Must be elastic to transfer the load to the fibres. ⢠Strength at ... 1.2.5.1 Amine based curing agents .... Today, CEs are established thermosetting resins for insulating high ..... and a significant difference is indicated between the first and the last test ... from the decrease in the entropic contribution to the free energy of mixing.
1
CHAPTER 1 INTRODUCTION
From pre-historic times, man has exploited for his own use the properties of natural polymers such as horns, waxes and bitumens. Over the years, it was gradually learnt that the properties of such materials could be improved by techniques, such as purification and modification with other substances. By the turn of the 19th century, with the explosion of scientific knowledge in fields such as chemistry and physics, coupled with the demands from industry for materials with properties which could not be found in nature, the scene was set for the development of a whole range of new materials; among them are the early synthetic polymers. Polymers have obviously not been discovered overnight. They came out of long and persevering studies by a host of motivated scientists, whose work has enriched human life. Today, polymers have become an indispensable material for us and it is difficult to think of daily life without them. Just as an architect chooses bricks, stones, logs of wood etc. in varying shapes, sizes and patterns to create various designs, a chemist produces innumerable plastics, rubbers, foams, fibers, adhesives, composites, etc. by judiciously combining various chemicals under desired conditions.
Thus, we can say that a polymer chemist is an “architect of molecules.”
2
Depending on its ultimate form and use, a polymer can be made available in many forms such as plastics, rubber, foam, adhesives, composites, etc. Out of all the forms, composites are widely used in high performance applications. High strength and light weight remain the winning combination that propels composite materials into new arenas, but other properties are equally important. Composite materials offer good vibrational damping and a low coefficient of thermal expansion, characteristics that can be engineered for specialized applications which translate into a finished product that requires less raw material, fewer joints and fasteners, and shorter assembly time. Composites have a proven resistance to temperature extremes, corrosion and wear, especially in industrial settings, where these properties do much to reduce product life-cycle costs. Composites differ from traditional materials in that the composite parts comprise two distinctly different components – fibres and a matrix material (most often a polymer resin) that, when combined, remain discrete but function interactively, to make a new material whose properties cannot be predicted by simply summing the properties of its components. In fact, one of the major advantages of the fibre / resin combination is its complementary nature. The structural properties of composite materials are derived primarily from the fibre reinforcement. High performance composites derive their structural properties from continuous, oriented high-strength fibre reinforcement – most commonly carbon, glass or aramid – in a binding matrix that promotes processability and enhances mechanical properties, such as stiffness, and chemical and hygroscopic resistance. The matrix material in the composite can be a polymer. In the last forty five years there has been a flurry of activity in the synthesis and development of high performance and high temperature polymers. This has
3
been, in large part, due to the need for advanced materials required for a diverse range of applications, including aerospace, automotive and microelectronic industries. These applications often demand a unique combination of properties, including high glass transition temperatures, toughness, good adhesion, oxidative and thermal stability, and low a dielectric constant. In this regard, a large number of polymers have been developed, which can be broadly categorized as either thermosets or thermoplastics. Examples of thermosets are unsaturated polyesters, epoxies, BMI, etc., while thermoplastics include PEEK (polyetherether ketone), polysulphones, polyether sulphones etc. Unsaturated polyesters are the most widely used matrix material in composite applications, but these resins are less resistant to moisture and have low performance properties, which limit their application in advanced composites. Epoxy thermosets have been widely used as matrix resins for advanced composites due to their outstanding mechanical and thermal properties. These properties include high modulus and tensile strength, high glass transition temperature, high thermal stability and moisture resistance. When cured, epoxy resins form a highly cross-linked three-dimensional infinite network, whose microstructure provides the desirable engineering properties. However, the highly cross linked micro structure also makes it brittle. It also absorbs moisture that acts as a plasticizer during service, and reduces the glass transition temperature leading to a decrease in dimensional stability. This undesirable property restricts epoxy thermosets from applications requiring high impact and fracture strengths.
4
Another important matrix material used for advanced composites is polyimide. Polyimides possess much better mechanical and thermal properties than epoxies, but the drawbacks of this resin are low moisture resistance and poor processability. Bismaleimides have better processability, but they are excessively brittle. Processabilty is another pivotal issue regarding the use of high temperature thermosetting polymers for composite applications. Generally, these systems require high temperatures for processing, and therefore, cannot be effectively used in conjunction with low cost processing techniques like resin transfer moulding. Therefore, attempts for developing novel high performance thermosetting polymers with easy processing and performance characteristics, have led to the development of cyanate ester resins. Cyanate ester resins are a novel class of thermosetting materials, which exhibit enhanced physical and thermal properties, when compared to traditional polymer systems like epoxies. These cyanate esters display features including high glass transition temperatures, low dielectric loss, low moisture absorption, low corrosion potential, and easy processing, and thus show promise in technical fields like aerospace and microelectronics. With the availability of functional materials, and the feasibility of embedding or bonding them into composite structures, new smart structural concepts are emerging to be attractive for potentially high-performance structural applications (Maugin 1988, Gandhi and Thompson 1992, and Srinivasan and McFarland 2001). A smart structure is one that has surface mounted or embedded sensors and actuators, so that it has the capability to sense and take corrective action. Numerous conferences, workshops, and journals dedicated to smart materials and structures, stand testimony to this growth. The technological implications of this class of materials and structures are immense: structures that monitor their own health, process
5
monitoring, vibration isolation and control, medical applications, damage detection, and noise and shape control. As applications of active vibration/deflection controls in aerospace, automobile industries and building applications, smart structures have received considerable attention (Lowey 1997). Vibration and the shape control of structures are essential to achieve the desirable performance in modern structural systems. Advances made in the design and manufacturing of smart structure systems, improve the efficiency of the structural performance. 1.1
MATRIX MATERIAL Although it is undoubtedly true that the high strength of composites
is largely due to the fibre reinforcement, the importance of the matrix material cannot be underestimated as it provides support for the fibres, and assists them in carrying the loads. It also provides stability to the composite material. The resin matrix system acts as a binding agent in a structural component in which the fibres are embedded. In a composite material, the matrix material serves the following functions•
Holds the fibres together
•
Protects the fibres from the environment
•
Distributes the loads evenly between fibres, so that all the fibres are subjected to the same amount of strain
•
Enhances the transverse properties of a laminate
•
Improves the impact and fracture resistance of a component
6
•
Helps to avoid the propagation of crack growth through the fibres, by providing an alternative failure path along the interface between the fibres and the matrix.
•
Carries interlaminar shear
The matrix plays a minor role in the tensile load-carrying capacity of a composite structure. However, the selection of a matrix has a major influence on the interlaminar shear, as well as on the in-plane shear properties of the composite material. Interlaminar shear strength is an important design consideration for structures under bending loads, whereas the in-plane shear strength is important for torsion loads. The matrix provides lateral support against the possibility of the fibre buckling under compression loading, thus influencing, to some extent, the compressive strength of the composite material. The interaction between the fibres and the matrix is also important in designing damage tolerant structures. Finally, the processability and defects in a composite material depend strongly on the physical and thermal characteristics, such as viscosity, melting point, and curing temperature of the matrix. 1.1.1
Properties of a Matrix The needs or desired properties of the matrix which are important
for a composite structure are as follows: •
Reduced moisture absorption
•
Low shrinkage
•
Low coefficient of thermal expansion
•
Good flow characteristics, so that it penetrates the fibre bundles completely, and eliminates voids during the compacting/curing process
7
•
Reasonable strength, modulus and elongation (elongation should be greater than the fibre)
•
Must be elastic to transfer the load to the fibres
•
Strength at elevated temperature (depending on application)
•
Low temperature capability (depending on application)
•
Excellent chemical resistance (depending on application)
•
Should be easily processable into the final composite shape
•
Dimensional stability (maintains its shape)
Out of the many matrix materials known, epoxy resin is a widely used matrix material in composite applications. 1.2
EPOXY RESIN Epoxy resin is defined as a molecule containing more than one
epoxide groups. The epoxide group, also termed as an oxirane or ethoxyline group, is shown in Scheme 1.1.
Scheme 1.1 Structure of Epoxide group These resins are thermosetting polymers, and are used as adhesives, high performance coatings, and potting and encapsulating materials. These resins have excellent electrical properties, low shrinkage, good adhesion to many metals, and resistance to moisture, and thermal and mechanical shock. Viscosity, epoxide equivalent weight and molecular weight are the important properties of epoxy resins.
