Modification of the Properties Biobased Thermoset Resin Using Cellulose Nano-Whiskers (CNW) As an Additive

2010:061 MASTER'S THESIS

Modification of the Properties Biobased Thermoset Resin using Cellulose Nano-whiskers (CNW) as an Additive

Anas Ibnyaich

Luleå University of Technology Master Thesis, Continuation Courses Advanced material Science and Engineering Department of Applied Physics and Mechanical Engineering Division of Wood- and Bionanocomposites

2010:061 - ISSN: 1653-0187 - ISRN: LTU-PB-EX--10/061--SE

Modification of the properties biobased thermoset resin using cellulose nano-

whiskers (CNW) as an additive

Anas Ibnyaich

Luleå University of Technology

Division of Manufacturing and Design of Wood and Bionanocomposites

Abstract

The aim of this work was to study how the addition of cellulose nanowhiskers (CNW) will affect the mechanical properties and rhelogical behaviour of the furfuryl alcohol (FFA) resin. The furfuryl alcohol is a biobased thermoset resin, based on sugarcane and it is water soluble and very brittle when cured. The used nanowhiskers were about 3 to 5 nm in diameter and around 200 nm in length. The nanocomposites were prepared by dispersion of aqueous nanowhisker suspensions (0.5, 1, 2, 3 wt%) in uncured resin using ultra sonication. The dispersion of CNW in FFA and the CNW- CNW as well as CNW-FFA interactions were studied through rheology measurements on neat FFA and FFA/CNW suspensions before curing. Thermogravimetric analysis was used to study the curing kinetics of the prepared materials. The material combinations were cured in different molds for further investigation, but also used as matrix for glass fiber composite. The nanocomposites were first pre-dried at room temperature in vacuum and then cured in an oven at 135°C and then post-cured in 100°C to remove the residual stresses from the materials. The time of each step of the curing cycle was determined for the each geometry. The cured composites microstructure was characterized with a scanning electron microscope. The nanocomposites thermo-mechanical properties were studied by dynamic mechanical thermal analysis and flextural properties of the hybrid composites (glass fiber-fufuryl alchohol-cellulose nanowhiskers) were determined in longitudinal and transversal direction.

Acknowledgements

This study was carried out Division of Manufacturing and Design of Wood and Bionanocomposites at Luleå University of Technology. This work is part of a larger EU project (WOODY) financially supported by the European commission, which is gratefully acknowledged.

Swerea SICOMP is greatly acknowledged for giving us access to rheology measurement equipments and other working facilities.

I would express my gratitude to Professor Kristiina Oksman Niska for offering me the opportunity to do this project and for her valuable suggestions and inspirations, through which I discovered another aspect of the nanocomposite materials and their processing.

I am also thankful to Dr. Aji P. Mathew for her continuous help and support during every step of the project, as well as her encouragement and kindness, she was never short of time and advice when I needed.

A special thanks for Dr. Gong Guan from whom I got a lot of help and assistance during the rheology measurement and valuable advices.

I express my special thanks to Mrs. Birgitha Nyström for providing great working atmosphere, hospitality and help during each of my visits at Swerea SICOMP.

I would like to thank Mr. Tord Gustafsson form APC Composite for providing me with the furfural resin and for his excellent organization of the woody meeting.

A huge thanks to Anusha, Natalia, Martha, Maiju, Guan, Sandra, Gilberto, Ahmad, Mohamed, Musa, Abelelkhader and Göran for making my stay in Lulea agreeable as well as to all my friends in the other part of the worlds.

Finally, I would like to express my gratitude to my family “especially to my Mother”, for their love, support and encouragement.

TABLE OF CONTENTS

I INTRODUCTION ...... 8

THERMOSET POLYMERS ...... 8 Epoxy resins ...... 9 Phenolic resins ...... 9 Polyurethanes ...... 10 Vinyl ester resins ...... 11 Unsaturated polyester resins ...... 11 Soya oil based polyesters ...... 12 Furfuryl alcohol resins ...... 13

NANOMATERIALS ...... 16 Carbon nanotubes ...... 17 Nano clays ...... 17 Nano silicas ...... 18 Cellulose nanowhiskers ...... 19

PREPARATION METHODS OF NANOCOMPOSITES ...... 20 In-situ polymerization of thermoset nano-composite...... 21 Mixing techniques ...... 21 Chemical modification of nanomaterials ...... 23

EFFECT OF NANOPARTICLES ON VARIOUS PROPERTIES OF THERMOSETS: ...... 25 Mechanical properties ...... 25 Electrical properties ...... 26 Tribological properties ...... 27 Thermal properties ...... 27 Fire retardancy and thermal stability ...... 28

II EXPERIMENTAL ...... 29

MATERIALS ...... 29

PROCESSING ...... 29 Molding of the pure FFA ...... 29 Mixing of FFA-CNW ...... 31 Composite preparation methods ...... 34 Characterization ...... 36

RESULTS AND DISCUSSIONS ...... 38 Thermogravimetric analysis (TGA) ...... 38 Rheology results: ...... 39 The drying-curing parameters ...... 45 Macroscopic appearance : ...... 48 Flexural properties of the multi-scale composites ...... 51 Microstructure ...... 53

CONCLUSIONS ...... 53

SUGGESTION FOR THE FUTURE WORK ...... 54

REFERENCES ...... 55

I INTRODUCTION

Since the invention of the first composite brick, formed from straw and mud, the incorporation of short fiber or other types of reinforcement in a matrix had always been an effective way to obtain a composite material with enhanced properties (1).

In recent years however, the traditional composite made out of the micro-sized reinforcement have reached their limits, since the properties achieved usually involves compromises. Generally, micrometric filler could provide an increase in thermal stability, stiffness or strength of the polymer matrix at the cost of impact strength, fracture toughness or optical clarity. To overcome these disadvantages, a new approach has been developed that suggests the use of nano-sized particles as reinforcement (2).

The incorporation of nano-sized materials in thermoset polymer matrix can enhance the mechanical properties and thermal stability of the resin as well as reduce the weight of the material. Due to the extremely high specific surface for a given volume of the nanoparticles, the degree of property enhancement depends not only on the intrinsic properties of the matrix and the nanoparticles, but also on the type of the interfacial interactions between the matrix and nanoparticles which play a key role for this new class of materials (2).

THERMOSET POLYMERS

Thermoset polymers consist of a three-dimensional macromolecular network, where the bridging between chains prevents its sliding. The application of heat will strengthen the network bonds and stiffens. However, under very high temperatures; thermoset polymers deteriorate (carbonization). The transformation of a thermoset resin during heating is irreversible and therefore these materials can not be recycled. Thermoset polymers have significant advantages over thermoplastics such as, good affinity to heterogeneous materials, creep resistance, and higher operating temperatures. The most common used thermosetting polymers are unsaturated polyesters, epoxies, vinyl esters and phenolics. But most recently, there are different attempts to develop new bio-based thermoset resins (3).

A bio-based resin is simply a polymeric resin made of substances derived from biological sources such as plants or vegetable oils instead of petrochemicals. The major advantages of these polymers are renewability, low environmental impact and reduction of the dependence on the limited resources of petrochemicals. They absorb the carbon from the atmosphere and avoid the use of volatile toxic chemicals such as formaldehyde that causes health and safety problems. Several bio-based thermoset

resins have already been developed, such as polyfurfuryl alcohol,,pine oil and soya oil based polymers (4).

Epoxy resins Epoxy resins are used in many industrial areas such as microelectronics, glues, adhesives, paints, and composites. In most of the cases, the formation of the epoxy network is done by condensation reaction between an epoxy monomer and a hardener. It is also noted that one of these precursers should have functionality greater than 2. The diglycidyl ether of bisphenol-A (DGEBA) is the most currently used epoxy monomer. Acid anhydrides, aromatic or aliphatic polyamines, and imidazoles are utilized as hardeners but diamine is the most preferable hardener. An appropriate selection of the hardener determines the final thermo-mechanical properties of the epoxy as well as the curing conditions (temperature profile and curing rate) (5).

Figure 1: Polycondensation reaction of an epoxy monomer and an amine hardner (5) A master thesis by Jin Zeng (6) mechanical properties of cellulose nanowhisker (CNW) reinforced epoxy nanocomposites was reported several type of composites using different dispersion methods were prepared. First, solvent exchange was used, where the water was substituted by an organic solvent which is more compatible with the hydrophobic matrix. This method was supposed to enhance the dispersion of the CNWs. However, it was seen that the acetone affected positively the thermo mechanical properties of the epoxy and when the acetone was evaporated, the CNWs were too agglomerated and formed entirely a separate phase. In second and third methods, composites were prepared without using any solvent; by dispersing respectively freeze dried CNWs and the super critical dried CNW aerogels. The composites with freeze dried CNWs showed better mechanical properties in comparison to those that have been prepared by using acetone, whereas the non- reproducible results showed slight improvement in the tensile properties of the super critical dried CNW aerogels (6).

Phenolic resins Phenolics are also thermoset resins, obtained by polycondensation of a phenol and an aldehyde with elimination of water and formation of a three-dimensional network. Phenol and formaldehyde are the commonly used monomers, but the use of

a modified phenol (cresol, resorcinol, etc) can lead to specific resins. The phenolic resins are used by many industries in form of resin (liquid solution or solid), molding materials, semiconductor products or composites (7).

Figure 2: Polycondensation of phenol formaldehyde in an alcalin media (7). Polyurethanes Polyurethanes result from the chemical reaction of a polyisocyanate with groups containing labile hydrogen such as hydroxyl, amine group and even water (Fig.3). This exothermic reaction may release a gas which leads to more or less marked expansion of the polymer. Other reactions can also take place which may lead to a structured polymeric network. The polymerization reactions may be performed very quickly in the presence of catalysts at room temperature, which results in mass production of the material.

