Preparation, characterization and biodegradability of acrylate graft rice husk/ lignin reinforced PBAT

The increasing worldwide waste of polymers and agricultural products has prompted a pressing need for biodegradable plastic to help protect the environment. Hence, this research focuses on polybutylene adipate-co-terephthalate (PBAT) biodegradable polymers containing between 30–50 percent of filler to achieve an optimal balance of mechanical properties and biodegradation. Given its high lignin content, rice husk (RH) has displayed comparable properties to those of commercial lignin, and thus, the lignin from the rice husk was modified with acrylic acid grafting to facilitate the composite fabrication process. Fourier transform infrared (FTIR) analysis was used to demonstrate the grafting synergy and verify that the lignin/rice husk had been successfully grafted with the acrylic acid grafting process. Subsequently, the acrylic acid grafted lignin/rice husk was incorporated into PBAT to create a PBAT composite containing 30–50% AA lignin. The results revealed that the lignin: PBAT ratio at 30:70 (P70L30) resulted in the most optimum mechanical properties and biodegradability. These included tensile strength of 19.48 MPa, elongation at break of 20.26 MPa, Young’s modulus of 1913.62%, and crystallinity of 31.45%. The biodegradation of P70L30 was 57.36% over the 6 months, which was faster than the 9.2% degradation of PBAT. Additionally, the water absorption of P70L30 was two times more compared to PBAT, indicating the change in hydrophobicity of PBAT to a hydrophilic composite. Agricultural waste-rice husk displayed identical properties to lignin when incorporated into PBAT and resulting in the mechanical properties of strength, elongation at yield, and Young’s modulus of 19.09 MPa, 21.87%, and 1936.98 MPa. The mechanical properties and biodegradability of P70L30 on both lignin/rice husk in PBAT composite demonstrated the potential of affordable materials to create sustainable packaging solutions with good biodegradability and added features.


Introduction
According to United States Environmental Protection Agency (EPA) and the European Union (EU), plastic accounted for around 15.8% of total garbage generated in 2018 [1,2].Around 8.3 billion metric tonnes of virgin plastics have been created since 1950, with approximately 6.3 billion metric tonnes of plastic garbage generated as of 2017 [3].The fact that only 9% of the 350 million tonnes of plastic garbage produced worldwide each year is recyclable or biodegradable is alarming [4].Polymer waste can be addressed in four different ways.The first most efficient strategy to reduce polymer waste is to practise the three Rs: reduce, reuse, and recycle.This will contribute to resource conservation and waste reduction.The second way is by implementing effective waste management methods.The report "Solid Waste Management in the World's Cities" by the World Bank 377 Page 2 of 14 highlights the significance of waste management systems in tackling waste and its effects on the environment and human health [5].Third, public education and outreach can be useful in promoting environmentally friendly behaviours and reducing waste, particularly polymer waste [6].Finally, the potential of biodegradable polymers to solve plastic waste and lessen the negative environmental impacts of plastic via technological avenues of development and application of biodegradable polymers [7].
Biodegradable polymers offer numerous environmental benefits, including reductions in greenhouse gas emissions, carbon footprints, and energy consumption during production [8].Biodegradable and biobased polymers such as polybutylene adipate co-terephthalate (PBAT), polyhydroxyalkanoates (PHA), polylactic acid (PLA), starch, protein, chitin, chitosan, and polybutylene succinate (PBS) are currently available in the market [4].Hence, PBAT is one of the most widely used biodegradable polymer thanks to its high thermal, mechanical, and flexibility capabilities, making it a good option for situations in where environmental performance is crucial [9].There are several key aspects of environmental performance that can make a biodegradable polymer like PBAT a good option, such as PBAT is a biodegradable polymer that can be broken down by microorganisms in the environment, hence reducing the risk of pollution.In the recent research, PBAT using biobased butanediol (BDO), which means that it is made from renewable resources, such as plants, making it a more sustainable option than traditional fossil fuel-based plastics, making it good option for environmental performance [9].Unfortunately, the cost of PBAT makes it less practicable to use on a big scale than other polymers like PLA, PBS, and low-density polyethylene (LDPE) [10].Under typical environmental conditions, PBAT has a poor biodegradation and is difficult to decompose.[11].In an effort to reduce costs and improve biodegradability, alternative agricultural fillers, such as cellulose, lignin, rice husk, starch, hemp, and jute, have been used to reinforce PBAT composites.These fillers are a feasible alternative for many applications such as biodegradable packaging, injection-moulded products, fibres and nonwovens applications, automotive parts and many more due to their low cost, outstanding tensile qualities, and good biodegradability [12].
The grafting process of PBAT requires the creation of synergy, which refers to the enhanced qualities of the combined substance that results from the bonding of two dissimilar materials.The important findings of grafting on PBAT are described in Table 1, which compares the mechanical properties of various PBAT composite materials.Table 1 shows the synergy of silane grafting has been shown to provide higher tensile strength mechanical properties than other processes, such as the addition of dopants or the methylation process.Additionally, it has been observed that biodegradation rates are higher after silane grafting, which helps to address the slow biodegradation of PBAT as mentioned earlier.Grafting PBAT with lignin has the potential to enhance the synergistic effects and favourable properties of PBAT.The utilization of lignin as a composite material is an excellent concept due to its cost-effectiveness, accessibility, and efficiency compared to petroleum-derived alternatives [13].This can result in a material with greater strength, stiffness, hardness, toughness, flexibility, and as well as better thermal stability and biodegradability which can be useful in a range of applications [14].

