Agri-Biodegradable Mulch Films Derived from Lignin in Empty Fruit Bunches

Abstract

Mulch films increase soil temperature, maintain soil moisture, improve water and fertilizer absorption, and reduce weed growth. This work studied a mulching film made using polyvinyl alcohol (PVA) and lignin extracted from empty fruit bunches (EFBs). The mulch films were investigated for opaqueness, biodegradation, water-solubility, absorption, and mechanical properties. Life cycle assessment (LCA) and cost estimate analysis were conducted. The composite mulch film-PVA solution was blended with 6% EFB lignin in dimethyl sulfoxide (DMSO) solution using five different amounts (0, 20, 40, 60, or 80 wt% lignin). The results showed that increasing the amount of lignin increased the film’s water solubility, moisture content, and biodegradability. At the same time, water absorption tended to decrease. Consequently, the light transmittance of the film was reduced, which had a positive effect on preventing soil weed growth. Tests of the mechanical properties showed that 60% lignin in the PVA film had the highest tensile strength (16.293 MPa). According to the LCA studies and cost estimation, the lignin-mixed PVA film had the lowest impact and was cheaper than the commercial mulching film. The results suggested that it is possible to blend polyvinyl alcohol polymer with lignin to improve biodegradability up to 25.47% by soil burial and 32% by water solubility.

1. Introduction

Agricultural mulch film is used to increase agricultural productivity, shorten harvesting time, control the weed content in the soil, reduce the use of pesticides, maintain soil temperature, improve water and fertilizer absorption, and minimize the use of soil irrigation by up to 30%. In addition, agricultural mulches are suitable for early crop production as soil temperatures rapidly rise. Agricultural mulch film accounted for 41% of the 3.6 million t of agricultural plastic globally consumed in 2007 [1], reaching 4.4 million t in 2012 and 7.4 million t in 2019 [2]. In China, the 2020 consumption of mulch film reached 1.357 million tons, covering an area of 1.74 × 107 hm² [2]. However, plastic mulch film must be properly disposed of by collecting and recycling or, if this is not possible, used as a landfill or incinerated with energy recovery, which has an uncontrollable environmental impact. In addition, the recycled film is contaminated with biological waste soil. Generally, mulch film contamination ranges from 50% to 75% of the initial weight. Therefore, biodegradable mulch is the most environmentally friendly and free disposal method [3].

The polymer used as a mulching film should have excellent mechanical properties and be low-cost and biodegradable. Polyvinyl alcohol (PVA) is water-soluble, and its semicrystalline plastic properties are widely utilized and reasonably priced. Furthermore, it has excellent solvent resistance, mechanical performance, and biocompatibility [4]. However, after crop harvesting, used mulch films pollute the soil, contribute to waste, and are time-consuming to discard. Therefore, a film that soil microorganisms can degrade was developed, using polymers, such as lignocellulosic biomass, lignin [5], and chitosan, to form films. Lignin is suitable for incorporation into polymer films due to its dark, opaque properties that can prevent UV radiation in sunlight from entering the soil and thus reduce weed growth. It also helps to control humidity, maintain soil temperature, and avoid water penetration into the ground. In addition, lignin has antimicrobial properties and antioxidants and can increase biodegradation using soil microorganisms and the thermal degradation rate of the film.

Empty fruit bunches (EFBs) from the palm oil industry are one form of residue that amounts to about 1 million t per year [6]. The main components of EFB are cellulose, hemicellulose, and lignin. Cellulose and hemicellulose can be used to produce bioethanol, while lignin is removed and discarded. Current research investigated value-added lignin as a natural and biodegradable raw material for human applications [7].

Lignin is a compound of carbon, hydrogen, and oxygen combined into several subunits, resulting in a complex with a high molecular weight. It has natural hydrophobicity and is dark brown when treated with acid or alkali. Lignin is a phenolic polymer formed from monolignols (monomers): p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. The phenolic compounds (hydroxyl and methoxyl groups) in lignin are biologically active, having antioxidant and antimicrobial activities [8,9]. Furthermore, the antimicrobial property of lignin depends on the type of plant from which the lignin has been sourced [10].

