- Email: [email protected]
Recycling of polymer waste: Part II—Stress degraded polypropylene
Polymer Degradation and Stability 20 (1988) 63-73
Recycling of Polymer Waste: Part II--Stress Degraded Polypropylene A. Valenza & F. P. La M a n t i a Istituto di Ingegneria Chimica, University of Palermo, Viale delle Scienze, 90128 Palermo, Italy (Received 27 July 1987; accepted 11 August 1987)
ABSTRACT Rheological and mechanical properties of blends of polypropylene and degraded polypropylene have been studied. Both properties, and particularly the elongation at break, depend on the composition and on the extent of degradation. The unusual behaviour of the elongation at break is correlated with crystalline phase segregation which appears with decreasing molecular weight of the degraded component.
INTRODUCTION The recycling of plastic waste is commonly carried out in many industries. This plastic waste can be conveniently reused because the extent of degradation undergone by the material is generally small and thus the structure and properties remain practically unchanged. 1 However, in some cases, because of the severity of the processing operations due to higher and higher flowrate outputs and because the same materials may be reused many times, the degradation undergone by the polymer can be extensive, resulting in significant changes in the structure of the material. In these cases, the blends made with these degraded materials often show properties different from those of the virgin material and strongly dependent on their molecular weight and chemical structure. 1-4 In this work, blends of virgin polypropylene with the same polymer degraded in different ways have been prepared in order to study rheological and mechanical properties as a function of the extent of degradation. 63
Polymer Degradation and Stability 0141-3910/88/$03-50 © Elsevier Applied Science Publishers Ltd, England, 1988. Printed in Great Britain
64
A. Valenza, F. P. La Mantia
Although, in some cases, the degradation is very extensive and probably unrealistic compared with the usual industrial situation, this work suggests some general points about the recycling of degraded materials and, in particular, about the properties to be expected for blends with a degraded homopolymer.
EXPERIMENTAL
Materials and preparation of blends The materials used in this work were two samples of isotactic polypropylene manufactured and kindly supplied by Himont (Italy). The properties of the raw materials, s and their sample codes are recorded in Table 1. These samples were subjected to thermomechanical degradation by processing in two different kinds of apparatus with different processing conditions, in order to obtain materials with different molecular weights and molecular structures. In particular one of these samples (PP2) was extruded in a Brabender laboratory extruder (D = 19mm, L/D=25) at a die temperature of 240°C and at 250 rpm. The corresponding output flowrate was 0"98 g/min. Both samples (PP2 and PP3) were processed in a Brabender Plasticorder equipped with a mixing chamber. A temperature of 200°C and a residence time of 15 min were maintained in all the tests, while the mixing speed was varied between 75 and 250 rpm. The sample codes of all the degraded materials and the processing conditions are reported in Table 2, together with their MFI values, and the number of carbonyl groups formed during the processing. The blends were prepared in the same Brabender laboratory extruder. The die temperature was 240°C, the rotational speed was 60 rpm and the output flowrate was 0.32 g/min. TABLE 1 Physico-chemical Characteristics of the Raw Materials Sample code
[r/] (dl/g) °
M F I (g/lOmin) b
Mw x 10 -3c
fld
PP2 PP3
2,00 2"65
3"5 0"39
390 680
8'6 9"7
a Intrinsic viscosity measured in tetrahydronaphthalene at 135°C. b Melt flow rate, according to the ASTM D 1238-73 method, procedure B. c Weight average molecular weight, by light scattering in ~t-chloronaphthalene at 150°C. d Polydispersity ratio, by GPC in orthodichlorobenzene at 135°C.
