Mechanical behavior of HDPELDPE blends
The mechanical properties of blends of virgin HDPE and LDPE and plastic wastes are below those expected on the basis of an additive rule.22-29 In Figure 5
- Figure 5. Polar representation of mechanical behavior of HDPE/LDPE system containing either virgin polymers or composition from urban wastes.
a polar representation was plotted showing the mechanical properties of HDPE/LDPE system inside the contour lines assigned for homopolymers. Also the contour lines for the wastes were plotted. As can be seen, the values are inside the range for homopolymers, indicating a mechanical behavior typical for material which does not degrade. Thus the recyclability of these plastic wastes can be attained without a danger of degradation.
The stress-strain curves of semicrystalline polymers show, at low strain rate, a sharp drop in stress after the yield point. After the neck formation, and during a certain time period, the stress does not change appreciably with further strain. Finally, there is a slight increase in stress and then the specimen breaks. Such behavior is typical for HDPE but not for LDPE. The drop in stress after the neck formation in LDPE is very small. HDPE-rich blends show a stress-strain behavior similar to that of pure HDPE, whereas the stress-strain curves of LDPE-rich blends resemble that of the pure LDPE. Figure 6 reports the tensile strength values at yield and at break for the HDPE/LDPE system and for wastes. The values relative to a blend from wastes are very similar to those of a blend containing 15% of virgin HDPE.
As known, the crystalline polymers are composed of lamellae containing folded chains. The lamellae are held together by the tie molecules which extend from one crystalline layer to another. Molecular imperfections (e.g., branches in polyethylene) tend to reside in the amorphous portion between crystallites, which suggests that a different behavior of HDPE and LDPE is related to their crystalline behavior. According to these observations, three interfacial modifiers of HDPE/LDPE system were chosen based on their chemical similarity to ethylene units but having different crystallization capabilities. An ethylene-vinyl-acetate amorphous copolymer (EVA), chlorinated polyethylene (ClPE) having 10-15% of residual crystallinity, and a low molecular weight polyethylene (LMWPE) practically without branches but having a very high crystallinity were used. These additives were also incorporated into the wastes.
- Figure 6. Tensile strength: at yield (—) and at break (—) for HDPE/LDPE system containing interfacial modifiers.
Table 3
Mechanical properties of HDPE/LDPE and HDPE/waste blends obtained by injection molding with and without interfacial agents (IA)
Table 3
Mechanical properties of HDPE/LDPE and HDPE/waste blends obtained by injection molding with and without interfacial agents (IA)
|
Concentration |
Tensile |
Impact |
Def. (m103) |
(MPa) | |||
|
HDPE |
IA |
E (MPa) |
% |
strength (MPa) |
strength (KJm2) | ||
|
none |
993 |
820 |
17 |
10.4 |
4.2 |
3.8 | |
|
100% |
1% EVA 1% ClPE |
387 346 |
700 750 |
18 16 |
11.0 15.4 |
3.9 3.2 |
|
|
1% IMV |
245 |
750 |
18 |
21.3 |
3.6 |
5.8 | |
|
none |
375 |
610 |
17 |
9.2 |
3.8 |
3.9 | |
|
85% |
1% EVA 1% ClPE |
408 316 |
700 700 |
23 14 |
9.6 9.5 |
3.8 2.6 |
4.0 5.5 |
|
1% IMW |
246 |
730 |
24 |
13.5 |
2.7 |
5.2 | |
|
none |
375 |
522 |
18 |
6.3 |
3.2 |
3.2 | |
|
50% |
1% EVA 1% ClPE |
346 280 |
610 604 |
18 19 |
7.3 16.0 |
3.7 4.4 |
|
|
1% IMW |
198 |
690 |
17 |
22.0 |
5.1 |
5.9 | |
|
none |
184 |
340 |
12 |
11.1 |
4.8 |
3.2 | |
|
14% |
1% EVA 1% ClPE |
173 158 |
340 370 |
12 12 |
|
5.2 7.6 |
2.9 4.0 |
|
1% IMW |
163 |
500 |
12 |
26.0 |
6.8 |
4.9 | |
|
none |
151 |
212 |
10 |
oo |
oo |
oo | |
|
0% |
1% EVA 1% ClPE |
155 100 |
150 155 |
9.5 |
23 |
5.4 |
5.0 |
|
1% IMW |
164 |
280 |
11 |
40 |
6.2 |
5.0 | |
|
none |
240 |
230 |
12 |
4.0 |
2.6 |
2.3 | |
|
wastes |
1% EVA 1% ClPE |
300 251 |
340 300 |
11 11 |
3.5 6.4 |
2.5 2.1 |
2.4 4.4 |
|
1% IMW |
228 |
330 |
12 |
5.2 |
2.0 |
3.5 | |
- Figure 7. Elongation at break of HDPE/LDPE system in a presence of the interfacial agents.
Table 3 contains data on mechanical properties of HDPE/LDPE system and wastes, both containing 1% of each interfacial modifier. Figures 7 and 8 give the relative values of elongation at break and impact strength, respectively, for modified systems. The elongation at break (Figure 7) is especially improved by incorporation of a low molecular weight polyethylene, which gives the most significant improvement when the matrix is composed of LDPE. The elongation at break of the film plastic wastes is improved by about 30-50% due to the action of additives. A surface improvement of the injected specimens can also be achieved by adding the additives. The blend with a low molecular weight polyethylene as an interfacial agent shows better properties.
The impact strength of the HDPE/LDPE system and wastes is also improved by the presence of additives (Figure 8). Similarly, the best results are obtained with a low molecular weight polyethylene.
On the contrary, these additives give rise to a lower elastic moduli compared with data for the unmodified system. It is, however, evident that it is possible to
- Figure 8. The impact strength of HDPE/LDPE system at -30oC in a presence of interfacial agents.
markedly change, and in general to improve, the mechanical behavior and the surface quality of plastic waste fractions by adding an interfacial modifier.
- Figure 9. Micrographs of HDPE/LDPE system. Magnification 500x.
- Figure 10. Micrographs of the etched samples of the same composition as in Figure 9.
- Figure 11. Micrographs of 85/15 (left) and 15/85 (right) HDPE/LDPE ratio with 1% EVA copolymer.
Figure 12. Micrographs of 85/15 (left) and 15/85 (right) HDPE/LDPE ratio with 1% ClPE.
Figure 12. Micrographs of 85/15 (left) and 15/85 (right) HDPE/LDPE ratio with 1% ClPE.
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