The efficiency of agricultural mulch film with PVA could be increased by adding lignin to enhance opacity properties and biodegradation and to improve water solubility [11]. Therefore, the present work investigated PVA agricultural mulch film incorporated with lignin. Then the opaqueness, biodegradation, water solubility, absorption, and mechanical and chemical properties were analyzed. In addition, an environmental impact assessment throughout the manufacturing process was undertaken regarding mulch film using the “cradle-to-the-grave” approach. Finally, life cycle assessment (LCA) was evaluated based on the production steps in the lignin extraction process and the whole life cycle of the mulch film compared to other commercial synthetic and biodegradable mulch films, as well as cost estimation analysis.

2. Materials and Methods

2.1. Materials

The EFB was kindly provided by the Suksomboon Group (Chonburi province, Thailand). The EFB was boiled and sun-dried at 65 °C for 2 days and then crushed. Next, the EFB was cut (0.25–0.42 mm) and dried in an oven (105 °C) for 3 h or until the weight was constant [12]. After that, the EFB composition was analyzed according to the method by Goering and Van [13]. Polyvinyl alcohol (PVA; MW = 146,000–186,000, 99% hydrolyzed) was purchased from Sigma Aldrich, Taufkirchen, Germany. Commercial alkaline lignin powder, sodium hydroxide (pellet), and sulfuric acid (98%) were purchased from Tokyo Chemical Industry Co., Japan, Kemaus, Australia, and QRec, New Zealand. Dimethyl sulfoxide (DMSO) (99.9%) was obtained from Merck, Germany. All chemicals except lignin were analytical grade.

2.2. Lignin Extraction

The EFB was pretreated using acid hydrolysis to remove hemicellulose and cellulose by boiling in an autoclave using an 8% H₂SO₄ solution at a ratio of EFB-to-H₂SO₄ of 1:10 at 121 °C for 1 h [14]. Then, the EFB was delignified in 2.5% NaOH solution at a ratio of EFB-to-NaOH of 1:10 at 121 °C for 1 h. The mixture was filtered to remove fiber residues, and the black liquor was collected for further two-step precipitation using lignin separation. In the first step, 50% H₂SO₄ was gradually added to adjust the black liquor from alkaline (pH 13) to neutral (pH 7). The black liquor was left at room temperature for 4 h to precipitate the solids containing silica sludge and other impurities. Next, the solid impurities were removed, and 50% H₂SO₄ was added to the solution until it reached pH 2 before being kept at room temperature for 8 h. Finally, the solid residues and lignin precipitate were washed with distilled water until the lignin precipitate had a pH equal to distilled water (pH 5) and was dried in the oven at 60 °C. The EFB pretreatment and delignification followed the method detailed by Kingkaew [15]. The amounts of lignin were determined for acid-soluble and -insoluble content [16].

2.3. Composite Films Preparation

The composite films were prepared using 3 g PVA dissolved in de-ionized water of 100 g solution under magnetic stirring at 100 °C for 1h and then at 30 °C for 15 min, after which it was left until the temperature had reduced to the ambient temperature. Lignin at 6% w/v was separately dissolved in DMSO. The PVA solutions were mixed using the different amounts of lignin in the ratios 0, 20, 40, 60, and 80% w/v. Then, each mixture was poured into separate plastic Petri dishes and placed in the oven at 60 °C until dry. The samples were moved to an aluminum tray and kept under ambient conditions (25–30 °C and 50–55% RH) until use. The average thickness of the dry films was approximately 0.125 mm.

2.4. Transparency Properties of Films

The film’s light transparency was measured using a UV-Vis spectrophotometer (Anthekie Advanced, Secomam, France) in the ultraviolet-visible range (200–900 nm), where the UV range was 200–400 nm, and the visible light range was 400–800 nm. The UV-Vis test aimed to determine the transmission characteristics and the effect of lignin on the anti-UV properties of the mulch film samples [17].

2.5. Film Contact with Water

2.5.1. Moisture Content

The moisture content of each sample (approximately 0.5 g) was placed in an oven (500, Memmert, Germany) at 100 °C for 24 h [18]. The procedure was performed in triplicate for each piece, and the moisture content was calculated using Equation (1):

$$\mathrm{Moisture} \mathrm{Content} (\%)=\frac{{\mathrm{W}}_{\mathrm{i}}-{\mathrm{W}}_{\mathrm{f}}}{{\mathrm{W}}_{\mathrm{i}}}\times 100\%$$

(1)

where W_i and W_f are the initial and final weights of the sample, respectively. The results were expressed as mean ± standard deviation [18].