PBAT-Polybutylene adipate co-terephthalate; VTMS-Vinyltrimethoxysilane
Rice husk known as agricultural waste is an abundant byproduct of rice production in Southeast Asia and has long been valued for its biodegradable properties.[18].Rice husk contains a significant amount of lignin, a complex, rigid polymer which provides mechanical strength and rigidity.Through alkaline treatment, the lignin present in rice husk can be extracted, providing a source of lignin [19].Both lignin and rice husk are examples of natural polymers that have been employed in the creation of a variety of products, including biodegradable plastics and polymer composites.Therefore, the high content of lignin and other biomaterials, such as silica, cellulose and hemicellulose, make rice husk a low-cost source of renewable carbon and a viable substitute for lignin [20].It's worth mentioning that the features and possible applications of substituting rice husk for lignin are almost comparable due to the similar chemical structure.The aim of this research is to explore the feasibility of utilizing acrylic acid grafting to create cost-effective lignin filled PBAT composites while maintaining its desired mechanical properties and biodegradability rate.The grafting process of acrylic acid provides an ideal structure that is similar to that achieved through silane grafting.The organic acid nature of acrylic acid enables a hydrophilic grafting process that not only improves the biodegradability of the PBAT/lignin composite but also allows for more filler-loaded PBAT to be created whilst retaining its mechanical properties and promoting natural breakdown [21].To achieve this, commercially available lignin and lignin obtained from rice husk will be compared to evaluate their performance.The outcome of this research could potentially lead to the development of a process that enables the acrylic acid grafting of lignin onto PBAT at a reduced cost while still providing its desired mechanical properties and promoting natural breakdown.

Polymer composite preparation
Typically, 20 g lignin and 75 ml benzene were added slowly into a mixture of 75 ml acrylic acid, 2 ml sulfuric acid and 2 g hydroquinone then reflux for 5 h at 80 °C with stirring.After completion, the acrylic grafted lignin/RH was filtered, washed by diluted acetone and dried.Figure 1 shows the schematic diagram of the preparation of acrylic grafted lignin/PBAT composite.Subsequently, PBAT was mixed with different ratios of dried acrylic grafted lignin (30wt%, 40wt%, and 50wt%) and acrylic grafted rice husk (30wt%), along with 0.05wt% dicumyl peroxide (DCP).The components were thoroughly blended by treating it in a water bath at 70 °C for 40 min, ensuring a uniform coating of lignin and DCP on the surface of the PBAT pellet.After the drying process of the mixture, single screw extruder is used to fabricate the PBAT composite.The extruder was set at 115-135 °C with 40 rpm to extrude pellets of PBAT composite.Table 2 summaries the composition of the PBAT composites prepared in this study.
In adherence to safety protocols, the experiment was conducted in controlled and secure environments, utilizing enclosed systems equipped with appropriate ventilation, such as fume hoods and exhaust systems.This precautionary measure significantly minimizes the risk of inhaling toxic vapours, including hazardous substances like benzene.Upon completing the experiment, the acrylic grafted lignin/RH underwent careful filtration, followed by a washing process with diluted acetone, and subsequent drying.The initial dilution of acetone served a dual purpose: it not only mitigated potential detrimental effects on the environment but also aided in removing any excess hazardous chemicals.Additionally, the volatility of acetone ensured rapid evaporation in a room environment, leaving no residue on the material.These steps were taken to uphold a safe and environmentally friendly approach throughout the entire experimental process.Moreover, hazardous waste generated during the experiment is diligently managed in strict accordance with local regulations for hazardous waste disposal.Licensed waste disposal facilities collect and treat the waste to prevent any potential harm to the environment and public health.This responsible waste management approach ensures the safeguarding of both the environment and public well-being.