65
Recycling of polymer waste: Part H TABLE 2
Processing Conditions and Final Properties of the Degraded Polymers Sample code
PP2E PP2M75 PP2M100 PP2MI50 PP3M250
Apparatus Mixing speed (rpm)
Extruder Mixer Mixer Mixer Mixer
MFI (g/lO min)
C=O index
4"5 6-9 12"3 17 37
0"08 0"09 0"10 0.12 0.16
250 75 100 150 250
For most systems (PP2/PP2E, PP2/PP2M75, PP2/PP2M150, PP2/PP3M250), blends with a weight fraction, ~k, o f 0, 10, 25, 50, 75, 90 and 100% o f virgin PP2 were prepared. For the system PP2/PP2M100 only the blend with a 50% PP2 content was prepared. Rheologieal
measurements
The rheological measurements were carried out using a constant rate capillary viscometer, the Rheoscope 1000 by C E A S T (Italy). The viscometer was equipped with a 1 m m diameter capillary, having a length/diameter ratio of 40. The test temperature was 220°C and the investigated range o f apparent shear rate was 6-1000 s-1. Lower shear rates were sometimes necessary in order to reach the Newtonian viscosity: in such cases a constant force rheometer, a modified version o f a C E A S T melt indexer, was used equipped with the same capillary. Because o f the large L / D ratio, Bagley's correction for entrance effects has been neglected. The Rabinowitsch correction for the shear rate was applied in all cases. The calculations were performed using an apt computing program on a desk-top computer. In a few cases the zero shear viscosity was not obtained. In these cases, use was made o f Ferry's relation to evaluate r/o :6 1 1 - = - - + b~
r/
r/o
(1)
where r/is the shear viscosity at shear stress r. Mechanical
measurements
Tensile tests were carried out using a Universal Tensile Testing Machine, Instron model 1121. The elongational velocity was 10 cm/min and the initial gauge length, 3 cm.
A. Valenza, F. P. La Mantia
66
Structural determinations The calorimetric tests were performed using a Perkin-Elmer differential scanning calorimeter, DSC 4, linked to a Data Station Perkin-Elmer model 3600. The heating rate was 20°C/min. Infrared spectra were determined using a Perkin-Elmer infrared spectrometer model 1420 which was linked to the same Data Station. The carbonyl index was evaluated as the ratio between the absorbances at 1720 and 2730cm- 1.7 The morphology of some blends was observed under cross-polarized light with a Leitz microscope. The samples were sections of tensile specimens obtained with the aid of a microtome.
RESULTS A N D DISCUSSION The effect of degradation during melt processing is well demonstrated by the MFI data reported in Table 2. During extrusion, the increase in the melt flow index, and thus the reduction of the molecular weight, are not particularly large. On the contrary, processing performed in the mixer gives rise to a strongly degraded material. The extent of degradation rises with the rotational speed and with the molecular weight of the sample. The C~---O group data, also shown in Table 2, indicate that the extent of oxidation is very small under most conditions. Only the more highly degraded material shows a moderate presence of carbonyl groups. All the data indicate that industrial plastic waste differs from virgin materials mainly in molecular weight while the chemical structure is not significantly modified. Figures 1 and 2 show the flow curves for the PP2/PP2E and PP2/PP3M250 blends. As previously mentioned the decrease in viscosity is very large for PP3M250 and small for the sample degraded in the extrusion process. In both cases, however, the viscosities of the blends are between those of the parent polymers. Moreover, the differences between the viscosities of the homopolymers and blends are reduced on increasing the shear rate. In fact, the samples with high Newtonian viscosity show a more accentuated non-Newtonian behaviour and the flow curves approach one another with increasing shear rate. The influence of composition on the viscosity of the blends is shown in Fig. 3 for the same systems. Comparisons are made at the same shear stress values. The PP2/PP2E blends show viscosities between those of the homopolymers, very similar to, but less than, those expected by an additive rule. The PP2/PP3M250 blends also show viscosity values less than those
Recycling of polymer waste: Part H
Ul
PP21PP2E
67
,
lp
:1OO 90
a
Q.
o
50 25 10
•
1(;
10
l
10
10 ~
Fig. 1.
%
75
I
I
I
1
10
10 2
10
Viscosity versus shear rate for the system PP2/PP2E.
lO~
!
PP2~P3M250
lO' ¢0
o.
lO ~
~,
10
! -2
10
Fig. 2.
-1
10
$ 1
I
I
I
1
10
10 z
10 3
Viscosity versus shear rate for the system PP2/PP3M250. Key of symbols as in Fig. 1.