2.5.2. Water Solubility

The dissolution percentage of the films was determined. First, approximately 0.5 g of each sample was dried in an oven at 100 °C for 24 h [18]. After that, its initial mass was recorded before placing it in 50 mL of distilled water at room temperature for 24 h. Then, the solution was removed, and the remaining films were placed in an oven at 100 °C for 24 h. Finally, its final weight was recorded, and the water solubility percentage was calculated using Equation (2):

$$\mathrm{Water} \mathrm{Solubility} (\%)= \frac{{\mathrm{W}}_{\mathrm{i}} {- \mathrm{W}}_{\mathrm{f}}}{{\mathrm{W}}_{\mathrm{i}}} \times 100\%$$

(2)

where W_i and W_f are the initial and final weights of the sample, respectively. The experiment was conducted in triplicate, and the results were expressed as mean ± standard deviation [18].

2.5.3. Water Absorption

Samples of approximately 0.5 g were weighed and dried in a circulating air oven at 100 °C for 24 h [18] to remove existing moisture and then immersed in water at room temperature for 24 h. After that, the samples were weighed again. The weight gain was recorded for 24 h. The percentage of water absorption was calculated using Equation (3):

$$\mathrm{Water} \mathrm{Absorption} (\%)= \frac{{\mathrm{W}}_{\mathrm{i}} {- \mathrm{W}}_{\mathrm{f}}}{{\mathrm{W}}_{\mathrm{f}}} \times 100\%$$

(3)

where W_i and W_f are the initial and final weights of the sample, respectively. The results were expressed as mean ± standard deviation. The method was conducted according to the ASTM D570 [19].

2.6. Soil Biodegradation

Film degradation was determined by weight loss and surface morphology changes using FE-SEM (JEOL Ltd., Tokyo, Japan). The film sample was placed in a tray filled with sandy loam soil at a soil surface depth of 1–2 cm to ensure aerobic conditions for decomposition at room temperature (30 °C). The relative humidity was maintained at 50% by periodically adding water. The films were removed from the soil after 0, 7, 14, 21, 28, and 35 days. The differences in film weights were measured to evaluate their degradation based on weight loss. Then, mulch film was dried in an oven at 100 °C for 24 h. The film weight loss (WL) was used as an indicator of degradation based on Equation (4) [20]:

$$\mathrm{Weight} \mathrm{Loss} (\%)= \frac{{\mathrm{M}}_0 {- \mathrm{M}}_{\mathrm{f}}}{{\mathrm{M}}_0} \times 100\%$$

(4)

where M₀ and M_f are the initial and final weights of the sample, respectively. Visual scoring was used to record changes in color, morphology, and the surface integrity of the films [21].

2.7. Mechanical Properties

Mechanical properties (tensile strength, elongation at break, Young’s modulus) of the film were tested according to ASTM D882 (the standard used to test thin film samples with a thickness of up to 1 mm) using a universal testing machine (UTM) Hounsfiled, USA model H50KS. The test specimens were rectangular plates 10 mm in width and 100 mm in length. This test was repeated 5 times using a crosshead speed of 5 mm/min, with an initial clearance of 50 mm of adhesion. Samples were prepared at a temperature of 25–30 °C and 50% relative humidity for 48 h before testing.

2.8. Characterization of Mulch Film

2.8.1. Surface Morphology

The surface morphology on the top of each sample was observed using field emission scanning electron microscopy (FESEM; JEOL, JSM7001F, Peabody, MA, USA) with an acceleration voltage of 2 kV and resolution of 100 µm under a nitrogen atmosphere. The samples were dried and coated with gold before analysis. The image was analyzed at 250× magnification.

2.8.2. Fourier Transform Infrared Spectroscopy

The chemical characteristics and interaction of the mulch film PVA and lignin were characterized using Fourier-transform infrared spectroscopy (FTIR; PerkinElmer Inc., Waltham, MA, USA). The lignin powder samples were analyzed for the functional group using an attenuated total reflection FTIR instrument (ATR-FTIR; BRUKER., Alpha-E) at room temperature. The absorbance spectrum (4000–400 cm⁻¹) of each blend or PVA composite was acquired at 4 cm⁻¹ resolution, and the signal was averaged over 32 scans.