8
1.2.1
Types of Epoxy Resins There are two main categories of epoxy resins, namely, glycidyl
epoxy, and non-glycidyl epoxy resins. Glycidyl epoxies are further classified as glycidyl-ether, glycidyl-ester and glycidyl-amine. Non-glycidyl epoxies are either aliphatic or cycloaliphatic epoxy resins. Glycidyl epoxies are prepared via a condensation reaction of an appropriate dihydroxy compound, dibasic acid or a diamine, and epichlorohydrin, while, non-glycidyl epoxies are formed by the peroxidation of an olefinic double bond. Glycidyl-ether epoxies such as, diglycidyl ether of bisphenol-A (DGEBA) and novolac epoxy resins are the most commonly used epoxies. 1.2.2
Diglycidyl Ether of Bisphenol-A Diglycidyl ether of bisphenol-A (DGEBA) shown in Scheme 1.2 is
a typical commercial epoxy resin and is synthesised by reacting bisphenol-A with epichlorohydrin in the presence of a basic catalyst.
Scheme 1.2 Structure of the DGEBA The properties of the epoxy resins depend on the value of n, which is the number of repeating units commonly known as the degree of polymerization. The number of repeating units depends on the stoichiometry of the synthesis reaction. Typically, n ranges from 0 to 25 in many commercial products.
9
1.2.3
Novolac Epoxy Resins Novolac epoxy resins shown in Scheme 1.3 are glycidyl ethers of
phenolic novolac resins. Phenols are reacted in excess, with formaldehyde in the presence of an acidic catalyst to produce phenolic novolac resin. Novolac epoxy resins are synthesised by reacting phenolic novolac resin with epichlorohydrin in the presence of sodium hydroxide as a catalyst.
Scheme 1.3 Structure of novolac epoxy resin Novolac epoxy resins generally contain multiple epoxide groups. The number of epoxide groups per molecule depends upon the number of the phenolic hydroxyl groups in the starting phenolic novolac resin, the extent to which they reacted, and the degree of low molecular species being polymerised during synthesis. Multiple epoxide groups allow these resins to achieve high cross-link density resulting in excellent temperature, chemical and solvent resistance. Novolac epoxy resins are widely used to formulate the moulding compounds for microelectronics packaging, because of their superior performance at elevated temperature, excellent mouldability and mechanical properties, superior electrical properties, and heat and humidity resistance. 1.2.4
Curing of Epoxy Resins The curing process is a chemical reaction in which the epoxide
groups in the epoxy resin react with a curing agent (hardener) to form a highly crosslinked, three-dimensional network. In order to convert epoxy resins into
10
a hard, infusible, and rigid material, it is necessary to cure the resin with a hardener. Epoxy resins cure quickly and easily at practically any temperature from 5-150oC depending on the choice of the curing agent. 1.2.5
Curing Agents (Hardeners) A wide variety of curing agents for epoxy resins is available,
depending on the process and properties required. The commonly used curing agents for epoxies include amines, polyamides, phenolic resins, anhydrides, isocyanates and polymercaptans. The cure kinetics and Tg of the cured system, are dependent on the molecular structure of the hardener. The choice of the resin and hardeners depends on the application, the process selected, and the properties desired. The stoichiometry of the epoxy-hardener system also affects the properties of the cured material. Employing different types and amounts of hardener which, tend to control the cross-link density, vary the structure. The amine and phenolic resin based curing agents, described below, are widely used for the curing of epoxy resins. 1.2.5.1
Amine based curing agents Amines are the most commonly used curing agents for epoxy cure.
Primary and secondary amines are highly reactive with epoxy. Tertiary amines are generally used as catalysts, commonly known as accelerators for cure reactions. The use of an excessive amount of catalyst achieves faster curing, but usually at the expense of working life, and thermal stability. The catalytic activity of the catalysts affects the physical properties of the final cured polymer.
11
1.2.5.2
Phenolic based curing agents Epoxy resins when cured with a phenolic hardener, give excellent
adhesion, strength, and chemical and flame resistance. Phenolic novolaccured epoxy systems are mainly used for encapsulation, because of their low water absorption, and excellent heat and electrical resistance. The usefulness of epoxy resins in many engineering applications is often limited by their brittle nature and poor thermal conductivity 1.3
TOUGHENING OF EPOXY SYSTEMS The epoxy resin is the most widely used matrix material for many
structural composites. It has many good properties like stiffness, low shrinkage, good adhesion to glass/ carbon fibre etc. Unfortunately, the very factor contributing to its high stiffness and heat resistance leads to its main draw back, viz (lack of toughness), and that is its highly cross-linked structure. Aircraft structures are often subjected to impact loads during flight, which can cause severe damage to the structure by delamination. Toughening can help in developing damage tolerant structural components for aircraft. The term toughness is a measure of the material's resistance to failure, i.e., the total amount of energy required to cause failure.A rough idea about the fracture toughness of various materials is given in Table 1.1, which shows that rubber toughened epoxy resins can match engineering thermoplastics in fracture toughness values. Composites made of such matrix systems are expected to be tougher, and hence more useful.
12
Table 1.1 Fracture toughness of various materials (Bascom et al. 1989)
Material Inorganic glass
Fracture toughness range (kJ/m2) Very low
Epoxy/Polyester resins
10-1
Polysulphones/rubber toughened epoxy
100
Metals
>10
1.3.1
Various approaches to toughening epoxy resins
The need for toughening epoxy resins was felt in the mid 1960 s. Initially, solid rubbers like nitrile and polysulphide rubbers were found to flexibilise epoxy resin. Such resin systems were found to be suitable for adhesives as these rubber modified resins could sustain large deformations, and hence, gave better peel joints than the unmodified epoxy based adhesive. However, as they caused unacceptable levels of deterioration of stiffness and thermal properties for composites, better tougheners were needed. Another method employed the introduction of inorganic as well as organic filler materials. Of these, the organic systems, especially the one based on elastomers, have come to the attention of material scientists. In particular, the amine-, carboxyl- and, hydroxyl-terminatedpoly (butadieneco- acrylonitrile) systems (i.e., ATBN, CTBN and HTBN respectively) have found favour, as reports of inclusion of the one of these rubbers in epoxy matrix have indicated an increased toughness value at the expense of the modulus and glass transition temperature (Kunz et al. 1982 and Alan Meek 1974).
13
The next method is the inclusion of thermoplastics, such as polysulphones, polyether sulphones and polyether imides into epoxy networks (Nam Gyun Yun et al. 2004; Mimura et al. 2000; Hourtson et al. 1991; and Akay et al. 1993). These studies show a good improvement in thermal and mechanical properties. However, these thermoplastic polymers show poor solubility in organic solvents, leading to difficulties in processing. Yet another method is transverse stitching, z-spinning and 3D weaving or braiding of reinforcement (Velmurugan et al. 2007 and GwoChung Tsai et al. 2005). It is observed that these techniques increase the Mode I delamination toughness several times higher than that of woven fibre reinforcement composite specimens. It is very expensive when compared to the other toughening mechanisms. Later toughening of epoxy was tried with interpenetrating polymer networks (IPNs). Several studies have demonstrated that the fracture energy of brittle matrices may be significantly improved with the formation of interpenetrating polymer networks (Robert Vabrik et al. 1998; Harani et al. 1998 and Raymond and Bui 1998). Polyurethanes have excellent elasticity and high impact strength; therefore, in proper ratio with epoxy resins, materials with the desired mechanical and thermal properties can be produced by a blending technique utilizing the interpenetrating polymer networks (IPNs) of the two polymer components. However, these have poor mechanical and thermal properties. Another area which continues to attract research interest is the field of thermoset-thermoset polymer blends, in particular, the incorporation of inherently tough polymers into the brittle polymer systems, in order to impart improvements in fracture toughness in the resulting blends. Hence, among the different materials used for the modification of epoxy resins, cyanate esters
14
are expected to be the best material to improve the thermomechanical properties. As compared to epoxies, they are inherently tough, have significantly better electrical properties and lower moisture uptake. Cyanate esters also offer ease of handling and processing similar to that of epoxy resin systems. In addition the dicyanate monomers are expensive to prepare, and hence, blending a small amount of dicyanate with epoxy resin to get a polymer resin with superior properties seems to be the best way. Thus, blending of epoxy with cyanate ester resin continues to attract research interest in order to impart improvements in fracture toughness, without compromising other mechanical and thermal properties. 1.4
CYANATE ESTER RESIN The term “cyanate ester resin” is used to describe prepolymers and
cured resins, the former containing the reactive ring forming cyanate (-OCN) functional group. Attempts begun in 1857 to react unsubstituted phenoxide with cyanogen halides produced only mixtures of imidocarbonates and cyanurates. O-alkylated aryl cyanates were prepared by this method in 1960. Synthesis via the thermolysis of 1,2,3,4 thio-triazoles was also successful, but too expensive for commercial interests. Most alkylcyanates were found to readily undergo exothermic isomerization to the more stable isocyanate form. Exceptions to this are cyanates of bicyclic alcohols having the OH group in the bridgehead position, and acidic alcohols containing electronegative substituents such as halogens. Based on the cyanogen halide chemistry, the first simple route to the synthesis of cyanate esters by reacting phenol with cyanogen halide, was discovered by Grigat and Pütter in 1963.
