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Malaysian Polymer Journal, Vol. 5, No. 2, p 84-98, 2010 Available online at www.fkkksa.utm.my/mpj

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Development of Inter-Crosslinking Polymer Materials From DGEBA/PDMS-Epoxy Resin Systems: Processing And Application Stu dy

M Suguna Lakshmi, B S R Reddy∗

Industrial Chemistry Laboratory, Central Leather Research Institute, Chennai, 600 020, India.

ABSTRACT: Diglycidyl ether of bisphenol A epoxy resin was modified with diglycidyl terminated polydimethyl siloxane to improve the mechanical properties. Non-phase segregated intercrosslinking was achieved through processing of the blends. High mechanical strength, good solvent resistance, and good adhesion strength exhibited was attributed to the homogeneous network structures formed between PDMS-Epoxy and DGEBA. Keywords: epoxy resins, formulations, curing, mechanical properties, peel strength 1.0 INTRODUCTION DGEBA Epoxy resins and their formulations were known to exhibit excellent adhesion, high mechanical strength, high chemical and corrosion resistance, durability in harsh environments, high heat resistance and dimensional stability properties [1]. However, the drawbacks were less moisture resistance and low impact strength properties [2]. It’s long been known that, the epoxy resins modified with different kinds of thermoplastic polymers were effective in improving the mechanical properties [3-4]. However, these polymers showed poor solubility in organic solvents that lead to difficulties in processing. During the curing, the solvent trapped in the blends gets volatilizes and therefore results in voids and irregularities in the matrix. This had reflected in decreased mechanical properties of the cured materials. Nevertheless, these problems could be overcomed in the case of siloxane polymers incorporation by the solventless method. Siloxane polymers possess high moisture resistance, high impact strength, high heat resistance, high weathering resistance, good low temperature flexibility, excellent electrical insulation, and chemical resistance properties [5-8]. Serer et al [9] have reported on the, modification of bisphenol A epoxy with the copolymers of siloxane blockers which enhanced the compatibility reduced the brittleness. But, siloxane was phase-separated due to the difference in densities, interfacial energy, chain lengths and volume fractions. They exhibited multiple glass transition temperatures due to their inherent thermodynamic incompatibility. Cassimere et al [10] have reported on the formation of homogeneous cured network of PDMS-Epoxy with DGEBA with the aid of a coupling agent. While, we have developed, a homogeneous blends from PDMS-Epoxy and DGEBA without adding any coupling agents. Thus, the aim was to study, the effect of adding PDMS-Epoxy to DGEBA to apply as adhesive for leather bonding applications. Several workers have reported on the development of various polymers for leather applications [11-15]. While, we have developed a process, to prepare an inter-crosslinked polymer networks from DGEBA and siloxane in absence of coupling agents. We have studied the efficiency of processing and the effectiveness of PDMS-Epoxy as specialty adhesive for leathers. *Corresponding Author: BSR Reddy, E-mail : induchem2000@yahoo

Malaysian Polymer Journal, Vol. 5, No. 2, p 84-98, 2010

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2.0 METHODS & MATERIALS 2.1 Materials

