Influence of the Type of Epoxy Hardener on the Structure and Properties of Interpenetrated Vinyl Ester/Epoxy Resins

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1 Influence of the Type of Epoxy Hardener on the Structure and Properties of Interpenetrated Vinyl Ester/Epoxy Resins O. GRYSHCHUK, J. KARGER-KOCSIS Institute for Composite Materials, Kaiserslautern University of Technology, D Kaiserslautern, Germany Received 7 May 2004; accepted 19 June 2004 DOI: /pola Published online in Wiley InterScience ( ABSTRACT: The morphology toughness relationship of vinyl ester/cycloaliphatic epoxy hybrid resins of interpenetrating network (IPN) structures was studied as a function of the epoxy hardening. The epoxy was crosslinked via polyaddition reactions (with aliphatic and cycloaliphatic diamines), cationic homopolymerization (via a boron trifluoride complex), and maleic anhydride. Maleic anhydride worked as a dual-phase crosslinking agent by favoring the formation of a grafted IPN structure between the vinyl ester and epoxy. The type of epoxy hardener strongly affected the IPN morphology and toughness. The toughness was assessed by linear elastic fracture mechanics, which determined the fracture toughness and energy. The more compact the IPN structure was, the lower the fracture energy was of the interpenetrated vinyl ester/epoxy formulations. This resulted in the following toughness ranking: aliphatic diamine cycloaliphatic diamine boron trifluoride complex maleic anhydride. For IPN characterization, the width of the entangling bands and the surface roughness parameters were considered. Their values were deduced from atomic force microscopy scans taken on ion-etched surfaces. More compact, less rough IPN-structured resins possessed lower toughness parameters than less compact, rougher structured ones. The latter were less compatible according to dynamic mechanical thermal and thermogravimetric analyses Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: , 2004 Keywords: atomic force microscopy (AFM); epoxy resin; fracture mechanics; interpenetrating networks (IPN); structure-property relations; vinyl ester resin INTRODUCTION Correspondence to: J. Karger-Kocsis ( karger@ ivw.uni-kl.de) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, (2004) 2004 Wiley Periodicals, Inc. In contrast to other methods of optimizing the properties of thermosetting polymers (e.g., the tailoring of the crosslink network via suitable monomers and the in situ generation or incorporation of micrometer-sized particles), the blending of thermosets to form interpenetrating networks (IPNs) is an entirely different route. In IPN compositions, two or more chemically distinct polymer networks are held together exclusively by permanent mutual entanglements. 1 3 The degree of intermixing of IPNs is mainly affected by the thermodynamic miscibility of the components, 4,5 the relative rate of network formation, 6 8 the composition, 9,10 the degree of crosslinking, and the mobility of the polymer chains at the time of phase separation. 8,11 Understanding the IPN structure and its development is of paramount importance for further material development. This work was aimed at studying the morphology and properties of vinyl ester (VE)/cycloaliphatic epoxy (Cal-EP) hybrid resins with different curing agents (hardeners) for the epoxy (EP) resin. 5471

