3.1 Characterization of GAP waste
A GAP waste used for this study was characterized by 73.92 % of cellulose content with hemicellulose and lignin content of 14.08 % and 12 %, respectively. Cellulose extracted using ultrasonic homogenization treatment was remained to the great extent (~80 % of yield) as compared to other chemical treatment by Hietala et al. (2018)and Liu et al. (2020) which produced 53.4 % and 95.17 % of cellulose, respectively. Meanwhile, the lignin content also decreased from 12 % - 4.67 % along the treatment. Hydrolysis and depolymerization of lignin and the weak hemicellulose-cellulose hydrogen bonding had release the celluloses fraction in the lignocelluloses. This ultrasonic irradiation with high energy and temperature generates cavitation which sufficient to loosen the texture of paper waste by dislocated the lignin fiber, hence, exposed the cellulose layer (Abdullah et al. 2016; Soontornchaiboon et al. 2016). Oliva et al. (2020) stated the effect of acid chemical which hydrolyze the cellulose fraction and facilitate rapid dissolution that cause damage to the fiber length. On the contrary, mechanical shear and cavitation resulting from the ultrasonic homogenization process which led to partial dissolution of paper waste pulp also disrupt the particle size of the fibers but did not severely occur. Kumari & Singh (2020) stated the effect of harsh pretreatment on the amorphous fraction of the cellulose producing residual cellulose with more crystalline properties that later affected the integration of cellulose as a filler with other composite materials. This shows the ability of ultrasonic homogenization treatment on GAP waste for high yield and cellulose recovery.
Table 2
ANOVA table of ultrasonic homogenizer treated paper waste on process responses (particle size, crystallinity and thermal properties) F-value
3.2 Optimization of ultrasonic-homogenizer process condition on particle size, crystallinity, and thermal properties of GAP waste
3.2.1 Regression coefficient and statistical analysis of the model
The experimental results by the Box Behnken design were assessed by ANOVA to determine the treatment of ultrasonic homogenizer effects towards the particle size, crystallinity, and good thermal properties of GAP waste. Regression coefficients of the predicted second-order polynomial models are summarized in Table 2. The statistically significant independent variables were considered in the probability p-value < 0.1, which the regressions were considered significant at 90%. The F-value of particle size, crystallinity, and temperature decomposition were 2.57, 8.04, and 3.24, respectively, which are significant, as there was only 11.34 %, 0.59 %, and 6.79 % (of each response) chance of model F-value could occur due to noise. The lack of fit (LoF) for all the dependent variables was insignificant (p > 0.05). The goodness of fit of the polynomial model was examined based on the coefficient of determination (R2) between 0.76 - 0.91, which is in good agreement with the experimental and predicted values. The lower coefficient of variation (CV) of 0.71-8.62 values suggested the relationship of ultrasonic homogenizer treatment of GAP waste and responses were well described by the model. The fitted model was further verified for its adequacy by plotting the normal probability plot distribution graph of particle size, crystallinity, and temperature decomposition as shown in Fig. 1. The residual positive probability distribution explained the imitative effects of the model surface as the closer the expansion rate towards the probability distribution, the higher reliability of the model in predicting the optimum parameter and responses during the treatment process (Yang et al. 2020). The statistical analysis results can be used for the prediction of the optimized parameters of the GAP waste cellulose production process.
3.2.2 Response surface analysis
The interaction of independent variables on particle size, crystallinity, and temperature decomposition of pulp GAP waste in a 3D response surface plot is illustrated in Fig. 2. The influence of temperature, ultrasonic homogenizer power or intensity, and treatment time of particle size (Fig. 2a - 2c) can be reflected from the 3D surface of the graph with the minimum value of the response showed the lowest particle size of the pulp GAP waste achieved after the treatment process. The particle size reduced significantly from 688 nm to 448 nm with the increased in temperature (75 °C), ultrasonic homogenizer power level of 70 %, and treatment time (1.5 h) implying that the effect of particle size depended on all the variables studied. Defibrillation of pulp GAP waste with the aid of ultrasonication process breaks the microfibrils into individual fibers produced uniformly dispersed nanosized fibers (Asem et al. 2021). Cellulose fiber dissolution was achieved through the energy generated by ultrasonication in which the hydrogen bond gradually disintegrated the aggregations of the cellulose fibers (Mohd Ishak et al. 2020).
