Rapid evaluation of thermal aging of polymer composites

7 March 2014
Wei Fan and Jia-lu Li
A statistical model predicts the residual flexural strength and lifetime of polymer composites based on performance over several days.

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 CH2 were oxidized to carbonyl in the later stage of thermal aging when the aging temperature was at least the polymer glass transition temperature (Tg): 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 Tg: 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 Tg 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


Fourier transform IR spectra recorded on the surface of neat resin samples after aging at different temperatures (90, 120, and 150°C) for 1, 3, 6, or 13 days (1, 3, 6, or 13d).


Weight loss of neat resin (R) and the laminated epoxy composite (L) vs. aging time at different aging temperatures.


Plots of the glass transition temperature (Tg) of L at different aging temperatures against (a) aging time and (b) conversion rate of epoxide group.

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.


Scanning electron microscopy (SEM) images of fracture surfaces of unaged and aged R samples at 90°C for different aging times.


SEM images of fracture surfaces of unaged and aged L samples at different temperatures for periods up to 13 days.


Plots of (a) flexural strength retention rate and (b) flexural modulus retention rate of R and L samples aged at different temperatures, against aging time.

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

where 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

Wei Fan
Composites Research Institute of Tianjin University and Education Ministry Key Laboratory of Advanced Textile Composite Materials

Wei Fan is mainly engaged in research on the structure and thermal aging performance of 3D braided polymer composites.

Jia-lu Li
Composites Research Institute of Tianjin University and Education Ministry Key Laboratory of Advanced Textile Composite Materials

Jia-lu Li pioneered 3D braided composites in China, and is mainly engaged in research on the structure and performance of 3D braided composite material.


References

  1. M. Akay and G. R. Spratt, Evaluation of thermal ageing of a carbon fibre reinforced bismalemide, Compos. Sci. Technol. 68, pp. 3081-3086, 2008.

  2. C. Damian, E. Espuche and M. Escoubes, Influence of three ageing types (thermal oxidation, radiochemical, and hydrolytic ageing) on the structure and gas transport properties of epoxy-amine networks, Polym. Degrad. Stabil. 72, pp. 447-458, 2001.

  3. T. K. Tsotsis, S. Keller, K. Lee, J. Bardis and J. Bish, Aging of polymeric composite specimens for 5000 hours at elevated pressure and temperature, Compos. Sci. Technol. 61, pp. 75-86, 2001.

  4. J. Kim, W. I. Lee and S. W. Tsai, Modeling of mechanical property degradation by short-term aging at high temperatures, Compos. B 33, pp. 531-543, 2002.

  5. S. Ciutacu, P. Budrugeac and I. Niculae, Accelerated thermal aging of glass-reinforced epoxy resin under oxygen pressure, Polym. Degrad. Stabil. 31, pp. 365-372, 1991.

  6. W. Fan and J.-L. Li, 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

  7. 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