Effect of the addition of nanoscale cellulose fibres from bagasse on the characteristics of biofoam from avocado seed starch

. Starch-based bio-foam material, as an alternative to styrofoam, has shortcomings in mechanical properties and water resistance, so it needs a filler in the form of cellulose fibre, which is insoluble in water and has strong properties. It has been studied that nanoscale fibres have an excellent mechanical property. This study aims to determine the effect of adding cellulose fibre and cellulose nanofiber (CNF) from bagasse on the characteristics of biofoam made of avocado seed starch. The manufacture of biofoam is conducted over the thermopressing method. The added cellulose fibres varied from 0%, 1%, 3%, and 5% w/w, and cellulose nanofibers ranged from 3%, 5%, 10%, 15%, and 20% w/w by weight of starch The results of testing biofoam from two different types of fillers showed that adding 5% of both fillers produced biofoam with the most excellent properties. Biofoam with 5% cellulose fibres from bagasse has a tensile strength value of 382.32 KPa, a water absorption capacity of 11.08%, and can degrade 10.94% in a specified time of 8 days. Adding 5% CNF from bagasse produced biofoam with a tensile strength of 385.02 Kpa, water absorption of 5.96%, and biodegradability value of 17.74% within eight days. It can be summarised that nanoscale fibres can increase the water resistance, mechanical properties, and biodegradability value of biofoam made with avocado seed starch.


Introduction
The functional use of plastics not only makes human life convenient but also is harmful to the environment because of its hazardous waste [1].One of the ubiquitous plastics is styrofoam, which has a wide range of applications but also generates many waste problems because it cannot be degradable [2,3].These problems of styrofoam encourage interest in looking for alternative biobased and biodegradable materials to achieve sustainable growth in the plastic industry [4].One of the biodegradable materials as an alternative to styrofoam is biofoam, which microorganisms can degrade with ease [5,6].
Biofoam is made from starch raw materials that are readily decomposed naturally [7].Starch has appealing characteristics and relatively low prices, making it a high-potential material for biofoam [8].Starch production is economically viable due to its renewable sources, such as fruit seeds, which are mostly underused.The avocado seeds are considered industrial waste, potentially an alternative source for extracting starch [9,10].
This research uses Avocado seed starch, which is prospective as the raw material of biofoam due to its high starch content of 79.45%, consisting of 29.55% amylose and 49.90% amylopectin [11].High levels of amylopectin in starch can reduce starch solubility, giving advantage to the characteristics of biofoam produced [12].
Despite the advantages, starch by itself is relatively weak in terms of mechanical properties and water resistance [13].Thus, it is necessary to add filler, including cellulose fibres, which are insoluble in water, easily degraded, and can form strong fibres [14].Adding fibres will compact the structure of biofoam so that it will look sturdy and dense [15].Furthermore, cellulose fibres will reduce the water absorption of biofoam [16].
Natural cellulose fibres are plentiful and can be easily extracted from plants [17].Bagasse fibres are prospective as filler for biofoam because it is an underutilized waste residue from sugarcane processing [18].It contains 52.42% cellulose, 21.69% lignin, and 25.8% hemicellulose [19].This study uses bagasse fibres as a filler because it can be a clean production alternative.
Cellulose nanomaterials or cellulose nanofibers (CNF) have excellent physical and chemical properties compared to cellulose fibres.The elasticity and tensile strength of CNF is quite large, namely 130-150 GPa, in addition to its high specific surface area and stable mechanical structure [20].
In this study, cellulose nanofiber from bagasse, a nanofiller material, is incorporated into biofoam from avocado seed starch to enhance its characteristics.Nanofiller materials derived from biomass, such as cellulose nanofibers (CNF) materials, are suitable for the development of bio-based nanomaterials because of their low density, high aspect ratio, large surface area, non-toxicity, and, most importantly, biodegradability and environmental-friendly [21].

Materials
The materials used for starch extraction include avocado seed and 3000 ppm sodium metabisulfite.The materials for fibres extraction include bagasse, 12% sodium hydroxide, 10% hydrogen peroxide, and 64% sulfuric acid.The materials for biofoam manufacture include avocado seed starch, bagasse CNF, bagasse, distilled water, polyvinyl alcohol (PVA), and magnesium stearate.Bacteria EM-4 is used in this study for biodegradability test.

