Fabrication and Characterisation of Cross-linked Cellulose hydrogel from Residual papers
Hillary Kiprotich*, Esther W. Nthiga, Douglas O. Onyancha
Chemistry Department, Dedan Kimathi University of Technology, Private Bag – 10143,
Dedan Kimathi, Nyeri – Kenya.
*Corresponding Author E-mail: hillary.kipro998@gmail.com
ABSTRACT:
Comprising hydrophilic polymer chains that have undergone physical, chemical, or polymerization cross-linking, hydrogel is a three-dimensional network structure1-3. They inflate in the presence of water and have the ability to store and absorb enormous amounts of water in their polymeric networks.4-8 Nowadays, synthetic polymers like polyacrylic acid (PAA), polyacrylamide (PAAm), and polyvinyl alcohol (PVA) dominate hydrogel fabrication research and industry9,10.
Because these synthetic polymers aren't biodegradable and are derived from petrochemicals, they pose serious environmental concerns. Finding a sustainable substitute for the high-performance hydrogel production process that makes use of polysaccharides is therefore essential11,12. The compelling features of cellulose-based hydrogels such as biodegradability and availability, motivate researchers to create novel materials for use in a range of domains, including tissue engineering, wound dressing, drug delivery, smart polymer development, sanitary products, waste water treatment and agricultural advancement.4,13-17
Since office wastepaper contains 78–80% cellulose (w/w), it is a valuable source of cellulose for the creation of modified cellulose products such cellulose hydrogels18. Every year, a variety of waste papers are created in great quantities, and because customers dispose of them by land filling on the spot and burning it afterwards, this presents an intricate environmental problem19,20. Because recycling waste paper shortens the fibre length and lowers the quality of the paper compared to paper created from raw pulps, recycling waste paper using other methods is essential21. By recycling the waste paper for cellulose hydrogel synthesis, it promotes a sustainable waste management practice reducing the burden waste on landfills18. Not only is cellulose renewable, but it also has various other distinguishing qualities like as biodegradability, biocompatibility, and solubility in most solvents that scientists utilise in the manufacture of cellulose hydrogel22.
The hemicellulose and lignin cementing matrix reduces cellulose fibre accessibility and complicates the cellulose isolation process hence need to be eliminated in order to optimise the performance of extracted cellulose23. It has been shown that utilising two-stage chemical pre-treatments, the combination of an acidic and an alkaline pre-treatment is very effective in removing hemicellulose and lignin levels23,24. The process of forming a covalent connection between two or more molecules is known as cross-linking25. Chemical crosslinking is necessary to maintain the hydrogels' network structure when they are completely surged and provides the synthesised hydrogels with superior thermal, mechanical, chemical, and surface properties2,26,27. Cellulose is reacted with low and high molecular weight cross-linkers in turn to create cross-linked networks of relatively short and long chains. This process creates chemically cross-linked cellulose hydrogels28. Because it is inexpensive, highly reactive, easily processed, soluble in water, less poisonous, and has a sufficient crosslinking capability, glutaraldehyde is the preferred cross-linking agent29. Because of its capacity to covalently attach to several cellulose molecules to form a three-dimensional hydrophilic network, it is frequently used as a cross-linker for cellulose to provide resilience and dimensional stability. Its structure contains the di-functional group CCO23,30,31.
MATERIALS AND METHODS:
Reagents and Chemicals:
The purity of all the chemicals utilised in this study was 98.9%, implying they were all analytical grade. Hydrochloric acid, sodium hydroxide, urea, thiourea, carboxymethyl cellulose and glutaraldehyde were sourced from Sigma Aldrich (Kobian, Nairobi).
Collection and pre-treatment of waste papers:
White office printing papers were collected randomly at Dedan Kimathi University of Technology. The papers were sorted and categorized according to their type and quality. The papers were then shredded and soaked in water to remove water soluble inks and dirt in order to facilitate subsequent processing.
