Sheet lining is a method developed to apply stable linings to large equipment such as towers, tanks, and large-diameter pipes. The process involves bonding thick sheets to metal containers via glass cloth, followed by welding the seams with a welding rod (ribbon) for protection. PTFE (Polytetrafluoroethylene) (※2) is commonly used for the thick sheet material, while PFA (Perfluoroalkoxy) (※3) is used for the welding rod. In the welding process, the joints between the sheets are grooved and then thermocompression bonding is performed using a hot air welding gun and a welding rod (ribbon).
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Application of Optical Coherence Tomography (OCT) for Nonintrusive Inspection of Fluoropolymer Linings | Daikin's New Technology Development
FEATURE
2025.01.23
Daikin Industries has developed a "nonintrusive inspection method for fluoropolymer linings" utilizing Optical Coherence Tomography (OCT) technology. This new method replaces traditional nonintrusive inspection techniques such as visual inspection, tap testing, and spark testing and enables the detection of internal defects in linings with high precision.
The OCT technology detects signs of deterioration in fluoropolymer linings (※1) inside tanks and pipelines used to store and transport high-purity chemicals. Using this unique method, a world's first, areas of concern, including previously undetectable defects inside the lining, can now be identified and immediately repaired prior to lining penetration.
In this article, Masao Noumi of Daikin's Technology and Innovation Center (TIC) reveals the purpose, background, and narratives regarding the development of this technology.
(※1) Definition of a lining
A lining is a surface treatment technology aimed at protecting objects and improving their functionality. It is a method that involves thickly coating a specific material on the surface or interior of an object. Generally speaking, a coating with a thickness greater than 0.3mm is referred to as a "lining," whereas anything thinner is called a "coating."
The OCT technology detects signs of deterioration in fluoropolymer linings (※1) inside tanks and pipelines used to store and transport high-purity chemicals. Using this unique method, a world's first, areas of concern, including previously undetectable defects inside the lining, can now be identified and immediately repaired prior to lining penetration.
In this article, Masao Noumi of Daikin's Technology and Innovation Center (TIC) reveals the purpose, background, and narratives regarding the development of this technology.
(※1) Definition of a lining
A lining is a surface treatment technology aimed at protecting objects and improving their functionality. It is a method that involves thickly coating a specific material on the surface or interior of an object. Generally speaking, a coating with a thickness greater than 0.3mm is referred to as a "lining," whereas anything thinner is called a "coating."
What is OCT? From Ophthalmology to Industrial Technology
——
OCT: A Technology That Uses Coherence of Light for Interior Image Samples
"OCT" stands for Optical Coherence Tomography, a technology that uses the coherence of light to create images of the internal microstructure inside a measurement target. OCT technology is based on methods that precisely measure the distance light travels in a given direction (which also contributed to determining meter-length units using a Michelson interferometer).
The characteristics of OCT include high speed, high resolution, noninvasiveness, and real-time internal observation of samples. As a result, it has been widely adopted for medical diagnoses in ophthalmology.
——Use of OCT Beyond Ophthalmology: Skin Cancer Screening and Dental Resin Examination
Outside of ophthalmology, OCT has been explored for uses such as skin diagnosis (including skin cancer screening), resin examination in dentistry, and catheter examinations to see inside blood vessels. In addition, it has also been considered for use in laboratory analytical instruments for scientific research.
(Left) Fluoropolymer lined storage containers for high-purity chemicals(Right) Principle of the Michelson interferometer
Challenges of Conventional Fluoropolymer Lining Inspection and Technology Development
——Challenge of Sheet Lining: Knowledge Transfer of Skills from Experienced Welders
Fluoropolymer linings are primarily used for storing and transporting ultra-pure chemicals, making long-term performance maintenance critical.
Welding process for fluoropolymer lining
Details of the welding process (partial)
——Development of Fluoropolymer Lining Inspection Technology: Inspired by Company Health Examination!
I joined Daikin Industries as a mid-career hire. As a university student, I majored in analytical chemistry and often had to confront the challenge of "nonintrusive testing" in my assigned department. Over time, I began receiving inquiries from other departments within the company regarding nonintrusive testing, and through this process, the idea of applying Optical Coherence Tomography (OCT) technology to nonintrusive inspection was born.
