Fabrication of advanced aerogels from pineapple leaf fiber for oil spill cleaning, heat and sound insulation

VIETNAM NATIONAL UNIVERSITY – HO CHI MINH CITYHO CHI MINH CITY UNIVERSITY OF TECHNOLOGY ---DO NGUYEN HOANG NGA FABRICATION OF ADVANCED AEROGELS FROM PINEAPPLE LEAF FIBERS FOR OIL-SPILL

VIETNAM NATIONAL UNIVERSITY – HO CHI MINH CITY

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

-DO NGUYEN HOANG NGA

FABRICATION OF ADVANCED AEROGELS FROM PINEAPPLE LEAF FIBERS FOR OIL-SPILL CLEANING,

HEAT AND SOUND INSULATION

Major: Chemical Engineering

Code: 8520301

MASTER THESIS

Ho Chi Minh City, September 2020

This research was conducted at: Vietnam National University – Ho Chi Minh City University of Technology and National University of Singapore.

Supervisor 1: Assoc Prof Phung Kim Le

Supervisor 2: Dr Son Truong Nguyen

Reviewer 1: Assoc Prof Quyen Ngoc Tran

Reviewer 2: Dr Viet Tan Tran

The master thesis was defended at Vietnam National University – Ho Chi Minh City University of Technology, September 07th, 2020.

Members of examining committee including:

1 Assoc Prof Long Quang Nguyen

2 Assoc Prof Quyen Ngoc Tran

3 Dr Viet Tan Tran

4 Dr Phuong Thi Hong Pham

5 Dr Lien Thi Le Nguyen

Confirmation of the Examination Chairman and the Head of Faculty after the thesis has been corrected (if any).

Vietnam National University – Ho Chi Minh City SOCIALIST REPUBLIC OF VIETNAM

MASTER THESIS TASK DESCRIPTION

I THESIS TITLE:

FABRICATION OF ADVANCED AEROGELS FROM PINEAPPLE LEAF FIBERS FOR OIL-SPILL CLEANING, HEAT AND SOUND INSULATION

TASKS AND CONTENT:

1 Synthesis of cellulose aerogels from pineapple leaf fibers.

2 Investigation of physical and chemical properties of synthesized cellulose aerogels.

3 Evaluation of potential applications of cellulose aerogels in oil-spill treatment and insulation.

II DATE OF ASSIGNMENT: 10/02/2020

III DATE OF COMPLETION: 30/07/2020

IV SUPERVISORS:

1 Assoc Prof Phung Kim Le

2 Dr Son Truong Nguyen

Ho Chi Minh City, ……/……/2020

DEAN OF FACULTY

I would like to express my gratitude to everybody who has directly or indirectly helped me

in completing my dissertation work Although it is just my name on the cover, many peoplehave contributed to the research in their own particular way and for that, I want to give themspecial thank

First and foremost, I wish to express my heartfelt gratitude to my supervisors, Assoc Prof.Phung Kim Le (HCMUT), Assoc Prof Hai Minh Duong (NUS), and Dr Son Truong Nguyen(HCMUT), for their invaluable guidance and constant support throughout my time as theirmaster student I have been extremely lucky to have those supervisors who cared so much about

my work, and who responded to my questions, and queries so promptly

I would like to convey my appreciation to my fellow group members, Mr Duyen Khac Le,

Mr Quoc Ba Thai, Ms Thao Phuong Luu, and technical staffs in Materials Laboratory National University of Singapore for their devoted help and technical assistance

-I also wish to thank my family and my beloved for their understanding, encouragement, andsupport Without their help, it would not have been possible to complete my graduate work.Finally, I would like to acknowledge the support of time and facilities from Ho Chi MinhCity University of Technology (HCMUT), VNU-HCM for this study

Over twenty million tons of pineapples have been produced annually worldwide After harvestingthe fruits, the pineapple waste is mostly discarded to be decomposed or burnt In this thesis, pineapplefibers (PF) from the pineapple leaf waste have been recycled into novel eco-friendly and high-valueengineering aerogels for the first time by using polyvinyl alcohol (PVA) as a cross-linker and a cost-effective freeze-drying process The developed PF aerogels have highly porous structures withporosity of 96.98 – 98.85%, extremely low density of 0.013 – 0.033 g/cm3, and super-hydrophobicitywith water contact angle of approximately 140° after being modified with methyltrimethoxysilane(MTMS) They present very low thermal conductivity of 0.030 – 0.034 W/m.K, indicating their greatapplication in insulating fields Experimental results demonstrate that the hydrophobic PF aerogelscan quickly adsorb motor oil (5w30) up to 37.9 g/g in only 1 minute, approximately two times greaterthan commercial polypropylene and polyurethane sorbents For the oil adsorption kinetics of the PFaerogels, the pseudo-second order model can provide a good fit with regression value close to 1 Incomparison to commercial acoustic absorber Basmel®, the 2.0 wt.% PF aerogel with a thickness of

30 mm exhibits greater noise reduction coefficient of 0.52, indicating the competitiveness ofdeveloped aerogel with present product on the market

TÓM TẮT

Hơn 20 triệu tấn dứa được sản xuất hàng năm trên toàn cầu Sau mỗi vụ thu hoạch, phần lớn phụphẩm dứa được bị thải bỏ để tự phân hủy hoặc đốt trên đồng ruộng Trong luận văn này, sợi lá dứa(PF) từ phụ phẩm lá dứa lần đầu tiên được tái chế thành vật liệu aerogel thân thiện với môi trường và

có giá trị kỹ thuật cao bằng cách sử dụng polyvinyl alcohol (PVA) như một chất liên kết chéo và quytrình sấy thăng hoa tiết kiệm chi phí Vật liệu PF aerogel có cấu trúc rỗng xốp cao với độ rỗng từ96,98 đến 98,85%, khối lượng riêng siêu thấp từ 0,013 đến 0,033 g/cm3, và tính siêu kị nước với gócdính ướt khoảng 140° sau khi được biến tính với methyltrimethoxysilane (MTMS) Vật liệu có độdẫn nhiệt rất thấp khoảng 0,030 – 0,034 W/m.K, chứng tỏ tiềm năng ứng dụng vào lĩnh vực cáchnhiệt Kết quả thí nghiệm chứng minh rằng PF aerogel kị nước có thể hấp phụ dầu nhớt 5w30 rấtnhanh đến 37,9 g/g trong vòng 1 phút, gấp hai lần tấm hấp thụ thương mại polypropylene vàpolyurethane Về động học hấp phụ dầu của PF aerogel, mô hình giả bậc hai cho thấy sự tương thíchtốt nhất với giá trị hồi quy gần bằng 1 Khi so sánh với vật liệu cách âm thương mại Basmel®, PFaerogel chứa 2,0% sợi với bề dày 30 mm có giá trị của hệ số tiêu âm cao hơn đạt 0,52, chứng tỏ tínhcạnh tranh của vật liệu aerogel với sản phẩm hiện có trên thị trường

I hereby declare that this thesis is my original work and it has been written by me in its entirety I have duty acknowledged all the sources of information which have been used in the thesis.

This thesis has also not been submitted for any degree in any university previously.

