### Rapid evaluation of thermal aging of polymer composites

Polymer composites (PMCs) based on high-temperature thermosetting resins show promise as next-generation materials for aerospace applications.^{1} Of particular interest is a series of composite materials consisting of epoxy resin and carbon fibers, which have excellent bonding, thermal, and mechanical characteristics.^{2} However, aerospace applications require long-term stability, and thus it is necessary to understand how the materials will behave throughout their intended service life.

The long-term performance of PMCs at elevated temperature is dictated by their thermal and oxidative stability.^{3} Previous research has mainly focused on the thermal aging mechanism and mechanical properties of PMCs, with only a few articles concerned with service life.^{4,5} We have developed a degradation model of thermal aging of a laminated composite as a function of time and temperature, based on experimental data.^{6}

We exposed samples of an epoxy (bisphenol A) neat resin (R) and a ‘T700-12K’ plain-weave carbon fiber fabric/epoxy 10-ply laminated composite (L) in fan ovens at 90, 120, and 150°C for up to 13 days. We analyzed the composites' flexural properties and weight loss and further studied the thermal degradation mechanism using chemical analysis, differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and surface morphology.

Fourier transform IR spectra showed that postcuring reactions occur in the initial aging period, and also that C–H and CH_{2} were oxidized to carbonyl in the later stage of thermal aging when the aging temperature was at least the polymer glass transition temperature (T_{g}): see Figure 1. Plotting weight loss against aging time for R and L at different aging temperatures revealed that weight loss increases in a simple exponential fashion and increases with temperature: see Figure 2. For the same conditions, R loses more weight than L, which indirectly illustrated that the weight loss occurred predominantly within the matrix of the composites. DSC analysis showed that postcuring of the matrix material raises the composite T_{g}: see Figure 3(a). We know from chemical analysis that many bonds were oxidized after aging of R at 150°C. Consequently, we assume that the reduction in T_{g} is caused by chain scissions in the polymer. We calculated the conversion rate of the epoxide group by analyzing Fourier transform IR spectra taken over time: see Figure 3(b).^{7}

Figure 1.

Figure 2.

Figure 3.

Higher aging temperatures and longer aging times increase the brittleness of the resin and cause the fiber/matrix interface in L to deteriorate: see the SEM images in Figures 4 and 5, respectively. The flexural strength and the flexural modulus retention rates of L were greater than those of R: see Figure 6. The flexural properties of L may be largely controlled by thermal aging of the matrix material.

Figure 4.

Figure 5.

Figure 6.

We developed two statistical models to rapidly predict the flexural strength and lifetime of the laminated composite in a certain temperature range. We fitted the degradation of the flexural strength observed in Figure 6 using the following equation:^{6}

*y*is residual flexural strength in MPa;

*t*is aging time in days;

*k*is an aging rate constant with temperature dependence as shown in the Arrhenius-type equation (2);

*B*is the initial strength; and

*a*is constant between zero and 1. where

*T*is the temperature in kelvin, and

*A*and

*E*are constants.

^{4}

Combining Equations (1) and (2) predicts the lifetime of L as about 14 years at normal atmospheric temperature (*T*=298K), assuming that the residual flexural strength is equal to 80% of its initial strength (i.e., *y*=80% *B*) as the critical value.

In summary, we have developed a model that quantifies the thermal aging degradation of a laminated epoxy resin/carbon fiber composite by its residual mechanical properties after accelerated aging. We have used the model to predict the lifetime of the composite. It can be applied to a range of many commercial PMCs. As a next step, we will use the model to predict the lifetimes of 3D braided polymer composites and also prolong the aging time to further verify our equations' reliability.

## Authors

## References

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*Rapid evaluation of thermal aging of a carbon fibre laminated epoxy composite*,**Polym. Compos.**, 2013. Article first published online: 29 October 2013. doi:10.1002.pc.22743 - X. Colin, C. Marais and J. Verdu,
*Kinetic modelling of the stabilizing effect of carbon fibres on thermal ageing of thermoset matrix composites*,**Compos. Sci. Technol.****65**, 2005.

**DOI:** 10.2417/spepro.005232