Unfolding complex thermal degradation mechanisms in polymer composites

25 April 2018
Muhammad Azeem Arshad and Abdelkrim Maaroufi
A new modeling method enables a kinetic interpretation of the complicated multistep thermal degradation mechanisms in polymer composites.

The integration of metal particles into polymer composites has established an interesting class of materials that is economical, environmentally friendly, and multipurpose. Among this class, composites of urea-formaldehyde cellulose (UFC) filled with metal particles exhibit a number of invaluable characteristics (e.g., higher thermal stability and interesting electrical/dielectrical properties), making them suitable for a wide range of applications.1–3

UFC/metal composites can be categorized into (electrically and thermally) insulating and conductive types. Insulating composites are potentially useful as thermal grease, thermal-interface materials, and electric cable insulation. Conductive composites, on the other hand, can be used for thermoelectrical and thermomechanical applications, and in organic photovoltaics.1–3

Kinetic modeling of thermally stimulated condensed phase processes can determine the activation parameters of polymer/metal composites and is therefore an extremely useful tool in the development of these materials. Kinetic interpretation enables transition states and process mechanisms within materials to be analyzed. The kinetic parameters of a material are physically meaningful because their alteration can enable processes (e.g., thermal decomposition of organic/inorganic materials, polymerization/depolymerization and cross-linking reactions in polymers, and thermal/thermo-oxidative degradation of polymeric materials) to be controlled, the efficiency of the material to be optimized, and the thermal stability (i.e., the lifetime)—even of materials outside the experimental range—to be determined. However, thermally activated condensed phase processes are renowned for their complexity. Even an apparently simple reaction might consist of several steps. We therefore developed an advanced approach for kinetically interpreting thermally stimulated condensed-phase processes.4 By applying this approach, we were able to investigate the degradation behaviors of epoxy composites filled with tin particles5 and to obtain profound insights into their thermal degradation pathways.5, 6

In our most recent work, we investigated the applicability of our advanced kinetic approach4 for predicting the complex reaction mechanisms of polymer composites. We also investigated the ability of our kinetic approach (and the mechanistic information thus obtained) for predicting the thermal degradation of urea-formaldehyde cellulose (UFC) composites filled with metal particles. To prepare the UFC for our UFC/tin composites, we used commercial grade urea-formaldehyde resin (supplied by Aicar S.A., Cerdanyola del Vallès, Spain) filled with α-cellulose, with a density of 1.38g/cm3 and an electrical conductivity of around 1×10−13S/cm at standard temperature. We incorporated 30wt% α-cellulose into the UFC resin and used a commercial powder of β-tin—delivered by Panreac (Castellar del Vallès, Spain)—with a purity of about 97%, a density of 7.29g.cm−3, an average particle size of 15±10μm, and an electrical conductivity of the order of 104S.cm−1. Prior to fabrication of the composite, we dried both the resin and the metal powder thoroughly at 60°C for 48h. We then prepared tin-filled UFC composites via blending and hot pressing, as described in our previous studies.1–3

We subjected the UFC/tin composites to thorough structural characterization. For this purpose, we selected a pair of insulating and conducting UFC/tin composites (with 5 and 35vol.% tin, respectively). We analyzed the dispersion of the metallic particles inside the composites, the polymer-metal interphases, the number/types (i.e., crystallinity and amorphicity) of the internal phases present in the composites, and the nature of the bonding in and between the composites' constituents by scanning electron microscopy (SEM), x-ray diffraction (XRD), and Fourier transform infrared (FTIR) analyses. The cross-sectional SEM images of UFC and UFC/tin composites, shown in Figure 1, reveal the morphologies of the composites and the influence of tin within the UFC. Our SEM, XRD (see Figure 2), and FTIR results suggest that the composites are fairly homogeneous. Furthermore, the XRD and FTIR spectra of pristine UFC and UFC/tin composites demonstrate fairly similar results,1 which suggests that the interactions between UFC and tin are unlikely to be chemical in nature.1–3


Scanning electron micrographs of (a) pure urea-formaldehyde cellulose (UFC), (b) insulating composite (UFC/Sn5vol.%, i.e., UFC with 5vol.% tin), and (c) conducting composite (UFC/Sn35vol.%, i.e., UFC with 35vol.% tin). (d) Energy dispersive x-ray diffraction spectra for UFC/tin composites.


Energy dispersive x-ray diffraction spectra for UFC/tin composites.

Once we had confirmed (by structural analysis) that the composites were suitable for further characterization, we subjected them to thermogravimetric analysis (TGA). We collected the thermoanalytical data of the composites by using a TGA analyzer (in nonisothermal experiment mode) at different heating rates. We also performed a thermal degradation kinetic study of the composites at a range of temperatures (from 30 to 600°C), under nitrogen flow. The apparent reaction profiles of pure UFC and UFC/tin composites thus obtained are shown in Figure 3, wherein the multistep thermal degradation of pure UFC and UFC/tin composites is described. In particular, the thermal degradation behaviors of the insulating and conducting UFC/tin composites could be attributed to the variation in the heat capacity and catalytic activity of tin (in the UFC resin) with temperature.


