In this study, a novel approach to address the pressing issue of plastic waste in the biosphere that leads to ecological hazards and threatens living beings is presented. Herein, a set of biodegradable composites has been developed through the melt blending of polybutylene adipate-co-terephthalate (PBAT) and rice husk (RH), aiming to discover effective surface modification techniques for enhancing mechanical properties while maintaining biodegradability above 90%. This research studied the diverse surface treatment methodologies applied to raw RH, including alkaline treatment, acetylation, and silane treatment. The novelty of this study lies in its focus on evaluating how these treatments distinctly influence the mechanical properties and biodegradability of RH. Additionally, it seeks to understand the underlying mechanisms driving these performance changes. To further improve the compatibility between hydrophobic PBAT and hydrophilic RH, compatibilizers such as maleic anhydride (MAH) and dicumyl peroxide (DCP), were incorporated. A range of analytical techniques, including scanning electron microscopy (SEM) for morphological analysis, tensile testing to evaluate the mechanical properties of PBAT composites and soil burial test to investigate the biodegradability of the composites. The results indicate that the PBAT/Silane RH/MAH composite exhibited exceptional mechanical properties, with a tensile strength of 22.49 MPa, strain at break of 41.83%, and young modulus of 187.60 MPa. Furthermore, the composites developed exhibited 90% mass loss during a six-month soil burial test, confirming their remarkable biodegradability. The findings present an innovative and practical solution for utilizing RH waste in a wide range of applications, particularly in the production of molded products such as straws.
Research Article
Surface Alterations on Agro-Waste Filler and their Effect on the Properties of Biodegradable Polybutylene adipate-co-terephthalate (PBAT)
https://doi.org/10.21203/rs.3.rs-4208294/v1
This work is licensed under a CC BY 4.0 License
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Polybutylene adipate-co-terephthalate (PBAT)
rice husk (RH)
acetylation
silane
alkali
maleic anhydride
Plastics are man-made polymers derived from petroleum or oil that have proliferated over the years. Chemical and biological inertness, high molecular weight, lightweight, cheap cost, hydrophobicity and resistance to light, bacteria, and water characteristics enable plastics employed in various sectors including textile, automotive, packaging, and construction. Production of plastics began in the 1950s, which leads to the global manufacturing of plastic increasing gradually by reaching 368 Mt year-1 in 2019. Plastic production is expected to double and quadruple by 2050 [1,2]. Excessive usage of plastic results in a significant amount of non-biodegradable garbage, which has detrimental consequences on the environment. Plastic decomposed into microplastics and nanoparticles which is more toxic than macroplastics under the influence of sunlight, heat, oxygen, mechanical stress, and enzymes via abiotic or biotic degradations [3].
Apart from sustainable plastic consumption and use, as well as improved recycling and waste management, the quest for biodegradable alternatives is a viable approach for reducing and eventually eradicating the global plastic waste problem [4]. As an outcome, biodegradable polymers (BP) have been developed and presented as greener alternatives. BP can degrade into biomass, carbon dioxide, and water in the environment [4–6]. Polysaccharides (starch, cellulose, carrageenan), proteins (whey protein, casein, gluten), lipids (fats and oils), poly (hydroxybutyrate) (PHB), and polylactic acid (PLA) are bio-based BP [7]. Polycaprolactone (PCL), polybutylene adipate-co-terephthalate (PBAT), and polybutylene succinate (PBS) are petroleum-based BP [7]. In summary, the best tensile properties (strain at break > 200%) and thermal qualities (Tm of 120°C and Tg of -17°C) make PBAT most desirable for short-lifespan applications such as disposable medical supplies, disposal tableware, and biodegradable shopping bags [7–9]. Despite its advantages, high manufacturing cost of PBAT restricts its wide spread of uses in the cost-competitive commodity plastics market [10]. Thus, filler addition and fabrication of composites is the potential to alleviate the production cost and low modulus of PBAT while maintaining its biodegradability [8]. Hence in this study, filler with 30% loading was corporate into PBAT to alleviate production costs and increase the modulus of PBAT. The melt blending technique was anticipated as a successful tactic to reduce material processing costs [11].
Researchers explored the possibility of utilizing agro-waste as fillers in composite manufacturing will lower costs and solve waste disposal problems in addition to advancing Sustainable Development Goal 12, which guarantees sustainable patterns of consumption and production [11–14]. Thus, in this study rice husk (RH) was applied as filler for PBAT. Rice is a predominant source of food for over half of the world’s seven billion inhabitants [15,16]. RH is an abundantly available source as it comprises approximately 20% of the 500 million tonnes of rice produced globally per year [17]. RHs are hydrophilic, the introduction of hydrophilic RH into hydrophobic PBAT weakens the qualities of composites owing to poor miscibility [18]. Poor RH dispersion into the PBAT matrix, agglomeration during processing, and low moisture resistance are all results of the incongruity between hydrophilic RH and hydrophobic PBAT. This challenge overcomes with; (1) the addition of compatibilizer; maleic anhydride and (2) surface modification on RH boosts the interfacial properties of PBAT and RH. The addition of compatibilizer reduces filler and matrix interfacial adhesion deficiency by creating covalent links [19,20].
