Why do ordinary plastic sheaths shatter in a liquid nitrogen environment (-196℃)?
In a liquid nitrogen environment (-196^\circ\text{C}), the plastic jackets of ordinary fiber optic patch cords (such as PVC, PE, or LSZH, etc.) will experience severe brittle failure. This is primarily determined by the physical properties of polymer materials and thermodynamic stress.
1. Why do ordinary plastic jackets shatter in liquid nitrogen?
The main reasons can be summarized as follows:
A. Glass Transition and Embrittlement
Plastics (polymer materials) have their specific glass transition temperature (T_g).
- At room temperature, plastics are in a highly elastic or semi-crystalline state, allowing for free movement of macromolecular chain segments, thus exhibiting good flexibility and bending resistance.
- When the ambient temperature drops below its T_g, the movement of macromolecular chain segments is “frozen,” and the material’s mechanical state transitions from the highly elastic state to the glassy state.
- The glass transition temperature of ordinary PVC is approximately between -10^\circ\text{C} and -50^\circ\text{C} (depending on the type and proportion of plasticizers); although the embrittlement temperature of high-density polyethylene (HDPE) is low, in the liquid nitrogen environment of -196^\circ\text{C}, all ordinary plastics are far below their glass transition temperatures. Plastics in the glassy state are extremely hard and brittle, having almost no capacity for plastic deformation, and are highly prone to brittle fracture (Shattering) under minimal external force.
B. Significant Coefficient of Thermal Expansion Difference and Internal Stress (Thermal Stress)
During the extreme temperature drop from room temperature (approx. 20^\circ\text{C}) to liquid nitrogen (-196^\circ\text{C}), a temperature difference of about 216^\circ\text{C}:
- The coefficient of linear thermal expansion (CTE) of ordinary plastics is typically as high as (50 \sim 150) \times 10^{-6}/\text{K}.
- The coefficient of linear thermal expansion of quartz fiber (silica) is extremely small, about 0.5 \times 10^{-6}/\text{K}.
- During rapid cooling, the outer plastic jacket tends to contract intensely. However, the internal quartz fiber (or metal strength members) contracts very little, which restricts the thermal contraction of the plastic jacket.
- Since the plastic is already frozen into a highly brittle glassy state, immense thermal stress rapidly builds up internally. Ultimately, this causes the plastic jacket to crack and shatter on its own, without any external force being applied.
2. Engineering Solutions for Extremely Low (Cryogenic) Temperature Environments
To achieve stable optical signal transmission in cryogenic environments such as liquid nitrogen (-196^\circ\text{C}) or even liquid helium (-269^\circ\text{C} / 4\text{K}), traditional ordinary polymer plastic protective sheaths must be completely abandoned.
OFSCN® has designed high-performance fiber optic patch cords without plastic jacketing, featuring seamless steel tube armoring for protection in alternating high and low temperature and extreme cryogenic environments:
Recommended Solution 1: OFSCN® 200℃ Fiber Optic Patch Cord
- Temperature Range: -200^\circ\text{C} to +200^\circ\text{C} (safe for use in liquid nitrogen environments).
- Technical Features: Completely eliminates ordinary plastic jacketing, using a 0.9\text{mm} seamless stainless steel tube for mechanical protection, with an inner layer of high- and low-temperature resistant polyimide-coated fiber. Because the metal steel tube maintains excellent toughness and mechanical strength at extremely low temperatures, it can perfectly withstand the cold shock of liquid nitrogen.
- Product Structure Diagram:
Recommended Solution 2: OFSCN® 300℃ Fiber Optic Patch Cord
- Temperature Range: -270^\circ\text{C} to +300^\circ\text{C} (can perfectly cover the extreme low-temperature range from liquid helium to liquid nitrogen).
- Technical Features: Uses a 0.9\text{mm} seamless stainless steel tube and special 300^\circ\text{C} polyimide fiber, maintaining stable optical and mechanical performance even in deep cryogenic environments near absolute zero.
- Product Structure Diagram:
Recommended Solution 3: OFSCN® 700℃ Fiber Optic Patch Cord
- Temperature Range: -270^\circ\text{C} to +700^\circ\text{C}.
- Technical Features: Uses a 0.9\text{mm} seamless stainless steel tube and Gold-coated Optical Fiber. Since the gold (Au) metal coating does not undergo glass transition over an extremely wide temperature span from deep cold (-270^\circ\text{C}) to high temperatures (+700^\circ\text{C}) and consistently maintains metallic ductility, this is the top-tier engineering solution with the highest resistance to embrittlement and cracking in deep cold and high/low-temperature cycling environments.
- Product Structure Diagram:
