Thermal Analysis Labs

Thermal Expansion / Dilatometry

Determining Glass Transition Temperature of PET

Polyethylene Terathalate (PET or PETE), is among the most used types of plastics in industry. It is used in drink and frozen food packaging and as a synthetic fibre under the common name “polyester.” As a result of its wide application, particularly in food applications, strict quality control is needed over batches. Measurement of glass transition temperature is one way in which quality of a batch of PET may be assessed.

Glass transition temperature (Tg) is a characteristic of some amorphous materials, including plastics. At the glass transition temperature, the material converts from a hard and brittle “glassy” state to a soft and rubbery, semi-molten state. At temperatures above this point, the plastic has a compromised ability to retain form and structure, unless it undergoes a recrystallization at a higher temperature. A literature survey for PET’s glass transition temperature shows it to be between 69°C and 85°C, depending on the grade examined.1 The C-Therm Dilatometer (DiL), which uses a horizontal Linear Variable Displacement Transducer (LVDT)-based technology compliant to ASTM E228 can be used to observe this phenomenon.

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Figure 1. C-Therm DiL High Precision Dilatometer

The glass transition of a polymer is evident via dilatometry as an inflection point in the thermal expansion curve, in between two regions of linear thermal expansion – the exact temperature of the transition is then determined using the two-line method. For added convenience, consistency and confidence, an advanced software package such as CALISTO data processing software may be used to automate analysis of the Tg point, thus eliminating possible user bias.

Measurement of Glass Transition Temperature
A 50mm piece of PET was cut from a thin rod, and the end surfaces were polished to achieve a smooth contact between the sample and the pushrod. A tipped pushrod was used with a cross-sectional area of 1 mm2. The minimal force employable by the DiL was used upon the sample to minimize contact force and reduce the risk of sample deformation.

The operation of the dilatometer was conducted in compliance with known measurement standards for thermal analysis.2 When thermally analyzing thermoplastics, one must be mindful of the effect of thermal history of the sample. This may be addressed by pre-conditioning the sample at a temperature above its glass transition temperature, taking care not to exceed the dilatometric softening point or recrystallization temperature, depending on the thermoplastic in question. The heating rate should not exceed 5°C/min. This preconditioning process normalizes the thermal history of the sample, enabling repeatable data collection. The measurement may then be conducted either upon the cooling from the temperature above the glass transition temperature, or with a second heating cycle.

PET is a material which is widely known to have morphology and thermal properties which are sensitive to its thermal history. Therefore, for this work, the sample was allowed to soak for 30 minutes at this 30°C to normalize the initial condition of the measurements. The sample was then heated at a rate of 5°C/min to 130°C then cooled. Measurement was conducted on the cooling cycle. The resulting measurement profile on the cooling part was obtained and analyzed using the Calisto software. Figure 2 shows the profile obtained and the glass transition temperatures on the first test.

PET 1

Figure 2. Glass Transition Temperature Determination: First run Tg=75°C

To assess repeatability of the analysis, the test was repeated upon the same sample material, using identical testing parameters and temperature profile. Figure 3 shows the data from this second test.

PET 2

Figure 3. Glass Transition Temperature Determination: Second run Tg=75.7°C

The repeatability is remarkable with an RSD of 0.7%. The average of the two measurements, at 75.35°C falls well within the expected range of 69°C-80°C.

In conclusion, glass transition temperature can be measured in a repeatable manner using horizontal pushrod dilatometry, provided the thermal memory of the sample is normalized through the heating profile.

References
1“Crystallization Behaviour of PET Materials”, Demirel B., et al, BAU Fen Bil. Enst. Dergigi C.H., Vol 13(1), pp 26-35, 2011; Polímeros vol.23 no.3 São Carlos 2013 Epub May 28, 2013; Misumi Corporation, “Glass transition temperature of plastics,” Plastic Moulding Tutorial, http://www.misumi-techcentral.com/tt/en/mold/2011/12/106-glass-transition-temperature-tg-of-plastics.html; Sepe, Michael. “Know How: PBT & PET Polyester: Part 2 The Performance Factor”Plastics Technology, November 2012. http://www.ptonline.com/columns/pbt-pet-polyester-part-2-the-performance-factor;
2“Standard Test Method for Linear Expansion of Solid Materials with a Push Rod Dilatometer”, ASTM E228-11