CHAPTER I Introduction 1.1 Thermosets Thermosetting

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CHAPTER I

Introduction

1.1 Thermosets

Thermosetting polymers/resins are pre-polymers in a soft solid or viscous liquid state. They change irreversibly into an infusible, insoluble three dimensional polymer network of covalent bonds as a result of a chemical reaction taking place in presence of a curing agent/hardener. The resultant structure is known as a thermosetting plastic or thermoset. They can be heated and shaped only once. If re-heated they cannot soften as polymer chains are interlinked. Thus, these polymers are synthesized as a final material of desired shape (polymerization and final shaping is done in the same process or mould is the true chemical reactor). They offer high thermal stability, good rigidity, hardness, and resistance to creep [1]. A good solvent can swell the thermoset but cannot dissolve it. Thermosets are usually amorphous (crosslinks inhibit the movement of polymer chains to pack or crystallize and restrict ordering of network structure) in nature. Thermosets do often exhibit high glass transition temperatures. Their glass transition temperature is higher than the temperature at which they are used (Tuse), therefore, they behave as glasses during their use [2]. The main thermosetting resins are epoxy, melamine formaldehyde, polyester, polyurethane and urea formaldehyde. Thermosets find applications where thermoplasfics cannot compete because of their properties and cost. When fire resistance is required, phenolics are the first choice. Urea formaldehyde is the material used as an adhesive for wood. Melamine formaldehyde is an excellent quality and low cost material used for furniture coating. Polyester resin is used to make fiber reinforced structural parts and epoxy resins constitute a very important class of thermosets used in aeronautical applications. The annual global market for different thermosets can be represented by a Pie chart as shown in the Figure 1.1 [3].

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% Market Share

• Polvurethane

• Urea-Fomuldehyde

• Phenolic resins 1 T • Unsaturated Polyester L 1^^"^^^ ^ • Epoxies % JL—^ • amino resins V ^^^^

Figure 1.1: Global market for different thermosets

1.2 History and Global Market of Epoxy Resin.

Epoxy was first synthesized by Pierre Castan of Switzerland and S.O. Greenlee of United States in late 1930s. The epoxy resin was first marketed by CIBA Company in 1946. By late 1960 almost 25 types of epoxies were available. The present and fiiture global epoxy market by Zion research analysis is as shown in the Figure 1.2.

Global bpoxy Kesiiis Market, 2014 - 2020 (Million Tons) (USD Billion) I

unTfl2014 2015 2016 2017 2018 2019 2020 i i^"Volume -•-Revenue

Sow«;ZionRM*afch An*lysii 2015

Figure 1.2: The present and future global epoxy market.

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Epoxy resins find applications in areas like coatings, composites, electrical and electronic insulation, wind turbines, construction and adhesives. The global demand of epoxy resin is represented as a pie chart (Figure 1.3) on the basis of applications by different sectors [3].

Figure 1.3: The global demand of epoxy resin by different sectors. (PAC, paint, adhesive, cements; AT, automotive and transportation; BC, building and civil engineering; E, electricity and electronics; F, furniture; C, consumer goods; MI, mechanical and industrial; P, packaging; O, others)

The consumption of epoxy resin by different countries (by HIS market) is presented by a pie chart in Figure 1.4. China is the leading consumer of epoxy resin in the world.

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World Cor»sump>tion (xf Ep>oxy Resins 2013

Other Asia V Other Canada-14'^ livdia Certral'South America I^ntral' East«rr Europe Taiwan Chiria Japan MiMeEast

Rep. of Koreai

Western Europe

Figure 1.4: The consumption of epoxy resin by different countries

1.3 Properties of Epoxy Resin

Epoxy resins exhibit outstanding adhesion to various substrates due to the presence of polar hydroxyl groups and ether Hnkages. They show low shrinkage upon cure as epoxy molecule has a rather small reorientation during the curing process as compared with other polymers and as a result, contact between the epoxy resin and the substrate is not disturbed by tensions. The surface tension of epoxy is most often less than the critical surface energy for most materials which also result in good adhesion. Mechanical properties of epoxy are better in comparison to other plastics due to its low shrinkage upon cure. Epoxy resin shows high corrosion and chemical resistance especially to alkali. They have good electrical insulating properties; their volume resistivity is normally lO'^ Q cm"'. Epoxies with high aromatic content are sensitive to light in the UV range. Direct irradiation with ultraviolet light quickly causes yellowing. Even normal sunlight contains enough ultraviolet radiation for yellowing to occur. Aliphatic epoxy resins with anhydride or amine hardener are U.V. resistant. They have ability to be processed under a variety of conditions. Epoxy

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composites provide high strength to weight ratios, good dimensional stability and good moisture as well as thermal resistance.

Electrical properties of epoxy resin Epoxy is electrically insulating in nature. Considering insulating properties of epoxy, this thesis focuses on the improvement in this property. Inducing electrical conductivity in such a versatile material would extend its application potential and will combine electrical properties of metals with advantages of polymers such as light weight, resistance to corrosion and chemicals, and lower cost. If electricity is induced in epoxy resin it can find applications such as protection of aircraft against lightning strike and electromagnetic interference of aircraft as epoxy is used as an adhesive and matrix material in the aerospace and automotive industry. Electrically conductive epoxy also finds application in fire retardant anfistatic regulation for mining equipments and storage of oil and gas and their transportation [4]. Electrically conductive epoxy adhesives are widely used in electronic packaging applications [5].

