Agricultural waste is a still untapped source of materials that can, in case of proper utilization, significantly improve the sustainability of polymers and their composites. In this work, polymer composites based on isotactic polypropylene were produced incorporating ground sunflower husk in the amount of 10 wt% and 20 wt%. The work’s main objective is to evaluate how preliminary fractioning of this agricultural waste filler affects the thermomechanical properties, microstructure and surface topology of polypropylene-based injection molded composites. The composites were analyzed for mechanical properties (tensile, impact strength and hardness), thermomechanical properties (Vicat softening point VST, heat deflection temperature HDT, and dynamic thermomechanical analysis DMTA) with reference to morphological changes evaluated using scanning electron microscopy (SEM). The quality of the produced composites was assessed on the basis of the analysis of the surface topology of the injected composites. It has been shown that the larger particle size of used filler has a direct impact on increasing composite stiffness in the room and elevated temperature. Moreover, a relationship was demonstrated between the size of the filler and the deterioration of the tensile strength in the case of composites with a higher content of filler. The results show that the addition of sunflower husk as a particle-shaped waste filler is an effective method to increase sustainability of polypropylene-based green composites with beneficial thermomechanical properties and to reduce the residue of sunflower husk from industrial oil production.
Due to their affordability and advantageous functional properties, polymers have a significant share in all industries’ materials recently. Due to its good mechanical properties, superior chemical resistance, price and availability, polypropylene (PP), as the second most frequently processed polymer, is used in different branches of the industry, including automotive, civil engineering, food and packaging [
Application of lignocellulosic fillers instead of inorganic fillers such as glass fibers make limitations of total ecological footprints of final products compared to pure polymer [
In addition to improving mechanical properties, an important aspect behind the use of lignocellulosic fillers are the benefits of obtaining a specific aesthetics of the final products (obtaining a wood-like effect) [
Fractionated and properly selected sieving procedure may cause beneficial results in the mechanical properties and the quality of injection molded parts made of the thermoplastic polymeric matrix and lignocellulosic filler. However, using the waste fillers should be characterized by simplifying all preliminary processes and minimization of operations, which are necessary to achieve the final products. Therefore, the studies’ realization, including complex evaluation of thermomechanical properties of the polypropylene-based composites filled with sunflower husk as agriculture waste filler with different maximum particle size resulting from the pre-preparation procedure, seems to be justified in relation to the research works published so far. This study presents the complex evaluation of mechanical properties, microstructure and surface quality analysis of polypropylene green composites made using ground sunflower husk as a filler, considering the use of different fractions and content of the filler particles.
The commercial injection molding grade isotactic polypropylene (iPP) Moplen HP500N, with a melt flow rate (MFR) of 12 g/10 min (230°C, 2.16 kg) from the Basell Orlen Polyolefins (Poland) was used in our experiments. The selected polymeric matrix was characterized by a low modification level, including a lack of nucleating agents and lubricants.
Sunflower husk (SH) delivered by a local supplier (Graterpoland, Poland) was preliminarily dried for 3 h at a temperature of 105°C before being milled. The milling process was conducted by means of Retsch Grindomix GM200 knife high-speed grinder. Each grinding process was performed with a cutting rotary knife speed of 5000 rpm in a time of 5 min. Fractionation of ground sunflower husk was proceeded by means of Firtsch Analysette 3 Pro sieve shaker equipped adequately with sieves with 200 µm, 400 µm and 800 µm mesh size. The sieving process was realized with an amplitude of 2 mm in a time of 20 min. The used in this study sunflower husk chemical composition is as follows: Cellulose (28.6 wt%), lignin (24.1 wt%), holocellulose (55.2%), mineral substances (3.9 wt%) and extractive substances, including oil (32.3 wt%).
Before melt-compounding, iPP pellets were milled into powder in a Tria high-speed grinder. After that iPP and adequately fractionated SH powders were premixed using a high-speed rotary mixer. Application of preliminary dry blending guarantees proper feeding of the material during the melt mixing process and constant share of the filler in the whole compounded material series. After physical premixing, the blends were dried in a vacuum at 105°C for 3 h using Chemland laboratory vacuum chamber Model DZ 1 BC II. The procedure allows obtaining the reduction of the moisture in the filler from 5.41 wt% to 0.7 wt%, verified by means of technical moisture analyzer Axis BTS110. After that, all blends were mixed in a molten state using a Zamak EH-16.2D twin-screw extruder that operated at temperature profile 185-185-182-180-178-175-170-160°C and 140 rpm, and pelletized after cooling in a water bath. Samples described in ISO 527 were prepared with Engel ES 80/20HLS injection molding machine with a processing temperature of 190°C. The injection molding process was realized with the following parameters: mold temperature Tmould = 25 °C/min, injection speed V = 80 mm/s, forming pressure Pf = 5.5 MPa and cooling time t = 40 s.
In further descriptions of the composites containing sieved organic filler with specified filler fraction, its’ names will contain suitable 200/400/800 annotation before the information about the filler content. For example, composites containing 10 wt% of the SH fractionated with a sieve with 400 µm mesh will be coded as 10/400.