15
ArOH
+
ClCN
+ Base
ArOCN
- Base.HCl
Scheme 1.4 Synthesis of cyanate esters Among the various reported routes for the Scheme 1.4 synthesis of cyanate esters, the first reported methods were not quite as simple and resulted in relatively low yields. Aryl dicyanates were found to undergo nearly quantitative conversion to cross-linked cyanate homopolymers by cyclotrimerization to Striazines. Tractable prepolymers of up to 50% conversion, produced commercially by thermally quenching the cyclotrimerization reaction, were found to be superior for pressure lamination than the low viscosity molten monomers. It was discovered that blends of cyanate esters and epoxides coreacted to form cost-effective hybrids. The complex reaction pathway, involving cyanate trimerization, epoxide insertion and ring cleavage with additional epoxide to form substituted oxazolidinones, was not investigated until much later. Nevertheless, blends of bis-phenol A dicyanate prepolymer with 45-55% epoxy resin were the predominant cyanate ester, of the 1980s. With the advancement of integrated circuit technology, interest was generated in the 1970s, for resins with low dielectric constants to achieve faster signal propagation via reduced resistance to the passage of electromagnetic fields. Increased glass transition temperature, Tg, for matching molten solder temperatures (~250°C) was also of interest for improving the dimensional stability and reliability of multilayer circuit boards. Bayer AG developed a prepolymer of bis-phenol A dicyanate and commercialized this laminating solution as Triazine A resin in 1976. Two
16
years later, Bayer exited this business, citing a premature market. A private communication suggested that carbamate impurities resulting from the use of an aqueous base during monomer synthesis and zinc co-ordination metal catalysts, may have contributed to the severe blistering of some laminates during the solder application. Bayer AG licensed their CE technology to Mitsubishi Gas chemical in 1978 and to Celanese in 1984. Mitsubishi Gas Chemical commercially introduced BT resins, blends of bisphenol. A dicyanate and or its prepolymers with bismaleimides BMI in 1978. The basic patent describes gel times for CE blends with several BMIs, including a Michael addition product of methylene dianiline and excess BMI monomer derived from the same aromatic amine. Although isoureas produced from the reaction with cyanates and secondary aromatic amine are known cyclotrimerization catalysts, no supporting evidence was offered for the proposed heterocyclic rings reported in subsequent publications, from the cycloaddition reactions between cyanates and maleimide C=C unsaturation. Mitsubishi Gas Chemical Co. supplied BT resins in addition to the formulated prepreg and clad laminate to circuit board manufacturers. Celanese Corp. (now Ciba-Geigy) initiated research in 1981 that utilized chemically tailored bisphenols and polyphenols to expand CE capabilities in the directions to lower dielectric loss properties, increased hydrolytic and thermal stabilities, lower cure temperatures and liquid monomer forms. These efforts combined with those of Dow Chemical Co. and Allied Signal have expanded the family of commercially available CE monomer, to the current seven structurally distinct monomers and numerous other speciality monomers and prepolymers. Beginning in 1979, considerable research was directed at toughening CE resins by alloying them with initially soluble engineering
17
thermoplastics that phase separated into continuous morphologies during cure. These efforts have produced composites for aircraft primary structures with melt processability similar to that of thermosetting epoxies, but which pass the wet condition tests for supersonic aircraft rated at 177°C skin temperatures. The same concept has been extended to ketone laminating solutions for toughened circuit board laminates with improved drilling characteristics and thin (50-100
m) double sided circuits with robust
handling properties for multichip module construction (Treliant Fang and David A. Shimp 1995). Today, CEs are established thermosetting resins for insulating high speed, high density electronic circuitry, as matrix resins for aircraft composites, geostationary broadcast satellites, radomes and antennae, and have potential as versatile adhesives as well as passive wave guides or active electrooptic components for processing light signals in fibre optic communication. 1.4.1
Curing of Dicyanate Cyanate ester resins are bisphenolic or polyphenolic derivatives
containing the ring forming cyanate (-OCN) functional groups. Chemically, this family of thermosetting monomers and their pre-polymers are esters of bisphenol (or polyphenol) and cyanic acid. The esters cyclotrimerize to form substituted triazine rings on heating. Cross linking of cyanate esters occurs via cyclotrimerization to form 3-dimensional networks of oxygen linked triazine rings. The curing reaction is classified as an addition polymerization and occurs without the emission of volatile by-products.
18 OAr
-
-
NCO
Ar
OCN
Heat/
N
N
Catalyst -
n[
ArO
-
N
OAr
-
-
Three Dimensional Polycyanurate N
Scheme 1.5 Formation of three dimensional networks by cyclotrimerization Pure cyanate ester monomers will cure relatively slowly. However, the addition of a catalyst exponentially increases the cure rate. Effective catalysts recommended by Rhone-Poulenc for cyclotrimerization were a combination of transition metal complexes and active hydrogen, non-volatile, liquid phenolic compounds, eg. nonyl phenol, that serves as a co-catalyst. The latter also promoted enhanced solubility of the metal catalyst in the cyanate resin. In general, organic soluble compounds of most coordination metals serve as potent trimerization accelerators. Zinc, copper, manganese and cobalt compounds were preferred over the more reactive transition metals like iron, tin, titanium, lead and antimony. This solution is consistent with the observation that the latter group promotes the undesirable hydrolysis of an otherwise stable cyanate ester linkage.
Specifically, chelated metals of
acetylacetonates are less active hydrolysis catalysts (relative to the metal carboxylates, octoates and naphthenates) and also have the added advantage of increased latency (less trimerization activity at room temperature per unit of elevated temperature activity) due to the lower activity of the chelate.
19
1.4.2
Copolymerization with Epoxides Early reports on the epoxide–cyanate co-reaction noted the
formation of oxazole (5-membered ring) structures without elaboration on the mechanism or the complete reaction scheme. Publications in the period 1987 –90 noted that the trimerization of the cyanate group preceded reactions involving epoxide, and demonstrated that cured hybrid properties were independent of the CE prepolymerization, and thus proposed additional epoxide consumption via polyetherification catalyzed by the triazine ring. Monofunctional epoxy and CE model compounds were first employed in 1988 to separate and identify reaction products by GPC and IR. Publications by Bauer and later by Shimp, elucidated and supported a complex reaction pathway partially described in Scheme 1.6. The sequence of CE trimerization followed by epoxide insertion, isomerization of glycidyl cyanurates to isocyanurates and isocyanurate ring cleavage / rearrangement with additional epoxide to form substituted oxazolidinones, can be observed by heating the trimer of a mono cyanate with monoepoxide. The complicated insertion – rearrangement – cleavage sequence is expected to occur only at higher temperature, due to both the lack of homoconjugation in the alkyl cyanurates and the high activation barrier to breaking the isocyanurate ring. (1)
Trimerization OAr
Trimerization
N
N
3 Ar' OCN Aryl cyanate
ArO
N Aryl cyanurate
OAr
20
(2)
Epoxide Insertion OAr'
OAlk
N
N
N
+ N
Ar'O
3 Ar"
O
CH
AlkO
Epoxide
CH2
CH2
CH
N
CH2 O
OAr'
Aryl Cyanurate
Alk =
CH2
N
OAlk
Alkyl Cyanurate
Ar"
O
OAr'
(3)
Rearrangement O
OAlk Alk N
N
AlkO
Alk N
N
OAlk
N
N
O
Alkyl Cyanurate
O
Alk Alkyl Cyanurate
(4)
Ring cleavage and reformation O Alk
3 Ar"
Alk N
O
CH2
CH
CH2 O
N
3 Ar"
+ O
N
O
O
CH
CH2 O
N
Epoxide Alk Alkyl Cyanurate
CH2
O
Alk
21
(5)
Direct Ring Formation
+
Ar' OCN
Ar"
O
CH2
Aryl cyanate
CH
CH2 O
Ar"
O
CH
CH2 O
CH2 N
Epoxide
Alk O
Scheme 1.6 Proposed cyanate – epoxy reaction pathway Reactions (1) and (5) are the major oxazolidinone reactions occurring at lower temperatures (T < 150°C) (Martin et al. 1999). 1.5
SMART STRUCTURES A smart structure involves distributed actuators and sensors, and one
or more microprocessors that analyze the response from the sensors, and use the distributed parameter control theory to command the actuator to apply localized strains. A smart structure has the capability to respond to a changing external environment (such as loads and shape changes) as well as to a changing internal environment (such as damage or failure). Many types of actuators and sensors are being considered, such as piezoelectric materials, shape memory alloys, electrostrictive materials, magnetostrictive materials, electrorheological fluids and fiber optics. These can be integrated with the, main load carrying structure by surface bonding or embedding without causing any significant changes in the structural stiffness of the system. Among these materials, the piezoelectric are the most popular. They undergo surface elongation (strain) when an electric field is supplied across them, and produce a voltage when surface strain is applied, and hence can be used both as actuators and sensors. In an applied field these materials, however, generate a very low strain but cover a wide range of actuation frequency. The most widely used piezoceramics (such as Lead Zirconate Titanate) are in the form of thin sheets, which can be readily embedded or attached to a composite structure.
