With the rapid development of communication technology in the last decade or so, the field of application of fibre optic cables has been expanding. As the environmental requirements for fibre optic cables continue to increase, so do the requirements for the quality of materials used in fibre optic cables. Fiber optic cable water-blocking tape is a common water-blocking material used in the fiber optic cable industry, the role of sealing, waterproofing, moisture and buffer protection in fiber optic cable has been widely recognized, and its varieties and performance have been continuously improved and perfected with the development of fiber optic cable. In recent years, the “dry core” structure was introduced into the optical cable. This type of cable water barrier material is usually a combination of tape, yarn or coating to prevent water from penetrating longitudinally into the cable core. With the growing acceptance of dry core fibre optic cables, dry core fibre optic cable materials are rapidly replacing the traditional petroleum jelly-based cable filling compounds. The dry core material uses a polymer that quickly absorbs water to form a hydrogel, which swells and fills the water penetration channels of the cable. In addition, as the dry core material does not contain sticky grease, no wipes, solvents or cleaners are required to prepare the cable for splicing, and the cable splicing time is greatly reduced. The light weight of the cable and the good adhesion between the outer reinforcing yarn and the sheath are not reduced, making it a popular choice.
2 The impact of water on the cable and water resistance mechanism
The main reason why a variety of water-blocking measures should be taken is that water entering the cable will decompose into hydrogen and O H- ions, which will increase the transmission loss of the optical fibre, reduce the performance of the fibre and shorten the life of the cable. The most common water-blocking measures are filling with petroleum paste and adding water-blocking tape, which are filled in the gap between the cable core and sheath to prevent water and moisture from spreading vertically, thus playing a role in water blocking.
When synthetic resins are used in large quantities as insulators in fibre optic cables (firstly in cables), these insulating materials are also not immune to water ingress. The formation of “water trees” in the insulating material is the main reason for the impact on transmission performance. The mechanism by which the insulating material is affected by water trees is usually explained as follows: due to the strong electric field (another hypothesis is that the chemical properties of the resin are changed by the very weak discharge of accelerated electrons), water molecules penetrate through the different numbers of micro-pores present in the sheathing material of the fibre optic cable. The water molecules will penetrate through the different number of micro-pores in the cable sheath material, forming “water trees”, gradually accumulating a large amount of water and spreading in the longitudinal direction of the cable, and affecting the performance of the cable. After years of international research and testing, in the mid-1980s, to find a way to eliminate the best way to produce water trees, that is, before the cable extrusion wrapped in a layer of water absorption and expansion of the water barrier to inhibit and slow down the growth of water trees, blocking water in the cable inside the longitudinal spread; at the same time, due to external damage and infiltration of water, the water barrier can also quickly block the water, not to the longitudinal spread of the cable.
3 Overview of the cable water barrier
3. 1 Classification of fibre optic cable water barriers
There are many ways of classifying optical cable water barriers, which can be classified according to their structure, quality and thickness. In general, they can be classified according to their structure: double-sided laminated waterstop, single-sided coated waterstop and composite film waterstop. The water barrier function of the water barrier is mainly due to the high water absorption material (called water barrier), which can swell rapidly after the water barrier encounters water, forming a large volume of gel (the water barrier can absorb hundreds of times more water than itself), thus preventing the growth of the water tree and preventing the continued infiltration and spread of water. These include both natural and chemically modified polysaccharides. Although these natural or semi-natural water-blockers have good properties, they have two fatal disadvantages: 1) they are biodegradable and 2) they are highly flammable. This makes them unlikely to be used in fibre optic cable materials. The other type of synthetic material in the water resist is represented by polyacrylates, which can be used as water resists for optical cables because they meet the following requirements: 1) when dry, they can counteract the stresses generated during the manufacture of optical cables; 2) when dry, they can withstand the operating conditions of optical cables (thermal cycling from room temperature to 90 °C) without affecting the life of the cable, and can also withstand high temperatures for short periods of time; 3) when water enters, they can swell rapidly and form a gel with a speed of expansion. 4) produce a highly viscous gel, even at high temperatures the viscosity of the gel is stable for a long time. The synthesis of water repellents can be broadly divided into traditional chemical methods – reversed-phase method (water-in-oil polymerization cross-linking method), their own cross-linking polymerization method – disk method, irradiation method – “cobalt 60” γ-ray method. The cross-linking method is based on the “cobalt 60” γ-radiation method. The different synthesis methods have different degrees of polymerisation and cross-linking and therefore very strict requirements for the water-blocking agent required in water-blocking tapes. Only very few polyacrylates can meet the above four requirements, according to practical experience, water-blocking agents (water-absorbing resins) can not be used as raw materials for a single part of the cross-linked sodium polyacrylate, must be used in a multi-polymer cross-linking method (i.e. a variety of part of the cross-linked sodium polyacrylate mix) in order to achieve the purpose of fast and high water absorption multiples. The basic requirements are: the water absorption multiple can reach about 400 times, the water absorption rate can reach the first minute to absorb 75% of the water absorbed by the water resist; water resist drying thermal stability requirements: long-term temperature resistance of 90°C, the maximum working temperature of 160°C, instantaneous temperature resistance of 230°C (especially important for photoelectric composite cable with electrical signals); water absorption after the formation of gel stability requirements: after several thermal cycles (20°C ~ 95°C) The stability of the gel after water absorption requires: high viscosity gel and gel strength after several thermal cycles (20°C to 95°C). The stability of the gel varies considerably depending on the method of synthesis and the materials used by the manufacturer. At the same time, not the faster the expansion rate, the better, some products one-sided pursuit of speed, the use of additives are not conducive to hydrogel stability, the destruction of the water retention capacity, but not to achieve the effect of water resistance. 3. 3 characteristics of the water-blocking tape As the cable in the manufacturing, testing, transportation, storage and use of the process to withstand the environmental test, so from the perspective of the use of optical cable, the cable water-blocking tape requirements are as follows: 1) appearance fiber distribution, composite materials without delamination and powder, with a certain mechanical strength, suitable for the needs of the cable; 2) uniform, repeatable, stable quality, in the formation of the cable will not be delaminated and produce 3) high expansion pressure, fast expansion speed, good gel stability; 4) good thermal stability, suitable for various subsequent processing; 5) high chemical stability, does not contain any corrosive components, resistant to bacteria and mould erosion; 6) good compatibility with other materials of optical cable, oxidation resistance, etc.