1.4 Chemistry of Epoxy Resin

Epoxy resins contain an oxirane ring in their structure [Figure 1.5]. Diglycidyl ether of bisphenol A (DGEBA) is the most common type of epoxy resin. It has glycidyl group in it [Figure 1.5].

-CH2— Oxirane ring Glycidyl group

Figure 1.5: Oxirane ring and glycidyl group.

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DGEBA is synthesized by condensation reaction of bisphenol A and epichlorohydrin in presence of a base such as NaOH, as presented in Figure 1.6. An epoxy resin system requires two components, a diepoxide or equivalent (epichlorohydrin is equivalent to a diepoxide since another epoxide group is formed upon destruction of the first one) and a reactive diol. Epichlorohydrin is the most readily available and cheapest diepoxide equivalent. Bisphenol A is a reactive diol, widely used as its aromatic nature enhances the hydroxyl reactivity and adds strength to the resin formed. DGEBA has two terminal epoxide groups.

0 CH3 /\ NaOH OH -fyA-\j—0E__ + 2CICH2 ^ ^ •

CH3 Bisphenol A Epichlorohydrin

I CH3 -HCl ClCH2CHCH20-^-C—Q-OCH2CHCH2CI

OH CH3 OH

CH3 CH3 0 _ / \ ^^CH^fo^Q-C-QoCH^CHCH^ ^0©—?—QoCH^"^^ CH3 L CH3

Diglycidyl ether of bisphenol A (DGEBA)

Figure 1.6: Reaction of bisphenol A and epichlorohydrin giving diepoxide

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The reaction involves nucleophilic attack of phenolic -OH on the least substituted carbon of the epoxide as shown in the Figure 1.7. Internal ring closure reforms the epoxide ring.

Figure 1.7: Nucleophilic attack of phenolic -OH on epoxy ring

Epoxy resins are available in two broad catagories: solid resins, which are usually used in coating applications and their degree of polymerisation i.e. n value is between 1 to 20. The other variety is available in liquid form having n value 0 /I. The commercial version of resin is the one having molecular weight of 380 ( n = 0). They find applications in structural plastics (reinforced plastics, adhesives and casting). Pure DGEBA is a solid (m.p. 43 °C). The amount of epoxide groups present in the resin is usually expressed as the weight per epoxide (WPE) or epoxide equivalent [6]. Epoxide equivalent is defined as the weight of the resin in grams which contain one gram equivalent of epoxide. For unbranched diepoxide, the WPE will therefore be half the number average molecular weight.The amount of epoxide present in the resin can also be expressed as 'epoxide content'. It represnts amount of epoxide present as equivalents per kilograms of resin. Epoxide content is reciprocal of WPE, multiplied by 1000. The other epoxy resin types are novolacs, peracid resins, hydantoin etc.

1.5 Curing Agents for Epoxy Resin

The low molecular weight, viscous liquid epoxy resins find applicafions when cured with appropriate curing agent (hardener). In most of the applicafions of epoxide

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resins, they are converted into a three dimensional infiisible network held together by covalent bonds, by initiating a cross linking reaction. In context to epoxy resin technology, this conversion from liquid or friable brittle solid into a tough cross- linked polymer is known as curing or hardening. The curing agents are broadly of two types viz. polyfiinctional and catalytic.

1.5.1 Polyfunctional curing agents Polyfiinctional curing agents have fiinctional groups that can react with glycidyl type of epoxide. On curing, a three dimensional network structure is obtained. They are used in stoichiometric or near-stoichiometric amounts. The cross linking is facilitated by opening of the epoxide ring usually through ionic mechanism. They cure usually by polyaddition reaction. Normally, no byproduct is formed in curing reaction and it is an exothermic process. The resulting structure involves cross links formed by curing agents as the means of holding the resin molecules together. Polyfunctional curing agents are of two types. i) Basic type Basic type of curing agents can be aliphatic or aromatic type of polyamines. Aliphatic polyamines are room temperature curing agents having low viscosity and low cost. They are usually used for curing of glycidyl ether type of epoxy resins. Aliphatic amines constitute largest group of curing agents. Some examples are as follows (Figurel.8)

H2N' - ^ N H

Triethylenetetramine (TETA) Diethylenetriamine (DETA)

Figure 1.8: Aliphatic polyamines type of curing agents

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Aromatic amines need elevated temperature for curing as their reaction at room temperature is slow. They are solids at room temperature. They impart improved chemical and thermal resistance to cured system. Following are important aromatic polyamine type of curing agents (Figure 1.9).