In enhancing the qualities of RH, surface modification techniques play a crucial role. Alteration of hydroxyl group, OH on the RH surface is vital to overwhelm the aforementioned restrictions [21]. Several studies investigating the influence of different treatments on tensile qualities and biodegradability of RH composites were summarized in Table 1. As indicated in Table 1, the common treatments used to enhance the properties of filler are alkali and silane treatment. Alkali treatment tends to increase the surface roughness of fiber by eliminating non-cellulosic compounds such as lignin, wax, and oil thus, disclosing far more hydroxyls, OH on the surface of the cellulose unit for mechanical interlocking between polymer and filler [22]. On the other hand, silane treatment on filler improves fiber functionality by reducing cellulose hydroxyl groups and rendering them compatible with polymers. When moisture is present, hydrolyzable alkoxy groups lead to silanol formation. The silanol will react with the fiber’s hydroxyl groups to form a stable, covalently bonded structure with the cell wall which results in the fiber and polymer matrix cross-linked to form a network [23]. Acetylation treatment on filler is relatively scarce. Acetylation treatment is an esterification modification that results in the filler being hydrophobic by substituting the hydroxyl functional groups in fillers with acetyl groups. This is usually achieved by treating the filler with an acetylation agent, acetic anhydride, or acetic acid, in the presence of a catalyst like sulfuric acid or a base like sodium hydroxide. In summary, while these treatments are known to modify RH’s surface properties, the detailed mechanisms by which they affect the mechanical properties and biodegradability of RH composites, as well as their interplay with various polymers, are not fully understood and represent significant gaps in the existing literature. Moreover, a deterioration in the mechanical properties were observed as the filler loading increased from 10% to 60% thus, the optimum filler loading for polymer composition was discovered 30%. Most of the literature in this domain does not emphasize biodegradability aspects and lacks an in-depth discussion of the mechanisms behind surface modification techniques.
Therefore, in this study, we investigate the effects of alkaline, silane, and acetylation treatment of RH surfaces, specifically with a 30% filler loading. The underlying mechanisms by which these treatments improve both the mechanical properties and biodegradability of RH are delineated. The most effective surface modification technique for RH that not only enhances mechanical properties but also adheres to biodegradability standards is recommended. The findings of this investigation are anticipated to broaden the current understanding of surface modification techniques on waste material and offer essential information for producing environmentally friendly molded products, such as biodegradable straws, which can help reduce the environmental footprint.
Table 1 Summary of Various Chemical Treatments of Fillers in Polymer Composites.
|
Author |
Composites |
Ratio
|
Treatment on fillers |
Tensile strength (MPa) |
Young Modulus (MPa) |
Elongation at break (%) |
Soil Burial test (%) |
Reference |
|
Hu et al. (2007)
|
PLA/Hemp fiber
|
70/30 60/40 50/50 70/30 60/40 50/50 |
- - - Alkali (6% solution of NaOH)
|
41.14 44.63 43.73 39.29 54.60 41.77 |
5652.5 7399.5 7030.8 7631.0 8489.6 7012.3 |
N/A
|
N/A
|
[24]
|
|
Fávaro et al. (2010) |
rHDPE/RH |
95/5 90/10 95/5 90/10 |
- - Acetylation (acetic acid, acetic anhydride, and 0.1% sulphuric acid) |
20.0 19.5 19.5 19.4 |
470 570 495 525 |
N/A |
N/A |
[25] |
|
Ragunathan et al. (2011) |
PP/NBRr/RHP |
70/30/5 70/30/10 70/30/15 70/30/30 70/30/5 70/30/10 70/30/15 70/30/30 |
- - - - Silane (3 wt% of γ- aminopropyltrimethoxysilane) |
13 12.9 12.3 10.5 17.6 16.8 15.9 13.5 |
1070 1075 1120 1140 1315 1350 1348 1430 |
6.2 5.5 4.4 4.3 4.6 5.0 3.7 3.3 |
N/A |
[26] |
|
Yeh et al. (2015)
|
PP/RH
|
50/50 50/50 50/50 |
- Alkali (10 wt% NaOH) Silane (3 wt% of 3-aminopropyltriethoxysilane) |
17.76 18.68 17.74 |
2134 2124 2110 |
N/A
|
N/A
|
[27]
|
|
Yeh et al. (2015)
|
PP/RH/PP-g-MA
|
50/50/2 50/50/2 50/50/2 |
- Alkali (10 wt% NaOH) Silane (3 wt% of 3-aminopropyltriethoxysilane) |
24.46 22.47 21.92 |
2135 2362 2284 |
N/A |
N/A |
[27] |
|
Zahari et al. (2015) |
PP/ijuk fiber |
100/0 90/10 80/20 70/30 90/10 80/20 70/30 |
- - - - Silane (vinyltrimethoxysilane) |
17.45 17.57 18.36 19.71 20.21 20.27 23.00 |
298.14 361.32 720.24 1052.40 1051.84 1066.30 1098.51 |
0.106 mm 0.079 mm 0.059 mm 0.052 mm 0.084 mm 0.065 mm 0.051mm |
