Synthesis and Characterization of Phenoxy Modified Epoxy Blends

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1 Available online at Synthesis and haracterization of Phenoxy Modified Epoxy Blends Varun Dixit, Arun Kumar Nagpal, Reena Singhal.* Department of Plastic Technology, arcourt Butler Technological Institute, Kanpur , India. ABSTRAT: The blends of Tetraglycidyl-4,4 -Diaminodiphenyl Methane (TGDDM) and Diglycidyl Ether of Bisphenol-A (DGEBA) type of epoxy resin synthesized by using different amount of engineering thermoplastic phenoxy resin (20 and 30 phr.). These blends were cured with aromatic amine type curing agent i.e. 4,4 -Diaminodiphenyl Methane (DDM) and 4,4 -Diaminodiphenyl Sulphone (DDS). The resulting cured blend was characterized by Scanning Electron Microscopy (SEM) and Fourier Transform Infra Red Spectroscopy (FTIR). The activation energy (Ea) for thermal degradation, thermal stability, Limiting xygen Index (LI) values and degradation kinetics of the blends were studied by Thermo gravimetric analysis (TGA) using oats and Red-fern method. The experimental results showed that the glass transition temperature (Tg) and activation energy (Ea) of different DGEBA/phenoxy cured blends increased when the phenoxy content was increased from 20 and 30 parts per hundred resin (phr.). DS and SEM analysis of different blends showed that phase separation occurred in all TGDDM/phenoxy cured blends; as two glass transition temperatures were observed in these blends. Keywords: epoxy resin, phenoxy resin; glass transition temperature; blends; phase separation; thermal stability. 1.0 INTRDUTIN Epoxy resins, one of the most important thermosetting polymers, are currently used in advanced composites, coatings, structural adhesives, and microelectronics, due to their high stiffness, high strength, good chemical resistance, dimensional stability and excellent electrical insulation properties [1,2]. ptimum performance and properties are obtained by crosslinking the epoxy resin into a three-dimensional, insoluble, and infusible network by reacting it with a suitable curing agent such as Phthalic Anhydride (PA), 4,4 - Diaminodiphenyl Methane (DDM) and 4,4 -Diaminodiphenyl Sulphone (DDS) etc. A major limitation of neat epoxy resins, particularly those for high-temperature application is their inherent brittleness arising from the cross linked structure. To circumvent such shortcoming, toughening of brittle epoxy resin has been undertaken extensively by incorporating liquid rubber such as carboxyl terminated butadiene acrylonitrile copolymers into the cross linked polymer [3]. Recently new attempts have been utilized in toughening the epoxy resin by using rigid thermoplastic materials like poly (ethylene oxide) (PE), poly (ether ether ketone) (PEK-) and polyesters [4-6] etc. which can enhance their toughness without sacrificing glass transition temperature and other desirable properties. Bucknall and co-workers were probably the first to initiate this interesting area of research by blending poly (ether sulphone) with epoxy resin networks [7]. The Epoxy/phenoxy blends are an interesting area of research since phenoxy resin contains same repeating unit as present in epoxy excepting oxirane ring. orresponding Author: reena_singhal123@rediffmail.com 69

2 Figure 1 shows the structures of epoxy and phenoxy resin and two curing agents i.e. DDM and DDS. Phenoxy resins are proton donating, high molecular weight thermoplastic polymer having pendent hydroxyl groups in their backbone chain. They are thermally stable material that can be processed at high temperature. Their ether linkages and pendent hydroxyl groups promote wetting and bonding to polar substrates and fillers. It is well known that epoxy resins may be cured by many types of hardeners, among which aromatic amines retain a prominent position due to enhanced environmental stability [1]. The primary aromatic amine type curing agents are more thermally stable than aliphatic amines due to the stable benzene ring which is tightly crosslinked into the cured structure. At a stochiometric ratio of epoxy resin and amine, reaction between the oxirane ring and primary amine group produce chain growth. Later reactions with secondary amines build chain branches and finally the system will gel and form a network n 3 Diglycidyl Ether of Bisphenol-A 2-2 N- -N Tetraglycidyl-4,4'- Diaminodiphenyl Methane n Phenoxy N- -N N- S -N 4,4'-Diaminodiphenyl Methane 4,4'-Diaminodiphenyl Sulphone Figure 1: Structures of Raw Materials The main objective of this research work was to examine the effect of phenoxy content on blending with two type of epoxies i.e. DGEBA and TGDDM. The resulting blends were cured with a stoichiometric amount of curing agents i.e. DDM and DDS. The FTIR analysis has been done to evaluate the possible changes in functional groups of resulting blends. The glass transition temperatures were determined for different epoxy/phenoxy cured blends through DS thermograms. The morphological studies of different epoxy/phenoxy cured blends after curing were carried out by SEM to investigate the phase behavior of blends. Thermogravemetric analysis (TGA) was used to determine the thermal stability, LI values and degradation kinetics of different epoxy/phenoxy cured blends. 70

