Tensile and impact behavior of polypropylene/low density polyethylene blends

Transcription

1 Polymer Testing 24 (2005) Material Properties Tensile and impact behavior of polypropylene/low density polyethylene blends R. Strapasson, S.C. Amico*, M.F.R. Pereira, T.H.D. Sydenstricker Mechanical Engineering Department, Federal University of Paraná (UFPR), P.O. Box , Curitiba-PR, Brazil Received 18 November 2004; accepted 7 January Abstract Blends of polypropylene (PP) and low-density polyethylene (LDPE) may contribute to make recycling more economically attractive. The aim of this work was to make PP/LDPE blends (0/100, 25/75, 50/50, 75/25 and 100/0 w/w) via injection molding carried out under various injection temperatures and to evaluate their tensile and impact properties. The blends yielded tensile stress strain curves very dependent on their composition, especially regarding elongation at break and the presence of necking. An irregular behavior for the 50/50 w/w blend is reported. Nevertheless, a linear variation of the yield strength and elastic modulus with the blend composition was observed. The behavior of the blend was also very dependent on processing temperature. Addition of 25% of LDPE to the PP may result in similar degradation of its mechanical properties to that caused by a 10 8C processing temperature increase. Statistical analyses proved valuable when reporting results concerning blends. q 2005 Elsevier Ltd. All rights reserved. Keywords: Low-density polyethylene; Polypropylene; Blends; Tensile and impact properties 1. Introduction As the economy achieves global status, many factors regarding the competitiveness of a nation come under investigation. More recently, together with important areas such as technology advancement and technology transfer, issues related to sustainable development and environment preservation are receiving increasing attention from the world community. Advantages of the mechanical recycling of polymers include reduction of oil and energy consumption compared with the synthesis of virgin polymers, reduced disposal of plastic waste in municipal garbage and generation of employment and income. The recycling of industrial scrap is an ongoing successful practice due to the low level of contamination. However, recycling of municipal plastic waste is often an arduous task due to the fact that this * Corresponding author. Tel.: C ; fax: C address: amico@ufpr.br (S.C. Amico). material is usually a mixture of several polymers, which makes processing more difficult and also limits the number of potential applications [1]. The most abundant plastics in Brazilian municipal waste are polypropylene (PP), poly(ethylene-terephthalate) and polyethylene (PE), the latter being available in different grades such as low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE). A few methods have been used to classify plastics from municipal post-consumer waste. With the flotation method, two fractions are obtained: a lighter fraction, floating on water, and a heavier fraction. The former is essentially constituted of LDPE, HDPE and PP [2], polyolefins that exhibit of similar density. It is uneconomic to separate them using alcohol solutions in a subsequent stage. Thus, the usual practice in small recycling units in Brazil is to indiscriminately mix different amounts of PE and PP during recycling, leading to incompatible blends of varying and poor properties. Aiming at good performance materials, it is important to consider processing conditions, blend composition and its /$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi: /j.polymertesting

