Dual Cure Phenol - Epoxy Resins: Characterisation and Properties

Dual Cure Phenol - Epoxy Resins: Characterisation and Properties Dual Cure Phenol - Epoxy Resins: Characterisation and Properties K. Ambika Devi, C.P. Reghunadhan Nair* and K.N. Ninan Propellant and Special Chemicals Group, Vikram Sarabhai Space Centre, Trivandrum-695 022, India Received: 19 December 2002 Accepted: 24 July 2003 SUMMARY Reactive blends of 2, 2'- diallyl bisphenol A (DABA) and a novolac epoxy resin (EPN) were investigated for their cure behaviour, and their rheological, physical, mechanical and thermal properties. Cure characterisation was by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The system underwent dual curing through a sequential phenol-epoxy reaction and allyl polymerisation. The former reaction was catalysed by triphenyl phosphine (TPP). Whereas the phenol-epoxy reaction was completed, the allyl polymerization was limited to 40% by the regulation of cure conditions. Increase of epoxy concentration in DABA-EPN blends led to an improvement in the tensile strength and flexural strength of the neat castings. The flexural strength and interlaminar shear strength of the glass laminate showed an improvement with an increase in EPN concentration. Although the crosslink density of the neat casting was enhanced by epoxy-concentration, this did not result in any significant variations in the glass transition temperature (T g ) of the cured matrix. The T g was in the range 78-82 C. Complete polymerization of all allyl groups resulted in an increase in the T g and thermal stability while the mechanical properties of the neat system remained practically unaltered. The dual curing of the matrix system resulted in considerable improvement in the flexural properties of their glass composites. INTRODUCTION Epoxy resins are the most versatile class of thermosetting polymers among the contemporary plastics. The use of epoxy resins in high performance structural materials has been increasing recently. Multifunctional epoxies are used in high performance adhesives and advanced composite matrix materials in aerospace and electronic industries. In combination with a large number of curing agents, epoxy resins find extensive use in a multitude of applications 1,2. A combination of high modulus and high strength at relatively high temperatures makes them suitable for advanced applications. Such applications include composite structures for satellites, rocket motor cases and high temperature adhesives for aerospace applications. However, their brittle nature and low glass transition temperature (T g ) demand matrix modification to widen the area of utility. The former can be overcome by toughening the matrix with an *Author for communication, E mail:cprnair @eth.net impact modifier. The toughening of the epoxy resins by carboxyl- terminated butadiene -acrylonitrile and other rubbers as well as thermoplastics has been studied extensively by researchers 3-6. Epoxy matrix properties are conveniently tunable by proper selection of curing agents from among amines, acids, anhydrides, phenols etc. Although less popular, polyphenols are used because the addition-cured, void-free product is comparatively tough, thanks to the formation of a flexible ether network 7,8,9. Epoxy resins cured with speciality phenolic hardeners have also been reported 10,11,12. One way to enhance the Tg is to use phenolic hardeners capable of inducing multiple curing reactions. In this perspective, we have examined 2,2'-diallyl bisphenol- A (DABA) as a dual curative for a novolac epoxy resin (EPN-1139). The system could cure by the phenolepoxy reaction as well as by the addition polymerization of allyl groups. The allyl group also offers the possibility of further matrix modification by co-reaction with other vinyl monomers. This Polymers & Polymer Composites, Vol. 11, No. 7, 2003 551