Fourier Transform Infrared Spectroscopy (FT-IR)
The prepared composite was analysed with a FT-IR using a Perkin-Elmer Frontier 2000 FTIR spectrometer (Bruker, Germany).The scanning range varied from 4000 to 400 cm −1 with a resolution of 16 cm −1 .The raw lignin, acrylic grafted lignin, rice husk and acrylic grafted rice husk were mixed with KBr powder and then pressed to form disc-shape samples for testing.

Mechanical properties
Tensile tests were conducted at a speed of 5 mm/min, according to ASTM D638 [22].The samples were hot pressed and dumbbell-shaped, with standard Type V samples having a thickness of 4 mm.After shaping, the samples were sealed and packed in plastic bags and conditioned at room temperature for 24 h before testing.Five samples of each composition were characterized, and the average value and standard deviation were recorded.

Thermal behaviour
Thermogravimetric Analysis (TGA) was performed on the samples PBAT, P70L30, P60L40 and P50L50 using the Mettler Toledo 851e equipment.The analysis aimed to investigate the thermal behaviour of the materials and understand their decomposition characteristics.The TGA measurements were carried out over a temperature range from 28 to 800 °C, with a heating rate of 10 °C/min, and an air flow rate of 40 mL/min.The weight changes of the samples were continuously monitored as the temperature increased, and the obtained data provided valuable insights into the thermal stability and decomposition processes of the materials.The degree of crystallization (Xc) kinetics was investigated using a Perkin Elmer Pyris 6 DSC tool.The samples were weighed and heated at a rate of 10 °C/min from 20 to 190 °C.The samples were kept at the max temperature for 5 min to remove thermal history and then cooled down to 20 °C at a rate of 10 °C/min.Then, the samples were second heated to 190 °C at a rate of 10 °C/ min to analyze the crystallization characteristic after heating.The percent crystallinity of the PBAT and PBAT composite was calculated using Eq. ( 1): where Xc is the degree of crystallization of polymer, ΔHf is the enthalpy of fusion, ΔH °m is the theoretical enthalpy of fusion of 100% crystalline PBAT (114 J/g), w is the weight fraction of the acrylic grafted lignin. (1)

Morphology analysis
The morphology of the cryo-fractured specimen was observed using scanning electron microscopy (SEM) operated at an acceleration voltage of 5 kV for secondary electron imaging and 15 kV for back-scattered electron imaging.Before testing, the samples were vacuum coated with a thin layer of gold palladium.The presence of elements in the PBAT composites were further confirmed with the Energy Dispersive X-ray spectroscopy (EDX).

Wettability analysis
The water contact angle (WCA) were measured by the sessile drop method with a droplet volume of 5 µL using a goniometer (Rame hart Instrument.Co, USA) on one side of the sample.The samples were placed in a Contact Angle Goniometer, attached to an Image analyzer.Each sample was subjected to 10 measurements.DROPimage Advanced software was used to obtain the WCA.

Water absorption test
The dried sample's initial mass was calculated by weighing it at first.The sample was then immersed in water.At regular intervals, it was removed from the water, and any extra water on the surface was wiped off with paper.The weight of the wet material was then determined.At that point, the water absorption rate could be calculated using the following Eq.( 2): where, W f represented the weight after soaking the sample at fixed intervals, and W i is the initial sample weight. (2)

Soil burial degradation test
The ability of the prepared samples to biodegrade was determined by tracking the weight loss of the samples after they had been buried in soil that was obtain from the agricultural field at the Universiti Sains Malaysia engineering campus.The dried samples were weighed (4 g) and then buried in the ground at a depth of 15 cm for 6 months.The buried samples were unearthed at predetermined intervals, cleaned with distilled water, dried under vacuum, and allowed to condition in a desiccator until their weight remained constant.The percentage of weight loss was calculated using Eq. ( 3): where W i and W f are the initial and final dry weights of the samples after degradation at any given time, respectively.

