Analysis of incoming waste flows, selection, mixtures and thermal impact on final performance

The polypropylene is a family of polymers widely used for the production of articles in the most varied applications, as it combines resistance, ease of coloring and ease of use through different thermal processes such as injection, blow moulding, extrusion and thermoforming.

It is also a polymer that lends itself easily to compounding operations, through which additives can be mixed that induce changes to the structure, thus increasing the final performance of the product, making it more rigid or more flexible or more performing to compression, traction or shear stresses.

By virtue of its ductility and ease of production, the waste that is collected, to then be recycled mechanically, presents a heterogeneity of compounds that it is important to know, in order to prevent any qualitative errors on the secondary raw material that will be produced.

First of all, let's see how a normal mechanical recycling process of polypropylene waste takes place.

The polypropylene waste which is sent for recycling can take the form of rigid waste, for example crates fruit and vegetables, pallets, bumpers, bottles, or in the form of flexible waste, such as bags, Big Bags, sheets and films in the packaging sector.

All of this waste must first be mechanically separated, so as to create an input of waste with a rigid consistency and one with flexible consistency, so as to initiate them to different manufacturing processes.

After making an initial summary selection by macro-category, an attempt is made to separate the waste based on the initial product type, for example the bottles will separate from the buckets, the pallets from the pharmaceutical products, the fruit and vegetable crates from the tubes and so on.

Even with regard to flexible waste, an attempt will be made to separate the different types of sheets, based on the type of packaging for which they were intended, the processes for which they are been subjected to and the products with which they have been in contact.

This second selection is aimed at creating a possible homogeneity among the families of waste selected, in order to make their recycling as simple and qualitative as possible.

The further selected waste will then be washed, with settling and mechanical processes, in order to minimize the contamination present on the flakes, which could compromise its mechanical qualities and the aesthetic aspect.

After the washing process the rigid waste will be dried, while the flexible one will pass through the densifier to agglomerate the light parts, so that it is more workable in the extrusion.

Later this semi-finished product will be used as feed for the extruders in the preparation of recipes for new recycled granules, recreating the virtuous circle of the economy circular.

Briefly described the mechanical recycling process of polypropylene let's see what the most common problems can be and how to solve them.

The first thing to verify, in the mechanical recycling of polypropylene, is the technical knowledge of the differences, in the incoming waste streams, on the various molecular structures of the polymer.

In fact, the molecular weight, its crystallinity and its origin, between homopimer and copolymer, can influence the physical-mechanical qualities of the final product.

For example, containers or buckets for storing lubricants or paints are commonly made from block copolymer, which has a good balance between impact properties and stiffness.

Other polypropylene containers, such as bottles for hygiene and cleaning products or containers for dairy products, can also be made of random copolymer or homopolymer, therefore , the melting temperature difference varies between homopolymers (160-165°C) and copolymer polypropylene (135-159°C).

If these different origins and characteristics of the material were combined during mechanical recycling, it would result in a recycled granule of lower quality than the same product through a selection of more careful refusal.

The second thing to keep in mind is the possible contamination of polypropylene with other common plastics such as PE.

Among the many polymers, HDPE is the one that most often creates a possible contamination, if not previously separated in the incoming waste stream, in fact PP and HDPE, both of the polyolefin family, have a great similarity in their structure and have a density lower than 1, so they float in the washing water.

Furthermore, during the extrusion phases, PP and HDPE have different melting temperatures, between 160 and 170 °C for polypropylene and 130 ° C for HDPE, leading the latter to possible thermal degradation, which manifests itself in the formation of black particles that can be imprinted on the final products, with aesthetic deficiencies.

It is therefore advisable to limit the presence of HDPE below the 5% threshold, to reduce the negative impact on products made with recycled raw materials.

The third thing to consider, as we mentioned before, is the fact that PP lends itself easily to compounding operations, therefore the waste could contain fillers such as talc, calcium carbonate, glass fibre, metals or particularly aggressive colours.

Knowing that the various compound additives have different physical and mechanical behaviors, both during the transformation of the raw material and from an aesthetic and performance point of view on the finished product , it is important to proceed with the analysis of the contents, with laboratory tests, to understand how to use the waste with additives during the recycling phases.

The fourth thing to keep in mind is the degradation of the polymer, not just the one we mentioned concerning the thermal phase- extrusive to produce the granule, but also what we can define as photo-oxidative, whereby a plastic product exposed to light and heat generates a decline in its performance due to the weakening and modification of its chains.

In fact, oxidative degradation can be generated not only by thermal degradation, induced by solar radiation, but also by high mechanical stresses. When the polymer degrades, the oxygen present in the plastic material disintegrates the molecules and creates free radicals, which react rapidly in a chain with the oxygen.

It can therefore be remembered that polypropylene, in the context of mechanical recycling, is a polymer with a marked property of thermal degradation compared to other types of plastic, both during its life cycle (mainly by photo-oxidation), both during processing and recycling.

Heat, mechanical stress and ultraviolet radiation strongly modify the structure and morphology and, consequently, the characteristics and properties of recycled polypropylene.

Both elongation and impact strength are the properties most affected by the phenomenon of degradation, in addition to discoloration failures and other aesthetic damage that must be taken into account into account.

As a last aspect, among many others that can be illustrated, I would mention the odor problem that can accompany polypropylene waste from post consumption.

The smell in the waste input can form due to the mixing of plastics that have contained aggressive liquids or solids, or caused by the biological fermentation of food waste or by the presence of chemical compounds, such as surfactants, which can impregnate the polypropylene.

The washing phases, even very accurate ones, can generally reduce the odorous impact but are unlikely to solve the problem.

Since the presence of odor in post-consumer recycled plastics is unpleasant for the final products, and since there is currently no definitive removal system, it makes necessary to have to separate the incoming waste streams, through an analytical verification, among those that are contaminated by unpleasant chemical compounds.

This operation is carried out quickly, precisely and analytically, with a test on the incoming waste sample, using ion mobility gas chromatography, which consists in inserting a small fragment of plastic waste into a test tube, then loading it into the laboratory machine which will give us the curve of the odorous chemical compounds present in the sampled waste.

By doing so, without a shadow of a doubt, we will have full knowledge of which smells and of which intensity our granule that we are going to produce will be composed.

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