Fracture toughness of α- and β-phase polypropylene homopolymers and random- and block-copolymers

doi:10.1016/S0032-3861(02)00590-6

Polymer

Volume 43, Issue 24, November 2002, Pages 6505-6514

https://doi.org/10.1016/S0032-3861(02)00590-6 Get rights and content

Abstract

The fracture and failure mode of α- and β-phase polypropylene homopolymers (PP-H), block- (PP-B) and random-type (PP-R) copolymers with ethylene were studied in high speed (1.2 m/s) flexural tests and compared. The crystallinity of the α- and β-modifications was assessed by wide-angle X-ray scattering and differential scanning calorimetry. The linear elastic fracture mechanical parameters, viz. fracture toughness (K_c) and fracture energy (G_c), were determined at room temperature and T=−40 °C on notched Charpy specimens. β-Phase PP-H and PP-B showed superior toughness to the α-versions. On the other hand, K_c and G_c were similar for PP-R in the temperature range studied for both α- and β-modifications. Fracture surfaces of the broken specimens were inspected in scanning electron microscopy and the related failure mode concluded. A model was proposed to explain the toughness improvement via β-crystallinity by considering all proved experimental findings.

Introduction

β-Nucleated isotactic polypropylene homopolymers (PP-H) have received considerable interest recently. This interest is mostly due to the peculiar thermal and mechanical performance of the β-crystalline PP-H [1], [2], [3]. The toughness of β-phase PP-H is markedly higher than that of the α-modification, both below and above the glass transition temperature (T_g). This has been demonstrated in several works adopting the methods of linear [4], [5], [6] and elasto-plastic fracture mechanics [7], [8], [9], [10]. Note that fracture mechanics is the right tool when a toughness comparison between various PP modifications is targeted. Concepts of the fracture mechanics, in fact, may yield an inherent material parameter which is independent of the test configuration. Attention should be paid to the fact that a break-thorough in the research and application of β-crystalline PPs occurred when highly selective β-nucleants became available [1], [3], [11].

Interestingly, the mechanisms of toughness improvement are still the topic of intense debates [5], [7], [10]. There are no doubts, however, about the role of the microstructure (lamellar ordering) and loading-induced β–α polymorphic transition. Therefore the dispute is focused on which are the causes and consequences of the toughness enhancement and how to distinguish between them. The most comprehensive review on β-phase PPs by Varga [1] highlights that the effect of β-crystallinity on the toughness of random- (PP-R) and block-type PP copolymers (PP-B) was less studied [12], [13], [14], [15]. Further, authors of the related works have used non-selective β-nucleants with the only exception of Zhang and Shi [12]. In addition, for the toughness determination of rubber-toughened PPs (PP melt blended by rubbers) only Grein et al. [15] used fracture mechanical methods. By contrast, the melting and crystallization characteristics of β-nucleated PP-R and PP-B systems have been well explored. It was shown that PP-R has a reduced tendency to β-crystallization [2], [16], [17].

Therefore the aim of this paper was to determine the fracture mechanical parameters of β-crystalline PP-R and PP-B systems produced by highly selective β-nucleants and to compare the related values with those of the α-modifications. In order to get a more complete picture, the work was extended also for α- and β-phase PP-Hs of extremely high molecular weight (MW). It is worth noting that the toughness improvement through β-crystallinity augments with increasing MW of the PP-H resin [1], [18], [19]. A further aim of this study was to assess the failure mode by fractography and thus to contribute to some open questions related to the toughness improvement caused by β-crystallinity.

Section snippets

Specimens and their characteristics

The basic properties of PP-H, PP-B and PP-R are listed in Table 1. The characteristics of the PPs in Table 1 already indicate that the fracture response of the PP-B and PP-R systems can only be compared with each other. There are some other aspects (β-nucleant, specimen preparation) besides the MW (cf. Table 1) due to which the fracture behavior of the α- and β-phase PP-Hs should be treated separately. On the other hand, a common discussion is straightforward as the polymorphic composition of

α- and β-crystallinity

Fig. 1 shows the WAXS patterns for the α- and β-crystalline PPs studied. The intense peak at 2Θ=16.2° and the less intense one at 2Θ=21° are assigned to the 300 and 301 planes of the β-crystals. The peak at 16.2° is widely used to detect the β-content of the polymorphous PP via the K-value of Turner Jones et al. [24]: $K= I_{(300)β} I_{(300)β} +I_{(110)α} +I_{(040)α} +I_{(130)α}$ Accordingly, K=1 for the fully β- and 0 for the fully α-crystalline PP. The overall crystallinity (X_c) was determined by: $X_{c} = A_{c} A_{c} +A_{a}$ where A_c

Conclusions

Based on this work performed on the fracture mechanical characterization of α- and β-crystalline PP homopolymers (PP-H), block (PP-B) and random copolymers (PP-R), the following conclusions can be drawn:

(i)
β-Crystallinity strongly increases the toughness of PP-H and PP-B both below and above T_g.
(ii)
β-Crystallinity may suppress the effect of rubber dispersion in PP-R, however, without affecting the overall toughness response. This finding holds likely for tests both below and above T_g.
(iii)
The energy

Acknowledgements

This work was supported by the DAAD, through sponsoring the research stay of H.B. Chen. J. Varga acknowledges the support of the Hungarian Scientific Foundation (OTKA:T-034230).

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Fracture toughness of α- and β-phase polypropylene homopolymers and random- and block-copolymers

Abstract

Introduction

Section snippets

Specimens and their characteristics

α- and β-crystallinity

Conclusions

Acknowledgements

Scr Metall Mater

Polymer

Polymer

Polymer

Polymer

Polymer

Polymer

Polymer

Polymer

Polymer

J Macromol Sci Phys

J Appl Polym Sci

Macromol Symp

Polym Engng Sci

J Appl Polym Sci

Eur Phys J E

J Polym Sci B: Polym Phys

J Appl Polym Sci

Adv Polym Technol