Considerations for the production and use of PO (PP/PE) granule

Non-aesthetic finished products, mainly intended for the market disposable, they were normally made with polypropylene compound, made up of a mixture of PP and PE (polypropylene + polyethylene) coming from the granulation of waste from separate collection.

If we consider plastic pallets or spacers for rebar or fruit and vegetable crates, to give just a few examples, the mix between the two families of polymersmade it possible to produce compounds whose percentage of PP in the mixture could vary from 30-40% to 60-70%, depending on the envisaged recipe.

The melting index at 230°/2.16 kg. it varied from 3 to 6 if the product had no added mineral fillers.

The characteristics of the granule produced and, consequently of the final product, express a good performance with regards to resistance to compression and a less excellent in terms of flexural strength.

As regards the ability to receive the colors in the extrusion phase of the granule or during the molding phases of the final product, yes may note that the medium-dark color range is the most appropriate, also by virtue of the fact that the base of the post-consumer semi-finished product to be transformed into granules is usually dark grey.

Today the PO, which identifies the polyolefin mixture coming from separate waste collection, has taken on a different composition compared to the past due to the greater performance of municipal waste separate collection systems, which tend to maximize the removal of the polypropylene fraction from the PP/PE mix.

This happens because the demand for polymers on the market tends to favor single compounds whether they are PP or HDPE or LDPE.

The production trend described above involves having to work on a PP/PE mix which is qualitatively less performing than in the past, because they have been the balance between the three families, PP, HD, and LD which constituted the PO in the past has been altered.

Furthermore, the increase in the production of both the waste to be processed and the demand for a granule from PP/PE compound has pushed some plastic waste treatment plants to accelerate the washing phase to recover productivity, decreasing the quality of the densified and ground material needed to produce the granule.

We can list some critical issues in the production of PO compounds:

• increase in LD% at the expense of HD in the polyolefin mix

• worsening of the quality of the incoming wash due to the increase in volumes to be treated and the different % of polymers in the recipe

• increase in the presence of bioplastics within the selected fraction which causes problems in the quality of the granule

• increase in the use on the market of packaging made with mixed plastics involving a greater percentage of multilayer materials, such as some labels, which difficult to coexist with traditional PO .

Compared to these changes in the basic composition of the PO and its processing , we will have to face problems to manage in the granule production phase and in the molding phase, in order to minimize the negative impacts of the quality of which the granule is composed.

 Regarding production, action should be taken:

• on washing and drying times of the semi-finished product

• on the size of the washing tanks

• on water management (cleaning and replacement)

• on the recipe of the PO compound for granulation

• on filtration

As regards the molding phase, the following should be done:

• on machine temperatures

• on the granule drying phase

• on verifying mold cooling

The technical intervention on these critical issues leads to the following improvements:

• greater resistance to bending of the final product

• improvement of aesthetic surfaces with reduction or disappearance of flashing on the finished product

• improvement of color homogeneity

• reduction of the bad odor of the granule and the finished product

• increase in the life of screws and cylinders in the granulation phase and in injection molds

• healthier workplaces during the plastic melting phases