22
1.5.1
Piezoelectric Materials Piezoelectric materials have become very popular in the dynamic
sensing, actuation and control of active structural and mechanical systems. They can act both as actuators and sensors. In the development of intelligent structural systems, piezoelectric are widely used as sensors and actuators for the dynamic monitoring and control of structural and mechanical systems. In general, piezoelectric materials respond to force/pressure and generate a charge/voltage; this is referred to as the direct piezoelectric effect; on the other hand, the material exhibits stress/strain changes when a strong external voltage/charge is applied, and this is called the converse piezoelectric effect. In addition, the material would also respond to a temperature fluctuation and generate a charge/voltage, and this is referred to as the piezoelectric effect. It has been over 120 years since the first discovery of piezoelectric materials. However, it is only relatively recently that engineers and scientists have again recognized the potential of piezoelectric materials in distributed
actuators
microelectromechanical
and systems,
sensors, and
high adaptive
precision
systems,
structural
systems.
Consequently significant efforts have been devoted to the research and development of piezoelectric technologies in recent years. Besides mechanical and electrical coupling and interaction, temperature can also influence the performance of piezoelectric devices. For example, temperature variation can introduce voltage/charge generation in piezoelectric sensors. In addition, control voltage can cause a temperature rise in piezoelectric actuators. 1.6
SCOPE AND OBJECTIVES In recent years, a new damping material has been developed on the
basis of a new damping mechanism. For some specialized composites, mechanical vibrating energy is first transmitted to piezoelectric ceramic
23
powders and converted into an alternating potential energy by the piezoelectric effect. Such an energy transferring effect is referred to as the piezo-damping effect (Sheng et al, 2008). Lead Zirconate Titanate (PZT) is usually used as piezoelectric ceramic fillers to construct the piezo-electric units in the polymer matrix. It is well known that reinforcement can improve the mechanical property of the composite greatly. The damping property of the composite may also be improved, if the reinforcement is chosen properly and combined with the matrix in a special way, to make varied damping mechanisms playing roles together in the composite. Thus the work throws light on the mechanical and damping properties of the quarternary composite with varying cyanate and PZT loading, and on analyzing the effect of cyanate and PZT loading and on the properties. In view of the challenges associated with the manufacturing of the high performance composites discussed above, the present research proposal is focused on the, Development of new matrix material Determination of the mechanical properties of the prepared composites Application of newly developed composites The objectives of the present work are given below. 1.
Development of the matrix material. Arocy b10 (bis phenol dicyanate) is chosen to prepare blends in different loading levels of 0%, 20%, 40% and 60% with commercial epoxy resin system (LY556) using DDM(HT972) as curing agent.
24
A ceramic material, namely, lead zirconate titanate is loaded in different loading levels of 10%, 20% and 30% with the best cyanate loaded epoxy resin system. 2.
Fabrication of composite plates The fabrication of the Piezothermoelastic polymer matrix E-glass fibre composites using various blends with the combination of the following matrix materials:
3.
a.
Epoxy Resin
b.
Cyanate Ester
c.
Lead Zirconate Titanate
Testing and determination of the Mechanical properties Determination of the following mechanical properties:
4.
a.
Tensile Strength
b.
Tensile Modulus
c.
Flexural Strength
d.
Flexural Modulus
e.
Fracture Toughness
Vibration analysis of the fabricated composites with and without the piezoelectric material.(Experimental and Finite Element Analysis) Study of the natural frequency, damping factor, stiffness, storage modulus and loss modulus
5. Performance and emission characteristics of the lead zirconate titanate loaded cyanate modified epoxy coated combustion chamber in a diesel engine
25
Study of the brake fuel consumption, brake thermal efficiency, pressure, heat release rate, hydrocarbon emission, oxides of nitrogen, carbon monoxide emission and exhaust gas temperature. The following are the contributions of this thesis: I
Scientific contribution i)
Development of a Piezothermoelastic cyanate based epoxy composite material.
II
Technical contribution i)
Application of an analytical expression to determine the damping factor and modulus of the developed composite material.
ii)
a. Fabrication of the Piezothermoelastic epoxy-cyanate ester matrix glass fibre composite. b. Determination
of
the
tensile
properties,
flexural
properties and fracture toughness of the fabricated composite. c. Determination of the vibration characteristics of the fabricated composites. III
Contribution towards practical application i)
Use of the developed composite for high temperature application in an engine, which can be extended to aircraft also.
26
1.7
LITERATURE SURVEY
1.7.1
Epoxy Toughening Kinloch et al (1983) studied the microstructure and fracture
behaviour of an unmodified and a rubber-modified epoxy. Values of the stress intensity factor, KIc, at the onset of crack growth, the type of crack growth, and the detailed nature of the associated fracture surfaces have been ascertained. Both materials exhibit essentially the same types of crack growth, but the values of KIc for the rubber-modified material were usually significantly higher than those for the unmodified epoxy. The mechanisms for this increased toughness have been considered, and one that accounts for all the observed characteristics, has been proposed. Smiley and Pipes (1987) have investigated the rate effect on Mode I interlaminar fracture toughness in graphite/PEEK and graphite/epoxy composites. The tests were performed over a range of crosshead speeds from 4.2 x 10-6 to 6.7 x 10-1 m/s. The results indicate that the toughness of both material systems is rate sensitive. The Mode I interlaminar fracture toughness of the graphite/PEEK material decreases from 1.5 to 0.35 kJ/m2 over five decades of the opening rate. The fracture toughness of the graphite/epoxy material decreases from 0.18 to 0.04 kJ/m2 over four decades of the loading rate. Chu (1989) has made a study of the non-linear effect on Mode-I interlaminar fracture toughness. Large deflections occur on testing thinner specimens or thicker specimens with larger delamination lengths using a linear analysis; the G IC value increases with increasing delamination growth, and a significant difference is indicated between the first and the last test point. However, a non-linear analysis shows much better results than a linear analysis.
27
Madhu S.Madhukar and Drazal (1990) have studied the fiber-matrix adhesion and its effect on composite mechanical properties for Mode-I and Mode-II fracture toughness of graphite/epoxy composite. This method demonstrated that there is a strong dependency of the composites interlamination fracture toughness and failure modes on fiber-matrix adhesion. The full potential of a composites inter laminar fracture toughness can be realized only when the fiber matrix interface is strengthened to its maximum level. Verchere et al (1990) studied the effect of a bisphenol-A diglycidylether based epoxy cured with a cycloaliphatic diamine (4,4 diamino-3,3 -dimethyldicyclohexylmethane, 3DCM), in the presence of an epoxy terminated butadiene-acrylonitrile random copolymer (ETBN). Results showed that the vitrification is slightly delayed with the rubber addition. The maximum Tg of the rubber-modified matrix does not depend on the cure temperature, but decreases with the initial rubber concentration. This implies that a significant amount of rubber remains in the solution in the continuous phase. This explains the delay in vitrification. Bazhenov (1991) has made a study on strong bending in the DCB interlaminar test of thin, E-glass woven –fabric-reinforced laminates. In this case only the critical crack extension force needs to be measured to determine the fracture toughness. The interlaminar fracture toughness of fabricreinforced laminates is significantly higher than that of non-woven unidirectional composites. The crack propagation between the fabric layers is unstable, whereas it is stable if the crack intersects some fabric layer. Verchere et al (1991) studied the mechanical properties of a system consisting of a bisphenol A diglycidylether epoxy, cured with a cycloaliphatic diamine (4,4 -diamino-3,3 dimethyldicyclohexyl-methane, 3DCM), in the presence of an epoxy-terminated butadiene-acrylonitrile random copolymer (ETBN), as a function of the cure schedule and the initial rubber
28
concentration. Fracture toughness (KIc) and fracture energy (GIc) were increased, while Young's modulus and yield strength decreased slightly with increasing volume fraction of the dispersed phase. There is no significant influence of the precure schedule and of the various observed particle diameters on the mechanical properties for a constant rubber volume fraction.The main deformation process in the rubber-modified epoxy networks is shear yielding, while cavitation is negligible. Hourston and Lane (1992) prepared a series of blends by adding a polyetherimide, in varying proportions, to a trifunctional epoxy resin, triglycidylparaaminophenol, cured with 4,4 -diaminodiphenylsulphone. All the materials showed a two-phase morphology when characterized by a dynamic mechanical thermal analysis and scanning electron microscopy. Addition of the thermoplastic resulted in improved fracture properties (K1C and G1C), as measured by three-point bending experiments, although no obvious correlation with blend morphology was observed. Roxana et al (1993) used castor oil (CO) to replace polydisperse commercial rubbers (carboxy- or epoxy-terminated butadiene-acrylonitrile random copolymers, CTBN or ETBN) in model systems, developed to analyse the origin of the phase separation process in rubber-modified thermosets. Mixtures of CO with an epoxy resin based on the diglycidyl ether of bisphenol A showed a higher miscibility than a typical CTBN (18% acrylonitrile)-DGEBA system. Percentage conversions at the cloud point during the DGEBA-EDA polymerization were experimentally determined, and compared with theoretical predictions using the pseudo-binary approach in the framework of the Flory-Huggins lattice model. Reasonable agreement was found, giving direct evidence of the fact that phase separation results from the decrease in the entropic contribution to the free energy of mixing during polymerization.
