Quality Control of Carbon Fiber-Reinforced Polymers Using TGA

It is crucial to determine fiber volume fraction for the quality control of carbon fiber-reinforced polymers (CFRP). This is done with a view to ensuring their favorable mechanical and physical properties for use in sectors such as aerospace, wind energy and automotive.

Published by Dominik Grund in the journal “Polymer Testing”, one of the newest and novel thermogravimetric testing methods considers heating rate, analysis atmosphere and sample size to efficiently ensure the high quality of the material. Here, the steps for the use of TGA in the quality control of carbon fiber-reinforced polymers are summarized.

The three sectors of aerospace, wind energy and automotive have identified the various advantages of carbon fiber-reinforced polymers (CFRPs) for their products. These include low density, high specific properties as well as corrosion resistance, which boost the usage of CFRPs in a number of applications. What is also crucial is the choice of the unique mix of matrix material, fiber length, orientation and fiber content. As a result, the composite can be tailored to be used in specific areas of application.

Vf, which stands for fiber volume fraction, accounts for the mechanical and physical properties of the material. Thus, Vf becomes the most crucial indicator in the quality control of CFRPs. In March 2019, Dominik Grund, Manuel Orlishausen and Iman Taha published an article in the journal “Polymer Testing”1, thus forming the base for this article.

There are Better Alternatives to Wet Chemical Methods

Commonly applied in the determination of fiber volume fraction, the wet chemical method exposes the CFRP sample to acid that is highly oxidizing at elevated temperatures. In an effort to circumvent the various health, safety and environmental concerns – as well as ensuring ensure high quality of CFRPs in a more efficient manner – the article’s authors suggest that incineration methods using TGA provide significant advantages over all alternatives. This is because the different fiber types can be separated based on differences in their thermal stabilities.

Determination of Vf Using a Two-Stage Thermo-Gravimetrical Procedure

One of the primary challenges faced by the author was to mitigate the overlap between resin and fiber degradation process. It was found that a two-stage analysis procedure using TGA gives the most accurate value of fiber volume fraction of CFRPs. The first step of dynamic heating is carried out in an inert atmosphere, which thereby decomposes the resin of the milled CRFP sample.

Following this, the sample is cooled to room temperature. The second heating step then follows, with heating done under MRC (mass-rate-controlled) conditions in technical air in order to decompose the resin char.

Two-stage TGA of milled CFRP (P6), including dynamic heating in inert atmosphere (first heating stage), followed by MRC heating in technical air (second heating stage).

Figure 1. Two-stage TGA of milled CFRP (P6), including dynamic heating in inert atmosphere (first heating stage), followed by MRC heating in technical air (second heating stage).

TGA measurement of milled CFRP1

Why a Two-Stage Analysis Using TGA Provides the Best Results

There are three key parameters in the procedure that need to be brought in line, to arrive at a meaningful degradation curve that can be used to determine Vf.

Atmosphere

Two degradation stages can be observed in an atmosphere of technical air. In fact, the second stage can be attributed to the thermal degradation of carbon fibers. Having said that, the mass loss in the first stage points to a degradation of both resin and fiber, which resolves in an overlap in the temperature range (300 – 700 °C), and thus prohibits the separation of the effects.

In an inert atmosphere, the matrix was able to decompose completely without any thermal effects on the carbon fibers. Nevertheless, significant char content was left behind. Thus, by combining both atmospheres, the separation of the degradation of individual components can be undertaken. Indeed, this was the most crucial step that the author took into account.

Heating Rate

With a constant heating rate of 5 K/min, the decomposition is linear. However, the MRC decomposition causes a more detailed signal, thus providing a clearer separation between the decomposition phases and can directly be used to calculate Vf.

Sample Size

When combined with a MRC heating rate, an atmosphere of technical air 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. This is because the signal displays a relatively broad plateau between 450 °C and 600 °C.

Conversely, a milled sample shows a smooth thermogravimetric curve with a notable peak on the first derivative curve. As emphasized by the author, milled samples also have a lower standard deviation in Vf and residual mass. Thus, this makes it more favorable to calculate fiber volume fraction.

TGA Procedure Reveals Equal Fiber Volume Fraction

The authors validated the TGA procedure against the commonly used wet chemical analysis. Results showed a difference of 1.1% in Vf between the two methods, a fact that can be attributed to the slight degradation of carbon fibers during TGA. However, for the wet chemical analysis, such deviation falls within the acceptable range of error.

Dominik Grund’s procedure using thermogravimetric analysis provides an approach for the determination of the fiber volume friction in order to efficiently control the quality of CFRPs. Moreover, the process of TGA can be automated and does not require constant supervision.

References

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.

This information has been sourced, reviewed and adapted from materials provided by NETZSCH-Gerätebau GmbH.

For more information on this source, please visit NETZSCH-Gerätebau GmbH.

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