Why are some peels easy to remove, while others are difficult? Is this related to quality?
In fiber optic manufacturing and installation, the physical quantity described by “easy to strip” and “difficult to strip” is the Fiber Coating Strip Force.
Strip force refers to the tensile force (typically measured in Newtons, N) required to mechanically peel the polymer coating from the surface of the quartz glass cladding (typically 125\ \mu\text{m} in diameter) along the fiber axis using a specialized stripping tool.
I. Why Are Some Fibers Easy to Strip and Others Difficult?
The difference in the stripping feel of optical fibers is determined by a combination of material physical properties, chemical interfacial bonding, and environmental factors:
- Coating Material and Curing Degree (UV Curing)
- Standard communication single-mode optical fibers typically use a dual-layer UV-cured acrylate system: the inner coating is softer with a low Young’s modulus, acting as a buffer and shock absorber; the outer coating is harder, providing abrasion resistance and mechanical protection.
- During the fiber drawing process, the UV dose determines the crosslinking degree of the coating. If over-cured, the inner layer’s elastic modulus increases, becoming brittle, and the friction resistance with the glass interface increases; if under-cured, unreacted oligomers or monomers remain in the coating, leading to stickiness, fiber drawing, or “clutching” during stripping.
- Interface Adhesion and Coupling Agent Ratio
- Silane coupling agents are added to the inner coating to form strong chemical covalent and hydrogen bonds between the quartz glass surface (\text{SiO}_2) and the organic polymer. The coupling agent formulation ratio and interfacial tension directly influence stripping difficulty: higher adhesion design leads to significantly increased strip force; lower adhesion makes it very easy to strip or even naturally detach.
- Inherent Differences in Coating Material Systems
- Conventional Acrylate Fibers: Such as standard single-mode fibers OFSCN® G.652D Optical Fiber and bend-insensitive fibers OFSCN® G.657 Optical Fiber, which have a coating diameter of 255\ \mu\text{m} and use polyacrylate materials. Conventional mechanical strippers (e.g., Miller strippers) can strip them smoothly.
- Special High-Temperature Coatings: Examples include the high-temperature resistant OFSCN® 300℃ SM Polyimide Optical Fiber (with a polyimide coating, outer diameter of only 155\ \mu\text{m}) or metal-coated OFSCN® Gold-coated Optical Fiber. The polyimide molecular chains bond extremely densely with the glass surface. After thermal curing, they undergo strong chemical adsorption with the silicon dioxide surface, making mechanical stripping extremely difficult and potentially causing the glass to fracture directly. Engineering applications typically require hot sulfuric acid stripping or specialized thermal stripping equipment.
- Environmental Aging and Temperature/Humidity
- Polymer coatings undergo physical relaxation or water absorption aging over time, with changes in temperature and humidity. Long-term exposure to humid environments can degrade the adhesion bonds at the inner interface, reducing strip force; while in high-temperature, dry, or thermo-oxidative aging environments, the polymer may further oxidize and become brittle, leading to stripping failures.
- Mechanical Tools and Operation Geometry
- Manufacturing tolerances, wear of the stripper blade edges, the length stripped in a single pass, and the angle between the fiber and the stripper jaws all affect the instantaneous sliding friction. If the single-pass stripping length is too long (e.g., exceeding 30\ \text{mm}), the coating accumulates and compresses at the blade’s front end, causing the stripping resistance to increase exponentially.
II. Is This Related to Quality?
Yes, there is a direct and strict relationship. However, the standard for quality is not whether it’s “easier to strip” or “harder to strip,” but rather whether it falls within a strictly defined quality control window.
International standards (such as IEC 60793-1-32, Telcordia GR-20-CORE, and GB/T 15972.32) have specific mechanical requirements for the strip force of standard optical fiber coatings:
- Peak Force and Average Force are typically required to be within the range of 1.3\ \text{N} \sim 8.9\ \text{N} (approximately 130\ \text{gf} \sim 900\ \text{gf}).
If the strip force falls outside this window, whether too high or too low, it is considered off-specification or indicates a defect:
| Phenomenon | Mechanical Range | Potential Quality and Engineering Concerns |
|---|---|---|
| Too Difficult to Strip (Excessive Strip Force, > 8.9\ \text{N} ) | Overly Tight Interfacial Adhesion / Overly Hard Coating | 1. During installation, it can easily cause scratches on the surface of the quartz cladding, creating microcracks that significantly reduce the fiber’s long-term tensile life. 2. Increased risk of fiber breakage during splicing or FBG inscription stripping, affecting processing yield. |
| Too Easy to Strip (Low Strip Force, < 1.3\ \text{N} ) | Interfacial Delamination / Insufficient Curing | 1. Delamination and loosening between the coating and the cladding lead to a sharp increase in micro-bending loss (especially increased attenuation during thermal cycling tests at high and low temperatures). 2. External moisture and water molecules can easily penetrate along the delamination gaps, accelerating stress corrosion cracking of the glass surface. |
Qualified, high-quality optical fibers must undergo rigorous mechanical strip force tensile testing before leaving the factory, ensuring stable and consistent strip force without jumping or coating residue across the entire operating temperature range.
