This blog is based on the article "Hydrogen Permeability of Thermoplastic Composites and Liner Systems for Future Mobility Applications" By Jan Condé-Wolter and team. Jan Condé-Wolter, along with his esteemed team, delves deep into the realm of thermoplastic composites, exploring their potential in the hydrogen economy. With a focus on hydrogen permeability, this comprehensive research provides pivotal insights that are set to shape the future of mobility applications. Drawing from rigorous experimentation and analysis, Condé-Wolter's work stands as a testament to the promise and challenges of thermoplastic composites in the rapidly evolving world of hydrogen storage and transportation. This article is a must-read for industry enthusiasts and researchers aiming to stay at the forefront of composite material innovations.

Exploring Hydrogen Permeability

Simplified Explanation of Hydrogen Permeability

Hydrogen permeability in materials, especially thermoplastic composites, is a critical aspect to explore, particularly in the context of storage and transportation applications. The permeability of hydrogen through a material involves several mechanisms, including the absorption of the gas into the material, diffusion through it, and desorption out of the material.

The article discusses the use of materials with low solubility for the gas to be stored, which reduces the processes of absorption and desorption, thereby reducing overall permeation. Additionally, diffusion can be influenced by adding impermeable fillers like fibers or nanoparticles, which reduce the surface to be permeated and cause tortuous paths, increasing the length of the diffusion path and thus reducing permeation.

Importance in Thermoplastic Composites

The article conducts high-pressure hydrogen permeation tests and measures specific permeation rates of various thermoplastic matrix materials as well as continuous fiber-reinforced thermoplastic composites (such as PA6, PA12, PA410, PPA, PPS), considering non-crimp and crimp textile architectures. Furthermore, liners based on thermoplastic high-barrier films made of EVOH (ethylene vinyl alcohol copolymers) were applied on PA6 composites, and their effect on permeation rate was investigated. EVOH is highlighted as one of the best-performing polymeric materials regarding permeation barrier due to its higher polyvinyl alcohol (PVA) content, which increases its polarity, decreasing the permeability of non-polar gases like hydrogen by limiting their solubility. However, as EVOH tends to swell and absorb water, which affects permeability negatively, it is often used in a multi-layer design to reduce water uptake and maintain permeation properties.

Technical Insight

• Hydrogen Permeation Tests: The article emphasizes conducting high-pressure hydrogen permeation tests to evaluate the permeation rates of various thermoplastic matrix materials and continuous fiber-reinforced thermoplastic composites.
• EVOH as a Barrier: EVOH, due to its high PVA content, limits the solubility of non-polar gases like hydrogen, thereby reducing their permeability. However, its polarity makes it susceptible to water absorption, which can negatively impact its permeability. Thus, EVOH is often utilized in a multi-layer design to mitigate water uptake while preserving its permeation properties.
• Permeation Measurement: The measurement process for permeation is automated and involves a marker fluid in graduated capillary tubing. The permeated hydrogen volume is detected in a measuring compartment, and according to the permeated hydrogen volume, the marker fluid is pushed along the tubing. The measurement process involves a float gauge on the marker fluid, which passes a total of three photoelectric barriers, indicating the time needed for the permeation of a specific volume of hydrogen.

Delving into the Research

Highlight of Key Samples and Their Significance

The research meticulously explores various thermoplastic matrix materials and continuous fiber-reinforced thermoplastic composites, conducting high-pressure hydrogen permeation tests to measure specific permeation rates. Some of the key samples investigated include:

• #4-PA410-CF: A sample with specific properties and measurements related to hydrogen permeability.
• #5-PPS-CF: Noted that measurements were not applicable as samples are not leak-tight.
• #6-PPA-CF-A, #7-PPA-CF-B, #8-PPA-CF-C: Similarly, these samples were not applicable for measurement due to not being leak-tight.
• #9-PA-6, #10-PPS, #11-PPA: These samples were measured and analyzed for properties like permeation rate.

These samples were subjected to various tests and analyses to understand their hydrogen permeability under different conditions and configurations. The research also explores the impact of using liners based on thermoplastic high-barrier films made of EVOH on PA6 composites and investigates their effect on permeation rate.