^^^\ / ^^2 \ /~^^i NH2

m- Phenylenediamine (MPD) Diaminodiphenyl methane (DDM )

Figure 1.9: Aromatic polyamine type of curing agents

Stoichiometric relation between DGEBA and TETA Stoichiometric relation between DGEBA and TETA depends on epoxy equivalent of DGEBA and amine equivalent weight per active hydrogen of TETA [1]. The ftinctionality of amino group towards epoxide ring depends on the number of active amino hydrogen atoms as one active amino hydrogen is responsible for the opening of each epoxide ring. For triethylenetetramine (TETA) there are 6 active hydrogen atoms per molecule. Molecular weight of TETA is 146. Thus, amine equivalent weight per active hydrogen of TETA is 146/6 = 24.3. If epoxy equivalent of DGEBA is 190 (380 g/mole/2 eq/mole), then 190 g of DGEBA will require 24.3 g of TETA to provide one hydrogen atom for each epoxide group. The amount of TETA required to provide one active hydrogen is then expressed as parts of curing agent per hundred parts of resin (phr) on the weight basis, the amount will be 24.3 x 100/190= 12.8 phr.

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ii) Acidic type The important acid curing agents have cycloaliphatic structure. They lead to systems having good mechanical, electrical and thermal properties than amine cured systems. They require a long and high temperature cure schedule. An accelerator (catalyst) is required to overcome this problem. They are converted into compounds that have labile hydrogen in the course of the reaction and react in 1:1 basis with epoxide group. Following are examples of acid curing agents (Figure 1.10).

O 0 0 II 0 \ T p I /" ^^

Figure 1.10: Acidic type of curing agents

1.5.2 Catalytic curing agents Catalytic curing agents achieve cross linking by initially opening the epoxide ring followed by homopolymerisation (self- polymerization) of resin resulting in a polyether structure. The amount of catalyst used with epoxy resins is usually decided empirically. Relatively low concentration (0-5 phr) of catalytic curing agents is required. They can be used as an accelerator to anhydride system. Lewis bases and Lewis acids constitute important catalytic curing agents.

i) Lewis base catalyst The Lewis bases are electron pair donors. The typical examples are imidazoles and tertiary amines such as 2-[(Dimethylamino) methyl] phenol (DMP -10) as shown in the Figure 1.11.

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Immidazole 2-[(Dimethylamino)methyl]phenoi

Figure 1.11: Catalytic type as curing agents (Lewis bases)

ii) Lewis acid catalyst They are electron pair acceptors. The examples are aluminium chloride (AICI3), stannic chloride (SnCb), titanium tetrachloride (TiCU)) and boron trifluoride (BF3). They act as curing agent by coordinating with epoxide oxygen, facilitating transfer of proton. They are effective in catalyzing glycidyl ethers and cycloaliphatic and linear epoxides.

1.5.3 Curing using amine curing agent Curing is generally facilitated by using amine curing agent. Cross-linking takes place due to reaction between the epoxide groups of DGEBA and nitrogen of an amine. One active hydrogen on the nitrogen atom of amine is responsible for the opening of one epoxide ring. The reaction of DGEBA and amine curing agent is described as in Figure 1.12 [6].

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OH o 1 p >Ti_r + / \ 1 a R.NH.CH2CH—

Primary Epoxide Secondary amine amine ring

OH 1 OH CH2.CH— 1 0 R.NH.CH2CH— + ^ Iv.lM -^ b A- CHo.CH—

Secondary Epoxide amine ring Tertiary amine

OH 1 O.CH2.CH — OH 0 1 1 c CH.CH2 + * " CH.CH2 ~

Secondary Epoxide Secondary alcohol ring alcohol

Figure 1.12: Reaction of epoxide group of DGEBA with (a) primary amine (b) secondary amine and (c) secondary alcohol

The above reaction (a), gives rise to the formation of secondary hydroxy] group and secondary amine. The secondary amine possesses one active hydrogen and opens one more epoxide ring giving rise to another secondary hydroxyl and tertiary amine (b). The tertiary amine formed in the above reaction can act as a catalyst for epoxide homo polymerisation (depending on its structure). There is a possibility of reaction between secondary hydroxyl group and epoxide ring leading to cross linking via formation of another secondary hydroxyl group which may get involved in opening of another epoxide ring in similar way (c). The resulting network is usually insoluble except strongest chemical reagents.

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1.5.4 Structure property relation in amine cross-linked epoxy structure

OH ^^3 NI —

N — Adhension

Good corrosion resistance, thermal propertis and hydropliobic nature, poor UV stability

Figure 1.13: Structure property relation in amine cross-linked epoxy structure

The structure property relation in amine cross-linked epoxy is shown in the Figure 1.13.The plurality of -OH groups facilitates hydrogen bonding, usefiil for adhesion to polar surfaces like glass, wood, etc. The rigid aromatic rings in DGEBA render corrosion resistance, thermal stability and hydrophobic nature but gives poor UV stability as presence of aromatic rings increase UV absorption of the resin making it more susceptible to degradation process [7]. The aliphatic linkages are the source for early thermal and thermo oxidative degradation of a cured epoxy system. Undesirably, the rigid structures also drastically reduce the processability of a resin due to reduction in solubility in common solvent and increase in viscosity. The

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mechanical properties of epoxy resin are good as a result of large cohesive forces in cured epoxy [7]. 1.5.5 Curing using anhydride curing agent Anhydrides are important curing agents for epoxy. Majority of anhydride curing agents are liquids. The curing reaction takes place at elevated temperature. Tertiary amines are usually used as accelerators for these reactions. Secondary alcohol from the epoxy backbone reacts with anhydride to form monoester. The carboxyl group of monoester then reacts with an epoxide ring to form a diester as shown in the Figure 1.14.