2 R. Strapasson et al. / Polymer Testing 24 (2005) behavior in the solid state from the melt. While the crystallization of a homopolymer is controlled by nucleation, spherulite growth, rate of cooling and degree of super cooling, crystallization behavior of polymer blends is more complex due to the existence of a second component, usually resulting in an incompatible mixture. Blends of PP and LLDPE (20/80 w/w), for instance, are partially miscible and its crystallization is controlled by nucleation and diffusion [3]. The incompatibility between LDPE and PP has already been reported by various authors [4,5], following microscopy and calorimetric studies. In LDPE rich blends, a heterogeneous PP dispersion in the LDPE matrix produces two phases in the melt. The low interfacial adhesion between the phases is responsible for a decrease in mechanical properties especially related to its morphology, including impact strength, strain at break and ductile to brittle transition. According to Shanks [6], the immiscibility between the phases makes the rule of mixtures ineffective in predicting some properties of interest. To overcome this difficulty, the use of various coupling agents have been reported. Amongst others [7 9], Yang [10] showed that the addition of a commercial ethylene/ propylene block copolymer improved the ductility of LDPE/PP blends, particularly for PP rich blends. Bertin [5] studied and characterized virgin and recycled LDPE/PP blends and the use of compatilizing agents, such as ethylene-propylene-diene monomer copolymer (EPDM) or PE-g (2-methyl-1,3-butadiene)graft copolymer, to enhance their impact strength and elongation at break. Although this may solve the compatibility problem, the use of compatibilizers adds cost to the recycled product, usually resulting in loss of interest from the recycling sector. In this work, the evaluation of tensile and impact properties of PP/LDPE blends was carried out to investigate the composition range for better mechanical performance and also to define the impact of PE addition on PP for composition adjustment of blends used in a commercial recycling unit in Almirante Tamandaré/PR, Brazil. 2. Materials and methods Polypropylene (H301-Braskem) and low density polyethylene (BC 818-Braskem) were used. The specific gravity of the PP is and that of the LDPE is g/cm 3, with melt flow index of 10.0 and 7.5 g/10 min, respectively. Pure PP, pure LDPE and their blends were processed in an injection-molding machine with various PP/LDPE weight contents, namely 100/0, 75/25, 50/50, 25/75 and 0/100. These blends are called PP100, PP75, PP50, PP25 and PP0, respectively, throughout the text, and each of them was processed at several injection temperatures (170, 180, 190 and 200 8C). For the evaluation of the blend mechanical properties, tensile tests were performed on an EMIC universal testing machine (Model DL10000), in general accordance with ASTM D638. Data for yield strength, elastic modulus and elongation at break were obtained in tests carried out at a crosshead speed of 5 mm/min. For low elongations, an EMIC extensometer having a gage length of 25 mm was used. Impact tests were performed on a PANTEC equipment (model PW-4), in general accordance with ASTM D256. Between 10 and 20 measurements were taken for each experimental condition, and the reported results include the mean values and their standard deviations. The statistical analysis of variance of tensile and impact results has been carried out using commercial software. A one-way ANOVA and a series of Tukey HSD post hoc were used to check for statistical difference among groups (for p!0.05). 3. Results and discussion 3.1. Tensile tests The stress strain curves for the various blends injected at 170, 180, 190 and 200 8C are shown in Figs. 1 4, respectively. The 170 and 200 8C curves for the PP0 (pure LDPE) were not included since there were flow difficulties relating to the high viscosity of the melt for the former and severe degradation for the latter, both resulting in nonhomogeneous test specimens. Analysis of these figures shows the importance of controlling the injection temperature, for example, elongation at break for pure polypropylene (PP100) decreases from 650% to less than 10%, when the temperature reaches 190 8C, indicating severe degradation. In fact, Figs. 3 and 4 demonstrate PP degradation with severe changes on curve profiles and on yielding. Therefore, the retrieved data for the 190 and 200 8C injection temperature should not be directly compared to data obtained for the other injection temperatures. Observation of these figures also shows for the PP50 a very distinct behavior. Yielding is not seen for any injection temperature, and very low yield strength and elongation at break are found, in comparison to the other blends. Fig. 1. Stress strain curves for the different blends injected at 170 8C.

