Epoxy Composites: Impact Resistance and Flame Retardancy

Expert overviews covering the science and technology of rubber and plastics

ISSN: 0889-3144

Volume 16, Number 5, 2005

Debdatta Ratna

Epoxy Composites: Impact Resistance and Flame Retardancy

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Item 1Macromolecules33, No.6, 21st March 2000, p.2171-83EFFECT OF THERMAL HISTORY ON THE RHEOLOGICAL BEHAVIOR OF THERMOPLASTIC POLYURETHANESPil Joong Yoon; Chang Dae HanAkron,UniversityThe effect of thermal history on the rheological behaviour of ester- and ether-based commercial thermoplastic PUs (Estane 5701, 5707 and 5714 from B.F.Goodrich) was investigated. It was found that the injection moulding temp. used for specimen preparation had a marked effect on the variations of dynamic storage and loss moduli of specimens with time observed during isothermal annealing. Analysis of FTIR spectra indicated that variations in hydrogen bonding with time during isothermal annealing very much resembled variations of dynamic storage modulus with time during isothermal annealing. Isochronal dynamic temp. sweep experiments indicated that the thermoplastic PUs exhibited a hysteresis effect in the heating and cooling processes. It was concluded that the microphase separation transition or order-disorder transition in thermoplastic PUs could not be determined from the isochronal dynamic temp. sweep experiment. The plots of log dynamic storage modulus versus log loss modulus varied with temp. over the entire range of temps. (110-190C) investigated. 57 refs.GOODRICH B.F.USA Accession no.771897

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ISBN: 978-1-84735-065-7

Epoxy Composites: Impact Resistance and Flame

Retardancy

Debdatta Ratna (IPTME, Loughborough University)


1

Contents

1. Introduction ................................................................................................................................................32. Thermosetting Composites .........................................................................................................................33. Epoxy Resins ...............................................................................................................................................3 3.1 Chemorheology and Curing of Epoxy ..............................................................................................54. Epoxy Composites ......................................................................................................................................5 4.1 Fracture Testing ................................................................................................................................7 4.2 Fracture Mechanism .........................................................................................................................75. Impact Resistant Epoxy Composites ...........................................................................................................86. Modification of Epoxy Matrix ....................................................................................................................8 6.1 Flexibilisation of Epoxy ...................................................................................................................8 6.2 Toughening of Epoxy .......................................................................................................................9 6.3 Liquid Rubber Toughening ...............................................................................................................9 6.3.1 Reaction-induced Phase Separation .....................................................................................9 6.3.2 Mechanism of Toughening .................................................................................................11 6.3.3 Morphological Parameters .................................................................................................11 6.3.4 Recent Advances ...............................................................................................................13 6.4 Toughening by Preformed Particle .................................................................................................15 6.5 Thermoplastic Toughening .............................................................................................................16 6.6 Rigid Particle Toughening ..............................................................................................................177. Nanoreinforcement of Epoxy ....................................................................................................................17 7.1 Clay Reinforced Epoxy ..................................................................................................................17 7.2 CNT-Reinforced Epoxy ..................................................................................................................208. Simultaneous Nanoreinforcement and Toughening .................................................................................219. Fire Retardant Epoxy Composites ............................................................................................................22 9.1 Flammability and Smoke Tests ..........................................................................................................22 9.1.1 UL-94 Flammability Test ...................................................................................................22 9.1.2 Cone Calorimetry ...............................................................................................................22 9.1.3 LOI Test ............................................................................................................................2210. Fire Retardant Resin Compositions ..........................................................................................................23 10.1 Halogenated Flame Retardants .......................................................................................................23 10.2 Phosphorus Containing Flame Retardants ....................................................................................23 10.3 Nanoclay-Based Flame Retardants ...............................................................................................24 10.4 Combination of Organoclay and Other Flame Retardants .............................................................26 10.5 Intumescent Fire Retardants ..........................................................................................................2611. Summary and Outlook .............................................................................................................................2612. List of Abbreviation and Acronyms ..........................................................................................................27


2

The views and opinions expressed by authors in Rapra Review Reports do not necessarily reflect those of Smithers Rapra Technology or the editor. The series is published on the basis that no responsibility or liability of any nature shall attach to Smithers Rapra Technology arising out of or in connection with any utilisation in any form of any material contained therein.

13. Additional References ...............................................................................................................................27Subject Index ....................................................................................................................................................99Company Index ...............................................................................................................................................115


3

1 Introduction A composite is defined as a combination of two or more materials with a distinguishable interface. The oldest man-made composite is concrete, which is associated with a macrolevel reinforcement. The urge to improve the properties of composite materials, has prompted material scientists to investigate composites with lower and lower reinforcement size, leading to the development of microcomposites and the recent trend in composite research is nanocomposites (with nanometer scale reinforcements). On the basis of the nature of the matrices, composites can be classified into four major categories: polymer matrix composite (PMC), metal matrix composite (MMC), ceramic matrix composite (CMC) and carbon matrix composite or carbon carbon composites. PMC can be processed at a much lower temperature, compared to MMC and CMC. Depending on the types of polymer matrices, PMC are classified as thermosetting composites and thermoplastic composites. In the present review, thermosetting composites with epoxy matrices will be discussed in detail.

2 Thermosetting Composites Over the last three decades, the use of PMC, has increased tremendously and this dramatic growth is expected to continue in the future. The composites possess many useful properties such as high specific stiffness and strength, dimensional stability, adequate electrical properties and excellent corrosion resistance. The implications are easy transportability, high payload for vehicles, low stress for rotating parts, high ranges for rockets and missiles, which make them attractive for both the civil and defense applications (186, 32). The composite industry is currently dominated by thermosetting resins namely epoxy, vinyl ester, unsaturated polyester, phenolic, polyimides, cyanate ester and so on. This is because of their availability, relative ease of processing, lower cost of capital equipment for processing and low material cost (a.1). The thermosetting resins are available in oligomeric or monomeric low viscosity liquid forms, which have excellent flow properties to facilitate resin impregnation of fibre bundles and proper wetting of the fibre surface by the resin. They are characterised by a crosslinking reaction or curing, which converts those into a three-dimensional (3D) network form (insoluble, infusible). Because of the crosslinked structure, thermoset composites offer better creep properties and environmental stress cracking resistance compared to many thermoplastics e.g., polycarbonate.