HO 1 0 0 pu-^C- " 0^ s, »1^0-'^'''» O--CH0 \ H ROH 'COR /^COR

Anhydride Monoester Epoxide ring Diester

Figure 1.14: Reaction of epoxide group of DGEBA with anhydride curing agent

1.6 Methods for Epoxy Toughening

Epoxy resins constitute a class of thermoset polymers with a high crosslink density of covalent networks. Crosslink density is defined as the number of polymer chain segments between crosslinks. The dense crosslinks lead to high glass transition temperatures (Tg), superior thermal resistance, good mechanical performance, excellent bonding, dielectric and ageing characteristics. However, an increase in crosslink density causes low fracture toughness (fracture energy) [8]. Toughness is a measure of a material's resistance to fracture when stressed or it is the amount of energy per unit volume that a material can absorb before rupturing. The toughening

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process leads to improvement in energy absorption capacity of material resulting in improved fracture toughness and impact resistance. By improving its impact strength, it is made useful for development of materials with high performance character. Figure 1.15 shows fracture energy of epoxy resin in comparison to different materials. Epoxy is slightly less brittle than inorganic glasses and needs toughening. Unmodified epoxy shows low fracture resistance.

10000(680)- 1 7075T6 Aluminium

1 Polysulphone thermoplastic

1 J Elastomodified epoxies E ^ 1000(68)— 1 Polymethyl 6* methacrylate ;> S

s Unmodified epoxied,polyester and ^ 10(0.68)- polyimide resin

Inorganic 100(6.8)— glasses

Figure 1.15: Comparison of fracture energy of epoxy resin with different materials

Methods for toughening of thermosets can be summarized as [9]. 1) Plasticization - Involves addition of plasticizer. Plasticizers are additives added to a synthetic resin to produce or promote plasticity and flexibility and to reduce brittleness. 2) Single phase toughening - Using long chain curing agent, the crosslink density is lowered.

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3) Two phase toughening - Adding second elastomeric phase in thermoset material such as carboxy-terminated butadiene acrylonitrile (CTBN), amine-terminated butadiene acrylonitrile (ATBN), unsaturated polyester resin etc. The increase of fracture toughness is due to the second phase acting as a step to crack propagation. 4) Chemical modification - Chemical modification is done by modifying the rigid epoxy backbone to a more flexible backbone structure or by lowering the crosslink density by increasing the molecular weight of the epoxy monomers and/or decreasing the functionality of the curing agents. Apart from these methods, formation of interpenetrating polymer networks (IPNs) of epoxy are studied nowadays for achieving improved toughening [10].

1.7 Interpenetrating Polymer Networks

A macroscopic homogeneous mixture of two or more different polymers may be defined as a polymer blend. Blending of polymers, results in the formation of new materials which combine useful properties of each constituent. The demerit of a polymer is compensated by blending it with other polymer rich in the aspect lagging in it. The blends provide cost performance ratio and reinforcement of properties. An interpenetrating polymer network (IPN) is defined as a blend of two or more polymers in a network form, at least one of which is synthesized and/or cross-linked in immediate presence of other (s). IPN networks are not covalently bonded with each other and have partial or total physical interlocking between them. If an IPN is formed as a combination of network of monomer A and monomer B during its formation, monomers of A react only with monomers of A and not with B and vice- a-versa. The two networks cannot be separated unless chemical bonds are broken. The IPNs are distinguished from the polymer blends, block copolymers and graft copolymers in two ways (1) an IPN swells but does not dissolve in solvent (2) creep and flow are suppressed. Creep is the tendency of a solid material to move slowly or deform permanently under the influence of mechanical sfress. The properties of IPN depend on the nature of individual polymer and the way they are combined. The two

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polymers forming IPN have similar kinetics and they are not dramatically phase separated. 1.7.1 Classification oflPNs Based on the chemistry involved, IPNs are classified as [11].

Simultaneous IPNs - They are formed by simultaneous polymerization of two monomers. The initiator and curing agent for both monomers act separately and simultaneously. They do not interfere in crosslinking of other monomer. Sequential IPNs - If the IPN involves polymer network of two monomers, then polymer network of one polymer is synthesized first and then swollen into monomers of other polymer along with its curing agent and initiator. In situ polymerization of other monomer is carried out. On the basis of structure IPNs are classified as [12] Full IPN - Any material containing two or more polymers in which there are no induced crosslinks between the individual polymers. Semi-IPN - Sequential IPN in which one of the IPN is cross linked and other is linear. The Full and Semi-IPN can be shown as Figure 1.16.

