Hyperbranched polymers - A novel class for high performance coatings

Excerpt: Development of high performance resin for low VOC coating is of great interest due to the adverse effect on health, safety and environmental issues.

Tirthankar Jana Berger Paints India Limited 14 & 15 Swarnamoyee Road, Howrah 711103, India


THE conceptual idea from Nature is not new; many scientific approaches have been inspired by observing different natural phenomena. One unique example is enzyme, which is very specific as well as effective in their function, is due to their control three dimensional structural symmetry. Development of hyperbranched polymer is one such attempt of the scientists to achieve better performance from any polymer. The concept of three dimensional structural growth of polymer was first introduced by Flory[1] In 1943, Flory used the term network cell, which is the most fundamental unit for growth of any macromolecular network structure. Recently, a significant volume of research work is in progress on this particular subject in different areas to meet the market demand.

Hyperbranched polymers are a class of macromolecular compounds having imperfectly branched or irregular structures with a large number of reactive groups.[2]

The term dendrimer is a Greek word; dendron means tree like and meros meaning units or parts. Thus dendrimers have tree like structure, free from major defects[3]. Hyperbranched polymers are also highly branched macromolecules but have less regular structural architecture and contain a large number of structural defects and missing branches.

    Dendritic and hyperbranched polymers are novel and versatile due to their unique structural features of highly branched but less entangled with large number of active surface functionalities. The hyperbranched polymers also possess a large variety of unusual properties like low viscosity, high reactivity, interior cavity etc.The chemical modification or functionalization in such type of polymer results in a wide range of variation in physical and chemical properties. These unique characteristics of these macro molecules render them in a wide range of valuable applications like medicine, adhesive and coatings, gene therapy and chemical sensors, drug delivery systems, electronic materials etc[4].

Structural aspects of dendritic polymer

The dendritic polymer consists of both dendrimer and hyperbranched polymer. These polymers have three architectural features in their chemical structure. These are i) central multifunctional core ii) interior layers composed of repeating units or propagating units attached to the central core and iii) exterior layers or surface of terminal functionalities, attached to the outer generation.

The core unit consists of multifunctional moiety, which affects the HIGH PERFORMANCE COATINGS shape of hyperbranched structure. This core may be small or large depending on the functionality of the central molecule.

The interior layers are produced by the repetitive attachment of the repeating or propagating units or building blocks to the core unit. The terminal or surface functional groups are exposed in the outer layer. The number of such terminal group depends on the multiplicity of the unit.

Classification of dendritic polymer

     There is no single well accepted way for classification of the dendritic polymers. An attempt has been made to classify the dendritic polymers into different categories based on their mode of synthesis i.e. divergent or convergent approach, accelerated and single step approach etc., presence of main functional linkages in the back bone chain i.e. hydrocarbon, amide, ester, urethane type etc., physical properties and applications i.e. photo active, redox active, liquid crystalline, catalytic, bioactive etc.[3]

Methodology for synthesis of dendritic polymer

In the divergent approach, dendrimer starts from the central core having more than two same types of functional groups which can be used for the branch growth. The central core thus diverges in stepwise manner to the periphery through the repetitive activation of the surface functional groups and addition of branching monomer units[5] . The divergent approach is most attractive for production of few dendrimers such as poly (amido amine), poly (propylene imine) etc. which are commercially available[6,7] . In convergent approach, the dendrimer structure starts from periphery and proceeds to the central core which is opposite to the divergent approach. However, convergent approach provides more precise control of structure over divergent approach. The main disadvantage of convergent step is that it requires more number of steps to build up a large structure compared to divergent approach as a result the final yield of the product in convergent approach is less than that of divergent approach.

The techniques for synthesis of hyperbranched polymers can also be divided into two major categories. One is single monomer methodology (SMM), in which hyperbranched macromolecules are synthesised by polymerization of ABX type of monomers e.g. polyethers , polyesters, polyamides, polycarbonates etc. The second category is double- monomer methodology (DMM), in which direct polymerization of two types of monomers or a monomer pair generates hyperbranched polymers. The classical examples are polymerization of A2 + B3 monomers are also called 'A2 +B'3 methodology[8] . Three main polymers like polyamides, polycarbonates and polyureas are synthesised by using A2 +B3 methodology. Couple-monomer methodology (CMM), is the combination of the basic SMM and DMM, used to prepare many types of hyperbranched polymers such as poly (sulfone amine)s, poly(ester amine)s, poly(urethane)s etc.

