Influence of alkali treatment on coir-reinforced polyvinyl alcohol/polyethylene glycol blends

Abstract

The utilization of natural fibers in polymer biocomposites is on the rise owing to their ready availability, lightweight nature, excellent mechanical strength and eco-friendly characteristics. In this study, a solution casting method was performed to synthesize biodegradable composite films based on polyvinyl alcohol (PVA) with 10 to 30% polyethylene glycol loading (PEG, matrix) and 10% untreated or alkali treated coir fiber (reinforcement). SEM, FTIR, XRD, mechanical properties and contact angle studies were used to examine how the alkaline treatment affected the structure and characteristics of the synthesized films. Alkali treatment on coir fiber removed hemicellulose, lignin, and pectin and significantly improved its crystalline nature from 38.3% to 69.5%, confirmed by the XRD test. Adding 10 wt% polyethylene glycol with alkali treated coir fibers attained the highest tensile strength which was 38.6% greater than untreated coir fiber and demonstrated the best reinforcing effects of treated coir on these biocomposite films. According to our research, coir fiber’s alkali treatment considerably enhances its ability to interact with polyvinyl alcohol polymer and polyethylene glycol polymer blends, making it possible to fabricate composite films with improved mechanical properties.

Keywords

Polyvinyl alcohol polyethylene glycol composites fibers mechanical properties blends

Introduction

An increase in plastic waste generated by the modern-day human beings has led researchers to develop natural fiber-reinforced polymer composites.¹ While synthetic fiber-based polymer composites possess good mechanical properties. These fibers have a significantly negative impact on the surroundings due to their hazardous nature, non-biodegradability, waste generation. In contrast, natural fiber bio-based polymer composites are environmentally friendly, low-cost, lightweight, non-toxic and recyclable.² Various natural fibers, including ramie, coir, hemp, jute, banana leaf fiber, consist of cellulose, hemicellulose, lignin, pectin, wax.^3–5

Adeniyi AG et al.⁶ reported the coir fiber consists of cellulose (36%–43%), lignin (41%–45%) and hemicellulose (0.2%) and due to the high lignin content, it is durable, relatively water resistant and can be chemically modified. Coir is the seed-hair fiber obtained from the exterior covering or husk of the coconut.⁷ Its cross-section has 30-300 cells on average. Crystalline cellulose is arranged helically in a matrix to form coir cells. There is also a noncrystalline cellulose-lignin complex.⁸

Moreover, it has high durability, low cost and outstanding biodegradability with many more advantages than other natural fibers. Coir has a higher extension at break than the other natural fibers. This property allows it to absorb strain more than other natural fibers. However, a disadvantage is that coir has a relatively high moisture intake, resulting in inadequate interfacial bonding with water-resistant matrix polymers.⁹ India and Sri Lanka are the leading exporters of coconut fibers accompanied by Thailand, Vietnam, Philippines and Indonesia. Around 5,00,000 tonnes of coconut fibers are produced annually worldwide with India and Sri Lanka being the main producers, according to the official Web site of the International Year for Natural Fibers 2009. More than 90 countries grow coconuts. The Philippines, Indonesia, India and Sri Lanka produce around 78% of the global supply.^10,11

Pretreatment is an essential step in transforming cellulose. The main objective is to remove the noncellulosic components by breaking the lignin seal. Silane treatment, acetylation treatment, isocyanate treatment, benzoylation treatment, alkaline treatment, and permanganate treatment are some treatment techniques. Alkali treatment is regarded as a potential pretreatment technique that modifies the structural characteristics, such as crystallinity and availability of the fiber surface area, which enables its compatibility with the matrix material to make the biocomposite film. The most common alkaline treatment uses sodium hydroxide (NaOH).^12,13

Among available biodegradable polymers such as polyhydroxyalkanoates,¹⁴ polyvinyl alcohol (PVA),¹⁵ polylactic acid (PLA),¹⁶ polyhydroxy butyrate (PHB),¹⁷ polycaprolactone (PCL),^18,19 starch,²⁰ the PVA is a synthetic polymer derived from petroleum resources. Similarly, PCL is synthetic and even PLA is highly synthesized polymer. However, they are considered biodegradable because they can be broken down by microorganisms such as bacteria²¹ and fungi into smaller, less complex molecules. When PVA comes into contact with water and microbes, it goes from a polymer to a monomer (single molecules that can eventually biodegrade). This process typically occurs in environments where these microorganisms are present in significant numbers, such as in soil, compost, or certain aquatic environments. Enzymatic degradation is reported by Ullah M et al.²² on PVA, Vikman M et al.²³ on PLA and Bubpachat T et al.²⁴ on thermoplastic starch-polycaprolactone blend. Chiellini E et al.²⁵ reported PVA biodegradation by fungi and yeasts, under composting conditions, in soil, in aqueous environments (aerobic) and under anaerobic conditions.

Polyvinyl alcohol has strong hydrogen bonding, high mechanical properties, significant film-forming ability, excellent thermostability, physical properties, non-toxic and high crystallinity.^26–28 Due to strong hydrogen bonding, it reacts efficiently with the hydrophilic surfaces of the biomaterials and is processed with suitable combinations with cellulosic and nanocrystalline cellulosic materials to synthesize biocomposites.²⁹ Owing to excellent properties, PVA composite films have many applications in food packaging, pharmaceutical packaging and biodegradable packaging.^30,31 PVA composite films are often used in food packaging due to their excellent barrier properties against oxygen and carbon dioxide, which help to preserve food freshness. In pharmaceutical packaging, due to their non-toxicity and biocompatibility, are used to package drugs,³² especially those that need to be delivered at a controlled rate. PVA composite films are ideal for creating biodegradable packaging materials.

