Identification of Thermoplastic Polymers: Decomposition Analysis by TGA

Two different temperature programs were used to perform TGA measurements on 22 different polymers. The content of volatile components (usually water), the mass loss and the temperature of the pyrolysis reaction, the content of carbon black produced in the pyrolysis, and the ash content were determined. We show how these quantities differ for the polymers we investigated. We also discuss how the quantities determined from the TGA measurements can be used to identify the composition of polymeric materials.

Introduction

Thermal analysis techniques are often employed to identify polymeric materials. Examples of the use of DSC are discussed in reference [1]. Thermogravimetric analysis (TGA) is a standard method for the determination of the composition of polymeric materials. The schematic curve of a TGA measurement is shown in Figure 1. The sample is first heated in an inert atmosphere (nitrogen) and the loss of mass measured. Volatile components such as moisture, solvents, monomers, and plasticizers vaporize at relatively low temperatures (1). In the next step (2), the polymer decomposes in a pyrolysis reaction. This event usually exhibits the greatest mass loss. The residue that remains consists of inorganic constituents, the so-called ash, and carbon black that was added as a stabilizer or filler or as carbon fibers, together with carbon black produced during the pyrolysis of the polymer. The carbon content is measured by switching the method gas to oxygen or air and measuring the loss of mass in the combustion step. An alternative method is to first heat the sample under inert conditions until pyrolysis is completed and then to cool the sample to a temperature at which combustion of carbon does not occur (e.g. 300 °C). In the third step, the atmosphere is changed from inert to reactive (preferably air) and the combustion of the carbon on heating measured. This method allows different types of carbon to be distinguished, for example pyrolysis carbon black, carbon black as an additive or carbon fibers because the particles with the larger relative surface area burn first. A temperature program like this is used for the TGA measurements for the Reference Library for Thermoplastic Polymers [2].

To correctly interpret the TGA curves of polymeric materials (which can consist of chemically modified polymers, polymer blends, fillers and fibers and other additives), we need to know the decomposition behavior of the “pure” polymers. In this article, we present the results of TGA measurements performed on 22 polymers.

_{Identification of Thermoplastic Polymers: Decomposition Analysis by TGA | Thermal Analysis Application No. UC 515 | Application published in METTLER TOLEDO Thermal Analysis UserCom 51}