Thermal Analysis of Polymers. Part 1: DSC of Thermoplastics

Introduction

Thermal analysis encompasses a group of techniques that are used to measure the physical properties of a substance as a function of time while the substance is subjected to a controlled temperature program. The techniques include DSC (Differential Scanning Calorimetry), TGA (Thermogravimetric Analysis), TMA (Thermomechanical Analysis) and DMA (Dynamic Mechanical Analysis).

Thermal analysis is employed in research and development, process optimization, quality control, material failure and damage analysis as well as to analyze competitive products. For example, the influence of moisture content, additives, plasticizers or fillers, and the content of impurities and contaminants can be determined from thermal measurements.

Furthermore, the different methods yield information about the processing, thermal history and pretreatment (storage and use), mechanical stress or strain, and dimensional changes.

This first article describes how DSC is used to analyze a thermoplastic, PET (polyethylene terephthalate), as comprehensively as possible [1]. The results of the various methods are compared with one another. The main topics discussed are:

• Glass transition
• Cold crystallization
• Recrystallization
• Melting
• Thermal History

PET

PET was chosen to represent the group of thermoplastic polymers. It is a polyester produced in a polycondensation reaction between terephthalic acid and ethylene glycol. Its structure is shown in Figure 1.

PET is used for many different applications. One of the most well known is the manufacture of plastic bottles in the beverage industry. It is also used as a fiber in the sports clothing industry because of its excellent crease-, tear- and weather-resistance properties and low water absorption.

Films of 1 to 500 µm are used for packaging materials, for the manufacture of furniture, sunshades, and so on. The finished films are often coated or laminated with other films and are widely used in the food industry, for example for packaging coffee or other foodstuffs to prevent the loss of aroma. The characterization of the properties of the material is therefore very important in order to guarantee constant quality.

Experimental Details

The DSC measurements described in this article were performed using a DSC 1 equipped with an FRS5 sensor and evaluated with the STARe software.

PET samples weighing about 3 to 10 mg were prepared and pretreated depending on the application. In general, samples should have a flat surface and make good contact with the crucible. The bottom of the crucible should not be deformed by the sample material when it is sealed.

Oxidative Stability (OIT/OOT)

Finally, we would like to briefly explain two DSC methods known as OIT and OOT that are used to measure the oxidative stability [10, 11] of polymers and oils. The methods simulate the accelerated chemical aging of products and allow information to be obtained about their relative stability. For example, different materials can be compared with one another or samples of the same material containing different additives can be analyzed to determine the influence of an additive. In practice, the method is widely used for PE (polyethylene). The application example described below also uses a sample of PE because the decomposition of PET is overlapped by melting and re-esterification and cannot be clearly identified.

The OIT (Oxidation Induction Time) measurement of PE (Figure 10) is often performed in crucibles made of different metals in order to determine the influence of the particular metal on the stability of the PE. In this example, the measurement was started in a nitrogen atmosphere according to the following temperature program: 3 min at 30 °C, heating at 20 K/min from 30 to 180 °C, then isothermal at 180 °C. After 2 min the gas was switched to oxygen. The measurement was stopped as soon as oxidation was observed. The OIT is the time interval from when the purge gas is switched to oxygen to the onset of oxidation. Measurements were performed in open 40-µL aluminum and copper crucibles for comparison. Oxidation clearly takes place much earlier in the copper crucible than in the aluminum crucible. The copper acts as a catalyst and accelerates the decomposition of PE.

The oxidative stability of samples can also be compared by measuring the Onset Oxidation Temperature (OOT). In this method, the sample is heated in an oxygen atmosphere and the onset temperature at which oxidation begins is evaluated. Since OIT measurements are easy to perform and do not take much time, they are often used in quality control to compare the stability of products.

Part 2 of this series of articles dealing with TGA, TMA, and DMA measurements of thermoplastics will appear in UserCom 32.

^{Thermal Analysis of Polymers. Part 1: DSC of Thermoplastics | Thermal Analysis Application No. UC 311 | Application published in METTLER TOLEDO Thermal Analysis UserCom 31}