2.9. Life Cycle Assessment

The different mulch films with PVA and lignin were assessed for their environmental impact throughout the product life cycle. The evaluation began by determining the raw material input, output, and energy consumption per functional unit using the Simapro software. Then, the Life Cycle Impact Assessment procedure, based on the Ecoinvent database, was applied to assess the environmental impact using the ReCiPe 2016 Midpoint/Endpoint (H) V1.04/World (2010). Finally, the life cycle of a mulch film was evaluated based on EFB lignin extraction, mulch film production, transportation, and disposal.

2.9.1. Goal and Scope Definition

The LCA aimed to evaluate the environmental performance of the mulch films with PVA and lignin compared to commercial synthetic mulch film and commercial biodegradable mulch film. The functional unit of the mulching film was 1 t. The Life Cycle Impact Assessment procedure, based on the Ecoinvent database, was applied to assess the environmental impact using the ReCiPe 2016 Midpoint/Endpoint (H) V1.04/World (2010). The system boundaries of the mulch film were “cradle-to-grave” for raw material extraction (polymer, resin, lignin extracted), mulch film production (film manufacturing process), transportation (distribution and final use), and end-of-life (landfill).

2.9.2. Life Cycle Inventory (LCI)

The life cycle inventory data, including all production phases of mulch film with PVA and lignin to the disposal phase, are shown in

Table 1. Life Cycle Inventory of mulch film with PVA and lignin.

Input/Output	Materials/Energy	Weights	Unit	Source
Inputs	EFB Lignin	0.4507	t	(Sirivechphongkul et al., 2022)
	Poly (vinyl alcohol)	0.3755	t	(Jungbluth et al., 2012)
	water	11.467	t	Ecoinvents 3
	Dimethyl Sulfoxide	0.2253	t	Ecoinvents 3
Output	Mulch Film	1	t
Waste	Wastewater	10.9374	t
Electricity	Stirrer/Extrusion	307.46	kWh

. The selected commercial synthetic mulch film contained 55% linear low-density polyethylene (LLDPE), 43% co-polymer, 2% UV stabilizer, and carbon black [22,23]. The commercial biodegradable mulch film comprised 23% potato starch, 7% plasticizer (ethylene glycol), and 70% polymer (PVA) [24]. The thickness of the mulch film with PVA and lignin was an average of 0.125 mm in this study [20,25]. All mulch films were produced through blown film extrusion. The weight of the mulch film was 1 t. The electricity consumption was about 0.0418 kWh per kg of mulch film [22]. The transportation (distribution and final use) was based on 500 t-km using a single diesel engine truck, and disposal was 100% landfill.

2.10. Cost Estimation of Mulch Film

Cost was investigated for the mulch film with PVA and lignin and was compared with commercial mulch film (synthetic and biodegradable). The cost estimation consisted of material, operational, and utility costs based on 1 t [26].

3. Results and Discussion

3.1. Effect of Initial Lignin Content

The compositional analyses of raw and pretreated EFB are listed in

Table 2. Composition of raw and pretreated EFB.

Composition	1 EFB
	Raw EFB	Pretreated EFB
Cellulose (%)	60.084 ± 0.98	48.46 ± 1.87
Hemicellulose (%)	21.20 ± 1.42	1.097 ± 0.41
Lignin (%)	12.84 ± 0.41	42.79 ± 1.28
Ash (%)	0.59 ± 0.15	0.65 ± 0.18
Others (%)	5.29 ± 0.46	6.99 ± 0.15

¹ EFB = Empty Fruit Bunches.

. The pretreated EFB was obtained after pretreatment using acid hydrolysis. The amounts of cellulose and hemicellulose (60.08 and 21.20%, respectively) substantially reduced after acid hydrolysis (48.46 and 1.097%, respectively). In contrast, lignin increased from 12.84 to 42.79% because the acid broke the ether bond between lignin, cellulose, and hemicellulose [27]. Consequently, the acid could digest the hemicellulose and some cellulose into smaller molecules but not the lignin (lignin will be only digested by alkaline solution); thus, the percentage of lignin increased.

Table 3. Composition of extracted and commercial Lignin.

