The first step of separation of plastic wastes from the overall waste stream was successfully achieved in an industrial plant having a capacity of 7 × 10 4 tons/year which was designed by ENADIMSA (Spain) and operated in Madrid according to the layout given in Figure 1. The economic evaluation, once the recyclability O. Laguna Castellanos et al. 61 Figure 1. Flow diagram of treatment and separation plant for municipal solid wastes. was determined, was made taking into account the actual sale price of the wastes based on the amortization costs and profitability of the plant during the 1983-86 period. A preliminary economic study was conducted using a classic method based on Engines and Machinery. The results obtained were then taken as starting data for more realistic evaluation, based on the state of the Spanish polymers market in the 1985-1988 period. Only the main results of these works will be presented here, with the goal of showing that the plastics recycling busi- ness is also very attractive from an economical point of view. EXPERIMENTAL Materials The film plastic wastes were supplied by ENADIMSA from the Municipal Treatment Plant of Urban Solid Wastes in Valdemingómez (Madrid). The iden- tification of polymers present in plastic wastes was carried out by IR and DSC methods. The results are compiled in Table 1. A flotation method was applied in order to separate ninety percent of the polyolefins present in the film plastic wastes fraction from rejectable materials. The physical properties of virgin polymers supplied by Repsol Química S.A. (wastes, and of polymers chosen as interfacial modifiers) are collected in Table 2. Procedures and Utilities The study of the rheological behavior of wastes and HDPE/LDPE system was carried out using a torque-rheometer. 16 62 Management of Plastic Wastes Table 1 Average composition of the plastic film wastes of urban origin (1985) Polymer % Remarks PVC 4 half is removed by flotation in water HDPE 20 LDPE 68 Solid waste 8 insoluble solids Blends, having 15/85, 50/50, and 85/15 of HDPE/LDPE ratio, were prepared according to an experimental design method by Box-Wilson. The experimental design consists of thirteen experimental combinations dis- tributed as follows: • four combinations corresponding to the factorial 2 2 • four combinations to obtain the central design rotability (star combina- tions) • five experimental pointsinthe center tocalculate the experimental error. The amounts of HDPE in a blend and the shear rate were chosen as independ- ent variables in this study. Experiments were carried out in the melt state at 190 o C. A Goodrich method 17 was used for determination of effective instrument dimensions, which allows one to employ the Daane et al. 18 procedure in order to relate torque-rheometer data to more fundamental rheological values. The same HDPE/LDPE ratios were used in the study of mechanical behavior of HDPE/LDPE system of injected specimens. As said previously, DSC thermograms were used to determine weight concentration of two crystalline components both in HDPE/LDPE blends and wastes. Tensile and impact tests were carried out according to the UNE standards and in agreement with the ISO standards. The preparation of the specimens was O. Laguna Castellanos et al. 63 Table 2 Physical properties of polymers and wastes used in this work Material Molecular weight []η Melt index Density M w M n dl/g T ( o C) P (kg) (g/10 min) kg ⋅ m -3 HDPE 141,000 19,000 0.87 190 2.160 6.7 916 LDPE 114,700 22,000 1.84 190 5.000 2.3 945 Wastes 0.76 190 2.160 1.1 941 LMWPE 0.74 125 0.325 1.1 924 EVA (24% av) 190 2.160 3.0 1,000 ClPE (36% Cl) 1,160 done by conventional processing methods: blending in a two-roll mill, followed by the injection molding of the materials. Thin sections, around 20 µ m in thick- ness, from injected parts were cut by a microtome. Samples were taken from the nearest to, and farthest from, the gate. The study of a microstructure of different systems was made by interference and phase contrast microscopy. SEM was also employed for observations of a fracture surface of specimens from the impact tests. An etching of samples was carried out according to Olley et al. 19 64 Management of Plastic Wastes Figure 2. IR spectra of HDPE and LDPE homopolymers and film plastic wastes. RESULTS AND DISCUSSION Identification of polymers present in the film plastic wastes and the rheological behavior of the HDPE/LDPE system In Figures 2 and 3 IR plots and DSC curves of HDPE, LDPE, wastes, and some blends are reported. Two re- marks can be made concerning these data: • the film plastic wastes from Ma- drid constituted, in 1985,mainly of HDPE and LDPE, having 20/70 HDPE/LDPE ratio deter- mined by the peak area method. 20 • no co-crystallization was found in HDPE/LDPE blends. 