Polyurethanes have been used in variety of industrial and domestic applications due to their relative ease of processing and wide range of properties (hardness and densities). These products are very flexible, lightweight (flexible foam) but can also be highly rigid and stiff (part of artificial heart, cars structure etc). Flexible foams are generally used in furniture, cars seats where rigid foam is used for thermal insulation purposes (8).

Figure 3: Isocyanate with groups containing labile hydrogen to form a urethane group (8).

Vinyl ester resins Vinyl esters are obtained by polycondensation reaction of an unsaturated carboxylic acid and an epoxy resin (Fig.4). The cross linking of the epoxy-vinyl-ester is done through the co-polymerization reaction of the styrene. However for more specific properties (viscosity, processability), other type of co-monomers such as methyl methacrylate and alkyl is used. Due to the viscosity range, and adjustable reactivity of vinyl ester resins through formulations, they are suitable for all transformation processes of thermosetting resins. Furthermore, the vinyl ester resins present a higher chemical, fire, and thermal resistance as well as enhanced mechanical proprieties over epoxy and polyester resins. Thus, vinyl esters are used in all application areas where good mechanical properties and/or chemical resistance higher than those of the polyester resins or conventional steels are required (9).

Figure 4: Epoxy vinyl ester polycondensation reaction (9).

Unsaturated polyester resins Unsaturated polyester (UP) resins are obtained by polycondensation of diacid anhydrides with glycols (Fig.5) (at least one constituent should contain ethylene double bond that may react later on with a vinyl, acrylic or allylic compound). The polyester resin is usually delivered in a liquid phase dissolved in a copolymerizable solvent, such as styrene. The transformation into a thermoset object is caused by a radical copolymerization of the monomer (usually styrene) with the double bonds of the prepolymer, resulting in a three-dimensional polyester chains linked by small polystyrene chains (3 to 5 units in general). The curing rate and the reaction temperature are chosen depending on the catalyst system used that consists of a blend of peroxide catalyst, inhibitor and accelerator. Since 1950, the unsaturated polyesters have been used mainly in the buildings, transport, and electronic industries. Recent attempts have made to develop a biobased polyester polymer as shown in the next paragraph (10).

Figure 5: Polymerization reaction of unsaturated polyester (10).

Soya oil based polyesters Vegetable soybean oil is abundant and renewable product. It is mainly composed of triglycerides (esters of glycerol and fatty acids). Each fatty acid arm contains 0 to 3 double bonds which can undergo an oxidative cross-linking, however the reaction is too slow for an industrial use. Besides this, the double bonds are free radicals (un- reactive); they have limited industrial applications as a monomer.

Figure 6: Reaction mechanism chemical modification of soy oil (11). Therefore, chemical modification of soybean oil is often used to increase its reactivity. Examples include epoxidized soybean oil (ESO), which offers opportunity for further modification by reacting with acrylic acid to form acrylated deoxidized soy oil (AESO). The obtained resin is then cured by free radical and cationic initiation mechanisms. The reaction mechanisms of these chemical modifications are presented in Fig.6 (12).

The very first study to investigate the possibility of using acrylated epoxidized soybean oil Tribest resin in a composite material as a matrix was performed by D,

Åkesson (2006) (13). In this study, some of the proprieties of Tribest resin were determined. It was found that by blending the resin with 1% of 1, 1-di-(tert- butylperoxy)-cyclohexane, and the polymerization reaction were completed in few minutes at the minimal curing temperature of 150°C. Furthermore, DMTA (dynamic mechanical thermal analysis) analysis was conducted at 20°C and showed that the cured polymer had a storage modulus (E’) of 890 MPa, a loss modulus (E’’) of 100

MPa and glass transition temperature Tg of 74°C (13).

In a another investigation made by the same group in the year 2008, they successfully cured the Tribest resin within 8 hours at room temperature by using 30% styrene as a solvent 0,2% peroxide Andonox KP-9 as initiator and 2% cobalt as accelerator. This new method was advantageous and was more cost effective compared to other curing methods. However, the use of styrene as a solvent made this process less environmentally friendly (14).

In a study performed at Swerea SICOMP, to characterize the Tribest resin, no solvent was used to get a long processing time and low viscosity at 50°C the concentrations of the catalyst, hardener and accelerator were carefully chosen as, 2.25% peroxide (hardener/initiator) Benox L40 and 0.6% accelerator Norpol 9826.

The DSC (differential scanning calorimetry) and rheology studies showed that Tg fluctuates around 38°C and the minimum viscosity of the Tribest resin at 50°C was around 0.5 Pa.s (15).

Furfuryl alcohol resins

Furfuryl alcohol (FFA) is an organic compound and is dark brown liquid in its normal condition. It is soluble in many organic solvents and water and is manufactured from hemi-celluloses of different types of agricultural products such as sawdust, wheat or corn. (Fig.7) The manufacturing mechanism is made in two steps, Firstly; the hemi-cellulose pentosane will undergo hydrolysis under the influence of an acid catalysis to produce the furfural. Secondly, the aldehyde group is reduced to alcohol through a catalytic hydrogenation using high pressure, Co-Mo and B and Cu- MgO as a catalyst, forming the furfuryl alcohol (16). .

Figure 7: Examples of natural products (corn, oats or wheat) used for furfural production (16). The FFA undergoes a very exothermic self-condensation reaction in the presence of acid (p-toluenesulfonic acid and acetic acid), leading to linear or branched polymer and water as a reaction product (Fig.8). The polymerization reaction can be controlled by an appropriate choice of the acidity and temperature of the system, and can be stopped by neutralization at any desired viscosity (16).

Figure 8: Polymerization FAA-resin (16). In the setting (heat hardening) reaction, the cross-linking of the polymer, is done through the reaction of the double bonds of a furan ring in one chain with the double bonds of a furan ring in a neighboring chain according to Diels-Alder mechanism (Fig 9). The cured polymer exhibits excellent chemical and fire resistance, which explains its use in the foundry and refractory industry (17).

Figure 9: Chemical structure of the cured FFA (17). The particularity of the FA alcohols is that the uncured resin is highly hydrophilic; Furfuryl alcohol pre-polymer contains 15 - 20 wt% of water. Therefore, if the curing is too fast, as the exothermic reaction takes place, the boiling point of water is reached. Its evaporation produces bubbles that become trapped in the viscous reaction and results in the expansion of polymer until gelation takes place. Thus, a heat treatment

is required to permit the physical water as well as the reaction water to escape as vapor before the final "heat hardening" takes place (18).

A preliminary study was performed by Swerea SICOMP (19), in order to characterize the processibility of FFA resin. The properties such us curing, glass transition temperatures, curing time, viscosity, morphology, shrinkage were determined using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), rheometry and other techniques. The main conclusions of this study were:

 DSC measurements conducted on the neat resin, aimed to study the kinetics of curing reaction, showed that α75%, 80% and 90% after 1, 2 and 4h respectively at 115°C.  From DMA measurements on the cured sample, it was observed that the glass transition was constantly increasing during the post curing process. The evolution of Tan(δ) peak indicates whether the resin was not fully cured or the reaction of carbonization of the polymer takes place at high post curing temperatures.  Through the viscosity measurement, it was shown that the viscosity decreases when the temperature or water content increases.  Depending on the curing cycles, the shrinkage of the cured samples varied between 2 and 7%.

250

200

150

100 Post cure at 135°C Post cure at 115°C

Tg from max tan(delta) max from Tg 50

0 0 100 200 300 400 500 600 Post curing time

Figure 10: The effect of the curing time on the Tg of the FFA polymer (time in minutes) (19).

 Since the purchased resin contains 20% of water a drying step is required before curing, otherwise the residual water will affect the shrinkage of the material after curing.

 After trying differing drying (curing) cycles, it was concluded that all physical water is gone after less than one hour at 115°C.

3,5 3,5

3 3

2,5 2,5

2 2

1,5 1,5

1 1 Dynamic viscosity at 50°C viscosity (Pas) at Dynamic Absolute Pressure (bar) and Viscosity (Pas) and (bar) Pressure Absolute 0,5 0,5

0 0 20 40 50 60 70 80 100 120 8 10 12 14 16 18 20 130 mbar & 350 mPas 300 mbar & 100 mPas Water content (%) Temperature (°C) Furolite 080101 Boiling point of water Viscosity Furolite 080101

Figure 11: The effect of the water content and the temperature on the viscosity of the resin (19). As an application of the FFA in nanocomposites, Pranger and Tannenbaum established the feasibility of producing furfuryl alcohol reinforced by cellulose nano whiskers (CNW) and montmorillonite mineral using in-situ polymerization without using any solvent or surfactants (20). They found sulfonic acid residues at the CNW surface, which were left over from the acid hydrolysis treatment, acted as catalysis for the polymerization reaction of furfuryl alcohol and showed that the CNW increased the thermal stability of FFA resin. The increase thermal stability was explained that CNW restricted the thermal motion of polyfurfuryl macromolecules (20).

In another study, Nathanael Guino et al. have prepared a FFA/SiO2 hybrid material by conduction simultaneously the polymerization and the in-situ silica incorporation using sol-gel process (21).

NANOMATERIALS

A nanomaterial is defined as a particle of size that is <100 nm in at least one dimension as shown in (Fig.12). The nanomaterials are grouped into three classes such as equi-axed, fibrous, and plate-like nano-materials depending on their geometry (22). These nanoscale particles have a huge surface area for a given volume because the surface area per unit volume of a particle is inversely proportional to the particle diameter or thickness, thus all the chemical and physical interactions that are governed by surface properties will be substantially affected which explains that the nanomaterials show significant remarkable properties if compared to that of the micrometer-scale counterpart. Few examples of nanomaterials currently in use are carbon nanotubes, nano clays, nano silicas and cellulose nanowhiskers.