3 2.0 EXPERIMENTAL 2.1 Materials Malaysian Polymer Journal, Vol. 5, No. 2, p 69-83, 2010 The epoxy resins used in this study were Tetraglycidyl-4,4 - Diaminodiphenyl Methane (TGDDM), XR-23, (weight per epoxide g) supplied by M/s Atul Ltd., (Gujarat, India) and Diglycidyl Ether of Bisphenol-A (DGEBA), PG-100 (weight per epoxide 175) supplied by M/s Parikh Resin Ltd., (Kanpur, India). Phenoxy resin (PKM- 301) was supplied by M/s In hem. Inc. (Rockhill, S). The 4,4 -Diaminodiphenyl Methane (DDM) and 4,4 -Diaminodiphenyl Sulphone (DDS) were obtained from M/s Aldrich hemical o. The structures of raw materials used in this study are shown in Figure Synthesis of Blends The TGDDM/phenoxy and DGEBA/phenoxy blends were prepared by melt blending technique; and their compositions are shown in Table 1. The obtained blends were cured with aromatic amine curing agent DDM and DDS in stoichiometric ratio of 30 per hundred parts of epoxy resin. The reaction was carried out in a thermostatic oil bath to maintain the constant temperature. Initially the phenoxy resin was weighed and meltdissolve (no solvent) in pre-determined quantity of liquid epoxy resin at temperature of for 2 hours [13]. After this the desired amount of curing agent (dried at 80 0 ) was added into the synthesized blend and mixed with help of stirrer because polymer starts to become gel above this temperature, until ensuring the homogenous and visually transparent mixture. Table 1: Feed omposition of DDS & DDM-cured TGDDM / Phenoxy blends in varying amount of phenoxy content Blend System Ratio Amount of Epoxy (gm) D1 DGEBA/phenoxy/DDS 100:20:30 50 D2 D3 D4 T1 T2 T3 T4 DGEBA/phenoxy/DDM DGEBA/phenoxy/DDS DGEBA/phenoxy/DDM TGDDM/phenoxy/DDS TGDDM/phenoxy/DDM TGDDM/phenoxy/DDS TGDDM/phenoxy/DDM 100:20:30 100:30:30 100:30:30 100:20:30 100:20:30 100:30:30 100:30: Amount of Phenoxy (gm) Amount of ardener (gm) 2.3 asting of Test Specimen: After ensuring the complete dissolution of curing agent, the blend was immediately poured into a specially designed Teflon and steel mold. The steel base plate was highly polished with hard-chrome layer. The blend were cured at for 2 hours and for 2 hours and post-cured at for 5 hours [9]. This might be attributed to step increase in cure temperature is given in order to avoid excessive heat of cure generation resulting in crack. After this samples were stored in desiccators for evaluation of various properties. 71