Polyvinyl alcohol has been chosen as a thermoplastic for developing blends³³ and composites with polyethylene glycol (PEG), chitosan, starch, alginate, and tara gum and different compositions of various polymer blends which have resulted in the desired properties of the material.³⁴ Polyethylene glycol (PEG) has a strong plasticization effect on PVA. Therefore, the PVA properties can be improved by blending with PEG.³⁵ Due to its high affinity for cellulose fibers, PEG functions as a binding mediator to encourage effective contact with the reinforcing material and polymer matrix.³⁶ According to Falqi FH et al.,³⁷ making films was not possible at greater than 40% PEG in the PVA/PEG blend due to phase separation. Faradilla RF et al.³⁸ showed that the PEG molecular weight significantly affected the properties of banana pseudostem nanocellulose films.

This study investigates, how 5% NaOH treatment affects the coir fiber (reinforcement) with polyvinyl alcohol and polyethylene glycol polymer blends (matrix). The morphology, crystallinity and mechanical properties of alkali-treated coir composite films with various PEG were examined and compared with composite films synthesized with untreated coir fibers with PEG from 10 to 30%.

Experimental Methods

Materials and chemicals

Coir was collected from the fibrous husk of the coconut procured from local suppliers near the Durga Kund temple, Varanasi, Uttar Pradesh, India. Polyvinyl alcohol was from HiMedia, 86%–89% hydrolyzed and polyethylene glycol with a 6000 average molecular weight, both procured from Merck. Laboratory-grade sodium hydroxide was purchased from “Indiamart” in pellet form and distilled water required for this research work was generated in our research laboratory through a distilled water unit.

Isolation of cellulose fibers

In this process, coir fiber was cut and washed with distilled water. The coir fibers were exposed to sunlight for 4 to 8 days to dry completely. To remove residual moisture, the fibers were further dried in an oven, at 60°C for 24 h. The dried coir fibers were crushed with a Wiley mill. Crushed coir fibers obtained from the Wiley mill were powdered with a grinder (Philips HL7536/01-500 W) and sieved through a 120 mesh sieve to get the untreated coir fiber.

Pretreatment (Alkali) of coir fibers

Powdered coir fibers (25 g) were immersed in 500 mL of 5% (w/v) sodium hydroxide (NaOH) solution and heated on a magnetic stirrer for 45 min at 80°C and 250 r/min. Then, vacuum filtration was used to wash the coir fibers with distilled water until a pH 7 was achieved. After being washed, coir fibers were completely dried in a 60°C oven. Treated coir fibers that had been dried further were powdered with the grinder (Philips HL7536/01-500 W) and sieved through 120 mesh sieves. Figure 1 shows the alkali treatment process.

Figure 1.

Process diagram illustrating the alkali treatment of coir fiber.

Fabrication of biocomposite films

Solution casting was used to fabricate the biocomposite films. The alkali treated coir (TC), untreated coir (UC), PVA and PEG polymer blend were used to synthesize the composite films. PVA (5g) and different amounts of PEG were mixed in double-deionized water so that the PVA/PEG mass ratio was 80/10, 70/20 and 60/30 in the mixture. The contents were stirred vigorously at 125°C at 250 r/min using a heating stirrer (magnetic) for 1 h to prepare the PVA/PEG blends. Table 1 shows the sample codes and compositions.

Table 1.

Sample formulations of biocomposite films.

Code	PVA (wt.%)	PEG (wt.%)	TC (wt.%)	UC (wt.%)
PVA/PEG10/TC	80	10	10	-
PVA/PEG20/TC	70	20	10	-
PVA/PEG30/TC	60	30	10	-
PVA/PEG10/UC	80	10	-	10
PVA/PEG20/UC	70	20	-	10
PVA/PEG30/UC	60	30	-	10

Further, 10 wt% untreated coir fiber was added in each prepared PVA/PEG10, PVA/PEG20 and PVA/PEG30 blend solutions in a 80/10/10, 70/20/10 and 60/30/10 mass ratio. Similarly, 10 wt% alkali treated coir fiber was added in each prepared PVA/PEG10, PVA/PEG20 and PVA/PEG30 blend solutions in a 80/10/10, 70/20/10 and 60/30/10 mass ratio (Table 1). The prepared polymer blend solutions with untreated and alkali treated coir fibers were stirred continuously at 125°C at 250 r/min for 2 h using a magnetic mixer until homogeneous solutions were formed. The glass casting plates (120 mm × 120 mm × 2 mm) were used to cast the solutions. The casting plates were left to dry for 2 days at ambient temperature until the composite films were completely dry and ready to peel off from the glass casting plates.³⁹ Figure 2 shows the coir fiber reinforced PVA/PEG blend composite film process.

Figure 2.

Process diagram illustrating the fabrication of coir fiber reinforced-PVA/PEG composite films.

Characterization of biocomposite films

X-ray diffraction (XRD) analysis

The XRD analysis analyses crystalline and amorphous nature of a material. Hence, the crystallinity of the untreated coir fiber, alkali treated coir fiber, PVA/PEG/UC and PVA/PEG/TC composite film was investigated using X-ray diffractometer (XRD, model Ultima IV, Rigaku, Japan). In this test, copper was used as the target material with a known wavelength (1.5481 Å) to produce monochromatic X-rays. These X-rays were directed on the sample, and the reflected X-rays were recorded. The patterns were run at 40 kV and 40 mA and an analysis was carried out at 2°/min from 5° to 80°.

The crystallinity for untreated and alkali treated coir fiber was calculated using Segal’s method⁴⁰ as follows:

C r I (%) = \frac{I_{200} - I_{a m}}{I_{200}} \cdot 100

where,

$C r I (%)$	=	percentage crystallinity index
$I_{200}$	=	maximum intensities
$I_{a m}$	=	minimum intensities

Scanning electron microscopy (SEM) studies

The morphological analysis of the untreated coir fiber, alkali treated coir fiber, PVA/PEG/UC, PVA/PEG/TC composite films was done with scanning electron microscopy (SEM, Model: EVO - Scanning Electron Microscope MA15/18, Company: CARL ZEISS MICROSCOPY LTD) at a 20 kV voltage with magnification at 300× and 6000×.