Figure 12: Schematic of nanoscale materials. Carbon nanotubes Carbon nanotubes (CNTs) are fibrous nanomaterials, defined as cylinders made of graphitic sheet of covalently bond carbons that have diameters ranging from one to ten nanometers and lengths up to centimeters. CNTs are mainly produced by direct-current arc discharge, laser ablation or chemical vapor deposition (CVD).These carbon nanotubes are classified into two main types, the single-walled carbon nanotubes (SWNT and the multi-walled carbon nanotubes MWNT (22). The SWNT consists of a layer of graphite cylinder where the MWNT are made out of two or more concentric cylinder shells of graphite with a interlayer separation of (0,34 nm) (2).

(C)

Figure 13: Schematic of: (a) single-wall (23)and (b) multiwall nanotube (24) (c) TEM image of Single wall nanotubes made by arc discharge (25). Carbon nanotubes are distinguished by their high flexibility, low density, and large aspect ratio (typically ca. 300-1000). Some CNTs can be stronger than steel, lighter than aluminum, and more conductive than copper. The theoretical and experimental results of individual single-wall carbon nanotubes (SWNT) have shown a very high tensile strength (150-180 GPa) and an elastic modulus up to 1 TPa (26). This exceptional combination of physical, mechanical, thermal, and electrical properties makes CNTs a potential candidate for many applications such as conductive and high strength nanocomposites, energy storage, and energy conversion devices, sensors, and semiconductor devices (26).

Nano clays Clay minerals are hydrated phyllosilicates which are very small crystals having dimensions in the order of micrometer. These crystals are made up of several layers by superimposition. Each layer is approximately 1 nm thick and posses a lateral dimension raging for 30 nm to several microns which may lead to an aspect ratio ranging from 10 to 1000 and a surface area of 750 m2. The layer structure consists of

one or two sheets of silica tetrahedra (tetrahedron of oxygen atoms with an atom silica center) joined to an octahedral alumina or magnesia sheet (octahedron of oxygen atoms with an atom of aluminum or magnesium in the center) with interlayer space called gallery in between (27). The clay minerals are categorized into four major groups depending on their structure including smectite, illite, vermiculite and kaolinite-serpentine. However, only the smectite type is used for nanocomposite preparation, and particularly the montmorillonites due to their high surface area and high surface reactivity. Before mixing with the polymer, these nanoclays are usually subjected to an organic treatment in which the cations (Na+ or Ca2+) inside the galleries are substituted by a cationic surfactant in order to convert the normally hydrophobic silicate surface to organophilic one, thus increasing the miscibility with the polymer (22).

Figure 14: SEM image of nanoclay (26), basic structure of clay minerals (27).

Nano silicas Nanosilicas are equiaxed nanoparticles that are produced via two methods. The first method is the high-temperature hydrolysis of silicone tetrachloride in an oxygen- hydrogen gas flame that results to silica nano-particle spherically shaped, with a diameter ranging from 7-40 nm and a specific surface area ranging between 50 and 380 m2\g. The second method is a sol-gel process in which the tetraethoxysilane (TEOS) is hydrolyzed in the presence of acidic or basic catalytic system in an ethanol solution to form the nanosilica (28). In this process, the pH of the solution has a large influence on the microstructure of the produced silica particles. In fact, during the base-catalyzed reaction, the rate of condensation is faster than hydrolysis, thus resulting in highly condensed species that could agglomerate into fine particles. On the other hand, in the acid catalyzed reaction, the rate of the hydrolysis step is faster than the condensation step, resulting in the more extension of the silica and less branched network structure. By using the sol-gel method, silica-thermoset nanocomposite can be produced from a liquid starting materials, where the production of the nanoparticles and polymerization reaction are performed simultaneously, thereby limiting their agglomeration. (29).

Figure 15: Scaning electron micrograph of silica nanoparticle (30) nanosilica synthesis via sol- gel methods mechanism (31).

Cellulose nanowhiskers Cellulose nanowhiskers (CNWs) are obtained from the native cellulose by acid hydrolysis, which removes amorphous part of cellulose. The remaining crystal particles are called whiskers or crystals and are of nanosize. The natural plants contains different types of components, other than crystalline cellulose, thus several separation treatments such as treatment with solution of sodium, bleaching and sulfuric acid hydrolysis are needed to isolate the cellulose nanowhiskers.

Figure 16: AFM image of cellulose nanowhiskers (32), cellulose acid hydrolysis (6). After several isolation steps, the whiskers are separated from the biomass by removing all the other components. The dispersion of these whiskers in the solvent and later in the matrix is an essential step for their use in composites. Dispersion is mainly done by providing mechanical energy, most often by ultrasound mixing devices. The goal of the dispersion step is to obtain a stable suspension of whiskers and to avoid the formation of aggregates (33).

Bondeson et al optimized the isolation process of CNW from microcrystalline cellulose by acid hydrolysis with a sulfuric acid (34), The different steps of the process are shown in (Fig 17). They reported that the isolation process resulted in CNW with a length between 200 and 400 nm and a width less than 10 nm was

produced with a yield of 30% by using 63.5% (w/w) sulfuric acid solution during 2 hours (34).

Figure 17: Preparation procedure for cellulose nanowiskers (35).

PREPARATION METHODS OF NANOCOMPOSITES

The production of composites based on thermoset resins is mainly done trough the in-situ polymerization processing. The dispersion of the nanoparticles, as individual particles, in the polymer matrix is considered as a main challenging step to manufacture a thermosetting nanocomposite with the desired properties. The nanoparticles can either be in the form of powders or dispersed in a solution. In both the cases, nanoparticles may attach together and form agglomerates due to their high inter-particles adhesive forces, which make the dispersion very difficult into the matrix.

There are different approaches to incorporate nanoparticles in a polymer matrix to obtain well dispersed and separated nanoparticles. First, approach is chemical technique that aims to produce the nanoparticles directly (in-situ) in the matrix during the polymerization step, this method is mainly used in the case of nanosilicas (31) and nanoclays (22) and thermoset resins. The second approach is to disperse dry nanoparticles in a liquid polymer resin, using mechanical mixing in which high shear forces are applied during the dispersion in order to break up agglomerates and have a homogeneous distribution of the nanoparticles. Furthermore, the application of ultrasound combined with the mechanical stirring is proven to be successful for the dispersion of CNT in epoxy resin (36). The ultra-sound mixing technique causes the breakage of the agglomerate particles as well as will mix the components in a molecular level. The third method is to apply a special chemical pre-treatment of nanoparticles by adding surfactant or by grafting before the mechanical dispersion process. A chemical modification of a nanoparticles surfaces aims to enhance the composite’s properties by increasing the compatibility between the matrix and the nanoparticles (37).

In-situ polymerization of thermoset nano-composite The most successful approach to produce thermoset polymer based nanocomposites is the in-situ polymerization, in which the nanoparticles are first dispersed in the thermoset resin precursor (monomer or oligomer). At this stage, the resin has a low viscosity, so an even dispersion can be achieved easily. In some cases, the use of a solvent and\or surfactant is also required. Once the nanoparticles are well dispersed in the resin precursor, the polymerization reaction is initiated by heating, adding the hardener, or subjecting the blend to UV light depending on the chosen curing mode (38). Furthermore, the addition of a good solvent for the resin monomers is believed to have a positive influence on the nanoparticle dispersion into the matrix. Since, it dilutes the mixture and reduces its viscosity. Generally, the solvents with lower boiling points are used because it is easy to remove these from the composite. As evidence, the study conducted by Liao et al. (39) pointed out that the use of acetone as a solvent combined with tip sonication enhances the storage modulus of a SWNT/epoxy nanocomposite. In a recent study, Jin Zeng et al. (6) dispersed CNW/epoxy via organic solvents, namely EGME (ethylene glycol mono- methyl ether) then the mixture was rotoevapored to remove the EGME. During the evaporation process, the cellulose was re-agglomerated and formed a gel instead of being dispersed in the epoxy. The results showed that the addition of EGME solvent was not a suitable way to disperse the CNW in the epoxy resin.

Another possibility is to synthesize a thermoset based nanocomposite material by chemical route. The sol-gel technique, aims to produce the nanoparticles directly (in situ) in the matrix during the polymerization step. This method is mainly used in the case of nanosilica or nanoclay based thermoset nanocomposites (28).

Mixing techniques Effective mixing is one of the most important steps in the nanocomposite manufacturing procedure. Mixing can be defined as a process by which the different components are carefully blended to achieve a proper dispersion and a uniform distribution of the nanoparticles in the matrix. The four possible states of mixtures that could be obtained after a mixing process of two materials are schematically represented in the Fig.18. In order to overcome the problems that are encountered in regard to the dispersion of nanoparticles as mentioned earlier in this report, several mixing techniques can be used such as high shear mixing, ultrasonic mixing, and mechanical stirring (1).

Figure 18: Schematically illustrates before mixing (a), poor dispersion and non uniformed distribution (b), poor dispersion but uniform distribution (c), good dispersion and good distribution (d), good dispersion but bad distribution (e). Mechanical mixing

For the thermoset based nanocomposite processing, there are two main types of mechanical mixing methods, mechanical stirring and high shear mixing as shown in the Fig.19. The mechanical stirring is mainly used for distributive mixing purposes, since its dispersive capacity is very poor and it is done by simple magnetic or electric agitator. The high shear mixing is a dispersive and distributive mixing method. In this type, high shear forces are applied during dispersion process to break up the agglomerates, separate and distribute the individual nanoparticles homogeneously in the resin. The high local shear forces can be realized by using several techniques such as rotating discs(dissolver), rotor-stator-systems, grinding effect of moving ceramic balls (ball mill) or by three roll mill. The choice of the dispersing technique is determined by the properties of the matrix and the nanomaterial such as viscosity and polarity (40).