Master student

Do Nguyen Hoang Nga

LIST OF ABBREVIATIONS i

LIST OF TABLES ii

LIST OF FIGURES iii

CHAPTER 1 INTRODUCTION 1

1.1 Situation of pineapple leaf waste in Vietnam 1

1.2 Oil-spill accident and current solutions 2

1.3 Noise pollution and acoustic insulation for construction 3

1.4 Food packaging and challenges 4

1.5 Aerogel 5

CHAPTER 2 LITERATURE REVIEW 8

2.1 Pineapple leaf fibers 8

2.2 Cellulose aerogel 10

2.2.1 Dispersion of cellulose 11

2.2.2 Gelation 12

2.2.3 Drying techniques 12

2.2.4 Modification of aerogels 15

2.3 Cellulose aerogel from agricultural waste 16

CHAPTER 3 EXPERIMENTS AND CHARACTERIZATION 22

3.1 Materials 22

3.2 Experimental techniques 22

3.2.1 Preparation of pineapple fiber 22

3.2.2 Preparation of PVA solution 23

3.2.3 Fabrication of the PF aerogels 23

3.2.4 Development of the hydrophobic PF aerogels 24

3.2.5 Fabrication steps of the thermal insulated jacket development 24

3.3 Characterization 26

3.3.1 Density and porosity 26

3.3.2 Surface-area, pore size and pore volume determination 26

3.3.3 Field emission scanning electron microscopy (FE-SEM) 26

3.3.4 X-ray diffraction (XRD) analysis 27

3.3.5 Compression test 27

3.3.6 Thermal conductivity 28

3.3.7 Thermal gravimetric analysis (TGA) 29

3.3.8 Water contact angle 29

3.3.9 Oil adsorption 30

3.3.10 Sound absorption 31

CHAPTER 4 RESULTS AND DISCUSSION 32

4.1 Characterization of developed PF aerogels 32

4.1.1 Morphologies and structure of the PF aerogels 32

4.1.2 X-ray diffraction (XRD) of the PF aerogels 34

4.1.3 FTIR spectroscopy of the PF aerogels 35

4.1.4 Water contact angle of the PF aerogels 35

4.1.5 Mechanical properties of the PF aerogels 36

4.2 Heat insulation of PF aerogels 39

4.2.1 Thermal conductivity 39

4.2.2 Heat insulation performance of PF aerogel-insulated jacket 40

4.2.3 Thermal stability 42

4.3 Acoustic insulation 43

4.4 Oil-spill cleaning up 44

4.4.1 Maximum oil adsorption capacity 44

4.4.2 Oil adsorption kinetics 45

4.4.3 Organic solvent adsorption 46

CHAPTER 5 CONCLUSION AND FUTURE WORK 48

5.1 Conclusion 48

5.2 Future work recommendations 48

REFERENCES 50

LIST OF PUBLICATIONS 63

APPENDIX 64

SHORT CURRICULUM VITAE 69

LIST OF ABBREVIATIONS

LIST OF TABLES

Table 1.1 A general summary of aerogel properties and potential applications 7

Table 2.1 Properties of natural cellulose aerogels from agricultural waste .11

Table 3.1 Diameter and length of pineapple fibers 22

Table 4.1 Morphologies summary of PF aerogels 33

Table 4.2 Crystalline and amorphous content of PF, MTMS-uncoated aerogel and MTMS coated PF aerogels 34

Table 4.3 Water contact angles of PF aerogels 36

Table 4.4 Young’s modulus of PF aerogels 38

Table 4.5 Summary of the maximum oil adsorption capacities and the adsorption rate constants of the hydrophobic PF aerogels using the pseudo first- and pseudo second-order models .45

LIST OF FIGURES

Fig 1.1 Pineapple leaf waste being burnt on-site 1Fig 1.2 Low-value engineering applications of pineapple waste such as fabrics (a), interiordecoration (b), animal feed (c), and vermicompost (d) .2Fig 1.3 An oil spill from the explosion of the Deepwater Horizon drilling rig in the Gulf of Mexico 2Fig 1.4 Physical remediation method including (a) boom, (b) skimmer, and (c) hydrocarbonabsorbers .3Fig 1.5 Common packaging for food delivery such as corrugated box, ice pack, and insulated bag(from left to right) .4Fig 1.6 Aerogel with different shape: (a) monolith, (b) particle, and (c) film 6Fig 2.1 Production of pineapple leaf fibers: sequential (a) plantation of pineapple, (b) fruit ofpineapple, (c) extraction of fibers from pineapple leaves, and (d) bunches of PF .8Fig 2.2 PF extraction by novel decortication 9Fig 2.3 Main structure of plant cell wall in lignocellulosic biomass containing lignin, hemicellulose,and cellulose 9Fig 2.4 Intramolecular (blue dots) and intermolecular (red dots) hydrogen bonding networks incellulose structure .10Fig 2.5 The primary mechanism for the dissolution of cellulose at the cellulose-solvent interface.(a) Cellulosic solid phase in contact with a solvent, (b) swelling of the solid phase, (c) point ofdisentanglement, (d) relocation of cellulosic chains from swelled phase to the solvent phase, and (e)advancement of solubilization front 12Fig 2.6 A typical phase diagram for two different drying techniques 13Fig 2.7 Processing of cellulose aerogel: (a,b) mechanism of cross-linking cellulosic chains, (b,c)freeze drying to obtain highly porous aerogel .13Fig 2.8 Comparison of aerogel fabrication strategies showing typical transitions into an aerogel 14Fig 2.9 Morphology of the recycled cellulose aerogel (a) before MTMS coating and (b) after MTMScoating 16Fig 2.10 Procedure for fabricating cellulose aerogels and carbon aerogels from corn bracts 17Fig 2.11 SEM images of cellulose aerogels (a, b and c) and carbon aerogels (d, e and f) from cornbracts .18Fig 2.12 The images (a and f) reveal the volume of carbon tetrachloride before and after filtration

in cylinder with the enlarged images The images (b–e) show the process of oil/water separation 18Fig 2.13 SEM images of cellulose aerogel from tea stem wastes .19

Fig 2.14 (a) The ultra-light weight sugarcane bagasse aerogel, (b) its very flexible nature, and (c)

the formation mechanism between sugarcane fibers and PVA crosslinker 20

Fig 2.15 SEM image on raw sugarcane bagasse (a), sugarcane fibers after treatment process (b), and 3D structure of sugarcane bagasse aerogel (c) 21

Fig 3.1 Microscope images of pineapple fibers (a) – before blending; (b) – after blending 23

Fig 3.2 Procedure of making PF aerogel from pineapple leaf fibers 23

Fig 3.3 MTMS modification of hydrophillic PF aerogels 24

Fig 3.4 Materials of (a) nylon, (b) cellulose aerogel, (c) neoprene are used for the insulated jacket design, and (d) the sandwich structure consists of neoprene as the outmost layer, the cellulose aerogel sheet within and nylon as the innermost layer .25

Fig 3.5 (a) Sketch of the insulated jacket and (b) final insulated jacket on the military canteen bottle 25

Fig 3.6 FE-SEM equipment with (a) FE-SEM Hitachi S4300 and (b) Cressington Sputter Coater 108 Auto 27

Fig 3.7 Instron 5500 Microtester 28

Fig 3.8 TCi Thermal Conductivity Analyzer 28

Fig 3.9 DTG60H thermogravimetric analyzer 29

Fig 3.10 VCA Optima goniometer 30

Fig 3.11 BSWA-Impedance Tubes for Acoustic Measurements 31

Fig 4.1 (a) Recycled PF from the pineapple leave waste, (b) well-controlled shape of the developed PF aerogel and (c) the flexible large-size (A4 size) PF aerogel 32

Fig 4.2 SEM images of PF aerogels with increasing PF concentration (x80 magnification) 33

Fig 4.3 SEM images of PF aerogels with increasing PVA concentration (x80 magnification) 33