N/A |
[28] |
|
Moustafa et al. (2017) |
PBAT/CG |
100/0 90/10 80/20 70/30
90/10 80/20 70/30 |
- - - -
Torrefaction at 270°C |
14.3 ± 21.2 11.1 ± 1.6 7.4 ± 2.1 6.8 ± 1.7
18.2 ± 1.4 12.9 ± 1.6 8.7 ± 2.2
|
N/A |
1545 ± 63 340 ± 4.2 220 ± 5.1 98 ± 7.4
1150 ± 7.9 910 ± 5.9 580 ± 8.4 |
N/A |
[29] |
|
Moura et al. (2018) |
PHB/RH |
100/0 90/10 80/20
90/10 80/20 |
- - -
Hot water treatment |
9.5 ± 3.11 14.0 ± 1.63 13.2 ± 2.17
11.1 ± 3.43 16.1 ± 5.08 |
732 ±104 844 ± 43 760 ± 59
759 ± 54 754 ± 40 |
5.67 ± 0.46 7.00 ± 0.53 5.57 ± 0.40
6.69 ± 0.42 6.95 ± 0.62 |
N/A |
[30] |
|
Royan et al. (2018) |
rHDPE/RH
|
70/30 70/30 70/30
70/30 |
- Alkali (0.5 M NaOH) Acid (3:1 (v/v) ratio of sulphuric and nitric acids) UV/O3 |
17.8 17.9 17.7
18.3 |
175 220 218
210 |
N/A
|
N/A
|
[22]
|
|
Sumrith et al. (2020) |
Bio-based epoxy/WHF |
100/0 70/30 70/30 70/30 |
- - Alkali (5% solution of NaOH) Silane (1% solution of 3-Aminopropyltriethoxysilane) |
64.6 61.4 64 65.3 |
1868.9 1844.5 2105.8 2162.2 |
N/A |
N/A |
[31] |
|
Xiong et al. (2020) |
PBAT/lignin |
100/0 60/40 50/50 40/60
60/40 50/50 40/60 |
- - - -
Methylation |
23.70± 1.51 14.41 ± 0.66 13.33 ± 0.58 12.78 ± 0.36
19.48 ± 0.71 13.89 ± 0.86 13.73 ± 0.53 |
N/A
|
816.49 ± 52.95 378.94 ± 10.21 55.12 ± 8.75 37.81 ± 5.91
545.08 ± 56.78 308.21 ± 42.68 144.12 ± 49.25 |
N/A |
[32] |
|
Jaya et al. (2020) |
LDPE/RH |
100/0 95/5 90/10 85/15 |
- Alkali (5 wt% NaOH) |
7.8 9.0 9.8 10.2 |
50 64 77 82 |
148 68 52 50 |
N/A |
[33] |
|
Azam et al. (2020) |
rHDPE/RH
|
50/50 50/50
50/50
50/50 |
- Alkali (0.5M NaOH)
Acid (3:1 (v/v) ratio of sulphuric and nitric acids) UV/O3 |
5.8 10.83
6.0
7.0 |
418 858
485
530 |
N/A |
N/A |
[34] |
|
Nanni et al. (2021)
|
PBS/GS
|
90/10 90/10
90/10
|
- Acetylation (1:10 (v/v) ratio of pyridine and acetic anhydride) Silane (1 wt% of MPTMS) |
25.4 ± 1 26.8 ± 1
26.3 ± 1 |
722 ± 2 732 ± 6
724 ± 5 |
168 ± 109 274 ± 32
153 ± 98 |
N/A
|
[35]
|
|
Zaman et al. (2021) |
PP/BBF |
100/0 90/10 80/20 70/30 60/40 50/50 |
- Alkali (5% solution NaOH) |
28.9 32 34 36 38 37 |
575.7 590 800 920 1150 1050 |
N/A |
N/A |
[36] |
|
Zaman et al. (2021) |
PP/BBF |
100/0 90/10 80/20 70/30 60/40 50/50 |
- Acetylation (toluene, acetic anhydride, 60% perchloric acid) |
28.9 33 35 39 43 41 |
575.7 675 825 1015 1475 1235 |
N/A |
N/A |
[36] |
|
Yang et al. (2023) |
PLA/WFs |
75/25 75/25 75/25 75/25 |
2wt% KH550 4wt% KH550 6wt% KH550 8wt% KH550 |
26.5 23.5 28 27.5 |
765 780 790 795 |
5.7 4.6 4.8 5.0 |
N/A |
[37] |
|
Yang et al. (2023) |
PLA/WFs |
75/25 75/25 75/25 75/25 |
2wt% KH560 4wt% KH560 6wt% KH560 8wt% KH560 |
26.0 29.0 26.5 25.0 |
835 825 830 820 |
4.7 4.9 4.8 5.1 |
N/A |
[37] |
|
Yang et al. (2023) |
PLA/WFs |
75/25 75/25 75/25 75/25 |
2wt% KH570 4wt% KH570 6wt% KH570 8wt% KH570 |
26.5 27.0 26.5 25.4 |
820 820 805 795 |
5.1 5.3 5.1 5.2 |
N/A |
[37] |
|
Perumal et al. (2023) |
PBAT/CS |
100/0 60/40 60/40 |
- - Acetylation (acetic anhydride and 50% sodium hydroxide) |
28 8.4 7.28 |
22 95 101.01 |
2500 975 107.5 |
N/A |
[9] |
|
Perumal et al. (2023) |
PBAT/CS/MAH |
60/40/2 |
Acetylation (acetic anhydride and 50% sodium hydroxide) |
15.47 |
95.30 |
170.81 |
N/A |
[9] |
|
Yang et al. (2024) |
PVC/Wheat straw
|
50/50 50/50 50/50 |
- Alkali (3% NaOH) Alkali (6% NaOH) |
26.8 28 27 |
N/A |
N/A
|
N/A
|
[38]
|
|
|
PBAT/RH
|
100/0 70/30 70/30
70/30 70/30 |
- - Acetylation (acetic anhydride and 2% sulfuric acid) Alkali (4 wt% NaOH) Silane (2 wt% of 3-aminopropyltriethoxysilane) |
23.29 ± 1.15 10.59 ± 0.76 15.08 ± 0.33
13.49 ± 0.92 18.83 ± 0.23 |
29.80 ± 3.00 84.68 ± 1.37 157.79±7.42
114.93±3.83 188.24±3.11 |
983.05 ± 111.67 61.04 ± 10.67 86.41 ± 8.66
34.62 ± 1.78 40.12 ± 0.04
|
1.7 96.4 95.2
94.6 95.2 |
This work
|
|
|
PBAT/RH/MAH |
69/29/2
69/29/2 69/29/2 |
Acetylation (acetic anhydride and 2% sulfuric acid) Alkali (4 wt% NaOH) Silane (2 wt% of 3-aminopropyltriethoxysilane) |
20.65 ± 1.35
14.97 ± 0.19 22.49 ± 0.85
|
165.06±6.15
97.16±5.85 187.60±1.93 |
88.41±7.69
34.89±2.65 41.83±4.52 |
93.1
92.6 92.1 |
This work |
Abbreviation: PLA, polylactic acid; rHDPE, recycled high-density polyethylene; NBRr, recycled acrylonitrile butadiene rubber; RHP, rice husk powder; PP, polypropylene; PP-g-MA, polypropylene-grafted maleic anhydride; CG, coffee grounds; PHB, polyhydroxybutyrate; WHF, water hyacinth fibers; LDPE, low-density polyethylene; PBS, polybutylene succinate; GS, grape stalks; BBF, banana bunch fiber; WFs, waste corrugated paper fibers; CS, corn starch; PVC, polyvinyl chloride; NaOH, sodium hydroxide; MPTMS, 3-trimethoxysilylpropyl 2-methylprop-2-enoate; KH550, 3-aminopropyltriethoxysilane; KH560, 3-glycidoxypropyltrimethoxysilane; KH570, 3-trimethoxysilylpropyl methacrylate.