K. Ambika Devi, C.P. Reghunadhan Nair and K.N. Ninan paper concerns the reactive blending of EPN-1139 and DABA. The cure characteristics and mechanical and thermal properties of the DABA-cured neat resin and glass laminates are examined and the composition dependency of these properties is discussed. EXPERIMENTAL Materials DABA was synthesized from bisphenol A by a previously reported procedure 13. The epoxy resin, EPN 1139 (Ciba Geigy, Mumbai) with epoxy equivalent weight of 175 and epoxy functionality of 5.2/kg was used as received. Plain weave, silane treated E-glass fabric (Unnathi Corporation, India, thickness = 0.25 mm) was used as received. Triphenyl phosphine (TPP, E-Merck) and acetone (SRL, India) were also used as received. Preparation of Laminates The DABA-EPN blend was dissolved in acetone. Glass fabric prepregs were prepared by dipping the fabric tape in acetone solution and then drying in air for 18 h at room temperature. They were cut into pieces of dimensions 120 mm x 100 mm, stacked and moulded in a hydraulic press between thick metallic platens to achieve the proper number of plies and thickness. The above heating schedule was adopted for the composites too. A pressure of 3.6 MPa was applied after the gelation of the resin at 150 C. Mechanical Testing of Neat Resin and Composites The tests were performed in accordance with ASTM testing standards using an Instron Universal Testing Machine, Model 4202. Instruments FTIR spectra were recorded with a Nicolet 510 P instrument. The cure characteristics of the resins were studied by DSC using a Mettler DSC-20 analyzer at a heating rate of 10 0 C/min in a nitrogen atmosphere. Thermogravimetry was performed on a Du Pont 2000 thermal analyzer in conjunction with a 951 thermogravimetric analyzer in a nitrogen atmosphere at a heating rate of 10 C/min. Dynamic Mechanical Analysis (DMA) of the cured polymer was performed using a Du Pont DMA-983 in a nitrogen atmosphere at a frequency of 1 Hz. Dynamic mechanical analyses were done with a Reologica StressTech Rheometer. Samples were analyzed in the oscillation mode using a parallel plate assembly (20 mm dia) with a gap of 0.5 mm in a controlled strain mode. Preparation of Neat Resin Castings Neat resin castings of DABA and EPN were prepared by curing the resin in rectangular aluminium moulds. TPP (0.5% of the total mass) was dissolved in DABA at 100 C, to which EPN was added and mixed well. The mixture was deaerated in vacuum at 50 C and poured into rectangular aluminium moulds (150 mm x 100 mm). Curing was done by stepwise heating in accordance with the following time-temperature schedule. Temperature ( o C) 100 150 200 250 Hold Time (min) 30 30 30 120 (240 or 360) The specimens were machined from the slab for evaluation of mechanical properties. Tensile tests: Flexural strength: Inter laminar shear strength (ILSS): ASTM D638 (neat resin) ASTM D790 (neat resin and composite) ASTM D2344 (composite) RESULTS AND DISCUSSION The EPN was cured by the DABA reaction proposed in Scheme 1 14. The system cures by a phenol-epoxy reaction and an allyl polymerisation. The scheme has been proposed based on the known mechanisms of these reactions. The former reaction shifted to a lower temperature regime on adding TPP. DSC showed the two-step curing as seen in Figure 1. The second step, corresponding to the allyl polymerisation is a low enthalpy process. The DMA (also shown in Figure 1) substantiated the DSC observation of two-step curing, the temperature zones in both corresponding to each other. The DMA confirmed that the allyl curing leads to a significant modulus increase. Cure Optimisation An earlier study showed that the phenol-epoxy reaction is complete in a very short time at temperatures above 150 C 15. The allyl curing is a slow process and was done at 250 C. The storage modulus time plot from isothermal DMA analysis at 250 C indicated that after 2 hours, the allyl polymerisation is completed to the extent of 40% which was calculated from the ratio of the storage modulus after two hours to the final stagnating storage modulus. The rheogram shown in Figure 2 552 Polymers & Polymer Composites, Vol. 11, No. 7, 2003

Dual Cure Phenol - Epoxy Resins: Characterisation and Properties Scheme 1 Co-curing of DABA and EPN Figure 1 DMA and DSC thermograms of the DABA-EPN 1:1 blend (in presence of TPP). Heating rate 10 C/min Polymers & Polymer Composites, Vol. 11, No. 7, 2003 553

K. Ambika Devi, C.P. Reghunadhan Nair and K.N. Ninan Figure 2 Isothermal DMA of DABA/EPN 1:1 blend at 250 C Table 1. Properties of DABA-EPN neat resin and composites (40% allyl curing) DABA/ EPN equivalent ratio 1:0.8 1:0.9 1:1.0 1:1.1 1:1.2 Flexural Strength (MPa) 77 91 113 106 116 Neat Tensile Strength (MPa) 81 105 118 131 130 casting Elongation (%) 3.5 3.0 3.6 4.0 3.6 T g ( C) M c (g/mole) 77 81 79 78 82 58 41 19 17 26 ILSS (MPa) 30-29 - 34 Composite Flexural Strength (MPa) 361-369 - 438 indicated that the cure completion requires heating for 5 to 6 hours at this temperature. Hence, in selected cases, the ultimate curing was done by heating the system at 250 for 6 hours. The completion of the cure reaction was also ascertained from the FTIR spectra of the cured product (for 1:1 equivalent composition), from the disappearance of the peak at 910 cm -1 due to both the allyl and epoxy groups. For compositions with excess epoxy, the possibility of homopolymerisation of the epoxy groups (initiated by the hydroxyl groups generated by the phenolepoxy reaction) exists. Mechanical Properties of Neat Castings The mechanical properties of various compositions are compiled in Table 1. Both the flexural and the tensile strength (F.S & T.S) of the resins increased with increasing EPN-content whereas the elongation (E) was practically unchanged. This is because, increasing the epoxy-content enhances the overall cohesion in the matrix through enhanced dipolar interactions. The presence of allyl groups tends to reduce the overall polarity, although their polymerisation adds to the crosslinking. 554 Polymers & Polymer Composites, Vol. 11, No. 7, 2003

