- The mechanical recycling of plastic waste
- How contamination of plastic materials occurs
- What possible elements of contamination during mechanical recycling
- Consequence of contamination during mechanical recycling on finished products
A multitude of pollutants can affect the quality of plastic recycling.
Mechanical recycling is an activity complex as it deals with a high variety of different types of plastics, and with them, products that can pollute the recycling processes reducing their quality are to be managed.
The factors and products that can degenerate and compromise recycling operations are many, diversified from each other and which must be paid close attention in order not to produce mediocre materials.
The mechanical recycling process must be managed in such a way as to be able to produce a recycled raw material that is the best possible in order to be, when possible, a valid alternative to virgin polymers.
The higher the quality of the recycled material, the greater the replacement of the raw material resulting from oil refining.
Therefore, to compete with virgin resin, the technical processability requirements of the various recycled plastics require that they have a high degree of purity, free from contamination problems , which is even more critical in the applications of containers that will be in contact with food.
The phenomena and materials that can pollute the processes are many and of different types, so let's go step by step to illustrate the main ones.
CONTAMINATION FROM OTHER PLASTICS
Today it is common to find different types of resins in the same application. For example, in the market for cleaning and personal hygiene products, plastic containers have components made of various materials, such as polypropylene (PP), high density polyethylene (HDPE), PVC and PET, which generate great problems during the recycling phases.
Among the main problems encountered there is the different chemical structure between the materials, as well as the sliding behavior of the molten plastic, very different from each other, which lead to highlight the heterogeneity and incompatibility between the different materials.
PET AND PVC
One of their applications is the production of shampoo containers, with the common feature that both are transparent, so they often contaminate each other.
This can present the difficulty of being separated with conventional flotation methods, due to the very similar density between the two (1.30-1.35 gr / cm3).
But if PET is contaminated by PVC, even at low concentrations, PVC degrades at the processing temperature of PET (around 260-280 ° C) , forming acids that break up the chemical-physical structure of PET and generate a progressive chemical change, with a friable behavior of PET.
Minimum quantities of 100 ppm of PVC cause discoloration of the PET during the drying phase and the generation of black points during extrusion.
PET and HDPE
Due to the high consumption volumes of both in their packaging application, the likelihood of mixing increases.
These two plastics are incompatible in the molten state, remaining independent once solidified. Portions of contaminating HDPE are visually imperfect and can cause mechanical brittleness and pollution of the PE.
Furthermore, there are problems related to melting temperatures, as PET does not melt at HDPE temperatures, risking to obstruct the channels and the nozzle of injection.
PP and HDPE
Polypropylene is often used in the production of caps and closures for HDPE containers, with applications in detergents, bleaches and shampoos.
PP and HDPE are inseparable with physical methods, due to their very similar specific gravity. During the melting of both plastics there is a problem of incompatibility, which is reflected both in the extruded HDPE products and in the containers obtained by blowing, which show deformations.
Furthermore, an accentuated presence of PP in a mixture with a prevalence of HDPE used to create bottles, creates a brittleness on the sealing line of the bottle itself. This, in most cases, when the bottle is filled and placed on the pallets, perhaps with other pallets of superimposed materials, a crack on the welding point with the leakage of the contents.
CONTAMINATION DURING PROCESSING
Metal contamination
During the processing of plastics, metal contamination can be caused by the presence of metal fragments or burrs, which could be generated by the malfunctioning of equipment, such as aluminum extruders, mills or adapters.
Their wear caused by continuous use can lead to the loss of small fragments that mix with the plastic materials to be used for blow molding, molding or extrusion.
These small fragments can scratch the extruder barrel or block the nozzles in injection molding machines, as well as produce extruded or injection molded items with defects.
Pollution from degraded polymers
Frequently, during the processing of both virgin resin and recycled HDPE, black dots or streaks may be present on the surface, as a manifestation of a partially oxidized material or degraded that has been charred, becoming trapped in rough surfaces or cavities.
These impurities can be present in the cylinder and on the surface of the screw or in the heads of the extrusion-blow molding systems, for a prolonged time, with consequent generation of defects in the final product.
Likewise, contaminants in plastics such as dirty grinds, foreign materials and different colors, as well as those materials with a lower melting temperature, are also causes of blackheads. Frequently, such contamination can also appear yellow, brown or amber, depending on the extent of the degradation.
Gel contamination
The gels (commonly called fisheyes), in the shape of an elongated ellipse, are evidence of quality problems in both the transparent and colored film, visible with a thickness less than 130 microns.
Gels are mainly visual defects, which reflect and transmit light differently from the rest of the material, caused by several reasons:
Moisture contamination
Water or humidity are contaminants that induce the breakdown of the hydrolytic chain, so the materials must be strictly dry before being processed.
In the case of a hygroscopic resin, such as PET, the recycled flakes must be dried at temperatures of 160-180 ° C to lower the moisture content to 50 ppm, necessary for injection-blow molding processing suitable for PET preforms and containers, in order to avoid a reduction of the molecular weight.
In any case, even for less noble materials or applications such as detergents or cosmetics or food products, it is a good idea to previously dry any recycled plastic material which must be used as a raw material, avoiding in any case qualitative reductions of the finished products.
Due to the great diversity of polluting sources, the range of effects attributed to the pollution problem can be:
Therefore, converters working with recycled materials must set increasingly stringent limits on the contamination of their incoming materials and for their products, carefully monitoring all input to be processed.
From the above it can be concluded that the two main factors that amplify the effect of pollutants are, the heterogeneity and incompatibility of the chemical nature of the materials recycled plastics, which give negative qualitative performances, resulting in a low added value of the reworked product.
Three important elements to consider when monitoring the quality of recycled materials with respect to the presence of contamination:
Automatic translation. We apologize for any inaccuracies. Original article in Italian.
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