EFFECTS OF MICROCRYSTALLINE CELLULOSE ON SOME PERFORMANCE PROPERTIES OF CHITOSAN AEROGELS

The aim of this research was to investigate the effect of the microcrystalline cellulose reinforcement on some physical, mechanical, thermal, and morphological properties of the chitosan aerogels. The bio-based chitosan aerogels were produced using chitosan as a matrix and the microcrystalline cellulose as a reinforce material through the freeze-drying method. The aerogel suspensions were prepared in five different ratios to investigate the effect of microcrystalline cellulose content. The density, porosity, thermogravimetric analysis, and compressive resistance tests were conducted according to relevant standards. Morphological properties were investigated using a scanning electron microscope. The introduction of microcrystalline cellulose significantly improved the compressive resistance, thermal properties (Tonset and T%50) of the chitosan aerogels. The optimum performance properties determined as 0,12 MPa for compressive resistance, 0,27 MPa for compressive modulus, 292,45 °C for Tonset and 365 °C for T%50. According to scanning electron microscope images, aerogels showed microporous structure as expected. As a result, the bio-based chitosan aerogels reinforced with microcrystalline cellulose were successfully manufactured. The mechanical and thermal properties including compressive resistance, compressive modulus, Tonset and T%50 of chitosanmicrocrystalline cellulose aerogels found promising.


INTRODUCTION
The petroleum-based plastics; bottles, cups are carried to the oceans through the winds. These plastics chemically degrade once exposed to UV radiation. Also, the shear forces created by the waves break plastics down into tiny pieces. These tiny polystyrene based pieces were swallowed by fishes and other sea animals (Smith et al. 2018). Polystyrene, one of the major raw material used for manufacturing plastics, are not full recyclable and creates land-fill (Mathias et al. 2011). In this regard, bio-based polymers are getting more attractive for researchers and manufacturers as an alternative to polystyrene based products (Gomes et al. 2017).
Microcrystalline cellulose (MCC) is mostly used as a filler because of its outstanding properties such as high crystallinity, high strength, low thermal expansion coefficient, low cost, low density, biodegradability, and no environmental pollution after disposal (Chen et al. 2020). MCC can be produced from any material having a high content of cellulose by acid and alkali hydrolysis (Trache et al. 2016), ultrasonic and extrusion methods (Abdullah et al. 2016), steam explosion and radiation enzymatic process (Henriksson et al. 2007) in order to use in pharmaceutical, cosmetic, food, packaging and polymer composite industries. These reported methods are expensive as the energy consumption for the treatment is high in pressure processes like a steam explosion and pressure extrusion (Katakojwala and Mohan 2020). The production cost and less yield might adversely affect the MCC market price development (Sundarraj and Ranganathan 2018). MCC is available at the market price of ≈ 4 $/kg, which is comparable to or less than some materials like glass or aramid (Trache et al. 2016). In this study MCC preferred because of its high crystallinity, high availability, biodegradability and compara-ble price. The crystallinity of commercial MCC reported as 78 % by El-Sakhawy and Hassan (2007) and 85 % by Adel et al. (2011). The crystallinity affects the thermal stability of the final product as reported by Wang et al. (2017). There are also limited studies focusing on MCC based composites.
Chitosan as a bio-based polymer has many application areas including but not limited to biomedical, water purification, insulation, and packaging. However, its weak performance properties require to be reinforced with materials having higher thermal and mechanical properties. Many studies focused on blending chitosan with cellulose and its derivatives. For example: Wang et al. (2017) produced chitosan and nanofibrillated cellulose (NFC) based nanocomposite foam with different ratios by freeze-drying technique. The chitosan-NFC nanocomposite foams showed a highly efficient oil/water separation capacity. Olorunsola et al. (2017) evaluated chitosan microcrystalline cellulose blends as direct compression excipients. Stefanescu et al. (2012) produced blend membranes using chitosan and cellulose in trifluoroacetic acid solution. They reported that chitosan/cellulose membranes can be used for wound dressing as it has the potential to prevent wound based on antimicrobial capability against Escherichia coli and Staphylococcus aureus. Altuntas and Aydemir (2019) investigated effect of wood flour on the performance properties of poly (L-lactic acid)-chitosan biopolymer composites. They reported that wood flour improved all the mechanical and thermal properties of PLA-chitosan. Kim et al. (2011) produced bacterial cellulose (BC)-chitosan scaffolds composite by submerging wet bacterial cellulose pellicle in chitosan solution followed by freeze-drying. Bhandari et al. (2017) produced CNF aerogels by using freeze-drying method and introduced as new possible carriers for oral controlled drug delivery system. Mishra et al. (2017) produced and characterized CNF by using protein-based biomass. On the other hand, gelatin, starch and cellulose were extensively studied to produce bio-based materials for many applications. Demitri et al. (2014) developed sodium salt of carboxymethyl cellulose (CMCNa) and polyethylene glycol diacrylate (PEGDA700) based foams. They reported that foams with better mechanical properties and water absorption capacity can be produced using 20 % Poly (ethylene glycol) diacrylate (PEGDA). Mishra et al. (2019) studied toxicity-related issues of CNF. Czaja et al. (2007) used bacterial nanocellulose (BNC) membranes for severe second-degree burns. They found that BNC membranes healed the wounds faster than conventional wound dressings. Yildirim et al. (2014) produced biodegradable foam using cellulose nanofibrils (CNF) and industrial cornstarch by the freeze-dying technique. They reported that optimum performance properties were determined when 6 % starch + 1,5 % CNF added.
In this research, MCC reinforced chitosan aerogels were designed and manufactured using the freeze-drying technique. The aim of this research was to investigate the effect of the MCC reinforcement on some physical, mechanical, thermal, and morphological properties of the chitosan aerogels.

