FLAME-RETARDANT CELLULOSE FIBRE/FIBRIL BASED MATERIALS VIA LAYER-BY-LAYER TECHNIQUE Oruҫ Köklükaya

FLAME-RETARDANT CELLULOSE FIBRE/FIBRIL BASED MATERIALS VIA LAYER-BY-LAYER TECHNIQUE

Oruҫ Köklükaya

Doctoral Thesis

KTH Royal Institute of Technology

School of Engineering Sciences in Chemistry, Biotechnology and Health

Department of Fibre and Polymer Technology

Stockholm, 2018

ISBN 978-91-7729-728-4 TRITA-CBH-FOU-2018:8 ISSN 1654-1081

Supervisor Prof. Lars Wågberg

AKADEMISK AVHANDLING som med tillstånd av Kungliga Tekniska Högskolan i Stockholm framlägges till offentlig granskning för avläggande av teknisk doktorsexamen fredagen den 4 maj 2018, kl. 10.00 i sal F3, Lindstedtsvägen 26, KTH, Stockholm.

Fakultetsopponent: Professor Gero Decher, Université de Strasbourg, Frankrike.

Avhandlingen försvaras på Engelska

ABSTRACT

According to an analysis conducted by the Swedish Chemicals Inspectorate in 2006, the approximate numbers of fire injuries per year in Sweden are 100 deaths, 700 major and 700 minor injuries.1 Observations also show that there has been an increase in the number of house fires during recent years. One possible explanation can be the increased use of plastics in the building industry and in furniture. The advantages of easy processing, light weight and low cost make plastic materials most prevalent in the market. However, plastics behave significantly differently from natural materials in the case of fire. Polymeric materials, including rigid polyurethane foams (PU) which are widely used in the building industry due to their insulating properties, are highly flammable and they release heat at a very high rate. In addition, polymeric materials release more harmful smoke, toxic gases and combustion products than natural materials. A house fire typically starts with the ignition of a combustible material. Flames then spread to nearby materials and shortly thereafter the heat radiation generated reaches a point where the contents of the room suddenly and simultaneously ignite. This stage is called a flash over. After this stage, the fire is fully developed and it continues until everything is consumed. The higher rate of heat and smoke production from plastic materials reduces the time to flash over and hence the time to escape from a fire. The traditional flame-retardant treatments are based mainly on halogenated compounds which are classified as gas phase flame-retardants. The halogenated flame-retardants are under severe investigation due to their adverse effect on health and on the environment since they release toxic gases during combustion and they may leach out and accumulate in the food chain.2-3 The restrictions due to growing environmental concerns have been a driving force to develop alternative flame- retardants by using natural and renewable resources. In recent years, the layer-by-layer (LbL) technique has been used as a simple and versatile surface engineering technique to construct functional nanocoatings through the sequential adsorption of polyelectrolytes and charged nanoparticles in an effort to impart flame-retardant characteristics by inhibiting the combustion cycle.4-5 This thesis presents the physical modification of cellulose fibre/fibril based materials as a means of improving flame- retardant properties.

In the first part of work described in this thesis, the adsorption of polyelectrolyte multilayers onto pulp fibres was investigated as a way to impart flame-retardant characteristics to paper-based materials. It was found that intumescent nanocoatings consisting of nitrogen and phosphorus containing polyelectrolytes such as chitosan

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(CH) and poly(vinylphosphonic acid) (PVPA) were able to significantly improve the thermal stability and flame-retardant properties of sheets made of LbL-treated fibres, and were able to self-extinguish the flame in the horizontal flame test (HFT). High magnification images revealed that this improvement in flame-retardancy was due to the formation of a coherent char layer on the fibres (Paper I).6 In addition to imparting flame-retardancy by the LbL-coating of polyethylenimine (PEI) and sodium hexametaphosphate (SHMP), it was also possible to improve the mechanical properties of the paper material with this treatment (Paper III).7

In the second part of the work, wet-stable porous cellulose fibril-based aerogels were developed by freeze-drying and used as a template for the build-up of intumescent nano-brick wall assemblies. The formation of multilayers of CH, PVPA and montmorillonite clay (MMT) was investigated as a function of solution concentration, and it was found that five quadlayers (QL) of CH/PVPA/CH/MMT treated aerogels using 5 g/L solutions of the respective components were able to self-extinguish the flame in HFT and that they showed no ignition under the heat flux of 35 kW/m2 used in cone calorimetry (Paper II).8 In a different application, a novel low density, porous, wet-stable cellulose fibre network was developed using chemically modified cellulose fibres by solvent exchange from water to acetone followed by drying at room temperature. The fibre networks (FN) were modified using the LbL technique to construct a flame-retardant nanocoating consisting of CH, SHMP, and inorganic particles (i.e., MMT, sepiolite (SEP), and colloidal silica (SNP)). The influence of the shape of the nanoparticles on flame-retardancy was investigated and it was found that plate-like and rod-like clays with a high aspect ratio showed self-extinguishing behaviour in HFT. A 5 QL of CH/SHMP/CH/SEP reduced the peak heat release rate and total smoke release by 47% and 43%, respectively, with an addition of only ~8 wt% to FN (Paper IV).

Finally, non-crystalline cellulose gel beads were used as a substrate for the LbL assembly of CH and SHMP in model studies aimed at identifying the molecular mechanisms responsible for the fire-retardant properties of the LbL structures. The beads were formed by precipitating the dissolved cellulose-rich fibres according to an earlier described procedure,9 and it was shown that these smooth cellulose beads can be utilized as a model substrate to study the influence of LbL chemistry and nanostructure on flame-retardancy. These new types of model systems thus constitute a new important tool for clarifying the mechanism behind flame-retardant nanocoating systems (Paper V).

SAMMANFATTNING

Enligt en analys som utarbetats av Kemikalieinspektionen 2006 är det ungefärliga antalet brandskador per år i Sverige 100 dödsfall, 700 allvarliga skador och 700 mindre skador.1 Observationer visar också att antalet bränder har ökat under de senaste åren. En möjlig förklaring kan vara en ökad användning av plast inom byggindustrin och i möbler. Enkel bearbetning, låg vikt och låg kostnad är de fördelar som gör plastmaterial så vanligt förekommande på marknaden. Plast uppträder dock väsentligt annorlunda än naturliga material vid brand. Polymermaterial, inklusive styva polyuretanskum (PU), som på grund av dess isolerande egenskaper används inom byggbranschen, är mycket brandfarliga och släpper ut värme i mycket hög grad. Dessutom frigör polymermaterial en större mängd skadlig rök, giftiga gaser och förbränningsprodukter jämfört med naturliga material. En husbrand börjar typiskt med antändning av ett brännbart material, flammor sprider sig sedan till närliggande material och kort därefter når den genererade värmestrålningen en punkt där rummets inventarier plötsligt och samtidigt antänds. Det sista steget kallas övertändning. Efter detta steg är branden fullt utvecklad och fortsätter tills allt brännbart material är förbrukat. På grund av den stora mängd värme och rök som produceras från plastmaterialen minskar tiden det tar att nå det övertända tillståndet. Detta minskar därmed också tiden som finns tillgänglig för att fly undan branden. De traditionella flamskyddsmedlen baseras huvudsakligen på halogenerade föreningar som klassificeras som gasfasflamskyddsmedel. De halogenerade flamskyddsmedlen är under kraftig undersökning på grund av dess negativa inverkan på hälsa och miljö då de släpper ut giftiga gaser vid förbränning och för att dess giftiga ämnen kan läcka ut och ackumuleras i livsmedelskedjan.2-3 De begränsningar som uppstått på grund av växande miljöhänsyn har varit en drivkraft för att utveckla alternativa lösningar till de befintliga giftiga flamskyddsmedelen genom att använda naturliga och förnybara resurser. Under senare år har multilagertekniken (eng. Layer-by-Layer technique) använts som en enkel och mångsidig teknik för att konstruera funktionella nanoytskikt genom en sekventiell adsorption av polyelektrolyter och laddade nanopartiklar i ett försök att ge upphov till flamhämmande egenskaper genom att störa förbränningscykeln.4-5 Denna avhandling presenterar tillvägagångssätt för att fysiskt modifiera material baserade på cellulosafibrer/fibriller med syfte att inkorporera flamhämmande egenskaper.

I den första delen av avhandlingen undersöktes adsorption av polyelektrolyt-multilager på massafibrer som ett sätt att ge flamskydd till pappersbaserade material. Det visade sig att de svällande nanoytskikten bestående av kväve och fosforinnehållande

polyelektrolyter såsom chitosan (CH) och poly(vinylfosfonsyra) (PVPA) signifikant kunde förbättra värmestabiliteten och inkorporera flamhämmande egenskaper hos ark framställda av LbL-behandlade fibrer. Under det horisontella flamtestet (HFT) var elden självutplånande och högupplösta bilder visade att detta flamskydd berodde på bildandet av ett sammanhängande karbonskikt på pappret (Paper I).6 Förutom att ge flamskyddande egenskaper genom LbL-deposition av polyetylenimin (PEI) och natrium hexametafosfat (SHMP), var det också möjligt att förbättra pappersmaterialets mekaniska egenskaper med denna behandling (Papper III).7

I den andra delen av avhandlingen utvecklades våtstabila porösa aerogeler av cellulosafibriller genom frystorkning och användes som en mall för uppbyggnad av svällande nanotegelväggar. Uppbygganden av multilager av CH, PVPA och montmorillonitlera (MMT) undersöktes som en funktion av lösningskoncentrationen. Det visade sig att aerogeler behandlade med fem quadlager (QL) av CH/PVPA/CH/MMT (5 g/L) kunde självutplåna elden i det horisontella flamtestet och visade ingen antändning i konkalorimetritestet under ett värmeflöde av 35 kW/m2 (Papper II).7 I en annan applikation utvecklades ett nytt lågdensitets, poröst, våtstabilt cellulosafibernätverk genom användning av kemiskt modifierade cellulosafibrer som fått genomgå ett lösningsmedelsbyte från vatten till aceton följt av torkning i rumstemperatur. Fibernätverket (FN) modifierades genom användning av LbL- tekniken för att konstruera ett flamhämmande nanoytskikt bestående av CH, SHMP, oorganiska partiklar (dvs MMT, sepiolit (SEP) och kolloidal kiseldioxid (SNP)). Studien inkluderade hur formen på nanopartiklarna påverkade materialets flamskyddande egenskaper. Man fann att de plattliknande och stavliknande nanopartiklarna med högt längd till bredd förhållande visade ett självutplånande beteende i det horisontella flamtestet. Ett 5 quadlager av CH/SHMP/CH/SEP reducerade toppvärmefrisättningshastigheten och den totala rökavgivningen med 47% respektive 43% med en viktökning på endast ~8 vikt% till FN (Paper IV).

Slutligen användes icke-kristallina cellulosagelkulor som substrat för LbL-deposition av CH och SHMP i modellstudier som syftade till att identifiera de olika molekylära mekanismerna ansvariga för de flamhämmande egenskaperna hos de bildade LbL- strukturerna. Kulorna bildades genom utfällning av upplösta cellulosarika fibrer enligt ett tidigare beskrivet förfarande från vårt team.8 Det visades att dessa släta cellulosakulor kan användas som ett modellunderlag för att studera påverkan av LbL- kemi och nanostruktur på flamskydd. Dessa nya typer av modellsystem utgör därför ett nytt viktigt verktyg för att förtydliga mekanismerna bakom olika flamskyddsmedel (Papper V).

LIST OF PUBLICATIONS

This thesis is based on the following papers, referred to in the thesis by roman numerals, and appended in the final section of the thesis.

I. Flame-retardant paper from wood fibers functionalized via layer-by- layer assembly Oruҫ Köklükaya, Federico Carosio, Jaime C. Grunlan and Lars Wågberg ACS Applied Materials and Interfaces (2015), 7, 42, 23750-23759 II. Superior flame-resistant cellulose nanofibril aerogels modified with hybrid layer-by-layer coatings Oruҫ Köklükaya, Federico Carosio and Lars Wågberg ACS Applied Materials and Interfaces (2017), 9, 34, 29082-29092 III. Tailoring flame-retardancy and strength of papers via layer-by-layer treatment of cellulose fibers Oruҫ Köklükaya, Federico Carosio and Lars Wågberg Cellulose (2018), accepted for publication doi.org/10.1007/s1057 IV. Development of hybrid coatings to reduce flammability of low density cellulose fiber networks via layer-by-layer assembly Oruҫ Köklükaya, Federico Carosio, Verónica Lopéz Durán and Lars Wågberg Manuscript V. A study of layer-by-layer nanocoatings on model cellulose gel beads to clarify their flame-retardant characteristics Oruҫ Köklükaya, Rose-Marie Pernilla Karlsson, Federico Carosio and Lars Wågberg Manuscript

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CONTRIBUTIONS TO THE PAPERS

The author’s contributions to the appended papers are as follows:

I. Most of the experimental work and preparation of the manuscript

II. Most of the experimental work and preparation of the manuscript

III. Most of the experimental work and preparation of the manuscript

IV. Most of the experimental work and preparation of the manuscript

V. Most of the experimental work and preparation of the manuscript

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RELATED MATERIAL

In addition to the appended papers, the work has resulted in the following presentations:

- Flame-Retardant Coating on Wood Fiber-Based Materials via Layer-by-Layer Assembly O. Köklükaya, F. Carosio, J. C. Grunlan and L. Wågberg American Chemical Society (ACS) meeting, Dallas, USA (2014)

- Flame-Retardant Coating on Wood Fiber-Based Materials via Layer-by-Layer Assembly O. Köklükaya, F. Carosio, J. C. Grunlan and L. Wågberg FRPM 2015 - European Meeting on Fire Retardancy and Protection of Materials, Berlin, Germany (2015) (Poster presentation)

- Flame-Retardant Paper from Wood Fibers Functionalized via Layer-by- Layer Assembly O. Köklükaya, F. Carosio, J. C. Grunlan and L. Wågberg American Chemical Society (ACS) meeting, San Diego, USA (2016)

- Flame-Retardant Paper from Wood Fibers Functionalized via Layer-by- Layer Assembly O. Köklükaya, F. Carosio, J. C. Grunlan and L. Wågberg International workshop on Nanostructured Materials and Their Use in Fire Retardancy Applications, Stockholm, Sweden (2016).