Starch extraction
Starch extraction was performed based on the method by Afif et al. [11].Avocado seeds are peeled, grated, and soaked in 3000 ppm sodium metabisulfite solution at neutral pH for 24 hours.The starch was extracted by crushing the raw material with 3000 ppm sodium metabisulfite solution in a blend until a slurry formed, then squeezed and filtered.The filtrate was precipitated for 24 hours.The precipitate was dried in an oven at 90°C for 6 hours to form starch powder, which was sieved through 60 mesh and stored in a clean container under refrigeration.

Fibres extraction
The extraction of cellulose fibres from bagasse was performed based on the method by Coniwanti et al. [6], which is modified as follows.Bagasse is soaked for one day, then washed and dried.Combing is carried out to remove corks that are stuck to the fibres.The bagasse fibres are ground and then sieved with a 60-mesh sieve.Delignification was done using 12% NaOH solution at a temperature of 90-95 o C for three hours, then rinsed until the pH was neutral.Then, bleaching was carried out using 10% H2O2 at a temperature of 80-90 o C for three hours, then rinsed until the pH was neutral.Bagasse cellulose fibres is dried in an oven at 55 o C for four hours.
The preparation of cellulose nanofibers (CNF) was not much different from the preparation of cellulose fibres from bagasse, with additional processes, namely hydrolysis and ultrasonication.Hydrolysis used 64% H2SO4 solution at 40 o C for 45 minutes, then rinsed until the pH was neutral.Then, ultrasonication was carried out at 65 o C for 95 minutes.Bagasse CNF was dried in an oven at 55 o C for four hours.

Manufacture of biofoam
The manufacture of biofoam begins by dissolving polyvinyl alcohol (PVA) with a PVA:water ratio of 1:3.Then raw materials, including avocado seed starch, cellulose fibres, and magnesium stearate as addictive according to the formulation, are mixed using a mixer with a rotation speed of 65 rpm, at a temperature of 100 o C for five minutes.The dough is put into the mold, then put into a thermopressing machine with an upper machine temperature of 160 o C and a lower machine temperature of 180 o C for 15 minutes.The formulation of biofoam was made by varying the mass of bagasse cellulose fibre by 0, 1, 3, and 5% w/w and varying the mass of bagasse CNF by 3, 5, 10, 15, and 20% w/w.

Characteristics test of biofoam
In this study, the characteristics of biofoam were analysed using Scanning Electron Microscopy (SEM), tensile strength test, water absorption test, and biodegradability test.The surface morphology of biofoam was analysed using Scanning Electron Microscopy (SEM) analysis.Tensile strength test using a Universal Testing Machine (UTM).The water absorption test was conducted using the ABNT NBR NM ISO 535 method.The biodegradability test was carried out by burying the sample in the soil for eight days.The soil was mixed with EM-4 bacteria to accelerate the rate of sample decomposition [22].

Avocado seed starch characteristics
In this study, the starch yield from the extraction of avocado seeds using sodium metabisulfite was 5.8%, which is not significantly different from those of Afif et al. [11].Starch consists of amylose and amylopectin, with different structures and functionality [9].The test results for starch, amylose and amylopectin content in avocado seed starch are shown in Table 1.The difference in starch content between this study and Afif et al. [11] is due to the difference in drying temperature of the avocado seed flour.Based on the study by Afif et al. [11], the drying temperature used was 50°C, whereas the drying temperature of 90°C was used in this study.The higher the drying temperature, the lower the avocado seed starch yield [10].

Fibres characteristics
This study's fillers of biofoam are bagasse cellulose fibres and cellulose nanofibers (CNF).Bagasse cellulose fibres were obtained through two stages: delignification and bleaching.On the other hand, cellulose nanofibers (CNF) were obtained from bagasse through multiple steps such as delignification, bleaching, hydrolysis, and sonication.The 12% NaOH delignification process aims to dissolve lignin into bagasse to facilitate the lignin-fibre separation [23].The bagasse cellulose obtained after the delignification process is still brown, indicating that the cellulose still contains residual lignin.A bleaching process with a 10% H2O2 solution was performed to remove residual lignin [24].
Using acid hydrolysis to extract bagasse CNF with 64% H2SO4 aims to break down the amorphous part of the cellulose chains and reduce fibre size [25].Ultrasound aims to stretch and break the cellulose chains so that the particle size distribution is perfected and smaller particle sizes are created [26].Surface morphology and size of bagasse cellulose nanofibers (CNF) were analysed using SEM as shown in Figure 1.Based on Figure 1, at 1000 times magnification, the morphology of bagasse CNF is rodlike and irregularly arranged, but forms lumps (agglomeration).The particle size of bagasse CNF ranged from 8.121 to 26.8 μm with an average particle size of 15.27 μm.Cellulose nanofibers are typically measured in nanometres (nm) with lengths of 10-100 nm and diameters of 1-100 nm.Nevertheless, the fibres obtained in this study still have the unique properties of microstructure materials, which have small cell sizes (1-100 μm) and high density (≥10 8 cells/cm 3 ) (1,2).This material can give an advantage to forming biofoam with high impact strength, low thermal conductivity, long fatigue life and good thermal stability [27].

