Elsevier

Polymer

Volume 49, Issue 12, 10 June 2008, Pages 2964-2973
Polymer

Effect of small amount of ultra high molecular weight component on the crystallization behaviors of bimodal high density polyethylene

https://doi.org/10.1016/j.polymer.2008.04.050Get rights and content

Abstract

In order to clarify the effect of high molecular weight component on the crystallization of bimodal high density polyethylene (HDPE), a commercial PE-100 pipe resin was blended with small loading of ultra high molecular weight polyethylene (UHMWPE). The isothermal crystallization kinetics and crystal morphology of HDPE/UHMWPE composites were studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM), respectively. The presence of UHMWPE results in elevated initial crystallization temperature of HDPE and an accelerating effect on isothermal crystallization. Analysis of growth rate using Lauritzen–Hoffman model shows that the fold surface free energy (σe) of polymer chains in HDPE/UHMWPE composites was lower than that in neat HDPE. Morphological development during isothermal crystallization shows that UHMWPE can obviously promote the nucleation rate of HDPE. It should be reasonable to conclude that UHMWPE appeared as an effective nucleating agent in HDPE matrix. Rheological measurements were also performed and it is shown that HDPE/UHMWPE composites are easy to process and own higher melt viscosity at low shear rate. Combining with their faster solidification, gravity-induced sag in practical pipe production is expected to be effectively avoided.

Introduction

Since its first introduction in pipe application over 40 years ago, polyethylene (PE) has been taking a growing place in transportation and distribution of water and gas. PE pipes offer distinct advantages compared with other piping materials because they are lightweight, corrosion-free, exhibit very high ductility and allow all-welded construction [1]. First- and second-generation PE pipe resins for water and gas distribution are known as PE-63 and PE-80, which must withstand a minimum required hoop stresses of 6.3 MPa and 8.0 MPa for up to 50 years at 20 °C, respectively. Driven by the advancement in catalyst and polymerization technology, the third-generation PE pipe resins called “bimodal” resins have emerged since 1990s, and are classified to PE-100 (i.e., pipe must withstand hoop stress of 10 MPa for up to 50 years at 20 °C, see ISO 12162). The molecular feature that allows these bimodal resins to exhibit improved properties is not the shape of the molecular weight distribution (MWD) but the preferred incorporation of comonomers in the long polymer chains within the second reactor during a cascade polymerization process. This way a polymer blend is formed with low-molecular weight ethylene homopolymer components and high-molecular weight ethylene-1-alkene copolymer components [2], [3], [4]. Bohm [3] has recently concluded that in a bimodal PE-100 resin, the high-molecular weight copolymers form amorphous regions and act as tie molecules that connect the crystal lamella mainly formed by the low-molecular weight homopolyethylene. In this manner, a physical network is formed and thus the mechanical properties of the polymer are greatly improved.

However, compared with unimodal resins, bimodal PE-100 resins lack the melt elasticity due to the absence of both long chain branching (LCB) and the very high molecular weight tail [4]. The result is that PE-100 resins are limited in producing large diameter, thick wall pipes because the molten polymers tend to sag before hardening. Another problem with pipe fabrication is the thermal gradient across the pipe wall during extrusion process. The outside and inside surfaces are cooled down by a water spray and solidified quickly. But the crystallization (shrinkage) of the core region is much slower which causes residual stress within a pipe [5]. Industrially, it has been widely accepted that accelerating the crystallization rate of PE pipe resins could effectively avoid sag due to the fast hardening of the molten polymers. Meanwhile, if the overall crystallization rate is increased, the residual stress within a pipe will decrease because the differences in shrinkage between surface and core region of the pipe wall become inconspicuous.

The crystallization of polymer is an important physical process and remains a hot issue. Many methods, models as well as technologies have been invented aiming to describe the whole process of crystallization from the very beginning to the late stage and clarify the mechanism behind it. Recently, the early stages of crystallization in polyethylene have been explored by some researchers [6], [7], [8] using various techniques and Bassett [9] studied a new linear nucleation and oriented crystallization of PE. It is widely accepted that the crystallization of polymers such as PE and polypropylene (PP) is mainly controlled by the nucleation stage. Using some specific nucleating agents to shorten the inducing time of crystallization and accelerate the formation of crystalline nuclei is a technique that is commonly applied in polymer industry. However, unlike PP, there are few commercial nucleating agents for PE. Therefore, how to promote the crystallization of bimodal PE pipe resins has been a challenging problem both for academe and industry.