3 470 R. Strapasson et al. / Polymer Testing 24 (2005) Fig. 2. Stress strain curves for the different blends injected at 180 8C. Surprisingly, further increase in the LDPE content causes a recovery of elongation at break yielding for the lower temperatures. Thus, the elongation at break (see Table 1) did not follow the rule of mixtures and also showed a minimum for PP50 for the temperatures of interest. Other observations from Figs. 1 4 include: (i) The C injection temperature range was shown to be the most suitable to maintain the PP properties, whereas the C temperature range was the most suitable for pure LDPE processing; (ii) The stress strain tensile curves were shown to be very dependent on the composition of the blends, with very distinct curve shapes regarding yielding, modulus and elongation at break; (iii) For all compositions, the stress strain curves were very dependent on the temperature and the elongation at break decreased with the temperature; (iv) Elongation at break evidenced blend incompatibility, unambiguously seen for the 50% polypropylene content blend, which showed the lowest elongation at break of all compositions studied. Also, no yielding was observed for this blend at any temperature; (v) Figs. 1 and 2 show that a 25% replacement of PP by LDPE (i.e. the PP75) causes a severe change of elongation at break, with a decrease in ductility, and (vi) Fig. 2 shows that a 25% replacement of LDPE by PP (i.e. the PP25) also changes the stress strain curve and causes an increase of elastic modulus and yield strength, as was observed by Bertin [5], with yielding only for the 170 and 180 8C temperatures. Fig. 3. Stress strain curves for the different blends injected at 190 8C. Fig. 4. Stress strain curves for the different blends injected at 200 8C. Table 1 compiles the yield strength and elastic modulus results for the different blends. The results were in the same range of those reported by Albano [11] for pure LDPE values at 180 8C and those by Bertin [5] for their LDPE/PP (90/10) blends. From the statistical analysis presented in Table 1, it can also be concluded that if pure PP is processed at 190 8C, only 10 8C higher than the optimum temperature, a yield strength and a elastic modulus reduction similar to that obtained with the inclusion of 25% of LDPE (PP75) may result. Figs. 5 and 6 show the variation of yield strength and elastic modulus, respectively, for the different materials. Fig. 5 shows a linear variation with the composition for the 170 and 180 8C injection temperatures, with high coefficient of determination (R-squared) values of 0.98 and 0.95, respectively. This suggests that yield strength varied according to the rule of mixtures, i.e. the yield strength is basically dependent on the blend composition. This of course was not seen for the temperatures at which polymer degradation was the determinant factor. Further observation of this figure also reveals that the values for 170 and 180 8C are the highest, being statistically similar (see Table 1) for all compositions except for the PP50, again showing that this blend has a particular behavior, being more prone to degradation. Every polymer has its optimal processing temperature profile to maximize a certain property and the same occurs for the different blends. Furthermore, the PP optimal temperatures appear to be dominating the blend behavior, i.e. 170 and 180 8C give the best results for all compositions, followed respectively by 190 and 200 8C, although for pure LDPE (PP100), 180 and 190 8C give the best results. The variation of elastic modulus followed the same trends as yield strength, namely the modulus varied linearly with the LDPE weight fraction (R 2 of 0.98 for both 170 and 180 8C), suggesting agreement with the rule of mixtures. The PP50 here again shows a different trend to the other blends, where the 190 8C result differed from the other temperatures. It can be said from the results that a replacement of 25% of PP with LDPE (PP75), even for the best temperatures,

4 R. Strapasson et al. / Polymer Testing 24 (2005) Table 1 Influence of the composition and the injection temperature on the tensile properties Injection Composition (%) Yield strength (MPa) Elastic modulus (MPa) Elongation at break (%) temperature (8C) PP PE Mean a SD b Mean a SD b Mean SD b a ag 187 O b b c c d d Irregular injection a ag b b e ce d d f f b ab g b g e f d f fh c g e ab f dh dh Irregular injection a Different letters mean statistically significant differences at 5% confidence level. b Standard deviation. causes a statistically significant reduction in yield strength and elastic modulus, but, from the slope of the linear trends of Figs. 5 and 6, one may infer that a replacement of around 10% PP with LDPE may not cause significant variations in these properties. This finding may be of interest to recycling companies to lower the cost of the product by partial replacement of PP by LDPE. The observed results of this work, namely, a linear variation of elastic modulus and yield strength with the blend composition and a minimum for the elongation at break for the PP50 blend, are in agreement with those from Kolarik [12]. This researcher found that the modulus and the yield strength upper boundary are monotonic functions of the blend composition and the yield strength lower limit shows a minimum near 50/50. However, there are other reports found in the literature. For instance Liang [13] reported an increase of elastic modulus with the PP content according to a logarithm rule of mixture. The yield strength increased in an irregular way with the PP content, whereas the elongation at break showed a minimum at 80%PP. The same author found a distinct behavior for another PP (with a lower melt flow index) for which the elongation at break showed two minimums, one for 20%PP and another for 100%PP. Tselios [14] found a minimum for the tensile strength and elongation at break for 25%PP. Yang [15] found a monotonic variation of the yield strength and elastic modulus. The elongation at break and tensile strength, however, reached a maximum for 30 50%PP blends, with the material behavior changing from ductile to brittle. Possible explanations for the apparent divergence may reside in the processing history and in the incompatibility of these blends, and consequent morphological Fig. 5. Variation of yield strength with blend composition. Fig. 6. Variation of elastic modulus with blend composition.

