Mechanical Recycling

Mechanical recycling can be described as the processing of plastic waste into secondary raw materials or products without any significant change in the chemical structure of the material. The mechanical recycling version is usually characterized by the following sequences of individual steps. They are carried out as part of the recyclate preparation and production process, some of which may be carried out simultaneously [24]:

Collection → identification → crushing →-washing

→ drying → sorting → agglomeration → extrusion

or preparation of mixtures → granulation

In this form of recycling, it is necessary to divide plastics into three groups: – Soft plastics (LDPE, LLDPE, HDPE, PP, PET, PVC,

etc.); – Hard plastics (ABS - acrylic-butadiene-styrene, PS,

PA, etc.); – Reinforced plastics (epoxy EP with reinforcing filler,

PP with reinforcing filler, polyurethane PU with

reinforcing filler, etc.).

Raw Material - Chemical Recycling

The principle of this form of recycling is the conversion to monomer or the production of new raw materials, which is carried out by changing the chemical structure of plastic waste by cracking, gasification or depolymerisation without energy recovery and burning [25].

Various processes known from the petrochemical industry are used to convert some plastics into basic monomer chemical components or carbohydrate fractions. This allows chemicals to be used either as a raw material for polymerization or in other chemical processes, for example for catalytic depolymerisation. [26]. This is a unique technology based on the catalytic transformation of the material without the presence of air. Under the influence of high temperature and atmospheric pressure, plastic material based on PE and PP, for example, is decomposed based on polymer degradation. It means that the result is a completely new product, synthetic oil.

Biological - Organic Recycling

This method of recycling is based on aerobic treatment, i.e. composting or on anaerobic treatment, i.e. digestion of biodegradable plastic waste. Biological recycling must take place under controlled conditions using microorganisms that produce stabilized organic residues, carbon dioxide and water in the presence of oxygen or stabilized organic residues, methane, carbon dioxide and water in the absence of oxygen. Biodegradation is a viable way of processing plastics, but not all types of plastic waste are recycled in this way. Biodegradable waste is referred to as BDP and is divided into three basic types, namely [18, 22]: – BDP on a synthetic basis (oil, natural gas, etc.); – BDP based on renewable resources (starch, cellulose,

polyhydroxyalkanoates, etc.); – GDP combined.

Analysis of the Technological Procedures of Valorisation and Finalization of the Product from the Recycling of Plastic Waste

The recovery of plastic waste in a specific company is carried out on recycling lines. The description of plastic waste recycling can be summarized in the following points:

– reduce the surface residual moisture of the re-

granulate by vibrating; – as a vibrating feeder, it serves to transport the re-

granulate to the centrifuge, which is mainly used

for the second stage of dewatering the re-granulate

and for feeding it to the injector, through which it

is pneumatically transported to the cyclone with the

help of a fan, from where the cooled re-granulate is

discharged at a temperature of 20-30ºC at certain

time intervals into big bags or sacks and then stored.

The dependence of selected indicators during recycling on the quality of plastic waste achieved in two different types of recycling lines during 24 hours on selected days is shown in the following tables (Table 1 and Table 2).

The values in Table 3 show the indirect proportionality between the data on the poor quality of recycled waste with a high waste rate as well as the performance of the line and downtimes.

For faster orientation in the values ​of Table 3 and for the purpose of transparency and emphasis on direct proportionality, several indicators are selected to describe the relationship mentioned earlier.

In addition to the selected indicators described in the previous tables, low-quality plastic waste (Fig. 5) has a significant negative impact on the most important indicator, namely the economic result.

The mentioned impact is shown in Table 5. It’s based on the results in 2022, and at the same time, in the calculations in columns 2 and 3 with lower and higher performance, we also pointed out the immediate development of the management action depending on the deteriorated quality of the waste.

Fig. 1.

Low-quality plastic waste.

Technological Procedure and Analysis of the Finalisation of Re-Granulates as a Final Product from the Recovery of Plastic Waste - Film Production

The company ensures the finalization of its own as well as purchased re-granulate or original primary granulate on four blowing lines (Fig. 6) with extruders with a diameter of 55, 65 and 80 mm, based on LDPE, LLDPE polymers and a small proportion of HDPE.