Fig 4.4 XRD patterns of the PF, MTMS-uncoated aerogel and MTMS-coated PF aerogels 34

Fig 4.5 FTIR spectra of MTMS-uncoated and MTMS-coated PF aerogels 35

Fig 4.6 Water contact angle on external surface and cross-sectional area with (a) increasing PF concentration and (b) increasing PVA concentration .37

Fig 4.7 Stress and strain curve of PF aerogels with (a) increasing PF concentration and (b) increasing PVA concentration 38

Fig 4.8 Thermal conductivity of the PF aerogels with (a) increasing PF concentration and (b) increasing PVA concentration .40

Fig 4.9 Heat insulation performance of PF aerogel-insulated bottles and uninsulated bottles during 6 hours testing with (a) ice slurry and (b) hot water .41

Fig 4.10 Thermal stability of the PF aerogels having various PF concentrations with (a) TGA and (b) DTA curves .42

Fig 4.11 Sound absorption performance of the PF aerogels (a) The acoustic insulation of the PFaerogels having different PF concentrations of 0.5 wt.%, 1.0 wt.% and 2.0 wt.% compared to that ofcommercial acoustic foam Basmel with the same thickness of 30 mm, (b) the effect of variousthickness on sound absorption of the PF aerogels having the same 2.0 wt.% PF concentration 44Fig 4.12 Oil adsorption kinetics of the MTMS-coated PF aerogels with (a) increasing PFconcentration and (b) increasing PVA concentration .45Fig 4.13 Various organic solvent adsorption capacities of the MTMS-coated aerogels having thesame 0.5 wt.% of the PF .46

The explosion of several updated technologies is opening a new era of industrial revolution 4.0.Following to the global trend, new materials with high performance and environmental friendlinessare being the topic which scientists and investors are very interested in In particular, aerogel known

as the lightest solid has been considered as a great potential material of the 21stcentury with superiorproperties such as ultra-low density, high porosity of nearly 99%, and large specific area As results,aerogel is being considered to replace traditional materials because of their practical applications such

as heat and sound insulation for construction; adsorption of oil, de-colorant, dye, and heavy metals inwastewater; active carriers in drug delivery; super-capacitors in electronics

Vietnam is an agricultural developing country with export turnover reaching more than 50 billionUSD However, the process of harvesting agricultural products on-site generates a large number ofby-products which are rich in biomass content This agricultural waste not only is not completelytreated, but also produces toxic compounds like CO2, CO, CH4and fine dust into the air when beingburnt directly on the field after harvest Thus, it pollutes the habitat of surrounding species like humanbeings and animals To the best of my knowledge, agricultural residues could be a natural resource

of useful organic ingredients such as bioactive chemicals like polyphenols, saponins, enzymes; humusfor fertilizer; high cellulose content for textile industry, etc

Therefore, the use of abundant agricultural by-products as materials for aerogel production isurgent now, in line with the trend of high-performance but eco-friendly materials The thesisFabrication of advanced aerogels from pineapple leaf fibers for oil-spill cleaning, heat and soundinsulation develops the process of converting pineapple leaf fibers into aerogels with effective heatand sound insulation, and oil adsorption

CHAPTER 1 INTRODUCTION

1.1 Situation of pineapple leaf waste in Vietnam

Agriculture is an important and indispensable economic sector in Vietnam With a GDP growthrate of 3.76% in 2018, Vietnam’s export value reached 40.02 billion USD – the highest value as of

2018 Therein, agricultural products are estimated 19.51 billion USD [1] In addition to the growth infarming, total annual biomass production can reach between 8 and 11 million tons In fact, among themajor crops of Vietnam, pineapple generates a huge number of by-products as 41,000 hectares ofcultivated area produced 567,100 tons of pineapple in 2017 In which, the pineapple residuesincluding unused skin, leaves, seeds, and meat account of 50% of the total weight of pineappleharvested [2] It is calculated that 1 hectare of pineapple destroyed for replanting after two harvestleaves nearly 50 tons of waste [3] At present, pineapple waste is mostly being turned into composts,landfilled or burnt [4-6] Crop waste burning in open air is a major source of air pollution, releasingpollutants such as incomplete combustion products including carbon monoxide, particulate matter,and polycyclic aromatic hydrocarbons into the environment (Fig 1.1) [6] These toxic chemicalscause health hazards to all living things as well as contribute to global warming [4, 7, 8]

Fig 1.1 Pineapple leaf waste being burnt on-site

However, value-added products could be produced from pineapple leaves: biodegradable fibersextracted from pineapple leaves can be used as fabrics in textile industry [9], or as mechanicalreinforcement in polymer composites such as biodegradable plastics and natural rubber [10], due tothe fibers’ good ultimate tensile strength (362 – 748 MN/m2), initial modulus (25 – 36 GN/m2), andelongation before breakage (2.0 – 2.8%) [11] Moreover, pineapple waste could be utilized to makeearthworm vermicompost, produce bioactive compounds (bromelain, citric acid, cysteine proteases),and animal feed (Fig 1.2) [5, 12] Toward a sustainable agricultural waste management, it ismandatory that agro-residues should be recycled and further processed to produce high-valueengineering products

Fig 1.2 Low-value engineering applications of pineapple waste such as fabrics (a), interior decoration (b),

animal feed (c), and vermicompost (d)

1.2 Oil-spill accident and current solutions

Today, it is undeniable that oil is very important for vehicles, machinery, equipment, etc In spite

of that, transportation and storage of oil might be a threat to the environment owing to oil-spillaccidents Since 1967, there have been over 10 oil pollutions with over 100,000 tons of oil spilled allover the world One of the biggest oil spills witnessed in history was the Deepwater Horizon disasterwhere about 4.4 million barrels of oil were spilled out across the ocean (Fig 1.3) [13] The resultedimpacts has seen on the environment as well as human health such as loss of habitat or shelter,deforestation, skin and eye irritation [14] A high concentration of low boiling saturated hydrocarbons

in oil spills can cause anesthesia and narcosis or even death of marine animals [15]

Fig 1.3 An oil spill from the explosion of the Deepwater Horizon drilling rig in the Gulf of Mexico

Some major technologies have been applied to deal with oil pollution including physical (booms,skimmers, and absorbents), chemical (dispersants, solidifiers), thermal (burning), and bio-remediation methods [16] Each cleanup method has pros and cons, however, as most of the oil spilloccurred during sea transportation, it is vital that oil is collected rapidly to prevent its spreading toother areas due to convection or diffusion Thus, physical absorption method, for example usinghydrophobic or oleophilic absorbents, is considered as an effective and low-cost solution (Fig 1.4)[17] Interesting, absorbed oil can be easily recovered by squeezing, compression, or distillationmethods [18-20].

