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

Non-aesthetic finished products intended for a disposable market were normally produced with PP compound made up of a mix of PP and PE (PO), coming from the granulation of waste from the selection of municipal waste.

If we take into consideration plastic pallets or spacers for construction or boxes for fruit and vegetables, to give just a few examples, the mix between the two families of polymers allowed the production of compounds whose % of PP within the mixture varied from 30-40% to 60-70% depending on the expected recipe.

The melt 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 article, showed a good performance as regards compressive strength and a less excellent one as regards bending resistance.

Regarding the ability to receive the colors in the granule extrusion phase or during the molding phases, I can say that, for as regards the dark color scale, the family of colors allowed a fair choice and the aesthetic appearance of the finished product was acceptable considering the product from which it started.

Today the so-called PO, which identifies the polyolefin blend coming from separate waste collection, has taken on a different average composition compared to the past due to of the increased performance of municipal waste selection plants which tend to maximize the withdrawal from the PP/PE mix of polypropylene, HD and LD, as the offer of separate inputs on the market allows a contribution margin on waste that is significantly higher than the sale of the original mix.

This, today, means having to work with a PP/PE mix that is qualitatively less performing than in the past as the balance between the three families, PP, HD, and LDs that made up the OP in the past have changed.

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

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

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

• Deterioration of the quality of the input washing due to the increase in the volumes to be treated and the different % of polymers in the recipe

• Increased presence of bio plastics within the selected fraction which cause problems in the quality of the granule

• Increased use on the market of packaging made with mixed plastics which involves a greater % of multilayer materials, such as certain labels, which are difficult to coexist with traditional PO .

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

As regards production, action should be taken:

• on washing times

• on the size of the tanks

• on water management

• on the PO compound recipe for granulation

• on filtration

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

• on machine temperatures

• on the granule drying phase

• on the verification of 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

• Improved color homogeneity

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

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

• healthier workplaces during the plastic melting phases.

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