Enhanced mechanical properties and improving wetting ability of polypropylene/cassava pulp composites

Abstract

Natural cassava pulp was selected as a bio-based reinforcement in plastic polymer composites to enhance mechanical and wetting properties as an eco-friendly product. This study developed reinforced polypropylene (PP) composites with cassava pulp (CP) to improve mechanical properties and wetting ability. The PP/CP specimens were fabricated via twin screw extrusion and injection molding. Tensile and flexural testing were performed using a universal testing machine, with wetting properties characterized by a contact angle goniometer. Incorporation of 10 wt% cassava pulp showed enhanced tensile strength (4.85%), Young’s modulus (14.38%) and flexural modulus (23.30%) compared with neat polypropylene, indicating higher stiffness of natural fiber-filled composites. Micropatterns were formed on the composite surfaces using the hot embossing technique. A superhydrophobic surface was achieved by designing micropattern geometry. Water contact angle of micropatterned neat polypropylene and polypropylene/cassava pulp composites increased compared to material with no pattern. Micropatterns on PP/CP composite surfaces can be used to develop new functional materials with high mechanical and superhydrophobic properties.

Keywords

cassava pulp polymer composites mechanical properties micropattern wetting properties

Introduction

Cassava pulp (CP) is a by-product of cassava starch processing, mainly comprising fibers, residual starch, and moisture. Cassava pulp is biodegradable, renewable, and inexpensive but contains up to 80 wt% moisture combined with high organic content that cause environmental problems on fermentation by emitting a strong and unpleasant odor. Cassava pulp is generally used to produce low value animal foods.^1–3 Recently, adding cassava pulp as a reinforcing filler in polymer material has attracted attention as a new product with high marketability and eco-friendly benefits. Cassava pulp contains cellulose, hemicellulose, lignin, and high amounts of starch,^2,4,5 mostly comprising fibrous materials, and can act as a reinforcement filler to enhance polymer tensile strength and Young’s modulus. Ruangudomsakul et al.⁶ stated that adding cassava pulp at 20 parts per hundred rubber (phr) in natural rubber biocomposites containing rubber grafted glycidyl methacrylate and sulfur improved tensile strength, while Jullanun and Yoksan⁷ reported that adding cassava pulp at 22.1 wt% enhanced tensile strength by up to 354% and Young’s modulus by up to 722% in thermoplastic cassava starch (TPS)/poly (lactic acid) (PLA) blend. They suggested that TPS/PLA/CP composites showed potential for use in industrial manufacturing of injection-molded products.

Polypropylene (PP) is a thermoplastic polymer widely used in many applications in the packaging, furniture, and automotive industries. Benefits of PP include its lightweight, low cost, and moldability.^8–10 Recently, PP/natural biopolymer composites have attracted increased attention as “green” or eco-friendly plastic product reinforcement. However, the mechanical properties of green products must meet customer criteria. Polypropylene/biocomposites combine low environmental impacts, economic profit, and acceptable mechanical performance.

Previous research^8,11 investigated the potential of PP blended with natural fibers as reinforcing filler. Nourbakhsh and Ashori¹¹ fabricated PP/poplar fiber composites using maleic anhydride grafted polypropylene (MAPP) as the coupling agent. Results showed that the tensile and flexural strength of PP improved with increased fiber content up to 40 wt%. They concluded that blending PP with natural biopolymer material offered many benefits such as improving mechanical properties and reducing production cost.

Functional surfaces provided polymer composite products with new properties and increased applications. Superhydrophobic surfaces of composites improved the wettability, offering unique products such as anti-fog material, anti-ice material, self-cleaning material, and material with reduced drag force.¹² Normally, hydrophilic surfaces are smooth and show high wettability with low water contact angle (WCA, θ < 90°), while hydrophobic surfaces have partial wettability and greater WCA (θ > 90°). Superhydrophobic surfaces have zero wettability with very high WCA (θ > 150°).¹³ To prepare this superior property, a micropattern was developed on the plastic surface.