2 5472 GRYSHCHUK AND KARGER-KOCSIS EXPERIMENTAL Resins and Curing A commercial bisphenol A based bismethacryloxy-type VE (Daron XP-45-A-2; styrene concentration 30 wt %), a product of DSM Composite Resins, was selected for this work. For its curing, 0.75 phr (parts per hundred parts of resin) dibenzoyl peroxide and 0.15 phr N,N-diethylaniline accelerator were used. The EP resin was a cycloaliphatic (Cal-EP) type, that is, 1,4-cyclohexanedimethanoldiglycidylether (Polypox R11, U. Prümmer Polymer-Chemie GmbH). Cal-EP was cured by an aliphatic diamine [Al-Am; 1,2-bis(2- aminoethoxy)ethane; Jeffamine EDR-148, Huntsman], a cycloaliphatic diamine [Cal-Am; 2,2 dimethyl-4,4 -methylene-bis(cyclohexylamine); HY2954, Vantico, Ltd.], a boron trifluoride complex [BF 3 ; trifluoroboron(4-chlorobenzeneamine); Anchor 1170, Air Products], or maleic anhydride (MAH; Aldrich). The EP/hardener ratio was stoichiometric for all EP formulations, except for EP/ MAH, for which it was set at 1/0.5. The VE/EP ratio was kept constant (1/1) for all hybrids. The resin mixtures were prepared by the mixing of VE with the EP resin first. Next, the peroxide was added, and the mixture was homogenized. Then, the EP hardener was added, and the mixture was homogenized again. Finally, the VE accelerator was added, and the resin mixture was poured into a paper beaker and degassed in vacuo before being poured into open poly(tetrafluoroethylene) molds. The cure regime of the plaques and specimens (3 mm thick) produced was as follows: room temperature for 12 h, 80 C for 3 h, and 150 C for 3 h. Material Characterization The resins were characterized with dynamic mechanical thermal analysis (DMTA). DMTA spectra were taken on rectangular specimens (50 mm 10 mm 3 mm) in the flexural mode at 10 Hz with an Eplexor 25N device of a Gabo qualimeter. DMTA spectra, that is, the complex modulus (E*) and its constituents [storage modulus (E ) and loss modulus (E )] and the mechanical loss factor (tan ), were measured and calculated as a function of the absolute temperature (T) from T 100 C to T 200 C at a heating rate of 1 C/min. The number-average molecular weight between crosslinks (M c ) was determined with an equation from rubber elasticity theory: M c 3q RT E r where q is the front factor (usually 1), is the material density, R is the universal gas constant, and E r is the modulus in the rubbery region. The E r values and corresponding T values were read from the DMTA traces as described in our earlier work. 17 Information on the morphology of the VE and VE/EP systems was expected from atomic force microscopy (AFM) studies. The polished surfaces of the specimens were eroded by Ar -ion bombardment. This occurred in a secondary neutral mass spectrometer (INA3, Leybold) operating at 500 ev. The beam was focused perpendicularly to the polished surface of the specimen. The overall ion dose was Ar /cm 2, which resulted in a surface roughening of about 200 nm (as discussed later). The surface profile was scanned by AFM (Digital Instruments) in the tapping mode, and the related amplitude- and height-contrast images captured. The fracture toughness (K c ) and fracture energy (G c ) of the resins were determined for compact-tension (CT) specimens in accordance with the ESIS testing protocol. 18 The CT specimens (35 mm 35 mm 3 mm) were notched by sawing. Their notch root was sharpened by a razor blade being pushed before their tensile loading (mode I type) at room temperature on a Zwick 1445 machine at a crosshead speed of 1 mm/min. The thermal stability of the hybrid resins was detected by thermogravimetric analysis (TGA) from 25 to 630 C at a heating rate of 10 C/min. RESULTS AND DISCUSSION Crosslinking Reactions The IPN structure in the 1/1 VE/EP formulation forms in situ (often termed simultaneous IPN formation). As VE crosslinks by radical copolymerization, which is a very fast process, the IPN architecture can likely be controlled by the EP component and its curing. Cal-EP, with the Scheme 1

