IEC TR 62959-2021 Optical fibre cables – Shrinkage effects on cable and cable element end termination – Guidance.
4 Abbreviated terms
CTE coefficient of thermal expansion
FMC field mountable connector
FMS fibre management system
HFFR halogen free flame retardant
LSZH low smoke zero halogen
ODFM optical distribution frame module
5 Characteristics of optical fibre cables
5.1 General
For continuously good optical cable performance, the materials, design and manufacturing of the cable should be optimised. Subclauses 5.1 to 5.5 give detailed information about these factors.
5.2 Cable materials
5.2.1 Plastic materials
Many different plastic materials, primarily thermoplastics, are optimised for commercially available extrusion processes. Some are specifically promoted as having a low post-extrusion shrinkage. Nonetheless, all extruded plastic materials expand and contract reversibly and shrink irreversibly.
It should be noted that plastic materials used for optical fibre cables have to meet many more requirements beyond shrinkage, depending on customer technical requirements and local market conditions and regulations. This can include, but is not limited to: free of hazardous substances and halogens, high tensile strength, good UV resistance, good weathering and abrasion resistance, high flame retardancy, high thermal stability, good bend behaviour, easy strippability of the cable sheath and fibre buffer and several other attributes.
5.2.2 Reversible thermal expansion and contraction
Temperature changes cause thermal expansion or contraction of materials. Each material has a certain linear coefficient of thermal expansion (CTE). Typical coefficients of ten materials are listed in Table 1 .
Because different materials are used within cables, when the temperature changes, the cable elements and the sheath expand or contract differentially. If the elements cannot move freely, forces are generated within the cable. If the fibre is stressed by such forces, then optical performance can degrade temporarily. After the temperature reverts to its original value, cable elements return to or close to their original lengths, unless they have undergone shrinkage or are restrained by internal coupling. This reversible thermal material dimension change is seldom independently addressed as a cable characteristic. Annex B describes a suitable test method for determination changes in cable sheath length, and optionally cable’s elements, on short cable samples during a climatic exposure test. Information about the thermal expansion and contraction can be helpful when classifying a cable and to understand the higher attenuation observed during climatic tests.
5.2.3 Irreversible thermal contraction (shrinkage) Irreversible thermal contraction is specifically relevant for extruded plastic materials in optical fibre cables. During the cooling stage of an extrusion process, the polymer orientation is “frozen”. If the extruded material is exposed to a high temperature, or kept for a long time at room temperature, the frozen-in polymer orientation can relax, and the extruded plastic material can shrink in direction of the extrusion in an irreversible way [6] 1 . The amount and speed of post-extrusion shrinkage can be influenced significantly by the process parameters during extrusion and by the choice of the base material. Zero or negligible shrinkage can be achievable in some cases.IEC TR 62959 pdf download.