Surface wettability is an important material property that depends on the chemical structure and micro-texture of the surface. Water contact angle is determined by the equilibrium between solid–vapor, solid–liquid, and vapor–liquid interfacial energies when a water droplet is positioned on the material surface. The basic theory of water contact angle on a smooth surface is described by Young’s equation as the relationship between the static contact angle (γ) of a water droplet and three interfacial surface tensions on a smooth surface, as shown in Figure 1, where γ_SV, γ_SL, and γ_VL are the interfacial tension of the solid–vapor, solid–liquid, and vapor–liquid, respectively.

Normally, surfaces are not perfectly smooth. Consequently, the Wenzel (W) and CB models were used to describe the micro-structured surfaces.^14,15 The Wenzel model hypothesizes that the water droplet entirely enters the cavities of the micro-structured surface. The CB model is based on the assumption that the water droplet is suspended on the top of the rough surface and the micro features prevent water penetration inside the grooves due to air bubble entrapment, suggesting that the water is only in contact with the top of the protrusions. The Cassie–Baxter model shows higher apparent water contact angle (WCA) than the Wenzel model and is applied for self-cleaning, water-repelling and anti-sticking properties. However, the Cassie–Baxter model is not always steady and water droplet properties can change from the CB to the Wenzel model due to vibrations and gravity.

Micropatterns are produced by various techniques such as hot embossing,^13,16,17 roll-to-roll hot embossing,¹⁸ injection molding,^19–22 and nanoimprint lithography.²³ Preliminary fabrication of micropatterns favors hot embossing due to high precision, uncomplicated operation, and high replication quality. In this process, the polymer substrate is heated to close to the softening temperature, and the microstructured master mold is pressed onto the polymer substrate for a sufficient time, followed by cooling and demolding.

Several publications^24,25 have reported on the development of polymer/natural fiber composites to improve mechanical properties. However, the combination of polymer composites with new functional surfaces has not been investigated. Therefore, this study developed reinforced PP/CP composites to achieve a green product and also created a micropatterned surface to enhance wetting ability toward superhydrophobic surfaces. Effects of geometry of the micropattern shape on wetting property were revealed and discussed.

Material and methods

Material

A commercial polypropylene block copolymer with high melt flow and good impact resistance at low temperature was used in this study. The PP had a melt flow index (MFI) of 30 g/10 min (2.16 kg/230°C) and was supplied by IRPC Public Company Limited, Thailand. Cassava pulp was obtained from a cassava starch processing factory in Thailand. Polypropylene-graft-maleic anhydride (PP-g-MA) obtained from Dow Chemical Company, Thailand was used as the compatibilizer.

Characterization of cassava pulp

Composition of as-received CP was determined according to the Grain and Feed Trade Association (GAFTA) methods. Contents of starch, fiber, and residual moisture were measured following GAFTA 23:0 (2014), GAFTA 9:0 (2014), and GAFTA 2:1 (2014), respectively.

Particle size analyzer

Particle size of ground CP powder was examined at 70% feed rate, at 3 bar pressure for 5 min, using a particle size analyzer (Mastersizer 2000 (laser diffraction) Malvern Instruments, Malvern Panalytical Instrumentation Company, UK).

Preparation of PP/CP composites

Cassava pulp was ground with a mechanical grinder at room temperature for 30 s and then dried in a convection oven at 80°C for 12 h to remove the residual moisture. As-received CP and the obtained ground CP were investigated for physical characteristics. For compounding, PP was blended with 10 wt% of ground CP and 3 phr of PP-g-MA. The blends were compounded using a co-rotating 20 mm twin screw extruder (Labtech Engineering Co, Ltd, Thailand). The screw diameter and L/D ratio were 20 mm and 32 mm, respectively. Extrusion temperature ranged from 150 to 200°C with screw speed 160 r/min and material side feed rate of cassava 5.1 r/min. Extrudates of the compounded PP/CP were cut into 3 mm-length pellets using a pelletizer (LZ-120/VS, Labtech Engineering Co, Ltd, Thailand). The pelletized PP/CP composites were dried in a vacuum oven at 80°C for 3 h and then converted to dogbone and rectangular specimens by an injection molding machine (Robotshot S-2000i 100 B, Fanuc, Japan) with temperature profile along the extruder barrel 150–200°C, cooling time 25 s and mold temperature 30°C.