Biofoam with filler of bagasse cellulose fibres
In this study, biofoam from avocado seed starch was made via the thermopressing method using a tool that can simultaneously press and heat the material [6].Polyvinyl alcohol (PVA) is added as a plasticizer to make the resulting biofoam more elastic.PVA has unique properties such as being soluble in water, non-toxic and can be degraded naturally [28].Magnesium stearate is also added to biofoam materials because it can form a hydrophobic film layer to prevent the biofoam created from adhering to the cast, thus enhancing the ability of biofoam to hold water.[15,29].
The number of fibres from bagasse is varied in this study to analyse the effect of the filler on the characteristics of biofoam.The resulting biofoam products in this study are brown and have a smooth texture on the top surfaces, while the bottom surfaces have rough and void textures, as shown in Figure 2. As shown in Figure 2, the more bagasse cellulose fibres added, the smoother the surface and the smaller the voids of the biofoam formed.This is due to the number of bonds between starch and bagasse fibres as filler increases, so the voids decrease.The effect of the level of bagasse fibres on the characteristics of biofoam was studied, including tensile strength, water absorption, and biodegradability, as shown in Figure 3.The mechanical properties, such as the tensile strength of biofoam, indicate how strong the biofoam is.The tensile strength testing was done because this biofoam will replace Styrofoam food packaging.Tensile strength is the breaking strength of a material calculated from the division of the maximum force a material can withstand by the cross-sectional area of the initial material [3].Based on Figure 3(a), the amount of bagasse fibres added is directly proportional to the tensile strength of the biofoam.This is due to the increased interaction between starch and fibres, which improves the mechanical properties of the biofoam.In addition, the cellulose fibres break down to form longer fibres that can bond with each other, resulting in higher tensile strength values [30].The highest tensile strength value is 382.32 kPa at biofoam with a 5% bagasse fibre composition.The obtained tensile strength values meet the standards compared to the commercial standards of Biofoam Synbra Technology.
The quality of biofoam can be influenced by its water absorption capacity.The higher the ability of biofoam to absorb water, the lower the quality of the biofoam because it is related to its durability when stored [11].This study conducted a water absorption test to determine the biofoam's resistance to water.Based on Figure 3(b), the concentration of bagasse fibres added to biofoam is inversely proportional to the water absorption capacity.The more concentration of bagasse fibres added, the lower the water absorption capacity of biofoam, which is similar to the result of Harunsyah et al. [3].These results follow the theory that the cellulose fibres characteristic, which is insoluble in water, will reduce the water absorption of biofoam [16].Besides, the more bagasse fibres added as filler, the denser the pores of the biofoam produced, which reduces the ability of the biofoam to absorb water.Adding 5% bagasse fibres produced a biofoam with a minimum water absorption value of 11.08%.This value meets the standard of water absorption for biofoam based on the Indonesian National Standard (SNI) of 26.12%.
A good characteristic of biofoam is its ability to degrade easily.The result of the biodegradability test in Figure 3(c) showed that the more bagasse fibres added, the lower the percentage of degraded biofoam.This is related to the characteristic of cellulose fibres, which are hydrophobic, resulting in microorganisms' difficulty in degrading the biofoam.The activity of these microorganisms requires water for metabolism, so the more bagasse fibres added, the lower the water absorption and slow down the degradation process in biofoam.In this study, adding a bagasse fibre composition of 5% resulted in the lowest value of biofoam biodegradability of 10.94% for eight days.In contrast, biofoam without adding bagasse fibres has the highest value of biodegradability of 14.71%.All values of biofoams meet the standard of Biofoam Synbra Technology.