LIST OF ABBREVIATIONS

AFM Atomic force microscopy BL Bilayer BTCA 1,2,3,4-Butanetetracarboxylic acid CH Chitosan CNF Cellulose nanofibril DMAc Dimethylacetamide EDX Energy dispersive X-ray analysis FE-SEM Field-emission scanning electron microscopy FIGRA Fire growth rate index FN Fibre network FTIR Fourier transform infrared spectroscopy HFT Horizontal flame test

HMw High molecular weight HRR Heat release rate KPVS Potassium polyvinyl sulphate LbL Layer-by-Layer LiCl Lithium chloride

LMw Low molecular weight LOI Limiting oxygen index MARHE Maximum average rate of heat emission MMT Montmorillonite MQ Milli-Q grade NMMO N-methylmorpholine-N-oxide OTB Ortho-toluidine blue PDADMAC Poly(diallyldimethylammonium chloride) PEI Polyethylenimine PET Polyelectrolyte titration

pkHRR Peak of heat release rate PTFE Polytetrafluoroethylene PU Polyurethane PVAm Poly(vinylamine) PVPA Poly(vinylphosphonic acid) RH Relative humidity SEP Sepiolite SHMP Sodium hexametaphosphate SHP Sodium hypophosphite SNP Colloidal silica TGA Thermal gravimetric analysis THR Total heat release TSR Total smoke release TTI Time to ignition QCM-D Quartz crystal microbalance with dissipation QL Quadlayer 3D Three-dimensional

TABLE OF CONTENT 1 INTRODUCTION ...... 1 2 BACKGROUND ...... 2 2.1 A brief introduction to cellulose and wood fibres ...... 2 2.2 Thermal degradation of cellulose ...... 3 2.3 Flame-retardant technology ...... 4 2.3.1 Mineral flame-retardants ...... 7 2.3.2 Halogenated flame-retardants ...... 7 2.3.3 Phosphorus-based flame-retardants ...... 8 2.3.4 Nitrogen-based flame-retardants ...... 10 2.3.5 Silicon-based flame-retardants ...... 10 2.3.6 Nanoparticles ...... 10 2.4 Layer-by-Layer assembly technique ...... 12 2.4.1 Flame-retardant coatings via layer-by-layer assembly ...... 13 3 EXPERIMENTAL ...... 15 3.1 Materials ...... 15 3.1.1 Wood-based cellulose fibres and fibrils ...... 15 3.1.2 Model surfaces ...... 16 3.1.3 Chemicals ...... 16 3.2 Methods ...... 18 3.2.1 LbL film deposition on flat model surfaces ...... 18 3.2.2 LbL assembly on fibres ...... 18 3.2.3 Laboratory sheet preparation ...... 18 3.2.4 Preparation of CNF aerogels ...... 19 3.2.5 Preparation of fibre networks ...... 19 3.2.6 LbL formation on cellulose fibre/fibril based porous substrates ...... 19 3.2.7 Preparation of cellulose gel beads ...... 20 3.2.8 LbL formation of thin film on cellulose gel beads ...... 21 3.3 Characterization techniques ...... 22 3.3.1 Quartz crystal microbalance with dissipation (QCM-D) ...... 22

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3.3.2 Atomic force microscopy (AFM) ...... 23 3.3.3 Field-emission scanning electron microscopy (FE-SEM) ...... 23 3.3.4 Energy dispersive X-ray analysis (EDX) ...... 24 3.3.5 Polyelectrolyte titration (PET) ...... 24 3.3.6 Nitrogen analysis (ANTEK) ...... 24 3.3.7 Thermogravimetric analysis (TGA) ...... 25 3.3.8 Flammability test ...... 25 3.3.9 Cone calorimetry ...... 25 3.3.10 Flame penetration test ...... 26 3.3.11 Limiting oxygen index (LOI) ...... 26 4 RESULTS AND DISCUSSION ...... 27 4.1 Flame-retardant paper (Papers I & III) ...... 27 4.1.1 The quantification of LbL film formation on model surfaces ...... 27 4.1.2 Morphology of multilayer thin films ...... 31 4.1.3 Multilayer thin film formation on wood fibres ...... 32 4.1.4 Thermal stability and flammability of LbL-treated fibres ...... 35 4.2 Flame-retardant cellulose-based porous materials (Papers II & IV) ...... 45 4.2.1 Multilayer thin film build-up ...... 45 4.2.2 Thermal degradation of LbL-treated porous substrates ...... 49 4.2.3 Flame-retardant properties of LbL-coatings on low density networks .... 51 4.3 Flame-retardant model cellulose gel beads (Paper V) ...... 57 4.3.1 LbL assembly build-up...... 57 4.3.2 Thermal analysis of LbL-treated cellulose beads ...... 60 5 CONCLUSIONS ...... 64 6 FUTURE WORK ...... 66 7 ACKNOWLEDGEMENTS ...... 68 8 REFERENCES ...... 70

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INTRODUCTION

1 INTRODUCTION

The need to achieve a cyclic green chemistry has meant there is a growing environmental demand for non-toxic chemicals to prepare, for example, flame- retardant materials. The traditional flame-retardant treatments were to a large extent based on halogenated compounds which are known to be toxic and to accumulate in the environment (i.e., in the food chain and in the bodies of mammals).2-3 In today’s society, this is simply not acceptable and new strategies are therefore needed. In recent years, the layer-by-layer (LbL) assembly technique10 has been exploited as an environmentally friendly method of imparting flame-retardant characteristics to substrates such as cotton textiles5 and polyurethane foams.11 In the present study, the first objective was to use the LbL technique to develop wood-based cellulose fibres while they are suspended in water in an effort to deposit flame-retardant multilayer, nanometer-thin films onto individual fibres prior to paper sheet preparation. The establishment of a sustainable method to inhibit the inherent flammable character of paper products will pave the way for many possible applications. The use of polyelectrolyte solutions or inorganic particles as layer constituents with a minimum environmental impact and the ability to achieve the desired flame-retardant properties with relatively few adsorption steps makes this technique interesting both from a fundamental point of view and from an upscaling perspective.12 It is of great interest to replace petroleum-based polymers with materials from renewable sources. For example, light weight green composites would have many possible applications as interior materials in the transportation and building industries if their flammability were suppressed. The second objective was to prepare porous materials from cellulose fibres/fibrils and to impart flame-retardant properties to these materials via the LbL technique using hybrid organic and/or inorganic building components. Finally non- crystalline cellulose gel beads were prepared and their smooth surfaces and defined shapes were utilized to provide a new insight to the action of flame-retardant nanocoatings that can lead to new strategies for cellulose-based materials.

BACKGROUND

2 BACKGROUND

2.1 A brief introduction to cellulose and wood fibres

Cellulose is the most abundant biopolymer on earth and cellulose fibres derived from wood have been the primary raw material for the pulp and paper industry for more than a century.13 Cellulose consists of repeating units of glucose. The native wood cellulose has a degree of polymerization of about 10 000 whereas a linear chain of cellulose in wood pulp contains 300-1700 D-glucose units linked together with β(1→4) glycosidic bonds.13 A bundle of linear cellulose chains assemble together under the action of inter- molecular and intra-molecular hydrogen bonding, due to the abundance of hydroxyl groups in each anhydroglucose unit, to form cellulose fibrils 2-3 nm in diameter and few micrometres in length.14-16 In wood, the cellulose fibrils aggregate and are embedded in a matrix of hemicellulose and lignin to form a fibre wall with a layered structure with an average thickness of 4 µm.17 The fibre wall consists of the primary wall (P) and secondary wall (S), and the secondary wall in turn consists of three layers; the outer layer (S1), the middle layer (S2) and the inner layer (S3). The wood pulp fibres are typically a few mm long and 20-50 µm in diameter.17 The hierarchical structure from the macroscopic tree to the cellulose molecules is schematically shown in Figure 1.

Figure 1 A schematic description of the hierarchical structure of tree to cellulose.

BACKGROUND

2.2 Thermal degradation of cellulose

In a real fire scenario at high temperatures, the polymeric materials undergo physical and chemical changes through the degradation of polymer chains into fragments and flammable/non-flammable species.18 The chemical structure of the polymeric materials influences the degradation kinetics and the resulting degradation products. Degradation mostly begins in the more accessible regions and in bonds having the lowest dissociation energies. Synthetic and natural polymers thermally degrade through mechanisms such as chain scission, chain stripping and crosslinking.19-20 These reactions are typically influenced by the presence of impurities and by the rate of heating.21 Cellulosic materials act like a thermoset material that does not melt but undergoes irreversible chemical changes when decomposing. A large number of studies have been published on the mechanism and kinetics of cellulose degradation and pyrolysis of cellulose using thermal gravimetric analysis which measures change in mass as a function of temperature.19, 22-27 Typically, the heating of cellulose to a moderate temperature (200-300 °C) causes a decrease in the degree of polymerization with the production of volatiles such as 1,6-anhydro-β-D-glucopyranose (levoglucosan) and a liquid pyrolysate as a result. At higher temperatures (400-600 °C), a further degradation of cellulose continues with the evolution of combustible gases and high boiling products.22, 28 In order to observe ignition and flaming combustion, a polymeric material must first decompose and break down into low molecular weight flammable volatiles and free radicals. At elevated temperatures (i.e., above the ignition temperature) or in the presence of an ignition source, these degradation products combust together with atmospheric oxygen.29 In order to understand the flammable character of cellulose, we must first investigate its decomposition. Shafizadeh et al. suggested a degradation mechanism for cellulose in which cellulose is transformed to a reaction intermediate called an “activated” cellulose species.23 It has been suggested that this intermediate is produced in an initial degradation step in which the degree of polymerization decreases but no mass loss is observed.26-27 This scheme of cellulose thermal degradation upon heating is shown in Figure 2.

BACKGROUND

Figure 2 Scheme of thermal and thermo-oxidative degradation of cellulose.21 The temperature is increasing from left to right.

Liu et al. proposed an improved mechanism for the formation of and consecutive evaluation of the active cellulose during cellulose pyrolysis.30 Price et al. also studied cellulose degradation via some form of activated cellulose species.31 This activated cellulose state is further degraded with increasing temperature through two competitive mechanisms.19, 22-24, 32 The pathway of thermal degradation and the composition of degradation products are both influenced by physical and chemical factors such as temperature,26 type of atmosphere,33 crystallinity,34 presence of impurities,35 size and texture of the cellulose sample.36 It is well accepted that cellulose undergoes degradation through two different pathways in particular dehydration and decomposition (i.e., depolymerization). In dehydration, the degradation reactions lead to the formation of anhydrocellulose and water (300-400 °C). The aliphatic char formed during degradation is further oxidized to aromatic char with a consecutive release of CO and CO2 (400-600 °C). This aromatic char exhibits thermal stability up to 800 °C.21 The depolymerization occurs in crystalline regions as well as in amorphous regions yielding levoglucosan through an unzipping mechanism37 where the chain scission of acetal bonds of the glycosidic units gives rise to levoglucosan formation.21 The levoglucosan is further degraded to low molecular weight products (i.e., furan and furan derivatives).25

2.3 Flame-retardant technology

The use of synthetic and natural polymeric materials over the past decades has drastically increased with advances in technology and an increase in the population. Nevertheless, in addition to being widely used, these polymeric materials, whether

BACKGROUND

natural or synthetic, are considered to be highly flammable due to their chemical structure.38 An increase in temperature of the polymeric material leads to endothermic bond scission reactions, and combustible volatiles diffuse into the air and create a highly flammable gaseous mixture which ignites when the activation energy of combustion is reached at a temperature defined as the ignition temperature. Combustion can however also take place at a temperature lower than the ignition temperature if there is an external energy source such as a flame or spark, etc. Combustion is defined as a self-sustained energy cycle. More specifically, the temperature of the polymeric material increases due to the combustion of gases and thus sustains the pyrolysis and enhances the production of combustible gas, which can also be described as a combination of various processes involving heating, pyrolysis, ignition, and propagation of thermal degradation. This combustion cycle is shown schematically in Figure 3. The increasing use of these flammable materials gave rise to a significant amount of research aiming at finding the most effective alternative flame- retardant system for these materials. The notion of a flame-retardant system is not new. Flame-retardants were first used in ancient Egypt and in the Roman empire,39 but their use in everyday products became popular in the 1970s.40 Later, during the twentieth century, many technologies aiming at obtaining flame-retardant properties were investigated. Most of the currently available flame-retardants used for textiles and fibres were reviewed by Weil and Levchik41 and a general overview of commercial flame-retardant technology was given by Morgan et al.42

Flame-retardant characteristics are achieved by interrupting the processes in the combustion cycle of the polymeric material, but flame-retardants which involve chemical or physical modifications of a material act differently depending on the chemical structure and burning behaviour of the polymeric material. Flame-retardants used for polymeric materials are typically active in the gas phase or in the condensed phase of the combustion cycle and they can be classified according to their mode of action, physical or chemical.43

BACKGROUND

Figure 3 Schematic description of the combustion cycle of a polymeric material.44

Physical flame-retardant system acts in the condensed phase by:

 Cooling the combustion area through endothermic decomposition to below the combustion temperature.  Formation of a protective layer which separates the combustion and thermal degradation phases and suppresses the release of combustible species.  Diluting the combustible gas mixture with the formation of an inert gas (e.g.,

H2O, CO2, NH3, etc.) which eliminates or delays the ignition of the polymeric material.

Chemical flame-retardant systems can act in the gas phase by:

 Scavenging free hydrogen and hydroxyl radicals from the flame zone to form less reactive or inert molecules which reduce the temperature and amount of flammable volatile species, or in the condensed phase by:  Promoting the formation of a carbonaceous layer (i.e., char) which acts as a physical insulating barrier between the gas and the condensed phase.

BACKGROUND

Flame-retardant additives that are commonly used include mineral flame-retardants, halogenated flame-retardants, phosphorus-based flame-retardants, nitrogen-based flame-retardants, silicon-based flame-retardants and nanoparticles, as reviewed by Laoutid et al.45

2.3.1 Mineral flame-retardants

Mineral fillers are active in both the gas and the condensed phases of combustion. At high temperatures, mineral fillers (e.g., aluminium hydroxide, magnesium hydroxide, carbonates) endothermically decompose and therefore cool the degrading polymeric material, and this consequently retards the thermal degradation. The release of non- flammable gas molecules (e.g., H2O, CO2) from the decomposition of filler dilute the combustible gas mixture and prevent or delay ignition. The non-flammable solid filler residues also enhance the formation of a protective ceramic char layer in the condensed phase.46

2.3.2 Halogenated flame-retardants

Halogenated flame-retardants are universally known to act in the gas phase. During combustion, the flame-retardant releases halogen radicals that then form hydrogen halides by abstracting a hydrogen atom from the degrading polymer.47 Hydrogen halides react with the gaseous free-radical species (e.g., hydrogen and hydroxyl radicals) that are necessary for sustaining combustion. Hydrogen radicals are used for the chain-branching free-radical reactions during thermal degradation48 and hydroxyl 20 radicals are used for the highly exothermic oxidation of CO to CO2. The reaction mechanisms for halogenated flame-retardants upon heating are shown in Figure 4. Halogenated flame-retardants dilute the combustible gas mixture by scavenging free- radical species and inhibit flame propagation. As shown in Figure 4, hydrogen halide reacts with hydrogen or hydroxyl radicals to form hydrogen gas and water. The halogen radicals formed can then be reused to further inhibit radical propagation. Chlorine-containing and bromine-containing compounds are the most commonly used halogenated flame-retardants because of their low bonding energy with carbon atoms.47

BACKGROUND

Figure 4 The reaction mechanisms for halogenated flame-retardant.

It has also been speculated that halogens can act in the condensed phase by promoting char formation.49 Despite their remarkable flame-retardant properties, halogenated flame-retardants are however undergoing worldwide investigation since they release toxic smoke and leach into the environment.2-3 These concerns have resulted in numerous studies focusing on the development of new flame-retardant chemistries with minimal environmental impact.