68
A. Valenza, F. P. La Mantia
10 4
3
10
2
10
10
0
50
100
Fig. 3. Viscosity versus composition at fixed values of the shear stress. Closed points refer to PP2/PP2E systems, open points to PP2/PP3M250. • ~ , z = 2- 103, • (D, z = 5.103, • I], z=1.104 , AA, z=3"104,VV,z=7"104.
expected by an additive rule, and a phase inversion is suggested by the Sshaped curve. In both cases the shapes of the curves do not change with shear stress. Some comments can be made for other blends not reported here. In particular, on increasing the extent of degradation of the recycled material, the r/-~k curves show a continuous change of shape from those of PP2/PP2E blends to those of PP2/PP3M250 blends. The mechanical properties, tensile strength and elongation at break, are reported in Figs 4 and 5 for all the samples. The tensile strength, a b, of the degraded samples decreases with the extent of degradation. All the blends show tensile strength values intermediate between those of the homopolymers. For the blends made with the more degraded materials, the tensile strength rises quickly with virgin PP content. For ~k > 5 0 0 , trb is almost independent of the composition and very similar to that of the pure polypropylene. All the systems show a minimum in the elongation at break-composition curve. This m i n i m u m depends strongly, however, on the extent of
Recycling of polymer waste: Part H 40
g
69
I
30
,
I
50
1 O0
2oJ 0
Fig. 4.
Tensile strength versus composition for all the blends. [] PP2/PP2E, • PP2/PP2M75, • PP2/PP2M150, • PP2/PP3M250, A PP2fPP2M100.
degradation o f the recycled polypropylene. In particular, P P 2 / P P 3 M 2 5 0 blends show a minimum at a b o u t ~, = 50% with a value of eb = 15%, while P P 2 / P P 2 M 7 5 blends show their minimum at a b o u t ~O= 25% with a value of eb - 2 0 % . P P 2 / P P 2 E blends show only a very slight minimum at a b o u t = 50% but with eb -~ 400%. In short, blends of virgin PP with degraded polypropylene show a minimum which becomes more and more strongly developed on decreasing the molecular weight o f this latter material. Only
. .n
10 0 Fig. 5.
• • L~ •
PP2/PP3M250 PP2/PP2M150 PP2/PP2MIO0 PP2/PP2M75
[] P P 2 / P P 2
| 50
E
100
Elongation at break versus composition for all the blends.
70
A. Valenza, F. P. La Mantia
for very high contents o f virgin polypropylene is it possible to achieve blends with high elongation at break values. It is thus possible to obtain ductile and fragile blends by changing the content and the molecular weight o f the recycled material. In particular, when the M F I decreases from 17 to 6 the virgin PP content necessary to achieve an elongation at break of 100% decreases from ,~ 60% to ,-~45 %. When the recycled polypropylene is slightly degraded (MFI = 4.5 as opposed to the original value of 3.5) the %-~, curve shows only a small m i n i m u m but all the blends have a ductile fracture. In order to give some quantitative measure o f the influence of the molecular weight o f the degraded c o m p o n e n t on the elongation at break of the blends, we have plotted eb as a function o f the melt flow index at various blend compositions, Fig. 6. We have preferred to use the melt index in order to have an easily obtained parameter which depends on the molecular weight. It is very obvious that l~or a virgin PP content o f 75%, eb is very high and
l I
I
I
I
I
I
,,~P : r s % 103
~
60
•
25
I
M FII 30
•
10 10
Fig. 6.
•
t 20
50
Elongation at break of different blends as a function of the MFI of the degraded component. Data for ~, = 60% are interpolated from the curve of Fig. 5.
Recycling of polymer waste: Part H
71
almost independent of the MFI value of the recycled material, but at lower contents of pure polypropylene the situation is completely different. In particular, on increasing the concentration of degraded material, the melt flow index necessary to obtain high elongation at break values strongly decreases and a sharp increase occurs in a narrow range of MFI values. In order to explain this unusual behaviour, calorimetric tests have been made and optical microscopy photographs obtained on samples with --50%. The calorimetric curves for the PP2/PP2E, PP2/PP2M75 and PP2/PPM 150 samples are illustrated in Fig. 7. It is very obvious that for the first sample, only the normal peak of the polypropylene is present, while for the last samples a new endotherrnic peak appears at about 130°C. The PP2/PP2M75 blend shows a very small peak at the same temperature. The presence of a new peak can be associated with the segregation of a new crystalline phase. This crystalline phase is present in significant amounts only when the second component of the blend has a very low molecular weight. Evidence of this new morphology is clearly seen in Fig. 8 where optical microscopy photographs of the same blends are presented. The most important feature is that the PP2/PP2E blend shows spherulites of about the same size throughout all the sample. On the contrary the PP2/PP2M150 blend presents islands of spherulites of different sizes. In particular,
-- PP2/PP2E
/i/ /!
