Why are
The ease of stripping the outer “protective skin” (i.e., the fiber coating, Coating) of an optical fiber is primarily determined by the Coating Strip Force. The magnitude of the stripping force is jointly influenced by factors such as the physicochemical properties of the coating material, the thickness of the coating layer, and the adhesion strength between the coating layer and the silica (glass) cladding.
Below, we will provide a detailed analysis from the perspectives of optical engineering and material science on why some optical fibers are easy to strip mechanically, while others require chemical solvents for stripping:
I. Why Are Common Optical Fibers “Easy to Strip”? (Mechanical Stripping Principle)
Common communication optical fibers, such as OFSCN® G.652D Optical Fiber or OFSCN® G.657 Optical Fiber, have coating layers with the following characteristics:
- Material Properties: They utilize Polyacrylate material. This is a high-molecular-weight UV-curable adhesive with good elasticity and moderate adhesion.
- Thickness Design: The outer diameter of the cladding for these types of fibers is 125\ \mu\text{m}, while the outer diameter of the coating is 255\ \mu\text{m}. This means the thickness of the coating on one side is approximately 65\ \mu\text{m}.
- Low Stripping Force Design: The adhesion force (interfacial shear strength) between the polyacrylate and the silica cladding is designed to be within a reasonable range, with a stripping force typically between 1\ \text{N} and 3\ \text{N}.
- Stripping Method: Due to the sufficient thickness and elasticity of the coating, standard mechanical stripping tools (like Miller strippers) can easily cut into the coating layer. When stretched, the coating layer shears and slides on the surface of the cladding, allowing it to be removed cleanly and completely without damaging the glass cladding.
II. Why Do Specialty Optical Fibers Require Chemical Solvents for Stripping? (Chemical Stripping Principle)
Specialty optical fibers designed for high-temperature or extreme environments, such as OFSCN® 300℃ SM Polyimide Optical Fiber or OFSCN® 200℃ Polyimide Optical Fiber, have completely different coating designs:
- Material Properties: They employ Polyimide (PI) material. Polyimide is a specialty engineering plastic with extremely high thermal stability and excellent mechanical properties.
- Extremely Thin Thickness: The outer diameter of the cladding for polyimide optical fibers is also 125\ \mu\text{m}, but their coating outer diameter is typically only 155\ \mu\text{m}. This means the thickness of the coating on one side is only about 15\ \mu\text{m}.
- Extremely High Adhesion and Stripping Force: Polyimide is formed by coating a liquid polyamic acid solution onto the fiber surface, followed by high-temperature thermal curing (imidization). It forms an extremely strong chemical and physical bond with the surface of the glass cladding (silica), resulting in a very high stripping force.
- Why Mechanical Stripping is Not Feasible?
- High Risk of Fiber Breakage: Because the PI coating is extremely thin (only 15\ \mu\text{m}) and very hard, the cutting edge of a mechanical stripper can hardly cut into the coating without contacting and scratching the 125\ \mu\text{m} glass cladding.
- Reduced Mechanical Lifespan: Even if one manages to scrape it off using mechanical force, the extremely high stripping force will cause microscopic scratches and micro-cracks on the glass surface. This drastically reduces the fiber’s tensile strength, making it highly susceptible to breakage during subsequent use, coiling, or splicing.
- Chemical Stripping Mechanism:
To remove the polyimide without stress and without damaging the glass cladding, chemical solvents must be used (such as concentrated sulfuric acid \text{H}_2\text{SO}_4 at temperatures between 100^\circ\text{C} and 150^\circ\text{C}, or specialized strong alkaline/organic solvent paint removers).- The chemical solvent attacks and breaks the imide rings within the polyimide molecular chains, causing the polymer network to degrade, swell, or completely dissolve.
- After the reaction, the originally tightly adhered PI coating softens, carbonizes, and peels off, achieving a perfect, damage-free strip without applying any mechanical stress to the glass cladding.
III. Other Harder-to-Strip Metal Coatings
In addition to polyimide, metallic coated fibers such as OFSCN® Gold-coated Optical Fiber have metallic layers (e.g., gold, copper) that exhibit metallurgical bonding or extremely strong physical adsorption with the silica cladding.
For such metallic fibers, conventional mechanical stripping is completely infeasible. It typically requires chemical etching solutions (e.g., using acid to etch the metal layer) or specialized methods like high-temperature electric arcs or fusion ablation to safely remove them.
Summary
Whether an optical fiber coating is easy to strip is essentially a trade-off between the coating material’s “temperature resistance and protective performance” and its “ease of processing.” Ordinary polyacrylate optical fibers are designed for easy mechanical stripping to facilitate rapid field installation; whereas specialty polyimide or metallic optical fibers, designed to operate in extreme temperature and strain environments, sacrifice ease of stripping and must rely on chemical stripping to ensure that the glass body is not subjected to mechanical micro-damage.