Degradation Kinetics of High-Performance Polymers and Their

Chapter 9

Degradation Kinetics of High-Performance Polymers and Their Composites 1

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J . M. Kenny and L . Torre

Downloaded by NORTH CAROLINA STATE UNIV on August 19, 2012 | http://pubs.acs.org Publication Date: October 13, 1995 | doi: 10.1021/bk-1995-0603.ch009

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Institute of Chemical Technologies, University of Perugia, Loc. Pentima Bassa, 05100 Terni, Italy Department of Materials and Production Engineering, University of Naples, P. Tecchio, 80125 Naples, Italy An experimental and theoretical study of the thermal stability of high performance thermoplastic matrices and their composites is presented. The study is focused on the degradation behavior, under different environments, of polyetheretherketone (PEEK), polyetherimide (PEI) and their continuous carbon fiber reinforced composites. The experimental results, obtained by thermogravimetric analysis performed in air and pure nitrogen, are the basis of the development of a phenomenology kinetic model which is able to predict the degradation rate under different environmental conditions. Moreover, the results of the model can be used to predict the onset of the degradation process at longer times and lower temperatures than those used in the experimental characterization. The experimental characterization has shown a good thermal stability of the materials studied with a better performance of PEEK with respect to PEI at high service temperatures.

Advanced thermoplastic polymers have been proposed in the last decade as a valid alternative to typical thermoset matrices in composites for aerospace applications. The most common thermoplastic matrices for thçse materials are the semicrystalline polyetheretherketone (PEEK) and the amorphous thermoplastic polyetherimide (PEI). Although cost considerations have not allowed an extensive utilization, thermoplastic matrix laminates normally offer higher toughness and damage tolerance properties than their equivalent thermoset based composites. As new projects in the aerospace industry are focused in the development of supersonic planes characterized by higher levels of the normal service temperatures, polymer matrix composites are being also considered as natural candidate materials as a consequence of their good thermal stability. This new application requires a reliable prediction of the long term behavior of such materials after exposure at elevated temperatures. The utilization of a material under critical temperature conditions requires the knowledge of its degradation kinetics which is related both to morphological changes and to chemical decomposition reactions. The degradation of polymeric composites has been studied mainly in relation with the reprocessability of thermoplastics after several cycles (1-3). For example, PEEK changes irreversibly its ability to crystallize if it is repetitively heated up to 400°C. However, chemical degradation is typically detected by monitoring weight changes occurring to a polymer exposed to high

0097-6156/95/0603-0140$12.00/0 © 1995 American Chemical Society

In High-Temperature Properties and Applications of Polymeric Materials; Tant, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


9. KENNY & TORRE

Degradation Kinetics of High-Performance Polymers

temperatures as a function of time. While in such situation metallic materials show generally a weight gain curve as a consequence of oxidation processes, polymers are characterized by weight loss processes that can be attributed to evaporation of solvents and/or plasticizers at relative low temperatures, and to chain scission with production of low molecular weight volatile molecules at higher temperatures. The particular mechanisms of degradation are a function of the polymer structure and can follow different steps because of the complexity of the polymeric chains (4-6). Functional group transfer, chain unzipping with elimination of volatiles, and the formation of intermediate compounds are the main decomposition mechanisms that contributes to complicate the development of a mechanistic kinetic model of the polymer degradation process. In particular, reported data on PEEK degradation indicated benzoquinone as the first gaseous product of the chain breaking reaction followed by the rearranging of the polymer chain (6,7). The effect of the environment is very important in polymer degradation. The presence of oxygen as reactant combined with heat causes very rapid degradation processes in polymeric chains. Therefore its very important to fix the environmental condition, when decomposition kinetics is studied. Moreover, in the particular case of polymer based composites, the presence of the carbon fibers must also be considered as they can affect the degradation process and degrade themselves in the presence of oxygen. Furthermore, carbon fibers can affect the thermal diffusion in the bulk. Thermogravimetric analysis (TGA) is commonly used to study the degradation of polymeric materials (8). With this technique the weight loss is measured as a function of time and temperature. Then, the degradation kinetics can be studied correlating the weight loss with a generic degree of degradation (9,10). Although T G A data may not give enough information about complex degradation mechanisms, it can be a very useful tool to analyze the overall behavior of the polymer in extreme conditions (4, 11, 12). When long term data are needed, the main experimental problem is the test duration. However, accelerated degradation experiments at high temperatures can be performed and the long term behavior can be extrapolated at lower temperatures . This procedure can be applied, under particular restrictions, assuming a simplified degradation process (9). In this work the results of several T G tests, performed under different environmental conditions, are used to analyze the thermal stability, and to develop a kinetic model of their degradation processes. The model can be applied to extrapolate the degradation behavior of the materials studied at long times. Experimental Procedures and Materials Degradation studies were performed on pellets of polyetheretherketone (PEEK) produced by ICI and polyetherimide (PEI-UltemlOOO) produced by General Electric, and on their commercial carbon fiber composites: APC2 and C E T E X respectively. The materials used in this research were kindly provided by Alenia. PEEK is a semicrystalline thermoplastic polymer that has been specially developed for high performance composites (13). It has a Tg of 147°C, a crystallinity content developed under normal processing conditions of 30%-35% with a melting point of 335°C and a processing temperature of 400°C. On the other hand PEI is an amorphous thermoplastic polymer with a Tg of 210°C and a processing temperature of 320°C. The environmental behavior of these two polymers has been mainly studied with reference of their solvent resistance (14). Weight loss determinations were performed in a T A T G A Thermogravimetric Analyzer model 911. Samples were tested both in air and in nitrogen atmospheres, in isothermal and dynamic modes, at different temperatures and heating rates in order to cover a wide range of thermal conditions. Data obtained from T G A experiments were analyzed using a computer program for non-linear regression analysis (Systat).