Figure 1,16: Interpenetrating polymer networks (a) full IPN and (b) semi-IPN

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Pseudo IPN - Simultaneous IPN in which one of the polymer is in network form and other is linear. Latex IPN - It is formed by mixture of two kinds of latex particles, followed by film formation and crosslinking of both the polymers. Thermoplastic IPN - It is two polymers IPN in which individual polymers are thermoplastics. One may be a block copolymer like SBS, rubber and other is typically a semi-crystalline or glossy polymer. Polymer may contain physical crosslinks arising from ionic groups and glossy domains. Millar IPN - The IPN in which two polymers are chemically identical. Gradient IPN - An IPN with non-uniform macroscopic composition and crosslink density, usually by non-equilibrium swelling in other monomer (monomer II) and polymerizing quickly before diffijsion equilibrium takes place.

L 7.2 Role oflPNs in improving mechanical properties Interpenetrating polymer networks (IPNs) prepared by blending two thermosets have been extensively used to improve mechanical properties of the thermosets and hence their applications [13]. The enhancement in mechanical properties is most probably due to high level of mixing in IPN and greater adhesion between dispersed and continuous phase due to permanent interpenetration at the phase boundaries [14]. Formation of epoxy-acrylate IPNs is reported to exhibit improvement in elongation at break, toughness, modulus, and tensile strength [15-17]. Epoxy/polydimethylsiloxane IPNs show potential toughening [18] and improved impact and thermal strength [19]. Epoxy/poly vinyl acetate IPNs are known for toughness [20]. Similarly, IPNs of polyurethane-polystyrene, polyacrylates, and polybenzoxazine, polymethacrylate, and epoxy-amine network are observed to have higher tensile strength and elongation at break, improved thermal and surface free energy, and exhibit gas barrier properties [21-23]. The performance of integrated IPN can be further improved by modifing them by using either organic or inorganic modifiers.

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Mechanical properties of epoxy/polyethersulfone IPNs are improved on addition of bismaleimide [24] as modifier, similarly, processability along with Tg, modulus, and fracture energy is higher in case of bisphenol A based bismaleimide resin modified by allyl fijncfionalized polyimide [25]. There is an improvement in the thermal and mechanical properties of siliconized epoxy modified with bismaleimide [26-28]. The polyester toughened epoxy systems are also further modified by bismaleimide to alter thermo-mechanical properties [29]. The improvement in the toughness of epoxy along with induction of electricity are the key objecfive of this thesis.The toughness has been improved by forming semi-IPN of epoxy and another thermoset UPR. The E-UPR IPN has been fiirher modified wih aromatic amies such as benzidine and diphenylamine to achieve the target. Unsaturated polyester resins/UPR (Figure 1.17) are in demand because of their relatively low cost, ease of processing, low densides, good corrosion resistance, excellent wetting properties with reinforcement and high strength to weight ratio [30]. They are available in a variety of grades. They are solutions of unsaturated polyester resin with an unsaturated reactant diluent like styrene, which is a vinyl monomer. The general purpose (GP) grade UPR is a blend of styrene with the condensation product of 1, 2 propylene glycol with mixture of maleic acid and phthaleic acid in the form of their anhydride. Curing of UPR involves formation of polystyrene crosslinks at the site of unsaturation in the polyester resin with the help of a catalyst and accelerator giving rise to cross linked three-dimensional structure.

Figure 1.17; Unsaturated Polyester Resin (UPR).

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Epoxy resins are highly miscible with UPR. The IPNs obtained by blending epoxy and UPR are important matrix material in high performance polymeric composites.

1.7.3 Importance of electrically conductive IPNs The intrinsically conductive polymers (ICPs) such as polyaniline (PANI) exhibits poor mechanical properties and also are difficult to process; hence it cannot be used for certain applications. In addition, they are prone to thermal degradation, very costly and have low glass transition temperature [31]. Thus, ICPs are blended with insulating polymer to get their interpenetrating polymer networks. PANI doped with acidic phosphate ester is blended with insulating polymer like melamine-urea resin to get semi-IPN with good mechanical and electrical properties [32]. Conductive PANI-epoxy interpenetrating polymer networks are also known [33]. Other way to induce electrical conductivity in IPNs is to add conductive filler to the matrix of interpenetrating polymer network. Polyurethanes (PU) - epoxy resin (E) interpenetrating polymer networks are made electro-conductive by incorporating nano-graphite (Nano G). This IPN also shows improved thermal stability and lap- shear strength [34]. Thus, by forming interpenetrating polymer network we can have tailor made electrically conductive polymer materials.

1.8 Composites

Composites are made by combining two dissimilar materials at a macroscopic level in such a way that the resultant material is conferred with properties superior than any of its components [35]. Two components neither take part in any chemical reaction nor do they dissolve or completely merge into each other. Even then, they remain strongly bonded with each other and maintain an interphase with each other. Interphase is the region where the reinforcement and the matrix phases are chemically and mechanically combined i.e. surface fiinctional groups on the reinforcement can

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react with the matrix chemically and/ or mechanically thereby determining composite properties. The reinforcement and matrix are complimentary to each other. They use each other's properties in such a way that the composite exhibits enhanced properties. The reinforcing material may be in the form of fibers, particles or flakes. The matrix constitutes a continuous phase.