Mechanical testing

Tensile test

The tensile test was carried out following ASTM D 638 at room temperature using a universal testing machine (Instron model 5967, Instron, USA), with a 30 KN load cell. The specimens were tested at crosshead speed of 100 mm/min. Five repeated tests were carried out to confirm the reliability of the results at room temperature.

Flexural test

The flexural test was carried out in accordance with ASTM D 790 at room temperature using a universal testing machine (Instron model 5967, Instron, USA), with a 30 KN load cell. The specimens were tested at crosshead speed and span of 1.38 mm/min and 5.1 mm, respectively. Five specimens were tested for each reported value.

Hardness test

The Rockwell HRRW scale hardness test was performed using a Hardness Rockwell Tester (Wilson 574T, Buehler, USA and Germany) with a 1/2” ball indenter, minor load 10 ± 0.2 kgf, major load 60 ± 0.45 kgf and dwell time 15 s. At least five specimens were tested for each reported value.

Scanning electron microscopy

Cross‐sectional view images of the specimen were observed for microstructure of blends using a Scanning Electron Microscope (SEM) (Hitachi/SU 1500, Hitachi, Japan). The sample was fractured by liquid nitrogen and the fractured specimens were coated with gold to prevent charging. The SEM was operated at an accelerating voltage of 15 kV.

Attenuated total reflectance Fourier-transform infrared spectroscopy

Attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) spectra of the samples were achieved by attenuated total reflectance Fourier-transform infrared spectroscopy (Thermo Scientific, Waltham, USA). Measurements were investigated in absorbance mode. The spectra were attained with frequency from 4000 to 400 cm⁻¹, spectral resolution 4 cm⁻¹ with 32 scans. At least three specimens were investigated.

Hot embossing

Master mold micropatterns were imprinted onto the PP/CP composite surfaces at 155°C. The PP/CP composites were preheated for 3 min and pressed at 5 MPa for 1 min. The sample was then removed to cool to room temperature for 3 min, followed by demolding. Figure 2 shows schematic micropatterns of the master mold with different geometries including square and circular arrays (patterns 1–4) with length 100 μm. The depth of the micropattern was 180 μm.

Surface profiles of micropatterned PP and PP/CP composites

Surface profiles and surface roughness of the PP test specimens and PP/CP compounds and mold patterns were measured with a laser scanning confocal microscope (LEXT OLS4100, Olympus, Japan). Arithmetic mean values of the surface roughness (R_a) and root mean square (S_q) values of mold patterns, molded PP and PP/CP were reported, with 3D surface textures of the test specimens also displayed.

Wettability testing

Water contact angle (WCA) of the specimen surface was recorded using a contact angle goniometer (Succasunna, Ramé-Hart Instrument Co, USA) attached to a high-resolution CCD camera. The WCA was tested using the sessile drop method at 25°C and 60% RH. After dropping the water on the composite surfaces for 15 s, the WCA was determined, with data averaged for five measurements.

Statistical analysis

One-way analysis of variance (ANOVA) with Minitab software for Windows, version 21 was used to analyze the statistical data. Results were compared by post‐hoc Tukey tests with significant difference at p < 0.05 (95% confidence interval), and reported as mean ± standard deviation.