2.3.3 Phosphorus-based flame-retardants

Phosphorus-containing flame-retardants are classified as gas and condensed phase 50 active compounds at high temperatures. The active radicals (PO2•, PO• and HPO•) released from phosphorus-based flame-retardants can enter the gas phase of the combustion cycle and react with hydroxyl and hydrogen radicals, limiting the exothermic oxidation reactions. Phosphorus-based flame-retardants also inhibit flame propagation in the condensed phase through the formation of phosphoric acid upon degradation. The phosphoric acid formed can further react with either another phosphoric acid molecule to form pyrophosphate through condensation45 or a hydroxyl- containing polymer through phosphorylation51-53 to release water. The water vapour released dilutes the combustible gas phase and limits the exothermic reactions. In particular, phosphorus-based flame-retardants are used with high oxygen-containing polymers (e.g., cellulose) due to their high efficiency in situations where the phosphorus flame-retardants react with the polymer forming a P-O covalent bond which involves charring of the polymeric material.42, 54 Organophosphorus compounds have been widely investigated as flame-retardants particularly for cellulose-based materials. It has been well documented that most phosphorus-based flame-retardants act by phosphorylation of cellulose at primary hydroxyls, preventing the formation of levoglucosan and subsequent flammable gases by pyrolyzing the cellulose.54 Many of these studies indicate that phosphorus-based flame-retardants are particularly effective when nitrogen is used in conjunction with phosphorus. It has been suggested that the

BACKGROUND

synergistic interaction of phosphorus with nitrogen increases the flame-retardant efficiency.55 Moreover, phosphoric acid can favour char formation of the condensed phase by catalysing the dehydration reaction. A thermally stable carbonaceous char layer isolates the condensed phase from the gas phase (where combustion occurs) and subsequently suppresses the emission of flammable volatiles as well as oxygen diffusion. More specifically, this condensed phase char formation concept has been investigated as an intumescent flame-retardant system.56-57 The word “intumescent” comes from Latin, meaning a swelling up or the state of being swollen. It has been suggested that intumescent flame-retardant additives undergo a thermal degradation upon heating and that this produces an expanded, multicellular thermally stable residue called intumescent char on the surface of the polymeric material.58 This swollen char layer provides insulation to the underlying degrading material, which reduces the heat and mass transfer between the flame and the burning material. Thermally insulating barriers also hinder the diffusion of combustible volatiles to the combustion zone.56, 59 Traditionally, intumescent systems consist of three components:56-57, 60

 an acid source  a carbon source  a blowing agent

An inorganic acid catalyses the dehydration of the carbon source and this promotes the formation of a char layer. The volatiles formed by the decomposing blowing agent then swell the carbon-rich char layer into a multicellular structure.57 To lead to intumescent behaviour, these three components must undergo thermal degradation in sequence to each other.45, 61-62 The behaviour of an intumescent flame-retardant is illustrated in Figure 5.

Figure 5 Schematic illustration of an intumescent coating.62

BACKGROUND

2.3.4 Nitrogen-based flame-retardants

Nitrogen-based flame-retardants consist mainly of compounds containing melamine and melamine derivatives.63 Melamine-containing flame-retardant additives act upon thermal degradation in a manner similar to that of a mixture of halogenated flame- retardants and inorganic flame-retardants. Melamine has a thermally stable crystalline structure which undergoes endothermic sublimation at low temperatures (~350 °C) and this cools the combustion phase.20 At high temperatures, the decomposing melamine evolves non-flammable volatiles (e.g. ammonia), which dilute the combustible gas mixture and leads to the formation of thermally stable condensates.64 Melamine salts also act as a condensed phase flame-retardant.20 The advantages of nitrogen-based flame-retardants are their low toxicity, low smoke release and recyclability.65 They are often used in combination with other flame-retardants such as phosphorus-based flame- retardants in order to exploit the P-N synergism mentioned earlier.63

2.3.5 Silicon-based flame-retardants

Silicon-containing chemical compounds (e.g. silicones, silicas, organosilanes, silsesquioxanes and silicates) have been considered as co-additives in flame-retardant systems.66 Naturally occurring inexpensive silicates are for example used in large quantities as fillers, resulting in a reduction in the amount of flame-retardant needed. Silicon-based flame-retardants can also be used as copolymers and as the main polymer matrix.67 In particular, silicones can be used as flame-retardants since they exhibit remarkable thermal stability and high heat resistance.45 The flame-retardant concept for silicon-based flame-retardant additives is the migration of silicone derivatives towards the surface of the degrading material during combustion to form a thermally stable char layer partially protecting the underlying material.66 The viscosity of the degrading polymer and of the silicon-based flame-retardant complex is the main parameter in the formation of the protective char layer.20, 45, 66

2.3.6 Nanoparticles

Nanocomposites have gained much attention as a novel and environmentally friendly alternative development in the area of flame-retardancy. Nanocomposites typically offer many advantages over conventional formulations that often require a high loading to achieve the desired performance.68 Nanocomposites generally consist of a polymer matrix (e.g., polystyrene (PS),69 ethylene-vinyl acetate (EVA),70 etc.) and nanometer

BACKGROUND

sized reinforcing elements (e.g., nanoclay, carbon nanotubes, oligosilsesquioxane, silica nanoparticles, etc.)45 that can be processed with techniques such as extrusion, injection moulding and casting.68, 71 Nanoparticles such as layered silicates with a high aspect ratio (i.e., two dimensional platelets) are commonly used to improve not only the flame-retardant properties but also the physical properties of a polymeric material. The exfoliation of the layered structure in these materials enhances the interfacial contact area with the polymeric matrix and hence reduces the amount of nanofiller required.72 Flame-retardant characteristics of nanocomposites can be classified as condensed phase active. More precisely, when polymer nanocomposites decompose upon heating, nanofillers form a protective barrier that consists of a multilayer carbonaceous silicate structure which has been described by Camino et al. as a labyrinth barrier formation.70 Gilman et al. suggested that the remarkable flame- retardant mechanism of nanocomposites is a consequence of the formation of a high performance char (i.e., carbonaceous-silicate) along the surface during combustion which insulates the underlying polymeric material and suppresses the rate of mass loss of the decomposed products.73 This silicate-reinforced char layer is formed when the polymer degrades and the silicate layers reassemble and then sinter to form a ceramic carbon layer.73 Although the thermal, mechanical and flame-retardant properties of nanocomposites are greatly influenced by their structure and geometry, it is also worth noting that their flame-retardant efficiency depends very much on the type of flame- retardant and on the flammability test used.20 The fire-retardant properties of a material are assessed by a variety of techniques. The most common methods that are used are the limiting oxygen index (LOI), the flammability test (UL-94), and cone calorimetry. The oxygen index measurement determines the minimum concentration of oxygen in a nitrogen-oxygen mixture that is required to sustain combustion,45 the UL-94 test measures the ignitability and flame-spread of materials exposed to a small flame and cone calorimetry measures the heat released in combustion through the use of oxygen consumption calorimetry.45, 74-75 The common flame-retardant effect of nanoscale particles assessed by cone calorimetry is a decrease in the heat release rate due to the formation of a sintered ceramic layer of nanoparticles and char which insulates the polymer from the heat flux. On the other hand, nanoparticles do not show similar flame-retardant properties in LOI and UL-94 tests, since the heat generated by flame is not enough to form and consolidate a protective layer of nanoparticles.20 It has been well investigated that concentration, dispersion/aggregation and aspect ratio are important parameters determining the ability of nanoparticles to improve the flame- retardancy of the material.20

BACKGROUND

Surface properties of polymeric materials play an important role in polymer flammability, since the surface separates the gas phase from the condensed phase and controls the combustion cycle by suppressing the heat and mass transfer. As described earlier, the flame-retardant performance of nanocomposites depends on the accumulation of nanoparticles, which are dispersed in the polymer matrix, on the surface as a thermally stable layer that acts as a barrier to inhibit combustion. Thus, considerable research is focusing on tailoring the surface properties of polymeric materials using nanotechnology in order to impart flame-retardant properties without altering any material bulk properties.76

2.4 Layer-by-Layer assembly technique

The layer-by-layer (LbL) assembly technique consists of a multistep deposition process of oppositely charged polyelectrolytes and/or nanoparticles which are capable of forming multilayer thin films on almost any charged surface10, 77-79 as shown in Figure 6.

Figure 6 Schematic representation of the layer-by-layer assembly technique, where a negatively charged substrate is immersed in the solution of cationic species and consecutively in the solution of anionic species with intermediate rinsing steps.

The principle of multilayer film formation of inorganic colloidal particles was initially presented by Iler et al.80 in 1966 and, in the 1990s, Decher et al.10 introduced the layer- by-layer technique which gained more attention after being developed and significantly

BACKGROUND

extended with the introduction of a broader range of polyelectrolytes and particles.79 The LbL technique was presented as a versatile method to fabricate multilayer thin films at the solid-liquid interface by sequentially adsorbing polymers or nanoparticles with tailored affinities to one another, usually by alternate dipping77, 80 of the substrate in solutions/dispersions or by direct spraying81 of the solution onto the surface. There are however other technologies to fabricate multilayers such as spin coating,82 electromagnetic deposition83 and fluidic assembly.84 These procedures can be repeated until the desired thickness and properties of the multilayer thin film are achieved. Richardson et al. have extensively reviewed various technologies used to fabricate the LbL assembly of nanofilms.85 Typically, a two-component LbL deposition process is shown by the formation of a single bilayer (BL) (i.e., one pair of complementary layers) (Figure 6) but the flexibility of the concept allows the repeating sequence to be extended to trilayers (TL)86 and quadlayers (QL).87-88 Over the past few decades, the layer-by-layer assembly of thin films has gained interest since it allows a bottom-up nanometer-scale control of the film thickness and because of the extensive choice of materials for coating including flat substrates,77, 80 particles,89 textiles,90 porous substrates16, 86 and wood fibres.91-92 In addition, this environmentally friendly surface engineering technique offers several multi-functionalities of the deposited coatings such as gas barrier,88 anti-bacterial,93 anti-reflection,94 super hydrophobicity,95 drug delivery,96 electrical conductivity97 and flame-retardancy.4-5

2.4.1 Flame-retardant coatings via layer-by-layer assembly

As previously described with respect to nanocomposite flame-retardants, a silicate filler (i.e., clay) can play an important role during combustion by forming an ordered structure composed of clay platelets and carbonized char which acts as a physical barrier (i.e., heat shield) at the flame-substrate interface, and thus protects the underlying material and limits the volatile emission.70 The first demonstration of a flame-retardant multilayer nanocoating using polyelectrolyte and nano-clay was inspired by this concept. The LbL technique has recently been demonstrated as a novel alternative technique to deposit thin film coatings consist of polymer (i.e., polyethylenimine (PEI)) and clay (i.e., montmorillonite (MMT)) on cotton textile in an effort to impart flame-retardant characteristics.98 Residues after the vertical flame test showed that 20 bilayers of this PEI/MMT thin film (~60 nm) partially protected cotton, preserved the weave structure and reduced the total heat release assessed by cone calorimetry as a result of a uniform and continuous char over the surface of the cotton fabric.98 Depending to their chemical structure, natural and synthetic polymeric materials behave differently when they are exposed to a flame. Thus, the LbL

BACKGROUND

assembled flame-retardant coatings need to be tailored for each specific substrate. It is possible to target different surface flame-retardant mechanisms using the versatile LbL technique which allows the chemistry and functionality of the coating to be altered to suit a specific substrate. Since the 1930s, intumescent chemistry has been used to confer flame-retardancy properties to metals, steel, wood and plastics. Traditional strategies and the latest developments in intumescent flame-retardants have been reviewed by Alongi et al.62 As mentioned earlier, intumescence is a chemical process that is activated by flame or heat application.57, 59, 61 The decomposition of substance, typically polymer, at the flame-substrate interface produces a swollen carbonaceous (i.e., char) layer (Figure 5) that thermally insulates the underlying substrate and suppresses the release of flammable volatiles as well as oxygen transfer between the gas and condensed phases, resulting in improved flame-retardancy.57, 59, 61-62 The translation of this chemistry into multilayer nanocoatings has been investigated and the LbL technique has been used to design intumescent nanocoatings by simultaneously providing the three main ingredients needed for intumescence (i.e., a carbon source, an acid source, and a blowing agent).99-102 Although one of the main characteristics of an intumescent flame-retardant is the formation of a macroscopic expanded char barrier, this behaviour is observed as a micro-scale bubbling in intumescent nanocoatings.103 Recently, Li et al. developed an LbL-assembled intumescent nanocoating consisting of poly(allylamine hydrochloride) (PAH) (i.e., blowing agent) and sodium hexametaphosphate (SHMP) (i.e., acid source) to create a char-forming coating where cotton cellulose acts as the carbon source. This thin film (~500 nm) self-extinguished the flame during vertical flame testing.99 This work became an inspiration for several other applications of intumescent coatings on different types of substrates such as ramie,104 polyester-cotton blend,101 polyethylene terephthalate (PET).105 In addition, various studies have been conducted using environmentally benign and sustainable building blocks (i.e., DNA,106 chitosan,4, 100 carbon nanotubes,11, 107 starch,108 etc.) to improve the flame-retardant properties of polyurethane foam (PUF),11, 86 melamine foam (MF),109 PET foam,110 polysiloxane foam (SiF),111 polylactide sheets (PLA),112 polycarbonate film (PC),113 polystyrene sheet (PS),114 and nylon (PA6).115

EXPERIMENTAL

3 EXPERIMENTAL

3.1 Materials

3.1.1 Wood-based cellulose fibres and fibrils

The fibres used in Papers I, III & IV were dried virgin softwood kraft fibres from SCA Forest Products (Östrand Mill, Sweden).The fibres were bleached according to a totally chlorine free (TCF) bleaching sequence (OO)Q(OP)(ZQ)(PO) where O= Oxygen, Q= Complexing stage containing chelating agents, P= Hydrogen peroxide, and Z= Ozone. The pulp was supplied as dry sheets which were disintegrated in deionized water. Before use, the fibres were washed and the counter-ions to the carboxyl groups in the pulp were exchanged to sodium ions according to a previously described procedure.116

Dissolving grade never-dried sulphite softwood fibres from Domsjö Fabriker AB, Örnsköldsvik, Sweden were used for the preparation of carboxymethylated fibres according to a method described earlier.117 Carboxymethylated fibres were used to prepare cellulose nanofibrils (CNF) (Paper II), paper sheets (Paper III), and cellulose gel beads (Paper V).

Before the carboxymethylation, the never-dried fibres were dispersed in deionized water at 10 000 revolutions using a laboratory slusher. After being solvent exchanged to ethanol, the fibres were impregnated with a solution containing 2 wt% monochloroacetic acid in isopropanol for 30 min. The impregnated fibres were then transferred to a solution of 3.24 wt% NaOH in methanol and mixed with isopropanol preheated to just below its boiling temperature (85 °C). The mixture was allowed to react for one hour while being refluxed. After carboxymethylation, the fibres were washed first with deionized water, then with acetic acid solution (0.1 M) and again with deionized water. The carboxyl groups on the fibres were converted to their sodium form by impregnating the fibres in NaHCO3 solution (4 wt%) for one hour. The fibres were finally washed to remove any excess salt. The charge density of the fibres was ~600 µeq/g as determined by conductometric titration. The carboxymethylation protocol was tailored to obtain the desired degree of substitution by altering the amount of monochloroacetic acid added. The carboxymethylated cellulose nanofilbrils117 used in Paper II were prepared at Innventia AB, Stockholm, Sweden (now RISE

EXPERIMENTAL

Bioeconomy) using high pressure homogenization at a ~2 wt% concentration following the above procedure.

3.1.2 Model surfaces

P-type, boron-doped, single side polished silicon wafers were purchased from MEMC electronic materials (Novara, Italy) or Addison Engineering, Inc. (San Jose, CA) and used as solid substrates for LbL deposition in model studies (Papers I, II, III, IV & V). The silicon wafer substrates were cleaned with a sequence of Milli-Q water, ethanol, Milli-Q water and then dried with a stream of nitrogen before use. In order to activate the surface, the wafers were plasma treated for 3 min at 30 W under reduced air pressure (PDC 002, Harrick Scientific Corp., NY, USA).