FT-IR analysis of lignin vs rice husk
The FTIR analysis shown in the Fig. 2 was carried out to determine the chemical structure of the acrylic grafted functional group.A broad absorption band appeared around 3431 cm −1 in the raw lignin FTIR spectrum (Fig. 2(a)), which corresponds to aromatic and aliphatic OH groups.
Peaks at 2933 and 2856 cm −1 demonstrate the vibrational stretching of CH 2 .The absorption bands in the region between 3000 and 2840 cm −1 are assigned to C-H stretching in methyl group [23].The peak at 1592 and 1508 cm −1 affirms the presence of typical aromatic ring vibrations of the phenylpropane (C9) skeleton [23].The peak at 1459 cm −1 is due to asymmetric C − H deformations in lignin in the methyl group [24,25].In the fingerprint region, the absorption band around 1275 cm −1 shows the C-O stretching vibration of secondary alcohol of the lignin structure [23].It continued to an absorption band around 1126 cm −1 attributed to the aromatic C-H in-plane deformations of the syringyl lignin structure [23].The peak at 1220 cm −1 is attributed to the C-O stretching guaiacyl (G) lignin structure.The C-H and C-O peaks indicate the presence of ether and phenolic hydroxyl groups in syringyl and guaiacyl compounds in the lignin structure [23].The absorption band around 1030 cm −1 corresponds to the aromatic C-H in-plane deformation of the guaiacyl structure.The aromatic C-H out-of-plane bending appears at 817 cm −1 for the raw lignin structure.
The FTIR spectrum of acrylic grafted lignin (Fig. 2(b)) shows most of the chemical groups of lignin is retained with few additional changes.In contrast to pure lignin, the peak associated with O-H stretching at 3431 cm −1 showed a shallow peak, indicating the creation of more intra-and intermolecular hydrogen bonds inside the grafted lignin [26].The C-H 2 stretching vibration in grafted lignin shifted to 2933 cm −1 from 2943 cm −1 compared to pure lignin.This is due to the lignin peak's overlap with the acrylic acid peak, causing the tiny left shift [27].The absorption peak of 2856 cm −1 shows C-H stretching vibration, which remains the same in both pure and grafted lignin samples.The absorption band 1730 cm −1 is related to the unconjugated carbonyl stretching and the absorption band 1649 cm −1 peak Fig. 3 Chemical structure of a lignin (guaiacyl and syringyl structures) and b acrylic grafted lignin that shows unique peaks in the FTIR refers to the C = O group stretching.These peaks indicate the availability of more C = O functional bonds after the grafting process, which eventually will assist in the bonding with PBAT [23].The peak at 1592 cm −1 in grafted lignin became less intense as it affirms that the typical aromatic ring vibrations of the phenylpropane (C9) skeleton have been modified with the C-O group [28].However, the absorption band around 1514 and 1459 cm −1 assigned to C-H deformation in methyl, groups remain the same.The peak at 1415 cm −1 corresponds to the -CH 2 vibration functional group that originates from acrylic acid [29].The peak attributed to C-O at 1275, 1183 and 1030 cm −1 groups retained as the one in pure lignin.However, an obscure absorption band at 1126 cm −1 is attributed to the aromatic C-H in-plane deformations of the syringyl lignin structure.This is due to the loss of the C-H bond while the bond is functionalized to the acrylic functional group.At the same time, the peak of 817 cm −1 of aromatic C-H out-of-plane bending remains unchanged.
The resemblance between the FTIR spectrum of alkaline treated rice husk in Fig. 2(c) and commercial lignin Fig. 2(a) indicates a significant presence of lignin in rice husk.The vibration around 3401 cm −1 is attributed to O-H bonds in Si-OH and the HO-H of water molecules adsorbed on the material surface [30].Rice husk has a lower absorption band compared to 3431 cm −1 on commercial lignin due to its higher molecular mass than commercial lignin.The presence of peaks at 2936 cm −1 corresponding to CH 2 stretching vibration aligns with commercialized lignin.A peak at 1640 cm −1 , representing the C-C bonds, was observed in rice husk.This peak indicates that lignin from rice husk is rich with cellulose [31].
The absorption bands at 1513 cm −1 (C-H) and 1269 cm −1 (CO) were comparable with raw lignin.The small peak at 1373 cm −1 shows unique cellulose C-H groups obtained from cellulose characteristics in rice husk, as reported by Ma'Ruf [32].Lastly, a broad absorption band around 1036 cm −1 were attributed to asymmetric Si-O-Si stretching vibration, from the structural siloxane bond of silica, proving that rice husk contains silica content [33,34].Hence, rice husks show reliable quantities of lignin, cellulose, and silica content after alkaline treatment.
In acrylic-grafted rice husk (Fig. 2(d)), the stretching vibration (1726 cm −1 ) of carbonyl groups (C = O) had shifted to the left due to the higher mass of RH compared to lignin [35].The absorption bands 1640 cm −1 , 1513 cm −1 , 1373 cm −1 , 1269 cm −1 , 1141 cm −1 and 1047 cm −1 are comparable to alkaline-treated rice husk that explained previously while the absorption band at the positions of 1460 cm −1 (C-H), 1425 cm −1 (CH 2 ), and 812 cm −1 (C-H) are comparable with acrylic grafted lignin with an additional peak of silica at 777 cm −1 .The results suggest that raw lignin and alkaline-treated rice husk exhibit comparable absorption bands, with the latter showing additional peaks related to silica and cellulose content.Consequently, the grafted lignin and grafted rice husk display similar peaks, indicating that rice husk is a viable substitute for lignin [30].
In summary, Fig. 3 illustrates the changes observed in the functional bonds of the FTIR spectrum during the production of grafted acrylic lignin.Notably, four new strong peaks appear at 1730 cm −1 , 1649 cm −1 , 1514 cm −1 , and 1415 cm −1 , while one peak diminishes at 1126 cm −1 , indicating the successful accomplishment of the acrylic grafting process.The hydrophilic properties of lignin and rice husk were enhanced by the creation of C = O bonding through acrylic grafting, which improved integration with the polymer matrix.Additionally, the grafting procedure offers a perfect structure comparable to silane grafting [15].Since acrylic acid is an organic acid, it may result in a hydrophilic grafting process that enhances the biodegradability of the incorporation process to create a composite.

