WAXD measurements at PETRA III: the kinetics of strain-induced crystallization in natural rubber (Sep. 2012)

Fig. 1

Fig. 1: Crystallinity vs. time for natural rubber filled with 20 phr carbon black N234 after strain steps (at t = 0) from 0% to 240%, 300%, 350%, 410% and 460 % (from bottom to top).

Fig. 2

Fig. 2: Strain and crystallinity vs. time for unfilled rubber during dynamic stretching between 0 % and 330 %.

Reasearchers from Leibniz-Institut für Polymerforschung Dresden, Technische Universität Dresden and DESY studied the strain-induced crystallization in cross-linked natural rubber by using time-resolved wide-angle X-ray diffraction at beamline P03 at PETRA III.

Strain-induced crystallization (SIC) is one of the key properties setting natural rubber apart from its synthetic competitors. Due to the reinforcing effect of the crystallites, the stress at break is elevated and the crack propagation rate considerably reduced. SIC has been studied for several decades, however the long exposure times of classic X-ray sources or 2nd generation synchrotrons, experiments were limited to quasistatic tensile tests [1,2]. Only limited work has been done so far to elucidate the crystallization process under dynamic loading [3,4]. However, in practice, numerous rubber products e.g. tires, are loaded dynamically, . Thanks to the high flux of beamline P03, for the first time it was possible to study SIC in situ during dynamic mechanical testing with a stretching frequency of approx. 1 Hz.

Two kinds of experiments were performed at beamline P03 (MiNaXS) in wide-angle diffraction (WAXD) setup. Firstly, dynamic mechanical tests with a frequency of 1 Hz were done with 50 Hz pattern acquisition rate. Secondly, tensile impact tests, stretching the sample by several hundred percent within less than 10 ms, were carried out and crystallization was observed during and after the impact with a frame rate of 140 Hz.

It could be shown that the crystallinity during cyclic dynamic stretching is significantly lower than during quasistatic stretching at a given strain. This is due to the non-instantaneous crystallization kinetics, which were studied in detail in the impact experiments (fig. 1 and 2). Only about half of the crystallization process is completed within 5 ms after the impact strain. In order to reach the same crystallinity as in a quasistatic test, a time regime of 1 to 10 s is needed. Since in dynamic testing at 1 Hz, the sample resides above the crystallization strain only for a fraction of a second, the maximum crystallinity in dynamic stretching is only about half of that in quasistatic stretching.
These results have direct effects on the stress-strain behavior of natural rubber at different strain rates, which in turn is critical to understand phenomenons such as the enormous resistance against quasistatic crack propagation, whereas the resistance is much lower under dynamic conditions.

[1] J. Katz, Naturwissenschaften 13, 410 (1925).
[2] S. Toki et al., Macromolecules 35, 6578 (2002).
[3] M. Acken et al., Ind. & Eng. Chem. 24, 54 (1932).
[4] B. Huneau et al., Proc. of ECCMR 7, 353 (2011).

(Info from authors)

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