Thermoplastics feel the heat

Uploaded 16 Mar @ 15:07pm

Thermal analysis is widely used in the polymer, petrochemical, chemical, pharmaceutical, automotive, microelectronics, food, and cosmetics industries as well as in scientific research. Here, Mettler Toledo explains the various effects that can be measured by thermal analysis to characterise a thermoplastic.

Polyethylene terephthalate (PET) is a polyester produced in a poly condensation reaction between terephtlialic acid and ethylene glycol. It 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 fibre in the sports clothing industry because of its excellent crease-, tear- and weather-resistance properties and low water absorption.

Films of 1 to 500 micron 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 characterisation of the properties of the material is therefore very important in order to guarantee constant quality.

Thermal analysis is employed in research and development, process optimisation, quality control, material failure and damage analysis as well as to analyse competitive products. For example, the influence of moisture content, additives, plasticisers 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 pre-treatment (storage and use), mechanical stress or strain, and dimensional changes.
It 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).

Focusing on DSC, a technique that measures the heat flow of samples as a function of temperature or time, this method allows physical transitions and chemical reactions to be quantitatively measured. Used to analyse PET the key measures include glass transition; cold crystallisation; recrystallisation; melting; thermal history; oxidation induction time, and decomposition.

The glass transition is a reversible transition that occurs when an amorphous material is heated or cooled in a particular temperature range. It is characterised by the glass transition temperature, T. On cooling, the material becomes brittle like a glass, and on heating becomes soft. In the case of thermoplastics, the glass transition correlates with the region above which the material can be moulded. The glass transition is exhibited by semi-crystalline or completely amorphous solids as well as by ordinary glasses and plastics (organic polymers).

Above the glass transition, glasses or organic polymers become soft and can be plastically deformed or moulded without breaking. This behaviour is one of the properties that make plastics so useful. Cold crystallisation is an exothermic crystallisation process. It is observed on heating a sample that has previously been cooled very quickly without time to crystallise. Below the glass transition, molecular mobility is severely restricted and cold crystallisation does not occur; above the glass transition, small crystallites are formed at relatively low temperatures. The process is called cold crystallisation.

Melting is the transition from the solid to the liquid state. An endothermic process, it occurs at a defined temperature for pure substances. The temperature remains constant during the transition: the heat supplied is required to bring about the change of state and is known as the latent heat of melting.
Recrystallisation is a type of reorganisation process in which larger crystallites are formed from smaller crystallites. The process is heating-rate dependent: the lower the heating rate, the more time there is for reorganisation.

Thermal history is often straightforward with a number of heating, slow or shock cooling and annealing profiles applied to samples. These can then be checked for changes in glass transition and/or cold crystallisation.

Finally, there are two DSC methods known as OIT (Oxidation Induction Time) and OOT (Onset Oxidation Temperature) that are used to measure the oxidative stability 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 analysed to determine the influence of an additive.

The OIT measurement is often performed in crucibles made of different metals in order to determine the influence of the particular metal on the stability. The OIT is the time interval from when the purge gas is switched to oxygen to the onset of oxidation. The oxidative stability of samples can also be compared by measuring the 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.

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