The isothermal degradation of some polyetherketones: a comparative kinetic study between long-term and short-term experiments

doi:10.1016/S0141-3910(01)00249-X

Polymer Degradation and Stability

Volume 75, Issue 3, 2002, Pages 465-471

https://doi.org/10.1016/S0141-3910(01)00249-X Get rights and content

Abstract

The kinetics of the isothermal degradation of three high thermally stable aromatic polyetherketones was studied by both a long-term (about 3 years) experiment at a temperature (270 °C) largely lower than the temperatures of fusion, and a set of short-term experiments at temperatures near the temperatures of fusion. An induction period was observed at 270 °C, followed by two degradation stages, the first with an exponential increase of the weight loss rate (V) as a function of heating time (t) and the final one showing constant weight loss rate. Short-term experiments showed similar behaviour, but no induction period was observed. The equations V=V_o×2^αt and V=K at various temperatures were determined for the exponential and linear degradation stages, respectively. The e-folding time α increased with temperature according to Arrhenius-type equations, by which the apparent activation energy values for degradation could be determined. The results are discussed and indicate that the kinetic parameters of degradation change with temperature.

Introduction

In recent years the interest of the aerospace industry has been devoted to the development of low density materials to be used in structural applications and, in particular, of high performance composites with polymer matrix. Themosetting matrices are widely used for this purpose, but their brittle nature to impact-initiated failure and the high production costs of their composites limit their use and drive the industries to examination of thermoplastics. Since these polymers can be subjected to high temperature not only in manufacturing, compounding and processing stages, but also in service or during repair, an important aspect of the development of modern engineering plastics is directed toward producing thermally stable thermoplastics keeping good physical and mechanical properties as well as good resistance to oxidative and chemical breakdown at high temperature. Since the thermal stability of a polymer is connected with the initial degradation temperature and the rate of degradation, the study of the kinetics of degradation and the determination of the associated kinetic parameters, particularly the apparent activation energy values, appear to be an interesting topic of research in this field.

We have in progress a comparative kinetic study on the degradation, in various environments and experimental conditions, of aromatic thermoplastic polyethers containing ketone and sulfone groups [1], [2], [3], [4], [5], [6], which are economically accessible polymers showing favourable properties for use as matrices for high performance composite materials, because of the presence of fully oxidized and flexibilizing ether and carbonyl groups [7], [8], [9], [10], [11].

Short-term isothermal experiments are usually performed to determine kinetic parameters associated with the thermal degradation of polymers, but the apparent activation energy values and the corresponding degradation curves thus determined might not be suitable to represent the behaviour of polymers in service because they are usually obtained at temperatures near (or, in some cases, above) the melting temperatures.

In this work we made a long-term isothermal degradation study on the following thermoplastic polyetherketones having very similar structures $− O − ArCOArCOArCOAr − −$ $− − OArO - ArCOArCOArCOAr − −$ $− − OArArO − ArCOArCOArCOAr − −$ (where: Ar=1,4-substituted phenylene).

The experiments were carried out in static air atmosphere for more than 3 years, at a temperature (270 °C) about 150 °C lower than the melting temperatures of the polymers. The results obtained were compared with those from a set of short-term isothermal measurements in a range (350–530 °C) more extended than that (470–530 °C) we used in the past for analogous measurements on the same polymers in the molten state [2]. In such a way we could consider also temperatures lower than the melting temperatures.

The purposes of this work were the following:

1.
to check if the apparent activation energy of degradation (E_a) is the same near the melting temperature and at largely lower temperatures;
2.
to find the V=f(t) equations at the various temperatures used for isothermal experiments;
3.
to verify if it is possible to predict similar equations at lower temperatures by experiments carried out at higher temperatures;
4.
to have information about the time needed to reach appreciable weight loss, to explore the possibility of performing degradation experiments below the glass transition temperature in reasonable times.

Section snippets

Materials

Polymers were synthesized according to the procedure previously reported [12] and were used in powder form. Polymers were dried under vacuum at room temperature and kept in a desiccator under vacuum until use.

Viscosity measurements

An Ostwald viscometer was used to determine the inherent viscosity (η_inh=lnη_r/C) of polymers. Measurements were performed at 30 °C (±0.1 °C), in sulfuric acid solution at a concentration of 0.5 g/dl.

Calorimetric measurements

A Mettler DSC 20 calorimeter coupled with a Mettler TC 10A processor as control and

Results and discussion

The melting temperatures of the polymers were first determined by DSC. Multiple unresolvable melting endotherms were observed in all cases, thus suggesting the presence of more than one crystal forms in the “as made” polymers. In particular, two DSC peaks were found for polymers 1 and 3, and three DSC peaks for polymer 2. The peak temperatures were taken as melting temperatures due to the impossibility of determining the onset values. The percentages of crystallinity were calculated by

Acknowledgements

The authors are grateful to the Italian M.U.R.S.T. for financial support.

References (23)

L Abate et al.
Polymer
(1999)
L Abate et al.
Polymer
(2000)
P.A Staniland et al.
Polymer
(1992)
D.J.T Hill et al.
Polym. Degrad. Stab.
(1993)
A Akbas et al.
Polymer
(1994)
L Abate et al.
Polym. Eng. Sci.
(1996)
L Abate et al.
Macromol. Chem. Phys.
(1997)
L Abate et al.
Polym. Eng. Sci.
(2000)
Abate L, Blanco I, Pappalardo A, Pollicino A. J Therm Anal Cal...
Critchley JP, Knight GJ, Wright WW. Heat-resistant polymers. New York and London: Plenum;...

G.R Belbin et al.

Phil. Trans. Roy. Soc. Lond.

(1987)

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The isothermal degradation of some polyetherketones: a comparative kinetic study between long-term and short-term experiments

Abstract

Introduction

Section snippets

Materials

Viscosity measurements

Calorimetric measurements

Results and discussion

Acknowledgements

Polymer

Polymer

Polymer

Polym. Degrad. Stab.

Polymer

Polym. Eng. Sci.

Macromol. Chem. Phys.

Polym. Eng. Sci.

Phil. Trans. Roy. Soc. Lond.