Crystallinity by WAXS

Crystallinity of all the fibres and their raw materials was determined using wide-angle X-ray scattering (WAXS) measurements. In the Fig. 3.11, the raw materials of virgin and recycled PP have four distinct diffraction peaks at 20 = 14.2°, 17°, 18.8°, and 21.4°/21.9°. According to the research of Huang et al. (2005), the 20 = 14.2°, 17°, 18.8°, and 21.4°/21.9° associated with the (110), (040), (130) and (111)/(041) planes, respectively. The peaks at 20 = 21.4° and 21.9° are the co-diffractions of b phase and a phase. The virgin and recycled PP fibres obtain five diffraction peaks at 20 = 14.2°, 17°, 18.8°, 25.5° and 28.4°.

The crystallinity values derived from the WAXS measurements are summarised in Table 3.7. As can be seen, the raw material of recycled PP has slightly higher crystallinity (51.2%) than that of the raw material of virgin PP (47.2%), because the raw material of recycled PP has lower molar mass, shorter molecular chains and more impurities. The lower molar mass and shorter molecular chains were easier to align and form into crystals, and the impurities greatly decreased the free enthalpy required for the formation of a critical nucleus, which further reduced the critical size of the nucleus and then led to the formation of heterogeneous nuclei (Aurrekoetxea et al. 2001). However, the hot drawing process can considerably improve the crystallinity of the virgin and recycled PP fibres to 82.7% and 81.7%, respectively. The a-crystallinity (given by Ka Xc) of the virgin and recycled PP fibres were significantly improved, while the b-crystallinity (given by K^Xc) kept similar to their raw materials.

WAXS profiles of the polypropylenes

Fig. 3.11 WAXS profiles of the polypropylenes

Table 3.7 Crystallinity of the polypropylenes

Composition

Crystallinity (Xc)

a-crystallinity (Ka Xc)

P-crystallinity (Xc Kp)

Raw material of virgin PP

0.472

0.40

0.077

100% virgin PP fibre

0.827

0.75

0.078

5:95 HDPE-virgin PP fibre

0.820

0.68

0.063

Raw material of recycled PP

0.512

0.43

0.078

100% recycled PP fibre

0.817

0.74

0.074

5:95 HDPE-recycled PP fibre

0.799

0.66

0.060

50:50 recycled-virgin PP fibre

0.819

0.75

0.071

As shown in Fig. 3.11, after the hot drawing process the virgin and recycled PP crystallised into the monoclinic a-form via diffractions at 29 = 14.2°, 17° and 18.8°, associated with the (110), (040) and (130) planes, respectively (Huang et al. 2005). On these facets the diffraction peak positions had no change, but the intensities of the peaks increased drastically. The intensity of the diffraction peak revealed the crystallinity and orientation of materials. It indicated that the hot drawing process did not affect the a-form crystal type, but significantly improved the crystallisation and orientation of the PP fibres. It was interesting to note that these three peaks overlapped to some extent, indicating that both crystal and mesomorphic phases co-existed in the fibres. The (111) and (041) doublet peaks moved to the off-axis and disappeared from the equator after the hot drawing process. On contrary, new peaks at 29 = 25.5°, 28.4° were visible, indicating the generation of new p-form crystals.

When 5% of HDPE was added into the virgin and recycled PP fibre, both their crystallinity were slightly decreased due to the heterogeneous structure. The crystallisation of HDPE restrained a- and p-crystallinity, as shown in Table 3.7. Therefore, the HDPE decreased the Young’s modulus of PP fibres, but increased the ductility of PP fibres as shown in Fig. 3.11. However, when 50% of virgin PP was mixed with the recycled PP, the a-crystallinity kept as high as the virgin PP fibre, but the p-crystallinity was limited to lower rate of 7.1% (in Table 3.7). Therefore, 50:50 virgin-recycled PP fibre obtained the highest Young’s modulus in all the PP fibres.

 
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