Depending on the fiber and polymer type, composition, treatment, processing conditions or the model used to determine this critical length, different Lc values were reported. As an example, Lc values ranging from 0.65 to 3.2mm were found for PP/HF composites using different methods. NF dimensions and mechanical properties of the composites are strongly influenced by the compounding conditions. Both the length and the diameter of the fibers are reduced during the melt processing of composites due to the fibers breakage and defibrillation. A four-times decrease of GF length was observed in the initial stage of compounding in a twin screw extruder followed by insignificant modification by the end of extrusion. A reduction of fiber length by a factor of 2 was reported for short hemp or flax fibers in polypropylene composites and by a factor of 3 for longer flax fibers when a similar extrusion process was used. Knowing that the length and the diameter of the fibers have an opposite effect on the mechanical properties, the processing conditions may be modified for enhancing defibrillation and reducing breakage. Besides the compounding conditions, the initial fiber length and the viscosity of the polymer matrix may influence the mechanical properties of final composites. In general, a higher initial fiber length or melt viscosity of the matrix leads to a better dispersion of the fibers. However, a higher melt viscosity results in difficult fiber wetting and may reduce the mechanical properties. Conversely, a low melt viscosity of the polymer matrix has as result a poor dispersion of fibers in the polymer due to the weaker shear forces.
A more intense fiber damage was observed in composites with a good interface, as the fibers are better bound to the matrix. For very weak interfaces,grow tent total debonding may take place and the amount of load transferred to the fiber will be less sensitive to the matrix viscosity. This is due to the contribution of interface friction and not interface bonding to the stress transfer. On the other hand, the increase of NF concentration in PP results in higher shear efforts in the concentrated composites and determines a more advanced decrease of the fiber length. Therefore, a suitable selection of the initial fiber length and viscosity of the matrix is crucial for the performance of the final composites. For automotive parts such as dashboard or door panel parts, PP/HF composites are usually fabricated by extrusion, using a twin-screw extruder, followed by injection molding. The influence of extrusion conditions on fibers damage and the mechanical properties of PP/HF composites was extensively studied, but the influence of extrusion – injection molding processing was not investigated so far. In addition, SEBS is more viscous compared to PP and it may induce a more intense mixing and a better dispersion of HF. Previous work has shown that SEBS addition in PP composites increases the toughness and enhances the reinforcing efficiency of HF. In this work, the control of the fiber length is proposed as a cheap and eco-friendly alternative to the expensive and aggressive chemical treatments of NF to improve the properties of PP composites. The main objective is to study the effect of the initial fiber length and their damaging during extrusion – injection molding on the thermal and mechanical properties of PP/SEBS/HF composites.
The addition of SEBS and the control of the fiber length using an automatic milling were considered in order to obtain balanced stiffness-toughness properties in PP/HF composites and preserve their environmentally friendly character, avoiding excessive fiber treatment. These composites are promising materials for electric vehicles parts.PP was mixed with 5 wt% MAPP and 15 wt% SEBS in a Turbula T2F mixer for 30min at room temperature and the granulate mixture was fed in a DSE 20 Brabender Twin Screw Extruder . The screws profile includes four intense mixing zones. The hemp fibers were dried in an oven at 90 ◦C for 4h prior to extrusion; they were added to the PP melt through a second feeder of the twin screw extruder to avoid the intense damage typical to the initial processing. PP blend and composites with 20 and 30 wt% HF were extruded at a constant screw rate of 150min−1 resulting filaments. The extruder barrel temperatures were 155–160–165–165–170–170 ◦C from the feeding to the die. The filaments were passed through a cooling bath and granulated with a pelletizer. Granulated blend and composites were dried in an oven at 80 ◦C for 4h prior to injection. Afterwards, they were injection molded in dumbbell shaped specimens for tensile tests using an injection molding machine at a temperature of 185 ◦C . The PP matrix modified with SEBS and MAPP was denoted as PPM. The composites made of 20 wt% and 30 wt% HF were denoted as PPM 20HF and PPM 30HF followed by , or depending on the type of HF used in the tests.The length of the fibers in PPM-HF composites was quantified to analyze the influence of the fiber length after extrusion and injection molding on the properties of the composites. Two methods were used for this purpose. In the first method, thin composite films, 40–50 m in thickness, were obtained by compression molding using an electrically heated press P200E . Small parts of injected specimens were pre-heated at 210 ◦C for 120 s and pressed at the same temperature with a pressure of 10 MPa for 90 s then cooled in a cooling cassette for 1min.