Silicon dioxide (SiO2) coated quartz crystal sensors (QSX 303, Q-Sense AB, Göteborg, Sweden) were used as substrate for the LbL deposition using Quartz Crystal Microbalance with Dissipation (Papers I, II, III, IV & V).

Model cellulose II surfaces were prepared following a procedure previously described118 where the pulp fibres were dissolved in a mixture of N-methylmorpholine- N-oxide (NMMO) and dimethylsulfoxide (DMSO). Non-crystalline cellulose surfaces were prepared following a procedure119-120 in which the fibres were dissolved in a solution of lithium chloride (LiCl, Merck) in N,N-dimethylacetamide (DMAc, >99.9%, Sigma Aldrich). The dissolved cellulose solutions were spin-coated using a KW-4A spin coater (Chemat Technology Inc., Northridge, CA) onto silicon wafer substrates which were pre-treated with polyvinylamine (PVAm). These cellulose surfaces were then placed in Milli-Q water to precipitate and remove any excess solvent before being blown dry with nitrogen.

3.1.3 Chemicals

In Paper I, cationic chitosan (CH) (Mw= 60 kDa, 95% deacetylation, GTC Union

Corp., Qingdao, China) and anionic poly(vinyl phosphonic acid) (PVPA) (Mw= 24 kDa, Polysciences Inc., Eppelheim, Germany) were used to prepare LbL-assembled thin films. The CH solution was prepared by adjusting the pH of Milli-Q water to pH 2 using hydrochloric acid (HCl, 6M) and 0.1 wt% CH was then added. The solution was stirred with a magnetic stirrer for 24 h to ensure complete dissolution. In Papers II, IV and V, 0.1 wt% or 0.5 wt% CH was added to 1 v/v% acetic acid solution and stirred with a magnetic stirrer for 24 h. The adsorption pH of the CH solutions was adjusted

EXPERIMENTAL

using sodium hydroxide solution (NaOH, 5M). In Paper III, poly(ethylenimine) (PEI)

(Mw= 25 kDa and 750 kDa, 50% aqueous solution, Sigma-Aldrich, Stockholm,

Sweden) and in Papers III & V, sodium hexametaphosphate (SHMP) (Mw=611.77 Da, +200 mesh, 96%, Sigma-Aldrich, Stockholm, Sweden) was used to form multilayers. Montmorillonite (MMT) (Cloisite Na+, average thickness of individual platelets is 1 nm, dimensions between 10 and 1000 nm and density of 2.86 kg/m3, BYK Additives and Instruments, Wesel, Germany) was used in Papers II & IV. MMT dispersions were prepared by first stirring the suspensions overnight using a magnetic stirrer, followed by high shear stirring with an Ultra Turrax mixer (IKA, T 25 Basic) at 15 000 rpm for 15 min, and by sonication with a Vibra-Cell ultrasonic processor (Sonics and Materials, Inc.) for 10 min. The dispersions were then centrifuged at 4 500 rpm for 1 h and the supernatant (0.7 wt%) was used to prepare clay dispersions. Sepiolite (SEP) is a complex magnesium silicate with a length of 10-5000 nm, a width of 10-30 nm, and a thickness of 5-10 nm (Sigma-Aldrich, Stockholm, Sweden). Sepiolite dispersions were also prepared following the procedure described for MMT, except that the pH of the suspension was set to pH 10 and suspension was centrifuged at 4 000 rpm. Colloidal silica (SNP) Bindzil 30/220 with a primary particle size of 12 nm, SiO2 content 30 2 wt%, Na2O content 0.3 wt% and a specific surface area of 220 m /g, was supplied by Eka Chemicals, Sweden. Sepiolite and colloidal silica were used in Paper IV. The polyelectrolytes and clays were used as received without further purification.

Polyvinylamine (PVAm) (Mw= 340 kDa, commercial grade Xelorex RS 1300, BASF, Ludwigshafen, Germany) was used as anchoring layer during model cellulose surface preparation (Papers I, III & V) and as a wet-strength additive during laboratory sheet preparation (Paper III). Before use, the PVAm was dialyzed and freeze-dried to remove excess electrolyte and possible contaminants. Sodium chloride (NaCl), sodium hydroxide (NaOH), hydrochloric acid (HCl), isopropanol, ethanol, methanol, acetone were purchased from VWR. Mono-chloroacetic acid, 1,2,3,4-butane tetracarboxylic acid, sodium hypophosphite, N,N-Dimethylacetamide (DMAc) and lithium chloride (LiCl) were purchased from Sigma-Aldrich. Sodium metaperiodate was purchased from Alfa Aesar and acetic acid was purchased from Acros Organics. All aqueous solutions were prepared using Milli-Q grade water (18.2 MΩ cm, Synergy 185, Millipore, Billerica, USA).

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3.2 Methods

3.2.1 LbL film deposition on flat model surfaces

LbL thin films were deposited on model surfaces (i.e., silicon wafer, model cellulose surfaces) by sequentially dipping the substrates into polymer solutions or nanoparticle dispersions, where a dipping robot (StratoSequence VI, nanoStrata Inc., Tallahassee, USA) was used for high numbers of BL depositions. In Paper II, CH, PVPA and MMT were used to fabricate a QL assembly of thin film where one sequence of CH/PVPA/CH/MMT is referred to a quadlayer (QL). The polyelectrolyte/nanoparticle concentration, salt concentration, pH of solution/dispersion, and adsorption times are given in the experimental part of each paper.

3.2.2 LbL assembly on fibres

The fibres were consecutively treated with cationic and anionic polyelectrolytes in an effort to achieve an LbL assembly of thin films according to the previously described procedure.91 In brief, the amount of fibres aiming at a grammage of 100 g/m2 was suspended in deionized water and the pH and the background electrolyte concentration were adjusted according to cationic adsorption. The cationic polyelectrolyte was added to the fibre suspension and allowed to adsorb for a certain period of time (10-30 min). The fibre suspension and polyelectrolyte mixture was then filtered using vacuum filtration and the fibre pad was rinsed with deionized water to remove excess polyelectrolyte. The fibre pad was then suspended in deionized water and the pH and background electrolyte were adjusted according to the next layer. The anionic polyelectrolyte was added and allowed to adsorb for the same length of time as the initial layer. The suspension was then again filtered and rinsed with deionized water. This adsorption sequence was referred as one bilayer (BL) and it was repeated until the desired number of BL was achieved.

3.2.3 Laboratory sheet preparation

Laboratory sheets with a targeted grammage of 100 g/m2 were prepared using the Rapid Köthen sheet preparation equipment (Paper Testing Instruments, PTI, Pettenbach, Austria). The sheets were dried at 93 °C and 95 kPa for 15 min and conditioned at 23 °C and 50% relative humidity before further testing. The grammage, thickness and density of the sheets were determined and dry tensile testing was

EXPERIMENTAL

performed using an Instron 5944 with a 500 N load cell. The strength properties of paper were evaluated by the tensile index, which is the maximum tensile force at break per unit width and grammage, and by the strain at break.

3.2.4 Preparation of CNF aerogels

Wet-stable CNF aerogels were prepared according to a method described earlier.16 The CNF gel (~2 wt%) was mixed with 1,2,3,4-butanetetracarboxylic acid (BTCA), used as a cross-linker, in a mass ratio of 1:1. Sodium hypophosphite (SHP) was used as a catalyst and was mixed with the CNF gel in a mass ratio of 1:2. The mixture was stirred for 20 min using an Ultra Turrax T25 (IKA, Germany) at 10 000 rpm. The gel mixture was then transferred to an aluminium mould and frozen using dry ice, followed by freeze-drying to obtain an aerogel. The dried aerogels were then heated to 170 °C for 5 min to ensure a covalent cross-linking reaction with the BTCA/SHP combination. Prior to any modification, the cross-linked aerogels were thoroughly rinsed with Milli- Q water and dewatered until the conductivity of the filtrate was below 5 µS/cm in order to remove any unreacted BTCA and SHP.

3.2.5 Preparation of fibre networks

The fibres were first washed and then suspended at a consistency of 1 wt% in 2- propanol solution (6.3 wt%). Sodium metaperiodate (2.7 g NaIO4/g fibre) was added to the suspension and the mixture was stirred for 2 h at room temperature. The reaction mixture was protected from light since the metaperiodate is light-sensitive. After 2 h, the fibre suspension was transferred to a container where the fibres were allowed to self-assemble during 24 h. The fibres were then solvent exchanged to acetone and rinsed with deionized water until a conductivity of 5 µS/cm was reached. Fibre network was finally rinsed with acetone in order to interlock the network structure which was then allowed to dry at room temperature for 24 h.

3.2.6 LbL formation on cellulose fibre/fibril based porous substrates

LbL-assembled thin films were deposited on porous cellulose substrates via the vacuum-assisted LbL-method.16 The porous substrate was first placed in a cationic polyelectrolyte solution for 5 min, and the substrate was then transferred to a Büchner funnel in order to force the excess solution through the porous network using vacuum.

EXPERIMENTAL

The substrate was then rinsed with an excessive amount of Milli-Q water to remove the weakly attached polymer while maintaining the vacuum. In order to form a BL, the substrate was then placed in an anionic solution/dispersion for 5 min, followed by filtration and rinsing. A schematic description of the LbL deposition is shown in Figure 7. In Paper II, the adsorption sequence of (CH/PVPA/CH/MMT) was used to deposit five quadlayers on CNF aerogels and in Paper IV, (CH/SHMP/CH/MMT), (CH/SHMP/CH/SEP), and (CH/SHMP/CH/SNP) sequences were used to deposit five quadlayers on the fibre networks. The adsorption time used was 5 min for the initial quadlayer in order to achieve a uniform deposition and 1 min for the next four quadlayers. The CNF aerogels were dried in an oven at 50 °C and the fibre networks were solvent exchanged to acetone and then dried at room temperature after the LbL treatment.

Figure 7 Illustration of the LbL build-up of quadlayer systems onto cellulose porous substrate by the rapid filtration technique.

3.2.7 Preparation of cellulose gel beads

Dissolving grade pulp received from Domsjö Fabriker AB, Örnsköldsvik Sweden was carboxymethylated to a degree of 795 µeq/g using the procedure described by Wågberg et al.117 1 g of the carboxymethylated pulp was then dissolved in 100 mL of a 7 wt% solution of LiCl/DMAc, following the steps previously described by Karlsson et al.9 and Carrick et al.121 The water in the pulp was then solvent exchanged, first by displacing the water with ethanol followed by an exchange to DMAc using a filtration procedure. Each solvent was displaced over a time of two days during which the solvent was changed at least twice per day. After this first step, the DMAc was dehydrated by preheating and kept for 30 min at a temperature of 105 °C. The LiCl was

EXPERIMENTAL

also dehydrated during 30 min by keeping it in an oven at 105 °C. After the dehydration, the DMAc was allowed to cool and the LiCl was added and dissolved during this time. The pulp was added to the DMAc/LiCl solution at a temperature of about 40 °C and then immediately placed in a 5 °C refrigerating room and stirred overnight. After about 24 h, the solution was clear and it was then filtered through a 0.45 µm PTFE syringe filter. The filtered solution was used to form the gel beads by being precipitated drop-wise through a needle of 0.64 mm into about 95 mL non- solvent consisting of 80 mL 0.03 M HCl (aq) with an addition of 15 mL ethanol. The gel beads formed were allowed to rest in the bottom of the beaker at 5 °C for 24 h. The non-solvent was then displaced with deionized water by decanting about 80 mL of the non-solvent four times during two days while stepwise decreasing the concentration of HCl. The gel beads were then washed with deionized water for one week. Prior to LbL treatment, the gel beads were dried at 23 °C and 50% RH.

3.2.8 LbL formation of thin film on cellulose gel beads

LbL thin films were deposited on the dried cellulose gel beads using a method similar to the vacuum-assisted LbL method. The cellulose beads were placed in a filtration set up, the first cationic CH solution (1 g/L, pH 5) was added and the polymer was allowed to adsorb for 5 min. The solution was then filtered and the cellulose beads were rinsed thoroughly using Milli-Q water at pH 5. For the second layer deposition, an anionic SHMP solution (5 g/L, pH 5) was added and allowed to adsorb for 5 min, followed by filtration and rinsing. This adsorption sequence is referred as a single BL and it was repeated with 1 min adsorption steps until the desired number of BLs was formed. A schematic description of the LbL deposition is shown in Figure 8.

EXPERIMENTAL

Figure 8 Schematic description of the LbL deposition on cellulose beads.

3.3 Characterization techniques

3.3.1 Quartz crystal microbalance with dissipation (QCM-D)

The deposition of the LbL assembled thin films was monitored in situ using an E4 QCM-D (Q-Sense AB, Göteborg, Sweden). The substrates used for model studies were either silicon dioxide coated quartz crystal or a model cellulose surface which was prepared according to the procedure described in section 3.1.2. The QCM-D was used to measure the adsorbed mass and visco-elastic properties of the each adsorbed layer during the build-up of the multilayer LbL thin film. The change in normalized resonance frequency of the quartz crystal is proportional to the adsorbed mass including the associated solvent. The adsorption onto the crystal is detected as a decrease in the resonance frequency and the mass deposited was calculated using the Sauerbrey relationship for flat, rigid and uniform conformations:

∆푓 ∆푚 = 퐶 (1) 푛 where ∆푚 is the adsorbed mass per unit area (mg/m2), 퐶 is the sensitivity constant (- 0.177 mg/Hz m2), 훥푓 is the change in crystal frequency (Hz), and 푛 is the overtone number. The energy dissipation of the adsorbed layer was determined from the decay in resonance amplitude with time after the driving voltage had been switched off. The energy dissipation (퐷) of the oscillating crystal is calculated according to:

EXPERIMENTAL

퐸 퐷 = 푑푖푠푠푖푝푎푡푒푑 (2) 2휋퐸푠푡표푟푒푑 where 퐸푑푖푠푠푖푝푎푡푒푑 is the energy dissipated during one oscillation period and 퐸푠푡표푟푒푑 is the energy stored in the oscillating system. The change in energy dissipation provides information about the visco-elastic properties of the adsorbed layer. Typically, a rigid thin layer shows a low or no dissipation change, while a mobile thick layer shows a larger change in dissipation.

3.3.2 Atomic force microscopy (AFM)

The dry thickness and the surface roughness defined as Rq of the LbL-deposited thin films on model surfaces were measured using an AFM Nanoscope IIIa equipped with E and J type piezoelectric scanners (Bruker AXS, Santa Barbara, CA). An E-type piezoelectric scanner was used for high resolution imaging and these images were used to determine the roughness. A J-type piezoelectric scanner was used to image scalpel- scratched areas and measure the thickness of the film. In Paper I, the silicon cantilever with a spring constant of 5 N/m (TAP 150, Bruker, Camarillo, CA) was used for tapping mode imaging in air. In Paper III, Scanasysts cantilevers with a spring constant of 0.4 N/m was used for tapping mode imaging in air. In Papers II, IV & V, the images were acquired in air using Scanasyst cantilevers with a nominal resonance frequency of 70 kHz and a spring constant of 0.4 N/m in Scanasyst mode.

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

The surface morphology of cellulose fibres (Papers I, III & IV), porous cellulose nanofibril aerogels (Paper II) and cellulose gel beads (Paper V) was investigated before and after the LbL assembly treatment using a Hitachi S-4800 field-emission scanning electron microscope (FE-SEM). Secondary electron images were acquired before and after the flammability test to study the micro-structure of the LbL coatings. The specimens were fixed to an aluminium imaging stub with a conductive adhesive tape and coated with ~5 nm thick platinum/palladium layer using a Cressington 208 HR high resolution sputter coater.