Fig 1.4 Physical remediation method including (a) boom, (b) skimmer, and (c) hydrocarbon absorbers

1.3 Noise pollution and acoustic insulation for construction

Living in a highly urban society, our environment is adversely affected by pollutions of manykinds, air pollution, water pollution, soil contamination, noise pollution, to name a few In which,noise pollution has become a serious threat in Vietnam According to a study conducted by theInstitute of Occupational Health and the Environment in 2018, about 10 – 15 million people working

in Vietnam have to deal with excessive noise [21] The research investigated noise levels on 12 mainstreets and junctions in Hanoi The results showed that an average value could reach to 78 decibelsduring the day and exceed limits by 20 – 40% at night

Some porous materials obtained from synthetic fibers, such as mineral wool or glass wool, arecommonly used to address noise pollution challenge because of their high performance and low-cost.Their diffuse-field sound absorption coefficient is very high at mid-high frequencies (500 – 2000 Hz).However, those materials have may cause health problems to our respiratory organs if the fibers areinhaled [22] Thus, it is necessary to have an eco-friendly and non-toxic sound insulation materialwhich can absorb most of the noise through the conversion of sound energy into thermal energy [23].One of the potential materials being researched at present is natural fibers like cotton, wool, kenaf,etc Those are lightweight, environmental-friendly, biodegradable, highly biocompatible, and showgood inherent acoustic insulation Pineapple leaf waste is made up more than 70% cellulose indicatingits great potential to be utilized as raw material for acoustic insulation material

1.4 Food packaging and challenges

Food delivery service is booming in Vietnam at present that many famous brands are competingfiercely like GrabFood, Now, Baemin, etc Customers can easily get their desired meals regardless ofwhere they are within a very short time since they finish ordering (usually about 20 – 30 minutes).When delivering perishable goods like meals, it is important for every business to ensure thefreshness, hotness (or coldness), and the original taste of food reach their foodies

Fig 1.5 Common packaging for food delivery such as corrugated box, ice pack, and insulated bag (from left

to right)

Several ways are currently being applied to keep the meal from getting spoiled such as usingcorrugated boxes, dry ice or frozen gel packs, and heavy insulated food delivery bags (Fig 1.5) Thoseitems have some limitations which need to be improved to enhance the efficiency of food deliveryservice For instance, the corrugated boxes may become deformed if they are exposed to extremepressure or when stacked They are not either weather-proof because water and other types of liquidcan saturate these boxes, even seep into the boxes and damage the contents For the packaging ofheavy items, they are not a good option because their endurance to mechanical stresses is relativelylow A traditional way to keep goods frozen while in transit is to use dry ice, which is solid carbondioxide Although it is capable of keeping the consignment very cold as low as -78 °C, it also has oneserious drawback When dry ice melts, it turns straight into carbon dioxide gas removing oxygen fromthe container and thus, this can make food get spoiled and release toxic chemicals Insulated box isnow an alternative to conventional preservation methods because of its safety and convenience forboth shippers and receivers In addition, it could be designed to transport temperature-sensitivemedications, protecting them from damaging temperatures, shocks, and light Some vaccines aredelicate biological substances that can lose part or all of their effectiveness if they are frozen, allowed

to get too hot, or exposed to bright light Those are reasons why they should be kept in the range of 2– 8 °C, from manufacture to use

is placed with a gas, without collapsing the gel-solid network [26] By sol-gel route and supercriticaldrying method to evaporate completely the liquid inside the gel without destroying the gel’s solidstructure, American scientist Samuel Stephens Kistler successfully synthesized aerogel for the firsttime in the year of 1931 [27] In the same year, Kistler et al prepared numerous aerogels fromalumina, nickel tartrate, stannic oxide, tungstic oxide, gelatine, agar, nitrocellulose, cellulose, and eggalbumin [27] Year after year with the development of science and technology, many different dryingmethods and fabrications of aerogels have been invented which allows humans to create aerogel-likematerials with porous structure preserved [28, 29].

There are several ways to classify aerogels Based on their shape after synthesis, aerogel is dividedinto 3 types: monolith, particle, and film (Fig 1.6) According to the synthesis method, there areaerogel, xerogel, and cryogel In addition, based on the pore size, they can be classified into micro-porous (< 2nm), mesoporous (2 – 50 nm), and mixed porous aerogel [30] Another way of classifyingaerogels which is recognized by most scientists is based on composition including single-componentaerogel and composite aerogel Some famous single-component aerogels are silica aerogel, celluloseaerogel, carbon aerogel, metal-oxide aerogel, and chalcogenide aerogel [31-34] Some typical aerogelcomposites are metal oxide-silica aerogels, metal oxide-RF aerogels, and silica-polymer aerogels[30] According to a new report by Grand View Research, Inc., from 2018 to 2025, the global aerogelmarket will rise with a high growth rate of 22.6% and will have reached an estimate of 3.29 millionUSD by 2025, to 785.3 million USD by 2022 by various market research groups, driven by demandsfrom several industries such as oil and gas, building and construction, automotive, and performancecoatings [35]

Fig 1.6 Aerogel with different shape: (a) monolith, (b) particle, and (c) film.

The high surface area and porosity make aerogels potential candidates for storage media, such asgas filters, absorption media, and hydrogen storage media [36] Silica aerogels were used as gas filters

to collect aerosols (viruses and bacteria in the size range of 20 – 2000 nm) for gas purification [37].Moreover, aerogels were studied as absorption media for wastewater treatment, since their highporosity leads to a large absorption capacity of 14 – 743 g/g [38] For example, carbon aerogels, silicaaerogels, and cellulose aerogels have served as absorbents for oils, other organic pollutants, and heavymetals [38, 39] One of the famous applications of aerogels is thermal insulation, as aerogels withextremely low thermal conductivity (< 0.05 W/m.K) are among the best-known thermal insulationmaterials [40, 41] Cohen et al reported a silica aerogel, fabricated by using separated catalysts forhydrolysis, condensation, and gelation steps, with a thermal conductivity of 0.0089 W/m.K underambient conditions [41] Many commercial thermal products have been developed from silicaaerogels, such as Thermal WrapTM, Compression PackTM, and LumiraTM[42, 43] Currently, NorthAmerican industrials Cabot Corporation and Aspen Aerogels are the two main players in the globalmarket for aerogel insulation, and their products are mainly sold in off-shore oil and gas,transportation, building insulation, high temperature insulation, cryogenic applications, and apparel[42] Aerogels also have other extraordinary properties, such as acoustic, optical, and electronicproperties as listed in Table 1.1 indicating their highly potential applications [44]

Table 1.1 A general summary of aerogel properties and potential applications.

Thermal conductivity · Best insulating solid

· Space vehicles and probes, casting moldsDensity/porosity · Lightest synthetic solid

special-Acoustic Lowest sound speed Impedance matchers for transducers, range

finders, and speakersMechanical · Elastic, flexible, or durable

· Lightweight

Energy absorbers, hypervelocity particle traps

Electrical · Lowest dielectric constant

· High dielectric strength

· High surface area

Spacers for vacuum electrodes, capacitors

In this study, pineapple leaf fibers (PF) are completely converted into cellulose-based aerogelsnamed PF aerogel by cross-linking with biodegradable polyvinyl alcohol and cost-effective freeze-drying method The effects of fiber and cross-linker concentration on the aerogels’ morphology,chemical structure, crystallinity, mechanical strength, thermal properties, acoustic insulation, and oiladsorption capacity are comprehensively investigated The heat insulation performance of PFaerogels is demonstrated with the thermal jacket for military water (ice slurry at -3 °C and hot water

at 90 °C)

CHAPTER 2 LITERATURE REVIEW

2.1 Pineapple leaf fibers

Pineapple leaf fibers obtained by mechanical extraction of pineapple leaf waste have a ribbon-likestructure and consist of vascular buddle system and present in the form of bunches of fibrous cells(Fig 2.1) [45] They are composed of many chemical constituents such as polysaccharides, lignin inmajor amount, and some miner chemicals like fat, wax, pectin, uronic acid, anhydride, pentosan, and

so forth [45] Previous studies showed that PF contain a huge amount of cellulose (70 – 82%), andarrangement of fibers is the same as cotton (82.7%) [46, 47] Cherian et al once reported that PF have81.27% cellulose, 12.31% hemicellulose, 3.46% lignin, and around 10% moisture content [48].However, composition of PF depends on the variety and environment where that pineapple is planted

Fig 2.1 Production of pineapple leaf fibers: sequential (a) plantation of pineapple, (b) fruit of pineapple, (c)

extraction of fibers from pineapple leaves, and (d) bunches of PF

Although there are numerous methods to extract PF from their leaves such as mechanicalextraction, retting, enzymatic degumming, or chemical treatment, commercial PF are mostlyproduced from decortication machine because of its low cost and high performance [9] As shown inFig 2.2, pineapple leaf is inserted between two blades and its upon waxy layer will be removed firstly.During the second step, when the leaf is pulled off, it will be ground one more time to remove theentire waxy layer left after the first step to obtain final product

Fig 2.2 PF extraction by novel decortication.