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HIGH-TEMPERATURE PROPERTIES AND APPLICATIONS OF POLYMERS

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T E M P E R A T U R E (°C) Figure 1 : Weight loss of P E E K in N2 and its derivative as a function of temperature obtained in dynamic TGA tests at different heating rates.

Figure 2: Weight loss of the PEEK matrix composite in N2 and its derivative as a function of temperature obtained in dynamic TGA tests at different heating rates.


9. KENNY & TORRE

Degradation Kinetics of High-Performance Polymers

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Results and Discussion P E E K Based Materials. Results of several thermogravimetric tests performed on the P E E K matrix at different constant heating rates, in nitrogen environment, are reported in Figure 1, in terms of percentage of residual weight as a function of time. The polymer is apparently stable below 450°C. At higher temperatures the sample weight begins to diminish in a degradation process which ends above 700°C where a stable degraded material is formed. The effect of the heating rate on the degradation kinetics is manifested in the shifting of Figure 1 curves: higher heating rate curves are shifted to higher degradation temperatures. This effect is mainly dependent on the activation energy of the degradation process. Similar degradation tests performed on the PEEK matrix composite are reported in Figure 2. In this case the effect of the carbon fibers is mainly manifested in the different residual weight of both samples. While in the first case this value (ca. 50%) is given by the remaining carbon structure of the polymer, the composite residue (ca. 83%) is also represented by the fibers that do not degrade in the nitrogen environment. A rough calculation gives, with the assumption that no carbon is lost during degradation, a fiber weight fraction of 66% which is close to the standard content (68% wt.). A second observation regards the difference between the form of the derivative curves between both materials. These curves, which can be considered as a representation of an overall degradation rate, show practically the same peak temperature. However, the degradation peak for the neat polymer is sharper than in the case of the composite. These two observations suggest a diffusion effect of the presence of the fibers which are not involved in the degradation process itself, but act as an inert filler only modifying the heat transfer in the material. As the service life of the materials studied involves normally the presence of oxygen, weight loss tests were also performed in normal air environment. The results of a dynamic T G A test performed at 10°C/min. are reported in Figure 3 where an evident difference with tests performed in nitrogen is observed. While only the matrix is degraded in nitrogen in a carbonization process, a complete degradation of the material is achieved in air. This difference is also manifested in the difference of the derivative curve obtained in air with respect to Figures 1 and 2 results. A single peak characterizes the degradation reaction under nitrogen while a double peak is observed when the process is conducted in the presence of oxygen. In the second case the first peak is located in the same temperature range and weight loss as the one observed in nitrogen, and can be attributed to matrix degradation, while the second peak can be easily attributed to the oxidation of the carbon fibers and the carbon structure of the degraded matrix. A similar weight loss and temperature degradation interval characterizes the degradation of the matrix in both cases suggesting that the first degradation process of the matrix is given mainly by thermally controlled dehydrogenation, without strong influence of oxidation processes on the reaction yield. While dynamic test results shown in Figures 1-3 give useful information on the temperature interval and total weight loss associated with the degradation reactions; isothermal test results are needed for kinetic analysis in order to decouple the contribution of temperature. These results are shown in Figures 4-6 for different materials and environmental conditions. The test temperatures were chosen following the dynamic results in the observed degradation interval. Higher values of the temperature produced very fast process with poor quality thermograms, and lower temperatures produced very slow processes with very long time experiments (>24 hours). By simple inspection of weight loss results the observations from dynamic tests are confirmed. A wider degradation interval is shown by the composite (Figure 5) compared with the neat polymer (Figure 4) and the degradation process in air (Figure 6) is faster than in nitrogen (Figure 5). It must be noted that while the presence of


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HIGH-TEMPERATURE PROPERTIES AND APPLICATIONS OF POLYMERS

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Degradation Kinetics of High-Performance Polymers and Their

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