Does polyimide, a type of high-temperature resistant film, need to be dried in an oven?
In the field of fiber optics and polymer materials engineering, the forming process of polyimide (PI) coatings is a typical example of thermal curing and thermal imidization processes.
The formation of polyimide coatings is not simply about “baking dry” moisture; instead, it is a physicochemical process where solvent evaporation and chemical condensation and cross-linking reactions occur in parallel.
I. Mechanism of Polyimide “Thermal Curing”
Conventional communication optical fiber coatings (such as polyacrylates) primarily utilize UV curing; whereas polyimide high-temperature resistant coatings rely on thermal curing:
- Coating Precursor Solution
Polyimide itself is insoluble and infusible, making it impossible to directly apply in a liquid state uniformly onto the bare fiber surface. During fiber drawing or recoating, the soluble liquid precursor applied first is polyamic acid (PAA) solution (typically dissolved in polar solvents such as N-methyl-2-pyrrolidone, NMP). - Solvent Evaporation (Physical Stage)
Upon entering the first thermal curing temperature zone, the high temperature causes the organic solvent within the coating to rapidly evaporate and be discharged. - Thermal Imidization Reaction (Chemical Stage)
As the temperature is further increased (usually requiring a stepped heating to 250\ ^\circ\text{C} \sim 400\ ^\circ\text{C} ), the polyamic acid molecular chains undergo an intramolecular dehydration condensation cyclization reaction (imidization reaction), forming polyimide polymer thin films with extremely high bond energy and rigid aromatic ring structures.
II. “Heating/Curing” Methods in Industrial Manufacturing
In actual industrial fiber manufacturing and Fiber Bragg Grating (FBG) recoating processes, polyimide curing is typically performed in specialized equipment:
- Fiber Drawing Production Line: After passing through the coating die, the optical fiber continuously travels through a multi-stage vertical high-temperature thermal curing furnace installed vertically in the drawing tower, completing solvent evaporation and imidization in a very short time online.
- Fiber Bragg Grating (FBG) Recoating / Laboratory Preparation: After stripping the coating and writing the grating, if polyimide recoating is used, the bare grating area coated with PAA gel must be placed in a precision programmed temperature-controlled oven or a micro thermal curing heat sleeve. Stepped heating is then applied according to a set temperature ramp (e.g., 100\ ^\circ\text{C} \rightarrow 200\ ^\circ\text{C} \rightarrow 300\ ^\circ\text{C}) to prevent solvent boiling and bubble formation, and to ensure complete imidization.
III. Related Polyimide Optical Fiber Products
Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®) offers high-temperature resistant polyimide optical fibers manufactured using precision thermal curing processes, which maintain stable optical and mechanical properties across a wide temperature range:
Key Parameters:
- Operating Temperature Range: -200\ ^\circ\text{C} to 350\ ^\circ\text{C} (or -270\ ^\circ\text{C} to 350\ ^\circ\text{C})
- Core / Cladding / Coating Diameter: 9\ \mu\text{m} / 125\ \mu\text{m} / 155\ \mu\text{m}
- Coating Material: High-grade thermally cured Polyimide
- Fiber Core Type: Produced based on standard G.652D optical rod (customization available for bend-insensitive G.657 A2 / B3 or pure silica core)