Thermoset composites form a major portion of the interior furnishings in todays commercial aircraft and in semiconductor devices (234, 245).

3 Epoxy ResinsEpoxy resins are a class of versatile polymer materials characterised by the presence of two or more oxirane ring or epoxy groups within their molecular structure. Like other thermosets they also form a network on curing with a variety of curing agents (115) such as amines, anhydrides, thiols etc. Amine curing agents are most widely used because of the better understanding/control of epoxy-amine reactions. The chemical structures of some commonly used curing agents namely: triethylene tetramine (TETA), 4,4 diaminodiphenyl methane (DDM), 4,4diaminodiphenyl sulfone (DDS), diethyl toluene diamine (DETDA), polypropyletheramine (Jeffamine) are presented in Figure 1.

Figure 1 Chemical structures of commonly used amine curing

agents


4

Undoubtedly, there exists more publications based on the basic and applied research on epoxy resins than that for any other commercially available thermosetting resins. The broad interest in epoxy resins originates from the versatility of epoxy group towards a wide variety of chemical reactions and the useful properties of the network polymers (a.2) such as high strength, very low creep, excellent corrosion and weather resistance, elevated temperature service capability and adequate electrical properties. Epoxy resins are unique among all the thermosetting resins due to several factors (a.1) (115) namely, minimum pressure is needed for fabrication of products normally used for thermosetting resins:

shrinkage is much lower and hence there is lower residual stress in the cured product than that encountered in the vinyl polymerisation used to cure unsaturated polyester resins.

use of a wide range of temperature by judicious selection of curing agent with good control over the degree of crosslinking.

availability of the resin ranging from low viscous liquid to tack free solid, etc. Because of these unique characteristics and useful properties of network polymers.

epoxy resins are widely used in structural adhesives, surface coatings, engineering composites, and electrical laminates.

The most commonly used epoxy resin is diglycidyl ether of bisphenol-A (DGEBA), which is characterised by two epoxy groups e.g., LY-556, GY-250 (Ciba Gigey). Multifunctional epoxies with functionality of three and four are also available. Chemical structures of DGEBA and multifunctional epoxies namely epoxy novolac (e.g., Dow DEN 438), tetraglycidyl ether of 4,4-diamino diphenyl methane (TGDDM) (e.g., Araldite MY-720, Ciba Speciality Chemicals), triglycidyl p-amino phenol (TGAP), (e.g. Araldite MY 0510, Ciba Speciality Chemicals), are given in Figure 2. The chemical nature and the amount used of curing agents or hardeners plays an important role in determining thermomechanical properties of the

Figure 2 Chemical structures of difunctional and multifunctional epoxies


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cured networks (243). A wide range of properties can be demonstrated and materials can be developed for extreme applications using the same resin, by judicious selection of curing agents. For example, DGEBA, when cured with an aromatic amine, produces a network with high glass transition temperature (Tg) and is used for high temperature composite applications whereas the same resin, on curing with Jeffamine (Mn > 800 g/mole) generates a flexible/rubbery network, which can be used for vibration damping applications (81).

High Tg epoxy matrices can be made by using an aromatic curing agent such as DDM, DDS, DETDA. The advantage of DETDA over others is that it is a liquid and offers better processability. The Tg can be further increased by using a higher functionality resin. Thermally stable high performance resins are required for use in composite structure for aerospace applications. Polyimides and cyanate ester reins are the leading candidates for exterior structural components. Conventional epoxies are generally not suitable for aerospace applications. However, multifunctional epoxies cured with a suitable aromatic amine, can offer thermal stability comparable to polyimides. Hence, the conventional epoxy resins, with the combination of a multifunctional component, can satisfy the thermomechanical properties, specified for aerospace application.

3.1 Chemorheology and Curing of Epoxy

From the application point of view, the effective use of any thermosetting system requires one to be able to predict the cure kinetics of the system to consistently obtain the maximum Tg and also to predict the flow behaviour of the curing resin, in particular to precisely locate when the sol-gel transitions occurs. This is because the polymer can be easily shaped or processed only before the gel point, where it can still flow and can be easily formed with the stresses applied relaxed to zero thereafter. Accurate knowledge of the gel point would therefore allow estimation of the optimum temperature and time for which the sample should be heated before being allowed to set in the mould.

The gel point of a crosslinking system is defined unambiguously as the instant, at which the weight average molecular weight reaches infinity and as such is an irreversible reaction. The crosslinked polymer at its gel point is a transition state between a liquid and a solid. The polymer reaches its gel point at a critical extent of crosslinking (agel). Before the gel point, that is at a < agel the polymer is called a sol, because it is typically soluble in an appropriate solvent. Beyond the

gel point, that is at a < agel, at least part of the polymer is typically not soluble in any solvent and is called a gel. Kinetically, gelation does not usually inhibit the curing process so the conversion rate remains unchanged. Hence, it cannot be detected by the techniques sensitive only to chemical reaction like differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). There are various methods to determine the gel point. The most sophisticated one is determination by using a rheological experiment. Another process, which a thermoset resin undergoes during cure, is vitrification. It is defined as the point at which the Tg of the network has become the same as the cure temperature. At this point, the material is transformed from a rubbery gel to a gelled glass.

Rheological measurement is generally carried out with a controlled stress rheometer using a parallel plate assembly. The dynamic viscoelastic test makes it possible to characterise the gelation and vitrification process during curing. Various parameters such as complex viscosity, storage modulus, loss modulus, loss factor at various frequencies (at a particular temperature) can be determined as a function of cure time. The point at which clear increase in G occurs is defined as gel point and G maximum is defined as the vitrification point. The gelation and vitrification are more clearly determined from a loss factor plot as shown in Figure 3 for a trifunctional epoxy system (154).

This experiment clearly demonstrates the time required for curing an epoxy system at a particular temperature to get a required Tg, which is very important for development of prepregs. It may be noted that when the Tg of the cured system reaches the cure temperature, the curing reaction becomes very slow. That is why when a multifunctional epoxy network, without a post curing treatment at a sufficiently high temperature, is subjected to dynamic mechanical analysis, it shows two loss peaks; one is for the partially cured network and other is for the fully cured network. Hence, it is very important to give a proper post curing treatment to an epoxy system (especially multifunctional epoxy system) to get a network with the desired Tg.

