Conversion of low-quality cotton to bioplastics

Extrapure DMAc (99%, A0403006) and anhydrous LiCl (99%, A0386841) were purchased from Acros Organics™ (NJ, USA). Glycerol (202397, certified ACS) and oleic acid (982133) were purchased from Fisher Scientific ™ (MA, USA). Low-quality cotton was collected from the Fiber and Biopolymer Research Institute (FBRI), Texas Tech University (Lubbock, TX, USA) and was scoured and beached following a general protocol reported in a previous study with a slight modification in caustic soda solution (concentration: 8%) (Abidi et al. 2007). The general scouring process uses 4% caustic soda, which could not completely clean low-quality cotton fibers with a high trash content. Therefore, the concentration of caustic soda was increased from 4 to 8% to remove the remaining trash content.

Cellulose films were conditioned in a controlled environment for 48 h (relative humidity of 65 ± 2% and temperature of  21 ± 1 °C) before characterization.

In this study, intact cotton fibers were dissolved using a protocol used to dissolve microcrystalline cellulose (Acharya et al. 2017). MCC is generally derived from wood. It has a low molecular weight and smaller particle size compared to cellulose derived from cotton fibers. As expected, this protocol did not successfully dissolve cotton fibers. At the end of 24 h, undissolved cotton fibers were still visible in the solution. Therefore, the dissolution was continued at 105 °C for an additional 12 h and was monitored using polarized light microscopy. Figure 1 shows polarized light microscopy images of 1% (w:v) cotton fibers in DMAc/LiCl solvent system at different times (0, 6, 9, and 12 h). According to these images, heating the solution at 105 °C improved fiber dissolution. After 12 h, only a few cotton fiber fragments were observed. Further heating at 105 °C led to discoloration of the solution.

Figure 2 shows the different steps followed during the conversion of cotton cellulose solution (1% w:v) into transparent and flexible films. After dissolution for an additional 12 h at 105 °C, the solution becomes viscous and clear (Fig. 2a). The regenerated cellulose hydrogel remains transparent and flexible (Fig. 2b). Glycerol plasticization does not cause drastic changes in hydrogels. However, plasticization with over 30% aqueous glycerol solution seems to affect the integrity of cellulose hydrogels. Cellulose hydrogels become increasingly fragile when the glycerol concentration exceeds 30%. Similarly, extending the plasticization period beyond 48 h also affects the integrity of hydrogels. These hydrogels tend to break during hot-pressing. The average thickness of cellulose hydrogels is approximately 2 mm. The physical and mechanical properties of dry cellulose films appear to be affected by plasticization and the drying technique. For example, cellulose hydrogels substantially shrink during air-drying and produce opaque, strong, nonflexible, and irregularly shaped materials (Fig. 2d).

Regenerated cellulose hydrogels also noticeably shrink during hot-pressing. The hot-pressed films show some degree of surface unevenness and stiffness but are transparent, smooth, and strong (Fig. 2e). Cellulose dissolution breaks hydrogen bonds that hold cellulose molecules together, and gelation and regeneration in deionized water allow hydrogen bonds to reform. As a result, cellulose chains aggregate and produce a hydrogel that often takes the shape of the mold that is used for casting the cellulose solution. Hot-pressing helps to stretch and rearrange cellulose chains and form a layered dense packing structure (Wang et al. 2013).

The use of a plasticizer improves the workability and flexibility of plastic films (Suyatma et al. 2005). The polymer–polymer interactions are reduced by the plasticizer; consequently, the rigidity or stiffness of the three-dimensional film structure is decreased. As a result, the plasticizer allows the deformation of bioplastics without rupture (Mekonnen et al. 2013). The plasticized and hot-pressed cellulose films exhibit improved flexibility and homogeneity compared to the properties of nonplasticized films (Fig. 2f). The plasticized and hot-pressed cellulose films are transparent and can be rolled without conspicuous breakage.