Self-condensing vinyl polymerization (SCVP) technique is applied for AB* type of monomers used to synthesize polystyrenes, poly(methacrylate)s or poly(acrylate)s.Multibranching ring- opening polymerization (SCROP) of latent AB monomers approach can be used to obtain polyamines, polyethers and polyesters.Proton - transfer polymerization (PTP) is used to formhy- perbranchedpolysiloxanes orpolyesters with epoxy or hydroxyl end groups.

The divergent and convergent methods are largely used to synthesize various dendritic polymers successfully, but both involve multisteps, time consuming and low yield process. Recently, a number of accelerated approaches have been used to synthesise hyperbranched polymers where less number of steps are involved. One such technique is single step approach, in which the number of steps for the synthesis of desired hyperbranched polymer is reduced and thereby reduces the cost of the materials[9] . The synthesis of hyperbranched polymer by single step approach is much simpler and does not require any protection or deprotection steps . However, some linear chains are formed by the side reactions which cause the possibility of formation of more defects in the hyperbranched polymer structure.

A comparison with linear and dendritic or hyperbranched polymers

Dendritic or hyperbranched polymers are non-entangled, uncoiled, highly branched and three dimensional architecture but linear polymers possess random coiled structure with high degree of conformational freedom. The hyperbranched polymers are generally having low poly dispersity index, whereas linear polymers have high poly dispersity index. It is also reported that the hydrodynamic volume of dendritic or hyperbranched polymer is much less than that of linear polymer[10] . This is due to the fact that these polymers have more compact, back folded globular structure.

Characterization techniques of dendritic and hyperbranched polymer

The characterization of dendritic or hyperbranched polymer is very difficult due to the presence of large number of functional groups at the periphery and also the structural defects formed during the generation of hyperbranched structure. For composition and functional group detection of anyhyperbranched polymer - element analyser, IR, UV-Visible spectroscopic techniques are generally used. GPC technique is being used for homogeneity, molecular weight and molecular weight distribution of the polymer[11] . The structure, degree of branching of hyperbranched polymer is characterized by 'H NMR,13C NMR spectroscopic techniques[12,13] .

   Some special techniques like small angle X-ray scattering (SAXS), dynamic light scattering (DLS), atomic forced microscopy (AFM), scanning tunnelling microscopy (STM) etc. are used for morphology study[14-17]. Thermal characterizations are generally carried out by thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC) techniques. The thermal stability along with the nature of degradation in different environmental conditions are HIGH PERFORMANCE COATING being studied by TGA technique[18,19]. DSC technique is used to study the glass transition temperature (Tg) and melting temperature (Tm) along with enthalpy changes during different transition or degradation stages[20].

Properties of dendritic and hyperbranched polymer

The significant properties of dendritic and hyperbranched polymers depend on the globular structure as well as core of the molecule, branching multiplicity and length of the branched segment.

The physical properties like solubility of any hyperbranched polymer is much higher compared to linear polymer of equivalent molecular weight[21] . The rise in viscosity of any hyperbranched polymer is less than that of linear polymers[22] . The density of hyperbranched polymer increases from the core to the periphery with the increase of generation number. In general, the density of the dendrimer varies just in reverse order as intrinsic viscosity with generation number[23] . The hyperbranched polymers are mostly amorphous, due to highly branched architecture without any long range order[24] . Some exceptions are liquid crystalline dendritic polymers which are used for electro-optic devices, thermo

chromic and fluid flow sensors for imaging devices and information storage[25-28] . The dendritic polymers having chiral moiety in their structure, display optical properties[29-33] unit may be present in the core unit, or branching blocks, or surface groups. The optical properties have potential applications in the field of chiral chromatographic materials, catalyst in asymmetric synthesis etc[34-36]. mechanical (load bearing) properties of conventional polymer are good. However, this property increases with increase of molecular weight and also chain entanglement of the polymer chain. Hyperbranched materials also have outstanding mechanical properties such as individual modulus, tensile strength, compressive modulus as well as flexibility due to their compact branched structure[37,38].

Scope of hyperbranched polymer for coating application

The low viscosity, high solubility, easy accessible to other substrate by multiple interactions through surface active groupsas well as various possibilities to derivatise the surface groups to obtain certain desired properties are the unique features of hyperbranched polymer for surface coating application.

