Tensile and Impact Behavior of Polypropylene/Low Density Polyethylene Blends

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Polymer Testing 24 (2005) 468–473 www.elsevier.com/locate/polytest 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 Parana´ (UFPR), P.O. Box 19.011, 81.531-990 Curitiba-PR, Brazil

Received 18 November 2004; accepted 7 January 2005

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 a108C 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 material is usually a mixture of several polymers, which makes processing more difficult and also limits the number As the economy achieves global status, many factors of potential applications [1]. regarding the competitiveness of a nation come under The most abundant plastics in Brazilian municipal waste investigation. More recently, together with important areas are polypropylene (PP), poly(ethylene-terephthalate) and such as technology advancement and technology transfer, polyethylene (PE), the latter being available in different issues related to sustainable development and environment grades such as low density polyethylene (LDPE), linear low preservation are receiving increasing attention from the density polyethylene (LLDPE) and high density polyethy- world community. lene (HDPE). Advantages of the mechanical recycling of polymers A few methods have been used to classify plastics from include reduction of oil and energy consumption compared municipal post-consumer waste. With the flotation method, with the synthesis of virgin polymers, reduced disposal of two fractions are obtained: a lighter fraction, floating on plastic waste in municipal garbage and generation of water, and a heavier fraction. The former is essentially employment and income. The recycling of industrial scrap constituted of LDPE, HDPE and PP [2], polyolefins that is an ongoing successful practice due to the low level of exhibit of similar density. It is uneconomic to separate them contamination. However, recycling of municipal plastic using alcohol solutions in a subsequent stage. Thus, the usual practice in small recycling units in Brazil is to waste is often an arduous task due to the fact that this indiscriminately mix different amounts of PE and PP during recycling, leading to incompatible blends of varying and * Corresponding author. Tel.: C55 41 361 3430; fax: C55 41 361 poor properties. 3129. Aiming at good performance materials, it is important to E-mail address: [email protected] (S.C. Amico). consider processing conditions, blend composition and its

0142-9418/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2005.01.001 R. Strapasson et al. / Polymer Testing 24 (2005) 468–473 469 behavior in the solid state from the melt. While the machine (Model DL10000), in general accordance with crystallization of a homopolymer is controlled by nuclea- ASTM D638. Data for yield strength, elastic modulus and tion, spherulite growth, rate of cooling and degree of super elongation at break were obtained in tests carried out at a cooling, crystallization behavior of polymer blends is more crosshead speed of 5 mm/min. For low elongations, an complex due to the existence of a second component, EMIC extensometer having a gage length of 25 mm was usually resulting in an incompatible mixture. Blends of PP used. Impact tests were performed on a PANTEC equipment and LLDPE (20/80 w/w), for instance, are partially miscible (model PW-4), in general accordance with ASTM D256. and its crystallization is controlled by nucleation and Between 10 and 20 measurements were taken for each diffusion [3]. experimental condition, and the reported results include the The incompatibility between LDPE and PP has already mean values and their standard deviations. been reported by various authors [4,5], following The statistical analysis of variance of tensile and impact microscopy and calorimetric studies. In LDPE rich blends, results has been carried out using commercial software. A a heterogeneous PP dispersion in the LDPE matrix produces one-way ANOVA and a series of Tukey HSD post hoc were two phases in the melt. The low interfacial adhesion used to check for statistical difference among groups (for between the phases is responsible for a decrease in p!0.05). mechanical properties especially related to its morphology, including impact strength, strain at break and ductile to brittle transition. According to Shanks [6], the immiscibility 3. Results and discussion between the phases makes the rule of mixtures ineffective in predicting some properties of interest. 3.1. Tensile tests To overcome this difficulty, the use of various coupling agents have been reported. Amongst others [7–9], Yang [10] The stress–strain curves for the various blends injected at showed that the addition of a commercial ethylene/ 170, 180, 190 and 200 8C are shown in Figs. 1–4, propylene block copolymer improved the ductility of respectively. The 170 and 200 8C curves for the PP0 (pure LDPE/PP blends, particularly for PP rich blends. Bertin LDPE) were not included since there were flow difficulties [5] studied and characterized virgin and recycled LDPE/PP relating to the high viscosity of the melt for the former and blends and the use of compatilizing agents, such as severe degradation for the latter, both resulting in non- ethylene-propylene-diene monomer copolymer (EPDM) or homogeneous test specimens. PE-g (2-methyl-1,3-butadiene)graft copolymer, to enhance Analysis of these figures shows the importance of their impact strength and elongation at break. Although this controlling the injection temperature, for example, may solve the compatibility problem, the use of compati- elongation at break for pure polypropylene (PP100) bilizers adds cost to the recycled product, usually resulting decreases from 650% to less than 10%, when the in loss of interest from the recycling sector. temperature reaches 190 8C, indicating severe degradation. In this work, the evaluation of tensile and impact In fact, Figs. 3 and 4 demonstrate PP degradation with properties of PP/LDPE blends was carried out to investigate severe changes on curve profiles and on yielding. Therefore, the composition range for better mechanical performance the retrieved data for the 190 and 200 8C injection and also to define the impact of PE addition on PP for temperature should not be directly compared to data composition adjustment of blends used in a commercial obtained for the other injection temperatures. recycling unit in Almirante Tamandare´/PR, Brazil. 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 2. Materials and methods break are found, in comparison to the other blends.