One day I happened to have a company health examination, and OCT technology was being used for the retinal examination. That sparked my curiosity about the underlying principles of the examination. Since the thickness of resin linings is about 3mm, I thought, "Why not apply the OCT technology used for retinal examinations to fluoropolymer lining inspections?" This became the starting point for development. I then began consulting with various individuals about OCT technology.
Fluoropolymer linings are primarily used for storing and transporting ultra-pure chemicals, making long-term performance maintenance critical.
In manufacturing, sheet lining welding heavily relies on the craftsmanship of veteran technicians, and there are significant challenges in passing on this technical knowledge, which can have a significant impact on company operations.
Additionally, in the welding process, if some unmelted areas remain at the weld fusion zone, fine voids may form. Moreover, the sheet material's specific gravity decreases when it is heated beyond the melting point of the welding rod (PFA) and then rapidly cooled. Excessive heat stress can lead to a significant reduction in specific gravity and material denaturation.These welding defects grow over time and manifest as quality failures after several years, potentially impairing the storage of ultra-pure chemicals and causing safety concerns.
However, traditional visual and tap tests can only detect clear anomalies, making it difficult to identify internal defects. This has made the development of nonintrusive sensing technology for construction sites a shared challenge across the industry.
Note (※2) PTFE (Polytetrafluoroethylene)
PTFE is widely used as an industrial material and has become a representative of fluoropolymers. Its stable molecular structure and excellent chemical resistance and heat resistance make it an ideal material for various resin lining applications. However, due to its difficulty in melting and forming complex shapes, it is mainly molded through compression or extrusion.
Note (※3) PFA (Perfluoroalkoxy)
PFA is a type of fluoropolymer that has thermoplastic properties, allowing it to be molded when heated. It features heat resistance, water repellency, chemical resistance, and non-stick properties, making it widely used in various industrial applications, especially for lining equipment that requires resistance to chemicals.
Welding process for fluoropolymer liningDetails of the welding process (partial)I joined Daikin Industries as a mid-career hire. As a university student, I majored in analytical chemistry and often had to confront the challenge of "nonintrusive testing" in my assigned department. Over time, I began receiving inquiries from other departments within the company regarding nonintrusive testing, and through this process, the idea of applying Optical Coherence Tomography (OCT) technology to nonintrusive inspection was born.
One day I happened to have a company health examination, and OCT technology was being used for the retinal examination. That sparked my curiosity about the underlying principles of the examination. Since the thickness of resin linings is about 3mm, I thought, "Why not apply the OCT technology used for retinal examinations to fluoropolymer lining inspections?" This became the starting point for development. I then began consulting with various individuals about OCT technology.
How OCT Technology Was Applied to Material Inspection
——Fluoropolymer Lining Inspection Technology: A Fusion of OCT Technology for Retinal Examinations with Telecentric Design and Fθ Lens Technology for Laser Printers
Originally, OCT technology was developed for ophthalmic examinations, focusing on high-precision imaging of very narrow areas, such as blood flow in the retinal layers.While OCT technology for eye exams focuses on high-precision imaging of small, narrow areas, fluoropolymer lining inspection requires observation of wide areas, often on a meter scale. Consequently, the technical requirements were the exact opposite.
Originally, OCT technology was developed for ophthalmic examinations, focusing on high-precision imaging of very narrow areas, such as blood flow in the retinal layers.While OCT technology for eye exams focuses on high-precision imaging of small, narrow areas, fluoropolymer lining inspection requires observation of wide areas, often on a meter scale. Consequently, the technical requirements were the exact opposite.
To better understand the principles of OCT technology, I researched patents from a company that developed an "optical coherence tomography" device used in health examinations. During my research, I came across another patent from the same company: a design for telecentric optical systems and an Fθ lens mechanism used in laser printers. (※4) This provided me with a moment of inspiration that this technology could be applied to our challenge of "wide-area observation" and "curved surface observation." In other words, I hypothesized that by combining the existing OCT technology with the telecentric design and Fθ lens technology could make fluoropolymer lining inspection possible.
Note (※4) Telecentric Design and Fθ Lens Mechanism
Telecentric design refers to an optical system in which the field of view remains constant (0 degrees) regardless of the distance between the object and the lens. In other words, it is an optical system where the principal rays are parallel. The Fθ lens mechanism is designed by altering the curvature of the two lens surfaces, ensuring that the scanning speed is uniform between the center and the periphery of the lens. This lens mechanism focuses incident light onto the sample surface.