2.1 Materials
Polybutylene adipate-co-terephthalate (PBAT) was purchased from ZhuHai WanGo Chemical Co., Ltd. (Zhuhai, China). Rice husk was obtained from Johor Bahru Flour Mill Sdn. Bhd. (Penang, Malaysia). Sorbitol (≥ 99%), maleic anhydride (≥ 98%) and dicumyl peroxide as radical initiator (˃ 97%), 3-aminopropyltriethyoxysilane (APTES) were obtained from Sigma-Aldrich (St. Louis, MO, USA). Acetic anhydride was purchased from Merck, KGaA (Darmstadt, Germany). Sulfuric acid (˃ 96%) was purchased from Chemiz (M) Sdn. Bhd. (Malaysia).
2.2 Surface Modification of RH
Prior to surface modification, RH was sieved with 800 mesh strainer and washed with tap water to get rid of dirt and dried at 80°C in hot air oven for 24 hours. For alkali treatment, RH was immersed in 4 wt% sodium hydroxide solution for 3 days. After 3 days of soaking, the modified RH was rinsed and filtered with deionized water via Buckner equipment. The sample was dried overnight at 80°C. For acetylation treatment, RH was acetylated by treated in a solution mixture of acetic anhydride and 2% sulfuric acid. The mixture was stirred using a magnetic stirrer for 4 hours at 90°C. After 4 hours of treatment, the modified RH was filtered and washed with distilled using the Buckner apparatus. The modified rice husk was dried at 80°C for 24 hours. Prior to silane treatment, 2 wt% of APTES was dissolved for hydrolysis in ethanol/water (80/20) solution for 2 hours. After 2 hours, RH was soaked into the prepared silane solution. The mixture was stirred using a magnetic stirrer for 5 hours at a room temperature. Finally, the modified RH was rinsed and filtered with deionized water to remove any unreacted silane and followed dried overnight at 80°C.
2.3 Polymer Composite Preparation
Table 2 summarizes the composition of PBAT biocomposites. Before composition, PBAT pellets, treated and untreated RH were hot air dried at 80°C for 24 hours. For treated or untreated PBAT/RH composites, PBAT pellets and treated or untreated RH were melted blended via HAAKE Internal Mixer with a rotor speed of 65 rpm at 175°C for 15 min. For treated or untreated PBAT/RH/MAH composites, PBAT pellets were melt blended with 1.6 wt% maleic anhydride and 0.4 wt% dicumpyl peroxide via HAAKE Internal Mixer with a rotor speed of 65 rpm at 175°C for 5 min and subsequently followed with melt blending with treated or untreated RH for 10 min. Obtained composites were shaped using compression moulding machine into 140 mm x 140 mm x 0.5 mm sheets at 160°C for 5 minutes of preheating, 2 minutes of heating, and 2 minutes of chilling. Table 2 summarizes the composition of PBAT biocomposites prepared in this work.
Table 2 Composition of PBAT Composites.
|
Samples |
PBAT (wt.%) |
RH (wt.%) |
Alkali RH (wt.%) |
Silane RH (wt.%) |
Acetylated RH (wt.%) |
|
PBAT |
100 |
- |
- |
- |
- |
|
PBAT/RH |
70 |
30 |
- |
- |
- |
|
PBAT/RH/MAH |
69 |
- |
29 |
- |
|
|
PBAT/Alkali RH |
70 |
- |
30 |
- |
- |
|
PBAT/Alkali RH/MAH |
69 |
- |
29 |
- |
- |
|
PBAT/Silane RH |
70 |
- |
- |
30 |
- |
|
PBAT/Silane RH/MAH |
69 |
- |
- |
29 |
- |
|
PBAT/Acetylated RH |
70 |
- |
- |
- |
30 |
|
PBAT/Acetylated RH/MAH |
69 |
- |
- |
- |
29 |
Abbreviation: PBAT, polybutylene adipate-co-terephthalate; RH, rice husk; MAH, maleic anhydride.
2.4 Characterization
PerkinElmer Frontier 2000 (PerkinElmer, USA) was used to analyze the Fourier Transform Infrared (FT-IR) spectra of treated and untreated RH as well as PBAT composites in a spectral range between 4000 and 500 cm-1 with a resolution of 16 cm-1. Field Emission Scanning Electron Microscope (FESEM) Zeiss Supra 35VP (Carl Zeiss, Germany) was used to examine the surface morphology and dispersibility of treated and untreated RH in PBAT composites. Prior to the analysis, a thin gold coating applied on the fracture surfaces. Using an Instron 3366 tensile tester with a speed of 5 mm/min, tensile properties of composites were tested at a temperature of 20-25°C. The samples with 0.5 mm thickness were injection molded with ASTM D-638 type V. For each composition, five samples were examined; the mean and standard deviation were noted. Biodegradation test is carried out according to standard ISO14855 by burying the sample beneath the natural soil at atmospheric conditions. Film samples (size 15mm×15mm×0.5mm) were buried at a depth of 15cm in the natural soil. The sample was withdrawn from the soil after 2, 4, and 6 months for weight measurement.
3.1 SEM
Figure 1 shows the surface morphology of raw RH, alkali RH, silane RH, and acetylated RH. Raw RH (Fig. 1a) possesses a smooth surface due to the presence of lignin, wax, and hemicelluloses [39]. The outer region of RH is made up of aligned and symmetrical bumps consisting mainly of silica onto the cellulose [25].