Diglycidyl ether of bisphenol A (DGEBA) (GY-250, EEW: 182-192) was purchased from Ciba-Geigy. Polydimethylsiloxane diglycidyl terminated (PDMS-Epoxy, EEW: 490, Viscosity 15,000 cST at 25ºC, d.0.99) and 2,4,8,10-tetra oxaspiro-[5.5]-undeccane-3,9-dipropanamine(TOSUP) were obtained from Aldrich chemical company. Diethylenetriamine (DETA) and Triethylenetetramine (TETA) were received from SD Fine Chemicals. Chrome re-tanned upper leathers were obtained from the tannery division of our institute. 2.2 Preparation of DGEBA/PDMS-Epoxy resin lami nates DGEBA was modified with different quantity of PDMS-Epoxy and DETA curing agent. Hundred parts of resin blends were prepared by mixing 10,30,50,70 and 90 weight parts of DGEBA with 90, 70, 50, 30 and 10 weight parts of PDMS-Epoxy (Table 1: I–V). The DETA quantity added was on the basis of equivalent weight of DGEBA and PDMS-Epoxy. Thus, the number of active hydrogen in DETA was equal to the number of epoxy groups in DGEBA and PDMS-Epoxy. The preparation of DGEBA/PDMS-Epoxy formulation was conducted in two steps. In the first step, PDMS-Epoxy and DETA were mixed at 28 ± 2 ºC to obtain a prepolymer stage. In the second step, DGEBA was added to the prepolymer at 28±2ºC and mixed thoroughly until it became homogeneous. The blends were degassed under vacuum to remove air bubbles and poured into a mould and cured at room temperature till it became hard. The hard materials were post cured at 70ºC for 5h. Similarly, the system VI and VII were prepared using TETA and TOSUP respectively. The materials prepared from the systems I-V was evaluated for spectroscopic analysis, mechanical properties, tensile strength, flexural strength, impact strength and hardness property. The samples of the systems V-VII were analysed for their cure behavior, thermal stability and fractured surface characteristics by DSC, TGA and SEM analysis. Table 1: Compositions of DGEBA/PDMS-Epoxy resin blends (I–VII) and their flow properties at the gel time.

Resin

Systems Gel Time

Min. є'(Storage Modulus)

[Pa]

є''(Loss Modulus)

[Pa]

η*(Complex Viscosity)

[P] I (10/90)* 194 1.50E+01 1.07E+03 1.07E+04 II (30/70)* 136 2.20E+01 1.82E+03 1.57E+04 III (50/50)* 119 1.36E+01 1.05E+03 1.01E+04 IV (70/30)* 81 9.02E+01 1.02E+03 1.20E+04 V (90/10)* 40 4.40E+05 8.78E+03 2.00E+04 VI (90/10)** 56 8.73E+03 8.65E+04 1.32E+04 VII (90/10)*** 105 6.99E+03 1.01E+04 1.32E+04

Resin systems: DGEBA/PDMS-Epoxy (w/w) (g) *DETA, **TETA and *** TOSUP amines mixed at stoichiometric ratios

2.3 Preparation of leather-to-leather adhered sampl es

Peel test specimens (Figure 1) were prepared from leather sheets of 150mm long × 28mm wide× 2mm height. Prior to adhesion, the flesh surface of the leather sheets were emeried with sand paper numbered 80 to make the surface rough to obtain better


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adhesion. The resin blend was then coated on the emeried area of 75 mm, and a 75 mm portion was left uncoated. Each leather sheet coated with the resin was placed face to face, and pressed at 5000psi for 5h at 28 ± 2ºC in the hydraulic press. They were post-cured in the oven at 70ºC for 5h. The obtained leather-to-leather bonded materials were evaluated for the peel strength property.

Figure 1: Schematic diagram of peel strength test specimen. 2.4 Characterization

Nicolet FT-IR spectrophotometer Model 20 DXB was used for recording the IR spectra using solid KBr pellets. Thermal properties were investigated using Mettler TA 4000 instrument. The differential scanning calorimetric data’s were obtained under nitrogen atmosphere using Perkin Elmer analyzer model, DSC 7. The mechanical properties were measured using universal testing machine, HTE-S-Series-H50K-S model, Hounsfield test equipments Ltd., UK. Tensile strength and tensile modulus properties were evaluated as per the ASTM D 3039. The Flexural strength and flexural modulus properties were carried out as per the ASTM 790. The hardness test was performed using Shore D hardness tester according to ASTM D785. Un-notched Izod impact strength was measured according to the ASTM D256-88 standards. Moisture resistance test was done for the specimens having 40 x 25 x 1.25 mm dimensions. The specimens were immersed in distilled water at 50ºC, 60ºC, 70ºC, 80ºC and 90ºC for a period of 100 hours and they were removed periodically for every 1hour, neatly wiped with tissue cloth, weighed and immediately placed back into the water. This procedure was repeated till the specimens reached the moisture saturation. The peel strength test of leather samples was done as per the ASTM D 4704-93 test method at 20ºC and 65% humid conditions. The ends of the un-bonded portions were clamped in a testing machine and the adhesion strength was measured by means of the force in N/mm that required for peeling off the leather sheets from each other. SEM analysis was done using Stereo Scan Model 440 (Oxford) at a magnification of 4000. The Viscosity measurements were carried out in Rheoplus instrument using parallel plate geometry at 25ºC with the angular frequency set at 1 Hz.