29
Dong Chen et al (1993) introduced butadiene-acrylonitrile random copolymers with various molecular weights and end-groups into a system consisting of the diglycidyl ether of bisphenol A and 3,3 -dimethyl-4,4 diamino dicyclohexyl methane (3DCM) cured at 50°C. They studied the influence of a low level of soluble additive on the polymerization rate, evolution of viscosity, gelation and vitrification times, and the final network in the case of different additive molecular weights and end groups of the additive. It was found that the additive influences the final network only by dissolving in the matrix, and that the dilution effect plays a minor role, but the catalytic effect of an additive can play a significant role on polymerization depending on the additive end groups. However, in all the rubber-modified systems, phase separation has no direct influence on the polymerization rate and the change of viscosity. Min et al (1993) used near infra-red spectroscopy techniques to study the cure reactions of various epoxy resin formulations based on the diglycidyl ether of bisphenol A resins cured with 4,4 -diaminodiphenyl sulfone
(DDS)
hardener.
Stoichiometric
and
non-stoichiometric
DGEBA/DDS resin formulations, using neat as well as thermoplastic toughened systems containing two phenolic hydroxyl terminated polysulfones with different molecular weights were involved in this study. This series of quantitative analysis of the major chemical groups in several resin systems, led to a clear understanding of not only the reaction mechanism in each system but also the cure kinetics. Cho et al (1993) used tetraglycidyl-4,4 -diaminodiphenyl methane based resin with 30 phr diaminodiphenyl sulfone as the curing agent and was toughened with poly(ether imide) (PEI). The effects of morphology on the fracture toughness of modified epoxy resins were investigated. Morphology was controlled by changing the curing conditions. The co-continuous
30
structure and morphology of the PEI spherical domain dispersed in the epoxy matrix were obtained. Phase-inversed morphology with PEI matrix was also obtained with 30 phr PEI content. The cured resin with phase-inversed morphology showed the highest fracture toughness. The modified epoxy resins with enhanced fracture toughness exhibited other improved mechanical properties such as flexural strength, flexural modulus and strain at break. Tzong-Hann Ho and Chun-Szhan Wang (1994) used dispersed acrylate rubbers to improve the toughness of cresol-formaldehyde Novolac epoxy resin cured with phenolic novolac resin for electronic encapsulation application. The effect of the alkyl group of acrylate monomer on the phase separation of the resultant elastomers from epoxy resin was investigated. The dispersed acrylate rubbers effectively improve the toughness of cured epoxy resins by reducing the coefficient of thermal expansion (CTE) and flexural modulus, while the glass transition temperature (Tg) was hardly depressed. Electronic devices encapsulated with the dispersed acrylate rubber-modified epoxy molding compounds have exhibited excellent resistance to the thermal shock cycling test and resulted in an extended device use life. ZhiQiang Cao et al (1994) studied several rubbers (acrylonitrile– butadiene copolymer) or thermoplastic (polyethersulfone) additives bearing different chain ends introduced into pure aromatic dicyanates. The influence of these initially miscible modifiers on the polymerization kinetics, as a function of their chemical structure and concentration, were also studied. It appears that apart from those bearing a labile hydrogen atom, the additives play almost no role on the polycyclotrimerization rate; neither does phase separation. However, the additives influence the structure of the final networks insofar as they partially dissolve in the matrix, and thus modify both the final Tg and the onset of vitrification, compared with the pure monomer.
31
Tzong-Hann Ho and Chun-Shan Wang (1994) investigated the use of dispersed silicone rubbers to reduce the stress of cresol–formaldehyde novolac epoxy resin cured with phenolic novolac resin for electronic encapsulation application. The effects of structure, molecular weight, and contents of the vinylsiloxane oligomer on reducing the stress of the encapsulant were studied. Morphology and the dynamic mechanical behavior of the rubber-modified epoxy resins were also studied. The dispersed silicone rubbers effectively reduce the stress of the cured epoxy resins by reducing flexural modulus and the coefficient of thermal expansion (CTE), whereas the glass transition temperature (Tg) was hardly depressed. Electronic devices encapsulated with the dispersed silicone rubber modified epoxy molding compounds have exhibited excellent resistance to the thermal shock cycling test and have resulted in an extended device use life. Philippe Bussi and Hatsuo Ishida (1994) studied the dynamic mechanical properties of blends of diglycidyl ether of bisphenol-A-based epoxy resin and internally epoxidized polybutadiene rubber. It is shown that the influence of the composition of the continuous phase and of the dispersed phase can be studied not only from the variations of the glass transition temperature but also from the changes in the apparent enthalpy of activation associated with this transition. As the initial rubber content increases, the composition of the dispersed phase remains practically constant while more rubber is able to dissolve in the continuous phase. Youjiang Wang and Dongming Zhao (1995) have studied the characterization of the interlaminar fracture behavior of woven fabric reinforced polymeric composites. A large displacement, small strain nonlinear beam model was used to calculate the interlaminar fracture toughness. The fabrics used included fiberglass and Kevlar woven structures with different weave patterns. This study shows that the improvement in the
32
interlaminar fracture behavior of laminated polymeric composites has generally focused on the matrix material, the reinforcement and the fibermatrix interface. Tsung-Han Ho et al (1996) used polyol or polysiloxane thermoplastic polyurethanes (TPU) to reduce micro-cracking in cresol– formaldehyde novolac epoxy resin cured with phenolic Novolac resin for electronic encapsulation application. A stable dispersion of TPU particles in an epoxy resin matrix was achieved via the epoxy ring opening with isocyanate groups of urethane prepolymer to form an oxazolidone. The effects of the structure and molecular weight of TPU in reducing the stress of the electronic encapsultant were investigated. The mechanical and dynamic viscoelastic properties and morphologies of TPU modified epoxy networks were also studied. Reza Bagheri and Raymond (1996) elucidated the role of particle cavitation in toughening through a comparative examination of epoxies modified by conventional rubber modifiers and hollow plastic particles. The results of this study illustrate that rubber particles with different cavitation resistance and pre-existing microvoids toughen the present epoxy matrix in the same manner. Therefore, they concluded that the cavitation resistance of the rubbery phase does not directly contribute to toughness, but instead simply allows the matrix to deform by shear. An additional mechanism of microcracking was observed when 40- m hollow plastic particles were employed. Despite the similar behaviour in fracture toughness testing, rubber particles and microvoids differ considerably in how they affect the compressive yield strength of the blend. The results of this study suggest the possible importance of inter-particle distance in the toughening of epoxies. This concept will be examined in part 2 of this study.