FT-IR analysis for PBAT composite
Figure 4 shows FTIR spectra of bare PBAT and its composites.In raw PBAT, the FTIR spectrum (Fig. 4(a)) starts with a vast and shallow absorption band at 3398 cm −1 , which is attributed to aromatic and aliphatic OH groups [36].This band represent the moisture on the PBAT material.The two peaks at 2926 and 2856 cm −1 corresponding to CH 2 stretching vibration [37].The other absorption bands in the region between 3000 and 2840 cm −1 are assigned to CH 2 stretching in methyl groups [23].The weak band at 1718 cm −1 is attributed to the carbonyl stretching vibrations of the carboxyl groups [20].The absorption band at around 1459 cm −1 assigned to C-H deformation in the methyl group [23].
The absorption band at around 1216 cm −1 ascribed to the O = C-O-C stretching of aromatic and aliphatic ester groups found in both butanediol (BDO) and adipic acid (AA) in PBAT [38].The absorption band at around 1115 cm −1 was assigned to -CH in-plane deformation [39].The peaks at 1033 cm −1 , 912 cm −1 and 732 cm −1 are attributed to -C-Ostretching vibrations, -C-OH blending, and = C-H bending of PBAT respectively [38].
The FTIR spectrum for PBAT composite with filler loadings of 30%, 40%, and 50%, are shown in Fig. 4(b), 4(c), Fig. 5 Chemical structure of a lignin grafted with acrylic acid b PBAT c acrylic grafted lignin incorporated PBAT and 4(d) respectively.The peak assigned to OH displays a left shift, indicating the intrinsic character of the acrylic grafted lignin, which becomes more prominent with higher lignin filler loading.Another changes in the PBAT composite are absorption band of 2953 cm −1 in P70L30/P60L40 and 2959 cm −1 band in P50L50, which are attributed to CH 2 stretching vibration.As the filler ratio increased to 50%, the peak broadened and intensified.This is attributed to the fact that the PBAT composite contains a higher amount of CH 2 .The absorption band at 1720 cm −1 related to carbonyl stretching vibrations in PBAT composite shows an increase in intensity as filler loading increases.This is due to the absorption band at 1730 cm −1 , corresponding to unconjugated carbonyl originating from acrylic-grafted lignin/ rice husk dominating the signal.In P50L50 of PBAT composite, an absorption band of 1514 cm −1 was assigned to the CH bond, showing typical aromatic ring vibrations of the phenylpropane (C9) skeleton.However, in P70L30 and P60L40, this peak was diminished.This demonstrates that P50L50 shows excess acrylic grafted lignin more than P70L30 and P60L40.Besides, the loss of the CH peak in P70L30 and P60L40 is also an indicator demonstrating all the bonds being crosslinked into PBAT [28].In summary, as illustrated in Fig. 5, the incorporation of acrylic grafted lignin into PBAT shows the loss of two peaks of 1216 cm −1 and 1514 cm −1 , corresponding to succesful formation of crosslinking bond within PBAT and acrylic grafterd fillers.