Preparation of the cellulose pulp:
Subsequently, to eliminate hemicellulose and leftover ink, 10g of the filtered material was treated with 12 weight percent NaOH while being continuously stirred for 24hours. The pre-treated sheets were then cleaned of excess NaOH using distilled water until a pH of neutral was achieved. To eliminate lignin, it was pre-treated once again with 3 weight percent HCl for two hours at 80°C. The cellulose fibres were then cleaned and dried for two hours at 105°C in an oven to produce cellulose pulp32.
Synthesis of cross-linked cellulose hydrogel:
A solution of NaOH/urea/thiourea (8: 6.5: 8, weight ratio) was prepared and precooled before use32,33. To create a self-standing cellulose hydrogel, 3g of pre-treated cellulose pulp was dissolved in the precooled NTU solvent for 1 hour while stirring. After adding 1.75g of carboxymethyl cellulose (CMC), the solution was stirred until all of the CMC had dissolved. After that, it was frozen for an entire night at -20°C to create an entirely independent hydrogel without the need for a cross-linker34. Then, for 25 minutes at room temperature, 5g of the synthesised uncross-linked cellulose hydrogel (UCH) was mixed with 5mL of acid-activated glutaraldehyde at 1800rpm until the mixture thickened into a gel. After that, in order to foster crosslinking and the creation of cross-linked cellulose hydrogels (CCH), the viscous gel was frozen for the whole night at -20 °C35. To remove excess glutaraldehyde, thiourea, urea, and NaOH, the resultant hydrogel was thoroughly washed with distilled water. It was then ground into a powder using a pestle and mortar after being oven dried for 24 hours at 60°C32,33.
Instrumentation:
The functional groups in the cellulose pulp, UCH and CCH, were identified using a Fourier Transform Infrared spectrometer (FT/IR-4700, JASCO, manufactured in Japan). The surface morphology of the hydrogels was examined using a scanning electron microscope, or FEI ESEM (Tescan Vega LMH).
FT-IR Characterization:
FT-IR analysis was performed to confirm that hemicellulose and lignin were successfully removed from waste papers in the cellulose pulp. It also determined how the addition of glutaraldehyde cross-linker affected the functional groups in the hydrogels. Infrared spectroscopy, when combined with the Attenuated Total Reflectance (ATR) methodology, is a versatile and non-invasive technique for obtaining the infrared spectrum that is suitable for the measurement of the infrared of solids with no sample preparation.
The sample is put in contact with the surface of an infrared transmitting crystal to begin the analysis. The infrared light is partially absorbed since it only enters the sample a short distance after reflecting off the crystal's interior surface. The sole prerequisite was to get the sample into close proximity to the crystal surface; no sample preparation was necessary. The samples' FT-IR spectra were acquired at a resolution of 4 cm−1 in the 4000 cm−1 to 400 cm−1 range using an Alpha Bruker Platinum-ATR spectrometer.
SEM Analysis:
The samples' surface characteristics were assessed using a scanning electron microscope FEI ESEM (Tescan Vega LMH). The dry SEM samples were affixed to individual specimen stub using adhesive carbon tape, ensuring secure and stable positioning and then placed on the sample holder and placed in the scanning electron microscope and analysed at 20Kv. In a scanning electron microscope (SEM), a high-energy electron beam is used to scan the material surface in a vacuum. The interaction between the electron beam and the sample produces a variety of signals, including X-rays, secondary electrons (SE), and backscattered electrons (BSE). These signals are useful in determining surface characteristics, chemical contrast and elemental composition of the sample.
Swelling ratio:
Distilled water was used to examine the hydrogels' swelling characteristics. After preparing 1g of dried UCH and CCH hydrogel samples, tea bags were filled with the samples, and 100mL of room-temperature distilled water was added. At the pre-determined times, the hydrogels' weights were noted. Equation 1 was used to determine the swelling percentage.
Wf – Wi
Swelling ratio (%) = ––––––––– × 100 ………………(1)
Wi
Where Wi denotes the hydrogel's beginning weight and Wf its weight upon inflating4,6,26,36.
RESULTS AND DISCUSSION:
Synthesis and characterisation of hydrogel:
Images of (a) wastepaper shreds, (b) extracted cellulose pulp, (c) uncross-linked cellulose hydrogels (UCH), and (d) cross-linked cellulose hydrogels (CCH) are presented in Figure 1.