The thin films were examined under 4× and 10× objective lenses using a B-150DBR optical microscope , equipped with LED light source and connected to a 3.14 megapixel digital camera, controlled by the OpticaScope 9.0 software. More than 20 images were taken for each sample. The length and diameter of the fibers were determined with ImageJ software using at least 150 measurements. In the second method, HF was extracted from the composites with p-xylene using a reflux extraction method. The extracted fibers were deposited on glass slides and examined in the same conditions with the films from the first method. The pristine fibers were also examined using ImageJ software and the images of the fibers were captured with a 13 megapixel Samsung digital camera. For this purpose, the dispersions in water of HF , and were deposited on glass slides and dried in an oven before examination. About 200 measurements were used for the determination of the weighted average length of each type of fibers. Attenuated total reflectance-Fourier transform infrared spectroscopy analysis was carried out using a TENSOR37 Spectrometer from Bruker. Duplicate samples were analyzed from 4000 to 400 cm−1, with a resolution of 4 cm−1 at 16 scans. The dispersion of the fibers in the polymer matrix was examined by SEM using composite specimens fractured in liquid nitrogen. The samples were sputter-coated with gold before examination. A Quanta Inspect F Scanning Electron Microscope with field emission gun having a resolution of 1.2nm was used for this purpose. Tensile properties of the composites were determined with an Instron 3382 Universal Testing Machine according to ISO 527. Tensile strength was determined on five specimens from each sample with a cross head speed of 50mm/min. Similarly, five specimens were used in each case to determine the modulus of elasticity with a cross head speed of 2mm/min, as stipulated by ISO 527. The significant differences were evaluated using the Student t-test. Thermogravimetric analysis of HF with different length and composites was carried out between 25 and 700 ◦C at a heating rate of 10 ◦C/min using a TA-Q5000 from TA Instruments. Measurements were done on duplicate samples weighing between 8 and 10mg. Nitrogen was used as a purge gas at a flow rate of 40mL/min.The microscopic investigation of hemp fiber extract shows extensive overlapping of individual fibers and large agglomerations. This suggests that minor amounts of polymer were not extracted by p-xylene. Indeed, a detailed investigation of the microscopic images after extraction shows that the fibers are covered by a thin layer of polymer which favors fibers agglomeration.
This is probably a result of the higher complexity of the polymer matrix containing SEBS and MAPP besides PP. To verify this observation, the most agglomerated fibers were investigated by ATR-FTIR . For comparison, SEBS, PP-MAPP and pristine HF were also analyzed. The spectroscopic results show that the agglomerated fibers contain un-extracted PP, probably MAPP which is tightly bound to the fibers. An interesting aspect was observed when the ATR-FTIR results of HF extracted from the composite were compared to pristine HF. Besides the peaks characteristic to PP, important changes in the bands ranging from 1100 cm−1 to 1000 cm−1 were observed after extraction. Previous works have shown that the natural fibers are not affected by solvents used to dissolve the polypropylene matrix. However the peaks characteristic to the C–O vibrations in cellulose alcohols from 1055 cm−1, 1030 cm−1 and 996 cm−1 are obvious after extraction but undifferentiated in the original HF. It may be supposed that hemicelluloses and lignin, with bands within 1100–1000 cm−1 , grow tent complete kit were partially removed. This may also contribute to supplementary defibrillation and changes in the size of the fibers. Besides, fibers agglomeration due to polymeric impurities makes the measurement of the fiber length extremely difficult using image analysis. Therefore, only the first method on compressed films was further used for the measurement of the fiber length after extrusion – injection molding of the composites. Representative optical microscope images of PPM 30HF composites with hemp fibers of different initial lengths are shown in Fig. 3. Shorter HF were observed in the case of PPM 30HF compared to the other two composites.
The attached histograms with the lengths of HF in composites show that almost 90% of the analyzed fibers have a length smaller than 1mm in the case of PPM 30HF and between 0.2 and 1.6mm in the case of PPM 30HF and . It is worth mentioning that the proportion of the fibers with a length smaller than 1mm is different in the last two composites, 72% for PPM 30HF and 47% for PPM 30HF. The weighted average length of each type of fibers in composites is shown in Table 1. The weighted average lengths of mechanically treated and fragmented HF, before to be inserted in PPM, were also given in Table 1 for comparison. A higher average length was reported for HF in PP/HF composites obtained by extrusion using a twin screw extruder. Berzin et al. started from hemp fibers with an initial length of 3mm and obtained a final length between 1.13mm and 1.66mm in PP/HF composites using a screw rate of 100min−1 or 200min−1 at different feeding rates.The greater reduction of fibers length in PP/SEBS/HF composites as compared to PP/HF published results has several reasons. Besides the different equipment and conditions, the presence of SEBS in our composites, with a higher melt viscosity compared to PP, is probably an important reason. A 5 times higher torque value was obtained for pristine SEBS compared to pristine PP , the raw materials used to fabricate the composites. A higher melt viscosity of the matrix induced by SEBS may influence the HF size and dispersion in the matrix after melt processing. Another difference is the further processing of composites by injection molding after extrusion, where higher shear rates are involved. In the extrusion process the shear rates are typically in the range of 80–220 s−1 and much higher, up to 100000 s−1 during injection molding. A more significant damage of the fibers and similar fiber length with that reported in Table 1 were reported in the case of polypropylene composites with flax and jute fibers obtained by a different method. The initial length of the fibers is an important parameter in PP/NF composites. In general, longer fibers break more and faster but, in the same conditions, longer initial fibers lead to higher fiber length in the composites after processing. A two times reduction in HF length was reported for PP/MAPP/20% HF composites obtained by extrusion and a more severe fiber breakage for PP/MAPP/30% HF composites obtained by melt compounding in an internal mixer and injection molding.