Morphology analysis
The effect of filler loading on the appearance of cryofractured PBAT composites was investigated using the tabletop scanning electron microscope and the results are shown in Fig. 6.According to Fig. 6(a), the morphology of PBAT is homogeneous and smooth, with no pores.Furthermore, the sample did not have any serious defects.However, the surface roughness of the PBAT composite (Fig. 6(b) to Fig. 6(d)), deteriorates significantly as the amount of acrylic-grafted lignin filler increases.This is demonstrated by the fact that the lignin filler in the PBAT matrix appears to have a structure that is both elongated and fibrillar (it appears as a white fibrous line).Besides, the image (insert of Fig. 6(d)) also shows that the compactness of AA lignin in the PBAT matrix increases as the filler loading increases from 30 to 50%.Hence, the strong interfacial bonding between the filler and the matrix at low filler loading would be affected as the filler increases to 50%.This is due to the limited matrix space available to intercalate the fillers as the filler loading increases.
Figure 7 shows the EDX spectrum and the back scattered electron image of PBAT and the composites.Elements such as carbon, hydrogen, and oxygen were anticipated in the observation due to PBAT, acrylic grafting, and lignin being organic in nature.The results from the EDX analysis showed a consistent composition in all samples, with aurum, carbon and oxygen being the only detected elements, which supported our initial assumption.Aurum was detected because of applying a thin layer of gold-palladium to create a conductive surface, which effectively reduced the charging effects during the analysis.However, detecting hydrogen was challenging using typical EDX detectors due to its low atomic number (Z = 1) and the emission of X-rays with very low energies.
The backscattered SEM images in Fig. 7 shows valuable insights into the grafting effect in the composite material.In backscattered SEM, the brightness of the image directly correlates with the intensity of backscattered electrons.Materials with higher densities tend to scatter more electrons, resulting in a greater intensity of backscattered electrons and, consequently, a brighter image.This result is in line with brightness observed in the backscattered SEM images with sequence of P50L5 0 > P60L40 > P70L30 > PBAT.This sequence reflecting effective grafting of lignin that results in denser materials which possess a higher number of atoms closely packed together, leading to increased electron scattering.

Thermal analysis
Figure 8 displays the TGA analysis results for PBAT, P70L30, P60L40, and P50L50.In the composite samples (P70L30, P60L40, and P50L50), a minor weight loss was observed in the temperature range of 100 °C to 230 °C.This weight loss is attributed to the evaporation of volatile components, moisture, and the initial stage of lignin degradation [40].Notably, the P50L50 sample exhibited a more substantial weight loss compared to P70L30 and PBAT, indicating that it is more hydrophilic, capable of absorbing greater moisture, and contains a higher lignin content.At 320 °C, a significant weight loss was initiated as the temperature increased, indicating the disintegration of PBAT [41].Between 230 °C and 500 °C, the second stage of lignin breakdown occurred, indicating primary lignin pyrolysis.During this process, the inter-unit connections in lignin were broken down through pyrolytic degradation, releasing monomeric phenols into the vapor phase [40].The weight of the sample reached stabilization at the highest temperature of 800 °C, signifying the completion of significant breakdown reactions.The presence of aromatic rings in lignin resulted in a gradual reduction process for the composites, corresponding to the third step of lignin breakdown [40].Consequently, only about 18% of the lignin in the experiment remained undecomposed for P60L40 and P50L50, which had a higher amount of lignin, while only 8% remained undecomposed for P70L30, which contained a lesser amount of lignin.
The composite was analysed using DSC at 20 to 190 °C and then cooled to 20 °C to understand its thermal properties further.The DSC heating and cooling data of the PBAT and PBAT composites with different filler loadings was investigated to collect Tm, Tc, and Xc data.Table 3 shows the degree of crystallinity between heating and cooling in the DSC.The data collected from the DSC heating and cooling cycles of the bare PBAT and PBAT composites with different filler loadings are analysed.The crystallization temperature (Tc) of the pure PBAT was 78.55 °C and the melting temperature (Tm) was 106 °C, which is in accordance with the reported work by Chen [35,36].For the filler-loaded PBAT composite, there is an increase in melting temperature (Tm), in the range of 123.63 °C to 127.33 °C from 10 °C.The crystallization temperature (Tc) of P70L30, P60L40 and P50L50 are comparable to PBAT, having 76.54 °C, 79.21 °C and 80.03 °C.This is due to filler loading with different Tm and Tc causing the increase of over PBAT composite [40].
As the temperature of crystallization rises, the degree of crystallization increases proportionally [40].This is due to lignin filler accelerating nucleation and a filled polymer beginning to crystallize, causing the X c to increase [41].Therefore, PBAT composites with acrylic-grafted lignin loading show higher crystallinity than PBAT.For an instant, the crystallinity data (X c ) shows a two-fold increase in crystallinity at 50% of filler loading compared to pure PBAT.The higher lignin filler can act as a nucleating site to boost the degree of crystallinity of composite samples [42].As a result, the sample with higher filler loading is brittle than PBAT.