Mechanical properties of pure iPP and iPP-SH composites were realized in the static tensile test according to standard ISO 527 by means of Zwick Roell Z020 TH ALLround Line universal testing machine with 20 kN nominal force strain gauge and video extensometer applied for the determination of the strain in the range of the 0.2% corresponding to yield stress of the tested material. Tests were performed with 50 mm/min cross speed for 15 specimens of each series.
The impact strengths of the unnotched samples wits 10 × 4 × 15 mm dimensions were measured by the Dynstat method (DIN 53435). The presented impact strength values are the mean value of the 15 measurements for each series.
The hardness evaluation was conducted using a KB Prüftechnik apparatus with a ball indentation hardness test according to ISO 2039 standard. The min. 30 measurements were taken for each material series.
Thermomechanical tests in static conditions were realized by the evaluation of Vicat softening temperature (VST) and heat deflection temperature (HDT). Measurements were performed using an HDT/Vicat testing machine RV300C (Testlab, Poland). Measurements were performed in an oil bath following ISO 306 the ISO 75 standards. The VST evaluation was realized in A120 variant, while the HDT type experiment was prepared load 1.8 MPa load. Both testes were realized with a heating rate of 120°C/h.
The DMTA test was performed using the Anton Paar MCR 301 rheometer equipped with a torsion DMA measuring tool. Investigations were carried out with a constant frequency of 1 Hz and a strain of 0.01%. All 50 × 10 × 4 mm3 samples were cooled down to –100°C and heated up to 150°C with a temperature ramp of 2°C/min.
The samples’ fractures were examined and digitally captured using a scanning electron microscope Zeiss Evo 40. The electron accelerating voltage of 12 kV was applied. Prior to the tests, all the specimens were sputtered with a layer of gold. The magnification of 2000x was used.
The topography of the samples’ surface was evaluated by means of Veeco NT1100 optical profiler operating at vertical scanning mode according to ISO 4287, equipped with a 5x magnifying objective. The dimensions of the tested area were 0.9 × 1.2 mm2. The amplitude 2D roughness parameters of Ra and Rz were evaluated, while for to determine the topological surface’s properties, values of Root Mean Square Deviation of the Surface Sq and skewness of Topographic Height Ssk parameters were determined.
In
Material | Tensile strength | Elasticity modulus | Elongation at break | Impact strength | Ball indentation hardness |
---|---|---|---|---|---|
[MPa] | [GPa] | [%] | [kJ/m2] | [MPa] | |
iPP | 32.1 ± 0.4 | 1.30 ± 0.01 | 162.3 ± 63.9 | 38.3 ± 1.1 | 53.6 ± 2.5 |
10/200 | 27.6 ± 0.3 | 1.30 ± 0.02 | 26.4 ± 4.1 | 15.3 ± 2.1 | 46.6 ± 1.4 |
20/200 | 29.9 ± 0.2 | 1.30 ± 0.01 | 17.9 ± 6.1 | 14.8 ± 1.9 | 47.6 ± 2.3 |
10/400 | 28.1 ± 0.2 | 1.36 ± 0.05 | 13.9 ± 0.9 | 12.8 ± 2.3 | 49.5 ± 1.9 |
20/400 | 24.0 ± 0.2 | 1.47 ± 0.01 | 8.9 ± 1.1 | 17.5 ±3.6 | 46.5 ± 3.4 |
10/800 | 28.6 ± 0.1 | 1.47 ± 0.01 | 9.0 ± 0.7 | 11.2 ± 1.1 | 48.1 ± 2.9 |
20/800 | 24.6 ± 0.3 | 1.55 ± 0.02 | 5.6 ± 0.2 | 9.5 ± 0.9 | 53.1 ± 2.4 |
To evaluate the thermomechanical properties of the obtained composites, the results of DMTA analysis and the VST and HDT measurements were taken into account.
Material | VST | HDT | G’–40°C | G’20°C | G’ 80°C | Tg | tanδ at Tg |
---|---|---|---|---|---|---|---|
[°C] | [°C] | [Pa] | [Pa] | [Pa] | [°C] | [–] | |
iPP | 152.0 ± 0.1 | 60.6 ± 0.3 | 1.92·109 | 9.54·108 | 2.97·108 | 12.1 | 0.0752 |
10/200 | 148.4 ± 0.3 | 57.6 ± 1.0 | 2.15·109 | 9.30·108 | 3.47·108 | 5.4 | 0.0742 |
20/200 | 148.9 ± 0.7 | 57.9 ± 0.1 | 1.93·109 | 8.57·108 | 2.97·108 | 6.3 | 0.0695 |
10/400 | 150.4 ± 0.1 | 59.8 ± 0.4 | 1.91·109 | 8.29·108 | 3.13·108 | 8.0 | 0.0789 |
20/400 | 147.1 ± 1.1 | 60.2 ± 0.1 | 2.25·109 | 1.05·109 | 3.96·108 | 5.2 | 0.0650 |
10/800 | 152.2 ± 1.3 | 60.9 ± 0.3 | 1.89·109 | 9.52·108 | 3.41·108 | 8.8 | 0.0670 |
20/800 | 148.7 ± 3.5 | 65.3 ± 0.7 | 2.06·109 | 1.04·109 | 4.00·108 | 8.8 | 0.0661 |
The comparison of DMTA results for iPP and iPP/SH composites is presented in
It should be noted that the percentage of filler for these tests was much higher and ranged from 30% to 60%, which partly explained the storage modulus increase. More reliable measurement results were presented by Garcia-Garcia et al. [
Summarizing the results of thermomechanical analyzes, it should be stated that the addition of sunflower husk particles does not significantly influence the thermal resistance of iPP. Interestingly, there are no visible differences between composites with 10% and 20% filler content. The conclusions that can be drawn from this type of observation prove the low reinforcement efficiency obtained by the addition of sunflower husk particles. However, their addition does not lower such thermomechanical factors, such as the VST and HDT.