3 INTERPENETRATED VINYL ESTER/EPOXY RESINS 5473 Scheme 2 buildup given in Scheme 1, was selected because of its high inherent flexibility. The working hypothesis is that its curing via polyaddition, homopolymerization, and dual crosslinking reactions alters the IPN morphology and thus the fracture mechanical properties as well. The polyaddition-type curing of EP occurs with diamines (Al-Am and Cal-Am; cf. Scheme 2). The amine hardener may be involved in a side reaction with styrene (cf. the Michaels reaction in Scheme 3). As a result, parts of both styrene and diamine become deactivated and thus do not participate in the related crosslinking reactions. This may result in a lower crosslinking density of the EP component. The BF 3 complex works as a catalyst for the homopolymerization of EP, which yields a highly crosslinked structure (cf. Scheme 4). MAH is a dual crosslinking agent in our case. As the related recipe contains no tertiary amine compound (which is the usual catalyst for EP curing with anhydrides), the first reaction is likely between the anhydride groups of MAH and the secondary OOH groups of VE 19,20 [cf. Scheme 5(a)]. This product may copolymerize with styrene or VE, as depicted in Scheme 5(b). The developed carboxyl groups may react with the EP ones of the EP resin [cf. Scheme 5(c)]. Note Scheme 5(a,b) may occur in another sequence; that is, MAH, VE, and styrene copolymerize before the anhydride (MAH) hydroxyl (VE) reaction takes place (cf. Scheme 6). Regardless of whether Scheme 5 or 6 holds, the outcome is the same: a grafted IPN structure between VE and EP. It is intuitive that these crosslinking reactions should affect the morphology of the IPN-structured resins. To detect the supposed changes, we subjected the ion-eroded polished surfaces of the VE and VE/Cal-EP hybrid resins to AFM investigations. IPN Morphology Figure 1 clearly shows the nodular structure (also called microgel) of VE. Accordingly, VE exhibits a two-phase structure. More or less spherical VE nodules are dispersed in a poly(styrene-co-vinyl ester) matrix. The size of the VE nodules is nm. The presence of such a two-phase dispersion was shown also by Mortaigne et al., 21 who used laser ablation for the physical etching of VE. The effect of the EP hardener on the AFM images is displayed in Figures 2 5. In all cases, an IPN structure is present. It has been reported 22,23 that in the VE/EP combinations used, the softer EP phase is markedly less resistant to ablation than VE. Considering the effect of the diamine hardeners (cf. Figs. 2 and 3), we note that the IPN structure is far less compact in Cal-EP Al-Am than in Cal-EP Cal-Am. On the basis of the related morphology, we can predict lower stiffness but higher toughness for the hybrids whose EP has been hardened by Al-Am instead of Cal-Am. The IPN structures in Figures 3 (Cal-Am) and 4 (BF 3 ) are quite similar, but crosslinking by BF 3 Scheme 3

4 5474 GRYSHCHUK AND KARGER-KOCSIS a straightforward mathematical characterization of the IPN structures revealed (perhaps some kind of fractal analysis would be useful). Therefore, the IPN structures have been characterized by the apparent band width of the entanglements (cf. Table 1). The band width decreases according to the following ranking: Al-Am Cal-Am BF3 MAH Scheme 4 results in slightly more pronounced compactness. An ever more compact IPN morphology has been found for the VE/EP hybrid with MAH crosslinking (cf. Fig. 5). Unfortunately, we are not aware of This ranking suggests a strong effect of the hardener on the ideal EP network unit 23 and IPN grafting. Coming back to Figure 5, we note the onset of a layered structure (see the steps in Fig. 5). This may be traced to the presence of a highly grafted IPN. Because of the cross reactions between VE and EP, as suggested by Schemes 5 and Scheme 5

5 INTERPENETRATED VINYL ESTER/EPOXY RESINS 5475 Scheme 6 Figure 1. Height (left) and amplitude (right) AFM images taken from the ion-etched surface of VE.

6 5476 GRYSHCHUK AND KARGER-KOCSIS Figure 2. Height (left) and amplitude (right) AFM images taken from the ion-etched surface of VE/Cal-EP Al-Am. Figure 3. Height (left) and amplitude (right) AFM images taken from the ion-etched surface of VE/Cal-EP Cal-Am. Figure 4. Height (left) and amplitude (right) AFM images taken from the ion-etched surface of VE/Cal-EP BF 3.