33
King-Fu Lin and Yeow-Der Shieh (1998) employed a two-stage, multistep soapless emulsion polymerization to prepare various sizes of reactive core–shell particles (CSPs) with butyl acrylate (BA) as the core and methyl methacrylate (MMA) copolymerizing with various concentrations of glycidyl methacrylate (GMA) as the shell. Ethylene glycol dimethacrylate (EGDMA) was used to crosslink either the core or shell. The number of epoxy groups in a particle of the prepared CSP measured by chemical titration was close to the calculated value, based on the assumption that the added GMA participated in the entire polymerization unless it was higher than 29 mol %. Similar results were also found for their solid-state
13C
-NMR
spectroscopy. Tsung-Han Ho and Chun-Shan Wang (1999) synthesized a series of phenol-based and naphthol-based aralkyl epoxy resins by the condensation of p-xylylene glycol with phenol, o-cresol, p-cresol, or 2-naphthol, respectively, followed by the epoxidation of the resulting aralkyl novolacs with epichlorohydrin. The incorporation of stable dispersed polysiloxane thermoplastic polyurethane particles in the synthesized epoxy resin's matrix was achieved via epoxy ring-opening with the isocyanate groups of urethane prepolymer to form an oxazolidone. The mechanical and dynamic viscoelastic properties of cured aralkyl novolac epoxy resins were investigated. A seaisland structure was observed in all cured rubber-modified epoxy networks via SEM. The results indicate that a naphthalene containing aralkyl epoxy resin has a low coefficient of thermal expansion, heat resistance, and low moisture absorption, whereas phenol aralkyl type epoxy resins are capable of imparting low elastic modulus, resulting in a low stress matrix for encapsulation applications. Shun-Fa Hwang and Bon-Cherng Shen (1999) have studied the opening-mode interlaminar fracture toughness of interply hybrid composite materials. This method demonstrated that crack growth in the three types of
34
specimens is dominated by the opening mode and the Mode-I interlaminar fracture toughness can be approximated. For hybrid composite specimens, the effect of geometrical non-linearity should be included. This study clearly shows that the effect of geometric non-linearity increases with an increase of the crack length. Hua and Hu (2000) have investigated a new kind of simultaneous interpenetrating polymer networks (SINs) composed of epoxy resin (epoxy) and urethane acrylate resin (UAR) having various amounts of hard segment and prepared with poly(oxypropylene) polyol (PPO) having different molecular weights, and the relationship between the morphologies and mechanical properties of these SINs were investigated in detail. It was found that the different morphologies of these SINs were related to various structures of the UAR network. The morphology of such SINs not only depends on the compatibility between the poly(methyl methyacrylate) segments existing in the UAR network and epoxy network, but is related to the microphase separation of the UAR network as well. Kessler and White (2001) have studied the self-activated healing of delamination damage in woven E-glass/epoxy composites. With the ultimate goal of self-healing in mind, two types of healing processes are studied. Healing efficiencies relative to the virgin fracture toughness of up to 67% are obtained when the catalyzed monomer is injected and about 19% for the selfactivated materials. Hsieh et al (2001) have investigated polyurethanes (PU) based on poly(butylene adipate) [PU(PBA)] and poly(oxypropylene) [PU(PPG)] polyols as a graft agent to prepare interpenetrating polymer networks of urethane-modified bismaleimide (UBMI) and the diglycidyl ether of bisphenol A (Ep) (UBMI/Ep graft-IPNs). The UBMI was introduced and partially grafted to the epoxy by PU graft agents, and then the simultaneous
35
bulk polymerization technique was used to prepare the graft-IPNs.All the PU graft agents were characterized by infrared (IR). The tensile strength of both the UBMI/Ep graft-IPNs with PU (PBA) and PU (PPG) graft agent systems increased to a maximum value with increasing UBMI content in the system, and then decreased with further increasing the UBMI content. For both kinds of PU with various molecular weights in the UBMI/Ep graft-IPNs, the Izod impact strength increased with the UBMI contents increasing. The better compatibility of PU (PBA)-based UBMI/Ep graft-IPNs led to higher impact strength. Fellahi et al (2001) used kaolin as a modifier at different contents to improve the toughness of diglycidyl ether of bisphenol A epoxy resin with polyamino-imidazoline as a curing agent. The chemical reactions suspected of taking place during the modification of the epoxy resin were monitored and evaluated with the Fourier transform infrared spectroscopy. The glasstransition temperature (Tg) was measured with differential scanning calorimetry. The mechanical behavior of the modified epoxy resin was evaluated in terms of the Izod impact strength (IS), the critical stress intensity factor (KIC), and the tensile properties at different modifier contents. Scanning electron microscopy (SEM) was used to elucidate the mechanisms of deformation and toughening in addition to other morphological features. Finally, the adhesive properties of the modified epoxy resin were measured in terms of the tensile shear strength (TSS). Dispenza et al (2001) chose a high molecular weight acrylonitrile /butadiene /methacrylic acid (Nipol 1472) rubber to control the processability and mechanical properties of a TGDDM (tetra glycidyl diphenyl methane) based epoxy resin formulation for aerospace composite applications. The physical blend of rubber and epoxy resin, achieved by the dissolution of all the components in a common solvent, forms a heterogeneous system after
36
solvent removal and presents coarse phase separation during cure, which impairs any practical relevance of this material. A marked improvement of rubberepoxy miscibility is achieved by the reactive blending (‘pre-reaction’) of the epoxy oligomer with the functional groups present in the rubber. Shangjin He et al (2001) synthesized two kinds of reactive toughening accelerators for epoxy resin, amine-terminated chain-extended urea (ATU) and imidazole-terminated chain-extended urea (ITU) from polyurethane prepolymer. Compared with the unmodified system, the curing activity, dynamic mechanical behavior, impact property and fracture surface morphology of the modified systems were systematically investigated. Results show that the curing activity of the modified epoxy resin E51/dicyandiamide (dicy) systems is so greatly enhanced that the apparent activation energy of the curing reaction decreases from 130.2 kJ/mol for the unmodified system to 75–85 kJ/mol for the modified systems. The curing reaction mechanism of the E-51/dicy system accelerated by ITU is different from that of the system accelerated by ATU, and a little different from that of the system accelerated by imidazole. Furthermore, the impact strength of the cured systems modified with ITU is 2–3 times higher than that of the unmodified system, while the glass transition temperatures are a little altered, and the fracture surfaces of all modified systems display tough fracture feature. Fábio et al (2002) employed hydroxy-terminated polybutadiene functionalized with isocyanate groups and in preparation of a block copolymer of polybutadiene and bisphenol A diglycidyl ether (DGEBA)based epoxy resin. The block copolymer was characterized by the Fourier transform infrared (FTIR) spectroscopy and size-exclusion chromatography (SEC). Cured blends of epoxy resin and hydroxy-terminated polybutadiene (HTPB) or a corresponding block copolymer were characterized by
37
differential scanning calorimetry (DSC), dynamic mechanical analysis (DMTA), and scanning electron microscopy (SEM). All modified epoxy resin networks presented an improved impact resistance with the addition of the rubber component at a proportion of up to 10 wt % when compared to the neat cured resin. Raffaele Mezzenga et al (2002) studied the effect of the structural buildup during the reticulation of thermoset systems containing reactive modifiers to strongly influence the final properties of such blends. This was studied by considering the rheological behavior during the cure of an epoxy/amine thermoset system blended with reactive dendritic hyperbranched polymers (HBPs). Depending on the chemical structure of the HBP used in the blend, a phase separation could be observed. The onset and offset of the phase separation process could be detected by observing the evolution of the viscoelastic properties. The phase separation onsets obtained by rheological measurements were compared with the values obtained by traditional cloud point observations. A good agreement between the two techniques was observed. Kim et al (2002) synthesized amine-terminated poly(arylene ether sulfone)–carboxylic-terminated
butadiene-acrylonitrile–poly(arylene
ether
sulfone) (PES-CTBN-PES) triblock copolymers with controlled molecular weights of 15,000 (15K) or 20,000 (20K) g/mol from amine-terminated PES oligomer and commercial CTBN rubber (CTBN 1300x13). The copolymers were utilized to modify a diglycidyl ether of bisphenol A epoxy resin by varying the loading from 5 to 40 wt %. The epoxy resins were cured with 4,4 -diaminodiphenylsulfone and subjected to tests for thermal properties, plane strain fracture toughness (KIC), flexural properties, and solvent resistance measurements. The fracture surfaces were analyzed with SEM to elucidate the toughening mechanism. The properties of the copolymer-
38
toughened epoxy resins were compared with those of the samples modified by the PES/CTBN blends, PES oligomer, or CTBN. The PES-CTBN-PES copolymer (20K) showed a KIC of 2.33 MPa m0.5 at 40 wt % loading while maintaining good flexural properties and chemical resistance. Giannotti
et
al
(2004)
modified
epoxy–aromatic
diamine
formulations simultaneously with two immiscible thermoplastics (TPs), poly(ether imide) (PEI) and polysulfone (PSF). The epoxy monomer is based on the diglycidyl ether of bisphenol A and the aromatic diamines (ADs) are either
4,4 -diaminodiphenylsulfone
or
4,4 -methylenebis(3-chloro
2,6-
diethylaniline). The influence of the TPs on the epoxy–amine kinetics is investigated. It is found that PSF can act as a catalyst. The presence of the TP provokes an increase of the gel times. Valéria et al (2005) prepared composites using epoxy resin (ER), carboxyl-terminated butadiene acrylonitrile copolymer (CTBN) and hydroxylterminated polybutadiene (HTPB), in different proportions. A chemical link between the HTPB and the epoxy resin was promoted employing tolylene diisocyanate (TDI). The reactions between elastomers and epoxy resin were followed by FTIR. The mechanical properties of the composites were evaluated and the microstructure was investigated through scanning electronic microscopy (SEM). The results showed that the impact resistance of the CTBN-modified ER was superior to that of the pure epoxy resin. For the composites with HTPB, the impact resistance increased with elastomer concentration of up to three parts per hundred parts of resin (phr). Higher concentrations of HTPB resulted in larger particles and gave lower impact values. Maity et al (2007) modified the diglycidyl ether of bisphenol-A (DGEBA) resin with amine functional aniline acetaldehyde condensate (AFAAC), and cured with an ambient temperature curing agent triethylene tetramine. The resulting networks displayed significantly improved fracture
39
toughness. The AFAAC was synthesized by the condensation reaction of aniline and acetaldehyde in the acid medium (pH 4) and characterized by FTIR and NMR spectroscopy, elemental analysis, viscosity measurements, and mole of primary and secondary amine analysis. The DGEBA and AFAAC
were
molecularly
miscible,
but
developed
a
two-phase
microstructure upon network formation. Epoxy/AFAAC compositions were systematically varied to study the effect of AFAAC concentration on the impact, adhesive, tensile, and flexural properties of modified networks. The dynamic mechanical analysis and scanning electron microscopy studies showed two phase morphology in the cured networks, where AFAAC particles were dispersed. The AFAAC modified epoxy network was thermally stable up to around 280°C. 1.7.2
Epoxy and Cyanate Ester Blends and Composites Ian Hamerton et al (1998) presented a review article on recent
technological developments in the field of cyanate ester resin. In this article, recent developments in the processing, toughening properties and applications of the cyanate ester resin are reviewed. Hwang et al (1999) studied the effect of the composition of polysulphone (PSF) on the cyanate ester system. From the study it is found that homogeneous bisphenol A dicyanate (BADCy) / PSF blends with a low content of PSF (less than 10 wt%) are cured isothermally, and blends are phase separated by nucleation and growth mechanism to form the PSF particle structure. But with more than 20 wt% of PSF content the BADCy / PSF blends are phase separated by spinodal decomposition to form the BADCy particle structure, and when the PSF content was 15 wt% the blends are phase separated by nucleation and growth, and spinodal decomposition resulting in the formation of a combined structure having both PSF and BADCy particle structure.