Design and manufacturing of aerogels
Medium molecular weight chitosan and microcrystalline cellulose (MCC) were used to manufacture biobased aerogels. The reinforcement was performed in five (5) different ratios as shown in Table 1.
The manufacturing method reported by Svagan et al. (2008) and Yildirim et al. (2014) was used with slight changes. For each individual test sample, 2 g chitosan was dissolved in 1% acetic acid-water solution using a magnetic stirrer at 41,8 rad/s for 2 hours. Then, the MCC was added to the suspension depending on pre-determined ratios and shear-mixed for another 30 minutes at 31,4 rad/s. The final suspensions were poured into the petri cups and located in the freezer at -80 °C for 24 hours. The frozen suspensions were sublimated under 800 mbar pressures for 72 hours using Thermo ModulyoD brand freeze-dryer. The demonstration of the manufacturing process is given in Figure 1.

Density determination
The six (6) specimens with 150 mm x 150 mm x 25 mm dimensions from each group were prepared for the measurements (ASTM C303-10e1 2016). The measured mass (m, kg) was divided to the measured volume (v, m 3 ) to calculate the density (Equation 1).

Porosity determination
The porosity (void fraction), the ratio of pore volume to the total volume of aerogels, was calculated using the porosimetry method (Gibson and Ashby 1997) as provided in Equation 2.

1
100 Where, I: porosity, P bulk: density of foam, P particle: density of particle

Morphological structure investigation
The morphological properties of the aerogels were investigated using JSM-7600F brand scanning electron microscopy (SEM). The samples were resized using a razor blade and pasted onto stubs. The gold-palladium (Au: Pd) sputter coating was applied to the samples and imaging was performed using an acceleration voltage of 15 kV.

Compression tests
A total of five (5) samples with 60 mm diameter and 30 mm thickness from each group were prepared and tested as shown in Figure 2. Each sample was compressed at a rate of 1,2 mm/min for 90 seconds (ASTM C165-07 2017). Compressive resistance (at 40 %) and compressive modulus (at 40 %) were calculated. The compression tests were conducted under laboratory conditions (25 ºC ± 2 °C temperature and 50 % relative humidity). The test samples were conditioned (20 ºC ± 2 °C temperature and 65 % relative humidity) to reach the constant moisture (hygroscopic equilibrium of 12 %) prior to testing.

Thermogravimetric analysis (TGA)
The total of five (5) samples from each group grinded into powders having total weight values vary between 6 mg to 10 mg. Once the samples prepared, the Thermogravimetric analysis was performed from 38 °C to 600 °C with a 10 °C increase per min. using PerkinElmer TGA 4000 Thermal Gravimetric Analyzer. The weight loss of samples as a function of temperature was recorded.

Statistical analysis
The density, porosity, compressive resistance and compression modulus data were compared by performing one-way Means/ANOVA to control if there was a significant difference (alpha = 0,01) or not. Significant differences among groups were calculated by using Tukey-Kreamer Honestly Significant Differences (HSD) test (alpha= 0,05).

RESULTS AND DISCUSSION
The results followed by discussions are provided under following subsections.