4 Epoxy CompositesA wide variety of composites can be made using epoxy as a matrix as shown in Table 1. They can be broadly grouped into fibre reinforced plastic (FRP) composites, particulate composites and nanocomposites (246, 277, 289, 294). Epoxy-based FRP composites can be tailor-made by judiciously selecting the resin compositions,


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fibres and by designing the interface. The FRP composites are used in aerospace, automotive and other structural applications (222, 225, 232). Epoxy-based FRP composites (for general purpose use) are made by the wet-lay up technique and followed by compression moulding. On the other hand, the prepreg route (autoclave curing) is the standard production method within the aerospace sector for manufacturing high fibre volume fraction, void free composite materials with good mechanical properties (233). Similar laminated composites, produced by a vacuum impregnation resin transfer moulding (RTM), offer cost savings, health

and safety benefits. However, the main drawback of RTM is the lower fibre volume fraction due to the low pressure involved in the process. Moreover, the property enhancement using 3D woven fabric due to through-the-thickness reinforcement and limiting crack propagation is not achieved in RTM (218), although a similar effect is achieved for wet lay-up with the autoclave curing route. The reason is thought to be due to the 3D weave involved, having a high crimp and low compressibility, hence, higher consolidation pressure than the pressure provided by RTM, is required to realise the actual advantages of a 3D weave.

Figure 3 Loss factor versus time plots of toughened trifunctional epoxy at 140 C using various frequencies

Reprinted with permission from D. Ratna, R. Varley and G.P. Simon, Journal of Applied Polymer Science, 2003, 89, 9, 2339. 2003, John Wiley and Sons

Table 1 Various types of epoxy composites and the manufacturing processesComposites Reinforcement ProcessFibre reinforced plastic composite

Glass fibre, carbon fibre, kevlar fibre, basalt fibre

Wet lay-up and compression moulding, prepreg lay-up with vacuum bagging and autoclave curing, filament winding with oven curing, pultrusion, resin transfer moulding (RTM), liquid composite moulding, structural reaction engineering moulding

Particulate microcomposite Silica, carbon black, calcium carbonate, glass beads, glass balloons, silicon carbide

Mechanical mixing and casting, compression moulding, matched-die moulding

Nanocomposite Nano silica, nanocalcium carbonate, nanoclay, carbon nanofibres, carbon nanotubes

Mechanical mixing and sonication followed by casting or compression moulding


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However, the epoxy-based composites are known to be highly susceptible to internal damage caused by a low velocity impact due to inherent brittleness of the cured resin, which may lead to severe safety and reliability problems (226, 237). Thus, for high performance applications the improvement of damage tolerance of epoxy composites by enhancing their impact strength is essential and has been the subject of investigation throughout the world (280, 285).

4.1 Fracture Testing

Toughness of a material is defined as the energy absorbed by a material before fracture. Toughness is a very important property in applications where the material has to encounter a lot of mechanical shock and vibration. Fracture toughness (KIc) is one of the most important properties of a material, which is used to design materials for dynamic applications. It basically describes the resistance of a material with a crack to fracture. Since it is almost impossible to make a material for practical purposes without cracks/defects, fracture toughness analysis is extremely important for design applications. The critical stress intensity factor, KIc and impact energy, GIc are determined using ASTM D5045-99 (a.3). The tests were carried out using an Instron machine using a flexure or tensile mode. An initial crack was machined in a rectangular specimen, and tapping on a fresh razor blade placed in the notch generated a natural crack. The parameters can be expressed mathematically as follows:

K PBW f xIC

Q= 1 2/ ( ) (1)

G E KIC IC=

1 2 2 (2)

Where: is Poisson's ratio PQ is the critical load for crack propagation, B and W are the thickness and width of the

specimen, E is elastic modulus, and f (x) is a nondimensional shape factor given

by:

f x x x x x xx( )( . ( ) ( . . . )

(= +

+6 1 99 1 2 15 3 93 2 71 2

2

))( ) /1 3 2 x (3)

The thickness of the specimen should be higher than the critical value below which the material shows plane stress behaviour. The KIc and GIc values of a given

material are a function of testing speed and temperature. Furthermore the values may be different under cyclic load. Therefore application of KIc and GIc in the design of service components should be made considering the differences that may exist between the test condition and the field condition.

Apart from the fracture toughness analysis, various impact tests are used for quick assessment of the behaviour of a material under dynamic loading conditions. The impact tests are used to determine the behaviour of a material subjected to shock loading in bending, tension and torsion. Mostly, the Charpy and Izod impact and occasionally tensile impact tests are used. These Izod/Charpy tests are widely applied in industries due to the ease of sample preparation and it is possible to generate comparative data very quickly. Both the tests are done as per ASTM D256 (a.4). In the Charpy test the specimen is supported as a simple beam whereas in Izod test it is supported as a cantilever. The apparatus consists of a pendulum hammer swinging at a notched sample. The energy transferred to the material can be inferred by comparing the difference in height of the hammer before and after the fracture. Depending on the instrument, the impact energy (J/m) or the load history during the impact event can be recorded. The test can be carried out using different combinations of impactor mass and incident impact velocity to generate the data on damage tolerance as functions of impact parameters, which are helpful for design of composite materials for a particular application (177).

Another test, which is used to evaluate epoxy composites, is the falling dart impact test. In this test a dart is allowed to fall on a specimen kept in a fixture with an annular hole typically ranging from 2.5 to 7.5 cm. The output of the load transducer can be directly fed into a signal processor and the impact energy or the entire loading history can be recorded. It can also be coupled with ultrasonic c-scan (nondestructive inspection technique composites in which a short pulse of ultrasonic energy is incident on a sample) and microscopic techniques to study the post failure analysis (258, 260). It may be noted that for critical applications, rigorous testing has to be carried out considering the service conditions, to ensure the specified service life (255).

4.2 Fracture Mechanism

The mechanical property of a composite depends on the nature of the matrix, reinforcement and the interface. Hence, the design of interface and processing play an important role in deciding the final properties of a composite. Once the optimum interface is designed and


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the processing is perfected, the mechanical properties can be predicted from the known value of mechanical properties of the individual components. The modulus of a composite material can be approximately calculated from simple linear or logarithmic relationships:

log Ec = log Ee (1-Vr) + log Er (Vr) (4)

Ec = Ee (1-Vr) + Er Vr (5)

Where: Ec, Ee and Er are the bending moduli of composite, epoxy and reinforcement, respectively, and

Vr is the volume fraction of reinforcement in the composite.