For each step of the cellulose modification process, the images serve as a preliminary observation. The cellulose pulp extracted from waste papers was fluffy and white in colour. The self-standing UCH obtained from dissolution of cellulose pulp was 2.8g and translucent white in colour. Chemical cross-linking of the UCH with glutaraldehyde gave 3.5g CCH. The colour of the obtained CCH was translucent light yellow and it retained the same colour after oven drying and pulverization.
Figure 1: The images of (a) wastepaper shreds, (b) extracted cellulose pulp (c) uncross-linked cellulose hydrogels (UCH) and (d) cross-linked cellulose hydrogels (CCH)
During alkali hydrolysis of waste paper, NaOH breaks down the ester linkages and glycosidic bonds present in hemicellulose and the hydrogen bonds present between the hemicellulose and the cellulose fibres dissolving it out. Subsequently, acid hydrolysis with HCl creates an acidic environment that promotes the cleavage of bonds within the lignin structure causing it to be solubilized or converted into smaller fragments that can be washed away from the cellulose fibres resulting in a more pure white cellulose pulp32. In the presence of acidic conditions, glutaraldehyde undergo hydration to form geminal diol intermediates. These diols are more reactive and easily undergo subsequent reactions with nucleophilic groups such as hydroxyl (-OH) groups. A hemiacetal intermediate is formed by condensation processes during the crosslinking of UCH, where the hydroxyl groups of cellulose is nucleophilically attacked by the acid-activated glutaraldehyde. After that, this intermediate is dehydrated, which causes an acetal bond to develop between the cellulose chain and glutaraldehyde, giving the hydrogel a three-dimensional structure37. These crosslinks introduce structural integrity and stability to the cellulose hydrogel, enhancing its mechanical properties and responsible for the yellow colour of CCH38,39.
Figure 2: ATR-FT-IR Spectra of (A) wastepaper shreds and (B) cellulose pulp
Figure 3: ATR-FT-IR Spectra of (A) uncross-linked cellulose hydrogel (UCH) and (B) cross-linked cellulose hydrogel (CCH).
Figure 2 (A) and (B) show the FT-IR spectra of cellulose pulp and waste papers, respectively.
The broad bands at 3338 and 3346 cm−1 display the O–H stretching vibration of the hydrogen-bonded hydroxyl group in cellulose40-42. The absorption band at 2957 cm−1 and 2886 cm−1 in spectrum (A) is caused by the elongation of aromatic sp3-hybridized C–H bonds of hemicellulose and lignin present in the waste paper23,43. The O-H group's bending vibration can be observed by the strong transmittance peaks at 1641 cm−1 in (A) and 1624 cm−1 in (B), which result from the cellulose's absorption of moisture44,45. The bending vibrations of the OH groups are shown by the peaks at 1319 cm−1 in (A) and 1387 cm−1 in (B)46. The C-O-C stretching of the glycosidic rings is indicated by the peak values at 1156 cm-1, 1023 cm-1 in (A) and 1057 cm-1 in (B)47,48. The aromatic ring C=C found in lignin is indicated by the peaks in (A) at 1430 cm-1 and 1514 cm−1.49 Therefore, the absence of peaks at 2957, 2886, 1513 and 1430 cm−1 in spectrum (B) denotes successful removal of lignin and hemicellulose by double alkali and acid hydrolysis. As a result, the FTIR spectrum confirms that the obtained cellulose pulp is pure.
Figure 3 displays the ATR-FTIR spectra of (A) uncross-linked cellulose hydrogel (UCH) and (B) cross-linked cellulose hydrogel (CCH).
