Pollutec Recycling

Mechanical Properties

The mechanical properties of all the investigated materials are reported in Figures 2-5 as a function of the mixed plastics waste concentration.

The experimental results are reported in dimensionless form, i.e., the value relative to each blend is divided by the corresponding value of the unblended matrix. These latter values are reported in Table 2 for three matrices. As expected, all the mechanical properties, except for the elastic modulus, decay on increasing the MPW content. The extent of this deterioration, however, is strongly dependent on the investigated property and on the nature of the matrix.

Figure 2. Dimesionless modulus vs MPW content.

Figure 3. Dimensionless tensile strength vs. MPW content. 1,2 1,0 0, 8 0,6 0,4 0,2 0,0

Figure 3. Dimensionless tensile strength vs. MPW content. 1,2 1,0 0, 8 0,6 0,4 0,2 0,0

Figure 4. Dimensionless elongation at break vs. MPW content.

Table 2

Mechanical properties of the three matrices

Figure 5. Dimensionless impact strength vs. MPW content.

As already mentioned, the elastic modulus (see Figure 2) increases with the mixed plastics waste content. Moreover, the improvement is larger for the blends containing LDPE and recycled polyethylene as a matrix. This behavior can be attributed to the high modulus of the polymers forming the MPW mixture. The remarkable increase of the modulus found for blends containing LDPE and RPE can be explained by considering their relatively low moduli compared

Figure 6. Stress-strain curves for LDPE and its blends with 10 and 50% MPW.

with polymers of the MPW phase which have considerably higher moduli). On the contrary this improvement is less pronounced for the blends containing HDPE as a matrix, because of its higher elastic modulus, similar to that of the PE component in the MPW phase.

As demonstrated in the previous work,2 the values of the modulus are lower than expected on the basis of an additive rule. It is due to the incompatibility among the different polymeric phases present in the blends.

Tensile strength (Figure 3) is only slightly influenced (especially for LDPE and RPE) by adding mixed plastics waste but an elongation at break decreases rapidly even at low MPW content (Figure 4).

The stress-strain curves of some blends are reported in Figure 6. The blends with MPW fail, in general, at a deformation in proximity of elongation at yield of the matrix when the stress reaches a plateau, extending almost up to the break point. It is probably due to this reason that the tensile strength slightly depends on the MPW content.

The elongation at break (Figure 4) is low for all blends due to the incompatibility between the various phases formed in the blends. In Figure 7, the SEM micrograph of a sample of a blend of LDPE and MPW (75%) highlights the

Figure 7. SEM micrograph of LDPE/MPW blend (MPW = 75%).

multiphase morphology of these materials and the poor adhesion among the different components. The lack of adhesion gives rise to microdefects in the structure of these material inducing a significant fragility. The blends containing HDPE have also poor properties.

Impact strength (Figure 5) is also significantly influenced by the presence of a heterophasic dispersed plastics waste, but the matrix seems to play a more important role. The impact strength of LDPE slightly decreases on increasing the MPW content within investigated composition range. Blends with HDPE and RPE show a different behavior. The samples prepared with the recycled polyethylene as a matrix do not show any change in the impact strength when MPW content is lower than 30%, which rapidly decreases at the MPW concentration of 50%. The blends containing HDPE show a remarkable decrease of the impact strength also at the low MPW concentrations.

These latter results are difficult to explain on the basis of the nature of the components ofthese blends. Only a better adhesion between the LDPE continuous matrix and the different phases of the MPW can be considered. This fact is, however, quite unexpected considering that the PE phase in the plastics waste is mostly HDPE.

Figure 8. Modulus of blends containing 50% MPW vs. CaCÛ3 content.

Figure 9. Tensile strength of blends containing 50% MPW vs. CaCÛ3 content.

Figure 9. Tensile strength of blends containing 50% MPW vs. CaCÛ3 content.