Hence, composites show a modulus value, which is in between that of an epoxy and a reinforcement. Interestingly, the impact strength of an FRP composite does not follow the rule-of-mixture (the average strength calculated from individual components) and the value is found to be much higher than the impact strength values of individual components. As for example, the impact strength of pure epoxy network (DGEBA cured with TETA) is about 20 J/m and impact strength of glass is less than that of epoxy, however, the impact strength of an epoxy/glass fibre composite is 950 J/m which is about 50 times more. Various models have been proposed to explain the toughness of the brittle matrix/brittle fibre composites (249, 251). The models considers various energy absorbing processes, which occur during the fracture of composites like fibre pull-out, fibre kinking, stress redistribution, creation of new surface through fibre, matrix and the interface, fiber debonding and so on (205, 268, 298, 299). Since fibre pull out and fibre debonding, largely contribute in energy dissipation during fracture, very high interlaminar shear strength (ILSS) is detrimental for toughness of composites. Thus, the interface has to be designed for an optimum ILSS to get the best mechanical property and toughness.

5 Impact Resistant Epoxy CompositesImpact resistance or damage tolerance of epoxy composites can be improved by improving the resin toughness, using high strain fibres and by designing the interfaces. Design of interface is particularly important for continuous FRP composites, where the impact resistance of a composite can be significantly improved, by controlling and manipulating the microstructure of the interfaces (259, 296). Generally, three techniques are used for such modification:

a) incorporation of discrete layers of tough resin known as interleaving (290, 292)

b) introduction of z-directional fibre (stitching) (153, 267), and,

c) addition of whiskers or short fibres to the interlaminar zone (supplementary reinforcement) (248, 262, 263, 280).

Wimolkiatiask and Bell (275) electropolymerised a high temperature thermoplastic (3-carboxy phenyl maleimide-styrene copolymer) interphase onto a graphite fibre and evaluated the fibre-reinforced epoxy composites. The improvement in critical strain energy factor of about 100% and notched impact of about 60% were achieved while maintaining the interlaminar shear strength at around the same value as for a controlled composite. Cox and co-workers (257) investigated the tensile behaviour of graphite epoxy composites with 3D woven interlock reinforcement and reported the contributions of various mechanism of fracture. The impact strength of an epoxy composite with a particular matrix can be improved by using a high strain fibre or fibre hybridisation (228, 231, 239). However, high strain fibres often show lower modulus and thus cannot satisfy the requirement for dimensional stability in high performance engineering applications (297). This approach is particularly exploited for the application of composites under high-incident-energy conditions, as for example ballistic applications (242, 258). Using a particular reinforcement, low-velocity impact resistance (desirable for structural applications) of a particulate or FRP composite can be improved to a great extent by increasing the toughness of the matrix (291) as discussed next.

6 Modification of Epoxy Matrix

6.1 Flexibilisation of Epoxy

Unlike thermoplastics, in which the fracture toughness and processability can be improved significantly by blending a plasticiser (nonreactive low molecular weight compound) or by physical blending with a ductile polymer, the plasticisation or compatible blending strategies are not successful in thermosets such as epoxies. This is because as a result of curing, the modifier either exudes out from the matrix or undergoes macro-phase separation. Moreover, the accumulation of free liquid plasticiser molecules at the fibre surface can act as a weak boundary layer and cause a substantial decrease in ILSS.


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Epoxies are flexibilised by using reactive diluents, which are basically mono epoxide compounds or by using long chain hardeners (287). The basic idea is to reduce the effective crosslink density (Xc), thereby making the networks less tight. Epoxy resins can also be chemically modified to extend the chain length between the two epoxy groups leading to the increase in molecular weight between crosslinks (Mc) and development of a tougher (ductile) network (141). A general scheme for chemical modification of epoxy resin is shown in Figure 4.

The major downside of this approach is that the modification is associated with a drastic reduction in Tg, which restricts the use of such materials for high temperature applications. This arises due to the typical plasticisation phenomena observed in case of compatible blending of a rigid plastic with a flexible polymer or low molecular weight plasticiser. This problem can be overcome by using the second phase toughening technology where the modifier is incorporated as a separate phase.

6.2 Toughening of Epoxy

A basic difference between toughening and flexibilisation is that in flexibilisation, the improvement in toughness

is associated with a significant deterioration in thermomechanical properties (especially Tg) whereas for toughening, the same is achieved without significant deterioration in thermomechanical properties. The difference arises due to differences in blend morphology. Flexibilisation is associated with single-phase morphology whereas toughening arises from a two-phase morphology. Unlike flexibilisation where the modifier becomes a part of the epoxy phase, the modifier forms a separate phase in toughening and thus the bulk thermomechanical properties of epoxy matrix, are retained. Depending on the second phase used, epoxy toughening can be grouped into four types: liquid rubber toughening, core-shell particle toughening, thermoplastic toughening and rigid particle toughening.

6.3 Liquid Rubber Toughening

6.3.1 Reaction-induced Phase Separation

Liquid rubber toughening is one of the most successful methods for improvement in toughness of epoxy resins and mostly exploited in the field of FRP composites and adhesives technology (253, 254). Unlike thermoplastics, where the toughening is achieved by a simple physical blending, in an epoxy resin, the same is achieved exclusively through the chemistry and is more challenging to the expertise of a polymer scientist. The basic criteria for a modifier to be a toughening agent for epoxies are:

a) the modifier should be a low molecular weight liquid to ensure miscibility with the epoxy resin,

b) it must have functionalities like carboxyl, amino etc., which can react with the epoxy resin, and,

c) it must have borderline miscibility so that before curing it remains miscible with the epoxy and undergoes a reaction-induced phase separation with the advancement of curing reaction, leading to the formation of a two-phase microstructure (223, 271).

The phenomenon can be explained more clearly by considering the thermodynamics of mixing.

