Spotlight: How to… simulate and optimise debinding processes

Materials World magazine
,
27 Nov 2019

NETZSCH Product Manager, Elena Moukhina, explains how the Kinetics Neo software helps perform polymer debinding processes more quickly and efficiently.

In ceramic firing and sinter metallurgy, the quality of the product depends on the temperature profile, particularly the heating rate. In the initial stage of the heating process, the polymer binder is carefully removed via thermal decomposition. But the evolving of gas should not be too intensive to prevent the formation of micro-cracks and ensure the structure of the original material is not destroyed. Therefore, in order to obtain the best product quality, this heating stage to achieve polymer decomposition should not be performed too quickly.

On the other hand, heating that is excessively slow increases the process time, which could be overly expensive and ecologically unfriendly as well as increasing production costs. The main objective is to create an optimal temperature profile having balanced heating to ensure the best quality material in the shortest time. In order to achieve this, it is necessary to know what happens with the material in the furnace during firing. It is recommended to use the simulation method for the debinding rate for the given temperature profile of the furnace.

Data for analysis

Polymer binders lose mass during thermal decomposition, which can be easily measured by thermogravimetry. However, chemical reactions are kinetic processes depending not only on temperature but also on time. So, at a constant temperature, the reaction will run and mass will change, yet the same mass can occur at different temperatures. The only items independent of the temperature profile are the chemical properties of a reaction, such as stoichiometric coefficients, orders of reaction and activation energies. With a binder, the components of the polymer mixture often decompose independently of each other. In this case, the initial and final material compositions are usually independent of the temperature profile too.

In order to find those parameters of chemical reactions that are independent of the temperature profile, it is necessary to carry out several thermogravimetric laboratory measurements under different temperature conditions, namely, under different heating rates. The typical shape of thermogravimetric curves for decomposition shows the temperature dependence on the measured mass for different heating rates. The measurements presented here were carried out on a NETZSCH TG 209 F1 instrument.

Kinetic analysis

In a polymer binder, the polymers usually decompose independently of each other, and the measured mass loss presents the mixture’s decomposition. Each component in the mixture can break down in several individual decomposition steps, thus, the individual steps in this process can appertain to the different polymers or to the same polymer. Kinetic analysis allows the kinetic parameters of the observed process, which are independent of the temperature profile, to be found. These parameters are the activation energy and reaction order for each visible decomposition step, as well as the contribution of each reaction step to the total decomposition process.

There are two different approaches for the kinetic analysis of measured data – the first is model-based according to the real chemistry of the process and considers the total process as the sum of the independent decomposition processes of different polymers. The decomposition of each polymer is considered as a series of consecutive individual reaction steps. Here, each reaction step has its own stoichiometry and activation energy, both of which maintain constant values from the beginning of the reaction step to its end. This approach describes the debinding process explicitly and in a manner that is very close to reality, but it requires time for analysis and construction of the kinetic model from parallel and consecutive reaction steps.

The second, more approximate approach is called model-free, where the whole process is considered as a one-step reaction in which activation energy and pre-exponential factors change with the reaction’s progress. This type is very fast for a process with consecutive steps, but also has some restrictions, for example, it cannot describe the decomposition of a mixture with parallel reactions or with reactions having significant overlapping.

NETZSCH Kinetics Neo software serves both analysis methods, which is an advantage over single method softwares. NETZSCH Kinetics Neo was used for the analysis shown here of thermogravimetric data by the model-based method, with the result showing a kinetic model portraying three consecutive reaction steps with their kinetic parameters. This is independent of the temperature programme, and may be used for the simulation of decomposition processes for other user-defined temperature programmes.

If the simulation is performed exactly for the same temperatures used during the experiment, then the simulated curves must fit the experiment if the model is correct. This fit is seen in the image, where experimental data for different heating rates are marked with symbols, and all simulated data based on the same kinetic model with the same set of kinetic parameters – but for different heating rates – are shown as the solid curves. This means the kinetic model was constructed correctly and the kinetic parameters were found to be correct, so this model may be used for the future modelling of binder decomposition inside the furnace where it is not possible to measure the mass loss.

Prediction and optimisation

The obtained kinetic model consisting of three individual consecutive reaction steps allows for the prediction of mass loss for the temperature programme given by the user. Therefore, knowing how hot it is inside the furnace allows for simulation of the debinding progress. For example, this model allows for simulation of the mass loss of the material in the tunnel kiln. In the case of a change in heat, the software calculates a new mass loss curve for the new temperature programme in each zone.

The decomposition rate depends not only on the temperature, but also on the current value of the conversion. Under the constant heating rate, there are ranges in the mass loss curve where this process is fast and ranges where the process is slow. Those parametres with the high reaction rate are the risk zones where the material structure can be damaged. The ranges with the low reaction rate result in unreasonable time loss and energy loss, and therefore in costs of the final product being too high.

For the optimisation process, it is necessary to find those temperature profiles where the mass loss rate will be constant, in order to find optimal product quality for the shortest time. Without the simulation possibility, such temperature profiles would have to be created by the chemical engineer via the trial and error method – this would require considerable time and generate substantial costs. Using Kinetics Neo software, the new temperature profile was calculated for the given mass loss rate of 0.05%/min.

In industrial processes characterised by some restrictions in the heating rates, this software can help to find the optimal temperature profile to obtain a simulated mass loss rate that is very close to the constant value. For instance, German company Haldenwanger required the software to optimise the temperature profile for ceramics firing with regard to its new foam ceramics, the quality of which is very sensitive to the debinding rate. This process contained two parts, debinding and then sintering. Optimisation of the temperature profile was carried out for both parts and the production time was reduced by more than 50%.

Kinetic analysis software applications

The application field of kinetic analysis and simulation is not limited to the debinding process during the production of ceramics or in sinter metallurgy. Such simulation is necessary, for example, for determining the lifetime of packaging materials or for in-process operations at high temperatures.