Thermogravimetric (TG, continuous lines) and derivative-thermogravimetric (DTG, dashed lines) curves of pure UFC, and insulating and conductive UFC/tin composites.

In the next step, we further probed the thermal behavior of the UFC/tin composites by carrying out kinetic modeling on the thermoanalytical data. We used the basic condensed-phase kinetic equation to model the thermally stimulated process. This equation can be used to determine the reaction rate from the gas constant, the pre-exponential factor (which describes the collision frequency of particles that are involved in the formation of the activated complex), the activation energy barrier of the reaction, and the function of degree of conversion (which describes the reaction mechanism).7 Several different kinetic methods can be used to determine the activation energies of pure UFC and UFC/tin composites. However, the results from our generalized method4, 7—see Figure 4—show that the description of thermal degradation mechanisms in UFC and UFC/tin composites cannot be given by a single-step kinetic equation. We therefore applied the advanced reaction determination methodology4, 7 to our thermoanalytical data for the pure UFC and UFC/tin composites to evaluate the reaction models. The results that we obtained (shown in Figure 5) suggest that the thermal degradation of pure UFC and UFC/tin composites follows highly complicated nucleation/growth mechanisms. Moreover, our detailed interpretations of the obtained kinetic parameters3 have enabled us to determine that our kinetic approach can be applied to other UFC-based composites (i.e., UFC/aluminum1 and UFC/zinc2) to reveal useful mechanistic information.


The variation in activation energies with respect to the degree of reaction advancement, derived by kinetic analysis, of pure UFC and insulating and conductive UFC/tin composites.


Reaction models of the thermal degradation of pure UFC and UFC/tin composites, evaluated by our advanced reaction model determination methodology. α: Degree of reaction advancement. f(α): Reaction model. df/dT: Temperature derivative of reaction model.

In summary, we have developed and explored the use of an advanced reaction model determination methodology, and have found it to be suitable for kinetically interpreting simple as well as complex thermal degradation mechanisms in polymer composites. We have recently used this approach to generalize the degradation of polymer solar materials and solar cells,8 with the aim of eventually predicting their lifetime. Furthermore, results from a recent kinetic study that we carried out on the dynamic percolation mechanisms in polymer composites will be published shortly.


Authors

Muhammad Azeem Arshad
Laboratory of Composite Materials, Polymers and Environment Faculty of Sciences, Department of Chemistry, Mohammed V University

Muhammad Azeem Arshad is a PhD research fellow working under the supervision of Abdelkrim Maaroufi. His research focus includes the kinetics of thermally stimulated solid-state processes.

Abdelkrim Maaroufi
Laboratory of Composite Materials, Polymers and Environment, Faculty of Sciences, Department of Chemistry, Mohammed V University

Abdelkrim Maaroufi is a professor whose research interests are focused on the electrical and thermomechanical properties of polymers and phosphate composite materials.


References

  1. M. A. Arshad, A. Maaroufi, R. Benavente and G. Pinto, Thermal degradation of urea-formaldehyde cellulose composites filled with aluminum particles: kinetic approach to mechanisms, J. Appl. Polym. Sci. 134, pp. 1-13, 2017.

  2. M. A. Arshad, A. Maaroufi, R. Benavente and G. Pinto, Kinetics of the thermal degradation mechanisms in urea-formaldehyde cellulose composites filled with zinc particles, J. Mater. Sci. Mater. Electron. 28, pp. 11832-11845, 2017.

  3. M. A. Arshad, A. Maaroufi, R. Benavente and G. Pinto, Predicting thermal degradation mechanisms in urea-formaldehyde cellulose composites filled with tin particles, Polym. Compos., 2017.

  4. M. A. Arshad and A. Maaroufi, An innovative reaction model determination methodology in solid state kinetics based on variable activation energy, Thermochim. Acta 585, pp. 25-35, 2014.

  5. M. A. Arshad and A. Maaroufi, Predicting thermal degradation mechanisms in polymer composites, Soc. Plast. Eng., 2015.

  6. M. A. Arshad, A. Maaroufi, R. Benavente and G. Pinto, Thermal degradation mechanisms of epoxy composites filled with tin particles, Polym. Compos. 38, pp. 1529-1540, 2017.

  7. M. A. Arshad and A. Maaroufi, Recent advances in kinetics and mechanisms of condensed phase processes: a mini-review, Rev. Adv. Mater. Sci. 51, pp. 177-187, 2017.

  8. M. A. Arshad and A. Maaroufi, Kinetic approach to degradation mechanisms in polymer solar cells and their accurate lifetime predictions, J. Power Sources, 2018. Accepted.

DOI:  10.2417/spepro.006991