1.8.1 Classification of composites Natural biological materials such as wood, bone and teeth are made up of at least two constituents and are composite materials. Usually, they show significnt anisotropy. Anisotropy means the properties of the material vary significantly when measured in different directions. Depending on matrix phase, the composites are classified as Metal Matrix Composites (MMCs), Ceramic Matrix Composites (CMCs) and Polymer Matrix Composites (PMCs).

Metal Matrix Composites (MMCs): They are composite materials with minimum two constituents. One consfituent is necessarily metal, which acts as a matrix and the other material is reinforcement such as different metal or ceramic material or organic compound. When more than two materials are present in a composite, they are known as hybrid composites. Aluminium, magnesium and titanium are usually used as matrix materials. They are ductile, light in weight and isotropic metals. Carbon or boron fibers, silicon carbide, alumina are some of the reinforcements used. Reinforcement improves the physical properties such as wear resistance, fiiction co efficient or thermal conductivity.

Ceramic Matrix Composites (CMCs): They consist of ceramic fibers embedded in ceramic matrix which results in the formation of ceramic fiber reinforced ceramic material (CFRC). Ceramic is an inorganic, nonmetallic solid material comprising metal, nonmetal and metalloid atom primarily held in ionic and covalent bonds. Ceramics have high strength and stiffhess even at very high temperature. They are

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chemically inert and have low density. They lack in fracture toughness and resistance to thermal shock. These properties can be improved by incorporating fibers in them. The most common matrices are carbon, alumina, silicon carbide, aluminium nitride, zirconia etc. The most common fibers used as reinforcement are carbon, silicon carbide, alumina, and mullite. The name of the CMC includes combination of type of fiber and type of matrix e.g. c/c or carbon/carbon indicates carbon fiber reinforced carbon.

Polymer Matrix Composites (PMCs): PMCs are the most important composites as they are light weight and have potential to replace conventional materials like steel, aluminum etc. Polymer matrix composites contain reinforcing material embedded in a polymer matrix. The reinforcement can be fibers, particulates (loose powder) or whiskers (fine single crystal in loose aggregates). The reinforcement provides stiffness, strength, thermal stability, fire retardancy, corrosion resistance and processability. Depending on the mechanical properties such as strength and stiffiiess, two categories of polymer composites are identified as reinforced plasfics and advanced polymer composites. Reinforced composites are cheaper e.g. polyester reinforced with glass fibers. Whereas, the advanced polymer composites are expensive. Usually matrix material is epoxy in advanced composites. They have superior strength and sfiffiiess. They are used in aerospace industry. The polymer matrices are either thermosetting resins or thermoplastic type. Thermosetting resins: The most commonly used thermosetting resins as matrices in PMCs are epoxy, unsaturated polyester and vinyl ester. The thermoset is allowed to crosslink during formation of composite. The mechanical properties depend on the type of thermoset and length and density of cross-links. Thermosets are brittle materials. Epoxies have good resistance to heat distortion and they show less shrinkage during curing than polyesters [36]. Epoxies can be partially cured and thus pre-pregs can be supplied. Pre-pregs are tape or sheet of fibers impregnated with resin.

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Thermoplastic resin: Polyether ether ketone (PEEK), polysulphones, polysulphide and polyimides are usually used as matrix materials. Most of these are amorphous polymers. They show less thermal stability and chemical resistance in comparison with thermosets but show more resistance to cracking and impact damage. They undergo large deformation before final fracture and their mechanical properties depend on temperature and applied strain rate. They are not creep resistant. Their processing is reversible. The matrix material binds the reinforcement together and provides rigidity and shape to the structure. They isolate the reinforcement so that it can act separately. Matrix gives good surface finish quality to polymer composite. Matrix provides protection fi-omchemica l attack and mechanical damage. Development of high performance composites requires improvement of polymer matrix properties. The matrix with desired properties can be formulated by preparing a special blend, IPN.

1.9 Electrically Conductive Polymer Composites (CPCs)

The conductive polymer composites (CPCs) have many promising industrial applications such as antistatic coating [37-39], electromagnefic interference shielding [37- 40], "smart" windows [40], sensing [41], flexible solar cell electrodes [42], field emission [43], electrochemical displays [44], catalytic and redox capacitors [45-46], electroluminescent walls [46], etc. Thus, as compared to use of ICPs or their blends or composites, electrically conductive polymer composite filled with conducfive filler, is rather an easy way to obtain electrically conductive polymeric materials. They are known as extrinsically conducting polymers and are widely used for large range of applicafions [47]. The minimum concentrtion of the filler required for conductiviy is called percolation threshold. The filler that percolates have more surface area, more porosity ad filamentous nature due to which they can enhance the conducting properties.Conductive filler provide a contineous conductive channel and makes it an