Lignin Content	Lignin
	EFB Lignin	Commercial Lignin
Acid Soluble Lignin (ASL) (wt%)	0.66 ± 0.010	0.53 ± 0.027
Acid Insoluble Lignin (AIL) (wt%)	68.50 ± 1.78	47.085 ± 2.56
Total ASL+AIL (wt%)	69.16 ± 1.78	47.62 ± 2.50
Lignin Extracts (g/kg of Pretreated EFB)	122.33 ± 3.98	-
Actual Lignin Content from Extracts (g/kg of Pretreated EFB)	84.60 ± 2.75	-

shows the composition of extracted lignin from this study compared to commercial lignin. The extracted lignin was obtained after EFB acid pretreatment, alkali delignification [8,28], and acid precipitation. Therefore, it is the so-called EFB lignin in Table 3. This lignin portion can be divided into acid-soluble and -insoluble lignin. Generally, the lignin composition may vary depending on the natural features of different biomass sources [29].

Even though the extracted lignin using alkali delignification was high at 122.33 g/kg, the actual lignin content after precipitation was only 84.6 g/kg. Nevertheless, the total ASL+AIL lignin content was 69.16%, or an extraction efficiency of 42.79% from the EFB acid pretreatment. Therefore, increasing the extraction efficiency would improve the overall lignin extraction process in the future.

3.2. The Composite Film

The composite films of PVA/lignin at 0, 20, 40, 60, and 80% (w/v) were uniform, transparent, and had a smooth texture (homogeneous), as can be seen in Figure 1. The average thickness of the PVA and lignin and PVA only films was 0.125 mm. The color of the composite films was dark brown or black except for neat PVA. Obviously, their color was darker, and their transparency dramatically decreased with increasing lignin content (Figure 1). Figure 1e shows the rough surface caused by the air bubble during drying. In addition, due to film peeling from the glass dishes, wrinkled surfaces were found in every sample.

The SEM observations (Figure 2) were aimed at determining the surface structure and homogeneity of the film mixture. In most cases, the film had a smooth, uniform distribution with no punctures or cracking, especially the neat PVA film. However, a lumpier, rougher film occurred at higher lignin contents, especially in the 20PVA/80LN samples with large lumps on the surface. These might be due to the incomplete dissolving of the lignin and PVA due to incompatibility or different polarities [30].

3.3. Transparency Properties of Films

When the mulch films were exposed to UV-Vis light based on the percentage of transmittance at wavelengths of 200−900 nm (UVC = 200–280 nm, UVB = 280–320 nm, UVA = 320–400 nm, and visible light = 400–800 nm), the determination of light permeability can provide data on the extent to which the mulch film containing PVA with lignin could prohibit weed growth for light transmittance below 25% [20]. As shown in Figure 3, light transmittance was continuously reduced with the addition of lignin to the film, protecting against UVA, UVB, UVC, and visible light (200–800 nm) compared with the neat PVA film.

The absorbance of UV light by the lignin produced the anti-UV properties of the mulch films. Phenolic hydroxyl groups on the outer surface of the lignin particles (after self-assembling from aqueous solutions) and others, such as ketones and other chromophores in lignin, were the primary causes of UV absorption [4,31]. The results showed that increasing the lignin concentration in the film effectively produced higher UV protection performance. In particular, the 20PVA/80LN and 40PVA/60LN samples had less than 25% film light transmittance in the 400–800 nm wavelength range. This result could be interpreted as indicating weed growth suppression.

3.4. The Film Contacts with Water

Agricultural mulch film helps to maintain soil moisture by reducing the rate of water evaporation. Therefore, the selected films should be hydrophobic, have low moisture absorption, and be biodegradable [18]. Lignin is naturally hydrophobic and can be mixed with other plastics to enhance this property. The water-polymer interaction was studied based on the moisture content, absorption, and solubility. The result of water interaction on films is shown in Figure 4. It was found that increasing the lignin concentration increased the water solubility of the film as well as the film moisture tendency, while the moisture absorption tended to decrease. The film with the highest lignin concentration (20PVA/80LN) had the lowest water absorption. Therefore, it was more water-resistant, which may have been caused by a reduced reaction of the lignin film with water. The decrease in moisture content with a higher lignin content was due to the reduction of the phenolic OH group [32], which makes the film more water-soluble. These results suggested a more substantial effect of polar lignin on the molecular weight as the increased concentration of lignin made the film more water-soluble.