Dual Cure Phenol - Epoxy Resins: Characterisation and Properties Dynamic Mechanical Properties of Cured System The T g of the cured matrices deduced from the tand peaks in the DMA curves did not show any significant variation on changing the ratio of the reactants. The average molecular weight between cross links (M c ), indicative of the inverse of the crosslink density, was calculated from the storage modulus (E ) corresponding to the rubbery region (T = 110 C) using the relationship M c = 3d RT / E where d is the density and R the universal gas constant. The crosslink density showed a systematic increase with increase in equivalent ratio up to 1:1. Further epoxy groups decreased the crosslinking, implying that the excess epoxy groups homopolymerise, leading to a marginal decrease in crosslink density. Since mechanical strength was not decreased with more epoxy, it is possible that the epoxy homopolymer segments added to the cohesive interaction in the matrix. Thermal Characteristics The thermal stability of the co-cured systems was investigated by thermogravimetry. The thermograms of the 40% allyl cured system showed a two-stage degradation. The first step occurred at a lower temperature and could be attributed to the degradation of the pendant allyl groups, which are not polymerized. The presence of uncured allyl groups adversely affected the thermal characteristics of the system. On effecting the complete polymerization of the allyl groups, the first degradation step disappeared and the overall thermal stability increased. Typical thermograms of 40% and 100% allyl cured systems are shown in Figure 3. Properties of Glass Laminates The utility of the present system for making composites was examined by processing laminates with glass cloth. The allyl polymerization was limited to 40%. The mechanical properties of the laminates are shown in Table 1. It was found that there was an increase in the interlaminar shear strength. The ILSS of the composite increased from 30 to 34 MPa as the Figure 3 Thermograms of partially and fully cured blend in nitrogen. Heating rate 10 C/min Polymers & Polymer Composites, Vol. 11, No. 7, 2003 555

K. Ambika Devi, C.P. Reghunadhan Nair and K.N. Ninan resin accommodated epoxy beyond the stoichiometric equivalent. A similar trend emerged for the flexural strength, which showed an increase from 369 to 438 MPa. As in the case of neat resin, the excess epoxy increased the flexural strength of the composite, possibly through the influence of epoxy homopolymer segments. The polar EPN and its cured product could promote fiber wetting and enhanced resinreinforcement interactions as well as increasing matrix cohesion. Since the trend in composition dependency of the mechanical strength of both neat resin and the composites was similar, it can be concluded that the composite failure was associated with the matrix. The enhanced cohesion of the epoxy-dominated matrix leads to a stronger composite. Effect of Absolute Allyl Polymerization In a typical system of 1:1 EPN/DABA, the allyl groups were completely polymerized by extending the cure time to six hours, and the effect on mechanical properties and T g was monitored. The cure completion was ascertained from the FT IR spectrum. The mechanical properties of the 40% and 100% allylcured neat castings and their glass laminates are given in Tables 2 and 3 respectively and their storage modulus versus temperature curves are given in Figure 4. The storage modulus and Tg of the 100% Table 2. Effect of extent of allyl curing on properties of polymer (1:1 composition) Property Tensile Strength (MPa) Elongation (%) Flexural Strength (MPa) T g ( C) 40% allyl cured Neat 118 3.6 113 79 Table 3. Effect of extent of allyl curing on properties of composite (1:1 composition) Property ILSS (MPa) Flexural Strength (MPa) T g ( C) 40% allyl cured Composite 29 369 86 100% allyl cured Neat 77 3.3 110 91 100% allyl cured Composite 32 458 107 allyl-cured system were found to improve. While the decrease in the polymer s tensile strength can be assigned to the practical difficulties in measuring very precisely the tensile strength of such highly cross linked systems, the almost invariant flexural Figure 4 Dynamic mechanical spectra of 40% and 100% allyl cured resin system 556 Polymers & Polymer Composites, Vol. 11, No. 7, 2003

Dual Cure Phenol - Epoxy Resins: Characterisation and Properties strength implies that the allyl curing has not resulted in any significant brittleness. However, completing the allyl polymerization enhances the ILSS and flexural strength of the composite. The increased ILSS and improved flexural strength at 100% allyl cure point to the consolidation of the interphase by enhanced crosslinking. The similar trend in ILSS and flexural strength again confirm the possible failure of the composite at the interphase. This improved interphase strength is achieved on effecting 100% allyl polymerization. The tanδ curves of 40%, 60 % and 100% allyl cured neat castings and that of 100% allyl cured composite given in Figure 5 show that the T g shifted to a higher temperature with increased allyl curing. The T g of the composite also improved significantly over that of the neat casting. 4. CONCLUSIONS The blend of a novolac epoxy resin and DABA coreacts to form crosslinked networks whose mechanical, physical, thermal and laminate composite properties depend on their composition. TPP shifted the phenol epoxy reaction to lower temperature. An increase in the epoxy concentration in the blend resulted in an improvement in the mechanical performance of both the neat castings and the glass laminates. The crosslink density of the system increased with epoxy content up to 1:1 stoichiometry. Complete allyl polymerization resulted in an improvement in Tg and thermal stability, without affecting the flexural properties of the neat system, while it improved the flexural strength and T g of the composite. ACKNOWLEDGEMENTS VSSC s permission to publish this article is gratefully acknowledged. K. Ambika Devi is grateful to the Director, VSSC and Deputy Director, Propellants, Chemicals and Materials Entity for permission to pursue doctoral studies. Figure 5 Tanδ curves of DABA/EPN 1:1 blend systems cured for various time intervals. N - neat casting. C - composite. Frequency, 1 Hz, Heating rate 10 C/min Polymers & Polymer Composites, Vol. 11, No. 7, 2003 557

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