On the other hand, alkali RH (Fig. 1b) possesses a rougher surface as the cellulose microfibrils of RH were revealed owing to the dissolution of lignin, hemicellulose, and other impurities during the alkaline treatment [39]. Similarly, after silane treatment (Fig. 1c), a rough surface structure was presented. The result is in agreement with the finding of Liu et al. [40], which showed jagged surface structures developed on silane-treated corn stalk fiber as a result of the ethanol/water solution removed hemicellulose and pectin. Similarly, acetylated rice husk (Fig. 1d) possesses a rough surface and loses its original structure. The result is coherent with studies by Trela et al. [41], as acetylated starch with a degree of substitution, DS= 1.35 possessed a rough surface as the starch granules lost their structure. The incorporation of ester groups results in granule deformation by increasing internal molecule volume and decreasing intermolecular tension caused by hydrogen bonds, resulting in granule structure opening [41]. The jagged and rough surface of the fiber was most desirable, as it could result in mechanical bonding and thus notably improve the compatibility between the fibers and matrix, which predicted to influence the mechanical properties of the biocomposites [40].
Fig. 1 SEM images of exterior surface morphology of (a) raw RH, (b) alkali RH, (c) silane RH, and (d) acetylated RH with magnification of 1000X.
Figure 2 shows the fracture morphology of neat PBAT and its composites. A smooth surface morphology was observed for bare PBAT (Fig. 2a) while rough surface morphologies were observed in PBAT composites (Fig. 2b-2i) due to the dispersion of RH particles. Poor dispersion of RH particles with higher filler pull-out and voids was observed for untreated RH composites (Fig. 2b). This indicates weak chemical bonding between RH and PBAT which may result in a deterioration in the properties of the composites.
Better dispersion of RH in the matrix with reduced filler pull-outs and voids was observed for treated RH composites indicating improved adhesion and interfacial strength. The incorporation of alkali-treated RH in PBAT (Fig. 2c) exhibits lesser filler pull-outs and voids when compared to PBAT/RH composite (Fig. 2b). Alkaline treatment modifies the surface chemistry of the RH particles, rendering them rougher and more compatible with the PBAT matrix. In contrast to alkali and acetylated RH, silane-treated RH (Fig. 2d) displayed a more compact structure without any voids, attributed to the strong bonding of the hydroxyl groups on the RH surface with the polymer matrix. Additionally, the incorporation of acetylated RH in PBAT (Fig. 2e) results in reduced filler pull-outs. Like alkali treatment, acetylation treatment altered the surface properties of RH, making them rougher and hydrophobic, thereby facilitating covalent bonding with the PBAT matrix.
Moreover, the incorporation of compatibilizer in untreated or treated RH to enhance further the adhesion between the filler and matrix while facilitating a uniform dispersion of RH particles was investigated. The untreated RH with MAH (Fig. 2f) shows less clustering of RH particles on the fractured surface. In fact, better improvement in the homogeneity was found for the treated RH composite reinforced with MAH (Fig. 2g-i). Among them, silane and acetylation treated RH composite with MAH (Fig. 2h and 2i) demonstrate a more consistent distribution of RH particles without voids or filler pull-outs when compared to other MAH-reinforced RH composites. It can be concluded that synergic effect of surface modification and MAH which fosters a stronger cross-linking between silane- and acetylated treated RH with the PBAT matrix, leading to a more effective distribution of stress across the across the composite and thereby presume to enhance its mechanical properties [11].
Fig. 2 SEM images of fracture morphology of (a) PBAT, (b) PBAT/RH, (c) PBAT/Alkali RH, (d) PBAT/Silane RH, (e) PBAT/Acetylated RH, (f) PBAT/RH/MAH, (g) PBAT/Alkali RH/MAH, (h) PBAT/Silane RH/MAH, (i) PBAT/Acetylated RH/MAH with magnification of 400X.
3.2 Tensile Properties
Table 3 represents the tensile properties observed for biocomposites with treated and untreated RH. Pure PBAT exhibited exceptional ductility with a strain at break of 983.05%; meanwhile had tensile strength, 23.29 MPa and young modulus, 29.80 MPa. These parameters of PBAT are almost similar to S. J. Xiong et al.(2020), with tensile strength and strain at break of 23.70 MPa and 816.49%, respectively [32]. Both tensile strength and strain at break significantly reduced upon addition of 30% RH to 10.59 MPa and 61.04%, respectively. Lack interfacial adhesion between the lignocellulosic RH and PBAT might explain the cause. Besides, SEM evidence of inadequate filler dispersion result weak points formation in the composites which inducing the premature break of the sample.
Nonetheless, an enhancement in the tensile properties was noted for treated RH composites when compared to untreated RH composites. The treatment processes introduced functional groups on the surface of RH, thereby improving the interfacial bonding between RH and PBAT matrix. This strengthened bonding prevents the formation of weak interfaces, resulting in more effective stress transfer and improved tensile strength. The inclusion of alkali treated RH in PBAT led to a 27% increase in tensile strength. The dissolution of hemicellulose after alkaline treatment, enhancing the interfacial adhesion between the fibers and matrix [42]. Similarly, the incorporation of acetylated RH in PBAT showed a 42% increase in tensile strength. This could be attributed to the acetylation process introducing acetyl groups that form chemical bonds with the polymer matrix, resulting in stronger adhesion between the two materials. Furthermore, the existence of phase adhesion between acetylated RH and PBAT limited the fracture development during tensile test [25]. The result contrasts with studies by Favaro et al. [25], where the incorporation of acetylated RH into polyethylene (PE) resulted decrease in tensile strength. This decrease is explained due to dewetting effect where the stress in the fiber/matrix boundary region focused around the reinforcement particle resulting in the particle-matrix interaction being weaker which leads to debonding at the boundary region. On the other hand, the inclusion of silane treated RH in PBAT resulted in a 78% increase in the tensile strength. The increase in tensile strength might due to the alignment of the organofunctional groups of the silane molecule with the polymer matrix, creating a more favorable interfacial environment and promoting adhesion [26]. A similar observation was made by N. Vaneewari et al. [43], where the inclusion of silane coupling agent affected the tensile qualities of Sugarcane bagasse/PP composites. The tensile strength of Sugarcane bagasse/PP observed an increase from 20.83 MPa to 23.42 MPa as the coupling agent loading increased from 10 to 20 wt%.