40
Roman et al (2000) investigated the effects of temperature and moisture on the thermal and mechanical properties of thermoplastic and elastomer toughened high-temperature cyanate ester composite material. The thermoplastic modified cyanate ester showed increased thermal stability. The elastomer modified cyanate ester showed the highest mode I fracture toughness values, primarily because the toughener did not phase separate. Kimo Chung et al (2001) evaluated the thermomechanical property changes of carbon fibre/cyanate ester composites by DMA through timetemperature equivalence. From the study a modelling methodology was developed which quantitatively provided an understanding of the ageing process of fibre reinforced composites in isothermal environments. Ian Hamerton et al (2002) prepared carbon fibre impregnated tape from a range of prepolymers comprising several different blends of akenyl functionalized cyanate ester monomers with commercial cyanate ester and BMI monomers and blends. Incorporation of akenyl-functionalised cyanate ester monomers into commercial cyanate ester/ BMI blends raised the Tg value while maintaining GIC and other mechanical properties.
This
enhancement in neat resin fracture toughness was translated into the corresponding composites. Jerome Dupuy et al (2002) investigated the thermophysical properties (heat capacity, thermal conductivity) and modelled neat resin and glass fibre composites. The models are used to stimulate the thermal transfers in an instrumented heated mould. The calculated local temperatures and surface heat fluxes appear to be in very good agreement with measurements for both the neat resin and the composite. The moisture absorption studies of cyanate ester modified epoxy resin matrices under constant hydrothermal conditions are an attempt to understand the so-called “reverse thermal effect.” From the swelling study it is argued that in the initial stage of the absorption
41
process the water diffuses into the regions of volume equal to or greater than the volume of water molecules, which does not result in swelling. In the later stage, the water molecules penetrate the regions with a volume less than that of the volume of water molecules, with molecular reorganization of the resin network, resulting in swelling (Sunil K. Karad et al 2002). Shinn-Gwo Hong et al (2003) studied the effect of copper oxide on the thermal degradation of bis-maleimide triazine (BT) prepreg with IR, ATR and TGA. The results indicate that the thermal degradation in the bulk BT is mainly from the epoxy constituent while that in the copper oxide contacted BT happens not only from the epoxy but also from the more stable cyanate ester constituent. Baochun Gwo et al (2003) investigated the chemical nature of the changes in a cyanate ester-novolac epoxy resin blend caused by hygrothermal ageing, and the effects of the residual reaction in the blends on the hygrothermal ageing resistance. The results of the study indicate that the long-term hygrothermal ageing may cause substantial changes in the chemical nature of the blends when the cure extent is not sufficiently high. Tim J. Wooster et al (2003) studied the effect of filler incorporation on the thermal, mechanical and conductivity properties of cyanate ester composites. From the study it is concluded that silica filler increased the thermal conductivity, Young’s modulus and dielectric constant (slightly) and decreased thermal expansion. It is also found that the addition of silica resulted in a marginal decrease in strength. Sabyasachi Ganguli et al (2003) prepared nanocomposites of cyanate ester by dispersing organically modified layered silicate into the resin. The inclusion of only 2.5 % by weight of organically modified layered silicates showed a 30% increase in both the modulus and toughness.
42
1.7.3
PZT Modified Polymer Composites Edward et al (1987) presented analytical and experimental studies
on piezoelectric actuators as elements of intelligent structures with highly distributed actuators, sensors and processing networks. Static and dynamic analytical models are derived for segmented piezoelectric actuators that are either bonded to an elastic substructure or embedded in a laminated composite. Lee (1991) developed a piezoelectric laminate theory that uses the piezoelectric phenomenon to effect distributed control and sensing of bending, torsion, shearing, shrinking and stretching of a flexible plate. The reciprocal relationship of the piezoelectric sensor and actuators is unveiled. Dimitriadis et al (1991) analytically investigated the behavior of two dimensional patches of piezoelectric material bonded to the surface of elastic distributed structures, and used as vibration actuators to the supported elastic structure. The theory is then applied to develop an approximate dynamic model of the vibration response of a simply supported elastic rectangular plate excited by a piezoelectric patch of variable rectangle geometry. Woo-Seok Hwang et al (1993) presented a finite element formulation for the vibration control of a laminated plate with piezoelectric sensors/actuators. The classical lamination theory with the induced strain actuation and Hamilton’s principle are used to formulate the equations of motion. The total charge developed on the sensor layer is calculated from the direct piezoelectric equation of motion. Ghosh and Batra (1995) showed the deflection of the centerline of a simply supported plate and the tip deflection of a cantilever plate, both
43
deformed quasistatically, that can be controlled by applying suitable voltage to the PZTs. The first order shear deformation theory is used to study the infinitesimal elastic deformations. The adhesive between the PZT and the plate is assumed to be of negligible thickness, and the displacement and surface tractions across the interfaces between the PZTs and the plate were taken to be continous. Paul Heyliger and Saravanos (1995) developed exact solutions for predicting the coupled electro mechanical vibration characteristic of simply supported laminated piezoelectric plates composed of orthorhombic layers. The three dimensional equations of motion and the charge equations are solved using the assumptions of the linear theory of piezoelectricity. Dimitris A Saravanaos et al (1997) presented the structural mechanics for the analysis of laminated composite plate structures with piezoelectric actuators and sensors. The theories implement the layer wise representation of displacements and electric potentials, and can model both the global and local electromechanical response of smart composite laminates.Finite element formulations are developed for the quasi-static and dynamic analyzing of smart composite structures containing piezoelectric layers. Vardarajan et al (1998) discussed the shape control of a laminated composite plate with integrated piezoelectric actuator. The effectiveness of the piezoelectric actuators and position sensors is investigated for shape control under the influence of quasi statically varying unknown loads. Hernandes et al (2000) investigated the free vibration behavior of thin composite plates with surface bonded piezoelectric patches, including stress stiffening effects. A finite element formulation is presented, based on the Reissener mindlin theory and including non-linear strain displacement relations to formulate a free vibration Eigen value problem in the presence of
44
a geometric stiffness matrix. The case of the symmetric laminate and ideal linear behavior is assumed for the piezoelectric actuation. Hori et al (2001) developed new types of piezoelectric damping materials,
piezoelectric
ceramic
(PZT)
powder/carbon
black
(CB)
powder/epoxy (EP) resin composites, and studied their mechanical and damping properties. Here, the mechanical energy of vibrations and noises were transformed into electric energy (current) by PZT, and the electric current was conducted to an external circuit through CB powders and then dissipated as thermal energy through a resistor. When CB was added to PZT/EP (70/30 in wt%), the mechanical loss factor (h), a measure of the mechanical and damping intensity, showed a maximum value of 0.08 at the CB content of 0.51 wt%, at which the CB particles electrically just contact each other. In this work, it was found that the PZT/CB/EP composite of 90.0/0.5/9.5 shows a large h value of 0.15. Hajime Kishi et al (2004) characterized the damping properties of carbon fiber-reinforced interleaved epoxy composites. Several types of thermoplastic-elastomer films, such as polyurethane elastomers, polyethylenebased ionomers and polyamide elastomers were used as the interleaving materials. The damping properties of the composite laminates with/without the interleaf films were evaluated by the mechanical impedance method. Also, the effects of the lay-up arrangements of the carbon-fiber prepregs on the damping properties of the interleaved laminates were examined. The viscoelastic properties of the interleaved polymer films were reflected in the damping properties of the corresponding interleaved laminates. The loss tangent of the interleaf films at the test temperature played an important role in the loss factor of the interleaved laminates. Also, the stiffness of the films at the resonant frequency of the laminates was another important parameter that controlled the loss factor of the interleaved laminates.