EXPERIMENTAL

3.3.4 Energy dispersive X-ray analysis (EDX)

An energy dispersive X-ray spectrometer (Inca Oxford Instruments, X-MAX, N80) was used for the elemental analysis of the LbL-treated fibre network (Paper IV) and LbL-treated cellulose gel beads (Paper V). The beam voltage was 5kV and the specimens were cut with a razor blade and mounted on aluminium stubs with adhesive copper tapes. Samples for EDX were not sputter coated with Pd/Pt.

3.3.5 Polyelectrolyte titration (PET)

Polyelectrolyte titration was used to determine the charge density of the polyelectrolytes/nanoparticles and the surface charge density of the wood fibres. PET was also used to determine the amount of polyelectrolyte and nanoparticles adsorbed onto wood fibres/CNF aerogels. In brief, the cationic polyelectrolytes were allowed to adsorb onto wood fibres (Papers I & III) and CNF aerogels (Paper II) and the residual amount was determined by titrating with potassium polyvinyl sulphate (KPVS, Wako Pure Chemical Industries, Osaka, Japan) in the presence of an ortho-toluidine blue indicator (OTB, VWR, Stockholm, Sweden). The colorimetric end point was detected with the optical two-beam method in which the photoelectric detector unit (BASF AG, Ludwigshafen, Germany) was connected to an auto titration unit (716 DMS Titrino, Metrohm AG, Switzerland).122 Poly(diallydimethylammonium chloride) (PDADMAC, Sigma-Aldrich, Stockholm, Sweden) was used to determine the amount of consecutively adsorbed negatively charged polyelectrolytes/nanoparticles using a rapid particle charge titration unit (Particle Metrix Stabino, Meerbusch, Germany).

3.3.6 Nitrogen analysis (ANTEK)

The nitrogen analysis was used to determine the nitrogen content of the paper sheets prepared with LbL-treated fibres (Paper III), LbL-treated fibre networks (Paper IV) and LbL-treated cellulose beads (Paper V), and thus to determine the amount of polymer adsorbed. The technique is based on the combustion of the sample (5-10 mg) in an oxygen-poor atmosphere at 1050 °C. At high temperature, nitrogen is oxidized to nitrogen oxides and then converted to excited nitrogen dioxide through an ozone treatment. The photomultiplier tube in the nitrogen analyser ANTEK 700 (Antek

Instruments, Houston, TX, USA) detects the light emitted as the excited NO2 returns to its non-excited state and the result is presented as the number of counts that corresponds to the emitted light. Calibration curves based on known amounts of

EXPERIMENTAL

polyelectrolytes were used to determine the amount of adsorbed polyelectrolytes on fibres, fibre networks and cellulose beads.

3.3.7 Thermogravimetric analysis (TGA)

Thermogravimetric analysis (TGA) was used to study the thermal and thermo- oxidative degradation of cellulose-based materials under nitrogen and air atmosphere. Thermal analyses for Paper I & II were conducted with ca. 10 mg sample in open alumina pans using a TA Instruments Q500 (South Carolina, USA). The samples were heated from 50 °C to 800 °C at a rate of 10 °C/min under a constant gas flow of 60 mL/min. The thermal stability of paper sheets prepared from LbL-treated fibres (Paper III), LbL-treated fibre networks (Paper IV), and LbL-treated cellulose beads (Paper V) was investigated using a Mettler Toledo TGA/DSC equipment (Stockholm, Sweden) under nitrogen atmosphere. The samples (ca. 10 mg) were placed in alumina crucibles and heated from 40 °C to 800 °C at a rate of 10 °C/min under a nitrogen flow of 50 mL/min.

3.3.8 Flammability test

The flame-retardant properties of LbL-treated paper sheets (Paper I & III), CNF aerogels (Paper II), and fibre networks (Paper IV) were assessed by flammability tests. Paper samples (30 × 100 mm2) were tested on a metal frame in horizontal configuration (Paper I) and in both horizontal and vertical configurations (Paper III). CNF aerogels (Paper II) samples (20 × 80 × 20 mm3) and fibre networks (Paper IV) samples (30 × 70 × 10 mm3) were placed on a wire mesh in a horizontal configuration. All the samples were exposed to a methane flame following the UL-94 standard for sample positioning and flame characteristics. The burning time, burning behaviour, and amount of residue were investigated. The tests were repeated three or four times in order to assess their reproducibility.

3.3.9 Cone calorimetry

The flame-retardant characteristics of cellulose-based materials (i.e., paper, CNF aerogel, and fibre network) were also assessed by cone calorimetry. The cone calorimeter is an intermediate scale flammability testing instrument based on the oxygen consumption during the combustion of the specimen. The sample is exposed to a constant heat flux of 1-100 kW/m2 generated by a conical heater and the combustion is then triggered by an electric spark. The oxygen consumption in the combustion gases

EXPERIMENTAL

was measured and used to calculate the heat released per unit time and surface area, known as the heat release rate (HRR, kW/m2). The calculation is based on the proportionality of energy release for every gram of oxygen consumed. The universally accepted proportionality constant for organic compounds is 13.1 kJ/g.123 In general, the HRR and in particular the peak of heat release (pkHRR, kW/m2) generated by combustion of the tested specimen are the most important parameters for evaluating the flammability of a material. In addition, the cone calorimeter test provides other fire parameters such as time to ignition (TTI, s), total smoke release (TSR, m2/m2), total heat release (THR, MJ/m2), and maximum average rate of heat emission (MARHE, kW/m2). The cone calorimetry tests were conducted in accordance with the international standard (ISO 5660) four times for each sample to assess the reproducibility and significance of the data. Before each test the samples were conditioned for at least 48 h in a climate chamber (23 °C and 50% RH). The uncertainty was evaluated as the standard deviation.

3.3.10 Flame penetration test

The resistance to severe flame conditions of LbL-treated CNF aerogels was evaluated by a flame penetration test in which the centre of the specimen (50 × 50 × 10 mm3) was subjected to a butane flame from a distance of 100 mm. The temperatures on the surface of the aerogel exposed to the flame and on the rear side of the specimen were measured throughout the test by thermo-couples (Stainless steel sheathead, k-type, 1.5 mm, Milano, Italy) kept in contact with the surfaces. The flame was continuously applied up to ~3 min and monitored with a camera in real time and the tests were repeated four times to assess the reproducibility.

3.3.11 Limiting oxygen index (LOI)

The principle of the limiting oxygen index measurement is based on the determination of the minimum concentration of oxygen in a nitrogen/oxygen mixture required to sustain the combustion of a vertically positioned specimen. In Paper III, paper samples were subjected to LOI measurements following the ASTM D 2863 standard, using a FIRE oxygen Index apparatus. A low LOI value indicates that the material is highly flammable in contrast to a high LOI value which generally indicates that the material is classified as less flammable or flame-retardant, since a high concentration of oxygen is required to sustain the combustion.

RESULTS AND DISCUSSION

4 RESULTS AND DISCUSSION

4.1 Flame-retardant paper (Papers I & III)

4.1.1 The quantification of LbL film formation on model surfaces

In Paper I, the adsorption of multilayer thin films of cationic chitosan (CH) and anionic poly(vinylphosphonic acid) (PVPA) on model cellulose surfaces was characterized using QCM-D. The change in the oscillation frequency and energy dissipation due to the adsorbed mass of the films of polyelectrolytes on the quartz crystal were monitored as a function of time. The normalized change in frequency and energy dissipation for the third overtone for the build-up of nine bilayers of CH/PVPA thin film are shown in Figure 9.

a) b)

Figure 9 The build-up of 9 bilayers of (CH/PVPA) multilayer film on a model cellulose surface a) without additional background electrolyte and b) with the addition of 10 mM NaCl, monitored by QCM-D as the normalized frequency shift for the third overtone and the change in energy dissipation. The CH and PVPA concentrations were 1 g/L and the pH was constant at pH 4.

The pH of the polyelectrolyte solutions and rinsing solutions was adjusted to pH 4 in order to achieve good solubility of chitosan throughout the LbL build-up. Figure 10 shows the continuous build-up of multilayers on model cellulose surfaces both with and without background electrolyte, where the adsorbed mass associated with solvent was calculated using the Sauerbrey equation (Eq. 1). The mass of multilayer adsorbed was larger with 10 mM NaCl than without electrolyte, and it was suggested that polymer chains are more densely packed on the surface in the presence of the NaCl electrolyte. The low energy dissipation of thin film with 10 mM NaCl further indicates

RESULTS AND DISCUSSION

the formation of a more compact and rigid layer, which is probably due to relaxation and reconformation of polyelectrolyte chains towards the solid/liquid interface.

Figure 10 The adsorbed mass of 9 bilayers of (CH/PVPA) with 10 mM NaCl and without background electrolyte including immobilized water calculated using the Sauerbrey equation for the third overtone. The CH and PVPA concentrations were 1 g/L and the pH was constant at pH 4.

The thickness of the 20 BLs of dry thin film on the silicon oxide surface (~25 nm) was also measured using both ellipsometry and AFM by scanning a thin film scratched with a scalpel (Figure 11). From these results, it has been suggested that continuous film formation was observed after deposition of a minimum of 10 BLs of CH/PVPA.

RESULTS AND DISCUSSION

Figure 11 a) The thickness of a dry thin film as a function of the number of bilayers deposited measured by ellipsometry and b) the thickness of 20BLs of (CH/PVPA) thin films deposited on a silicon oxide surface measured by AFM on a scratched film. The CH and PVPA solutions were 1 g/L with 10 mM NaCl at pH 4.

In Paper III, two different molecular masses of PEI (i.e., LMw= 2kDa and HMw= 750kDa) and SHMP were used to construct multilayer thin films on model cellulose surfaces with two different charge densities, and the sequential deposition was monitored in situ using QCM-D. Figure 12 shows as a function of layer number the frequency shift, the change in energy dissipation and the adsorbed mass calculated using the Sauerbrey equation.

RESULTS AND DISCUSSION

a) b)

Figure 12 LbL assembly of PEI/SHMP thin film on model cellulose surfaces and model carboxymethylated cellulose surfaces as a function of layer number. a) Normalized frequency shift, b) change in energy dissipation, and c) adsorbed mass during the build-up of 3.5 BLs of multilayer for the third overtone. The PEI and SHMP concentrations were 1 g/L in a 10 mM NaCl aqueous solutions at pH 9 and 4, respectively. The sequence used was 10 min of adsorption followed by 20 min of rinsing for each layer.

The degree of dissociation and charge density of PEI naturally depend on the pH and salt concentration.124 The difference in pH of the polyelectrolyte solutions (i.e., PEI at pH 9 and SHMP at pH 4) in the adsorption steps alters the charge density of the previously adsorbed layer and naturally affects the adsorption in each step. An odd- even effect was observed for the build-up of both low and high molecular mass PEI on both of the model cellulose surfaces and this is also in accordance with earlier results for the adsorption of polyallylamine hydrochloride (PAH) and polyacrylic acid (PAA) which was ascribed to the formation of a less rigid film when the cationic PAH was adsorbed.92, 125 A larger frequency shift was observed during the adsorption of PEI layers, and this translates into a higher adsorbed amount when PEI was in the outermost layer. The adsorbed amount was higher on the higher charge density model

RESULTS AND DISCUSSION

cellulose surface than on the low charge density model surface and it was also found that the HMw-PEI system showed a significantly higher adsorption than the LMw-PEI system on both model surfaces. The low dissipation values for the LMw-PEI system indicate the formation of a rigid film which is closely associated with the surface, whereas the higher dissipation values for the HMw-PEI system indicate the formation of a softer viscoelastic film.

4.1.2 Morphology of multilayer thin films

In Paper I, the surface morphology and roughness of the dry CH/PVPA multilayer films deposited on silicon oxide surfaces were characterized using AFM. Figure 13 shows the AFM tapping mode 3D-height images of different number of bilayers of CH/PVPA films. The images show an uneven film with an average roughness of 6.8 nm at 5BL. When the number of bilayers deposited was increased to 20 BL, the films exhibited a more continuous structure with a somewhat lower roughness of 2 nm.

Figure 13 AFM tapping mode 3D-height images of dry (CH/PVPA) films on silica wafers with increasing number of bilayers. The CH and PVPA concentrations were 1 g/L with 10 mM NaCl at pH 4. The images are 1 × 1 µm2.

In Paper III, the height images of two types of multilayer film consisting of LMw-PEI or HMw-PEI combined with SHMP adsorbed on model cellulose and carboxymethylated cellulose surfaces were obtained. These are shown in Figure 14. The average roughness was approximately the same for the model cellulose and CM- cellulose surfaces (i.e., 8.5 ± 0.4 nm and 8.0 ± 0.1 nm, respectively). The multilayer films did not change the morphology of the model cellulose surfaces, but on the CM- cellulose surface, the multilayer films changed the structure, and the measured roughness of LMw-PEI and HMw-PEI systems was determined to be 11.8 nm and 25.5 nm, respectively.

RESULTS AND DISCUSSION

Figure 14 AFM tapping mode height images of 3.5 bilayers of LMw or HMw PEI and SHMP deposited on a) a model cellulose surface and b) a model CM-cellulose surface. The PEI and SHMP concentrations were 1 g/L in 10 mM NaCl aqueous solutions at pH 9 and pH 4, respectively. The scanned areas were 2 × 2 µm2 and the z-range of the films is indicated in the scale bar to the right of the images.

4.1.3 Multilayer thin film formation on wood fibres

In Paper I, the saturation adsorption of CH was determined by polyelectrolyte titration (PET). A salt concentration of 10 mM NaCl was used since the model studies with QCM-D showed a higher adsorbed amount and one aim was to increase the adsorbed amount with as few layers as possible. The adsorption isotherm showed that the adsorption of CH reached a plateau at 2.6 mg CH/g fibre in the first layer. The amount of polyelectrolyte adsorbed in each consecutive layer was then determined by titrating the filtrates obtained from the dewatered fibre suspensions. The results are presented in Figure 15. As indicated by the QCM-D measurements, there is a continuous build-up of multilayers on wood fibres showing that is possible to increase the adsorbed amount simply by increasing the number of BLs.

RESULTS AND DISCUSSION

Figure 15 Amount of CH and PVPA adsorbed per gram of fibres, determined by PET. The fibres were treated alternately with 10 mg/g fibre addition of CH and PVPA solutions at pH 4 for 10 min, with 10 mM NaCl salt concentration in each layer.

In Paper III, the PET was used to investigate the influence of molecular weight of cationic PEI on the amount adsorbed. The adsorption isotherms for fibres with two different charge densities (i.e., cellulose fibres and carboxymethylated cellulose fibres) were determined for up to two layers using LMw-PEI, HMw-PEI, and SHMP at different polymer concentrations, as shown in Figure 16. As in the model experiments with QCM-D, the CM-cellulose fibres with the higher charge density showed a significantly higher adsorption capacity than the unmodified cellulose fibres.

Furthermore, the results in Figure 16a show that the amount of HMw-PEI adsorbed onto the cellulose fibres was higher than that of LMw-PEI. This is in agreement with the QCM-D measurements.

RESULTS AND DISCUSSION

a) b)

Figure 16 Adsorption isotherms for LMw-PEI, HMw-PEI and consecutive SHMP layers adsorbed onto a) cellulose fibres and b) CM-cellulose fibres determined using PET. The PEI addition was in 10 mM NaCl at pH 9 and SHMP addition was in 10 mM NaCl at pH 4.

