Synergistic toughening of polypropylene random copolymer at low temperature: β-Modification and annealing

https://doi.org/10.1016/j.msea.2011.05.030Get rights and content

Abstract

The synergistic effect of β-modification and annealing on the impact toughness of a commercially available polypropylene random copolymer (PPR) was investigated. Interestingly, the impact toughness of β-nucleated PPR after annealing at low temperature (0 °C) was almost five times as high as that of the virgin PPR without annealing. The crystalline structure, supermolecular structure, phase morphology, the relaxation of chain segments and fracture behavior of matrix were investigated to explore the toughening mechanism related to the β-modification and annealing. It was found that annealing improved the mobility of chain segments in amorphous phase as well as the strength of ligament of PPR matrix with profuse β crystals having the intrinsic low plastic deformation resistance, responsible for the superior toughness achieved. This work provides a possible method to toughen semi-crystalline polymers at low temperature by combination of β-modification and suitable annealing.

Highlights

► The synergistic effect of β-modification and annealing on the impact toughness of a commercially available polypropylene random copolymer (PPR) was investigated. ► The impact toughness of β-nucleated PPR after annealing at low temperature (0 °C) was almost five times as high as that of the virgin PPR without annealing. ► Annealing improved the mobility of chain segments in amorphous phase as well as the strength of ligament of PPR matrix. ► This work provides a possible method to toughen semi-crystalline polymers at low temperature by combination of β modification and suitable annealing.

Introduction

Polypropylene (PP) is one of the most widely used thermoplastic; but its application in some fields is limited especially at low temperature, due to its low impact resistance. The fracture resistance of this semi-crystalline polymer shows dramatic dependence on the ability of shear yielding and crazing [1], [2]. The way of promoting extensive shear yielding and/or multiple crazing in the polymer matrix is much beneficial for toughening. As demonstrated in the past studies, various elastomers are thought to be the most efficient toughening agents for PP; but the improvement of fracture toughness is at the cost of deterioration of stiffness and strength [3], [4], [5]. Instead of adding elastomers, copolymerization of propylene with ethylene or other olefins is another useful and effective method to produce high-performance polypropylene copolymers, PPR [6], [7], [8], [9], [10], [11]. In this copolymer, the homopolymer sequences are semi-crystalline and form a compound crystalline-amorphous biphase; while the propylene-ethylene random segments with high ethylene content tend to coalesce together to form a rubbery phase. It is indeed a multiphase polyolefin system with an excellent rigidity-toughness balance: the crystalline phase of homopolymer sequences guarantees the moderate strength and modulus, and the well-dispersed rubbery domains offer superior toughness. Therefore, polypropylene random copolymers (PPR) constitute an important class of plastic resins that are widely used as matrix components in pipe, automobile parts, furniture, and other industrial uses in the past two decades, based on the excellent mechanical properties and relatively low production cost [12], [13], [5]. Even so, it is also found that the toughness of PPR at low temperature (below Tg) is still too low and thus limits its further applications.

On the other hand, polypropylene is a polymorphic material with at least four crystalline forms, namely, the monoclinic α-form, trigonal β-form, orthorhombic γ-form, and mesomorphic smectic form [14], [15]. The differences in the supermolecular structures and the different crystalline states of PP exhibit different mechanical features. For example, the α-PP shows excellent modulus and tensile strength but inferior fracture toughness because the presence of interlocking effect of the radial lamellae by the tangential crystallites makes the plastic deformation very difficult [16], [17]. On the contrary, β-PP without cross-hatching allows the initiation and propagation of plastic deformation more easily and then enhances the energy dissipation [18], [19], [20], [21]. Especially, the enhanced toughness of β-PP can be attributed to a stress-induced transformation from less dense (β-phase) to more dense (α-phase) crystalline structure at the root of a growing crack [22], [23]. An additional factor that should be responsible for the reducing resistance to plastic flow initiation is the crystallographic symmetry of the hexagonal β phase with three equivalent glide planes. This indeed offers a great probability of favourably orientated crystals for slip with regard to the principal shear stress, and allows a more uniform deformation of the β lamellae at reduced yield stress. The amorphous phase has also been supposed to take part in the plastic modification of PP containing β phase crystals through higher intercrystalline tie chain density [24], [25]. On the basis of crystallization kinetics and chain-folding regularity, the amorphous phase is believed to provide an uniform stress distribution over the crystalline lamellae in the case of the β phase. A consequence of this is strong strain hardening for the β-phase that may notably account for the enlargement of the plastic zone at the crack tip. But, only controlling the amount of β-PP in the materials is not enough to obtain the high toughness. What is worse, some works found that the presence of β-phase had little effect at low temperature (below Tg) for isotactic polypropylene [26]. Thus, the combination of rubber particles and β-modification has been considered. Largely improved fracture resistance, as well as the shift of brittle-ductile transition to lower rubber particles volume fraction, has been reported in the literatures [27], [28], [29], [30].