Meanwhile, both temperature (p < 0.04) and treatment time (p < 0.01) are the most governing factors for the increment of crystallinity index (CrI) of the treated pulp GAP waste (p < 0.01) followed by ultrasonic power level factors (Fig. 2d - 2f). The CrI of treated pulp GAP waste is one of the important features influencing the mechanical and thermal properties. Increasing temperature and extending the ultrasonication time intensified the treatment and enhanced the disruption of the amorphous region, thus exposed the crystalline fraction of the treated pulp GAP waste. Increment in crystallinity from 57.23 % – 71.85 % was observed at 75 °C with 1.5 h time using 70 % of power level. The decreasing trend of CrI was observed at higher temperatures with extended ultrasonication treatment time. The CrI was reduced from 70.3 % to 56.8 % with the increase of temperature from 75 °C to 90 °C and sonication time up to 2 h (Fig. 2f). Temperature provides heat energy, which enhances the protonation reactions (Bello and Chimphango 2021) and facilitates the action of ultrasonic homogenizer in breakage of amorphous region for high crystallinity of pulp GAP waste. At longer treatment time, ultrasonication with high energy break the intermolecular hydrogen bonds of pulp GAP waste cellulose and become non-selective by removing both amorphous and crystalline regions causing the collapse of crystal structure that explained the reduction in CrI of treated pulp GAP waste.
The increase in treated pulp GAP waste thermal stability from 345.8 °C to 355.3 °C with increase in temperature to 75 °C and power level of 70 % was expected from ultrasonic pretreatment (Fig. 2g - 2i). The increase in thermal stability of the cellulose was directly proportional to the CrI values. As the high amorphous region was sensitive towards high-temperature degradation, the crystalline fraction was on contrary contributed to the greater thermal properties of the cellulose (Yacob et al. 2019; Qu et al. 2021). It was observed that the extent of thermal stability gradually increased with the treatment temperature from 60 °C to 75 °C and ultrasonic power of 60 % to 70 %. However, as the incubation time increased, the treated pulp GAP waste showed a reduction in its thermal properties, in which attributed to the depolymerization and decomposition of the glycosidic linkages of cellulose and the formation of char residue (Kassem et al. 2020). Within this range, the cellulose crystal was loosened, and rearrangement of cellulose crystallite occurred yielding a more compact crystal structure with higher decomposition temperature. Nevertheless, if the ultrasonic time was taken into consideration, a de-structuring process of cellulose of treated pulp GAP waste might take place at a longer treatment time thus will lower the crystallinity and lead to easier degradation of cellulose and observation of low degradation temperature (Mohd Ishak et al. 2020).
3.2.3 Validation of pretreatment condition
While keeping the independent variables (temperature, ultrasonic power, and treatment time) within the set range, the optimum ultrasonic homogenization treatment conditions were determined by minimizing the response of particle size and maximizing the crystallinity index and temperature decomposition of the pulp GAP waste (Table 3). The optimum value of the particle size, CrI, and temperature decomposition predicted by the numerical RSM model was 516.4 nm, 71.51 %, and 355.20 °C, respectively at 75.82 °C with 70.27 % of ultrasonic power level for 1.42 h of treatment conditions with a high desirable function of 0.84.