FT-IR spectroscopy analysis

Fourier-transformed infrared spectrophotometer was used to characterize the PVA/PEG/UC and PVA/PEG/TC composite films. All spectra were recorded using a Thermo-Nicolet’s iS5 model from 4000-500 cm⁻¹ and at 4 cm⁻¹ resolution. The resulting FTIR spectra were compared to evaluate the effects of untreated and alkali treated coir filling in the PVA/PEG films based on the vibrational bands intensities and shift.

Film thickness

The composite films thickness was measured using a nickel-plated brass screw gauge (0–25 mm/0.01 mm). To ensure accuracy and account for any potential variations in the composite film’s thickness, measurements are taken at five different points on each composite film. The five measurements are then averaged to obtain a single, representative thickness value for each composite film. Average thickness values with standard deviation were reported.

Swelling behaviour

To study the composite film’s water absorption properties, a swelling behaviour test was performed. A 20 mm × 20 mm square was cut from the dried film and dipped in distilled water at 35°C for 20 s before each measurement. Excessive moisture on the film surface was wiped away with filter paper and immediately weighed. Five samples of each composite film were tested and average values with standard deviation were reported.

The percentage of swelling was determined as follows:

S (%) = \frac{w_{i f} - w_{i o}}{w_{i o}} \cdot 100

where,

$w_{i o}$	=	initial weight measurement
$w_{i f}$	=	final weight measurement

Contact angle

In order to determine the hydrophilic behaviour of films, contact angle measurements of PVA/PEG/UC and PVA/PEG/TC composite films were examined by sessile drop technology (KRUSS DSA25 Series, Germany) at room temperature. In this measurement, the film surface was exposed to a water droplet and given one second to interact with the water drop. Following this, the water contact angle was captured immediately. This procedure was repeated five times, and the average contact angle with standard deviation was calculated.⁴¹

Mechanical properties

The mechanical testing was done with a Universal Tensile Machine (model number AEC112-ACD), Asian Engineers Company, India. The mechanical testing was performed using ASTM D882 with 80 mm × 20 mm samples, at 2 mm per second. The samples were less than 1 mm thick. Five film samples were tested and average values with standard deviation were reported.

Results and Discussion

X-ray diffraction analysis (XRD)

Figure 3(a) shows the XRD diffractogram for the alkali treated and untreated coir fibers powders. The crystallinity can be seen in the treated coir XRD, which shows the characteristic peaks related to increase cellulosic fibers crystallinity with peaks at 2θ = 20.8° and 26.6°.⁴² The untreated coir with a peak at 21.28° is related to lignocellulosic fibers with the rest of the region being amorphous. According to Ayeni et al.,⁴³ who showed the 2θ = 22.50° peaks for 5% NaOH treated coir fiber correspond to cellulose while at 17° and 35° for untreated coir fiber indicate amorphous material. Figure 3(b) shows XRD studies of PVA/PEG/UC and PVA/PEG/TC films. The PVA/PEG10/UC peak at 2θ = 19.84° signifies its crystalline nature. This is consistent with El-naggar et al.,⁴⁴ who confirmed the peak at 2θ = 19.5° for 10 wt% PEG in 70%PVA/20%PVP/10%PEG blend with various loading of SnS₂/Y. This crystalline nature decreased as the PEG loading increased for PVA/PEG20/UC depicted by the 18.7° and 23.02° peaks due to hydrogen bonding between the constituents and the rest of the region was amorphous. The peak for PVA/PEG30/UC film at 23.02° shows a decrease in crystallinity and the amorphous nature was the 18.7° peak.

Figure 3.

(a) XRD patterns of untreated and treated coir fibers, (b) XRD patterns of synthesized PVA/PEG/UC and PVA/PEG/TC films.

Similarly, PVA/PEG10/TC 19.52° peak depicts the highly crystalline behaviour⁴⁵ compared to all PVA/PEG/UC and PVA/PEG/TC films due to the excellent chemical hydrogen bonding of treated coir fiber, PVA and PEG, which resulted in a highly ordered structure in the film. For PVA/PEG20/TC film, the 2θ = 28.88° peak represents the film crystalline nature^46,47 and the amorphous region is at around 2θ = 19.68° and 20.78°. At 30 wt% PEG, the peaks at 2θ = 18.96°, 22.84° and 26.34° showed the PVA/PEG30/TC composite film semicrystalline behaviour.⁴⁸ The crystallinity index is:

• 38.3% for UC

• 69.5% for TC

• 67.5% for PVA/PEG10/UC

• 62% for PVA/PEG20/UC

• 59.6% for PVA/PEG30/UC

• 85.4% for PVA/PEG10/TC

• 73.7% for PVA/PEG20/TC

• 61% for PVA/PEG30/TC.

Morphological analysis of biocomposite films

Figure 4 shows SEM images for untreated and 5% NaOH treated coir fiber. The untreated surface is covered with noncellulosic components such as lignin, pectin, wax and impurities (Figure 4(a)). Ng YR et al.⁴⁹ reported a similar observation that impurities such as wax, pectin, oil and lignin covered the untreated coir. Figure 4(b) shows that after alkaline treatment, the surface is clear as these impurities were removed. The surface pores are where the surface impurities were removed from the surface.

Figure 4.

SEM micrographs of (a) Untreated coir fiber, (b) Treated coir fiber with 5% NaOH.

