Molecular Orientation by FTIR
As shown in Eq. (3.1), the closer to zero the absolute value of f is, the less orientation the molecules have, and vice versa. As can be seen in Fig. 3.7a, the recycled PP raw granules in the parallel and perpendicular directions showed similar intensities of absorption brands at 998 and 1153 cm-1. The f998 and f1153 were calculated as -0.09 and 0.05, respectively. Therefore, the raw materials had
Fig. 3.7 FTIR spectra of a recycled PP raw granules and b recycled PP fibres on the parallel and perpendicular directions
not obvious molecular orientation. However, after the melt spinning and hot drawing process, the intensities of absorption brands at 998 cm-1 of the recycled PP fibres exhibited significant difference on the parallel and perpendicular directions as Fig. 3.7b. Thef998 was calculated as -0.5, thus the crystal phase of the recycled PP fibres exhibited considerable molecular orientation.
Crystal Structure and Crystallinity by DSC
The crystal structure of the PP fibres was studied by DSC. As shown in Fig. 3.8a, b, the raw material of recycled PP has a broader melting endotherm than that of the virgin PP, which indicates that the raw recycled PP has more crystals of different sizes, and the raw virgin PP has more uniform crystal sizes. From the Table 3.4 the raw recycled PP exhibits higher heat of fusion (AH) than that of the raw virgin PP, indicating the raw recycled PP has higher crystallinity. This is why the raw recycled PP has higher mechanical properties than the raw virgin PP as Table 3.1. As Table 3.1, the raw recycled PP has much higher MFI than that of the raw virgin PP, indicating that the raw recycled PP has lower molar mass and shorter molecular chains. In the processing stages and service history of the recycled PP, chain scissions frequently occurred, thus significantly reducing the molar mass and shortening the molecular chains. The lower molar mass and shorter molecular chains are easier to rearrange, orient and form into crystals than the longer molecular chains of the raw virgin PP (Gonzalez-Gonzalez et al. 1998).
As shown in Table 3.4, the AH and crystallinity of the raw virgin PP are very small (only 76.2 J/g and 36.8%, respectively). However, after the melt spinning and hot drawing process, the AH and crystallinity of the virgin PP fibre are increased dramatically to 106.7 J/g and 51.5%, respectively. Similar with the recycled PP, the process improved the crystallinity of the recycled PP from 41.5% to 50.9%.
Fig. 3.8 DSC heating curves of the PP fibres and their raw materials (First round of heating from 30 to 220 °C)
Table 3.4 Peak temperatures of melting and heat of fusion for the a- and p-form crystals (first heating from 30 to 220 °C)
Furthermore, as Fig. 3.8c the virgin and recycled PP fibres exhibit a double melting endotherm at around 154 °C and 168 °C. The peak located at 154 °C is ascribed to the p-form crystals and the other is to the a-form crystals (Tabatabaei et al. 2009). According to Huo et al. (2005), the oriented molecular chains formed a row-nuclei and the oriented a row-nuclei can induce to form p-crystals. Somani et al. (2005) reported that the PP fibres which have a double melting endotherm show a specific shish-kebab structure. The shish in PP had a melting temperature of about 5-10 °C higher than that of the kebabs and about 15-20 °C higher than that for spherulites.
On the a-form crystallisation in Table 3.4, the virgin PP fibre exhibited higher peak melting temperature (Tm) than that of its raw material. As Zhao and Ye (2011), the high-temperature endotherm represents the melting of the highly chain-extended and the highly oriented crystalline blocks formed during the hot drawing process, while the low-temperature endotherm is due to the melting of strained non-crystalline region and some partially oriented lamellar. According to Elias et al. (2000), the hot drawing process can result in a connectivity of molecular chains in the shish or fibrils, thus increasing the crystal thickness. Therefore, a higher melting point was obtained by the virgin PP fibre. However, the recycled PP fibre had slightly lower peak temperature of melting than that of its raw material probably because of molecular defects and lower purity.
As seen in Table 3.4, the recycled PP fibre and virgin PP fibre have similar DHs (105.4 and 106.7 J/g, respectively) and similar crystallinity (50.9% and 51.5%, respectively). However, as Fig. 3.8a, b the recycled PP fibre has much higher DH on p-form crystals than that of the virgin PP fibre. The p-form crystals are much less stable than the a-form crystals (Hirose et al. 2000). Due to a large amount of p-form crystals, the recycled PP fibre showed lower tensile strength. Although the oriented a row-nuclei of the virgin PP fibre induced some p-crystals, the a-form crystals were still dominant and the DH of p-form crystals was very low in the virgin PP fibre. Therefore, the virgin PP fibre had a large amount of stable a-form crystals, thus showing higher tensile properties.
When 5% of HDPE was mixed with the virgin PP fibre as Fig. 3.8a, the DH of a-form crystals was decreased and the a-form crystallisation was significantly affected by the heterogeneous structure between HDPE and virgin PP. Moreover, the HDPE offered nucleation sites for p-form crystals, thus improving rate of the p-form crystals. Therefore, the tensile strength and Young’s modulus of the virgin PP fibre was weakened by the HDPE. On the contrast, the 5% of HDPE restrained the formation of p-form crystals in the recycled PP fibre, more stable a-form crystals were produced by adding the HDPE as Fig. 3.8b. The recycled PP normally has more molecular defects and lower purity due to the degradation from its service and processing history. The HDPE acted as a compatibiliser, which changed rough phase structure of the molecular defects and some impurities, thus more stable a-form crystals were obtained.