Table 3 Optimization parameter limit (actual and optimized) values and desirability towards responses (particle size, crystallinity, and thermal properties)
Variables
|
Target
|
Experimental limit
|
Optimum value
|
Desirability
|
Model
|
|
|
|
0.84
|
A: Temperature (°C)
|
In range
|
60-90
|
75.80
|
|
B: Power (%)
|
In range
|
60-80
|
70.27
|
|
C: Time (hour)
|
In range
|
1-2'
|
1.42
|
|
Response
|
|
Experimental range
|
Predicted
|
Actual
|
Particle size (nm)
|
Minimum
|
448.27 - 688.82
|
516.4
|
522.03
|
Crystallinity (%)
|
Maximum
|
55.48 - 73.02
|
71.51
|
71.76
|
Temperature decomposition (°C)
|
Maximum
|
343.64 - 356.54
|
355.19
|
355.47
|
A validation experiment was conducted using the predicted treatment conditions and the obtained results of 522.03 nm of particle size, 71.76 % of CrI, and decomposition temperature of 355.50 °C agreed with the value of RSM regression study. This suggested the reliability of the Box-Behnken experimental design in optimizing and understanding the individual and interaction effect of the parameters during the ultrasonic homogenization treatment process of paper waste.
3.3 Composite cellulose GAP waste reinforced starch (cGAP-st) film
Ultrasonic homogenizer paper waste comprises 70 % - 80 % of cellulose content with smaller particle size with acceptable values of crystalline and thermal resistance properties. However, owing to the reduction in fibers quality and hydrophilicity properties, its potential to be recycled is inevitably difficult. The adsorption of water from the environment due to hydrophilicity features decreases the efficiency of the cellulose barrier’s properties against water and gasses, especially for its application as food packaging film materials (Balasubramaniam et al. 2020). Therefore, the cellulose-GAP waste was modified using H3PO4 (cGAPH3PO4) hydrolysis and TEMPO (cGAPTEMPO) oxidation before being reinforced with starch (hereafter known as cGAPH3PO4-st and cGAPTEMPO-st) to improve the film hydrophobicity, strength, and quality. The dissolution mechanisms of paper waste cellulose in H3PO4 were initiated by the esterification reaction between H3PO4 and the hydroxyl group of cellulose in which the hydrolysis reaction occurred yielding the regenerated cellulose (Kassem et al. 2020). Incorporating and impregnation of cellulose with starch is a suitable approach owing to its similar structure and could attain poor mechanical, physical, and chemical properties of starch as a composite film for the packaging industry (Liu et al., 2021).
3.3.1 Surface characteristic and wettability analysis
The wettability properties of neat starch, cGAPH3PO4-st and cGAPTEMPO-st films were determined by contact angle analysis (Fig. 3). Wettability is the ability to withhold liquid on the surface of the composite films, which also determines the hydrophilicity properties of the films (Megashah et al. 2020). Cellulose-based coating films have the ability to absorb moisture and swell at relatively high humidity that influence the barrier performance as packaging materials (Nuruddin et al. 2021). The contact angle of the neat starch film was 72.2° and the contact angle of cGAPH3PO4-st and cGAPTEMPO-st films increased to 74.8° and 79°, respectively, corresponded to an improvement of 4 % -10 % (Fig. 3a).
Increased in the degree of contact angle indicates a reduction in hydrophilicity of both cGAPH3PO4-st and cGAPTEMPO-st films attributed to the action of phosphoryl and carboxyl groups during dissociation of H3PO4 hydrolysis and TEMPO oxidation process, which occupy a large number of hydroxyl group of the cellulose and starch structure. The hydrophobic backbones were exposed, and the hydrophilic hydrocarbon region of the film matrix was removed which was reflected by the increase in contact angle degree (Chowdhury 2019). The TEMPO-mediated oxidation process was targeted to separate the paper waste fibers and facilitate its mechanical disintegration (Kassab et al. 2020). The strong interfibrils interactions was favored by the oxidized groups with a few ·OH available (Chiulan et al. 2021). In addition, cellulose GAP waste might contain a trace amount of lignin that contributed to an increase in contact angle of the starch-cGAP network, as lignin is more hydrophobic than cellulose (Hietala et al. 2018). However, it did not exhibit a significant difference as compared to the number of hydroxyl group in the structure network.
