Recycling of Plastic Materials Part 4 ppt

Thermal analysis The melting and crystallization behaviors were studied in a Mettler TA 3000 DSC. The non-isothermal crystallization was performed as follows: heating at 20 o C/min up to 200 o C, 3 min. of dwelling time, cooling down at cooling rates vari- able from 30 to 1 o C/min. RESULTS AND DISCUSSION Rheology The rheological behavior of polyolefin blends has been widely studied by many authors. It was found that the shear viscosity exhibits maxima and minima when plotted as a function of composition. In general, the viscosity vs. shear stress trends for the binary blends, which present negative (PS/LDPE, PS/PP, PS/HDPE) 16-21 and positive (PP/HDPE) 22 deviations, cannot be super-imposed into a master curve by a simple horizontal shift. In other words, the shape of de- pendence changes with composition. E. Gattiglia et al. 43 Figure 1. Viscosity of homopolymers vs. shear rate at 190 o C. In our case, also considering the rather complicate composition of the mix- tures, we do not intend to fully studythe rheological characteristics of thesema- terials, but simply to check whether such blends may present melt properties which can prevent processability with usual machinery. Figures 1 and 2 show the viscosity, η , vs. shear rate, & γ , of homopolymers and their blends, respectively. The addition of PP and PS reduces the viscosity of the blends in the range of 44 Recycling of Plastics from Urban Solid Wastes Figure 2. Viscosity of blends vs. shear rate at 190 o C. Table 3 Density and melt flow index measured at 190 o C and calculated by the additivity law Mixture MFI (g/10 min) Density experimental calculated (g/cm 3 ) Mix 1 1.19 1.57 0.9234 Mix 2 1.31 2.04 0.9261 Mix 3 1.81 2.01 0.9261 LDPE CF/LDPE EF = 77.8/22.2 1.71 0.92 - shear rates under investigation and then increases the value of the melt flow in- dex. In Table 3 experimental MFI values are compared with calculated values assuming the additivity law. The experimental values are lower than calculated suggesting that weak interactions among components exist, due to a similar chemical structure of blend ingredients. Therefore, it seems that the processability of blends does not present particular problem. Density In Table 3 the density values of the Mix 1, 2, and 3 are presented. These values are about 0.5% higher than those calculated according to the (additivity rule) weighted average of contributions of several components, assuming that the de- gree of crystallinity of crystallizable polymers is not affected by the blending. It can be thus deduced that the blends have a compact structure, without holes or voids. Morphology The morphologies of the fracture surfaces of the Mix 2, Mix 3, and Mix 4 are shown in Figures 3, 4, and 5. The morphology of Mix 1, not reported here, looks E. Gattiglia et al. 45 Figure 3. SEM micrograph of cold fractured surface of LDPE/HDPE/PP/PS = 81/9/8/2 blend. 46 Recycling of Plastics from Urban Solid Wastes Figure 4. SEM micrograph of cold fractured surface of LDPE/HDPE/PP/PS = 63/7/24/6 blend. Figure 5. SEM micrograph of cold fractured surface of LDPE/HDPE/PP/PS = 54/6/32/8 blend. guishable in such composition. As it will be discussed later, HDPE certainly gives rise to homodomains inside the LDPE matrix because it crystallizes before LDPE. However, from the point of view of the morphology LDPE/HDPE 90/10 mixture will be considered homogeneous. Mix 2 presents a ductile fractured PE matrix with inclusions of PP and PS not easily recognizable. The same morphological characteristics were observed in the case of Mix 5 from plastic wastes. The morphology of Mix 3 (Figure 4) pres- ents clearly visible PS and PP domains. PS domains break with a typical rough, globular surface while PPdomains,whosesize distribution is broad,on fracture, expose a circular cross-section with smooth surface. On increasing the amount of PP (Figure 5) to 32% (Mix 4) this component almost tends to form a co-contin- uous matrix with PE’s; PS is still visible as separated spherical particles. It is important to notice that in all blends PS and PP domains are well locked into the matrix. The interfacial adhesionis good and no voids are observed at the phase boundary. Of course the structures of the blends are not in thermody- namic equilibrium, as can be seen by melting and molding the samples in differ- ent processing conditions. However, no morphological modifications or shrinkage phenomena were observed by annealing of the mixtures at about 100 o C for more than two months. E. Gattiglia et al. 47 Figure 6. Optical micrograph of LDPE/HDPE/PP/PS = 63/7/24/6 blend at 180 o C. Parallel Nicols. Magnification 200 × . like a single phase material, meaning that the two components are indistin- Phase segregation is present also in the melt, as can be observed by the optical microscope. Figure 6 clearly shows segregated PS droplets at 180 o C, even at a few percent of PS as in Mix 3. On the contrary, PP domains in the molten state are visible only when its content is higher than 10 wt%. However, cooling down slowly one can follow the crystallization of PP as shown in Figure 7 for Mix 