The thermodynamic condition for compatibility is that the free energy change of mixing (Gm) at constant pressure (P) and temperature (T), should be negative (a.5):

Figure 4 Chemical modification of epoxy resin


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(Gm) P,T < 0 (6)

Combining the Flory-Huggins equation and the Hildebrand equation, the free energy of mixing can be expressed as:

Gm /V = e r (de - dr)2 + RT(e/Ve . ln e + r / Vr . ln r) (7)

where e, r are the volume fractions, de, dr are the solubility parameters, Ve and Vr are the molar volume of epoxy and rubber, respectively, V is total volume, R is universal gas constant and T is temperature.

Since both e, r are fractions, the second term is always negative. High temperature favours miscibility. For a fixed epoxy/rubber composition, Gm at constant temperature, depends on dr, i.e., the chemical nature of rubber and Vr which is dependent on the molecular weight of the rubber. A toughening agent has to be designed in such a way that the free energy of mixing is marginally negative at the curing temperature. Then the rubber will be compatible with epoxy before the curing but with the advancement of curing reaction, Ve and Vr will increase due to the increase in molecular weight of the rubber and the epoxy and at a certain stage Gm will become positive. At that point rubber starts undergoing phase separation and it is called the cloud point. The process continues until the gelation point, where the phase separation is arrested due to the

tremendous increase in viscosity. The final network is obtained after a heat treatment called post curing. The process is shown schematically in Figure 5.

If (de - dr) is very low, then Gm will be highly negative and entropy change (during curing) cannot make Gm equal to - ve, before gelation, leading to the formation of a single phase morphology. Again if (de - dr) is very high, then Gm will be positive at curing temperature and rubber will be immiscible at the initial stage itself, leading to a macro level phase separation. As an example, carboxyl-terminated polybutadiene is not a suitable toughening agent for epoxies as the solubility parameter of butadiene is much lower than that of epoxy and it is immiscible with epoxy. However, a carboxyl-terminated copolymer of butadiene and acrylonitrile (CTBN) with 20 to 30 wt.% of acrylonitrile (polar acrylonitrile increases the solubility parameter) has a solubility parameter, which is close to the solubility parameter of the epoxy and is an effective toughening agent for the epoxy. The copolymers are commercially produced by the Goodrich Company and are known as Hycar CTBN. The amine-terminated copolymers of butadiene and acrylonitrile are known as ATBN. CTBN or ATBN with a higher acrylonitrile content, have a better miscibility with the epoxy resin in terms of solubility parameter and undergo phase separation at a later stage of curing and result in lower amount of phase separated rubber. CTBN with an acrylonitrile content of more than 30 wt.%, results in a single-phase

Figure 5 Description of reaction-induced phase separation in rubber-modified epoxy system


11

morphology and the modifier acts as a flexibiliser rather than a toughening agent.

The morphology is controlled by the initial cure temperature and the post curing condition (although it affects the final properties), has no role in morphology development (272). The time from cloud point to gelation is the effective phase separation time (tps). For complete phase separation, the tps has to be higher than the time required for diffusion of rubber from epoxy medium (tdiff). The diffusivity is the controlling factor of phase separation if tdiff is greater than tps. The diffusivity of rubber in epoxy medium (Dr) is considered to be proportional to the temperature viscosity ratio through the Stokes-Einstein equation (a.6):

Dr = kT/6pRr he (8)

Where: k is the Boltzman constant, Rr is the radius of rubber adducts, he is the viscosity of the medium, and, T is absolute temperature.

The characteristic time scale for diffusion in two dimensions is:

tdiff = L2 / 2 Dr (9)

A length scale (L) can be assigned from the average two-dimensional distance between domain centres obtained from micrographs of the cured specimen.

6.3.2 Mechanism of Toughening

The rubber modified epoxy with two-phase microstructure shows improved fracture toughness (178, 272, 279) as the rubber particles, dispersed and bonded to the epoxy matrix act as the centres for dissipation of mechanical energy. A number of theories have been proposed to explain the toughening effect of rubber particles on the brittle epoxy matrix, based on the fractographic features and fracture properties of the rubber toughened epoxy networks. According to recent theories, the most accepted mechanism for rubber toughening is rubber cavitation followed by shear yielding. In rubber modified plastics, under triaxial tensile stresses, voids can be initiated inside the rubber particles. Once the rubber particles are cavitated, the hydrostatic tension in the material is relieved, with the stress state in the thin ligaments of the matrix between the voids being converted from a triaxial to a more uniaxial tensile stress state. This new stress state is favourable for the initiation of shear bands. In other words, the role of rubber particles is to cavitate internally, thereby relieving the hydrostatic tension and

initiating the ductile shear yielding mechanism (269). At the crack tip where the hydrostatic tensile component is large, the magnitude of the concentrated hydrostatic stress in the vicinity of rubber particles is insufficient to promote shear yielding.

The internal cavitations of the rubber particles relieve the plain strain constraint by effectively reducing the bulk modulus and then the magnitude of the concentrated deviatoric stress is sufficient for shear yielding. The voids left behind by the cavitated rubber particles act further as stress concentrators. Li and co-workers (a.7) studied the fracture behaviour of unmodified and CTBN modified epoxies under hydrostatic pressure. They found that when rubber cavitations were suppressed by superimposed hydrostatic pressure, the fracture toughness of CTBN modified epoxy was no higher than that of the unmodified epoxy. This implies that the stress concentration by rubber particles alone will not necessarily induce massive shear yielding and increase the fracture toughness. Hence, rubber cavitations are very important to the toughening of rubber modified epoxies; without cavitations these rubber particles can still cause stress concentration but they are not effective in toughening.

Dompas and Groeninckx (264) developed a criterion for the internal cavitations. The condition has been treated as an energy balance between the strain energy relieved by cavitations and the surface energy required to create a new surface and given by the following equation:

Utotal = Ustrain + Usurface = - p/12 Kr 2 do3 + gp2/3 do2 < 0 (10)

Where: Kr, , do, g are the rubber bulk modulus, relative volume strain, rubber particle diameter and surface energy per unit area, respectively.

Accordingly, the cavitation of rubber is dependent on the elastic and molecular properties of rubber, rubber particle size and on the applied volume strain, which again depends on the difference in Poissons ratio between the matrix and the rubber. Cavitation resistance increases with the decrease in particle size and difference in Poissons ratio.