Figure 19: (a) Rotor-stator-systems (41) (b) combination of disolver and ball mixing technology, (c) three roll mill (42).

Ultrasonic mixing

In ultrasonic mixing, ultrasound energy will be used. Sonification shown in Fig.20 and are considered to be very effective ultrasonic mixing processes to disperse the nanoparticles such as CNT, CNW in water, organic solvents, and resins. In the sonification process, the probe generate ultrasounds or sound waves with a frequency above 19 KHz that propagate into the liquid media by alternating a high and low-pressure cycles that causes ultrasonic cavitations. The dispersion by sonification uses liquid jet streams resulting from ultrasonic cavitations to break the bonding forces between particles (hydrogen bonds and / or forces of Vander Waals) without breaking the covalent bonds within the particles (43). This separates the agglomerated particles as individual particles, improves the dispersion and the stability of suspensions. However, the use of ultrasound results in a rapid heating of the suspension. Thus, the duration of treatment should be limited to several minutes in ice bath.

Figure 20: Sonification processor used in this study. In most of the cases, more than one method is used. In such cases, the respective advantages of each method combine synergistically. For instance, Zhou et al used high-speed mechanical stirrer in addition to the dispersion by ultra-sound for developing multi-walled carbon nanotubes (CNTs) reinforced epoxy composite. First, the CNTs were mixed with epoxy resin using high-intensity ultrasonic for an hour and then the curing agent was added and mixed using a high-speed mechanical stirrer for about 10 minutes. They concluded that sonification was very efficient method for infusion of carbon nanotubes into epoxy resin up to a concentration of 0. 3% (above this value the CNTs start agglomerate) (44).

Chemical modification of nanomaterials The main objectives of the chemical modification are to enhance the compatibility of the nanomaterials with the matrix and to improve the dispersion in the polymer. In case of CNWs, the aim of the chemical treatment is to replace a number of hydrophilic OH-groups on the surface of the nanowhiskers by more hydrophobic groups, without changing the morphological or crystalline state of the CNW. This

modification can be achieved through different chemical treatments such as esterification, oxidation or by grafting silane groups on the surfaces (33).

 The esterification reaction involves the condensation of the cellulosic hydroxyl groups with an acid derivative (anhydride acid, acid chloride or ester of this acid) to form an ester and water (33).

 The oxidation of the cellulosic –OH group forms an aldehyde or acid. This reaction could be performed by several processes. However, only few are used for selectivity reasons or operating conditions. For example, the oxidation catalyzed by the radical 2, 2, 6, 6-tetramethylpiperidine-1-oxyl or TEMPO which is performed in aqueous solutions, oxidized only the primary alcohols without affecting the secondary alcohols (33).

 In an aqueous medium, silanes alkoxy groups undergo hydrolysis before they condensate with a hydroxyls group of cellulose, forming a covalent bond between wood and silanol (wood-O-Si) or to auto-condensate another silanol group Si-OH (33, 37).

In the case of CNTs, Virtanen et al. reported two different methods for cutting physically the graphitic materials including CNTs by using ultrasounds frequencies 20 kHz to 1MHz or by electromagnetic radiation (45). The cleavage was immediately followed by chemical treatments of highly reactive nascent radical site that resulted from bonds breakage in order to graft different groups such as amino, hydrazine, sulfur, hydrogen sulfite, mercapto, acrylic, acrylonitrile or epoxy compound. Thus, during the curing process these grafted functions will contribute to the polymerization process, creating covalent bond. It has been shown that this mechanical-chemical treatment lead to a strong hybrid structure between the matrix and the CNTs, that is mainly due to the enhancement of CNTs dispersion and to the reaction of the grafted functions with the monomers during the polymerization process during the curing, thus creating the covalent bond between the matrix and filler (46).

Figure 21: Surface modification procedure of a graphitic materials (46).

EFFECT OF NANOPARTICLES ON VARIOUS PROPERTIES OF THERMOSETS:

The incorporation of nanoparticles in a thermoset resin is expected to have significant effect on the polymers mechanical, barrier, thermal, electrical, and other properties as illustrated in the Fig. 22.

Figure 22: Effect of nanoparticles on various properties of thermosets (47). Mechanical properties By the addition of conventional high modulus reinforcements into a polymer, usually the modulus and strength of the composite are improved, while the ductility and the impact strength are decreased. In the case of well dispersed nanoparticles strength, modulus as well as the ductility of the composite can be improved since they do not create high stress concentrations due to their small seize (2). There are two different mechanisms by which the mechanical properties of the composites are improved due to the addition of nanoparticles. The first mechanism is load transfer mechanism and the second is restriction of the polymer molecular chain mobility which will results in the increased glass transition temperature. Ho et al. showed an increase in the tensile strength and the Vickers` hardness by the addition of nanoclay to the epoxy using the mechanical stirring method (48). Weiping et al. reported that nanoclays could increase the fracture toughness of epoxy (43). Singh et al. observed a significant increase in fracture toughness by the incorporation of nanometer-sized aluminum particles into thermosetting unsaturated polyesters (21). Furthermore, it is clearly evident that the addition of organomontmorillonite (OMMT) to polyurethane improves the strain to failure by factor of two as shown in Fig. 23. This effect is explained by the plasticizing effect of the alkyl-ammonium ions, which was not accompanied by the decrease in strength and modulus. Hence, it can be summarized that these intercalated clays are much smaller than the critical crack size for polymers, thus they enhanced both the toughness and strength of the polymers (2).

Figure 23: Stress strain curves for (A) a pristine polyurethnane elastomer,(B) a polyurethane- clay nanocomposite prepared from OMMT (5wt%) (2). In case of epoxy nanocomposites, nanoparticles have a quite favorable effect on the impact behavior both at lower and higher volume fractions of the filler materials. (Fig10). It seems that the lower content of filler is effective to obtain the strong reinforcement. Especially for the small particles of Al2O3, and TiO2, a maximum of impact energy is reached in the range of 1 to 2 vol%. The nanocomposites containing the sub-micron TiO2, (300 nm) exhibits a maximum of the impact energy at 4-5 vol%. Kornmann et al. found that the fracture toughness of unsaturated polyester is doubled by adding 1.5 vol% silicate to form an exfoliated structure (49). Zilg et al. (29) proposed that the tactoids of intercalated organo-clay act as a toughening phase, resulting in shear yielding of the polymer matrix (29).

Figure 24: Improvement of the impact toughness of epoxy resin using different nanoparticles (29). Electrical properties The majority of the thermoset polymers do not posses good electrical properties. So in order to achieve the required electrical property according to the application, nanoparticles could be used due to their quantum effect and smaller particle size by which the percolation can occur at lower volume fraction since the inter-particle spacing decreases. This effect would be more significant if the aspect ratio of the nano-materials is high. 26

For example, CNTs increase the conductivity of the epoxy resin by nine orders of magnitude when added in a range of 0–4 wt% CNTs, as a result of percolation phenomenon (43). The degree of alignment along the measurement direction has also an important influence on the electrical conductivity of the nanocomposite. For example, in a CNT\ epoxy nanocomposite, when half of the CNT sheets are aligned along the measurement direction, conductivity is 13,084 S/m, whereas the random orientation of CNT in the epoxy gives conductivity below 5 S/m (50).

Tribological properties The incorporation of nanoparticles has significant effect on tribological properties of thermosets. For example, the addition of 3 wt% nanoclay to unsaturated polyester resulted in 85% improvement in wear resistance and 35% decrease in coefficient of friction (50). For the epoxy based coatings, the introduction of the pretreated nanoclays did not only improve the scratch resistance performances but they also maintained the transparency of nanocomposite coatings (51). Furthermore, Al2O3 nanoparticles proved to be quite effective in lowering frictional coefficient and wear rate of epoxy composites sliding against steel. A decreased wear rate of the epoxy by 97% was obtained by the incorporation of 0.24 vol% of pretreated alumina nanoparticles Al2O3–c-PAAM/epoxy (52). Incorporating nano-TiO2 (300 nm) into epoxy composites is also proven to improve the wear resistance especially at high contact pressure and high sliding speed (53).

Thermal properties It is also very important to develop a thermoset based nanocomposite material with good thermal properties among which the thermal conductivity plays a key role. This needs to be in a high level for the composite to be very promising for various applications especially in the fabrication of electrical devices. One of the several studies that have been carried out in this regard and have successfully shown an increase of 40 % of the thermal conductivity of an epoxy resin or polyurethane loaded with 0.5 wt% of CNTs, as shown in Fig. 25 (54).

Figure 25: Evolution of the thermal conductivity of chemically modified MWNTs PU based nanocomposite (54). Fire retardancy and thermal stability In addition to those above mentioned properties, the thermal stability and fire retardant properties of a composite material are also essential to be studied and improved specially for applications such as electrical and thermal insulators. These properties could be enhanced by addition of plate-like nanomaterials such as nanoclays particles. For example, the organically modified nanoclays or organoclays added to the PU act as a thermal insulator and mass transport barrier (Fig.26) to the volatile products generated during the decomposition as well as the penetration of oxygen necessary for the oxidation reaction through silicate layers which protect the bulk of the thermoset matrix (34). Apart from the nanoclay, the use of TiO2 and SiC particles as reinforcement could also enhance the thermal stability as reported by Mahfuz et al (55).