4 2.4 ARATERIZATINS Malaysian Polymer Journal, Vol. 5, No. 2, p 69-83, Fourier Transform Infrared Spectroscopy A Perkin-Elmer FTIR spectrophotometer was used to record the IR spectrum ( cm -1 ) of monomers and cured samples. Analysis were done by using solid potassium bromide pellets Differential scanning calorimetry DS was carried out using a Perkin Elmer (Pyris Diamond) differential scanning calorimeter under nitrogen atmosphere. Scans were obtained under dynamic conditions with heating rates of 10 /min. from 100 to Scanning electron microscopy The surface morphology of synthesized amine cured epoxy/phenoxy blend samples was examined under scanning electron microscope (SEM). The epoxy/phenoxy blend samples were coated with a thin layer of pure gold in S0 Sputter oater, and imaged in a SEM (LE Electron microscopy, England) Thermogravemetric Analysis Thermogravimetric analyzer of Perkin Elmer s (Pyris Diamond) was used to study the thermal degradation pattern and thermal stabilities of the cured epoxy specimens at a heating rate of 10 0 min -1 in a nitrogen atmosphere. Thermogravimetric analysis was also used as a fast and exact method for determination of the kinetics of thermal degradation of polymers. The activation energy and order of reaction (n) were evaluated by using integral equation of oats and Redfern [8]. (1) For all value of n except n=1; for the situation w here n=1, this equation becomes... (2) Where a is the fraction decomposed, b is the heating rate, T is the absolute temperature in Kelvin. Ea is the activation energy, R is gas constant, n is the order of reaction, Z is the pre-exponential factor. Thus, a plote of ether..(3).. (4) The activation energy was calculated from the slope of the plote.. (5) 72

5 3.0 RESULTS AND DISUSSIN Malaysian Polymer Journal, Vol. 5, No. 2, p 69-83, Fourier Transform Infrared Spectroscopy The FTIR spectra of neat DGEBA, neat phenoxy resin, and DDM-cured DGEBA/phenoxy blend (30 phr. phenoxy) are shown in Figure 2, in the range of cm -1. The FTIR spectrum of neat DGEBA resin exhibited a peak of oxirane group at cm -1 and another small peak of hydroxyl group at cm -1, shown in Fig.2a. Phenoxy resin is high molecular weight, thermoplastic polymer having pendant hydroxyl groups and no oxirane ring. ence a broad peak of hydroxyl group was observed at cm -1 in FTIR spectrum of neat phenoxy resin, shown in Fig.2b. The FTIR spectrum of DDM-cured DGEBA/phenoxy blend (D4) (Figure 2c) exhibited a depleted peak of oxirane group at cm -1. This peak was shifting in lower side at cm -1 probably due to ydrogen bonding between epoxy ring and hydroxyl group of phenoxy cm cm cm cm cm -1 Figure 2: FTIR spectrum of (a) Diglycidyl ether of bisphenol-a (DGEBA) (b) Phenoxy resin (c) DGEBA/phenoxy/DDM blend (D4 The FTIR spectrum of DGEBA shows the peak of oxirane ring at cm -1.The hydroxyl group of phenoxy resin reacts with oxirane group of epoxy resin and form linear chain compound of epoxy/phenoxy blend. The synthesized blends were cured with aromatic primary amine to produces a hydroxyl group which was exhibited a peak at cm -1, attributed to stretching vibration of the - group [Scheme 1] and this reaction is a typical curing reaction of epoxy resin with an amine hardener [1]. The Scheme 1 describes the blending of epoxy and phenoxy resin and curing reaction of epoxy resin by using aromatic amine separately. In FTIR spectrum of present blend (D4) (Figure 2c), the peak of hydroxyl group was observed at cm -1. The peak of hydroxyl group in the blend (D4) shifts from cm -1 to cm -1 i.e. in lower side with high intensity in comparison to neat DGEBA and neat phenoxy resin. This shift in lower side is due to hydrogen bonding 73