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electrically conductive composite. The filler can be incorporated in the polymer matrix by melting or solution mixing [48]. The filler dispersed materials are then molded into different shapes by using readily available processing technique. Electrically conductive polymer composites obtained in this way serve as an inexpensive approach to obtain conductive materials for applications where metals are not suitable. The conductivity and mechanical properties can be easily tailored by incorporation of different types, grades, amount of conductive fillers [47-48]. Depending on the types of filler particles, electrically conducfive polymer composites can be categorized as metal-based and carbon-based composites. A majority of research activifies in this area are focused on carbon-based polymer composites as carbon fillers provide lower density, are available in large varieties, and can be tailored to obtain desired particle size and morphology. On the contrary, the metal particles themselves offer higher electrical conductivifies than carbon fillers, but they have higher density, and are prone to corrosion. Corrosion, in turn, reduces their electrical conductivity. Nanocomposite materials based on carbon nano fillers show novel properties on nano and macro scales [49-50]. They provide better option for energy-storage, biomedical, nano-electronics and thermal-interface materials. Most important factors effectively influencing the conducfivity of CPC are: concentration, average size and type of filler particles, and values of three types of interactions: macromolecule-macromolecule, macromolecule-filler, and filler-filler. The loading of filler beyond percolafion threshold weakens the mechanical properties of the composite [51].

1.9.1 Polymer nanocomposites (PNCs) The polymer nanocomposites were introduced by Toyota group for the first time in the year 1993 [52]. They consist of nano fillers dispersed in polymer matrix. Nanocomposites are gaining importance as these composites exhibit unique properties and find many potenfial applications in automotive, aerospace, construction and electronic industry [53].

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The advantage of polymer nanocomposites is that, they provide value added properties to the original polymer without altering its prossessability, mechanical properties and light weight. Polymer nanocomposites show significant enhancement in its properties at much lower loading of the filler than polymer composites with conventional micro scale filler (such as glass or carbon fiber), which ultimately result in lower component weight and can simplify processing [54]. The enhanced properties result in increase in the range of their applications. They are endowed with high-tech applications and tailor made properties. Nanocomposites have reinforcement having nano-scale structure (at least one dimension less than 100 nm) with high aspect ratio (surface to volume ratio) [55]. Smaller size of the filler leads to exceptionally large interfacial area between the matrix and reinforcement phase(s) than conventional composite materials in the nanocomposite [56]. The interface controls the degree of interaction between filler and matrix and thus controls properties of the composite. The reinforcement can be particles, sheets, platelets, spheroids or fibers. The design and properties of PNCs are affected by size and interphase between nano filler and matrix [57]. In general, nano fillers are classified on the basis of their geometries broadly as particle, layered, and fibrous materials [58]. The examples of particle type nano fillers are carbon black, metals (Al, Fe, Au and Ag), metal oxides (ZnO, AI2O3 and Ti02), silica nanoparticle, polyhedral oligomeric sislesquioxanes (POSS). Carbon nanotubes are examples of fibrous materials. Filler with a nanometer thickness, a high aspect rafio (30-1000) and plate like structure, is classified as a layered nanomaterial (such as an organosilicates, graphite nano platelets, graphite nanosheets) [59]. Depending on the nature of the components (type of nano filler and polymer matrix) and the method of preparation, significant differences in composite properties may be obtained.

1.9.2 Carbon allotropes used as conductive fillers Carbon is the lightest element in Group IV in the periodic table with some unique properties. Under ambient conditions, sp^ bonded graphite is the ground state

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phase of carbon [60]. Small carbon clusters viz ftillerenes discoverd in 1985 by Kroto et al. and carbon nanotubes discovered in 1991 by lijima [60] have become potential nanofillers for rendering electrical conductivity to polymer composites. The physical reason for the formation of these nanostructures is that a graphene layer, defined as a single layer of 3D graphite, of finite size has many edge atoms with dangling bonds, which correspond to high energy states. Therefore, the total energy of a small number (30-100) of carbon atoms is reduced by eliminating the dangling bonds, even at the expense of increasing strain energy, thereby promofing the formation of closed cage fuUerene molecules, the most stable being C60, Under slightly different growth conditions, carbon nanotubes are formed. The structure of a variety of forms of carbon is shown in the Figurel .18.

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Figure 1.18: Carbon allotropes; a) graphite, b) fullerene C60, and c) carbon Nanotube

A carbon nanotube (CNT) can be seen as a cylinder rolled from a graphene sheet, capped at both ends by hemispheres of flillerenes. The high curvature of the graphene sheets increases the total energy of the tubules per carbon atom. Carbon nanotube research is probably the most active research field in carbon science currently. The fundamental carbon nanotube is a single-wall structure which has three basic

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geometries of edge states as shown in Figure 1.19, graphene ribbons terminated by armchair edges, zigzag edges and chiral edges respectively [60].

Figure 1.19: Schematic models for (a) armchair, (b) zigzag and (c) chiral single-wall carbon nanotubes

Carbon black Another important carbon filler used to confer electrical conductivity to the polymer composite is carbon black. Typical carbon blacks are composed of nearly pure carbon in colloidal entities of aciniform morphology. The term aciniform, means "clustered like grapes", it refers to the characteristic appearance of the colloidal entities composed of spheroidal particles fused together in clusters of branched, irregular shape as presented in the Figure 1.20 [61-62].

