In this work, a bromide initiator was firstly grafted on the surface of the cellulose fiber (CF) through esterification. Then, the cellulose fiber-based macroinitiator (CF-MI) was used to initiate the SET-LRP of the stearyl acrylate monomer as exposed in Scheme 1. The SET-LRP was performed in the presence of a copper wire and a tetradendate tertiary amine ligand (Me6TREN). Once the polymerization was completed, the modified cellulose fibers (CF-PSA) were extruded with the HDPE and the resulting filament was process through 3D printing.
The initial and unmodified cellulose fiber, the cellulose fiber-based macroinitiator and the Poly(stearyl acrylate) grafted CNF (CF-PSA) were analyzed by solid state 13C CP/MAS NMR and ATR-FTIR spectroscopies. The results are exposed in Figure 1 and Figure 2.
The main characteristic peaks from the cellulose could be identified on the NMR spectra (Figure 1), for all the samples CF, CF-MI, and CF-PSA. Moreover, some additional peaks (20 ppm, 56 ppm and few weaker signals from 100 to 160 ppm) could be also observed, and were attributed to some residual lignin. It is to be noted that the peak of the carbons 4 and 6 are split in two, due to a difference in chemical shift between crystalline and amorphous cellulose(Park et al. 2010). In addition to those characteristic peaks, the CF-MI spectrum show the appearance of a new peak at 171 ppm, respectively attributed to the carbonyl of the ester group. This carbonyl could also be observed on the FTIR spectra (Figure 2) with the apparition of a new band localized at 1733 cm-1. The solid state 13C CP/MAS NMR and ATR-FTIR characterization confirmed that the cellulose fiber-based macroinitiator was successfully synthesized.
After the synthesis of the macroinitiator (CF‑MI), stearyl acrylate (SA) was grafted onto the surface through SET‑LRP (CF-PSA). The NMR signal of the CF-PSA showed a peak localized at 177 ppm attributed to the C12 sites (carbonyl bond, ester group) and strong peaks localized between 40-20 ppm, which are attributed to the carbon chain of the SA unit (C13 to C29). The FTIR spectra of the CF-PSA (Figure 2) also show a large increase of the C=O band localized at 1730 cm-1, as each SA monomer grafted on the macro-initiator add an ester function. In addition to this, the C-H bands around 2900 cm-1 are much stronger and more distinct, due to the addition of the long carbon chains from SA. From the FTIR and NMR spectra, it was concluded that the grafting of PSA on the surface of the macro-initiator was successful.
The next step of the composite preparation was the compounding of the obtained CF‑PSA with the thermoplastic matrix HDPE. The compounding was done with either 10 or 20 % (w/w) of CF‑PSA (respectively labelled CF-PSA10%@HDPE and CF-PSA20%@HDPE). The composites were produced with a counter rotating twin-screw extruder, and the obtained bio-based filament was either (a) directly 3D print, or (b) processed into tensile test samples specimen using injection molding for their mechanical characterization. The mechanical properties were characterized (tensile strength and Young’s modulus) and the cross-section of the broken specimen was further analyzed with a field emission scanning electron microscope. For better comparison, several references samples were also produced: neat HPDE, HDPE with 10% of unmodified fibers (CF10%@HDPE), and finally, HDPE with 7% of unmodified fibers and 3% of maleic anhydride grafted polyethylene (MAPE). MAPE was also selected as it is one of the most widely used additives, e.g. coupling agent, for increasing the compatibility, adhesion between cellulose fibers and a thermoplastic matrix(Seo et al. 2020). The coupling reaction between the cellulose fiber and the MAPE is exposed in Scheme 2. The reaction was performed directly in the counter rotating twin-screw extruder by simply mixing the cellulose fiber, the MAPE and the HDPE.
