NIET JOURNAL oFENGINEERING &TECHNOLOGY Winter 2011 co N co L!) I Q) N N N Z C/) C/) Study on the curing kinetics of epoxy resins using diorganotin dichlorides Vinay Kumar' Abstract Curing kinetics of diglycidyl ether of bisphenol-A (DGEBA) in the presence of Phenylmethyltin dichloride (PMTC), Phenylethyltin dichloride (PETC), butylmethyltin dichloride (BMTC) and dimethyltin dichloride (DMTC) were investigated by the dyanamic differential scanning calorimetery. Keywords: Diglycidyl ether of bisphenol-A (DGEBA), dichlorides, curing kinetics, thermal stability. 'Jagdishprasad Jhabarmal Tibrewala University, Jhunjhunu Rajasthan 333001 'Jagdishprasad Jhabarmal Tibrewala University, Jhunjhunu Rajasthan 333001 'Department of Chemistry, IILM AHL CET, Greater Naida 201306 Lalit Kuma/ Gagan Deep] Introduction Epoxy resins have been used in many industrial applications such as in surface coating, adhesive, structural composites, printed circuit boards and insulating materials for electronic devices because of their good chemical resistance and superior mechanical and electrical properties[1 ,2]. These resins also possess excellent process ability. However, the conventional epoxy resins are inefficient to satisfy the required properties in the field of advanced materials which require higher thermal and flame resistance [3,4]. Several approaches were used in the past to improve the thermal stability and flame retardance of the epoxy resins. Improvement in the thermal properties of the epoxy resin by the incorporation of polynuclear aromatic structures such as naphthalene, biphenyl, anthracene, pyrene etc [5-7]. or phosphorus containing moieties [8-11] has been reported in literature. Another approach used to develop flame resistant epoxy formulation involves the incorporation of additives having flame- retarding elements such as halogens, boron, silicon, tin, arsenic or antimony compounds or their combinations [12-14]. Apart from the structure of curing agents, properties of epoxy formulations can also be varied to a large extent by the choice of curing conditions. Kinetic characterization of thermoset resins is therefore of great importance in understanding the structure-properties-processing relationship in
Transcript
NIETJOURNALoFENGINEERING&TECHNOLOGY
Winter 2011coNcoL!)
IQ)NNNZC/)C/)Study on the curing
kinetics of epoxy resinsusing diorganotin dichloridesVinay Kumar'
Abstract
Curing kinetics of diglycidyl ether of bisphenol-A
'Department of Chemistry, IILM AHLCET, Greater Naida 201306
Lalit Kuma/ Gagan Deep]
Introduction
Epoxy resins have been used in manyindustrial applications such as in surfacecoating, adhesive, structural composites,printed circuit boards and insulating materialsfor electronic devices because of their goodchemical resistance and superior mechanicaland electrical properties[1 ,2]. These resins alsopossess excellent process ability. However, theconventional epoxy resins are inefficient tosatisfy the required properties in the field ofadvanced materials which require higherthermal and flame resistance [3,4]. Severalapproaches were used in the past to improve thethermal stability and flame retardance of theepoxy resins. Improvement in the thermalproperties of the epoxy resin by theincorporation of polynuclear aromatic structuressuch as naphthalene, biphenyl, anthracene,pyrene etc [5-7]. or phosphorus containingmoieties [8-11] has been reported in literature.Another approach used to develop flameresistant epoxy formulation involves theincorporation of additives having flame-retarding elements such as halogens, boron,silicon, tin, arsenic or antimony compounds ortheir combinations [12-14]. Apart from thestructure of curing agents, properties of epoxyformulations can also be varied to a large extentby the choice of curing conditions. Kineticcharacterization of thermoset resins is thereforeof great importance in understanding thestructure-properties-processing relationship in
order to manufacture high performancematerials.
In view of the above, it was thought worthwhile todevelop flame resistance epoxy resins and tostudy the curing and thermal behaviour of epoxyresin (DGEBA) in the presence of differentdichlorides containing tin.
Experimental
Materials
Diglycidyl ether of bisphenol-A (DGEBA, GradeLY556 having an epoxy equivalent 177) wasprocured from Hindustan Ciba Geigy Ltd. Methylethyl ketone, PhSnCI3, Et4Sn, Me4Sn, Me3SnCIand BuSnCI3 (Aldrich) have been used as such.PhMeSnCI2,PhEtSnCI2, Me2SnCI2 andBuMeSnCI2 have been prepared as reported inliterature [15].
Curing Studies
TA 2100 thermal analyzer having a 910 DSCmodule was used to record DSC scans at aheating rate of 5, 10, 15 and 200C/min. For curingstudies, samples were obtained by mixingDGEBA with stoichiometric amounts ofunsymmetrical diorganotin dichloridesdissolved in methyl ethyl ketone. After thoroughmixing, the solvent was removed under vacuumand the freshly prepared samples were used forrecording DSC traces in static air atmosphere ata programmed heating rate from roomtemperature to 300°C. 5 ± 2 mg of samples wereused in each experiment. The curing mixtures ofDGEBA and diorganotin dichlorides (PMTC,PETC, DMTC and BMTC) have been designatedas DPMTC, DPETC, DDMTC and DBMTCrespectively.
