Optical fiber loose tubes are a key structure that protects fibers from external stress and ensures stable transmission performance. Material selection directly determines the mechanical reliability and service life of optical cables.
Why PBT is Preferred
Polybutylene terephthalate (PBT) has a typical elastic modulus of around 2–3 GPa, higher than that of PA12 (polyamide 12), which is approximately 1.2–1.8 GPa. This means lower deformation under the same load and better resistance to lateral compression.
Its coefficient of linear thermal expansion is approximately (6–10) × 10⁻⁵ /°C, providing excellent dimensional stability, which helps control fiber excess length and reduces microbending risks under temperature variations.
In addition, low moisture absorption, good chemical resistance, and moderate cost make PBT one of the mainstream materials for loose tube applications.
It should be noted that PBT is a semi-crystalline polymer, and its crystallinity strongly depends on extrusion processing conditions. Proper process control is critical to achieving stable performance.
Three Key Control Parameters
The performance stability of loose tubes depends on strict control of three key parameters, each directly affecting long-term cable performance:
Melt Flow Index (MFI):
It reflects the extrusion flowability. For loose tube-grade PBT, it is typically controlled at 7.0–15.0 g/10 min. It must be well matched with processing equipment; otherwise, tube formation quality may be affected.
Shrinkage:
Thermal shrinkage behavior affects fiber excess length distribution inside the tube, which in turn influences microbending loss and low-temperature performance. It is a critical factor for stable optical transmission.
Hot Water Aging Resistance:
Ester bonds in PBT molecular chains may undergo hydrolysis under high temperature and high humidity, leading to performance degradation. Accelerated aging using pressure vessel tests, evaluating intrinsic viscosity and mechanical property retention, is commonly used to assess long-term reliability. This is also one of the reasons why PBT is widely used in underground and harsh-environment optical cables.
Alternative Materials and Modifications for Special Applications
Not all applications are suitable for pure PBT. Depending on environmental requirements, alternative materials and modification technologies are used as complements:
PP (Polypropylene):
PP offers better hydrolysis resistance and good flexibility. However, due to its low polarity, compatibility with filling compounds depends on specific formulation systems and must be carefully evaluated.
PA12 (Polyamide 12):
PA12 was used in early loose tube designs, but due to its lower modulus and higher cost, it has been largely replaced in mainstream applications. It is now mainly used in niche applications requiring high flexibility.
Modification Approaches:
The most common improvement in anti-bending performance comes from blending PBT with TPEE (Thermoplastic Polyester Elastomer). The hard-segment/soft-segment structure improves repeated bending resistance, meeting requirements for cable jointing and dynamic routing.
In addition, PET/PBT blending systems are also being explored to balance performance and cost.
Key Performance Requirements of Filling Compounds (Cable Jelly)
The filling compound inside the tube is a critical protective medium for optical fibers, and its performance is mainly evaluated by the following:
Thixotropy:
It behaves as a low-viscosity fluid under shear stress for easy filling, and then quickly returns to a gel state when static, providing long-term cushioning and mechanical protection for fibers.
Hydrogen Evolution (Hydrogen Generation Level):
Hydrogen ingress into optical fibers increases transmission loss. Therefore, filling compounds must exhibit very low hydrogen generation. High-end products may include hydrogen scavengers to further reduce risk.
Cleanliness and Compatibility:
The compound must be uniform, free of impurities and air bubbles, and chemically compatible with fiber coatings and tube materials to avoid degradation or interaction effects.
From crystallization control of PBT, to optimization of modification technologies, and finally to filling compound performance, every step must be precisely controlled to ensure long-term stable optical transmission and provide a reliable foundation for communication networks.
Post time: May-28-2026