T
0
50
i
i
100
150
°C
2 O0
Fig. 7. Calorimetric curves of the blends: PP2/PP2M150, PP2/PP2M75, PP2/PP2E for ¢, = 5 0 % .
72
A. Valenza, F. P. La Mantia
(a)
(b)
(c) Fig. 8. Optical microscopy photographs of microtomed sections: (a) PP2/PP2M150; (b) PP2/PP2M75; (c) PP2/PP2E.
Recycling of polymer waste: Part H
73
spherulites of large size are seen near spherulites of smaller size and similar to the spherulites of the PP2/PP2E sample. Finally, the PP2/PP2M75 blend shows spherulites of slightly different size, and a small amount of segregated phase which becomes large when the difference in molecular weight between the two components increases. As noted by other authors, a probably high melting temperature crystalline regions of higher molecular weight polypropylene act as a nucleating agent for the lower molecular weight polypropylene. Of course the very unusual morphology shown by the PP2/PP3M250 sample induces brittle fracture because of the weak interspherulitic boundaries both among the large spherulites and at the boundaries between the zones with crystallites of different size.
ACKNOWLEDGEMENT This work has been financially supported by M.P.I. Thanks are due to 'Istituto per la Chimica e la Tecnologia dei Materiali Polimerici' for allowing use of the Rheoscope 1000.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
J. Leidner, Plastic Waste, Marcel Dekker, New York (1981). K. B. Abbas, A. B. Knutsson and S. H. Berglund, Chemtech., 8, 502 (1978). K. B. Abbas, Polym. Eng. Sci., 20, 376 (1980). A. Valenza and F. P. La Mantia, Poly. Deg. and Stab., 19, 135 (1987). D. Romanini and G. Pezzin, Rheol. Acta, 21, 699 (1982). J. D. Ferry, J. Am. Chem. Soc., 6, 1330 (1942). A. Garton, D. J. Carlsson and D. M. Wiles, J. Polym. Sci. Chem. Ed., 16, 33 (1978). B. L. Deopura and S. Kadam, J. Appl. Polym. Sci., 31, 2145 (1986).
Recycling of Polymer Waste: Part II--Stress Degraded Polypropylene A. Valenza & F. P. La M a n t i a Istituto di Ingegneria Chimica, University of Palermo, Viale delle Scienze, 90128 Palermo, Italy (Received 27 July 1987; accepted 11 August 1987)
ABSTRACT Rheological and mechanical properties of blends of polypropylene and degraded polypropylene have been studied. Both properties, and particularly the elongation at break, depend on the composition and on the extent of degradation. The unusual behaviour of the elongation at break is correlated with crystalline phase segregation which appears with decreasing molecular weight of the degraded component.
INTRODUCTION The recycling of plastic waste is commonly carried out in many industries. This plastic waste can be conveniently reused because the extent of degradation undergone by the material is generally small and thus the structure and properties remain practically unchanged. 1 However, in some cases, because of the severity of the processing operations due to higher and higher flowrate outputs and because the same materials may be reused many times, the degradation undergone by the polymer can be extensive, resulting in significant changes in the structure of the material. In these cases, the blends made with these degraded materials often show properties different from those of the virgin material and strongly dependent on their molecular weight and chemical structure. 1-4 In this work, blends of virgin polypropylene with the same polymer degraded in different ways have been prepared in order to study rheological and mechanical properties as a function of the extent of degradation. 63
Polymer Degradation and Stability 0141-3910/88/$03-50 © Elsevier Applied Science Publishers Ltd, England, 1988. Printed in Great Britain
64
A. Valenza, F. P. La Mantia
Although, in some cases, the degradation is very extensive and probably unrealistic compared with the usual industrial situation, this work suggests some general points about the recycling of degraded materials and, in particular, about the properties to be expected for blends with a degraded homopolymer.