Moreover, an increase in young modulus was observed when both treated and untreated RH were integrated into the PBAT matrix. This could be attributed to the rigid and fibrous characteristics of RH, which facilitate a more even distribution of stress throughout the material, thereby enhancing its load-bearing capacity. This finding aligns with a study conducted by Lule et al.[44], where the young modulus of coffee husk composites was observed to be twice that of neat PBAT. This improvement may be explained by the enhanced stress transmission at the interface of the composite components.
Furthermore, an increase in the strain at break was observed for PBAT/Acetylated RH composite. This could be attributed to the presence of acetyl groups on the surface of the RH particles, enhancing interfacial adhesion with the polymer matrix. This improved bonding at the interface contributes to the formation of a more ductile and resilient material, thereby allowing for an increased strain at break. Additionally, the better distribution of RH particles within the matrix, coupled with lesser filler pull-out and voids observed for PBAT/Acetylated RH (Fig. 2h), exerted a positive influence on mechanical qualities [25].
The tensile properties of both treated and untreated PBAT/RH composite were observed to be enhanced with the inclusion of 1.6 wt% MAH and 0.4 wt% DCP. The order of tensile strength for composites containing MAH is as follows: PBAT/Silane RH/MAH has the highest tensile strength, followed by PBAT/Acetylated RH/MAH, then PBAT/Alkali RH/MAH, and PBAT/RH/MAH has the lowest.The PBAT/Silane RH/MAH composite in this work exhibited notable tensile strength with 22.49 MPa. This enhancement can be attributed to a crosslinking reaction between the polymeric molecules and the silane-treated RH, promoting improved interfacial adhesion [45]. Besides, the homogeneous dispersion of the RH phase, along with the absence of filler pull-outs and voids observed have a great contribution in improving the tensile strength. Similar result were observed by Dammak et al. [46], the presence of grafted maleic anhydride (MA) improves the tensile strength and elongation at the break of PBAT/TPS composites by forming ester linkage between grafted MA onto PBAT and TPS.
Table 3 Tensile Properties of PBAT/ RH Composites.
|
Composites |
Tensile Strength (MPa) |
Strain at break (%) |
Young Modulus (MPa) |
|
PBAT |
23.29 ± 1.15 |
983.05± 111.67 |
29.80 ± 3.00 |
|
PBAT/RH |
10.59 ± 0.76 |
61.04 ± 10.67 |
84.68 ± 1.37 |
|
PBAT/RH/MAH |
11.53 ± 1.51 |
36.73 ± 1.94 |
81.47 ± 11.22 |
|
PBAT/Alkali RH |
13.49 ± 0.92 |
34.62 ± 1.78 |
114.93 ± 3.83 |
|
PBAT/Alkali RH/MAH |
14.97 ± 0.19 |
34.89 ± 2.65 |
97.16 ± 5.85 |
|
PBAT/Silane RH |
18.83 ± 0.23 |
40.12 ± 0.04 |
188.24 ± 3.11 |
|
PBAT/Silane RH/MAH |
22.49 ± 0.85 |
41.83 ± 4.52 |
187.60 ± 1.93 |
|
PBAT/Acetylated RH |
15.08 ± 0.33 |
86.41 ± 8.66 |
157.79 ± 7.42 |
|
PBAT/Acetylated RH/MAH |
20.65 ± 1.35 |
88.41 ± 7.69 |
165.06 ± 6.15 |
3.3 FT-IR
Figure 3 shows the IR spectra of untreated and treated RH. Figure 3(a) shows the IR spectra of raw RH. The wide broadband peak at 3401 cm-1 is ascribed to the stretching vibration of O-H groups in cellulosic fibres as well as to the adsorbed moisture [47]. Peak at 2922 cm-1 represent aliphatic asymmetrical stretching of -CH3 groups meanwhile peak at 2850 cm-1 represents aliphatic symmetrical stretching of -CH2 groups. Peak at 1728 cm-1 represents the vibration of carbonyl groups of the hemicellulose [27]. Absorption peak at 1654 cm-1 corresponds to deformation vibrations of -OH groups [48]. The peak at 1515 cm-1 is attributed to the vibration of C=C bonds in lignin. The peak at 1423 cm-1 represents the bending vibrations in -CH3 groups of lignin, cellulose, and hemicellulose. The peaks at 1373 cm-1 and 1323 cm-1 represent the bending vibrations of -CH2 groups in lignin, cellulose, or other organic compounds. Peaks at 1236 cm-1, 1158 cm-1,and 1104 cm-1 are attributed to C-O stretching vibrations of hemicellulose and cellulose. Peak at 1056 cm-1 corresponds to C-O and C-C stretching vibrations [22]. The peak at 897 cm-1 represents the Si-C stretching vibrations. The peak at 794 cm-1 reflects the Si-O-Si bending vibrations of silicates (SiO2) present in RH.
Figure 3(b) shows the IR spectra of alkali treated RH. The peaks observed for alkali-treated RH at 3401 cm-1, 2922 cm-1, 2850 cm-1, 1654 cm-1, 1515 cm-1, 1373 cm-1, 1423 cm-1, 1323 cm-1, 1158 cm-1, 1104 cm-1, 1056 cm-1, and 897 cm-1 remain the same as raw RH. However, the bands at 1723 cm-1, 1235 cm-1 and 794 cm-1 disappeared after the treatment. The peak at 1723 cm-1 disappeared indicating the removal of hemicellulose [27]. The peak at 794 cm-1 disappeared indicating some of the cellulose protected by the silicon-cellulose barrier on the RH was degraded [27]. Additionally, the band at 1235 cm-1 indicates the lignin’s acetyl group’s C-O stretching disappeared upon alkaline treatment [49].