Polypropylene (H301-Braskem) and low density polyethylene (BC 818-Braskem) were used. The speciﬁc gravity of the PP is 0.905 and that of the LDPE is 0.918 g/cm3, 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, Fig. 1. Stress–strain curves for the different blends injected at tensile tests were performed on an EMIC universal testing 170 8C. 470 R. Strapasson et al. / Polymer Testing 24 (2005) 468–473

Fig. 2. Stress–strain curves for the different blends injected at Fig. 4. Stress–strain curves for the different blends injected at 180 8C. 200 8C.

Surprisingly, further increase in the LDPE content causes a Table 1 compiles the yield strength and elastic modulus recovery of elongation at break yielding for the lower results for the different blends. The results were in the same temperatures. Thus, the elongation at break (see Table 1) did range of those reported by Albano [11] for pure LDPE not follow the rule of mixtures and also showed a minimum values at 180 8C and those by Bertin [5] for their LDPE/PP for PP50 for the temperatures of interest. (90/10) blends. Other observations from Figs. 1–4 include: (i) The 170– From the statistical analysis presented in Table 1, it can 180 8C injection temperature range was shown to be the also be concluded that if pure PP is processed at 190 8C, most suitable to maintain the PP properties, whereas the only 10 8C higher than the optimum temperature, a yield 180–190 8C temperature range was the most suitable for strength and a elastic modulus reduction similar to that pure LDPE processing; (ii) The stress–strain tensile curves obtained with the inclusion of 25% of LDPE (PP75) may were shown to be very dependent on the composition of the result. blends, with very distinct curve shapes regarding yielding, Figs. 5 and 6 show the variation of yield strength and modulus and elongation at break; (iii) For all compositions, elastic modulus, respectively, for the different materials. Fig. the stress–strain curves were very dependent on the 5 shows a linear variation with the composition for the 170 temperature and the elongation at break decreased with and 180 8C injection temperatures, with high coefficient of the temperature; (iv) Elongation at break evidenced blend determination (R-squared) values of 0.98 and 0.95, respect- incompatibility, unambiguously seen for the 50% poly- ively. This suggests that yield strength varied according to propylene content blend, which showed the lowest the rule of mixtures, i.e. the yield strength is basically elongation at break of all compositions studied. Also, no dependent on the blend composition. This of course was not yielding was observed for this blend at any temperature; (v) seen for the temperatures at which polymer degradation was Figs. 1 and 2 show that a 25% replacement of PP by LDPE the determinant factor. Further observation of this figure also (i.e. the PP75) causes a severe change of elongation at break, reveals that the values for 170 and 180 8C are the highest, with a decrease in ductility, and (vi) Fig. 2 shows that a 25% being statistically similar (see Table 1) for all compositions replacement of LDPE by PP (i.e. the PP25) also changes the except for the PP50, again showing that this blend has a stress–strain curve and causes an increase of elastic modulus particular behavior, being more prone to degradation. and yield strength, as was observed by Bertin [5], with Every polymer has its optimal processing temperature yielding only for the 170 and 180 8C temperatures. 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 (R2 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. Fig. 3. Stress–strain curves for the different blends injected at It can be said from the results that a replacement of 25% 190 8C. of PP with LDPE (PP75), even for the best temperatures, R. Strapasson et al. / Polymer Testing 24 (2005) 468–473 471

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 PP PE Meana SDb Meana SDb Mean SDb (8C) 170 100 0 24.8 a 0.9 1327 ag 187 O800 – 75 25 22.4 b 0.9 1143 b 109 170 7.1 50 50 16.7 c 0.8 731 c 95 17.7 2.2 25 75 11.5 d 0.8 436 d 43 400–512 – 0 100 Irregular injection 180 100 0 25.1 a 0.8 1304 ag 172 600–700 – 75 25 22.8 b 1.1 1149 b 137 100 – 50 50 13.8 e 0.8 845 ce 141 3.7 0.7 25 75 11.4 d 0.6 435 d 54 400–529 – 0 100 7.6 f 0.2 157 f 24 111 13 190 100 0 23.4 b 1.3 1240 ab 101 5.7 1.9 75 25 19.6 g 1.5 1085 b 153 4.9 1.7 50 50 10.9 g 2.4 932 e 184 2.1 0.9 25 75 7.9 f 0.6 404 d 53 4.1 0.9 0 100 7.9 f 0.3 191 fh 32 66 24.1 200 100 0 16.9 c 1.1 1452 g 258 1.9 0.4 75 25 14.2 e 2.3 1209 ab 133 1.9 0.6 50 50 7.5 f 0.7 403 dh 36 3.7 0.8 25 75 6.9 0.7 350 dh 51 25 3.2 0 100 Irregular injection