The PET results for the cellulose fibres indicate that LMw-PEI is not able to penetrate into the nanoporous fibre wall and that multilayer films are formed only on the external layers of cellulose fibres. However it was noted that, in the case of CM-cellulose fibres, there was no significant difference observed in the adsorbed amount of PEI. The adsorption isotherm of HMw-PEI show a steeper slope than that of LMw-PEI and no true plateau was achieved for several of the adsorption isotherms. The higher degree of swelling of CM-cellulose fibres probably allows the LMw-PEI to penetrate into the fibre wall, resulting in a higher adsorbed amount. The adsorption isotherms of SHMP adsorbed onto the pre-adsorbed layer of PEI show a higher adsorbed amount than the

LMw-PEI and HMw-PEI layers. The adsorption of SHMP is much larger on the CM- cellulose fibres, which is in agreement with QCM-D measurements. The total amount of PEI adsorbed on fibres, after the adsorption of 3.5 BL multilayer films, was also determined using nitrogen analysis. The adsorbed amount of PEI is plotted in Figure 17 for two different types of cellulose fibres, showing higher adsorbed amounts for the

HMw-PEI systems. This result is consistent with the adsorption of a single layer of PEI on fibres characterized with PET and with the model studies showing how the model studies can be used to eliminate the influence of the more complex fibre structure in order to clarify how the interaction between the polyelectrolytes can be used to optimize the BL formation.

RESULTS AND DISCUSSION

Figure 17 Adsorbed amount of PEI on cellulose fibres and CM-cellulose fibres determined using ANTEK nitrogen analysis. The fibres were treated with 3.5 BL of PEI/SHMP. The PEI addition was in 10 mM NaCl at pH 9 and the SHMP addition was in 10 mM NaCl at pH 4.

4.1.4 Thermal stability and flammability of LbL-treated fibres

The thermal stability of paper sheets prepared from cellulose fibres and LbL-treated cellulose fibres under nitrogen and air atmosphere was characterized using thermogravimetric analysis. In Paper I, paper sheets prepared from different numbers of bilayers of CH/PVPA treated fibres were studied. Figure 18 shows the weight loss and derivative weight loss results, and Table 1 summarizes the data from this figure.

RESULTS AND DISCUSSION

a) b)

c) d)

Figure 18 TG and dTG of untreated and CH/PVPA LbL-treated cellulose fibres under a, c) nitrogen and b, d) air atmosphere.

RESULTS AND DISCUSSION

Table 1 TGA data for untreated and CH/PVPA LbL-treated fibres under nitrogen and air atmosphere.

T Residue at Sample onset 10% T [°C] T [°C] [°C] max1 max2 600 °C [%] Nitrogen Paper 323 365 - 13 1 BL 332 365 - 13 5 BL 332 371 - 13 10 BL 329 354 - 20 20 BL 328 350 - 25 Air Paper 310 341 471 1 1 BL 316 341 476 1 5 BL 317 341 503 2 10 BL 314 332 508 6 20 BL 318 332 521 9

The thermal degradation of cellulose in nitrogen resembles the pyrolytic reaction occurring at the flame substrate interface. Figure 18a shows that cellulose undergoes pyrolysis in a single step through two competitive pathways, i.e. depolymerisation and decomposition. Although the initial degradation of LbL-treated fibres occurs at a slightly higher temperature (Tonset 10%), the temperature for the maximum mass loss

(Tmax1) decreases with an increasing number of deposited BLs. The increase in residue at 600 °C also indicates that the LbL treatment enhanced the char-forming ability of cellulose. In air, the cellulose undergoes a two-step degradation (Figure 18b). In the first step (300-400 °C) volatiles are released and aliphatic char is formed and these components are then further oxidised to an aromatic char and CO/CO2 (400-600 °C).

Under an air atmosphere, Tonset 10% increased, Tmax1 reduced, the second degradation temperature (Tmax2) increased and the residue at 600 °C increased with increasing number of deposited BLs.

The change in thermal degradation of cellulose fibres can be explained by the enhanced char formation due to the presence of the phosphorus-containing polymer within the LbL coating. At high temperatures, phosphoric acid is formed and this favours the dehydration of both cellulose and chitosan and subsequently results in the creation of a protective char on the surface of each LbL-treated fibre. The flame-retardant characteristics of the untreated and LbL-treated fibres were assessed using flammability tests in horizontal configuration in which the ignition and propagation of fire on the specimen after exposure to a flame were recorded. Table 2 shows the

RESULTS AND DISCUSSION

flammability results and Figure 19 shows photographs of different samples during and after the flammability test.

Table 2 Flammability data for untreated and CH/PVPA LbL-treated fibres in the presence of 10 mM NaCl at pH 4.

Burning time After-glow time Burning rate Sample Residue [%] [s] [s] [mm/s] Control 60 ± 6 5 ± 1 1.5 ± 0.1 - 1 BL 57 ± 2 7 ± 2 1.6 ± 0.1 - 5 BL 52 ± 2 - 1.9 ± 0.1 9 10 BL 53 ± 5 - 1.9 ± 0.2 11 20 BL 38 ± 8 - 1.7 ± 0.1 70

Figure 19 Photographs of flammability test and the residue at the end of the test for untreated and CH/PVPA LbL-treated fibres. The number of bilayers is shown in the figures.

RESULTS AND DISCUSSION

The reference sample was completely consumed by the flame and by the after-glow. On the other hand, as the number of adsorbed BLs increased, the amount of coherent char residue also increased, reaching as high as 70% for 20 BLs. Solid state combustion (i.e., after-glow) was eliminated with a minimum of 5 BLs of CH/PVPA. The greater flame-retardant properties of LbL-treated fibres can be ascribed to an alteration in the pyrolysis of cellulose occurring adjacent to the flame. More specifically, the LbL coating generates a char layer which is capable of suppressing the mass and heat transfer between the flame and the fibres. The LbL coating also releases a non-flammable gas mixture which dilutes the combustible gas mixture and lowers the temperature. The fibres treated with 20 BLs of CH/PVPA self-extinguished the flame indicating that the surface coverage achieved with the coating was sufficient to promote the homogeneous char formation. SEM micrographs of the residues (Figure 20 f-j) reveal that the dimensional and structural losses of the fibres caused by the combustion are prevented by increasing the number of bilayers. The self-extinguishing 20 BL treated fibres retained their original shape showing the formation of a protective char layer.

Figure 20 SEM images of untreated and (CH/PVPA) LbL-treated fibres before and after the flammability test (inset images, scale bar 10 µm ); (a,f) reference, (b,g) 1 BL, (c,h) 5 BL, (d,i) 10 BL, (e,j) 20 BL and (k) cross section of 20BL.

The combustion of paper sheets before and after the LbL treatment was investigated using cone calorimetry with a heat flux of 35 kW/m2 in a horizontal configuration. The results of these tests are recorded as time to ignition (TTI), total heat release (THR), peak heat release rate (pkHRR), and residue. They are summarized in Table 3. The TTI

RESULTS AND DISCUSSION

of paper samples showed no significant difference after LbL treatment which is in agreement with TGA results where Tonset was similar for all the samples. The pkHRR was gradually reduced when the number of deposited BLs was increased, and 20 BL treated fibres showed a significant reduction in the pkHRR by 49% relative to that of the reference fibres. This can also be ascribed to the improved char-forming ability of LbL coating, which also increased the residue collected at the end of the test. The fire growth rate index (FIGRA) was introduced as an alternative parameter to assess the burning propensity of a material when exposed to a heat flux. It is determined by dividing the pkHRR with the time at pkHRR. The FIGRA value was reduced by increasing the number of BLs deposited, as shown in Table 3.

Table 3 A summary of the cone calorimetry data for untreated and CH/PVPA LbL-treated fibres.

pkHRR FIGRA Residue Sample TTI [s] THR [MJ/m2] [kW/m2] [kW/sm2] [%] Control 38 ± 7 1.2 ± 0.2 83 ± 5 2.1 ± 0.4 - 1 BL 32 ± 4 1.2 ± 0.3 76 ± 3 2.1 ± 0.2 1 ± 1 5 BL 37 ± 5 0.8 ± 0.3 65 ± 11 1.7 ± 0.5 6 ± 1 10 BL 39 ± 3 0.6 ± 0.3 59 ± 14 1.4 ± 0.3 9 ± 2 20 BL 32 ± 12 0.8 ± 0.2 42 ± 7 1.3 ± 0.6 12 ± 1

In Paper III, the flame-retardant properties of paper sheets prepared from untreated fibres, LbL-treated fibres and LbL-treated carboxymethylated fibres were compared. In addition, a wet-strength paper sheet was prepared by treating the fibres with a wet- strength additive (i.e., PVAm) prior to sheet preparation and this paper was LbL-treated with the same LbL coatings by sequential dipping in oppositely charged polyelectrolyte solutions starting with the anionic polyelectrolyte solution since the PVAm-treated fibres exhibited a positive surface potential. The LbL treatments of the fibres coated with HMw or LMw-PEI and SHMP were investigated. All the untreated reference paper sheets showed a single step degradation with similar amounts of residue under nitrogen atmosphere, as shown in Figure 21a, and all the applied LbL treatments showed a reduction in the degradation temperature of cellulose detected as a decrease in the Tonset

10 % and T max, as shown in Table 4.

RESULTS AND DISCUSSION

Table 4 TGA data for untreated and 3.5 BLs of PEI/SHMP LbL-treated fibres in nitrogen.

T [°C] T [°C] Sample onset 10% max Residue [%] Cellulose fibre 328 363 13

LMw 319 345 14

HMw 301 311 34 CM-cellulose fibre 328 363 15

LMw 310 327 24

HMw 301 310 34 Wet-strength paper 319 363 13

LMw 310 354 17

HMw 310 345 18

a) b)

c) d)

Figure 21 Weight loss for a) the reference samples, b) paper prepared from (PEI/SHMP) 3.5 BLs treated cellulose fibres, c) paper prepared from (PEI/SHMP) 3.5 BLs treated CM-cellulose fibres and d) (PEI/SHMP) 3.5 BLs treated wet-strength paper. The TGA measurements were performed in N2 at a heating rate of 10 °C/min.

RESULTS AND DISCUSSION

LbL coatings with HMw-PEI showed a larger reduction in degradation temperature, especially when the coating was applied on individual fibres than the wet-strength paper where the coating was applied to the already formed, dried and re-wetted paper. This can be expected since the amount of SHMP adsorbed was higher in the case of

HMw-PEI than in the case of LMw-PEI, as shown by QCM-D measurements and PET results. The homogeneous film formed with HMw-PEI led to the formation of a continuous and stable char which increased the thermal stability of the cellulose fibres. Paper sheets prepared with LbL-treated fibres showed a significantly larger amount of residue than the LbL-treated wet-strength paper sheets. As previously suggested, this behaviour can be ascribed to the promotion of char formation by the presence of phosphorus in the coating. However, the residual amount was less presumed due to the insufficient multilayer thin film formation on wet-strength paper because of the lack of fibre-fibre contact areas for the LbL-coating. The reaction of the LbL-treated samples to a direct flame was investigated by flammability tests in both horizontal and vertical configurations, Figure 22 shows photographs of the residues after the flammability tests.

RESULTS AND DISCUSSION

Figure 22 Photographs of paper sheets prepared from untreated and 3.5 BLs of LMw or HMw- PEI/SHMP treated cellulose fibres and CM-cellulose fibres compared with untreated wet- strength paper and 3.5 BLs of LMw or HMw-PEI/SHMP treated wet-strength paper after flammability tests. a) Vertical flame test and b) horizontal flame test.

Untreated samples immediately ignited and were completely consumed by the flame and by the after-glow. LbL treatment on fibres and on wet-strength papers eliminated the after-glow and increased the amount of residue in both horizontal and vertical configuration. In fact, HMw-PEI/SHMP coating on fibres showed self-extinguishing behaviour in HFT, which can be attributed to the increased formation of thermally stable char due to the higher adsorbed amount of SHMP. However the coating with

LMw-PEI was not able to achieve self-extinguishing behaviour. This is in agreement with the TGA results. In VFT, all the LbL-treated samples completely burned but the amount of residue was higher for the samples coated with HMw-PEI, and LbL treatment on fibres with HMw-PEI formed a coherent residue. The results obtained with flammability tests showed that the coating efficiency was greater when HMw-PEI was used, and this was associated with an increase in LOI value. The SEM images of HFT

RESULTS AND DISCUSSION

residues show the formation of micro-bubbles on the HMw-PEI/SHMP treated fibre surface, typically observed for intumescent coatings as a result of char formation (Figure 23).

Figure 23 SEM images of the residues of LbL-containing paper samples after HFT for the 3.5 BLs of LMw or HMw PEI, in combination with SHMP treated fibres/papers. a) Cellulose fibres, b) CM-cellulose fibres, and c) wet-strength paper. Micro-bubbles are marked with yellow arrows. (Left LMw-PEI and right HMw-PEI treated samples)

The mechanical properties of LbL-treated paper sheets are presented in Figure 24. Neither tensile index nor strain at break showed any significant change when the fibres were treated with LMw-PEI/SHMP. The low degree of polymer entanglement between

LMw-PEI and SHMP show no improvements in the fibre joint strength. A HMw-PEI coating permits for a greater intermixing of polyelectrolytes at the fibre-fibre interface, which could be explained by the higher water content of the HMw-PEI LbLs detected by the QCM-D measurements.

RESULTS AND DISCUSSION

a) b)

Figure 24 a) Tensile index and b) strain at break values for untreated and LbL-treated paper samples. 3.5 BLs of LMw or HMw PEI, in combination with SHMP, treated fibres/papers (i.e., cellulose fibres, CM-cellulose fibres, and wet-strength paper). The PEI addition was in 10 mM NaCl at pH 9 and SHMP addition was in 10 mM NaCl at pH 4.

4.2 Flame-retardant cellulose-based porous materials (Papers II & IV)

4.2.1 Multilayer thin film build-up

In Paper II, multilayer thin films of CH/PVPA/CH/MMT were assembled onto silicon oxide substrates to study the fundamental build-up of the LbL films. The influence of the concentration of polyelectrolyte and nanoparticle solutions on the formation of LbL thin films was investigated. The film build-up was monitored using QCM-D and the results are shown in Figure 25. Two different solution concentrations (0.1 and 0.5 g/L) were used with a salt concentration of 10 mM NaCl for adsorption and a larger frequency shift was observed in the case of 0.5 g/L concentration indicating a higher adsorbed amount of polymers including immobilized water which was calculated using the Sauerbrey equation. The high energy dissipation can be ascribed to the formation of a viscoelastic thin film for both the concentrations used. Despite the similar increase in energy dissipation values observed in the MMT adsorption after the deposition of the third QLs, the calculated amount adsorbed was larger for 0.5 g/L due to the adsorption kinetics. This can be attributed to the formation of a more expanded and more flexible structure in the outer layers in the case of 0.1 g/L concentration and a more even packing in the adsorbed layers at 0.5 g/L concentration.

RESULTS AND DISCUSSION

a) b)

Figure 25 QCM-D data for the build-up of 5 QLs of CH/PVPA/CH/MMT assemblies at pH 5 with 0.1 and 0.5 g/L solution concentrations a) Frequency shift and energy dissipation as a function of time, and b) adsorbed mass calculated using the Sauerbrey equation in the presence of 10 mM NaCl. The rinsing solution was Milli-Q water at pH 5 and with the addition of 10 mM NaCl. The adsorption sequence used was 10 min of adsorption followed by 10 min of rinsing for each layer.

