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polyethylene blends
Polymer Degradation and Stability 16 (1986) 347-359
Thermoplastic Elastomers: Part 2 Effect of Natural Rubber/Polypropylene as Solid Phase Dispersant in Polypropylene/Polyethylene Blends S. A1-Malaika & E. J. Amir* Department of Molecular Sciences, Aston University, Aston Triangle, Birmingham B4 7ET, Great Britain (Received 22 July 1986; accepted 2 August 1986)
ABSTRACT The role of natural rubber/polypropylene ( NR/ PP) thermoplastic elastomer was examined as a solid phase dispersant (SPD) in the incompatible binary blend of polypropylene/low-density polyethylene (PP/LDPE). The mechanical performance of the resultant blend was compared with that of the same binary blend but in the presence of ethylene propylene diene rubber (EPDM) which was shown from previous studies to be a good SPD for this blend. The evidence presented here shows that the overall mechanical performance of the polyblend is effectively improved in the presence of NR/PP. Although the improvement in impact properties is not as high as that offered by EPDM, it is shown that the use of NR/PP as SPD offers the advantages of higher tensile strength and modulus to the blend. Both EPDM and NR/PP sensitise photo-oxidation of the polyblend, NR/PP showing a higher level of sensitisation. This difference in behaviour is attributed mainly to the difference in the level and position of the reactive unsaturated sites in the two SPD systems.
INTRODUCTION It has been shown that the incorporation of polypropylene (PP) in lowdensity polyethylene (LDPE) by conventional melt processing olSerations is detrimental to the mechanical behaviour o f the latter, x This was * Present address: Research Institute for Estate Crops, Bogor, Indonesia. 347 Polymer Degradation and Stability 0141-3910/86/$03"50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain
348
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attributed to the higher susceptibility of PP to thermal- and photooxidation. The addition of elastomeric solid phase dispersant (SPD is a copolymer containing segments compatible with the incompatible phases of the blend leading to 'compatibilisation' by more effectively dispersing the second phase in the matrix) such as ethylene-propylene diene rubber (EPDM) markedly improves the initial mechanical properties of such blends although it adversely affects their photo-oxidative stability. 2 Both chain branching (in PP) and unsaturation (in EPDM) have been shown to be responsible for this oxidative instability. 2 The purpose of this investigation is to explore the possibility of using natural rubber (NR)/polypropylene (PP), thermoplastic elastomer at a weight ratio of 60/40, as the SPD in PP/LDPE blends, and to compare its behaviour with that of EPDM (the mechanochemical formation and performance of this NR/PP blend has been discussed recentlya). The photo-oxidative stability of the blend in the presence of both SPD systems was also investigated.
EXPERIMENTAL
Materials Polypropylene (HWM25) and low-density polyethylene (Alkathene30) were supplied by ICI Ltd, natural rubber (SMR5) by MRPRA and ethylene propylene diene rubber by Exxon Chemicals Ltd under the trade name Vistalon 6505. Cyasorb UV531 and Cyasorb 1084 were supplied by American Cyanamid and the hindered piperidine, Tinuvin 770, by Ciba Geigy.
Polymer processing and testing The polyblend used, PP/LDPE (50:50 weight ratio), contained up to 30% NR/PP or EPDM of the total formulation. Blending was carried out at 180°C in a Hampden-RAPRA torque rheometer for 5min in a closed mixer. The processed blend samples were then compression moulded at 180°C into sheets 200#m thick. The NR/PP (60:40 weight ratio) blend was processed with 0.3% dicumyl peroxide in the torque rheometer at 160°C for 10 min (closed mixer) as described previously. 3 Tensile tests were carried out on an Instron tensile tester (Model TMTS) using a crosshead speed of 3cm per rain at 20°C. The falling dart method described previously4 was used to measure the impact strength of the polymer blends.