Figure 3a shows the FTIR spectra acquired from cotton fibers and nonplasticized cellulose films. Dissolution of cotton fibers followed by regeneration created major changes in the FTIR spectra. The characteristic peaks of native cellulose at 3334 and 3293 cm⁻¹, assigned to OH stretching vibrations, provided information regarding the intra- and intermolecular hydrogen bonds (Carrillo et al. 2004; Abidi et al. 2010a). These OH stretching vibrations in the spectra of the regenerated cellulose film became broader and shifted to a higher wavenumber (3350 cm⁻¹). This suggested that the crystalline structure of cotton cellulose was disrupted during the dissolution process, and therefore, the regenerated samples have an abundance of free hydroxyl groups (Ciolacu et al. 2011; Lan et al. 2011). These changes were also accompanied by a pronounced increase in peak intensities at 1640 cm⁻¹ and 897 cm⁻¹, which were attributed to O–H bending of adsorbed water and β-linkage of cellulose, respectively. This is because dissolution followed by regeneration increases the amorphous region of cellulose, which in turn facilitates water adsorption and accessibility of β-linkage by the IR beam (Ciolacu et al. 2011; Liyanage and Abidi 2019). Similar changes in the IR band at 897 cm⁻¹ have been reported due to the regeneration of native cellulose (Ciolacu et al. 2011; Dissanayake et al. 2019). In addition, the vibration at 1427 cm⁻¹ (CH₂ scissoring referred to as a crystalline absorption band of cellulose) became broad and shifted to 1439 cm⁻¹ in the regenerated film, suggesting the destruction of intramolecular hydrogen bonds and the crystalline structures of cellulose (Nelson and Mares 1965; Zhou et al. 2002; Zhang et al. 2002). In addition, the IR vibrations of native cellulose located at 1314, 1108, 1054, and 985 cm⁻¹, originating from C–H bending, asymmetric ring stretching, C–O stretching, and ring stretching mode, respectively, were not visible in the spectra of regenerated cellulose films. The disappearance of the absorption bands of native cellulose suggests alterations of the crystalline structure of cellulose (Ciolacu et al. 2011).

Figure 3b shows the FTIR spectra of hot-pressed cellulose films prepared from nonplasticized and glycerol plasticized hydrogels. The plasticized cellulose films show major IR bands originating from glycerol (e.g., 2934, 2880, 1415, 1157, 1030, 993, 922, and 851 cm⁻¹). In addition, some IR vibrations (e.g., 1108 and 897 cm⁻¹) assigned to cellulose disappeared when glycerol was applied. The OH stretching vibrations at 3284 cm⁻¹ became broad and intense in the plasticized cellulose films, implying that the intra- and intermolecular bonding of cellulose changed due to glycerol plasticization. The peak intensities of several IR bands seemed to change with increasing concentrations of glycerol (see supplementary materials-Fig. S3).

As discussed above, the changes in the peak intensities of several characteristic IR vibrations indicated the disruption of the crystalline structure of cellulose (Zhang et al. 2002; Oh et al. 2005; Gwon et al. 2010; Ciolacu et al. 2011). The peak intensity ratio of 1429/897 cm⁻¹ was termed the IR lateral order index (Hurtubise and Krassig 1960). It serves as empirical crystallinity index of cellulose (Abidi et al. 2014) and is positively correlated to the change in cellulose crystallinity (Liyanage and Abidi 2019). Due to dissolution and regeneration, the intensity of the vibration at 1429 cm⁻¹ decreased, and the intensity of the vibration at 897 cm⁻¹ increased. As a result, the IR lateral order index was significantly reduced from 2.28 ± 0.35 to 0.48 ± 0.03 (n = 6 and p ≤ 0.05).

Figure 4a shows the XRD patterns of cotton fibers and cellulose films plasticized with different concentrations of glycerol. The XRD pattern of cotton fiber showed typical diffraction peaks of cellulose I at 2θ = 14.7°, 16.5°, 22.7°, and 34.4° corresponding to crystal planes (hkl) of \(1\overline{1}0\), 110, 200, and 004, respectively (French 2014). In X-ray diffractograms of regenerated cellulose films, sharp crystalline peaks at 2θ values of approximately 14.7°, 16.5°, and 22.7° were absent, and instead, a broad and weak crystalline peak was observed around the 200 (hkl) plane. The XRD patterns of these cellulose films appear identical to those of amorphous cellulose structures (Ciolacu et al. 2011). XRD data were smoothed using PDXL software (Rigaku, Japan) and exported to PeakFit software; peak fitting was performed using Gaussian functions (Fig. 4b). Cotton fibers showed a 78% crystallinity index, and cellulose films showed much lower crystallinity indices that ranged between 32 and 42%. As expected, this change in crystallinity was attributed to the disruption of well-organized native cellulose microstructures followed by the development of more amorphous cellulose structures in bioplastic films. The presence of amorphous cellulose is highly advantageous for film flexibility and elongation. In addition, amorphous cellulose has more accessible OH groups that could serve as active sites for material functionalization to introduce new functionalities or to improve their mechanical properties.