7 INTERPENETRATED VINYL ESTER/EPOXY RESINS 5477 Figure 5. Height (left) and amplitude (right) AFM images taken from the ion-etched surface of VE/Cal-EP MAH. 6, the Ar -ion bombardment does not erode the EP phase selectively, and thus the strongly grafted IPN breaks in a stepwise manner. Figure 6(a,b) shows the stiffness ( E* ) and tan as a function of temperature for the thermosets studied. E* below the -relaxation transition [glasstransition temperature (T g )] follows this range: MAH Cal-Am BF3 Al-Am This agrees with the change in the entanglement band width; that is, the smaller the band width is of the IPN-structured resin, the higher its E* value is (cf. Table 1). To quantify the overall crosslinking degree of the resins, we have calculated the apparent M c values (M c,app ; see Table 1). DMTA Response Although Figure 6(a) already displays the strong effect of the hardener, it becomes even more pronounced in the tan T traces in Figure 6(b). T g of VE is reduced by the IPN structuring with EP, as the latter has a markedly lower T g than VE (not shown here). It is a general rule for IPN systems that the resulting T g is between those of the components, and T g can reliably be estimated by the Fox equation. 24 A further general aspect of IPNstructured resins is the appearance of a broad T g ( -relaxation peak). In this relaxation, a shoulder can be resolved for the diamine (Al-Am and Cal- Am) hardeners. This is usually a hint of worse compatibility between the resins. The -relaxation peak is very prominent for the Al-Am hardener; in addition, it shifts toward higher temperatures in comparison with those of other EP curing formulations. The pronounced peak may be traced to the chair boat conformation transition of the cyclohexylene unit in this Cal-EP/Al-Am system. According to the corresponding idealized network, the chair boat transition of the cyclohexylene unit of Cal-EP is not hampered in con- Table 1. Characteristics of the Crosslinked and IPN Networks (Hybrid) Resin Entanglement Band Width (nm) R z (nm) R max (nm) Density (g/cm 3 ) M c,app (g/mol) K c (MPa m 0.5 ) G c (kj/m 2 ) Peak Tan Peak VE 60 a b b VE/Cal-EP Al-Am VE/Cal-EP Cal-Am VE/Cal-EP BF VE/Cal-EP MAH a Nodular structure. b Not measured.

8 5478 GRYSHCHUK AND KARGER-KOCSIS Figure 6. (a) Traces of E* versus T and (b) traces of tan versus T for the IPNstructured VE/EP hybrid resins. trast to all other formulations, for which it requires cooperative motions. 23 Morphology Toughness Relationships The magnitude of the relaxation is often quoted to reflect the toughness (e.g., ref. 25 and references therein): the higher the relaxation is, the greater the toughness is. The related tan data in Table 1 seem to correlate with both the band width and M c,app. The K c and G c values are also listed in Table 1. Unfortunately, the limited amount of data available does not allow us to check whether G c changes linearly with the square root of M c,app according to the theory of Lake and Thomas. 26 On the other hand, attempts have been made to correlate G c with some roughness parameters of the AFM topology. The mean roughness depth (R z ) and the maximum roughness depth (R max ) have been determined along a diagonal scan line of the

9 INTERPENETRATED VINYL ESTER/EPOXY RESINS 5479 Figure 7. K c and G c (a) as a function of R z and (b) as a function of R max. height-modulated AFM images shown previously. R z is the mean of five roughness depths of five successive sample lengths of the roughness profile. R max is the largest of the five roughness depths. Figure 7(a,b) shows that both K c and G c increase with increasing R z and R max. This finding corroborates that the morphology of the IPN controls the fracture mechanical response. With increasing roughness, both K c (to a smaller extent) and G c (to a larger extent) increase. Attention should be paid to the fact that the roughness parameters change linearly with the band width. TGA Response The compatibility of the VE/EP formulations should also be shown in TGA measurements. Figure 8(a) demonstrates the weight-loss curves of VE and VE/EP formulations. For the hybrid resins, the thermal decomposition starts earlier than for VE. Thus, it is in concert with the ablation behavior. EP is markedly more etchable by Ar bombardment than VE. Furthermore, there is a strong effect with respect to the type of EP hardener. In case of the diamine hardeners, two de-

10 5480 GRYSHCHUK AND KARGER-KOCSIS Figure 8. TGA traces of the VE and VE/EP formulations: (a) weight loss versus temperature and (b) temperature derivative. composition steps appear [cf. Fig. 8(b)]. This reflects some IPN incompatibility already detected by AFM in the form of wider entangling bands. Better compatibility between the resins and forced grafting result in IPN-structured systems of improved thermal resistance [cf. Fig. 8(a)]. The thermal decomposition of such systems occurs in one major step, to which a second one is superposed as a shoulder [cf. Fig. 8(b)]. CONCLUSIONS Based on this work, devoted to the study of the effects of EP hardeners on the structure toughness relationships of VE/EP (1/1) hybrid resins, the following conclusions can be drawn: The type of EP hardener strongly affects the IPN morphology. The latter is characterized