The use of NR/PP as solid phase dispersant
349
Six samples were tested for each measurement of tensile properties and impact strength. Dynamic mechanical properties were measured on a dynamic viscoelastometer (Rheovibron, R) as described previously. 5 Melt flow index (MFI) measurements were carried out on a Davenport melt flow indexer at 230°C using a load of 50 Newton. Calculated MFIs are the average of five measurements for each sample. Scanning electron microscopy (using a Cambridge S150 scanning electron microscope) was carried out on fractured surfaces of polymer blends (after a Charpy impact test). The test specimen was cut with a sharp knife to give an 0"05-cmthick sample which was mounted on a stub and vacuum-coated with a 100-,~-thin layer of gold. RESULTS
Effect of solid phase dispersants on mechanical performance of PP/LDPE blends Figure 1 compares the effect of NR/PP as a solid phase dispersant with that of EPDM on the mechanical performance of PP/LDPE blends. Impact strength and elongation at break were found to increase with increasing concentration of both dispersants although the increase is more marked in the presence of EPDM. It is known that toughening is achieved by transferring large amounts of the externally applied energy from the matrix to the rubber particles; hence the formation of cracks in the rubber phase. 6 Examination of scanning electron micrographs of fractured surfaces of PP/LDPE blends containing different concentrations of NR/PP (Figs 2-6) shows clearly that increasing the concentration of the SPD causes the fracture (where the blend PP/LDPE fails during impact) to occur in the rubber particles instead of the PP/LDPE matrix, leading to higher impact strength (see Fig. 1 inset). The increase in impact strength is parallelled by a concomitant decrease in tensile strength and modulus (Fig. 1). In this case, however, NR/PP offer higher tensile properties at all concentrations when compared to EPDM. Figure 7 compares the dynamic mechanical properties of the polyblend in the presence of NR/PP and EPDM. The mechanical damping (tan c~at 20°C) increases with increasing concentration of both SPDs while the complex modulus (E*) decreases. The addition of rubber, which has a lower glass transition temperature than the thermoplastic components, is expected to affect the viscoelasticity of the blend. The rubber will also increase the flexibility of the blend and, consequently, decrease the modulus (E*). It was shown previously that, for high-impact polystyrene, increasing amounts of the polybutadiene rubber cause an
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Fig. 2. SEM micrograph of a fractured surface of PP/LDPE (50:50 weight ratio) blend.
Fig. 3.
SEM micrograph of a fractured surface of PP/LDPE containing 5% NR/PP.
Fig. 4. SEM micrograph of a fractured surface of PP/LDPE containing 10% NR/PP.
Fig. 5. SEM micrograph of a fractured surface of PP/LDPE containing 20% NR/PP.
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SEM micrograph of a fractured surface of PP/LDPE containing 25% NR/PP.
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Fig. 8. Efectof NR/PP and EPDM on the melt flow index of PP/LDPE blends. increase in the mechanical damping and a decrease in the complex modulus. 5 This may explain the present observation that the use of EPDM (which has higher rubber content) in PP/LDPE blends gives higher tan (9 and lower E* values when compared with the use of NR/PP as a solid phase dispersant. Furthermore, blends containing EPDM show lower MFI values than the corresponding NR/PP-containing blends (Fig. 8); this indicates a higher viscosity of blends containing EPDM, which is attributed to higher rubber content.
Effect of SPD on photo-oxidative stability of P P / L D P E blends Figures 9 and 10 compare the behaviour of different concentrations of EPDM and NR/PP on the photo-oxidative stability of PP/LDPE blends. It is clear that, although both exert pro-oxidant effects, stability is more adversely affected by NR/PP. Blends containing NR/PP show a much higher build-up of carbonyl functional groups (a consequence of higher levels of hydroperoxides in the blend), which is parallelled by a faster rate of decay in impact strength with increasing irradiation time as compared to analogous EPDM-containing blends. Figure 11 shows the effect of commercial uv stabilisers on the photo-oxidative stability of PP/LDPE blends in the presence of 20% of SPD. The use of a combination of commercial uv stabilisers, e.g. substituted 2-hydroxy benzophenone (Cyasorb UV531) and a nickel complex (Cyasorb 1084) affords good stability to both systems but the commercial hindered piperidine, Tinuvin 770, does not improve substantially the stability of blends containing NR/PP. Changes in impact strength of the same samples follow similar trends.
The use of NR/PP as solid phase dispersant
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DISCUSSION The results show clearly that the mechanochemically-produced thermoplastic elastomer N R / P P can be used as SPD for P P / L D P E blends. All the initial mechanical properties of the blend are markedly improved although enhancement in impact strength is not as high as that obtained with EPDM. However, N R / P P has the advantage of contributing higher tensile strength and modulus to the blend and is much cheaper and easier
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Fig. 11. Photo-oxidative stability of PP/LDPE blends containing 20% of each SPD system (NR/PP, - - - ) and (EPDM, - - - ) in the absence and presence of uv stabilisers. Numbers on curves represent the type of uv stabiliser, Tinuvin 770, Cyasorb UV531 and Cyasorb 1084. Inset shows the corresponding decay in impact strength of the same films.
