Mechanical Properties of PP Fibres

As can be seen in Fig. 3.6, all the fibres produced by the melt spinning and hot drawing process showed a brittle mode of failure, with a short elastic period of steep slope and a regime of sharply rising specific stress until fracturing occurred at strains between 8 and 16%. Table 3.3 presents average of the tensile strength, Young’s modulus and elongation at break of these fibres and their standard deviations. Compared with the mechanical properties of virgin and recycled PP raw materials in Table 3.1, the melt spinning and hot drawing process offered the PP fibres much higher mechanical properties. As shown in Table 3.3, the virgin PP fibre obtained the highest tensile strength at 457.1 MPa and high Young’s modulus at 7526 MPa, while the recycled PP fibre exhibited 341.6 MPa of tensile strength and high Young’s modulus of 7115 MPa.

As shown in Table 3.1, the melt flow index (MFI) of raw recycled PP is 13 dg/min, which is much higher than that of raw virgin PP (only 3.5 dg/min). This means that the fibre from recycled PP has much lower molar mass and hence shorter molecular chains than those of virgin PP fibre (da Costa et al. 2007). During the service history, the recycled PP materials normally have natural aging from long exposure to the air, light, moisture, temperature and weathering (Villain et al. 1995). Moreover, the multiple processes under high shear forces and temperatures, and the presence of impurities and oxygen severely damage molecular chain of PPs, including crosslinking, chain scission and formation of double bonds (Hinsken et al. 1991). The chain scissions and degradation make the molecular chain easier to

Typical stress-strain curves of the PP fibres

Fig. 3.6 Typical stress-strain curves of the PP fibres

Table 3.3 Tensile properties of the PP fibres

PP compositions

Tensile strength (MPa)

Young

(MPa)

s modulus

Elongation at break

(%)

Mean

Standard

deviation

Mean

Standard

deviation

Mean

Standard

deviation

100% virgin PP fibre

457.1

31.7

7526

2011

10.6

1.4

5:95 HDPE-virgin PP fibre

436.0

23.2

6837

1538

16.5

2

100% recycled PP fibre

341.6

29.3

7115

2083

8.4

2.2

5:95 HDPE-recycled PP fibre

341.9

27.6

6467

2326

9.4

1.7

50:50 virgin-recycled PP fibre

435.5

26.5

9016

1919

8.1

1.4

be pulled out, forming disentanglement and promoting nucleation of micro-voids and micro-cracks (Bahlouli et al. 2006). Therefore, the recycled PP fibre had much lower tensile strength and elongation at break than those of virgin PP fibre.

When 5% HDPE was mixed with both the recycled and virgin PP fibre, the Young’s modulus of both fibres were decreased and the elongation at break were increased compared with the 100% recycled and virgin PP fibre. This indicated that the HDPE brought ductility to the PP fibres. However, when 50% of the virgin PP was mixed with 50% of the recycled PP, the Young’s modulus was considerably increased to 9016 MPa, which was even higher than that of virgin PP fibre, and the tensile strength remained as high as the virgin PP fibre at 435.5 MPa. Therefore, the recycled PP fibre can obtain good ductility from the 5% of HDPE and can be well modified to obtain high tensile strength and Young’s modulus by mixing with 50% of virgin PP.

It is worth to note that there is a high variation on the Young’s modulus of all the fibres. The Young’s modulus of PP fibre is very sensitive to its crystallisation. As macromolecule polymer, the location, length and entanglement of the molecular chains of PPs are not even through the whole PP materials, thus the crystallisation of PP is not consistent, especially for the PP fibres. Virgin PP materials have more homogeneous molecular chains, thus showing lower variation than that of recycled PP fibres, which have some degradations from service history and repetitive processing.

 
Source
< Prev   CONTENTS   Source   Next >