—— The Uniqueness of This Technology: 1) Measures under External Disturbances such as Temperature and Vibration, 2) Measures Wide Areas and Curved Surfaces, 3) Produces High Resolution of 2D/3D Images
Typical precision instruments, such as physical and chemical devices, are used in stable environments free from external influences (such as temperature, humidity, and vibration). In contrast, the fluoropolymer lining inspection process takes place in outdoor environments with high temperatures and vibrations.
Therefore, a new inspection device must have an optical design that cancels out external disturbances, such as temperature and vibration, which are unique impediments compared to what is seen for the conventional OCT technology used in retinal examination devices.By adopting the principles of the Fizeau interferometer (※5), we were able to perform tomographic imaging using OCT technology by directly contacting the objective lens with the sample. In other words, this method effectively prevents the influence of external optical noise.
Note (※5) Fizeau Interferometer
A Fizeau interferometer is an interferometer that converts light from a point source into a collimated beam, which is directed onto the test surface after passing through a reference surface (half mirror). The reflected light from the test surface is then interfered with the reflected light from the reference surface. The interference pattern is guided to the observation surface via a beam splitter. Compared to the Michelson interferometer, the Fizeau interferometer is less affected by vibration and temperature changes, allowing for stable measurements. It is particularly suited for observing the surface flatness of reflectors and can evaluate the precision of optical components with high resolution.
(Source: The Japan Society of Mechanical Engineers > https://www.jsme.or.jp/jsme-medwiki/doku.php?id=15:1011059)
(Left) Principle of the Fizeau interferometer(Right) How to use the developed OCT inspection device (probe part) and observation image of the welded part
The adoption of the Fθ lens allows for linear scanning of the sample surface in accordance with the rotation of the mirror inside the OCT system. As a result, it is possible to observe the sample surface over a wide area, enabling the observation of curved surfaces. In medical diagnostic applications, the allowable light output is strictly limited due to the risk of tissue damage. On the other hand, in industrial applications, such as fluoropolymer lining inspection, the intensity of the irradiated light can be set higher, and no attenuation of the input light source is necessary, resulting in higher resolution for the output 2D/3D images.
The measurement depth of OCT systems is primarily limited by the absorption and scattering of light from the material being measured. In retinal examinations, near-infrared light with a wavelength of 800–900 nm is used to reduce the absorption of water by the vitreous body. In contrast, the OCT developed by Daikin Industries uses near-infrared light with a wavelength of 1310 nm, enabling measurement of deeper parts of resin linings (approximately 5 mm). Defect sizes can be detected at the μm level, and defects of approximately 100 μm can be reliably detected even under noisy conditions.
Regarding intellectual property, patents in Japan have already been granted (Patent No. 7128430, Patent No. 6922965), and overseas patents are expected to be granted in the three regions of North America, Europe, and China.
Overview of OCT inspection system for fluoropolymer liningChallenges and Behind-the-Scenes of OCT Technology Development
—— Development of the new optical theory resulted from independent research and concentrated thought in consultation with others
This research and development started from the ground up and initially only involved me. I then searched for OCT experts to help me move the development forward. I consulted with medical professionals who were knowledgeable about OCT technology, but at first, no one seemed to fully understand the differences in usage environments between medical devices and industrial applications.
At the same time, I read numerous medical OCT papers and reviewed complex physical equations. From the elemental technology of laser printers (such as the Fθ lens), I derived ideas and considered ways to make the system usable outdoors. I eventually developed a unique optical theory that could cancel out disturbances such as temperature and vibrations.
—— Prototyping: A Turning Point Through Collaboration with Universities
By the time I had a clear image of the equipment in my mind, I struggled with procuring various devices and assembling the prototype. At that point, I received support from the Institute of Laser Engineering (ILE), Osaka University, with whom we have an extensive collaborative agreement. With the support of their network, I was able to carefully select and source components from around the world to assemble the prototype. During the early stages of prototyping, the system did not operate as expected, and it was a continuous trial-and-error process. However, the moment the system worked as I envisioned, I felt the excitement of realizing that my efforts and emotions had shaped the research into a tangible result.
After completing the prototype, I presented the results, but on-site workers evaluated it as "technically interesting, but lacking in practicality." The main criticism was that the system required manual focus adjustment, and the usability from the field perspective was limited, meaning the product was not yet easy for workers to use.