358
s. AI-Malaika, E. J. Amir
to produce than EPDM. It was shown i that the main factor necessary to achieve good impact resistance and elongation to break of polymer blends is the presence of a rubbery interphase between the heteropolymers. However, tensile strength depends more on the dispersant properties of the third component. The higher impact strength offered by EPDM to the blend (Fig. 1 inset) is most certainly due to the fact that EPDM has a higher rubber content than NR/PP; hence more energy is transferred to, and dissipated by, the rubber phase. Moreover, the structure of the constituents of EPDM bears a resemblance to both PP and LDPE in the blend, and this contributes to greater compatibility between the two phases and hence results in a tougher (higher impact strength) blend. The processing operation has some effect on the overall performance of the blend when processed in t h e presence of different dispersants. The lower melt flow index values in the polyblend in the presence of all concentrations of EPDM compared to that of NR/PP (Fig. 8) indicates higher viscosity in the former case which may contribute to less efficient dispersion during the shearing process, hence the observed lower tensile strength and modulus values (see Fig. 1). It is clear that at the concentrations at which dispersants have an appreciable, beneficial effect on mechanical performance (Fig. 1), they have a deleterious effect on the long-term photo-oxidative stability of the blend (Figs 9 and 10)" It has been shown 5'7 that photo-oxidation in polymer blends does not occur uniformly; oxidation occurs much more rapidly in the rubber phase than in the saturated polymers. The main reason for the high susceptibility of the rubber phase to photo-oxidation is the presence of olefinic unsaturation which is an effective pro-oxidant even when present in minor proportions. This was shown 4 to be due to the formation of hydroperoxides which not only lead to sensitising oxidation of the saturated polymer but are also the cause of rapid changes in mechanical behaviour due to concomitant crosslinking and chain scission reactions leading to the loss of the useful impact properties of the materials. The difference in the extent of the pro-oxidant effect shown by EPDM and NR/PP (Figs 9 and 10) is due to differences in the compositions; N R has a much higher unsaturation level than EPDM (14.3% isoprene in N R compared to only 0.6-1.0 mole per cent diene in EPDM s); hence, higher levels of hydroperoxides are expected to build up in the former which are mainly responsible for the higher photo-oxidative instability in the case of blends containing NR/PP. Furthermore, differences in the location of the unsaturation sites for the two SPD samples must also contribute to the observed differences in photo-oxidative stability of the blends. In the case of EPDM, unsaturation is located on the side chains pendant to the main chain but their effect on the properties of the blend is expected to be
The use of NR/PP as solid phase dispersant
359
minimal. In NR, however, unsaturation is distributed throughout the main chain, The primary function of photostabilisers in polymer blends is to inhibit the destruction of the SPD which is responsible for the toughness of the blend. Figure 11 shows clearly that a synergistic commercial uv stabiliser system enhances the photo-oxidative stability of blends containing both types of solid phase dispersant. The behaviour of the commercial nitroxyl precursor, Tinuvin 770, is different; it stabilises the EPDM-containing blends to the same extent as the uv absorbers but does not offer any appreciable stabilising effect to the NR/PP-containing blends. The combination of the much cheaper uv stabilisers (UV531 + 1084) is, therefore, a more cost-effective stabiliser system for P P / L D P E blends containing SPD systems based on E P D M or NR/PP.
A C K N O W L E D G E M ENTS We are grateful to the Indonesian Government and the Research Institute of Estate Crops for financial support and study leave for one of us, E.J.A.
REFERENCES 1. A. Ghaffar, C. Sadermohaghegh and G. Scott, Europ. Polym. J., 17, 941 (1981). 2. C. Sadermohaghegh, G. Scott and E. Setoudeh, Poly. Deg. and Stab., 3, 469 (1980-81). 3. S. AI-Malaika and E. J. Amir, J. Natural Rubber Research 1, 104 (1986). 4. G. Scott and M. Tahan, Europ. Polym. J., 13, 981 (1977). 5. A. Ghaffar, A. Scott and G. Scott, Europ. Polym. J., 11, 271 (1975). 6. C. B. Bucknall and R. R. Smith, Polymer, 6, 437 (1965). 7. M. Ghaemy and G. Scott, Poly. Deg. and Stab., 3, 233 (1981). 8. J. P. Daugherty, Ethylene-propylene rubbers, Enjay Polymer Lab., Linden, New Jersey, 3 0969).