21 The rheological study of HDPE/LDPE system and wastes was conducted to confirm the processability of wastes and their thermomecha- nical stability during the processing operations. The re- sults obtained demonstrated a good processability of wastes. Torque, shear stress, and shear rate of pure components, blends, and wastes are reported in Figure 4. No thermal-oxi- dative degradation of the wastes was detected during and at the end of the processing. The flow curve of the HDPE sample is higher than that of LDPE and the flow curves of blends and wastes fall in between. The curve for wastes is between the curves for blends containing 85 and 50% of O. Laguna Castellanos et al. 65 Figure 3. DSC thermograms of HDPE and LDPE and their blends obtained from waste materials. HDPE. This is because of a presence of ten percent of inorganic solid particles in the wastes which increases their rheological response in terms of viscosity. Mechanical behavior of HDPE/LDPE 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 66 Management of Plastic Wastes Figure 4. Rheological properties of HDPE/LDPE blend and 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 in- side 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,atlow 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 O. Laguna Castellanos et al. 67 Figure 5. Polar representation of mechanical behavior of HDPE/LDPE system containing either virgin polymers or composition from urban wastes. 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 crystal- line behavior. According to these ob- servations, three interfacial modifiers of HDPE/LDPE system were chosen based on their chemical similarity to ethylene units but hav- ing different crystallization capabili- ties. An ethylene-vinyl-acetate amorphous copolymer (EVA), chlori- nated 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. 68 Management of Plastic Wastes Figure 6. Tensile strength: at yield (—) and at break (—) for HDPE/LDPE system containing interfacial modifiers. O. Laguna Castellanos et al. 69 Table 3 Mechanical properties of HDPE/LDPE and HDPE/waste blends obtained by injection molding with and without interfacial agents (IA) Concentration E (MPa) % Tensile strength (MPa) Impact strength (KJm 2 ) Def. (m ⋅ 10 3 ) F max (MPa) HDPE IA 100% none 993 820 17 10.4 4.2 3.8 1% EVA 387 700 18 11.0 3.9 4.0 1% ClPE 346 750 16 15.4 3.2 6.1 1% IMV 245 750 18 21.3 3.6 5.8 85% none 375 610 17 9.2 3.8 3.9 1% EVA 408 700 23 9.6 3.8 4.0 1% ClPE 316 700 14 9.5 2.6 5.5 1% IMW 246 730 24 13.5 2.7 5.2 50% none 375 522 18 6.3 3.2 3.2 1% EVA 346 610 18 7.3 3.7 3.1 1% ClPE 280 604 19 16.0 4.4 5.3 1% IMW 198 690 17 22.0 5.1 5.9 14% none 184 340 12 11.1 4.8 3.2 1% EVA 173 340 12 14.0 5.2 2.9 1% ClPE 158 370 12 22.1 7.6 4.0 1% IMW 163 500 12 26.0 6.8 4.9 0% none 151 212 10 ∞∞∞ 1% EVA 155 150 9 ∞∞∞ 1% ClPE 100 155 9.5 23 5.4 5.0 1% IMW 164 280 11 40 6.2 5.0 wastes none 240 230 12 4.0 2.6 2.3 1% EVA 300 340 11 3.5 2.5 2.4 1% ClPE 251 300 11 6.4 2.1 4.4 1% IMW 228 330 12 5.2 2.0 3.5 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 sig- nificant 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 70 Management of Plastic Wastes Figure 7. Elongation at break of HDPE/LDPE system in a presence of the interfacial agents. [...]... Figure 9 73 74 Management of Plastic Wastes 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 Microstructural aspects of HDPE/LDPE blends Figures 9 to 10 show micrographs of the microtomed sections from injection molded bars The different microstructure of HDPE/LDPE blends, having... inside the cavity of a mold and near the gate, show differences in O Laguna Castellanos et al Figure 13 Micrographs of 85/ 15 (left) and 15/ 85 (right) HDPE/LDPE ratio with 1% LMWPE Figure 14 SEM micrographs showing fracture surface of HDPE (left) and LDPE (right) Magnification 30x 75 76 Management of Plastic Wastes Figure 15 SEM micrographs showing the fracture surface of the film plastic wastes Magnification:... 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 72 Management of Plastic Wastes Figure 9 Micrographs of HDPE/LDPE system Magnification 50 0× O Laguna Castellanos et al Figure 10 Micrographs of the etched samples of the same... of the film plastic wastes Magnification: (left) 30×, (right) 50 × response of the material under polarized light When HDPE is the matrix, smooth surface and high birefringence are found, as in the case of pure HDPE, with ringed and impinged macroaggregates of crystals also visible These features indicate that the microstructure of these materials is basically due to HDPE In Figure 10 the above observations... appearance of the injected bars The changes in the size of the O Laguna Castellanos et al 77 Figure 16 SEM micrographs of HDPE/LDPE system showing differences in fracture mechanism due to different matrix Figure 17 SEM micrographs showing brittle fracture (HDPE) and ductile fracture (LDPE) macroaggregates of crystals and the presence of the interfacial additives enhance the stretchability of the compatibilized... confirmed for samples which have undergone a chemical attack directed to the amorphous zones of blends Figures 11-13 show micrographs of HDPE/LDPE blends containing 1% of EVA copolymer, chlorinated polyethylene, and a low molecular weight polyethylene, respectively The changes on the microstructural level of these specimens, compared to the unmodified HDPE/LDPE system, previously discussed, are evidently . 346 750 16 15. 4 3.2 6.1 1% IMV 2 45 750 18 21.3 3.6 5. 8 85% none 3 75 610 17 9.2 3.8 3.9 1% EVA 408 700 23 9.6 3.8 4.0 1% ClPE 316 700 14 9 .5 2.6 5. 5 1% IMW 246 730 24 13 .5 2.7 5. 2 50 % none 3 75 522. EVA 155 150 9 ∞∞∞ 1% ClPE 100 155 9 .5 23 5. 4 5. 0 1% IMW 164 280 11 40 6.2 5. 0 wastes none 240 230 12 4.0 2.6 2.3 1% EVA 300 340 11 3 .5 2 .5 2.4 1% ClPE 251 300 11 6.4 2.1 4.4 1% IMW 228 330 12 5. 2. copolymer. Figure 12. Micrographs of 85/ 15 (left) and 15/ 85 (right) HDPE/LDPE ratio with 1% ClPE. O. Laguna Castellanos et al. 75 Figure 13. Micrographs of 85/ 15 (left) and 15/ 85 (right) HDPE/LDPE ratio