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Figure 1.20: Carbon black

The entities are generally called aggregates. Within each aggregate, the carbon atoms are arranged in imperfect graphite layers. The arrangement of carbon atoms and layers is referred to as microstructure. The layers are arranged more or less concentrically within each particle or growth center, with a fair degree of parallelism between adjacent layers in small regions or crystallites. The layers are continuous from one particle to the next within the aggregate. Proper dispersion is essential because in the dry state, carbon black aggregates are always agglomerated or reversibly associated with many other aggregates. It is essential to separate the individual aggregates from each other without fracturing them.

Layered nanomaterials Different types of composites of layered filler 'clay' are shown in Figure 1.21. Depending on the interaction between the organic polymer and layered filler, three types of nanocomposites exist [63].

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Intercalated Nanocomposites Conventional Exfoliated Xanocomposires Nanocomposites

Figure 1.21: Layered silicate polymer nanocomposites

When the polymer is unable to intercalate (or penetrate) between the silicate sheets, a phase-separated composite is obtained, and the properties stay in the same range as those for traditional microcomposites (conventional composite). In an intercalated structure, where a single extended polymer chain can penetrate between the silicate layers, a well-ordered multilayer morphology results in alternating polymeric and inorganic layers (intercalated composite). When the silicate layers are completely and uniformly dispersed in a continuous polymer matrix, an exfoliated or delaminated structure is obtained (exfoliated composite). The physical properties of each type of composite are significantly different

Graphite Nanoplatelet/sheets-reinforced systems Graphite has similar structure as nanoclay and thus clay polymer reinforcement concept is applicable to them. Natural flake graphite (NFG) is also composed of layers. Nanosheets where carbon atoms present on the NFG layer are covalently bonded, while those present in adjacent planes are bound by much weak van der Waals forces. The weak inter planar forces allow certain atoms, molecules and ions to

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intercalate into the inter planar spaces of the graphite, resulting in increase in inter planar spacing. The original graphite flakes with a thickness of 0.4-60 mm may expand up to 2- 20,000 mm in length [50]. These sheets/layers get separated down to Inm thickness, resulting in high aspect ratio (200-1500) and high modulus (~lTPa) [64]. Furthermore, when dispersed in the matrix, the nanosheet exposes an enormous interface surface area (2630 m g"') and plays a key role in the improvement of both the physical and mechanical properties of the resultant nanocomposite [65]. Expanded graphite (EG) can be easily prepared by rapid heating of graphite intercalation compound (GIC) [50]. These are shown schematically in Figure 1.22; the black lines shown in the figure represent the graphite sheets when they are viewed from a direction parallel to the sheets. As it has a high expansion ratio, the galleries of EG can be easily intercalated through physical adsorption.

Figure 1.22: Schematic diagram showing expansion of GIC to EG

Dispersion of the nanoparticle and adhesion at the particle-matrix interface play crucial roles in deciding the mechanical properties of the polymer nanocomposite (PNCs) reinforced with fibrous or particle-reinforcement. In absence of proper dispersion, the nanomaterial will show poor mechanical properties in comparison to conventional composites. By optimizing the interfacial bond between the particle and

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Ph.D. Thesis rf ?l Savitribai Phule Pune University

chemical route. Room temperature curing was attempted using triethylene tetramine as a curing agent. Semi-IPNs with 11.1 % UPR is found to exhibit best mechanical properties. Further, semi-IPNs were also prepared by adding aromatic amines such as (DPA, secondary amine) and benzidine (Bz, primary amine). Structural elucidation of the samples through identification of functional groups was carried out with the help of FTIR spectroscopy. The mechanical properties such as hardness, izod impact and tensile strength of blends were compared. Thermal properties were studied by thermogravimetric (TGA) and differential scanning calorimetric (DSC) analysis. Scanning electron micrographs (SEM) was used to study the morphology of the samples.

The above semi-IPNs have been used as matrix material for the preparation of nanocomposites in the further work.

Part II - Synthesis of electrically conductive nano composites using E-UPR semi- IPNs /modified semi-IPNs as matrix for nano fillers

Epoxy resin nanocomposites containing graphite nanosheets (GNS) and unsaturated polyester resin (UPR) were prepared by incorporating GNS in semi interpenetrating polymer network (semi-IPN) of epoxy resin and unsaturated polyester resin (E-UPR). GNS are prepared by rapid heating of sulphuric acid- graphite intercalation compound (GIC) followed by ultra-sonication. The E-UPR- GNS semi-IPNs were further modified with aromatic amines such as benzidine (Bz) and (DPA). All the samples were prepared by mixing triethylenetetramine (TETA) as the curing agent at two different temperatures viz. room temperature (RT; 30 °C) and low temperature (LT; 5 °C). The morphology, mechanical, thermal and electrical properties were evaluated using various analytical techniques. Positron annihilation lifetime spectroscopy (PALS) was carried out to correlate the measured properties with the free volume and free volume hole size distribution. The results are consistent with the reported studies.

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