AFM height images of the LbL films are shown in Figure 26. The thin films adsorbed on silicon oxide surfaces show similar surface roughness after drying, regardless of the solution concentration used. This can be ascribed to the consolidation of clay- containing water-rich layers which lead to the formation of a highly oriented smooth dry film. Despite the similar surface roughness obtained for the higher solution concentration the film was thicker than that made at low concentration, which is supported by the higher adsorbed amount shown by QCM-D.

RESULTS AND DISCUSSION

Figure 26 AFM height images of 5 QLs of (CH/PVPA/CH/MMT) deposited on a silicon wafer from a) 1 g/L and b) 5 g/L solutions in the presence of 10 mM NaCl at pH 5, c) the thickness was measured by scanning the scalpel-scratch area of 10 × 10 µm2, and d) the roughness was calculated by scanning an area of 2 × 2 µm2.

In Paper IV, the effect of the shape of the nanoparticles on the flame-retardant characteristics was investigated. Nanoplatelet MMT, spherical colloidal silica (SNP) and rod-like clay sepiolite (SEP) were used as building blocks during the build-up of QLs with cationic CH and the phosphorus-containing oligomer SHMP. The build-up of nanocoatings on the silicon oxide surface was monitored using the QCM-D. Figure 27 summarizes the results, showing the change in frequency normalized with respect to the third overtone as a continuous linear growth for the MMT and SNP containing QLs. The low dissipation value of SNP indicates a thin, rigid film with less associated water. On the other hand, the high energy dissipation observed for the MMT and SEP containing QLs indicates rather water-rich, soft films. It has been suggested that the drastic decrease in the frequency of SEP after the second QL is due to the high volume fraction of SEP which inhibits the QCM sensor from directly sensing the adsorbed mass.

RESULTS AND DISCUSSION

a) b)

Figure 27 QCM-D data for the formation of the 5 QLs of (CH/SHMP/CH/MMT), (CH/SHMP/CH/SNP), (CH/SHMP/CH/SEP) nanocoatings in the presence of 10 mM NaCl at pH 5. a) Change in normalized frequency, and b) energy dissipation as a function of layer number.

AFM height images were used to measure the thickness and roughness of nanocoatings that were dried with nitrogen gas (Figure 28). Table 5 shows the thickness and roughness of each nanocoating as a function of the number of QL deposited. Despite the difference in thickness, MMT and SNP showed similar surface roughness values, whereas nanocoating with SEP gave a roughness of 17 nm.

Figure 28 AFM height images of 5 QLs of a) (CH/SHMP/CH/MMT), b) (CH/SHMP/CH/SEP), and c) (CH/SHMP/CH/SNP) deposited in the presence of 10 mM NaCl at pH 5. The images are 5 × 5 µm2, and the z-range is indicated in the scale bar to the right in the figure.

RESULTS AND DISCUSSION

Table 5 Thickness and roughness of LbL nanocoatings.

Sample Thickness [nm] Roughness [nm] 5 QL 10 QL 5 QL 10 QL CH/SHMP/CH/MMT 8.0 ± 0.1 25 ± 0.4 4.0 ± 0.4 5.0 ± 0.4 CH/SHMP/CH/SNP 23 ± 3.4 41 ± 2.6 6.0 ± 0.5 8.0 ± 1.2 CH/SHMP/CH/SEP 21 ± 2.0 75 ± 2.5 17 ± 0.3 32 ± 0.9

4.2.2 Thermal degradation of LbL-treated porous substrates

Although the LbL coatings on the CNF aerogels led to a reduction in the cellulose degradation temperature, as shown in Figure 29, all the LbL-treated samples yielded a higher amount of residue than the untreated sample under both nitrogen and air atmosphere. This behaviour can be ascribed to the presence of phosphorus and inorganic clay within the LbL coating in which PVPA acts as acid source that dehydrates the cellulose and favours cellulose char formation during decomposition, while MMT platelets reinforce the char and improve its barrier properties which as a result suppresses the release of combustible volatiles. During thermal degradation in air, the char formed in the first step experiences a rapid oxidation (400-500 °C) due to a failure of the coating in the case of the 1 g/L solution, while the larger adsorbed amount achieved using a 5 g/L solution, shown by weighing the aerogels before and after the LbL treatment and also by QCM-D model studies, produced a more stable charred structure.

RESULTS AND DISCUSSION

a) b)

c) d)

Figure 29 Untreated and 5 QLs of CH/PVPA/CH/MMT LbL-treated CNF aerogels in the presence of 10 mM NaCl at pH 5, TG and dTG plots under (a, c) nitrogen and (b, d) air atmosphere.

In Paper IV, the thermal stability of LbL-treated FNs was also evaluated using TGA under a nitrogen atmosphere. The data are summarized in Table 6. Thermal degradation of LbL-treated FNs occurred in a single step where the T onset 10% showed a reduction which can be attributed to the presence of phosphorus. At 800 °C in nitrogen, the percentage weight residue for each LbL-treated FN was greater than the percentage weight gain due to the LbL coatings. More specifically, the untreated FN left 21 wt% residue and after the addition of 0.9 wt% for 5 QLs of CH/SHMP/CH/MMT coating, the residue amount increased to 30 wt%, which implies that the LbL-coating enhances the char formation of FN.

RESULTS AND DISCUSSION

Table 6 TGA data for untreated and LbL-treated FN under nitrogen atmosphere.

T T Residue at 800 °C Weight Sample onset 10% max [°C] [°C] [%] gain [%] Control-FN 265 324 21.0 ± 0.3 - CH/SHMP/CH/MMT-5QL 250 325 30.6 ± 1.2 0.9 ± 0.3 CH/SHMP/CH/SNP-5QL 249 324 26.4 ± 0.7 1.0 ± 0.6 CH/SHMP/CH/SEP- 5QL 243 324 29.0 ± 3.7 7.9 ± 3.3

4.2.3 Flame-retardant properties of LbL-coatings on low density networks

The application of a direct methane flame immediately ignited the untreated CNF aerogel and the flame propagated along the surface and then extinguished due to the formation of a dense char layer upon collapse of the porous network, leaving 82 wt% of residue. The aerogels coated with 5 QLs of (CH/PVPA/CH/MMT) from 1 g/L solutions self-extinguished the flame immediately after removal of the ignition source, but the after-glow consumed the whole sample due to an insufficient amount of coating, leaving 10 wt% of residue. This behaviour is in agreement with the TGA results in air (Figure 29d) where the second step degradation led to the highest weight loss for the samples coated with 1 g/L solutions. In contrast, LbL coating from 5 g/L solutions self-extinguished the propagating flame, eliminated the after-glow phenomenon and increased the amount of residue to 98 wt%. A possible explanation for the better flame-retardant behaviour with the 5 g/L solution could thus be the improved barrier properties of the LbL coating. The residues of the flammability test are shown in Figure 30.

Figure 30 Photograph of residues after the flammability tests of a) untreated and 5 QLs of (CH/PVPA/CH/MMT) treated CNF based aerogels from b) 1 g/L and c) 5 g/L solutions in the presence of 10 mM NaCl at pH 5.

RESULTS AND DISCUSSION

Unlike CNF aerogels, the untreated FNs were completely consumed by the flame leaving only ash as residue. All the applied nanocoatings suppressed the after-glow and slowed down the spreading of the flame and 5 QLs of CH/SHMP/CH/MMT and CH/SHMP/CH/SEP were able to self-extinguish the flame. Even after a second application of flame, no ignition was observed. In comparison, the nanocoating with SNP showed self-extinguishment for only 50% of the test samples. The higher aspect ratio clay in the nanocoating enhanced the barrier properties of the char layer and this improved the flame-retardant characteristics. The flammability test residues of FNs are shown in Figure 31.

Figure 31 Photographs of the residues from the flammability test on a) untreated FN, b) 5QLs of (CH/SHMP/CH/MMT), c) 5QLs of (CH/SHMP/CH/SNP) and d) 5QLs of (CH/SHMP/CH/SEP) treated FNs in the presence of 10 mM NaCl at pH 5.

Under the heat flux of 35 kW/m2 in cone calorimetry, the untreated CNF aerogel ignited and combusted, releasing heat which was calorimetrically measured by the consumption of oxygen. In contrast, the LbL-treated aerogels did not ignite. Instead they glowed leaving oxidation to occur in the solid state without a clear flame. Since this process consumed oxygen, the heat released was also measured, giving a very low HRR signal, as shown in Figure 32. The pkHRR of the untreated aerogel was 149

RESULTS AND DISCUSSION

kW/m2 and it was reduced to 40 and 31 kW/m2 by 5QLs of nanocoating from 1 and 5 g/L solutions, respectively.

Figure 32 Heat release rate curves of untreated and 5 QLs of (CH/PVPA/CH/MMT) treated CNF aerogels with 1 g/L and 5 g/L solutions in the presence of 10 mM NaCl at pH 5.

When the FNs were exposed to a heat flux (35 kW/m2), the combustion and after-glow consumed the untreated FN, resulting in a pkHRR of 169 kW/m2 (Figure 33). All the nanocoatings applied were able to delay the time to reach the pkHRR value and suppress the post-incandescence.

RESULTS AND DISCUSSION

Figure 33 Heat release rate curves for untreated and 5 QLs of (CH/SHMP/CH/MMT), (CH/SHMP/CH/SEP), and (CH/SHMP/CH/SNP) treated FNs in the presence of 10 mM NaCl at pH 5.

A summary of the cone calorimetry data is given in Table 7. The most commonly used parameter to predict the flammability of materials and the fire risk is the heat release rate. The deposition of nanocoatings of MMT, SEP and SNP resulted in remarkable reductions in both pkHRR and MARHE.

Table 7 Cone calorimetry data for untreated and 5 QLs of (CH/SHMP/CH/MMT), (CH/SHMP/CH/SEP), and (CH/SHMP/CH/SNP) treated FNs in the presence of 10 mM NaCl at pH 5.

Weight Weight pkHRR THR TSR MARHE Sample residue gain [%] [kW/m2] [MJ/m2] [m2/m2] [kW/m2] [%] Control-FN - 169 ± 27 4.9 ± 0.7 8.2 ± 2.4 70 ± 11 - MMT-5QL 0.9 ± 0.3 95 ± 8 3.8 ± 0.9 9.1 ± 4.3 42 ± 6 3 ± 1 SEP- 5QL 7.9 ± 3.3 90 ± 4 4.2 ± 0.2 4.7 ± 1.9 42 ± 2 9 ± 1 SNP- 5QL 1.0 ± 0.6 119 ± 6 4.2 ± 0.4 6.0 ± 2.2 52 ± 4 1 ± 0.5

The MARHE value is defined as the total heat release normalized with respect to the time of combustion, and it is used to assess the main fire risk in developing fires. Nano clays with high aspect ratio improved the barrier properties of the coating much more than the colloidal silica, which resulted in an increase in the residue at the end of the test. Smoke release during a fire incident could be a possible cause of death due to

RESULTS AND DISCUSSION

toxic combustion products. A flame-retardant that can reduce the smoke release is therefore desired. Nanocoating with SEP reduced the total smoke release by 43% relative to that of the uncoated FN. This effect can be ascribed to the strong adsorption property of the mesoporous structure of the sepiolite.

Flame-retardant barrier properties of composite materials are typically assessed by the flame penetration test which can also be used to classify materials in terms of their insulation abilities. A schematic description of the tests and photographs of the tested aerogels are shown in Figure 34. Flame was able to penetrate through the untreated aerogel, but 5 QLs of (CH/PVPA/CH/MMT) treated aerogels from 5 g/L solutions were capable of withstanding exposure to the flame for 200 s, thermally insulating the unexposed side of the aerogel. This hybrid coating achieved a temperature drop of 650 °C across the thickness of 10 mm aerogel that can be attributed to the improved formation of a thermally stable char layer.

Figure 34 a) Schematic description of the flame penetration test, b) photographs of the front and rear sides of CNF aerogels and temperature measured during the flame penetration test on untreated and 5 QLs of CH/PVPA/CH/MMT-5 g/L treated CNF aerogels in the presence of 10 mM NaCl at pH 5.

During combustion of the untreated CNF aerogel, the aerogel partially lost its integrity and its porous structure as shown in Figure 35. SEM images show the smooth but damaged charred layer formed upon dehydration and decomposition of cellulose. In contrast, the aerogel coated with 5QLs of CH/PVPA/CH/MMT-1 g/L partially maintained its porosity and the coating affected the final morphology of char layer. The

RESULTS AND DISCUSSION

coating protection efficiency increased with increasing solution/dispersion concentration to 5 g/L. When the aerogel was coated with 5QLs of CH/PVPA/CH/MMT-5 g/L, the porosity and integrity of the aerogel were completely preserved. A larger amount of clay in the coating reinforces the char layer and prevents cracking on exposure to a flame, resulting in a self-extinguishing behaviour. The coating was more brittle when a concentration of 1 g/L was used for LbL deposition.

Figure 35 SEM images of aerogels after the horizontal flame test (a, d) untreated, (b, e) (CH/PVPA/CH/MMT)-5QL-1 g/L treated, (c, f) (CH/PVPA/CH/MMT)-5QL-5 g/L treated aerogels in the presence of 10 mM NaCl at pH 5.

Nanocoatings of MMT, SEP, and SNP preserved their unique textures and nanostructures even after exposure to the flame as shown in Figure 36. All the nanocoated FN systems maintained their fibrous structure and MMT and SEP coated FNs showed self-extinguishing behaviour. It was not possible to sustain the flame due to the remarkable barrier properties of the high aspect ratio clays. SEM images of HFT residues show hollow fibres, indicating that the underlying cellulose was completely dehydrated and decomposed on exposure to the flame, leaving coating and char as a shell.

RESULTS AND DISCUSSION

Figure 36 SEM images of the residues from the horizontal flame test of 5 QLs of a) (CH/SHMP/CH/MMT), b) (CH/SHMP/CH/SEP), and c) (CH/SHMP/CH/SNP) treated FNs in the presence of 10 mM NaCl at pH 5. The colours indicate the higher magnification images.

4.3 Flame-retardant model cellulose gel beads (Paper V)

4.3.1 LbL assembly build-up

QCM crystals were used as a flat substrate to prepare model cellulose surfaces having the same charge density as the specially prepared cellulose gel beads, in order to be able to investigate the principles behind the multilayer formation and to better understand the formation of the LbLs on the beads. Figure 37 shows the frequency shift and the change in dissipation for the third overtone for the successive adsorption steps. During the adsorption of cationic CH, there was an increase in frequency shift together with a decrease in energy dissipation. A possible explanation for this behaviour is that the release of the counter-ions within the cellulose film when the CH was adsorbed led to a reduction in osmotic pressure which in turn led to a deswelling of the cellulose film. Earlier studies have shown a deswelling of cellulose film when a polyelectrolyte of opposite charge to the cellulose is adsorbed.126 On the other hand, the adsorption of SHMP showed a continuous build-up with a decrease in frequency and an increase in energy dissipation. The low molecular weight SHMP is evidently adsorbed onto the CH-covered surface and is also able to increase the swelling of the adsorbed layer, detected as an increase in energy dissipation. A loosely associated SHMP layer could be a possible explanation of these results but, since the results are basically the same after rinsing with MQ, this is not a plausible explanation.

RESULTS AND DISCUSSION

a) b)

Figure 37 QCM-D data for LbL build-up of 5 BLs of (CH/SHMP) nanocoating on model cellulose surfaces (i.e., 795 µeq/g) a) change in normalized frequency shift and b) change in energy dissipation for the third overtone. The polyelectrolyte concentration used for CH was 1 g/L and for SHMP was 5 g/L, both in a 10 mM NaCl aqueous solution at pH 5. The rinsing solution was Milli-Q water at pH 5. The adsorption sequence and rinsing were continued until a steady state signal was reached.