Thermoplastic Elastomers: Part 2 Effect of Natural Rubber/Polypropylene as Solid Phase Dispersant in Polypropylene/Polyethylene Blends S. A1-Malaika & E. J. Amir* Department of Molecular Sciences, Aston University, Aston Triangle, Birmingham B4 7ET, Great Britain (Received 22 July 1986; accepted 2 August 1986)
ABSTRACT The role of natural rubber/polypropylene ( NR/ PP) thermoplastic elastomer was examined as a solid phase dispersant (SPD) in the incompatible binary blend of polypropylene/low-density polyethylene (PP/LDPE). The mechanical performance of the resultant blend was compared with that of the same binary blend but in the presence of ethylene propylene diene rubber (EPDM) which was shown from previous studies to be a good SPD for this blend. The evidence presented here shows that the overall mechanical performance of the polyblend is effectively improved in the presence of NR/PP. Although the improvement in impact properties is not as high as that offered by EPDM, it is shown that the use of NR/PP as SPD offers the advantages of higher tensile strength and modulus to the blend. Both EPDM and NR/PP sensitise photo-oxidation of the polyblend, NR/PP showing a higher level of sensitisation. This difference in behaviour is attributed mainly to the difference in the level and position of the reactive unsaturated sites in the two SPD systems.
INTRODUCTION It has been shown that the incorporation of polypropylene (PP) in lowdensity polyethylene (LDPE) by conventional melt processing olSerations is detrimental to the mechanical behaviour o f the latter, x This was * Present address: Research Institute for Estate Crops, Bogor, Indonesia. 347 Polymer Degradation and Stability 0141-3910/86/$03"50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain
348
S. AI-Malaika, E. J. Amir
attributed to the higher susceptibility of PP to thermal- and photooxidation. The addition of elastomeric solid phase dispersant (SPD is a copolymer containing segments compatible with the incompatible phases of the blend leading to 'compatibilisation' by more effectively dispersing the second phase in the matrix) such as ethylene-propylene diene rubber (EPDM) markedly improves the initial mechanical properties of such blends although it adversely affects their photo-oxidative stability. 2 Both chain branching (in PP) and unsaturation (in EPDM) have been shown to be responsible for this oxidative instability. 2 The purpose of this investigation is to explore the possibility of using natural rubber (NR)/polypropylene (PP), thermoplastic elastomer at a weight ratio of 60/40, as the SPD in PP/LDPE blends, and to compare its behaviour with that of EPDM (the mechanochemical formation and performance of this NR/PP blend has been discussed recentlya). The photo-oxidative stability of the blend in the presence of both SPD systems was also investigated.
EXPERIMENTAL
Materials Polypropylene (HWM25) and low-density polyethylene (Alkathene30) were supplied by ICI Ltd, natural rubber (SMR5) by MRPRA and ethylene propylene diene rubber by Exxon Chemicals Ltd under the trade name Vistalon 6505. Cyasorb UV531 and Cyasorb 1084 were supplied by American Cyanamid and the hindered piperidine, Tinuvin 770, by Ciba Geigy.
Polymer processing and testing The polyblend used, PP/LDPE (50:50 weight ratio), contained up to 30% NR/PP or EPDM of the total formulation. Blending was carried out at 180°C in a Hampden-RAPRA torque rheometer for 5min in a closed mixer. The processed blend samples were then compression moulded at 180°C into sheets 200#m thick. The NR/PP (60:40 weight ratio) blend was processed with 0.3% dicumyl peroxide in the torque rheometer at 160°C for 10 min (closed mixer) as described previously. 3 Tensile tests were carried out on an Instron tensile tester (Model TMTS) using a crosshead speed of 3cm per rain at 20°C. The falling dart method described previously4 was used to measure the impact strength of the polymer blends.
