Effect of Resin Formulation on Thermoset Viscoelasticity

[+] Author and Article Information
K. M. B. Jansen

 Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlandsk.m.b.jansen@3me.tudelft.nl

J. Eng. Mater. Technol 128(4), 478-483 (May 08, 2006) (6 pages) doi:10.1115/1.2345437 History: Received August 26, 2005; Revised May 08, 2006

Viscoelastic relaxation curves of thermoset resins may change considerably for relatively small changes in properties of the initial monomer mixture or final conversion level. In order to be able to predict the effect of such changes, a model is proposed which relates the properties of the initial monomer mixture such as the functionality, mixing ratio, and conversion level to changes in the relaxation curves. It basically consists of two parts. Firstly, the crosslink density is calculated based on the exact composition of the monomer mixture and, secondly, the effect of this crosslink density on the position of the glass transition and the rubbery modulus was calculated. The model was tested with a series of Novolac epoxies in which the functionality, mixing ratio, and conversion level were varied systematically. It turned out that changes in the relaxation curves due to variations in conversion level could be predicted quite accurately from the shape and shift factor of the fully cured mastercurve. The agreement between relaxation curve predictions for the series with changing functionality and mixing ratio was only moderate, which was ascribed to errors in the prediction of correct values for the crosslink density.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

Structure formulas, molecular weight M, and functionality f of: (a) epoxy monomers, (b) monomer with missing epoxy group, and (c) bisphenol-A

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Figure 2

Storage modulus of standard epoxy system (fully cured, stoichiometric EPN1180 epoxy+BPA). Symbols: mastercurve after frequency-temperature superposition; thick full line: fit to mastercurve (Eq. 11). (b) Shift factor aT corresponding to mastercurve of (a). Symbols: data points; full lines: fit functions.

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Figure 3

Measured versus predicted crosslink density for series with changing conversion, functionality, and mixing ratio. Points on dashed line agree exactly with predictions.

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Figure 4

DMA glass transition temperature versus crosslink density. Dashed line: prediction according to Eq. 14 with B=0.028Cm3∕mol

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Figure 5

Mastercurves of conversion series for a reference temperature of 120°C. Full lines are predictions based on the reference curve and predicted crosslink density.

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Figure 6

As in Fig. 5, but now for the functionality series. The fA=3.6 data corresponds to the series pA=0.97 in Fig. 5 (standard).

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Figure 7

As in Fig. 5, but now for the mixing ratio (rA) series. The rA=1.0 data corresponds to the series pA=0.97 in Fig. 5 (standard).



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