Process Analytical Technology

Thermal Interface Materials (TIMs) have been used in the electronic industry for decades from CPUs to phone batteries. However, as electric vehicles (EVs) become increasingly prominent, the volume of TIMs required for adequate battery thermal management has increased dramatically. With this new volume, new problems are being encountered by both thermal adhesive manufacturers and adhesive dispensing groups. 

In particular, particle sedimentation has proven to be a challenge for these groups. In order to increase the adhesive’s thermal conductivity, conductive fillers are added to the mixture. However, given enough time, these conductive particles will succumb to settling as gravity draws them out of suspension towards the bottom of their storage tank, causing drastically uneven thermal properties. This problem is magnified given the volume requirement of EVs, as well as the safety and liability risk of having poor thermal management.

The Modified Transient Plane Source (MTPS) technique is able to rapidly monitor particle settling for quality control applications. As conductive fillers settle towards the bottom of the storage tank, the thermal conductivity will increase from their presence. Consequently, the thermal conductivity at the top of the adhesive tank will diminish. The MTPS can be retrofitted for in-line process monitoring, using thermal conductivity and effusivity measurements. Data for this application can be seen below: 

As can be seen, the MTPS sensor at the bottom of the adhesive sample shows a significantly higher thermal conductivity over time, increasing as the weight percentage of filler increases. This shows the effects that particle settling has on thermal adhesives, and how the MTPS sensor is able to monitor this for when the adhesive is out of spec.

In the Powder Metallurgy (PM) process, it is crucial to have consistent and uniform products. A big part of achieving this goal is ensuring a homogenous powder blend. However, a challenge with this is creating a process that achieves good homogeneity and operates efficiently. Currently, a common approach for determining mixture homogeneity is to compare the final blend properties with the predicted values required. However, this method is unable to determine how long the blend will take to homogenize; therefore, process optimization is often done through trial and error.

Thermal effusivity is a material property that combines thermal conductivity, density, and heat capacity, meaning that it can differentiate between solids, liquids, and powders based on their heat transfer properties. In PM mixtures, the effusivity will change as the blending process reaches steady state, and will exhibit noticeable differences as a response to non-typical events such as particle segregation.

Figure 1: Various blend scenarios as characterized by thermal effusivity measurements. Steady state conditions show stable, homogeneous blend.

The measurement set up four in-line Modified Transient Plane Source (MTPS) sensors embedded in the lid of the blending unit. During the blending process, the powder will be “quasi-stationary” for a time, providing good contact for the sensors. During dynamic blend trials, the stable region of blend homogeneity can be identified based on thermal effusivity as seen in Figure 2.

Figure 2: Thermal effusivity versus blend time. Note the steady state region of thermal effusivity, indicating a completed blending. Continuing blending caused large deviation in RSD, indicating over blending and particle separation.

The use of in-line MTPS sensors was shown to have strong correlations between various phenomena that occur while blending including changes in apparent density, over blending, and mixing time optimization.

This case highlight is take from the paper Monitoring of Powder Homogeneity During Double-Cone Blending, which can be read here.

Lubricants play an important role in the pharmaceutical industry. In particular, magnesium stearate (MgSt) is a common lubricant, however its properties can vary significantly depending on the way it was produced. These can impact the rate at which active ingredients are released, meaning that determining the optimal mixing time and state are crucial.

Thermal effusivity can be used to quickly and non-destructively monitor blending uniformity for pharmaceutical ingredients. By embedding multiple Modified Transient Plane Source (MTPS) sensors in the lids of the V-type mixer, thermal effusivity can be measured, giving insight to blend quality and lubricant adhesion levels, as seen in Figure 1:

Figure 1: The change in thermal effusivity by blending, reflects the changing adhesion of the added MgSt

These results allow for the determination of the optimal blending time, regardless of the particular physicochemical properties. This study concludes that in-line thermal effusivity testing can effectively determine uniformity without intricate sampling processes, and serve as an efficient monitoring tool for pharmaceutical ingredients.

This case highlight is from a study called Evaluation of physicochemical properties on the blending process of pharmaceutical granules with magnesium stearate by thermal effusivity sensor and can be read here.