6 between various groups of phenoxy and epoxy as well as oxirane ring. The probable curing reaction involved between (a) epoxy with amine [1] and (b) epoxy with phenoxy [9] is shown below in scheme-1. (a) R'N R R'N 2 R 2 R R'N 2 R 2 R (b) PENXY + (Phenoxy resin) 2 Epoxy resin PENXY 2. (Epoxy resin) Scheme 1: Probable curing reaction of (a) epoxy with amine, (b) epoxy with phenoxy. 3.2 Differential scanning calorimetry In this work, DS was used for determine the glass transition temperature of different epoxy/phenoxy blends. Typical dynamic DS scans at 10 0 min -1 of DGEBA/phenoxy blends (D1, D2, D3 and D4) and TGDDM/phenoxy blends (T1, T2. T3 and T4 respectively) with different amount of phenoxy content are shown in Figure 3 and 4 and summarized in Table 3. The DS thermograms of DGEBA based blends revealed that the Tg. increased from to in DDS-cured blends (D1 & D3) and to in DDM-cured blends (D2 & D4) which are shown in Figure 3. In comparison to the DDS, the DDM-cured blends exhibited lower Tg s indicating better flexibility of synthesized blends. The DS thermograms of TGDDM based blends are shown in Figure 4, for T1 & T3 (DDS-cured blends) and T2 & T4 (DDM-cured blends) which exhibited opposite trends in comparison to DGEBA based blends. From Figure 4, it is clear that all TGDDM based blends exhibited two Tg s, indicating phase separation in the blend. The high viscosity of TGDDM resin was due to its high molecular weight which responsible for poor mixing with phenoxy resin leading to phase separation. In DDS-cured blends (T1 & T3), the first Tg. increased from to with increasing phenoxy resin from 20 to 30 phr. The first Tg. for DDM-cured blends (T2 & T4) also increased greatly from to with increasing phenoxy at the same ratio. The second Tg also increased from to for DDS-cured blends (T1 & T3) and to for DDM-cured blends (T2 & T4) with increasing phenoxy from 20 to 30 phr. An opposite pattern was observed in comparison to DGEBA based blends that DDM-cured blends exhibited higher Tg s, in comparison to DDS-cured blends. 74

7 In a first case all of the DGEBA/phenoxy blends were miscible and no phase separation occurred due to secondary hydrogen bonding interactions. (d) (c) (b) (a) Figure 3: DS thermograms of DDS and DDM-cured DGEBA/phenoxy blends (a) D1 (b) D2 (c) D3 (d) D4, at heating rate of 10 0 /min. Figure 4: DS thermograms of DDS and DDM-cured TGDDM/phenoxy blends (a) T1 (b) T2 (c) T3 (d) T4, at heating rate of 10 0 /min. 75

8 Available online at Table 2: Decomposition behavior of dds and ddm-cured epoxy/ phenoxy blends at a heating rate of 10 0 min. -1 Sample Designation % of Phenoxy ontent (phr.) Initial Decomposition Temp. (Ti) ( 0 ) Maximum Decomposition Temp. (Tm) ( 0 ) Final Decomposition Temp. (Tm) ( 0 ) har Yield (Yc) (%) at LI (%) Activation Energy (Ea) (K.J./mole) D D D D T T T T

9 Table 3: Glass transition temperature of dds and ddm-cured epoxy/ phenoxy blends at a heating rate of 10 0 c min. -1 Sample % of Phenoxy Glass Transition temperature ( 0 ) Designation ontent (phr) Tg1 Tg2 D D D D T T T T The DS study showed that each DGEBA/phenoxy blend had single compositiondependent Tg thus indicating single phase nature which is also proved by SEM micrograph of the blend shown in Figure 5a. In a second case, all of TGDDM/phenoxy blends were partially miscible which produces phase separated networks. Some amount of phenoxy was not dissolved in TGDDM epoxy resin due to the high viscosity of TGDDM resin. The results proved that each TGDDM/phenoxy blend had two Tg s indicating phase separated network and partially miscible blend as shown in Figure 5b. (a) (b) Figure 5: Scanning electron micrographs of (a) DGEBA/phenoxy/DDM blend (D2) (b) TGDDM/phenoxy/DDM blends (T2). 3.3 Scanning electron microscopy The morphology and phase separation of various DGEBA/phenoxy and TGDDM/phenoxy cured by DDS was studied by SEM. The Scanning electron micrograph of surface of DGEBA/phenoxy/DDS cured blend (D2) at magnification-200x is shown in Figure 5a and TGDDM/phenoxy/DDS blend (T2) at similar magnification-200x is shown in Figure 5b. Figure 5a reveals that (D2) blend (30 phr. phenoxy content) has homogenous morphology as phenoxy was miscible in this blend possibly due to strong molecular interactions between the hydroxyl groups and oxirane ring. Guo et al. [10] investigated the effect of different type of curing agents on phase behavior of epoxy/phenoxy blends and found that phenoxy was miscible with uncured bisphenol-a type epoxy resins i.e. diglycidyl ether of bisphenol-a as shown by existence of single Tg. The miscibility due to hydrogen bonding was also indicated by FTIR studies, as discussed earlier. The SEM of TGDDM/phenoxy/DDS blend (T2) with 30 phr. phenoxy is shown in Figure 5b indicates that phenoxy was partially miscible and phase separation 77