6.3.3 Morphological Parameters

Rubber toughened epoxy networks display a discrete morphology, which consists of spherical particle dispersed in the epoxy matrix. The morphology is characterised by polarised optical microscope, scanning electron microscopy (SEM), atomic force microscopy and transmission electron microscopy (TEM). A typical SEM


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microphotograph for rubber toughened epoxy network is shown in Figure 6. The morphological parameters dictate the fracture properties of the toughened epoxy networks (145). Various morphological parameters and the molecular parameters, which control the morphology,

are listed in Table 2. It is very difficult to study the effect of an individual parameter on toughening effect as the parameters are interrelated. For example if we want to study the effect of functionality of rubber by changing the functionality, this changes the solubility parameter difference and affects the other morphological parameters.

6.3.3.1 Rubber Content

The impact energy of rubber-toughened epoxy systems increases with increase in rubber volume fraction in the epoxy network due to dissipation of mechanical energy by the rubber particle by various mechanisms as discussed in Section 6.3.2. However, beyond an optimum rubber concentration the impact energy decreases due to rubber agglomeration and phase inversion. Generally, optimum rubber concentration is found to be 10 to 15 wt.%. The effect of rubber content on impact strength of carboxyl-terminated poly(2-ethyl hexyl acrylate) (CTPEHA) rubber (220) is presented in Figure 7.

Table 2 Parameters influencing rubber toughening

Molecular Parameters Morphological Parameters

Processing ParametersEpoxy RubberMatrix ductility Polarity Rubber volume fraction Initial cure temperatureFunctionality Molecular weight. Particle size Post cure temperature and timeMolecular weight Functionality Particle size distributionCuring agent (type and concentration)

Concentration Matrix ligament thickness

Viscosity Viscosity Particle to matrix adhesion

Figure 6 SEM picture of rubber-toughened epoxy

Figure 7 Effect of acrylate rubber content on impact strength of modified networks

Reprinted with permission from D. Ratna, A.K. Banthia and P.C. Deb, Journal of Applied Polymer Science, 2000, 78, 4, 716. John Wiley and Sons


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6.3.3.2 Particle Size and Distribution

The origin of this size dependence on toughening behaviour arises from the role played by the particles, which is governed by the size of the process zone. Large rubber particles (> 5 mm) lying outside the process zone are only able to act as bridging particles, which provide only a modest increase in fracture energy. Small rubber particles, which lie in the process zone are forced to cavitate by the large hydrostatic stress component that exists in the process zone and contributes to the increase in fracture energy. However, very small particles (< 0.2 mm) cannot cavitate (cavitation resistance increases with decreasing particle size - see Equation 10) and cannot effectively toughen the epoxy matrix.

6.3.3.3 Matrix ligament thickness

Wu and Mongolina (a.8) proposed that the matrix ligament thickness (MLT) i.e., surface-to-surface, interparticle distance is the more fundamental parameter. For effective toughening, the average matrix ligament thickness (t) should be less than that of a critical value (tc) where brittle-tough transition occurs. The tc is independent of rubber volume fraction, particle size and characteristics of the matrix alone at a given test temperature and rate of deformation. For blends with dispersed spherical particles, the tc can be related to the rubber particle size and rubber volume fraction (r) by the following equation:

tc = do [k (p/6r )1/3 - 1] (11)

Where: k is a geometric constant and do is the particle diameter.

The existence of critical matrix ligament thickness for an effective rubber toughening, can be explained (203) in the light of two basic mechanisms, namely rubber cavitations followed by formation of a shear band and crazing. The low MLT maintains the connectivity of the yielding process, which then propagates over the entire deformation zone and makes the blend tough. This happens when t is less than tc. In the cases where crazing is the major energy dissipating mechanism the high MLT causes the formation of secondary crazes at the highly stressed region of the ligament which then propagate rapidly leading to the catastrophic failure of the materials.

For a given volume fraction of rubber, the critical MLT is achieved by decreasing the particle size and by improving the dispersion. For most systems, this concept works very well and a decrease in particle

size corresponding to a lower brittle-tough transition temperature. However, it has been found in a number of systems that there exists a minimum particle size below, which the brittle-tough transition no longer shifts to lower temperatures. As a possible explanation for the peculiar behaviour, it has been suggested that particles which are too small are not able to cavitate and therefore do not release the hydrostatic tension in the material to promote ductile shear yielding.

6.3.3.4 Matrix Particle Adhesion

Matrix-rubber particle adhesion is an important parameter for rubber toughening. For effective rubber toughening, the rubber particles must be well bonded to the epoxy matrix. The poor intrinsic adhesion across the particle-matrix interface causes premature particle debonding, leading to the catastrophic failure of the materials (289, 300). Most of the studies, reported in the literature, have been concerned with reactive groups terminated rubbers (functionality = 2 eq/mole) as toughening agents for epoxies, which results in dispersed rubbery particles having interfacial chemical bonds as a consequence of chemical reactivity. It was observed that further increase in functionality of rubber, improved the toughening effect up to an optimum value of functionality (2.3-3 eq/mole). The toughening effect decreases beyond the optimum value of functionality, due to the formation of a single phase morphology (203).

6.3.4 Recent Advances

The commercial toughening agents used widely are CTBN and ATBN from the Goodrich Company. A multifunctional liquid rubber (nitrile-diene-acrylamide terpolymer) for toughening epoxy composites and coatings, was developed by Wolverine Gasket division of Eagle Picher Industries, Inkster, MI, USA (295). A new additive that increases the strength and toughness of amine cured epoxy resins, has been commercialised by Uniroyal Ltd., of Elmira, Canada (a.9). NASA has discovered (279) that fibre-reinforced epoxy composites can be made tougher by incorporating a bromine containing additive which resulted in substantial increase in flexural and impact strength. Addition of small amount of CTBN or ATBN further improved these properties.

In the last two decades a lot of work has been done and various issues in the field have been addressed as listed in Table 3. The commercially available liquid rubber (CTBN, ATBN) toughened epoxy often shows


14

outstanding fracture properties and the technology is exploited in the field of engineering adhesives (124). However, since the butadiene component of the elastomer contains unsaturation, it would appear to be a site for premature thermal and/or oxidative instability and such modified resins are not suitable for application at high temperatures. One would imagine that excessive crosslinking could take place with time, which would detract from otherwise desirable improvements accomplished with these structures. Secondly, there

is some limitation in its use due to the possibility of the presence of traces of free acrylonitrile, which is carcinogenic. Hence, considerable efforts have been made to develop saturated liquid rubber alternative to CTBN.