For example, the hydroxyl functional groups of intermediate resin when reacts with unsaturated fatty acid, form alkyd resin, different acrylic monomers when grafted with unsaturated fatty acid form acrylated resins, hydroxyl or acid functional groups may react with epoxy terminated polymers or may be transformed into ionic groups for water solubility or may be mixed with non- reactive groups for viscosity, polarity and compatibility adjustment[39] the importance of hyperbranched polymers are being investigated in a broad range of coating applications and several types of hyperbranched polymers are now commercially available[40-42].

Another important aspect of hyperbranched structures is the non- Newtonian relationship between viscosity and molecular weight, these rheological properties i.e. low viscosity at high molecular weight affect other characteristics such as reactivity, chemical resistance, mechanical properties etc. Since viscosity of hyperbranched polymer is low so for low VOC coating formulation, use of such of polymer is advantageous[42]

Commercially available hyperbranched Polymers for coating application

One of the commercially available hyperbranched aliphatic polyester for coating application is Boltron manufactured by Perstrop Speciality Chemicals. Boltron consist of multifunctional core from which branches are extended to form a highly branched inherent structure with large number of hydroxyl groups[41]. The AB2 type of monomer 2,2dimethylolpropionic acid is used toform such type of hyperbranched polymer. The geometric structure is spherical consisting of three different regions viz. interior core, intermediate layer and shell. The interior ester groups are shielded by the dense tree structure for higher hydrolytic and chemical stability [39]. Such types of polyesters are used as coating for flexible packaging, radiation curing applications etc.

A second class of commercially available hyperbranched polymer is polyesteramides. In polyesteramide the pendent hydroxyl and carboxyl groups can be modified with a broad range of functional groups such as tertiary amines, alkyl chains and unsaturated groups. The rearrangement of these reactive species leads to a complex branched polycondensation product [43]. Such type of polyesteramide can be used as crosslinkers for coatings, toner resins, high solid air drying alkyds and surfactants[43].

Polyethylene imines is a special type of hyperbranched resin, based on homopolymers of ethyleneimine having general formula – (-CH2 -CH2 - NH-)n – where, n= 10 – 106[44] . Such type of resin has highly branched, spherical with well- defined ratio of primary, secondary and tertiary amine functionswhich is about 1:1:1 ratio. These functionalised grades are commercially available as a good dispersants for pigments as well as adhesion promotes to any polar surfaces [45].

Recent research trend for development of hyperbranched polymer for coating application

The recent research trend for the development of hyperbranched polymer mainly focused on high solid hyperbranched air drying alkyd, high solid hyper branched alkyd for one pack polyurethane system, high solid hyperbranched alkyd/polyester for two pack polyurethane, hyperbranched polyester amide, hyperbranched polyester resin for water dispersible system, hyperbranched polyester for UV curable system, Castor oil based hyperbranched polyurethane resin etc. The main objectives of these developments are to develop high performance coating with respect to properties i.e. mechanical, chemical, outdoor durability as well as minimization of VOC.

High solid hyperbranched air drying alkyd

Manczyk et.al.reported a high solid hyperbranched resin synthesised from trimethylolpropane and dimethylol propionic acid[46] . In this polyester resin the terminal hydroxyl end groups undergoes esterification reaction with high linoleic acid content unsaturated fatty acid and form hyperbranched alkyd. It is also reported that with increasing degree of branching, the viscosity of the resin decreases, as a result solvent intake of the resin is low i.e. VOC is less.The silicone modified high solid hyperbranched alkydresin was reported by Brostowet. al[47]. . The silicone modified hyperbranched alkyd was synthesised by the etherification reaction between a hyperbranched alkyd resin and a silicone intermediate resin (Z- 6018, Dow Corning). This silicone modified hyperbranched resin was characterized by NMR spectroscopy and GPC technique. It is reported that the hydroxyl value of the resin decreases with increasing silicone content but hydrodynamic volume and glass transition temperature, Tg of the resin increases with increase in silicone content in the resin. The film properties of silicone modified hyperbranched resins are better than those of conventional alkyds with respect to adhesion (stronger interfacial interactions), drying time, hardness, gloss and gloss retention. The hyperbranched phenolic modified air drying alkyd was reported by Murillo et. al[48]. . Here, tall oil fatty acid (TOFA) based hyperbranched polyester polyol of fourth generation was modified by phenol (as blocking agent).It is reported that the gloss and hardness values of phenol modified hyperbranched resin increases with increase in phenol content. The glass transition temperature, Tg, thermal decomposition modified resin is better than that of conventional resin.Kumar and Mahato have synthesized a hyperbranched soya alkyd modifiedwith silicone intermediate (Z - 6018, Dow Corning) and 2-hydroxyethyl methacrylatefor long life exterior coating. 