EXPERIMENTAL
Materials and preparation of blends The materials used in this work were two samples of isotactic polypropylene manufactured and kindly supplied by Himont (Italy). The properties of the raw materials, s and their sample codes are recorded in Table 1. These samples were subjected to thermomechanical degradation by processing in two different kinds of apparatus with different processing conditions, in order to obtain materials with different molecular weights and molecular structures. In particular one of these samples (PP2) was extruded in a Brabender laboratory extruder (D = 19mm, L/D=25) at a die temperature of 240°C and at 250 rpm. The corresponding output flowrate was 0"98 g/min. Both samples (PP2 and PP3) were processed in a Brabender Plasticorder equipped with a mixing chamber. A temperature of 200°C and a residence time of 15 min were maintained in all the tests, while the mixing speed was varied between 75 and 250 rpm. The sample codes of all the degraded materials and the processing conditions are reported in Table 2, together with their MFI values, and the number of carbonyl groups formed during the processing. The blends were prepared in the same Brabender laboratory extruder. The die temperature was 240°C, the rotational speed was 60 rpm and the output flowrate was 0.32 g/min. TABLE 1 Physico-chemical Characteristics of the Raw Materials Sample code
[r/] (dl/g) °
M F I (g/lOmin) b
Mw x 10 -3c
fld
PP2 PP3
2,00 2"65
3"5 0"39
390 680
8'6 9"7
a Intrinsic viscosity measured in tetrahydronaphthalene at 135°C. b Melt flow rate, according to the ASTM D 1238-73 method, procedure B. c Weight average molecular weight, by light scattering in ~t-chloronaphthalene at 150°C. d Polydispersity ratio, by GPC in orthodichlorobenzene at 135°C.
65
Recycling of polymer waste: Part H TABLE 2
Processing Conditions and Final Properties of the Degraded Polymers Sample code
PP2E PP2M75 PP2M100 PP2MI50 PP3M250
Apparatus Mixing speed (rpm)
Extruder Mixer Mixer Mixer Mixer
MFI (g/lO min)
C=O index
4"5 6-9 12"3 17 37
0"08 0"09 0"10 0.12 0.16
250 75 100 150 250
For most systems (PP2/PP2E, PP2/PP2M75, PP2/PP2M150, PP2/PP3M250), blends with a weight fraction, ~k, o f 0, 10, 25, 50, 75, 90 and 100% o f virgin PP2 were prepared. For the system PP2/PP2M100 only the blend with a 50% PP2 content was prepared. Rheologieal
measurements
The rheological measurements were carried out using a constant rate capillary viscometer, the Rheoscope 1000 by C E A S T (Italy). The viscometer was equipped with a 1 m m diameter capillary, having a length/diameter ratio of 40. The test temperature was 220°C and the investigated range o f apparent shear rate was 6-1000 s-1. Lower shear rates were sometimes necessary in order to reach the Newtonian viscosity: in such cases a constant force rheometer, a modified version o f a C E A S T melt indexer, was used equipped with the same capillary. Because o f the large L / D ratio, Bagley's correction for entrance effects has been neglected. The Rabinowitsch correction for the shear rate was applied in all cases. The calculations were performed using an apt computing program on a desk-top computer. In a few cases the zero shear viscosity was not obtained. In these cases, use was made o f Ferry's relation to evaluate r/o :6 1 1 - = - - + b~
r/
r/o
(1)
where r/is the shear viscosity at shear stress r. Mechanical
measurements
Tensile tests were carried out using a Universal Tensile Testing Machine, Instron model 1121. The elongational velocity was 10 cm/min and the initial gauge length, 3 cm.
A. Valenza, F. P. La Mantia
66
Structural determinations The calorimetric tests were performed using a Perkin-Elmer differential scanning calorimeter, DSC 4, linked to a Data Station Perkin-Elmer model 3600. The heating rate was 20°C/min. Infrared spectra were determined using a Perkin-Elmer infrared spectrometer model 1420 which was linked to the same Data Station. The carbonyl index was evaluated as the ratio between the absorbances at 1720 and 2730cm- 1.7 The morphology of some blends was observed under cross-polarized light with a Leitz microscope. The samples were sections of tensile specimens obtained with the aid of a microtome.
