In many cases the mass loss profiles of polymers are complex and consist of several overlapping steps. One way of overcoming this is to decrease the heating rate so as to achieve more complete separation of each stage of the decomposition process. Such an approach naturally increases the time for the experiment to be carried out. An alternative approach can be used whereby instead of the usual linear temperature ramp a strategy is adopted whereby the rate of rise of temperature is slowed or even suspended as soon as some predetermined rate of mass loss is detected. Once this has occurred, heating is recommenced until a further mass change is detected. This technique is known as Sample Controlled Thermogravimetry (SCTG).
In its simplest implementation, the temperature program can be arranged to alternate between linear heating at a constant rate of temperature rise interspersed by isothermal segments when mass loss is occurring. This has been termed "step-wise isothermal" heating. The advantage with this approach is that the temperature profile can be recorded and used to specify conditions for future experiments on similar samples (for use in quality control). Another approach is to use a dynamic heating rate whereby the rate of temperature change is gradually reduced from an initial rate (possibly even resulting in the sample being cooled) as the rate of mass loss increases. This approach has been commercialised as "high resolution thermogravimetry" by TA Instruments (HiRes™ TGA).
The curves below show the mass and rate of mass loss of a sample of a laminate of nylon-6 and low density polyethylene (LDPE) obtined at a heating rate of °C/min. The weight losses of each component are almost completely overlapping and it is impossible to use the data for a quantitative assay of the sample.
The next set of curves were obtained using a variable heating rate technique. Although there is still incomplete separation of the degradation of the two components, measurement of the mass change between the plateaux either side of the saddle in the derivative mass loss curves allows quantitative analysis of each of the components which agrees well with theoretical calculations based upon the thickness of each layer. Ancillary experiments on the pure components show that the nylon-6 decomposes before the LDPE.
The penalty for this particular approach is the extended experimental time needed to obtain such data indicated by the curves shown in below comparing the tie taken for each experiment. This is an extreme case to illustrate the use of sample controlled thermogravimetry. For less overlapping weight losses often the variable heating rate program can be quicker than a conventional linear ramp and always with improvements in resolution of weight losses.
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