Figure 26: Diffusion mechanism of barrier property. Summary & Scope of Work The development of new materials that not only display better mechanical, physical and chemical properties but also presents recyclability, biodegradability and environmentally friendly characteristics is a real challenge.

Although there is a significant amount of research carried out to study the property enhancement with the incorporation of nanomaterials into the conventional thermoset resins as well as to develop a new manufacturing processes of nanocompsites, these studies do not cover all the possible combinations of nanomaterials – thermosets. Furthermore, the processing difficulties of the nanocomposites such as agglomeration of nanoparticles are still to be resolved due to which their use has been limited in commercial applications up to now.

The newly developed bio-derived thermoset materials such as furfural alcohol resin (FFA) can be a good alternative for the conventional petroleum based thermosets. However, the high fragility of FA and its poor fracture toughness property would definitely prevent its development into a commercial product. Thus, the incorporation of CNW into FA seems to be promising route to enhance the 28

mechanical property of this thermoset while maintaining the bio-based character of the nanocomposite. Hence, the main goal of this work is to develop a feasible process of manufacturing a cellulose nanowhisker-furfural alcohol based nanocomposite. The main objectives of this work are as follows:

 Determination of the curing parameter of the pure resin;  Identification of the optimal volume of nano-material and resin matrix;  To find a suitable process to obtain a uniform dispersion of the nanomaterial into the resin matrix;  To investigate the dispersion state of the nanomaterials into the resin matrix using rheology tests;  To study the mechanical properties of the developed nanocomposite.

II EXPERIMENTAL

MATERIALS

Matrix: Furfural alcohol (FFA) Furolite /Biorez 080101 with 20% water content mixed with S-type catalyst resin was used as a matrix as received form Transfurans chemicals, Belgium. The FFA has a low viscosity (100 to 200 cP at 25°C), according to the manufacturer’s technical data sheet Reinforcement: Unidirectional glass fiber (GF) fabric Dewold L 500 E 11-1, was provided by Swerea SICOMP. Nano-reinforcement: An aqueous suspension of 3 wt% CNWs that had a crystal size about 3 to 5 nm in diameter and around 200 nm in length was used in this study as nano-reinforcement.

PROCESSING

Molding of the pure FFA The molding of the FFA resin is done on two steps, first drying and then curing. The used FFA resin contains 20% water, thus when the resin is dried and cured in the vacuum oven, four processes take place simultaneously at different rates: 1- Water (solvent) evaporation; 2- Polymerization reaction proceeds by which molecular weight of the polymer increases and a three-dimensional network is formed; 3- Water elimination .as a side product of the polymerization reaction 4- Increases of the viscosity of the system and the glass transition temperature of the formed polymer as a result of the crosslinking reaction and the decrease of the water (solvent) content in the system.

When the rate of polymerization is higher than the solvent evaporation rate, a solid film will be formed on the top of suspension (low water content at the surface thus of high Tg), which significantly reduces the evaporation rate of water resulting in void (bubbles or foaming) formation in the sample due to the water retention. In order to avoid this problem the molding should be done at least in 2 steps:

1) Drying step: Aims to take out all the physical water (solvent) without initiating the polymerization, since the polymerization reaction is thermally activated, a maximum vacuum is needed for water evaporation with a minimum temperature possible. (Fig. 27) 2) Polymerization step: In this step the temperature is increased gradually since the polymerization reaction should proceeds at a low rate that will allow the evacuation of the chemical water (Fig.28), i.e. the water that is formed during the polymerization reaction as a side product (Fig.9).

Vacuum Solvent

FA aqueous Dried resin suspension

Figure 27: Schematic presentation of the drying step.

Heat Chemical water

Dried FA Cured FFA resin

Figure 28: Schematic presentation of chemical water evaporation during the curing step.

The FFA resin was molded in tree molds with different geometries. An aluminum petri-dish, polypropylene syringe tube (for which the open end orifice was blocked) and handmade open mold (using a large aluminum band as the mold’s wall and PET sheet as a mold’s base) were used to make respectively a thin films ~1-2 mm, long cylinders~ 4-7 cm and thick plates ~ 1-1.5 cm. During this part, the main purpose was to define for each geometry the right pressure and temperature cycles in order to get a sample free of defect within a minimum time. Starting with a high vacuum and low temperature and then temperature was increased and the vacuum decreased gradually step by step. In each step the samples are weighted regularly and when the weight of the suspension stopped decreasing we move to the next step. All the parameters (time, temperature, weight, pressure) were recorded. According to water 30

phase diagram (Fig.29), during the drying step in order to accelerate the evaporation of water we need to be in the water vapor zone or at least in the liquid-gas equilibrium line that is equivalent to the boiling point. However, during the curing step it is recommended to be just above the liquid-gas equilibrium to avoid bubbles formation that will lead to surface defects, since at this stage the system don’t recover it shape like a liquid any more thus the bubbles shape will remain on the surface of the sample after the curing.

Figure 29: Water phase diagram (56). Mixing of FFA-CNW The main aim of this part was to find a suitable way to disperse the CNW into the FFA resin matrix. Thus different ways were explored in order to ensure homogenous dispersion of CNW into the resin. The concentrations of CNW ranged from 0.5 % to 3 % by dried weight of the final composite. Therefore the dispersion process was carried out in the following ways:

Dispersion using aqueous solvent In this trial the water was introduced in order to dilute the FFA, since the FFA resin is relatively viscous, before mixing it with CNW aqueous suspension. This was expected to facilitate the incorporation of CNW in the FFA. However when more water was added in a proportion more than ~40 % to the polymer a phase separation was observed. In addition, when the resin is mixed with a large quantity of water it becomes more sticky and difficult to be mixed using magnetic stir. The conclusion was that the water and the FFA have limited miscibility. Furthermore, the addition of water will extend the time of drying.

Figure 30: Photographs shows (a) phase separation water-FFA (b) the change of the flow properties of the resin, the mixture become more sticky.

Dispersion without additional solvent In this method 3 wt% CNW water suspension and FFA were mixed using a magnetic stir for 30 min and sonified for 10 min with an amplitude of 70% for cycle of 0.5 using UP 200 S ultrasonic processor. Since CNW are in water suspension with 3 wt% concentration, thus higher loading of CNW into FA resin means that we introduce large quantities of water in the mixture, this water creates similar problems as those observed in the dispersion using an aqueous solvent, thus limiting the loading at which we have a homogenous mixture to 1%wt CNW.

Table 1 Material compositions without solvents Final CNW Water Dry FA Dry CNW FA H O composition suspension 2 content (g) (g) (g) (g) (wt%) (g) (wt%) 0,5 9.95 0.05 12.44 1.67 4.11 0.29 1 9.90 0.10 12.40 3.33 5.73 0.36 2 9.80 0.20 12.25 6.67 8.91 0.47

Dispersion using ethanol as a solvent Since water is not a good solvent for FFA, one option was to substitute partially the water in CNW aqueous suspension by a good solvent of FFA and CNW like ethanol in order to minimize the negative effect of water. Thus our approach was first to concentrate the CNW up to 15 wt% by dialysis against polyethylene glycol (PEG), and then disperse it in ethanol using magnetic stir for 15 min and sonified for 5 min. The obtained solution had a concentration of 3% CNW, was than mixed with FA resin using magnetic stir for 15 min and sonification for 5 min. In this case the all suspensions looked very homogenous and stable without any phase separation.

Table 2 Compositions with ethanol as solvent. Composition Weight of Weight of Weight of Weight of Weight of Weight of

of the dried dried FFA dried FFA CNW H2O ethanol composite (g) CNW(g) solution(g) suspension(g) (g) (g)

0,5 wt% 9.95 0.05 12.44 1.67 0.72 3.39 1 wt% 9.9 0.1 12.4 3.33 1.00 4.72 2 wt% 9.8 0.2 12.25 6.67 1.56 7.35 3 wt% 9.7 0.3 12.125 10 2.12 10.00

However, during the last stages of curing process, it was observed that the sample contracted in the tree dimension and cracked in several small pieces.

Figure 31: Photographs showing the (a) tree dimensional contraction of the FFA during the curing inside the syringe mold, (b) fractured pieces of the sample, (c) cross-sectional view showing the cracks.

Dispersion using a concentrated suspension of CNWs To minimize the problems caused by the water or ethanol, the concentrated 15 wt% CNW was tested. In this method, a master batch was prepared by mixing the 15 wt% CNW aqueous suspension directly with FFA-resin in order to obtain a master batch with 3 wt% CNWs. Since the 15 wt% suspension was very viscous, several mixing steps were required to obtain a homogenous suspension. In the first step, the FFA was gradually mixed to concentrated CNW suspension using a mortar, until the FFA was incorporated into the CNW suspension. In the second step, a high shear mixer was used until the suspension the CNW aggregates disappeared. At this stage CNW suspension formed small droplets dispersed in the FFA resin. Therefore to further improve the dispersion the mix was sonified for 5 min, and stirred for 10 min to make sure that the dispersion was homogeneous (Fig.32).

The master batch was stored in refrigerator for following use. Suspensions with CNW concentrations of 0.5, 1 and 2 wt% based on the dried weight were obtained by diluting the master batch using FFA resin. The diluted suspension were mixed mechanically for 10 min and then ultrasonified for 3 min to make sure that the CNW were homogenously dispersed. All the obtained suspensions were very homogenous and stable even after storage for several days in refrigerator.

Figure 32: Preparation of 3 wt% CNW master batch mixing & dispersion method.

Since the last method described in this section proved to be the most promising in the dispersion of the CNW, it have been selected as preparation procedure for the CNW-FFA nano suspension tested in the rest of this work.