10 occurred in synthesized blend due to high molecular weight and high viscosity of TGDDM resin [10]. A non uniform granular structure was also observed due to high molecular weight of TGDDM resin. The effect of curing agent on morphology of TGDDM/phenoxy/DDS cured blend with different amount of phenoxy content from 10 to 40 phr was investigated by Woo et al. [11]. They observed that all the blends had phase separation, and the blend with highest phenoxy content exhibited the highest phase separation. 3.4 Thermal stability of epoxy/phenoxy blends Generally, it is accepted that the reliable degradation temperature and kinetic parameters, such as initial decomposition temperature (Ti), the temperature of the maximum degradation (Tm), and the activation energies for decomposition (Ea) can be used to assess a material s lifetime [12]. owever, the initial decomposition temperature is an important factor for determination of thermal stability of any system. The initial decomposition temperature (Ti) generally represents the thermal stability of polymers. The initial decomposition temperature is to be considered at 2.5 % weight loss of epoxy/phenoxy blends. Thermal stability of epoxy/phenoxy system is of great importance in part of chemical structure and bonds as well as the crosslink density of the system. huan Shao Wu et al. [13] investigated the thermal stability of epoxy resin containing flame retardants based on silicon, phosphorus and melamine. They assessed two parameters: initial decomposition temperature (IDT) and integral procedure decomposition temperature (IPDT). Thermal degradation behavior of DDS and DDM-cured DGEBA/phenoxy blends and TGDDM/phenoxy blends was studied by thermogravimetric analyzer as shown in Figure 6 and 7. In all epoxy/phenoxy blends, one step degradation mechanism was observed during the examination of thermograms. It is apparent that the shape of thermograms is not linear throughout the degradation reaction in all cases. In the initial stages (1-2.5 % weight loss), the rate of weight loss was comparatively very slow from the initial degradation temperature 337 0, 353 0, and at 2.5% weight loss for the blends D1, D2, D3, and D4 and 250 0, 264 0, 263 0, and at 2.5% weight loss for the blends T1, T2, T3, and T4 respectively. In final stage, rate of weight loss ( %) was significant at higher temperatures. While at higher weight loss (above 85 %), the rate of weight loss again very slow up to final decomposition temperatures of 494 0, 510 0, and for the samples D1, D2, D3, and D4 and 517 0, 469 0, and or the samples T1, T2, T3 and T4, respectively. These key facts indicate that, in the initial stage of polymer decomposition, the extent of crosslinking is greater than the extent of bond breaking, while at middle stage of decomposition rate is highest. owever, in the last stage of decomposition, a similar trend was observed like as initial stage of decomposition. The temperature of initial decomposition (Ti), the temperature of maximum decomposition (Tm), the temperature of final decomposition (Tf), and the percentage of char yield (Yc) at were determined from TGA curves as shown in Figure 6 and 7. From the data of DGEBA/phenoxy blends and TGDDM/phenoxy blends it is clear that thermal stability increased with increasing phenoxy content, but TGDDM/phenoxy blends showed lower thermal stability than DGEBA/phenoxy blends, due to poor miscibility and less chemical interactions between TGDDM and phenoxy. It is clear from Figure 6b that a regular and homogenous morphology was observed in DGEBA/phenoxy blends. Park et al. [14] observed that the Diglycidyl ether of bisphenol-a/tetraphthalic anhydride/n,ndimethylbenzylamine system showed higher initial decomposition temperature than that of the Diglycidyl ether of bisphenol-a/phthalic anhydride/n,n-dimethylbenzylamine system, due to relative low crosslink density of the DGEBS/DMBA system. 78