Figure 5 has the SEM images of PVA/PEG, PVA/PEG/UC and PVA/PEG/TC film surfaces. The SEM micrographs demonstrated that the film surface had altered from smooth to uneven due to apparent crosslinking between the constituents.⁵⁰ The SEM images of PVA/PEG10 and PVA/PEG20 films resulted in a smooth surface while the surface of PVA/PEG30 film was rough. The PVA/PEG20/TC film showed a smooth surface due to excellent compatibility among the constituents, while the PVA/PEG10/TC film resulted in significant pores on its surface. Considerable phase separation was evident in the PVA/PEG30/TC films.⁵¹ Compared to PVA/PEG20/UC and PVA/PEG30/UC films, PVA/PEG10/UC film was smooth as a result of fatty, oily and waxy substances present in coir fiber. The PVA/PEG20/UC film indicated lower compatibility with PVA, thus slightly decreasing tensile strength compared to the PVA/PEG10/UC film. Whereas on further increasing the amount of PEG, similar significant phase separation and pores can be seen in PVA/PEG30/UC film like PVA/PEG30/TC film, observed with a significant decrease in tensile strength.⁵²

Figure 5.

SEM micrographs of synthesized film (a) PVA/PEG10, (b) PVA/PEG20, (c) PVA/PEG30 (d) PVA/PEG10/UC, (e) PVA/PEG20/UC, (f) PVA/PEG30/UC, (g) PVA/PEG10/TC, (h) PVA/PEG20/TC, (i) PVA/PEG30/TC.

Fourier-transform infrared spectroscopy analysis

Figure 6 is the FTIR spectra for PVA/PEG/UC and PVA/PEG/TC films from wave number (4000 to 500 cm⁻¹). The PVA/PEG/TC and PVA/PEG/UC films spectra demonstrate significant hydrogen bonding from the 3500-3100 cm⁻¹ wavenumber range. In the PVA/PEG10/UC and PVA/PEG10/TC films, the wave numbers for the OH group stretching vibrations are around 3276 cm⁻¹ and 3270 cm⁻¹.⁵³ FTIR spectra showed nearly identical absorption bands across the entire wavelength range. However, there was a modest shift in the peak location and the comparative magnitude for the OH group stretching vibration for PVA/PEG/UC and PVA/PEG/TC films, which is due to the hydrogen (-H) chemical bonding reaction between PVA, PEG and untreated coir fiber for PVA/PEG/UC films³⁷ and between PVA, PEG and treated coir fiber for PVA/PEG/TC films at different PEG loading. The absorption peaks for all the synthesized films were seen in the 2910-2880 cm⁻¹, 1088 cm⁻¹, 1455-1200 cm⁻¹ and 1711 cm⁻¹ wave number bands corresponding to -CH stretching,⁵⁴ -C-O stretching, CH, OH bending and -C=O stretching, respectively.^53,55–57 The 2906 cm⁻¹ (-CH stretching) and 2925 cm⁻¹ (-CH₂ stretching) peaks for PVA/PEG10/UC shifted to 2904 cm⁻¹ and 2927 cm⁻¹ for PVA/PEG10/TC film. The 1088 cm⁻¹ (C-O stretching) peak was somewhat broader in PVA/PEG10/TC film than for the PVA/PEG10/UC film. The 1088 cm⁻¹ (-C-O stretching) peak for PVA/PEG20/UC moved to 1091 cm⁻¹ for PVA/PEG20/TC film.

Figure 6.

FTIR spectra of synthesized PVA/PEG/UC and PVA/PEG/TC films.

Film thickness

Table 2 presents the average thickness for each film in mm with standard deviation. The thickness doubles as the PEG goes from 10 to 30%. A slight variation was seen in the average thickness of the treated and untreated coir films due to the fact that the fiber content was fixed in each film. However, a significant increase in film thickness was observed due to increased PEG content in each film.

Table 2.

The average thickness and standard deviation of the synthesized biocomposite films.

Synthesized biocomposite film	Average thickness (mm)	Std dev
PVA/PEG10/TC	0.08	0.031
PVA/PEG20/TC	0.10	0.028
PVA/PEG30/TC	0.17	0.044
PVA/PEG10/UC	0.14	0.033
PVA/PEG20/UC	0.29	0.030
PVA/PEG30/UC	0.33	0.047

Swelling behaviour

Figure 7(a) shows the percentage swelling of the PVA/PEG/UC and PVA/PEG/TC films. The treated coir films swelled more significantly than the untreated coir films (Table 3) due to removing the lignin content and, impurities, and opening the pores, and changed pore size. Mahato DN et al.⁵⁸ investigated the increase in water content on alkali treatment of coir fiber. The percentage swelling is highest for PVA/PEG10/TC film than PVA/PEG20/TC and PVA/PEG30/TC films indicating the strong attraction of water molecules for -OH groups existing in the PVA/PEG10/TC film whereas PVA/PEG20/UC film resulted in the lowest percentage swelling which might be the result of excellent reactivity of PVA, PEG and untreated coir fiber.⁵⁹ It can be seen that the hydroxyl groups present in the PVA, PEG and untreated coir fiber were involved in the intermolecular hydrogen bonding, hence PVA/PEG/UC films led to less hydroxyl groups causing decreased percentage swelling than with the PVA/PEG/TC films.^60,61 Nazi et al.⁶² reported that the NaOH treatment of coir pith resulted in increase in water absorption rates versus raw coir pith.

Figure 7.

(a) Percentage swelling behaviour of the synthesized PVA/PEG/UC and PVA/PEG/TC films with various PEG loading, (b) Effect of treated coir, untreated coir and polyethylene glycol on contact angles of the synthesized composite films.

Table 3.

Contact angle and percentage swelling average, and standard deviation.