4 Optical cable water barrier performance standards
A large number of research results show that unqualified water resistance to the long-term stability of the cable transmission performance will produce great harm. This harm, in the manufacturing process and factory inspection of optical fiber cable is difficult to find, but will gradually appear in the process of laying the cable after use. Therefore, the timely development of a comprehensive and accurate test standards, to find a basis for the evaluation of all parties can accept, has become an urgent task. The author’s extensive research, exploration and experiments on water-blocking belts have provided an adequate technical basis for the development of technical standards for water-blocking belts. Determine the performance parameters of the water barrier value based on the following:
1) the requirements of the optical cable standard for the waterstop (mainly the requirements of the optical cable material in the optical cable standard);
2) experience in the manufacture and use of water barriers and relevant test reports;
3) research results on the influence of the characteristics of water-blocking tapes on the performance of optical fibre cables.
4. 1 Appearance
The appearance of the water barrier tape should be evenly distributed fibres; the surface should be flat and free from wrinkles, creases and tears; there should be no splits in the width of the tape; the composite material should be free from delamination; the tape should be tightly wound and the edges of the hand-held tape should be free from the “straw hat shape”.
4.2 Mechanical strength of the waterstop
The tensile strength of the waterstop depends on the method of manufacture of the polyester non-woven tape, under the same quantitative conditions, the viscose method is better than the hot-rolled method of production of the product tensile strength, thickness is also thinner. The tensile strength of the water barrier tape varies according to the way the cable is wrapped or wrapped around the cable.
This is a key indicator for two of the water-blocking belts, for which the test method should be unified with the device, liquid and test procedure. The main water-blocking material in the water-blocking tape is partly cross-linked sodium polyacrylate and its derivatives, which are sensitive to the composition and nature of water quality requirements, in order to unify the standard of the swelling height of the water-blocking tape, the use of deionised water shall prevail (distilled water is used in arbitration), because there is no anionic and cationic component in deionised water, which is basically pure water. The absorption multiplier of water absorption resin in different water qualities varies greatly, if the absorption multiplier in pure water is 100% of the nominal value; in tap water it is 40% to 60% (depending on the water quality of each location); in seawater it is 12%; underground water or gutter water is more complex, it is difficult to determine the absorption percentage, and its value will be very low. To ensure the water barrier effect and life of the cable, it is best to use a water barrier tape with a swelling height of > 10mm.
Generally speaking, the optical cable does not contain the transmission of electrical signals of the metal wire, so do not involve the use of semi-conducting resistance water tape, only 33 Wang Qiang, etc.: optical cable water resistance tape
Electrical composite cable before the presence of electrical signals, specific requirements according to the structure of the cable by the contract.
4.4 Thermal stability Most varieties of water-blocking tapes can meet the thermal stability requirements: long-term temperature resistance of 90°C, maximum working temperature of 160°C, instantaneous temperature resistance of 230°C. The performance of the water-blocking tape should not change after a specified period of time at these temperatures.
The gel strength should be the most important characteristic of an intumescent material, while the expansion rate is only used to limit the length of initial water penetration (less than 1 m). A good expansion material should have the right expansion rate and high viscosity. A poor water barrier material, even with a high expansion rate and low viscosity, will have poor water barrier properties. This can be tested in comparison with a number of thermal cycles. Under hydrolytic conditions, the gel will break down into a low viscosity liquid which will deteriorate its quality. This is achieved by stirring a pure water suspension containing swelling powder for 2 h. The resulting gel is then separated from the excess water and placed in a rotating viscometer to measure the viscosity before and after 24 h at 95°C. The difference in gel stability can be seen. This is usually done in cycles of 8h from 20°C to 95°C and 8h from 95°C to 20°C. The relevant German standards require 126 cycles of 8h.
4. 5 Compatibility The compatibility of the water barrier is a particularly important characteristic in relation to the life of the fibre optic cable and should therefore be considered in relation to the fibre optic cable materials involved so far. As compatibility takes a long time to become apparent, the accelerated ageing test must be used, i.e. the cable material specimen is wiped clean, wrapped with a layer of dry water-resistance tape and kept in a constant temperature chamber at 100°C for 10 days, after which the quality is weighed. The tensile strength and elongation of the material should not change by more than 20% after the test.