When 50% of virgin PP was mixed with 50% of recycled PP as Fig. 3.8c, a double-melting endotherm was found on the a-form crystallisation. One peak is located on 167 °C, which is close to the Tm of a-crystals of recycled PP fibre; the other peak at 169.8 °C, which is similar with the Tm of a-crystals of virgin PP fibre.
Moreover, the DH of b-form crystals on 154.3 °C is much lower than that of recycled PP fibre and slightly higher than that of virgin PP fibre. Therefore, the 50% of virgin PP not only retained high crystallinity and crystal structure, but effectively restrained the formation of b-form crystals in the recycled PP fibre. Finally, the 50:50 virgin-recycle fibre showed very high tensile strength and Young’s modulus as shown in Table 3.4.
The samples were then held at 220 °C for 5 min, and cooled from 220°C to 30 °C at a scan rate of 10 K/min. As seen in Fig. 3.9a, b, and Table 3.5, the raw material of recycled PP has lower Tc (114.8 °C) than that of the raw virgin PP (119.8 °C), indicating that the raw recycled PP is easier to crystallise and can crystallise at a lower temperature. This is due to the lower molar mass and shorter molecular chains of recycled PP, which can be reflected by its lower MFI in Table 3.1. According to Horvath et al. (2013), the shorter molecular chains are easier to be released from the strained or entangled macromolecules, thus the crystallisation can be further developed by the rearrangement of these freed macromolecules segments. The heat of fusion of raw recycled PP is 87.9 J/g, which is lower than that of the raw virgin PP (88.4 J/g), indicating that the raw recycled PP released less thermal energy, formed fewer crystals and less perfect crystals than the raw virgin PP, because the raw recycled PP has more defective molecules and impurity.
As Table 3.5 and Fig. 3.9, after the melt spinning and hot drawing process, both the virgin PP and recycled fibres obtained much higher Tc (124.8 °C) and more
Fig. 3.9 DSC heating curves of the PP fibres and their raw materials (cooling from 220 to 30 °C)
Table 3.5 Peak temperature of crystallisation and heat of fusion (cooling from 220 to 30 °C)
narrow crystalline peaks than those of their raw materials, indicating the hot drawing process highly oriented and aligned the crystal structures. When the temperature cooled down from the 220 °C, the ordered molecular structure was more active at higher Tc and thus formed more perfect crystals than the raw materials, which led to significantly improved mechanical properties of the fibres.
The heat of fusion of recycled PP fibre was also improved to 89.4 J/g, because its short oriented molecular chains were crystallised easily and quickly. However, the virgin PP fibre obtained lower heat of fusion (87.6 J/g) than that of its raw material. The oriented molecular chains of virgin PP fibre is more likely to form perfect crystals than its raw material, but it needs more time due to the long molecular chains. However, because of the same cooling rate in the DSC tests, the virgin PP fibres did not have enough time to form perfect crystals, thus obtained lower heat of fusion (Zhang et al. 2011). Moreover, the long molecular chains were easier to entangle and entwist together, thus deceased molecular order degree and crystallinity (Zhong and Mao 2009).
When 5% of HDPE was mixed with the recycled and virgin PP fibres, their DHs were improved to 93.8 J/g and 97.8 J/g, respectively, indicating the HDPE had heterogeneous nucleation effect on the PP crystallisation. The crystallisation of HDPE, which can be seen on the small peaks around 118.8 °C in Fig. 3.9a, b, also contributed to the DHs. When 50% of the virgin PP was mixed with 50% of the recycled PP, the Tc and DH was slightly decreased probably due to compatible problems between the virgin and recycled PPs.
Thermal history of all the samples was eliminated through heating and cooling processes, and then the samples were reheated from 30 °C to 220 °C. As seen in Fig. 3.10a, b, the raw materials still have broad peaks, which are related to three-dimensional crystals known as spherulites and/or rows of lamellae (Somani et al. 2005). Further, the raw material of the recycled PP has a broader peak than that of the raw material of the virgin PP, which indicates the crystals of recycled PP have broader size distribution. However, all the fibres show narrow peaks, because the highly chain-extended and the highly oriented crystalline blocks formed fibrils during the hot drawing process. The thermal history of highly extended and oriented molecular chain is hard to be fully eliminated in the first round of heating and cooling (Zhao and Ye 2011). Small melting peaks from the HDPE can be found for the 5:95 HDPE-virgin PP fibre and 5:95 HDPE-recycled PP fibre. As Table 3.6 the Tms of all the fibres are only about 165 °C, which are much lower than the Tm of
Table 3.6 Peak temperature of melting and heat of fusion (second round of heating from 30 to 220 °C)
Fig. 3.10 DSC heating curves of the PP fibres and their raw materials (second round of heating from 30 to 220 °C)
a-form crystals from the first heating in Table 3.4, indicating that the hot drawing process highly extended and oriented crystalline blocks. Furthermore, the DHs of all the fibres in Table 3.6 are much lower than those in Table 3.4, indicating that the hot drawing process significantly improved the crystallinity of the fibres.