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3.0 RESULTS AND DISCUSSION 3.1 Processing conditions for making formulation s DGEBA/PDMS-Epoxy formulations were prepared using DETA, TETA and TOSUP curing agents by varying the DGEBA and PDMS-Epoxy ratios. The PDMS-Epoxy and the respective curing agents were premixed together, before being combined with DGEBA. The premixing helped the epoxy molecules to face higher interaction with the curing agents and facilitated the maximum networking between PDMS-Epoxy and amine. The PDMS-Epoxy linked with amine enabled the strong compatibilization with DGEBA as showed in Scheme 1.

Scheme 1: Curing reaction of DGEBA/PDMS-Epoxy with DETA, TETA and TOSUP. Further, it was necessary to keep the resin formulations in the flowing stage during the coating. Therefore, rheological studies were done to find out the optimum time at which the flow of the resin would be suitable to apply on leather. It was observed that, the viscosity seemed to be rising suddenly during the gelation time, i.e., the period of time it took to reach the gelation state, which is denoted as the “gel time”. During the gel time, the reaction proceeded fast and the molecular weight increased rapidly and therefore the flow of the resin was reduced due to the increase in viscosity. The gel time was steadily increasing when the PDMS-Epoxy quantity was increasing in the formulation blends. This was due to the presence of larger PDMS-epoxy backbone that hindered the terminal epoxy group to come closer to react with amine molecules. Thus, the gel time increased with increase in the PDMS-epoxy quantity for the formulations from V to I, was due to the slower mobility of the PDMS-Epoxy resin. Therefore, nonreactive bulky groups present in their backbone hindered the diffusion of unreacted monomer into the prepolymer formed. The reaction between PDMS-Epoxy and amine was mainly diffusion controlled reaction. In the case of DGEBA and amine, the reaction was favoured due to the short back bone present in DGEBA. Thus, the IPNs formed due to the complete miscibility because of the intermolecular hydrogen bonding interactions occurred between PDMS-Epoxy and amine [16]. The formulated resin systems were applied on leather just prior to the gel time at which the flow of the resin formulation was optimum. From the rheological studies it was observed that the system contained lowest PDMS-Epoxy (V, VI & VII) had exhibited shortest gel time. During the gel time, the other gelation properties like, η* (Complex viscosity), є' (Storage modulus) and є'' (Loss modulus) were reached their maximum.


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Thus, the high η*, є' and є'' observed at the gel time was due to the formation of densed cured structures. In system V-I the formation of inhomogeneous networks was increased when PDMS-Epoxy quantity was increased. Only the formulations V, VI and VII had 10 parts of PDMS-Epoxy could produce homogeneous networks. 3.2 FTIR Spectral analyses The spectra of DGEBA/PDMS-Epoxy resins cured with DETA (A), TETA (B) and TOSUP (C) were given in Figure 2. The hydroxyl group formed due to the reaction of epoxy with amine was identified from the appearance of bands at the regions between 3200 and 3650 cm-1. Similarly appearance of -NH stretching peak of amine around 3300 and 3400 cm-1 overlapped with the –OH stretch bands.

Figure 2: FT-IR Spectra of DGEBA/PDMS-Epoxy with DETA (V), TETA (VI), and TOSUP (VII).

