EXAMPLES OF POLYAMIDE-BASED COMPOUNDS FOR "METAL REPLACEMENT" AND THEIR ANALYSIS BY SCANNING ELECTRON MICROSCOPY WITH ELEMENTS MICROANALYSIS

One of the peculiar characteristics of thermoplastic polymeric materials (such as aliphatic polyamides and semi-aromatic polyamides) and of compounds based on these polymers, is that they have a density much lower than that of metals and metal alloys (in table 1 are reported, for example, densities of some compounds reinforced with 50% glass fiber by weight in comparison with some metals or metal alloys).
This characteristic, combined with the possibility of obtaining compounds (that can be defined as structural ones) with significantly increased mechanical performance compared to the polymer as it is, for example through the use of reinforcements such as glass fibers or carbon fibers, makes these compounds (also together with other characteristics of polymeric materials) suitable to be considered in place of metals and metal alloys in many applications.
So, in general terms we can speak of "metal replacement" and, among the most important compounds for this replacement, we can certainly include specific compounds based on PA6, PA66, blend of PA66 with semi-aromatic amorphous polyamide and those based on semi-aromatic polyamides.

Table 1: density of some metal alloys (or metal) compared to compounds reinforced with 50% of glass fiber

The target, using these compounds, is so to replace the metal while maintaining the same functionality of the part: it is therefore necessary, as a starting point, a careful analysis of the application and its requirements, that is to say the analysis, as complete as possible, of the environment in which the application operates, for example in terms of temperatures (peak and continuous), of loads (static, dynamic or cyclic mechanical stresses), of times in which and for which these various factors act, presence of chemicals, humidity, UV radiation, etc.
It must be said that, also on the basis of this analysis, it is almost never feasible to replace the metal directly and immediately with a thermoplastic material-based compound for a part produced in metal (or metal alloys): a redesign, broadly speaking, of the part is generally necessary, for example for what concerns the thicknesses (modified), the use of ribs (for example in a grid) and the need to avoid, in the design of the part made by a thermoplastic compound based material, sharp corners and geometric discontinuities (specifically if the part in thermoplastic material is produced by injection molding).
From this point of view, namely that of an appropriate redesign, the use of CAE analysis (FEM), structural calculation software, as well as the use of process simulation software (such as, for example, Moldflow) come into play and are of great support.

As mentioned, the density of polyamide-based compounds is decidedly lower than that of metals and metal alloys and this is certainly one of the factors that allows the necessary redesign (for example through an increase in thickness) of the part so that it is possible to produce the finished part with a compound instead of metal.
Indeed, if we consider as a specific property (normalized with respect to density) the tensile stress at break for the compounds indicated in table 1 (for non-conditioned materials, or DAM i.e., "Dry As Moulded") we can see that these compounds have decidedly high specific stress values (see Figure 1) compared to some metal alloys (or metal).

Figure 1: specific stress at break for some metal alloys (or metal) compared to compounds reinforced with 50% glass fiber. Specimens are dry as moulded (DAM)

The advantages of "metal replacement", which therefore "push" and direct towards this replacement are many: first of all, there is a considerable advantage in terms of cost reduction and specifically cost per liter [€ / lt], that is the cost per volume of material (and not simply of cost per kg since it is with a given volume of material that a certain number of pieces or parts are produced).
There is clearly an advantage in terms of weight (and therefore of consumption related to this).
These advantages are directly linked to the density of polyamide-based compounds (including those based on polyamide blend and semi-aromatic polyamides).
There is also a reduced environmental impact (from the LCA, "Life Cycle Assessment"), considerable design flexibility, the possibility of integrating functions, excellent aesthetics, easier assembly and reduced post-processing, specific chemical resistances (for example corrosion) and better processability (also keeping in mind process temperatures involved).
But which are compounds that can be considered for this replacement?
Some are those indicated in table 1 (and Figure 1) but even more specific compounds can be considered as, for example, those indicated and described below, in table 2.

Table 2: specific polyamide (and blends) based compounds that can be considered for metal substitution

In table 3, for the materials indicated in table 2, in addition to density, also a series of mechanical properties are reported such as the tensile modulus, the tensile stress at break, the tensile strain at break and the flexural stress at break (for materials conditioned at 23 ° C / 50% RH for 24h).

Table 3: density and mechanical properties of some polyamide (and blends) based compounds that can be considered for metal substitution (materials conditioned at 23 ° C / 50% RH for 24h)

Data reported in table 3 were obtained from materials (compounds) with composition as indicated in table 2: tested materials were produced with the same extruder and the specimens, used for the evaluation of mechanical performance, were obtained using the same press. injection.
Furthermore, mechanical tests, as well as all the microscopic analysis below listed, were performed with the same instruments and by the same operators.
An evaluation of the morphological and dimensional characteristics of the reinforcements contained in these compounds (reinforcements which, as mentioned, allow to obtain high mechanical performance) can, for example, be carried out by scanning electron microscopy with elements microanalysis.
Hereafter some images obtained with this technique on injection molded plates for some of the mentioned compounds.

Image 1: dimensional (punctual) analysis and morphological analysis carried out on part of an injection moulded specimen made with a PA66, PA6 and semi-aromatic amorphous PA (PA6I / 6T type) blend-based compound with 70% special glass fiber (ellipsoidal section) defined as flat fiber

In this image some peculiarities of the special glass fiber used can be highlighted and verified: for example, it is possible to evaluate the specific morphology of the fiber (flat fiber) as well as verify, albeit punctually, the dimension of the major axis (which is about 26- 27 microns compared to a nominal value of 28 microns.

Image 2: dimensional (punctual) analysis, morphological analysis and mapping of elements carried out on part of an injection moulded specimen made with a PA66 and semi-aromatic amorphous PA (PA6I / 6T type) blend-based compound with 65% glass fiber having a diameter of 6 microns

From this image it is possible to verify that the average diameter of the used glass fiber is actually of the order of 6 microns (consistent with the nominal value) and, from the mapping of the elements, to clearly visualize the glass fibers in the matrix.

Image 3: dimensional (punctual) analysis and morphological analysis carried out on part of an injection moulded specimen made with a PA66 and semi-aromatic amorphous PA (PA6I / 6T type) blend-based compound with 40% carbon fiber

From this image, in addition to displaying the carbon fibers present, it’s possible to get an idea of the average diameter of the carbon fiber contained in the compound that is to say 6-7 microns (compared to a nominal value of 7 microns).

Image 4: dimensional (punctual) analysis, morphological analysis and mapping of elements carried out on part of an injection moulded specimen made with semi aromatic polyamide (PA6T / 6I) based compound with 55% glass fiber and high thermal stabilization

From this image, it is possible to assess that the average diameter of the glass fiber used is of the order of 10 microns (value in line with the nominal value of the fiber, that is, 10 microns).
Also in this case, by elements mapping, it is possible to clearly visualize the glass fibers in the matrix.

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

Ing. Luca Ciceri - RDLAB137 srl

Last revision: 29/08/2022