Table 6 Materials composition with concentrated CNW. Used matrix Composition Wt% CNW based on Batch Name %FFA %CNW %H2O the dried composite Pure FA 80 0 20 0 0 wt% 73 0 27 0 0,5wt% 72.635 0.365 27 0.5 1 wt% 72.365 0.73 27 1 2 wt% 71.54 1.46 27 2 3 wt% 70.81 2.19 27 3

Composite preparation methods After that the drying-curing cycle was determined for the pure resin and the dispersion process of the CNW in FFA was developed, samples of nanocomposite with the different geometries as well as laminate of FFA-CNW-GF multiscale composite were prepared.

Nanocomposites The prepared FFA-CNW suspensions were held in the long cylinder and molded using a mold of metal sheet and cured according to the optimized curing cycle. Some 34

adjustments in the curing time since the mixed samples contained more solvent than pure FFA resin. One difficulty that remained was related to nature of the used resin. In fact, samples with exact dimensions in one molding step were not possible to produce. Primarily, the made samples were not flat, and the bending was increased with the storage time even for the tick samples up to 2 cm. Secondly, the resin with 20 % water content in addition to water produced during the polymerization reaction made the preparation more difficult. Therefore thicker and larger samples were made to be able to grind and polish the samples for required dimensions.

Figure 33: Photographs showing a (a) polished flat sample (b) bended sample.

Multi-scale composite The preparation of the multi-scale composite was done at Swerea SICOMP by hand lay-up. Glass fiber fabric was cut into six pieces 16*16 cm2 and put in metallic mold. Then pure FFA and previously prepared nanosuspensions were applied on each GF fabric cloth using a brush and a roller to work it into the glass fiber. After that all the composite laminate were kept at room temperature for 12 hours, placed into an oven at 80°C for 24h, and heated up to 135°C for 15 min. An average composition of the prepared lamination is presented in the Table.7.

Figure 34: Schematic presentation of the multi-scale composite.

Table 7 Composition of the prepared laminate. Laminate CNW content Used compositions Matrix Glass name (wt%) FFA% CNW (%) H2O(%) (wt%) fiber (wt%) Pure FFA - 80 - 20 46 54 0 wt% - 73 - 27 45 55 0,5wt% 0.5 72.63 0.36 27 43 57 1 wt% 1 72.36 0.73 27 43 57 2 wt% 2 71.54 1.46 27 43 57

3 wt% 3 70.81 2.19 27 44 56 .

Characterization Thermogravimeteric analysis Thermogravimetric analysis (TGA) of the uncured resin with different CNW concentrations (nano-dispersion using ethanol as a solvent) were carried out using a TGA 550, TA Instruments. The heating rate was 10°C/min and the samples were heated from the room temperature to 200°C. Two samples were tested form each suspension to ensure accuracy of data.

Rheology experiment A stress-controlled dynamic rheometer C-VOR (Bohlin) which was provided by Swerea SICOMP was used with the cone-and-plate geometry, to measure the rheological properties of FFA/CNW suspensions before curing. The cone angle was 4°, the diameter of the plate was 40 mm, and the gap distance was 150 µm. Two kinds of dynamic rheology measurements, i.e. steady shear test and small-amplitude- oscillatory-shear test, were carried out at a controlled temperature of 25±1°C using an oil bath.

Figure 35: (a) C-VOR (Bohlin) Rheometer (b) schematic presentation of the cone-plate configuration.

Steady shear test: Steady shear tests were carried out to measure the dependence of shear viscosity and shear stress on shear rate, respectively. The shear rates were increased from 0.1 s-1 to 1000s-1.

Small-amplitude-oscillatory- shear test: Small-amplitude-oscillatory-shear tests were performed to indicate more information about the dispersion of CNW in FFA and the interaction between CNW and FFA. Frequency sweeps, which were performed in the linear viscoelastic region, 36

were carried out to determine the dependence of dynamic moduli on frequency of all the suspensions. Therefore, an amplitude sweep, from 0.02984 Pa to 100 Pa at a fixed frequency of 1 Hz, was performed before frequency sweep to determine the stress amplitude used in frequency sweep. Then, frequency sweeps were carried out for all the suspensions from 0.01 to 100 Hz at a fixed stress of 0.03 Pa.

Flexural test The multi-scale composite laminates samples of 25 mm in width and 50 mm in length in both fiber direction and cross fiber direction were measured using Shimadzu AG-X universal testing machine in flexural mode according to ASTM D790M with a crosshead speed 1,7 mm/min. The length used between the support span was 32 mm.

Figure 36: Shimadzu AG-X universal testing machine in flexural mode. Dynamic mechanical thermal analysis Dynamic mechanical thermal analysis (DMTA) was performed using a Rheometric Scientific TA Q800 DMA device in the 3 point bending mode using 50 mm clamp. The samples of the cured nanocomposite and pure FFA were prepared by cutting stripes measuring 60x12.5x3 mm3 from the different laminates. The measurements was carried out at a constant frequency of 1 Hz, different strain amplitudes (10, 15, 20 %) were used in a temperature range of 25-260°C, a heating rate of 3°C/min and gap distance of 50 mm.

Figure 37: TA Q800 DMA device in the 3 point bending mode. 37

Scanning electron Microscopy The microstructure of the cured samples of both the pure FFA resin and 3 wt% FFA-CNW nanocomposite was investigated using scanning electron microscope (JSM-6460/6469LV JEOL, Japan) with an acceleration voltage of 15 kV in secondary electronic imaging (SEI) mode. The specimen’s surface was scanned under vacuum with a high energy electron beam that interacts with the material. As a result of this interaction different signals are emitted secondary electrons (SE), back-scattered electrons (BSE), X-Rays and light. Through SE detector the SEM can visualize the specimen surface topography with a high resolution and a wide range of magnification.

Primary electronic beam RX

BSE SE

Sample

Figure 38: Signals emitted trough the interaction of electron beam with the composite sample.

RESULTS AND DISCUSSIONS

Thermogravimetric analysis (TGA) In order to examine the curing reaction of FFA-CNW nanocomposite and the evaporation of the solvents, a thermogravimetric analysis of the pure furfural and its nanocomposites with CNW content of 0.5, 2 and 3 wt% was performed at temperature ranging from 25°C to 200°C. The samples were taken from the nano suspension prepared by using ethanol as a solvent, than a calculated amount of water and ethanol was added to each sample to make the water and ethanol contents relatively equal for all samples. In all the cases, weight loss takes place in five stages. Pure furfuryl alchohol and its nanocomposites with the different CNW loading shows similar curve pattern. The observed vertical shift is mainly due to difference of water and ethanol that could not get it equal for all the samples.

The first stage between 0 to 80°C corresponds mainly to the loss of ethanol as it has a lower ebullition point than water. In the second stage at 80°C to 100°C, the evaporation of ethanol continues. In the third stage the 100°C to 135°C the evaporation of water starts, In third fourth stage 135°C to 150°C is mainly due to the polymerization reaction where in the loss in last stage we couldn’t exactly figure it 38

out, but it could be some crosslinking reaction or maybe the start of degradation or even a carbonization of the material.

Figure 62: Thermogravimetric analysis of the FFA and its nanaocomposites with different CNW loadings.

Rheology results: Homogeneous dispersion of CNW in FFA and good interaction between CNW and FFA are necessary to guarantee the good properties of the final nanocomposites. The changes of mechanical properties of neat polymer with the addition of nanoparticles are usually used to indicate these two key factors. However, we have found that it is very difficult to produce large amount of FFA/CNW nanocomposites which can be used for maco-mechanical tests. On the other hand, the rheological properties of complex fluids are very sensitive to the inherent structures of materials. Therefore, rheology measurement has been widely used in nanocomposites to indicate the dispersion of nanoparticles and the interaction between nanoparticels and polymer matrices.

The dispersion of CNW and the interaction between CNW and FFA are not changed significantly before and after curing. Therefore, dynamic rheology measurements on the FFA/CNW suspension before curing were carried out. For each material, the same experiment was repeated at least twice, and the average values were reported.

Steady shear test Steady shear tests were performed to show the influence of CNW on the flow properties of FFA. Repeated steady shear tests for each material to reveal the shear stress and viscosity as a function of shear rate, were shown in Figure 39.

5x10-1 103 -1 (1a) (1b) 4x10

-1 3x10 102

2x10-1 101

10-1 Viscosity (Pa.s) Viscosity (Pa.s) 0

Shear stress (Pa) 10

10-1 101 102 103 101 102 103 -1 Shear rate (s-1) Shear rate (s )

(2a) (2b) 100 102

101 Viscosity (Pa.s) 10-1 Shear stress (Pa) 100

1 2 3 101 102 103 10 10 10 -1 Shear rate (s-1) Shear rate (s )

101 103 (3a) (3b)

102

100

101

Viscosity (Pa.s) Shear stress (Pa)

10-1 100 101 102 103 101 102 103

Shear rate (s-1) Shear rate (s-1)

3 101 10 (4a) (4b)

10 102

Viscosity (Pa.s) Shear stress (Pa)

10-1 101 1 2 3 101 102 103 10 10 10 -1 Shear rate (s-1) Shear rate (s )

Figure 39 Repeated tests (open and closed symbols) of (a) viscosity and (b) shear stress as a function of shear rate for (1) neat FFA; (2) FFA/0.5% CNW; (3) FFA/2.0% CNW and (4) FFA/3.0% CNWs The curves of viscosity~shear rate and shear stress~shear rate for each material in repeated measurements almost superpose with each other, indicating that the dispersion of CNW in FFA is uniform and all the suspensions are stable.