The entanglement of neat starch, cGAPH3PO4-st and cGAPTEMPO-st were observed using SEM (Fig. 3b). A strongly bonded fiber with densely packed composite film was observed due to the close intertwining of the cellulose GAP fiber with the starch material (Megashah et al. 2020). In comparison with neat starch film, cGAPH3PO4-st and cGAPTEMPO-st film exhibited a flat and slightly smoother surface with fewer voids. The cGAPTEMPO-st film revealed micro and nano-size porous structures and bumpier surface as compared to cGAPH3PO4-st that are rather even and spotless. These occurrences may be due to mild process condition of cellulose treatment using H3PO4 as compared to TEMPO. Cellulose GAP waste with smaller diameter and chain length was generated after the H3PO4 hydrolysis and TEMPO oxidation process, which provided a better interaction among the cellulose fibers and the starch particles. The alteration on surface morphologies demonstrated that cGAPH3PO4-st and cGAPTEMPO-st were homogeneously incorporated within the starch matrix and provided more compact structure as compared to neat starch. Since there was no clear discern individual cellulose GAP waste, the length and diameter of the fiber structures were not measured during the SEM studies.
3.3.2 Mechanical properties
Fig. 4 demonstrates the mechanical properties of neat starch, cGAPH3PO4-st and cGAPTEMPO-st. The stress-strain curves exhibited an extraordinary combination of high tensile (71.32 MPa) and elongation (up to 13 %) for cGAPTEMPO-st film sheet which was higher than the corresponding value of neat starch and cGAPH3PO4-st.
Higher tensile strength was attributed to the surface modification of the hydroxyl group that increased the compatibility of the treated cGAP with the starch matrix (Takagi et al. 2013). The elongation-at-break and Young Modulus values were plotted in Fig. 4b and Fig. 4c. The cGAPTEMPO shows high elongation without losing the tensile strength associated to the highly disordered of the fibers that formed interfibril and intrafibril hydrogen bonds linkages that allowed efficient sliding during tensile analysis. However, a slight reduced value of Young Modulus obtained was caused by the slippage and porosity of the composite as pores will act as energy dissipators under applied tension (Chen et al. 2020). The cGAPH3PO4-st composite showed tensile strength of 35.48 MPa with the highest Young Modulus of 1.9 GPa and elongation-at-break of 1.93 %.
The decrease in tensile strength and elongation at break were due to the non-homogenous dispersion of cGAPH3PO4 in the starch matrix and the probability of agglomeration (Kassab et al. 2020). Despite reduction in tensile strength and elongation at break, the increment in Young Modulus value by more than 100 % exhibited the higher stiffness as compared to the neat starch film. Stiffness of the cellulose matrix was contributed by the interfibril hydrogen bond density (Wang et al. 2015). It can be seen from the dense and packed fiber of cGAPH3PO4 from the SEM images correlated to the high interfibril hydrogen linkages, explaining the higher Young Modulus value as compared to the more porous structure of neat starch and cGAPTEMPO. Application of cellulose as fillers demonstrated high reinforcing ability with other matrix due to the nanometric size of the cellulose particles, the functionalized surface modification by solvent, and the aspect ratio of the cellulose fiber, which enhanced the support and interfacial interaction between the polymer chains that significantly increased its mechanical properties (Kassab et al. 2020).