      Initially temperature of phenol , silicone acrylate monomer was prepared by reacting hydroxyl terminated silicone intermediate and 2-hydroxyl ethyl methacrylate (HEMA) (scheme 1 and scheme 2 ). This silicone acrylate resin was used for the modification of novel soya alkyd resin. The silicone acrylate modified alkyd resin wascharacterized by FTIR spectroscopy and C NMR spectroscopy. It is reported that such silicone acrylate–soya alkyd resin based polymer film performsexcellent mechanical properties and exterior durability compared to silicone modified alkyd resin.

     Bat et.al. reported a hyperbranched alkyd resin synthesised by the reaction of vegetable fatty acids, dipentaerythritol, dimethylol propionic acid and benzoic acid as chain terminator . Here, dipentaerythritol was used as the core molecule of the resin, which was esterified with dimethylol propionic acid and again esterified with castor oil fatty acids. The hydroxyl groups present in the castor oil fatty acids were then reacted with linseed oil fatty acids and benzoic acid. The final characterization of the resin was done by FTIR spectroscopy and thermal properties by DSC technique. The performance of the resin in coating application was satisfactory with respect to drying, gloss, hardness, flexibility, adhesion and impact resistance.

High solid hyperbranched alkyd for one pack polyurethane system

Naik et. al. reported the synthesis of a hyperbranched polyol using dipentaerythritol as a core material and 2,2-bis (methylol) propionic acid as a chain extender[51] . This hyperbranchedpolyol was reacted with different amount of soya fatty acid to form hyperbranched alkyd. Such type of hyperbranched alkyd contain unreacted hydroxyl groups which can react with isophoronediisocyanate (IPDI) at NCO/OH ratio of 1.6:1 and form high solid hyperbranched urethane alkyd resin. The excess of NCO groups in the urethane alkyd resins are cured with atmospheric moisture. The free hydroxyl groups and branching structure of the resins were characterized by FTIR and 13C NMR spectroscopic analysis. It is also reported that at an opptimum ratio of fatty acid and isocyanate, the coating performs optimum solvent resistance, mechanical and weathering resistance properties. Such types of coating are particularly useful in the areas where surface preparation and moisture removal are very difficult. These coatings are generally applied under humid condition. The mechanical and gloss retention properties of such hyperbranched urethane alkyd based coatings are better than conventional alkyds due to higher degree of crosslinking and compact structure. Recently, Raju et.al. reported a series of hyperbranched polyurethane – urea coatings synthesised by two step reaction process . Initially, isocyanate terminated polyurethane prepolymers were prepared from a hyperbranched polyester polyol with isophoronediisocyanate (IPDI) at NCO/OH ratio of 1.6:1 for 5 hrs. at 70- 80°C. The excess isocyanate after the synthesis of isocyanate terminated hyper branched poly urethane prepolymer was reacted completely with atmospheric moisture (scheme 3 ).The extent of urethane urea bonds in hyperbranched urea network have been characterized quantitatively by FTIR spectroscopy. From thermogravimetric analysis it was reported that hyperbranched polyurethane – urea coating showed higher glass transition temperature and two steps decomposition with good thermal stability but the conventional polyester sample showed single step decomposition profile.

High solid hyperbranched alkyd/polyester for two pack polyurethaner

Naik and Ratna reported a high solid hyperbranched polyurethane alkyd formed by mixing hyperbranched alkyd and isocyanate trimer[53] .They synthesised a second generation hyperbranchedpolyol by using dipentaerythritol as core material and di methylol propionic acid as chain extender. This hyperbranched polyol was reacted with linseed oil fatty acid to make a series of hyperbranched alkyd ( scheme 4 and scheme 5 ). These hyperbranched alkyds contain varying amount of hydroxyl groups which were cured with HDI (Desmodur N 3390) depending on NCO : OH ratio. The performance of the coating was evaluated by measuring adhesion, tensile strength, scratch hardness, abrasion resistance, flexibility, impact resistance etc. The weathering properties, resistance to corrosion, salt spray, sea water immersion and humidity were also performed on the coated specimen. It is reported that, at an optimum ratio of NCO:OH the cured film perform excellent enhancement of mechanical properties and weathering resistance.