Code	Contact angle (°)		Time	Swelling (%)
	Average	Std dev	(sec)	Average	Std dev
PVA/PEG10/TC	35.9	2.02	20	59.5	5.03
			40	158.5	4.96
			60	239.7	4.80
			80	321.1	4.06
			100	402.3	4.05
			120	483.1	4.58
PVA/PEG20/TC	36.3	2.14	20	47.8	4.96
			40	111.3	4.82
			60	179.9	4.93
			80	265.6	4.95
			100	346.3	4.53
			120	428.9	4.28
PVA/PEG30/TC	43.5	2.03	20	35.3	4.51
			40	75.4	3.94
			60	125.5	3.85
			80	203.4	4.47
			100	275.2	4.28
			120	350.2	4.92
PVA/PEG10/UC	49.6	1.80	20	24.0	3.98
			40	45.5	4.80
			60	71.2	4.88
			80	90.3	4.79
			100	109.2	4.82
			120	127.9	4.19
PVA/PEG20/UC	59	2.48	20	21.5	3.18
			40	36.4	4.25
			60	51.0	4.76
			80	64.6	4.88
			100	79.4	5.23
			120	95.5	4.32
PVA/PEG30/UC	45	1.90	20	26.6	3.59
			40	50.5	4.50
			60	68.5	4.79
			80	93.8	4.71
			100	118.0	4.75
			120	142.7	4.85

Contact angle

Figure 7(b) shows the surface hydrophobic property of the composite films determined by the water contact angle measurement. The treated coir composite films contact angles are lower than untreated composite films due to destruction and lignin removal, the increased pores available after the coir fiber treatment of and also the multiple hydroxyl groups to interact with water molecules. The swelling test also confirmed this. According to Abdul Khalil et al.⁶³ lignin is present in coir in higher amount (43%–49%) than other components such as cellulose (32%–43%) and hemicellulose (10%–20%) etc. Rozman et al.⁶⁴ showed that the greater lignin in coconut fiber decreases the water absorption of the composites. Treating fibers with alkali reduces the composite hydrophilic nature. This is typically caused by the removal of lignin, hemicelluloses and the reaction of cellulose hydroxyl groups with NaOH, which lowers the fiber’s capacity to absorb water and thus increases the contact angle. This study also showed that alkali treated coir composite films resulted in a lower contact angle than the untreated coir composite films. Therefore, alkaline treatment shows two different effects: first, it increases the water sorption after lignin removal and second, it reduces water absorption by reacting the NaOH with OH- groups present in the cellulose. Hence, due to the higher lignin content than the cellulose in untreated coir, the PVA/PEG/UC films showed a decrease in percentage swelling (Figure 7(a)). Thus, the contact angles increased. Treating coir with alkaline removed the lignin and increased the water absorption⁶⁵ and accordingly, for its composites. Therefore, the contact angles for treated coir composite films were lower (Table 3).

Mechanical behaviour of synthesized films

Figure 8(a) illustrates the tensile strength (TS) and Figure 8(b) shows the elongation at break (EB). The mechanical properties are higher for treated coir fiber films than untreated coir fiber films up to 20 wt% PEG loading (see Figure 8). At 30 wt% PEG in PVA/PEG/UC and PVA/PEG/TC films the TS decreases due to poor interaction.⁶⁶ The highest TS (46.6 MPa) and EB (4%) was for PVA/PEG10/TC film due to better interlocking among the PVA, PEG and treated coir fiber which is higher than the PVA/PEG10/UC film. A slight decrease in TS from 33.7 MPa to 32.7 MPa and 46.6 MPa to 40.3 MPa was seen for PVA/PEG20/UC and PVA/PEG20/TC films compared to PVA/PEG10/UC and PVA/PEG10/TC films, while the EB increased from 2.5% to 3.3% and 4% to 4.3% was reported for the same (Table 4). Chemical compositions, mainly lignin and cellulose in the coir fiber greatly influence the composite’s film mechanical properties. Therefore, higher lignin content than the cellulose in untreated coir fiber makes it less hydrophilic than the treated coir due to lower -OH groups present in cellulose.^64,67 The lower hydrophilic nature of untreated coir fiber and the greater hydrophilic nature of the PVA/PEG matrix can be considered as two different phases, resulting in weak bonds.⁶⁸ Hence, the mechanical properties of the untreated coir PVA/PEG10/UC and PVA/PEG20/UC films are lower than treated coir PVA/PEG10/TC and PVA/PEG20/TC films.

Figure 8.

(a) Tensile strength, (b) Elongation at break of the synthesized PVA/PEG/UC and PVA/PEG/TC films.

Table 4.

Mechanical properties: Tensile strength and elongation at break average, and standard deviation.

Code	Tensile strength (MPa)		Elongation at break (%)
Code	Average	Std dev	Average	Std dev
PVA/PEG10/TC	46.6	1.11	4.0	0.25
PVA/PEG20/TC	40.3	0.95	4.3	0.28
PVA/PEG30/TC	19.8	1.13	3.5	0.26
PVA/PEG10/UC	33.7	1.10	2.5	0.24
PVA/PEG20/UC	32.7	1.03	3.3	0.27
PVA/PEG30/UC	29.6	1.10	5.7	0.25

Conclusions

In the present work, the biocomposite films were fabricated successfully with untreated, treated coir fiber and polyvinyl alcohol/polyethylene glycol blends via solution casting. XRD confirmed that the alkaline treatment of coir fiber greatly influences the crystallinity of the synthesized films. Hence, an increase in crystallinity by 81.5% with respect to untreated coir was observed after pretreating the coir fiber with an alkaline (5% NaOH) solution. Also, the PVA/PEG10/TC film resulted in highest crystallinity index. The FTIR spectra and SEM images showed significant cross-linking via hydrogen bonding between the coir fibers and polymer blends. Alkaline treatment effectively removed the coir fiber impurities, as seen in the SEM images. The contact angle and percentage swelling test confirmed that the hydrophobicity of the treated coir composite films decreased due to the lignin removal from coir fiber and the presence of hydrophilic PVA and PEG polymers. The mechanical characteristics showed that adding NaOH treated coir fiber with the PVA/PEG blend gave the higher tensile strength versus the untreated coir with the PVA/PEG blend films. Hence, adding treated coir to PVA/PEG10 blend increased the tensile strength 39% compared to untreated coir, which was noticeably more than reported by other published research. Consequently, a blend of PVA and PEG (up to 20 wt%) with added treated coir, facilitates the composite film synthesis with significant improvement in mechanical properties and characteristics.

