reduce the dispersion of process heat and the material will be in motion, so that during the transit phase inside the hopper it can be hit with jets of hot and dehumidified air.

The dryer will produce a constant flow of hot and dry air which will have the ability to significantly reduce the internal humidity of the hygroscopic polymers.

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