Several liquid rubbers have been investigated as alternatives to CTBN. The chemical structures of some useful toughening agents are shown in Figure 8. Saturated rubbers namely polyacrylates, polyurethane,

Table 3 Issues involved in toughening research and addressed in the last two decadesIssue SolutionToxicity and poor oxidative stability of commercial toughener (CTBN, ATBN)

Use of saturated modifier e.g., polyacrylates, polyurethanes, polysiloxane, etc.

Depression of epoxy Tg Use of high molecular weight rubber and optimum cure temperature

Processability (high viscosity) Use of hyperbranched polymer based toughening agentsReduction in modulus Use of liquid crystalline modifiers and nanoreinforcement

Figure 8Chemical structures of some useful toughening agents. ESO = epoxidised soybean oil


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polysiloxane, and epoxidised soybean oil (ESO) (142, 220, 230), offer better oxidative stability and better performance of such materials. Complete phase separation can be achieved for such liquid rubbers by increasing the molecular weight of liquid rubber and decreasing the cure temperature within the processing window (the theoretical reason for the effect of these two parameters have been discussed in Section 6.3.1). However, beyond a certain molecular weight, the liquid rubber undergoes phase separation at a very early stage leading to agglomeration and macrophase separation. The optimum molecular weight for difunctional modifier is reported to be about 7000 g/mole (220).

The modification of epoxy with linear elastomers, as discussed in previous sections, is associated with a considerable increase in viscosity, which is disadvantageous as far as processing is concerned. The problem can be overcome by using dendritic hyperbranched polymer (HBP) based toughening agents (168, 202). Due to the compact 3D structure of dendritic polymers, these molecules mimic the hydrodynamic volume of spheres in solution or melt and flow easily past each other under applied stress. This results in a low melt viscosity, even at high molecular weights, due to a lack of restrictive interchain entanglements (a.5). Indeed, dendritic polymers have been shown to exhibit melt and solution viscosities that are an order of magnitude lower than linear analogues of similar molecular weight (a.5) (168). The high density of functional terminal groups on dendritic polymers also offers the potential for tailoring their compatibility either through conversion of dendritic polymer end groups to chemically suitable moieties or through in situ reaction to form covalently bound networks. These two properties: low viscosity and tailorable compatibility, make HBP excellent candidates as flow additives that could act simultaneously as toughening agents. These polymers are commercially available e.g., Boltron.

The fourth issue is reduction in modulus of cured epoxy as a result of incorporation of rubber. It is necessary to couple a strengthening mechanism with the toughening process to get really tough and strong materials. The successful approach is nanoreinforcement which will be discussed in the following sections.

6.4 Toughening by Preformed Particle

The phase separation, in the case of liquid rubber toughening depends upon the formulation, processing and curing conditions. Incomplete phase separation can result in a significant lowering of the epoxy Tg. Moreover, the rubber phase that separates during the

cure, is difficult to control and may result in uneven particle size distribution. The differences in morphology and volume of the separated phase affect the mechanical performance of the product. The factors that affect the fracture toughness of the modified epoxy such as morphology, particle size and composition are interdependent and hence, it is very difficult to study the effect of the individual parameters. These problems can be minimised by using an insoluble preformed particle directly (265). Since the size, morphology and composition, shell thickness and crosslink density of the rubbery cores can be controlled separately by using emulsion polymerisation techniques, the effects of various parameters on the toughening of epoxies can be investigated.

The control of the particle parameters by emulsion polymerisation has been extensively studied, and various efficient technologies have been developed (283). Monodisperse latex particles with a diameter from submicron to micron range can be prepared. Cohesive strength, which is influenced by crosslink density of the rubber phase, can be controlled by the conversion of polymerisation and the amount of crosslinking agent (a.10). Interfacial architecture can be controlled by changing the following parameters:

1. thickness of the shell which depends on the ratio of the shell-core materials and polymerisation mechanism,

2. chemical bonding and physical interaction between particles and matrix which can be enhanced by introducing functional groups onto the surface of the shell,

3. grafting between the shell and core, and,

4. molecular weight of shell materials. Various morphologies of the composite such

as core shell, occluded or multilayer can be achieved through two or multiple stage emulsion polymerisation (278). The preformed particles are incorporated into the epoxy matrix by mechanical mixing. The dispersibility of the particles can be improved by:

(1) introducing crosslinking into the shell or

(2) using comonomer like acrylonitrile or glycidyl methacrylate (GMA), which increases the interfacial adhesion by polar or chemical interactions (208).


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6.5 Thermoplastic Toughening

Rubber toughening can dramatically increase the fracture resistance of cured epoxy castings and composites. However, the presence of the rubber phase does somewhat decrease the modulus and thermal stability of the materials and increase the tendency of water absorption with an accompanying loss of properties at elevated temperatures. Moreover, the reactive rubbers have been reported as ineffective modifiers for a highly crosslinked system based on an epoxy having a functionality of more than 2 (i.e., 3 or 4). This is because the rubber rich particles, as stress concentrator, induce the plastic deformation of a highly crosslinked matrix to a far less extent and dissipation of fracture energy by the enlargement of deformation zone can hardly be attained (272). The search for an alternative method to rubber toughening led to the development of thermoplastic toughened epoxy (84, 125, 256). Because of their high modulus and high Tg of engineering thermoplastics, the modified epoxy resin will reach or even exceed the corresponding values for the unmodified resin. Unlike the rubber toughening where significant reduction in stiffness and modulus was observed at an ambient temperature, in case of thermoplastic toughened epoxy, reduction in stiffness becomes significant only at temperatures near to the Tg of the thermoplastic moiety.

The initial studies on thermoplastic toughened epoxy were carried out by using unreactive low molecular weight thermoplastics like polyether sulfone) (PES), polyether imide. No significant increase in fracture toughness was observed due to such blending of thermoplastic modifier with difunctional and multifunctional epoxies (302). It was concluded that the lack of improvement of fracture toughness observed in these systems may be due to the fact that cured epoxy resins high crosslink density inhibited the primary toughening mechanism namely the formation of shear bands.