In industry, the extracted fibers are in textile grade that have been used as material for fabrications

of fabrics, ropes, handkerchiefs, knitting shirts, blankets, etc [49] According to some studies in thepast, PF have been used as reinforcing composite matrix because of their excellent specific strengthwith a modulus ranging from 34.5 to 82.52 GN/m2, a tensile strength of 413 – 1627 GN/m2, and anelongation at breakpoint ranges from 0.8 to 1.6 % [45] The superior mechanical properties of PF areassociated with their high cellulose content and comparatively low microfibrillar angle (14°) [48] Inaddition to high strength and stiffness, PF have low density of 1.07 – 1.526 g/cm3which is suitablefor synthesis of cellulose-based aerogels and other natural lightweight materials [45]

Because of high cellulose content, PF becomes the ideal material for cellulose extraction in highpurity In the structure of plant cell wall, the lignin/hemicellulose complex plays a key role to holdcellulose fibers together as described in Fig 2.3 Based on this binding function, lignin provides thestiffness, compressive strength, and water impermeability to the cell wall Moreover, hemicelluloseadheres to the cellulose fibrils through hydrogen bonds and Van der Waal’s interactions and alsocross-link with lignin

Fig 2.3 Main structure of plant cell wall in lignocellulosic biomass containing lignin, hemicellulose, and

cellulose

Cellulose is a linear homopolysaccharide in which D-anhydroglucose units are linked via b-1,4glycosidic bonds with the repeating unit of cellobiose The monomer of cellobiose consists of threehydroxyl groups which form strong hydrogen bond with the adjacent glucose unit in the same chainand with the different chains, called as intramolecular and intermolecular hydrogen bondingnetworks, respectively (Fig 2.4) [50] These hydrogen bonding networks are strong and tightlypacked in the crystalline parts of cellulose fibrils which lead to the tough, strength, fibrous, insoluble

in water, and high resistant to most organic solvents in plant cell wall [51] With its plentiful carbon,hydroxyl groups, monomers of glucose, cellulose is the most natural source for the production ofcarbon-based materials, valuable chemicals, textiles, papers, and so on [52]

Fig 2.4 Intramolecular (blue dots) and intermolecular (red dots) hydrogen bonding networks in cellulose

structure

In order to obtain highly pure cellulose, outer components including lignin and hemicelluloseshould be removed by physical (milling, grinding, high-shear homogenization, ultrasonication),chemical (acidic and/or alkaline hydrolysis, oxidation, use of ionic liquids or organic solvents),multiple or combined (steam explosion, microwave, hydrothermolysis), and biological methods(microbiology, micro-aerobic pretreatment) [48, 51, 53] Each method has its own pros and consalong with different extraction efficiency and production cost Depending on the component to berecovered and the required purity, an appropriate treatment is applied To maintain inherentmechanical strength of PF as mentioned and save production cost as well as being environmentallyfriendly, in this study, the fibers are only mechanically pre-treated to produce fibers of smallerstrength and increase their contact area while interacting with other chemicals in aerogel fabrication.2.2 Cellulose aerogel

In recent years, developments of high-performance materials from agricultural waste haveattracted attention from the research community and government authorities, because of theirabundant sources, biodegradability, and relatively pollution-free of the agricultural waste A lot ofgreen materials have been developed, such as concrete mixtures from banana leaves ashes [54], glass-

ceramics from rice husks and sugarcane leaves [55], cellulose aerogels from papers and fabrics [56].Thanks to their ultra-light 3D porous networks with porosity of 84.0 – 99.9% and density of 0.0005– 0.35 g/cm3, their large specific surface area (10 – 975 m2/g), their biocompatibility, their lowthermal conductivity, and their excellent stability [57], cellulose aerogels have distinguishedthemselves among eco-friendly materials A highly porous but stable structure of cellulose aerogelscan be created with a good mechanical strength (compressive modulus of 5.2 kPa – 16.67 MPa) andhigh absorption capacity, with the use of a suitable adhesive agent [28] Cellulose aerogel is generallyprepared in three steps: dissolving/dispersing cellulose or cellulose derivatives (precursors), formingcellulose gel by the sol-gel process, and drying cellulose gel while basically retaining its porousstructure [28] These step-by-step procedures are usually followed in the production of naturalcellulosic aerogel.

Because of those extraordinary properties, cellulose-based aerogels are a type of friendly and multi-functional material that has great potential in the application of adsorption,oil/water separation in oil-spill cleaning up; heat and acoustic insulation; biomedical devices in drugdelivery; metal nanoparticle/metal oxide carriers; and many other areas [28] Because of diversematerials for aerogel fabrication, properties of natural cellulose aerogel are also varied as shown inTable 2.1

environmentally-Table 2.1 Properties of natural cellulose aerogels from agricultural waste

Materials (mg/cmDensity3) Porosity(%) Specific surfacearea (m2/g) modulus (kPa)Compression Reference

Fig 2.5 The primary mechanism for the dissolution of cellulose at the cellulose-solvent interface (a)Cellulosic solid phase in contact with a solvent, (b) swelling of the solid phase, (c) point of disentanglement,(d) relocation of cellulosic chains from swelled phase to the solvent phase, and (e) advancement ofsolubilization front.

In the last decade, many scientists have reported some feasible solvents for cellulose dissolutionlike 1–butyl–3-methylimidazolium chloride (BmimCl), ionic liquids, tetrabutylammoniumfluoride/dimethyl sulfoxide mixture, sodium hydroxide/urea solution, N-methylmorpholine N-oxide(NMMO), etc [61, 63, 64] However, those solvents have several drawbacks such as toxicity, highcost, insufficient solvation control, volatility, and trouble in solvent recovery [65]

2.2.3 Drying techniques

The last step of aerogel fabrication is to remove the filler in the gel by drying Because theproportion of the solid dispersed phase in the gel is usually low (1 – 3 wt.%), it can cause severeshrinkage of the gel during normal drying and affect the overall performance of the aerogel [70] Tosolve this problem, unique drying methods including supercritical and freeze drying (lyophilization)are used to remove the solvent inside the gel to produce stable aerogels (Fig 2.6) [71] Under the

SolventCellulosic

chains

supercritical conditions, the gas-liquid interface can be eliminated and the shrinkage caused bycapillary force during drying can be prevented [70] Often, there is a solvent exchange step withethanol and CO2 liquid after gelation to provide better fluids for supercritical drying The obtainedaerogels usually show high porosity, high specific surface area and low density While being a highlyeffective method for producing aerogels, supercritical drying takes several days, requires specializedequipment, and presents significant safety hazards due to its high-pressure operation [71].