Results and discussion

Preparation of cassava pulp

As-received cassava pulp was a coarse accumulation of large particle size, as shown in Figure 3(a). Before compounding, the cassava pulp was ground using a mechanical grinder for 30 s. The ground cassava pulp is shown in Figure 3(b), with particle size distribution presented in Figure 4, ranging 3–1700 μm with average 285 μm. Cassava pulp composition was characterized according to GAFTA methods. Starch was over 40 wt%, followed by 16 wt% cellulose fiber with residual moisture at around 11 wt%. Therefore, a drying step was required to reduce moisture content before blending CP with the polymer using the melt extrusion process.

ATR-FTIR

Figure 5 shows the ATR-FTIR spectra of PP, PP/CP composites, PP-g-MA, and CP. The CH- asymmetric (2950 cm⁻¹) and symmetric stretching (2870 cm⁻¹) were seen in polypropylene. Peaks for bending vibration of CH₃ were located at 1452 and 1377 cm⁻¹, while peaks located at 1170 and 838 cm⁻¹ were attributed to C-H vibration and the peaks at 804 cm⁻¹ corresponded to C-C groups.²⁶ For CP, the peak located at 1110 cm⁻¹ was assigned to C-O-H. For PP-g-MA, the characteristic peaks of grafted anhydride at 1743 and 1712 cm⁻¹ were assigned to carbonyl (C = O) of the anhydride and acid groups, respectively.²⁷ For PP/CP containing PP-g-MA as a compatibilizer, a characteristic peak of grafted anhydride too small to observe was found at 1743 cm⁻¹, corresponding to C = O and indicating the presence of PP-g-MA in the PP/CP system.

Mechanical properties of PP and PP/CP composites

The polymer composites were fabricated using an injection molding machine. To investigate the mechanical properties of the composites, dogbone, and rectangular specimens were prepared. Table 1 shows the mechanical properties of neat PP and PP with 10 wt% of CP content. The neat PP showed tensile strength, Young’s modulus and elongation at break of 27 MPa, 813 MPa, and 244%, respectively. After blending PP with 10 wt% CP, tensile strength and Young’s modulus of the PP/CP composites increased by 4.85% and 14.38% to 28 MPa and 930 MPa, respectively. Improvement of tensile strength and Young’s modulus values of the composites was explained by the reinforcing effect of CP fibers.⁷ In this study, CP contained 16 wt% of cellulose fiber. This result concurred with previous reports.^3,28 The fiber content in CP is normally between 15 and 30 wt%. Jullanun and Yoksan⁷ reported that using CP as the reinforcing filler improved both strength and modulus of poly (lactic acid)/thermoplastic starch composites. The CP fiber increased the ability to transfer stress across the interface between the fiber and polymer matrix^24,29 In this research, PP-g-MA was used as a compatibilizer to improve the compatibility between the PP and CP phases, as confirmed by SEM. The SEM image in Figure 6(a) shows the neat PP droplet phase in the polymer matrix. The droplet in commercial PP grade is an impact modifier additive incorporated in PP material from polymer manufacture. These results agreed with Wang et al.³⁰ who described many large droplets dispersed in the PP matrix due to impact modifier additive. In terms of PP/CP composites, Figure 6(b) and (c) show the CP fiber phase in the polymer matrix with no phase separation between PP and CP, suggesting that using CP in the presence of PP-g-MA showed good compatibility between PP and CP. Hujuri et al.³¹ reported that PP-g-MA compatibilizer increased the flexural strength of PP composites added with 5–20% pineapple leaf fiber. CP enhanced the stiffness of the PP matrix²⁵ but elongation at break, as the ability of the material to stretch before breaking, decreased. A 10% reduction in elongation at break of PP/CP was recorded when CP was added due to reduced deformability of the rigid interface between CP and the PP matrix, indicating that PP was more flexible than the PP/CP composites. Similar results were observed by Nithikarnjanatharn and Samsalee.¹ They reported that tensile strength and elongation at break of poly (butylene-succinate) (PBS) blended with CP biocomposites decreased with an increment of CP content. The biocomposites had poor interfacial adhesion in the polymer matrix due to agglomeration of fiber particles and restricted motion in the polymer chain.

