XPS measurement of the homogeneity of a PDMS plasma treatment on carbon fibers

Carbon fiber has become one of the most attractive new materials of the 21st century due to its mechanical properties, thermal stability and corrosion resistance. These interesting characteristics coupled with a high lightness make it an interesting material for many sectors.

The modification of the initial properties of the carbon fiber (thermal conductivity, adhesion, porosity, hydrophobicity...) via a surface treatment allows to widen its range of application.

Often used in the aerospace industry, they are used to manufacture aircraft components such as wings and fuselages, reducing their weight and increasing their energy efficiency. In the automotive industry, carbon fiber composites are used in parts such as body panels and brake components, where they help improve performance and reduce fuel consumption. They are also used in sporting goods such as golf clubs, tennis rackets and bicycles, as well as fishing rods and archery equipment. Carbon fibers are also becoming increasingly popular in the construction industry as a reinforcing material for concrete. Carbon fibers, like many other materials, can be treated with a coating to change their appearance or surface properties to suit specific conditions of use.

The recovery rate of a treatment is a key data for many manufacturers to control their coating processes.

In this application note, we will take the example of determining the recovery rate of plasma treated carbon fibers to impart hydrophobic properties.

The methodology followed is easily transposable to all types of natural or synthetic fibers. First, a tracer element must be determined to measure and quantify its presence after treatment. In this case, fluorine was chosen as the tracer element because of its presence in the carbon fiber manufacturing process as an extrusion agent.

X-ray photoelectron spectroscopy (XPS) analyses can be carried out in several acquisition modes. The grid mode is perfectly adapted to the statistical characterization of the surface of a natural or synthetic fiber.

Giving access to a data matrix spread over a length of several cm, this mode allows to evaluate with precision the homogeneity of a treatment.
In XPS, the surface is irradiated by X-ray photons from a monochromatic source. The atoms of the sample emit photoelectrons with energies specific to each element and its environment. The spectra obtained most commonly present the number of photoelectrons detected as a function of their binding energy (difference between the energy of the X-ray photons and the kinetic energy of the emitted electrons).
Classically, the surface analyzed by XPS is of the order of a few hundred µm². The linescan mode, based on a perfectly adapted acquisition software, allows to cover a much larger surface spread over a length of several cm.

TESCAN ANALYTICS has an expertise of more than 20 years in the use of XPS on all types of materials, insulating or conducting... With the latest generation instruments, our team of experts works with all industrial sectors.

 

Figure 1: Superposition of XPS spectra of untreated (blue), PDMS treatment 1 (red) and PDMS treatment 2 (green) carbon fibers


Table 1: Elemental compositions (atomic %)

In Figure 1, the XPS spectra of the untreated carbon fibers as well as those of the fibers that underwent PDMS treatments 1 and 2 have been superimposed.

The recovery rate of the treated carbon fibers can be estimated from the difference in atomic concentrations of fluorine, the tracer element of the fiber, before and after PDMS plasma treatment, from the following formula:

with :
[F]_{average reference}: average atomic % of fluorine measured on reference carbon fibers
[F]_{treated fibers}: average atomic % of fluorine measured after PDMS treatment

The elemental compositions of the fibers are presented in Table 1.

The elemental analyses show the detection of fluorine up to 7.9 atomic % on the extreme surface of the untreated carbon fibers. This fluorine is related to residues of fluorinated extrusion agents used in the carbon fiber forming process. After plasma treatment, a decrease in fluorine content is measured for treatment 1 (7.9 % before treatment to 2.9 % after treatment 1). Treatment 2 leads to a total masking of fluorine ([F] = 0 % after treatment 2).

These results indicate a total coverage (100%) with treatment 2, the absence of fluorine indicates a PDMS layer thickness greater than 10 nm (XPS analysis depth). Treatment 1, on the other hand, shows 63% coverage, indicating a thickness < 10nm. The statistical approach reveals a better homogeneity with treatment 2 for which lower standard deviations are measured.

In this example, the contribution of the linescan mode for statistical quantification of carbon fiber recovery from PDMS plasma treatment was demonstrated.

Linescan XPS is the method of choice for statistical monitoring of the recovery rate of a substrate by a treatment over a length of several cm.

This methodology can be transposed to many applications in order to study the efficiency and homogeneity of a treatment.

In other work, the strength of XPS has been demonstrated for evaluating the homogeneity of an atmospheric plasma chemical functionalization treatment on polymer films.

For more examples of the application of XPS or our other analysis techniques, click here.

Other complementary techniques can be used to study the coating of a substrate by a treatment (TOF-SIMS in imaging mode; TEM or SEM depending on the thickness of the treatment).