The technological process of production consists in the preparation of the necessary deposit of input re-granulate or granules depending on the ratio of the prescribed components such as: – Re-granulate by type (LD, LLDPE); – Dyes; – Additives (antistat, roughening agent, UV stabilizer,

Fig. 3.

Metamax 80 blow molding lines.

etc.).

Following the previous step, the prepared mixture is filled into the hopper before entering the extruder. After heating the individual parts of the extruder and the blow mould to the required operating temperature (180-200 ºC) and then turning on the drive of the extruder screw, the melt is extruded through the moulding head and the following main parts of the machine (Fig. 7) are gradually included in the production process to create the final product: – Guide basket; – Support plates - guide cylinder; – Filer; – Pick-up cylinders; – Guide rollers; – Ionizer; – Towing cylinders; – Paring knives or carving knives; – Winding unit.

Fig. 2.

Film blowing scheme.

The melt in the extruder is heated, depending on the size of the machine, in five or seven zones by electric resistance band heaters with a power of 15-31 kW. The temperature in the individual zones is maintained using automatic regulation, while 1 kW fans are used for cooling in each zone, so that the melt does not overheat or that the cooling time is as short as possible.

Device performance, i.e. the amount of blown film in kg/h is regulated by the speed of the extruder and the thickness of the film in the endless hose using the air supplied from the blower with an adjustable amount on the suction side or with the speed of the blower as well as by regulating the speed between the take-off roller and the take-off rollers.

The method of setting the geometric parameters of the foil and hose is shown in Table 6 depending on the regulation of individual aggregates on the blowing line.

From the endless hose, by cutting one or both ends of the fleece before winding it into a roll, we distinguish the resulting product according to types as follows (Fig. 8): – Hose; – Semicolon; – Flat film, one or two layers.

The company uses approx. 65% of re-granulate from its own recycling of plastic waste in the production of films. The remaining 35% consists of 20% from the primary polymer and 10% from external re-granulates, respectively 5% from additives (paints, additives, etc.). occurring non-meltable foreign admixtures such as: paper, wood, metal, foreign polymer (BOPP, PP, etc.), which cause a high waste rate of up to 12%. Technological downtime at the expense of deterioration of the time utilization of the machine is below 90%. Line performance, product quality and, the deterioration of production efficiency is as shown in the numbers according to the results achieved.

The multi-species nature of plastic waste and the subsequent imperfect sorting and re-sorting before recycling in order to guarantee the required productivity of the recycling line causes that, due to the inhomogeneity and non-uniformity of the polymers, the produced re-granulate in one batch has different quality parameters, for example MFI - the value of the flow index, which causes uneven strength parameters of the manufactured film, which is manifested by a paper effect in the tangential direction in an endless hose.

Technological Procedure and Analysis of the Finalization of Re-Granulates as a Final Product from the Recovery of Plastic Waste - Production of Bags

Film from our own production is used as part of further recovery with a share of 66% for sale and 34% for our own consumption in the production of the following types of bags (Fig. 9):

The same characteristic steps in the production process for both types of bags are: – Unwinding the film from the roll; – Sealing the hose in the transverse direction in order

to close the lower part of the bottom of the bags

according to the set length of the bags.

In the case of rolled bags, compared to the production into bundles, the hose when unrolling from the roll is folded in the direction of production to the maximum width of the turret head of the magazine up to 800 mm, and the number of bags in the roll as well as the spacing of the perforations are adjusted depending on the set length of the bags, facilitating tear-off for individual bags from rolls when used by customers.

The difference in stacked - folded bags compared to bags in rolls is that in stacked - folded bags, after setting the length of the bags (instead of perforation) after sealing the bottom of the bags, they are separated automatically by cutting the hose behind the sealed weld.

Fig. 4.

Distribution of types of foil windings.

Fig. 5.

Stacked and rolled bags.