Fig 2.6 A typical phase diagram for two different drying techniques

Another approach is freeze drying which is known as an outstanding technique for the rejection ofsolvent from wet gels or hydrogels to attain dried cellulose aerogels (Fig 2.7) The process starts withthree necessary steps of the lyophilization cycle: i) to decrease the temperature in a specificenvironment, normally below the triple point of the pore-filled liquid, succeeded by ii) the use ofvacuum and ultimately iii) controlled sublimation following isobaric conditions as shown in Fig 2.6

Fig 2.7 Processing of cellulose aerogel: (a,b) mechanism of cross-linking cellulosic chains, (b,c) freeze

drying to obtain highly porous aerogel

After the liquid is removed, the solid skeleton structure in the gel can be kept unbroken, leadingthe formation of robust aerogels with high porosity and low density The growing ice crystal duringfreezing step is related to the development of a dendritic network typically in the range of few tens

of micrometer size Consequently, liquid crystallization and growth of ice crystals play a significantpart in the formation of porous framework (pore distribution and pore morphology) of celluloseaerogels Additionally, the rate of sublimation is affected by several parameters (i.e., temperature, thesize and shape of gel, and concentration of cellulose) Slow cooling (below -20 °C) gives large pores,abnormal structure and solvent segregation after ice-sublimation Fast cooling of solvent by directlysubmerged in liquid nitrogen (-196 °C) for specific time interval results in smaller ice-crystals andcan keep the structure of the initial dispersion Moreover, morphologies of resulted aerogels depend

on various factors such as the precursor concentration, type of liquid, temperature of freezing, andfreezing container [72-74] At the end of this process, the final remaining water content in aerogelremains at inadequate levels ranging from 1 to 4% which shows an isotropic foam-like behavior,usually designated to as cellulosic aerogels comprising a hierarchical porous structure Aerogelsproduced through freeze drying often experience some shrinkage and cracking while also producing

a non-homogenous aerogel framework [75] Fig 2.8 illustrates the differences in aerogel structurebetween supercritical drying and freeze-drying methods Compared to supercritical drying, freeze-drying technique takes advantage of being more economical, safer, and has been applied in massproduction

Fig 2.8 Comparison of aerogel fabrication strategies showing typical transitions into an aerogel

2.2.4 Modification of aerogels

Depending on the specific applications, for example, oil-spill cleaning up, the developed aerogelsare further modified with an appropriate chemical and method There are a large number of hydroxylgroups on the surface of cellulose aerogels that are amphiphilic and associated with poor oil/waterselective adsorption capacity [76, 77] By increasing the surface roughness of aerogels or introducingsubstances with low surface energy, the hydrophobicity and lipophilicity of aerogels can be improved,thus significantly enhancing the oil/water selective adsorption capacity of the aerogels Commonlyused methods for hydrophobizing cellulose aerogels include chemical vapor deposition (CVD) usingcoupling agents such as trimethylchlorosilane (TMCS), methyltrimethoxysilane (MTMS),methyltrichlorosilane (MTCS), and octadecyltrimethoxysilane (OTMS); cold plasma treatment;surface flourination or esterification [78, 79] After modification, the water contact angles of theaerogels are usually larger 135° and their adsorption performance regarding oil and organic solvents

is generally in the range of 80 – 95 g/g [56, 80, 81]

Surface modification using cold plasma technology has been developed to introducehydrophobicity onto surface of material Interestingly, the depth of plasma-induced deposition that isonly a few nanometers; this means inner structure of a sample still remains hydrophilic while itssurface becomes hydrophobic Therefore, the whole structure of modified material might not behydrophobic, thus the oil or organic solvent adsorption capacity of the material decreases AlthoughCCl4, CF4, SF6, etc are generally deemed as the ideal plasmas [82, 83], there are several associatedproblems, such as high discharge power, long modification period and medium hydrophobic effect,needed to be solved

The developed aerogels might undergo carbonization which is the formation of carbon aerogel bycarbonizing aerogel at a high temperature under a flowing nitrogen atmosphere In the carbonizationprocess, the aerogel is heated to a high temperature (≥ 600 °C) under the inert atmosphere of N2or

Ar, oxygen and hydrogen moieties are decomposed into gases through high temperature reaction andescaped from the aerogel to form carbon structure [84] The physical properties of aerogels aresignificantly changed in this stage The carbonization of cellulose aerogel causes significant decreases

in the volume and mass, because of the removal of O and H atoms [85] In addition, the activationprocess can increase the specific surface area of carbon aerogels and improve their pore structure.The specific surface area and average pore size of carbon aerogels derived from cellulose aerogelsare 100 – 1364 m2/g, and 2 – 100 nm, respectively Up to now, those carbon aerogels have been used

in lots of the remediation processes based on their adsorption and catalysis properties, like oil/waterseparation [86, 87], removal of heavy metal ions [88], clean-up of volatile organic compounds [89],

CO2capture [90], removal of nitrogen oxide [91] and so on

2.3 Cellulose aerogel from agricultural waste

Studies on the synthesis of cellulose aerogels from agricultural by-products are currently not ofinterest to domestic scientists, although cellulose aerogel exhibits characteristic properties such ashigh biological compatibility, biodegradability, and especially are fabricated from low-cost materialsbecause of an abundant source In 2013, Nguyen Truong Son et al synthesized cellulose aerogel (Fig.2.9) from paper waste and modified products to handle oil spill [80] Cellulose fibers were separatedfrom paper waste and dispersed in to a mixture of sodium hydroxide, urea, and water The researchteam selected the freeze-drying method to remove the liquid in the gel and further modified thesurface of obtained aerogel with MTMS After freeze-drying, the sample was placed in the big glassbottle containing a small open vial of MTMS The container was then capped and heated in an oven

at 70 °C for 2 h for the silanization reaction Thereafter, the coated sample was placed in the vacuumoven to remove the excess coating agent until the pressure reached 0.03 mbar MTMS was chosenbecause of its low price and common being used for development of hydrophobic and oleophillicaerogels The results showed that developed aerogels have low density of 0.04 g/cm3, porosity of97.3%, and the highest amount of oil adsorbed about 20.5 g/g at ambient condition [80] In addition,

a high contact angle of 145° was found for the cellulose aerogel modified with MTMS, indicating thehydrophobicity of the material

Fig 2.9 Morphology of the recycled cellulose aerogel (a) before MTMS coating and (b) after MTMS

coating

In the same year, dos Santos et al conducted a two-step chemical treatment of pineapple leavesthat is alkaline treatment with sodium hydroxide (NaOH) 2% within 4 hours at 100 °C, followed bybleaching with mixture of NaOH, acetic acid (CH3COOH), and sodium chlorite (NaClO2) at 80 °C

in the next 4 hours The refining efficiency was 40% and purity of final product was 74.5% [92] Theoptimum condition for cellulose recovery from pineapple leaves with the highest content wasdetermined to be using NaOH 25 g/L at 95 °C with extraction time of 6 hours, resulting in a samplewith cellulose content of 85% However, the disadvantage of this study is the use of toxicenvironmental chemicals such as NaClO2 to handle lignin and hemicellulose elimination Chlorite

(ClO2-) can form chlorine free radicals (Cl·) which react with the lignocellulosic compounds inpineapple leaves and generate organochlorine products with high toxicity [93] Therefore, the aim ofusing eco-friendly chemicals is of interest to scientists One of potential approaches is to convert rawmaterials (pineapple leaves) directly into cellulose-based aerogels.