The use of NR/PP as solid phase dispersant
349
Six samples were tested for each measurement of tensile properties and impact strength. Dynamic mechanical properties were measured on a dynamic viscoelastometer (Rheovibron, R) as described previously. 5 Melt flow index (MFI) measurements were carried out on a Davenport melt flow indexer at 230°C using a load of 50 Newton. Calculated MFIs are the average of five measurements for each sample. Scanning electron microscopy (using a Cambridge S150 scanning electron microscope) was carried out on fractured surfaces of polymer blends (after a Charpy impact test). The test specimen was cut with a sharp knife to give an 0"05-cmthick sample which was mounted on a stub and vacuum-coated with a 100-,~-thin layer of gold. RESULTS
Effect of solid phase dispersants on mechanical performance of PP/LDPE blends Figure 1 compares the effect of NR/PP as a solid phase dispersant with that of EPDM on the mechanical performance of PP/LDPE blends. Impact strength and elongation at break were found to increase with increasing concentration of both dispersants although the increase is more marked in the presence of EPDM. It is known that toughening is achieved by transferring large amounts of the externally applied energy from the matrix to the rubber particles; hence the formation of cracks in the rubber phase. 6 Examination of scanning electron micrographs of fractured surfaces of PP/LDPE blends containing different concentrations of NR/PP (Figs 2-6) shows clearly that increasing the concentration of the SPD causes the fracture (where the blend PP/LDPE fails during impact) to occur in the rubber particles instead of the PP/LDPE matrix, leading to higher impact strength (see Fig. 1 inset). The increase in impact strength is parallelled by a concomitant decrease in tensile strength and modulus (Fig. 1). In this case, however, NR/PP offer higher tensile properties at all concentrations when compared to EPDM. Figure 7 compares the dynamic mechanical properties of the polyblend in the presence of NR/PP and EPDM. The mechanical damping (tan c~at 20°C) increases with increasing concentration of both SPDs while the complex modulus (E*) decreases. The addition of rubber, which has a lower glass transition temperature than the thermoplastic components, is expected to affect the viscoelasticity of the blend. The rubber will also increase the flexibility of the blend and, consequently, decrease the modulus (E*). It was shown previously that, for high-impact polystyrene, increasing amounts of the polybutadiene rubber cause an
350
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Fig. 2. SEM micrograph of a fractured surface of PP/LDPE (50:50 weight ratio) blend.
Fig. 3.
SEM micrograph of a fractured surface of PP/LDPE containing 5% NR/PP.
Fig. 4. SEM micrograph of a fractured surface of PP/LDPE containing 10% NR/PP.
Fig. 5. SEM micrograph of a fractured surface of PP/LDPE containing 20% NR/PP.
The use of NR/PP as sofid phase dispersant
Fig. 6.
353
SEM micrograph of a fractured surface of PP/LDPE containing 25% NR/PP.
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S. AI-Malaika, E. J. Amir
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Fig. 8. Efectof NR/PP and EPDM on the melt flow index of PP/LDPE blends. increase in the mechanical damping and a decrease in the complex modulus. 5 This may explain the present observation that the use of EPDM (which has higher rubber content) in PP/LDPE blends gives higher tan (9 and lower E* values when compared with the use of NR/PP as a solid phase dispersant. Furthermore, blends containing EPDM show lower MFI values than the corresponding NR/PP-containing blends (Fig. 8); this indicates a higher viscosity of blends containing EPDM, which is attributed to higher rubber content.
Effect of SPD on photo-oxidative stability of P P / L D P E blends Figures 9 and 10 compare the behaviour of different concentrations of EPDM and NR/PP on the photo-oxidative stability of PP/LDPE blends. It is clear that, although both exert pro-oxidant effects, stability is more adversely affected by NR/PP. Blends containing NR/PP show a much higher build-up of carbonyl functional groups (a consequence of higher levels of hydroperoxides in the blend), which is parallelled by a faster rate of decay in impact strength with increasing irradiation time as compared to analogous EPDM-containing blends. Figure 11 shows the effect of commercial uv stabilisers on the photo-oxidative stability of PP/LDPE blends in the presence of 20% of SPD. The use of a combination of commercial uv stabilisers, e.g. substituted 2-hydroxy benzophenone (Cyasorb UV531) and a nickel complex (Cyasorb 1084) affords good stability to both systems but the commercial hindered piperidine, Tinuvin 770, does not improve substantially the stability of blends containing NR/PP. Changes in impact strength of the same samples follow similar trends.
The use of NR/PP as solid phase dispersant
iii
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356
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DISCUSSION The results show clearly that the mechanochemically-produced thermoplastic elastomer N R / P P can be used as SPD for P P / L D P E blends. All the initial mechanical properties of the blend are markedly improved although enhancement in impact strength is not as high as that obtained with EPDM. However, N R / P P has the advantage of contributing higher tensile strength and modulus to the blend and is much cheaper and easier
The use of NR/PP as solid phase dispersant
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Fig. 11. Photo-oxidative stability of PP/LDPE blends containing 20% of each SPD system (NR/PP, - - - ) and (EPDM, - - - ) in the absence and presence of uv stabilisers. Numbers on curves represent the type of uv stabiliser, Tinuvin 770, Cyasorb UV531 and Cyasorb 1084. Inset shows the corresponding decay in impact strength of the same films.