Thermal Stability
Thermal stability of the resins cured isothermallyby heating at 100 ± 20°C for 2h in an air oven inthe presence of various diorganotin dichlorideswas evaluated by recording TG / DTG traces innitrogen atmosphere (flow rate 60cm3/min) usinga Rheometric Thermal Analyzer. A heating rate of10°C rnin and powdered samples of 10 ± 2 mgwere used in each experiment
Results and Discussions
In the DSC scans of all the samples, a broad
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exothermic transition was observed. The curingexotherm was characterized by noting thefollowing temperatures:
Tj = kick-off temperature, where the curing starts
Tonse! = temperature where the first detectableheat is released. It was obtained by extrapolationofthe steepest portion ofthe curve
Tp= temperature of peak position of exotherm
T, = temperature of the end of curing exotherm
6H = heat of curing, calculated by measuringarea under the exothermic transition
A single exotherm was obtained in all samples.The peak exotherm temperature was higher inthe case of DBMTC as compared to sampleDPMTC, DPETC and DDMTC. The heat ofpolymerization was decreased as the heatingrate was increased.
All the characteristic curing temperatures i.e. T,To, T, and T, showed the effect of the dichloridesstructure. The following trend was observed inthe peak exotherm temperatures:DPMTC < DPETC < DDMTC < DBMTC. Thecuring characteristics were expected to bedependent upon the electrophilicity of the tin andinduction of the alkyl or aryl group bonded to tinin the various dichlorides. The alkyl groups have+ ve induction (+ I) where as aryl groups(benzene) have a - ve induction (- I). On thebasis of the electrophilicity and induction onewould have expected the highest curingtemperatures with BMTC.
DGEBA was cured isothermally in the presenceof different dichlorides by heating in an air oven(100 20°C for 2 h) to determine the glasstransition temperature. In the DSC scans ofisothermally cured samples, no exothermictransition was seen thereby indicating thecomplete cross-linking reaction. The glasstransition temperature is dependent on therigidity of polymer backbone as well as on thecross link density. Highest value of Tg wasobtained for cured resin DPETC.
Curing Kinetics
Kinetic parameters of the curing reaction can beobtained from dynamic DSC scans of isothermalexperiments. The dynamic method was used in
NIETJOURNALoFENGINEERING&TECHNOLOGY
the present study and the OSC scans wererecorded at different heating rates for thesamples. The characteristic curing temperaturefor the OGEBA at different heating rates in thepresence of different dichlorides are
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activation energy was calculated from the slopeof the graphs obtained and results aresummarized in Table 1.
Activation energy of curing was lowest in case of
Sample Molar ratio of Heating Tj Tonset Tp Tr ~H Ea TgDesignation DGEBA: Rate (0C) (0C) (0C) (0C) (Jig) (KJ/mol) (0C)
The activation energy ofthe curing reaction wascalculated using Ozawa's method [16-17]assuming that:
1. The peak exothermic temperature (Tp)represents a point of constant conversion
2. The reaction follows the first order kinetics
3. The temperature dependence of the reactionrate constant obeys Arrhenius' equation
The data from dynamic OSC measurements areanalyzed in accordance with the followingequation:
R t:.log KEa
0.4567 t:. (llTp)
Where K is the heating rate, Ea the activationenergy, R the gas constant. Assuming a constantconversion at the peak exotherm temperature,plots of log K vs. 1/Tp were obtained for all theresin samples and are shown in Fig. 3. The
(OPMTC) and was highest in case of (OBMTC).Thus the activation energy of curing showed thefollowing trends in the presence of variousdiorganotin dichlorides.
OBMTC >OOMTC>OPETC >OPMTC
Thermal Stability
The relative thermal stability of the cured resinswas evaluated by comparing initialdecomposition temperature (lOT), temperatureof maximum rate of mass loss (Tmax), finaldecomposition temperature (FOT) and per centchar yield at 800°C. The results are summarizedin Table 2. All the samples were stable up to235°C. A single step degradation was observedin all the samples. The char yield was lowest incase of resin cured with OMTC and highest incase of resin cured with PETC. Thus thefollowing trend was observed in char yield of thecured resins:
OPETC>OPMTC> OBMTC>OOMTC
Char yield can be used as a criteria forevaluating limiting oxygen index (LOI) of theresins in accordance with Van Krevelen andHoftyzer equation [18].
Table 2: Results of thermal stability of isothermalcured epoxy resins
(DGEBA: R1R2SnCI2) at 1DOC heating rate innitrogen atmosphere
All the samples had LOI values calculated basedon their char yield higher than 28. These resultsthus clearly showed that flame resistant DGEBAresin can be obtained by using curing agentshaving a combination of tin and chlorine as flameretarding elements.
Conclusions
Thermal stability of the cured resin was found tobe dependant on the structure of the network.Curing agents having Sn and CI elementsshowed the effect of structure on the curingkinetics as well as on the thermal stability andhave increased the flame retardancy of the epoxyresins. 00
ReferencesLee H & Neville K Eds, Handbook of Epoxy Resins,McGraw-Hili, New York, 1972.
2 Lubin G Ed & Nostrand Van Reinhold, Handbook ofComposites, New York, 1982.
3 Iko K, Nakamura Y, Yamaguchi M and Imamura N,IEEE Elect Insul Mag, 6 (1990) 25.
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