Figure 5 shows the SEM morphology of a mulch film (40PVA/60LN) before and after 24 h of water solubility. The sample exhibited numerous ripples, lines, and holes after 24 h of immersion, indicating partial solubility of the polymer and fiber. In addition, the presence of lignin forming a microscopic phase separation structure in the PVA matrix substantially increased the water molecules penetrating within the nanocomposite film, ultimately increasing the hydrophilic properties [31].

3.5. Soil Biodegradable

Figure 6 shows the effect of lignin content on soil biodegradability of mulch film made of PVA/lignin at 7, 14, 21, 28, and 35 days of degradation. During 35 days of burial time in soil, the specimens gradually disintegrated. The increasing lignin content of the PVA film increased the biodegradation rate when embedded in the ground, with an increase in the percentage of weight loss. The 40PVA/60LN and 20PVA/80LN samples showed tremendous weight losses of 25.46 and 24.88%, respectively. In general, the highest increases in weight loss were observed during the first 7 days, followed by a slighter increase and then leveling off in all composite films after 7 days. The rapid weight loss of PVA lignin mulch films at the early burial time might be due to the availability of water molecules from the PVA structure supporting microbial degradation.

As shown in Figure 7, significant changes were observed in the soil’s weight reduction and morphology of the mulching film surface (40PVA/60LN) at 0, 21, 28, and 35 days. Initially, the film had a smooth surface (0 days), and this began to change after 21 days of burial, with blisters forming and many ripples and holes after 35 days of decomposition, indicating partial biodegradation of the PVA and lignin. However, the deterioration of fibers took longer. Therefore, it could be used for crops with a longer growing period than 35 days. During burial and soil biodegradation, some characteristic changes occurred, such as increased fragility and turbidity due to the fluctuating temperature and humidity during burial testing [11]. In addition, other biological factors could affect film degradability, such as soil conditions and microorganisms in the soil [33].

3.6. Mechanical Properties

Figure 8 shows the effect of the proportion of lignin content in the PVA mulch film on the tensile strength, elongation at break, and Young’s modulus. The PVA mixed with the lignin films 40PVA/60LN and 20PVA/80LN were selected due to their light transmittance of less than 25% and having the highest biodegradability and water solubility compared to 100% PVA films. A slight increase in the tensile strength was observed when the lignin content was increased to 60%. However, 80% had the lowest due to the low elasticity of lignin. In addition, while the elongation at the break was the highest at 60%, lignin content was found to add lignin 60%, which maximized these values.

Young’s modulus values of the neat PVA, 40PVA/60LN, and 20PVA/80LN were 257.77, 208.9, and 140.1 MPa, respectively, as shown in Figure 9. These results indicated that there was no effect of the inclusion of lignin with PVA in both the tensile strength and modulus. However, small lignin particles would reinforce the polymer. Thus, the composite PVA polymer had better mechanical properties. The higher the modulus and tensile strength, the stronger the film [31]. This behavior could be explained by forming strong hydrogen bonds between the lignin and PVA, contributing to the lignin’s high surface adhesion and distribution [11]. This resulted in a significant reduction in the transformability of PVA. Additional considerations are that lignin can act as a solid structural component and as a crosslinker in PVA with a linear structure [32].

3.7. Fourier Transform Infrared Spectroscopy

The chemical structural changes in the mulch film PVA and PVA/lignin blends were investigated using Fourier Transform Infrared Spectroscopy Analysis (FT-IR), as shown in Figure 10. The typical peaks of lignin powder were observed at 3390 cm⁻¹ (3030–3690 cm⁻¹), 2923 cm⁻¹, 1707 cm⁻¹, and 1220 cm⁻¹ corresponding to -OH, C-H, C=O, and C-O stretching, respectively. The bands of the aromatic structure and the vibration of C-H of the syringyl form were present at 1451–1601 cm⁻¹ and 838.7 cm⁻¹. The syringyl breathing band with a C-O stretching signal was observed at 1322 cm⁻¹. A band at 1118 cm⁻¹ represented the aromatic C-H in plain deformation of the syringyl unit [34].