Easy-to-Understand Summary of the Methodology

The methodology adopted in the research is thorough and systematic, ensuring accurate and reliable results. Here’s a simplified breakdown:

• Manufacturing Process: The specimens were manufactured with attention to various parameters like temperature, pressure, and pre-drying. The process and parameters were iteratively adjusted, especially for high-temperature tape materials or new injection molding compounds. Quality assurance tests were conducted, including a specially developed pressure tightness test, micrographs of crosscuts, and CT-Scans for samples showing noticeably high leakage rates.

• Permeation Measurement: The measurement of permeation is automated and involves a marker fluid in graduated capillary tubing. The permeated hydrogen volume is detected in a measuring compartment, and according to the permeated hydrogen volume, the marker fluid is pushed along the tubing. The measurement process involves a float gauge on the marker fluid, which passes a total of three photoelectric barriers, indicating the time needed for the permeation of a specific volume of hydrogen.

• Evaluation of Data: The research also involves evaluating permeation data, utilizing a method involving a sigmoid function and regression fit to collected data. It also discusses the steady state and its calculation. An exemplary leakage rate data and evaluation of samples like #1 P6-CF-3, #8PPA-CF, #9 PA6, and #13 PA6-CF + M100 were conducted, classifying measured leakage rates according to whether they are in the steady-state and contribute to the mean value or they are located in the increase during the lag-time.

Methodology

Evaluation of Permeation Data

The research meticulously evaluates permeation data, ensuring a comprehensive understanding of the hydrogen permeability of various materials and configurations. The time between passing light barriers is measured, and since the volume in between is constant, a leakage rate Q (in ml/h) is measured directly. The duration of the experiment and the beginning of the steady state were determined manually by a test engineer. To reduce human bias, a uniform evaluation method was applied to all measured raw data, and no mean value was calculated over the manually determined steady-state. An example of this method is shown with the data of the #10 PPS sample in Figure.

Utilization of Sigmoid Function and Regression Fit

A sigmoid function with its coefficients a, b, and c is a regression fit to the collected data using the least square method:

Each regression fit shows a sufficient coefficient of determination (r-square) above 0.91. As the function passes through the zero point and asymptotically approaches a limit value, it shows two physical characteristics of leakage measurements over time and is a suitable and simple approximation. The beginning of the steady-state was defined as the first data point reaching 95% of the steady-state value of the regression fit, which can be represented by the fraction of coefficients a and b:

Data points in the lag time before reaching the steady state are marked with red X as non-steady-state values, while all measuring points in the steady state are marked with a green circle. This methodology, involving the evaluation of permeation data and the utilization of a sigmoid function and regression fit, ensures a thorough and systematic analysis of the hydrogen permeability of various thermoplastic composites and liner systems, providing valuable insights and data for further research and application in future mobility applications.

Key Takeaways

Summarizing Major Findings

The research embarked on a comprehensive exploration of the hydrogen permeability of various thermoplastic composites and liner systems, providing pivotal insights for future mobility applications. The study involved a meticulous analysis of several samples, such as #4-PA410-CF, #5-PPS-CF, #6-PPA-CF-A, #7-PPA-CF-B, #8-PPA-CF-C, #9-PA-6, #10-PPS, and #11-PPA, under high-pressure hydrogen permeation tests. The findings reveal that thermoplastic composites exhibit sufficient hydrogen barrier properties and can be amalgamated with robust thermoplastic liner materials. The results also underscore the importance of investigating the damage behavior of thermoplastics, as some samples exhibited transverse cracks without external load, leading to significant leakage rates.

Potential Implications in the Industry

The findings from the study have several potential implications in the industry, particularly in the development and optimization of materials and configurations for hydrogen storage and transportation applications. Here are a few key points:

• Promising Liner Material: The research supports the concept of thermoplastic multi-cell vessels, identifying thermoplastic composites as promising liner material for future hydrogen pressure vessels due to their sufficient hydrogen barrier properties.
• Sustainable Element: The good mechanical properties, efficient manufacturing processes, and improved recyclability of thermoplastic composites compared to thermoset composites make them a promising and sustainable element in the future hydrogen economy, warranting further investigation.
• Addressing Challenges: The study highlights the need for further investigation into the damage behavior of thermoplastics, as some samples exhibited transverse cracks without external load, leading to significant leakage rates. This is pivotal for ensuring the reliability and safety of thermoplastic composites in real-world applications.
• Innovation and Development: The insights derived from the study can fuel innovation and development in the composites industry, guiding researchers and engineers in developing and optimizing materials and configurations that minimize hydrogen permeability while ensuring reliability and safety in hydrogen storage and transportation applications.

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