As shown in Fig. 39 (1a), even the neat FFA did not behave as absolutely Newtonian fluid since the viscosity is shown to be dependent on shear rate. It is because that in order to keep identical water content in all suspensions, additional amount of water is introduced in neat FFA, which results in a compound system since water is not miscible with FFA resin. The dependence of viscosity on shear rate increases with increasing CNW loadings as shown in Fig. 39 (2a)~(4a). Since the water content in all suspensions were the same, the significant non-Newtonian effect is attributed to CNW. Stronger shear thinning behavior, i.e., viscosity decrease with increasing shear rate, was observed in the suspension with higher CNW concentration,indicating that CNW are more aligned along the flow direction.

Through the comparison of shear viscosity (Figure 40) and shear stress (Figure 41) for all the suspensions at different shear rates, we find that both shear viscosity and stress increase with increasing CNW loadings. The viscosity of FFA/3.0% CNW is almost 2 orders of magnitude higher than that of neat FFA, and the shear stress of FFA/CNW 3.0% CNW is more than 1 order of magnitude higher than that of neat FFA. Two kinds of networks are speculated to form in the FFA/CNW suspensions. One is the CNW-CNW networks through physical connection and hydrogen bonds of CNW. The other one is CNW-FFA networks due to the strong interactions between hydroxyl-rich surface of CNW and polar FFA. Therefore, the results in Figure 40 and 41 reveal that stronger shear is needed to break the networks in the suspensions with more CNW.

Pure PFA PFA/0.5% CNWs 10 PFA/1.0% CNWs PFA/2.0% CNWs PFA/3.0% CNWs

1 Viscosity (Pa.s) Viscosity 0.1

0.01 10 100 1000

-1 Shear rate (s ) Figure 40 Comparison of shear viscosity~shear rate plots of neat FFA and FFA/CNW suspensions

1000

100

Pure PFA Shear stress (Pa) Shear stress PFA/0.5 % CNW 1 PFA/1.0 % CNW PFA/2.0 % CNW PFA/3.0 % CNW

10 100 1000

-1 Shear rate (s )

Figure 41 Comparison of shear stress~shear rate plots of neat FFA and FFA/CNW suspensions Small-amplitude-oscillatory-shear tests Stress sweep tests at a fixed frequency of 1 Hz were carried out before frequency sweep tests to determine the linear viscoelastic region, as exemplified for neat FFA and FFA/3.0% CNW suspension shown in Figure 42. It is shown that for neat FFA, G’ is almost independent of stress in the whole sweeping range of stress, while for FFA/3.0% CNW suspension, G’ keeps decreasing in the whole stress sweeping region and a quasi-plateau can only be seen at lowest stress amplitude. Therefore, the controlled stress amplitude used in frequency sweeping tests for all suspensions is set as 0.03 Pa.

1000

100

0.1 G'(Pa) 0.01

1E-3

1E-4 0.1 1 10 100 Shear stress (Pa)

Figure 42 The dependence of elastic modulus (G’) on sweeping stress of neat FFA (■) and FFA/3.0% CNW (w/w) (□) at a fixed frequency of 1.0 Hz to measure the linear viscoelastic region Figure 43 shows the dependence of elastic modulus (G’) and viscous modulus (G’’) on frequency of neat FFA and FFA/CNW suspensions with different CNW loadings, separately. For neat FFA in low frequency region, G’ is lower than G’’, while the increase of G’ with increasing frequency is faster than that of G’’, resulting in a crossing of G’ and G’’ at a certain frequency after which G’ is higher than G’’ (Fig. 43 (a)). It indicates that the neat FFA behaves as viscous liquid. With the addition of CNW, G’ is almost higher than G’’ in the whole frequency region, which indicates that the elasticity of the FFA is increased by CNW (Fig. 43 (b)~(d)).

106 106 5 a b 10 5 10 104 4 103 10 2 3 10 10 101 102 100 1 10-1 10 -2 10 100

10-3 Dynamic modulus (Pa) Dynamic modulus (Pa) -1 -4 10 10 10-5 10-2 10-2 10-1 100 101 102 10-2 10-1 100 101 102 Frequency (Hz) Frequency (Hz)

5 5 10 10 c d 4 4 10 10 103 103 102 102 101

101

0 Dynamic modulus (Pa)

Dynamic modulus (Pa) 10

-1 100 10 -2 -1 0 1 2 10-2 10-1 100 101 102 10 10 10 10 10 Frequency (Hz) Frequency (Hz)

Figure 43 Comparison of elastic modulus (G’, ■) and viscous modulus (G’’, □) of neat FFA and FFA/CNWs suspensions with different CNW loadings: (a) neat FFA; (b) FFA/0.5% CNWs (w/w); (c) FFA/1.0% CNWs (w/w); (d) FFA/2.0% CNWs (w/w); (e) FFA/3.0% CNWs (w/w). Figure 44 compares the elastic modulus of neat FFA and FFA/CNW suspensions with different CNW loadings as a function of frequency. It is clearly shown that G’ of FFA is increased significantly with the addition of CNW, especially in the low frequency region, which demonstrates that the long-range structure of the suspensions is remarkably changed by CNW. For instance, G’ of the suspension with only 0.5% w/w CNW is almost 2 orders of magnitude higher than that of neat FFA at the frequency of 1 Hz. When the CNW concentration was 3% w/w, the G’ of the suspension was almost 4 orders of magnitude higher than that of neat FFA at the frequency of 1 Hz. On the other hand, the dependence of G’ on frequency in low frequency region was weakened with the addition of CNW. When the CNW loading was up to 3% w/w, an apparent plateau of G’ in the low frequency region is observed. It indicates a pseudo-solid like behavior of the FFA/CNW suspensions even the concentration of CNW is only 3% w/w.

105

103

101

10-1 PFA

PFA/0.5% CNW Storage modulusStorage (Pa) 10-3 PFA/1.0% CNW PFA/2.0% CNW PFA/3.0% CNW 10-5 10-2 10-1 100 101 102 Frequency (Hz) Figure 44 Comparison of the elastic modulus (G’) as a function of frequency for neat FFA and FFA/CNW suspensions with different CNW loadings.

Figure 45 compares the viscous modulus of neat FFA and FFA/CNW suspensions with different CNW loadings as a function of frequency. G’’ of FFA is also significantly enhanced with the addition of CNW, and the dependence of G’’ of the suspension also decreases with increasing CNW contents. A quasi-plateau is observed when the CNW concentration is up to 3% w/w. The results shown in Fig. 44 and Fig. 45 clearly indicate the network-structured suspensions due to the strong CNW-CNW and CNW-FFA interactions, even at low concentration of CNW (only 3% w/w).

106

105

104

103

102

101

100 PFA

Viscousmodulus (Pa) 10-1 PFA/0.5% CNW PFA/1.0% CNW 10-2 PFA/2.0% CNW PFA/3.0% CNW 10-3 10-2 10-1 100 101 102 Frequency (Hz) Figure 48 Comparison of the viscous modulus (G’’) as a function of frequency for neat FFA and FFA/CNW suspensions with different CNW loadings.

The drying-curing parameters The thin film In the first case thin films ~1mm tick were made by solution casting (FFA with 20% of water) in aluminum Petri dish, according to this curing-drying cycle(See Table 8).

Table 8 Curing cycle for thin film. Curing cycle for thin film t (h) T(°c) V(mbar) 3 40 -800 1 50 -800 2 80 -500-0 2 100 0 1 115 0 5min 135 0

It was found that the use of very low pressure (high vacuum) in this case was not recommended. In fact, during the first step for pressure value lower than -800 mbar, the suspension started bubble and went out of the Petri dish since it height is not enough to contain the bubbles. In addition, in the third step, at 80°C the suspension was very viscous and a rubbery film was form thus under pressure values lower than -500 mbar, could (partially) detaches the film from the surface of the Petri dish .Thus when vacuum is taking out the film regain the Petri dish due to gravitational force, but not in the same place leading to folds as shown in (Fig. 50.a) the obtained film was very fragile, to a point that it broke when it was unmolded. Thus it was not possible to

cut these to obtain samples for mechanical testing. After storage for a couple of days in a desiccator the prepared film starts to bend due to the high shrinkage (Fig.50.b)

Figure 50: Photographs of FFA thin films, a) shows the folds and b) shows the bending.

The long cylinder Since the first geometry was considered impracticable for the specimen preparation. Another option was to make a long cylinder (l=5 cm) specimen for mechanical compression test. The easiest way to do it was to use PP syringe tube (for which the open end orifice was blocked) as open mold. A valid specimen was obtained according to this curing-drying cycle(see Table 9):

Table 9 Curing cycle for long cylinder Curing cycle for long cylinder t (h) T(°c) V(mbar) 53 40 -900 5 50 -800 4 80 -500-0

4 100 0 2 115-135 0 4 135 0

During the drying-curing process, similarly to the thin film in the first step for pressure value lower than -900 mbar, the suspension start bubbling and go out of the tube. Another problem was the formation of rubbery film on the top due to the decrease of the water content which decreased the evaporation rate as mentioned earlier, thus pulses of depression time to time were required in order to create a stronger bubbles and break the film. The obtained samples were looking good and compact, there was relatively little bubbles trapped and no foaming was observed. But when the samples were cut we observed a lot of internal cracks (Fig.51) that led to a void in the centre of the cylinder. These cracks were probably initiated due to thermal residual stress. In order to eliminate these residual stresses the cooling rate

after curing was slowed down by turning off the oven and keeping the samples inside over night. After unmolding, the specimen was subjected to an annealing heat treatment of 4 hours at 100°C. Thus the thermal residual stresses were eliminated a specimen free of cracks was obtained.

Figure 51: Cross-sectional view of FFA long cylinder (a) with and (b) without the annealing treatment.