Density and porosity
The density and porosity results and their statistical comparisons are given in Table 2. Values put in the parentheses: The coefficient of variation. A, B, C, D letters indicate the significant differences between the groups.
The density of chitosan-MCC aerogels was determined to change between 30 kg/m 3 -110 kg/m 3 . There is a considerable effect of solid content on the final density ( Table 2). As expected; an increase in solid content within the same volume increased in the density. The addition of MCC decreased the porosity from 94,73 % to 62,30 %.

Morphology
The MCC SEM images and the particle size measurements were provided in Figure 3. The MCC particle sizes determined ranging between 45,8 µm to 257 µm ( Figure 3). As given in Figure 3, MCC is composed of aggregate bundles of multi-sized cellulose micro particles that are strongly bonded to each other. The morphological properties of chitosan-MCC aerogels were investigated to understand their microstructure (Figure 4). The aerogels microstructure showed porous structures interconnected through cell walls (sheets). As shown in Figure 4c

Compressive resistance and modulus
The compressive resistance and modulus of chitosan-MCC aerogels and relevant studies for comparison purposes are given in Table 3. Values put in the parentheses: The coefficient of variation. A, B, C, D letters indicate the significant differences between the groups. CNF: Cellulose nanofibril, CMF: Cellulose microfibril, MCC: Microcrystalline cellulose. I Yildirim et al. (2014), II Sehaqui et al. (2011), III Ali and Gibson (2013), IV Liu et al. (2017).
The addition of MCC increased the compressive resistance and compressive modulus. However, there are no statistical differences between the compressive resistance of Group 3 and Group 4. The highest compressive resistance obtained from Group 5. The lowest compressive resistance obtained from pure chitosan aerogels as expected.

Thermogravimetric analysis
The Thermogravimetric analysis of the chitosan-MCC aerogels are shown in Figure 5. According to TGA curve of chitosan aerogels (Group 1), the weight loss occurred at three steps with a maximum rate of weight loss at 341,3 °C. The onset of the peak at around 272,2 °C showed the starting point of decomposition. Typically, TGA curves of chitosan-MCC aerogels, the weight loss occurred at three steps with a maximum rate of weight loss at 365 °C. The degradation occurred between 200 °C and 400 °C. The first loss of weight of aerogels is due to the evaporation of adsorbed water. The introduction of MCC consistently increased T onset and T %50 of chitosan aerogels as shown in Table 4. The highest T onset (292,45 °C) and T %50 (365 °C) determined when 8 g MCC introduced (Table 4.).
Derivative (DTG) curves were obtained from TGA curve as a function of time. As shown in Figure 6, two peaks were recorded for chitosan-MCC aerogels as expected. The thermal properties of chitosan-MCC aerogels and other relevant studies are compared in Table 4.  Yildirim et al. (2014), II Petersson et al. (2007), III Kim et al. (2015) CNF: Cellulose nanofibril, CMF: Cellulose microfibril, MCC: Microcrystalline cellulose.
The results obtained from thermogravimetric analyses found comparable with relevant literature studies. The T %50 found between 341,3 °C and 365 °C. Wang et al. (2017) found the T %50 between 300 °C and 350 °C for chitosan-CNF foams. Yildirim et al. (2014) found the results between 300 °C and 335 °C for CNF-starch foams. Wang et al. (2017) found the first dTGA temperature between 300 °C and 359 °C for chitosan-CNF foams. Similarly, the first dTGA temperature of chitosan-MCC aerogels found between 293,99 °C and 316,18 °C. In the other study, performed by Lim et al. (2015) the onset temperature value was found 250 °C. The variety in the onset and decomposition temperatures can be explained with the different sources of the used materials.

CONCLUSIONS
This research focused on the design and development of bio-based aerogels. Due to chitosan's low performance properties, another biopolymer; MCC, was used as a reinforcing material. As a result, chitosan and MCC found miscible blends to an extent. The randomly dispersed MCC powders were clearly observed on the aerogels reinforced with 8 g MCC. The introduction of MCC significantly improved the compressive resistance, modulus and T onset , T %50 of the chitosan aerogels. However, increasing the MCC amount from 4 g to 6 g didn't change the compressive resistance and compression modulus of the aerogels. The optimum performance properties were determined when 8 g MCC introduced to the chitosan matrix. The aerogels produced from chitosan and MCC may have a wide range of applications, including as rigid packaging material. The use of biomaterials may reduce the use of petroleum-based products and reduce the landfill.