One domestic research in 2018 was published by Huynh Minh Dat and his colleagues onfabrication of cellulose aerogel from agricultural waste The group took advantage of rice straw asmaterial to recover cellulose and to develop aerogel Straw was pulverized and underwent removal

of wax on its surface with a mixture of toluene-ethanol in a Soxhlet extraction system within 6 hours.The material was continually treated with sodium hydroxide, followed by hydrogen peroxide Thecellulose extracted was dispersed into water with polyvinyl alcohol and the mixture was dried toproduce cellulose aerogel exhibiting very low density of 0.041 – 0.047 g/cm3 and porosity up to96.9% [94]

A porous superhydrophobic biomass carbon aerogel was developed in 2019 from corn bracts forselective oil/water separation [95] During the fabrication, the flexible and ultralight aerogel wasobtained by corn bracts as raw material via simple alkalization, bleaching, freeze-drying andcarbonization as illustrated in Fig 2.10 The resultant aerogels that constituted by interconnected cornbracts-based carbon fibers exhibit a three-dimensional hierarchical porous structure in microscale ascan be seen from Fig 2.11

Fig 2.10 Procedure for fabricating cellulose aerogels and carbon aerogels from corn bracts

The aerogels have large specific surface area (675.85 m2/g), mesoporous structures (3.92 nm onaverage), and super-hydrophobicity (water contact angle > 150°) Thus, the aerogels can absorb abroad variety of oils and organic solvents with high selectivity and excellent absorption capacities(77.67 – 143.63 g/g) thanks to the highly porous structure and rough surface of biomass carbon fibers.Interestingly, the carbon aerogels can be used as an efficient filter for continuous oils separation due

to their super-hydrophobicity and porous structure The oil separation process by using carbonaerogels as filter is presented in Fig 2.12 No significant difference was found in the volume of carbontetrachloride before (5.00 mL) and after (4.63 mL) filtration, indicating the potential application ofthe carbon aerogels in oil/water separation

Fig 2.11 SEM images of cellulose aerogels (a, b and c) and carbon aerogels (d, e and f) from corn bracts.

Fig 2.12 The images (a and f) reveal the volume of carbon tetrachloride before and after filtration incylinder with the enlarged images The images (b–e) show the process of oil/water separation

Recently, a cellulose aerogel isolated from tea stem wastes has been developed by Kaya and Tabak

in 2020 [96] In this study, firstly, following delignification and removing hemicellulose by alkaliprocess, bleaching and hydrolysis, pure raw cellulose fibers were isolated using tea stem waste Next,cellulose hydrogel was prepared by dispersion of fibers in sodium hydroxide/urea solution, followed

by sonication The hydrogel was then placed in a freezer for more than 24 h to occur gelation Afterthat, the sample was thawed at room temperature before being submersed into 99% ethanol forcoagulation within 4 h After coagulation, solvent exchange process was made by immersing the gel

in de-ionized water for 2 days The final product – cellulose aerogel was produced via freeze-dryingfor 24 h at -98 °C after pre-freezing the sample at -12 °C for 12 h The morphological and structuralfeatures of cellulose aerogels were revealed in Fig 2.13 Overall, the surface of aerogel with a widelyporous and network structure is comprised of cellulosic micro-fibers The porosity comes fromremoving of frozen water in the course of the freeze-drying process Moreover, an irregular and

complicated porous structure was formed based on interconnections between the cellulosic pores.Therefore, it can be said that cellulose aerogel can be used as a potential adsorbent due to tunnel-likespaces inside the network scaffold structure BET results showed the aerogel with average porediameter and total pore volume as 5.45 nm and 0.475 cm3/g, respectively According to specificsurface area analysis, a value of 327.42 m2/g was recorded for the aerogel.

Fig 2.13 SEM images of cellulose aerogel from tea stem wastes

Sugarcane bagasse, one of the largest agricultural residues, has been also converted successfullyinto biodegradable aerogels by Quoc et al in 2020 [33] Sugarcane bagasse was washed and groundinto fibers The fibers were then treated with sodium hydroxide 1% (wt/vol) at 120 °C for 2 h Thedried alkali-treated fibers were immersed in a hydrogen peroxide 10% (vol/vol), and its pH wasadjusted to 12 with sodium hydroxide Finally, the mixture was soaked with deionized (DI) wateruntil neutralized and then drained at 70 °C for 24 h The recycled fibers and PVA were dispersed in

DI water, followed by sonication until forming the homogenous suspension PVA was used as binder

to cross-linked cellulose chains in the fibers based on hydrogen interactions between hydroxyl groups

to form a stable matrix In the past, PVA was introduced to make a combination with cellulose fibersthat lead to the formation of biodegradable composite films [97] The obtained products also showedimproved strain at break, preserving transparency and flexibility of original films It is also stated thatthe dispersion of fibers in the matrix and the adhesion between the matrix and fibers are the crucialfactors for the efficiency of enhancing mechanical properties of the films The strong interaction of

the PVA matrix with the dispersed cellulose phase, mainly via hydrogen bonding or bond network[98].

Then, a freeze-drying process was conducted for two days at -70 °C to produce aerogels Theresulting aerogels was additionally cured at 80 °C for 3 h to promote more cross-linking of PVAmolecules and the final products were obtained (Fig 2.14) This environmental-friendly recycledaerogel has an ultra-low density (0.016 - 0.112 g/cm3), a high porosity (91.9 - 98.9%), and a very lowthermal conductivity (0.031 - 0.042 W/m.K) The FE-SEM images and surface morphologies ofsugarcane bagasse before and after chemical treatment are presented in Fig 2.15a and b The rawsugarcane fibers exhibit a complex matrix of cellulose, and many impurities identified ashemicellulose and lignin; therefore, sugarcane fibers cannot be seen clearly from there figures Fig.2.15c shows a 3D porous network structure of sugarcane bagasse aerogel, implying that it is self-assembled through hydrogen bonding Furthermore, the aerogels after coating with MTMS haveshown their super-hydrophobicity properties and maximum oil absorption capacity (up to 25 g/g).The biodegradable aerogel has a Young's modulus of 88 K Pa and can be bent without breaking,demonstrating its high flexibility as shown in Fig 2.14b

Fig 2.14 (a) The ultra-light weight sugarcane bagasse aerogel, (b) its very flexible nature, and (c) the

formation mechanism between sugarcane fibers and PVA crosslinker

Fig 2.15 SEM image on raw sugarcane bagasse (a), sugarcane fibers after treatment process (b), and 3D

structure of sugarcane bagasse aerogel (c)

In general, although there are many studies on cellulose aerogels from agricultural residues, thesynthesis of aerogel still undergoes many chemical pre-treatment steps with sodium hydroxide andhydrogen peroxide or sodium hypochlorite which increase the production cost Moreover, the solutionobtained after each pre-treatment step has not been completely managed, mostly is released into theenvironment The efficiency of cellulose recovery from by-products is relatively low that causes theprice of final product to increase significantly To the best of my knowledge, the pineapple leaveshave not been utilized to fabricate advanced aerogel for high-value engineering applications such asheat and sound insulation or treatment of oil spill

CHAPTER 3 EXPERIMENTS AND CHARACTERIZATION

This chapter presents information about the materials, the fabrication approaches, and thecharacterization methods used in the thesis After the materials are listed, the fabrication approaches

of pineapple fiber aerogel will be discussed Following the discussions on fabrication approaches, thehydrophobic modification method used will be described Finally, the characterization methods, such

as Brunauer-Emmett-Teller analysis, field emission scanning electron microscopy X-ray diffraction,scanning electron microscopy, water contact angle measurement, thermal gravimetric analysis,thermal conductivity analysis, oil adsorption and sound absorption will be presented