Figure 3(c) shows the IR spectra of silane RH. Silane treated RH showed high intensity peak at 3401 cm-1 and 1655 cm-1. The increase in peak intensity at 3401 cm-1 indicating the sygnetic effect of -OH group and -NHgroup. The high intensity of the peak at 1655 cm-1 indicated an increase in the amount of water absorbed in cellulose following removal of wax silane treatment. The peak at 1723 cm-1 assigned to the stretching vibration of C=O group disappear after silane treatment indicating the removal of hemicellulose structures [31]. New bands observed at 1457 cm-1 referred to deformation vibrations of -NH2 groups [31]. The peak at 1158 cm-1 corresponds to the Si-O-Si linkage of triethoxysilane [31]. Peak at 1230 cm-1 associated to Si-O-C vibration indicating triethoxysilane react with hydroxyl groups on the RH surface [31]. The peak in the range of 1100-1200 cm-1 also attributed to the silanol (Si-OH) groups on the silane treated RH.
Figure 3(d) shows the IR spectra of acetylated RH. Most peaks remain the same as raw RH, except a new peak at 1743 cm-1 indicating a new acyl group was added to lignocellulosic RH with a predicted acetyl group (-C=O) vibration [25]. Introduction of such acetyl groups on the surface of RH can form hydrogen bonds with functional groups in PBAT. This molecular interaction could increase the filler's adherence and bonding to the polymer matrix. Additionally, a slight increase in peak intensity at 1235 cm-1 and 803 cm-1 was observed. These peaks are ascribed to stretching vibrations of C-O and C-C, respectively.
Fig. 3 IR spectra of (a) raw RH, (b) alkali RH, (c) silane RH and (d) acetylated RH.
Figure 4 show the IR spectra of neat PBAT and PBAT biocomposites. In Figure 4(a), the peak at 2952 cm-1 represents the vibration of the methylene (CH3) and the peak at 2856 cm-1 attribute the vibration of methyl (CH2) groups [50]. The peak at 1718 cm-1 is assigned to stretching vibrations of carbonyl group (C=O) in ester linkage. The bands at 1456 cm-1 and 1389 cm-1 attributes the in-plane bending and out-of-plane bending modes of the -CH2 respectively. The peaks at 1267 cm-1, 1167 cm-1, and 1103 cm-1, attribute to C-O stretching [50]. Peaks at 1016 cm-1, 936 cm-1, 873 cm-1, and 727 cm-1 were attributed to the in-plane and out-of-plane bending mode of =C-H in the benzene ring, respectively [51].
Figure 4(b)-(e) show the IR spectra of untreated and treated PBAT/RH composites. The peaks observed for treated and untreated PBAT/RH composites at 2952 cm-1, 2850 cm-1, 1712 cm-1, 1456 cm-1, 1389 cm-1, 1267 cm-1, 1167 cm-1, 1103 cm-1, 1016 cm-1, 936 cm-1, 873 cm-1, and 727 cm-1 remain the same as neat PBAT. However, slight increase in the peak intensity at 1103 cm-1 was observed. The peaks correspond to C-O stretching. It deduced that some -OH groups of treated rice husk react with the -COOHgroup of PBAT to form ester bond.
Figure 4(f)-(i) shows the IR spectra of untreated and treated PBAT/RH/MAH composites. Nevertheless, new peaks were observed at 1504 cm-1, 1410 cm-1 and 1118 cm-1 for PBAT/RH/MAH composites. The peak at 1504 cm-1 correspond to the stretching vibration of C=C bonds in ester groups. The peak at 1409 cm-1 associated with the deformation vibrations of CH2 groups. Additionally, the band at 1118 cm-1 attributed to C-O stretching vibrations in ester groups. In addition, the peak intensity at 726 cm-1, 873 cm-1, and 1016 cm-1 were observed to increase for treated and untreated PBAT/RH/MAH. MAH contains reactive functional groups, specifically anhydride groups (C2H2O3), which can undergo reactions with various functional groups present in PBAT, treated or untreated RH, or both. These chemical reactions can result in the formation of new chemical bonds and changes in the molecular structure of the composite. The increase in intensity at 726 cm-1, 873 cm-1, and 1016 cm-1 may be associated with vibrational modes of bonds formed due to these reactions. Additional peak observed at 2920 cm-1 which correspond to the asymmetric stretching vibration of methylene, CH2 groups. This deduced that MAH react with PBAT by opening the anhydride ring, leading to the introduction of carboxylic acid or other functional groups into the polymer. These functional groups may exhibit characteristic FTIR peaks that contribute to the appearance of the new peak. Alteration of functional groups following MAH corporation in PBAT/RH composites, bear significance for understanding molecular changes and their consequential impact on the mechanical properties.
Fig. 4 IR spectra of neat PBAT and PBAT biocomposites.
3.4 Mechanism Between MAH, DCP, PBAT and Treated Rice Husk.
Figure 5 represents the possible mechanism between alkali RH and PBAT. In step I, the RH undergoes alkali treatment with 4 wt% sodium hydroxides, NaOH. NaOH reacts with the -OH group of RH to form H2O and -O-Na+. Such reaction showed a reduction of the -OH group on the RH surface which incline with a decrease in -OH peak intensity at 3400 cm-1 (Fig. 3b). In step II, the -OH group of RH react with the -COOH group of PBAT. Peak at 1267 cm-1 (Fig. 4c) indicating the stretching of C-O group in ester group indicating the formation of covalent bonds between the RH and PBAT. These interactions improve the adhesion and compatibility between the RH and PBAT, potentially leading to enhanced mechanical properties and material performance.
Fig. 5 Possible mechanism of interaction between alkali RH and PBAT.