a Different letters mean statistically significant differences at 5% confidence level. b Standard deviation. causes a statistically significant reduction in yield strength reported an increase of elastic modulus with the PP content and elastic modulus, but, from the slope of the linear trends according to a logarithm rule of mixture. The yield strength of Figs. 5 and 6, one may infer that a replacement of around increased in an irregular way with the PP content, whereas 10% PP with LDPE may not cause significant variations in the elongation at break showed a minimum at 80%PP. The these properties. This finding may be of interest to recycling same author found a distinct behavior for another PP (with a companies to lower the cost of the product by partial lower melt flow index) for which the elongation at break replacement of PP by LDPE. showed two minimums, one for 20%PP and another for The observed results of this work, namely, a linear 100%PP. Tselios [14] found a minimum for the tensile variation of elastic modulus and yield strength with the strength and elongation at break for 25%PP. Yang [15] blend composition and a minimum for the elongation at found a monotonic variation of the yield strength and elastic break for the PP50 blend, are in agreement with those from modulus. The elongation at break and tensile strength, Kolarik [12]. This researcher found that the modulus and the however, reached a maximum for 30–50%PP blends, with yield strength upper boundary are monotonic functions of the material behavior changing from ductile to brittle. the blend composition and the yield strength lower limit Possible explanations for the apparent divergence may shows a minimum near 50/50. However, there are other reside in the processing history and in the incompatibility of reports found in the literature. For instance Liang [13] these blends, and consequent morphological

Fig. 5. Variation of yield strength with blend composition. Fig. 6. Variation of elastic modulus with blend composition. 472 R. Strapasson et al. / Polymer Testing 24 (2005) 468–473

Table 2 Inﬂuence of the composition and the injection temperature on impact strength

Injection tempera- Composition (%) Energy (J) Impact strength (J/ ture (8C) m)a PP PE Mean SDb 170 100 0 0.12 0.01 12.2 ae 75 25 0.26 0.10 26.1 b 50 50 0.42 0.08 41.7 c 25 75 O4.0 – Partial breakage 0 100 – – Irregular injection 180 100 0 0.10 0.01 10.3 ae 75 25 0.26 0.10 25.8 b 50 50 0.37 0.06 37.3 cd 25 75 – – Partial breakage 0 100 O4.0 – No breakage 190 100 0 0.09 0.01 8.5 ae 75 25 0.25 0.09 25.2 b 50 50 0.28 0.06 28.0 d 25 75 O4.0 – Partial breakage 0 100 – – No breakage 200 100 0 0.05 0.01 4.5 a 75 25 0.16 0.11 16.3 e 50 50 0.29 0.07 28.5 bd 25 75 O4.0 – Partial breakage 0 100 – – Irregular injection

a Different letters mean statistically signiﬁcant differences at 5% conﬁdence level. b Standard deviation.

(microstructural) changes that occur upon blending, with the Combining the results of tensile and impact tests it may different phases assuming distinct behavior for different be said that, although the addition of LDPE in the PP compositions, as discussed in detail by Tselios [14]. 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 3.2. Impact tests with the addition of a compatibilizer such as in Hope [17] and Yang [10]. The latter reported the use of ethylene/ Table 2 shows the results for the impact tests. The pure propylene block copolymer to increase the ductility of these LDPE injected samples did not fracture at the test conditions blends by improving the interfacial adhesion without used, even for a 4J hammer. significant loss in elastic modulus, especially for blends When PP is incorporated in the LDPE, however, there is with high PP content. 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 4. Conclusions from ductile to brittle in the range of 25 to 50%PP (PP25 to PP50), due to the low interfacial adhesion and, conse- The widespread presence of polypropylene and low- quently, low stress transfer between the phases. This density polyethylene in municipal wastes and their common behavior was observed for all temperatures, namely, the combined use by the recycling industry makes the study of higher the PP content, the lower the impact strength, in the mechanical behavior of these blends valuable for agreement with Tselios [14]. practical every-day use. For pure PP, degradation is evident only at 200 8C, thus The stress–strain tensile curves were very dependent impact strength was found to be less sensitive to the on the composition of the blends, with the curve shapes injection temperature than the tensile properties, this trend being very distinct as regards yielding, modulus and being followed by the intermediate composition blends. elongation at break. 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