Biofoam with filler of bagasse cellulose nanofibers
Just like biofoam with bagasse fibres, biofoam with bagasse cellulose nanofibers (CNF) as filler is brown but brighter.The results of biofoam with bagasse CNF are shown in Figure 4.The top surface of the biofoam with 3% bagasse CNF addition has a smooth but slightly brittle texture, and the bottom surface is slightly rough, has pores, and has a very brittle texture.On the other hand, the top surface of the biofoam with 5% bagasse CNF composition had a smooth texture with small pores, while the bottom surface had a rough texture with larger pores.
Based on Figure 4, as the amount of bagasse CNF added increased, the top and bottom textures of the biofoam became uneven.When 20% bagasse CNF was added, the biofoam cracked on the surface, forming holes in some places.The higher the number of cellulose nanofibers, the more hygroscopic the fibres, so the fibres will absorb the water for mixing the ingredients.This leads to uneven mixing of the materials so that the dough printed in thermopressing does not evenly expand and will create cracks and holes in the biofoam.Bagasse cellulose nanofibers addition on biofoam also varied in this study.The characteristics of biofoams with varied bagasse CNF are shown in Figure 5.The tensile strength of the biofoam increased with the addition of bagasse cellulose nanofibers (CNF) to 758.53 kPa with a composition of 15% bagasse CNF.The addition of bagasse CNF increases the cellulose content and tensile strength value [6].Furthermore, adding small-sized fillers increases the surface area and interactions between materials, thereby improving the mechanical properties of biofoam.However, adding 20% bagasse CNF decreased the tensile strength of the biofoam.Adding bagasse CNF as filler can increase tensile strength values, but when the filler composition exceeds the optimum point, the filler particles agglomerate, reducing tensile strength values.
The water absorption capacity of the biofoam increases when more bagasse CNF is added, in contrast with bagasse fibres (without nanoscale).The increase in water absorption value occurs because the bagasse CNF is hygroscopic, so more water can be absorbed by biofoam.However, the lack of bagasse CNF as filler also causes an increase in water absorption capacity.When adding 3% bagasse CNF, the water absorption value increases because the water content in the biofoam dough is high and makes the biofoam dough aqueous.As a result, excessive expansion occurs so that the resulting biofoam structure has many cavities and thin walls [30].
Figure 5(c) shows the correlation between the length of the burial time and the percent biodegradability of biofoam.The biodegradability of biofoam made with bagasse cellulose nanofibers (CNF) is higher than its counterpart without nanoscale.As seen in Figure 5(c), the biofoam with the highest level of biodegradation was found in biofoam with a 20% bagasse CNF of 23.78%, while the lowest level of biodegradation was with a 5% bagasse CNF of 17.74%.The more bagasse CNF added, the easier the biofoam to decompose, as shown in Figure 5(c).This is because bagasse CNF interacts more readily with water and microorganisms and is susceptible to physicochemical attack [31].In this research, the biofoam produced still meets commercial standards for Synbra Biofoam technology, where biofoam could be degraded within eight days, with the lowest biodegradation rate of 17.74%.Based on Synbra Technology's commercial standards, the biofoam will be degraded entirely within six weeks [32].

Biofoam morphology
Morphological tests were carried out to determine the morphological structure of the biofoam produced.Morphological tests were conducted on biofoam with bagasse cellulose fibres of 5% and cellulose nanofibers (CNF) of 5% using Scanning Electron Microscopy (SEM) with results shown in Figure 6.Biofoam with 5% bagasse CNF as filler produces smaller cavities than biofoam with 5% bagasse fibres.This happens because the bagasse CNF can be distributed uniformly so that good interphase bonds can be formed between the bagasse CNF and starch and PVA.Also, bagasse CNF can absorb water to produce an optimum biofoam dough.As a result, the amount of water from the biofoam dough decreases, and the holes formed decrease.

Conclusion
Biofoam from avocado seed starch was successfully made with bagasse cellulose fibres and cellulose nanofibers as filler.Biofoam, with the addition of 5% bagasse fibres, has the best characteristics, which include a tensile strength value obtained of 382.32 KPa, a water absorption capacity of 11.08%, and biodegradability of 10.94% within 8 eight days.The addition of bagasse cellulose nanofibers (CNF) affects the characteristics of biofoam, where the best characteristics were found in the addition of 5% CNF including a tensile strength value of 385.02 Kpa, water absorption capacity of 5.96%, and biodegradability of 17.74% within eight days.

Table 1 .
Chemical content of avocado seed starch.