Mixed Plastics From Household Waste

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A study of this type of a plastic waste has been performed at CSI using samples that came from two different experimental collection systems, the first in plastic boxes (internal volume: 3 m3), and the second in arranged plastic areas inside

Figure 1. Mix composition.

the dump. The two "city" samples showed two different compositions, as reported in Figure 1. The polyolefin fraction (PP and PE) falls in the range of about fifty to fifty five percent, while large differences between the two samples are in the PET and PS percentages.

A direct solution to plastic disposal problems can be the reuse of a heterogeneous mixture of plastics, directly obtained from a separate urban collection. The processing behavior of the heterogeneous mixture has been preliminary investigated to establish the critical technological points and a more suitable machinery. In particular, injection molding and extrusion (single and twin screw) have been considered in this investigation.

Afterwards, the mechanical properties evaluation has been performed to identify potential applications of plastic waste.

As a result of this research step, it has been demonstrated that the heterogeneous mixture can be processed through extrusion or injection molding technologies using traditional machineries. In the case of extrusion, it has been found that the use of a twin counter-rotating screw extruder guarantees greater efficiency than a single screw extruder. In addition, a degassing unit on the extruder is advisable to remove the gas that is eventually generated by residual traces of organic materials following the washing step. Due to the presence of PVC resin, the melting temperature must be kept below 483 K and the barrel residence time must not exceed 6 min. Furthermore, the relatively high content, in the mixture, of semicrystalline polymers like PET, whose melting point is above the processing temperature, influences either the technological solution or the final properties of the manufactured product. Larger injection gates and mould channels must be used in order to avoid undesirable occlusions in the channels.

A micro-grinding of the semicrystalline polymers seems to be the most suitable solution to produce thin wall articles with improved aesthetics and mechanical properties.

Nevertheless, because of the incompatibility of the various plastic materials, the mechanical properties ofmolded or extruded products are rather poor. Some mechanical properties of a molded sample have been measured and the results are reported in Table 1.

Table 1

Mechanical properties of heterogeneous mix

Table 1

Mechanical properties of heterogeneous mix

Izod impact strength unnotched (KJ/m )

Tensile properties

Elastic modulus (MPa)

Elongation at break (%)

11.5

950

7

A sample of the heterogeneous plastic mixture derived from the city 2 collection, having the composition reported in Figure 1, have been evaluated through mechanical impact (Izod) and tensile tests. According to the ASTM methods, several specimens have been molded, employing a traditional injection molding machine. The melt temperature during the plastic mixture processing was held at 463 K and the barrel residence time was about 3 minutes. The mechanical testing showed wide numerical distribution of the values, but a more important aspect was a very poor tensile or flexural strength. This means a strong limitation of applications, in particular in the case of thin walls and manufactured products that have to work under flexural or tensile stresses. Therefore, by adding to the mixture specific components, like other polymers from homogeneous recycling, fillers (talc), fibers or promoters that increase the compatibility, it is possible to improve the tenacity or stiffness, product aesthetics, and processability.

Table 2

Improvement of mechanical properties

Table 2

Improvement of mechanical properties

Material

Formulations

1

2

3

4

5

Heterogeneous mix

100

100

100

100

100

Talc

20

Glass fiber

30

SBS finaprene

10

LDPE from agricultural recycling

20

Flexural evaluation

Properties

Strength

at yield

16.5

19.0

(MPa)

at break

20.5

16.5

24.5

Elongation

at yield

8.0

11.2

(%)

at break

6.8

4.3

2

Elastic modulus (MPa)

950

1250

2800

700

820

Table 2 reports some mechanical property improvements obtained through an addition of several components.

As shown, the glass fibers allow the product to obtain a very high stiffness, higher than that with talc and far better than that of the original mixture, while the SBS and PE improve the tenacity. The final choice of a type and an amount of these components is based on their cost as well as their efficiency. Finally, specific "aid-processors" can be employed to increase the processability range of the heterogeneous mixture of plastics.

The philosophy of our research is to consider the heterogenous mixture as a new plastic base for different formulations to be correlated with application requirements. Starting from this concept, some formulations have been employed to produce several manufactured products for different application areas. Figure 2 shows some samples of extruded profiles that can be employed to build benches, garden tables, bicycle racks, fences, and playing facilities for parks.

Figure 2. Extruded profile sample. Figure 3. Coextruded samples

Table 3

Mechanical properties of PO fraction/heterogeneous mix

Table 3

Mechanical properties of PO fraction/heterogeneous mix

IZOD impact strength

Tensile properties

Material

Unnotched (KJ/m2)

Notched (J/m)

Elastic modulus (MPa)

Elongation at break (%)

PO fraction

no break

80

450

108

Heterogeneous mix

11.5

-

950

7

Coextrusion technology is the best way to improve the surface properties like puncture, impact and weather resistance, as well as the appearance. Samples obtained through coextrusion are shown in Figure 3. One interesting application for mixed plastics, because of their large market volume, is the production ofinjected tiles for paving. In order to demonstrate the feasibility ofthis application a large parking area has been built at CSI employing specially designed molded tiles (Figure 4).

A wide spread solution, in terms of application and market volume, can be the recycling of single materials or homogeneous fractions obtained from a separation process of the mixture. In fact, the samples obtained from single homogeneous fractions show a general performance far greater than that of samples

Figure 4. Parking area.

Table 4

Mechanical properties ofPVC fraction/virgin PVC

Table 4

Mechanical properties ofPVC fraction/virgin PVC

Material

IZOD impact strength (notched) (J/m)

Tensile properties (elastic modulus) (MPa)

PVC fraction Virgin PVC

60 40-90

2810 3000

produced from mixed plastics. Currently we are working on a procedure that separates four fractions of different materials. These are: polyolefins, PS, PVC, and PET. As shown in Table 3, mechanical properties of polyolefin-based fraction are far better than those of the heterogeneous mixture.

The improvement of tenacity is, in particular, evident when considering impact resistance. Samples subjected to impact tests show an increase in elongation at the breaking point from 7 percent to above 100 percent. As well as for polyolefins, the properties ofrecycled PVC fraction are comparable with those of a common virgin PVC, as shown in Table 4.

With regard to PET fraction, the potential applications are strongly dependent on its purity. Applications like films, fibers, or straps are not recommended when a high concentration of impurities are present. In this case, PET fraction can be employed for structural applications as an engineering polymer. To this aim an appropriate mixing process of PET with other components, like glass fibers, impact modifiers and/or nucleating systems must be developed. However, the reuse of the PET fraction implies that the amount of residual PVC must be kept below 50 ppm to avoid undesirable polymer degradation. This degradation determines poor surface appearance and loss of mechanical properties of the manufactured products.

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