4 Results and discussion
elasticity, while the two straight lines serve to read the corresponding values of the rupture point from the scales. At the first maximum of the graph both tensile and yield strength can be read from the derived stress-strain diagram. Accordingly, it refers to the stress where the plastic deformation, which occurs as necking of the material, begins.
Subsequently, the material starts to flow, because the long polymer chains disentangle, slide past each other and align parallel under constant load. If the elongation speed is low enough, the creep zone may expand to the whole gage section of the specimen. By contrast, lower strain at break is observed for higher rates as the macromolecules have less time to rearrange under load and they react more brittle.
Figure 17: Force-strain diagram of recycled and uncontaminated PC/ABS
Tensile modulus
The results of the determination of tensile modulus are presented in Figure 18. Regarding the label-free measurement series, the median of the elastic modulus is 1905 MPa, it reduces as labels are added to the pure material. Concerning the paper series, the modulus of elasticity fluctuates, but shows no clear tendency. The brown label residues on the surface of the manufactured bars highlight that polymer blend and paper are incompatible, which is why the determined product properties are not predictable.
As opposed to this, PP1 labels lead to a minor but gradual reduction in the modulus of elasticity to 1890 MPa, 1874 MPa and 1831 MPa with rising contamination. Plotting the modulus of elasticity as a function of the median of the PP1 concentration, as illustrated
in Figure 19, gives a straight line characterized by a high coefficient of determination (R2), which is 0.9954. This linear correlation may imply compatibility of PP and PC/ABS, leading to a superposition of the properties. Tensile modulus of pure polypropylene typically ranges between 1100 and 1300 MPa [21]. Adding PP stickers would consequently lessen the high rigidity of PC/ABS, as can be observed in the diagram.
Figure 18: Results for tensile modulus
Figure 19: Tensile modulus as a function of label concentration
According to literature sources, the compatibility between PP and PC/ABS is poor [50], but since the concentration of polypropylene is very low, both components may be
1650 1700 1750 1800 1850 1900 1950 2000
label-free 0.17%
paper
0,35%
paper
1.03%
paper
0.17%
PP1
0.35%
PP1
1.03%
PP1
1.03%
PP2
Tensile modulus[N/mm²]
1820 1830 1840 1850 1860 1870 1880 1890 1900 1910
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Tensile modulus[N/mm2]
Label concentration [wt.-%]
paper PP1 PP2
miscible. PP2 stickers cause the least decline of the modulus of elasticity to 1893 MPa referred to pure PC/ABS. Contamination with each 1.03% of PP1 and PP2, respectively, seems to have different effects. That is because these stickers are not chemically identical and the pure polymer types are most likely to have different properties.
Unlike the label-free series, the values of the other fractions are distributed broadly. This might be due to the reason that impurities, that affect tensile modulus, vary with respect to frequency of occurrence, so that some test bars are more contaminated than others.
Tensile strength
Figure 20 shows the tensile strength as investigated in the tests, where the pure reference fraction shows a median of 52.34 MPa. In short, tensile strength decreases the more, the more labels are added.
Figure 20: Results for tensile strength
Generally, paper worsens this property the most, this becomes particularly visible at its highest concentration. The addition of 0.17%, 0.35% and 1.03% wt.-% paper labels leads to a decrease in tensile strength to 51.63 MPa, 51.02 MPa and 48.70 MPa, which is a nearly linear correlation according to Figure 21. Since the adhesion between the two materials is poor, the paper particles interfere with the closed PC/ABS matrix. The inclusions lower the contact surface between the polymer chains and therefore the intermolecular forces, that make the material resistant towards acting forces, deteriorate.
47 48 49 50 51 52 53
label-free 0.17%
paper 0,35%
paper 1.03%
paper 0.17%
PP1 0.35%
PP1 1.03%
PP1 1.03%
PP2
Tensile strength [N/mm²]
Figure 21: Tensile strength as a function of label concentration
What is more, the boxplots show that these three series are distinguished by a wide variation of the readings as the number and size of paper residues in the test bars varies.
As opposed to this, tensile strength reduces only to 50.70 MPa for contamination with 1.03% PP1 and declines even fewer to 51.49 MPa when adding the same amounts of PP2 labels. In contrast to paper, polypropylene melts and is dispersed finely during the processing, so that the morphologic structure of the product is more homogeneous.
Figure 21 represents a linear correlation between the concentration of PP1 and tensile strength, R2 equals 0.9976. As already discussed above, in case of PC/ABS, tensile strength is equivalent to yield strength. In practice, this means that the plastic deformation of the material, containing impurities, already starts at lower loads.
Strain at break
Strain at break is the property which is most strongly affected by the sort of contaminants, as can be seen from Figure 22 and Figure 23.
Ultimate elongation drops from more than 120% for the label-free bars to less than 5%
for each of the paper series. The lack of compatibility between the paper and the polymer matrix gives rise to early material break. In fact, the sole presence of only a few of these particles already has a detrimental effect, as they cause crack initiation. Consequently, the location of the paper contaminants in the tensile bar determines where the break occurs, the exact concentration of these impurities in the polymer plays a minor role. The
48 49 50 51 52 53
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Tensile strength[N/mm2]
Label concentration [wt.-%]
paper PP1 PP2
median of the strain at break decreases from 4.8% to 3.7%, which seems negligible when considering that the concentration of paper almost increases six-fold.
Figure 22: Results for strain at break
A higher contamination with PP1, however, improves strain at break. If the previously stated theory of good compatibility between PC/ABS and PP in small concentrations applied, the opposite trend should be observed. In this case, higher PP concentration would result in bigger reduction of ultimate elongation. Specimens with 1.03% PP1 even exhibit a higher strain at break than those containing 1.03% PP2.
Figure 23: Strain at break as a function of label concentration 0
20 40 60 80 100 120 140
label-free 0.17%
paper 0,35%
paper 1.03%
paper 0.17%
PP1 0.35%
PP1 1.03%
PP1 1.03%
PP2
Strain at break [%]
0 20 40 60 80 100 120 140
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Strain at break[%]
Label concentration [wt.-%]
paper PP1 PP2