Mechanical properties
The data of typical stress-strain curves for PBAT, pure lignin with PBAT, acrylic grafted lignin incorporated with PBAT, and acrylic grafted rice husk with PBAT are shown in Table 4.There are differences between the stress-strain of PBAT and PBAT composites.The tensile strength, Young's modulus, and elongation at break of bare PBAT were found to be 25.19 MPa, 19.56 MPa and 2601.38%,respectively.Three dumbbell strips are used for each characterization test, showing the average result of the mechanical properties.
From Table 4, the tensile strength of PBAT composites decreases from 25.19 MPa to 13.77 MPa, respectively, when filler loading increases from 30wt% to 50wt%.Although the degree of crystallinity increases in PBAT composites, the tensile strength is reduced due to lignin's stiffness and brittleness, which results in the steric hindrance effect attributed to the cross-linked aromatic structures of PBAT [38].Hence P70L30 is optimal filler loading into PBAT.The tensile strength of acrylic-grafted rice husk (P70L30RH) is 19.09MPa, which is equivalent to commercial lignin while being somewhat lower.This might be due to the complex structure of rice husk, the formation of weak points in the matrix, and anisotropic crystalline structure of cellulose, hence reducing tensile strength [43].The degree of crystallisation also rises as the temperature of crystallisation does [40].This is because the filled polymer is starting to crystallise and the lignin filler is accelerating nucleation, which raises the Xc [41].Therefore, PBAT composites with acrylic-grafted lignin loading showed higher crystallinity than PBAT.For instant, the crystallinity data (Xc) shows a two-fold increase in crystallinity at 50% of filler loading compared to pure PBAT.The higher lignin filler can act as a nucleating site to boost the degree of crystallinity of composite samples [42].As a result, the sample with higher filler loading is more brittle than PBAT.

Wettability of PBAT and PBAT composite
The wetting properties of the samples were assessed via contact angle measurements.Pure PBAT exhibited hydrophobic characteristics, with a water contact angle of 86°, as confirmed by Moustafa [46].As more filler was introduced through acrylic grafting, the surface of the samples became more hydrophilic.In the case of the sample with a 50% loading of acrylic grafted lignin (P50L50), the water contact angle decreased to 39°.For samples with 40% and 30% loading, the water contact angles were 45° and 52°, respectively.This is due to the hydrophilic nature of acrylic grafting, which favours intermolecular hydrogen bonding and improves the water absorption of the composite [47], resulting in a smaller contact angle.