Footnotes

Acknowledgments

The authors express sincere gratitude to the Department of Chemical Engineering & Technology, Indian Institute of Technology (BHU) Varanasi, India, for offering a laboratory facility to conduct the research work.

Author contributions

Vandna: methodology; data collection; material preparation; writing the original draft; conceptualization; V. L. Yadav: supervision; validation of results; reviewing and editing.

Declaration of conflicting interests

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

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Vijay Laxmi Yadav

Data availability statement

All data used during the study appear in the submitted manuscript.*

Biographies

Vandna, obtained B.Tech. in Chemical Engineering at Bundelkhand Institute of Engineering & Technology, Jhansi, 284128, UP, India, and M.Tech in Chemical Engineering at Indian Institute of Technology (BHU), Varanasi, 221005, UP, India, currently, she is a research scholar at Department of Chemical Engineering and Technology, Indian Institute of Technology (BHU), Varanasi, 221005, UP, India. She is interested in natural fiber-based polymer composites and polymer blends.

Dr. Vijay Laxmi Yadav, obtained her B.Tech., M.Tech and Ph.D. in Chemical Engineering at Indian Institute of Technology (BHU), Varanasi, 221005, UP, currently, she is a professor in the Department of Chemical Engineering and Technology, Indian Institute of Technology (BHU), Varanasi, 221005, UP, India, She is interested in Polymer Technology, Transfer Processes, Chem. Reaction Engg, Chemical Technology, Polymer composites, Utilization of waste, Wastewater treatment.

References

Terzioğlu

Güney

Parın

, et al. Biowaste orange peel incorporated chitosan/polyvinyl alcohol composite films for food packaging applications. Food Packag Shelf Life 2021; 30(October 2020): 100742.

Kabir

Wang

Lau

, et al. Chemical treatments on plant-based natural fibre reinforced polymer composites: an overview. Compos Part B Eng 2012; 43(7): 2883–2892. DOI: 10.1016/j.compositesb.2012.04.053.

Pappu

Pickering

Thakur

. Manufacturing and characterization of sustainable hybrid composites using sisal and hemp fibres as reinforcement of poly (lactic acid) via injection moulding. Ind Crops Prod 2019; 137(May): 260–269. DOI: 10.1016/j.indcrop.2019.05.040.

Sood

Dwivedi

. Effect of fiber treatment on flexural properties of natural fiber reinforced composites: a review. Egypt J Pet 2018; 27(4): 775–783. DOI: 10.1016/j.ejpe.2017.11.005.

Dixit

Joshi

Kumar

, et al. Novel hybrid structural biocomposites from alkali treated-date palm and coir fibers: morphology, thermal and mechanical properties. J Polym Environ 2020; 28(9): 2386–2392. DOI: 10.1007/s10924-020-01780-1.

Adeniyi

Onifade

Ighalo

, et al. A review of coir fiber reinforced polymer composites. Compos Part B Eng 2019; 176(July): 107305. DOI: 10.1016/j.compositesb.2019.107305.

Ali

Liu

Sou

, et al. Mechanical and dynamic properties of coconut fibre reinforced concrete. Construct Build Mater 2012; 30: 814–825. DOI: 10.1016/j.conbuildmat.2011.12.068.

Yan

Chouw

Huang

, et al. Effect of alkali treatment on microstructure and mechanical properties of coir fibres, coir fibre reinforced-polymer composites and reinforced-cementitious composites. Construct Build Mater 2016; 112: 168–182.

Obada

Kuburi

Dauda

, et al. Effect of variation in frequencies on the viscoelastic properties of coir and coconut husk powder reinforced polymer composites. J King Saud Univ - Eng Sci 2020; 32(2): 148–157. DOI: 10.1016/j.jksues.2018.10.001.

10.

Kong

Ting

Shang

, et al. Study of properties of coconut fibre reinforced poly (vinyl alcohol) as biodegradable composites. ARPN J Eng Appl Sci 2016; 11: 135–143.

11.

Verma

Gope

. The use of coir/coconut fibers as reinforcements in composites. Biofiber Reinforcements in Composite Materials. University of Toronto Canada.: Woodhead Publishing House, 2015, pp. 285–319.

12.

Yadav

. Nano-clay treatment of jute fiber for reinforcement in polymer composites: mechanical, morphological, and thermo-kinetic analysis. J Reinf Plast Compos. 2024.

13.

Dixit

Yadav

. Comparative study of polystyrene/chemically modified wheat straw composite for green packaging application. Polym Bull 2020; 77(3): 1307–1326. DOI: 10.1007/s00289-019-02804-0.

14.

Albuquerque

PBS

Malafaia

. Perspectives on the production, structural characteristics and potential applications of bioplastics derived from polyhydroxyalkanoates. Int J Biol Macromol 2018; 107(PartA): 615–625. DOI: 10.1016/j.ijbiomac.2017.09.026.

15.

Kalambettu

Damodaran

Dharmalingam

, et al. Evaluation of biodegradation of pineapple leaf fiber reinforced PVA composites. J Nat Fibers 2015; 12(1): 39–51.

16.

Getme

Patel

. A review: bio-fiber’s as reinforcement in composites of polylactic acid (PLA). Mater Today Proc 2019; 26: 2116–2122. DOI: 10.1016/j.matpr.2020.02.457.

17.

Melo

JDD

Carvalho

LFM

Medeiros

, et al. A biodegradable composite material based on polyhydroxybutyrate (PHB) and carnauba fibers. Compos Part B Eng 2012; 43(7): 2827–2835. DOI: 10.1016/j.compositesb.2012.04.046.

18.

. Physical properties and biodegradability of maleated-polycaprolactone/starch composite. Polym Degrad Stabil 2003; 80(1): 127–134.

19.

Cava

Giménez

Gavara

, et al. Comparative performance and barrier properties of biodegradable thermoplastics and nanobiocomposites versus PET for food packaging applications. J Plast Film Sheeting 2006; 22(4): 265–274.

20.