3.3.3 XRD, FTIR spectroscopy and thermal stability analysis
The neat starch, cGAPH3PO4-st and cGAPTEMPO-st films were evaluated for its crystallinity pattern using XRD analysis. As shown in Fig. 5a, the diffractograms at sharp peaks of 2q= 17.8, 22.7 showed a typical crystal lattice of cellulose II polymorph with CrI of St-CelPW-H3PO4 and St-CelPW-TEMPO were calculated as 59.6 % and 47.66 %, respectively. A reduction in CrI by 16 % - 33.5 % was observed after surface modification processes by H3PO4 and TEMPO owing to the removal of amorphous and partly of crystalline components due to the strong chemical reactions. Nigam et al. (2021) reported on high CrI was observed due to the removal of amorphous, which was regarded as hemicellulose and lignin structures during the treatment process. Compared with the original starch film, XRD pattern of the cGAPH3PO4-st and cGAPTEMPO-st revealed a localization change of the main diffraction peaks at 22.2°, corresponding to the crystallographic planes of cellulose II, which was a more stable structure. This was in agreement with Kassem et al. (2020) who observed a great impact of the solvent selection on CrI values. The sharp decreased may be due to the breakage of hydrogen bonding among the hydroxyl group of the cellulose and substituted by negatively charge phosphoric groups and carboxyl group moiety on the surface of the cellulose backbone (Joshi et al. 2019). Plus, acid hydrolysis and TEMPO oxidation process disrupted the intermolecular chain interactions, thus caused swelling of the crystalline region of cellulose in treated paper waste (Ramadoss and Muthukumar 2015). A low crystallinity signified an active and stabilized surface charge suspension of cGAPH3PO4-st and cGAPTEMPO-st, which led to a more homogenized mixture and facilitated uniform network of the starch-cGAP film.
In the FTIR analysis (Fig. 5b), the prominent peak at 3292 cm-1 was attributed to the stretching vibration of O-H hydrogen bond, whereas the two associated peaks at 2915 cm-1 and 2846 cm-1 indicated CH2 stretching vibration which is the distinguished features of cellulose (Ramadoss and Muthukumar 2015; Rosli et al. 2021). The peak absorption at 1649 cm-1 belonged to O-H stretching vibration of water, which characterized the hydrophilic features of the cellulose samples. Two intensified peaks (1538 cm-1 and 1543 cm-1) were attributed to the asymmetric stretching vibration of COOH groups that was only shown in cGAPTEMPO-st. A similar observation was reported by Qu et al. (2021) on the overlapping peaks during TEMPO oxidation and the O-H bending vibration of the absorbed water. The bands at 1259 cm-1, 1364 cm-1, 1366 cm-1, and 1433 cm-1 in the spectrum were related to the symmetric bending of CH2 in which the specific band between 1415 cm-1 - 1420 cm-1 was attributed to the crystalline structure of the cellulose (Joshi et al., 2019). The FTIR spectrum of two peaks at a shoulder of 1085 cm-1 showed −CH2−O−CH2− stretching of the methyl group characteristics and particularly prominent in cGAPTEMPO-st, implying methyl group presence from TEMPO oxidation process, whereas peak at 1056 cm-1 was assigned to a skeletal vibration of the C-O-C pyranose ring present in cellulose (Baruah et al. 2020). Cellulose characteristic band at a sharp peak at 992 cm-1 depicts the cellulose spectrum corresponding to β-glycosidic bond, between the sugar units in hemicellulose and cellulose (Ramadoss and Muthukumar 2015). Meanwhile, the peak at 900 cm-1 was assigned to the amorphous region of cellulose, was similarly reported by Poletto et al. (2014). No other derivational functional group appeared during the H3PO4 hydrolysis and TEMPO oxidation process of cellulose paper waste.
