ANALYSIS OF THE MOST COMMON USED REINFORCEMENT IN THERMOPLASTIC COMPOUNDS

In the context of compounds based on thermoplastic polymers (such as PA6, PA66 and semi-aromatic polyamides), those defined as reinforced are particularly common and widespread.

The purpose of using reinforcements, as the term itself indicates, is to obtain significantly increased mechanical performance compared to that of a thermoplastic material as it is (see also the article at the link https://rdlab137.it/it/2019-06-06-09-01-40/esempi-di-compound-base-poliammide-per-il-metal-replacement-e-loro-analisi-tramite-microscopia-elettronica-a-scansione-con-microanalisi-degli-elementi.html )

Among the most used reinforcements in compounds there are certainly glass fiber and carbon fiber, so we will focus attention on these types of reinforcement.

Considering compounds obtained by extrusion (among the most used extruders there are co-rotating twin-screw ones) the fibers present in the compound are defined as "short", that is with length within a certain value (generally under a millimeter). This value, for a given initial length of the fibers, is determined by the production process (in fact there is a reduction in the initial length of the fiber due to the breaking, "milling", of the fiber itself during the extrusion process).

Often, for a variety of reasons, it can be useful (or necessary) to evaluate in quantitative and / or qualitative terms the type of reinforcement contained in a given compound.

Some cases, among the many, in which this evaluation is relevant can be, for example:
- check of the reinforcement content (both in terms of quality control and counter-typing)
- verification of the type of reinforcement (discrimination of the reinforcement)
- verification of dimensional or morphological characteristics of the reinforcement (e.g., due to non-conformity of a part or control of specific type of reinforcement)

But which analysis techniques are indicated (and effective) to evaluate the used reinforcement?

One of the simplest and most common is incineration in air which allows to evaluate the quantity of glass fiber present in the compound: at high temperature, 650-800 ° C, in air, the polymer degrades (decomposes) completely and only the glass fiber, that does not decompose at these temperatures, remains (standard glass fiber used for compounds - type E glass, where E stands for “Electric” - softens at about 845 ° C).

However, when dealing with carbon fiber it is more suitable, to obtain reliable results, to resort to thermo-gravimetric analysis (TGA) which allows the use of different types of atmosphere (oxidant, such as air or oxygen, or inert such as nitrogen) and therefore to discriminate with better precision the carbon fiber from the glass fiber (possibly changing the type of atmosphere during the test).
In fact, it should be considered that in an oxidizing atmosphere or in air, such as during incineration, the carbon fiber is oxidized and there is no residue and even using (in incineration) reduced times and temperatures it is much more difficult to obtain precise and repeatable results.

The TGA also allows to evaluate weight losses associated with some specific additives or fillers (such as melamine cyanurate or PTFE) and therefore, clearly, it is an analysis that provides more information than simple incineration although its cost (in terms of equipment and times) is much greater.

Some example TGA thermograms are shown below.

Figure 1: TGA thermogram of a PA66 based compound with (nominal) 50% glass fiber by weight (test conducted in N2 with a heating ramp of 20 ° C / min).

From figure 1 it can be seen that the final residue by weight (at about 800°C) is 48.84% and it is therefore consistent with the nominal value indicated of 50%.
The fiber content in a compound can in fact have a variation, albeit limited, linked to the process (e.g., to the dosage of the fiber), around the nominal value: in general, a fluctuation of 1.5-2.0% is reasonable (and allowable) for materials containing more than 30% by nominal weight of glass fiber.

It should also be added that the TGA analysis, being carried out on a small amount of material, can accentuate even a minimal inhomogeneity of the material itself (and for this reason it is an analysis that should be repeated more than once on different samples of the same material).

 Figure 2: TGA thermogram of a PA66 based compound with 40% (nominal) carbon fiber by weight (test conducted in N2 with a heating ramp of 20 ° C / min).

From the thermogram shown in figure 2 it can be seen that the final residue (37.34%) is substantially consistent with a nominal value of 40% (for what has already been said previously on the fluctuation of the reinforcement content value and homogeneity, on a reduced scale, of the material).

Another analysis of considerable importance for the dimensional and morphological evaluation of the reinforcements is scanning electron microscopy with microanalysis of the elements (SEM-EDX).
This (punctual) analysis allows to understand whether the fibers present are glass fibers or carbon fibers and to evaluate the critical dimensions (for example the diameter) that influence the reinforcement effect exerted by the fiber.

Below are some images and punctual dimensional analyses obtained on compounds containing carbon fiber and glass fiber.

Figure 3: SEM image (left) and mapping of elements (EDX) (right) of a PA6 based compound with 20% milled glass fiber.

From this image (figure on the left) it is possible to have an idea (albeit punctual) of the dimensions (for example diameter) of the fiber used (about 20-21 microns against a nominal value of 20 microns for the used glass fiber).
Clearly, for a statistically significant evaluation of the size and morphology of the type of used glass fiber, a repetition of the measurement in several points of the part / specimen is necessary (and certainly recommended).

From mapping of the elements (figure on the right) it is also possible to clearly visualize the fibers in the polymer matrix.

 Figure 4: SEM image of a PA66 + PA6I / 6T blend based compound with 40% carbon fiber.

In this case too (Figure 4) it is possible to evaluate the dimensions (diameter) and morphology of the fibers (in this specific case carbon fibers) present in the compound.

If possible, for example if it’s necessary to directly analyze a piece or finished part, a relatively simple analysis, also complementary to those mentioned (such as incineration and TGA), especially when it’s known, at least as a rough idea, the type of polymer matrix and reinforcement you are dealing with, it’s the determination of density.
From the value of the density, it is in fact possible to calculate the reinforcement content albeit in an approximate way (also due to the possible effects of the transformation process used): often this calculation is a good check and confirmation of what has been determined with other analysis techniques.

In the specific case, however, in which it is not known whether glass fiber or glass spheres (or a combination of glass fibers and spheres) are present in the material, neither the TGA, nor the incineration nor the density allow to discriminate it with precision.
In this situation, scanning electron microscopy with microanalysis of the elements can certainly be of great use as evident from the images below:

Figure 5: SEM image of a PA6 based compound with 30% glass beads (left) and a PA6 based compound with 15% glass fibers and 15% glass beads (right).

From these images it is possible to clearly identify and distinguish the glass fibers and the glass spheres.

Obviously, also on the post incineration residue (if present) it is possible to carry out a series of analyses such as: Fourier transform infrared spectroscopy, optical microscopy and scanning electron microscopy with microanalysis of the elements to evaluate the type of reinforcement (or filler) present.

RDLab137 is equipped to perform directly, or through the extensive network of laboratories with which it collaborates and interacts, a whole series of analyses such as incineration, density determination, TGA (also at high resolution), Fourier transform infrared spectroscopy, optical microscopy, etc.

In RDLab137 is also available a latest generation electron microscope that has been
used to obtain the images shown and which can be used for evaluations of this type and for further evaluations related to the potential of the technique (see also at the link https://rdlab137.it/it/laboratorio/microscopiaelettronica-sem-edx.html ).

Ing. Luca Ciceri - RDLAB137 srl

Last revision 28/11/2022