The ground and fractionated sunflower husk sieved with suitable 200 µm to 800 µm sieves were analyzed by scanning electron microscopy. SEM images of the powdered organic fillers have been presented in
Moreover, no evidence of particle pull-out holes effect was observed on the analyzed surface. This indicated that plasticization effects related to oil migration from sunflower husk to the polypropylene matrix resulted in improved matrix-filler interactions. Achieved results may be related to effects described by Faludi et al. [
The selected 2D roughness parameters, i.e., Ra and Rz, reflecting the average roughness and mean peak-to-valley height, respectively, as well as roughness evaluated by Sq surface topography 3D parameter and surface skewness Ssk of injection molded samples are collectively presented in
Amplitude parameters | ||||
---|---|---|---|---|
Material | 3D paramaters | 2D paramaters | ||
Sq | Ssk | Ra | Rz | |
[µm] | ||||
iPP | 3.44 | 1.33 | 2.65 | 32.32 |
10/200 | 2.90 | 0.69 | 2.25 | 37.66 |
20/200 | 3.67 | 1.27 | 2.68 | 31.32 |
10/400 | 3.34 | 1.22 | 2.55 | 28.33 |
20/400 | 3.79 | 1.15 | 2.74 | 35.87 |
10/800 | 3.13 | 1.00 | 2.39 | 26.39 |
20/800 | 3.44 | 0.21 | 2.58 | 34.02 |
All samples showed a relatively high value of the Sq, which may result from the injection cavity roughness. Compared to other studies concerning the application of lignocellulosic fillers with the same filler content, the values of Ssk are similar, while there is a lack of visible difference between the composite grades. For both parameters Ra and Sq, which represents the mean roughness of the sample, the increased amount of the filler results in the higher roughness. The used for evaluation of surface quality Ssk topography parameter describes whether the valleys or peaks dominate in the tested area. Negative values of Ssk suggests valleys dominance, while positive values show the domination of asperities. All samples showed positive and comparable values of Ssk. The only exceptions are the composites assigned as 10/200 and 20/800, which showed significantly lower values. The lowest value was observed for the sample with the highest content of the filler characterized by the biggest particles; therefore, it can be stated that despite that the polymer covered the external layer of the injection molded part, the share of the large particles near the surface influences the surface quality. Based on realized evaluations, it can be stated that for manufactured composites, the SH particle size did not induce negative changes of the surface roughness and no visible tendency was observed, whereas the increasing filler content in the composite results in the negative effect of increased surface roughness.
As part of the realized work, polypropylene composites filled with ground sunflowers husk were produced and tested. The use of sunflowers husk as a waste filler allowed to increase the sustainability of the polypropylene. Despite the slight deterioration in polymer composites’ mechanical properties compared to the unmodified polymer matrix, the changes are not so significant that they could limit the application in the production of less mechanically loaded parts. The use of larger fractions of the filler allows obtaining increased stiffness of composites both at room temperature and at elevated temperatures. There were no significant changes in roughness caused by the addition of the natural waste filler. Despite the lack of filler surface modification, no adhesion defects were observed at the polymer-filler interface. In the case of all composites, correct saturation of the filler with polymer was observed, and the SEM images made suggest the participation of the filler in the load transfer during composite load for composites filled with SH achieved by the sieving with the use of 800 µm sieve. It should be emphasized that in the case of introducing 10 wt% filler and more, despite the tensile strength was comparable to the unmodified polymer, the significant reduction of the elongation at break and ductility occurs as well as the aesthetics and lowering the price of the composite material. The final properties of the composites manufactured using SH results from the complex modification effects of reinforcing behavior of lignocellulosic filler and plasticizing behavior of the oil residues migrating from the agricultural waste filler.
dynamic thermomechanical analysis
heat deflection temperature
isotactic polypropylene
melt flow rate
polylactide
polyhydroxybutyrate-co-valerate
average roughness
mean peak-to-valley height
scanning electron microscopy
sunflower husk
surface topography 3D parameter
surface skewness
glass transition temperature
Vicat softening temperature
wood flour