5 472 R. Strapasson et al. / Polymer Testing 24 (2005) Table 2 Influence of the composition and the injection temperature on impact strength Injection temperature (8C) Composition (%) Energy (J) Impact strength (J/ PP PE Mean SD b m) a ae b c O4.0 Partial breakage Irregular injection ae b cd Partial breakage O4.0 No breakage ae b d O4.0 Partial breakage No breakage a e bd O4.0 Partial breakage Irregular injection a Different letters mean statistically significant differences at 5% confidence level. b Standard deviation. (microstructural) changes that occur upon blending, with the different phases assuming distinct behavior for different compositions, as discussed in detail by Tselios [14] Impact tests Table 2 shows the results for the impact tests. The pure LDPE injected samples did not fracture at the test conditions used, even for a 4J hammer. When PP is incorporated in the LDPE, however, there is a significant impact strength reduction (as in Yang [15] and Wang [16]), shown by the partial specimen fracture. Higher PP content causes a further reduction until 10 J/m is achieved by the pure PP, with the fracture mode changing from ductile to brittle in the range of 25 to 50%PP (PP25 to PP50), due to the low interfacial adhesion and, consequently, low stress transfer between the phases. This behavior was observed for all temperatures, namely, the higher the PP content, the lower the impact strength, in agreement with Tselios [14]. For pure PP, degradation is evident only at 200 8C, thus impact strength was found to be less sensitive to the injection temperature than the tensile properties, this trend being followed by the intermediate composition blends. Similiarly, it can be noticed that the 170 and 180 8C temperatures appear to be more suitable for the processing of these blends. Combining the results of tensile and impact tests it may be said that, although the addition of LDPE in the PP improves the impact characteristics, it has a detrimental effect on the tensile properties. Therefore, it appears that the only way of improving these two blend properties would be with the addition of a compatibilizer such as in Hope [17] and Yang [10]. The latter reported the use of ethylene/ propylene block copolymer to increase the ductility of these blends by improving the interfacial adhesion without significant loss in elastic modulus, especially for blends with high PP content. 4. Conclusions The widespread presence of polypropylene and lowdensity polyethylene in municipal wastes and their common combined use by the recycling industry makes the study of the mechanical behavior of these blends valuable for practical every-day use. The stress strain tensile curves were very dependent on the composition of the blends, with the curve shapes being very distinct as regards yielding, modulus and elongation at break. The linear law of mixtures was obeyed for all blends regarding elastic modulus and yield strength, except those in which polymer degradation was the determinant factor. Elongation at break, however,