Water absorption test
The absorption of water by PBAT and PBAT composite at a specific time is shown in Table 5. Bare PBAT has the lowest water absorption capacity, which can be attributed to its hydrophobicity [44].PBAT's water absorption capacity was increased with the addition of acrylic grafted lignin filler.The water contact angle findings demonstrate that acrylic acid grafting lignin is more hydrophilic than PBAT, confirming that the grafting process is effective [45].This is due to acrylic acid grafting having the functional group of carboxyl (-COOH) that is versatile and hydrophilic [46].Hence, this proves that the acrylic grafting shows hydrophilicity functionalities that agree with Zhou [47].This is consistent with the organic compound of acrylic-grafted lignin's hydrophilic character.The percentage of water absorption P50L50 is around 3.08% compared to PBAT of 0.337%.In the first 24 h, the contribution of water absorption to the presence of micro-voids at the reinforcement/ matrix interface [48].Water is directly absorbed when it enters the composite through small gaps in polymer chains, and it is then distributed through capillary transport into these gaps, where water is stuck between the fiber and matrix interfaces.Besides, the swelling of a polymer occurs at the cross-link junctions where each network strand is being stretched and untangled [49].As a result, the ability of the PBAT composite to absorb water improves dramatically during the first 24 h of immersion and then decreases after that time.After 5-10 days, the sample's water absorption capacity reaches its equilibrium or saturation point.Therefore, the water absorption behavior for all the samples can be modeled as a diffusion process type Fickian [50].This shows that the incorporation process of P50L50 degraded completely from initial compared to pure PBAT of 9.2%.This confirms that using lignin as filler improves the degradation process [28].In 6 months, the degradation breaks down the PBAT composite sample into very tiny pieces, while PBAT still retain the original shape.Hence, biodegradation depends on the characteristics of the composite and microorganisms in the soil.Therefore, an increase in organic compounds leads to a higher degradation rate [51].Hence, proving from Kikku that organic grafted improve in degradation rate [52].The observation of biodegradation increases drastically when more filler loading (acrylic grafted lignin) is added to incorporating PBAT.Furthermore, when determining the weight loss of a material specimen, thickness can be an essential component.The thickness decrease for PBAT is 29.5% as a result of the degradation of thickness, whereas the thickness reduction for P70L30, P60L40, and P50L50 is more than 70% and breaks into pieces.Hence, the result of thickness and weight also indicates that the utilisation of grafted lignin is very likely to improve the biodegradation rate of PBAT.

Conclusion
In conclusion, PBAT composites made with lignin and rice husk that have been grafted with acrylic are safe and effective.Both the grafting and the incorporation processes are tested by FTIR characterization.Rice husk has been shown by FTIR to possess silica, cellulose, and lignin levels that are very similar to lignin.The SEM microstructure studies show the incorporation of lignin with fibrous structure in the PBAT composite.The variations in the grafted lignin content have influenced the mechanical properties and biodegradability of the composite.Additionally, SEM BSE supported FTIR's findings regarding the good incorporation of lignin and PBAT.The thermal analysis revealed that that acrylic-grafted lignin provides huge amounts of crystallinity toward the PBAT composite.The grafting of PBAT is optimum at 30wt% due to the mechanical properties and biodegradability still considerable.The mechanical properties, such as tensile strength, are maintained at 19.48 MPa while having an elongation at a yield of 1913.38%.Besides, the biodegradability of P70L30 is 57.36% over the 6 months that is faster compared to PBAT of 9.2% degradation.Thus, the P70L30 is used to achieve the proper balance in biodegradability, cost and mechanical properties.Acrylicgrafted lignin incorporates PBAT-proven changes to hydrophilicity.The incorporation of 30% acrylic grafted rice husk shows comparable mechanical results toward P70L30 with a tensile strength of 19.09 MPa and elongation at a yield of 1936.98%.Therefore, it is concluded that rice husk can be used as a replacement for commercial lignin in order to lessen agro waste while still keeping the polymer's mechanical and biodegradable qualities.The composite made from rice husk and PBAT, is projected to have uses including packaging and are practical usage in different industrial applications due to their similar features and low cost.

Fig. 2
Fig. 2 FT-IR spectra of a raw lignin, b acrylic grafted lignin, c rice husk and d acrylic grafted rice husk

Fig. 4
Fig. 4 FTIR spectra of a PBAT b P70L30 c P60L40 d P50L50 e acrylic grafted lignin

Fig. 6
Fig. 6 Cross section morphology via secondary electron mode with 1 k magnification imaging for a PBAT b P70L30 c P60L40 d P50L50 composites

Fig. 7 Fig. 8
Fig. 7 EDX and SEM back scattered electron mode morphology showing the elemental analysis spectrum and image brightness, respectively for a PBAT b P70L30 c P60L40 d P50L50 composites

Table 1
Comparison between PBAT and its composite material with 30% filler on mechanical properties and biodegradability

Table 3
Degree of crystallinity for heating and cooling curve a The temperatures at the peak points in the 2nd heating curves of DSC b The temperatures at the peak points in the 1st cooling curves of DSC

Table 4
Mechanical Properties of PBAT and PBAT/AA Lignin composites

Table 6
displays the weight loss percentage for PBAT and PBAT composite employing the soil burial degradation test over a period of six months.The weight loss of PBAT after six months revealed a 9.2% degradation.The PBAT composite showed in Table6that P70L30, P60L40, and P50L50 had respective weight loss percentages of 57.4,69.8, and 100 during a six-month period.

Table 5
Percentage of water absorption of PBAT/PBAT composite at predetermined time points

Table 6
Weight loss percentage of PBAT and PBAT composite over the time of 6 months