RESULTS A N D DISCUSSION The effect of degradation during melt processing is well demonstrated by the MFI data reported in Table 2. During extrusion, the increase in the melt flow index, and thus the reduction of the molecular weight, are not particularly large. On the contrary, processing performed in the mixer gives rise to a strongly degraded material. The extent of degradation rises with the rotational speed and with the molecular weight of the sample. The C~---O group data, also shown in Table 2, indicate that the extent of oxidation is very small under most conditions. Only the more highly degraded material shows a moderate presence of carbonyl groups. All the data indicate that industrial plastic waste differs from virgin materials mainly in molecular weight while the chemical structure is not significantly modified. Figures 1 and 2 show the flow curves for the PP2/PP2E and PP2/PP3M250 blends. As previously mentioned the decrease in viscosity is very large for PP3M250 and small for the sample degraded in the extrusion process. In both cases, however, the viscosities of the blends are between those of the parent polymers. Moreover, the differences between the viscosities of the homopolymers and blends are reduced on increasing the shear rate. In fact, the samples with high Newtonian viscosity show a more accentuated non-Newtonian behaviour and the flow curves approach one another with increasing shear rate. The influence of composition on the viscosity of the blends is shown in Fig. 3 for the same systems. Comparisons are made at the same shear stress values. The PP2/PP2E blends show viscosities between those of the homopolymers, very similar to, but less than, those expected by an additive rule. The PP2/PP3M250 blends also show viscosity values less than those
Recycling of polymer waste: Part H
Ul
PP21PP2E
67
,
lp
:1OO 90
a
Q.
o
50 25 10
•
1(;
10
l
10
10 ~
Fig. 1.
%
75
I
I
I
1
10
10 2
10
Viscosity versus shear rate for the system PP2/PP2E.
lO~
!
PP2~P3M250
lO' ¢0
o.
lO ~
~,
10
! -2
10
Fig. 2.
-1
10
$ 1
I
I
I
1
10
10 z
10 3
Viscosity versus shear rate for the system PP2/PP3M250. Key of symbols as in Fig. 1.
68
A. Valenza, F. P. La Mantia
10 4
3
10
2
10
10
0
50
100
Fig. 3. Viscosity versus composition at fixed values of the shear stress. Closed points refer to PP2/PP2E systems, open points to PP2/PP3M250. • ~ , z = 2- 103, • (D, z = 5.103, • I], z=1.104 , AA, z=3"104,VV,z=7"104.
expected by an additive rule, and a phase inversion is suggested by the Sshaped curve. In both cases the shapes of the curves do not change with shear stress. Some comments can be made for other blends not reported here. In particular, on increasing the extent of degradation of the recycled material, the r/-~k curves show a continuous change of shape from those of PP2/PP2E blends to those of PP2/PP3M250 blends. The mechanical properties, tensile strength and elongation at break, are reported in Figs 4 and 5 for all the samples. The tensile strength, a b, of the degraded samples decreases with the extent of degradation. All the blends show tensile strength values intermediate between those of the homopolymers. For the blends made with the more degraded materials, the tensile strength rises quickly with virgin PP content. For ~k > 5 0 0 , trb is almost independent of the composition and very similar to that of the pure polypropylene. All the systems show a minimum in the elongation at break-composition curve. This m i n i m u m depends strongly, however, on the extent of
Recycling of polymer waste: Part H 40
g
69
I
30
,
I
50
1 O0
2oJ 0
Fig. 4.
Tensile strength versus composition for all the blends. [] PP2/PP2E, • PP2/PP2M75, • PP2/PP2M150, • PP2/PP3M250, A PP2fPP2M100.
degradation o f the recycled polypropylene. In particular, P P 2 / P P 3 M 2 5 0 blends show a minimum at a b o u t ~, = 50% with a value of eb = 15%, while P P 2 / P P 2 M 7 5 blends show their minimum at a b o u t ~O= 25% with a value of eb - 2 0 % . P P 2 / P P 2 E blends show only a very slight minimum at a b o u t = 50% but with eb -~ 400%. In short, blends of virgin PP with degraded polypropylene show a minimum which becomes more and more strongly developed on decreasing the molecular weight o f this latter material. Only
. .n
10 0 Fig. 5.
• • L~ •
PP2/PP3M250 PP2/PP2M150 PP2/PP2MIO0 PP2/PP2M75
[] P P 2 / P P 2
| 50
E
100
Elongation at break versus composition for all the blends.