Table 1.

Mechanical properties of all specimens.

Sample	PP	PP/CP (10 wt%)
Tensile stress (MPa)	26.8 ± 0.4^b	28.1 ± 0.3^a
Young’s modulus (MPa)	813.4 ± 14.2^b	930.4 ± 13.8^a
Elongation at break (%)	244.4 ± 50.2^a	10.1 ± 1.0^b
Flexural strength (MPa)	31.9 ± 0.3^b	36.5 ± 0.4^a
Flexural modulus (MPa)	928.6 ± 20.6^b	1145.0 ± 23.7^a
Rockwell hardness (HRRW)	65.8 ± 1.4^b	78.9 ± 4.3^a

Data are reported as mean ± standard deviation. Lowercase superscripts letters (a–c) show significant differences (p < 0.05) in each row.

The Rockwell hardness testing results are shown in Table 1. When adding 10 wt% of CP in PP, the hardness of PP/CP composites increased by almost 20% compared to neat PP. The CP acted as reinforcement and increased the rigidity of the PP matrix,³² as also shown by the surface morphology of the composites (Figures 6(b) and (c)). Fractured PP/CP composites did not show phase separation and had improved hardness. The PP matrix and CP were incompatible due to hydrophobic and hydrophilic characteristics. Interfacial adhesion was improved by the incorporation of a coupling agent as PP-g-MA.

Surface profiles of micropatterned PP and PP/CP composites

In the hot embossing process, characteristics of the microstructured master mold with different micropattern geometries (pattern 1 (square shape), pattern 2 (circular shape), pattern 3 (mixed square-circular shape), and pattern 4 (mixed square-rhombus shape)) are shown in Figure 2. The PP and PP/CP composites were embossed using embossing temperature of 155°C and applied pressure of 5 MPa. The optical images and surface profiles of the micropattern arrays on the sample surfaces were investigated using a 3D laser confocal microscope, as shown in Figure 7. Average roughness (R_a) and root mean square roughness (S_q) are shown in.Table 2 Low R_a and S_q values indicated a smooth sample surface. The R_a and S_q values of PP and PP/CP specimens increased with all fabricated microstructures compared to specimens without patterns. Results revealed that the R_a and S_q values of PP with pattern 3 (mixed square-circular shape) and pattern 4 (mixed square-rhombus shape) were higher than the square pattern 1 and circle pattern 2. This indicated that the mixed patterns increased R_a and S_q because of increased complexity of the square and circle patterns. PP/CP composites showed the same trend as neat PP. The R_a and S_q values of PP/CP were higher than PP. In Figure 8, the SEM images displayed the top view of the micropattern on neat PP and PP/CP composites with pattern 1. Small particles of CP were found on the top of the PP/CP composite micropattern, thereby increasing the surface roughness.

Table 2.

Average roughness (R_a), root mean square roughness (S_q), and water contact angle of all specimens.