Hedrick and co-workers (301) considered poor interfacial adhesion to be the main reason for the inability of commercial thermoplastics (nonreactive) to improve the toughness of epoxy resin. They used phenolic -OH ended bisphenol-A based PES and amine-terminated PES oligomers as toughening agents and claimed that this approach resulted in remarkable increase in fracture energy. The theory is similar to liquid rubber toughening. The thermoplastic modifier having reactive end groups, reacts with the epoxy resin. The presence of an excess of epoxy resin essentially produces an epoxy end-capped thermoplastic modifier. The modified and unmodified epoxy resin further reacts with the curing agent and produces the toughened network. Initially

the thermoplastic is compatible with the epoxy resin but as the molecular weight increases due to the curing reaction, the homogeneous mixture starts undergoing phase separate by a spinodal decomposition, resulting in the development of a two-phase microstructure (107). Gorbunova and co-workers (107) reported that incorporation of polysulfone into epoxy networks, resulted in a considerable increase in impact strength and cross-breaking strength of epoxy composites.

In contrast to the liquid rubber modified epoxy systems, which displays a simple particulate microstructure, the thermoplastic-toughened epoxy networks produce different morphology/microstructures at different modifier concentrations (266). Initially at low concentrations of thermoplastic, the thermoplastic becomes miscible in the epoxy matrix and generates a single-phase morphology. On subsequent increase in thermoplastic concentration, the phase separation occurs leading to the development of a two-phase morphology and the microstructure changes from particulate to cocontinuous and finally to the phase-inverted microstructure. The mechanisms responsible for the toughening of epoxy by thermoplastics, are reported to be plastic yielding or drawing and tearing of thermoplastic rich phases.

Once the -OH- terminated bis phenol-A (BPA) based PES was found to be successful for toughening the epoxy matrix, efforts were concentrated on developing amine-terminated oligomers of PES, which can be synthesised by adding a stoichiometric amount of amino phenol as the end-capping agent (282). The oligomeric amines can be reacted with epoxy and DDS to give the toughened thermoset. Pak and co-workers (274) have used PES with pendent amino groups as the modifier for epoxy resin. Like amine-terminated PES, they can be used as such or after modification with maleic anhydride. It has been found that with increase in -NH2 content, the toughness increases initially, passes through a maximum and then decreases. The initial increase in fracture toughness with increase in -NH2 content is due to increase in interfacial adhesion between the epoxy matrix and the dispersed PES particles, which prevents the debonding of particles. The decrease in fracture energy above an optimum -NH2 concentration can be attributed to the formation of a single-phase morphology (undesirable for toughening) as a result of higher miscibility of the PES, containing higher -NH2 content, with the epoxy resin.

Crystalline thermoplastics have also been utilised for toughening epoxy resins (238). Polyethylene oxide (PEO) was reported to be an effective toughening agent for epoxy (284). The -OH groups of PEO react


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with epoxy at an elevated temperature and form a compatible blend with single-phase morphology or two-phase microstructure depending on the molecular weight of PEO and the curing condition. Nichols and Robertson (a.11) reported a systematic exploration of the relationship between thermal history, morphology and mechanical properties of polybutylene terepthalate (PBT)/epoxy blends. They found that 5 wt.% of a thermoplastic PBT was able to increase the fracture energy (GIc) of a brittle anhydride cured epoxy from 180 to 2000 J/m2 with proper control of morphology. The exceptional higher toughening ability of PBT in comparison to Nylon 6, can be attributed to phase-transformation of PBT at the crack tip (a.11) as observed in toughened ceramics.

6.6 Rigid Particle Toughening

The fourth approach generally taken to improve crack resistance of an epoxy resin, is the incorporation of rigid inorganic fillers such as alumina, silica, glass beads, etc., into glassy epoxy matrix (276, 281, 286). The mechanisms proposed for improvement of fracture resistance are shear yielding and crack pinning. However, unlike rubbery filler which can cavitate, get stretched and remain bonded due to the chemical interaction to sustain the imposed load, the rigid filler cannot deform or cavitate and easily get debonded from the matrix leading to the catastrophic failure of the material. It is difficult also to achieve a good dispersion of such materials in the epoxy matrices. The combinations of glass beads and rubbery fillers have been tried out (253, 273, 276) and it was reported that the fracture energies displayed a strong improvement and a synergistic effect due to the presence of both kinds of particles in the hybrid composites (270, 271).

7 Nanoreinforcement of EpoxyNanoreinforced composites or nanocomposites offer a great potential for novel properties because the distinct inorganic organic component properties can be combined in a single material with a uniformity of dispersion at the nano level. Such reinforcement often offers synergistic improvement in properties when the component sizes approach the nano scale (77). The concept of nanoreinforcement arises from the knowledge that control of structure/interactions at the smallest scales and the systematic nanometer by nanometer construction of composites provide the best chance to control the macroscopic properties

(207). An effective exploitation of nanoreinforcement requires an understanding of nonscale structure-property-processing relationships of nancomposites to select the right nancomponent and to process them properly for the target properties (214). The main advantage of the nanocomposite is the property tradeoffs associated with conventional composites. For example, nanomodification can improve the stiffness without sacrificing toughness, can enhance barrier properties without sacrificing transparency and offer flame retardency without deteriorating mechanical property and colour (100, 190, 191). Applications of nanocomposites have been proposed for ballistic armour, capable of withstanding small arms fire (7.62 mm bullets) (209). If the nanoreinforcement effect can be synergistically coupled with the effect of other additives/microfillers, the resultant composite materials will find wider applications leading to the more and more replacement of conventional metallic materials for critical applications (22, 102).

Various nanofillers namely, nanosilica (204) and nano calcium carbonate (33), polyhedral oligomeric silsesquixane (24, 31), have been used to make epoxy-based nanocomposites as described in Table 1. Nanocomposites are now no longer only restricted to the laboratory, they are found in the real world (62). Recently, two new polyamide-6 nanocomposites (NanoTuff and Nanoseal), have been introduced by the Nylon Corporation of America (21). Two technologies namely the technology of polymer/clay and technology of polymer/carbon nanotube (CNT) nanocomposites, will be discussed briefly in the following sections.

7.1 Clay Reinforced Epoxy

Polymer-clay nanocomposite (PCN) is one among very few areas in the field of polymer technology which has drawn considerable interest in recent years (42, 146, 151, 152). The main attractions of PCN are the low cost of clay and the well developed intercalation chemistry, which makes it possible to achieve a nanostructure from a micron size filler. Thus, PCN technology can avoid the potential health hazards involved in other nanomaterials technology.

The clays are made