The wide peaks at 3321 and 3307 cm−1 were caused by the stretching vibration of the hydroxyl groups, which included intramolecular and intermolecular hydrogen bond vibration in the hydrogels4,50-52. The cellulose matrix's free hydroxyl groups were consumed by glutaraldehyde when it was added to create acetal linkages, which caused the absorption band of hydrogen-bonded O-H stretching vibrations in uncross-linked cellulose hydrogel (3326 cm-1) to shift to a lower frequency in cross-linked cellulose hydrogel (3307 cm−1). The hydroxyl groups found in the O-H bending vibrations of the hydrogels were associated with the absorption band seen at 1616 cm−1. The hydroxyl group is reduced during the cross-linking activity to generate acetal linkages, which accounts for the modest decrease in peak intensity at 1616 cm-1 for CCH53. At 1569 cm-1, the peak for CCH is caused by the asymmetric deformation modes of acetal linkages, or -COO-.54,55 The absorption band at 1107 cm-1 and 1126 cm-1 displays the C-O stretching vibration, which is associated with the C-O-C and C-OH in the pyranose rings of UCH and CCH. The CCH bands at 1107 cm-1 shifted to a higher frequency than the UCH bands (1126 cm−1) due to the O-C-O bond between GA and cellulose23. The changes in the peak intensities and shifts from the findings relative to existing literature offer insights into successful structural modifications induced by glutaraldehyde cross-linker into the cellulose matrix38,56-58.
SEM characterisation:
SEM micro-images of waste paper and cellulose pulp are presented in figure 4 (A) and (B)
SEM was used as an analytical method to describe the materials' morphological characteristics. Numerous images were captured using the secondary electron (SE) and backscattered electron (BSE) detector to show the surface topology and chemical contrast of different sections of the sample. A more intuitive investigation of the alterations in the surface morphology of the cellulose fibres after double alkali and acid hydrolysis treatment was made possible by the use of SEM to image the samples and produce micro-images of the fibres59. The observation made from surface characterization by use of the secondary electron detector indicates that the sample is made of fibres entangled together. The images from different sections of the samples show a similar fibrous topography of the sample surface. The cellulose fibres in the cellulose pulp appeared as long fibres with homogenous surface4. The hydrolysis of lignocellulosic microfibrils resulted in the cleavage of their amorphous portions, which formed crystalline mesh-like particles32,62,53,54.
Figure 4: SEM micro-images of (A) waste paper and (B) cellulose pulp
The SEM micro-images of uncross-linked cellulose hydrogel and cross-linked cellulose hydrogel are presented in figure 5 (A) and (B)
Figure 5: SEM micro-images of (A) uncross-linked cellulose hydrogel and (B) cross-linked cellulose hydrogel
The samples were imaged using a SEM to create micrographs of the fibres, which provided insight into the changes in the hydrogels' surface morphology following cross-linking with GA. Both samples have a surface morphology that resembles a sponge, with open channels mixed in with smoother connecting domains. This is most likely due to the crystalline nature of the samples63. The uncross-linked cellulose hydrogel sample exhibited significant porosity than cross-linked cellulose hydrogel. Porosity refers to the amount of empty space within a material and is a fraction of its total volume. Pore sizes influence how cells migrate and can even determine their orientation. Large pores weaken the overall structure of the hydrogel if there are not enough crosslinks holding it together hence UCH has low mechanical strength 55. Notably, the pores in the sample were predominantly in the nanoscale range but were unevenly distributed throughout. The smaller and evenly distributed pores in CCH is due to the crosslinks which form covalent bond between the layers of cellulose pulling the chains nearer to each other and counteract the repulsive force between the chains 64.
Figure 6 displays the SEM micro-images that illustrate the dimensions of the crosslinks in the cross-linked cellulose hydrogel.
Figure 6: SEM micro-images showing dimension of crosslinks in cross-linked cellulose hydrogel.
The cross-linked cellulose sample shows the presence of the crosslinks as depicted in figure 5. The CCH sample had crosslinks that were oriented in different directions and intertwined to form a mesh-like structure. This network is responsible for the material's mechanical qualities, including strength and flexibility65,66. There was homogeneity observed in the sample since the cross-linked cellulose fibres had the same pattern throughout the sample. The fibres were in the nanoscale as shown above.
Swelling properties:
The graph showing the swelling degree of each hydrogel is shown in figure 7.
