Aerospace, wind energy and automotive sectors have discovered the various advantages of carbon fiber-reinforced polymers (CFRPs) for their products. Low density, high specific properties and corrosion resistance boost the usage of CFRPs in numerous applications. The choice of the unique mix of matrix material, fiber length, orientation and fiber content is crucial. Thus, the composite can be tailored to specific areas of application. The fiber volume fraction (Vf) accounts for the material’s mechanical and physical properties and is therefore the most crucial indicator in quality control of CFRPs. The following editorial refers to an article published by Dominik Grund, Manuel Orlishausen and Iman Taha in the journal “Polymer Testing” in March 20191. The comprehensive overview of thermoanalytical methods to determine Vf has made it for us “NETZSCH – Paper of the Month May 2019”.
There are better alternatives to wet chemical methods
The wet chemical method that is commonly applied to determine fiber volume fraction exposes the CFRP sample to highly oxidizing acid at elevated temperatures. In order to circumvent the various health, safety and environmental concerns and to ensure high quality of CFRPs in a more efficient manner, the author of the article highlights that incineration methods using TGA provide significant advantages over all alternatives as the different fiber types can be separated based on differences in their thermal stabilities.
Determination of Vf using a two-stage thermogravimetrical procedure
The author was faced with the challenges to reduce the overlap between resin and fiber degradation process and found that a two-stage analysis procedure using TGA provides the most accurate value of fiber volume fraction of CFRPs. The first dynamic heating step is carried out in an inert atmosphere, which decomposes the resin of the milled CRFP sample. Cooling the sample to room temperature is followed by a second heating step under MRC (mass-rate-controlled) conditions in technical air in order to decompose the resin char.
Why a two-stage analysis using TGA provides the best results
The procedure has three key parameters that need to be brought in line in order to arrive at a meaningful degradation curve that can be used to determine Vf.
In an atmosphere of technical air two degradation stages can be observed. The second stage can be attributed to the thermal degradation of carbon fibers. However, the mass loss in the first stage indicates a degradation of both resin and fiber, which resolves in an overlap in the temperature range (300 – 700 ° C) and does not allow the separation of the effects.
In an inert atmosphere the matrix completely decomposed without any thermal effects on the carbon fibers. However, significant char content was left behind. Combining both atmospheres allows the separation of the degradation of individual components. That is indeed the most crucial step that the author took into account.
- Heating rate
The decomposition with a constant heating rate of 5K/min is linear. However, the MRC decomposition leads to a more detailed signal, which provides a clearer separation between the decomposition phases and can directly be used to calculate Vf.
- Sample size
An atmosphere of technical air combined with a MRC heating rate revealed that the incineration of the resin and the beginning of the decomposition of the fibers could not be clearly distinguished for a solid sample. The reason behind this is that the signal shows a relatively broad plateau between 450 °C and 600 °C. In contrast, a milled sample shows a smooth thermogravimetric curve with a notable peak on the first derivative curve. As the author emphasizes, milled samples also have a lower standard deviation in Vf and residual mass and are therefore more favorable to calculate fiber volume fraction.
TGA procedure reveals equal fiber volume fraction
The TGA procedure was validated against the commonly used wet chemical analysis by the authors. There is a difference of 1.1 % in Vf between the two methods, which can be attributed to the slight degradation of carbon fibers during TGA. However, the observed deviation lies within the acceptable range of error for the wet chemical analysis.
The procedure using thermogravimetric analysis by Dominik Grund provides an approach for determining the fiber volume friction in order to efficiently control the quality of CFRPs. Furthermore, the TGA process can be automated and does not require constant supervision.
1 Grund, Dominik & Orlishausen, Manuel & Taha, Iman. (2019). Determination of fiber volume fraction of carbon fiber-reinforced polymer using thermogravimetric methods. Polymer Testing. 75. pp. 358 – 366. 10.1016/j.polymertesting.2019.02.031.