Recently, Hu and Zhang synthesized an aliphatic polyester based polyurethane elastomer with hyperbranched segments from polyester diol , hydroxyl terminated hyperbranched polyester (Boltorn HB 20), IPDI and 1, 4 - butane diol (BDO) (Scheme 6)[54]. The degree of hydrogen bonding, microstructure, morphologies of these polyurethane materials were characterised by FTIR, WAXD and DSC techniques. It is reported that the polyurethane elastomers containing small amount of BoltornHB 20, enhances hydrogen bonding and mechanical properties compared to conventional polyurethane resin.

Recently, Karak et.al.synthesised a highly branched polyester resin by using monoester of nahar seed oil (Mesuaferrea L. seed oil), phthalic anhydride, maleic anhydride as A2 monomer andtrimellitic anhydride as B3 monomer[53] . A clear lacquer was made with this resin by mixing the resin in ultra sonicatoralong with styrene as reactive diluent, MEKP as initiator and cobalt octoate as activator. Such mixed lacquer when applied over panel, cured film was formed . The performance of the cured film was satisfactory with respect to impact resistance, gloss, scratch hardness, chemical resistance and anticorrosive property. Karaket.al.also reported aMesuaferrea L. seed oil based highly branched thermostable polyester nano clay composite material. Such type of highly branched polyester resin was synthesised by the condensation of dimethylol propionic acid and carboxy terminated prepolymer of Mesuaferrea L. seed oil[56] . The polyester nano clay composite was prepared by the dispersion of different dose level of nano clay into the polyester resin ( scheme 7 ). It is reported that at an optimum dose of nanoclay loading to the mixture of polyester, epoxy and hardener - the tensile strength, elongation at break, scratch hardness of the cured film was maximum. Such type of composite coating films have improved thermal resistance, mechanical strength, gloss and impact resistance property. These special properties are most suitable for development of any advance surface coating material.

Karak et. al. also reported the synthesis of hyperbranched polymer from the monoglyceride of Mesuaferrea L. seed oil, poly(caprolactone) diol, 2,4 – toluenediisocyanate and glycerol by A2+ B2 approach ( scheme 8 )[57] . The 3 synthesised hyperbranched polymer structure was characterized by FTIR, 1 HNMR and UV spectroscopic technique. It is also reported that the hyperbranched polyurethane with 30 % hard segment perform optimum properties with respect to tensile strength, impact resistance, hardness, Raju et. al. reported the polyurethane adhesion, flexibility, gloss, elongation at coating based on hyperbranched break and chemical resistance.

Raju et. al. reported the polyurethane coating based on hyperbranched polyester polyol[58] . Different hyperbranched polyester polyol were synthesised by varying the hydroxyl terminated precursors like pentaerythritol, trimethylol propane and glycerol. The degree of branching of the hyperbranched polyester was for interaction between nanoclay and polyester matrix characterized by 13C NMR spectroscopy. NCO terminated PU prepolymers were synthesised by reacting hyperbranched polyester with isophoronediisocyanate at NCO/OH ratio of 1.6:1. The excess NCO content in the NCO capped PU pre polymer was reacted with atmospheric moisture.

Synthesis of HBPU – Urea coatings from hyperbranched polyol

Hyperbranched polyester amide

   Karaket . al . synthesised hyperbranched polyester amide by using N,N- bis(2-Hydroxy ethyl) castor oil fatty amide, maleic anhydride, phthalic anhydride and isophthalic acid as A2monomers and diethanol amine as B3 monomer[53] . The chemical structure of the resinwas characterized by FTIR,'H NMR and 13C NMR spectroscopic technique. It is reported that the degree of branching and degradation temperature increases with the increase of diethanol amine in the resin formulation. Such type of hyperbranched polyester amide when mixed with epoxy resin, epoxy cured films were formed. This epoxy coating system performed very good adhesion, scratch hardness, abrasion resistance, impact resistance, solvent and chemical resistance. These epoxy systems can be used for the development of advance coating material. Karaket.al.also reported epoxy cured poly (ester amide) thermosetting material by using poly(amido amine) as hardener( scheme 9 )[60] . Such system performs better properties than cycloaliphatic amine hardener cured system. It is also reported that the use of poly (amido amine) hardener perform better thermal stability than cycloaliphatic amine hardener cured system. The chemical resistance particularly alkali resistance of epoxy poly(amido amine) cured poly(ester amide) resin is much superior over epoxy cycloaliphatic amine system.