The epoxide group cleavage was confirmed by the absence of bands around 1230 cm-1 and 830cm-1, which accounts for the symmetrical and asymmetrical stretch respectively. The presence of Si–C and Si–O– linkages were confirmed by the absorption peaks occurred at 1255-1257 cm–1 and 1000-1100cm–1. The FTIR spectral analysis showed that the DGEBA/PDMS-Epoxy formulations have been completely crosslinked by the respective amine curing agents. 3.3 Thermal properties The thermal stability was evaluated for DGEBA/PDMS-Epoxy (I-V) systems prepared using DETA (Fig.3). All the systems had exhibited single stage decomposition temperature. This observation was related to the homogenized segments formed by the polymer system. The thermal stabilities were evaluated by comparing their initial decomposition temperatures (IDT). The IDT of all the systems exhibited around 200ºC was due to the scission of CH2 bond in amine molecules. However, the variation in the composition weight ratio of DGEBA and PDMS-Epoxy did not affect the thermal stability.


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The thermogravimetric curves of DGEBA/PDMS-Epoxy cured with DETA (V), TETA (VI) and TOSUP (VII) in stochiometric ratios were given in Figure 4. The IDT values of V, VI and VII were around 200ºC. The temperature at which 10% weight loss occurs, Td10, was higher for the system VII when compared to the systems V and VI as shown in figure 5. This was attributed to the strong influence of the thermally stable oxygen linkages in TOSUP enhanced the resistance to degradation. The Td10 was higher for the system V compared to system VI. This was due to the less aliphatic groups present in DETA (system V) which helped to form close networks and improved the thermal stability. A decrease in the decomposition temperature with respect to increase in the chain length could be attributed to the enhanced molar volume and chain flexibility that gives higher relaxation time in the cured networks. The dependence of decomposition temperature on alkyl chain length have been interpreted in terms of the thermal stability. The comparison study of the maximum decomposition temperatures were in order, VII > system VI > system V.

Figure 3: Thermogravimetric curves of DGEBA/PDMS-Epoxy with DETA (I-V).


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Figure 4: Thermogravimetric curves of DGEBA/PDMS-Epoxy with DETA(V), TETA(VI) and TOSUP(VII).

Figure 5: Percentage Weight losses Vs Temperature for DGEBA/PDMS-Epoxy with DETA (V), TETA (VI) and TOSUP (VII).


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3.4 Differential scanning calorimeter The thermal behaviour studies of the polymer blends could be one of the most effective methods for determining the miscibility of blends. The extent of cured network structures formed could also be directly read from the glass transition temperatures of the cured resins. When two polymer blends were miscibilised at the molecular level, a single and composition dependent Tg between the Tg’s of the blend components was observed [17]. Accordingly, DSC scans (Figure 6) of the cured polymers V, VI and VII show a single Tg values which confirmed that, these blends have formed homogeneous networks. The Tg values for the formulations V, VI and VII were found to be 69.4, 95.4 and 98.6ºC respectively. The comparison of the Tg values were in the order V < VI < VII. The extent of forming dense structures was highly dependent on the nucleophilicity of the amines used. The highest Tg value observed in VII compared to V and VI was due to the tetraoxaspiro unit present in the TOSUP that hindered the segmental motions and therefore reflected as raise in Tg value. System VI had showed higher Tg value compared to V. This was due to the presence of high NH groups that had accelerated the curing rate and extended more links to form a higher dense polymer networks and that has resulted in high Tg.

Figure 6: DSC Curve of DGEBA/PDMS-Epoxy with DETA (V), TETA (VI) and TOSUP (VII).