11 INTERPENETRATED VINYL ESTER/EPOXY RESINS 5481 by the entangling band width and surface roughness parameters (R z and R max ) derived from AFM scans taken on ion-etched surfaces of the resins. Increasing the band width and roughness increases the K c and G c values. G c is more sensitive to the IPN morphology than K c. Cross reactions between the VE and EP phases (the formation of grafted IPN) and EP homopolymerization yield a compact IPN structure with a low band width and roughness. The related hybrids show improved stiffness, though at the cost of toughness, in comparison with the other formulations. The authors thank the German Science Foundation for its financial support of this project (DFG FOR 360; Ka 1202/9). Thanks are also due to DSM Composite Resins and U. Prümmer Polymer-Chemie GmbH for material samples and to Fonds der Chemischen Industrie for financial support. REFERENCES AND NOTES 1. Sperling, L. H. Polymeric Multicomponent Materials; Wiley: New York, 1997; Chapter Sperling, L. H. Interpenetrating Polymer Networks and Related Materials; Plenum: New York, Frisch, H. L.; Zhou, P. In Interpenetrating Polymer Networks; Klempner, D.; Sperling, L. H.; Utracki, L. A., Eds.; Advances in Chemistry Series 239; American Chemical Society: Washington, DC, Zhou, P.; Frisch, H. L.; Ghiradella, H. J Polym Sci Part A: Polym Chem 1992, 30, Frisch, H. L.; Frisch, K. C.; Klempner, D. Pure Appl Chem 1981, 53, Tabaka, M. T.; Widmaier, J. M.; Meyer, G. C. In Sound and Vibration Damping with Polymers; Corsaro, R. D.; Sperling, L. H., Eds.; ACS Symposium Series 424; American Chemical Society: Washington, DC, Dadbin, S.; Burford, R. P.; Chaplin, R. P. Polym Gels Networks 1995, 3, Dadbin, S.; Burford, R. P.; Chaplin, R. P. Polymer 1996, 37, Xue, Y.; Frisch, H. L. J Polym Sci Part A: Polym Chem 1994, 32, Hourston, D. G.; McCluskey, J. A. J Appl Polym Sci 1986, 31, Edbon, R. J.; Hourston, D. J.; Klein, P. G. Polymer 1984, 25, Ferry, J. D. Viscoelastic Properties of Polymers; Wiley: New York, Levita, G.; De Petris, S.; Marchetti, A.; Lazzeri, A. J Mater Sci 1991, 26, Varley, R. J.; Hodgkin, J. H.; Simon, G. P. J Appl Polym Sci 2000, 77, Un, B. K.; Young, J. J., Kim, J.; Lee, S.-S.; Park, M.; Lim, S. J Appl Polym Sci 2001, 79, Crawford, E.; Lesser, A. J. J Polym Sci Part B: Polym Phys 1998, 36, Karger-Kocsis, J.; Gremmels, J. J Appl Polym Sci 2000, 78, Williams, J. G. In Fracture Mechanics Testing Methods for Polymers Adhesives and Composites; Moore, D. R.; Pavan, A.; Williams, J. G., Eds.; ESIS Publication 28; Elsevier: Oxford, 2001; p Lees, H.; Neville, K. Handbook of Epoxy Resins; McGraw-Hill: New York, Shechter, L.; Wynstra, J. Ind Eng Chem 1956, 48, Mortaigne, B.; Feltz, B.; Laurens, P. J Appl Polym Sci 1997, 66, Karger-Kocsis, J.; Gryshchuk, O.; Schmitt, S. J Mater Sci 2003, 38, Karger-Kocsis, J.; Gryshchuk, O.; Jost, N. J Appl Polym Sci 2003, 88, Fox, T. G. Bull Am Phys Soc 1953, 1, Karger-Kocsis, J.; Kuleznev, V. N. Polymer 1982, 23, Lake, G.; Thomas, A. G. Proc R Soc London Ser A 1967, 300, 108.