Hyperbranched polyester resin for water dispersible system

Murillo et. al. synthesized different water borne hyperbranchedacrylated- maleinized alkyd (HBRAAM) resins by modifying a hyperbranchedalkyd resin (HBRA) with butylmethacrylate – maleic anhydride copolymers (BMA - MA) in presence of p-toluene sulfonic acids [61].

These hyperbranchedacrylated- maleinized alkyd can be made water emulsifiable by using diethanol amine ( scheme 10 and scheme 11 ). Such types of waterborne hyperbranchedacrylated – maleinized alkyd resin when used in coating application performs very good drying, adhesion, flexibility, gloss, hardness and chemical resistance property.

Hyperbranched polyester for UV curable system

Shi et.al.have synthesized a series of UV curable hyper branched polyurethane acrylates by modifying the hyperbranched polyester encapped with hydroxyl groups using urethane acrylate prepared from toluene diisocyanate and 2-hydroxylethyl acrylate[62] . It is reported that, the hyperbranched polyurethane acrylates are promising oligomer for UV curable coatings, inks and adhesives.

The water borne hyperbranched polyester containing multihydroxy functional aliphaticpolyestercore, which is encapped with an epoxy group of Cardura E 10 and salt like groups in different ratios was synthesised by Baoet. al. ( scheme 12 )[66] .The core is second generation hyperbranched polyester containing Boltorn H20 with approximately 16 hydroxyl groups. Here the concentration of salt like groups in the resin, controls the water solubility of the resin. Such kinds of resins are mostly used for UV curable wood coating application.

Shi et al. also reported the same type of UV curable water borne hyperbranched polyester from multi hydroxyl functional hyperbranched aliphatic polyester core encapped with methacrylic acid[63].

Castor oil based hyperbranched polyurethane resin

Karak et.al.have synthesized a series of castoroil based hyperbranched polyurethane resin by A2+B3 approach. Here castor oil or monoglyceride of castor oil used as the hydroxyl containing B3 reactant and toluene diisocyanate (TDI) or diphenyl methane diisocyanate (MDI) as an A reactant, 1,4- 2 butane diol or polyethylene glycol (PEG) as the chain extender and poly(caprolactone)diol as a macroglycol [64,65]( scheme 13 ). The resin structures werecharacterized by FTIR and NMR spectroscopic analysis, the morphology of the resin was characterized by wide angle X ray diffraction and Scanning electron microscopic study. The degree of branching of the polymer was calculated by 'H NMR spectra. The hard segment content in the polymer structure, affect thermal degradation and crystallization of the hyperbranched polymers. Such type of hyperbranched urethane resin when used in surface coating application perform good gloss, tensile strength, elongation at break, scratch hardness and chemical resistance property.


Hyperbranched polymers are novel and versatile due to their unique chemical and physical properties as well as their potential applications in different fields. The unique architectural features of highly branched structures with large number of surface active functional groups, less chainentanglement, high surface functionalities help to possess a large variety of unusual properties like low viscosity, high reactivity, compact structure etc. The chemical modification or functionalization of hyperbranched polymers help to develop silicone modified hyperbranched air drying alkyd, silicone acrylate modified alkyd for long term exterior coating application, moisture cured one pack high solid polyurethane resin, high solid two pack polyurethane resin for high performance protective coatings, high performance nanocomposite coating, waterdispersible and UV curable coating systems etc. Commercially available hyperbranched polymers like polyester, polyesteramide and polyethylene imine can be chemically modified to develop a range of pigment dispersants and stabilizer at low viscosity level.

Finally, it is one of the most exciting and young field of research area and there is lot of scope for developmental work as well as commercialization of any type of new product in this field.


The author would like to thank Mr. B. Bera , Mentor , Research and Development, Mr. T. K. Dhar, Vice President, Research and Development, for their inspiration to publish this article. The author is also grateful to the colleagues, Mr. S. Ray and Mr. S. Chakrabarti for their support to prepare this paper.


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