3 at 140 o C. Crystallization behavior Since the crystallization process is very important in controlling the morphol- ogy and thus the mechanical properties, we will discuss in more detail the melt behavior during cooling. In Figures 8 and 9 the effect of cooling rate on the crys- tallization temperature, T c , of the LDPE, HDPE, and PP pure and in mixture, is shown. The point of interest here is the fact that for cooling rate higher than 1 o C/min. the T c of HDPE is higher than PP and both are, as known, well above that of the LDPE. Only at verylow cooling rates, PP crystallizes before HDPEand probably at very fast cooling rate (> 40 o C/min), they crystallize at the same time. In Figure 8, the crystallization peaks of PP and HDPE components of Mix 2 merge 48 Recycling of Plastics from Urban Solid Wastes Figure 7. Optical micrograph of LDPE/HDPE/PP/PS = 63/7/24/6 blend at 140 o C. Crossed Nicols. Magnification 200 × . E. Gattiglia et al. 49 Figure 8. Crystallization temperatures of pure homopolymers and LDPE/HDPE/PP/PS = 81/9/8/2 blend as a function of cooling rate. Figure 9. Crystallizationtemperaturesofpurehomopolymersand LDPE/HDPE/PP/PS =54/6/32/8 blend as a function of cooling rate. multaneous crystallization of HDPE and PP is only due to particular conditions of cooling and blend compositions. In fact, increasing the PP content (Mix 4) two peaks occur, when the cooling rate is less than 10 o C/min (Figure 9). A comparison between the crystallization behaviors of the Mix 5, prepared from the light fraction of plastic wastes, and Mix 2, from virgin polymers only puts into evidence that LDPE of Mix 5 crystallizes earlier than LDPE of Mix 2. This agrees well with the smaller crystalline grains observed in optical micro- scope and may be attributed to some nucleating powerof the present impurities. The T c ‘s of PP and HDPE in the blends are lower than those of the pure homopolymers. On the contrary the T c of LDPE in the blend is a few degrees higher than that of the single component and the difference increases on in- creasing the cooling rate. This behavior is understandable considering the fa- vorable effect ofthe already crystallized HDPEand PP on the nucleationprocess of the LDPE. Therefore, the LDPE matrix crystallizes always when HDPE and PP are solid and PS is below its glass transition. During this crystallization a volume reduction occurs and the matrix shrinks over the domains of the dis- persed phases clinging them together very solidly. This is the main reason of a good contact between the matrixand the different polymer domains, observed in the electron microscope analysis. Mechanical properties Tensile behavior Values of tensile modulus, E, yielding stress, σ y , tensile strength, σ b , and elon- gation at break, ε b , of the homopolymers and mixtures are presented in Table 4. Modulus values of the homopolymers scatter by about ± 5% whereas for blends by about ± 8%. The scatter of σ b and σ y data ranges from ± 12% for homopolymers to about ± 18% in the case of blends values; ε b results have wider scatter ranging from ± 20 to ± 27%. Reducing the amount of LDPE in a blend, the modulus and the yield stress in- crease, whereas σ b does not practically change and ε b seems to reach a maximum for the Mix 3 having 70% PE. The Mix 5, prepared with the light fraction of plas- tic wastes, shows practically the same tensile modulus and strength as pure homopolymers (Mix 2); however, samples break before reaching the high defor- mation of Mix 2, probably due to defects created by impurities not completely re- moved during the flotation process. The increase of the E modulus, reducing the percentage of LDPE, is well below the additivity rule prediction, as shown in 50 Recycling of Plastics from Urban Solid Wastes into one and only one T c is detected in the range of cooling rate examined.This si- Figure 10, even if the morphology does not reveal any kind of holes or voids be- tween the dispersed phase domains and the matrix. If we consider the mixtures as a matrix-filler composite in which the matrix is the LDPE/HDPE blend andthe fillers are PP and PStaken together, it should be possible to compare our mechanical data with the model developed for poly- mer-filler systems by Nielsen. We recognize that this approximation is quite simplistic, because the difference between the moduli of LDPE and PP-PS com- ponents is not as high as in the case of a polymer matrix and an inorganic filler. Nevertheless, we think that this approach can be attempted making the follow- ing assumptions: • the matrix is a mixture of LDPE and HDPE in the constant weightratio 9/1 for all blends, and its modulus is that experimentally measured for theMix 1( ≈ 151 MPa) • the filler consists PP and PS ina constant ratio 4/1 and its modulus is taken as the weighted average ( ≈ 739 MPa) of the moduli of two components. E. Gattiglia et al. 51 Table 4 Tensile characteristics of injection molded specimens Sample E (MPa) σ y (MPa) σ b (MPa) ε b (%) LDPE CF 119 - 12 613 LDPE EF 143 - 13.8 548 HDPE 640 - 30.4 960 PP 704 - 25.2 720 PS 850 - 15.1 60 Mix 1 151 10.6 13.4 353 Mix 2 196 11.5 13.5 377 Mix 3 264 16.1 14.4 440 Mix 4 330 17.9 13.9 147 Mix 5 190 - 12.0 80 Mix 6 1026 - 22.2 - For a polymer-filler system, the Nielsen model 23,24 is described by the following equation: E=E(1+AB )/(1B ) b m ff φ−φψ [1] where: A=K f 1 E − [1a] B= (E E 1) /(E E + A) f m f m //− [1b] ψ =1+(1 )/( ) max max 2 f −⋅φΦΦ [1c] 52 Recycling of Plastics from Urban Solid Wastes Figure 10. Comparison between experimental and calculated tensile moduli of the mixtures. [...]