Like many researchers, I initially focused on theories and relationships but had neglected the practical application of the technology. While theoretical aspects are important, balancing them with field usability is key to success. The feedback from the field was blunt but honest. Through mutual trust and collaboration with the workers, we were able to continuously improve the system, resulting in a product that is now highly accepted by the field.
—— Skills Transfer Challenges: Driving OCT Development
Within the company, there was a serious issue with passing on lining skills from experienced workers to younger employees. While measures such as "recording work on video" were implemented, there was no fundamental solution to this challenge.
There are only a limited number of workers skilled in sheet lining welding, and the need for "transferring skills" has become increasingly more urgent. This sense of urgency pushed forward the development of the OCT system.
As a response to feedback from the field, an autofocus function was added to the system. Additionally, feedback pointed out that the visibility of the touch panel display decreased significantly under direct sunlight, leading to the implementation of an anti-glare treatment on the panel surface.
Field Tests and Results from Actual Use in a Plant
Chemical plants operate 24/7 to increase efficiency, conducting regular maintenance and repairs to prevent issues. During the scheduled maintenance period, the prototype was brought into an actual plant as part of a field test to confirm operation.
Field test results in actual plantPreviously, inspections were conducted through tapping and visual methods. With the new prototype, the inspection results are visualized in images. As a result, the following positive feedback was received from the business division:
(1) The inspection can be completed with one click using a single button. It's easy for anyone to use, and no qualifications are needed for the operator.
(2) It eliminates dependence on the individual worker's skills, reducing the risk of personalized inspections.
(3) The image clearly shows the condition and size of defects, making the inspection results more convincing to third parties.
(4) The system allows for immediate decision-making regarding whether to replace or repair the entire container, or just monitor the defect based on the visible defect size.
(5) With data accumulation, the degree of degradation can be assessed, enabling estimates of the asset's remaining lifespan.
Future Prospects and Further Developments
—— Exploring Applications for Other Equipment and Infrastructure Deterioration Diagnosis
Originally, this system was developed for internal use in facility maintenance. However, due to its unique technology, we are now considering applying it to other customer equipment. Through degradation diagnosis and lifespan predictions, we aim to drive demand for our products, such as fluororesin. Furthermore, this technology may prove useful for rust prevention, corrosion protection, and maintenance solutions for social infrastructure like roads, bridges, and tunnels, addressing aging infrastructure issues.
In particular, industries that use long-lasting (20–30 years of operation) materials or multi-layer structures with hidden substrates are expected to find the technology valuable.
Additionally, OCT technology's image data is well-suited for information technology, and we aim to integrate it with AI and machine learning to expand its application. To accelerate market expansion, we plan to proactively utilize trade shows and forums to disseminate technology and create a forum for the exchange of information that transcends the boundaries of existing industries.
—— The Daikin Environment: A Place for Challenge and Growth
Daikin is known for being a company that encourages challenge and growth, especially for those who have a strong desire to accomplish something. This was certainly true for the development of this OCT system. I started from nothing, and through theoretical validation, I was able to turn a list of equations written in my notebook into an actual device. This device did not exist in the world before, and being able to contribute to the advancement of chemical infrastructure maintenance and inspection was incredibly fulfilling as a researcher.
I also believe that Daikin's flat organizational structure is one of its great qualities. There are few barriers between departments, and if you encounter any difficulties, you can easily consult with others, including executives. A corporate culture has been created that if you put your heart into your actions, you can always receive the support of many people who are always willing to lend a helping hand.This environment has been a great source of joy to me personally, and I truly feel that Daikin's atmosphere fosters challenges and growth.
※The information and profiles are based on the time of the interview.
Masao Nomi
Technology and Innovation Center
Joined in June 2012. Originally from Fukuoka Prefecture.
Responsible for exploring and introducing new analytical and evaluation technologies.
"The deeper you go into optical technology, the more fascinating it becomes. I aim to continue creating technologies and ideas that illuminate the future."
Technology and Innovation Center
Joined in June 2012. Originally from Fukuoka Prefecture.
Responsible for exploring and introducing new analytical and evaluation technologies.
"The deeper you go into optical technology, the more fascinating it becomes. I aim to continue creating technologies and ideas that illuminate the future."
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