3.5 Mechanical properties The mechanical properties of neat resin laminates for the system V, VI and VII were given in Figure 7. The flexural properties of system V and VI were higher when compared to system VII. This was due to the lesser –CH2- linkages in DETA (V) and TETA (VI), which had increased the crosslinking density. Similarly, the tensile strength and hardness properties of system V and VI should have been higher than system VII, since system V and VI have a tendency to increase higher cross-linked density compared to system VII. Nevertheless, the higher tensile strength and hardness values exhibited by the system VII, compared to system V and VI were due to the reinforcing ability of the


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oxaspiro units present in the TOSUP. An increase in the impact strength by more than 50% was observed for the system VII compared to V and VI could be attributed to the presence of high -CH2- linkages.

Figure 7: Mechanical properties of DGEBA/PDMS-Epoxy with DETA (V), TETA (VI) and TOSUP (VII). 3.6 Hygrothermal properties The neat resin specimens made from the systems V, VI and VII were evaluated for the hygrothermal properties. The diffusion pattern of the moisture into the conditioned specimens showed in Figure 8, was found to be linearly proportional to the time and the temperature. The absorption rate was fast and increased linearly with the time. This confirmed that, these results were in agreement with Fick’s diffusion theory. During the latter hours of the absorption, they exhibited a steady state increase in moisture gain instead of reaching the saturation level. The non-Fickian anomaly behaviors found in epoxy systems have been reported [18, 19]. The absorption levels continued to rise but the rate of absorptions was slow and steady after the linear increase of M (Saturation) with t1/2. Further, the moisture diffusion pattern of the specimens conditioned at 60ºC and 90ºC were not deviated from Fick’s law. While, the moisture diffusion pattern of the specimens conditioned at 70ºC and 80ºC were deviated from Fick’s law around their Tg temperatures. This kind of non-Fickian behaviour [20-21] was due to the relaxation of polymer segments arised by the internal moisture concentration. The moisture absorption of the epoxy materials usually will be slow at the saturation level even if the conditioned temperature was above the Tg temperature [22]. Surprisingly, the moisture diffusion rate observed for the specimens conditioned at 90ºC was very high. This was because the specimens had spent comparatively lesser time at 90ºC to reach the near saturation level and therefore it did not experience the polymer segment relaxation phenomena. It was also observed that the specimens that had spent long time at their Tg temperature environment were prone to yield to the non-Fickian behaviour.


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Figure 8: Hygrothermal properties of DGEBA/PDMS-Epoxy with DETA (V), TETA (VI) and TOSUP (VII).

The moisture diffusivity in epoxy specimens can be determined using Fick’s second law with a constant diffusivity D, by plotting percentage water gain (Mt) Vs temp (t1/2) curve [23]. π h 2 Mt2 – Mt1

2

16 Mm √t2 - √t1

Where t is exposure time, Mm is the maximum moisture gain, and h is the sample thickness.

Further, using Arrhenius theory, the activation energy (Q) and the diffusion constant (D0) could be determined by plotting ln D Vs 1/T. The D0 (coefficient of Diffusivity

D = Do exp Q

RT

constant) and the Q values obtained were very low as showed in Table 2. It was not possible to get linear regression fit curve for ln D Vs 1/T to obtain better values.

D=


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Table 2: Moisture diffusivity (D), Activation energy (Q) and Maximum moisture gain (Mm) value of the specimens made from DGEBA/PDMS-Epoxy cured with DETA (V), TETA (VI) and TOSUP (VII).

