Thursday, 14th january 2021 | Technical information
Marco Arezio - Consulente materie plastiche - Post-Consumer Recycled LDPE: 60 Types of Odors Hinder Sale

The separate collection of plastic packaging is a modern achievement that allows, through recycling, the reuse of exhausted packaging with the double advantage of reducing the 'carbon footprint and the withdrawal of natural resources from the earth to create new products.

However, much still needs to be done in the recycling sector as the share of plastic that is collected and reused is still far less than what is produced every day.

This quantitative imbalance between what is recycled and what is produced again has many causes:

• Limited diffusion of separate waste collection in the world

• Difficulty in recycling many multilayer plastic packaging

• Low quality of recycled raw material

• Lack of a culture of recycling

In countries where separate collection has started and is working stably, the production of recycled raw material suffers from a quite negative judgment on the quality of the same, caused by factors that also, but not only, depend on the mechanical recycling chain.

This negative evaluation has a significant impact on the sales of the recycled raw material, relegating its use only to certain sectors of use, reducing hence the salable quantities and lowering the average price per ton, which in turn entails a low economic margin for companies that recycle.

Furthermore, the less recycled granule you sell , the less plastic waste you can recycle and the greater the problem of its disposal becomes, risking to landfill the precious raw material that could be reused.

Among the problems suffered by the recycled raw material, despite the enormous plant development in the sector, that of odor is among the most felt by customers who could use it to produce films, rigid packaging, materials for the construction sector, for the automotive sector, gardening, furniture and many other products.

To date, the perception of the smell of a plastic raw material coming from post-consumption is entrusted, in a completely empirical way, to a nasal sensation of those who produce it and those who use it, who evaluate both the type and intensity of odors present in the recycled plastic in an extremely subjective way.

Evaluation that you can then collide with the end customer who will buy the product and will give a further personal evaluation of the smell.

The human nose is certainly an excellent tool, but each person perceives odorous solicitations in a completely personal way, which is why in particular cases, groups of people are hired who together make assessments on the odors to be intercepted.

If we take for example the plastics recycling chain, starting from separate collection, we have seen that LDPE bags and flexible packaging that go to recycling, bring with them a very high number of chemicals that generate odors in the recycling chain.

The detection of the sources of odors has not been studied through empirical sensory methods, therefore through the human nose, but through a chemical investigation carried out by one laboratory machine which consists of a gas chromatograph with an ion mobility spectrometer.

This tool analyzed the chemical components, within a large sampling of recycled LDPE from separate waste collection, identifying 60 types of substances chemicals that generate odors.

The sampling analyzed came from the traditional mechanical recycling cycle in which the material is selected, shredded and washed with a stay in water of about 15 minutes .

The most common smells perceived by the human nose of this sampling were:

• Molds

• Urine

• Cheese

• Earth

• Fecal

• Soap

• Coffee

• Sweaty

• Pepper

These families of perceived odors are created by approximately 60 chemical compounds that are associated during the collection and processing of recycled plastic. Some critical points have been identified:

The separate waste collection bag which contain the domestic plastic packaging to be selected in which we find different types of polymers, may contain residues of substances such as detergents, food, oils, disinfectants, chemicals, creams and many others. This mixture of different chemical elements can bind to the surface of the plastic but, depending on the time of association, it could also penetrate inside.

The selection between the various plastics , through optical reader machines, creates a certain percentage of error which translates into the possibility of have quantities of mixed plastics within the selected fraction.

The washing phase of the plastic ground has the function of further dividing, by density, the plastics introduced and has the purpose of clean them of any product residues that the packages have contained or have come into contact with. With the exception of PET, the other polymers from separate collection are generally washed in cold water, a process that does not significantly affect the cleaning process in order to reduce odors.

The extrusion phase of the washed material, for the formation of the granule, could lead to a degradation of the raw material in which they are present fractions of polymers other than the main one which will then melt at different temperatures. This can cause the formation of chemical elements that will give rise to odors.

Intervening on these phases would lead to significant improvement in the quality of post-consumer polymers produced, not only through a reduction in the types and intensity of odors, but it would also improve their technical performance.

The analytical control of odors , through tools that detect their chemical genesis, can help not only in the certification phase of the odor level of the final raw material in an unequivocal and no longer empirical way, but would also give important support in the creation of recipes on the types of raw materials to be used during the recycling phases of plastic waste, on the identification of the best sources and on the results of production processes in the plant (selection, washing and extrusion).

Reducing odors and improving the quality of the post-consumer granule would lead to the opening of new markets in which the material could be used first recycled instead of virgin with an environmental, economic and industrial advantage.

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

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