However, the small part of specimen split in two pieces after storage for a couple of days in a desiccator and after one week in room condition without any external applied stress (see Fig.52). This is believed to be related to the continuation of the evaporation of water after the curing. This evaporation of water might lead to gradient of water content in the sample that can generate some cracks or helps in the crack propagation and the growth of some micro cracks that were caused by the cutting or the unmolding of the sample.

Figure 52: Cross-section of fractured surface FFA after (a, b) before (c) storage in the desiccator.

Due to these problems and to the fact that making such sample was very time consuming process since the contact surface area with air trough which the evaporation occur was very limited, the long cylinder was also considered impracticable for the specimen preparation.

The thick plate The mold that was used to make the thick plate was designed in way that its wall is long enough to contain the bubbles when drying under vacuum, and present a

high contact surface area that will allow an increase in the evaporation rate. The cure cycle is given in Table 10.

Table 10 Curing cycle for thick plate.

Curing cycle for the thick plate t (h) T (°C) V (mbar) 12 40 >-1000 3 50 -900 4 80 -500 2 100 0 2 115\135 0 40 min 135 0

By using this geometry, the drying curing time was substantially reduced.

Macroscopic appearance : The nanocomposite: From the comparison of the macroscopic appearance of the FFA nancomposite with 3% CNW and the pure FFA cured under the same conditions with similar water content in (Fig.53). We can clearly conclude that the addition CNW increase the water retention of the polymer. This is due mainly to hydrophilic character of CNW. Even a larger drying step was not enough to extract all the water in the resin before that the curing stage was reached.

Figure 53: (a) Pure furfural (b) 3 wt% nanocomposite cured under the same conditions.

The multiscale composite: Visual inspection of prepared glass fiber fabric furfurylalcohol-cellulose nanowhiskers composite laminates can be very useful to get a preliminary evaluation of the quality of the composite. Fig.54 shows photographic pictures of the prepared laminates. The index in the bottom of each picture presents the CNW content in the matrix. The water content in the used suspensions was 30% for all samples except for “pure furfural” sample for which the water content was of 20%.

Figure 54: Appearance of the prepared multiscale composite laminates with different CNW contents. Multiscale composite with CNW showed an increase in the porosity of the laminate with CNW content of the matrix. This is due to the increase in the viscosity of the FFA by the addition of CNW.

Dynamic mechanical properties: DMTA measurement is one of the methods used to determine the thermal behavior of the FFA thermosetting polymer. In this type of thermoset resin system the molecular chain mobility is very low, which give very small transitions that are not easily detectable by using DSC. Fig. 55 shows the loss factors (tan δ) curves up to 250°C, obtained for the pure furfural cured in both 115°C and 135°C as well as for 3 wt% FFA-CNW. At the temperature for which the loss factor passes through a maximum, the tan delta peak was determined to be 190°C, 185°C and 177°C which means that either an increase in the curing temperature by 20°C, or the 3 wt% CNW do have a insignificant influence on the tan delta factor of the furfural resin. The noise (irregularity) of the curves observed at temperatures beyond the 190°C is caused by the variation of the value of Young’s modulus of the polymer in the rubbery state form the value that was putted in the system during the setup of the experiment.

Figure 55: DMTA curves loss factor (tanδ) of pure FFA resin and it´s nanocomposite.

By using dynamical mechanical thermal analysis (DMTA) the mechanical behavior of the previous materials was also studied. Fig.56 shows the storage modulus as a function of the temperature curves of the pure furfural cured in both 115°C and 135°C. The curves of pure furfural and 3 wt% FFA-CNW are presented in Figure 56, left. All samples did not display a typical behavior of thermosetting polymer and the glassy plateau where modulus had to be constant through the whole temperature range below 190°C was not observed. The storage modulus showed significant temperature dependence in the glassy state. Similarly the rubbery plateau was not observed. In fact the modulus increases with the temperature after the relaxation peak, due to the generation of further cross linking reaction indicating that a total conversion of FFA was not reached during the curing process.

Figure56: DMTA curves bending storage modulus as a function of the temperature curves of (right) pure FFA resin and (left) its nanocomposites.

The fact that the curve of the 3 wt% CNW nanocomposite superpose to the curve of the pure FFA in the glassy and rubbery regions shoes that a loading 3 wt% CNW have no significant influence in bending storage modulus. Furthermore in the rubbery state for temperatures beyond 200°C the CNW has already thermally degraded thus it does not affect the cross-linking reaction rate. From (Fig.56 b) we can see that, an initially higher storage modulus of 135°C cured sample (7,3 GPa) than the sample cured in 115°C (5,6 GPa). Thus the high curing temperature increased the crosslinking density in the FFA, leading to a stiffer material.

Flexural properties of the multi-scale composites The mechanical properties of the multi-scale composites FFA\GF\CNW with different CNW loadings were examined by measuring the flexural properties in the longitudinal and transversal directions. Fig. 57 shows the test samples.

Figure 57: The flexural samples of prepared multiscale composites (a) transversal, (b) longitudinal, (c) cross-section of the longitudinal samples.

The tests were conducted in other to see if the cellulose nanowhiskers influence the mechanical properties of a glass fiber reinforced FFA. But, one have to take into account that the addition of CNW into the furfural resin decreased the processability therefore samples with higher CNW content contains more voids than the others. Moreover, it was difficult to obtain a similar value for the four samples within the same laminate since the void sizes and distribution varies within the same laminate. Thus, we presented only the values of the best one from each laminate, assuming that it is the one with the less void content.

Figures Fig.58 and Fig.59 present respectively the typical flexural strain-stress curves obtained in longitudinal and transversal directions for the different composites. Figure.60 summarizes the evolution of the transversal and longitudinal flexural strength and modulus with an increasing CNW content.

Figure 58: Typical flexural stress-strain curves obtained in the longitudinal direction

Figure 59: Typical flexural stress-strain curves obtained in the transversal direction

Figure 60: Transversal and longitudinal flexural modulus and strength of multi scale composites with different CNW contents.

The flexural properties in the longitudinal direction depend mainly on how the matrix wet and how the moisture penetrates into the glass fiber bundles, and also the type of interaction between furfural, glass fiber and cellulose nanowhiskers. As we

can see form the Fig.60 the addition of CNWs improved slightly the flexural properties of the composites; this improvement can be attributed to the reinforcing effect of the CNW. The flexural properties in transversal direction however depend more on the mechanical properties of the matrix. In the transversal direction the matrix in between glass fiber buddle acts as the weak links in the composite. It might be possible that make its thickness inhomogeneous and increase the void content in this regions since it doesn´t flow easily due to its high viscosity.

Microstructure The investigation of cryogenic bending fracture surfaces in the SEM gives information about the relation of the microstructure and to mechanical behavior. A comparison of (Fig 61.a) and (Fig 61.b) reveals the fractured surfaces of 3 wt% CNW-FFA nanocomposite and of the pure FFA. A flat and smooth surfaces characteristic of brittle fracture were observed in both cases.

Figure 61: SEM images of fractured surfaces of (a) the nanocomposite with 3 wt% CNW and (b) the pure FFA.

In addition, the micrograph of the nanocomposite did not show any clear indication of micro-scale aggregation of the cellulose nanowhiskers, suggesting a good dispersion of the cellulose nanowhiskers in the furfural resin was obtained by used dispersion method. Furthermore, the absence of the micro voids indicates that the drying step was successful to take out the chemical water.

CONCLUSIONS

The goal of this work was to produce nanocomposites based on furfuryl alchohol (FFA) and cellulose nanowhiskers (CNW). For the preparation of the nanocomposites, a mixing and dispersion methods that combined mortar, high shear mixing and sonifcation was developed. The nancomposites were molded in open molds inside in a vacuum-oven. Temperature and vacuum cycles were determined in order to obtain materials free from void and defects.

The rheology measurement was carried out on the neat FFA and FFA/CNW suspensions before curing to study the dispersion of CNW in FFA and the interaction between CNW-CNW and CNW-FFA. The results showed that even small amount of CNW changed the flow properties of FFA significantly due to the uniform dispersion of CNW and the strong interaction between CNW-CNW and CNW-FFA. The steady shear viscosity and shear stress increased with increasing CNW contents, being almost 2 orders of magnitude higher at 3 wt% of CNW compared to the pure FFA. The dynamic elastic modulus and viscous modulus of the suspension containing 3% w/w CNW were almost 4 orders of magnitude higher than those of neat FFA, and pseudo-solid like behavior was clearly observed when the concentration of CNW was higher than 1% w/w.

The DMTA results showed that the nanocomposite with 3 wt% CNW showed no significant increase in storage modulus compared to the pure FFA. It was also seen that the high curing temperature increased the crosslinking density in the FFA, leading to a stiffer material. Finally the results indicated that a complete conversion was not reached during the curing.

The macroscopic observation of the FFA/CNW nanocomposite and multiscale composite with CNWs showed poor surface quality and higher porosity compared to the non-reinforced ones. This is expected to depend on the increased viscosity and water retention of the FFA when CNWs were added.

The flexural properties in the different multiscale composite laminates showed that the addition of CNW improved the multiscale flexural properties slightly in the fiber direction. However it decreased in the transversal direction which indicate that the mechanical properties of the matrix were not improved by the incorporation of the CNWs. Nevertheless the fact that the thickness, void content and distributions varies in between the samples and within each of the samples, makes the data unintentionally inaccurate, thus the conclusions drawing from it not reliable.

SUGGESTION FOR THE FUTURE WORK

In order to improve the understanding of this system as well as the properties of the final nanocomposites further studies are required.

 Explore other ranges of the nanocomposite compositions

 Use the two component FFA resin system in which the monomer and the catalyst are provided separately, will allow us to heat the system during the mixing process without initiating the polymerization reaction.

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