In the mulch film, the neat PVA had an absorption band at 3259.88 cm⁻¹ in the hydroxyl stretching region. At the same time, the PVA–lignin film band shifted to an upper wavenumber region at 3279.715 cm⁻¹, similar to the lignin powder wavelength of 3390 cm⁻¹ following Sirivechphongkul and Srinophakun [7]. The results indicated the presence of the formation of hydrogen bonds between the hydroxyl groups of the lignin and the PVA matrix [4]. The band at 1139.14 cm⁻¹ represented the aromatic C-H in plain deformation of the syringyl unit on the PVA–lignin film, which was consistent with the band of lignin powder at 1118 cm⁻¹. The bands were considered a C-O stretching vibration at 1085.2 cm⁻¹ (neat PVA) and 1090.87 cm⁻¹ (PVA/lignin). The results indicated the presence of hydrogen bonding interactions between hydroxyl groups in the PVA and hydrophilic polar groups (hydroxy and carboxyl functional groups) in the lignin [31].

3.8. Life Cycle Impact Assessment (LCIA)

This evaluation considered the inputs and outputs of raw materials and the energy consumption per functional unit using the Simapro software. In addition, the scope of this study covered information for the environmental impact assessment of the mulch film consisting of PVA with lignin (40PVA/60LN, as the best condition), which was performed using the ReCiPe Midpoint/Endpoint (H) model from the Ecoinvent database.

3.8.1. Environmental Impact of PVA–Lignin Film

The environmental performance assessment of the PVA–lignin film involved the following components: PVA film production, lignin extraction from EFB, solvent DMSO, water consumption, energy used in film-forming, transportation, and end-of-life disposal (Figure 11). The highest environmental impact was attributed to the PVA polymer step, followed by the lignin extraction, transport, and landfill steps (end-of-life disposal). This impact caused marine and freshwater ecotoxicity; other effects included human non-carcinogenic and carcinogenic toxicity.

Table 4. Characterization of mulch film PVA with lignin (40PVA/60LN) using ReCipe Midpoint (H).

Impact Category	Unit	Polyvinylalcohol	Lignin
Global warming	kg CO2 eq	3087	508
Terrestrial ecotoxicity	kg 1,4-DCB	15,234	3111
Freshwater ecotoxicity	kg 1,4-DCB	307	192
Marine ecotoxicity	kg 1,4-DCB	387	99
Human carcinogenic toxicity	kg 1,4-DCB	66	42
Human non-carcinogenic toxicity	kg 1,4-DCB	3427	1562

shows the environmental impacts of mulch film PVA with lignin. The PVA polymers had the highest impact on marine ecotoxicity (387 kg 1,4-DCB) due to vinyl acetate and polyvinyl chloride in PVA production. In producing these substances, toxic substances are released into freshwater sources, affecting the water system for underwater plants and freshwater animals. The freshwater ecotoxicity was caused by using the PVA polymer (307 kg 1,4-DCB) and lignin (192 kg 1,4-DCB). Large amounts of NaOH and sulfuric compounds are used in the production of lignin, which result in acidic and alkaline chemicals entering the seawater and destroying marine ecosystems; both types of chemicals can cause coral bleaching and the death of corals and marine life. Wastewater pollution can also alter ocean temperature, pH, salinity, and oxygen levels.

In the damage assessment based on endpoint categories (Figure 12), long-term impacts from midpoint categories in human health had the highest impact. The life cycle of the mulch film made from PVA and PVA–lignin had the highest overall impact on human health impact mainly caused by the adverse effect of global warming on human health, followed by particle matter formation, driven primarily by chemical use in PVA production (vinyl acetate, polyvinylchloride, and NaOH), electrical energy, lignin production, and sanitary waste landfill.

3.8.2. Environmental Impact of PVA–Lignin Film Compared to Commercial Synthetic and Biodegradable Mulch Films

The environmental performance of the PVA mulch film with lignin was assessed against other mulch films (commercial synthetic and biodegradable), as shown in Figure 13. The results showed that commercial biodegradable mulch film had the highest environmental impact. In contrast, PVA–lignin and commercial synthetic mulch films had similar impacts causing marine and freshwater ecotoxicity, with other effects including human non-carcinogenic and carcinogenic toxicity, respectively.

Table 5. Characterization of mulch film PVA with lignin (20PVA/80LN) compared with PVA mulch film, commercial synthetic, and biodegradable mulch films using ReCipe Midpoint (H).

