LDPE Recycled from Post-Consumer: 60 Types of Odors Obstruct Sales

The separate collection of plastic packaging, especially for those in LDPE , is a modern achievement which allows, through recycling, the reuse of exhausted packaging with the dual 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 lower than that which 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 recycling culture

In countries where separate waste collection has started and functions stably, the production of recycled raw material suffers from a fairly negative judgment on its quality, caused by factors that also depend, but not only, 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 some sectors of use, thus reducing the salable quantities and lowering the average price per ton, which in turn leads to a low economic margin for companies that recycle.

Furthermore, the less recycled granules sold , the less plastic waste that can be recycled and the greater the problem of its disposal becomes, risking the precious raw material that could be reused ending up in landfill.

Among the problems that recycled raw materials suffer from, 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 automotive, gardening, furniture and many other products.

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

An evaluation which can then clash with the end customer who will buy the product created and give a further personal evaluation of the smell.

The human nose is certainly an excellent tool but each person perceives odorous stimuli in a completely personal way, and this is why, in particular cases, groups of people are hired to together evaluate the odors to be intercepted.

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

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

This tool analyzed the chemical components within a large sampling of recycled LDPE coming from separate waste collection, identifying 60 types of chemical substances 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 approximately 15 minutes.

The most common odors perceived by the human nose in this sampling were:

• Mold

• Urine

• Cheese

• Earth

• Fecal

• Soap

• Coffee

• Sweaty

• Pepper

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

 The separate waste collection bag containing 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, chemical products, creams and many others. This mixture of different chemical elements can bind to the surface of the plastic but, depending on the association time, it could also penetrate inside it.

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

 The washing phase of the ground plastic has the function of further dividing, by density, the plastics introduced and has the aim of cleaning them from the residues of products that the packaging has contained or has come into contact with. With the exception of PET, the other polymers coming from separate waste 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 there are fractions of polymers other than the main one which will therefore 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 a significant improvement in the quality of the post-consumer polymers produced, not only through a reduction in the types and intensity of odors, but would also improve their technical performance.

The analytical control of odors , through tools that detect their chemical origins, can help not only in the certification phase of the odorous level of the final raw material in an unequivocal and no longer empirical way, but would also provide important support in the recipe creation phase 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 the production processes in the plant (selection, washing and extrusion).

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

Category: news - technical - plastic - recycling - LDPE - post-consumer - odors

See more information on LDPE recycling