How to recycle a multi-purpose material from the packaging, construction and food sectors

The EPS or more commonly called expanded polystyrene, is obtained from polystyrene by means of a polymerization process which occurs through a chemical reaction of styrene.

In the polymerization phase, expanding additives such as pentane are added to the polystyrene, favoring the birth of EPS, which comes in small balls with a glassy appearance and of different granulometry.

By then bringing the balls to a temperature of about 90 °C through the use of steam, the gas contained in them triggers their volumetric expansion equal to 20 - 50 times the volume of the same.

After the expansive phase, we move on to the sintering of the balls, which consists, again through the use of steam at 110 - 120 ° C, in their ability to agglomerate with each other, with the possibility of creating monolithic blocks.

The EPS thus produced is used in many sectors, such as insulation in the building industry, for the protection of objects during the packaging, and in the food sector for the production of containers of various types.

This very large multi-sector use, leads to the creation of a large amount of waste which must be properly managed, sending it for recycling, as EPS can be a circular product.

How to recycle EPS with the mechanical system

The first criticality encountered speaking of recycling EPS is its volume in relation to its weight , two elements that determine costs for the storage of waste and for its transport. In fact it is a very light material, about 15-25 Kg. /m3 and very voluminous.

For these reasons, the first phase of EPS recycling lies in its volume reduction, through shredding mechanical waste, in order to obtain irregular pieces with dimensions from 2 to 10 cm.

After the crushing phase, we move on to that of grinding, which consists in using hammer mills or knife mills with shafts counter-rotating, which have the ability to reduce the EPS to the desired size.

As an alternative to grinding, the crushed EPS scarts can be compacted with specific presses, so as to monolithically reduce their volume , bringing the specific weight between 300 and 800 Kg/m3.

If you opt for grinding waste, you get a raw material that can be used for the extrusion stages, then creating a granular crystal polymer with a high fluidity, around 14-18, usable for injection moulding.

To extrude the EPS it is necessary to have a forced feeding system as the material is very light, it is also advisable to have a degassing to remove gases present within the cellular structure.

If the ground or compacted waste comes from separate collection, therefore post-consumption, it is advisable to insert a magnet on the conveyor belt that can intercept any metallic elements present in the ground.

It is also always advisable to sift the ground in order to eliminate any impurities consisting of wood, paper, non-ferrous elements which are not intercepted by the magnets.

There are other non-mechanical recycling systems for EPS which can be listed below:

• Thermomechanical molecular cracking system

• Microwave and infrared system that generates a controlled pyrolytic process

• Liquid dissolution system that allows the recovery of uncontaminated EPS

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