Recently, the microstructure and mechanical behaviors of β-PP have also been comparatively researched through annealing process at the elevated temperature between glass transition temperature (Tg) and melting temperature (Tm) [31], [32], [33], [34]. Compared to α-PP with the interlocked structure, it seems more likely that β-PP has more potential to be toughened by secondary crystallization occurring in the amorphous phase during annealing. Some works have proved that, at a certain annealing temperature (130 °C), the fracture resistance of β-PP can be largely improved [34]. With the improvement of the crystals induced by secondary crystallization, including the degree of crystallinity, molecular arrangement, and lamellae thickness, the fracture toughness in the bulk crystals increases because more fracture energy is required to destroy these improved crystal structure. In addition, the number of chain segments in the amorphous region decreases and some microvoids form, making the lamellae structures looser and more available to slip and/or elongate along the impact direction [34], [35]. Although annealing is a useful way to improve the impact strength of β-PP at room temperature, the effect of annealing on the low temperature toughness of β-PP is few at hand.

In current study, we attempted to improve the toughness of PPR at relatively low temperature by combination of β-nucleating agent and annealing. The injection-molded bars of PPR with a fixed β-nucleating agent content of 0.1 wt.% were prepared at 240 °C, followed by annealing at 110 °C for 2 h. It was found that the synergistic toughening effect at low temperature (0 °C), through β-modification and annealing, was achieved. Numerous characterizations, including polarized light microscopy (PLM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), optical microscopy (OM), scanning electron microscopy (SEM) and dynamic mechanical analysis (DMA), were performed to disclose the underlying toughening mechanism.

Section snippets

Materials and sample preparation

Polypropylene random copolymer (PPR) used in this study was a commercial grade R200P supplied by Hyosung (Korea), with Mw = 72.2 × 104 g/mol, Mw/Mn = 4.5 and density of 0.91 g/cm3. The mass percentage of ethylene component was as low as 3.8 wt.%. A small amount of antioxidant (Irganox 1010) was added into PPR to prevent the thermal decomposition during melt processing. The rare earth β-nucleating agent, marked as WBG, was kindly supplied by Winner Functional Materials Co. (Foshan, Guangdong, China).

The

Toughness and fracture morphology

On the basis of the research goal of this study, the notch Izod impact strengths for PPR, PPR-A, β-PPR and β-PPR-A were studied at 0 °C, and the values are plotted in Fig. 1. It is found that the sample of PPR shows relative low impact strength at this testing temperature. It also shows that the impact strength of PPR has been improved a little by either annealing or β-modification. However, the sample of β-PPR-A shows obviously high impact strength (30.2 kJ/m2), which is almost five times as

Conclusion

In this work, excellent low temperature impact toughness of polypropylene random copolymer (PPR) was obtained by combining β-modification and annealing. On the one hand, the decrease in the crystalline size and formation of β form reduce the plastic resistance of crystals. On the other hand, because of the second crystallization, the strength of amorphous phase enhances and can transmit stress to the adjacent crystallites. What is more, the enhanced molecular mobility in the amorphous phase is

Acknowledgement

We would like to express our great thanks to the National Natural Science Foundation of China for financial support (50973065, 50873063, 20874064).

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