Sample	Shape parameters	PP			PP/CP composites
Sample	Shape parameters	R_a (μm)	S_q (μm)	Water contact angle (°)	R_a (μm)	S_q (μm)	Water contact angle (°)
No pattern	—	0.18 ± 0.03^cA	0.36 ± 0.01^cB	82.2 ± 4.7^cA	0.21 ± 0.08^cA	0.50 ± 0.08^eA	72.8 ± 4.2^cA
Pattern 1	Square arrays (square block size 100 μm, height 100 μm and pitch distance 200 μm)	10.51 ± 0.44^aA	11.94 ± 0.17^aB	142.2 ± 5.1^aB	11.19 ± 0.47^aA	12.24 ± 0.07^cA	152.2 ± 2.8^aA
Pattern 2	Circular arrays (circular block size 100 μm, height 100 μm and pitch distance 200 μm)	6.53 ± 0.23^bA	8.10 ± 0.55^bA	114.2 ± 2.1^bA	7.44 ± 0.89^bA	8.73 ± 0.28^dA	133.5 ± 6.7^bA
Pattern 3	Mixed square-circular arrays (square and circular block size 100 μm, height 100 μm and pitch distance 200 μm)	10.56 ± 0.22^aB	13.70 ± 0.18^aB	142.8 ± 2.2^aB	12.06 ± 0.26^aA	14.20 ± 0.12^aA	152.9 ± 1.2^aA
Pattern 4	Mixed square-rhombus arrays (square and rhombus block size 100 μm, height 100 μm and pitch distance 200 μm)	11.89 ± 1.39^aA	12.69 ± 1.84^aA	152.7 ± 2.4^aA	11.44 ± 0.53^aA	13.02 ± 0.39^bA	155.0 ± 2.4^aA

Data are reported as mean ± standard deviation. Lowercase superscripts letters (a–c) show significant differences (p < 0.05) along columns, while uppercase superscripts (A–C) show significant parameter differences (p < 0.05) in each row.

Wettability of micropatterned surfaces

The water contact angle was used to evaluate the superhydrophobic surface between the PP surface and the PP/CP specimen. Figure 9 and Table 2 show water contact angles of PP and PP/CP specimens with and without patterns 1 to 4. Water contact angles of PP and PP/CP without pattern were 82.2° and 72.8°, respectively, implying that the hydrophilicity of the PP/CP specimen was higher than the PP specimen owing to the hydrophilic cassava pulp.³³ To improve the hydrophobicity of the material, microstructures on plastic surfaces with different topographies were fabricated using the hot embossing technique. When comparing PP with and without micropatterns, the former exhibited higher hydrophobicity,³⁴ as shown in Table 2. Polypropylene with no pattern showed a water contact angle of 82.2°, whereas PP with square pattern 1 had a high water contact angle of 142.2°. Micropatterns had different surface profiles related to roughness. The existence of micro/nano-roughness drastically changed the wettability properties of the polymer materials by increasing the contact angle, resulting in low energy surfaces. As previously described,³⁵ non-wettable surfaces normally exhibit low surface energy. The roughness of the surfaces (roughness average, R_a, is the arithmetic average of the absolute values of profile heights over the evaluation length, and S_q is the root mean square average of the profile heights over the evaluation length) impacts the wettability.

To systematically investigate the effects of wettability on geometrical parameters of micropillars on the substrate surface, PP substrates with various micropattern geometries were investigated. The dependence of the geometrical parameter is shown in Figure 9 and Table 2. In Figure 9, the lowest contact angle was observed in pattern 2 (circular shape) due to the round corners of the micropillar. Moreover, lowest surface roughness (R_a = 6.53 ± 0.23 μm and S_{q =} 8.10 ± 0.55 μm) of pattern 2 was observed resulting in low water contact angle. Comparing between pattern 1 (square shaped arrays) and pattern 2 (circular shaped arrays), the water contact angle of PP with square shaped arrays (pattern 1) was higher than the circular shaped arrays (pattern 2). The mixed pattern between square-circular shaped arrays showed a mid-value between pattern 1 and pattern 2. The superhydrophobic surface of the PP substrate improved with the mixed pattern of square-rhombus shaped arrays of pattern 4 to 152.7° because the edge of the hybrid square-rhombus in pattern 4 acted as a pinning point of the water drop. Air pockets preventing water penetration into the microroughness were observed in PP with micropattern 4, representing the Cassie–Baxter model.