TGA thermograms and first derivative thermogravimetry of cotton fiber and cellulose films plasticized with different concentrations of glycerol are shown in Fig. 5. TGA thermograms of nonplasticized cellulose films were identical to those of cotton fibers and showed two main weight loss regions (37–100 °C and 250–450 °C). In addition to these two weight loss regions that were attributed to the removal of adsorbed water and decomposition of cellulose (Abidi et al. 2008; Acharya et al. 2017), the films treated with glycerol showed an additional weight loss region between 100 and 250 °C. The weight loss between 100 and 250 °C was attributed to the decomposition of glycerol (thermogram of glycerol is shown in supplementary materials—Fig. S4). Regenerated cellulose films showed significantly higher weight loss between 37 and 100 °C (p ≤ 0.05) compared to cotton fibers (Fig. 5c). This is because cellulose films have more amorphous regions with free hydroxyl groups that can adsorb more water molecules though hydrogen bonding (Acharya et al. 2017). The effect of glycerol treatment on weight loss (%) was also significant, probably due to the hygroscopic nature of glycerol (p ≤ 0.05). However, there was no significant effect of glycerol concentration on the adsorbed water content of cellulose films (p ≤ 0.05).

The peak decomposition temperature of cotton fibers was observed at approximately 381 °C (Fig. 5d). All regenerated films showed significantly lower decomposition temperatures than cotton fibers (~ 345 °C) (p ≤ 0.05). This could be associated with the low crystallinity of cellulose films. Statistical analysis did not reveal a significant effect of glycerol concentration on the thermal decomposition of cellulose films. TGA thermograms of cellulose films prepared from low-quality cotton fibers appear identical to those prepared from pretreated MCC (Ma et al. 2011) and from wood and bamboo pulps (Wang et al. 2013). The hot-pressed cotton cellulose films showed comparable decomposition temperatures to those films prepared by hot-pressing wood and bamboo pulp hydrogels (Wang et al. 2013) and significantly higher degradation temperatures compared to the degradation temperatures of MCC composite films (294–304 °C) (Ma et al. 2011). High temperature tolerance is very important in the field of plastics and for different industrial applications.

Figure 6 shows the deformation recovery angles (DRAs) of cellulose films plasticized with different concentrations of cellulose. The DRA of nonplasticized cellulose film is approximately 18°, and it is significantly increased due to plasticization (p ≤ 0.05, n = 72). Recovery from deformation was significantly increased as the glycerol concentration increased up to 30%. Further increase in glycerol concentration did not significantly increase the DRA but affected the integrity of films and made films sticky with poor deformation recovery. Plasticizers create intermolecular hydrogen bonds with cellulose (Xiao et al. 2003), and the inter- and intramolecular hydrogen bonds of cellulose molecules become weaker at high concentrations of plasticizer (Hosokawa et al. 1990; Park et al. 1993; Xiao et al. 2003; Hongphruk and Aht-Ong 2010). The weaker polymer–polymer interaction and/or stickiness could impact the recovery from the deformation of cellulose films treated with > 30% glycerol. Therefore, 30% glycerol was selected as the optimal concentration of glycerol to produce flexible cellulose films from cotton fibers.

Cellulose itself is a hydrophilic polymer, but the incorporation of glycerol drastically increased the hydrophilicity of cellulose films. Therefore, during water adsorption experiment conducted in a relatively humid environment of 65 ± 2% humidity, plasticized cellulose films rapidly absorbed moisture from the environment compared to nonplasticized films (Fig. 7). After 24 h, glycerol-treated films showed approximately 40% weight gain, whereas nonplasticized films showed only ~ 12% weight gain. Therefore, it is highly desirable to impart hydrophobic characteristics to films to prevent adverse effects of water adsorption in humid environments.