11 Ti Tm Tf Figure 6: Thermal degradation by thermogravemetric analysis in nitrogen atmosphere of DDS and DDM-cured DGEBA/phenoxy blends (a) D1 (b) D2 (c) D3 (d) D4, at heating rate of 10 0 /min. Figure 7: Thermal degradation by thermogravemetric analysis in nitrogen atmosphere of DDS and DDM-cured TGDDM/phenoxy blends (a) T1 (b) T2 (c) T3 (d) T4, at heating rate of 10 0 /min. In present investigation the char yield was tending to be dependent on the structure of epoxy/phenoxy cured blends. The char yield for DGEBA/phenoxy blends (D1, D2, D3, and D4) was calculated as 12.7, 12.2, 14.6 and 12.6 respectively. The char yield for TGDDM/phenoxy blends (T1, T2, T3, and T4) was also calculated as 21.92, 25.03, and respectively. The char yield was found higher for TGDDM/phenoxy blend due to large and bulky molecule of TGDDM resin and increased with increasing phenoxy content in TGDDM/phenoxy cured blends. The char yield can be used as criteria for evaluating limiting oxygen index (LI) of the resin in accordance with Van Krevelen and oftyzer equation []. LI = Yc (6) Where Yc = char yield 79

12 The LI values for blends calculated by the above equation were found to be more than 22.38, as shown in Table Degradation kinetics of epoxy/phenoxy blends TGA was also used to study the thermal degradation kinetics of polymers. The statistical analysis of degradation behavior was studied by integral method of oats and Redfern [8] equation, done by linear regression, provided by the best fit value of n, where n was taken as 0, 0.5, 0.67, 1, and 2. The degradation kinetics was analyzed by using the above order of reaction for different blends in the range of 2.5 to 85 % weight loss of thermograms. The results revealed that degradation follows first order of reaction with correlation coefficient values of , , , and , for the blends D1, D2, D3, and D4 and , , , and , for the blends T1, T2, T3, and T4 respectively. From the value of correlation coefficients, close to unity indicate the best fit. The graphs plotted between the log10 [1-(1-a) / T 2 ] and 1000/T for first order of reaction are shown in Figure 8 and 9. From the slope of plots and its intercept value, the activation energy (Ea) was calculated and summarized in Table 2. In DDS-cured DGEBA/phenoxy blends, the activation energy drastically increased from to K.J./mole. Similarly it increased from 1.96 to K.J./mole in DDM-cured DGEBA/phenoxy blends. The opposite pattern was observed in all TGDDM/phenoxy blends. The activation energy decreased from to K.J./mole for DDS-cured TGDDM/phenoxy blends (T1 & T3). Similarly activation energy decreased from to K.J./mole for DDM-cured TGDDM/phenoxy blends (T2 & T4) with increasing phenoxy from 20 to 30 phr. Figure 8: Plot for decomposition of DDS and DDM-cured DGEBA/phenoxy blends (a) D1 (b) D2 (c) D3 (d) D4, at heating rate of 10 0 /min. 80

13 Figure 9: Plot for decomposition of DDS and DDM-cured TGDDM/phenoxy blends (a) T1 (b) T2 (c) T3 (d) T4, at heating rate of 10 0 /min. Table 2 reveals an interesting result that energy of activation for DGEBA based blends was nearly double than that of observed for TGDDM based blends. This indicate that DGEBA based blends are thermally more stable due to better miscibility and chemical reaction/ crosslinking between epoxy and phenoxy resin. Another finding is that DDScured DGEBA/phenoxy blends had higher activation energy than DDM-cured DGEBA/phenoxy blends. Adversely, the TGDDM based blends exhibited the higher activation energy with DDM. 4.0 NLUSIN From the above study of synthesized various DDS and DDM-cured epoxy/phenoxy blends, it can be concluded that: The DGEBA/phenoxy blends have single Tg but in all TGDDM/phenoxy blends, oppositely two Tg s were observed. The DDM-cured TGDDM/phenoxy (30 phr. phenoxy) blend has the highest Tg value. The DDM-cured DGEBA/phenoxy blend (D2) was found to be miscible and shows the homogenous morphology but in DDM-cured TGDDM/phenoxy blend (T2), phenoxy was partially miscible and phase separation occurred. The temperature of initial degradation was significantly higher for DGEBA based blends in comparison to TGDDM based blends. The activation energy ( K.J./mole) was observed the highest for DDS-cured DGEBA/phenoxy blend (D3). The char yield (26.28%) was the highest for DDS-cured TGDDM/phenoxy blend (T4). Acknowledgement The authors would also like to thank Dr. Pankaj Verma (Atul Ltd., Gujarat) for providing us some required raw materials. The authors gratefully acknowledged to Mr. Anil Kumar Saini and Smt. Rekha, II; IIT, Roorkee for the thermal analysis and SEM 81

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