Araya

Esquivel

Jimenez

, et al. Antimicrobial activity and physicochemical characterization of thermoplastic films based on bitter cassava starch, nanocellulose and rosemary essential oil. J Plast Film Sheeting 2022; 38(1): 46–71.

21.

Suzuki

Ichihara

Yamada

, et al. Some characteristics of pseudomonas O–3 which utilizes polyvinyl alcohol. Agric Biol Chem 1973; 37(4): 747–756.

22.

Ullah

Sun

, et al. Polyvinyl alcohol degradation by Bacillus cereus RA23 from oil sludge sample. 3 Biotech 2019; 9(10): 350. DOI: 10.1007/s13205-019-1882-6.

23.

Vikman

Hulleman

SHD

Van Der Zee

, et al. Morphology and enzymatic degradation of thermoplastic starch-polycaprolactone blends. J Appl Polym Sci 1999; 74(11): 2594–2604.

24.

Bubpachat

Sombatsompop

Prapagdee

. Isolation and role of polylactic acid-degrading bacteria on degrading enzymes productions and PLA biodegradability at mesophilic conditions. Polym Degrad Stabil 2018; 152: 75–85. DOI: 10.1016/j.polymdegradstab.2018.03.023.

25.

Chiellini

Corti

D’Antone

, et al. Biodegradation of poly (vinyl alcohol) based materials. Prog Polym Sci 2003; 28: 963–1014.

26.

Xiao

Gao

, et al. Poly(vinyl alcohol) films reinforced with nanofibrillated cellulose (NFC) isolated from corn husk by high intensity ultrasonication. Carbohydr Polym 2015; 136: 1027–1034. DOI: 10.1016/j.carbpol.2015.09.115.

27.

Ibrahim

El-Zawawy

Nassar

. Synthesis and characterization of polyvinyl alcohol/nanospherical cellulose particle films. Carbohydr Polym 2010; 79(3): 694–699. DOI: 10.1016/j.carbpol.2009.09.030.

28.

Mandal

Chakrabarty

. Studies on the mechanical, thermal, morphological and barrier properties of nanocomposites based on poly(vinyl alcohol) and nanocellulose from sugarcane bagasse. J Ind Eng Chem 2014; 20(2): 462–473. DOI: 10.1016/j.jiec.2013.05.003.

29.

Asad

Saba

Asiri

, et al. Preparation and characterization of nanocomposite films from oil palm pulp nanocellulose/poly (Vinyl alcohol) by casting method. Carbohydr Polym 2018; 191(December 2017): 103–111. DOI: 10.1016/j.carbpol.2018.03.015.

30.

Tan

Zhang

, et al. High-Performance biocomposite polyvinyl alcohol (PVA) films modified with cellulose nanocrystals (CNCs), tannic acid (TA), and chitosan (CS) for food packaging. J Nanomater 2021; 2021: 1–9.

31.

Tan

Ching

Poh

, et al. A review of natural fiber reinforced poly(vinyl alcohol) based composites: application and opportunity. Polymers 2015; 7(11): 2205–2222.

32.

Feldman

. Poly(Vinyl alcohol) recent contributions to engineering and medicine. J Compos Sci 2020; 4(4): 175.

33.

Kittikorn

Chaiwong

Stromberg

, et al. Enhancement of interfacial adhesion and engineering properties of polyvinyl alcohol/polylactic acid laminate films filled with modified microfibrillated cellulose. J Plast Film Sheeting 2020; 36(4): 368–390.

34.

Guimarães

Botaro

Novack

, et al. High moisture strength of cassava starch/polyvinyl alcohol-compatible blends for the packaging and agricultural sectors. J Polym Res 2015; 22(10): 192.

35.

El-naggar

Heiba

Mohamed

, et al. Structural, linear and nonlinear optical properties of poly (vinyl alcohol) (PVA)/polyethylene glycol (PEG)/SnS2:Y nanocomposite films. Optik 2022; 258(March): 168941.

36.

Sharma

Beniwal

Toor

. The effect of rice straw derived microfibrillated cellulose as a reinforcing agent in starch/polyvinyl alcohol/polyethylene glycol biocompatible films. Mater Chem Phys 2022; 291(July): 126652.

37.

Falqi

Bin-Dahman

Hussain

, et al. Preparation of miscible PVA/PEG blends and effect of graphene concentration on thermal, crystallization, morphological, and mechanical properties of PVA/PEG (10wt%) blend. Int J Polym Sci 2018; 2018: 1–10.

38.

Faradilla

Lee

Sivakumar

, et al. Effect of polyethylene glycol (PEG) molecular weight and nanofillers on the properties of banana pseudostem nanocellulose films. Carbohydr Polym 2019; 205(October 2018): 330–339.

39.

Srivastava

Dixit

Pal

, et al. Effect of nanocellulose on mechanical and barrier properties of PVA–banana pseudostem fiber composite films. Environ Technol Innov 2021; 21: 101312. DOI: 10.1016/j.eti.2020.101312.

40.

Caliari

ÍP

Barbosa

MHP

Ferreira

, et al. Estimation of cellulose crystallinity of sugarcane biomass using near infrared spectroscopy and multivariate analysis methods. Carbohydr Polym 2017; 158: 20–28.

41.

Tsuchiya

Ifuku

Koyama

, et al. Development of regenerated silk films coated with fluorinated polypeptides to achieve high water repellency and biodegradability in seawater. Polym Degrad Stabil 2019; 160: 96–101.

42.

Olonade

Savastano Junior

. Performance evaluation of treated coconut fibre in cementitious system. SN Appl Sci 2023; 5(8): 218. DOI: 10.1007/s42452-023-05444-2.

43.

Ayeni

Mahamat

Bih

, et al. Effect of coir fiber reinforcement on properties of metakaolin-based geopolymer composite. Appl Sci 2022; 12(11): 5478.

44.