The mechanical properties of the different samples were investigated by tensile testing and the results are exposed in Figure 3. During the characterization, the modulus of elasticity (MOE) of the bio-based composite were determined with an elongation speed of 1 mm.min-1. The pure extruded HDPE was tested with an elongation speed of 20 mm.min-1 as it could undergo strong thinning and elongation before breaking. At the same speed, CF‑PSA@HDPE broke without any significant deformation after only 4 mm of elongation. For this reason, the elongation speed was lowered to 1 mm.min-1.
The maximal elongation before breaking, as exposed in Figure 3 has been taken as a qualitative sign of the compatibility. Indeed, pure HDPE can undergo a very high strain event at high elongation speed (Figure 4). This is not the case for HDPE mixed with the modified cellulose fibers. Therefore, in cellulose/HDPE composite, it is the HDPE that is responsible for the supported strain, while the cellulose is reducing it – but the Fiber increase the stiffness of the composite. The more compatible the cellulose is, the more it will have interactions with HDPE and restricts its ability to rearrange. This leads to a decrease of the supported strain.
The modulus of elasticity and the yield strength values of the different samples are listed in Table 1. The addition of the unmodified fiber to the HDPE matrix lead to a significant loss of 22% of the polymer strength while the stiffness of the material did not improve (MOE remain the same). A similar strength loss (15%) was observed for the polymer modified samples: the CF-PSA and the CF-MAPE (10% wood fiber). However the Young´s modulus increase by roughly 25%, leading to a notable stiffer material. The increase of the CF-PSA amount to 20 % in the composite, did not have the expected, estimated result as the stiffness of the material did not increase, if compare the same sample with 10 % content.
Finally, the mechanical properties of the CF-PSA and the CF-MAPE were comparable, leading to a similar increase of the compatibility, adhesion between the cellulose fibers and HDPE. However, CF-PSA was produced through SET-LRP, meaning that the reaction condition, e.g. choice/addition of the second monomers can be optimized and therefore new functionality can be introduced onto the hydrophobic fiber.
Table 1: Mechanical properties from sample, obtained by tensile testing, using the EN ISO 527-1 Norm
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MOE (MPa)
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Yield Strength (MPa)
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HDPE
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800 ± 60
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21.5 ± 0.5
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CF-PSA(10%)@HDPE
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1037 ± 41
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18.2 ± 0.2
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CF-PSA(20%)@HDPE
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1002 ± 38
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17.5 ± 0.2
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Fiber(7%)+MAPE(3%)@HDPE
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1049 ± 33
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18.2 ± 0.2
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Fiber(unmodified)@HDPE
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797 ± 66
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16.8 ± 0.2
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The initial and unmodified cellulose fiber, the CF-PSA such as the cross-section of the broken CF-PSA specimen (after tensile testing), were analyzed with a field emission scanning electron microscope. The FE-SEM pictures are exposed in Figure 5 and Figure 6.
As seen in Figure 5, the aspect ratio of the cellulose fibers used in this study is relatively low. This could be a possible explanation for the yield strength diminution and the relative low gain in Young´s modulus for both the CF‑PSA and the CF-MAPE composites. The reinforcing properties of the wood fibers also depends of their aspect ratio as reported elsewhere(Peltola et al. 2014).
The modified CF-PSA show a different surface (Figure 5, bottom), as for the CF surface, which one is fully embedded in the grafted SA polymer.
Regarding the CF-PSA composite samples, it could be seen from the FE-SEM pictures exposed Figure 6 that the cellulose fibers are evenly spread within the composite. Moreover, the modified fiber were not pull-out from the composite after reaching the fracture point during the tensile testing, since no voids in the matrix were observed. This showed a good compatibility of the modified fibers with the HDPE matrix.
Finally, the last step was to 3D print an object with our CF‑PSA(10%)@HDPE composite filament. The major issues with this step were the difficulties inherent to the printing of HPDE, such as shrinkage or warping, leading to the irreversible deformation of the printed object. The 3D printed object are exposed in Figure 7. The printability of the composite could be performed without warping and with a better layer adhesion if compared to the printed object made of pure HDPE.