The TGA and DTG curves of the neat starch, cGAPH3PO4-st and cGAPTEMPO-st which determined the thermal properties of starch-cellulose films are presented in Fig. 5c. Generally, two phases of thermal degradation patterns were observed between 30 °C -150 °C that corresponded to the initial weight loss due to the water and other volatile organic matters evaporation that were contained in the starch-cellulose matrix. Then, it was followed by polymer degradation temperature at 250 °C - 400 °C, which attributed to the cellulose, hemicellulose, and lignin fraction (Kassab et al. 2020). The cGAPTEMPO-st showed a higher decomposition temperature (Tmax) of 313.5 °C and a maximum weight loss (63.31 %) as compared to the neat starch film (306.3 °C) and cGAPH3PO4-st (217.4 °C). Meanwhile, the rate of film decomposition showed in DTG peaks exhibited rapid decomposition of neat starch film (124.1 µg/min) as compared to cGAPH3PO4-st (105.6 °C) and cGAPTEMPO-st (110.9 µg/min) at a higher temperature. The Tmax of cGAPTEMPO-st was 11.7 %, slightly lower than cellulose-paper waste after ultrasonic homogenization process and it may be due to the crystallinity reduction, suggesting a greater loss in thermal stability of the cellulose. Also, hydrogen bond intermolecular chains breakage during TEMPO oxidation process and the OH group substitution of the cellulose with the carboxylic groups from the TEMPO catalyst are the possible causes of the decrease in decomposition temperature of the cGAPTEMPO-st. Meanwhile, cGAPH3PO4-st had a lower decomposition temperature with 48.2 % of weight loss as compared to the starch film control sample due to the functionalized surface. Dissolution of cGAP in the H3PO4 induced a rearrangement and reformation of its hydrogen bonding network with respect to grafted phosphate groups (Kassem et al. 2020). In addition, formation of higher char was observed and was due to the presence of phosphate groups characterized by its flame retardant properties due to its ability in char formation (El Achaby et al. 2018).
3.3.4 Transparency and UV shielding
Transparency of coated packaging materials originated from the nanofiber size homogeneity and degree of fibrillation between the cellulose paper waste with starch matrix components. Transparency of the film materials depends on the diffuse transmittance of wide and narrow-angle light scattering which plays a role in customer’s visual inspection of the products (Nuruddin et al. 2021). In Fig. 6a, the neat starch, cGAPH3PO4-st, and cGAPTEMPO-st films transmittance spectra were determined and the average percentage of transmittance in the UV-Vis region (UV A: 320-400 nm; UV B: 280-320 nm; and UV C: 190-280 nm). The neat starch film displayed a high transmittance in both visible light area (transmittance of 51.1 %) and UV-Vis region, UV A, UV B, and UV C of 46.9 %, 36.8 %, and 29.3 %, respectively. The cGAPTEMPO-st films with moderate transparency showed a decrease in transmittance (40.9 %) in visible light area and transmittance reduction of 32 % -38 % in the UV-Vis region. The TEMPO treated cGAP waste produced nano particle size, negatively charge groups, good dispersion, and better interfacial interaction with starch macromolecular chains, yielding better quality of transparent film as compared to cGAPH3PO4-st. The lowest transparency was observed in cGAPH3PO4-st with only 30 % of transmittance detected in the visible light area, whereas, the transmittance results of UV A, UV B, and UV C were 11.5 %, 4.4 %, and 1.3 %, respectively, due to agglomeration and poor dispersion of cGAPH3PO4 in the starch matrix (Kassab et al. 2020). All synthesized films displayed smoother surfaces and high clarity in the order of neat starch > cGAPTEMPO-st > cGAPH3PO4-st (Fig. 6b).
The TEMPO-oxidized treatment cellulose paper waste was attributed to smaller size with high homogeneity and led to a higher transparency of cGAPTEMPO-st films. Kaffashsaie et al. (2021) explored on the electro-statistical forces created by TEMPO oxidation process to produce elementary fibrils of individual and bundles form, thus produced better dispersion and avoided agglomeration in the cGAPTEMPO-st films. Meanwhile, the cGAPH3PO4-st film with poor transparency was attributed by the presence of residual ink of the paper waste that was incompletely removed by only using the H3PO4 hydrolysis treatment. A high transparency of the cGAPTEMPO-st film showed its positive effect and suitable candidate as a packaging application material. The above results indicated that the proposed process for cellulose extraction for packaging purposes is technically feasible. However, more studies on the economic perspective and environmental impact need to be carried out for better understanding and improvement, especially during manufacturing process.