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s. AI-Malaika, E. J. Amir
to produce than EPDM. It was shown i that the main factor necessary to achieve good impact resistance and elongation to break of polymer blends is the presence of a rubbery interphase between the heteropolymers. However, tensile strength depends more on the dispersant properties of the third component. The higher impact strength offered by EPDM to the blend (Fig. 1 inset) is most certainly due to the fact that EPDM has a higher rubber content than NR/PP; hence more energy is transferred to, and dissipated by, the rubber phase. Moreover, the structure of the constituents of EPDM bears a resemblance to both PP and LDPE in the blend, and this contributes to greater compatibility between the two phases and hence results in a tougher (higher impact strength) blend. The processing operation has some effect on the overall performance of the blend when processed in t h e presence of different dispersants. The lower melt flow index values in the polyblend in the presence of all concentrations of EPDM compared to that of NR/PP (Fig. 8) indicates higher viscosity in the former case which may contribute to less efficient dispersion during the shearing process, hence the observed lower tensile strength and modulus values (see Fig. 1). It is clear that at the concentrations at which dispersants have an appreciable, beneficial effect on mechanical performance (Fig. 1), they have a deleterious effect on the long-term photo-oxidative stability of the blend (Figs 9 and 10)" It has been shown 5'7 that photo-oxidation in polymer blends does not occur uniformly; oxidation occurs much more rapidly in the rubber phase than in the saturated polymers. The main reason for the high susceptibility of the rubber phase to photo-oxidation is the presence of olefinic unsaturation which is an effective pro-oxidant even when present in minor proportions. This was shown 4 to be due to the formation of hydroperoxides which not only lead to sensitising oxidation of the saturated polymer but are also the cause of rapid changes in mechanical behaviour due to concomitant crosslinking and chain scission reactions leading to the loss of the useful impact properties of the materials. The difference in the extent of the pro-oxidant effect shown by EPDM and NR/PP (Figs 9 and 10) is due to differences in the compositions; N R has a much higher unsaturation level than EPDM (14.3% isoprene in N R compared to only 0.6-1.0 mole per cent diene in EPDM s); hence, higher levels of hydroperoxides are expected to build up in the former which are mainly responsible for the higher photo-oxidative instability in the case of blends containing NR/PP. Furthermore, differences in the location of the unsaturation sites for the two SPD samples must also contribute to the observed differences in photo-oxidative stability of the blends. In the case of EPDM, unsaturation is located on the side chains pendant to the main chain but their effect on the properties of the blend is expected to be
The use of NR/PP as solid phase dispersant
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minimal. In NR, however, unsaturation is distributed throughout the main chain, The primary function of photostabilisers in polymer blends is to inhibit the destruction of the SPD which is responsible for the toughness of the blend. Figure 11 shows clearly that a synergistic commercial uv stabiliser system enhances the photo-oxidative stability of blends containing both types of solid phase dispersant. The behaviour of the commercial nitroxyl precursor, Tinuvin 770, is different; it stabilises the EPDM-containing blends to the same extent as the uv absorbers but does not offer any appreciable stabilising effect to the NR/PP-containing blends. The combination of the much cheaper uv stabilisers (UV531 + 1084) is, therefore, a more cost-effective stabiliser system for P P / L D P E blends containing SPD systems based on E P D M or NR/PP.
A C K N O W L E D G E M ENTS We are grateful to the Indonesian Government and the Research Institute of Estate Crops for financial support and study leave for one of us, E.J.A.
REFERENCES 1. A. Ghaffar, C. Sadermohaghegh and G. Scott, Europ. Polym. J., 17, 941 (1981). 2. C. Sadermohaghegh, G. Scott and E. Setoudeh, Poly. Deg. and Stab., 3, 469 (1980-81). 3. S. AI-Malaika and E. J. Amir, J. Natural Rubber Research 1, 104 (1986). 4. G. Scott and M. Tahan, Europ. Polym. J., 13, 981 (1977). 5. A. Ghaffar, A. Scott and G. Scott, Europ. Polym. J., 11, 271 (1975). 6. C. B. Bucknall and R. R. Smith, Polymer, 6, 437 (1965). 7. M. Ghaemy and G. Scott, Poly. Deg. and Stab., 3, 233 (1981). 8. J. P. Daugherty, Ethylene-propylene rubbers, Enjay Polymer Lab., Linden, New Jersey, 3 0969).