For micropatterned PP and PP/CP, contact angle of the PP/CP specimen was higher than the PP specimen with the same pattern. For instance, PP/CP with pattern 1 showed water contact angle of 152.2°, whereas neat PP with pattern 1 revealed low water contact angle of 142.2°. Water contact angle of the material surface increased due to surface roughness. Small particles of CP on the top of micropatterns on PP/CP composites induced high surface roughness (R_a and S_q) resulting in high water contact angle. Due to micropattern geometrical characteristics and the surface roughness profile, superhydrophobic surfaces of the PP/CP specimens (large water contact angle >150°) were achieved in all patterns, excluding PP/CP with pattern 2.

For geometrical parameters, water contact angles of micropatterned PP/CP specimens showed the same trend as the micropatterned PP system. Largest water contact angle was detected in square-rhombus arrays (155.0 ± 2.4°), followed by mixed patterns between square-circular shaped arrays (152.9 ± 1.2°), square shaped arrays (152.2 ± 2.8°), and circular shaped arrays (133.5 ± 6.7°), respectively. The edge of the square-rhombus arrays in pattern 4 acted as a pinning point of the water drop, leading to highest water contact angle. Lowest contact angle was observed in pattern 2 due to the round micropillar corners and lowest surface roughness (7.44 ± 0.89 μm for R_a and 8.73 ± 0.28 μm for S_q).

Micropatterns with different surface profiles generated diverse surface roughness (R_a and S_q). Relationships between the surface profiles of micropatterns and wetting properties of PP/CP composites are shown Figure 10. The water contact angle increased with surface roughness (R_a and S_q). The root mean square roughness (S_q) values of patterns 1, 2, 3, and 4 were 12.24 ± 0.07, 8.73 ± 0.28, 14.20 ± 0.12 and 13.02 ± 0.39 μm, respectively, whereas S_q was 0.50 ± 0.08 μm on the flat surface. Water contact angle increased from 72.8 ± 4.2° to 155.0 ± 2.4° with surface roughness but remained constant at surface roughness higher than 11 μm for R_a and 12 μm for S_q. Thus, surfaces with different topographies and roughness were crucial parameters for enhancing superhydrophobic properties.

Conclusions

Cassava pulp was used as a bio-based reinforcement in polymer composites to increase the mechanical and wetting properties of plastic products. Tensile strength, Young’s modulus, and flexural modulus of polypropylene/cassava pulp composites increased with an increment of 10 wt% cassava pulp, while elongation at break of the polypropylene/cassava pulp composites decreased. Micropatterns were fabricated on the composite surfaces using a master mold with various geometries. A superhydrophobic surface was produced by designing the micropattern geometry. Water contact angle of micropatterned neat polypropylene and polypropylene/cassava pulp composites increased compared to material with no pattern. Largest water contact angle was detected in square–rhombus arrays, followed by mixed patterns between square–circular shaped arrays, square shaped arrays, and circular shaped arrays, respectively. Water contact angle of polypropylene/cassava pulp composite surfaces was higher than polypropylene surfaces with the same pattern. Small particles of cassava pulp on the top of polypropylene/cassava pulp composite micropatterns enhanced surface roughness (R_a and S_q), leading to high water contact angles. Both micropattern topographies and surface roughness were essential for enhancing water contact angle toward material development with desirable mechanical and superhydrophobic properties. Adding 10 wt% of CP in PP improved the mechanical properties of the composites. Future studies are required to investigate CP and compatibilizer content addition to enhance polymer mechanical properties.

Footnotes

Acknowledgements

The authors gratefully acknowledge thankful for research equipment support provided by the National Metal and Materials Technology Center (MTEC) and the National Electronics and Computer Technology Center (NECTEC), the National Science and Technology Development Agency (NSTDA) and the Ministry of Higher Education, Science, Research and Innovation, Thailand. The authors are grateful to Prof. S. Seraphin, Professional Authorship Center, NSTDA, for fruitful discussion on manuscript preparation.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: NSRF via the Program Management Unit of Human Resources & Institutional Development, Research, and Innovation [grant no. B16F640115].

ORCID iD

Charinee Winotapun

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