... plastics As a consequence, plastics offer many advantages in packaging materials • The total amount of plastics present in municipal solid wastes is estimated to be currently about 7% of total waste The present paper shows the scheme followed in the development of a strategy of study and determination of recycling feasibility of the plastic waste fraction Recycling of urban plastic wastes 1 The initial... Data of mixtures containing less than 50% LDPE (Mix 3 and Mix 4) indicate very poor impact properties suggesting that 54 Recycling of Plastics from Urban Solid Wastes Table 5 Flexural modulus and impact strength of injection molded specimens Sample Flexural modulus IZOD (J/m) o (MPa) -23 C 0C o 30 C o LDPE CF 182 n.b n.b n.b LDPE EF 216 n.b n.b n.b HDPE 1035 196* 212* 268* PP 1 143 4 24 33 PS 1709 44 53... wastes” Three kinds of wastes are generated: municipal, agricultural, and uncontrolled plastic wastes The last group of plastic wastes must not be considered in the sense of a technical problem because they can only be avoided due to common consent of users of public 60 Management of Plastic Wastes facilities The agricultural plastic wastes can be collected and well classified in the place of their generation... general agreement has been reached on the real recycling possibilities of plastic wastes, considering the place and the manner in which wastes are generated and restriction to thermoplastic polymers The basic principles of recycling are included in studies conducted during the 1980s which considered the technical validity of recycling of various plastic wastes Most of these studies have been carried out by... costs of the packaging industries would be increased by approximately four hundred percent by the weight if non -plastic materials were used • An increase by two hundred and fifty percent by volume of wastes would become real if plastic packaging materials were not used • Approximately two hundred percent increase in energy consumption and costs of materials is feasible for packaging without plastics... Their knowledge on the recyclability of their industrial wastes was applied to solve the early 13-15 steps of the recyclability of the products manufactured by these companies It is important to mention that economic aspects play a secondary role under the expectations of regulations and laws which control recycling of plastic wastes The second and the biggest source of plastic wastes generation, which... and K Ito, Proceed Intl Conf Polym Process., Cambridge, Mass (1977) 23 T B Lewis and L E Nielsen, J Appl Polym Sci., 14, 144 9 (1970) 24 L E Nielsen, Mechanical Properties of Polymers and Composites, Vol 2, Marcel Dekker, New York, (19 74) 57 O Laguna Castellanos et al 59 Management of Plastic Wastes: Technical and Economic Approach O Laguna Castellanos, E Pérez Collar, and J Taranco González Instituto... depending on the geometry and size of the filler particles, as observed from the SEM morphology; f is the correction factor related to the Poisson’s ratio, ν, of the matrix; φf is the volume fraction of the filler; Φ max is the maximum packing fraction of the filler The SEM pictures offer evidence that the geometry of the dispersed phase is complex, due to the presence of ellipsoid and sphere shaped domains... question regards the composition of the source: • if plastics are mixed with other materials (glass, paper, organic), a separation is needed • if plastics are dirty (clays or similar contaminants), wastes must be cleaned • if the plastic waste consists of a polymer blend, the situation is much more complex The problem was studied by comparison between the polymer blend of the plastic waste fraction and polymer... 29, 2117 (19 84) D W Clegg, A A Collyer, and K Morton, Polymer Comm., 24, 10 (1983) R Wycisk, W M Trochimczuk, and J Matlys, Eur Polym J., 26, 5 (1990) L Bohn, Rubber Chem Technol., 41 , 49 5 (1968) S Astengo, Thesis, University of Genoa, (1989) A Serra, Thesis, University of Genoa, (1991) C Perrone, Poliplasti (Milan), 5, 72 (1987) Y Shimomura, J E Spruiell, and J L White, Polym Eng Rev., 2, 41 7 (1983) . 12 613 LDPE EF 143 - 13.8 548 HDPE 640 - 30 .4 960 PP 7 04 - 25.2 720 PS 850 - 15.1 60 Mix 1 151 10.6 13 .4 353 Mix 2 196 11.5 13.5 377 Mix 3 2 64 16.1 14. 4 44 0 Mix 4 330 17.9 13.9 147 Mix 5 190 -. fractured surface of LDPE/HDPE/PP/PS = 81/9/8/2 blend. 46 Recycling of Plastics from Urban Solid Wastes Figure 4. SEM micrograph of cold fractured surface of LDPE/HDPE/PP/PS = 63/7/ 24/ 6 blend. Figure. crystallization peaks of PP and HDPE components of Mix 2 merge 48 Recycling of Plastics from Urban Solid Wastes Figure 7. Optical micrograph of LDPE/HDPE/PP/PS = 63/7/ 24/ 6 blend at 140 o C. Crossed