El-naggar

Heiba

Mohamed

, et al. Improving the optical characteristics of PVA/PVP/PEG blend via loading with nano SnS2/Y. J Mater Sci Mater Electron 2022; 33(16): 12783–12795. DOI: 10.1007/s10854-022-08224-7.

45.

Mohammed

El-Sayed

. PEG’s impact as a plasticizer on the PVA polymer’s structural, thermal, mechanical, optical, and dielectric characteristics. Opt Quant Electron 2023; 55(13): 1141. DOI: 10.1007/s11082-023-05420-5.

46.

Lian

. Effect of PEO on the network structure of PVA hydrogels prepared by freezing/thawing method. J Appl Polym Sci 2013; 128(5): 3325–3329.

47.

Liang

Tian

, et al. High-strength cellulose/poly(ethylene glycol) gels. ChemSusChem 2008; 1(6): 558–563.

48.

Nangia

Shukla

Sharma

. Frequency and temperature-dependent impedance spectroscopy of PVA/PEG polymer blend film. High Perform Polym 2018; 30(8): 918–926. DOI: 10.1177/0954008318774837.

49.

Shahid

SNAM

Niaa

. The effect of alkali treatment on tensile properties of coir/polypropylene biocomposite. IOP Conf Ser Mater Sci Eng 2018; 368(1): 012048.

50.

Kumar

Tanwar

Gupta

, et al. Pineapple peel extract incorporated poly(vinyl alcohol)-corn starch film for active food packaging: preparation, characterization and antioxidant activity. Int J Biol Macromol 2021; 187(July): 223–231.

51.

Rolim

Pieretti

Renó

DLS

, et al. Antimicrobial activity and cytotoxicity to tumor cells of nitric oxide donor and silver nanoparticles containing PVA/PEG films for topical applications. ACS Appl Mater Interfaces 2019; 11: 6589–6604.

52.

Liu

Chen

Liu

, et al. A novel poly (vinyl alcohol)/poly (ethylene glycol) scaffold for tissue engineering with a unique bimodal open-celled structure fabricated using supercritical fluid foaming. Sci Rep 2019; 9(1): 9534. DOI: 10.1038/s41598-019-46061-7.

53.

Kochkina

Butikova

. Effect of fibrous TiO2 filler on the structural, mechanical, barrier and optical characteristics of biodegradable maize starch/PVA composite films. Int J Biol Macromol 2019; 139: 431–439. DOI: 10.1016/j.ijbiomac.2019.07.213.

54.

Sajid

Azmami

Ahmadi

ZEL

, et al. Extraction and application of cellulose microfibers from Washingtonia palm as a reinforcement of starch film. J Plast Film Sheeting 2021; 37(4): 529–558.

55.

Rani

Ahamed

Deshmukh

. Dielectric and electromagnetic interference shielding properties of carbon black nanoparticles reinforced PVA/PEG blend nanocomposite films. Mater Res Express 2020; 7(6): 064008.

56.

Chen

Zong

Wang

, et al. Microfibrillated cellulose reinforced starch/polyvinyl alcohol antimicrobial active films with controlled release behavior of cinnamaldehyde. Carbohydr Polym 2021; 272(April): 118448. DOI: 10.1016/j.carbpol.2021.118448.

57.

Gulati

Lal

Diwan

, et al. Investigation of thermal, mechanical, morphological and optical properties of polyvinyl alcohol films reinforced with buddha coconut (Sterculia alata) leaf fiber. Int J Appl Eng Res 2019; 14(1): 170–179, https://www.ripublication.com

58.

Mahato

Mathur

Bhattacherjee

. Effect of alkali treatment on thermal stability and moisture retention of coir fibre. Indian J Fibre Text Res 1995; 20(4): 202–205.

59.

Priya

Gupta

Pathania

, et al. Synthesis, characterization and antibacterial activity of biodegradable starch/PVA composite films reinforced with cellulosic fibre. Carbohydr Polym 2014; 109: 171–179. DOI: 10.1016/j.carbpol.2014.03.044.

60.

Mittal

Chaudhary

. Experimental study on the water absorption and surface characteristics of alkali treated pineapple leaf fibre and coconut husk fibre. Int J Appl Eng Res 2018; 13(15): 12237–12243, https://www.ripublication.com

61.

Yang

Ching

Chuah

. Applications of lignocellulosic fibers and lignin in Bioplastics: A Review. Polymers. 2019; 11(5): 1–26. DOI: 10.3390/polym11050751.

62.

Nazi Mwambegu

Gnanamoorthy

. Water absorption in alkaline-treated coir pith – for use as reinforcement material in polymer matrix composites. Mater Today Proc. 2023. DOI: 10.1016/j.matpr.2023.04.235.

63.

Abdul Khalil

HPS

Bhat

Ireana Yusra

. Green composites from sustainable cellulose nanofibrils: a review. Carbohydr Polym 2012; 87(2): 963–979. DOI: 10.1016/j.carbpol.2011.08.078.

64.

Rozman

Tan

Kumar

, et al. Effect of lignin as a compatibilizer on the physical properties of coconut fiber-polypropylene composites. Eur Polym J 2000; 36(7): 1483–1494.

65.

Kocaman

Karaman

Gursoy

, et al. Chemical and plasma surface modification of lignocellulose coconut waste for the preparation of advanced biobased composite materials. Carbohydr Polym 2017; 159: 48–57. DOI: 10.1016/j.carbpol.2016.12.016.

66.

Ali

Yong

Ching

, et al. Effect of single and double stage chemically treated kenaf fibers on mechanical properties of polyvinyl alcohol film. Bioresources 2015; 10(1): 822–838.

67.

Onuaguluchi

Banthia

. Plant-based natural fibre reinforced cement composites: a review. Cem Concr Compos 2016; 68: 96–108. DOI: 10.1016/j.cemconcomp.2016.02.014.

68.

Martinelli

FRB

Ribeiro

FRC

Marvila

, et al. A review of the use of coconut fiber in cement composites. Polymers 2023; 15(5): 1309.