UHMWPE is one of the leading plastics that have been developed in recent decades. The outstanding properties of UHMWPE, such as toughness, high wear strength, and abrasion resistance, provide not only new utilities but also scientific interests. UHMWPE has been widely used to optimize the property of polymers such as PE, PP, ethylene-propylene-ternary (EPT) rubber, polyaniline (PANI) and polyacrylate (PA) [10], [11], [12], [13], [14], [15]. Recently, Busby et al. [16] obtained a novel nanostructured polymeric composite of polycarbonate (PC) and UHMWPE via a supercritical-fluid route. With regard to the blending of HDPE and UHMWPE, many research works have already been reported elsewhere [17], [18], [19], [20], [21], [24], [25]. For example, Tincer and Coskun [17] have prepared the blends of HDPE/UHMWPE at different compositions and mixing rates to study their mechanical properties, thermal oxidative degradation and morphologies. Lim et al. [20], [21] have studied the suitability of HDPE/UHMWPE composites as biomaterials. However, most of these works were focused on mechanical or processing properties. As UHMWPE owns a high melt viscosity and can be drawn even from a melt, special morphology such as shish–kebab can be formed during the crystallization of UHMWPE [22], [23]. Accordingly, whether UHMWPE will have a special influence on crystallization behaviors of bimodal HDPE is an interesting problem and has not been well understood up to date, although some investigations have paid attention to the crystallization behavior of HDPE/UHMWPE composite under shear [26], [27], [28].

In our previous studies [29], [30], using self-consistent mean field theory, it has been found that, for the case of a binary polymer blend, broadening MWD would decrease the energy barrier of nucleation and theoretically the high molecular weight component has the capability of inducing nucleation. Therefore, if UHMWPE could act as a certain nucleating agent for bimodal HDPE pipe resins and accelerate their crystallization rates, we may develop a potential preparation technique for producing high performance pipe materials with improved sag-resistance. However, few relevant experimental research works have been reported.

In the present study, we introduced a small amount of UHMWPE into a commercial bimodal PE-100 pipe resin by melt blending. The aim of this work is to clarify the effect of ultra high molecular weight component on the crystallization of bimodal HDPE. The isothermal and nonisothermal crystallization behaviors of physically blended HDPE/UHMWPE composites were studied with differential scanning calorimetry (DSC). The DSC thermograms provided necessary crystallization kinetics data which were further analyzed by the Avrami method. Using successive self-nucleation and annealing (SSA) thermal fractionation technique, changes in chain structures induced by UHMWPE were obtained. The spherulitic morphologies of neat HDPE and HDPE/UHMWPE composites during isothermal crystallization were observed by polarized optical microscopy (POM). Rheological measurements were also performed to evaluate the processing property of the composite.

Section snippets

Materials and sample preparation

HDPE 4803T, a commercial bimodal PE-100 pipe resin provided by Yangzi Petrochemical Co., SINOPEC (Nanjing, China) was used in this study. Its melt flow rate (MFR) is 0.04 g/10 min (190 °C, 2.16 kg load) with a density of 0.948 g/cm3. UHMWPE powders with molecular weight Mw = 3.5 × 106 g/mol were kindly provided by Second Auxiliary Factory of Beijing (China).

The HDPE granules and UHMWPE powders were melt-blended by a Brabender Mixer (PLE651) at a speed of 45 rpm and a mixing temperature of 165 °C for 7 min.

Thermal properties of HDPE/UHMWPE composites

The thermal properties of neat HDPE and HDPE/UHMWPE composites were measured by a standard heating–cooling–heating procedure at an identical rate of 10 °C/min. The melting temperature (Tm), the heat of fusion (ΔHm), the initial crystallization temperature (Tci) and the crystallinity (χc) are shown in Table 1. For samples UHMWPE-0.5–UHMWPE-3.0, as the content of UHMWPE was pretty low, the melting temperatures and the enthalpies of fusion have not substantially changed compared with neat HDPE.

Conclusion

In this study, UHMWPE in a bimodal HDPE pipe resin acted as a kind of nucleation agent that promotes the crystallization behavior of HDPE, at a very small loading of UHMWPE, was demonstrated. The isothermal crystallization behaviors of neat HDPE and HDPE/UHMWPE composites were studied. The crystallization kinetics analyzed by the Avrami method reveals that the introduction of a small amount of UHMWPE to neat HDPE could obviously accelerate the isothermal crystallization rate. In terms of LH

Acknowledgements

We gratefully acknowledge the financial support from the National Basic Research Program of China (G2005CB623803), the National Natural Science Foundation of China (Grant No. 50673021), the Hi-Tech Research & Development Program of China (2007AA03Z450) and the Natural Science Foundation of Shanghai (Grant No. 06ZR14007).

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