Densification of polypropylene: the reduction of product humidity must take into account packaging and storage problems

The densification of polypropylene from post consuption, is an operation that allows to use a waste consisting mainly of food packaging, in which the average component of polypropylene above 85 allows the production of many non-aesthetic finished products.

The separate collection that is carried out in our homes involves the separation of mixed plastics from paper, glass, metals and paper.

Mixed plastics are started at the selection plants of materials that are tasked with separating the various types of plastics present in the collected bags. The predominant quantities are represented by HDPE, PET, Polypropylene, mixed plastics and polystyrene.

The separation takes place by loading the contents of the bags on the conveyor belts, which is initiated to the separation through optical-read machines, allow a family division of homogeneous plastics.

One of these families is polypropylene food packaging that is separated from other materials and started at the recycling stage.

These operations involve the shredding of the material and the subsequent washing,through the centrifuge and decanting in the tub of polypropylene, with the purpose of separating by mechanical action and by different pollutants or plastics not intercepted by optical readers.

The next step is the densification of the material, which aims to drastically reduce the amount of water present in the ground polypropylene, with the consequence of allowing the extrusion of the material but also a reduction in the overall weight per cubic meter.

The densification of polypropylene takes place through the clutch on the cochlea or counter-rating screws,which perform an action of plasticization and drying, if there is no dedicated plant for this purpose, and the subsequent phase of briquette of the material.

If the densified material is not automatically funneled into an extruder to produce grain, but is bagged in Big Bags for later use or because the densified will be sold as well, it is important to follow a few steps:

• If the material were sold for compounds, the size of the piece should be, preferably, between 10 and 12 mm., thus allowing a simpler mixing action with other polypropyls in the form of densified or ground. This could result in a re-windowing of the densified to reduce its size.

• You have to be very careful to measure the temperature of the densified before bagging it, as it is easy that an operation to fill the Big Bags with hot material, can induce phenomena of internal self-combustion of the material. This phenomenon can happen because the heart of the material in the Big Bag hardly cools, indeed, it tends to accumulate heat risking to enter the phase of self-combustion. To avoid this phenomenon, it is important that the densifier exit provides air cooling of the material and, if possible, avoid direct bagging until the material has cooled evenly.

• The right average humidity percentage of each Big Bag should be measured with cold and seemingly dry material, if the destination of polypropylene is to be sold to make compounds. It is not only a matter of weight, in fact a higher percentage of humidity corresponds to a greater weight of the load not liked by the customer, but, much more important, a higher humidity could result in problems during extrusion and possible aesthetic defects of the finished products made.

• One last note concerns the percentage of polypropylene that your densified, since the selection of the material upstream in the waste selection centers, will determine its value. If the customer has special needs in terms of the minimum percentages of polypropylene in the densifier that he will purchase, it is important to regularly check this value through the SDC’s test of the various batches of selected waste received.

Post-consumption polypropylene densified it can be, as we have seen, used for the production of granules with the product such that, for the realization of compounds in granules, mixing post-industrial waste, and finally can also be used for direct molding, for non-aesthetic products, through molds with injection points suitable for the size of the scales.