6 R. Strapasson et al. / Polymer Testing 24 (2005) demonstrated incompatibility for this blend, unambiguously seen for the 50% PP content blend, which showed the lowest elongation at break of all compositions studied. For each composition, the behavior was very dependent on the processing temperature, and the blends showed an optimum injection temperature around C. The replacing of 25% of PP by LDPE may be as harmful to the mechanical properties as the use of an injection temperature 10 8C higher than the optimum temperature range. For a partial substitution of polypropylene without a statistically significant loss in elastic modulus or yield strength, an LDPE content of around 10% may be allowed in the mixture. When polypropylene is added to the polyethylene, there is a significant reduction in impact strength, with partial sample fracture for the 25%LDPE content blend. Further PP addition makes the blend behavior change from ductile to brittle. References [1] H.P. Blom, J.W. Teh, A. Rudin, PP/PE blends. IV. Characterization and compatibilization of blends of postconsumer resin with virgin PP and HDPE, J. Appl. Polym. Sci. 70 (11) (1998) [2] N.T. Dintcheva, F.P. La Mantia, F. Trotta, M.P. Luda, G. Camino, M. Paci, L. Di Maio, D. Acierno, Effects of filler type and processing apparatus on the properties of the recycled light fraction from municipal post-consumer plastics, Polym. Adv. Technol. 12 (9) (2001) [3] J. Li, R.A. Shanks, Y. Long, Miscibility and crystallisation of polypropylene-linear low density polyethylene blends, Polymer 42 (5) (2001) [4] J.W. Teh, A. Rudin, J.C. Keung, A review of polyethylenepolypropylene blends and their compatibilization, Adv. Polym. Tech. 13 (1) (1994) [5] S. Bertin, J.J. Robin, Study and characterization of virgin and recycled LDPE/PP blends, Eur. Polym. J. 38 (11) (2002) [6] R.A. Shanks, J. Li, F. Chen, G. Amarasinghe, Timetemperature-miscibility and morphology of polyolefin blends, Chin. J. Polym. Sci. 18 (3) (2000) [7] G. Radonjic, N. Gubeljak, The use of ethylene/propylene copolymers as compatibilizers for recycled polyolefin blends, Macromol. Mater. Eng. 287 (2) (2002) [8] A.G. Andreopoulos, P.A. Tarantili, P. Anastassakis, Compatibilizers for Low Density Polyethylene/Polypropylene Blends, J. Macromol. Sci. Pure 36 (9) (1999) [9] E. Vaccaro, A.T. Dibenedetto, S.J. Huang, Yield strength of low-density polyethylene-polypropylene blends, J. Appl. Polym. Sci. 63 (3) (1997) [10] M.B. Yang, K. Wang, L. Ye, Y.W. Mai, J.S. Wu, Low density polyethylene-polypropylene blends Part 2 strengthening and toughening with copolymer, Plast. Rubber Compos. 32 (1) (2003) [11] C. Albano, J. Reyes, M. Ichazo, J. Gonzalez, M. Hernandez, M. Rodriguez, Mechanical, thermal and morphological behaviour of the polystyrene/polypropylene (80/20) blend, irradiated with gamma-rays at low doses (0 70 kgy), Polym. Degrad. Stabil. 80 (2) (2003) [12] J. Kolarik, Simultaneous prediction of the modulus and yield strength of binary polymer blends, Polym. Eng. Sci. 36 (20) (1996) [13] J.Z. Liang, C.Y. Tang, H.C. Man, Flow and mechanical properties of polypropylene low density polyethylene blends, J. Mater. Process Tech. 66 (1-3) (1997) [14] C. Tselios, D. Bikiaris, V. Maslis, C. Panayiotou, In situ compatibilization of polypropylene-polyethylene blends: a thermomechanical and spectroscopic study, Polymer 39 (26) (1998) [15] M.B. Yang, K. Wang, L. Ye, Y.W. Mi, J.S. Wu, Low density polyethylene-polypropylene blends. Part 1 ductility and tensile properties, Plast. Rubber Compos. 32 (1) (2003) [16] Z. Wang, Toughening and reinforcing of polypropylene, J. Appl. Polym. Sci. 60 (12) (1996) [17] P.S. Hope, J.G. Bonner, A.F. Miles, A study of compatibilisers for recovering the mechanical-properties of recycled polyethylenes, Plast. Rubber Compos. 22 (3) (1994)