Fracture behavior of bimodal polyethylene: Effect of molecular weight distribution characteristics
Graphical abstract
Introduction
Since the application of polyethylene (PE) in pipes over 40 years ago, more and more attention has been paid to reach even higher performance standards. Numerous novel polymerization technologies have been developed in order to tailor the architecture of PE to obtain materials with higher properties [1], [2]. It is well known that the mechanical properties of PE are usually influenced by molecular weight (Mw) and molecular weight distribution (MWD), as well as other microstructural features, such as the comonomer type, comonomer content, short chain branch (SCB) and its length etc. [3], [4], [5], [6], [7].
As for Mw, unimodal polyethylenes made by traditional catalysts have conflicts between their mechanical properties and processing behaviors, that is, improving mechanical performances by increasing Mw usually deteriorates the processing properties owing to high melt viscosity. Recently, based on metallocene catalysts, some novel synthetic techniques have been developed to produce bimodal polyethylene (BPE) with particular double peaks in the MWD curve. The bimodal material consisted of both high molecular weight fraction and low molecular weight fraction provides more balanced performances to processing and mechanical behaviors, because the high molecular weight fraction brings excellent mechanical properties and the low molecular weight fraction allows the melt to flow easily.
The BPEs with different MWD characteristics (such as peak position and peak height) show great variations in their properties, which are controlled by both the catalysts and the process [8], [9], [10], [11], [12]. Obviously, the understanding of the relationship between MWD and properties of BPE has great value and significance for its preparation and application. Some researchers [7], [13], [14] have already studied the melting behaviors and crystallization kinetics of BPE with different Mw and Mw/Mn. DesLauriers et al. [10] used some BPE pipe resins with different Mw and Mw/Mn to compare their yielding stress, tensile strength and rheology parameters. However, there are few reports particularly focusing on how the fracture behaviors of BPE are influenced by the MWD characteristic.
Usually single stage process, two stage cascade polymerization process and melt mixing are used to produce BPE, and in essence, two kinds of polyethylenes with different Mw and MWDs are mixed together using these methods [15], [16]. Some literatures [2], [7], [17] have reported that the high molecular weight fraction of BPE usually consists of α-olefin copolymers and the low molecular weight fraction is generally homopolymer. For the benefits of preparing BPE samples with desired MWDs, we used melt blending method to prepare samples due to the difficulty in obtaining BPEs with regular variable MWDs from commercial channels. So a homo-polyethylene was used to blend with a commercial BPE. The samples made by this procedure have the same double peak positions but different peak heights in their MWD curves. In this study the influences of bimodal characteristic on the fracture behaviors of BPE will be discussed.
In the industrial practice, after a long period of service, brittle fracture can be observed for many PEs although under loads well below their yield stress [18], [19]. Usually common PEs used for pressure pipes have defects such as low environmental stress cracking resistance and lack of the ability to stop the crack effectively, which may cause severe economic and environmental losses by leakage. Considerable efforts have been paid to study the influences of structural factors, including Mw, SCB content, length of SCB and distribution of SCB, on the fracture behaviors and long term properties of materials [20], [21], [22], [23], [24], [25], [26], [27], [28]. Undoubtedly, evaluation of the cracking resistance of BPE is especially important for its application as pressure pipes. In this article, one available method, namely essential work of fracture (EWF) has been used to assess the BPEs’ fracture toughness. This method has been developed greatly and become much attractive in evaluating the fracture behavior of ductile polymers, particularly due to its validity and ease of implementation [29], [30], [31], [32], [33], [34], [35], [36], [37]. Meanwhile, based on the successive self-nucleation and annealing analysis (SSA), the reason for the varied fracture behaviors of BPEs with different bimodal characteristics was discussed and the underlying mechanism was elucidated reasonably.
Section snippets
Materials and sample preparation
Two kinds of parent polymers named bimodal PE100 (Shanghai Petrochemical Co.; ρ = 0.949 g/cm3, Mw = 5.9 × 105 g/mol) and unimodal PE2911 (Fushun Petrochemical Co.; ρ = 0.960 g/cm3; Mw = 1.7 × 105 g/mol, ) were used in this paper. The samples were prepared by blending PE100 and PE2911 with the mass ratio of 100/0, 92/8 and 84/16 in a TSSJ-25 co-rotating twin screw extruder twice, L/D = 33. and they were labeled as S1, S2 and S3, respectively. The temperature profiles were 130 °C,
MWD characteristic and compatibility
Fig. 1a presents the MWD curves of BPE samples S1–S3 prepared by blending unmodal PE2911 and bimodal PE100. As can be seen, the relative intensities of the two peaks in the GPC MWD curves vary regularly. The average molecular weight, , and molecular weight distribution width, , are listed in Table 1. It is seen that the does not show obvious variation and the decrease of is also slight. The main differences exist in the intensities of the upper (the high molecular weight
Conclusion
It has been demonstrated that the MWD characteristic parameter (AL/U, the ratio of LPF to UPF) effectively influences the properties of BPE materials, especially the fracture toughness. Increasing the value of AL/U leads to obvious improvement of the fracture toughness we, namely, the enhancement of resistance to crack propagation.
As verified by SSA fractionation, the crystal perfection of BPEs is intensely affected by AL/U. With AL/U increasing, the thickness and amount of thick lamellas
Acknowledgements
We gratefully acknowledge the financial support from the National Natural Science Foundation of China (50873068, 51073109 and 20734005).
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