D was studied at 60, 70, 80 and 90ºC The siloxane modified epoxy systems exhibited low moisture absorption properties because of the inherent hydrophobicity of the siloxane molecules. These siloxane functional groups would usually present at the surface of their composites due to the conformational freedom of the backbone chains that allowed methyl groups on silicone atom to align at the surface. In this way, the methyl groups in the air/PDMS surface showed mostly wax like surface and therefore repelled the water molecules [24]. While, the high conformational freedom arised due to the flexible Si-O-Si groups in PDMS increased the internal free volume of the materials. Thus, the absorbed water molecules were easily diffused and therefore the quick pick up was observed. The kind of moisture pick up could be mostly the “unbound” type. The “bound” type normally disrupts the interlinked hydrogen bonding that would end up with the loss of weight. However, the “bound” type phenomena were not observed in our specimens. 3.7 Peel strength test Leather to leather adhered specimens of the systems I to V were evaluated for the peel strength properties. The adhesion strength (Figure 9) was in the increasing order with respect to decreasing in the PDMS-Epoxy quantity in the blends. Thus, system I and II showed lower peel strength, while III, IV and V showed higher peel strength. The system V showed highest peel strength compared to the rest of the systems. This was due to high concentrations of OH groups that anchor to the leather surface which had exhibited in high bonding strength resulted in high bond strength. Therefore, peel strength property increased as the composition of siloxanes to the epoxy decreased. Hence, the system V with DGEBA/PDMS-Epoxy 90:10 ratio was found to be an excellent ratio for leather to leather bonding. The comparison of peel strength property of V, VI and VII showed in Figure 9 (b) shows that the system V possess higher peel strength compared to VI and VII. The peel strength properties were in the following order VII < VI< V. The incidence of the cohesive failure was observed visually at the adhesive interface in the tested leather specimens.

Figure 9: Peel strength properties of leather specimens for adhesion using DGEBA/PDMS-epoxy with DETA (I-V) and TETA (VI) and TOSUP (VII).

Sample D60

(mm 2s-1) D70

(mm 2s-1) D80

(mm 2s-1) D90

(mm 2s-1) Q(kcal /mol)

V 5.905 X 10-6 7.749 X 10-7 5.604 X 10-7 1.407 X 10-7 0.02 VI 4.033 X 10-4 5.862 X 10-6 1.430 X 10-4 5.700 X 10-5 1.05


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3.8 SEM analysis of cured resin specimens The fractured surfaces (Figure 10) of the cured resins were homogeneous and did not have any distinct phase domains. This confirmed the homogenized IPN networks formed between DGEBA and PDMS-epoxy (Figure 11). The crack propagation studied in all the systems was of a shear, stable and in continuous pattern. The presence of lines that emanating from the crack point was more like a shear fracture. This was evident in the case of TOSUP due to their inherent flexibility characteristics. The uniform river flow pattern observed in V, VI and VII was attributed to the high compatibility of the blends. The single Tg values obtained was due to the participation of OH molecules of PDMS-Epoxy in the reaction which was confirmed by the appearance of homogenised structure morphology of the cured materials.

Figure 10: SEM analysis of DGEBA/PDMS-Epoxy with DETA (V), TETA (VI) and TOSUP (VII).

Figure 11: Schematic diagram showing PDMS-Epoxy/ DGEBA/ DETA (V) appearing in a homogenous state.


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3.9 SEM Analysis of peeled leather surface The bonding strength property of epoxy resins on leather-to-leather substrate was found to be good (Figure 12). The OH group generated during the reaction between epoxy and amine had attributed to the cohesive interaction of the resins with leather. The significant cohesive failure occurred at the peeled leather pieces was observed in systems V and VI could be due to the good coupling interaction between the resins and leather. The low concentration of the ruptured leather fibrils in system VII compared to system VI and V was attributed to the non-reactive PDMS segments present in TOSUP.

Figure 12: SEM micrographs of failed surface of the leather samples prepared using DGEBA/PDMS- Epoxy blends V, VI and VII. 4.0 CONCLUSIONS

DGEBA modified with PDMS-Epoxy had showed an outstanding performance as an adherent to the leather-to-leather bonding. The toughness properties of DGEBA was enhanced by the inclusion of PDMS-Epoxy whose tendency to remain phase separated was controlled by optimizing the processing conditions, composition ratio, and the curing conditions. The strong mechanical phase interaction brought by the chemical bonding with leather interface was confirmed by peel test and SEM analysis. The influence of DETA, TETA and TOSUP curing agents on processability, gelation, mechanical properties, structure morphology, and adhesion strength were studied. REFERENCES [1] Gutman, I., Hartings, R., Matsuoka, R., and Konda, K., “Experience with IEC 1109

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