The thickness and roughness of the dried nanocoatings on the model cellulose surfaces were investigated using AFM. The results in Figure 38 show two regimes for the growth of the multilayer with a linear build-up up to 20 BL deposition after which the growth is super-linear.127 The exact reason for this change in LbL-formation is not clear, but it has been suggested that it could be due either to a diffusion in and out of the formed film of at least one component of the LbL 120 or to an island growth of the formed layers.128 Considering the low molecular mass of the SHMP, it is not unlikely that this polymer can diffuse into the LbL structure and, since the roughness of the LbLs is decreased with increasing BLs, it seems that the diffusion model is most applicable for this LbL construction, although more work is necessary to establish this.

RESULTS AND DISCUSSION

a) b)

Figure 38 AFM characterisation of CH/SHMP nanocoatings on model cellulose surfaces as a function of the number of BLs a) thickness and b) roughness. The CH and SHMP concentrations were respectively 1 g/L and 5 g/L in the presence of 10 mM NaCl at pH 5.

The LbL coatings were successfully deposited on cellulose gel beads using the same solution conditions which were used for QCM-D meaurements. The build-up of multilayers on cellulose beads was conveniently followed using nitrogen analysis since CH has a natural content of nitrogen. The results in Figure 39a show the continuous increase in the amount of CH per gram of cellulose as a function of the number of bilayers deposited. The chemical nature of the LbL coating on dry cellulose beads was further characterized using FTIR spectroscopy. Figure 39b shows the spectra of untreated and LbL-treated beads where the appearance of new peaks is attributed to the presence of the coating constituents. The two peaks at 1635 and 1528 cm-1 are ascribed + to asymmetric and symmetric vibrations of NH3 groups of CH and the two strong peaks at 1250 and 865 cm-1 are ascribed to the stretching of P=O and P-O-P groups of SHMP.

RESULTS AND DISCUSSION

a) b)

Figure 39 (a) Adsorbed amount of CH determined by using destructive nitrogen analysis (ANTEK) and (b) FTIR spectra of cellulose beads and CH/SHMP treated beads. The CH and SHMP concentrations were respectively 1 g/L and 5 g/L in the presence of 10 mM NaCl at pH 5.

4.3.2 Thermal analysis of LbL-treated cellulose beads

The effect of an LbL coating on the thermal degradation of cellulose beads was investigated using thermal gravimetric analysis in nitrogen (Figure 40).

a) b)

Figure 40 (a) TG and (b) dTG of untreated cellulose beads and CH/SHMP LbL-treated cellulose beads. The CH and SHMP concentrations were respectively 1 g/L and 5 g/L in the presence of 10 mM NaCl at pH 5.

The LbL coating did not change the degradation of the cellulose beads, where the initial weight loss due to dehydration was observed at 100 °C. The most significant mass loss was observed at 246 °C, which is ascribed to the decomposition of non- crystalline cellulose and to the loss of entrapped water within the beads. The rate of

RESULTS AND DISCUSSION

weight loss at 246 °C, where the cellulose is usually degraded by a depolymerisation reaction, was less indicating that the LbL coating forms an insulating layer which protects the cellulose from degradation.129 The residual amount at 800 °C had increased to 29% with 100 BLs of coating, which implies that phosphate groups present within the LbL coating phosphorylate cellulose and that this subsequently favours the dehydration of cellulose towards the formation of a greater residual amount of a thermally stable char layer.

The reaction of untreated and LbL-treated cellulose beads to heating was further investigated using a ceramic heating element with a simultaneous recording of the degradation with a high speed camera. The morphology of the cellulose beads was investigated using SEM and EDX before and after heating the beads to ~400 °C. Untreated cellulose beads had a smooth morphology, but after the deposition of the multilayers, the morphology appeared to be wrinkled, due to the stress created by the difference in modulus between the nanocoating and the cellulose surface (Figure 41) which also clearly indicate that a film is indeed formed on the surfaces of the beads.130 The increase in thickness of the coating with increasing number of deposited bilayers resulted in the formation of fewer but larger wrinkles. After the heat application, the morphology changed drastically. The appearance of cracks and voids in the untreated cellulose beads showed the degradation of cellulose in the beads. Conversely, the appearance of small bubbles on the LbL-treated cellulose beads indicated the formation of a swollen char layer which can be attributed to instumescent char formation by the nanocoating.99 These model experiments also showed that the LbL treatment first creates a dry secondary structure on the cellulose that is dependent on the thickness of the layers and on the difference in mechanical properties between the underlying structure and the film. When exposed to heat, a typical bubble pattern was formed on the surface of the beads. The size of the bubbles was dependent on the thickness of the LbLs and it can be hypothesized that, as the pressure builds up within the cellulose bead due to the emission of volatiles, the LbL structure will yield and that the size of the yielded structure will be dependent on, among other things, the mechanical properties of the film at the time of creation of the bubble. As the bubbles are formed, the pressure inside the bead will also temporarily decrease. The size of the bubbles appears to be larger when the number of BLs deposited is larger which can be due to an increase in strength of the film with a larger number of bilayers, i.e. a higher pressure inside the heated cellulose bead is necessary to deform the film and due to the thicker film the bubbles will be larger.

RESULTS AND DISCUSSION

Figure 41 SEM images of a) an untreated cellulose bead, and of CH/SHMP assemblies of b) 10 BL, c) 20 BL, d) 50 BL, and e) 100 BL. Corresponding high magnification images (middle) and high magnification images after heat application (right). The CH and SHMP concentrations were respectively 1 g/L and 5 g/L in the presence of 10 mM NaCl at pH 5.

RESULTS AND DISCUSSION

The EDX spectra of cellulose beads show the elemental composition in which phosphorus could be detected only in the LbL-treated beads, indicating the presence of SHMP in the thin film. Figure 42 shows the elemental mapping of a cross-section of a bead where phosphorus was distributed along the surface of the bead. Despite thermal degradation, phosphorus is still present in the swollen char layer along the surface.

Figure 42 Cross-section SEM images and corresponding images from EDX elemental mapping of a) 100 BL treated bead before heating and b) 50 BL treated bead after heating. The blue colour shows the presence of phosphorus.

CONCLUSIONS

5 CONCLUSIONS

The aim of the work described in this thesis was to impart flame-retardant characteristics to cellulosic-based materials using the layer-by-layer assembly (LbL) technique, and multilayer thin films of polyelectrolytes and inorganic particles were deposited onto cellulose substrates such as cellulose-rich fibres, wet-stable CNF aerogels, wet-stable fibre networks and cellulose gel beads. A novel approach was presented for the production of flame-retardant paper using the LbL technique, and model cellulose surfaces have been exploited to establish a basic understanding of the thin film build-up of CH and PVPA in which the adsorbed amount increased with the addition of 10 mM NaCl as a background electrolyte. The following conclusions can be drawn from the results:

To be able to form a coherent thin film it was necessary to deposit at least 10 BLs. A multilayer thin film has been successfully formed on cellulose-rich wood fibres through sequential adsorption and washing steps. A 20 BLs film was capable of increasing the amount of residue to 25% at 600 °C in nitrogen. The paper sheets prepared from 20 BLs of CH/PVPA treated fibres self-extinguished the flame during the horizontal flame test. The LbL-treated fibres maintained their shape due to the enhanced char formation. Cone calorimetry revealed that a phosphorus-containing multilayer film was able to reduce the pkHRR of paper by 49% relative to that of untreated fibres. Since the application of 20 BLs is practically difficult in large-scale paper production, similar flame-retardant treatments were studied with the aim of depositing fewer bilayers while maintaining the same reduction in flame-retardancy. The enhanced thermal stability of lignocellulosic fibres coated with 3.5 BLs of HMw-PEI/SHMP was demonstrated by thermal gravimetric analysis, which showed that the residual amount at 800 °C under nitrogen atmosphere was 34%. After the LbL-treatment the paper sheet was however still burning when tested in the vertical configuration but a more coherent char residue remained than with the reference sheet. A self-extinguishing behaviour was achieved in HFT where FE-SEM images of the burned regim showed the formation of a swollen char on the fibres. The flame-retardancy of LbL coatings on fibres was further characterized using LOI, which showed that more oxygen was required to sustain the flame. In addition to the flame-retardancy, thin films of 3.5 BLs of (HMw-PEI/SHMP), with HMw-PEI in the outermost layer significantly improved the paper strength, probably due to an increased degree of contact in the fibre/fibre joints as well as a larger number of fibre-fibre joints per sheet volume.

CONCLUSIONS

Wet-stable cellulose nanofibril-based aerogels were prepared using freeze drying and a consecutive covalent crosslinking. The high porosity and high surface charge of the CNF aerogels were utilized to tailor the thermal stability and flame-retardant properties using a rapid LbL-filtration method. Aerogels coated with 5 QLs of CH/PVPA/CH/MMT from 5 g/L solutions/dispersions had an amount of residue after thermal degradation in nitrogen of 42% compared with 24% for the untreated aerogel. LbL-treated aerogels not only self-extinguished the flame during HFT but also withstood the penetration of butane flame, with a temperature drop of 650 °C between the exposed side and the rear of the aerogel. The results also showed that exposure to a 35 kW/m2 heat flux was not able to ignite the LbL-treated aerogel whereas combustion occurred in the solid state with a decrease in pkHRR value of 79% relative to that of the untreated aerogel. This remarkable flame-retardant behaviour can be ascribed to both the presence of phosphorus which favours char formation and to the thermal barrier effect of the MMT clay. Moreover, a new type of porous substrate was prepared using cellulose-rich fibres through periodate modification, and these wet-stable FNs were then used as a substrate to construct hybrid multilayers using CH as a carbon source/blowing agent, SHMP as an acid source, and inorganic particles (e.g., SEP, MMT, and SNP) as reinforcing agent in an effort to impart flame-retardancy. Pyrolysis conditions in TGA showed that the amount of residue at 800 °C increased with all the LbL systems where MMT-5QL resulted in a 30% residue compared with 21% for the reference. FNs coated with MMT-5QL or SEP-5QL containing nanocoatings self- extinguished the flame, but a SNP-5QL nanocoating was unable to stop the flame propagation during HFT. All the quadlayer systems showed a reduction in THR. More specifically, SEP-5QL reduced the pkHRR by 47% and TSR by 43% relative to the reference FN. The high aspect ratio clay with higher adsorbed amounts formed a thermally stable char layer which acted as a barrier preventing heat and mass transfer. The smoke produced upon combustion was also entrapped within the inherent free volume of clay.

Finally, the smooth surface of the non-crystalline cellulose gel beads was utilized to investigate the detailed effects of multilayer nanocoatings on the thermal stability and flame-retardancy of cellulose. It has been shown that at 800 °C in nitrogen 10 BLs of intumescent nanocoating increased the amount of residue to 21% compared with 6% for the reference. However, since a further increase in the number of BLs (20, 50, and 100 BL) increased the residual amount to only 25%, 26%, and 29%, respectively, it can be concluded that the LbL technique has been exploited to design an efficient and sustainable fire/flame protection that can be used to impart unprecedented flame- retardant characteristics to cellulose-based materials.

FUTURE WORK

6 FUTURE WORK

The primary aim of the project was to develop sustainable and scalable processes to produce light weight and flame-retardant organic/inorganic foams and composites using cellulose in the form of nanofibrils or fibres. It was suggested that the new sandwich structure of the FiReFoam project, financing this work, should consist of tailored cellulosic foam as the foam core with a paper sheet as the surface material (Figure 43).

In this respect, the following steps are suggested:

Freeze-cast foams; Ultra-light weight cellulose nanofibril-based foams with lamellar pore channels were prepared by a recently developed freeze-casting process. By changing the CNF concentration and cooling rate, the pore thickness can be tailored. Due to the directional growth of ice crystals, freeze-cast foams exhibit a low thermal conductivity in the radial and a high strength in the axial direction. Preparing wet- stable freeze-cast foams will make it possible to tailor the surface properties of lamellar channels using the LbL technique to improve the anisotropic thermal and mechanical properties of these CNF-based foams as core material.

Upscaling of the LbL assembly; An XPM experimental paper machine trial in MoRe Research in Örnsköldsvik showed that it is possible to produce flame-retardant paper using a novel in line LbL-addition set up. More specifically, pulp fibres were LbL treated in a continuous manner prior to sheet forming. The paper sheet prepared using LbL-treated fibres self-extinguished the flame in HFT. This LbL set up can be utilized to evaluate and optimize other possible multilayer systems.

Other possible ways of using combinations of inorganic particles and polyelectrolytes; Fibre/fibril based porous substrates were utilized as template for the rapid LbL assembly of multilayers consisting of polyelectrolytes/nanoparticles. It would be interesting to investigate particles and polyelectrolytes other than the ones studied in this thesis.

Sandwich structure; Sandwich structures prepared using different LbL modified papers, CNF aerogels, FNs and freeze-cast foams should be further investigated in order to establish an understanding as to how to utilize cellulose fibres and fibrils to improve thermal stability and mechanical properties as well as flame-retardant characteristics.

FUTURE WORK

LbL-modified sandwich structures should also be evaluated with respect to their commercial relevance regarding cost, efficiency, toxicity and flame-retardant performance.

Another future step would be scaling up the sandwich structure production process.

Figure 43 Schematic representation of the FiReFoam sandwich structure

ACKNOWLEDGEMENTS

7 ACKNOWLEDGEMENTS

First of all I would like to thank and present my sincere gratitude to my supervisor Prof. Lars Wågberg for his guidance, endless support, and believing in me. I would like to thank you for giving me the opportunity to work in your group. I truly appreciate your optimism, enthusiasm and inspiring ideas. I especially thank you for creating a positive research environment.

I would also like to thank Dr. Federico Carosio for all the support, fruitful discussions and encouragement during these years. Without your help, this thesis would not be possible.

I would like to thank all my co-authors for your help and invaluable comments. Prof. Jaime C. Grunlan is acknowledged for great collaboration.

The Swedish Foundation for Strategic Research (SSF) is acknowledged for the financial support.

I would like to thank all the members of FiReFoam Project, particular thanks to Prof. Lars Berglund and Prof. Lennart Bergström for valuable discussions.

Prof. Giovanni Camino is acknowledged for all the invaluable scientific discussions.

I would like to thank Assoc. Prof. Anthony Bristow for the linguistic review of all the scientific papers and the thesis.

Mia, Mona, Inga, Daniel, and Thèrése are thanked for organizing all the administrative things and creating an excellent working environment.

I would like to thank Prof. Mikael Hedenqvist for reviewing my thesis.

Thanks to all former and present friends and colleagues at Department of Fibre and Polymer Technology.

I would like to thank present and former members of Fibre Technology group for creating an excellent workplace.

ACKNOWLEDGEMENTS

I would like to particularly thank Verónica for great collaboration and all the help. I would like to extend my thanks to Maryam for the great collaboration in FiReFoam project.

Finally and most importantly, I would like to thank my dear mother, father, brother, grandmother and my aunts for your loving hearts and endless moral support despite the distance.

(Son olarak ve en önemlisi, aramizdaki mesafeye ragmen beni her zaman seven ve sonsuz destek olan canim anneme, babama, abime, ananeme ve teyzelerime tesekkur etmek istiyorum.)

My final and deepest thanks to my colleague, my co-author, my best friend, my sambo and my love Pernilla. You mean everything to me. I would like to thank you for all the support and being always there for me. I love you!

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8 REFERENCES

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