Figure 6 represents the reaction mechanism between silane RH and PBAT. Silane coupling agents act as bridges by forming covalent bonds between the PBAT and RH. In step I, the ethoxy group (–OC2H5)of APTES, which was connected to Si, undergoes hydrolysis in an ethanol solution to form silanol groups, Si–OH. This evident an increase in the amount of water absorbed in cellulose at 3401 cm-1 and 1655 cm-1 (Fig. 3c). In step II, the Si-OH underwent a condensation reaction with hydroxyl groups of RH fibre to form covalent bonding which Si-O-C showed by 1230 cm-1 (Fig. 3c). In step III, the silanol groups of silane RH reacts with the -COOH group of PBAT to form ester bond.
Fig. 6 Possible mechanism of interaction of silane RH and PBAT.
Figure 7 represents the reaction mechanism between acetylated RH and PBAT. In step I, the hydroxyl groups on RH surface react with acetyl groups of acetic anhydrides to form a new acyl group. This is conformed with the presence of C=O peak at 1743 cm -1. Then in step II, some hydroxyl groups presence in acetylated RH reacts with the -COOHgroup of PBAT to form ester group.
Fig. 7 Possible mechanical interaction of acetylated RH and PBAT.
Figure 8 shows the mechanical interaction of PBAT composites filled with silane RH, MAH and DCP. Silane treatment was selected among other treatments due to its high performance in the mechanical properties. In step I, DCP broke down into radicals. These radicals are highly reactive species with unpaired electrons which attract a H atom from the PBAT backbone and produced PBAT radicals. These PBAT radicals are chemically active and serve as reaction intermediates. In step II, PBAT radicals react with anhydride group of MAH, leading to the grafting of MAH molecules onto the PBAT backbone creates grafted PBAT-MAH molecules. In step III, the grafted PBAT-MAH radicals may transfer hydrogen from silane RH at the termination step to form PBAT-MAH-RH. Anhydride groups (C2H2O3) of MAH, which undergo reactions with -OH functional groups present in RH, and -CH2 group present in PBAT result in the formation of new chemical bonds and changes in the molecular structure of the composite. The increase in intensity at 726 cm-1, 873 cm-1, and 1103 cm-1 may be associated with vibrational modes of bonds formed due to the reactions between MAH, PBAT, and silane RH. Additionally, the band observed at 1118 cm-1 attributed to C-O stretching vibrations in ester groups.
Fig. 8 Possible mechanical interaction of PBAT, DCP, MAH and Silane RH.
3.5 Biodegradation Test
The macroscopic appearance of treated and untreated PBAT/RH composites at different burying times is shown in Table 4. The mass loss (%) increased with the increased burial time for the entire sample, confirming the biodegradability of the PBAT/RH composites. All samples demonstrated gradual fragmentation during their decomposition over a six-month period, resulting in minimal remaining material after this duration.Top of FormTop of Form The mass loss rate observed for neat PBAT after 6 months was 1.7%. This is due to the PBAT composed of butylene adipate and terephthalate units which are relatively stable under certain conditions contribute to a low degradation rate when exposed to common environmental conditions.
The degradation rate of PBAT was observed increase upon the addition of untreated RH. The mass loss (%) observed for PBAT/RH composite was 96.4%. This is due to RH containing cellulose and lignin, which are organic components that microorganisms can break down. A similar trend was observed when treated RHs incorporated into PBAT matrix. However, a slight decrease in mass loss (%) rate observed when compared to untreated PBAT/RH composite. This might be due to the enhanced compatibility and adhesion result in more stable and resilient composite structure, making it less susceptible to rapid degradation.
The mass loss (%) observed for treated and untreated PBAT/RH composites reinforced with MAH were much lower when compared to other composites. MAH improves the adhesion between the treated and untreated RH and PBAT matrix. By enhancing compatibility, MAH can create a stronger bond between the components, potentially reducing the ingress of environmental factors that promote degradation. Similar observation was observed by Dammak et al. (2020), where the extent of mineralization of PBAT/TPS dropped by 72-74% in the presence of maleated PBAT (PBATg-MA), indicating a decrease in the rate of CO2 generation.[46] The inclusion of PBATg-MA creates a chemical continuity between the PBAT and the TPS phase, resulting in an interfacial domain that is more difficult for bacteria and enzymes to hydrolyze starch to breakdown. Nonetheless, most of the composites have undergone over 90% degradation within 6 months, meeting the standard criteria for degradability.
Table 4 Biodegradability of PBAT/RH Composites at Different Burying Times.
This study highlights the substantial progress made in the development of sustainable polybutylene adipate-co-terephthalate (PBAT) composites using chemically treated rice husk (RH) as a reinforcing agent. The treatment of RH with silane process enhanced the compatibility with PBAT, leading to improved dispersion and coalescence within the composites. Tensile testing showed that the inclusion of compatibilizers; maleic anhydride (MAH) and dicumyl peroxide (DCP) enhanced mechanical properties. The significance of these findings lies in the development of environmentally friendly materials with improved mechanical and thermal characteristics, making them highly suitable for diverse applications, with the added advantage of excellent biodegradability, contributing to sustainable and eco-conscious product development.
Acknowledgments
The work was supported by the Ministry of Higher Education of Malaysia under the Fundamental Research Grant Scheme (FRGS) with Project Code: FRGS/1/2021/TK0/USM/01/1.
Author Contributions
Niresha Perumal: Methodology, Writing – Original Draft, Visualization, Investigation. Srimala Sreekantan: Conceptualization, Methodology, Validation, Investigation, Supervision, Project Administration, Funding acquisition, Writing – Review & Editing. Zuratul Ain Abdul Hamid: Conceptualization. Kesaven Bhubalan: Conceptualization. Muhammad Fadhirul Izwan Abdul Malik: Investigation, Formal analysis. Anwar Iqbal: Investigation, Formal analysis. All authors have given approval to the final version of the manuscript.
Funding
This work was supported by the Ministry of Higher Education of Malaysia for the Fundamental Research Grant Scheme (FRGS) with Project Code: FRGS/1/2021/TK0/USM/01/1.
Data availability
The datasets generated during and/or analyzed during the current study cannot be shared at this time as the data forms part of an ongoing study and due to legal reasons.
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
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No competing interests reported.