3.1 Materials

Pineapple fibers were purchased from Vanilla Silk Company with a diameter of about 100 mm andlength of about 60 cm MTMS, PVA flakes and organic solvents (ethanol, acetone, and isopropanol)are purchased from Sigma-Aldrich Motor oil 5w30 used during oil adsorption test was purchasedfrom the commercial market PVA solution is prepared from dry PVA flakes and deionized (DI)water All the solutions are made with deionized (DI) water For acoustic insulation test, Basmel ispurchased from Acousti-teq Asia PTE Ltd (Singapore) All water used was deionized (DI)

3.2 Experimental techniques

3.2.1 Preparation of pineapple fiber

Pineapple fibers with 2 meters length were cut into pieces of 0.2 – 0.5 cm length These smallpieces were blended by the blender within 1 minutes The size of pineapple fiber before and afterblending is investigated by microscopy It is shown that the morphology of material is changed (Table3.1 and Fig 3.1) Particularly, the diameter of fibers reduces below 10 mm and their length hits a low

of 251.24 mm

Table 3.1 Diameter and length of pineapple fibersSample Before blending After blendingDiameter (mm) 25.61 – 76.36 9.57 – 19.48Length (mm) 200,000 – 500,000 251.24 – 528.51

(a) (b)Fig 3.1 Microscope images of pineapple fibers (a) – before blending; (b) – after blending

3.2.2 Preparation of PVA solution

PVA flakes were added into DI water in a ratio of 5g (PVA):100mL (DI water) The mixture washeated to 80oC and stirred with a hot plate magnetic stirrer for 3 hours to dissolve the PVA flakes.The concentrated 5 wt.% PVA solution was left to cool and kept for use later

3.2.3 Fabrication of the PF aerogels

Fig 3.2 Procedure of making PF aerogel from pineapple leaf fibers

Initially, the PF were blended for 2 minutes in a blender to reduce their diameter to below 20 mm.The pre-treated PF were dispersed into the PVA solution in a desire ratio of PF and PVA The mixturewas sonicated for 15 minutes at (250 – 300) W (UIP2000hdT, Hielscher-Ultrasound Technology,Teltow, Germany) for homogenization and removal of air The cross-linking reaction between PVAand the PF was carried out in the oven at 80oC for 2 hours The suspension was subsequently pre-frozen at -18oC for 4 hours before being freeze-dried to obtain the PF aerogel A Toption TPV-50F

Vacuum freeze dryer was used at 0.5 mbar for 48 hours The PF aerogels are fabricated from various

PF concentrations (0.5, 1.0, 2.0 wt.%) and PVA concentrations (0.1, 0.2, 0.4 wt.%)

3.2.4 Development of the hydrophobic PF aerogels

Following the procedure described in section 3.2.3, the aerogels fabricated are hydrophilic due tothe surface hydroxyl groups inherited from the PF and PVA The PF aerogels were then coated withMTMS to form the hydrophobic PF aerogels, for oil and organic solvent adsorption applications.MTMS is chosen as the coating agent because of its popularity for fabrication of hydrophobic aerogel,with simple coating procedure, and its low cost [99, 100] The PF aerogels were placed in an airtightcontainer together with an open glass vial of MTMS The container was then heated in an oven at

70oC for 24 hours to promote the silanization reaction [101] The excessive MTMS was removed in

a vacuum oven until the pressure reached 0.03 mbar The chemical reaction is illustrated in Fig 3.3

Fig 3.3 MTMS modification of hydrophillic PF aerogels

3.2.5 Fabrication steps of the thermal insulated jacket development

The thermal insulated jacket is designed according to a sandwich structure in which PF aerogel isencapsulated between an internal nylon layer and an external neoprene layer (Fig 3.4) Among thefabrics, nylon in Fig 3.4a is chosen over the others due it having the lowest thermal conductivityvalue, high durability, low cost, high availability and smooth texture The heavy-duty fabric ofneoprene in Fig 3.4c is chosen as the external layer of the thermal jacket due to it having low thermalconductivity, excellent durability, mostly weather-proof properties, high abrasion properties and tearresistance [102] These materials are cut into specific shaped panels and sewn together by straightstich A zipper is applied to form the final thermal jacket Fig 3.4d shows the proposed sandwichlayer using different materials forming the insulated jacket

Fig 3.4 Materials of (a) nylon, (b) cellulose aerogel, (c) neoprene are used for the insulated jacket design,and (d) the sandwich structure consists of neoprene as the outmost layer, the cellulose aerogel sheet within

and nylon as the innermost layer

In addition, the prototype fabrication has the following steps: (i) Cutting the inner and outer fabricinto specifically-shaped panels as shown in Fig 3.5a, (ii) sewing the inner fabric pieces together toform an inner jacket, (iii) wrapping the aerogel securely around the inner jacket, (iv) sewing the outerfabric pieces and zipper together over the aerogel layer and with the inner jacket, and (v) sewing thecollar onto the neck of the assembly to complete the product Straight stitch should not be applied asstraight stitch can tear the fabric easily because neoprene and nylon are stretchable Hence, the zigzagstitch method is preferred The zipper is applied to allow the military canteen bottle to be inserted andremoved easily from the insulated jacket The collar is used to reinforce the neck area Thecomponents come together to form a final thermal jacket are shown in Fig 3.5b

Fig 3.5 (a) Sketch of the insulated jacket and (b) final insulated jacket on the military canteen bottle

For experiments with low temperatures, an ice slurry containing a mixture of crushed ice and liquidwater at a ratio of 2:1 is used to test their heat insulation performance Ice slurry is preferred overcold water as the consumption of ice slurry can adjust body temperature more effectively than that ofchilled water due to its higher cooling capacity For experiments with high temperatures, 90 °C hotwater is used as a content The water canteen without thermal jacket is use for a comparison Bothexperiments are conducted in laboratory under static condition with an ambient temperature of 24 °Cfor 6 hours Each experiment is measured three times and an average result is calculated The mainfocus of this work is to study the effects of the structures and morphologies of the cellulose aerogels

on the heat insulation of the water canteen bottle

3.3 Characterization

3.3.1 Density and porosity

Densities of the PF aerogels were determined via measurements of the weight and the volume ofthe aerogel samples (Eq 3.1) The porosities of the PF aerogels were calculated from Eq 3.2

24

D h

rp

3.3.2 Surface-area, pore size and pore volume determination

The Brunauer-Emmett-Teller (BET) theory studies the physical adsorption of gas molecules onthe surface of solids, which is widely accepted in specific surface-area analysis [103] This theoryadopts the Langmuir mechanism with three simplifying hypotheses: (i) the heat of adsorption is equal

to the condensation molar heat in all layers except the first layer; (ii) the evaporation-condensationconditions of all layers are identical except the first layer; (iii) the number of adsorbed layers is infinite[104] In this thesis, the surface area, pore size, and pore volume of the PF aerogels were determined

by nitrogen physisorption measurements with a Nova Station A (Quantachrome) All the samples aredegassed in vacuum at 80oC for 24 hours before measurement

3.3.3 Field emission scanning electron microscopy (FE-SEM)

FE-SEM Hitachi S4300 (Fig 3.6a) was used to analyze the morphology and microstructure of theaerogels FE-SEM employs a high-energy beam of electrons to scan through the surface and processesproducing signals to derive information about the sample’s surface topography, composition andother properties such as electrical conductivity [105] The aerogels were cut into smaller pieces and