70
A. Valenza, F. P. La Mantia
for very high contents o f virgin polypropylene is it possible to achieve blends with high elongation at break values. It is thus possible to obtain ductile and fragile blends by changing the content and the molecular weight o f the recycled material. In particular, when the M F I decreases from 17 to 6 the virgin PP content necessary to achieve an elongation at break of 100% decreases from ,~ 60% to ,-~45 %. When the recycled polypropylene is slightly degraded (MFI = 4.5 as opposed to the original value of 3.5) the %-~, curve shows only a small m i n i m u m but all the blends have a ductile fracture. In order to give some quantitative measure o f the influence of the molecular weight o f the degraded c o m p o n e n t on the elongation at break of the blends, we have plotted eb as a function o f the melt flow index at various blend compositions, Fig. 6. We have preferred to use the melt index in order to have an easily obtained parameter which depends on the molecular weight. It is very obvious that l~or a virgin PP content o f 75%, eb is very high and
l I
I
I
I
I
I
,,~P : r s % 103
~
60
•
25
I
M FII 30
•
10 10
Fig. 6.
•
t 20
50
Elongation at break of different blends as a function of the MFI of the degraded component. Data for ~, = 60% are interpolated from the curve of Fig. 5.
Recycling of polymer waste: Part H
71
almost independent of the MFI value of the recycled material, but at lower contents of pure polypropylene the situation is completely different. In particular, on increasing the concentration of degraded material, the melt flow index necessary to obtain high elongation at break values strongly decreases and a sharp increase occurs in a narrow range of MFI values. In order to explain this unusual behaviour, calorimetric tests have been made and optical microscopy photographs obtained on samples with --50%. The calorimetric curves for the PP2/PP2E, PP2/PP2M75 and PP2/PPM 150 samples are illustrated in Fig. 7. It is very obvious that for the first sample, only the normal peak of the polypropylene is present, while for the last samples a new endotherrnic peak appears at about 130°C. The PP2/PP2M75 blend shows a very small peak at the same temperature. The presence of a new peak can be associated with the segregation of a new crystalline phase. This crystalline phase is present in significant amounts only when the second component of the blend has a very low molecular weight. Evidence of this new morphology is clearly seen in Fig. 8 where optical microscopy photographs of the same blends are presented. The most important feature is that the PP2/PP2E blend shows spherulites of about the same size throughout all the sample. On the contrary the PP2/PP2M150 blend presents islands of spherulites of different sizes. In particular,
-- PP2/PP2E
/i/ /!
T
0
50
i
i
100
150
°C
2 O0
Fig. 7. Calorimetric curves of the blends: PP2/PP2M150, PP2/PP2M75, PP2/PP2E for ¢, = 5 0 % .
72
A. Valenza, F. P. La Mantia
(a)
(b)
(c) Fig. 8. Optical microscopy photographs of microtomed sections: (a) PP2/PP2M150; (b) PP2/PP2M75; (c) PP2/PP2E.
Recycling of polymer waste: Part H
73
spherulites of large size are seen near spherulites of smaller size and similar to the spherulites of the PP2/PP2E sample. Finally, the PP2/PP2M75 blend shows spherulites of slightly different size, and a small amount of segregated phase which becomes large when the difference in molecular weight between the two components increases. As noted by other authors, a probably high melting temperature crystalline regions of higher molecular weight polypropylene act as a nucleating agent for the lower molecular weight polypropylene. Of course the very unusual morphology shown by the PP2/PP3M250 sample induces brittle fracture because of the weak interspherulitic boundaries both among the large spherulites and at the boundaries between the zones with crystallites of different size.
ACKNOWLEDGEMENT This work has been financially supported by M.P.I. Thanks are due to 'Istituto per la Chimica e la Tecnologia dei Materiali Polimerici' for allowing use of the Rheoscope 1000.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
J. Leidner, Plastic Waste, Marcel Dekker, New York (1981). K. B. Abbas, A. B. Knutsson and S. H. Berglund, Chemtech., 8, 502 (1978). K. B. Abbas, Polym. Eng. Sci., 20, 376 (1980). A. Valenza and F. P. La Mantia, Poly. Deg. and Stab., 19, 135 (1987). D. Romanini and G. Pezzin, Rheol. Acta, 21, 699 (1982). J. D. Ferry, J. Am. Chem. Soc., 6, 1330 (1942). A. Garton, D. J. Carlsson and D. M. Wiles, J. Polym. Sci. Chem. Ed., 16, 33 (1978). B. L. Deopura and S. Kadam, J. Appl. Polym. Sci., 31, 2145 (1986).