Impact Category	Unit	PVA/Lignin Mulch Film	Commercial Synthetic Mulch Film	Commercial Biodegradable Mulch Film
Global warming	kg CO2 eq	4176	6289	6519
Terrestrial ecotoxicity	kg 1,4-DCB	20,009	24,544	32,242
Freshwater ecotoxicity	kg 1,4-DCB	561	501	665
Marine ecotoxicity	kg 1,4-DCB	567	637	830
Human carcinogenic toxicity	kg 1,4-DCB	118	114	137
Human non-carcinogenic toxicity	kg 1,4-DCB	6124	5120	8369

shows the environmental impacts of mulch film PVA with lignin compare with PVA mulch film, commercial synthetic, and biodegradable mulch film. The marine and freshwater ecotoxicity levels of the PVA–lignin mulch films (567 and 561 kg 1,4-DCB, respectively), commercial synthetic mulch film (637 and 501 kg 1,4-DCB, respectively), and bio-degradable mulch film (830 and 665 kg 1,4-DCB, respectively), could be mainly attributed to the use of polymers in the production process, followed by the processing of potato starch in the commercial biodegradable mulch film and the production of lignin in the PVA–lignin mulch films.

The environmental impacts on endpoint categories (Figure 14) had the highest impact on long-term human health. The commercial biodegradable mulch film had the most significant impact, followed by commercial synthetic mulch film and PVA/lignin film. The highest global warming impact on human health was due to fine particulate matter formation, human non-carcinogenic toxicity, and fossil resource scarcity, caused mainly by chemical additive/composite use in polymer production and electrical energy.

3.9. Cost Estimation

The estimated costs (

Table 6. Cost estimation of mulch film.

Material Cost	Amount	Cost	Cost/Unit
Lignin Extract	0.4506 t	USD 6858.132	USD15.22/kg [26]
Commercial Lignin	0.4506 t	USD 85.614	USD190/t
DMSO	0.2253 t	USD 450.6	USD2000/t
PVA	0.3755 t	USD 375.5	USD1000/t *
Water Utilities	11.467 t	USD 5.5617	USD0.0005/L [26]
Labor	-	USD 28.8	USD1.2/h, 3 people, 8 h [26]
Energy and Utility Cost	405.855 kWh	USD 64.937	USD0.16/kWh [26]
Equipment Cost per Operation Time		USD 136.876
Total Weight (Mulch Film with Lignin Extract)	1 t	USD 7920.407	USD7920.407/t
Total Weight (Mulch Film with Commercial Lignin)	1 t	USD 1147.889	USD1147.889/t
Commercially Available Synthetic and Biodegradable Mulch Film (same prices)	1 t	USD 3950	USD3.95/kg

* Source Polyvinyl Alcohol PVA PVOH PVAC Flakes ShuangXin 2699 100-78 on m.alibaba.com accessed on 20 September 2022.

) of raw materials throughout the production life cycle of mulch film with PVA and lignin, including water, utilities, and operational costs, involved electricity consumption of USD 0.16/kWh and utility water at USD0.0005/L. Therefore, the price of lignin extract was assumed to be USD15.22/kg, and for commercial lignin, USD190/t. For the raw material mixed with PVA for the mulch film, the cost was USD 1000/t. Therefore, the price of mulch film with lignin extract–PVA was USD7920.407/t, and for mulch film using commercial lignin–PVA was USD1147.889/t (Figure 15).

The cost of PVA–commercial lignin mulch film (USD1147.889) was lower than the commercial synthetic or biodegradable mulch film (same prices for both, USD3950). However, the mulch film of PVA–commercial lignin was the most suitable as it was more environment-friendly and cost-effective. In addition, the materials used were all bio-based and highly biodegradable.

4. Conclusions

Increasing the amount of lignin embedded in the PVA mulching films increased the biodegradability of the film, and the water solubility increased with the lignin content. In addition, the lignin reduced the UV light transmittance of the film, which helped prevent weeds from growing in the soil. Mechanical property testing showed that 60% lignin in the PVA film (40PVA/60LN) produced the highest tensile strength. Based on the LCA studies and cost estimation, the lignin-mixed PVA film had the lowest impact and was cheaper than commercial mulching film.