Figure 8 shows the dynamic contact angles of nonplasticized and plasticized cellulose films grafted with different concentrations of oleic acid. Water droplets placed on nonplasticized cellulose film showed an approximately 80° contact angle (Fig. 8a). Grafting with oleic acid did not change the surface hydrophobicity of nonplasticized films (Fig. 8c, e). In contrast, water droplets quickly spread on the surface of plasticized cellulose films due to the increased hydrophilicity associated with glycerol treatment. As shown in Fig. 8b, the initial contact angle of the water droplet on the plasticized film was ~ 30°, and it immediately dropped within a few seconds. Therefore, surface modification with a hydrophobic agent is desirable to overcome the surface hydrophilicity created by glycerol treatment while preserving the flexibility of films. Surface grafting with oleic acid at concentrations ≤ 0.4 mol/L seemed to minimize the spread of water droplets and increase dynamic contact angles. A higher concentration of oleic acid (> 0.4 mol/l) did not increase water contact angles. Figure 8f shows the change in water droplets on plasticized cellulose films grafted with 0, 0.4, and 1 mol/L oleic acid. A high concentration of oleic acid resulted in the formation of homopolymers, and the second plasma treatment led to the formation of COOH functional groups, which increased the surface hydrophilicity. Therefore, the optimal oleic acid concentration for surface grafting of plasticized cellulose films was 0.4 mol/L. Using this concentration, highly flexible plasticized cellulose films (contact angle ~ 5°) can achieve reasonable surface hydrophobicity (contact angle ~ 80°). This contact angle is comparable with that of nonplasticized cellulose film and therefore suggests that surface grafting with 0.4 mol/L oleic acid can overcome the hydrophilicity introduced by glycerol.

Polymer materials undergo different rates and types of deformation when a stress is applied. This behavior is extremely important for industrial applications of these polymer materials. In the current study, a large number of cellulose films were prepared to determine their behavior under applied stress. Dumbbell-shaped test specimens were prepared using an ASTM D638-14 type IV cutting die and a manual clicker press to minimize inconsistencies introduced during specimen cutting (see supplementary materials—Fig. S5). Figure 9 shows representative stress vs. strain curves of nonplasticized and plasticized cellulose films. Table 1 shows the tensile strength and elongation at break, the required energy to break, and Young’s modulus of the test specimens and their standard deviations. The results indicate that the two types of cellulose films show major differences in their stress vs. strain curves. Nonplasticized films display significantly higher tensile strength (i.e., stress at break) (average = 45–48 kPa) compared to plasticized cellulose films (average 20–23 kPa). In contrast, nonplasticized cellulose films exhibit significantly lower elongation at break (average = 28–38%) compared to plasticized cellulose films (average = 69–77%). Compared to starch-based films, these films show moderate tensile strength and excellent elongation. Sultan and Johari reported that the banana peel films with 4% corn starch had an average tensile strength of 34.7 Pa (Sultan and Johari 2017). Jiménez et al. and Oluwasina et al. reported that plasticized starch films had a higher tensile strength (9.2 MPa, 1.14 MPa), but poor elongation at break (8%, 0.22%) compared to these cellulose films (Jiménez et al. 2012; Oluwasina et al. 2019). Abral et al. reported that tapioca starch bioplastic films had a higher tensile strength (~ 1 MPa) and comparable elongation at break (~ 77%) (Abral et al. 2018).

The toughness of the material is defined as the energy required to break, which is calculated using the area under the load vs. displacement curve. A tough material will not necessarily be the strongest material. A material with lower tensile strength and very high elongation can absorb a significant amount of energy before it reaches the breaking point. In this study, the average area under the stress vs. strain curves of nonplasticized and plasticized cellulose films ranged between approximately 1120–1470 and 790–990 N. mm, respectively. Given the variability in each set of samples, both types of cellulose films prepared from low-quality cotton fibers required a comparable amount of energy to break. Nonplasticized cellulose films have a significantly higher Young’s modulus (average = 1400–2300 kPa) than plasticized films (average = 80–90 kPa). This means that for the same strain, significantly greater stress is needed for nonplasticized cellulose films compared to that needed for plasticized films. Statistical analysis showed no significant difference in tensile characteristics of cellulose films from different batches (p ≤ 0.05). The variability within each replication could be associated with irregularities in gelation or inconsistencies introduced during hot pressing.