AN EXAMPLE OF COMPOUND DE FORMULATION

It is a common need for those involved with thermoplastic compounds, whether in quality control or in research and development, to be able to know what the composition (or formulation) of an "unknown" compound (i.e., with unknown composition) is (albeit with varying degrees of precision and approximation).
This need may, for example, be related to a dispute where a manufactured part needs to be analyzed for compositional compliance (i.e., to understand whether the part is actually manufactured from the supplied/specified material) or to verify that something is (or is not) present in a material that should (or should not) be included in the formulation.
Another area in which the compositional evaluation of an unknown compound is of interest is, as mentioned, in product development, specifically in the definition of increasingly complex and targeted compound formulations: indeed, in this area we are often confronted with materials from various manufacturers whose composition is obviously not known (or at least not known at a sufficiently detailed level for product development purposes).
Assessing what is the composition of materials that are, for example, market references is indeed of considerable importance for the development and improvement of formulations dedicated to specific applications or application areas.
What we may call the de-formulation of a polymer-based compound is clearly directly and immediately linked to what are the identifying analyses of polymeric materials (https://rdlab137.it/en/archive/the-identity-card-of-a-polimeric-material.html).
It must also be added that the "world" of identification analyses (and not just those specific to polymeric materials) is extremely extensive and includes a decidedly wide variety of techniques, and combinations of techniques.
Having said that, it is evident that it is necessary to evaluate which analyses to start from in order to understand the composition of a compound and, on the basis of the results gradually obtained on the material under analysis, to evaluate how to proceed (i.e., which, if any, further targeted analytical techniques to consider): an analysis plan with a sequence (logical and chronological) of the analyses to be conducted must therefore be defined.
Clearly, if one has any indication whatsoever, albeit a rough one, of the unknown material to be analyzed (e.g., derived from a technical or safety data sheet), these can at least help to define which analyses to consider initially: if, for example, the material contains, or should contain, PTFE an analysis assessing fluorine content and/or a TGA can certainly be indicated from the outset.
It is also generally true that one analysis technique alone is practically never sufficient to identify a rough composition of the material and, as mentioned, a combination of techniques is essentially necessary.
As an example of de-formulation, let us now consider the case of a compound of unknown composition identified quite generically as a "material with high wear resistance," i.e., a material with tribological properties, whose formulation evaluation was required.
Differential scanning calorimetry (DSC) and infrared spectroscopy (FT-IR) were considered as starting analyses.
Note: For compounds based on thermoplastic polymers, these two analyses are definitely among the "basic" ones to be considered virtually always in the first instance (https://rdlab137.it/en/laboratory-tests/composition-thermal-analysis.html ; https://rdlab137.it/en/laboratory-tests/spectroscopy.html).

The DSC thermogram of the unknown material is shown below (Figure 1):

Figure 1 - DSC thermogram. Test conducted in nitrogen with 20K/minute heating ramp from 20°C to 360°C.

Two endothermic phenomena corresponding to the melting of two thermoplastic polymers in the material can be clearly seen: based on the melting temperatures, it can be reasonably assumed that these are Polyamide 66 and PTFE, respectively.
This assumption is confirmed by infrared spectroscopy: in fact, the FT-IR spectrum of the material shows the presence of peaks that can be clearly associated with Polyamide 66 (Figure 2a) and PTFE (Figure 2b) as well as additional specific peaks that can be associated with silicone (Figure 2c).
Note: Silicone (usually of high molecular weight) is often used, in percentages usually ranging from one to three percent by weight, in combination with PTFE in tribological compounds based on Polyamide 66.

Figure 2a - FT-IR spectrum of the compound (blue) and a standard Polyamide 66 (red spectrum). The area of the specific peaks of Polyamide 66 is highlighted (green box).

Figure 2b - FT-IR spectrum of the compound (purple) and a standard PTFE (red spectrum). The specific peak area of PTFE is highlighted (magnification below).

Figura 2c -FT-IR spectrum of the compound (purple) and a high molecular weight silicone (red spectrum). The area of the silicone-specific peaks is highlighted (magnification below).

At this point we already have a range of information about the composition of the material (base polymer and possible additives) but to better quantify the components, TGA (thermogravimetric analysis) was also considered.
The TGA confirms the presence of PTFE (and gives an indication of the amount of PTFE): indeed, in the thermogram (Figure 3) two very clear decomposition peaks can be seen, attributable to Polyamide 66 and PTFE.
The second peak (the one attributable to PTFE) also gives us an idea of the amount by weight of PTFE present of the compound (about 18-19% by weight).
An inorganic residue (about 40% by weight) is also clearly present.

Figure 3 - TGA thermogram. Test conducted in nitrogen with 20K/minute heating ramp from 20°C to 800°C.

In addition to the analyses described above, SEM-EDX (scanning electron microscopy with EDX probe, https://rdlab137.it/en/laboratory-tests/electron-microscopy-sem-edx.html) analysis was considered, which allowed for a more precise assessment of the filler/reinforcement type present and confirmed the presence of PTFE and silicone.

In fact, one can see from the electron microscope images (Figure 4) the presence of fibers that, based on the elements (such as, for example, silicon, calcium, aluminum) highlighted by the element microanalysis (element mapping on the left), appear to be glass fibers. This assessment is also consistent with an inorganic residue determined by TGA.
Of these fibers, it is also possible to get an idea (measurements on the right) of the diameter and, albeit with greater uncertainty, the length (post compound extrusion and injection molding process).

 Figure 4 -SEM-EDX images; element mapping (left), dimensional (specific point) and morphological analysis performed on specimen of the compound of unknown composition.

Both fluorine (present in PTFE) and silicon (present in both glass fiber and silicone) are also detected. Note: The high amount of fluorine measured seems reasonably due to the fact that PTFE, in general, in this type of compound tends to concentrate in the surface are of the part (or specimen).

Table of microanalysis of elements:

To complement these analyses, the density of the material was also measured lastly, which makes it possible in a simple but effective way to confirm a rough idea of the composition of the compound.
Assuming in fact, from the results of the analyses carried out, a composition such as PA66 + 40% glass fiber + 18% PTFE +2% silicone, a calculated density of 1.63 g/cm3 is obtained, in excellent agreement with that measured (1.62 g/cm3).
Thus, the combination of the analyses performed allowed the composition of the unknown compound under study to be defined with good approximation.
RDLab137 can support in defining the analyses and interpreting the results for evaluating the composition of unknown compounds.

Ing. Luca Ciceri- RDLAB137 srl
Last revision 13/01/2025