45
Botelho et al (2005) determined the viscoelastic properties, such as the storage modulus (E’) and loss modulus (E’’), for a glass fiber/epoxy composite, aluminum 2024-T3 alloy and for a glass fiber/epoxy/aluminum laminate (Glare). It was found that the glass fiber/epoxy (G/E) composites decrease the E’ modulus during hygrothermal conditioning up to the saturation point (6 weeks). However, for Glare laminates the E’’ modulus remains unchanged (49 GPa) during the cycle of hygrothermal conditioning. The outer aluminum sheets in the Glare laminate shield the G/E composite laminae from moisture absorption, which in turn prevent, to a certain extent, the material from hygrothermal degradation effects. Botelho et al (2006) investigated the viscoelastic properties, such as elastic (E’), and the viscous (E’’) responses were obtained for the aluminum 2024 alloy, carbon fiber/epoxy, glass fiber/epoxy and their hybrids aluminum 2024 alloy/carbon fiber/epoxy and aluminum 2024 alloy/glass fiber/epoxy composites. The experimental results were compared with the calculated E modulus values by using the composite micromechanics approach. For all the specimens studied, the experimental values showed good agreement with the theoretical values. The damping behavior, i.e., the storage modulus and the loss factor, from the aluminum 2024 alloy and fiber epoxy composites can be used to estimate the viscoelastic response of the hybrid FML Tsantzalis et al (2007) investigated the fracture toughness of carbon fiber reinforced polymer (CFRP) laminates doped with carbon nanofibers (CNF) and/or piezoelectric (PZT) particles. An increase of 100% in fracture energy was observed after the addition of 1% CNF in the matrix of the laminates. The investigation of the fracture surfaces showed extensive fiber bridging because of the presence of CNFs, which verifies the enhanced fracture properties. On the other hand, the introduction of PZT particles led to
46
a reduction in the fracture toughness, mainly due to the brittle character of the particle inclusions. Jian Gu et al 2007 prepared and characterized the high damping properties of the promising low density epoxy/fly ash composites, a series of epoxy composites filled with fly ash of a different volume fraction. Damping tests of cured epoxy composites are performed in the temperature range of 40 to 150 C and in the frequency range of 10 to 800 Hz, by using a tensioncompression mode. The results show that the values of tangent delta (tan ) reach their peak values at the glass transition temperatures for the composites with 30–50 vol.% fly ash, and the tan
values attenuate slowly with the
increase in frequency, which indicates that the damping properties of such composites are better than those of other composites. Scanning electron microscopy was used to observe the fractured surfaces of the composites, and to clarify the dispersion and distribution of fly ash particulates in the matrix. In addition, the thermogravimetric curves were also employed to characterize the heat-resistant performance of the composites. Tsantzalis et al (2007) studied vapor growth carbon nanofibers (CNF), lead zirconate titanate piezoelectric (PZT) particles, as well as a combination of these two added in an epoxy resin (EP), and their influence on the mechanical quasi-static properties. Moreover, the prepared samples were characterized by a dynamic thermal mechanical analysis, and optical and scanning electron microscopy. An enhancement of the mechanical properties was observed by the addition of the CNF. The uncured mixtures were also used as matrix material for manufacturing unidirectional carbon fiber reinforced laminates. In this paper, the author, Toshio Tanimoto, summarizes the previous works on the passive damping of carbon-fiber reinforced plastic (CFRP) cantilever beams using: (1) interleaving of viscoelastic thermoplastic films,
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(2) surface-bonded piezoelectric ceramics, and (3) dispersed PZT particle interlayers.
Introducing
polyethylene-based
film
interlayers
between
composite plies resulted in a significant increase in the vibration loss factor. It is also shown that the vibration damping of CFRP laminates can be improved passively by means of resistively shunted, surface-bonded piezoelectric ceramic, PbZrO3–PbTiO3 (PZT) sheets. Kostopoulos et al (2007) investigated the influence of carbon nanofibers (CNF) and/or piezoelectric (PZT) particles on the fracture behaviour of carbon fiber reinforced polymer laminates. For this purpose the fillers were added as dopants in the epoxy matrix of the laminates. An increase of 100% in the fracture energy was observed after the addition of 1% CNF in the matrix of the laminates, while the introduction of the PZT particles led a to reduction in the fracture energy, mainly due to the brittle character of the particle inclusions. In addition, the acoustic emission technique was used for monitoring the fracture process of the laminates. Rodríguez et al (2008) investigated the catalytic performance of 3 wt.% copper supported on carbon nanofibers (CNFs) in liquid phase oxidation, using a batch stirred tank microreactor in order to determine the decolorization and total organic carbon (TOC) removal efficiency in washing textile wastewater (WTW). A preliminary study was carried out in a temperature range of 120–160 °C and two oxygen partial pressures of 6.3 and 8.7 bar. TOC removal and toxicity reduction were as high as 74.1% and 43%, respectively at 140 °C and 8.7 bar, after 180 min reaction. The main intermediates detected in raw wastewater were decanoic acid, methyl ester and 1,2-benzenedicarboxylic acid, and they have been degraded by means of a Cu/CNF catalyst. The application of CWAO to the treatment of a textile effluent at 160 °C and 8.7 bar of oxygen partial pressure showed that the use of a Cu/CNF catalyst significantly improves the TOC and color removal
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efficiencies, and it can be considered as an option for a pretreatment step in the treatment of these industrial effluents. Sheng Tian et al (2008) prepared a new type of rigid piezo-damping epoxy-matrix composites, containing multi-walled carbon nanotubes(CNT) and piezoelectric lead zirconate titanate (PZT), and investigated their electrical and damping properties. The dynamic mechanical thermal analysis revealed that the loss factors of the composites were improved by the incorporation of PZT and CNT under the concentration above a critical electrical percolation. Based on this piezo-damping material, the PZT contributes to the transformation of mechanical noise and vibration energies into electric energy, while the CNT serves in the shorting of the generated electric current to the external circuit. An optimum formulation for the piezodamping epoxy based materials can be designed on the basis of the results of this study. 1.7.4
Engine Coating Dennis N. Assanis (1988) deals with the transient analysis of piston-
liner heat transfer in low-heat-rejection diesel engines.A two-dimensional finite element program has been developed to analyze the transient heat flow paths in low-heat-rejection engine combustion chambers. This analysis tool has been used to study the transient heat transfer performance of a ceramiccoated piston-liner assembly and compare it with the performance of baseline cast-iron components. The direction of the gas-to-liner and piston-to-liner heat flux, changes several times during the cycle, and these changes occur at different instants, as the distance from the cylinder fire-deck varies. In the conventional metal engine, heat flows from the relatively hotter piston to the liner, via the rings for most part of the cycle. In contrast, for the LHR configuration, the net heat transfer is from the liner to the rings, and thus to the piston.
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Miyairi (1988) deals with the computer simulation of a low heat rejection direct injection diesel engine using a two zone combustion model and Anand’s heat transfer model. Spray penetration, deflection, growth, and rate of entrainment were considered under the steady state condition. The combustion and heat transfer characteristics were studied in this literature by taking into account the high temperature swing in the low heat rejection engines. The heat transfer through the combustion chamber components was studied, by solving the unsteady conduction heat transfer equations. This literature is used in this work as a basic support for the formulation of various models for the combustion and heat transfer calculations. Randolph A Churchill et al (1988) studied a low heat rejection engine. Reducing heat losses from the engine cylinder makes minimal changes to the efficiencies of the existing engines. It reduces the need for cooling systems and their cost, reducing weight and reducing the complexity. Partially stabilized zirconia has been developed, that decreases the magnitude of the phase changes and is now considered a good candidate for engine use. Recovering the available exhaust heat will also aid in controlling pollution. It may be possible to build an unlubricated engine with 55% thermal efficiency. Jorge J.G.Martins (2004) deals with the thermodynamic analysis of an over expanded engine. The equation of an over-expanded engine is developed with the equation for the otto cycle, diesel cycle and dual cycle at part load condition. It is clear that the most efficient cycle under light and part load condition is the miller cycle with a fixed trapped compression ratio, particularly at very low loads (0.3 to 0.4), where it can achieve theoretical efficiencies approaching 73%. In the diesel cycle, the amount of intake air is the same as there is no restriction on the intake. So, the change in the cycle configuration to a lower load will be the reduction of heat supplied during the isobaric heating.
