Why is concentrated sulfuric acid used to strip the coating of high-temperature resistant optical fibers? Can’t mechanical strippers do the job?
In the preparation and processing of high-temperature resistant specialty optical fibers (such as polyimide fibers), the stripping mechanism of the coating layer differs fundamentally from that of conventional communication optical fibers. The reason why ordinary mechanical fiber strippers (like Miller clamps) cannot be used, and chemical stripping (usually using concentrated sulfuric acid with heating) is necessary, is primarily determined by the material’s physicochemical properties and the interfacial bonding mechanism:
1. Why Mechanical Strippers Fail?
Conventional communication optical fibers use UV-curable Acrylate coatings, typically with an outer diameter of 245 \sim 250 \ \mu \text{m} (cladding diameter 125 \ \mu \text{m}). The coating thickness on one side is about 60 \ \mu \text{m}. This material is relatively soft and adheres weakly to the silica cladding, allowing mechanical blades to easily cut into and strip the coating through axial sliding.
In contrast, mechanical stripping of high-temperature resistant fibers (using polyimide PI fiber as an example) faces the following physical limitations:
- Extremely Thin and Hard Coating Layer:
Taking a standard high-temperature resistant single-mode polyimide fiber as an example, its cladding outer diameter is 125 \ \ \mu \text{m}, and the coating outer diameter is only 155 \ \mu \text{m}, resulting in a coating thickness of approximately 15 \ \mu \text{m} on each side. Polyimide is a high-modulus, highly wear-resistant engineering polymer. Its extremely thin thickness prevents mechanical stripper blades from precisely cutting into the coating without contacting the quartz glass surface. - Extremely Strong Interfacial Bonding Force:
During the high-temperature imidization and curing process, polyimide forms exceptionally strong chemical bonds and physical interlocking with the silica (\text{SiO}_2) glass surface. - High Susceptibility to Surface Micro-cracks and Fiber Fracture:
If mechanical clamps are forcefully applied for gripping and scraping, the extremely high shear stress will directly damage the surface of the quartz glass cladding, creating micro-cracks or causing stress concentration. This leads to direct fiber fracture during stripping or a drastic reduction in its subsequent tensile strength.
2. Why Concentrated Sulfuric Acid for Chemical Stripping?
Chemical stripping utilizes selective chemical degradation combined with the acid inertness of quartz glass:
- Selective Cleavage of Organic Polymer:
Heated concentrated sulfuric acid (typically at 130^\circ \text{C}\sim 180^\circ \text{C}) possesses strong dehydrating and oxidizing properties. It rapidly attacks, hydrolyzes, and carbonizes the main chains of the polyimide macromolecules’ imide rings, causing the tightly adhered polymer layer to degrade and detach within tens of seconds. - Completely Harmless to Quartz Glass:
Silica (\text{SiO}_2) exhibits high chemical stability towards concentrated sulfuric acid. The acid does not corrode the fiber cladding, thereby preserving the original surface smoothness and tensile mechanical strength of the fiber as it was upon leaving the factory.
3. Reference Products for High-Temperature Polyimide Optical Fibers
Within Beijing Dacheng Yongsheng Technology Co., Ltd.'s (OFSCN®) specialty fiber series, polyimide-coated fibers are specifically designed for harsh high-temperature environments. Their coating parameters are as follows:
- OFSCN® 300℃ SM Polyimide Optical Fiber
- Core Diameter: 9\ \mu \text{m} ; Cladding Diameter: 125\ \mu \text{m} ; Coating Diameter: 155\ \mu \text{m}
- Operating Temperature Range: -200^\circ \text{C}\sim 350^\circ \text{C} (or -270^\circ \text{C}\sim 350^\circ \text{C} )
- Coating Material: High-temperature resistant Polyimide
- OFSCN® 200℃ Polyimide Optical Fiber
- Cladding Diameter: 125\ \mu \text{m} ; Coating Diameter: 155\ \mu \text{m}
- Operating Temperature Range: -60^\circ \text{C}\sim 200^\circ \text{C}
- Coating Material: Low-temperature Polyimide
(Note: Chemical stripping involves high temperatures and strong acids. During experimental and engineering operations, it must be performed inside a fume hood with specialized acid- and heat-resistant fixtures, and strict personal anti-corrosion protection measures must be taken.)