Figure 7: Graph of swelling degree verses time for uncross-linked cellulose hydrogel (UCH) and cross-linked cellulose hydrogel (CCH) at the first 8 h.
The Flory-Rehner proposed theory, which suggests that the equilibrium volume of the cellulose hydrogel networks is dictated by the balance between the osmotic pressure and the elastic restorative force, may help to explain the swelling behaviour in the synthesised cellulose hydrogels4,67. The hydrogels expand as a result of the polar groups' complete hydration, which exposes the hydrophobic groups68. Furthermore, these exposed groups combine with water molecules to create water that is hydrophobically bonded69,70. Initially, the UCH starts absorbing water rapidly and swells rapidly than CCH due to its porous structure as evidenced in figure 5. The voids available allows easy penetration of water molecules into the cellulose network. As more water is absorbed into the matrix the UCH hydrogel become filled up limiting further absorption leading a significant plateau as evidenced in the graph between 4-6hrs. The maximal swelling capacity of the UCH hydrogel is reached after 7-8 hours, after which the structure's mechanical strength decreases and the hydrogel degrades, disintegrates, and dissolves in water71. The weak forces of van der Waals and hydrogen bond collapse are also the cause of this because of the free chain ends or chain loops, which leads to inhomogeneities or network defects72-75.
For the CCH, initially its swelling ratio is lower than that of UCH since its less porous and the presence of crosslinks prevent the mobility of water molecules but with the increase in time it swells steadily. The presence of crosslinks ensure enhanced structural integrity by providing mechanical strength to the cellulose network enabling it to withstand the osmotic pressure generated during swelling and this allows the CCH to absorb more water without disintegrating and dissolution in water leading to a higher swelling ratio with time9,76. Also the acetal linkages made by GA creates a three dimensional network that traps water molecules, preventing them from escaping the cellulose matrix allowing for greater water retention hence higher swelling capacity31. Cross linking causes a decrease in the hydrogel's crystalline behaviour by interfering with the chain orientation, resulting in an elastic hydrogel that, unlike UCH, can withstand excess water. This is the reason why the CCH never collapsed after 7-8 hours74. High swelling capacity cross-linked cellulose hydrogels have great promise for usage in controlled-release fertiliser applications, where they can increase fertiliser utilisation efficiency and conserve water4. Glutaraldehyde-cross-linked cellulose hydrogels had a greater swelling capability than CCH cross-linked hydrogels employing other cross-linking agents, according to literature comparison. According to studies, cross-linked cellulose hydrogels expand more slowly because water molecules are unable to diffuse into the matrix as easily, which is consistent with the data that was collected77-80.
CONCLUSION:
In conclusion, cellulose was successfully isolated from waste paste papers by double alkali and acid hydrolysis. By comparing the ATR FT-IR spectra of waste papers and cellulose pulp, the absence of peaks at 2957, 2886, 1513, and 1430 cm−1 in the pulp spectrum confirmed the absence of lignin and hemicellulose in the pulp. A self-standing cellulose hydrogel without a cross-linker was generated from cellulose pulp by dissolution using NTU solvent and CMC as a gelling agent. Cross-linked cellulose hydrogel was successfully synthesised from UCH by crosslinking with glutaraldehyde. This was evidenced by data interpreted from ATR FT-IR which confirmed the formation of acetal crosslinks. It was also evidenced by a decrease in porosities as shown by SEM micro-images. Cross-linked cellulose hydrogels exhibited higher swelling capacity which can be attributed to better mechanical strength and water absorption as compared to uncross-linked cellulose hydrogel. The cross-linked cellulose hydrogel exhibits favourable features and a great swelling capacity, which suggest its prospective applications in heavy metal removal, drug delivery, and for agricultural application as water reservoirs.
CONFLICT OF INTEREST:
None of the authors of this study have a conflict of interest.
ACKNOWLEDGEMENT:
The authors express their gratitude to Dedan Kimathi University of Technology (DeKUT) for their invaluable assistance with this research.
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Received on 26.06.2024 Modified on 23.07.2024
Accepted on 21.08.2024 ©AJRC All right reserved
Asian J. Research Chem. 2024; 17(4):209-216.